Modern practices in electrophoretic deposition to manufacture energy storage electrodes

The applications of electrophoretic deposition (EPD) to the development of electrochemical energy storage (EES) devices such as batteries and supercapacitors are reviewed. A discussion on the selection of parameters for optimizing EPD electrode performance, such as light‐directed EPD, co‐deposition of active materials such as metal oxides and materials manufactured with high porosity and fibrous properties is highlighted. Additionally, means for overcoming obstacles in the improvement of the mechanical properties, conductivity and surface area of EES materials are discussed. The exceptional benefits of EPD such as low cost, small processing time, simple apparatus requirements, homogeneous coatings, binder‐free deposits and selective modification associated with thickness and mass loadings leading to effective EES electrode materials are highlighted. Finally, EPD processes have evolved as modern manufacturing tools to produce technologically improved solid electrolytes and separators for lithium‐ion and/or sodium‐ion batteries, and further research and development programmes are encouraged towards industrialisation.


| INTRODUCTION
Electrochemical energy storage (EES) plays a significant role at scales as large as electric grid balancing down to everyday power electronic devices, [1][2][3][4][5][6] in addition to the extensive application of batteries and supercapacitors in electric vehicle development over the years. 7,8 They are crucial for economies such as the United Kingdom to achieve sustainability and carbon emission reduction. 9 Similarly, the UNFCCC (during the Paris Climate Conference in 2015) has encouraged global leaders from the developing nations to consider reducing greenhouse gas emissions such that global average temperature is allowed to settle at well below 2 C above pre-industrial levels. 10 Therefore, much research work in recent years has focused on developing ground-breaking concepts for membranes and electrode materials for various EES devices. 11 These key components are designed in such manners that the mechanism of charge storage in batteries and supercapacitors are different; battery materials are aimed towards high energy density applications, whereas supercapacitor materials are designed for high power density use. 12 In all such cases, electrode materials play one of the most important roles in enhancing the performance of EES systems from the viewpoint of long cycling life, high specific capacity and/or high rate capability. [13][14][15][16] Electrodes for energy storage have classically been prepared in various ways in both academia and industry such as slot-die coating or slurry casting. 2 In these methods, electrode materials are dispersed/dissolved in a solvent to form a viscous slurry, and a film is obtained after coating and solvent evaporation. In spite of that, it is not easy to optimize thickness control or film assembly efficiency. Furthermore, conventional techniques suffer from significant drawbacks when it comes to preparing nanostructured energy materials owing to a lack of control in the effective dispersion of the nanoparticles in viscous slurry pastes leading to disadvantageous agglomeration. This is mainly applicable for the cases of chemical and thermal methods, which are susceptible to adulteration of the active materials with unwanted impurities, thereby decreasing electrode performance. 17,18 Alternatively, methods reported for manufacturing microbatteries, which include radio-frequency sputtering, physical and/or chemical vapour deposition (CVD) or pulsed laser deposition, suffer from high processing costs that affect their applicability at scale. 19 Considering the aforementioned approaches for manufacturing effective electrode materials, it is imperative to find versatile and cost-effective processes that do not compromise EES performance. In this regard, electrophoretic deposition (EPD) has been reported to possess several benefits, including scalability, facile and economical set-up, as well as the ability to prepare electrode materials in relatively small time scales. 20 The mechanism of EPD is summarised in Figure 1. 21 Electrophoresis occurs when charged suspended particles in a suitable solvent migrate towards an oppositely charged surface upon application of an electric field between two electrodes. The charged materials migrate towards the deposition electrode and aggregate into a relatively dense and uniform film (known as deposition). The EPD technique may, therefore, be applicable for any solid material in fine powder form (eg, ranging from micro-to nanostructures) or as a colloidal formulation that may involve polymers, metals, ceramics and glasses. 22 EPD was discovered in 1808 (see the following section for a historical overview) and was modelled for the first time by Hamaker in 1940. The requirement for stable suspensions to form an adhering sediment due to gravitation when allowed to stand still has since been defined mathematically for the EPD technique. 23 Several authors have explained how the positively charged particles move towards the cathode (negative electrode) in the EPD bath (colloidal solvent formulation), thereby overwriting the coagulation mechanism without increasing electrolyte concentration at the electrode. 24 This follows up with a distortion and a reduction in the diffuse double layer (known as lyosphere) thickness owing to the combined effects of the applied electric field and fluid dynamics leading to coagulation of particles. Figure 1 summarises the main steps in an EPD process. 21 The equation inset in the figure relates to the electrolyte solvent and the resultant particle mobility. The deposition process itself is more complicated, and different theories F I G U R E 1 Diagram showing the electrical double-layer distortion and thinning mechanism during an EPD process. An important equation is also displayed that summarizes the key parameters that affect EPD performance. Reproduced with permission from Elsevier 21 have been proposed; the distortion and thinning of the particle double layer, the flocculation by the accumulation of particles, neutralization of the particle charges, and several other postulations that are described elsewhere. 22,25 Since EPD is affected by several parameters that are commonly related to each other (eg, a change of the suspension medium will produce a change of the viscosity, but also a change of the zeta potential of the particles due to a different particle-medium interaction), a correlation of the experimental parameters and deposition results is not straightforward and can suffer from inconsistencies. For this reason, comprehensive investigations of the EPD process are a key factor to produce coatings with preferred features depending on the desired application.
Conventional EPD employs pure water 26 or aqueousbased solutions 27 as suspension media, because the high dielectric constant(s) of these solvents facilitates the development of charge on the dispersed colloidal particles and therefore stabilizes the suspension. 28 For industrial purposes, aqueous suspensions have the advantage of high solid loadings and low environmental impact. 29 However, these solvents may undergo electrochemical reactions, such as the formation of ions as well as hydrogen and oxygen evolution at the electrode surfaces, which interfere with the EPD of the particles. 28 The use of nonpolar media (usually organic) minimizes the interference of electrochemical reactions, for example, water electrolysis, thereby circumventing turbulent flow and other hydrodynamic effects of the solvent. Moreover, nonpolar media provide more control over the film thickness and the roughness of the film surface than aqueous solutions. 30 As a consequence, more and more research investigations involving organic colloidal formulations are gaining popularity within the scientific community. 28 EPD is strongly dependent on the presence of a stable colloidal formulation. 19 The stability of a suspension, adjustable with surfactants, is correlated with electrostatic repulsive and van der Waals attractive interactions. The stability of suspension/colloid is characterized by zeta potential, the value of which represents the magnitude of repulsive interaction while the sign represents the migratory direction of particles. Factors that influence zeta potential include electrolyte pH, temperature and surfactant type. Other factors that affect particle migration in the EPD solvent such as the voltage field or deposition time (also dependent on electrolyte viscosity) are discussed in Section 3. 31 A typical EPD system includes a power supply, a conductive substrate and a counter-electrode. As the voltage is applied, active material particles in the liquid suspension are deposited on the substrate. Electrolyte sedimentation may be prevented by adding dispersing agents, 32 while conductive additives like Super P (carbon black) or acetylene black are often added to achieve deposits with good electrical conductivity. 19 The utilization of EPD for preparing EES devices relies on the direct deposition of active particles on the current collector. 19 Employing conductive substrates is a prerequisite for both EES devices and the EPD process. Combining EPD in electrode fabrication for EES applications also reduces the interfacial resistance and increases the mechanical flexibility of the electrodes. 31 In addition, EPD of binder-free films for rechargeable batteries 33 or supercapacitors 34 has been reported, demonstrating a promising possibility to increase the energy density of such EES materials. 19 The cycling performance or conversion efficiency can also be improved due to the densely packed EPD coatings. 35 Furthermore, controllable nanostructures can be realized using EPD and such electrodes usually exhibit improved performances when applied as micro-energy storage devices. 19 Although research on electrode materials is fast developing, the fundamental mechanistic understanding regarding how the design of EPD and properties of nanoand/or micro-structured electrodes prepared by EPD can improve electrochemical reaction efficiencies is still not fully understood. To the best of our knowledge, this is the first review article that explains in-depth the advantages of applying the versatile EPD process for specifically making and applying energy storage electrodes with relevance for both academia and industry. This is likely to be so because a general search of the literature on EPD processes for energy storage and conversion results in the identification of two review papers published in the recent past focussing mainly on academic studies, 19,36 while this paper focuses more stringently on EES devices and how they can be of benefit for industrial type applications. The most recently published review paper on EPD focuses on the progress and challenges of carbon nanotubes (CNTs), 20 which once again confirms the timely impetus of our work presented below.
To systematically illustrate the production convenience, versatility and high performance of EES electrode materials produced by EPD, studies involving lithium-ion batteries (LiBs), supercapacitors, redox flow batteries (RFBs) and regenerative fuel cells (RFCs) are highlighted in this paper. The basic comparison of these EES devices is discussed in some depth. Also, a relevant perspective is drawn for the potential application of EPD in other emerging EES technologies such as sodium-ion capacitors, magnesium-/aluminium-ion batteries and batterysupercapacitor hybrids. Finally, the opportunities and challenges associated with scaling-up the EPD technique for the production of industrially viable electrode materials for EES devices are detailed. This review paper also shows early evidence of the evolution of EPD processes and manufacturing tools to produce technologically improved solid electrolytes and separators for LiBs, and further research and development programmes are expected to support their industrialisation.
This review is divided into eight sections. Section 1 introduces this work and Section 2 describes a brief history of the EPD process along with a useful timeline up to the point where EPD was first reported to produce an energy storage electrode. Section 3 discusses the key principles behind the application of EPD to prepare surface coatings. Section 4 covers some industrial studies on EPD. Section 5 provides an overview underlying the successful use of EPD for producing an energy storage electrode. Section 6 discusses various in-depth applications of energy storage electrodes produced by means of EPD with relevant examples of LiBs, RFBs and supercapacitors. Section 7 discusses the remaining challenges of EPD for producing energy storage electrodes with some applications for sodium and magnesium ion batteries. Section 8 concludes this review and provides a short perspective.

| BRIEF HISTORY OF EPD
The electrophoresis phenomenon was originally reported by Reuss in 1808 who observed the electrical migration of suspended clay particles in water. 24 Since then, the first quantification of the phenomenon for thin double-layer electrophoresis was reported by Smoluchowski. 37 Smoluchowski's equation was further improved by Henry by means of a fitting parameter as a function of Debye length and particle size for taking into account changes in double-layer thicknesses. 38 More complicated mathematical formulations were developed since then by various scientists. For instance, Overbeek modelled the electrophoretic motion of a spherical particle in his PhD thesis in 1941. 39 Other researchers like Booth developed mathematical relationships describing how the initial application of an electric field on a colloidal liquid suspension affects the electrophoretic retardation force and relaxation effect (see Figure 2A for a historical timeline on electrophoresis). 40,41 This was followed by independent studies by White as well as Ohshima and O'Brien, who enhanced the theories for regimes to account for higher concentrations of particle and larger zeta potentials in electrolyte suspensions. [42][43][44] Deposition mechanisms were also proposed in the 1940s and beyond, such as EPD yield quantification by Hamaker followed by the effect of deposition substrate and consequently the electrode materials (eg, planar, 2D fibre mats and/or 3D porous materials) on deposited films. 38 Despite such progress, the literature lacks a converging viewpoint on the actual mechanism for the EPD process. In general, the scientifically accepted consensus banks on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory that attempts to quantify colloidal stability by accounting for the interaction potentials between particles and the consequent influence of the applied electric field on the electric double layer. 38 In particular, DVLO accounts for the opposing influences of van der Waals attractions and electrostatic repulsions on particle deposition. 39 Three factors play a significant role as explained by the DLVO theory and these include the Hamaker constant (a function of van der Waals forces), particle surface potential (a function of electrostatic forces), and electrolyte concentration (accounting for electrostatic effects upon Debye length). 37 A successful EPD is postulated to be influenced by the electric field applied that disrupts the electrolyte balance close to the electrode materials-this occurs by means of an increase in concentrations of counter-ions along with a corresponding decrease in co-ions concentrations around the electrode surfaces. Therefore, the double layer associated with the colloidal particle as well as the Debye screening length changes significantly enough to cause colloidal destabilization and deposition. The accumulation of particles due to a flocculation phenomenon is also considered to be a significant theory reported in several EPD-based literature. 23 In particular, this postulation accounts for the fact that van der Waals forces between particles apply a considerable influence on the particles when they are within a certain critical range, resulting in flocculation of the particles and consequently deposition on a charged surface (see Figure 1). However, to bring the particles within the aforementioned van der Waals range, the particles need to be pressurized by means of electrostatic forces of attraction to overpower opposing repulsive electrostatic forces and compel them to move within the critical range for deposition to occur.
EPD has been successfully used in the past for various ceramics-based applications, 45,46 but has recently been employed for a range of different exciting applications (see Figure 2). Examples include porous electrodes and membranes for batteries, capacitors, and fuel cells [47][48][49] (see Figure 2B that illustrates the rapid growth in the publication of EPD for making energy storage electrodes up to 2020). For instance, Wang et al 50

