Electrode materials

Since dendritic growth of Li on graphite anodes is enhanced at larger current densities,^{Reference Liu, Du, Shen, Zuo, Cheng, Ma, Yin and Gao64,Reference Jansen, Kahaian, Kepler, Nelson, Amine, Dees, Vissers and Thackeray65} high-rate LIBs tend to use anode materials that can be cycled at higher potentials for increased safety. Crystalline materials on the binary Li₂O–TiO₂ composition line [e.g., the spinel form Li₄Ti₅O₁₂ (LTO)^{Reference Ferg, Gummow, de Kock and Thackeray66–Reference Ohzuku, Ueda and Yamamoto68}] have been used as an alternative anode material for high-rate LIBs due to their higher Li ion insertion potential of ∼1.5 V versus Li⁺/Li.^{Reference Sandhya, John and Gouri69–Reference Madian, Eychmüller and Giebeler71} Although the higher reaction potential decreases the energy density of the battery and in doing so increases the cost per kilowatt hour stored, it diminishes the risk of Li plating and dendrite formation during cycling at high rates. Additionally, electrolyte decomposition is thermodynamically less favorable at the higher voltages. This minimizes SEI formation and reduces impedance on charging/discharging thus improving rate capability. The higher voltage also permits the use of the Al as a current collector (which is lighter and cheaper than Cu) as Li alloying with Al does not occur until 0.3 V versus Li⁺/Li.^{Reference Wen, Boukamp, Huggins and Weppner72}

Spinel LTO has an excellent cycle life due to its thermal stability and negligible volume change on cycling.^{Reference Takami, Satoh, Hara and Ohsaki56,Reference Ohzuku, Ueda and Yamamoto68,Reference Yang, Choi, Kerisit, Rosso, Wang, Zhang, Graff and Liu70} It was the first commercialized non-carbonaceous anode material for LIBs, with Toshiba and Altairnano employing anodes comprising micrometer-sized secondary LTO particles for higher rates and longer cycle lives in their higher rate LIB products, which have achieved power densities of 4 kW/kg (6.5 kW/L).^{Reference Takami, Kosugi and Inagaki73–76} However, the specific reversible capacity of LTO electrodes is limited to less than 175 mAh/g (Li_4+xTi₅O₁₂, 0 ≤ x ≤ 3),^{Reference Yang, Choi, Kerisit, Rosso, Wang, Zhang, Graff and Liu70,Reference Ge, Li, Li, Dai and Wang77} with its charging/discharging rate being limited by the poor conductivity of the micrometer-sized particles used in the electrodes.^{Reference Colbow, Dahn and Haering67} In recent years, these issues have been addressed through use of C-coating and nanostructuring, allowing LTO capacities exceeding 150 mAh/g to be achieved at rates of 10C^{Reference Wang, Wang, Tang, He, Gan, Li, Du, Li, Lin and Kang78,Reference Odziomek, Chaput, Rutkowska, Świerczek, Olszewska, Sitarz, Lerouge and Parola79} ; however, these strategies negatively impact the volumetric capacity and may not be commercially viable for some applications.

Various crystalline TiO₂ polymorphs have also been explored as alternatives to LTO. Fully lithiated TiO₂ (i.e., LiTiO₂ corresponding to 100% reduction of Ti⁴⁺ to Ti³⁺ on lithiation) has a theoretical capacity of 335 mAh/g, which is only slightly less than that of graphite (372 mAh/g). Among the many crystalline TiO₂ polymorphs reported, rutile, anatase, brookite, and the “bronze” form TiO₂–B^{Reference Marchand, Brohan and Tournoux80} have been shown to have Li electrochemical activity, and all have theoretical volumetric capacities that significantly exceed that of LTO (see Table S1).^{Reference Yang, Choi, Kerisit, Rosso, Wang, Zhang, Graff and Liu70} However, full lithiation of these TiO₂ polymorphs can only be approached using nanostructures,^{Reference Yang, Choi, Kerisit, Rosso, Wang, Zhang, Graff and Liu70,Reference Madian, Eychmüller and Giebeler71,Reference Brumbarov, Vivek, Leonardi, Valero-Vidal, Portenkirchner and Kunze-Liebhäuser81,Reference Auer, Steiner, Portenkirchner and Kunze-Liebhäuser82} and it has been largely through the study of nanostructured TiO₂ polymorphs that differences in Li ion charging/discharging rates and capacities arising from crystal structure have been observed. Of particular interest, has been the high rate and cycle life reported for the bronze polymorph TiO₂–B.^{Reference Yang, Choi, Kerisit, Rosso, Wang, Zhang, Graff and Liu70,Reference Zukalová, Kalbáč, Kavan, Exnar and Graetzel83–Reference Dalton, Belak and Van der Ven86} The faster charge storage reported for this form has been attributed to freely accessible parallel channels for Li ion transport perpendicular to the (010) face^{Reference Zukalová, Kalbáč, Kavan, Exnar and Graetzel83} in the relatively open bronze polymorph.^{Reference Yang, Choi, Kerisit, Rosso, Wang, Zhang, Graff and Liu70}

