Functional Graphene Coatings in Electrochemical Energy Technology—Beyond Corrosion Protection ^†

Abstract

Coating the surfaces of active masses and auxiliary components in devices of electrochemical energy technology with graphene and closely related materials has been suggested and experimentally verified in numerous examples. The results in terms of improved performance are promising and suggest the need for further research and technological development. This report provides a complete overview, providing details that are relevant for understanding the way in which these coatings work. Suggestions and directions for further development are indicated.

1. Introduction

Graphene (GR) and its chemical relatives, graphene oxide (GO), reduced graphene oxide (rGO), and few-layer graphene (flG), have enjoyed enormous popularity for numerous purposes and applications, inspired by the knowledge of their highly attractive materials properties and many suggestive, perhaps sometimes slightly overoptimistic, research reports. An earlier overview on the use of graphene in electrochemical energy technology (EET) is available [1]. The sometimes encountered (but mostly not stated) assumption that rGO and graphene are the same (presumably based on the assumption that, e.g., reduced copper oxide and copper are the same), as found in, e.g., [2], is not correct, as discussed in [3].

The salient details of the members of this family of layered carbonaceous 2D materials, such as their molecular structures, are displayed in Figure 1. Graphene as well as flG, GO, and rGO are frequently exploited in different applications due to their tunable optical and electrical properties, very high specific surface area (~2600 m²·g⁻¹), and catalytical properties with various surface functional groups. Graphene, as a single-atom-thick carbon sheet of hexagonally arranged carbon atoms, is a pioneer member of this family. It can act as a building block for the formation of different distinct carbon-based structures, such as spherical fullerenes, carbon nanotubes, and flat graphite with a typical honeycomb lattice structure.

The essential property for electrochemical applications of such materials to be discussed later is their electronic conductivity (for an overview, see

Table 1. Literature values of electronic conductivity of graphene and related materials.

Material	Electronic Conductivity	References
graphite	3.14·103 S/cm 1	[4]
graphite	2.0·103 to 4.0·103 S/cm	[5]
graphene	6000 S/cm to 100 MS/m	various
graphene	1.46 ± 0.82·106 S/m.	[6]
single-layer graphene	7.14·105 S/cm	[4]
single-layer graphene	1.0·106 S/cm	[7,8]
few-layer graphene	1.22·105 to 5.26·105 S/cm 1,2	[4]
few-layer graphene	2.94·105 to 8.33·105 S/cm	[9,10]
graphene nanosheets	1.05 to 6.03 S/cm 1,2	[4]
graphene oxide	4.57 × 10−8 S/cm	[11]
reduced graphene oxide	2.3 ± 1.0 to 14.6 ± 5.5 S/m	[5]
reduced graphene oxide	4.21 × 10−5 S/cm	[6]

¹ Calculated values; ² Value depends on number of layers.

). However, the reported values vary widely in the literature; thus, only a qualitative comparison is provided. Few-layer graphene sheets have increased electrical conductivity up to ~10⁴ S·m⁻¹, which is comparable to that of pristine graphite. In turn, there is a big difference in electrical conductivity between GO and rGO. While GO shows insulating or semi-conducting properties with low conductivity, rGO shows rather high electrical conductivity of up to 10³ S·cm⁻¹. The introduction of heteroatoms (N, B, S, etc.) into the carbon network leads to additional free electrons or holes in the carbon structure, increasing the electrical conductivity and causing the appearance of new catalytic properties.

Beyond the use of these materials from the graphene family as active electrode masses in, e.g., secondary batteries and supercapacitors [3,12], they can be used as coating materials for active and auxiliary materials. Rather popular is the use of such coatings for corrosion protection, as reviewed elsewhere with respect to the contribution of such coatings toward slower aging in devices and systems of electrochemical energy technology (EET) [13,14]. An experimental transmission electron microscopy (TEM) platform for the study of nanobatteries, including the examination of graphene coatings, has been reported [15].

Among the numerous reports on the use of G, GO, rGO, and flG in EET, the majority deal with their use as active masses or as components in mixtures or composites (see, e.g., [3]). A significant minority of these publications describe their use as coatings. Certainly, such coated materials may be called composites, but unfortunately this handling and terminology result only in confusion (for examples, see [16,17,18]). Sometimes terminology oscillates confusingly between composite, embedding, and coating [19]. Reports are included here as long as coating-like effects can be assumed based on the reported evidence, whereas reports wherein the process of deposition of the active mass on some support or current collector is called a coating are not included (see, e.g., [20]). In the absence of any binding, or at least the generally accepted definition of the term “coating” (which seems not necessary when considering common language), it may be appropriate to assume that a coating makes up only a small gravimetric fraction of the material. Cases like ref. [21], with an optimum graphene content of 40 wt%, or with 35.3 at. % carbon content (including graphene) as in [22], may seem to stretch the term coating a bit. Sometimes care in reporting and proper use of terms appear to be less than convincing; although reduced graphene oxide and graphene may be considered from a chemical point of view as being identical, there seem to be significant differences [3]. Accordingly, in the following text, assignment of a coating to any of said materials is based on the provided experimental details, not on confusing terminology (for some illustration see Figure 2). Active masses deposited by layer-by-layer assembly, as in [23], may also be called coated in a slightly misleading way, making them not relevant in the present report. In another report about a layered electrode with active mass particles sandwiched between the current collector and a top graphene coating, the relevance of this report is certainly given [24]. Adding to this confusion is possibly the use of some other terms in the reports presented. A coating is a top layer of some other material applied (coated) either on the electrode or on the material later made into an electrode. A coating might indeed—depending on the method—cover only the apparent surface but not the complete particle. Encapsulation may be considered as a complete “all around” coating, possible only when it is either applied by a suitable method to particles made into an electrode or—less likely to succeed—by some immersive method to the electrode. Depending on the application, the distinction may be relevant or even important.