| PRINCIPLE OF ELECTROPHORETIC DEPOSITION (EPD) TO PRODUCE SURFACE COATINGS
In most metal-based applications, coatings are an essential part of the protection of the material's surface from wear and corrosion. Coating methods may be divided into two according to the phase from which the active material is deposited onto the respective material surface. Such coating processes are ubiquitous owing to the big diversity of applications. These processes contain various online/offline parameters that are functions of material microstructure, process effectiveness and other relevant factors. 51 A small selection of deposition techniques to produce surface coatings is summarised in Figure 3A. Some examples of gaseous phase depositions involve physical vapour deposition (PVD), plasma spraying and CVD, while a few examples of liquid phase deposition include thermal spraying, dip coating and electrochemical methods. 54 Among liquid phase surface coating processes, electrochemical deposition and EPD ( Figure 3B) ensure cost-effective and controlled material films (≈25-100 nm) at high deposition rates (≈240 μm thickness in 1 hour). The fact that EPD can be performed successfully to prepare highly functional materials on a range of different deposition substrates at room temperature and pressure displays its excellent features. 55,56 Therefore, in recent years, these deposition processes, using hard particle dispersion in the metallic matrix, have attained widespread importance in chemical, mechanical and electrical applications. 54 In other words, EPD as a suspension-based colloidal manufacturing process meets the criteria for inexpensive mass production of surface coatings and objects for additive manufacturing. 28 This technique offers a number of advantages including high efficiency, convenience and cost-effectiveness, and allows for versatile manipulation of micro/nanoparticles in a colloidal solution. 19 Surface coating thickness, particle size and inter structure of surface coatings produced by means of EPD are controllable by adjusting the suspension/process parameters. 57 Efficient production of surface coatings typically requires a good substrate. 19 With excellent conductivity and surface F I G U R E 3 (A) Flow chart showing some fabrication methods for thin-film surface coatings. With permission from InTech Open. 52 (B) Simplified diagrams of two types of cathodic electrodeposition processes: (A) electroplating and (B) electrophoretic deposition (EPD). Reproduced with permission from InTech Open 53 evenness, indium tin oxides (ITOs), metal layers and graphite sheets are commonly used as substrates. In addition, the standard of the deposited films correlates closely with the dimensions and therefore the surface features of EPD precursors. 58 Owing to the fact that EPD is a low-cost non-line-ofsight coating process, it is very likely to produce coatings of uniform dimensions on complex substrates with uneven morphology. In addition, high-quality film coatings with tailored morphology specific to the required application may be designed by means of EPD-the versatility of the process is highlighted by the fact that film thicknesses can vary from nano-to millimetre-scale that has a direct effect on the mass loadings. 59 Thus, this versatile process is capable of manufacturing polymer coatings for a wide range of applications, one example being the biomedical industry. 60 This is drawn from the simplicity of EPD to operate in mild aqueous/organic environments at low temperatures and pressures, 61 with excellent efficiency, 62 and short processing times 63 that facilitate a facile scale-up towards commercial production. 64 Despite the aforementioned exceptional features, EPD faces such drawbacks as poor film/coating adhesion, challenges in morphology control, and rates of deposition that are far from linear. 28 This is particularly influenced by the time dependency of crucial EPD processing parameters that include colloidal concentration, composition, electric field effects, pH, electrophoretic mobility, etc., which, in turn, severely impact the deposited surface films/coatings quality. 25 Besides these parameters that significantly impact upon the quality of surface coatings, there are several fundamental factors that need particular attention and are reviewed in detail in other relevant papers. 19,21,25,36

| CURRENT DEVELOPMENT IN EPD INDUSTRIAL ACTIVITIES
EPD technology is industrially scalable and involves simple processing methodologies to manufacture at scale. This technology has been employed for industrial electrophoretic painting purposes from 1970 onwards ( Figure 2A). The low levels of pollution, ease of digital automation, and the ability of EPD to produce homogeneous functional coatings are advantages that have enabled its widespread adoption in painting car bodies on an industrial scale. 65 It continues to excel in depositing films with controllable microstructures as a function of the processing conditions. Thus, there is great potential to apply this highly versatile technology for largescale processes despite a lack of a holistic understanding of the interaction of complex factors that influence EPD. This suggests an exciting opportunity for EPD technology to manufacture an uncountable number of electrode designs, for tailored energy storage performances that will be reviewed later.
There is a growing interest in using EPD as a manufacturing method because of its compatibility with a vast material set including metals, 66 ceramics, 24,67 biomaterials, 68 polymers, 69 semiconductors 70 and composites, 71 and its speed, with depositions, typically lasting from seconds to minutes that can yield composite functional materials with several centimetres of thicknesses. 72 It has been successfully used in a wide variety of applications such as solid-state lighting, 73 reactive materials, 74 energy storage, 75 drug delivery, 76 and photovoltaics. 77 Figure 4 highlights some of these applications, which are discussed in depth in a recent book chapter. 78 EPD can be extensively applied for industrial coatings onto conductive substrates, 79 such as for UV curable coatings. 65 EPD coatings on relevant substrates provide enhanced chemical shielding, thereby preventing corrosion and discoloration unlike what is noted for their anodic counterparts. 80 During the use of EPD for painting automobiles, entire car bodies or their assemblies are dipped into a liquid vessel in order to prevent the coated substrate from future corrosion. 81 However, there are several overhanging problems that need attention. For instance, the effect of trapped air bubbles during the dipping process may cause non-uniform coating owing to poor coverage of the EPD solvent on the charged body surface. Additionally, liquids that remain entrapped after completing the aforementioned process are likely to cause corrosion in followup processing steps. Moreover, automotive parts containing rubber, plastics and other components are deformed easily when subjected to high-temperature baking, which itself increases the manufacturing cost due to energy and resource wastage. As a consequence, new methods of drying such as UV curing have been applied successfully in several industrial EPD activities in the last decade. 65 UV curing has some unique advantages such as high-curing speed, solvent-free formulations (reduced emissions of volatile organic carbon), and lowtemperature processing with minimal energy consumption. [82][83][84] Some investigators applied EPD electrodes with special features such as the ability to change position during EPD and/or the ability to be reconfigured in order to form free-standing films or turbulent gradients that is normally not attainable under standard conditions. 67 Of particular note, Anné et al applied a series of four counter-electrodes to prepare a functionally graded netshaped femoral ball head. 85 Nold and co-workers were the first to explore the reconfigurable electrode concept when they reported the use of both a movable coaxial cable electrode set-up as well as an addressable electrode array to localize deposits onto a surface to create an EPD computer-aided manufacturing system. 86 Pascall and coworkers demonstrated the formation of composites with gradients both in-plane and along the deposition direction using light-directed EPD, 87 as discussed in the following paragraph.
As mentioned above, EPD is well-suited for multimaterial deposition and has been proven at industrial scale, but patterning of fine features and voids generally has not been possible. 67 Pascall and co-workers previously demonstrated a new additive manufacturing technique called 'Light-Directed EPD', which involves the use of photoconductive electrodes, laser-cut photomasks and UV light to locally control the electric field. 87,88 The use of laser-cut photomasks, while reconfigurable, limits the scalability, the ability to be automated, and the resolution of the technique to 100 seconds of μm. 67 A more recent work expands on that of Pascall and co-workers' by moving to a projection-based system that not only removes the need to produce physical masks, but additionally enables an adaptable depositing pattern to form as a function of processing parameters. This opens the door to rapid fabrication of complex structures over large build areas. Therefore, a free-standing bridge structure produced via EPD was demonstrated recently. 67 In another example of the industrial application of EPD, a jet engine used up about 1.8 tonnes of nickel alloys, which made it possible for the plane to complete nearly 20 000 flight hours without significant maintenance, a considerable achievement for EPD in the aerospace industry. 89 Besides the numerous applications of Ni, the destruction of these appliances (aircrafts, electromagnetic devices, supercapacitors, sensors, etc.) by wear, corrosion and fatigue is of a major concern. 54 Since the protection of these crucial components is critical, electrophoretically deposited Ni-carbonaceous coatings have presented glowing properties, which make them a reliable coating variant that can protect bare Ni during everyday use as well as in corrosive environments. 54 Another popular industrial application of EPD involves physical synthesis techniques for combining biochar with graphene for potential applications in supercapacitors. 90 For instance, for manufacturing graphenecoated biochar, the charged particles migrate under an electric field and deposit onto the electrode of the opposite charge. However, graphene is electrically neutral. Therefore, graphene oxide (GO) is used instead because it contains charged functional groups, mostly negatively charged. After the GO is electrophoretically deposited on the biochar, it is subjected to chemical or electrochemical reduction to obtain rGO, since GO is electrically insulating. In previous studies, GO has been electrophoretically deposited on CNTs and carbon fibres. 91,92 A more recent study aimed to improve the electrical conductivity of pinewood-derived biochar via EPD of GO followed by electrochemical reduction. 90 The loading amount of rGO was affected by the concentration of the GO suspension, while the electrochemical reduction voltage was varied to control the reduction degree. The electrode conductivity was influenced by rGO content as well as the quality of its reduced state. A specific capacitance of 167 F g À1 is reported after depositing rGO in comparison to 39 F g À1 for a supercapacitor using biochar electrodes in the F I G U R E 4 Some of the scalable applications of EPD for potential industrial use. Reproduced with permission from Elsevier 78 absence of rGO (>4 times higher capacitance in the presence of rGO). Furthermore, the supercapacitor can undergo 10 000 charge/discharge cycles without displaying capacitance fade at a relatively high current density of 5 A g À1 .
The next section reviews the application of EPD for energy storage systems. Following onwards, detailed aspects of three exemplary EES devices are considered including RFBs, lithium-ion technologies and supercapacitors.

