A. Interfacial charge transfer

The charge-transfer stage is a critical step where an ion transfers from a (typically) liquid phase into a solid phase at the same time that an electron is transferred into the solid phase. When transferring from a liquid electrolyte, the ion typically needs to lose its solvation shell ^{Reference Xu48} and, in the case of multivalent cations, other ions. ^{Reference Wan, Perdue, Apblett and Prendergast49} In theoretical descriptions of charge transfer, an intermediate, transient step is assumed between the solvated stage and the intercalated stage. The difference in energy between the solvated stage and the transient stage is the activation energy for charge-transfer during intercalation (E _a), and may be related to the charge transfer resistance (R _CT) ^{Reference Xu48} :

(2) $${1 \over {{R_{{\rm{CT}}}}}} = {A_0}{\rm{exp}}\left( { - {E_{\rm{a}}}/kT} \right)\quad .$$

Here, A ₀ is a pre-exponential constant, k is the ideal gas constant, and T is the temperature. The activation energy for charge transfer has been ascribed as the energy required to desolvate the intercalating cation. ^{Reference Xu48} Modification of the interlayer that allows for solvent co-intercalation or partial cation solvation in the structure can thus decrease the activation energy for charge transfer.

B. Ion transport in the interlayer

Once the ion has been intercalated, it undergoes solid-state diffusion due to the concentration gradient developed between the surface and the bulk of the material. Typically, diffusion is the rate-limiting step for ion intercalation in electrochemical energy storage ^{Reference Park, Zhang, Chung, Less and Sastry50} except in the case of intercalation pseudocapacitance, which is surface limited. ^{Reference Augustyn, Come, Lowe, Kim, Taberna, Tolbert, Abruña, Simon and Dunn38} In most layered compounds, the ions move via interstitial vacancies in a 2D plane. In a layered oxide with a modified interlayer, the ion may diffuse via different mechanisms. For example, in a hydrated, layered structure, the ion may move via the Grotthus mechanism responsible for rapid proton diffusion in water. ^{Reference Whittingham51} The use of interlayer solvation in multivalent ion intercalation is beneficial for screening the diffusing ion from the inorganic framework. ^{Reference Nam, Kim, Lee, Salama, Shterenberg, Gofer, Kim, Yang, Park, Kim, Lee, Chang, Doo, Jo, Jung, Aurbach and Choi52,Reference Song, Gillette, Lee, Noked, Rubloff, Gillette, Duay, Rubloff and Lee53}

The interlayer environment can lead to changes in the rate capability for ion intercalation due to its influence on the diffusion coefficient, D ^{Reference Van der Ven, Bhattacharya and Belak54} :

(3) $$D = p{\lambda ^2}{v^*}\,\exp \left( { - {E_{\rm{B}}}/kT} \right)\quad ,$$

where p is a geometrical factor related to the interstitial vacancy network (1D, 2D, or 3D), λ is the hopping distance, v* is the vibrational frequency, and E _B is the activation barrier for hopping, or the maximum energy point between the initial and final site of the diffusing ion. It is this activation barrier that will be affected by changes due to the interlayer environment, including the presence of interlayer molecules. As in the case of the activation energy barrier for charge transfer, the exponential dependence of the diffusion coefficient on E _B indicates that even small changes in the diffusion environment will lead to significant changes in the diffusion coefficient.

A. Structural water

Interlayer water molecules can be present within a layered structure by three different means: (i) as structural water present in the as-synthesized oxide; (ii) as a result of water diffusion into the interlayer spacing; and (iii) via electrochemical cycling in aqueous electrolytes. This section will focus on layered oxides with structural water, which include WO₃·nH₂O, ^{Reference Freedman58} MoO₃·nH₂O, ^{Reference Kuzmin and Purans59} V₂O₅·nH₂O, ^{Reference Augustyn and Dunn60} and birnessite (δ) MnO₂. ^{Reference Ghodbane, Pascal and Favier61} In particular, the molybdenum and tungsten oxides form a series of stable and metastable hydrous phases. The stable phases include monoclinic WO₃·2H₂O, WO₃·H₂O, MoO₃·2H₂O, and MoO₃·H₂O. The dihydrates and monohydrates of tungsten and molybdenum are isostructural; the structures of the tungsten oxide hydrates are shown in Fig. 4. These hydrates are built up of corner-sharing and tilted octahedra. In the case of the monohydrate, the water is located at the apex of the octahedra whereas in the dihydrate, the second water is hydrogen bonded within the interlayer.