Another strategy for increasing the energy density of Ti-based anodes without significantly compromising their rate capability is to incorporate additional transition metals.^{Reference Tian, Xiang, Zhang, Li and Wang87–Reference Takami, Ise, Harada, Iwasaki, Kishi and Hoshina93} Higher capacities are theoretically possible, especially if the valence of the added metal can change by more than one (e.g., through use of Mo, W, V, and Nb). The addition of heavier metals may not necessarily increase the specific energy density of the resulting cells;^{Reference Tarascon and Armand61} it is more likely to lead to gains in volumetric performance. Of particular interest has been the family of TiO₂–Nb₂O₅ (TNO) materials due to their high theoretical capacity (up to 400 mAh/g with multielectron redox; see Table S1) arising from the idealized Ti⁴⁺/Ti³⁺ and Nb⁵⁺/Nb³⁺ redox couples.^{Reference Tian, Xiang, Zhang, Li and Wang87,Reference Han, Huang and Goodenough88,Reference Griffith, Senyshyn and Grey92–Reference Wu, Miao, Han, Hu, Chen, Lee, Kim and Chen94} Toshiba have adopted this strategy in their high-power SciB cells, which employ a TiNb₂O₇ anode comprising C-coated micrometer-sized particles and an LiNi_0.6Co_0.2Mn_0.2O₂ (NCM) cathode to achieve power and energy densities of 10 kW/L (charging for 10 s at 50% state of charge) and ∼350 Wh/L, respectively.^{Reference Takami, Ise, Harada, Iwasaki, Kishi and Hoshina93} Although this energy density surpasses that of similar cells with LTO anodes (177 Wh/L),^{Reference Hesse, Schimpe, Kucevic and Jossen40} it is still only half that possible with state-of-the-art high-energy density cells.

This approach of using open structural motifs can present advantages over nanostructuring in achieving higher rate electrodes without compromising capacity because volumetric energy density is not reduced due to pore volume (i.e., they have greater tap density). Griffith et al. adopted this approach with Nb_xW_yO_z 3D open lattice structures. The multielectron redox capabilities of both Nb⁵⁺ and W⁶⁺ lead to theoretical capacities for Nb_xW_yO_z compounds of 300–400 mAh/g (similar to the titanium niobium oxides); however it remains open as to whether full two electron redox will be accessible under stable cycling conditions (especially at high rates) even though multielectron is achievable in both individual metal oxide families. With electrodes comprising Nb₁₈W₁₆O₉₃ micrometer-sized solid particles, capacities of ∼150 mAh/g at 10C and ∼75 mAh/g at 100C were realized (see Fig. 3).^{Reference Griffith, Wiaderek, Cibin, Marbella and Grey95} Nanostructuring or the use of amorphous forms of mixed metal oxides^{Reference Daramalla, Venkatesh, Kishore, Munichandraiah and Krupanidhi96} may provide paths to further increases in (gravimetric) capacity for these multiple transition metal anode materials.

Figure 3. Structure of the bronze-like Nb₁₈W₁₆O₉₃. (a) Crystallographic structure of the bronze-like Nb₁₈W₁₆O₉₃ showing open the structural motif that facilitates fast Li ion diffusion through the lattice, (b) electrochemical voltage profiles as a function of lithium concentration and C-rate, (c) derivative curve of the voltage profiles, (d) electron microscope image showing the micrometer-sized particles of the mixed metal oxide, and (e) gravimetric capacity as a function of C-rate for two different Nb_xW_yO_z materials and cycling limits (reproduced with permission from Griffith et al.^{Reference Griffith, Wiaderek, Cibin, Marbella and Grey95}).

