Non‐van der Waals 2D Materials for Electrochemical Energy Storage

The development of advanced electrode materials for the next generation of electrochemical energy storage (EES) solutions has attracted profound research attention as a key enabling technology toward decarbonization and electrification of transportation. Since the discovery of graphene's remarkable properties, 2D nanomaterials, derivatives, and heterostructures thereof, have emerged as some of the most promising electrode components in batteries and supercapacitors owing to their unique and tunable physical, chemical, and electronic properties, commonly not observed in their 3D counterparts. This review particularly focuses on recent advances in EES technologies related to 2D crystals originating from non‐layered 3D solids (non‐van der Waals; nvdW) and their hallmark features pertaining to this field of application. Emphasis is given to the methods and challenges in top‐down and bottom‐up strategies toward nvdW 2D sheets and their influence on the materials’ features, such as charge transport properties, functionalization, or adsorption dynamics. The exciting advances in nvdW 2D‐based electrode materials of different compositions and mechanisms of operation in EES are discussed. Finally, the opportunities and challenges of nvdW 2D systems are highlighted not only in electrochemical energy storage but also in other applications, including spintronics, magnetism, and catalysis.


Introduction
The depletion of fossil fuel reserves, the ever-increasing energy demand, and the crisis in energy supply chains threaten our energy security and the environment, arousing intense global concerns. [1] If no concrete steps are taken to offset this trend, world oil consumption will increase by 1.9 million barrels per day in 2023, with an average total consumption of 102.6 million barrels per day. Unplugging our civilization from its reliance on fossil fuels presents a formidable challenge for the coming decades, calling for development of alternative fuel production pathways from renewable sources, [2] such as solar and wind power. However, they are intermittent in nature requiring the practical storage of the harvested energy, such as in electrochemical energy storage (EES) devices, and then used at the time of peak demand or low power availability. Moreover, the need for decarbonizing the transportation sector and the broader use of mobile electronics, big data centers, and intelligent grids further increase the demand for sustainable mobile EES with transformative performance, safety, and low cost. [3] Thus, EES and related advanced materials are key enablers for this strategic transformation and the related global sustainability goals set by the United Nations for affordable, reliable, and sustainable energy for all.
To date, materials design and engineering with controlled dimensionality are essential for accessing properties not observed in their bulk counterparts. Over the past decades, the development, understanding, and application of lowdimensional materials in EES brought dramatic scientific and technological advances. [4] For example, confined LiFePO 4 nanoparticles display superior electrochemical capacity and ultrafast charging/discharging capability, bypassing the limitations of the bulk LiFePO 4 phase, offering new opportunities for designing high-energy and high-power hybrid supercapacitors. [5] The confinement of atoms and electrons in 0D (e.g., quantum-dots), in 1D (e.g., nanowires), or in 2D nanomaterials can fundamentally affect the structure (e.g., the dominance of selective or rare crystal planes, dangling bonds and defects) and the physicochemical properties, such as the electric and ionic conductivity, electronic structure, spin, and redox states, The development of advanced electrode materials for the next generation of electrochemical energy storage (EES) solutions has attracted profound research attention as a key enabling technology toward decarbonization and electrification of transportation. Since the discovery of graphene's remarkable properties, 2D nanomaterials, derivatives, and heterostructures thereof, have emerged as some of the most promising electrode components in batteries and supercapacitors owing to their unique and tunable physical, chemical, and electronic properties, commonly not observed in their 3D counterparts. This review particularly focuses on recent advances in EES technologies related to 2D crystals originating from non-layered 3D solids (non-van der Waals; nvdW) and their hallmark features pertaining to this field of application. Emphasis is given to the methods and challenges in top-down and bottom-up strategies toward nvdW 2D sheets and their influence on the materials' features, such as charge transport properties, functionalization, or adsorption dynamics. The exciting advances in nvdW 2D-based electrode materials of different compositions and mechanisms of operation in EES are discussed. Finally, the opportunities and challenges of nvdW 2D systems are highlighted not only in electrochemical energy storage but also in other applications, including spintronics, magnetism, and catalysis. and mechanical properties. Moreover, due to the high fraction of surface atoms, low dimensional materials are more susceptible to covalent and non-covalent chemical functionalization, or engineering of heterojunctions, doping, and defects. One of the most indicative examples is graphene, which has propelled intense research activity, particularly after the pioneering report and Nobel prize to Geim and Novoselov on the electronic properties of the atomically-thin, exfoliated graphite layer. [6] It's 2D open and freely available surface, the abundant reactive sites at edges and vacancies, the high electrical conductivity, and the ability for covalent or non-covalent integration with secondary electrode components with synergistic properties (e.g., redox-active moieties, ionic transport boosters) have made a paradigm shift in EES in batteries, supercapacitors, and hybrid devices. [7] The properties of this atomically-thin structure have triggered research toward the development and study of 2D materials beyond graphene, such as layered metal oxides, transition metal dichalcogenides (TMDs), 2D covalent organic, and metal-organic frameworks (MOFs), silicene, germanene, black phosphorene, arsenene, stanene, borophene, carbon nitride, hexagonal boron nitride, perovskites, and MXenes. [8] These materials range from insulators, metals and diamagnets to spin-active systems, often exhibiting properties that are not shared with their 3D counterparts, rendering them promising platforms in diverse applications, spanning from sensing, catalysis, energy storage, to magnetic and spintronic technologies. [9] For example, band structure tunability, higher carrier mobilities, longer carrier-diffusion lengths, and tunable optical bandgaps lead to improved optoelectronic devices, [10] with flexibility and mechanical strength. [11] In EES in particular, there are several examples of electrodes constructed from exfoliated layered materials with dramatically enhanced features with respect to their 3D counterparts. [12] Bulk MoS 2, which suffers from poor cycling stability, becomes substantially more stable and active in the form of nanosheets, due to improved and surface-controlled diffusion processes and accessible active sites for interaction with Li or Na atoms. [8d,13] Altogether, these features have shaped an exciting field of research, based on 2D materials originating from the exfoliation of their respective 3D layered phases, whereby the crystal planes are held together by weak van der Waals (vdW) forces.
Apart from such vdW-derived 2D crystals, there is an emerging and potentially wide family of non-van der Waals (nvdW) 2D solids, where the atoms in their parent bulk phases are connected together in all 3D with robust metallic, ionic, or covalent bonds. [9g] Therefore, nvdW 2D materials refer to 2D crystals originating from non-layered (nvdW) 3D solids which, unlike the vdW systems, are more resistant to exfoliation. However, recent works have shown that their exfoliation, or bottomup synthesis, forming nvdW 2D crystals are feasible. [14] Eminent members of this family are metal oxides that abundantly occur in natural ores, with bandgaps ranging from insulators to semiconductors, and applied in diverse fields including catalysis, sensing, EES, magnetic storage, and spintronics. [15] 2D nanoengineering of hematite to hematene leads to a remarkable saturation magnetization at room temperature and to the absence of the Morin transition. [16] In magnetene, the 2D form of magnetite, interesting low-friction properties evolve, not observed in the 3D counterpart. [17] In nvdW 2D chalcogenide AgCrS 2 , an increase in the ionic conductivity is revealed compared to the bulk form. [18] The layered ionic materials, such as layered double hydroxides (LDHs) and perovskite oxides, also categorized as nvdW materials when the adjacent layers are held together with strong ionic bonds, [8h,19] exhibit improved charge carrier lifetimes when they are delaminated into their 2D form. [8h] NvdW 2D materials also attract interest as electrocatalysts, due to the different electronic states and charge polarization phenomena witnessed on the surface metal atoms embedded in unique coordination environments. [20] In such systems, also known as metallenes, the metal atoms are often under-coordinated, and are employed in electrocatalysis due anisotropy, intrinsic strain influence, and versatility for chemical modifications. [21] The properties of nvdW 2D nanomaterials can be controlled not only by the structure and composition of the parent 3D solids, but also by heteroatom doping, vacancy, alloy and heterojunction/interface-engineering, modifying their electronic structure and enhancing the overall electrocatalytic performance in targeted reactions. [22] Electrocatalysis is also of high importance in EES technologies, involving metal-air, and metal-CO 2 chemistries. [23] During the last years, a boost in the number of reports on nvdW 2D materials in EES is witnessed, unraveling their defect-and active site-rich surfaces, which boost the energy carrier dynamics creating effective ion-transport pathways. For example, 2D FeF 3 and FeS 2 were recently synthesized and applied as battery electrodes, after their successful integration with carbon nanotubes to increase the final conductivity. The results demonstrated short solid-state diffusion lengths, leading to enhanced rate performance, [24] enabled by the rich chemistry and electronic properties of the surface-exposed transition metals, the quantum confinement effects and dangling bonds. The high surface area-to-thickness ratio of the 2D sheets maximizes the active site density, leading to faster interfacial charge transfer, which could be further improved by increased porosity [25] (holey structures).
In this work, we provide an overview of the latest advances in the rapidly evolving landscape of nvdW 2D materials, with a particular interest in the synthetic methods and their hallmark properties, concerning particularly EES applications. We describe the top-down and bottom-up approaches for obtaining the respective 2D sheets and highlight the opportunities for controlling their structural features and, thus, their properties and function in metal-ion batteries, and supercapacitors. [26] The up-to-date review articles on the EES field mainly focus on 2D materials from vdW precursors, and on few nvdW systems reported before 2019. [9g,12c,d,13b,23c] With the increasing interest in nvdW 2D materials in the recent years, this review brings a timely insight on the developments in the field related to their application in EES spanning from lithium-ion, and postlithium batteries, batteries based on conversion chemistries, air batteries, as well as supercapacitors and metal-ion capacitors (Figure 1). The properties of the 2D materials are discussed with respect to the properties of the bulk 3D counterparts, and a very brief overview on MXenes is also provided, since this class of 2D nvdW materials is already extensively reviewed regarding both the synthesis and EES applications. [27] Finally, the future directions and challenges toward new materials in this emerging field and their innovative applications are www.afm-journal.de www.advancedsciencenews.com discussed. The physicochemical properties of these 2D crystals are broadly tunable due to the large diversity of the parent chemical structures and the substantial impact that the exfoliation methods exert on the final properties. Therefore, the expected developments in monolayer nvdW materials will stimulate significant research and fruitful synergism between the distinct scientific communities working on fundamental and applied materials science, engineering, and technology.