FIG. 4. Relationship between crystal structure and temperature of WO₃ hydrates. MoO₃ hydrates are isostructural with the tungsten oxide hydrates.

The effect of structural water on electrochemical energy storage has been investigated with molybdenum oxide hydrates cycled in nonaqueous lithium ion electrolytes. Nazri et al. ^{Reference Yebka, Julien and Nazri62} reported that molybdenum oxide hydrates (MoO₃·H₂O and MoO₃·²/₃H₂O) exhibited higher capacity and better cyclability than anhydrous MoO₃. Kumagai et al. ^{Reference Kumagai, Kumagai and Tanno63} reported that the Li⁺ intercalated into the structure in between the hydrated layers, and, in the monohydrate, obtained capacities of up to 400 mA h/g at a current density of 0.2 mA/cm² between 3 and 1 V versus Li/Li⁺. Due to the potential range for intercalation (3.5–1.5 V versus Li/Li⁺) and lack of Li in the as-synthesized structure, the materials are best suited as cathodes for primary batteries or anodes for hybrid electrochemical capacitors.

In aqueous electrolytes, interlayer water molecules have been hypothesized to provide rapid diffusion channels for protons. The Grotthus mechanism of proton diffusion ^{Reference Whittingham51} has been proposed in hydrated tunnel structures, such as those formed by hexagonal MoO₃, and is expected to occur in layered structures as well. ^{Reference Muñoz-Santiburcio, Wittekindt and Marx64} This mechanism accounts for the rapid diffusion of protons in aqueous electrolytes via the formation and deformation of hydrogen bonds on water molecules. ^{Reference Winter and Brodd65} However, it is not clear whether the Grotthus mechanism occurs in all hydrated layered oxides. Recent density functional theory calculations indicated that protons do not intercalate via the water network in WO₃·2H₂O; instead, it was theorized that the mechanism of proton intercalation is binding to a bridging oxygen, the same as in WO₃. ^{Reference Lin, Zhou, Liu and Ozolins66} On the other hand, experimental results of electrochromic hydrated tungsten oxides show rapid coloration and bleaching in acidic electrolytes, which was ascribed to rapid proton diffusion via the Grotthus mechanism. ^{Reference McIntyre, Basu, Peck, Brown and Augustyniak67} The room temperature proton conductivity at ∼50% relative humidity of WO₃·nH₂O was determined to be ∼1 × 10⁻⁵ S/cm for the dihydrate and monohydrate, and the proton conduction was hypothesized to occur via the interlayer hydrogen bonded network as in the Grotthus mechanism. ^{Reference Li, Hibino, Miyayania and Kudo68} These diverse results underscore the need for in-depth understanding of transport mechanisms during electrochemical intercalation in hydrated structures.

In addition to the potential for rapid proton diffusion via the Grotthus mechanism, the presence of interlayer water molecules has been correlated with improved intercalation kinetics of multivalent ions. Multivalent ions are those with a formal charge greater than 1; of particular interest to energy storage applications is the storage of the divalent cation Mg²⁺. This is because Mg metal, with a theoretical capacity of 2205 mA h/g (for the reaction Mg²⁺ + 2e⁻ ↔ Mg) ^{Reference Walter, Kravchyk, Ibáñez and Kovalenko69} can be reversibly cycled in a suitable nonaqueous Mg-ion electrolyte without the formation of dendrites, as in the case of lithium metal. Thus the use of Mg metal as an anode allows for achieving both high energy density and safety. One of the challenges of Mg batteries, however, is finding a high voltage and high capacity cathode material. Intercalation of Mg²⁺ is not as facile as Li⁺ due to the sluggish solid state diffusion of Mg²⁺, which is due to (i) the Coulombic repulsion between the framework transition metals and the diffusing Mg²⁺ ions, (ii) the need for a transition metal to accept 2e⁻, which results in an increase in the ionic radius and thus unit cell volume, ^{Reference Levi, Levi, Chasid and Aurbach70} and (iii) the strong solvation ^{Reference Lapidus, Rajput, Qu, Chapman, Persson and Chupas71} and anion coordination ^{Reference Wan, Perdue, Apblett and Prendergast49} of Mg²⁺, which raises the activation energy for charge-transfer at the electrode/electrolyte interface.