| OVERVIEW OF EPD TO MANUFACTURE EES DEVICES
Classical electrode fabrication in any research area mainly relies on various deposition techniques such as spray deposition, sputtering (eg, radio-frequency sputtering), electrochemical deposition, CVD, EPD, atomic layer deposition, pulsed laser deposition, sol-gel, layer-by-layer deposition and spin-coating. 93 These physicochemical deposition methods result in simple and versatile means for manufacturing EES electrodes. Despite that, such deposition methods face several limitations such as complicated material handling procedures, and challenging thickness and uniformity control of thin films when used for on-chip integration and large-scale manufacturing applications. 94 Industrial EES devices are manufactured in general by means of high-speed roll-toroll processing, which commences with chemicals deposition, rolling of electrodes, roll slicing and then proceeds into the construction of the EES cell by combining the electrodes with a separator. The follow-up processes involve electrolyte addition and soaking of electrodes and separator, and ultimately packaging along with relevant incisions based upon commercial standards. 93 This elaborate process, which was originally adopted in the 1980s and 1990s from Japanese magnetic tape factories, requires significant amounts of times to manufacture an electrode and faces obstacles in scaling-up processes, including costly expansion of production lines. Automatic and sophisticated industrial production steps can significantly reduce the upfront investment and manufacturing time. 95 In other words, conventional electrode manufacturing steps such as blade-coating (whereby a slurry suspension is prepared by physically mixing the active material with binder and conductive agent, for application over a current collector surface) face limitations related to film assembly efficiency and thickness control. 96 Unfortunately, the EES device usually has lower energy and tap densities in addition to high interfacial resistance as a consequence of the presence of non-active substances. 19 Such disadvantages could easily be circumvented by manufacturing high-performance binder-free electrodes by means of EPD (with its unique advantage of ensuring decent control of surface coating thickness and mass loadings). 96 In fact, the need of using binders in the electrode production process adds an extra cost to the process. 97 Additionally, the drying process of the widely used solvent N-methyl pyrrolidone (NMP) is time-consuming and expensive, especially at large scale. Drying can take a long time for some electrodes and can vary between 12 and 24 hours at 120 C or 60 C under vacuum. 84,[98][99][100] In the EES manufacturing industry, recycling and recovery of the NMP solvent from the drying step is essential because the substance is not only dangerous to handle but also a potential threat to the environment (due to its toxicity). 101,102 High capital investments are also needed to introduce NMP recovery systems, which offset any economic benefits arising from recycling and re-use of NMP. 103 Such an extensive drying step is less important for electrodes made via EPD, especially if applying low boiling point organic solvents as the colloidal media.
One inferred disadvantage of EPD could be the increasing electrical resistance of the deposits that could ultimately stop the EPD process. This 'thickness limiting' effect is ideal for depositing onto the surface of an irregular 3D structure. 104 If the deposits were a film of porous layer (which is the case in battery and supercapacitor electrodes), the voltage drop across the layer will remain very low because of the presence of conductance pathways available throughout the accessible porous film. 105 If a sufficiently conductive electrolyte is used, the layer's electrical resistance will not limit its growth. This suggests the possibility of an unlimited layer thickness. Biest and co-workers have confirmed that porous deposits (10%-60% porosity usually) with thicknesses reaching several millimetres can be achieved with relative ease. 106 This suggests the manufacture of 100 mm thick electrodes for practical energy storage is encouragingly possible using EPD technology. [107][108][109] The process also offers improved electronic and physical connectivity to the current collector surface by creating an electrochemical bond unlike a physical one as in standard slurry castbased coating. 110,111 In conclusion to this section, it is noted that EPD for producing energy storage electrodes is a widely researched topic with some recent reviews. 20,112,113 Most of these academic assessments have tended to remain rather general, with a specific focus on the combination of both energy conversion and EES devices. The specific details related to improving the EPD parameters for energy storage electrodes have not been evaluated in much depth. Therefore, this elegant area of research is anticipated to undergo further investigations. The following section expands upon the various applications of EPD in specific EES devices.

| RESEARCH, DISCOVERY AND INNOVATION ACTIVITIES ON EPD FOR EES DEVICES
Since the 1990s, significant research studies on EES devices have been conducted with a special focus on advanced electrodes design. 114,115 The use of EPD for such purposes is gaining momentum in recent times due to good control of the electrode uniformity and thickness, resulting in highly structured porous electrodes with complex 3D geometry. 116,117 Furthermore, EPD can easily be scaled-up for meeting demands for large electrode sizes and volumes by effectively controlling the colloidal processes that influence the deposition process (see Section 3 for more details). 21,73 In recent times, studies for the design of advanced electrodes for EES devices have increased significantly because of the excellent processing techniques used in the manufacturing activities to enable high performance (see Figure 2B showing a rising trend in the number of publications in this field of research). 118 Therefore, there is a general scientific consensus that additional research on EES devices is necessary. 25 Needless to say that in this regard, EPD has produced promising results for EES devices. 19 Presently, such devices have been dynamically engaged for electric vehicle (EV) purposes, but EVs nonetheless need low-cost and robust energy sources and/or storage means to reduce operating costs. The development of the EES technology for diverse uses is a means to minimize such issues and EPD can play a major role in this regard. The following subsections discuss some of the advances made in the EPD of electrodes for some key applications.

| Lithium-ion batteries (LiBs)
LiBs are important energy storage devices for vehicle electrification and portable electronics. 119 Today's LiB electrodes are manufactured by a standard slurry casting process (also known as doctor blade method). The process involves mixing electrochemically active materials, electrically conductive carbon particles and binders in a liquid media, which are then physically cast as a slurry onto metal foils, such as Cu, stainless steel and Al, followed by drying and compression. Despite its significant application, it has been optimized by trial and error, which limits the quality of the manufactured electrodes.
For instance, they face drawbacks such as occlusion of the surface of active materials that result in an unwanted drop in the material conductivity and also reduce the gravimetric capacity. 120 As a commonly used binding material, polyvinylidene fluoride (PVDF) is not amenable and thus is unable to sustain wear and tear due to expansion and contraction processes as a consequence of LiB charge and discharge, resulting in a weakening of bonds between the conductive carbon material, active cathode material and the current collector. This, in turn, affects long-term performance such as capacity fade upon cycling. Furthermore, NMP solvent, which dissolves PVDF effectively, is costly, poisonous and may affect pregnant women adversely. 120 Additionally, binders like PVDF have been reported to catalyse thermal runaway reactions in LiBs. 121 Likewise, battery manufacturing costs require further reduction-therefore replacing NMP-based slurry casting techniques is an essential requirement. Especially, binder-free immobilization of the active materials on the electrode surface has great importance in reducing cost and the effects of impurities. 122 In standard studies involving flexible LiBs, the doctor blade technique, CVD, physical vapour deposition and nanofabrication, among others, are usually harnessed for preparing flexible electrodes. 123 These processes work well for lab-scale prototypes, but are found significantly lacking and also expensive when applied for industrialscale mass production of flexible electrode materials (that also need to be sufficiently adhesive to current collectors). 124,125 CVD is an expensive technique because thermal curing of the substrate during the course of deposition or annealing of the obtained coating is crucial for enhanced battery performance. 124 Among other solution-based electrode processing techniques, EPD is currently gaining much attention for formulating LiB electrodes mainly due to its simplicity of operation and relatively low cost in comparison to aforementioned manufacturing techniques. 124,126 Additionally, EPD ensures great flexibility in manufacturing different classes of materials, while preserving structure and particle size of the commencing powders in the deposited surface coatings. 126 Mass loadings and film thicknesses of EPD electrode materials may be simply controlled by means of adjusting the interaction between applied current or voltage, composition of electrolyte (and/or colloid) as well as the time of deposition. 124 Recent research has demonstrated that new manufacturing methods, which include EPD, can produce improved electrodes with more capacity and greater power, which could enable vehicles to travel longer distances and result in higher performances. 127,128 This section will report on the latest findings from the literature regarding the manufacturing research of electrochemical engineering approaches to produce improved electrodes by means of EPD. For the EPD process, formulation of the colloidal electrolytes can be formed according to the solvents. Three kinds of solvents (aqueous, organic polar and nonpolar) can be used to obtain a colloidal electrolyte. Although nonpolar solvent enables more homogenous depositions, it needs elevated applied potentials. On the other hand, aqueous solutions can result in hydrogen and oxygen evolution side reactions beyond certain potentials. Therefore, the choice of solvents mainly depends on the formulation of the film which will be formed on the electrode surface.
The EPD process can directly allow a controlled placement of different shapes, sizes and types of materials at different positions to produce non-monolithic electrodes. 19 This can be easily achieved by controlling the properties of charged materials and their electric fields. The ability to deploy dilute colloidal electrolytes (eg, using <10 g dm À3 of materials), as opposed to the viscous slurry casting technique, drastically increases the technological applicability of EPD to produce electrodes with a higher fraction of materials density. This is because typically <1 wt% of inactive materials (ie, charging agent and polymer binder) are required to provide surface charge on the materials and adhesion property to the current collector surface, respectively. Putting this in the context of LiBs means higher density of active material electrodes can be readily deposited because only relatively tiny quantities of inactive materials are required for processing, as opposed to at least 5 to 30 wt% of inactive materials, which are needed for slurry casting techniques. 2 Very recently, Lalau and Low (WMG) 2 reported that EPD of LiN 1/3 Mn 1/3 Co 1/3 O 2 (NMC111) was carried out at 80 V constant voltage applied for 5 minutes using an NMP-based colloidal coating medium. Industrial-level mass loadings of above 30 mg cm À2 and film thicknesses exceeding 150 μm were deposited successfully on 3D Al mesh, which was found to be twice the mass loading and film thickness in comparison to that on 2D Al foil current collector. The LiBs with EPD electrodes distinctively exhibited high capacitance retention, albeit for shortcycling, which was a decent step towards future scale-up studies, especially with electrodes possessing high materials mass loadings and density (90 wt%), as well as thick surface coatings that met industrial standards (as shown in Figure 5). In a separate investigation, Seo and coworkers produced CNT/graphene nanosheet composites by means of EPD, applying 300 mA of a constant current (DC) for 1 hours, and used it successfully as a negative electrode in a LiB. 129 Uceda and co-workers performed EPD with lithium titanate oxide (LTO) nanoparticles that were carbon-coated to prepare highly conductive but thick LTO electrodes with very good rate capability when compared with their traditionally fabricated LTO counterparts. 130 Other researchers 125 prepared electrodes with LTO coatings of 3.3 μm thickness for micro-battery applications by means of EPD employing a 95/5 vol% ethanol/water mixture, a charging agent comprising entirely of magnesium chloride, and a throbbing potential. 130 The Mg 2+ cations (generated by means of cathodic hydrolysis were reported by Benehkohal and co-workers in a separate work and for a different cation) 131 were co-deposited as magnesium hydroxide (this acted as a binding agent to improve the LTO conductivity) along with the LTO. The film, however, became non-uniform with extended time resulting in an effective thickness limited to 6.5 μm. 130 A recent publication was reported on an EPD process that is capable of producing strongly adherent, and highly conductive LTO-Carbon (Acetylene Black) composite electrodes by applying C-coated LTO nanoparticles. 130 This accomplishment was realised by F I G U R E 5 Galvanostatic charge/discharge cycling of a LiB whose electrode materials were produced by means of EPD at WMG. C-rate evaluation of EPD NMC111 electrodes deposited on (A) Al 2D foil and (B) 3D Al mesh current collectors. Charging occurred at a constant C/10 rate, while discharging was performed at a range of different C-rates as displayed. (C) NMC's successive cycling performance parameters at C/5. Reproduced with permission from Wiley 2 employing a water soluble binder, namely styrene butadiene rubber (SBR), which is already well established in the commercial battery manufacturing sector. SBR acts as both a binder for the electrode assembly as well as a stabilizing agent for the nano-C-coated LTO-Carbon formulation. However, applying a common solvent to disperse LTO and carbon (two non-similar materials) is challenging, especially when it comes to preparing a stable suspension. After a thorough evaluation of solvents Uceda et al settled for a 10 wt% water diluted acetonitrile medium for the purposes. 130 The addition of SBR enhanced the kinetics of deposition of LTO/carbon material on the current collector, producing high-quality electrodes for LiB applications. For instance, the conductivity, as determined by electrochemical impedance spectroscopy (EIS), of the compressed EPD electrodes was 15 times better when compared with their counterparts prepared using traditional slurry cast methods employing PVDF binding materials. 2 In addition, rate capability studies showed higher capacitances for electrodes prepared by means of EPD. 130 This, therefore, paves a way forward for EPD to apply standard industrially relevant materials to produce high-value EES electrodes for commercial LiB applications.
A number of studies concentrated on EPD of LFP for manufacturing LiB electrodes, besides the thesis referred above. 120,132,133 Even though successful results were reported from such investigations, the authors highlighted some useful areas for enhanced research and development on the EPD technique. Some complications were highlighted due to the development of uniformly charged colloidal formulations during EPD of LFP with conductive additives, which negatively affected the adsorption of dispersants and binders on the particles, thereby inhibiting the formation of adherent surface coatings. 134 Some authors overcame most of these issues by using aqueous-based polyelectrolytes successfully such as poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO-Na) and carboxymethyl cellulose sodium salt (CMC-Na) for the EPD of a range of different substances, like CNTs, oxides, polymers and hydroxides. 135,136 In a more recent study, the EPD LFP electrode, displayed a capacity of 146.7 mA h g À1 at a C-rate of C/10 along with steady long-term charge/discharge successive cycling performance. 134 Similarly, Cohen and co-workers prepared a membrane-electrode-architecture by coating Celgard separators with LFP (positive electrode) and LTO (negative electrode) by means of a concurrent EPD method. 124 The LiB with this three-layered electrochemical assembly delivered capacities ranging from 125 to 140 mA h g À1 at various C-rates, as shown in Figure 6.
Yilmaz and co-workers recently prepared a highly conductive three-dimensional cathode having an array of micropatterned vertically aligned CNTs on Al foil by means of EPD. 120 The 3D structure's practical usefulness was shown by processing commercially available LFP powder with two different methods-EPD and vibrationassisted drip-coating. EPD cathodes displayed a high specific capacity of 143 mAh g À1 at a C-rate of C/20 at a cell voltage of 3.4 V, which was better than those fabricated by means of vibration-assisted drip-coating. This was postulated due to a more uniform particle coating, and stronger particle attachment to the CNTs, leading to better electrochemical performance for EPD electrodes. 120 In summary, such investigations involving the EPD technique to apply LFP as a successful battery material for LiBs as well as microbatteries are given in Table 1. 132 It is also noted that some studies reported on binder-free EPD LiB electrodes, which is a decent step forward in terms of reducing operational costs. 137 Oltean et al 140 coated TiO 2 on alumina pillars via EPD and employed the rods as current collectors. Moreover, Pascall and co-workers 87 applied photoconductive electrodes with an EPD method to arbitrarily produce micropatterned LiBs. The conductive part on the electrode surface was patterned with the as-deposited multiple materials. 19 In particular, the surface resistance changed with illumination variations. Similar to these examples, traditional methods of fabricating 3D structures also provide insights into the efficient EPD of 3D micro-LiBs. 141 In another work, positive electrode architectures were effectively fabricated by means of the EPD of a functional coating comprising lithium iron phosphate (LFP)/carbon black/PVDF materials onto carbon fibre assemblies. 133 The electrochemical investigation on the EPD-coated carbon architectures exhibited that they operate nicely as positive electrodes, with a specific capacity ranging within 60 to 110 mAh g À1 , and 99.8% coulombic efficiency. Das and co-workers also recently showed the successful application of binder-free EPD of SbNP/rGO (antimony nanoparticles embedded in reduced GO) on Cu foil for enhanced LiB and Na-ion battery (SiB) performances at high C-rates for long cycles (as shown in Figure 7A). 138 This was a significant achievement because traditional doctor-blade slurry coating cannot prevent Sb delamination on Cu foil, whereas EPD was proven to be a viable technique to fabricate relatively rapid binder-free electrodes in about 3 minutes while also hindering Sb delamination. 142 Majumdar's group also prepared EPD anodes (bismuth iron oxide) that displayed exceptional performance in both LiBs and SiBs ( Figure 7B), thereby showing significant promise for advanced battery technologies. 143 Based upon such progress, it is, therefore, important to summarize the key contribution of EPD on LiB electrode fabrication in comparison to some standard electrode manufacturing processes (mentioned briefly in Section 3) and this is given in Table 2 to complete this important sub-section.
It is also worth mentioning that one of the latest publications on conversion-type anodes for lithium half-cell application 147 mentions about the drawbacks of such systems involving transition metal oxides. In particular, capacity fading is a serious issue suffered from nickel ferrite anode materials and therefore various approaches have been tried and tested in the literature to circumvent this, for example, encapsulation by means of carbonaceous substances in addition to applying a range of binders. 153 However, the complexity and expenses associated with scaling-up is not a practical solution for industrial applications. In contrast, EPD is a facile method that can be scaled-up on demand and can produce uniform deposition of LiB electrode materials with good porosity networks as reviewed very recently. 154 High porosity enables a decent buffering of volume changes in nickel ferrite anodic materials upon cycling, thereby enabling the maintenance of good performances. Similar to the above example, the porosity control advantage offered by EPD enables optimization for particular active materials, that is, allowing for expansion of high capacity materials like silicon, or chemical changes as for conversion-type cathodes such as those being used for lithium/sulphur chemistries. 155,156 Besides the above discussions on the application of EPD for LiBs, the importance of modifying electrode functional properties by means of nanomaterials, nanoarchitectures or nanocrystals is elaborated in the consequent sub-section. Additionally, EPD for manufacturing micro-LiBs is briefly touched upon to conclude this section, as this is a growing area of research.