2D Nanoengineering of nvdW Materials
Several synthetic strategies for nvdW 2D materials have been developed (Figure 2), showcasing that the technical aspects of the methodology and the selection of the involved reagents and precursors determine the structural and physicochemical features of the obtained 2D materials, as well as the yield and quality of the nanosheets. Similarly to the case of most nanomaterials, nvdW 2D materials are prepared by either top-down (from their non-layered 3D solids) or bottom-up methods from molecular precursors (Figure 2). Top-down exfoliation approaches refer to the mechanical exfoliation (e.g., scotch tape method, and dry shear-force delamination) and liquid phase exfoliation (LPE) methods, involving shear-assisted or sonication-assisted LPE, often accompanied by intercalation of molecules or ions. [28] Conversely, bottom-up synthesis embraces methods such as chemical vapor deposition (CVD), topochemical or template-assisted growth and molecular beam epitaxy. Employing liquid, electrochemical, or other wet or dry chemical exfoliation techniques, several 2D nvdW materials have been synthesized, such as noble metals (Ag, Au, Pd, Rh), metal oxides (TiO 2 , CeO 2 , SnO 2, and Fe 2 O 3 ), and metal chalcogenides (PbS, CuS, CuSe, ZnSe). [9g,13b,28,29] In the following, the primary methods and principles for the synthesis of 2D materials are described, and in each case specific examples for the production nvdW 2D crystals are reviewed in more detail. Finally, a critical assessment on the synthesis of nvdW 2D materials is provided.

Liquid Phase Exfoliation
One of the most widely applied and facile way to obtain singleor few-layered 2D materials is the LPE method, which involves a shear force (e.g., ultrasonication, roll mill) and a solvent. [28,30] Exfoliation of materials in an appropriate solvent in the presence of sonication to produce 2D derivatives exploits the energy of the ultrasonic waves to perturb the interaction forces between the layers or atomic planes of the bulk crystals. Nowadays, the LPE is perhaps the main top-down method to exfoliate layered materials such as graphene, phosphorene, hexagonalboron nitride (h-BN), TMDs (e.g., MoS 2 , MoSe 2 , WS 2 ), transition metal oxides (TMOs) and MOFs. [31] Undoubtedly, among the most critical factors in achieving high yield exfoliation is the selection of a suitable solvent in order to minimize the required energy input and overcome the interaction forces. [32] More specifically, matching the surface tension of the solvent and the target layered material is required to achieve a high yield. After overcoming the interaction forces, the flakes of the exfoliated material have to be and remain solvated and dispersible in the solvent, which depends on the specific molecular interactions between the solvent molecules and the 2D crystals. [32] For example, graphene and TMDs are efficiently exfoliated in N-methyl-2-pyrrolidone (NMP), and N,N-dimethylformamide (DMF), since the values of their surface tension and solubility parameters are well matched. [33] Bulk liquids have long been known to show short-range order. Notably, when the dispersed particle size is comparable to that of the ordered solvent layer, the ordering of the solvent molecules significantly differs from that in the bulk liquid and plays a crucial role in determining the dispersion stability. [34] Electrostatic forces can enhance such solvent ordering, and in this scope, Cullen et al. demonstrated a true ionic solution of 2D materials, such as MoS 2 , MoSe 2 and WS 2 , by forming layered material salts that spontaneously dissolve in polar solvents yielding ionic solutions. [35] They showed that intercalated 2D material salts could easily dissolve in polar aprotic solvents forming ionic solutions in an inert atmosphere. Moreover, the intercalation of alkali metals (Li and K) in liquid ammonia can control the intercalation process and prevent the degradation of samples. However, liquid ammonia is not particularly attractive as a solvent. NMP and DMF are also potent solvents for exfoliation but have been placed on the list of substances of very high concern (SVHC). Thus, the employment of green solvents for exfoliation has imminent importance Figure 1. NvdW materials applied in supercapacitors and batteries, embracing Lithium-ion (LIC) and metal sodium capacitors (MSC), lithium-ion batteries (LIB), lithium-sulfur batteries (Li-S), sodium-, and zincion batteries. Considering the defect-rich surfaces and surface-exposed metal atoms in such inorganic, atomically thin 2D phases, nvdW 2D materials are also expected to play an essential role in metal-air and metal-CO 2 batteries, where efficient catalysts are required for the oxygen reduction and evolution reactions, as well as for the respective CO 2 transformations.
in the coming years. [36] Another factor that influences the exfoliation procedure (i.e., the lateral dimensions and thickness of the nanosheets) is the ratio of in-plane-tearing energy (E E ) to out-of-plane-peeling energy (E S ), as defined by the equation L/t ∼ 2E E /E S (where L corresponds to the longest lateral dimension and t is the thickness). This equation shows a relation of mechanical (or bond-strength) anisotropy with the aspect ratio of the obtained nanosheets, corresponding to a fundamental relationship between nanosheet size and thickness, which is not significantly affected by the solvent choice or sonication conditions. [37] Solvothermal exfoliation is another, yet less common, LPE strategy, involving the treatment of the parent material and the solvent under high pressure and temperature, employing specific equipment, such as a high-pressure reactor. The process usually affords expanded or pre-exfoliated materials requiring a post-treatment step with ultrasonication rendering the solvothermal exfoliation method less preferred for large-scale use. [31a] However, the stronger forces developed during solvothermal processes could promote the exfoliation of certain materials, particularly in the case of nvdW ones, where the 3D bonds may limit the effectiveness of the LPE under normal conditions. Application of LPE faces challenges in nvdW systems because they do not possess a strong anisotropy between the in-plane and out-of-plane directions. Xie et al. explored a family of non-layered materials which still possess some degree of anisotropic character. In particular, they studied structures with chain-like morphology from the group of chalcogenides, such as tellurium and its alloys. [38] The bulk material consists of atomic chains connected with strong covalent intrachain bonds, while the adjacent chains are linked by weak vdW interactions, thus offering the opportunity to produce 2D exfoliated nanoflakes using LPE. Remarkably, 3D non-layered bulk Te particles could be exfoliated into 2D Te nanosheets, with a thickness of ≈5-6 nm after subjecting to sonication in isopropyl alcohol (IPA). Tellurium nanosheets exhibited improved electron-hole separation leading to superior photocurrent response behavior under visible simulated light, offering solutions for solar light harvesting applications. The same method was used to prepare 2D exfoliated selenium using different solvents (DMF, NMP, DMSO, and IPA). Among those, IPA and DMF were more effective for exfoliating bulk Se powder into 2D Se nanosheets of 3-6 nm thickness, due to the match between the surface energies of the solvents and the 2D Se nanosheets. [39] Similarly to bulk Se powder, the as-prepared 2D Se nanosheets demonstrated the typical trigonal phase (i.e., t-Se), confirming that the crystalline features of Se were retained during the exfoliation procedure. The resulted 2D Se nanosheets had a sizedependent band gap (Eg), varying from 1.98 to 2.31 eV as the lateral size decreased, strong photoluminescence, and robust chemical stability under ambient conditions. In another work, the LPE of hematite ore in DMF was successfully applied resulting to hematene. After sonication of 50 mg of hematite for 50 h, hematene was isolated by typical centrifugation to remove bigger non-exfoliated particles, followed by filtration of the supernatant to obtain a few mg of the product. [16] X-ray diffraction showed two distinct monolayer orientations corresponding to the (001) and (010) planes with a thickness of 3.98 and 3.2 Å, respectively (Figure 3). The successful LPE of hematene from hematite opened an avenue to explore the possibility of obtaining high-quality 2D nvdW iron oxide crystals from iron ores by exfoliation. Ilmenite (FeTiO 3 ) and chromite (FeCr 2 O 4 ) were successfully exfoliated in atomically thin sheets using the same method as above. [40] The resulting ilmenene and chromiteen sheets had a [001] orientation, corresponding to (001) faceted planes and (111) planes, respectively. Non-layered iron pyrite was also exfoliated using LPE by Kaur et al. [24b] whereby bulk iron pyrite (FeS 2 ), a naturally abundant non-layered metal sulfide, was dispersed in NMP, and the 2D platelets were obtained via probe sonication followed by centrifugation. The exfoliated material remained dispersed in the supernatant, with a reported yield of ≈3% with respect to the starting bulk material. The dangling bonds created from the exfoliation process, present on the surface of the platelets, were passivated by oxygen-containing groups forming oxides, hydroxides, and sulfates. The authors claimed that the material was susceptible to exfoliation due to an anisotropic bonding architecture. However, they noted a disagreement in the literature regarding the magnitudes and relative strengths of the iron−sulfur versus sulfur−sulfur bonds in pyrite. Other nvdW materials exfoliated via LPE under sonication, using organic solvents such as DMF or NMP (Table 1), include 2D manganese telluride, [41] SnP 3 , [42] FeF 3 , [24a] and goethite. [29a] The common characteristic of most LPE procedures is the use of organic solvents, such as NMP and DMF, which have been placed on the SVHC due to their toxicity and environmental issues. [36] In this context, Eltman et al. developed an environmentally friendly approach to synthesize V 2 O 5 from vanadium dioxide [43] (VO 2 (B), where B refers to the semi-metallic phase [44] ). A VO 2 (B) suspension in water was sonicated and refluxed for ≈7 days to achieve efficient exfoliation. XRD results showed that water intercalates in the bulk VO 2 (B), transforming it into ultrathin V 2 O 5 0.55H 2 O nanosheets. [43] Remarkably, electrochemistry experiments showed that the isolated fraction containing the population with the thinnest sheets demonstrated the best properties for electrochemical energy storage, as explained in more detail in chapter 3.1.
Adv. Funct. Mater. 2023, 33, 2209360 Figure 3. a) Schematic illustration of the liquid exfoliation in DMF of bulk hematite to hematene having two different crystallographic planes. The thickness and atomic arrangements of hematene in these two crystallographic planes are illustrated, where the red spheres correspond to oxygen, yellow spheres correspond to iron, and d is the sheet thickness. b) Bright-field TEM image of a single sheet, Scale bar, 0.5 µm (left) and high-magnification image of a monolayer and bilayer hematene, scale bar, 50 nm (right). Reproduced with permission. [16] Copyright 2018, Springer Nature.