Although this discussion has focused more on anode materials due to the need to find alternative safe anode materials for high-rate devices, achievement of high rate in cells requires that the Li insertion and extraction processes and electron transport are rapid in both the anode and cathode materials. Currently, most cathode materials are limited by the conduction of Li ions in the crystalline cathode material. Consequently, large concentration overpotentials occur at high lithiation rates, resulting in electrolyte oxidation, dissolution of cathode materials, and corrosion of the current collector.^{Reference Vetter, Novák, Wagner, Veit, Möller, Besenhard, Winter, Wohlfahrt-Mehrens, Vogler and Hammouche50,Reference Wohlfahrt-Mehrens, Vogler and Garche97} Metals released by cathode oxidative decomposition can diffuse through the electrolyte to deposit on the anode and/or contribute to SEI growth and further degrade battery performance.^{Reference Liu, Liu, Lin, Pei and Cui55,Reference Tarascon and Armand61} Collectively, these aging mechanisms result in increased impedance and reduced cycle life of the battery and may necessitate a narrowed cycling voltage (thus smaller utilized capacity) to be used for high-rate operation. The material strategies that have been discussed above could also be applied to the engineering of high-rate cathode materials, although additional research is required to understand the electrode degradation mechanisms with high rate usage at high oxidation potentials.

The rate capability of both anode and cathode materials can also be enhanced through the use of carbon coating and nanosizing. An often-quoted example of the value of carbon coating is the story of FePO₄, a material that was initially considered to be a low-rate cathode material due the poor electronic conductivity of both oxidized and reduced forms and poor ionic conductivity of micron-size particles.^{Reference Padhi, Nanjundaswamy and Goodenough98,Reference Kang and Ceder99} The use of nanomaterials decreases both the Li and electron transport lengths. Additionally it reduces the probability that Fe antisite defects block ion transport in the 1D tunnels (aligned in the [010] crystallographic direction). It was found that the rate of charging/discharging these electrodes was also dependent on the Li ion transport across the surface of the particles as the ions can only move into the bulk of the crystalline FePO₄ in 1D channels.^{Reference Kang and Ceder99–Reference Islam, Driscoll, Fisher and Slater101} By carbon coating the surfaces of the particles, charging capacity was significantly increased.^{Reference Ravet, Chouinard, Magnan, Besner, Gauthier and Armand102,Reference Doeff, Hu, McLarnon and Kostecki103} Herle et al. proposed that the increased rate capability was due to the formation of a highly conductive surface layer of iron phosphide (FeP) arising from the carbothermal reduction of Fe at C-contaminated FePO₄surfaces.^{Reference Herle, Ellis, Coombs and Nazar104} Kang and Ceder reported the fabrication of a conductive Li₄P₂O₇-like amorphous phase on the surface of the particles using a heat treatment to improve the rate capability of LFP cathodes. With this process, they were able to achieve electrode capacities of >100 mAh/g at 200C and 60 mAh/g at 400C, although using an electrode with a very dilute concentration of active material.^{Reference Kang and Ceder99} In view of these remarkable results, they concluded that electrode materials with extremely high-rate capability will “blur the distinction between supercapacitors and batteries,”^{Reference Kang and Ceder99} a point that we will return to later in this article. More recent work has shown that the high rates achieved in LFP are associated with the ability of the material to operate via a metastable Li_xFePO₄ solution during high-rate cycling; the classical model of intercalation in material that operates via a two-phase reaction, involving nucleation, growth, and movement of a phase boundary is no longer appropriate, at least at high rates.^{Reference Bai, Cogswell and Bazant105–Reference Liu, Strobridge, Borkiewicz, Wiaderek, Chapman, Chupas and Grey108}

Electrode morphology and structuring

Micro- and nanostructuring of electrode particles can be used to achieve higher capacity and rate capability in LIBs.^{Reference Aricò, Bruce, Scrosati, Tarascon and van Schalkwijk109,Reference Bandaru, Yamada, Narayanan and Hoefer110} In particular, it can (i) increase the electrode/electrolyte interface area thereby enabling increased charge storage; (ii) provide shorter path lengths for both electron and Li ion diffusion that can improve high-rate performance; and (iii) more readily accommodate the strain associated with Li insertion/removal, which can enable increased cycle life. However, set against these advantages are some disadvantages, which include (i) increased undesirable electrode/electrolyte reactions due to higher surface area leading to higher impedance, reduced rate, and diminished cycle life; (ii) less-dense packing of active material (i.e., lower “tap density”), leading to lower volumetric energy densities; and (iii) potentially more complex and/or costly electrode synthesis, which preclude use in large-scale battery manufacturing.^{Reference Aricò, Bruce, Scrosati, Tarascon and van Schalkwijk109} When optimizing electrode materials specifically for high-rate performance, the contribution of micro- and nanostructuring to the kinetics of charge storage can be complicated by how the porosity is introduced in the electrode. For electrode materials in which electronic conductivity and charge transfer reactions are not rate limiting, ion transport in the host materials and/or the percolating electrolyte of 3D porous materials can become limiting.^{Reference Bae, Erdonmez, Halloran and Chiang111}

