Designed Nanoarchitectures by Electrostatic Spray Deposition for Energy Storage

The development of advanced electrode materials for various energy‐storage systems, especially the fabrication of designed structures and morphologies of electrode materials, has attracted intense interest in both the academic and industrial fields. Among the various synthesis methods, electrostatic spray deposition (ESD) is a simple but versatile approach, by which materials can be fabricated with various morphologies, such as granular, dense, and porous, in an easily controllable manner. Herein, motivated by the rapid advancements of the given technology, a comprehensive introduction of ESD is provided, with emphasis on the kinds of materials and the types of morphology that can be obtained, along with the important control parameters. The progress on electrode materials, which are applied in a great variety of energy‐storage systems, such as Li‐ion batteries, Na‐ion batteries, supercapacitors, Li–S batteries, and Li–O2 batteries, prepared by ESD is also summarized. How the specific morphologies designed by ESD improve the electrochemical performance for different types of electrode materials is also discussed. The aim is to promote the collaborative efforts among different communities to optimize and develop the ESD technique and to explore advanced electrode materials for energy storage.


Introduction
Energy storage is ever more important with the universal recognition of environmental issues. There are major concerns with regard to the present fossil fuel-based energy economy, which leads to depletion of nonrenewable energy sources and presumably dramatic climate changes associated with high CO 2 emission. Therefore, the use clean and renewable energy is advised in order to reduce greenhouse-gas emissions. Solar and wind energy technologies are promising and fairly mature. However, the intermittence of these resources limits their application, which also highly depends on high-efficiency energystorage systems. Among the various types of available energy-storage modes, i.e., mechanical, electrical, chemical, and electrochemical modes, electrochemical energy storage based on batteries and supercapacitors is highly promising to effectively harvest these intermittent energy sources. [1][2][3] In principle, electrochemical-energy-storage systems are divided into two types: high-energy and high-power systems. Batteries show a high energy, but their power density is relatively low. Therefore they usually suffer from a drastic capacity drop during fast charge and discharge. On the contrary, supercapacitors exhibit high power capability, and they can realize fast charge-discharge processes. However, the stored energy of a supercapacitor per unit mass is rather poor. In addition, combining the advantages of both batteries and supercapacitors is also promising, i.e., developing electrochemical energystorage devices that can hold both high energy and high power. [4] Rechargeable lithium-ion batteries are one of the great successes of modern electrochemical-energy-storage devices, and they are widely used in cell phones, laptops, digital cameras, tablets, etc., owing to their high energy density (>180 Wh kg −1 , about 2-3 times the energy per unit weight and volume compared with other conventional rechargeable batteries). [5] Although lithium-ion batteries are also promising candidates for electric vehicles (EVs) and hybrid electric vehicles (HEVs) as the state-of-the-art technology, e.g., the lithium-ion battery in the Tesla, tremendous research efforts have been devoted to new energy-storage systems to overcome the shortcomings of the energy density of lithium-ion batteries for application in mobile transport. Lithium-sulfur (Li-S) batteries and lithiumair  batteries represent next-generation energy-storage systems with high energy density. Lithium-sulfur batteries can offer a high theoretical energy density of 2500 Wh kg −1 or 2800 Wh L −1 , and the abundance of sulfur in the earth crust also makes lithium-sulfur batteries a low-cost system. [6] On the other hand, Li-O 2 batteries provide the highest theoretical energy densities (3500 Wh kg −1 ) for all known battery systems, which is approaching the energy level of gasoline. [7] In addition, oxygen is an environmentally friendly, no-cost, and unlimited source, which makes this system more attractive. Although lithium-based batteries have pronounced advantages due to economic and availability issues, sodium-ion batteries are realistic promising alternatives and draw current worldwide interest. [8] Besides the various types of batteries, if we pursue a high power function, supercapacitors are a very suitable choice. [9] Various energy-storage systems are available; however, no matter whether batteries or supercapacitors, the electrochemical performance is mainly determined by the electrodes materials.
There are, in principle, four storage modes for the battery electrodes. [10] 1) Single-phase storage: In this mode, the storage is based on dissolution, and the phase stays constant. A pertinent example is lithium storage in LiCoO 2 and TiS 2 . [11,12] 2) Two-phase storage: In this case, the storage accompanies phase transformation, e.g., the LiFePO 4 /FePO 4 system. [13] 3) Conversion-reaction storage: [14][15][16][17] This storage mode accompanies phase changes as well; however, it will lead to phase decomposition, which is actually multiphase storage. 4) Interface storage: [18] Lithium can be stored at the interface or higher dimensional defects. No matter the kind of storage mode, the transport properties, such as electronic conductivity, ionic conductivity, and diffusion coefficient, are key parameters for high-performance battery electrodes. In the literature, it is clear that the diffusion coefficients of the most electrodes are fairly low, [19] although the reliable data on the transport properties are rather scarce. As a result, it is significant to overcome the ionic-and electronic-transport limitations by the following approaches: 1) Doping the electrodes: Doping the target material by foreign atoms can change the concentrations of electronic and ionic charge carriers, and hence leads to an increase in the electronic or ionic conductivity. [20] Establishing defect models, i.e., concentration of defects in dependence of control parameters such as temperature, partial pressure, doping concentration, etc., is very helpful for achieving rational control of charge carriers. [21,22] 2) Surface coating: Coating the electrode surface by electronically conducting materials, such as carbonaceous materials (amorphous carbon, [23] carbon nanotubes (CNTs), [24] graphene [25] ), metals or metal oxides (Ag, [26] RuO 2 [27] ), or conducting polymers. [28] This method is highly effective for improving the electronic conductivity of the electrodes. 3) Size-reduction: According to the formula τ = L 2 /2D (where τ is the diffusion time, L is the diffusion length, and D is the diffusion coefficient), due to the quadratic dependence of τ on L, reducing L can significantly shorten the diffusion time. 4) Morphology control: The transport properties and diffusion behaviors for each electrode materials are different. To design specific architectures for relevant electrode materials according to their transport properties is a highly effective approach. [19]

The Electrostatic Spray Deposition Technique
Since the invention of the ESD technique, which was first developed by Schoonman and co-workers at Delft University of Technology, [35] this approach has found a great number of applications for various energy-storage devices (Li-ion batteries, Na-ion batteries, supercapacitors, Li-S batteries, and Li-O 2 batteries), with different typical structures and morphologies, such as particles, dense and porous thin film, as shown in Figure 1. Figure 1 also exhibits the experimental set-up of ESD. In principle, a typical setup includes three parts: 1) a metal nozzle, which is connected with a syringe and provides the precursor solution; 2) a substrate, whose temperature can be controlled by a thermocouple; and 3) a high-DC voltage supply. Briefly, in the ESD process, owing to the high DC electric field between the nozzle and substrate, the precursor solution is atomized into an aerosol. Afterward, the charged aerosol (composed of charged droplets) is attracted and deposited on the substrate. The solvent is evaporated and the relevant chemical reactions occur due to the controlled high temperature. Finally, thin-film materials with various characteristic morphologies are obtained. [38] In addition, a particle suspension of the material can also be deposited onto a substrate to form a solid layer, besides forming a thin film from the precursor solution. [40] In a typical ESD process, there are several either sequentially or simultaneous physical and chemical process involved, including mainly five steps: [39] 1) spray formation, 2) droplet transport, evaporation, and disruption, 3) droplet preferential landing, 4) droplet spreading and penetration, and 5) decomposition, reaction, and surface diffusion. Each step is described below separately: 1) Spray formation: Due to the high voltage applied between the metal nozzle and the substrate, when the applied voltage is higher than a certain threshold, the precursor solution is drawn into a cone. The characteristic shape of the cone is often called the "Taylor cone." If the voltage is high enough, the electrostatic force overcomes the surface tension www.advmat.de www.advancedsciencenews.com of the precursor liquid, and a microjet will be extracted from the Taylor cone. This microjet elongates to a fine filament and disperses into small aerosol droplets. [41] Such a Taylor-cone-jet mode [42] is the most used, due to its capability to stably generate monodisperse particles. However, depending on the form of the meniscus, there are also several other modes in terms of the motion pattern of the jet, the way the jet disintegrates into droplets. For the cone-jet mode, the size of the droplets is usually determined by the following equation: [41]

