Lithium-Ion Batteries with High Rate Capabilities - ACS Sustainable

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Lithium-Ion Batteries with High Rate Capabilities Ali Eftekhari*,†,‡ †

The Engineering Research Institute, Ulster University, Newtownabbey BT37 OQB, United Kingdom School of Chemistry and Chemical Engineering, Queen’s University Belfast, Stranmillis Road, Belfast BT9 5AG, United Kingdom

‡

ABSTRACT: Rate capability has always been an important factor in the design of lithium-ion batteries (LIBs), but recent commercial demands for fast charging LIBs have added to this importance. Although almost all works devoted to the LIB electrode materials examine the rate capability somehow, there are growing efforts in the quest for high rate capability LIBs. Because this is only possible by a subtle design at the material architecture to maximize both the ion and charge transfer processes simultaneously, the achievements can be applied to various materials. Owing to the fact that the term high rate capability is a relative concept in the literature, the present paper aims to provide a general picture of the achievements in a comparative manner to highlight the most promising results reported in the literature. KEYWORDS: Lithium-ion battery, Cathode materials, Anode materials, High rate capability, Fast charging

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INTRODUCTION Since the late 1970s, lithium-ion batteries (LIBs) have found a central place in solid-state electrochemistry, but the commercial introduction by Sony in 1991 made them popular energy storage devices. After 3 decades, LIBs have become a vital component in everyday life, as the public expectation is rising. When buying common electronic devices such as laptops, mobile phones, and so forth, people pay particular attention to the LIB performance. As a matter of fact, the battery performance is now one of the key features in the advertisement of the portable electronic device. Although the first focus was to increase the capacity of the LIBs to last longer, a demanding request is to speed up the charging process, as the consumers wish to have the electronic device more portable by reducing the time it is connected to the electricity resource. High capacity LIBs need both the anode and cathode materials, with high capacities for hosting Li-ions. On the anode side, the metallic Li anode, which could deliver the highest specific capacity, is not practical because of the safety issues. Therefore, both anode and cathode are made of intercalation materials with high capacities. The intercalation material should ideally have a rigid cage-like structure to host Li-ions, but for a high specific capacity, this cage-like skeleton should be lightweight. Hence two requirements are necessary for a high capacity electrode material: the material architecture should be able to host Li-ions as much as possible, and the skeleton should be made of lighter elements to keep it lightweight. The most common cathode material, LiCoO2 has a high theoretical capacity, but only half of it can be practically achieved because half of the Li-ion atoms are indeed part of the base architecture. This means full extraction of the Li-ions results in the structural breakdown, which is characterized by a phase transformation. On the other hand, the requirement for lighter elements has © 2017 American Chemical Society

two consequences; the choices are more limited, and the spaces for hosting the Li-ions within the lattice are lower because of shorter chemical bonds. This is the reason that small lattices cannot reversibly accommodate K-ions for designing an analog potassium-ion battery.1 On the other hand, most of the electroactive materials are reduced to the elemental form at the potentials close to that of Li/Li+, which is required for the anode performance. Therefore, the majority of novel anode materials are based on conversion mechanisms in which there is no rigid lattice structure during the charging discharge.2 Hence, these materials are subject to severe volume changes during the lithiation/delithiation, which is also accompanied by huge capacity fading. This is a serious obstacle for the practical development. In any case, the commercial LIBs based on the conventional electrode materials have somehow reached their maximum capacity of about 372 mAh g−1.3−6 The research attempts for increasing the LIB capacity is mainly the quest for finding novel electrode materials with higher specific capacities. On the other hand, the demand for improving the rate capability of the LIBs has recently attracted considerable attention, and there are still more rooms for improving the current electrode materials to deliver faster performance. There are several factors, which limit the rate capability of a LIB electrode materials: Solid-State Diffusion. Because the diffusion of Li-ions is much slower in the solid electroactive materials as compared with the electrolyte solution, the solid-state diffusion lengths should be minimized. For instance, the rate capability can be improved by reducing the particle size, but this approach is not Received: January 5, 2017 Revised: February 16, 2017 Published: March 10, 2017 2799