Numerous approaches have been proposed to structure electrode materials at the microscale to increase electrolyte access. These include laser structuring,^{Reference Habedank, Kraft, Rheinfeld, Krezdorn, Jossen and Zaeh112,Reference Mangang, Seifert and Pfleging113} hard templating (e.g., colloidal crystal),^{Reference Petkovich and Stein114–Reference Sakamoto and Dunn116} and soft templating^{Reference Petkovich and Stein114,Reference Li, Liu and Zhao117} ; however, it is necessary to establish the C-rates over which the structuring or templating process effectively increases capacity or capacity retention with high rates.^{Reference Habedank, Kraft, Rheinfeld, Krezdorn, Jossen and Zaeh112} If the Li ion concentration becomes depleted at surfaces due to fast cycling, this can result in large concentration overpotentials, which limit the rate of the charge transfer reactions.^{Reference Habedank, Kraft, Rheinfeld, Krezdorn, Jossen and Zaeh112} At high charging/discharging rates, highly tortuous Li ion diffusion paths can restrict access to some regions of the current collecting surface, which can significantly curtail the capacity.^{Reference Delattre, Amin, Sander, De Coninck, Tomsia and Chiang118} One approach by which high capacity can be retained with high-rate cycling is to use a dual-scale porosity, which combines channels, coated with a porous matrix, aligned to be perpendicular to the current collector.^{Reference Bae, Erdonmez, Halloran and Chiang111} This arrangement can facilitate Li ion access to a 3D volume without depletion of ions at the electrode surface. It can also allow for thicker electrodes to be used for increased capacity.^{Reference Delattre, Amin, Sander, De Coninck, Tomsia and Chiang118} It is important that both electrodes in a device can support high charging and discharging rates. For example, although laser structuring of a graphite anode can increase capacity over the range of 1–3C, at faster rates, the device performance can become limited by the cathode.^{Reference Habedank, Kraft, Rheinfeld, Krezdorn, Jossen and Zaeh112}

Nanoscale structuring, through formation of nanoparticles, nanotubes, nanofibers, and nanowires, can also be used to increase Li ion flux at surfaces.^{Reference Aricò, Bruce, Scrosati, Tarascon and van Schalkwijk109,Reference Guo, Hu and Wan119–Reference Zhang, Porras-Gutierrez, Mauger, Groult and Julien127} Additional benefits can arise from (i) space charge effects at the electrode/electrolyte interfaces and (ii) spatial confinement, both factors having the potential to change the thermodynamics of charge transfer reactions from that observed in bulk materials.^{Reference Aricò, Bruce, Scrosati, Tarascon and van Schalkwijk109,Reference Palacin, Simon and Tarascon125,Reference Obrovac and Dahn128,Reference Delmer, Balaya, Kienle and Maier129} Nanoconfinement (see Fig. 4) can impact ion transport in the electrolyte, phase transitions, and solvation structures.^{Reference Palacin, Simon and Tarascon125,Reference Pean, Daffos, Rotenberg, Levitz, Haefele, Taberna, Simon and Salanne130–Reference Fichtner132} It has been shown to enhance charge storage in electrochemical capacitors, which have emerged as high-rate complementary energy storage solutions to batteries.^{Reference Salanne, Rotenberg, Naoi, Kaneko, Taberna, Grey, Dunn and Simon133–Reference Prehal, Koczwara, Jäckel, Schreiber, Burian, Amenitsch, Hartmann, Presser and Paris135}

Figure 4. Nanoconfinement of a hydrated ion in a 3D carbon nanopore. Schematics showing (a) a low level (i.e., retention of some hydration) and (b) a higher level of nanoconfinement. The light blue, red and dark blue circles represent the bare ion, the hydrated ion, and the cutoff radius for which the degree of confinement is determined, respectively. In this model, the ions are assumed to be hard spheres that can approach the electrode surface as close as their bare ion radius (reproduced with permission from Prehal et al.^{Reference Prehal, Koczwara, Jäckel, Schreiber, Burian, Amenitsch, Hartmann, Presser and Paris135}).