| Nanocrystals
In order to exploit the functional properties of nanomaterials in practical devices, nanocrystals (NCs) are commonly assembled into dense and yet larger superstructures. 157 A bottom-up method is used to prepare such compact structures and has the following advantages: (a) significant cost reduction in comparison to classical 'top-down' tactics such as CVD, and (b) unique properties that are yet to be explored indepth. 158 Furthermore, the nature of the assembly (eg, ordered arrays, chains) can be effectively controlled by modulation of NC shape and ligand environment. 159 EPD can be used to form NC films with advantageous properties such as high conductivity, and easily controllable thickness. 160,161 EPD offers several advantages over other methods of coating formation. Firstly, it is scalable; electrode size can vary from nanoscale to automotivesized. 162 Secondly, the deposited coating properties (thickness, density, and morphology) can be tweaked in a facile manner by changing essential EPD parameters that are discussed in Section 3 (such as deposition voltage, inter-electrode distance, deposition time, etc.). Finally, the formation of configured deposits is possible by means of electrode templates. As a consequence, thin films of NCs, 161,163,164 CNTs 160,165 and rGO 116 have been demonstrated. Robinson's group also revealed a significant enhancement in conductivity when comparing a film of Cu 2Àx S NCs deposited by means of EPD with one deposited by means of spin-coating ( Figure 8A). 161 The same group demonstrated in a later publication that they could successfully prepare robust LiB electrodes from copper sulphide, manganese sulphide and germanium nanoparticles by means of binder-free EPD techniques in addition to negating the use of conductivity enhancing agents (detailed in Figure 8B). 166 Furthermore, Ha and co-workers demonstrated an enhancement of mechanical adhesion of EPD films compared with the more conventional slurry or drop-cast-based deposition methods. 167 A further advantage of EPD is demonstrated in the case of deposition of nanorods (NRs). Here, the in-built dipole causes the NRs to orientate and deposit along the electric field lines, causing dense, uniform and aligned films of easily controllable thickness to be deposited successfully. 168 These coatings exhibit improved charge transfer when compared with spherical NCs. 169 This work, therefore, demonstrates an economical combination of the EPD and nano-synthesis processes.
When chalcogenide-based NCs are dispersed in any kind of non-polar solvents, an electric field (direct current or DC) corresponding to several hundred volts per centimetre is usually needed to ensure quick deposition. 161,170 In a recent publication by Bree and coworkers, a programmable dip coater in combination with a high voltage source was used to conduct EPD with stringent control over electrode spacing, electric field and deposition time. 171 Two copper electrodes were used to apply 300 V to a freshly prepared colloidal formulation of NCs (toluene was used as a solvent in the case of tin sulphide (SnS) and anhydrous hexane for Cu 2 ZnSnS 4 (CZTS)), as illustrated in Figure 9A. 171 Deposition was allowed to occur for 5 minutes, during which the liquid colour changed, and became colourless when all the NCs were deposited. For both materials reported by Ryan et al, the NCs were deposited on the positive electrodes alone ( Figure 9B). 171 The deposited electrode microstructure is shown in the cross-sectional SEM image in Figure 9C. 170,172,173 In another study by Bree et al, SnS nanocubes were used as LiB anodes. 171 The nanocubes were deposited as compact, lean conductive surface coatings with elevated gravimetric capacity by means of a binder-free EPD process. A discharge capacity of 552 mAh g À1 was obtained with a minimal loss of around 0.08% per cycle for over 400 charge/discharge cycles. Finally, as demonstrated in similar investigations for LiBs, 2 the application of 3D current collectors enabled higher mass loadings of SnS nanocubes in the EPD electrodes. 171

| 3D micro-LiBs
In a micropatterned EES device (battery or capacitor), large contact area is obtained by forming an interdigitated structure, 174,175 and such a structure improves the rate capability of LiBs and lithium-ion capacitors (LiCs). 19 In addition, studies also reported a 3D LiB configuration consisting of a number of micro-LiBs. 125 In such a battery, micro-LiBs made via EPD are connected in parallel onto a substrate. The entire micro-LiB possesses a sizeable electrochemical surface area owing to the fact that each of its electrode has decent electrolyte exposure. Therefore, the 3D micro-LiBs supply a large current and a decent-power density. In addition, brilliant rate capability and successive cycling operation are displayed by such 3D LiBs. 176 In EPD, a 3D arrangement may be obtained by shaping a conductive layer onto a resistive surface, and the process may be likened to integrated circuit plate designing. Mazor and co-workers performed EPD of a lithiatedgraphite anode layer along with LiFePO 4 on a perforated-F I G U R E 7 (A) Hydrothermal material synthesis and binder-free SbNPs/rGO electrodes fabricated on copper foil by EPD (a); galvanostatic discharge-charge cycling for SbNPs, SbNPs/rGO-1, SbNPs/rGO-1.5, and SbNPs/rGO-2.5 electrodes tested in LiBs (b); and galvanostatic discharge-charge cycling for SbNPs, SbNPs/rGO-1, SbNPs/rGO-1.5, and SbNPs/rGO-2.5 electrodes tested in SiBs. Numbers 1, 1.5 and 2.5 refer to the original concentration of GO (mg mL À1 ) in the reaction mixture for preparing the EPD electrodes (c). Image reproduction approved by Elsevier. 138,142 (B) Majumdar's group successfully prepared anodes via EPD that gave excellent rate performance ($465 and 151 mAh g À1 at current density of 0.1 and 6.4 A g À1 , respectively) and cyclability during lithium storage while giving highly reversible, stable discharge capacity of $223 mAh g À1 , maintained beyond 100 cycles with a constant coulombic efficiency of 98% during SiB applications. Reproduced with permission from Elsevier 143 T A B L E 2 EPD parameters for preparing LiB electrodes and key results reported in recent literature and compared against a few electrodes prepared via industrially approved processes silicon surface to function as the cathode. 177 Furthermore, a patterned gold-coated substrate was connected to 10 000 micro-LiB units per cm 2 to create a 3D design. 19 At 100% depth of discharge (DoD), the micro-LiB configuration exhibited a stable operation over 200 cycles.