Setup and Basic Principles of ESD
where d is the droplet diameter, α is a constant determined by the permittivity of the liquid, Q is the liquid volume flow rate, ε 0 is the permittivity of the free space, ρ is the mass density of the liquid, σ is the liquid conductivity, and γ is the surface tension of the liquid. The exponents a, b, c, d, and e in Equation (1) are constants, which vary slightly according to different authors. [41] A conclusion can be drawn that three main factors determine the droplet size, i.e., the electric-field strength, the liquid flow rate, and the properties of fluid (including viscosity, surface tension, conductivity, and relative permittivity). The droplet or spray formed by the ESD technique is generally smaller than other atomization methods, and it will not aggregate due to its unipolar charged property, leading to a much narrower size distribution. To achieve atomization by ESD, the precursor solution need to be electrically conductive, and a high DC voltage in the range of 2-15 kV is usually applied. 2) Droplet transport, evaporation, disruption. The charged droplets are attracted to the ground substrate by a Coulombic force due to the electric field. Solvent evaporation during flight will occur, especially under heating. When the droplets move to the heated substrate, a temperature gradient is built between the metal nozzle and the substrate. The evaporation of solvent leads to the size reduction of each droplet with unchanged charges; therefore, the surface charge density of the droplet increases. The point when the Coulombic repulsion of the surface charge is equal to the surface tension of the droplet is the so-called Rayleigh limit, which can be expressed as Equation (2).
where Q R is the maximum attainable charge density, m is the mass of the droplet, ρ is the true density of the droplet, r is the radius of a liquid droplet, and γ is the surface tension of the liquid with respect to the surrounding gas. A charged droplet maybe disrupted into several smaller droplets after reaching the Rayleigh limit. Note that the Coulombic repulsion among smaller droplets will prevent smaller droplets aggregating into big droplets, resulting in a uniform morphology. 3) Droplet preferential landing. The charge distribution is generally not uniform. The position relative to the nozzle, and especially the local curvature of the substrate surface will influence such a charge distribution. In general, the place with the greater curvature has more charge concentration, leading to a stronger electric field than other places. When a charged droplet moves to the substrate surface, it will be attracted more to the place the greater curvature (so-called preferential landing). This indicates that the morphology will be influenced by the surface condition of the substrate. 4) Droplet spreading and penetration. The type and dynamics of spreading depend strongly upon the spreading coefficients: [39] sg sl lg where γ sg , γ sl , and γ lg represent the interfacial tensions between the substrate and the ambient gas, between the substrate and the liquid drop, and between the liquid drop and the ambient gas, respectively. If S < 0, only partial wetting occurs, with equilibrium being reached at a finite contact area. If S ≥ 0, the drop spreads until it totally covers the surface. The value of S is related to the spreading rate. In principle, the substrate properties and the viscosity of the liquid are the main factors that affect the spreading rate. In general, the higher the viscosity, the lower the spreading rate. In addition, when defects (i.e., cracks or pinholes) are formed in the earlier deposited layer, the subsequent droplets may penetrate into them through capillary action. By this repair effect, a defect-free layer can be obtained by the ESD process. 5) Decomposition, reaction, and surface diffusion. Decomposition and reaction can occur before or after the droplets arrive at the substrate. If the surrounding temperature is high enough, the droplets will be dried before reaching the substrate. If the temperature is relatively low, the solution droplets will reach the substrate, and the wet-chemistry process of a solvent and metal salt precursor will determine the morphology of the layer. Generally speaking, in the ESD process, the final morphology of the thin-film layer depends on the relative rates of the different processes, i.e., spreading, precipitation, decomposition, and reaction.

Various Materials Prepared by ESD
For a typical ESD process, the precursor solution is prepared by dissolving metal nitrates or acetates in water, ethanol, ethylene glycol, propylene glycol, 1,2-propanediol, butyl carbitol, or their mixtures. During the ESD process, the precursors are easily decomposed and form various metal oxide thin-film layers in an ambient atmosphere. In fact, ESD is a very powerful technique to prepare different oxides. A large number of oxides have been fabricated by such techniques for various functions, e.g., CeO 2 , [43] LiCoO 2 , [44] [54] etc. However, the other compounds can also be prepared by the ESD technique with some modifications in the precursors. If the metal precursor contains a sulfur source (e.g., (NH 4 ) 2 MoS 4 for preparing MoS 2 ) or if additional sulfur sources such as (NH 4 ) 2 CS or L-cysteine are added in the precursor solution, various metal sulfides can be obtained as well.
The successful examples are CdS, [55] MoS 2 , [37] WS 2 , [56] ZnS, [57] SnS, [58] etc. Following the same method, selenides such as CdSe have been successfully prepared with an additional precursor (NH 4 ) 2 CSe. [59] By controlling the precursor on purpose with the target elements or units, a great number of compounds, i.e., carbides, [60] metals or alloys, [61] phosphates, [62] titanates, [63] or silicates [64] can be fabricated by the ESD technique as well (detailed www.advmat.de www.advancedsciencenews.com compounds can be seen in Table 1). In addition, besides the dissolved precursors, suspensions are also suitable to grow thinfilm layers by the ESD technique. Hence, some carbonaceous materials, i.e., CNTs or graphene have also been prepared. [65] Moreover, by using a mixture of different types of precursors, a great deal of composite thin films can be fabricated, [36,66] which can be seen in Table 1 as well.

Various Controllable Morphologies by ESD
The typical morphologies obtained by ESD can be divided into three main categories: granular, dense, and porous. One of the pronounced highlights of the ESD technique is that the morphology is easily controlled by the ESD experimental parameters, such as the voltage, the distance between the metal nozzle and the substrate, the concentration of the precursor solution, the flow rate, the substrate temperature, the deposition time, etc., which makes this technique a very powerful approach for nanostructure design. As mentioned earlier, the solution chemistry, for example, the relative rates of spreading, precipitation, decomposition, and reaction will also have an important influence on the final morphology. For instance, if the precipitation and decomposition are fast while the spreading rate is slow, a granular morphology will be formed. On the other hand, in the opposite situation, where the spreading of the droplets is fast, the solutes in solvent are sufficiently large, the droplets arriving at the substrate are still wet, then a dense film will be obtained. [39] In addition, as to optimizing the battery performance, a porous structure, especially a 3D crosslinked porous structure is important. A possible formation mechanism of this porous structure by ESD was proposed by Chen et al. [67] Assuming that the droplets reaching the substrate surface are still wet, simultaneous spreading of the solution and evaporation of the solvent occurs. Due to the higher local temperature at the edge part of a landed droplet than one in the middle part, nucleation and precipitation of the solute take place, first at the edge, with evaporation of the solvent occurring. This gives rise to many voids in the droplets, leading to a porous structure. It can be understood that the most important factors to influence the formation of the porous structure include the substrate temperature, the boiling point of the solvents and the surface tension and viscosity of the solution.
Understanding the rules as to how experimental parameters influence solution chemistry and how such parameters affect the final morphology and structure during ESD are very important to achieve rational morphology design by ESD. LiOH. [69] The reticular morphology obtained using LiNO 3 leads to a smaller pore diameter and thinner pore walls, which is different from LiOH and LiOAc sources. Li 2 CO 3 is the only lithium source of the four that does not give rise to a reticular structure. (iii) The Effect of Flow Rate: Chen et al. used ESD to prepare porous LiCoO 2 thin films and investigated the effect of flow rate on the final morphology. They found that at small flow rates, the average pore size was approximately proportional to the flow rate. [67] Adv. Mater. 2019, 31, 1803408

Advantages of ESD
There are several advantages of the ESD technique. First of all, ESD is a simple, cheap, and flexible method, by which morphology can be easily controlled at the nanoscale. Second, using this method, highly pure materials can be prepared. Third, ESD can produce extremely thin layers, and the quality of such thin films is high, which can be defect-free and more homogeneous compared to other approaches. Fourthly, the growth rate of the thin film by the ESD technique is relatively high compared to other thin-film techniques such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). [40] For instance, the deposition rate of a 1 µm thin-film layer is 0.1, 0.006-0.06, 0.02-0.05 µm min −1 by ESD, PVD, and CVD techniques, respectively. [40,73] Fifth, ESD can be carried out in an ambient atmosphere and at low temperature; especially, complex reactors and high-vacuum systems are unnecessary. In addition, the efficiency of the ESD process is very high, and almost 80-90% of base materials can be deposited onto the substrate. [40] Finally, if the ESD technique is applied to prepare electrodes for energy storage, one of the pronounced advantages is that the electroactive materials can be directly deposited on the various current collectors (also acting as a substrate) without any additional conductive additives and binders, which greatly simplifies the battery assembly process. Moreover, it can further increase the rate capability of electrodes (binders are usually nonconductive polymers) and improve the energy density of the electrode (without nonelectroactive parts).

New Trends of ESD
There are some new advances for the ESD technique. 1) Preparation of new compounds by ESD: As we discussed before, since ESD normally is carried out in an ambient atmosphere, it is a widely used method to fabricate various metal oxides. By carefully choosing appropriate precursors, new compounds, such as sulfides, have been prepared recently. [58] 2) Finding new applications: Since new compounds have been successfully prepared, the materials obtained by ESD have more and more applications. In terms of energy storage, previously, the materials prepared by ESD were mainly applied in Li-ion batteries and supercapacitors, while, lately they are also able to be applied in Na-ion batteries, Li-S batteries, and Li-O 2 batteries. 3 www.advmat.de www.advancedsciencenews.com ESD technique can be applied together with other synthesis methods, e.g., the templating method, [74] the sol-gel method, [48] the solvothermal method, [75] the electrospinning method, [76] etc., to fabricate new and more complex morphologies. For example, when ESD is applied together with the soft-chemistry method such as the sol-gel and solvothermal methods, the building units of the final materials can be modified. When ESD is combined with the template method, the porosity can be carefully controlled. When ESD is applied together with the electrospinning method, unique 1D-3D composites can be prepared. Detailed information on such combined techniques will be discussed for the specific materials in the following text. 4) Combination of ESD with other characterization methods: The ESD technique can be combined with other advanced characterization approaches to realize in-depth characterization. For instance, change of mass of LiMn 2 O 4 can be investigated in situ by electrochemical quartz crystal microbalance (EQCM) combined with ESD. [70]