The present report provides an overview as complete as possible of the reported applications of such coatings, their mode of coating, and the noticed benefits, including the proposed reasons for the improvements. This shall enable the reader to pick a suitable coating, method of applying it, and the expected benefits.

2. Applications

In this chapter, the reported applications of coatings with the materials introduced above are presented. To make the collected reports more accessible, they are grouped according to the three main fields of EET and the main components in these systems. In the case of multi-functional coatings, corrosion protection as one task out of several in these reports is covered in a separate review [25]. In case of doubt, the interested reader might wish to consult this source too.

2.1. Battery Electrode Coatings

Graphene or reduced graphene oxide coatings have been suggested and tested as the coatings of electrode materials in secondary batteries.

2.1.1. Aqueous Batteries

Coating of a copper electrode for an aqueous copper/aluminum battery with rGO resulted in corrosion protection and generally improved cell performance [26]. Whereas details of the corrosion protection action are provided, the reasons for the improved cell performance are not given.

2.1.2. Nonaqueous Lithium-Ion Batteries

• Negative electrode materials

Silicon as a promising (in terms of charge storage capability) material for the negative electrode in lithium-ion batteries suffers from poor electronic conductivity and major volume changes, resulting in mechanical fragmentation during cycling. In addition to the formation of composites (see, e.g., [27]), coatings have been tried. A graphene coating on tremella-like nanostructured silicon resulted in major performance improvements [11]. The volume changes were taken care of by the voids in the tremella-like structure, whereas the poor electronic conductivity was compensated by the coating. Silicon nanotube bundles were coated with graphene, resulting in a five-fold increase in achieved capacity when compared to the uncoated material [28]. The role of a mechanical protection layer was identified as the major role of the coating, providing mechanical stabilization and a stable solid electrolyte interphase (SEI). Graphene coated on porous silicon as an active mass improved the rate capability and storage capacity, and the stability during 100 cycles was also improved significantly when compared to the performance of the bare material [29]. Graphene bonded on an oxidized surface of submicron-size silicon nanoparticles by hydrogen interactions yielded a negative electrode material with remarkable rate capability and mechanical stability [30]. Surface treatment of silicon particles before coating was studied in [31]; thermal surface oxidation in advance of CVD formation of the graphene coating was found to be very advantageous in terms of almost no capacity losses during 300 cycles as compared to a steady decline without such thermal treatment. Using a mixture of CO₂ and CH₄ as source for graphene formation instead of a mixture of H₂ and CH₄ yielded better performing silicon electrode material [32]. flG coated by CO₂-enhanced CVD silicon nanoparticles was examined as a negative electrode material [33]. The addition of CO₂ as a mild oxidant to CH₄ as a graphene source resulted in a highly conformal coating; the role of CO₂ was elucidated using DFT calculations. Lithium ion migration in the silicon–graphene system was examined using theoretical tools [34]. Apparently, the coating helped this migration in a way not revealed so far. How this affected intercalation/deintercalation appeared to be beyond the scope of this study. Graphene sheets grown on silicon nanocones provided longer cycling life, better rate capability, higher Coulombic efficiency, and lower electrode polarization [35]. Silicon lithiation-enhanced charge transfer at the silicon–graphene interface was the subject of a first principles study [36]. Silicon particles trapped between a copper current collector and a soft graphene top coating were kept in place by this coating, successfully yielding an electrode with a capacitance that was stable for about 450 cycles [19]. Coating of silicon-polydopamine nanoparticles with graphene improved mechanical cycling stability and capacity by enhancing the electronic conductivity of the material [37]. Further applications of CVD-deposited graphene on nanostructured silicon were collected in [38]. Silicon nanoparticles were encapsulated into rGO bubbles, with the filled bubbles forming a film used as the negative electrode, and the product was sometimes called a composite [39]. The bubble film accommodated volume changes during cycling, stabilized the SEI, and supported ionic and electronic conduction. A stable SEI was also claimed as a major benefit of coating silicon nanoparticles with flG when using an ionic liquid as the electrolyte [40]. Silicon nanoparticles coated with crumpled rGO were examined as a negative electrode material [41]. The crumpled morphology of the coating accommodated the substantial volume changes of the silicon during cycling, contributing to significantly improved performance in terms of increased capacity, stability, and Coulombic efficiency. Silicon nanoparticles were coated with polyaniline and GO, yielding after pyrolysis a material with inner carbon and outer rGO coatings [42]. The dual coating accommodated volume changes during cycling, stabilized the SEI, and kept the silicon nanoparticles from agglomerating during 200 cycles (instead of 50 without the rGO coating). A porous Si-Sn-alloy prepared by high-energy ball milling coated with graphene and subjected to selective etching (which removed one of the alloy constituents) provided stable capacitance during 600 cycles when Si was left and 300 cycles when Sn was left [43]. The improvement was attributed to the voids left by the removal of one alloy constituent, leaving voids that accommodated volume changes during cycling.