Hierarchical micro-nanostructured electrodes can exploit the positive attributes of nanostructures for increased surface area, nanometer diffusion lengths, nano-confinement effects, and microstructures for improved stability and ease of handling during electrode preparation.^{Reference Guo, Hu and Wan119,Reference Yue and Liang136} They can be fabricated through the aggregation of nanosized crystallites to form larger, porous agglomerates, which can be then further grouped into micrometer-sized grains (e.g., see Fig. 5).^{Reference Odziomek, Chaput, Rutkowska, Świerczek, Olszewska, Sitarz, Lerouge and Parola79} The resulting multiscale porosity can enable effective penetration of the electrolyte, with the distance for Li ion diffusion lengths being minimized within the electrochemically active nanoparticles. The application of this hierarchical structuring process using LTO nanoparticles enabled the achievement of a capacity of 170 mAh/g at 50C with minimal capacity fading after 1000 cycles.^{Reference Odziomek, Chaput, Rutkowska, Świerczek, Olszewska, Sitarz, Lerouge and Parola79} The higher capacities observed in the hierarchically structured electrode in comparison with bulk LTO were partially attributed to the formation of a Ti-rich surface layer.^{Reference Odziomek, Chaput, Rutkowska, Świerczek, Olszewska, Sitarz, Lerouge and Parola79,Reference Wang, Gu, Guo, Li, He, Tsukimoto, Ikuhara and Wan137} This highlights that nanostructuring may also introduce benefits in terms of metal–oxide surface stoichiometry^{Reference Lu, Gu, Hu, Chiu, Li, Demopoulos and Chen138} in addition to improved electrolyte access and greater accommodation of strain. Understanding the surface atomic structure is also critical to the understanding of electrolyte decomposition side reactions that result in gas generation and SEI formation.^{Reference Lu, Gu, Hu, Chiu, Li, Demopoulos and Chen138}

Figure 5. Hierarchical micro-nanostructure through aggregation. Schematic depicting the hierarchical aggregation of crystalline nanoparticles into the progressively larger microparticles used for electrode fabrication (adapted from Odziomek et al.^{Reference Odziomek, Chaput, Rutkowska, Świerczek, Olszewska, Sitarz, Lerouge and Parola79} and reproduced under Creative Commons Attribution 4.0 International License).

Another common approach to fabricating hierarchical micro-nanostructures is to coat 3D microstructured current collectors (e.g., metal foams), which provide a percolating network for both electron and ion pathways with an electroactive nanomaterial.^{Reference Yue and Liang136} Coating can be achieved by thermal oxidation,^{Reference Wang, Liang, Wang and Yang139,Reference Choi, Park, Um, Yoon and Choe140} electrodeposition,^{Reference Cui and Zhang141} casting of a slurry,^{Reference Yue and Liang136,Reference Ji, Zhang, Pettes, Li, Chen, Shi, Piner and Ruoff142,Reference Yao, Okuno, Iwaki, Awazu and Sakai143} or surface anodization.^{Reference Bi, Paranthaman, Menchhofer, Dehoff, Bridges, Chi, Guo, Sun and Dai144,Reference Jiang, Hall, Song, Lau, Burr, Patterson, Wang, Ouyang and Lennon145} With processes where the active material is directly grown on the metal scaffold, the need for binder can be eliminated. Typical fluoropolymer binders are insulating and electrochemically inactive; they decrease gravimetric energy density and increase the resistive losses and hence power losses, especially under high-rate operating conditions. Binder material can also swell in commonly used organic electrolytes, leading to poor stability and irreversible capacity losses.^{Reference Yoo, Frank, Mori and Yamaguchi146,Reference Bülter, Peters, Schwenzel and Wittstock147} Anodization of metal foams in fluoride-containing electrolytes to form nanotube arrays on the foam surface is one approach that has been successfully used to fabricate hierarchically structured electrodes.^{Reference Bi, Paranthaman, Menchhofer, Dehoff, Bridges, Chi, Guo, Sun and Dai144,Reference Jiang, Hall, Song, Lau, Burr, Patterson, Wang, Ouyang and Lennon145} Figure 6 depicts the growth of TiO_2−x nanotubes directly on a Ti foam current collector. The foam allows increased Li ion access to the surface structures and the 20–50 nm thickness of the TiO_2−x nanotube walls improves electronic conductivity of the electrode.^{Reference Jiang, Hall, Song, Lau, Burr, Patterson, Wang, Ouyang and Lennon145}

Figure 6. Hierarchically structured TiO_2−x anode formed by anodization of a Ti foam current collector. (a) and (d) Low- and high-magnification images of the Ti foam, (b) scanning electron microscope image of amorphous TiO_2−x nanotubes formed on the foam surface, and (c) distribution of pore sizes as determined by X-ray computed tomography. Galvanostatic charge/discharge curves are shown in (e) and (f) for amorphous and crystalline TiO_2−x nanotube arrays on Ti foams, respectively (adapted from Jiang et al.^{Reference Jiang, Hall, Song, Lau, Burr, Patterson, Wang, Ouyang and Lennon145}).