Application Potential of ESD for Energy Storage
Generally speaking, frequently used approaches (e.g., doping, carbon-coating, size-reduction, and morphology control) that can improve the performance are all compatible with the ESD technique. First of all, rational doping is easily achieved during the ESD process. Second, ESD is an effective carbon-coating technique to coat various carbonaceous materials, such as CNTs, rGO, and amorphous or crystalline carbon. It is also an efficient method to prepare various electrode/carbon composites. Third, according to the principle of ESD, electrodes with nanoscale components are easily obtained. Finally, ESD is a simple and powerful technique that can easily control the morphology. Various morphologies such as dense, porous, and 3D porous tricontinuous composites, etc. obtained by ESD can just meet the different requirements of various energy-storage devices based on different mechanisms. In addition, ESD is a very suitable technique to fabricate self-supported thin-film or 3D electrodes without additional conductive additives and binders.
In terms of the design of nanoarchitectures, if we would like to improve the performance of electrode materials for different energy-storage devices (i.e., batteries and supercapacitors), the first step is to determine what kinds of structures or morphologies are preferred for various application aims. The energy-storage mechanisms of electrodes for batteries and supercapacitors can be generally divided into three types: the intercalation reaction, the alloying and conversion reaction, and the surface/interface reaction. 1) The intercalation reaction includes most cathode and some anode materials for lithium and sodium batteries. The transport properties, such as electronic conductivity, ionic conductivity, and chemical diffusion, are key issues for this type of reaction. The chemical diffusion coefficient is given by: [77] ion eon eon where σ ion , σ eon , and σ are the ionic conductivity, electronic conductivity, and total conductivity. c eon and c ion are the concentrations of the electronic and ionic carriers. The χ-factors describe the internal trapping reactions (usually crucial at the temperature of operation). For electrodes based on intercalation, pure materials with high crystallinity and an effective mixed conducting network are crucial for improving the performance.
2) The alloying and conversion reaction includes most anodes and some cathodes materials of lithium-and sodium-ion batteries, as well as Li-S and Li-O 2 batteries (the reactions from sulfur to Li 2 S and from O 2 to Li 2 O 2 can be considered as conversion reactions). Besides transport properties, how to buffer the huge volume change and prevent pulverization and cracks of the electrodes are highly significant and of wide concern for this type of reaction. As a result, the designed structures or morphologies of electrodes should have the abovementioned multifunctions (facile ion/electron transport, buffering of large volume changes, and preventing pulverization and cracks of the electrodes).
3) The surface/interface reaction relates to supercapacitors and Li-O 2 batteries. A high surface area is particularly important for both surface adsorption of supercapacitors and surface catalysis of Li-O 2 batteries. Structures that can improve the charge and mass transport and increase surface catalysis reactivity are desired. Table 3 summarizes the energy-storage mechanisms versus various devices, as well as different structure and morphology requirements for various batteries and supercapacitors.
In the following text, by collecting pertinent examples, we will show the great potential of ESD for energy storage. We will summarize the recent progress on electrode materials for Li-ion batteries, Na-ion batteries, supercapacitors, Li-S Adv. Mater. 2019, 31, 1803408 Table 3. Summary of the energy-storage mechanisms, systems, and requirements for high-performance electrodes.

Mechanisms
Devices and systems Requirements Intercalation reaction Cathodes and anodes for Li-ion batteries 1. Pure materials with high crystallinity; Cathodes and anodes for Na-ion batteries 2. An effective mixed conducting network.
Alloying and conversion reaction Cathodes and anodes for Li-ion batteries 1. An effective mixed conducting network; Cathodes and anodes for Na-ion batteries 2. Buffering of the huge volume changes; Li-S batteries 3. Preventing electrode pulverization and cracks.

Li-O 2 batteries
Surface/interface reaction Supercapacitors 1. High surface area; 2. Facile charge and mass transport; 3. Increase of the surface catalysis reactivity.
www.advmat.de www.advancedsciencenews.com batteries, and Li-O 2 batteries prepared by ESD. The various designed nanostructures fabricated by ESD and the role that the unique advantages and aspects of ESD plays on the electrochemical performance of various specific electrodes will be discussed and emphasized.

Cathode Materials for Lithium-Ion Batteries Prepared by ESD
Since the invention of the lithium-ion battery, LiCoO 2 has been the dominant cathode material. [78] In order to further decrease the high cost and environmentally unfriendly element Co, other layered transition-metal oxides materials with the formula of LiMO 2 (M: Mn, Co, Ni, or a mixture of them) or lithiumrich transition-metal oxides with the formula of xLi 2 MnO 3 + (1−x)LiMO 2 have also been investigated. Other developments regarding the cathodes have focused on lithium manganese oxides spinel and lithium transition-metal phosphate, such as LiFePO 4 . Because most lithium cathodes are based on the intercalation reaction, pure materials with high crystallinity and sufficient lithium diffusion are the most critical (Table 3). In this direction, ESD can be applied due to its impurity-free synthesis properties and its effective mixed-conducting-network solution.

Layered Lithium Transition-Metal Oxides
In the past two decades, intense effort has been devoted toward layered LiCoO 2 cathodes and relevant LiCoO 2 , LiMnO 2 , and LiNiO 2 systems. Replacing Co with relatively cost-effective and abundant Ni or/and Mn is a widely used method to increase the market potential. Research has been focused on increasing the power capability and cycling stability, especially on elevated temperatures or at higher potentials. The solutions include surface coating, [79,80] core-shell structures, [81] metal doping, [82] and new electrolytes. [83] ESD uses a precursor solution including Li and Co elements, which can be mixed on the atomic scale, resulting in final products with high purity and designed composition and morphology.
The ESD technique was first developed to prepare 4 V LiCoO 2 cathode materials by Schoonman and co-workers. [44] The morphology of LiCoO 2 can be easily controlled during the ESD process. LiCoO 2 with morphologies of dense layer, dense layer with incorporated particles, porous top layer with a dense bottom layer, and fractal-like porous layer can be obtained by carefully controlling the experimental parameters, such as the deposition time, deposition temperature, precursor solution concentration, electric-field strength, substrate, and solvent. [39] Especially, by using a solvent mixture of ethanol (15 vol%) and butyl carbitol (85%), LiCoO 2 thin films with a unique 3D crosslinked structure, having a high porosity and a narrow pore size distribution, are obtained. [84] The crystallinity of LiCoO 2 can be controlled during the ESD process by changing the substrate temperature and postannealing temperature. When the substrate is 280 °C, amorphous LiCoO 2 is formed; while above 340 °C, crystalline hexagonal LiCoO 2 is obtained. [44] For a Li cathode, a high crystallinity is important, as we discussed before. Yoon et al. found that a LiCoO 2 thin film by ESD, with a stable cycling performance, can be obtained by annealing at temperature as low as 600 °C. [85] ESD can also be applied as a coating technique to increase the performance of LiCoO 2 . For example, Yu et al. synthesized nano-SiO 2 -modified LiCoO 2 thin films by the ESD technique. [86] The texture of the film is porous, with a LiCoO 2 (shell)/SiO 2 (core) structure. This LiCoO 2 -SiO 2 (15%) exhibits the highest discharge capacity of 130 mAh g −1 at the current density of 0.1 mA cm −2 , and the lowest impedance. SiO 2 helps to improve the cycling stability, i.e., the capacity of LiCoO 2 -SiO 2 (15%) hardly fades over 60 cycles. In addition, an LiMn 2 O 4 -coated LiCoO 2 thin-film electrode has been successfully prepared as well. [87] In terms of electrochemical performance, a highly porous LiCoO 2 thin film without binder and conductive additives prepared by ESD is a good choice. [88] Koike et al. were able to use an aluminum substrate with a low temperature of 650 °C to prepare LiCoO 2 by ESD. Walls a few micrometers thick and ≈5 µm holes were observed, as shown in Figure 2a. The capacity of the first cycle is 140 mAh g −1 and the capacity is still 93% of the first cycle after 100 cycles at a 1C rate. The rate performance is good for this film, and the capacity is around 79% and 57% at high rates of 10 C and 20 C, respectively ( Figure 2b).