Among the TMOs, tungsten trioxide (WO 3 ) attracted particular interest due to its environmental friendliness, and earth abundance, together with unique characteristics developed when exfoliated into ultrathin nanosheets. [45] Guan et al. devised an effective strategy to exfoliate WO 3 with the assistance of bovine serum albumin (BSA) as an exfoliating agent in an aqueous solution at pH 4. [46] The electrostatic attraction of the protonated amino groups of BSA on WO 3 led to the formation of BSAbound nanosheets, achieving good and stable dispersion of BSAfunctionalized WO 3 ultrathin flakes. The 2D material exhibited superior performance and unique advantages in applications such as visible-light-driven photocatalysis, high-capacity adsorption of organic dyes, and fast electrochromic properties. The WO 3 nanosheets could photocatalytically degrade organic molecules substantially more efficiently than the non-exfoliated WO 3 .
LDH materials can be effectively exfoliated using LPE in the presence of appropriate ions and solvents that allow ion intercalation or ion exchange to weaken the bonding between the oppositely charged layers, expand the interlayer space, resulting in single-layered or few-layered 2D materials. [47] For example, Qin et al. reported the synthesis of 2D Co-LDH nanomesh by a multistep reaction of topochemical oxidative intercalation, anion exchange, and LPE, using mesoporous single-crystalline Co(OH) 2 as the precursor. [48] Briefly, the Co precursor was prepared via a softtemplate technique using an amphiphilic copolymer which can self-assemble into spherical micelles at a specific concentration and followed by a complex multistep procedure. The bulk LDHs could be delaminated into nanosheets after being dispersed in formamide and stirred in N 2 atmosphere. The final material showed remarkable catalytic properties in the oxygen evolution reaction.

Electrochemical Exfoliation
Electrochemical exfoliation is a popular method for producing 2D materials due to the low-temperature conditions, simple instrumentation, repeatability, high yield, and good quality of the obtained sheets. [52] The process involves the deposition of the 3D material on an electrode surface, which is then immersed in an aqueous or organic electrolyte. The application of a direct voltage drives the ions of the electrolyte to the electrode, promoting their intercalation between the planes of the solid, and weakening the interaction forces leading to separation and, eventually, releasing them into the electrolyte. [53] The setup consists of a working electrode, a counter electrode, and an electrolyte solution, while using a reference electrode is optional but required for recording the processes, such as possible redox events and reaction potentials. The working electrode (foil or rod) serves as an anode in the case of ions intercalation or a cathode in case of cations intercalation. The counter electrode is a metallic wire or foil, and the electrolytes are usually composed of an aqueous solution of acids, inorganic salts, or surfactants. [54] The type of solvent and electrolyte plays a crucial role not only in the efficiency of the exfoliation but also in the quality and properties of the exfoliated materials. For example, cathodic exfoliation requires a negative potential whereby the positive ions in the solution are most critical and can drive the expansion and exfoliation of the material through their intercalation between its layers. To this end, lithium ions are considered as an optimal choice due to the small radius, high diffusion coefficient, and good solubility of some lithium salts in non-aqueous solvents, [55] enhancing the intercalation ability of Li.
[56] On the other hand, for anodic exfoliation, a positive potential is applied to the material deposited on the working electrode, whereby negative ions (e.g., OH − , SO 4 2− , Cl − , Br − ) are attracted by the applied potential enhancing the intercalation. Electrochemical exfoliation can also be achieved by dispersing the bulk material in the electrolyte solution between two conductive electrodes (Pt foils are most commonly used). [57] The solvents and mixtures thereof with different anionic species involved contribute further to creating the appropriate conditions for exfoliation, which are related to the matching of the surface energies to that of the target materials for exfoliation in order to minimize the enthalpy change of the process. [58] Further aspects of a suitable electrolyte are high conductivity, the appropriate size of solvated ions to match well with the interlayer gap or pores of the bulk material, and redox processes that are required to produce gaseous species (from water electrolysis for example) that subsequently contribute to layer separation. Most exfoliation methods utilize aqueous or organic solvents, which might be limited by early oxidation/reduction, insufficient intercalation, or limited choices regarding the range of surface energy. For these reasons, exploring mixed solvent systems is an attractive direction. [59] Solvent mixtures have been used to exfoliate layered materials by combining two "mediocre" solvents to form a strong co-solvent. [60] In particular, Halim et al. used a simple water-alcohol mixture to exfoliate graphite and MoS 2 efficiently. With this approach, the use of highly toxic solvents could be avoided by replacing them with environmentally friendly solvents that attain similar surface tension properties and, simultaneously, reduce the costs of using expensive pure solvents.
In the case of nvdW materials electrochemical exfoliation is still not widely applied, nevertheless some interesting reports showed its potential. Yang et al. produced a few-layered 2D birnessite (MnO 2 , Figure 4a). [61] Although birnessite is a layered material, the interlayer forces are particularly strong; thus, birnessite could be considered a borderline material between vdW and nvdW solids. In this approach, manganese metal was applied as the working electrode. Then, upon the application of positive potential, a thin layer of birnessite intercalated with alkali metal cations (e.g., Na + , K + , or Li + ) was formed on the surface of bulk Mn through an anodic-dissolution cathodicdeposition mechanism. In the following, applying a highly negative charge, the newly formed birnessite layer on the Mn metal electrode was cathodically exfoliated into 2D birnessite. The Na-Bir-5 (the code name of the material which was exfoliated at a scan rate of 5 V s −1 ) exhibited transparent texture and displayed the largest and thinnest sheets, suggesting that the potential scan rate is an essential parameter for the microstruc-ture of the exfoliated products. The TEM images of Na-Bir-5 showed few-layered sheets with graphene-like wrinkles, and the interplanar spacing of the (001) lattice planes was 0.73 nm (Figure 6b,c).

Chemical Vapor Deposition
In certain fields of research and technologies, such as optoelectronics and spintronics, strict control of the crystallinity and defects is required. In such cases, LPE, although simple in principle, usually suffers from relatively low quality because of the high energy involved and applied in bulk without selectivity. To bypass such challenges, CVD is a bottom-up method that provides a scalable and controllable way to grow highquality and large-area 2D materials at a reasonable cost. CVD employs gaseous molecular precursors which react in the vapor phase or on substrates, thus forming the solid products that are deposited and stabilized on the substrates or grown on-surface. The size, morphology, orientation, and number of layers are controllable and influenced by the temperature, pressure, gas flow rate, source material concentration, and source−substrate distance and interactions. [9e] The desired 2D material dictates the selected precursors, which serve as reactants in the CVD process, where three reactions occur: thermal decomposition, chemical synthesis, and chemical transport. For example, graphene is mainly deposited using CH 4 and H 2 gases as precursors, whereby the strict control of the gas flow rate is decisive for the structural control of the finally obtained graphene. [62] Also, the temperature gradient over the reactants' pathway can affect the reaction of the decomposition or gasification of the precursors, controlling their atomization, their concentration, mass transport, and eventually the growth mechanism, thus defining the quality and composition of the final products. High-quality products are usually obtained at relatively high temperatures, however, at the expense of energy and costs. [9e,63] With this technique, the synthesis of high-quality 2D nvdW Cr 5 Te 8 with tunable thickness has been achieved. [ [61] Copyright 2021, Elsevier.
Cr 5 Te 8 flake of a single unit cell thickness and lateral size of tens of micrometers was obtained, showing metallic or ferromagnetic properties depending on the growth time which affected the thickness. The Cr 5 Te 8 flakes were stable during aging and deterioration testing which could be connected to the continuous layer structure instead of discrete islands (multidomain 2D crystals). Several nvdW materials of the Cr n X family (X = S, Se, and Te) have been also synthesized via CVD. [65] The synthesis of ultrathin nanosheets (along with thicker flakes) of non-layered γ-CuI (a p-type semiconductor with cubic zinc blende structure) on SiO 2 /Si substrate has been also reported using a physical vapor deposition process, with γ-CuI powder as the precursor and argon as the carrier gas at ca. 400 °C. [66] In another work, one unit cell-thick 2D crystals of cobalt ferrite (which in bulk crystallizes in a spinel-type nonlayered structure) were grown by CVD on mica substrates placed above the precursors. Ferric oxide, cobalt(II) oxide, ferric chloride, and sodium chloride powders were mixed along with molecular sieves to adsorb by-products and heated at 700 °C. Interestingly, the method could be extended to the synthesis of other spinel-type nanosheets, such as 2D manganese ferrites, [67] and non-layered 2D iron selenides with ferromagnetic (Fe 5 Se 8 ) and metallic (Fe 3 Se 4 ) properties. [68] Nevertheless, CVD can be effectively applied only in technologies where thin films of materials are required, such as in magnetic recording, spintronics, (optoelectronics) and field effect transistors.

Template-Assisted Growth
Template-assisted or topochemical growth of 2D materials is an attractive synthetic method wherein ionic or molecular precursors self-assemble into 2D nanosheets upon specific interactions with the surface of solid substrate or at dynamic interfaces (e.g., lamellar phases of liquid crystals) aiding or directing the planar growth of crystals. [69] The unique surface activities at different interfaces and crystal planes allow the morphological templates to drive the crystal growth of the precursors. The growth of the material may be governed by factors such as the adsorption energy barriers and catalytic processes. The 2D nanosheets may then be released from the template surface, for example, via chemical intercalation and exfoliation, stamping or via selective decomposition of sacrificial templates. Chen et al. produced nvdW quasi-2D holey Co 3 O 4 via a controlled template-directed strategy by growing cobalt oxide over graphene oxide (GO) surfaces. [70] Co 3 O 4 nanoparticles were formed during on-surface condensation on the free-standing GO sheets, modified with sufficient oxygen-containing functional groups for promoting the interactions and immobilization of the cobalt metal ions on its surface. The cobalt ions anchored on GO formed cobalt hydroxide precursors, and after their condensation via hydrolytic processes, they self-linked with each other forming holey Co 3 O 4 2D sheets. The final material satisfied several critical requirements for Li and Na storage (e.g., low volume expansion, high charge transport), not shared with its 3D counterpart in bulk or nanoparticulate form. In another interesting work, a novel synthetic strategy was applied toward the formation of 2D hexagonal transition metal oxides (h-MOs), such as TiO 2 , MnO, Fe 2 O 3 , CoO, Ni 2 O 3, and Cu 2 O, which displayed distinctly different charge distribution between the metal centers and oxygen anions from that observed in their 3D crystal structures. [71] These 2D oxide layers were obtained according to a controlled oxidation mechanism, whereby one oxygen atom is chemisorbed on a metal atom on the first exposed metal layer of a polished bulk metal surface. Then, a mirror charge was possibly induced in the metal atom to attract another oxygen atom symmetrically. Eventually, a depolarized monolayer oxide structure was formed, energetically allowed to adapt to the honeycomb-like planar hexagonal crystal coordination. [72] The low surface roughness of the polished bulk metal substrate facilitated the mechanical exfoliation by stamping, and also restricted the uncontrolled, defect-driven oxidation (Figure 5a). Microscopy analyses confirmed the layered nature of the exfoliated h-MO sheets, which were endowed with a high degree of smoothness and large lateral dimensions, while AFM evidenced sub-nanometer thickness, which could be considered as single unit cell layer (Figure 5b-e).