In addition to elemental silicon, its monoxide SiO has been suggested as a negative electrode material. Unfortunately, its use faces the same problems as silicon, as already addressed above. Coating with graphene yielded major improvements in remedying these shortcomings [44]. Enhanced performance was also observed after coating SiO_x particles with graphene sheets [45]. Similar results were reported elsewhere, with corresponding arguments regarding observed improvements [46,47].

Nanoparticles of Ge were coated with graphene (the title of the report is slightly confusing) [48]. The cycling stability during 90 cycles suggested some effect in suppressing pulverization as well as particle agglomeration.

Graphene coating of hollow nanospheres of SnO₂ yielded a negative electrode material with attractive and stable performance, which in the absence of data obtained with uncoated material could not be judged by comparison [49]. Coating of composites of SnO₂ and carbon nanotubes with graphene alleviated the drawbacks (poor electronic conductivity and cycling stability because of volume changes during cycling) of this otherwise attractive negative electrode material [50]. Microspheres of SnS₂ coated with rGO afforded better performance in terms of higher rate capability and better cycling stability than uncoated SnS₂ [51]. The improvements were attributed to faster lithium ion diffusion and higher ionic conductivity.

A “composite” of Li₄Ti₅O₁₂ with 0.5 wt% graphene showed improved performance, which was ascribed to accelerated lithium ion diffusion and ionic conductivity [52]. Given the very low graphene content, the term “coating” used elsewhere in the report appears to be more appropriate. Nanowires of surface-fluorinated Li₄Ti₅O₁₂ were combined with rGO into a composite for use as a negative electrode in lithium-ion batteries [12]. Elsewhere in the report, the beneficial effects of coating with graphene are discussed; apparently the authors considered both materials, graphene and rGO, to be the same. The addition of rGO resulted in increased capacitance and surface fluorination further helped. The increased electronic conductivity of the composite material and accelerated lithium ion diffusion were attributed to rGO addition. In another study, Li₄Ti₅O₁₂ coated with graphene or some carbon coating (the report is ambiguous in this detail) with and without nitrogen doping performed best when compared to material coated with whatever nitrogen-free material or not coated at all [53]. The coating—at this stage, the authors seemed to prefer graphene—derived from strong binding between the active material and the coating from DFT considerations, in turn stabilized the electrode/electrolyte interface and reduced the chemical activity of the mass. This may provide a way toward reducing the effects of volume changes (which have been claimed to be absent with so-called zero-strain materials). In addition, electronic conductivity was improved by the coating. These advantages were apparently enhanced by nitrogen doping.

Mesoporous particles of NiFe₂O₄ coated with graphene were examined as a negative electrode material for lithium-ion batteries [54]. The increased ionic conductivity and prevention of aggregation during cycling were attributed to the coating. Microrods of Co₃O₄ coated with graphene yielded significant improvements in terms of capacity and, to a lesser extent, cycling stability when compared to the performance of pristine material [55]. Nanosheets of Co₃O₄ grown on a thin layer of rGO (which was later called a coating) on nickel foam improved both Li⁺ and Na⁺ storage performance by enhancing adhesion to the current collector [56]. Electrophoretic co-deposition of Co₃O₄ and graphene nanoplates yielded a binder-free electrode material [57]. In the slightly confused description, the authors oscillated between co-deposition and coating; in any case, with graphene, the reported capacity was about 25% smaller. NiO, as an attractive negative electrode material, gained structural stability and sufficient electronic conductivity by CVD coating with graphene [58]. Coating of cobalt-free LiNiO₂, suggested as a positive electrode material, suppressed oxygen release at high states of charge during cycling, which in turn caused structural damage and performance degradation [59], with more details reported by these authors elsewhere [60]. Similar results were obtained with LiCoO₂ [61]. Oxygen release from the lithium-rich positive electrode mass could be suppressed by regulation of redox couples near particle surfaces by coating with fluorinated graphene [62]. Nanotubes of electrospun NiO/Co₃O₄ coated with rGO (not graphene as claimed in the title) showed significantly improved performance, which was attributed to increased electronic conductivity and suppression of the negative effects of volume changes by the tubular structure [63]. Microspheres of NiCoMnO₄ wrapped with graphene by electrostatic self-assembly were examined as an electrode material, with much improved performance in terms of storage capability and stability when compared to the uncoated material [64]. LiNi_0.5Mn_1.5O₄ particles were coated with sulfonated graphene, improving electronic conductivity and accelerating lithium ion transport [65]. Much longer cycling life, greater rate capability, and less growth of the SEI were stated as the noted improvements. Coatings of hollow and solid micropencils of Co₃V₂O₈ were considered in a report—not more [66].

The poor electronic conductivity of FeP and its volume changes during cycling as the negative electrode in lithium-ion batteries were ameliorated with a reduced graphene oxide rGO coating, obtained from a graphene oxide GO solution with subsequent reduction during further chemical treatment [67]. Ultrafine particles of FeSe were embedded in an elastic carbon structure and finally coated with graphene, yielding a stable, mechanical stress-resistant electrode material for lithium-ion and also for sodium-ion batteries [68]. Compared to plain FeSe, the capacity was almost doubled, and it actually improved during 300 cycles.