DOI: 10.1021/acssuschemeng.7b00046 ACS Sustainable Chem. Eng. 2017, 5, 2799−2816

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the SEI formation of (a) E-SnO2, (b) E-SnO2@C anode materials during lithiation, delithiation process and prolonged cycles. Reproduced with permission.137 Copyright 2015, Elsevier.

practical because of practical disadvantages. On the other hand, the diffusion coefficient should be increased by designing the most appropriate architecture providing spacious diffusion pathways. This includes both within the solid particles and between the particles within the electrode. A single crystal material can provide ordered channels for the diffusion within the solid, but long solid-state diffusion pathway has its own downside too. In fact, the architecture of the electrode should be subtly designed at both lattice and nano/micro scales. Electrical Conductivity. Diffusion is not the only ratedetermining factor in the redox system of the Li intercalation materials. The electron transfer between the redox sites and the current collector is usually an important issue because most of the available candidates for the LIB electrode materials have low electrical conductivity. The most common solution is to utilize a conductive agent to improve the interfacial connection and conductivity between the electroactive particles. On the other hand, the byproduct films formed on the electrode surfaces can be harmful not only for the cyclability but also rate capability because they slow down the diffusion process. Therefore, it is beneficial to have protective layers to avoid the direct contact of the electroactive material and the electrolyte species. This is the reason that carbon coating can both improve the electrical conductivity and the electroactive material protection. Figure 1 illustrates a schematic how a protective carbon layer can preserve the geometrical structure of a SnO2 anode material to control the geometry and size of the growing solid electrolyte interphase (SEI). In general, enhancing the rate capability of the LIB electrode materials needs a subtle design at the atomic scale to minimize the rate-determining obstacles rather than a case discovery. Because the concept of rate capability is relative (whereas some authors mean 50C by high-rate capability, others simply refer to 5C for the same material), and there is no standard for the definition of high rate capability LIBs, the present paper attempts to provide a general insight into the field by reviewing the current status and recent advancements. This can assist the researchers working on various parts of LIBs to plan better for enhancing the rate capability, which is indeed an important commercial demand. Here, the most promising materials, which can provide an overall insight into the concept of high rate charging/discharging are reviewed with a focus on the intercalation anode materials to gain their similarities with the

cathode materials to understand the requirements for fast charging better/discharging regardless of the operating voltage. The first part of the manuscript discusses the fundamental basics of rate capability and the factors affecting it. Then, available reports for the fast battery performance of common electrode materials (both anode and cathode) will be reviewed. This roughly should provide a general overview that which electroactive materials have the stronger capability for fast performance. Furthermore, reviewing the factors controlling the rate capability can assist researchers to design the material architecture for fast performing LIBs.

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FUNDAMENTALS OF RATE CAPABILITY C-Rate vs Applied Current. In electrochemistry, the galvanostatic rate is the applied current, which can be normalized with the active mass (A g−1), but the rate of charging/discharging is presented in C-rate. This notation, which has been borrowed from industry, subtly differentiate the rate capability from the capacity. Charging under that rate of 1C means that the electrode has been fully charged within 1 h, regardless of its capacity. The target capacity is usually the theoretical capability, which should be achievable. In many anode materials, the theoretical capacity is high (over 1 Ah g−1), but the reversible capacity achieved is far less than the theoretical capacity. Therefore, there is a tendency to report the applied current instead of C-rate (as can be seen in Table 1 and reset of the present manuscript). Because the theoretical capacity is actually an unattainable target, calculating the C-rate based on this value is not practically useful. Lattice Architecture. The rate-determining step in the lithiation/delithiation process is usually the Li diffusion, which is governed by the slow solid-state diffusion throughout the electroactive lattice. Therefore, shortening the solid-state diffusion length can increase the rate capability. Hypothetically, if depositing the electroactive material on the current collector with a thickness of one unit cell, the role of the solid-state diffusion is eliminated. However, even in this simplified case, the orientation of the unit cells plays a crucial role in the rate capability, as controls the accessibility of the Li redox sites. It has been recently discussed that the LIB performance of ordered mesoporous electroactive materials is controlled by the mesopore structure due to the ratio of the edge unit cells to the bulk lattice.7 2800