The use of sacrificial templates to synthesize nvdW 2D materials has also received attention. Peng et al. synthesized various 2D holey TMO nanosheets, such as ZnMn 2 O 4 (ZMO), using GO as a 2D template to immobilize the TMO precursors on the surface. The resulting TMO precursor/rGO was then annealed to remove the rGO template, forming crystalline TMO nanostructures chemically interconnected with each other, forming 2D holey nanosheets. [73] Defect-free phosphorene layers, produced by exfoliation from bulk phosphorous, have been also utilized as sacrificial templates to efficiently produce 2D transition metal phosphides, such as Co 2 P, Ni 12 P 5 , and Co x Fe 2−x P. [74] In another example, complex cubic MnSe 2 /Se precursors were prepared through a modified hydrothermal reaction using a 1:5 hydrazine-water solution. The cubes had a size of ≈15 µm and stripe-like surfaces. Thanks to its facile reducibility, elemental Se was selectively removed from the parent crystals with sodium borohydride, facilitating the formation of rectangular MnSe 2 nanosheets with a thickness of ≈22 nm. The 2D manganese selenide material was endowed with ambient temperature magneto-photoconductive properties, predisposing it as efficient semiconductor for optoelectronic and spintronic applications. [75] Xiao et al. published a salt-templated method for producing several 2D TMOs, such as h-MoO 3 , MnO, and h-WO 3 . [76] In this method the facets of the salt crystals served as surfaces for the directed growth of the deposited molecular or ionic precursors. When the crystal lattices of the salt and of the developing oxide match, planar growth is likely to take place. The dried salt crystals with the oriented precursors deposited were annealed at a high temperature producing 2D oxides on the salt crystal surfaces. Then, water or other solvents, in which the salt is dissolved, were used to release the 2D oxide structures. The rational design of holey 2D transition metal carbide/ nitride heterostructured nanosheets (h-TMCN) was reported via a bottom-up method, [77] by thermal annealing of the Mo/Zn bimetallic imidazolate frameworks (Mo/Zn BIFs). More specifically, 2D Mo/Zn bimetallic imidazolate frameworks were first synthesized via the coordination of Zn 2+ , [MoO 4 ] 2− and 2-methylimidazole. The formation of molybdenum carbide/nitride 2D nanocrystals was then performed by annealing the 2D Mo/Zn BIF in N 2 atmosphere at ≈800 °C to enable the pyrolysis of the organic linkers. This step facilitated the conversion into the www.advancedsciencenews.com final 2D holey inorganic heterostructured nanosheets without affecting the original 2D architecture. The as-obtained h-TMCN holey nanosheets exhibited homogeneity and were efficiently used as bifunctional catalysts for water oxidation and full water splitting. However, their thickness of ≈45 nm, suggesting the need of modifications if ultrathin 2D nvdW materials are targeted.
Turning to more dynamic interfaces for template 2D crystal growth, Zavabeti et al. developed a liquid metal-based method to produce thin sub-nanometer layers of 2D oxides (HfO 2 , Al 2 O 3 , and Gd 2 O 3 [78] ). Eutectic gallium-based alloys were used as a reaction solvent to develop alloys with the desired metals into the melt. At the surfaces of the alloys 2D metal oxide layers were formed and isolated either by stamping on substrates or by exfoliation in suspension. The structures generated were sub-nanometer thick (probably corresponding to a single unit cell), exhibiting translucent appearance and sporadic wrinkles when observed in HR-TEM. Crystalline 2D SnS films were also produced on a large scale by the interaction of liquid tin (Sn) with 50 ppm H 2 S gas mixed with N 2 an anoxic atmosphere containing. [79] A monolayer of SnS film with large lateral dimensions, no pinholes, and a low number of grain boundaries was created. Similarly, large-area p-SnO/n-In 2 O 3 metal oxide heterostructures [80] and 2D Ga 2 O 3 films [81] with a high degree of crystallinity were created at liquid metal interfaces.
Fully dynamic liquid crystal templates have been also employed for the bottom-up synthesis of 2D Co 3 O 4 nanosheets relying on the use of lamellar phases from pluronic polymeric surfactants. During a solvothermal step the ionic precursors formed a transient cobalt oxide 2D phase trapped between the polymeric lamellae, which was then calcined to deliver the final 2D Co 3 O 4 material. [82] The solvothermal step was carried out in an ethanol/water solvent with cobalt (II) acetate tetrahydrate as the metal precursor and hexamethylenetetramine as a base, in the presence of pluronic P123 and ethylene glycol as a cosurfactant. With the assistance of ethylene glycol, the pluronic surfactant forms an inverse micellar phase of a lamellar structure, where confined oligomerization reactions and assembly of the cobalt precursor in the 2D space took place. This promoted the formation of a material with nanosheet morphology and a thickness of ≈3-5 nm. An attractive approach for the synthesis of 2D indium tin sulfide (In 4 SnS 8 ) was also developed by controlling the isoelectric point of amino acids (L-cysteine) playing the dual role of sulfur source and structure-mediating agent. [83] In particular, at the isoelectric point of L-cysteine (pH = 5.05), the dissolved metal cations coordinated with the aminoacid's mercapto (SH) groups, forming electrically neutral coordination complexes. In the following, these complexes grew further under solvothermal conditions into thin sheets of In 4 SnS 8 , via a 2D nucleation and layer growth mechanism.

MXenes as 2D nvdW Materials
MXenes have attracted paramount interest as novel 2D transition metal carbides, nitrides, and carbon nitrides, particularly after the report on Ti 3 C 2 . [84] They represent a particular class of nvdW 2D materials with several review already Reproduced with permission. [71] Copyright 2021, Springer Nature.
articles describing their synthesis, structure, properties, and their application in EES technologies. [27,85] Therefore, we herein provide only a brief description focusing on the synthetic procedures. [8g,9c,86] The general formula of MXenes is M n+1 X n T x (n = 1-3), where M is an early transition metal (such as Sc, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, Mn), X is carbon and/or nitrogen, and T represents a surface termination unit such as hydroxyl, oxygen or fluorine. These surface termination groups are formed during the exposure of the chemically reactive surface metal atoms of the layers. The three MXene classes, according to their atomic lattices and composition can be classified as ordered mono-M, double-M, and solid-solution M elements. [87] Production of MXenes is a topochemical method, however, it is quite different from the other topochemical methods so far applied for nvdW 2D materials as described above, and requires the selective acidic etching of the MAX or non-MAX parent phases (where A represents Al, Si or Ga atoms). The etching method appears to be the most effective in terms of feasibility, yields, controllability, and cost-efficiency. With over 70 types of different MAX ternary carbides and nitrides the family of MXenes is very large and continuously growing. Thus far, over 30 kinds of MXenes have been experimentally obtained and many more are expected according to theoretical predictions. [86a,88] The first reported MXene, Ti 3 C 2 , was obtained from the Ti 3 AlC 2 MAX phase by selective etching of the Al atoms in aqueous hydrofluoric acid (HF) solution at controlled temperature, HF concentration, and etching time. [84] High HF concentration or long etching duration may lead to the complete dissociation and formation of carbide-derived carbons. This method is low-cost and high-yield and remains the primary synthetic approach for high-quality MXenes. [89] The key to this method's success is that the MA metallic bond is more susceptible to acidic hydrolysis than the MX bonds. [90] Thus, the exfoliation process requires the A-elements to react with HF and form fluorides (e.g., AlF 3 ) and gaseous hydrogen. After this step, the resulting M n+1 X n has an accordion-like structure (Figure 6a,b). In 2021, the Ti 3 C 2 MXene was prepared using in situ formed HCl to etch the Al atoms from the MAX phase and was further functionalized with amorphous MoO 3 through TiOMo covalent bonding, affording 2D nvdW heterostructures (Figure 6c). [26a] DFT calculations suggested that these nvdW heterostructures can firmly stabilize the aMoO 3-x phase. In particular, after the adsorption of the MoO 3+ cations onto the negatively charged MXene surface, the bonding lengths of all the related TiO bonds were found to increase, indicating the partial transfer of electron density from the TiO bonds to molybdenum atoms, forming TiOMo bonds (Figure 6d). Carbides are generally more susceptible to wet acid etching, while only a few nitrides have been successfully prepared via this method. [91] After the etching process, the unstable surface dangling metal bonds react with the aqueous solution and create surface terminations, such as O, OH, F, ascribing hydrophilic properties. The surface terminations also critically affect the electronic, optical, magnetic, and mechanical properties of MXenes. [92] MXenes were also recently obtained via acidfree etching. According to this, Ti 3 C 2 was produced in a fluoride-free approach via anodic etching in an alkaline aqueous electrolyte consisting of 1 m NH 4   rods were used as both precursors and electrodes. [93] With the application of +5 V, the bulk anode gradually detached, and the electrolyte turned from transparent to ash-grey. Afterward, the sediment and suspended powders were transferred into a 25 mass % TMAOH solution. This step promoted the delamination of the etched Ti 3 C 2 T x into individual 2D sheets because the positively charged TMA + ions intercalated into the interlayer space weakening the interactions between 2D layers. Moreover, in situ chemical transformations were proven to be an efficient method to synthesize termination-free MXenes. For example, hexagonal Mo 2 C nanosheets were produced by deoxygenation and carburization of the H 2 MoO 5 precursor. [94] In 2021, a MXene/LDH hybrid (NiFe-LDH/Ti 3 C 2 ) was synthesized, through a simple hydrothermal reaction, [95] whereby NiFe-LDH nanoflakes were uniformly aligned between the MXene nanosheets, inhibiting their self-stacking. In particular, Ni, Fe, and urea were added in a high-pressure reactor, and mixed into a homogeneous solution, whereby the Ni 2+ and Fe 3+ cations were attached on the negatively charged MXene nanosheets via electrostatic forces and then nucleated. In this topochemical method, MXenes played the role of the template for the nucleation and vertical growth of the nvdW 2D NiFe-LDH nanoflakes.