The presence of interlayer water molecules can enable the reversible intercalation of Mg²⁺ because these species can act as a solvation shell, and shield the diffusing divalent cation from the lattice anions and cations. ^{Reference Levi, Gofer and Aurbach72} For example, V₂O₅ aerogels are high-surface area oxide materials made via supercritical drying of a V₂O₅ gel. ^{Reference Chaput, Dunn, Fuqua and Salloux73,Reference Le, Passerini, Guo, Ressler, Owens and Smyrl74} Their short-range structure is similar to that of V₂O₅ xerogels, ^{Reference Petkov, Trikalitis, Bozin, Billinge, Vogt and Kanatzidis75} which consists of V₂O₅ bilayers separated from each other by a large interlayer spacing (∼11.5 Å) filled with water molecules. In the case of the aerogels, the nominal formula after supercritical drying is V₂O₅·2H₂O and ∼1.5 H₂O molecules are removed by heat treatment at 120 °C. ^{Reference Chaput, Dunn, Fuqua and Salloux73} The maximum amount of intercalation in the aerogel is ∼0.6 Mg²⁺ per V₂O₅ ^{Reference Tang, Sakamoto, Baudrin and Dunn76,Reference Le, Passerini, Coustier, Guo, Soderstrom, Owens and Smyrl77} which corresponds to a gravimetric capacity of 88 mA h/g. Figure 5(b) shows the cyclic voltammogram of a V₂O₅·0.5H₂O aerogel in Mg(ClO₄)₂ in propylene carbonate electrolyte. It should be noted that while Mg²⁺ intercalation is reversible in V₂O₅ aerogels, the separation between the cathodic (Mg²⁺ intercalation) and anodic (Mg²⁺ deintercalation) peaks is larger than in the case of Li⁺ intercalation [Fig. 5(a)], indicating that kinetics are still sluggish despite the benefit of interlayer water.

FIG. 5. Cyclic voltammetry of V₂O₅·0.5H₂O aerogel in (a) LiClO₄ in propylene carbonate and (b) Mg(ClO₄)₂ in propylene carbonate electrolytes at 0.1 mV/s. The aerogel can be reversibly cycled in both electrolytes due to the nanostructured morphology and presence of structural water. Reprinted from Ref. Reference Tang, Sakamoto, Baudrin and Dunn76, with permission from Elsevier.