Layered Transition-Metal Oxides
Among the layered transition metal oxides, great attention has also been paid to V 2 O 5 and other relevant compounds with a layered structure, due to their pronounced advantages, such as abundance, low cost, and ease of fabrication. In terms of the electrochemical performance, they have high energy efficiency and high theoretical capacity (≈294 mAh g −1 corresponding to 2 lithium insertion), which are comparable to other commonly used cathodes, such as LiCoO 2 . [89,90] The major concerns for these materials are the poor transport properties and the structure stability, which lead to limited rate performance and cycling stability. A great number of nanostructured designs have been devoted to these concerns, i.e., synthesizing nanotubes, nanofibers, nanoparticles, and so on. [91,92] ESD is a suitable technique to prepare porous V 2 O 5 , which greatly increases the transport properties. Kim et al. used the ESD technique to prepare a V 2 O 5 thin-film electrode on a platinum substrate. [93] The porous V 2 O 5 thin film showed a high capacity of 270 and 260 mAh g −1 at current densities of 0.2 C and 1 C, showing no fading after 25 cycles in the voltage range of 2-4 V. Furthermore, a unique 3D porous multideck-cage V 2 O 5 thin film with very high rate capability and cycling stability was achieved by Wang et al. by using the ESD technique. [49] The morphology-formation mechanism was investigated (Figure 3a). First of all, on a stainless-steel substrate, a 2D reticular layer is formed. As the deposition time increases, such 2D layers will stack together and the thickness will increase. Finally, the new spray droplets prefer to be deposited on some standingout spots to form spherically shaped structures, leading to 3D multideck-cage microspheres. Such 3D porous structures are very stable, even after annealing in air at 350 °C for 2 h. In the voltage range of 2.5-4.0 V, the ESD V 2 O 5 film will transform to LiV 2 O 5 with a capacity of 142 mAh g −1 , which is nearly the theoretical capacity. The rate capability is excellent: the capacities are 120 and 86.7 mAh g −1 with current densities of 8 C and even 56 C, respectively (Figure 3b). The cycling performance is good as well, i.e., it can at least maintain 110 mAh g −1 during 200 cycles (Figure 3c). This demonstrates that the 3D porous structure from ESD is effective to increase the transport of both electrons and ions.
ESD can be applied as a doping method to further increase the performance. Li et al. prepared a Fe 0.1 V 2 O 5.15 thin film by the ESD technique. [94] They found that Fe 3+ can act as a stabilizing agent in the layered V 2 O 5 and delay the ε → δ → γ phase transition at a potential close to 2 V, leading to better reversibility and cycling stability in the voltage range of 2-4 V. Porous Fe/V oxides, including crystalline Fe 2 V 4 O 13 and amorphous Fe 2 V 4 O 12.29 thin film, were prepared through ESD by the same group as well. [54] They found that amorphous Fe/V oxide shows better rate capability and cycling performance compared to its crystalline counterpart.
In addition, the ESD technique can be combined with the solvothermal method to further tune the morphology. For example, micrometer-sized agglomerates of primary V 2 O 5 particles have been prepared, for the first time, by an oil-bath method. Afterward, by using ESD and subsequent heat treatment, such particle suspensions are deposited on carbon plate substrates as a thin film comprising interconnecting walnutlike particles. [75]

Lithium Manganese Oxides Spinels
Lithium manganese oxide spinel (i.e., LiMn 2 O 4 ) is another attractive alternative to commercialized LiCoO 2 , due to its advantages such as abundance, low cost, environmental friendliness, high safety, and high power capability. However, the major issue of this material is the severe capacity fading with cycling. [95] The reasons for that are as follows: 1) dissolution of LiMn 2 O 4 into acidic electrolyte, especially at elevated temperatures, according to the disproportion reacting: 2Mn 3+ → Mn 4+ + Mn 2+ ; 2) in the high-voltage region, transformation of an unstable two-phase structure to a more stable single-phase structure via the loss of MnO; [96] and 3) some electrochemical reactions occurring in the electrolyte at a high voltage. There are some effective approaches that are applied to overcome such issues, for example, doping LiMn 2 O 4 with a lower valence state (such as Li or Al), [97] surface coating, [98] and choosing suitable electrolyte additives. [99] and 4 h, respectively). b) Rate capability, charged at a 0.5 C rate. c) Cycling stability at a 10 C rate. Reproduced with permission. [49] Copyright 2011, Royal Society of Chemistry. www.advmat.de www.advancedsciencenews.com parameters during ESD; namely, sponge-like porous (15% ethanol + 85% butyl carbitol), fractal-like porous (80% ethanol + 20% glycerol), and dense structures (pure ethanol). [68] Among these various morphologies, the sponge-like porous LiMn 2 O 4 cathode has a lower resistance and better rate capability compared to the dense film, which can be attributed to the large active surface area.
ESD offers the possibility of combination with advanced electrochemical techniques to achieve in-depth characterization. For the LiMn 2 O 4 cathode, it is important to investigate the surface films that surround the LiMn 2 O 4 particles in situ. The EQCM is an in situ mass-sensitive detector based on the measurement of resonant-frequency changes induced by mass or viscosity changes of a film attached to a quartz-crystal substrate during electrochemical measurement. [70] This EQCM technique can be applied to investigate the mass change of LiMn 2 O 4 due to the formation of a surface film. The key point of EQCM is the fabrication of LiMn 2 O 4 thin film on the EQCM electrode, which can be effectively achieved by the ESD technique at relatively low temperature without any conductive additives and binders. Shu et al. used EQCM combined with the ESD technique to investigate an electrode with a surface film of LiMn 2 O 4 . [70] They found that the mass of the surface layer around LiMn 2 O 4 was 1.2%.
In order to increase the energy density of LiMn 2 O 4 , ESD is also suitable for the preparation of phase-pure and high-voltage LiNi 0.5 Mn 1.5 O 4 thin-film electrodes, which deliver a capacity of 135 mAh g −1 . [100] It can be controllably deposited on various substrates, e.g., flat or 3D-architectured substrates, demonstrating the great potential to fabricate 3D all-solid-state batteries by the ESD technique.

Lithium Transition-Metal Phosphates
Lithium transition-metal phosphates such as LiMPO 4 (M = Fe, Co, Ni, Mn) and Li 3 M 2 (PO 4 ) 3 (M = V, Fe) have attracted great attention, owing to their competitive energy density and outstanding thermal stability. LiFePO 4 , first reported by Goodenough and co-workers, [13] has obtained great success in the last two decades, and was considered the safest cathode material for lithium batteries. Nonetheless, the major shortcomings of LiFePO 4 are its intrinsic sluggish mass and charge transport. [101,102] In order to overcome the electronic-and ionic-transport limitations, doping, [20,22,[103][104][105] particle-size reduction, [106,107] coating with electronic (or ionic) conductive materials, [23,108] and control of shape morphology [109,110] are widely used approaches. ESD is an effective coating and morphology-control approach to increase the transport properties of different lithium transition-metal phosphates.
Thin-film olivine structure phosphates have been successfully prepared. Ma  The poor electronic conductivity is a key issue for phosphates. ESD is powerful to realize carbon coatings or build phosphate-carbon composites by simply adding some carbon sources during the ESD process, which can reduce the particle size of the phosphate as well. Wang et al. used the ESD technique by adding glucose as a carbon precursor to prepare Li 3 V 2 (PO 4 ) 3 /C thin films. [113] The thickness of the film was about 3 µm, and it was composed of ≈10 µm walnut-kernellike clusters. Such clusters are further composed of small ≈50 nm Li 3 V 2 (PO 4 ) 3 crystals, which are distributed in a continuous carbon matrix. The carbon content is ≈10%. The rate capability of the Li 3 V 2 (PO 4 ) 3 /C thin film is excellent. In the voltage range of 3.0-4.3 V, the specific capacity is 118, 115, and 80 mAh g −1 at current densities of 1, 6, and 24 C, respectively. The Li 3 V 2 (PO 4 ) 3 /C thin film exhibits stable capacity retention, i.e., the capacity maintains 113 mAh g −1 (initially 118 mAh g −1 ) after 100 cycles. This demonstrates that the 3D porous structure of phosphate/carbon is a very useful approach to overcome the shortcomings of mass and charge transport for these materials.

Anode Materials for Lithium-Ion Batteries Prepared by ESD
Based on the electrochemical reaction mechanism, anode materials for lithium-ion batteries can be divided into three categories: intercalation-deintercalation, alloying-dealloying, and conversion reactions. Except for the intercalation reaction, which is similar to that for cathode materials, most anode materials suffer from a large volume change and cracks during the lithiation process ( Table 3). The ESD technique has great potential for the preparation of high-rate, long-cycle anode materials with various designed nanostructures, which are able to accommodate the huge volume change and prevent electrode pulverization.

Intercalation-Deintercalation
The classic electrochemical reaction mechanism for an anode material is intercalation and deintercalation, such as graphite, mostly used in the commercial lithium-ion batteries. TiO 2 is another promising anode based on this mechanism, where a phase-pure material and an effective mixed conducting network are very important. ESD is applied to carry out impurity-free synthesis and morphology control. Chen et al. used the ESD technique in combination with the sol-gel process to synthesize TiO 2 thin film with nanostructures on various substrates, which combined the advantages of both ESD and the sol-gel method. [48] Titanium tetraisopropoxide was dissolved in absolute ethanol. Afterward, the vessel was sealed and the solution was aged until it become a semitransparent sol. The sol was taken and diluted to prepare the precursor sol for ESD. Al www.advmat.de www.advancedsciencenews.com disks, ITO, and Pt disks were chosen as substrates. The deposition temperature determined the surface morphology of the ESD-derived TiO 2 films. For instance, at 100 °C, a very dense and smooth film can be prepared, which cannot be obtained with an ethanolic precursor solution by ESD; at 220 °C, densely packed submicrometer TiO 2 particles were obtained.
Li 4 Ti 5 O 12 is a promising zero-strain electrode material involving two phases with the same symmetry (Li 4 Ti 5 O 12 -Li 7 Ti 5 O 12 ), with the voltage ≈1.5 V. [114] This anode material delivers a specific capacity of 175 mAh g −1 with excellent cycling stability. It can be either coupled with a high-voltage cathode, such as LiMn 2 O 4 or LiCoO 2 , to offer a working voltage of ≈2.5 V, or coupled with LiFePO 4 to provide a cell with high safety. The main issues for this anode material are its low electronic conductivity and the low lithium-ion diffusion coefficient. ESD is an effective method to prepare high-performance Li 4 Ti 5 O 12 porous thin films to overcome these transport issues. Phase-pure Li 4 Ti 5 O 12 is obtained after annealing at 700 °C with a porous nanotree morphology by using lithium acetate and titanium butoxide as the precursor. [51] The Li 4 Ti 5 O 12 thin film exhibits a specific capacity of 150 mAh g −1 (close to the theoretical capacity) with a Coulombic efficiency nearly 100% and excellent capacity retention for 70 cycles.