Coating of graphitic carbon-coated Fe₂O₃ contained in conductive carbon nanofibers with onion-shaped graphene layers yielded a stable material for a negative electrode [69]. The graphene coating prevented peeling off of the particles of Fe₂O₃ from the carbon nanofibers and increased the electronic conductivity. The electrochemical performance as well as cycling stability of α-Fe₂O₃ nanoparticles was improved by coating with graphene [21]. At an optimum graphene content of 40 wt%, the term composite may be more appropriate, as was sometimes used by the authors in the report. Particles of a composite of carbon/Fe₃O₄ with a graphene coating acting as a buffering layer showed increased electronic conductivity and mechanical stability [70]. Coating particles of a composite of magnetite (Fe₃O₄) and N-doped carbon yielded an electrode material in which the coating prevented volume expansion during cycling, with the associated detrimental effects reducing the stability and cycling lifetime [71]. Hollow nanospheres of Cr₂O₃ wrapped in rGO showed significantly increased storage capability [2].

Porous particles of Mn₂O₃ were embedded in rGO, whereas elsewhere in the report they were coated with graphene [14]. The authors concluded that rGO provided high electronic conductivity and “prevented large volume expansion”. MnO₂ pre-intercalated with Na⁺ or ${\mathrm{NH}}_4^{+}$ and coated with rGO yielded an electrode material with increased electronic conductivity due to the rGO and better growth morphology because of the intercalated cations [72]. Porous nanospheres of CoMoO₄ coated with graphene showed higher electronic conductivity and structural stability during cycling as a negative electrode material compared to the uncoated material [73]. Better performance of CVD-deposited graphene coating on nanorods of MnO, suggested as a negative electrode material, than wrapping with chemically exfoliated graphene was reported [74]. The improved cycling stability and rate capability were attributed to better optimization of coating thickness and defect density.

Graphene-encapsulated particles of ZnO for use as a negative electrode material in lithium-ion batteries were prepared by simple spray-drying of a dispersion of GO and a zinc salt solution, followed by heat treatment [75]. The stability of the electrode material and rate capability were significantly improved.

A composite of nanoflowers of CuS and rGO avoided the observed drawbacks of the metal sulfide, i.e., structural deterioration and the “shuttling effect” of the sulfide ions during cycling [76]. Graphene, which actually encapsulated the metal sulfide, strongly adsorbed the soluble polysulfides. MoS₂ coated with graphene (the material was also called a composite) was studied, but the electrochemical performance was not reported [77].

Layered Ni-Co double hydroxides with an ultrathin conformal graphene coating prepared by electrodeposition (reduction of rGO) showed a major improvement in cycling stability as compared to the uncoated material [78].

A core/shell electrode material of graphene-coated graphite showed enhanced electronic conductivity and in turn better rate capability and capacity when used as a negative electrode material in potassium-ion batteries [79]. The stability and cycling lifetime were also improved.

A hydrophobic graphene coating of lithium metal helped to improve the moisture tolerance of the lithium electrode in lithium-air batteries [80]. A graphene-coated lithium foil used in a lithium metal battery enabled stable cycling for 470 cycles with 76% capacitance retention [81]. Spray-coating of lithium metal yielded a coating with rGO by spontaneous reduction, enabling a dendrite-free metal electrode [82]. Coating a porous copper current collector for the negative electrode in lithium metal batteries with N-doped graphene was proposed as a way to mitigate the effects of volume changes, low cycling efficiency, and safety concerns [83]. Uniform lithium deposition and uniform ion flux were observed. Coating of the lithium metal electrode with multilayer graphene and a Cs⁺ additive in the electrolyte solution improved the electrode performance significantly [84]. The coating separated the metal from the SEI and stabilized the Coulombic efficiency, while the Cs⁺ addition helped to suppress dendrite formation.

Coating of structured copper surfaces with stacked graphene protected the metal electrode from undesired chemical reactions with the electrolyte solution [85].

• Positive electrode materials

Pyrophosphate (Li₂FeP₂O₇) is an attractive positive electrode material for lithium-ion batteries. Unfortunately, low electronic conductivity hampers its successful use. A theoretical first principles-based study unraveled the details of the improved ion diffusion induced by the graphene coating of active material particles [86]. Particles of olivine-type LiFePO₄ were coated with GO, which was subsequently reduced by sintering of the active material [87]. A uniform spherical morphology was observed; at an optimum graphene content of 8 wt.%, stable electrode performance was recorded during 30 (!) cycles. Nanoparticles of LiFePO₄ were decorated via a catalytic process with graphene and, during the process, the formed graphene sheets formed a cross-linked network in the active mass [88]. The much increased rate capability and cycling stability were attributed to the increased electronic and ionic conductivities of the material. Nickel-doped LiFePO₄ coated with graphene showed increased conductivity and capacity in comparison to plain LiFePO₄, which was stable for 20 (!) cycles [89]. Nanostructured LiFePO₄ co-doped with Nb⁵⁺ and Ti⁴⁺ was coated with graphene (actually rGO according to the experimental description) [90]. At a nominal composition of Li_0.99Nb_0.01Fe_0.97Ti_0.03PO₄, storage close to the theoretical value was observed. As a major reason for the improvements, the establishment of a 3D conducting network was claimed.