One potential drawback of structural water is that it may be removed during electrochemical cycling, leading to changes in the energy storage behavior, crystal structure, and potential contamination of a non-aqueous electrolyte with water. ^{Reference Levi, Gofer and Aurbach72} The significance of this issue on the stability of interlayer structural water during electrochemical cycling varies. In the case of V₂O₅ aerogels, Le et al. ^{Reference Le, Passerini, Coustier, Guo, Soderstrom, Owens and Smyrl77} used chemical analysis to determine that structural water was retained after chemical and electrochemical intercalation of Li⁺ and Mg²⁺. On the other hand, Novák et al. ^{Reference Novák, Scheifele, Joho and Haas78} reported that structural water was removed upon repeated electrochemical intercalation and deintercalation of Mg²⁺ in hydrated layered vanadium oxide bronzes of the family MV₃O₈·nH₂O, where M = Li, Na, K, Ca_0.5, and Mg_0.5, and this group reported the same water loss in V₂O₅ xerogel. The decrease in structural water content was correlated with decreased capacity for Mg²⁺ in the materials. Recently, the decrease in capacity of V₂O₅·0.6H₂O with cycling in a Li⁺ nonaqueous electrolyte was correlated with accumulation of LiOH. ^{Reference Wangoh, Huang, Jezorek, Kehoe, Watson, Omenya, Quackenbush, Chernova, Whittingham and Piper79} In the case of MoO₃·nH₂O cycled in a nonaqueous Li⁺ electrolyte, the decrease in capacity with cycling of anhydrous MoO₃ was greater than that of the hydrous phases, MoO₃·H₂O and MoO₃·¹/₃H₂O. ^{Reference Yebka, Julien and Nazri62} Kumagai et al. ^{Reference Kumagai, Kumagai and Tanno63} proposed that electrochemically intercalated Li⁺ react with the interlayer water molecules of MoO₃·2H₂O because of the observed poor reversibility and decrease in interlayer spacing upon Li⁺ intercalation from ex situ XRD; the monohydrate was more stable. Based on these reports, it is possible that the removal and reactivity of interlayer water is more likely in structures where the water molecules are bound via hydrogen bonds to the interlayer, and not covalently bound as in the case of MoO₃·H₂O.

B. Solvent cointercalation & exchange

The previous section discussed the behavior of layered crystal structures with interlayer water molecules present in the as-synthesized state. For other layered structures, water and other polar solvent molecules may be intercalated during electrochemical cycling (so-called co-intercalation) or by solvent exchange. ^{Reference Schöllhorn, Kuhlmann and Besenhard57} Adapting the nomenclature established by Schöllhorn ^{Reference Schöllhorn, Whittingham and Jacobson80} these reactions can be written as:

(4) $${\rm{Cointercalation}}:\;{\rm{MO}} + x{{\rm{A}}^ + } + x{{\rm{e}}^ - } + y\left( {{\rm{solv}}} \right) \to {{\rm{A}}_x}{\left( {{\rm{solv}}} \right)_y}{\rm{MO}}\quad {\rm{,}}$$

(5) $${\rm{Solvent}}\;{\rm{Exchange}}:\;{\left( {{\rm{solv}}} \right)_y}{\rm{MO}} + y\left( {{\rm{SOLV}}} \right) \to {\left( {{\rm{SOLV}}} \right)_y}{\rm{MO}} + y\left( {{\rm{solv}}} \right)\quad ,$$

where MO is a layered oxide, A⁺ is metal cation, and SOLV and solv represent two different solvents. These reactions present an additional mechanism for the control of the interlayer of transition metal oxides for electrochemical energy storage.

Solvent cointercalation has been investigated for improving the kinetics of Mg²⁺ energy storage. Using first-principles calculation, Gautam et al. ^{Reference Sai Gautam, Canepa, Richards, Malik and Ceder81} found that the cointercalation of water with Mg²⁺ into xerogel V₂O₅ in nonaqueous electrolytes can increase the intercalation potential. Previously, Levi et al. ^{Reference Levi, Gofer and Aurbach72} proposed that the increase in potential occurs because the intercalating ion does not need to shed a solvation shell (or does so only partially) during the charge-transfer step. Solvent cointercalation can be used to synthesize electrochemically active materials, such as the transformation of spinel Mn₃O₄ into δ-MnO₂ in aqueous electrolytes. ^{Reference Nam, Kim, Lee, Salama, Shterenberg, Gofer, Kim, Yang, Park, Kim, Lee, Chang, Doo, Jo, Jung, Aurbach and Choi52,Reference Song, Noked, Gillette, Duay, Rubloff and Lee82} Such hydrated δ-MnO₂ has been investigated as a potential cathode material for Mg ion batteries. ^{Reference Nam, Kim, Lee, Salama, Shterenberg, Gofer, Kim, Yang, Park, Kim, Lee, Chang, Doo, Jo, Jung, Aurbach and Choi52,Reference Song, Noked, Gillette, Duay, Rubloff and Lee82,Reference Sun, Duffort, Mehdi, Browning and Nazar83} The improved intercalation kinetics and reversibility of Mg²⁺ in δ-MnO₂ have been ascribed to efficient Coulombic screening of the intercalating Mg²⁺ from the host structure by interlayer water molecules [Fig. 6(a)]. First, spinel Mn₃O₄ was cycled in an aqueous Mg²⁺ electrolyte to form hydrated, layered δ-MnO₂. Then, the material was cycled in a nonaqueous Mg²⁺ electrolyte with varying amounts of water. Compared with the completely anhydrous electrolyte, hydrated δ-MnO₂ cycled in a Mg²⁺ nonaqueous electrolyte with small amounts of water showed higher capacity and lifetime [Figs. 6(b) and 6(c)]. However, Sun, et al. found that the hydrated material undergoes a conversion reaction with Mg²⁺ in a nonaqueous electrolyte with the formation of MnOOH, MnO, and Mg(OH)₂; reversible intercalation was only observed in an aqueous Mg²⁺ electrolyte. ^{Reference Sun, Duffort, Mehdi, Browning and Nazar83}