Sn Prepared by ESD
Anode materials based on alloying-dealloying, such as Sn, Si, and Ge, are able to offer higher capacity compared to commercialized graphite anode (372 mAh g −1 ). For Sn anode materials, Li 4.4 Sn can be formed, with a corresponding capacity as high as 994 mAh g −1 , which is a serious candidate to replace the graphite anode. [115] However, it also has several problems for Sn anodes. When Sn reacts with Li to form Li 4.4 Sn, it will lead to large volume change (259%). [14] Pulverization and cracks of the Sn anode during cycling result in a poor cycling stability. [116] The main approaches to overcome such issues include reducing the size of the Sn to the nanoscale [117] and preparing Sn-carbon composites or Sn-metal alloys. [118] ESD is a suitable method to tune the nanostructure of Sn and achieve carbon-coated Sn composites, leading to high-capacity and long-cycling anode materials. Li et al. fabricated 3D porous core-shell Sn@carbon composite anodes directly on a Ni foam substrate by the ESD technique (Figure 4a). [61] Tin (IV) acetate and PVP was applied as the precursor. During the ESD process at 270 °C, tin (IV) acetate decomposes into SnO 2 , leading to PVP-coated SnO 2 . After annealing at 900 °C, SnO 2 was reduced to crystalline Sn metal and the PVP was transformed into a conductive carbon layer. The thickness of such a carbon layer is 3.2 nm, with the carbon content in the Sn-carbon being around 35% by weight. The capacity is 672 mAh g −1 at a current density of 25 mA g −1 , and the values of capacity retention are 86%, 77%, 71%, 61%, and 52% relative to the capacity at 25 mA g −1 at the current density of 100, 150, 200, 250, and 300 mA g −1 , respectively (Figure 4b). Such a designed 3D porous structure and carbon shell can buffer the large volume change during the charge-discharge process and enhance the electric conductivity, resulting in a high-performance Sn anode.

Si Prepared by ESD
Si is another promising anode material based on the alloying process, whose capacity is as high as 4212 mAh g −1 , almost 10 times higher than the commercial graphite anode. [119] However, the poor electronic conductivity of Si limits the power density; while the huge volume change (>300%) results in pulverization and electric disconnection of the Si particles, leading to poor cycling performance. In order to solve such issues, designing various nanostructures for Si anodes, such as nanoparticles, nanowires, and nanotubes, and porous or hollow structures are proved to be effective approaches to improve the performance. [120][121][122] Synthesis of Si/carbon composite is another widely used method as well. [123,124] ESD offers different ways to fabricate various Si/carbon composites, which can be used independently or applied together with other techniques to prepare high-performance Si anodes. Yin et al. used electrospray to synthesize Si/carbon nanoporous microspheres. [125] Si nanoparticles were dispersed into an aqueous solution of sodium alginate to form the precursor solution, which was electrosprayed in to CuCl 2 aqueous solution to form microspheres. After calcination, carbon coating, and a HF etching process, the target Si/carbon was obtained  www.advmat.de www.advancedsciencenews.com (Figure 5a). Such microspheres exhibited a pear-like morphology with a 30-70 µm diameter, with a surface area of 57 m 2 g −1 and a pore size of 30 nm (Figure 5b). The Si/C nanoporous microspheres exhibited a capacity larger than 1000 mAh g −1 and greatly improved cycling stability compared with pure nano-Si due to the controlled manner of ESD in terms of pore size, carbon coating, and porosity.
ESD can also be applied as a layer-by-layer synthesis technique to fabricate Si/C composites (Figure 5c,d). [126] First of all, a well-dispersed GO suspension is deposited on a copper substrate by ESD to form a thin film of GO sheets (A layer). Afterward, a mixture precursor of Si nanoparticles, PVP, CB, and MWCNTs is used to form a B layer by the same ESD technique. The A layer and B layer are deposited alternately, leading to a target thin film with alternatively stacked Si-porous carbon layers and graphene layers. By heat treatment, Si nanoparticles are embedded in the porous carbon layer composed of a nitrogen-doped carbon framework, carbon black, and carbon nanotubes, which are further sandwiched by flexible and conductive graphene sheets (Figure 5e,f). The porous carbon layer and graphene layer will accommodate the volume change of the Si particles, enhance the electronic conductivity, facilitate ion transport by the void space, and prevent the agglomeration of Si nanoparticles. Hence, such an electrode exhibits a reversible capacity of 1020 mAh g −1 with 75% capacity retention after 100 cycles and a good rate capability on the basis of the total electrode weight. [126] The combined ESD/electrospinning technique is an effective approach to fabricate Si/C composites with high performance. Currently, Si electrodes are prepared by casting slurries consisting of Si, conductive carbon, and a binder onto Cu metal current collectors. Since the Si loading is normally less than 1 mg cm −2 , in spite of the Si anode with a high reported capacity (>2000 mAh g −1 ), the overall capacity of the entire anode is less than 100 mAh g −1 , considering the weight of the current collectors. As a result, high-capacity Si and lightweight current collectors can be used to increase the overall capacity of the anode. Xu et al. prepared a 3D Si/C fiber paper electrode by a combined ESD and the electrospinning technique. [76] Such a paper electrode was prepared by simultaneously electrospraying a nano-Si-PAN dispersion and electrospinning a PAN solution to uniformly distribute PAN-coated nano-Si clusters into a PAN nanofiber paper, followed by heat treatment in Ar (Figure 6a). After carbonization, the paper electrode still shows good flexibility (Figure 6b). The PAN nanofiber with a diameter of ≈200 nm built a continuous carbon network with microsized void spaces, and Si nanoparticles were bonded together by coated carbon to form 1-2 µm clusters (Figure 6c). The continuous carbon fiber network provides a fast electronic pathway, while the void space is beneficial for ionic transport through the electrolyte. In addition, the unoccupied fiber cages offer free space to accommodate the volume change of the nano-Si and alleviate stress/strain, leading to a long cycle life. The Adv. Mater. 2019, 31, 1803408

Figure 5. a) Schematic illustration and b) SEM image of Si/C nanoporous microspheres. c) Schematic of the fabrication process for a layer-by-layer
Si-C/G electrode. d) Schematic of the structural changes of the electrode during the lithiation/delithiation process. e,f) SEM images of Si-C/G electrode on the surface and in cross-section. a,b) Reproduced with permission. [125] Copyright 2011, American Chemical Society. c-f) Reproduced with permission. [126] Copyright 2015, Elsevier.

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Si content in the paper electrode is ≈72 wt%. The electrode exhibits an overall high capacity of 1600 mAh g −1 and good capacity retention of 840 mAh g −1 after 600 cycles. The rate capability is good as well for such paper electrodes with a mass loading of 1.7 mg cm −2 , with a current density from 0.2 to 4 A g −1 (Figure 6d). The success of the Si/C electrode highlights the great potential of the combined electrospinning/ESD technology as a suitable mass-production technique with low cost for flexible batteries or electronics.

Oxides Prepared by ESD
SnO 2 -based oxide is another extensively investigated anode for lithium batteries based on the alloying reaction. SnO 2 will first react with lithium to form Sn metal and Li 2 O (step I, conversion reaction), and then the Sn metal will further react with lithium to form a lithium tin alloy (step II). Since step I is irreversible, tin-oxide-based anodes are, in principle, anodes based on alloying-dealloying. Besides Sn and Si anodes, ESD is an effective method to improve the performance of SnO 2 -based oxides or composites. Yu et al. fabricated a few nanostructured carbon-free anode materials, such as SnO 2 , Li 2 O-SnO 2 , CuO-SnO 2 , and Li 2 O-CuO-SnO 2 composite, by the ESD technique, which exhibit a special porous, multideck-cage morphology (Figure 7a,b). [66] The hollow porous spheres with diameter of ≈5 µm are randomly arranged, and the diameters of pores are in the range of 200 nm to 1 µm. The thickness of the interconnected piled-up porous spheres is ≈10-15 µm. Oxide anodes based on alloying, with various morphologies, crystallinities, and components, have been extensively investigated by the ESD technique; for example, amorphous tin oxide, [127] SnO 2 -CoO, [128] SnO 2 -MnO, [129] SnO 2 -iron oxide, [130] SnO 2 -Fe 2 O 3 , [131] SnO 2 -SiO 2 , [132] SnO 2 /graphene composite, [133] etc. Nanoporous tree-like SiO 2 film was prepared by the ESD technique combined with the sol-gel method. [45] Spherical SiO 2 with a size of 300-500 nm sized is monodisperse with pore size of ≈7.5 nm.