The interface between LiCoO₂ as the positive electrode and Li_1.5Al_0.5Ge_1.5(PO₄)₃ as the solid electrolyte was modified with a layer of graphene [91]. The flexible graphene layer acted as a buffer, reducing the contact resistance between the active mass and solid electrolyte. This generic problem of solid electrolytes has been addressed before [92,93].

LiMn_0.7Fe_0.3PO₄ was coated with electrochemically reduced GO, resulting in higher storage capacity and better stability [94]. Subsequent coating with the chemically reduced GO was less efficient, which was attributed to the formation of a conductive network in the former case. Nanoparticles of (Li_0.893Fe_0.036)Co(PO₄) were combined with graphene into a two-layer sandwich, which appeared to be more particle-like in the report [95]. The presence of graphene in the structure improved the rate capability, and stable cycling during 100 cycles was reported. Hierarchical flower-like particles of Li₂FeSiO₄ were coated with graphene (why this was called activation remains unclear) [96]. The obtained material formed a secondary structure with superior structural and thus cycling stability and improved transport properties. A DFT study of graphene-coated iron borate and lithium iron borate was reported [97]. The release of manganese ions by the dissolution of manganese oxide LiMn₂O₄ spinel-positive electrodes during cycling was inhibited by coating with single-layer graphene, resulting in greater capacitance stability [98].

Graphene coating of hollow spherical LiNi_0.5Mn_1.5O₄ suppressed structural deformation during cycling and prevented corrosion and the formation of surface defects, resulting in significantly improved performance, with 82.5% capacitance retention after 1000 cycles at the 20 C rate [99]. Micro-nanostructured LiNi_0.5Mn_1.5O₄ embedded in graphene was studied with respect to the question of how a graphene coating benefited this electrode material [100]. Protection against corrosion and stabilization against structural deformation (presumably phase transformation) were found as the main effects. Coating of cerium-doped LiNi_0.5Co_0.2Mn_0.3O₂ with graphene improved the electrochemical performance in terms of slightly increased capacitance and improved cycling stability [101]. The improved capacitance retention and rate capability were attributed to increased electronic conductivity and protection of the active mass particles against the electrolyte solution. Beneficial effects of a graphene coating were also observed with LiNi_1/3Mn_1/3Co_1/3O₂ [102]. Particles of nickel-rich LiNi_0.8Co_0.1Mn_0.1O₂ were coated first with perylene-3,4,9,10-tetracarboxylic dianhydride for better adhesion of the multilayer graphene sheets that were coated thereafter (“snowballing strategy”), yielding a positive electrode material with substantially enhanced storage capability and stability during 100 cycles [103]. Graphene coating (termed here encapsulation) of nickel-rich LiNi_xCo_yMn_1−x−yO₂ was suggested as a way to enhance cycling stability and rate capability [104]. These positive effects were attributed to limited access of the electrolyte solution to the active mass and stronger mechanical support mitigating effects of volume change during cycling. In order to achieve a maximum of volumetric as well as gravimetric charge storage capability of an electrode material, the fraction of added auxiliary materials for enhanced electronic conductivity and as a binder must be minimized [105]. Using a Pickering emulsion approach, a graphene coating of nickel-rich LiNi_0.8Co_0.15Al_0.05O₂ powder was prepared and used as a positive electrode, with only 0.5 wt% content of graphene being equivalent in terms of the conductivity effect to 5 wt% carbon black [106]. In addition to accelerating electron transport surface degradation, surface structural transformation was presumably slowed down. Coating of nickel-rich and magnesium-doped LiNi_0.8Co_0.1Mn_0.1O₂ with graphene afforded increased structural stability and improved electron and ion conductivities [107]. Coating of another nickel-rich positive electrode material, LiNi_0.84Co_0.11Mn_0.05O₂, with rGO decorated with V₂O₅ was reported [108]. rGO provided increased electronic conductivity whereas V₂O₅ facilitates lithium ion intercalation/deintercalation and stabilized the layered structure of the active mass. Improved capacity and better stability as compared to the uncoated material were found. Conformal graphene coating of nanoparticles of a nickel-rich positive electrode material was found to reduce cell impedance, enhance the packing density of the particulate electrode material, and finally increase the cell lifetime four-fold [109]. Conformal coating of nickel-rich NMC532 (LiNi_0.5Mn_0.3Co_0.2O₂) material slowed down interfacial chemomechanical deterioration [110]. The accelerated electrolyte solution decomposition, in particular at high states of charge, of this “high-voltage” material and chemomechanical strain were mitigated, with associated gains in cycling stability and Coulombic efficiency.

Oxygen loss from lithium-rich Li₂MnO₃ can cause structural degradation, which in turn results in low Coulombic efficiency, capacitance fade, and voltage decay. A defective graphene coating can help in stabilizing surface oxygen [111]. The mechanistic details of LiO₂ extraction from lithium-rich mixed cathode materials Li(Li_0.2Mn_0.54Ni_0.13Co_0.13)O₂ were elucidated using in operando X-ray absorption spectroscopy [112]. Further spectroscopic studies of this material by the same authors suggested that graphene coating suppressed the formation of the monoclinic phase, resulting in improved stability [113]. The same active mass was coated with graphene by a spray-drying method, yielding an electrode mass with less solid electrolyte interphase formation and increased electronic conductivity [114].

Coating of phosphorene with graphene improved the stability of this electrode material [115].