A key practical concern for all structuring methods is whether a process can be practically scaled to manufacturing and whether the improvements in rate or cycle life that can be achieved result in a commercial advantage. Nanostructuring generally reduces tap density and so it is necessary to consider volumetric as well as gravimetric energy and power densities.^{Reference Griffith, Wiaderek, Cibin, Marbella and Grey95,Reference Zhang, Cheng and Zhang148} Also, although the larger surface area introduced by nano/micro-structuring can be beneficial for high-rate performance, it can increase the probability of undesirable side reactions and consequently reduce cycle life. To minimize this possibility, as is the case for supercapacitors, highly pure materials will be required and this can increase battery cost.

Electrolyte selection

The rate and cycle life of LIBs depend strongly on the electrolyte used.^{Reference Logan, Tonita, Gering, Li, Ma, Beaulieu and Dahn149} Most currently produced LIBs utilize electrolytes comprising Li ion salts (e.g., 1.0 M LiPF₆) in a mixture of organic solvents. Common solvents used include ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate, propylene carbonate (PC), and ethyl methyl carbonate.^{Reference Xu62,Reference Nishi, Azuma and Omaru150} Organic solvents permit a voltage operating window that is approximately three times larger than that possible with equivalent salt concentrations in aqueous electrolytes due to their greater oxidative and reductive stability compared with water. This wider voltage window results in LIBs having enhanced gravimetric and volumetric energy densities compared with batteries that use aqueous electrolytes. However, the ionic conductivities of the organic electrolytes are lower (e.g., 1–12 mS/cm for 1.0 M LiPF₆ in EC/DMC^{Reference Xu62,Reference Valøen and Reimers151} ) than those of aqueous electrolytes [(e.g., 600–825 mS/cm for ∼1.2 g/cm³ H₂SO₄ (aq.) used in lead acid batteries^{Reference Pavlov, Naidenov and Ruevski152}]. To help compensate for the low conductivity of the electrolyte, high-rate Li-ion cells typically use thin electrode stacks (e.g., ≤100 μm thick).^{Reference Delattre, Amin, Sander, De Coninck, Tomsia and Chiang118}

One rate-limiting property of the organic electrolytes is their low Li⁺ transference number, t ₊, of <0.5.^{Reference Doyle, Fuller and Newman153,Reference Capiglia, Saito, Kageyama, Mustarelli, Iwamoto, Tabuchi and Tukamoto154} This means that the ionic current is carried predominantly by the counter ion and not the Li⁺ on charging and discharging. The low transference number arises from the preferential solvation of Li⁺ compared with its counter ion in carbonate solvents and results in a concentration gradient that limits the rate at which the battery may be charged or discharged. It also limits the charging voltage, and hence energy density of the battery, through an increased concentration overpotential and this, in turn, places an upper limit to the thickness of the electrodes used. Doyle et al. showed (by simulation) the importance of using an electrolyte with a high t ₊ for high-power batteries and demonstrated significant enhancement in terms of rate capability when using a t ₊ = 1.0 Li ion polymer electrolyte over an electrolyte with t ₊ ≈ 0.2 even with an order-of-magnitude decrease in electrolyte conductivity in the former.^{Reference Doyle, Fuller and Newman153} Improvements in energy density were especially evident at high charging/discharging rates when Li ions became depleted with low t ₊ electrolyte due to their slower transport to the electrode surface.

One approach to increasing the Li ion transference number in organic electrolytes is to use larger and hence lower mobility polymeric anions.^{Reference Diederichsen, McShane and McCloskey155} Design of such electrolytes requires maximizing both the free Li ion concentration and localized charge density of the anion groups on the polymer backbone, while keeping the polymer content as low as possible so as to not overly increase the electrolyte viscosity.^{Reference Diederichsen, McShane and McCloskey155} However, to-date, very few attempts at improving ion conductivity have been realized through this approach because the choice of anions suitable for Li ion electrolytes is limited^{Reference Xu62} and anion suitability also depends on their electrochemical stability at electrode surfaces. Instead, solvent composition tailoring has been the main tool for manipulating electrolyte ion conductivity due to the availability of a vast number of candidate solvents. For solvated Li ions to migrate quickly under an electric field, ideally the solvent molecules prevent the Li cations and the electrolyte anions from forming close ion pairs. Solvents with a higher dielectric constant screen ion interactions; however, these solvents typically have a higher viscosity, which acts to slow ion movement in an electric field. Consequently, solvent mixtures are used, with one component selected for ion shielding (i.e., high dielectric constant) and the other(s) selected for their low viscosity.^{Reference Xu62}