Conversion Reactions
Conversion-reaction anodes have pronounced advantages and disadvantages compared with the ones based on intercalation. [16,134] On the one hand, the theoretical capacity of a conversion anode is very high, but on the other, the kinetic problems associated with it are extraordinarily severe. We will take RuO 2 as a relevant example. It will reduce to Ru metal and a Li 2 O matrix according to the following equations: RuO 2 + 4Li → Ru + 2Li 2 O. [17] It is clear to note that it involves 4 Li per transition metal, compared to 1 Li per metal in LiFePO 4 (two-phase mechanism) and 0.5 Li per metal in LiCoO 2 (single-phase mechanism), which corresponds to a much larger theoretical capacity. According to reaction equation of the conversion reaction, the initial oxides, i.e., RuO 2 will decompose into complex phase mixtures of RuO 2 , Ru, and Li 2 O, which will lead to severe kinetic problems, not only on discharge, but even more strongly on charging, whereby nucleation and diffusion are necessary to form the electroactive material again. Usually, for the conversion reaction, polarization and reversibility are extremely Adv. Mater. 2019, 31, 1803408   Figure 6. a) Schematic illustration of the synthesis process of the flexible 3D Si/C fiber paper electrode. b,c) Photographs and SEM image of 3D Si/C fiber paper electrode. d) Voltage profiles at current density from 0.2 to 4 A g −1 for such an electrode. Reproduced with permission. [76] Copyright 2014, Wiley-VCH.
www.advmat.de www.advancedsciencenews.com serious. Due to the phase decomposition and multiphase mixture properties, a large volume change occurs during the charge-discharge process, which results in a poor cycling stability for the conversion reaction. Increasing the reversibility, cycling stability, and rate capability for high-energy conversion anodes is still a challenging task.

Oxides Prepared by ESD
Metal oxides based on the conversion reaction are considered as potential anode materials for Li-ion batteries. The ESD technique is an ideal approach to prepare carbon-free and binder-free conversion anodes in order to overcome the abovementioned issues. Yu et al. fabricated a reticular CoO-Li 2 O composite on a conducting nickel-foam substrate by the ESD technique (Figure 8a). [36] Such a porous nickel-foam substrate can make sure of the maximization of the contact area between the electrode and the electrolyte compared to a dense metal foil. The reticular film with the thickness of 30 µm had a good adherence to the substrate (Figure 8b). In the composite film, CoO is crystalline and Li 2 O is amorphous. Introduction of Li 2 O has several advantages. 1) Li 2 O limits the growth of CoO particles. 2) Li 2 O as an oxidant can convert CoO to the higher-valency Co oxide, which will lead to extra capacity. 3) Li 2 O can buffer the stress due to the volume change of CoO during the cycling, resulting in good cycling stability. The reticular composite film shows good cycling stability and rate capability compared to the dense film (Figure 8c,d). It delivers a capacity of 650 mAh g −1 at 5 C for the reticular film, while the value is 250 mAh g −1 for a dense film under the same conditions. In principle, pristine metal oxides normally have low electronic conductivity. In order to overcome this issue, a porous NiO-Ni nanocomposite can be prepared by ESD. [135] The introduction of Ni can offer a highly electronic conductive medium and facilitate a more complete decomposition of Li 2 O during charging.
In order to further improve the performance, ESD can be used together with other synthesis methods (i.e., solvothermal method) to prepare conversion anodes. [53] Cobalt oxide thin films composed of interconnecting hollow spherical Co 3 O 4 particles (≈300 nm) can be prepared through two steps: 1) Hollow cobalt alkoxide particles in a stable suspension are synthesized as a precursor by the solvothermal method from mixed polyalcohol solutions of cobalt acetate in an oil bath. 2) A thin film is prepared by ESD using the above precursors combined with heat treatment. Such a film shows a capacity of ≈1000 mAh g −1 up to 50 cycles and good rate capability.
A series of oxides anode based on conversion reaction have been successfully fabricated by ESD to enable fast electron/ ion transport and buffer the volume changes, leading to an improved performance, i.e., Co 3 O 4 -graphene composite, [136] 3D porous ZnO thin film, [137] porous Fe 2 O 3 thin film, [52] Fe 2 O 3 thin film, [138] Mn x O y nanoparticle, [139] 3D porous MnO thin film, [140] MnO/CNT composite film, [141] MnO/graphene composite  www.advmat.de www.advancedsciencenews.com film, [142] 3D porous ZnMn 2 O 4 thin film, [143] and 3D reticular pomegranate-like CoMn 2 O 4 /C. [144] Normally the ESD technique is a facile method to fabricate conductive additive-free and binder-free electrodes. However, on the other hand, a binder can also be added to prepare an electrode during ESD. For instance, metal-oxide/PVdF nanocomposites can also be obtained by electrospraying precursor solutions containing metal salts dissolved in NMP together with PVdF as binder. [145] In this way, small oxide nanoparticles can be prepared and dispersed in situ in the binder, creating oxide (e.g., Fe 2 O 3 , CuO, SnO 2 , CoO)/PVdF nanocomposites. [145,146] ESD together with electrochemical cycling can be applied to synthesize single-crystalline metal nanoparticles with various morphologies, based on the conversion reaction. [147] For a 3D reticular Li 2 O-Cu 2 O composite film prepared by ESD, if the cell of Li 2 O-Cu 2 O/Li is discharged, Cu nanospheres (On Ni foam) and pyramid-like Cu particles (on Cu foil) can be successfully obtained. Furthermore, the shape of the Cu particles can be controlled by changing the cycle numbers of the cell; for example, Cu pyramids can change to Cu truncated pyramidal nanoparticles after 50 cycles.

Sulfides Prepared by ESD
Besides metal oxides, metal sulfides have been extensively investigated as high-capacity lithium anodes as well. By using the ESD technique, various oxide-based conversion electrodes can be prepared easily, whereas it is not so straightforward to obtain conversion sulfide by ESD. Recently, by choosing an appropriate precursor of (NH 4 ) 2 MoS 4 , MoS 2 has been prepared by conventional ESD, according to: (NH 4 ) 2 MoS 4 + H 2 → MoS 2 + H 2 S + NH 3 . [37] In addition, a 3D porous interconnected nanocomposite was constructed by ESD, integrating nano-2D graphene, nano-1D CNT, and nano-0D MoS 2 (Figure 9a). [37] Depending on annealing or not, either amorphous MoS 2 (0.5-5 nm) or nanocrystalline few-layered MoS 2 (5-10 nm) can be obtained. Such 0D-1D-2D integration has several advantages. MoS 2 nanodots (0D) are beneficial for more reversible conversion reactions. 1D CNT as an electronic connector between MoS 2 and the carbon skeleton will also prevent the agglomeration of graphene. 2D graphene is a suitable matrix to constitute an effective, 3D, porous, and integrated conductive network. Finally, an efficient electrochemical circuit is constructed for both electrons and lithium ions by ESD. The composite shows an excellent rate capability, especially for annealing samples. The capacities are 949, 883, 858, 737, and 652 mAh g −1 at current densities of 100, 500, 1000, 5000, and 10 000 mA g −1 respectively (Figure 9b), and the good capacity retention is maintained for at least 100 cycles (Figure 9c).
For the purpose of the synthesis of MoS 2 , the key point is to choose the precursor of (NH 4 ) 2 MoS 4 , which limits the application to other metal sulfides. In order to solve this problem, we have also proposed a general strategy to fabricate 3D porous interconnected carbon-coated metal sulfides by ESD and without adding expensive carbon materials such as graphene and CNT. [58] Taking SnS/C as an example, the key strategy is to choose L-cysteine together with a SnCl 2 . The L-cysteine is Adv. Mater. 2019, 31, 1803408 www.advmat.de www.advancedsciencenews.com not only the sulfur source for the sulfide formation, but it also acts as a complexing agent for Sn 2+ ions in the 1,2-propanediol solvent. [58] The L-cysteine molecule has different functional groups, such as NH 2 , COOH, and SH, and has a strong tendency to coordinate Sn 2+ . During ESD and post-heat-treatment in Ar, the Sn 2+ -L-cysteine complex decomposes to form SnS, and the extra L-cysteine and 1,2-propanediol convert to amorphous carbon. In this way, small nanorods of SnS with a size of 10-20 nm embedded in amorphous carbon are formed, which further self-assemble into a 3D porous interconnected composite (Figure 10a,b). The nanocomposite shows excellent rate capability and cycling stability due to the 0D⊂3D porous interconnected structure, which improves the transport properties and buffers the volume change during cycling. The reversible capacity is around 953 mAh g −1 at 100 mA g −1 , and it still has a capacity as high as 329 mAh g −1 at very high current density of 10 A g −1 (Figure 10c). At a current density of 1 A g −1 , the capacity maintains 535 mAh g −1 , which is 80% capacity retention (Figure 10d).

Electrode Materials Prepared by ESD for Sodium-Ion Batteries
In view of the abundance and the low cost of elemental sodium, sodium-ion batteries are considered as a realistic promising alternative compared to its lithium counterpart. [8,148] However, due to the larger ionic radius and greater molecular mass, it leads to several disadvantages, such as lower theoretical capacity, lower cell voltage, less-rapid transport, more-severe volume change, and structure impact, when compared to its lithium analogues. In spite of such shortcomings, there are several arguments in favor of sodium storage, as well as in terms of kinetics. 1) For some specific materials, such as NASICON materials, sodium transport is even higher than lithium.
2) The charge-transfer reaction from liquid electrolyte to solid electrode is even easier for Na + due to the less extensive solvation of the bigger cation. [149] Referring to cathode materials for sodium batteries, layered transition-metal oxides, [150,151] polyanionic materials, [152][153][154][155] and Prussian Blue materials [156] have been extensively studied. For anode materials, hard carbon, [157] alloys, [158] conversion anodes, [159] and low-voltage transition metal oxides, and phosphates [160] are most commonly investigated anode materials. The ultimate goal for sodium storage is to improve the specific performance of sodium batteries, i.e., capacity, rate capability, cycling stability, etc., to compete or even exceed lithium batteries. As shown by various examples of lithium electrodes, ESD has several pronounced advantages. For example, it can prepare phase-pure materials with high crystallinity; it is an effective method to construct an efficient mixed conducting network; and it can build a robust and porous structure to accommodate the volume changes. Hence, more and more high-performance electrodes for sodium storage have been prepared by ESD recently.