The notorious polysulfide shuttle mechanism and the lacking electronic conductivity of sulfur have left this otherwise highly attractive positive electrode material for lithium-ion batteries as an ongoing challenge. Graphene-coated sulfur nanospheres showed 50% capacity retention after 100 cycles [116]. Wrapping, i.e., coating, poly(ethylene glycol)-coated sulfur particles with GO sheets subsequently decorated with carbon black nanoparticles yielded a positive electrode material with a capacitance that was stable for more than 100 cycles [117]. The stability was attributed to the “double coating” accommodating volume changes during cycling and keeping the polysulfides within. A graphene-sulfur composite with 82 wt% sulfur was studied as a positive electrode material [118]. Some graphene acting as a coating provided an electronically conducting matrix and prevented the dissolution of polysulfides. Graphene coating of composites of sulfur with mesoporous carbon yielded an electrode material with the coating accommodating volume changes during cycling, preventing the release of soluble polysulfides and increasing the electronic conductivity of the material [119]. With this material combination, similar improvements were reported elsewhere [120]. In another study, rGO-coated composites of mesoporous carbon and sulfur were examined [121]. The coating slowed down polysulfide diffusion and helped to increase the sulfur content in a way not revealed in the report. Coating of a carbon nanotube-sulfur composite with rGO restrained undesired polysulfide release from the positive electrode [122]. Particles of a composite of multiwalled carbon nanotubes and sulfur were coated (or encapsulated) with rGO, yielding an electrode material with remarkable improved capacity during 200 cycles and slightly better stability than the material without rGO [123]. Somewhat disturbingly, coating showed up only as a keyword together with graphene! This also applies to a report wherein studies of a vesicle-like composite of sulfur and rGO are described [124]. Again, graphene present as nanosheets improved the electronic conductivity and mitigated volume changes of the composite during cycling. A mixture of mesoporous carbon and graphene oxide was melt-infiltrated with sulfur after thermal treatment of the mixture in the presence of ammonia, yielding a mixture with graphene [125]. Whether the graphene acted as a coating (as sometime suggested) or just as a constituent remained unclear; anyway, the composite showed improved performance and capacity retention with current density, suggesting increased electronic conductivity. Polysulfides were trapped in the active mass and nitrogen doping (from the ammonia) enhanced polysulfide adsorption, further inhibiting the shuttle effect. Particles of a composite of carbon nanorods and sulfur coated with graphene showed improved performance due to the coating, which inhibited polysulfide diffusion and enhanced electron and ion transport [126]. Sulfur encapsulated in polypyrrole embedded or sandwiched between rGO showed some increase in capacity when compared with the material without rGO, with the mode of operation of the rGO as stated in preceding examples [127].

The separator inside lithium-sulfur batteries is certainly an auxiliary component; nevertheless, a paper separator coated with multiple CVD-deposited graphene layers inserted as an additional layer between the electrodes of such batteries is possibly better located in this section [128]. The improved performance in terms of increased cycling stability was attributed to unspecified interactions between polysulfide species and graphene. Nanosheets of CoS coated with rGO (elsewhere in the report graphene is mentioned) on a flexible carbon fiber support served as an interlayer in a lithium-sulfur battery, improving polysulfide conversion and Li₂S decomposition [129]. A coating with B/N-doped rGO and boron nitride nanosheets on the separator of a lithium-sulfur cell enhanced sulfur utilization, inhibited the shuttle effect, and prevented lithium corrosion by lithium polysulfides [130]. Low self-discharge and improved cycling stability of a lithium-sulfur battery was afforded by a sandwich interlayer of VS₂, carbon nanotubes, and carbon nanofibers coated with graphene [131]. The interlayer suppressed polysulfide shuttling and recovered inactivated sulfur species.

2.1.3. Auxiliary Materials

Coating of current collectors for lithium-ion batteries with flG resulted in lower contact resistance between the current collector and active mass [132]. Simple carbon coating of aluminum foil needed to alleviate, at least in part, poor adhesion, insufficient electrical contact, and localized corrosion initiated by contact with the electrolyte solution was claimed to be too heavy. Instead, coating with a graphene/Ketjen Black mixture or a mixture of graphene micro sheets and graphene was suggested [133]. With the latter coating, lower Ohmic resistance and better adhesion were found. Modification of aluminum foil with carbon black and graphene, used as the current collector for a positive lithium iron phosphate electrode, resulted in an improved rate capability, lower internal cell resistance, and increased cycling stability [134]. The coating increased the effective surface area of the foil, improved mechanical adhesion, and also suppressed corrosion. Graphene modification of copper foil, used as a current collector with Li₄Ti₅O₁₂ as the negative electrode material in a lithium-ion battery, improved overall performance [135]. The storage capability was increased by 32% in comparison to an uncoated electrode, which was attributed to improved electric contact between the active mass and current collector.

Coating of nickel foam, used as the current collector for a positive electrode, with nitrogen-doped graphene was suggested, and some performance improvements were noticed [136].

Coating of the aluminum surfaces of battery packs for heat dissipation with graphene-copper composites resulted in more even temperature and heat distribution [137]. For overviews on carbon and graphene coatings in thermal management, see [138,139].