In contrast to several top-down MXene preparation methods, only a few bottom-up methods have been reported. The crystal lattice and the multi-element nature of MXenes make the bottom-up approaches very demanding. However, high-quality, superconductive 2D α-Mo 2 C crystals were successfully synthesized via CVD at ≈1090 °C. At this temperature, Mo and Cu atoms formed a liquid alloy from Mo and Cu metal foils which were placed together. Subsequently, Mo atoms diffused from the interface to the surface of the liquid Cu to form Mo 2 C crystals by reacting with the carbon atoms delivered by the thermal decomposition of methane. [96] It was found that the key for ultrathin Mo 2 C 2D films was retaining a low concertation of methane. After growth, the 2D crystals were transferred to SiO 2 /Si substrates using ammonium persulfate, a mild Cu etchant.
In general, the as-prepared MXenes after etching, although already in a layered vdW state (the accordion structure), must be delaminated in a consequent step, which usually involves shear forces, further selective chemical etching and/or intercalation. For example, successful delamination can be achieved during exfoliation using lithium fluoride salts in an acidic environment (e.g., LiF+HCl). The small lithium cations can intercalate between the MXene layers leading to swelling and subsequent separation of the layers. [97] Mechanical forces such as sonication or organic polar solvents with bulky organic intercalants can also assist the delamination of MXenes, such as dimethyl sulfoxide (DMSO) with dissolved TMAOH. [98] Another challenging matter is the exfoliation and preparation of 2D nanomaterials from their bulk counterparts with controlled phases. [9a] Ajayan's group demonstrated a strategy to obtain 2D transition metal chalcogenides with well-defined phases by converting nvdW bulk solids, like MAX phases. [99] In particular, the strategy included the reaction of active M-A bonds in MAX phases with chalcogen-containing gases, resulting in products of AZ and MZ compositions (Z: sulfur, selenium, or tellurium). Thermodynamically, when the temperature is high enough, the post-transition metal species A (Si, Al, Sn, Ge) in MAX phases can be transformed into metal chalcogenide gases, thus facilitating the conversion of MAX phases into 2D MXene structures. Using this principle, they synthesized a broad library of 2D materials involving 13 transitionmetal chalcogenides.

Critical Assessment of the Synthetic Approaches
Intense research efforts are focused on studying the applicability of the already established methods for the exfoliation of vdW materials to the case of nvdW ones, but also on developing and optimizing new advanced synthetic methods to obtain 2D materials from non-layered solids with desired morphology and properties. NvdW 2D materials are more challenging to obtain than the vdW analogs due to the non-layered nature of the parent bulk materials with strong covalent bonds in all three dimensions. For nvdW solids, the cleavage of the covalent bonds via chemical reactions using reactive reagents maybe be a promising strategy, going beyond the role of a solvent for intercalation and exfoliation of layers. This strategy proved very effective for MXenes, however, has not yet been explored in other nvdW solids. Generally, a material's proclivity toward exfoliation is expressed in terms of the energy required to separate the crystal planes, defined as the energy difference between the relaxed 2D and bulk counterparts, which depends on various parameters, such as the type of atoms or the type of compound (e.g., binary or ternary). Friedrich et al. calculated the exfoliation energy for different nvdW materials (binary and ternary). They observed a significant decrease in exfoliation energy when they replaced one type of atoms in the binary system with another type to make it ternary. [100] For instance, the experimentally obtained exfoliation energy for Fe 2 O 3 was ≈0.16 eV Å −2 , whereas, in the case of the binary FeTiO 3 structure, the average exfoliation energy is ≈0.08 eV Å −2 . Further, the exfoliation energy in ternary compounds is smaller for those systems whereby the terminating atoms have a lower oxidation state, which might be related to the strength of Coulomb interactions between facets. For example, Ag, Na, and Cu (with oxidation state +1) ternary compounds show lower exfoliation energy than Fe, Co, Ni, and Mn (having oxidation state +2) ternary compounds. Interestingly, termination with active spin atoms (Fe 2+ , Mn 2+ , Ni 2+ , and Co 2+ ) offers exciting magnetic properties in the respective nvdW 2D materials. The exfoliation energy for different nvdW structures based on computational calculations and experimental evidence is given in Figure 7. [100] Undoubtedly, surface energy plays a critical role in the exfoliation proclivity of the materials; however, it is not the only parameter involved. The stability of the 2D sheets after exfoliation is also critical, and not meeting the conditions for it can lead to the relaxing of the structure back to the original one. An indicative example of this interplay of energetics is NaCl, which has low surface energy allowing its facile exfoliation but remains unstable as a 2D structure. [101] Therefore, compared to vdW 2D materials, exfoliation of nvdW 2D materials has a particular interest and different challenges that could convey remarkable properties exploitable in diverse applications. The high applicability potential stems from the properties of the surface-exposed atoms, which are terminated by dangling bonds and unsaturated www.advancedsciencenews.com coordination sites, offering a high degree of chemical activity, [29a] as well as due to the emergence of new and rare surface -or edge -spin states and band structures, being very sensitive to changes of the surrounding environment.
It is highly desirable to control the defects, domain, doping and surface chemistry in 2D materials to tune the physical properties. Zero-dimensional imperfections in 2D materials include vacancies, anti-sites, and substitutional atoms, whereas, examples of 1D defects include grain boundaries, twin boundaries, edges, and dislocations. Certainly, growth and assembly techniques are key to control these defects in 2D nanomaterials. Top-down approaches are attractive to create big and highquality (defect-free) 2D materials in the case of layered parent phases, but for nvdW systems it is more challenging to achieve similar results. Bottom-up approaches, such as well-designed CVD and topochemical strategies, might be able to address this challenge, since they are based on an atom-by-atom growth process.
Scrutinizing in more detail the individual methods, dry mechanical exfoliation methods can offer high-quality materials, but only on a small scale and with low efficiency. The 2D materials created still require to be transferred and stabilized, which calls for additional processing steps. [103] Only microscale yields of crystalline samples can be obtained via dry mechanical means, unless ball-milling or rolling shear delamination methods can be effectively developed for high-quality 2D crystal exfoliation. On the other hand, the application of mechanical exfoliation methods based on wet shear [32] and ultrasonic methods [104] allows the manufacturing of higher quantities of 2D materials. However, the control over the defects or the lateral size of the sheets can be an issue, since long sonication or high power to modify flake sizes and the yield of the method may have a detrimental effect on the crystallinity of products. [105] Topochemical synthesis is also a viable process for improved yields of 2D materials with prospects for obtaining high crystal quality. The quality of the 2D material produced using this method is significantly influenced by the structuredirecting effect of the templates used and by the growth conditions in which the atomic or molecular precursors are handled and assembled. Temperature, concentrations, solvents, purity, dissolved gasses, pressure can substantially affect the delicate self-assembly topochemical processes. If the appropriate conditions are identified, this scalable method can provide a high degree of control over the material's size, shape, and uniformity. For example, highly crystalline, single-domain micronsized monolayer 2D SnS films were grown at the interface of liquid tin (Sn) with an anoxic atmosphere of H 2 S. [79] Usually, during the interfacial synthesis with liquid metals, the high surface tension of the one phase compared to other allows the ideal growth of 2D materials at the interfaces, resulting in the fewer grain boundaries/dislocations. [80,81] Furthermore, due to the involvement of chemical reactions and restructuring of the precursors, the domain structure is also affected. For instance, when 2D (Na 0.5 Bi 0.5 ) 0.93 Ba 0.07 TiO 3 is prepared from the parent Na 0.5 Bi 4.5 Ti 4 O 15 structure, stripe-like domains are converted to lamellar domains. [106] Thus, by controlling the domains, the physical characteristics can also be tuned for different applications, such as for enhancing the piezoelectric performance. [107] 2D materials from wet topochemical processes typically exhibit surface flaws in the form of terminal groups, domains and crystallinity.
CVD, which can be considered as a gas phase topochemical synthetic approach, offers improved control over the shape, crystallinity, thickness and orientation of the 2D crystals. It is also flexible in terms of material doping. In CVD, the polycrystalline nature of the substrates and the transfer processes frequently result in imperfections like polycrystalline 2D sheets and cracks in the crystal structure, rendering the role of the substrate crucial for controlling the final properties of the 2D materials. By selecting substrates with different structural features, the domain organization can be controlled, generating continuous single-layer 2D films made up of many small or few large crystalline domains, when polycrystalline or monocrystalline substrates are used, respectively. A structural relationship is certainly developed between the 2D film and the substrate when strong interactions are present in between them, dependent on the matching of the lattice constants, surface energies and charges, promoting clearly defined crystallographic correlations. On the other hand, vdW epitaxy, i.e., weakly interacting substrates/films, causes strain and interfacial defects for the 2D Figure 7. Exfoliation energies of a) binary and b) ternary systems for two different functionals (PBE+U and SCAN). The dashed horizontal black lines correspond to the graphene exfoliation energy. [9b,102] For the ternaries, the underlined atom corresponds to the favorable termination atom (at the bottom axis). The inset indicates the two possible termination geometries. In the case of GeMnO 3 (marked by "*"), PBE+U favors Mn termination, while for SCAN Ge termination is preferred. Data points for Fe 2 O 3 and FeTiO 3 are realized experimentally. [16,40a] Reproduced with permision. [100] Copyright 2022, American Chemical Society. material and multidomain nature. [108] CVD can be effectively applied mainly in technologies where thin films of materials are required, such as in magnetic recording, spintronics, (optoelectronics) and field-effect transistors.
Printing techniques to create films from nvdW materials are particularly attractive due to their scalability and flexibility. In 2 O 3 [109] and Ga 2 O 3 [110] thin films have been prepared by flexography printing and polydimethylsiloxane printing, respectively, however the films were quite thick [109] or with many randomly oriented nanocrystalline domains. [109,110] Undoubtedly, advanced printing techniques with superior control over the quality (presence of defects and thickness) of 2D materials is very challenging, and its realization may generate substantial thrust in the field of thin film applications. Beyond thin films, printing multicomponent 3D architectures containing atomically thin layers interwoven with other nanomaterials in organized or random networks would revolutionize the utilization of nvdW 2D materials in applications where higher amounts of materials are required, as for example electrodes in EES devices.

2D nvdW Materials in Electrochemical Energy Storage
2D materials have attracted paramount interest as electrodes in EES devices, such as batteries and supercapacitors, due to their open morphology/architecture, tunable charge transport, and redox properties. These features improve diffusion, interactions with the energy carriers, electronic communication, and the electrodes' mechanical stability compared to their bulk counterparts. [12c] The large surface-to-volume ratio and open framework structure of 2D materials ensure an increased number of active surface sites and accessible diffusion channels, further promoting the charge transport and energy storage capacity. Graphene and its derivatives are thoroughly studied 2D materials for energy storage, [7a,c,111] and motivated significant research efforts to study and exploit a large variety of 2D materials beyond graphene. [112] The interest in pursuing further advances in the field and developing previously unexplored 2D vdW and nvdW materials is crucial for identifying the next generation candidates for electrochemical energy storage technologies.