| Lithium-ion capacitors (LiCs)
LiCs bridge the gap between LiBs and supercapacitors by employing an LiB anode with one supercapacitor cathode in the presence of a lithium-ion enriched electrolyte. 178 The electrolyte commonly used involves the dissolution of the lithium-based salt in an organic solvent like propylene carbonate due to a larger potential window than that of aqueous electrolytes, thus providing a higher energy density. 96 Since the capacity of the LiC electrode is much lower than that of the LiB electrode, the cathode is the key to improving the energy performance of LiCs. Activated carbon (AC) is a conventionally employed cathodic substance for LiCs as a consequence of its prominent specific surface area, decent electrical conductivity and modest price. 66 However, AC possesses relatively minimal specific capacity. 179 Therefore, to cultivate LiCs as a contesting candidate for EES applications, it is imperative to fabricate unconventional cathodic substances with decent specific capacities and excellent rate capabilities. Thus, Mandler et al recently used a combination of electrodeposition and EPD for assembling porous electrodeposited rGO (ErGO) as an LiC cathode and assessed its operation in both symmetric capacitors and lithium half-cells resulting in improved capacitance by compositing with V 2 O 5 nanomaterials. 96 This compositing process improved the capacitance of the LiC significantly. In particular, the ErGO/V 2 O 5 LiC displayed capacitance values as high as 168 F g À1 at 0.1 A g À1 , as illustrated in Figure 10. 96 Wu and co-workers initially synthesized 1D nitrogendoped carbon nanoparticle chains (CNCs) by means of thermal pyrolysis of a 3D polymer precursor in the form of zinc hexacyanoferrate without using any additional nitrogen, carbon and catalyst sources. 180 The CNC film was then deposited on a stainless steel (SS, 1 cm Â 1 cm) sheet by means of EPD. Then they compared the performance of the CNC electrodes with commercially sourced multiwalled CNT (MWCNT) electrodes. They found that the MWCNT electrodes stored Li + ions mainly by means of intercalation, whereas the CNC electrode accumulated Li + ions owing to a combination of the adsorption and intercalation phenomena. From these results, the authors F I G U R E 8 (A) EPD results in an order of magnitude enhancement in film conductivity ($75 SÁcm À1 ) over conventional spin-casting. 161  concluded that the CNC electrode had better super capacitive performance than the commercially sourced MWCNT electrode, mainly due to a lower internal resistance of the former. Specifically, the CNC electrode displayed a high capacitance of 680 F g À1 at 1 A g À1 at a working potential range of 0.01 to 3.50 V vs Li/Li + , in comparison to that of the MWCNT counterpart that had approximately three times less capacitance of 252 F g À1 . 180

| Lithium-sulphur (Li-S) systems
The lithium-sulphur battery chemistry, which utilises a sulphur cathode and lithium metal anode, has excellent potential to replace lithium-ion as the standard high energy density EES. The gravimetric capacity of the sulphur cathode (1675 mAh g À1 ) and lithium anode (4200 mAh g À1 ) far exceeds that of current generation electrodes. EPD has been successfully utilised in the manufacture of sulphur-based cathodes, largely through the preparation of suitable conductive films, which act as host materials for sulphur. For example, Chung and coworkers successfully used the EPD process without binding agents to decorate a carbon fibre paper (CFP) with carbonaceous nanomaterials to prepare cathodes for Li-S battery applications. 181 The S cathode modified with EPD of CNT on CFP (EPD:CFP/CNT) provided a capacity of around 2.2 times higher (upon completing 50 cycles) than that supplied by its counterpart without the EPD-CNT coating (CFP/S cell) at 0.1 C. Also, the CFP/S electrode's rate capability could be elevated by means of a combination of the EPD-CNT/carbon black and EPD-CNT electrode materials on the CFP surface. As such, the EPD of CNT and Pt nanoparticles resulted in an EES device that displayed a specific capacity of about 730 mAh g À1 at 1 C. 181 Finally, the EPD manufactured cathodes outperformed their counterparts prepared by means of the classical slot-die casting process, especially with regards to successive charge/discharge cycles, polarization and internal resistance. This was as expected and confirmed by other investigations for EPD to prepare high-quality LiB electrodes in comparison to those made by standard slot-die casting of slurry pastes. 2 In particular, the slot-die casting results in the formation of heterogeneous microstructures in the CNT film with blocked porous architectures therefore causing meagre interfacial connection between the CNT clusters. As a consequence, the slurry cast CNT film ended up hindering a uniform sulfide/ sulphur deposition during cycling, which resulted in poorer performance when compared with the excellent output from EPD electrodes.
Similarly, some researchers utilised EPD to fabricate a high-quality, conformal nanostructured carbon film, which was then loaded with sulphur through vapourphase sulfurization. 139,156 When tested as cathodes in Li-S batteries, these electrodes exhibited high capacities in excess of 1200 mAh/g À1 .
The major challenge to be overcome to commericialise lithium-sulphur battery is that known as 'polysulfide shuttling', whereby ions such as Li2S4 migrate to, and poison, the anode, leading to rapid capacity loss. 182 A unique application of EPD has been demonstrated to overcome this challenge. Blanga et al reported the formation of a polysulfide barrier through EPD. 183 A Li 10 SnP 2 S 12 (LSPS)/polyethylene oxide (PEO) film was deposited from acetone solution, and demonstrated high ionic conductivity. When coated on the surface of sulphur and lithium sulfide cathodes, the film enhanced electrode capacity and faradaic efficiency, while effectively mitigating the effect of polysulfide shuttling.
EPD has also shown promise in overcoming another challenge with Li-S batteries, specifically that of lithium dendrite formation at the anode. Zhang et al carried out the EPD of Ti 3 C 2 T x nanoflakes onto Cu foam. 184 The 3D structured current collectors demonstrated excellent Li plating/stripping efficiency with reduced overpotential, maintaining a uniform and dendrite-free Li film. The authors attribute this to both the 3D nature of the EPD electrode, which increased the effective surface area and the lithiophilic nature of the nanoflakes. Wang et al reported a similar effect from the EPD of black phosphorus (BP) onto Cu foam, whereby initial lithium nucleation was confined to the BP sites and thus controlled. 185 This effect brought about a significant increase in the plating/stripping efficiency and cycling stability.
Such exceptional results for the EPD electrodes, resulting in high capacities, could therefore transform into high energy densities of Li-S batteries, opening up the scope for scaling-up the EPD technique to commercial Li-S applications in future. 181

| Electrodes for flow batteries
Redox flow batteries (RFBs) have many advantages such as high-energy efficiencies (up to 80%), simple control and monitoring systems, long cycling lifetimes, relatively low maintenance costs, flexible design and an environmentally friendly structure. 14 From the environmental point of view, Fernandez-Marchante and co-workers showed the lowest environmental impact from 12 midpoint impact categories, including water footprint, and photochemical oxidation, with a longer lifetime for RFBs considering the re-use of vanadium electrolytes in comparison to conventional LiBs. 186 Although primary studies were done by Thaller in the early 1970s, 187 major improvements were obtained by Skyllas-Kazacos in the 1980s. 14,[188][189][190] This type of battery consists of inert electrodes, flowing electrolytes and membranes as its main structure. [191][192][193][194][195] Cycling life and the performance of the battery are also mainly related to the type of electrodes employed. 14,[196][197][198][199] Although graphite-based electrodes have been used for positive and negative electrode components, modification of this material by metal oxides and other carbon-based compounds has great importance. 14,197,[200][201][202] In this regard, the preparation of highperformance electrode materials for RFBs can be conducted by means of EPD and its associated advantages are discussed in this review. This is of utmost significance at this time because the application of RFBs for grid-scale load levelling as well as efficient and green storage of renewable energy is expected to increase in the near future. EPD, therefore, has an important role to play in terms of the electrode and, in the future, membrane modification or (even) manufacturing processes. In hindsight, the reported applications of EPD for RFB applications are low (as discussed briefly in the following paragraphs), and this review is expected to advocate its interesting properties for the scientific community to explore. rGO was deposited electrophoretically on carbon paper (using a horizontal EPD set-up in contrast to the standard vertical means) for use as a positive allvanadium RFB (VRFB) electrode. 203 An increase in the surface area by 24% was reported due to the deposition of rGO. A decrease in charge transfer resistance (R ct ) was also correlated to an increase in electrochemical surface area (ECSA). This investigation was followed up with a study on hydrogen/vanadium regenerative fuel cell (RHVFC) electrodes, 3 as well as on VRFB electrodes, 200 which were modified with rGO in both studies. The investigation on RHVFCs showed enhanced performance due to EPD of rGO on the carbon paper electrodes, especially for ones that were subjected to thermal treatment beforehand (Figure 11), 3 signifying the promise of this modification technology for flow battery scale-up applications. 204 Gonzalez and co-workers applied EPD on a graphene oxide (GO) suspension in water to coat graphite felt substrates. 204 From SEM and XPS evaluation of the GO decorated graphite felts, the establishment of a hybrid architecture possessing more oxygen-containing functional groups than standard felts was reported to enhance electrolyte wettability and consequently ECSA. More evidence of this effect was only confirmed by a higher capacitive current from cyclic voltammetric investigations in the same work. 205 The presumption was that graphenelike sheets of rGO (due to partial reduction of GO during heat treatment) may have improved electronic conductance, which was confirmed by separate studies. 99,[206][207][208] Recently, the EPD of nitrogen-doped graphene (N-G) on carbon paper electrodes (CP) has been reported by WMG to enhance the performance of non-aqueous VRFBs applying deep eutectic solvents (DESs) 113 as well as three different RFCs employing either highly acidic or alkaline electrolytes ( Figure 12A). 209 In the first investigation, the DES-based VRFB was subjected to about 20 cycles to evaluate its temporal fluctuation of efficiencies and utilization. 113 Figure 12B, which was reconstructed from x-ray tomographic scans, clearly shows the N-G occupying the porous CP electrode, which was considered responsible for the enhanced improvement of the N-G/CP electrode in comparison to the pristine CP electrodes in the DES-based VRFB. In this respect, Figure 12C displays the cycling curves that show a noticeable energy efficiency and electrolyte capacity utilization enhancement when using N-G/CP electrodes for the DES-based non-aqueous VRFB in a similar manner to aqueous-based RFCs. 209 The EPD method was mainly applied to carbon-based materials such as rGO and N-G in the literature. 113,206 In this respect, Table 3 displays a range of carbon-based electrode modifications reported in the literature for applications in selected RFB systems. 14,219 As can be noted in Table 3, EPD of carbonaceous materials has provided many benefits in comparison to standard electrode treatment methods such as thermal or chemical. However, this method can also be used for the deposition of other types of materials to improve the electrode F I G U R E 1 1 Application of rGO modified Freudenberg carbon paper (commercially available) as RHVFC positive electrodes to result in high peak power densities and energy efficiencies when the rGO modified side faced the membrane (rGO FM). The heat-treated CP that underwent EPD with rGO delivered the best performance (rGO HT FM). EPD was conducted with DMF as the solvent. HT, heat treated electrode; DMF, EPD performed with only DMF solvent without rGO dispersed in it; rGO FF, rGO deposited carbon paper electrode facing the flow fields of the RFB. Reproduced with permission from Elsevier 3 performance of the RFBs. Especially, metal-based materials such as TiO 2 , TiN, TiC, MnO 2 , etc. or metal-organic frameworks can be used as effective modifiers to the surface of the carbonaceous materials for both positive and negative electrodes. 211 Increased active sites due to the modification of RFB electrodes with such kind of materials via the EPD method can increase the electrocatalytic selectivity of the electrode materials for redox reactions associated with the flow battery type. Indeed, enhanced properties such as high discharge capacity, high energy and columbic efficiencies in battery properties can also be obtained by this modification process. 14 This, therefore, advocates a strong case to perform more extensive investigations on EPD for enhancing the electrodes and/or membranes of RFBs in future studies, with a particular emphasis towards advocating the widespread commercial adoption of this EES device. 220