Sodium Cathodes Prepared by ESD
NASICON-type Na 3 V 2 (PO 4 ) 3 (NVP) is a promising sodium cathode material due to its high sodium conductivity, high thermal stability, and energy density. [161,162] However, the main issue is its poor electronic conductivity, [163] which limits the chemical diffusion of this material. To construct an effective mixed conducting network, especially, an electronic conducting network is very important. A bicontinuous network (current collector, electrolyte) is an optimized solution, and for NASICON Adv. Mater. 2019, 31, 1803408 Figure 9. a) Schematic representation for MoS 2 -rGO-CNT nanocomposite prepared by ESD, demonstrating the functional integration of 0D, 1D, and 2D nanostructures. b) Rate capability of as-prepared and annealed composites. c) Cycling stability for the as-prepared sample, the annealing composite, and bulk MoS 2 prepared by ESD at current density of 1 A g −1 . Reproduced with permission. [37] Copyright 2014, Wiley-VCH.
www.advmat.de www.advancedsciencenews.com materials with high ionic conductivity, a tricontinuous network (current collector, electrolyte, electroactive mass) is even helpful. Besides the commonly used templated method [4] or templatefree approaches [164] to prepare 3D electrodes, the ESD technique is also a general strategy to prepare self-supported interpenetrating 3D tricontinuous cathodes. [62] By using this technique, we fabricated an interconnected 3D tricontinuous NVP-rGO-CNT, which was directly deposited on a stainless-steel current collector without any conductive additive and binder (Figure 11a-c). This electrode shows a much more ordered structure than the ones achieved by other template-free methods, which is even comparable to the tedious templating method. Such an electrode exhibits an excellent electrochemical performance when used for both cathode (2.3-3.9 V) and anode (1.3-2.0 V), as well as full NVP symmetric cell (1-2.2 V). For example, as far as for cathodes, up to 30 C, the specific capacity is still as high as 109 mAh g −1 , which is quite comparable to the theoretical capacity of NVP. Even at an ultrahigh C rate of 100 C (11 A g −1 , full charge-discharge in 36 s), the capacity still maintains 82 mAh g −1 , which is 70% of the theoretical capacity (Figure 11d). Such rate capability is quite comparable to supercapacitors, but with the much higher energy density of batteries. After 2000 cycles at 10 C, the capacity is still as high as 96% of its initial capacity, with high Coulombic efficiency (Figure 11e). For the anode part, such a 3D tricontinuous NVP:rGO:CNT can deliver 70% of its theoretical capacity at 20 C, and its capacity shows almost no decay (maintains 99%) after 2000 cycles at 10 C. A full sodium battery using 3D tricontinuous NVP:rGO:CNT as both the cathode and anode was assembled as well, which delivered a capacity of 90 mAh g −1 at 10 C with voltage of 1.7 V. The excellent performance is due to the unique 3D tricontinuous structure constructed by ESD, i.e., continuous electronic phase (rGO-CNT), continuous electroactive (here also ionic) phase (NVP), and continuous ionic phase (electrolyte in pores), which ensures fast electron and Na + transport in the entire electrode. Hence, the ESD technique is a simple but powerful method to prepare high-power-high energy 3D or 2D thin-film batteries.

Sodium Anodes Prepared by ESD
Besides NASICON-type sodium anodes such as Na 3 V 2 (PO 4 ) 3 (1.3-2 V), self-supported 3D porous reticular Nb 2 O 5 @carbon composites based on the intercalation reaction were also prepared by ESD. [165] The nanosized Nb 2 O 5 encapsulated in the carbon matrix facilitates the electrochemical kinetics between the Nb 2 O 5 and electrolyte. The 3D interconnected hollow structure buffers the volume change during cycling. Therefore, the Nb 2 O 5 @carbon obtained by ESD exhibits improved sodiumstorage performance.
An alloying anode such as Sb was also successfully fabricated by ESD to improve its performance. Sb is a promising anode candidate for sodium batteries due to its low reaction potential and high theoretical capacity of 660 mAh g −1 . [166] Nonetheless, similar to alloy anodes used for lithium batteries, the large volume change will lead to a poor cycling stability. To solve this problem, Sb nanoparticles encapsulated in a 3D reticular carbon  www.advmat.de www.advancedsciencenews.com network were prepared by ESD with a postheat treatment. [167] The diameter of pores in the reticular structure are 100 nm to 20 µm, and the width of the wall is ≈8 µm (Figure 12a). Sb nanoparticles are ≈20-40 nm, and they are embedded in the reticular carbon matrix (Figure 12b). The electrode exhibits a high rate capability up to 5 C (Figure 12c). At 0.2 C, it still delivers 404 mAh g −1 after 100 cycles (Figure 12d), which shows good cycling stability. Such a 3D porous structure effectively buffers the volume change during the sodiation/desodiation process.
ESD is a powerful method to prepare metal sulfides based on the alloying reaction for sodium storage as well. As we have already shown in the part on lithium anodes, a carbon-coated 3D porous interconnected SnS/C composite can been prepared by the ESD technique. [58] Such a SnS/C composite as an alloying anode also shows a good sodium-storage performance. The capacities are 419, 334, 310, 205, and 145 mAh g −1 at current densities of 100, 500, 1000, 5000, and 10 000 mAh g −1 . At 1 A g −1 current density, the capacity is still as high as 266 mAh g −1 , which maintains more than 80% capacity retention with almost 100% Coulombic efficiency.
Conversion anodes such as sulfides are also extensively investigated for sodium batteries. 3D porous WS 2 and 3D porous WS 2 /C nanocomposites was successfully fabricated by the ESD technique. [56] WS 2 /C nanocomposites include nano-0D WS 2 , nano-1D CNTs, and nano-2D rGO. Depending on annealing, WS 2 can be amorphous or nanocrystalline. An effective electrochemical integrated circuit was constructed by ESD with the combination of 0D-1D-2D nanostructures, leading to good electrochemical performance for sodium storage. It delivered capacity of ≈400 mAh g −1 at 0.2 C and 81 mAh g −1 at 10 C. After 300 cycles at 1 C, the capacity still maintained 219 mAh g −1 .

Electrode Materials for Supercapacitors Prepared by ESD
No matter whether for electrical-double-layer capacitors (EDLCs) or pseudocapacitors, electrode materials with a high surface area are extremely significant. At this point, the various porous structures obtained by the ESD technique are very suitable to improve the electrochemical performance of supercapacitors.

Electrical-Double-Layer Capacitor Materials Prepared by ESD
Various carbon materials with high surface area are widely used for EDLCs, which exhibit high power density compared to pseudocapacitors. Graphene is one of the materials of interest for supercapacitors owing to its high surface area and high electronic conductivity. Various morphologies constructed with graphene are achieved by ESD. A graphene-nanoplatelet thin film was prepared through the ESD technique by dispersing graphene nanoplatelets with a surface area of 600-750 m 2 g −1 and a diameter of 2 µm in 1,2-propenediol solvent as a precursor. [168] The film shows capacitances of 55 and 53 F g −1 for the 1 and 6 µm thick films, respectively, at a discharge rate of 1 A g −1 . A crumpled reduced graphene oxide (C-rGO) thin film was also fabricated by ESD. [169] The C-GO thin film was first prepared by dissolving GO in water/absolute ethyl alcohol solution and depositing them on the stainless steel by ESD. The C-GO film was further reduced to C-rGO by exposure to hydrazine hydrate vapor at 85 °C for 10 h. Such C-rGO shows a high capacitance of 366 F g −1 at 1 A g −1 in 6 m KOH aqueous solution and 108% capacitance retention to 40 000 cycles.
www.advmat.de www.advancedsciencenews.com Besides graphene, CNTs are also applied for supercapacitors. Compared to pristine CNTs, surface-modified CNTs have better dispersion stability in a polar solvent and better electrochemical performance due to the exposure of edge sites by the partial opening of the nanotube structure. [170] Through chemical etching by KMnO 4 , surface-unzipped CNTs with oxygen-containing functional groups were obtained, making them highly dispersible in a polar solvent. Hence, binder-free unzipped multiwalled CNT electrodes were prepared by ESD using this precursor, which delivered a capacitance of 133 F g −1 at a scan rate of 10 mV s −1 in 1 m H 2 SO 4 aqueous electrolyte.
In order to realize a unique morphology with high surface area, rGO/CNT composites can be prepared through ESD. Micropatterned interdigitated electrodes combining both rGO and CNT composite thin films by ESD have also been reported. [65] Single-layer GO and COOH-functionalized CNT were dispersed by sonication in 1,2-propanediol as a precursor solution. GO can be reduced to rGO during the ESD process at 250 °C without a further thermal or chemical reduction process. Figure 13a schematically describes the procedure to integrate supercapacitor electrodes on interdigital Ti/Au microelectrodes and how to fabricate microsupercapacitors by the ESD technique. As shown in Figure 13b,c, a typical microsupercapacitor with 20 in-plane interdigital microelectrodes (100 µm in width, 2500 µm in length, distance between adjacent microelectrodes 50 µm) was constructed. The obtained rGO-CNT hybrid electrode exhibits porous morphology, and CNTs uniformly appear between the rGO sheets, which prevents the stacking of the rGO sheets and maintains a porous structure (Figure 13d,e), thus offering a high surface area for the microelectrodes. Among the samples of rGO, rGO-CNT (9:1), and rGO-CNT (8:2), the best performance is achieved for rGO-CNT (9:1). The capacitance of 5.1 mF cm −2 at a 3 mA cm −2 current density is obtained, and it drops by about 30% at current density of 100 mA cm −2 (Figure 13f). Similar work combining rGO and CNT to prepare a thin-film electrode for supercapacitors was also performed using a mixed solution of water and ethanol (volume ratio 1:1) instead of high-boilingpoint solvent such as 1,2-propanediol at 300 °C by the ESD technique. [171]