2.1.4. Other Metal-Ion Batteries

Graphene-coated particles of FeS₂ further encapsulated in carbon fiber were examined as the negative electrode for sodium-ion and potassium-ion batteries [140]. High electrochemical reversibility and stability were noticed; for sodium ions, the graphene coating seemed to reduce the diffusion barrier between FeS₂ and graphene, according to theoretical calculations. The mechanism of capacity fading of micron-sized particles of FeS₂ was studied [141]. Graphene coating, proper choice of binder, and electrode potential control resulted in high capacity, extended cycling life, and increased rate capability.

Graphene coating of Fe₃S₄ was suggested as a way to alleviate the volume changes of the flower-like microstructure of the material and improve its electronic conductivity, resulting in much improved cycling stability [142]. Nitrogen-doped graphene coating of microspheres of FeS₂ yielded an electrode material for the negative electrode of sodium-ion batteries [143]. Taking into account also theoretical considerations, graphene improved the electronic conductivity of the otherwise poorly conducting sulfide, and it also accelerated sodium ion diffusion kinetics and supported mechanical stabilization. A graphene (actually, according to the report it was rGO) coating on graphite improved its performance as a negative electrode material for potassium-ion batteries [79]. The beneficial effect of the coating, which was evident in the improved cycling stability, was attributed to the buffering capability of the coating, which mitigated the volume changes of the graphite during cycling.

Graphene coating of Co-doped Na₃V_2−xTi_x(PO₄)₂F₃ yielded a negative electrode material for sodium-ion batteries [144]. The coating improved electrochemical kinetics. NASICON-type NaTi₂(PO₄)₃ conformally coated with graphene yielded a negative electrode material for sodium-ion batteries with high and long-term stable capacity [145]. Somewhat confusingly, the material was called a nanocomposite in the title of the report. Microspheres of Na₄Fe₃(PO₄)₂(P₂O₇), suggested as a positive electrode material for sodium-ion batteries, coated (decorated) with graphene and further embedded in a graphene network showed improved performance, which was attributed to increased electronic conductivity and mechanical stabilization of the material against volume changes during cycling [146].

A noteworthy example of a negative electrode material for sodium-ion batteries without any coating (graphene or other) appears to be SnS [147]. Graphene-encapsulated composites of SnO₂ and carbon nanotubes showed promising performance as a negative electrode material for sodium-ion batteries [50]. Placing a polypropylene separator coated with polydopamine and multilayer graphene on a sodium metal electrode enabled dendrite-free cycling without significant capacitance loss during 500 cycles [148]. GeP₃ in a carbon matrix coated with rGO (GeP₃/C@rGO), suggested as a sodium-storage material, showed higher electronic conductivity and larger surface area than plain GeP₃ and GeP₃ coated with graphene [149]. A fourth sample mentioned in the abstract remains a mystery. GeP₃/C@rGO performed best in terms of capacity, current capability, and long-term stability. rGO protected and restrained the material inside.

Wrinkled graphene sheet coating of hard carbon derived from coal liquefaction residues, as the negative electrode material for sodium-ion batteries, improved performance [150]. The improved performance was attributed to the enhanced electronic conductivity of the active mass by the coating.

The nucleation and growth dynamics of zinc deposition in a zinc metal-free zinc-ion battery were directed by applying a graphene coating on the copper current collector [151]. Further beneficial effects were achieved by coating a zinc electrode with carbon nanofibers and graphene acid (carboxyl-enriched graphene) [152]. The coating was zinc ion selective and prevented zinc corrosion. Coating graphene on the zinc electrode of a zinc-ion battery resulted in significantly improved cyclic reversibility [153]. Although dendrite formation was not inhibited, the coating apparently homogenized the electrode field in front of the electrode, affording even metal deposition.

A FeCo-catalyst for oxygen reduction in a zinc-air battery coated with graphene showed almost four-electron reduction of oxygen [154]. In addition to the remarkable activity stability, methanol tolerance was noted. A bifunctional oxygen electrode catalyst for zinc-air batteries containing B,N-doped graphene nanomesh decorated with Co₃O₄ was reported [155]. Why the term coating shows up remains unclear.

A coating of GO on various non-noble metal current collectors prevented electrolyte solution decomposition and their corrosion [156].

• Other battery electrode materials

Organic electrode materials suggested as negative electrode materials for aqueous lithium-ion batteries suffer from low electronic conductivity [157]. Coating with graphene significantly improves electrochemical performance.

Conceivable diffusion hindrance due to the steric hindrance established by the graphene coating may be avoided by coating with single walled carbon nanotubes [158]. An active mass of Fe₃O₄ was successfully employed as an example.

2.2. Supercapacitor Electrode Coatings

The advantageous properties of graphene coating on porous silicon, used as a supercapacitor electrode material, are summarized in [159]; for more similar observations, see [160]. Similar observations were reported for mesoporous silicon oxide wrapped into graphene and used as the negative electrode in a lithium-ion capacitor [161]. The positive effects of graphene coating on the performance of various anthracite-derived carbonaceous materials in lithium-ion capacitors were reported [162]. Particles of electrodeposited Ni(OH)₂ and Ni coated with graphene in a supergravity field showed increased capacitance and rate capability when compared to the uncoated material [163]. Stability was not examined. Coating of nickel foam with graphene before deposition of ZnCo₂O₄, as an active material for a redox supercapacitor, resulted in a substantially different morphology of the formed cobaltate, with significantly increased surface area and porosity and associated improvements in performance [164]. Oxygen vacancy birnessite-type Na_xMnO₂, to be used as a positive electrode in a redox supercapacitor, was coated with rGO [165]. The coating increased the electronic conductivity and stability of the birnessite nanosheets.