| Electrodes for supercapacitors
Supercapacitors represent a critical class of EES devices with vital advantages such as elevated power response and exceptional cycling stability owing to their lofty power density, rapid charge/discharge rate, and high coulombic efficiencies. 221 Supercapacitors are conventionally classified into two types that owe their distinction to contrasting energy storage features. 222 One such classification is known as the electrical double-layer capacitor (EDLC) that owes its performance uniqueness to ion  223 The other classification is known as a pseudocapacitor whose mechanism is dependent upon a rapid and reproducible faradaic process that occur as a consequence of electroactive species. 227,228 Similar to batteries, supercapacitors comprise of a couple of electrodes, a separator and an ionic solvent/electrolyte. 229 Symmetrical supercapacitors tend to consist of two duplicated electrodes fabricated from identical materials, whereas asymmetrical supercapacitors consist of two dissimilar electrodes prepared from varying capacitive and faradaic materials. The industrially approved standard method for manufacturing electrodes involves paste-coating that can cause binding agents (commonly employed) to cover the electrode surface or block the open porosity network.
Additionally, a crucial issue for a packed supercapacitor is the need to account for the complete system mass, which comprises electrode films, current collectors, binder, separators, electrolyte, connectors and packing materials. 230 This is also true for supercapacitors meeting the minimum industrially approved active material mass  loading standard of 10 mg cm À2 . In these cases, the active electrode mass is around 30% of the total device weight. 231 For such circumstances, the operation of the supercapacitor estimated from the electrode material has to be divided by 3 or 4. 232 The divisor is elevated to 30 if a slender electrode having a 1 mg cm À2 mass loading is used. 230 Therefore, adequate electrode mass loading is essential to diminish overheads due to inactive materials, which can effect a decent electrochemical supercapacitor output. Currently, EPD has been considered appealing for manufacturing electrodes owing to its ability to produce surface coatings without employing binders while ensuring quality control over the active material film thickness as well as mass loadings. The consequent paragraphs deliver a sufficient impression of current scientific and technological advances in EPD electrodes manufacturing and performances. 36 Rashti et al described a scalable, rapid and costeffective EPD method for manufacturing nickel cobaltite composite electrodes for supercapacitor applications. 233 The composite EPD electrode (NiCo 2 O 4 /PANI/rGO) with rGO and polyaniline (PANI) was post-treated thermally under a reducing nitrogen atmosphere to carbonize the PANI component. NiCo 2 O 4 platelets, dispensed on carbonized PANI-rGO architectures, were involved in charge storage with decent capacitance, exceptional rate performance (1235 F g À1 at 60 A g À1 ), and 3000 charge/ discharge cycles at 10 A g À1 . 233 The composite electrode in combination with AC in a symmetrical capacitor displayed a capacitance retention of 78% after 3500 cycles at an enhanced functioning potential of 1.5 V at a practical current density 1 A g À1 . This study indicated the significance of the EPD composite construction method that, in turn, influenced the aggregate microstructure. 233 Binder-free Ti 3 C 2 MXene (MXenes are 2D transition metal carbides, carbonitrides and nitrides)/CNTs (Ti 3 C 2 / CNTs) coatings were effectively decorated on a graphite paper substrate by means of EPD and applied as electrodes for a supercapacitor (Figure 13). 234 The manufactured Ti 3 C 2 /CNTs electrode displayed elevated specific capacitance, about 1.5 and 2.6 times than that of immaculate CNTs and Ti 3 C 2 coatings, respectively. Ti 3 C 2 /CNTs electrodes impressively gave excellent capacitance retention over 10 000 cycles owing to an effective hindrance to MXene restacking as a consequence of incorporating CNTs into MXene by means of the practical and effective EPD technique. 234 Graphene quantum dots (GQDs) are comparatively a contemporary genre of carbonaceous nanomaterials with various attractive properties such as photoluminescence and decent surface area. 235 Owing to its large surface area to volume ratio, GQD is an admirable intrant material for application in supercapacitors. 236 Tjandra et al showed an effective means of preparing GQDs by means of a hydrogen peroxide-assisted hydrothermal reaction and then applied it with carbon cloth to manufacture a flexible supercapacitor electrode possessing high rate capabilities and a relatively high areal capacitance of 70 mF cm À2 . 237 The authors also prepared full cell flexible supercapacitors using two EPD GQD electrodes ( Figure 14 shows the supercapacitor performance with an areal capacitance of 24 mF cm À2 ). 237 Wang with an rGO membrane-electrode-assembly manufactured by means of EPD to fabricate an all-solid-state supercapacitor. 228 The all-solid-state rGO supercapacitor demonstrates a volumetric capacitance of 108 F cm À3 in addition to an energy density of 7.5 Wh cm À3 at a power density of 2.9 W cm À3 . Furthermore, the supercapacitor is able to sustain 335 000 cycles successfully, which compares well with industrial-level EES devices. 228 Other workers fabricated micrometre-sized supercapacitors based on onion-like carbon (OLC) particles using EPD on interdigitated gold current collectors. 238 The as-prepared micro-supercapacitors had electrode thicknesses ranging into several micrometres. Additionally, this supercapacitor is capable of delivering over 10 000 stable cycles at 200 V s À1 with four orders of magnitude higher capacitance than standard electrolytic capacitors. 19 Additionally, their rates of discharge are about three orders of magnitude greater when assessed against standard supercapacitors. 239 Liu and co-workers applied an EPD method to fabricate GQDs-based micro-supercapacitors. 236 Both symmetric supercapacitor (with the GQDs as both the cathode and anode) and asymmetric supercapacitor (with GQDs as the anode and MnO 2 as the cathode) were made. These symmetric micro-supercapacitors gave excellent electrochemical performance such as high power response with a small time constant (τ 0 = 53.8 μs in ionic liquid electrolyte and τ 0 = 103.6 μs in aqueous electrolyte) and decent cycling stability. 239 The asymmetric supercapacitor exhibits two times higher specific capacitance and energy density than the symmetric micro-supercapacitor. Other researchers fabricated an MWCNTbased supercapacitor electrode by means of EPD. 240 These supercapacitors exhibit small equivalent series resistance yet high specific power density in addition to very good knee frequency reaction. 239 Niu et al combined photolithography with EPD to build micropatterned rGO interdigitated electrodes coated by gel electrolytes for an F I G U R E 1 4 (A) Galvanostatic charge and discharge curves of a symmetric supercapacitor at different areal currents; (B) Spatial capacitance of the supercapacitor against the spatial discharge current; (C) Ragone plot of the spatial energy and spatial power of the supercapacitor; and (D) Impedance graph along with an equivalent circuit layout and an inset figure magnifying the supercapacitor impedance in the high-frequency territory. Reproduced with permission from Elsevier 237 all-solid-state micro-supercapacitor application. 241 Compared with conventional capacitors, these microsupercapacitors exhibit a combination of enhanced specific capacitance, columbic efficiency and knee frequency due to effective utilization of the electrochemical surface area of rGO layers. 239 Table 4 shows the important processing parameters and obtained film characteristics in selected reports involving (mainly) binder-free EPD of different electrode materials for supercapacitors. Application of EPD for the production of the electrode materials has important advantages such as relatively quick processing times, as displayed in Table 3. Argüello and co-workers used EPD to obtain manganese oxide and graphene nanoplatelets (GNP) on graphite paper as electrode materials for supercapacitors. 242 They completed the deposition process in only 2 minutes. GNP + MnO 2 based electrode had relatively high specific capacitance of 422 F g À1 between 0 and 0.8 V potential range at 0.1 A g À1 charge/discharge current density. 242 Nguyen and co-workers also reported on the efficient application of EPD to produce MOF (metal-organic-framework)/Ni 3 (HITP) 2 based binder-free electrodes for supercapacitor applications. 243 Although the EPD process was completed in a relatively low potential of 0.5 V and in 10 minutes, the system maintained 15.69 mF cm À2 capacitance between 0 V and 1.0 V at a charge/discharge current density of 0.10 mA cm À2 . 243 Another example of a binder-free electrode fabrication process was achieved by Nguyen et al with the deposition of HRGO (holey reduced graphene oxide)-RuO 2 in 500 seconds at 4.0 V. The flexible electrodes manufactured, when applied in a functional supercapacitor showed 418 F g À1 specific capacitance in PVA/H 2 SO 4 electrolyte at an operational voltage of 0 to 1.0 V at 1.0 A g À1 current density. As can be noted from Table 4, the EPD method for the fabrication of electrodes using many different types of substances such as graphene, rGO, metal oxides and other materials has a significant potential for industrially scalable applications due to its easy, low cost and relatively rapid processing steps. Table 5 shows a comparison of several AC-based supercapacitors reported in the literature. One of the publications was reporting on commercial type supercapacitor electrode performances, but these were made by slurry casting. 252 In general, EPD of AC on metal substrates has not performed well for supercapacitor electrodes until Shreshta and co-workers managed to isolate a good recipe using IPA and ethyl cellulose (EC) to obtain a supercapacitor with a good gravimetric capacitance of 153 F g À1 . 247 According to the authors, this was the first time that an EPD-based AC electrode gave such a high capacitance. Additionally, in comparison to other reports on industrial type supercapacitors (made via slot-die casting), 230 the results presented by Shreshta et al are similar, as highlighted in Table 6.
It is interesting to note from Table 5 that most of the representative electrodes presented in the literature do not meet the commercial standards for deposition thickness and mass loading for supercapacitors. 230 To provide consistent electrochemical performance, a supercapacitor electrode is required to maintain a thickness and active mass loading analogous to electrodes meeting industrial standards, that is, 50 to 200 μm and 8 to 10 mg cm À2 , respectively. 171 Also, a good number of publications do not even mention the thickness of the active material (AC and carbon black usually) on the current collector. As a consequence, a comparison of these two important criteria for supercapacitor electrodes (by means of EPD only) reported in the literature is shown in Table 6. From several representative supercapacitors reported in the literature, average mass loadings of around 1 mg cm À2 or less (rarely higher) and deposited film thickness of around 1 to 10 μm (rarely 20 μm or above as shown in Table 6) is common. In comparison, a recent work by WMG provides much better mass loadings and film thicknesses, as can be gleaned from Tables 5 and 6. 246 Therefore, a versatile EPD method has been established in WMG for preparing high-performance supercapacitors that meet the industrial standards for good mass loadings and film thicknesses as shown in Figure 15. 248 The EPD conditions used for the study in question were very similar for producing high mass loadings (and decent coating thicknesses) for SiO 2 separators for LiB applications (shown as an example only in Table 6). 254 In a recently published work, a binder-free EPD procedure was used for preparing TiN nanocoatings with a rapid time constant of 10 ms (determined via impedance spectroscopy). 260 These nanocoatings were then sintered and resulted in high-performance supercapacitor electrodes with a capacitance of 120 F g À1 at a high current density of 2 A g À1 . The authors successfully compared the theoretical EPD kinetics, determined via Sarkar and Nicholson's method (Equations [1] and [2]), 24 with the actual kinetics and found the latter to have higher values, as shown in Figure 16. 260 They attributed this slight discrepancy to the fact that the model was not comprehensive enough to account for the full phenomena occurring at the solid/liquid interface or at the surroundings near the EPD electrode due to suspension flocculation. τ T A B L E 4 Important processing parameters and obtained film characteristics in selected reports involving the EPD of (mainly) binder-free electrodes for supercapacitors Estimated from a value given in the literature of 178 F g À1 at a current density of 0.5 A g À1 . b Estimated from a value given in the literature of 124 F g À1 at a current density of 0.5 A g À1 .