As mentioned in the earlier discussion of LIB Technology, an SEI typically forms at LIB anode surfaces due to reduction reactions involving the chemical species in the electrolyte. Migration of ions through this surface layer is typically the rate-limiting step for most LIB chemistries.^{Reference An, Li, Daniel, Mohanty, Nagpure and Wood46,Reference Peled and Menkin47,Reference Xu62} An analogous layer called the cathode–electrolyte interface (CEI) forms on cathodes by oxidation reactions involving electrolyte components at high potentials but the impact of this thin or possibly transient layer on battery cell performance is generally considered to be less significant.^{Reference Vetter, Novák, Wagner, Veit, Möller, Besenhard, Winter, Wohlfahrt-Mehrens, Vogler and Hammouche50,Reference Wohlfahrt-Mehrens, Vogler and Garche97} The nature of the formed SEI depends on many factors, including the anode material and electrolyte composition. When the SEI continues to grow on cycling, it increases the cell impedance and eventually consumes the finite quantity of Li, limiting the cycling life of the battery. Consequently, some control over SEI formation and growth is critical for the achievement of LIBs with high rate capability and cycle life. One option is to use electrode materials with a higher (i.e., less negative) reduction potential (e.g., those based on early transition metals, as discussed previously for electrode materials), so that the parasitic reactions and hence SEI formation can be minimized; however, the increased rate and cyclability are achieved at the cost of capacity due to the significantly reduced lower cell voltage. Furthermore, the less-passivated surfaces of the high potential anodes can give rise to challenges, such as gas generation and pressure buildup in the cell.^{Reference Buannic, Colin, Chapuis, Chakir and Patoux156–Reference Rodrigues, Kalaga, Trask, Shkrob and Abraham158}

Another option is to engineer an artificial SEI to achieve high and stable conduction of Li ions to the electrode surface.^{Reference Wang, Kadam, Li, Shi and Qi48} The SEI composition can be tuned by using additives in the electrolyte that will react at the surface before the electrolyte solvent and anion species.^{Reference Peled and Menkin47} Another option is to carefully select the choice of solvent as some solvent species have been shown to form more stable insulating layers without seriously impacting the Li ion conduction. Takenaka et al. confirmed, using atomistic simulations (see Fig. 7), that an EC-based SEI film can prevent excess reduction of electrolyte on a graphite anode surface while still allowing Li ions to pass through. In contrast, a PC-based film could not sufficiently insulate the anode surface, allowing continued electrolyte reaction despite PC only differing from EC molecules by the presence of their methyl side groups.^{Reference Takenaka, Suzuki, Sakai and Nagaoka159} This example highlights the need for a greater understanding of SEI formation and, in particular, use of simulations to guide the development of electrolytes for stable SEI layers with high conductivity for high-rate LIBs.^{Reference Peled and Menkin47,Reference Wang, Kadam, Li, Shi and Qi48,Reference Takenaka, Suzuki, Sakai and Nagaoka159}

Figure 7. Atomistic models of SEI films formed over a graphite anode. Models showing (a) an EC-based electrolyte, and (b) a PC-based electrolyte (green = SEI film; blue = Li⁺; gray = PF₃, C₂H₄, or C₃H₆; purple = EC or PC; orange = PF₆) (reproduced with permission from Takenaka et al.^{Reference Takenaka, Suzuki, Sakai and Nagaoka159}).