FIG. 6. (a) Proposed mechanism for the enhanced electrochemical energy storage of Mg²⁺ in layered δ-MnO₂. Reproduced from Ref. Reference Song, Noked, Gillette, Duay, Rubloff and Lee82 with permission of the PCCP Owner Societies. (b) Galvanostatic charge/discharge of δ-MnO₂ in Mg²⁺ non-aqueous electrolyte with different water content, and (c) capacity versus cycle number of the same system. Reprinted with permission from Ref. Reference Nam, Kim, Lee, Salama, Shterenberg, Gofer, Kim, Yang, Park, Kim, Lee, Chang, Doo, Jo, Jung, Aurbach and Choi52. Copyright 2015 American Chemical Society.

The benefits of water co-intercalation for the reversibility of multivalent ion intercalation are not limited to Mg²⁺. González et al. ^{Reference González, Nacimiento, Cabello, Alcántara, Lavela and Tirado84} reported on the reversible charge storage of Al³⁺ from an aqueous electrolyte into V₂O₅ xerogel and proposed a co-intercalation reaction mechanism:

(6) $${{\rm{V}}_2}{{\rm{O}}_5} + {x \mathord{\left/ {\vphantom {x 3}} \right. \kern-\nulldelimiterspace} 3}{\rm{Al}}\left( {{{\rm{H}}_2}{\rm{O}}} \right)_n^{3 + } \,+\, x{{\rm{e}}^ - } \leftrightarrow {\rm{A}}{{\rm{l}}_{{x \mathord{\left/ {\vphantom {x {\rm{3}}}} \right. \kern-\nulldelimiterspace} {\rm{3}}}}}{{\rm{V}}_2}{{\rm{O}}_5} \cdot n{{\rm{H}}_2}{\rm{O}}\quad {\rm{.}}$$

Since Al³⁺ is a small cation (ionic radius of 0.68 Å) with a large charge, it has a high standard hydration enthalpy, which means that it is very strongly hydrated. Therefore, it is reasonable to assume that solid state intercalation of Al³⁺ can only occur via solvent cointercalation, otherwise, the activation energy for charge transfer at the interface is too high.