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ACS Sustainable Chemistry & Engineering Table 1. Specific Capacities of Various Electrode Materials at Low and High Charge/Discharge Rates Material

Synthesis route

LiNi0.5Mn0.5O2 (Li0.2Ni0.2Mn0.6)O2 LiNi0.2Co0.3Mn0.5O2 LiNi0.5Co0.2Mn0.3O2 Li1.2Ni0.2Mn0.6O2

Two-step hydrothermal

Li(Ni1/3Co1/3Mn1/3)O2 LiFePO4/C

Nonhydroxide Hydrothermal

LiFePO4/C LiFePO4/C

Liquid-phase Microwave assisted solvothermal Sol−gel Hydrothermal

[email protected] LiMn0.4Fe0.6PO4 LiFe0.5Mn0.5PO4/C LiMn0.8Fe0.2PO4/C LiMnPO4·Li3V2(PO4)3/C Li3V2(PO4)3/C (nitrogendoped graphene) Li3V2(PO4)3/C V2O5 V2O5 (carbon-supported) Carbon C

Hydroxide coprecipitation Hydrothermal

Coprecipitation Solid-state reaction in molten hydrocarbon

Rheological phase Self-assembly Hydrothermal

Ge/C G@Fe3O4 C/MnOx/C MnO2/rGO MoO2−C

Thermal reduction

MoO2 NiO ZnO TiNb2O7

Hydrothermal Hydrothermal Hydrothermal Hydrothermal

TiNb2O7/graphene TiO2 bronze

Direct dispersion Hydrothermal

TiO2 C/TiO2

Hydrothermal

TiO2/SnO2 Amorphous TiO2/N-doped Carbon TiOF2 Li4Ti5O12 Li4Ti5O12

Hydrothermal Hydrothermal-calcining

Li4Ti5O12 Li4Ti5O12 Li4−xTi5O12 Li4Ti5O12/N-doped Carbon Li4Ti5O12 Li4Ti5O12/C FeS2@C Li3VO4/graphene Na0.95V3O8 MoS2 entrapped carbon sheath

Morphology Nano/micro hierarchical microspheres Nanoparticles Na-doped Ni-rich Hierarchical nano/ microstructure Mesoporous Microspherical mesoporous Nanocomposites Macro-mesoporous

Low rate

Hydrothermal Hydrothermal Sol−gel

Extremely high rate

ref.

152 @ 10C

25

238 136 163 240

183 @ 10C 89 @ 5C 121 @ 10C 186 @ 5C

39 138 139 40

@ @ @ @

1C 0.5C 1C 1C

60 @ 50C

195 @ 1C 140 @ 1C

175 @ 10C 86 @ 20C

140 50

143 @ 1C 129 @ 2C

104 @ 10C 111 @ 10C

141 55

164 @ 0.1C 155 @ 1C 157 @ 1C

127 @ 10C 129 @ 20C 115 @ 10C

56 65 142

161 @ 0.0C

113 @ 10C 101 @ 16C

192 @ 0.1C

161 @ 10C

Macroporous Nanowires Nanosheet-assembled Nanospheres Macro-mesoporous hollow

154 @ 1C 273 @ 0.1 A g−1 860 @ 0.5 A g−1

120 @ 10C 161 @ 2 A g−1 554 @ 3 A g−1

530 @ 2.5 A g−1

180 @ 60 A g−1

Nanoclusters Nanoparticles

1442 @ 0.1C 771 @ 0.2 A g−1 600 @ 0.5 A g−1 763 @ 2 A g−1 (50th cycle) 150 @ 0.5C 969 @ 0.5C 365 @ 1 A g−1 235 @ 1C