In recent years, 2D nvdW materials have been explored as electrode materials in EES and other applications, such as in catalysis for energy conversion via CO 2 reduction and transformation into chemical forms with high energy content or higher carbon content. [9g] Many vdW materials, such as MoS 2 , [8d,112a] WS 2 , [8e] boron nitride [8f ] are being investigated as battery electrodes. However, the relatively inert surfaces of vdW 2D sheets may limit the electrochemical activity of the electrodes. On the other hand, the defect-rich surfaces of 2D nvdW materials improve the interactions with the energy carriers (the charged species) and create effective ion-transport pathways. [26a] Thus, in recent years, nvdW materials have been considered and investigated as promising electrode materials for EES. Here we summarize the progress on nvdW materials studied as electrode materials in batteries and supercapacitors, a rapidly expanding area of research and development.

Batteries
2D morphology can enhance ion diffusion by providing shorter or open diffusion pathways in the 2D channels, contributing to improved kinetics, essential for effective battery charging and discharging at high rates. Fast kinetics may offer benefits because it can reduce the effects of parasitic reactions resulting in improved electrode stability and, thus life cycle of the battery. More particularly, 2D nvdW materials bring additional advantages because they open the doors to exploit surfaces with compositions and chemistries which have not been previously attainable. Their defect-rich 2D crystal facets, cleaved from the 3D bulk phase, give access to redox processes, sites for smooth/homogeneous metal-ion deposition and stripping, or create radical sites opening paths for more effective electronic communication. On the other hand, the highly active surface of 2D nvdW materials with many dangling bonds might require robust functionalization to improve the structural stability during electrochemical cycling. To overcome the stability issue Yan et al. developed 2D nvdW heterostructures as a LIB anode electrode by covalently grafting amorphous MoO 3-x on the highly conductive sheets of Ti 3 C 2 MXene (aMoO 3-x @MXene, Figure 8a and Table 2, MoO 3−x @Ti 3 C 2 -MXene material, entry 2). [26a] The electrochemical performance of this novel 2D nvdW heterostructure was compared with the vdW self-assembled MoO 3-x -Ti 3 C 2 MXene structure that was fabricated by physically mixing amorphous MoO 3−x nanosheets with MXene in tetrahydrofuran. The presence of Ti-O-Mo covalent bonding on the oxygen termination deficiencies on MXene surfaces in aMoO 3-x @Mxene provided high structural stability to the electrode, resulting in superior rate performance (≈60% capacity retention at 1 A g −1 with respect to the capacity at 0.1 A g −1 ) and high cycling stability (≈78% capacity retention for 500 cycles) than the physically mixed system. The origins of the superior stability and performance of the nvdW heterostructure were investigated by studying the role of the O deficiencies using DFT calculations. The analysis suggested that as the number of deficiencies in the nvdW heterostructures increased, their formation energy gradually decreased and reached a minimum at a certain critical point. This low formation energy was the origin of the high stability of the 2D nvdW heterostructure. The density of states calculations revealed an increased electron density at the Fermi level of the nvdW heterostructure, which was critical for improving the electronic charge transport kinetics of the material. Thus, the nvdW heterostructure showed capacitorlike behavior reflecting fast and effective charging-discharging kinetics (Figure 8b,c).
Efforts have also been applied for the straightforward exfoliation (like LPE) of earth-abundant nvdW minerals, which have some degree of anisotropy on their bond-strength architecture. Pyrite (FeS 2 ) is a low-cost and naturally abundant mineral with broad applicability and bond-strength anisotropy. [31c] 2D platelets of FeS 2 were prepared using a simple LPE method in N-methyl-2-pyrrolidone solvent. [24b] Although FeS 2 does not have high conductivity, its properties were effectively exploited via its integration with 20 mass % single wall carbon nanotubes (SWCNTs) (Table 2, FeS 2 /SWCNT, entry 1). Thus, the 2D FeS 2 / SWCNT electrode (active mass loading of 0.4 mg cm −2 ) showed good lithium storage capacity, reaching up to 900 mAh g −1 after www.advancedsciencenews.com kinetic activation, matching or slightly surpassing the theoretical capacity of the 3D FeS 2 of 890 mAh g −1 , which has not been achieved with 3D nanoparticles of the same material. [118] Monoclinic vanadium dioxide (VO 2 (B)), although less abundant than iron-based nvdW materials, is of high interest due to the remarkably versatile oxidation chemistry of vanadium. Therefore, the exfoliation of VO 2 (B) was studied in water affording hydrated vanadium pentoxide nanosheets with thickness between 4.0 and 4.3 nm, composed of 3-4 layers (   www.afm-journal.de www.advancedsciencenews.com and lithiation kinetics were investigated on free-standing electrodes prepared by depositing the V 2 O 5 ·0.55 H 2 O nanosheets on MWCNT paper by vacuum filtration. These binder-free electrodes preserve the conductivity of the MWCNT matrix and are favorable for providing short diffusion pathways. Thus, even a high-thickness electrode of 45 µm could deliver a capacity of 174 mAh g −1 at 10 mA g −1 , while electrodes of 4 µm delivered 480 mAh g −1 at 10 mA g −1 . In another work, Chen et al. prepared 2D holey Co 3 O 4 nanosheets using template-assisted synthesis (Figure 9a, Table 2, Co 3 O 4 , entry 7). [70] The 2D structure of this non-layered cobalt oxide showed excellent reversible capacities of 1324 mAh g −1 at a current density of 0.4 A g −1 and 566 mAh g −1 at 0.1 Ag −1 for Li and Na ion storage, respectively. The 2D holey Co 3 O 4 nanosheets with 10 nm holes showed excellent Li and Na storage performance compared to the sheets with 5 and 20 nm holes or non-porous nanosheets. Cycling the nanosheets with 10 nm holes against lithium and sodium showed higher capacity and cycling stability than the control samples (Figure 9b,c). The 10 nm pores in the 2D holey Co 3 O 4 nanosheets were critical for accumulating the volume expansion during lithiation, and particularly sodiation, due to the larger radius of sodium. Furthermore, the 2D holey Co 3 O 4 nanosheets could more effectively accommodate the volume expansion than conventional Co 3 O 4 particles because the Co 3 O 4 phase remained as ultrasmall and interconnected Co nanoparticles that could be effectively accumulated into the developing sodiated electrode without extensive cracking and failure of the electronic communication. The volume expansion during sodiation was studied using in situ TEM and HR-TEM imaging, which showed that the size of the nanosheets significantly enlarged after 65 min of sodiation (Figure 9d) when full sodiation was achieved. At this stage, the material preserved its 2D structure and porous morphology.  [70] Copyright 2017, American Chemical Society. they exhibited improved structural and cycling stability during the demanding processes of volume expansion upon sodiation.
Peng et al. prepared various holey nanosheets of mixed TMO using templated assisted synthesis with GO. [73] Among them, ZnMn 2 O 4 (ZMO) and NiCo 2 O 4 (NCO) were studied as LIB and SIB electrodes, respectively (Table 2, entries 5 and 6, respectively). The ZMO underwent alloying/dealloying reactions showing a specific capacity of 770 mA g −1 at the current density of 200 mA g −1 . It also exhibited improved cycling stability than the control samples of ZMO nanoplatelets without holes, and the ZMO+SP (i.e., the physical mixture of ZMO nanoplatelets with conductive carbon), prepared without a GO template (Figure 10a). The ZMO nanosheets maintained a stable specific capacity of 500 mAh g −1 after 50 cycles, whereas the control samples could deliver only 320 mAh g −1 (control ZMO+SP) and 100 mAh g −1 (control ZMO). The high capacity of the nanosheets was ascribed to the formation of a ZnLi alloy and to the reversible conversion of the Mn 2+ and Mn 3+ ions during electrochemical operation. Moreover, the holey NCO nanosheets showed excellent rate performance reaching 76% capacity retention after cycling at the current density of 3200 mA g −1 with high Coulombic efficiency of 98% and 96% in ZMO and NCO, respectively. As in the previously described work, the properties of the 2D NCO electrode were attributed to the 2D holey structure of the mixed TMO that can accommodate the volume changes during charging/discharging, as confirmed via in situ TEM studies. The latter unveiled a wellpreserved 2D and porous structure after lithiation/delithiation, as well as during and after the in situ stress tests (Figure 10b).
Another attractive 2D carbon/TiO 2 composite was developed by Yanjie Hu et al. [114] using multiple synthetic steps such as template-assisted, polymerization, carbonization, and wetchemical etching processes ( Table 2, entry 8). The resulting layered composite showed enhanced Li storage, delivering a high specific capacity of 510 mAh g −1 at a current density of 0.1 A g −1 .
It was ascribed to the large electrode-electrolyte interface and Figure 10. a) Cycling performance of the 2D holey ZMO nanosheets, control ZMO+SP, and control ZMO at the current density of 800 mA g −1 for 50 cycles. Coulombic efficiency of the 2D holey ZMO nanosheets. b) TEM images of the 2D holey nanosheets at different lithiation stages/times (first row). TEM images of the 2D holey nanosheets under press (second row). Scale bars, 100 nm. Red arrows point out the holey structures of the 2D ZMO nanosheets. c) Rate capability of the Na-Bir-5, Na-Bir-10, and Na-Bir-20 electrodes at current densities between 200 and 2000 mA g −1 . d) Charge-discharge profiles of the Na-Bir-5 electrode at current densities between 200 and 2000 mA g −1 . Reproduced with permission. [73] Copyright 2017, Springer Nature.
short diffusion paths, with the capacitive processes significantly contributing to the performance, even at lower scan rates. The capacitive contribution was 60% and 77% at the scan rates of 0.1 and 0.5 mV s −1 , respectively, and increased further up to 82% at the scan rate of 0.9 mV s −1 , suggesting the effective use of the composite as Li-ion battery/capacitor anode material.