Pseudocapacitor Materials Prepared by ESD
Conducting polymers and transition oxides are widely used examples for pseudocapacitive materials, which exhibit a promising energy density. In order to increase the surface area and control the porosity, ESD combined with a template method is promising. A CNT/PPy composite film with controlled pore sizes was prepared by this strategy. [74] CNTs and nanosize silica were first deposited by ESD, and then PPy was electrochemically deposited on to the CNTs to anchor them in an entangled structure. Finally, the silica was removed, leaving a 3D entangled structure of a CNT/PPy film. The pore size can be controlled by the amount of silica. The capacitance of such a film (80% PPy) is 250 and 211 F g −1 at scan rates of 10 and 500 mV s −1 , respectively, in 1 m KCl.
RuO 2 is one of the best electrode materials with high capacitance and long cycle life for supercapacitors, due to its high conductivity and electrochemical reversibility. Hydrous RuO 2 ·xH 2 O [172] and anhydrous RuO 2 [173] were successfully prepared by the ESD technique, with specific capacitances of Adv. Mater. 2019, 31, 1803408 Figure 12. a,b) SEM and TEM images of Sb in 3D reticular carbon. c,d) Rate performance and cycling stability (0.2 C) of Sb in 3D reticular carbon. Reproduced with permission. [167] Copyright 2015, Wiley-VCH.
www.advmat.de www.advancedsciencenews.com 650 F g −1 and 500 F g −1 , respectively, at a scan rate of 20 mV s −1 . The capacitance of the anhydrous RuO 2 can be further improved by ESD followed by electrochemical lithiation and delithiation. [174] Based on the reversible conversion reaction, the as-prepared ESD RuO 2 with primary particle sizes of 15-30 nm will be converted to nanostructured RuO 2 ranging from 2 to 5 nm. The specific capacitance will be increased to 653 F g −1 at 20 mV s −1 .
ESD allows homogeneous mixing of different precursors at the atomic scale, so that phase-pure metal oxides composites can be constructed by ESD for supercapacitors, and the performance can be improved with synergistic effects. [72] Benefiting from the high capacity of VO 2 and high stability of TiO 2 , a unique interconnected pore network of VO 2 /TiO 2 nanosponges was successfully prepared by ESD. The binder-free electrode exhibited a specific capacitance of 86.2 mF cm −2 (≈548 F g −1 ) and a good cycling stability with 84.3 retention after 1000 cycles.

The ESD Technique for Li-S Batteries
Li-S batteries are considered as one of the most promising next-generation batteries owing to their high theoretical energy density (2600 Wh kg −1 ) and low cost. [6,175] However, they still suffer from several serious issues. One disadvantage is the low electronic and ionic conductivity of sulfur, which results in low sulfur utilization and poor rate performance. The other problem is the shuttle effect of polysulfides. The produced intermediate polysulfides are easily dissolved in the organic electrolyte and then diffuse to the anode part to react with the Li anode, which leads to a worse cycling performance and a low Coulombic efficiency.
ESD can be applied to design a unique structure for the sulfur cathode in order to further improve the performance of Li-S batteries. For example, a 3D interpenetrating sulfurimpregnated MWCNT microball cathode was prepared by using the ESD technique as follows. [176] An MWCNT/organic binder compact sphere with a uniform size of 3-6 µm was prepared by the ESD process. After burning out the binder at 400 °C, mesoporous MWCNTs were obtained with a specific surface area of 175 m 2 g −1 . Finally, on impregnation of sulfur, a sulfurimpregnated MWCNT microball cathode was achieved. This cathode can improve the electronic and ionic transport of active materials and overcome the volume change during cycling, leading to good cycling stability and rate performance. [176] Suppressing the shuttle effect is the most important issue for Li-S batteries. As a thin-film preparation technique, besides the 3D porous structure, ESD can also prepare dense thin films. Hence, ESD is a suitable method to construct an interblocking-layer between the cathode and the separator to solve the shuttle effect. [177] As shown in Figure 14a, a multifunctional polysulfide blocking layer (MPBL) including a conductive polymer and carbon is coated on the cathode by ESD. On the one hand, PEDOT:PSS offers a high electronic conductivity and reacts with polysulfide by strong chemical binding. On the other hand, nanosized carbon black with a high surface area will increase the conductivity and reacts with the polysulfide by physical adsorption. An integrated and compact layer with a controlled thickness, by easily changing the spray time and concentration of precursors, can be achieved by ESD. As displayed in Figure 14b, the electrode is fully wrapped by a smooth and dense MPBL, whose thickness is only 100 nm, thinner than the ones prepared by other methods. The rate performance of such an MPBL-coated cathode is superior. It exhibits the capacity of 1034 mAh g −1 at 0.3 C, and the capacity is still 615 mAh g −1 when the current increases to 3 C (Figure 14c). The cycling stability of MPBL-coated cathode is also sufficiently improved compared to the PEDOT:PSS coated cathode and uncoated cathode. The capacities are 879, 645, and 473 mAh g −1 , respectively, after 200 cycles at 0.3 C (Figure 14d).

The ESD Technique for Li-O 2 Batteries
Li-O 2 batteries provide even higher theoretical energy densities (3500 Wh kg −1 ) compared to Li-S batteries, and represent a very promising system for various devices based on energy storage. [178] The design of highly effective oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts are key issues to improve the battery performance. [179,180] In order to increase the kinetics of the ORR and OER, the enhancement of the electronic conductivity and catalytic activity is crucial. The ESD technique can be applied to design suitable components and structures of catalysts. For example, a porous Co 3 O 4 / Ketjenblack cathode film was directly deposited on Ni foam by the ESD technique as the cathode of a Li-O 2 battery. [181] The optimized Co 3 O 4 /Ketjenblack (80%) composite film exhibited the best performance. The discharge capacity was 2044 mAh g −1 at a current density of 100 mA g −1 , with the lowest overpotential of less than 1.0 V. This unique cathode prepared by ESD has a high surface area and facile charge and mass transport, and hence results in increased surface catalysis reactivity.

Outlook and Conclusions
Fabrication of designed structures and morphologies for energystorage materials is essential to meet the targets of energystorage applications. Among the various synthesis methods, the ESD technique attracts intense interest and undergoes pronounced advances. ESD is a simple but versatile method, which can be applied to prepare a large number of compounds for various energy-storage systems (Li-ion batteries, Na-ion batteries, supercapacitors, Li-S batteries, and Li-O 2 batteries) with different energy-storage mechanisms (intercalation reaction, alloying and conversion reactions, and surface reaction), leading to a pronounced improvement of the electrochemical performance (i.e., capacity, rate capability, and cycling stability). The highlight of this technique is that it can offer various morphologies, such as granular, dense, porous, 3D interpenetrating Figure 14. a) Schematic of electrode configurations for a cathode coated with a multifunctional polysulfide blocking layer (MPBL) for a Li-S battery. b) SEM image of the MPBL-coated S/C cathode. c) Rate performance and d) cycling stability of the MPBL-coated, a PEDOT:PSS-coated, and an uncoated S/C cathode. Reproduced with permission. [177] Copyright 2016, Elsevier.
www.advmat.de www.advancedsciencenews.com Adv. Mater. 2019, 31, 1803408 tricontinuous, etc. with a controllable manner, easily achieved by adjusting the experimental para meters, such as the voltage, the concentration of the precursor solution, the flow rate, the substrate temperature, the deposition time, etc. In addition, ESD is also a fast thin-film deposition method compared to PVD or CVD, which can prepare an extremely thin and defectfree layer homogenously, only in an ambient atmosphere and at low temperature without complex reactors and high-vacuum systems. Furthermore, electroactive materials can be directly deposited on the current collector by ESD without any additional conductive additives and binders, which greatly simplifies the battery/supercapacitor assembly process and increases the rate capability and energy density of the electrodes.
However, several challenges remain for the ESD technique. At the moment, not all compounds with designed structures and morphologies can be prepared by the ESD technique. The detailed mechanisms as to how the experimental parameters of the ESD process affect the morphologies of specific materials are still not fully understood. In addition, scale-up of the ESD technique for industrial applications in terms of various energystorage systems is still a challenge. Great efforts still need to be devoted to the ESD technique in order to further improve this powerful and versatile method. All in all, pronounced advantages and the brilliant perspective of the ESD technique will make a great contribution to energy storage in both the academic and industrial fields in the future.