Material
T A B L E 6 A comparison of mass loadings and film thicknesses for different supercapacitor electrodes prepared by means of EPD as reported in the literature (representative sample) and compared with recent work in WMG 246

Active material EPD parameters
Mass loading (mg cm À2 ) EPD film thickness (μm) Ref. where m is the mass of deposits (g), m 0 the total suspension mass (g), t is deposition time (s), V is the suspension volume (mL), f is a theoretical factor used by Sarkar and Nicholson, 24 μ e is the electrophoretic mobility (cm 2 V À1 s À1 ), S is the EPD surface (cm 2 ) and E is the applied electric field (V cm À1 ). Finally, hybrid electrodes, which are assembled from a chemical (battery-related) compound and a physical F I G U R E 1 5 An effective EPD technique was developed at WMG to deposit activated carbon on aluminium foil current collectors at industrial-scale mass loadings and film thicknesses. These electrodes were tested in both coin cell and A7 pouch cell supercapacitors and gave very good performance-excellent capacitance retention is noted for both cells at 0.1 A g À1 current density. Reproduced with permission from RSC 248 (electrical double layer) storage material, have exhibited significant capabilities in battery-supercapacitor hybrid (BSH) compilations. In a current investigation by Mandler et al, an EPD-based approach is followed for assembling a binder-free, high-performance BSH device successfully. 244 The procedure applied NiCo 2 O 4 (NCO) as the chemical storage material, whereas Ketjenblack (KB) was employed as the physical storage compound. These materials were then dispersed in an electrolyte of Ni 2+ ions. Therefore, it was possible to ascribe a positive surface charge on the electrode materials that enabled a similar EPD rate of both NCO and KB materials on nickel foam (NF) substrates. This ensured a good manipulation strategy to be applied to the KB to NCO proportion in the composite electrode and optimize its operation. In this electrode architecture, the KB chains functioned as a rapid electron route enabling decent conductivity for the NCO electroactive compound ( Figure 17). 244 In particular, the KB chains maintained effective contact with NCO despite mechanical wear and tear throughout the electrochemical reaction. Hence, the robust hybrid electrode exhibited decent specific capacity (460 C g À1 at 1 A g À1 ) and enhanced cycling operation (82.5% retention despite undergoing 15 000 galvanostatic charge/discharge or GCD cycles). When the NF/NCO-KB electrode was used as the positive in combination with AC as negative electrodes, the BSH displayed an excellent cycling performance of 88.6% capacitance retention upon undergoing 10 000 cycles along with a high energy density of 53.0 W-h kg À1 at a power density of 746 W kg À1 (Figure 17). 244 7 | DEVELOPING RESEARCH AREAS ON THE APPLICATIONS OF EPD FOR ELECTROCHEMICAL ENERGY STORAGE

| Background
Carbon-based composites and carbon-containing materials have recently been successfully applied in a range of industrially relevant activities, which include energy storage devices, fuel cells and membranes. To augment the properties and operation of carbon comprising EES devices, polymeric electrodes may be appropriately altered by carbon-containing compounds and metal oxides by means of EPD to assemble a composite that supports the charge accumulation and gathering processes owing to enhanced electrode surface areas. The carbon-polymer nanocomposite architectures help circumvent issues associated with the maintenance of highperformance polymeric materials that consequently can be assembled into exceptional composites for application in outstanding EES devices. 261 This section briefly reviews some of the challenges still faced by conventional EPD techniques and also assesses some new methods being reported in the literature, such as the incorporation of advanced nanomaterials to prepare exceptional electrode materials. 262 For example, in Section 6.4, the decoration of carbon paper electrodes with nitrogen-doped graphene nanomaterials with EPD displayed the versatility of this automated process for enhancing hybrid flow cell performance. 25,263 In addition, EPD set-up is relatively facile and is able to deposit films that meet industrial standards for EES electrode thicknesses and mass loadings by regulating a range of practical factors of operation. 264 The electrolyte/electrode interaction in EES systems is an extremely important factor governing their performance. Although it is well known that EPD is used for producing superhydrophobic surfaces 265 for applications in aqueous-based electrolyte EES systems, it needs to be ensured that both the substrate on which deposition occurs as well as the colloidal particles embedded onto the substrate are hydrophilic in nature. This may be easily ensured by EPD because it is an in-situ means for preparing either hydrophilic or hydrophobic surface coatings with effective control over film porosity and tap density, which are direct functions of the deposition voltage (in contrast to slurry casting procedures that are dependent upon a secondary step like electrode calendaring to ensure effective tap densities). A very good example of the effective application of hydrophilic surface coatings involves the EPD of rGO on carbon paper electrodes for providing excellent electrochemical performance for applications in RFBs as discussed above. 203 Other EES devices use organic electrolytes, which are not so reliant on hydrophilic electrodes for generating decent performance, and thus it is not essential to be concerned about the wettability of the EPD electrodes. A reasonable example may be the application of LiB electrodes, which interact with the standard organic-based electrolyte consisting of a mixture of ethyl carbonate and dimethyl carbonate, containing 1 M LiPF 6 salt, to provide a reversible capacity of around 650 mAh g À1 at 0.5 A g À1 up to 150 cycles. 147 Despite the fact that the widespread commercial adoption of EPD for manufacturing EES devices is rather scarce, there is yet a significant increase in the number of patents filed recently for industrial applications of EPD for the large-scale production of electrode materials for batteries (see Table 7 for a summary). Among several different companies and academicians that applied for such patents, some examples are discussed herein. For instance, Golodnitsky and co-workers used a sequential EPD method on a single conductive substrate for developing three-layer lean-film battery (TFB) architectures. 266 In another patent application, bipolar electrochemical devices were made by means of electrophoresis. 267 Xiao patented a method that included EPD for the production of electrode materials for LiBs. 268 In this process, a potential was applied between a current collector and a counter-electrode submerged in a colloidal formulation consisting of GO, binding material and also other components to deposit the active materials such as rGO on the working electrode surface. 268 SiO-based carbon nanofiber composite anode materials produced on a nickel/copper catalyst by means of EPD were also patented for particular applications in lithium-based EES devices. 269 Ruoff and co-workers used the EPD method for effective reduction of GO to make graphene film on an electrode surface such as aluminium foil, copper plate, nickel plate and Si wafer substrates. 272 In another application, GO was deposited on a titanium sheet electrode surface by means of EPD and it was reduced by thermal treatment for the production of anode materials for LiBs. 273 A potential production  270 The use of EPD for the production of composite anode materials for LiBs was also patented. 271 Functionalized MWCNTs including active materials were electrophoretically deposited to form nanostructured composite electrodes for the use in batteries. 274 Gaben also applied for a patent for manufacturing all-solid-state batteries in a multi-coated architecture by means of a range of methodologies that comprised of EPD processes. 275 In short, the use of the EPD method played a key role for the production of anode materials for LiBs in many patent applications. 276,277 Adaptation of the EPD method towards industrial applications is a critical issue. In particular, the development of systems that will allow mass production in comparison to standard slot-die casting processes is vital for the use of the electrophoresis method in electrode production methods for EES devices. With this key point in mind, Oakes and co-workers published a crucial work about the use of EPD in a roll-to-roll production process for high-quality electrode materials. 278 Since the parameters such as low cost, homogeneous coating, reproducibility and simplicity are important for the production of such electrode materials, the published method heralds a new perspective in this important area of research. 278 Figure 18 shows the basic structure of the system for benchtop roll-to-roll EPD coater and the subsequently processed materials obtained by use of this process. 278 The authors illustrated a methodology involving the EPD of the hybrid assortments of CNTs, MoS 2 nanosheets, silicon nanoparticles, carbon nanohorns and graphene nanoplatelets without applying any surfactants. Indeed, production of the positive and negative electrode F I G U R E 1 8 Basic structure of the system for benchtop roll-toroll EPD coater and SEM picture of obtained electrode materials by means of this process. Reproduced with permission from ACS 278 materials of LiBs was completed in less than 30 seconds. 278 Pascall and Kuntz also patented the use of a light-directed EPD system for the production of flexible electrode web substrates. 279 Cathodic EPD was initially commercialized by PPG, which became an industrially relevant process for the corrosion protection of vehicle panels. Of late, PPG acquired financial support of $2.2-million from the US Department of Energy (DOE) for investigating the comparative application of EPD against standard slot-die casting for the preparation of LiB electrodes. 280 PPG's process concentrates on tackling engineering approaches of EPD to prepare sophisticated LiB materials and devices in collaboration with Oak Ridge National Laboratory in Tennessee.
The employment of EPD for EES devices has augmented in time and enhanced the operation of prevailing battery/supercapacitor electrodes considerably. More investigations are necessary to comprehensively study the properties and mechanism of the EPD method. An awareness of the effect of significant parameters such as deposition time, inter-electrode distance or applied voltage on colloidal properties during the EPD process is essential (discussed briefly in Section 3). This comprehension will facilitate improved manipulation of the structure and morphology of the nanomaterials and may result in the discovery of original functional composites. Furthermore, extensive EPD investigations may involve the effects of modulated electric fields, such as throbbing direct current 281 and alternating currents 282 to devise electrodes with diverse architectures for EES applications because EPD under modulated electric fields can convey enhanced traits in comparison to those acquired by traditional EPD, 283 in addition to well-orientated microstructures. 284 Furthermore, there is a tendency for decent reproducibility because automation of EPD operating with modulated fields is potentially facile. 281 Accordingly, attention should be paid to the design and manufacture of 2D and 3D architectures for the innovative application of progressive EES devices. Finally, potential future EPD investigations may focus on applying a range of unknown nanomaterials to design and develop electrodes for revolutionizing supercapacitors, batteries, and regenerative fuel cells 36 (or hybrid RFBs) such as the hydrogen/benzoquinone 5 and other similar RFCs as reported recently. 209 In addition, EPD is already used in industry on large scale such as car painting. Yet, we expect that it will also penetrate other areas, where large area electrodes are required (e.g., in the field of water electrochemical treatment and electrolyzing). The significant advantage of EPD stems from the ability to predesign and characterize the nanoparticles that are deposited, which maintain their performance. Hence, for example, ZnO nanoparticles that are used for the photodegradation of organic dyes can be synthesized with optimal photonic properties. More specifically, for EES, we anticipate that EPD will slowly take a lead and will be used for the mass production of electrodes. Besides its low-cost advantage, EPD can be integrated into line production quite easily. Yet, to become a leading manufacturing technique in electrode production for EES, EPD still has to prove that it can be used to form relatively thick coatings with good adhesion and reproducibility. Furthermore, EPD can readily be used for producing graded materials, which still have not entered EES. The fact that EPD is a 'binderfree' approach is still not enough to make it an appealing process in the industry. Moreover, the rate of deposition will have to increase to compete with the current methods. To conclude, when EPD demonstrates some unique advantages that cannot be obtained easily and for the same costs by the current techniques, EPD will pave its way to the most advanced production lines.