Superior Zn-ion storage was reported by exfoliating nvdW manganese metal into 2D Na-intercalated manganese oxide (birnessite) via electrochemical exfoliation under 5, 10, and 20 V s −1 (denoted as Na-Bir-5, Na-Bir-10, and Na-Bir-20; Table 2, MnO 2 material, entry 9). [61] It showed a Zn storage capacity of 325 mAh g −1 at a current density of 200 mA g −1 . The 2D Na-Bir-5 structure exfoliated at a slower scan rate achieved high cycling stability with 84% capacity retention after 1000 cycles and excellent rate performance, keeping 96% capacity after cycling back to 200 mA g −1 from a current density of 2000 mA g −1 (Figure 10c). The linear shape of the charge/discharge curve (characteristic of high capacitive contribution) revealed the fast kinetics of the 2D Na-intercalated manganese oxide (Figure 10d), leading to the overall improved rate performance.
Zhang et al. fabricated layered 2D metal sulfide using a surface-charge regulating strategy. [83] By adjusting the pH value to 5.05, 2D structures of In 4 SnS 8 and In 4 SnS 8 @graphene were obtained, whereby the layers are connected and assembled into nanosheet arrays (Figure 11a, Table 2, In 4 SnS 8 @Gr material, entry 10). The pristine In 4 SnS 8 was also prepared by adjusting pH to 2.35 and 8.00, producing microspheres (In 4 SnS 8 -2.35) and dispersed nanosheets (In 4 SnS 8 -8.00), respectively, to compare the performance with the larger lateral sized sheets of the 2D In 4 SnS 8 prepared at the pH of 5.05. The high-rate performance and long cycling stability of the 2D In 4 SnS 8 -5.05 suggested an essential role of morphology in the electrochemical behavior of the material. The In 4 SnS 8 @ graphene system was an auspicious anode material for SIBs, reaching a capacity of 840 mAh g −1 at a current density of 200 mA g −1 . Thanks to its connected layer morphology, the composite exhibited good charge transfer, thus leading to appreciable rate performance, keeping 58% of its capacity at a current density of 500 mA g −1 , as well as high cycling stability, retaining 70% of its capacity after 200 cycles at the current density of 500 mA g −1 . The system's behavior was studied by ex situ XRD analysis and operando synchrotron powder XRD to investigate the possible phase transformation mechanisms of the In 4 SnS 8 during sodiation/desodiation. During the first discharge, the initial intense peaks of the In 4 SnS 8 phase gradually disappeared due to the conversion reaction between In 4 SnS 8 and Na intercalation, which led to the production of metallic Sn, sulfides (such as In 2 S 3 ), and SnS, as confirmed by XRD (Figure 11b). Upon full discharge, all In 4 SnS 8 reflections disappeared due to the extensive conversion/alloying reactions, as confirmed by the emergence of diffraction peaks from metallic In and Sn species, metal sulfides (SnS 2 , SnS, In 2 S 3 , Na 2 S), and alloys with Na (Na 9 Sn 4 ). Upon recharging, de-alloying took place as suggested by the strengthening of the SnS reflection; however, the signal from the In 4 SnS 8 phase was not fully recovered.
Xia Huang et al. constructed ultrathin, carbon-wrapped titanium nitride nanomesh (Table 2, entry 11) synthesized via a nanoconfined topochemical conversion of polydopamine-coated exfoliated-Ti 3 C 2 T x (Ti 3 C 2 T x @PDA, Figure 11c). [115] TiN NM@C has a well-interconnected lamellar structure, visible in the SEM image of Figure 11d. Using high-resolution TEM (HR-TEM) imaging, the authors examined the nanomesh structure of TiN, being partially wrapped by amorphous carbon along with the representative TiN (111) plane (Figure 11e,f). The ammoniation of Ti 3 C 2 T x @PDA at temperatures such as 700, 800, and 900 °C provided TiN NM@C with different crystal sizes of TiN and affected the formation of pores. As the ammoniation temperature increased, larger crystals, more pores, and larger pore sizes were produced on the nanomesh. Further, TiN NM@C obtained at an ammoniation temperature of 800 °C was studied as the electrode material in a lithium-sulfur battery (LSB), and its electrochemical performance outperformed the respective from carbon-wrapped TiN nanoparticles (TiN NP@C) obtained by changing the reaction conditions. Due to the 2D morphology, the symmetric cell of TiN NM@C-based LSB achieved a high storage capacity of up to 582 mAh g −1 at 4 C. Upon cycling for a few initial cycles, the capacity peaked at 853 mAh g −1 during the kinetic activation of the electrode material and was maintained at 346 mAh g −1 after 1000 cycles showing its good cycling stability. Because of the intriguing, interconnected lamellar structure, high mass loaded freestanding electrodes could be prepared for TiN NM@C. Thus, freestanding electrodes were fabricated using higher sulfur loading of 7.3 mg cm −2 that achieved an areal capacity of 5.6 mAh cm −2 at the high current density of 6.1 mA cm −2 (0.5 C), keeping a capacity retention of 70% after 200 cycles.

Supercapacitors
Supercapacitors store energy based mainly on surface-controlled electrochemical phenomena. Thus, to improve the storage capacity, it is essential to enlarge the surface area of the electrode materials or, in general, to render the surfaces readily available to the energy carriers, i.e., the ions in the electrolyte. An accessible and substantial surface area ensures the availability of many active sites, improving the charge storage. The 2D morphology (mainly regarding few-layered and ultrathin nanolayered structures) is accepted to bestow large surface areas and short ion-diffusion paths through the electrode material. Hence 2D structures are promising for improving the storage capacity. However, restacking and inert surfaces are aspects and challenges that have to be considered achieving substantial improvements. Furthermore, research on enlarging the surface area and controlling the porosity has been extensively pursued in the last two decades, [119] and further developments based solely on such aspects will be hard to achieve. For example, activated graphene with 3100 m 2 g −1 has been reported achieving 165 F g −1 capacitance on a symmetric cell in an organic electrolyte (BMIM•BF 4 /acetonitrile). [119a] 2D materials derived from non-layered or nvdW solids could offer opportunities to bypass such limitations by providing a large number of active sites for sorption of electrolyte ions but also for pseudocapacitive charge storage due to the presence of defects, dangling bonds and redox-active metals on the surface. Moreover, they could be functionalized with appropriate www.afm-journal.de www.advancedsciencenews.com chemical groups, improving the structural stability and enhancing the charge transport properties of the materials.
Xia et al. synthesized ultrathin 2D nanoribbons of nvdW MoO 2 covered with polypyrrole (2D MoO 2 /PPy) using hydrothermal and liquid phase exfoliation methods, which were studied both for Li-ion storage and as electrodes in microsupercapacitor devices. [113] The Li-ion storage in the 2D MoO 2 /PPy composite was monitored ex situ with high-resolution XPS, and the Mo 3d spectrum of pristine 2D MoO 2 (at open circuit voltage, OCV) showed a characteristic peak of Mo 3d 5/2 at a Figure 11. a) Schematic illustration of the synthesis of In 4 SnS 8 /Graphene through the engineering of the isoelectric point of L-cysteine, which serves as both an orientation-directing agent and a sulfur source (Green ovals: metal ions, red: oxygen, blue: nitrogen, yellow: sulfur, grey: carbon, and pale grey: hydrogen). b) Ex situ XRD patterns of In 4 SnS 8 @Gr electrodes at different discharge/charge states. c) Schematic of the nanoconfined topochemical conversion from Ti 3 C 2 T x @PDA nanosheet to ultrathin TiN NM@C, d) SEM, e,f) HR-TEM images of TiN NM@C. Reproduced with permission. [83,115] Copyright 2021, Wiley-VCH; Copyright 2021, Wiley-VCH.
binding energy of 231.7 eV. After full lithiation, the spectrum showed peaks only for single species, which corresponded to the formation of Li-intercalated MoO 2 (Li X MoO 2 , Figure 12a). Further, the spectrum returned to its initial shape during delithiation, suggesting high reversibility of 2D MoO 2 , although some low-intensity peaks originated from oxygen deficiencies that developed during charging. The ultrathin 2D-MoO 2 electrodes demonstrate superior reversible capacity up to 1516 mAh g −1 after 100 cycles at 100 mA g −1 and 489 mAh g −1 after 1050 cycles at 1000 mA g −1 . As a micro-supercapacitor in a PVA/LiCl gel electrolyte, the 2D MoO 2 /PPy electrodes showed a significant improvement by a factor of 2.4 in charge storage with areal capacitance of 63.1 mF cm −2 at 0.1 mA cm −2 , better rate performance, and cycling stability compared to the bulk counterpart (Figure 12b). The 2D nanoribbon structure provided a large surface area helping to increase the capacitive contribution of 2D MoO 2 (Figure 12c, 77% vs 58%) compared to its unexfoliated analog, implying faster kinetics for charge storage. The dangling bonds and defects on the surfaces of 2D nvdW materials also pose a challenge due to their reactivity, which, if not exploited in a beneficial way, might lead to instability issues. For example, MoO 2 is susceptible to surface oxidation greatly hampering charge transport. In this case, this was mitigated by polypyrrole coating of the 2D MoO 2 /PPy composite, preventing surface oxidation. The XPS analysis confirmed that Mo 4+ was the dominant oxidation state, whereas in other MoO 2 systems surface oxidation states of Mo 5+ and Mo 6+ are present. The synthetic strategy involved a hydrothermal process yielding the Mo 6+ O 3 nanoribbons, which were then topotactically converted via reduction into single-crystal 2D-Mo 4+ O 2 with the assistance of the pyrrole monomer oxidation and polymerization, retaining fully the original nanoribbon morphology of  [113] Copyright 2018, American Chemical Society. d) Electrochemical performances of CoNi 2 S 4 //AC hybrid capacitor chargedischarge curves at various current densities, and e) Cycling performance of the CoNi 2 S 4 //AC hybrid capacitor for 10 000 cycles at a current density of 4 A g −1 , the inset show the comparative TEM images of CoNi 2 S 4 freestanding sheets before and after cycling. Reproduced with permission. [117] Copyright 2019, Elsevier. Data analysis of NiFe LDH/MXene, f) Log i versus log v plots, g) Capacitive contribution ratio at 2 mV s −1 , h) Capacitive and diffusive contribution ratio at respective scan rates. Reproduced with permission. [95] Copyright 2021, American Chemical Society.
MoO 3 . The work thus offered a strategy for improving the stability and performance of 2D nvdW structures via polypyrrole coating.