Solvent exchange of water for other polar molecules can be performed in structures that contain water in the as-synthesized state or in anhydrous layered oxides whose interlayer bonding is weak enough that solvent molecules may intercalate. Molybdenum oxides serve as model systems for investigating solvent exchange because both hydrated and anhydrous crystalline MoO₃ form layered structures, ^{Reference Chippendale, Cheetham, Braithwaite and Haber85} which expands the number of precursors that can be used for solvent exchanged MoO₃. Schöllhorn et al. ^{Reference Schöllhorn, Kuhlmann and Besenhard57} performed solvent exchange of Na_0.5(H₂O) _y MoO₃ bronze with dimethylsulfoxide (DMSO); the interlayer spacing increased from 11.41 to 16.92 Å. In a related experiment, Na_0.5(H₂O) _y MoO₃ bronze was used to intercalate various organic compounds; interlayer spacings as large as 37.9 Å were reported for MoO₃ intercalated with tetradecylamine. ^{Reference Tagaya, Ara, Kadokawa, Karasu and Chiba86} Chen et al. ^{Reference Chen, Liu, Yin, Li, Yang, Li, Fan, Wang, Zhang, Li, Xu, Lu, Yang, Sun and Gao87} utilized MoO₃·H₂O as the precursor for the solvent exchange of water with n-octylamines in ethanol. The reaction was described as an acid–base reaction between the water molecules and amines. The amines occupy the interlayer space in a bilayer arrangement with a 51° tilt angle and with an expanded interlayer spacing of 23.1 Å, as illustrated in Fig. 7; heat treatment of the materials at 550 °C yields anhydrous MoO₃. The electrochemistry of these materials was not reported and bears further investigation as such large interlayer spacings may be beneficial for multivalent and high-rate intercalation.

FIG. 7. Synthesis of a MoO₃-alkylamine hybrid starting from (a) MoO₃·H₂O precursor, (b) structure of the inorganic-organic hybrid including orientation of alkyl amine chains, (c) thermal heat treatment at 550 °C yields orthorhombic α-MoO₃. Reproduced from Ref. Reference Chen, Liu, Yin, Li, Yang, Li, Fan, Wang, Zhang, Li, Xu, Lu, Yang, Sun and Gao87 with permission of The Royal Society of Chemistry.

C. Cation exchange

Layered metal oxides may also undergo cation exchange. In transition metal oxides, the layers are negatively charged and can thus accommodate cationic interstitial guest species. For cation exchange, the starting material may contain the exchangeable ion in the as-synthesized state (e.g., LiCoO₂) or as a result of electrochemical cycling (e.g., Li _x V₂O₅). Cation exchange may occur by the exchange of only the cation or with solvent co-intercalation, as described above. A recent review summarized the general features of cation exchange in nanoscale materials. ^{Reference Rivest and Jain88}

The desire to improve the electrochemical behavior of lithium ion cathode materials has focused significant effort on the cation exchange of layered metal oxides. LiCoO₂ is a prototypical Li-ion battery cathode material whose structure consists of alternating layers of lithium and cobalt separated by oxygen layers in an …ABCABC… stacking sequence. ^{Reference Manthiram, Nazri and Pistoia89} A low-temperature route for the synthesis of LiCoO₂ and LiNiO₂ was proposed by Larcher et al. ^{Reference Larcher, Palacín, Amatucci and Tarascon90} who performed the cation exchange of NiOOH and CoOOH in a hydrothermal reactor in the presence of LiOH; Fig. 8 shows the topotactic nature of the cation exchange. The oxyhydroxides are similar in structure to the LiMO₂ (M = Ni, Co). In the case of LiCoO₂, the electrochemical behavior of the cation-exchanged phase was inferior to the high temperature phase due to lower coulombic efficiency. On the other hand, cation exchanged LiNiO₂ exhibited similar capacity and reversibility as the high temperature analog. The increased surface area of the cation exchanged LiNiO₂ did result in increased moisture sensitivity as compared to the high temperature phase, ^{Reference Larcher, Palacín, Amatucci and Tarascon90} thus exacerbating the well-known surface instability issues of LiNiO₂. ^{Reference Liu, Oh, Liu, Lee, Cho, Chae, Kim and Cho91}

FIG. 8. XRD patterns as a function of the extent of cation exchange of CoOOH (HCoO₂) to LiCoO₂. Reprinted with permission from Ref. Reference Larcher, Palacín, Amatucci and Tarascon90. Copyright 1997, The Electrochemical Society.

Cation exchange is also useful as a low temperature method to synthesize metastable phases. A well-known example is the cation exchange synthesis of LiMnO₂, reported by Armstrong et al. ^{Reference Armstrong, Bruce, Armstrong and Bruce92} and Capitaine et al. ^{Reference Capitaine, Gravereau and Delmas93} by stirring NaMnO₂ in LiCl or LiBr in alcohol. However, during electrochemical cycling, the obtained LiMnO₂ with an O₃ layered structure undergoes conversion to a more stable spinel phase. ^{Reference Quine, Duncan, Armstrong, Robertson and Bruce94} Therefore, one of the challenges of synthesis via cation exchange is that the structure may convert to a more stable phase during prolonged electrochemical cycling.