| Hybrid technology
Niu et al produced flexible micro-supercapacitors by means of a combination of photolithography and EPD to develop graphene microelectrodes. 241,285 These graphene microelectrodes facilitated the electrolyte ions transportation, allowing more accessible surface area. Also, the reported specific capacitance of the as-prepared graphene-based micro-supercapacitor was 285 F g À1 , nearly three times more as compared with conventional graphene-based supercapacitors (86 F g À1 ).
In other applications, a new type of 3D metal printing is known as laser-assisted EPD ( Figure 19). This method is based on an optical trap of locally concentrated nanoparticles in a solvent to which an electric field is applied to obtain a compact morphology. 288 A firmly focused laser beam is directed onto an electrolyte consisting of negatively charged metallic nanoparticles that are then subjected to optical traps that agglomerate particles in the focal spot of the laser beam. 287 The focal spot is used to gather a large number of charged particles in the region of interest for further processing, which includes EPD. 286 EPD compels the particles to coagulate and deposit on the surface possessing positive charges. 289 Crucially, this association of laser with EPD should be further investigated to develop electrode materials with high mass loadings and film thicknesses of electrode materials that meet industrial standards for future studies. For example, the LiB electrodes developed in WMG could be further enhanced by combining laser concentration approach with EPD for application in practical EES devices. 2 7.3 | EPD of solid electrolytes and separators for batteries: a developing area of research EPD has been employed for designing and manufacturing an innovative combination of electrodes and solid electrolytes with decent performances in EES devices when compared with those assemblies prepared by means of classical processes. 19 EPD of solid-state electrolytes is a relatively new topic for LiBs, and reports in the literature are rather scarce. 154,290 However, this topic appears to be more well established for solid oxide fuel cells. 19 Although not the focus of this review, some of the solid oxide fuel cell materials that have similar composition as LiB solid-state electrolytes are mentioned briefly herein to present a platform for researchers for taking up this important topic in future investigations. There are no such investigations for supercapacitors at present and may be an interesting topic of fundamental and applied research.
Among the scarce publications on EPD for solid electrolyte LiBs, 291 one work reports on the EPD of a PVDF-HFP {Poly(vinylidene fluoride-co-hexafluoropropylene)} membrane deposited on the anode. 292 The resulting separator/anode (see Figure 20A) facilitates a decent ionic conductivity of 8.1 Â 10 À4 S cm À1 , and when used in a LiB, produces 370 mA-h g À1 discharge capacity (99.5% of the theoretical capacity) at 0.1 C, a capacity retention of $100% in >300 charge/discharge cycles, and a rate capability of 270 mA-h g À1 at 1 C. Interestingly, the porosity of the polymer membrane is tuneable by choosing the appropriate solvent, resulting in an improvement of conductivity and durability of the separator/solid electrolyte film. In a separate investigation by Goodenough and coworkers an HFP separator is successfully impregnated by means of EPD with atomic interlamellar lithium-channel phyllosilicate clay. 293 This enhanced membrane architecture remarkably diminishes lithium dendrite formation resulting in an exceptional capacity of 152 mAh.g À1 at 0.5C as shown in Figure 20B.
Additionally, hybrid solid electrolytes combining ceramic and polymeric ionic conductors have been exhaustively investigated as benign electrolytes for employment in solvent-free rechargeable LiBs. 294 The unique benefits of such a combination of solid electrolytes include flame resistance, flexibility, more stable anode/electrolyte interfacial resistance compared with conventional liquid electrolytes, and low-cost design owing to the highly versatile EPD process. 295 For instance, EPD was used to prepare an aggregated solid ceramic/polymerized electrolyte loaded with lithium iodide. Even though the coarseness of polyethylene oxide (PEO)-LiAlO 2 composite coatings was larger than that of PEO depleted compounds, the deposits were comprised of densely packed decently separated nanoparticles in contradiction to the highly visible aggregation in polymer depleted deposited electrolytes, in which salt/ceramic grain-peripheries perform a principal part in the conveyance of Li + ions. The solid LiAlO 2 :LiI:P(EO) 3+x electrolyte film was uniformly deposited on the substrate with controllable thickness in the range of tens of micrometres and was characterized by means of a high, temperature-independent conductivity of 0.5 mS cm À1 . 296 It is imperative that ceramic particles are electrically charged to allow transport by EPD. 297 It should be noted that cathodic EPD is desirable to anodic EPD, owing to issues connected with anodic dissolution, corrosion of substrates and deposit adulteration. Therefore, as reported in a recent investigation on the EPD of NMC111 on 2D and 3D Al-based current collectors in WMG, 2 it is imperative to apply electrolyte additives like PDDA for ascribing positive charges on electrochemically active particles. Additionally, it is imperative for EPD to efficiently deposit uniform coatings of various battery-F I G U R E 2 0 (A) (a) Cartoon showing how the interaction of molecules within the PVDF-HFP/acetone colloidal solution may be exploited for EPD. Grey, white, red and cyan spheres are associated with carbon, hydrogen, oxygen and fluorine atoms, respectively. Blue surfaces display a charge gain of 0.15 eÅ À3 while yellow surfaces correspond to an equivalent charge loss. (b) EPD process used to fabricate the half LiB. The separator side and the anode side of the separatoranode structure and the EPD separator deposited on an Al substrate are shown, respectively. Reproduced with permission from RSC. 292 (B) A LiB coin cell assembled with Li-MMT/PVDF-HFP (PVDF-HFP/ Li-montmorillonite) separator (prepared via EPD) delivers an excellent discharge capacity of 152 mAh g À1 with a capacity retention of 98.5% after 300 cycles at 0.5C, which is beyond those delivered by PVDF-HFP (145 mAh g À1 ) and the common literature reported Celgard 2325 (138 mAh g À1 ) separators. Reproduced with permission from Elsevier 293 related substances, incorporating cathode and anode composite substances, electrolytes and separators. Cathodic deposition is preferred by employing analogous bath compositions. Undeniably, varying the course of the electric field as well as altering the solvent or additives for deposition of specific materials may cause the delamination of the previously deposited material coatings. 297 EPD of sub-micrometre powders enables compact assembly, reliable sinter-ability and uniform microstructure. 298 Despite that, since the sub-micrometre particles possess large surface areas, they have a compelling propensity to agglomerate. Decently scattered and steady suspensions of fine materials may be acquired by means of an appropriate choice of an efficacious dispersant. 299 A crucial issue to take into account involves the use of fine particles for EPD, which could render the coatings to become prone to cracking when being dried, and therefore the selection of the binding material becomes imperative to prevent this. The optimal quantity of binder is governed by particle size and surface area. 297 To effectively exploit the benefits of the methodology established in the EPD of solid electrolyte materials, 300 a selection of processing factors needs to be ascertained. The effect that electrolyte additives have upon deposition morphology and EPD rate is important to clarify. 297 Advanced studies are needed for optimization of colloidal bath composition to diminish the number of additives and attain homogeneous and robust deposits of manipulative thickness, as well as to hinder pinholes, cracks and further imperfections.

| Sodium-ion batteries and capacitors
New trends in EES technologies can also be useful application areas for EPD technology. Na-ion batteries (SiBs) and Na-ion supercapacitors (SiCs) are two important examples of these EES technologies. While the main structure of LiBs and SiBs is the same, SiBs consist of sodium-based material as cathode instead of lithium. 301 Although Na + ions are larger than Li + ions, Na + transport properties in a battery are highly connected with the crystallographic morphology of the electrodes for long cycle applications. 302 Here, electrode materials hold a significant function in the operation of the SiBs. Whereas layered transition metal oxides, polyanionic compounds can be used as electrode materials in SiBs, carbon-based materials, transition metal oxides, alloying materials, phosphorous-based materials and insertion materials (ie, MXene, titanate) may be employed in anode materials of SiBs. 303 As can be seen from the structure of the anodic and cathodic materials, the EPD method can be useful for the production of high surface area electrode materials without binders for SiBs. Dashairya et al used the EPD technique to prepare anodic materials consisting of Sb/rGO on Cu foil for use in SiBs. They obtained 350 mAh g À1 specific capacity at 0.2 C current rate up to 100 cycles ( Figure 7A). 138 SiCs are another important developing EES technology with its comparable properties such as high energy density, fast energy delivery and long cycle life, which have attracted attention to compete with LiCs. 304 In the classical anodic design of SiCs, battery-like anodic compounds, such as hard carbon materials, transition metal sulphides and selenides, Tiand Nb-based compounds consisting of various types of reactions (insertion, alloying and conversion) have been studied. 304 On the other hand, capacitive cathodic materials such as AC, graphene and CNTs can be used to improve the performances of SiCs. 304 As can be seen from the used electrode materials in SiBs and SiCs, the EPD method has great potential for use in the production of high-performance electrodes with its inherent advantages. Similar advantages for EPD could be envisaged for magnesium ion batteries, as reported recently, but this is still a developing area of research. 305 Finally, the potential for EPD to manufacture 2D MXene-based membranes with high sodium-ion selectivity could pave the way for highly effective means for sourcing sodium salts. For instance, in a recently published work, a rapid means for preparing a 2D membrane of two orders of magnitude larger area (575 cm 2 ) than those made via standard vacuum filtration was reported using EPD. 306 Around 99.5% recovery of sodium and other ions were reported and it is postulated that this technology could be extended to sourcing lithium ions for LiBs in future studies.

| CONCLUSIONS
Sustainable energy supply is getting more important day by day as a consequence of the increasing energy demand of today's knowledge-based societies. In this regard, the storage of energy obtained from different sources such as renewable and/or classical sources via EES devices plays a key role. In most EES devices such as LiB, supercapacitors and RFBs, electrode materials and their preparation have a critical influence on the cost and the performance of the devices. From this point of view, preparation of the electrodes in easy, economical, sustainable and industrially applicable ways is needed by the battery/supercapacitor manufacturers. Electrophoretic deposition is also increasingly being used to meet this demand since it has many advantages such as low cost, facile applicability, short formation time, simple apparatus requirement, homogeneous coatings preparation, binder-free processing and selective modification as a function of thickness and mass loading. Although this technology has been used for numerous academic studies, it also has great potential for industrial applications, especially in the manufacture of battery materials, as demonstrated in a recent publication by WMG. So far, most of the EPD activity has focused on the preparation of LiBs and LiCs. One common drawback usually noticed from academic publications on energy storage electrode materials is the substantially thin-film coatings that do not conform to industrial standards for, say, supercapacitor applications. Many of the literature reports address the enhanced specific capacity and power density but hardly discuss the mass loading and film thickness that are necessary for wide-scale commercial adoption. Additionally, the challenges that EPD as a process faces in meeting these industrially relevant standards while maintaining sufficient porosity and tortuosity of the asprepared films is of utmost importance that needs full attention to detail. Therefore, this review also attempts to highlight these key aspects of the development of the EPD process such that future research activities consider these practical aspects. Recently WMG reported the production of activated carbon-based supercapacitors meeting industrial standards for film thicknesses above 50 μm and mass loadings in excess of 8 mg cm À2 .
In addition to the above, there is a critical need to enhance the EPD technology for use in other emerging energy storage applications such as RFBs. Since this technology allows the production of electrode materials without the use of binders, it has the potential to reduce the cost of the battery materials by enabling the manufacturing of purer electrodes than conventional means. Hence, this review includes recently published studies, which emphasize the significant role of EPD in enhancing organic redox flow battery performance and discusses key processing information for researchers to follow up. It also opens a fresh perspective for the use of this technology in the electrode modification/production processes of highly economical and market-ready hybrid redox flow cells as reported in a recent study by WMG. Other potential uses of EPD electrodes may involve the recycling of valuable materials from lithium-ion processes for re-purposing as energy storage devices for efficient EV applications. Finally, this review shows early evidence in the evolution of EPD processes and manufacturing tools to produce technologically improved solid electrolytes and separators for LiBs, with future potential applications in both academic and industrial research. However, it is noted that almost no research outputs are reported on the application of EPD electrodes for aluminium and magnesium ion batteries, which could be a significant area of development in the near future. The facts and perspectives discussed herein are expected to raise more interest in this technology for more widespread industrial applications of EPD for EES devices over and above what is already known for solid oxide fuel cells.
Summarizing the EPD studies and the advantages that this technology offers leads to drawing some perspectives and predictions about future activities. It is evident that once thicker films are deposited on a routine base, the barrier towards commercialization will be lowered significantly, which will form strong feedback for further development and applications that are currently neglected. The good control of the EPD process will enable the formation of graded materials, and integration of the technology in flow systems, thus making it possible to form layered deposition very fast. Furthermore, as EPD takes place on electrically conducting surfaces, the selective deposition on conducting patterns and the build-up of high aspect ratio structures can also be foreseen. EPD can be combined in the future with microand nano-electrochemistry and with technologies such as scanning electrochemical microscopy, allowing highresolution patterning of surfaces. The spectrum of materials, in particular, for EES systems will be expanded and also the size and complexity of the building blocks. The optimal size of the building blocks in EES devices is not necessarily in the nanometer range and EPD will have to cope with this. Finally, new approaches will have to be developed for controlling the orientation of the deposited non-spherical materials, such as nanorods. This will give EPD an even stronger advantage over gaseous-based deposition methods and will make EPD an indispensable approach for the construction of highly sophisticated and efficient EES devices.

DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analyzed in this study.