The synthesis of ultrathin (2 nm) CoNi 2 S 4 nanosheets was also reported using a microwave anion-exchange method ( Table 2, entry 15). [117] The micron-sized lateral dimensions (thus high aspect ratio) and ultrathin layers were evaluated as electrodes in a hybrid/asymmetric supercapacitor leading to improved charge storage and transport at the electrodeelectrolyte interface. The CoNi 2 S 4 //(active carbon, AC) supercapacitor achieved a high specific capacitance of 85 F g −1 at a current density of 1 A g −1 , with excellent cycling stability (82% retention after 10 000 cycles, Figure 12d,e). The supercapacitor also demonstrates excellent energy density of 67.7 Wh kg −1 at a power density of 0.8 kW kg −1 . Structural analysis after cycling showed the preservation of the ultrathin nature of the 2D sheets, indicating the prevention of restacking. However, the micron-sized nanosheets cracked into smaller sheets during cycling (Figure 12e, inset TEM images). The authors related the cracking to the strongly alkaline environment (2 m KOH electrolyte) that could oxidize the CoNi 2 S 4 electrode.
A micro supercapacitor was reported based on nvdW ZnP ultrathin nanosheets using solvothermal and phosphorization methods. [120] ZnP nanosheets coated on laser-developed graphene foam (ZnP@LIG) achieved excellent stability (capacitance retention of 93%) after 10 000 cycles at the high current density of 3 Ag −1 . The nvdW metal phosphide nanosheets could provide good electronic conductivity with a stable structure leading to improved storage capacities. Chenyang Li et al. reported NiFe LDH nanoflakes hydrothermally deposited on MXene (NiFe-LDH/MXene). [95] The MXene/NiFe heterostructures were studied as anodes in LIBs and lithium-ion capacitors (LICs). The kinetic study of NiFe-LDH/ MXene demonstrated a dominant contribution of the capacitive charge storage. The obtained b-values from the kinetic study were 0.77 and 0.72, higher than a diffusion-controlled battery behavior (b = 0.5). At the same time, 76% capacitive contribution was calculated by quantitative analysis at a 2 mV s −1 scan rate, supporting the dominant role of the capacitive behavior (Figure 12f,g). It was attributed to the 2D heterostructure of the NiFe-LDH/ MXene that could provide large interfacial surface with redox potential variations between the different metal coordination environments, as well as to the conductivity of the MXene phase. As the scan rate increased from 0.1 to 2.5 mV s −1 the capacitive contribution increased from 41% to 81% (Figure 12h) indicating good rate performance of the NiFe-LDH/ MXene system and predisposing it for use in LICs. Thus, the LIC (NiFe-LDH/MXene// AC) delivered high specific energy up to 168 Wh kg −1 , and 53 Wh kg −1 , at a power 47 W kg −1 and 1158 W kg −1 respectively.
In order to provide exfoliated 2D materials in high yields, the development of scalable methods for straightforward translation into commercial applications is required. Jeong et al. synthesized a composite of ultrathin layers of birnessite (δ-MnO 2 ) and nitrogen-doped carbon using a fluid-dynamic reactor. [121] The bulk δ-MnO 2 was combined with dopamine (as a carbon precursor), and exfoliated inside the fluid-dynamic reactor. Fully functional electrodes of high mass loading up to 19.7 mg cm −2 were achieved, delivering a specific capacitance of 480.3 F g −1 at 0.5 mA cm −2 .
The obtained results are promising since several 2D nvdW materials have been reported so far showing improved electrochemical performance compared to their bulk counterparts. For example, pyrite nanoparticles with CNTs delivered Li-ion storage capacity around ≈700 mAh g −1 at a current density of 0.1 Ag −1 , [122] while the 2D nvdW platelets with CNTs delivered 800 mA hg −1 at the same current density. [24b] More indicatively and remarkably, the 2D holey Co 3 O 4 nanosheets [70] showed excellent Li-ion storage capacity of 1324 mA h g −1 at the current density of 0.4 Ag −1 , preventing restacking and withholding the volume expansions without structural and stability deterioration of the active electrode material. However, the Co 3 O 4 nanoparticles anchored on graphene showed a substantially lower capacity of ≈800 mA hg −1 , although recorded at an 8-fold lower current density of 0.05 A g −1 , under which conditions the capacities are usually higher. [123]

Summary and Outlook
Recent advances in nvdW materials have opened the doors for their application in electrochemical energy storage. They benefit from surface polarized and under-coordinated metal cations or light-element anions and the highly reactive surface chemical states offering new active sites for interaction with the electrolyte ions, thus improving the charge storage and charge diffusion processes, even inside the bulk of the electrode materials. The abundance of surface dangling bonds and unsaturated coordination sites contribute to developing chemical activity and zwitterionic paths that can be leveraged for improved charge-carrier dynamics. Additionally, the dangling bonds at the surfaces become redox-active sites which may be beneficial for extra charge storage both in batteries and in supercapacitors. Such active sites are also prone to functionalization with chemical groups or can facilitate interactions with other lowdimensional materials toward heterostructures with integrated and often synergistically improved properties. Such properties may allow the production of thick electrodes and thus practical devices with high energy densities, if, at the same time, the usually poor electronic conductivity can be tackled.
Research in nvdW materials is envisaged to play a pivotal role in post-lithium batteries (Na-ion, Zn-ion, K-ion, Mg-ion, Al-ion) and the modified lithium batteries (Li-S, Li-Si, Li-air, Li-Se, Li-CO 2 , along with their post-Li counterparts), which are currently hampered by critical issues, including the identification of effective materials as electrodes for the cathode and the anode. [3a,124] For instance, the problem of fast capacity loss in Li-S batteries due to lithium polysulfide dissolution could be tackled via the interaction with the reactive chemical states on the surface of 2D nvdW materials, restricting dissolution and loss of sulfur in the electrolyte. [125] Furthermore, nvdW 2D structures undergo strong vertical contraction and lateral expansion after exfoliation, which offers the opportunity to tailor the band gap over an extended range of values and with a variety of topological features. [100] This property, along with the abundant surface defects and surface-exposed metal atoms, can boost the electrocatalytic activity of nvdW-based electrodes, [126] which are vital but have not yet been exploited in metal-air and metal-CO 2 EES systems.
The application potential of nvdW 2D materials expands well beyond the field of energy storage, embracing areas such as spintronic and magnetic applications, [127] whereby metal ions offer spin-polarized surface states with ultrafast dynamics and significant magnetotransport effects. For example, the 2D monolayer formulation of iron oxide ores, like hematene (Fe 2 O 3 ), [16] showed ferromagnetic ordering of the spins, unlike the bulk counterpart, which is antiferromagnetic. Other examples include magnetene (Fe 3 O 4 ), with enhanced magnetization as the thickness of the sheets decrease, [128] chromiteen (FeCr 2 O 4 ), [17,40b] and ilmenite, which also exhibited an antiferro-to-ferromagnetic phase change after synthesis of the 2D counterpart (2D FeTiO 3 , ilmenene). [40a] It is also worth mentioning the cases of VO 2 [129] and MnSe, [75] where the 2D structuring leads to significantly enhanced ferromagnetic response. Low-dimensional 2D structuring also reveals many different properties, such as superior ionic conductivity in 2D AgCrS 2 , [18] whose investigation offers broad application potential, propelling new findings in various fields with both scientific and technological importance, such as in catalysis and electronics. Up-to-date, particularly in the field of catalysis, a plethora of nvdW 2D materials including metals, alloys, metal oxides, sulfides, metal nitrides and metal phosphides have shown superior activity for many electrocatalytic reactions, such as hydrogen and oxygen evolution, oxygen reduction, carbon dioxide and nitrogen reduction, as well as carbon monoxide oxidation. [20a] The properties of nvdW 2D materials have been increasingly exploited owing to their novel chemical and electronic properties stemming from the high concentration of surface unsaturated atoms and high energy surfaces as active sites. Furthermore, the open 2D structure promotes mass and ionic transport. However, the low electronic conductivity of the majority of nvdW 2D materials is one of the main issues for harnessing their full potential in both electrocatalysis and EES applications. The high carrier mobility and tunable bandgap via facile surface modification due to the abundance of dangling bonds also make them suitable for broadband photodetectors, sensors, and photovoltaic devices. The cases of 2D β-B, [130] Ti, [131] Ge, [132] and Sn [133] are similarly interesting examples due to their low cytotoxicity, which could be exploited in biomedical applications as in bioimaging and cancer treatment.
Finally, the formation of heterostructures between different nvdW materials or between nvdW and vdW ones opens new horizons and numerous possibilities for broadening the family of 2D materials, offering opportunities for tweaking their properties and applications. The presence of dangling bonds at the surface of nvdW 2D structures guarantees facile handles for developing interactions with different substrate materials modifying the band structure, the surface chemical, redox, and spin states, as well as spin−orbit coupling, polarizability, optoelectronic, and ferromagnetic transitions. [134] Amid the rapid progress in the field, there are critical challenges that should be addressed. The development of methodologies for selective plane cleavage and synthesis of nvdW 2D systems with controlled thickness, lateral and domain size, defects and surface terminations is very important for both bottom-up and top-down approaches. The ability to preselect the cleavage direction and the mechanism of formation is still under investigation and concerted research efforts are required to understand how to control the fragmentation of 3D solids into 2D crystals. Selective chemical cleavage of crystal planes via reactive-transformation pathways (as in MAX-MXene transformation) will also have to be substantially studied and developed along with strategies to control porosity, as in holey 2D sheets or via the formation of networks. Theoretical tools can also substantially contribute for the discovery of yet experimentally unknown nvdW 2D phases, such as the anti-MXenes 2D transition-metal compounds (from KFe 2 Se 2 -like nvdW bulk materials) with high metallicity and active basal planes with excellent potential as electrode materials in electrocatalysis and EES. [135] Surface group termination control for increased hydrophilicity or organophilicity, depending on the type of electrolytes, will depend on the development of effective covalent derivatization pathways. Moreover, there is an urgent need to identify non-toxic solvents for LPE from sustainable carbon sources, in view of regulations for green technologies and the upcoming restrictions in the use of powerful solvents, such as DMF and NMP. Furthermore, method scalability and low-cost production are also key topics that must be dealt with in the future for the translation of the technologies into industrial practice. In order to achieve a paradigm shift in electrochemical energy storage, the surface of nvdW 2D materials have to be densely populated with active sites for catalysis, metal nucleation, organic or metal-ion accommodation and transport, and redox -charge storage (from both metals cations and anions [136] ), and endowed with pronounced chemical and structural stability during electrochemical cycling. The study of 2D engineering in nvdW materials has begun, and although it is still at an early stage, the results indicate significant potential, with many and exciting phenomena to be discovered.