D. Polymers and small molecules

Some structures can exhibit very large interlayer spacings that can accommodate not just solvent molecules or cations, but polymers. Of interest for electrochemical energy storage are compounds that will increase the capacity by providing additional redox active sites or that will increase the electronic conductivity of the oxide to increase the rate capability. An example of a polymer that can provide both is polyaniline (PANI), which can be intercalated into V₂O₅ xerogels, with a resulting interlayer spacing of 14–19 Å. ^{Reference Ruiz-hitzky and Ruiz-Hitzky95} As shown in Fig. 9(a), the PANI is only present within the interlayer. Figure 9(b) shows the galvanostatic charge/discharge of nanostructured V₂O₅ and V₂O₅/PANI nanocomposite; the capacity of the polymer-intercalated oxide was approximately doubled at the same time that the electronic conductivity increased ∼50 times. ^{Reference Chen, Yang, Zhang, Yang, Hou and Zhu96}

FIG. 9. (a) TEM of a vanadium oxide/polyaniline composite, where the polyaniline is inserted into the interlayer spacing of the oxide. Reprinted from Ref. Reference Malta and Torresi97, with permission from Elsevier. (b) Galvanostatic charge/discharge of nanostructured V₂O₅ (curve “a”) and V₂O₅/PANI nanocomposite (curve “b”) in a non-aqueous Li⁺ electrolyte at a current density of 29.5 mA/g. Reproduced from Ref. Reference Chen, Yang, Zhang, Yang, Hou and Zhu96 with permission of The Royal Society of Chemistry.

E. Exfoliation

An important property of bulk layered oxides is that they can be completely exfoliated into mono- or few-layer sheets (‘nanosheets’). There has been significant interest in discovering the electrochemical energy storage properties of such materials because, in theory, they exhibit short diffusion distances and a large number of surface redox sites ^{Reference Peng, Zhu, Chen, Ruoff and Yu98,Reference Liu and Liu99} which could lead to high power and high energy density storage. In this regard, the methods of modifying the interlayer described above have all been utilized as a means to expand the interlayer spacing of layered oxides, which decreases the force needed to pull apart the layers into nanosheets. The applied force can be mechanical, ^{Reference Kalantar-Zadeh, Vijayaraghavan, Ham, Zheng, Breedon and Strano100} acoustic, ^{Reference Zhang, Gao and Gong101} thermal, ^{Reference Du, Zheng, Zhang, Wang, Chen, Xue, Dai, Ji and Cao102} or a combination of these. ^{Reference Aksit, Toledo and Robinson103} Figure 10 shows the general mechanism of exfoliation of layered materials into few-layer sheets. ^{Reference Nicolosi, Chhowalla, Kanatzidis, Strano and Coleman18} A recent review described synthesis methods for controlling nanosheet size, composition, and structure. ^{Reference Ma and Sasaki104}

FIG. 10. Schematic description of exfoliation mechanisms of layered materials to yield nanosheets: (a) intercalation followed by agitation; (b) ion exchange followed by agitation; (c) direct agitation via sonication. From Ref. Reference Nicolosi, Chhowalla, Kanatzidis, Strano and Coleman18. Reprinted with permission from AAAS.