Although currently the most popular current collector and mechanical support for electrodes in supercapacitors is aluminum foil, sometimes treated mechanically or chemically for slightly modified surface properties to improve the adhesion of the active mass and reduce electrical contact resistance (not electrochemical charge transfer resistance as sometimes erroneously claimed), other materials have been examined. Better mechanical properties and higher corrosion stability are among the arguments for proposing thin foils or meshes of stainless steel of various compositions. Coating with high-quality graphene by direct deposition in a CVD process has been studied [166]. Improved electric contact, i.e., lower Ohmic resistance, was observed as the desired effect of this surface modification, which was claimed as being suitable also for coating other current collector materials. As a consequence, all of the related performance properties of an EDLC supercapacitor were noticeably improved.

A coating with graphene or graphene oxide—the report remains ambiguous about this difference—on polyacrylonitrile fiber cloth was applied before carbonization [167]. The amount of initially deposited graphene oxide had some influence on the relative performance improvements of the prepared electrodes; in the absence of a blank (without such coating), the observed effects are hard to attribute to the coating in detail. The authors claimed a reduced specific surface area because of the “shielding effect” of the coating presumably during carbonization and subsequent chemical activation.

The capacitive behavior of carbon nanofiber cloth coated with GO was examined [168]. The somewhat ambiguous description does not reveal the intended function; the absence of data obtained without such a coating does not help.

Passivating the graphene coating of porous silicon to be used as an electrode material in supercapacitors has been reviewed, and higher electronic conductivity essential for this application was observed [159].

Graphene coatings of textiles were used to provide surface conductivity for subsequent deposition of MnO₂ in an otherwise barely comprehensible report on flexible supercapacitors [169]. Coatings of rGO were applied in supercapacitor-related studies reported in an otherwise mysterious report [170].

Various coatings of carbon cloth with carbon-coated aluminum foil, used as a support and current collector in supercapacitors, were compared, and coating with graphene yielded superior results in terms of recorded capacitance and internal cell resistance [171].

2.3. Coatings in Fuel Cells

The layered Ni-Co double hydroxides with an ultrathin conformal graphene coating already mentioned above as a negative electrode material for lithium-ion batteries [78] also provided significantly enhanced electrocatalytic activity in the oxygen evolution reaction.

Thermally annealed self-assembled three-dimensional graphene was proposed as a cheaper substitute of CVD-coated graphene on porous metal foam in PEM fuel cells, yielding improved cell performance [172].

Graphene coatings on metals, alloys, and other materials for bipolar plates for PEM fuel cells showed high corrosion protection [173], and their further advantages were reviewed [174,175]. The catalytic properties of the metal components of the alloys were effective through the very thin graphene coating. The degradation mechanisms of graphene coatings on bipolar plates for PEM fuel cells were reviewed [176]. Fabrication defects, acting as initiation sites for degradation, were highlighted. Degradation of graphene-coated copper in a simulated PEMFC environment was studied using spectroscopic methods [177]. Coated copper had significantly higher corrosion resistance after extended exposure to the simulated environment, confirming the corrosion-protective effect.

The catalytic effects of graphene coatings on various crystallographic platinum surfaces in the dioxygen reduction reaction were studied via DFT calculations [178]. Further enhancement of catalytic activity by nitrogen doping appeared to be feasible. Carbon nanotube and graphene coatings on stainless steel mesh, used as the positive electrode (dioxygen reduction electrode) in a microbial fuel cell, increased the power density and decreased the internal resistance of the cell [179].

Graphene-coated nickel foam, used as the negative electrode (the authors use the term cathode) in microbial electrosynthesis of acetate from CO₂, yielded a 1.8-fold increase in volumetric acetate production as compared to the uncoated nickel foam [180]. Both increased active surface area (see also [181,182,183]) and accelerated electron transfer were invoked as reasons.

2.4. Further Applications

With redox flow batteries (for an introduction, see [12]), graphene coating of the Nafion^® membrane to separate the two electrolyte solutions in the half cells was reported [184]. In a single-cell study, higher energy efficiency, power density, and discharge capacity were recorded. These benefits were attributed to reduced vanadium crossover and enhanced electrochemical activity. Both arguments remain mysterious in the light of a recently reported major survey [185]. Similar benefits were reported for devices employed in flow-electrode capacitive mixing [186].

3. Conclusions

Graphene and its chemical relatives, graphene oxide, reduced graphene oxide, and few-layer graphene (flG), coated by different methods on active masses and auxiliary components in systems for EET are suggested and examined. The beneficial effects and their conceivable reasons and modes of operation are reviewed. At very low actual contents of the coating material, the beneficial effects are established mostly by increased capacitance, higher current capability, and improved stability. The suggested reasons for the improvement of electrode and material performance and mode of operation are as follows:

• Protection of the active mass against dissolution and/or corrosion;
• Increased electronic conductance;
• Mitigation of volume change effects;
• Inhibition of pulverization and agglomeration.

Further research should aim at the optimization of coating procedures and the amounts of added material based on a deeper understanding of the mode of operation. Longer stability tests aiming at higher cycle numbers closer to commercial expectations are urgently needed.