Another electrolyte strategy that may offer a path toward longer cycle life and perhaps also higher rate is to use superconcentrated, or high ion-to-solvent ratio, electrolytes.^{Reference Wang, Yamada, Sodeyama, Chiang, Tateyama and Yamada160–Reference Zeng, Murugesan, Han, Jiang, Cao, Xiao, Ai, Yang, Zhang, Sushko and Liu162} Although lowering ionic conductivity,^{Reference Wang, Yamada, Sodeyama, Chiang, Tateyama and Yamada160} this newer class of electrolyte has been proposed to provide greater oxidative stability (allowing higher cycling voltages and commensurately larger energy densities), reduced volatility and flammability, and enhanced Li ion interfacial transport.^{Reference Drozhzhin, Shevchenko, Zakharkin, Gamzyukov, Yashina, Abakumov, Stevenson and Antipov161} A key reason for the continued use of the 1.0 M LiPF₆ electrolyte in commercially produced LIBs has been the ability for this salt to protect against the corrosion of the Al current collectors by forming an AlF₃ coating on the cathode through the reaction of hydrogen fluoride (HF, a breakdown product of PF₆⁻)Reference Sloop, Pugh, Wang, Kerr and Kinoshita¹⁶³ with Al.^{Reference Tarascon and Armand61,Reference Zhang and Devine164} However, HF also acts to dissolve the metal oxide cathode material,^{Reference Aurbach, Markovsky, Levi, Levi, Schechter, Moshkovich and Cohen165} releasing metal ions, which can subsequently deposit on the anode where they further catalyze the reduction of electrolyte solvent.^{Reference Kim, Pieczonka, Li, Wu, Harris and Powell166,Reference Pieczonka, Liu, Lu, Olson, Moote, Powell and Kim167} This degradation process consumes Li ions and thickens the SEI for graphite anodes and, in doing so, decreases both the rate capability and cycle life of the battery.

Wang et al. have demonstrated the use of the more stable LiN(SO₂F)₂ (LiFSA) salt mixed with DMC at very high salt:solvent molar ratios of 1:1.1 (compared with 1:10.6 for the 1.0 M LiPF₆ in EC/DMC) to achieve cathode stabilities at potentials of up to 5 V versus Li⁺/Li, promising long cycle life and high rate capability with enhanced safety. This superconcentrated electrolyte forms a 3D network of anions and solvent molecules that coordinates the Li ions and effectively inhibits the dissolution of both the Al cathode current collector as well as the transition metal oxides at potentials of up to 5 V.^{Reference Wang, Yamada, Sodeyama, Chiang, Tateyama and Yamada160} In electrolytes with salt-to-solvent molar ratios of >1:2, the solvent molecules are mostly coordinated by Li ions in the primary solvation shell, and this decreases the reactivity of the solvents molecules at the electrolyte surfaces.^{Reference Zeng, Murugesan, Han, Jiang, Cao, Xiao, Ai, Yang, Zhang, Sushko and Liu162} Although some sacrifice in electrolyte conductivity results with high salt-to-solvent electrolytes (∼1.1 mS/cm for LiFSA at 30 °C with 1:1.1 molar ratio)^{Reference Wang, Yamada, Sodeyama, Chiang, Tateyama and Yamada160} compared with ∼6 mS/cm (1:10.6 molar ratio) for 1.0 M LiPF₆ in EC/DMC,^{Reference Valøen and Reimers151} the improvement in cycle life that may result from the diminished reactivity at electrode surfaces may be valuable for services, such as grid stabilization, depending on the cost of the salt.

Strategies considered together

Typically, any high-rate LIB device design will draw upon more than one of the aforementioned design strategies; however, there are also interdependencies. For example, changes in electrolyte to optimize SEI formation need to take into account the materials used for the electrode. If an anode comprising an early transition metal is adopted, specifically for increased cycle life, then additives or specific solvent combinations to enhance the formation of an ion conductive SEI may not be required as the higher voltage minimizes reactions involving electrolyte species. Similarly, if electrode structuring is used, consideration should be directed at how a structured electrode volume with different diffusion length scales impacts the electronic and ionic conductivity of the electrode and even SEI formation. Identifying the rate-limiting steps and processes on charging and discharging both the anode and cathode becomes the key to optimizing battery cells for improved rate and cycle life.

This independency of strategies highlights the importance of improving our understanding of all mechanistic aspects of LIBs if we are to effectively engineer storage devices capable of high rates and very long cycle lives at costs that are competitive for grid stabilization services. The above discussion of SEI formation highlights the importance of simulation and modeling in this next phase of LIB design and evolution. The electrochemical reactions involved in these cells are complex, and greater understanding of the rate-limiting steps in fast charge transfer reactions is required. For many years, it has been assumed that Li ion diffusion limits charging/discharging rates^{Reference Jiang and Peng168}; however, recent reports are now suggesting that the charge transfer kinetics may also become limiting in cases where Li ion diffusion may be rapid.^{Reference Jiang, Hall, Song, Lau, Burr, Patterson, Wang, Ouyang and Lennon145,Reference Bai and Bazant169} This suggests that more advanced simulation models may be required to predict ionic and electronic flow though percolating porous networks and rates of charge transfer reactions.^{Reference Bai and Bazant169,Reference Yamada and Bandaru170}