Such exfoliated layered oxide nanosheets have primarily been investigated for use as electrochemical capacitor electrodes, and specifically as pseudocapacitors. ^{Reference Augustyn, Simon and Dunn25} This is because the nanosheet architecture would ideally expose all of the redox active sites directly to the electrolyte and enable rapid charge-transfer reactions with minimal solid state diffusion. As noted previously, MoO₃ forms both hydrous and anhydrous layered polymorphs and it is a good precursor material for the synthesis of nanosheets. Bulk crystalline MoO₃ can be exfoliated by sonicating a dispersion of the oxide in a solution of water and isopropanol ^{Reference Zhang, Gao and Gong101} ; the mechanism of exfoliation is the intercalation of isopropanol between MoO₃ layers and subsequent application of acoustic energy in the form of sonication. Hanlon et al. ^{Reference Hanlon, Backes, Higgins, Hughes, O'Neill, King, McEvoy, Duesberg, Mendoza Sanchez, Pettersson, Nicolosi and Coleman105} synthesized MoO₃ nanosheets via liquid-phase exfoliation that consisted of sonicating MoO₃ in isopropanol. Figure 11 shows the dispersion, absorbance, and TEM of the exfoliated nanosheets separated by size into small, very small, and large; the size selection was obtained by centrifuging the stock solution at different speeds and selecting the supernatant. The maximum capacitance (∼200 F/g¹ at 10 mV/s or 200 s) was obtained by combining the exfoliated MoO₃ nanosheets with carbon nanotubes.

FIG. 11. Synthesis of MoO₃ nanosheets via liquid exfoliation of bulk MoO₃: (a) dispersions of (left to right) very small, small, and large nanosheets; (b) measurement of sedimentation via absorption as a function of time; (c) TEM of very small MoO₃ nanosheets; (d) small MoO₃ nanosheets; and (e) large MoO₃ nanosheets. ^{Reference Hanlon, Backes, Higgins, Hughes, O'Neill, King, McEvoy, Duesberg, Mendoza Sanchez, Pettersson, Nicolosi and Coleman105} Reprinted with permission from Ref. Reference Hanlon, Backes, Higgins, Hughes, O'Neill, King, McEvoy, Duesberg, Mendoza Sanchez, Pettersson, Nicolosi and Coleman105. Copyright 2014 American Chemical Society.

The benefits of synthesizing nanosheets from exfoliated bulk oxides is that this technique is readily scalable. ^{Reference Nicolosi, Chhowalla, Kanatzidis, Strano and Coleman18} However, there are several challenges for using exfoliated nanosheets as energy storage materials for large-scale devices. ^{Reference Palacín, Simon and Tarascon44} The high surface area of nanosheets can lead to increased side reactions with the electrolyte, which have to be avoided for long cycle life and safety. Second, the high surface area can also lead to low volumetric capacity/capacitance as compared with the bulk materials. Third, such materials contain many defects that can lead to limited cycle life and barriers for ion diffusion. On the other hand, nanosheets can be highly advantageous for small-scale energy storage devices. ^{Reference Li, Shi, Lang, Wen, Li and Jiang106} Also, restacked exfoliated nanosheets have recently been reported as precursors for the synthesis of Li-ion battery cathode materials, presenting an interesting strategy for the development of metastable layered oxides. ^{Reference Cheng, Yang, Li, Li and Chan107}

F. Exfoliated metal oxide heterostructures

Exfoliated layered oxide materials can be assembled into new heterostructured layered oxide materials. ^{Reference Geim and Grigorieva108} The motivation for synthesizing such materials is that they can exhibit improved properties over the individual components. In the case of oxide materials, this typically means improving their electronic conductivity to enable higher rate capability by forming heterostructures with graphene. One example of this method is the synthesis of graphene/MnO₂ hybrids via co-exfoliation for use as electrochemical capacitors. ^{Reference Mendoza-Sánchez, Coelho, Pokle and Nicolosi109} The highest capacitance was obtained with the co-exfoliated materials (as opposed to the individual components); a volumetric capacitance of 200 F/cm³ was obtained at a sweep rate of 100 mV/s (charge/discharge time of 10 s). The synthesis of these heterostructures opens up the possibility of entirely new metal oxide/hybrid materials assembled at the atomic layer. Most of these composites; however, are synthesized via self-assembly ^{Reference Wang, Kou, Choi, Yang, Nie, Li, Saraf, Hu, Zhang, Graff, Liu, Pope and Aksay110,Reference Wu, Zhou, Yin, Ren, Li and Cheng111} as opposed to bulk exfoliation.