1122 @ 100C 392 @ 12C 444 @ 1 A g−1 168 @ 5 A g−1 573 @ 2 A g−1

640 @ 400C 118 @ 35C

150 151 152 153 154

129 606 230 138

129 @ 30C

155 118 156 157

Interconnected nanofibrous Nanotubular Nanosheet-assembled shells Beads

235 @ 0.2C

148 @ 20C

180 @ 100C 66 @ 100C

158 159

267 @ C/10 204 @ 1C

176 @ 10C 105 @ 20C

147 @ 25C

160 94

437 @ 1C 290 @ 1C

120 @ 10C 156 @ 10C

Nanocubes Nanosized Mesoporous

200 @ 1C 166 @ 1C 176 @ 0.2C

120 @ 20C 142 @ 10C 139 @ 50C

148 @ 0.2C 193 @ 1C 159 @ 1C

181 141 148 146 163

@ @ @ @ @

20C 10C 20C 10C 20C

148 521 247 117 563

@ @ @ @ @

30C 10 A g−1 8 A g−1 10C 10 A g−1

Nanocrystals Nanopowders Macro/nano hierarchical porous Nanorod-Nanoflake

Hollow spheres

Nanocone Hierarchical Hierarchical microspheres

Bamboo-slip Flower-like Crystalline particles Highly crystalline Well-aligned nanosheet arrays Hydrothermal/solid-state Porous nanowires Pomegranate-like Nanobelts Hybrid hollow spheres

2801

83 @ 50C

889 471 300 739

@ @ @ @

0.1 A g−1 0.2 A g−1 0.3C 3 A g−1

@ @ @ @

30C 10C 10 A g−1 10C

143 144 69

70 @ 50C

100 @ 50C

Carbothermal

Hydrothermal Hydrothermal Molten salt assisted selfassembly Hydrothermal Hydrothermal Ion exchange CVD Hydrothermal

High rate

182 @ 1C

145 146 147 148 149

161 162

123 @ 20C

141 @ 100C 60 @ 100C

78 @ 200C 125 @ 90C 142 @ 32 A g−1

97 163 164 165 136 112 166 106 116 167 121 168 123


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Figure 2. Schematic diagrams of (a) OMC-CP and (e) OMC-IP. TEM images of (b and c) OMC-CP and (f and g) OMC-IP, viewed from (b and f) the (110) and (c and g) the (001) directions. Discharge capacity over cycling of (d) OMC-IP and (i) OMC-CP electrodes under different current densities varying from 100 to 12 500 mA g−1. Reproduced with permission.12 Copyright 2016, Royal Society of Chemistry.

involvement of an addition mechanism of the Li storage through Li−C−H bond and metallic lithium clusters in the OMC microcavities.6,14,16 In the case of the OMC-CP, the mesoporosity is ordered in a single particle, but in the electrode structure where the particles are closely packed, the mesopores are not necessarily accessible in the right direction. The better rate capability of the OMC-IP can be attributed to the interconnection of the mesopores, as they are not limited to one entrance, and the electrolyte and Li ions therein can reach the mesopores from various points. For 1D nanomaterials, Tang et al. showed that the aspect ratio plays a particular role in the rate capability.17 For conversion-based materials (vs intercalation materials in which the lattice structure has empty sites for the Li accommodation), such mesoporous structures can provide a host matrix in which the electroactive material can be fairly spread.11 In this case, there is enough space to handle the volume expansion. For instance, Li cannot guarantee diffusion into/from the germanium via a conventional solid-state diffusion, and the anode material loses its capacity after a few cycles. However, Ge nanoparticles distributed within an OMC can maintain a high capacity over hundreds of cycles.18 Pseudocapacitance vs Battery Performance. Owing to the practical potentials and market demands for supercapacitors, pseudocapacitive behavior has attracted considerable practical interest during the past decade. The mechanism of pseudocapacitance is somehow similar to the redox systems of battery materials but normally undergoes a different pathway. Although the redox sites in an ideal battery material participate in the electrochemical reaction at a specific potential (i.e., the formal redox potential), there are redox sites with various reactivities in a pseudocapacitor. Because the redox sites with different energies are spread across the electroactive material in the latter case, there is less competition between the electroactive species to participate in the redox reaction at each potential. Therefore, pseudocapacitive behavior has a higher rate capability in comparison with battery performance. This is indeed the characteristic feature of supercapacitors. Another key difference is that battery performance is mainly conducted by solid-state diffusion, but the pseudocapacitive

The lattice orientation, as will be discussed in details later, can be well understood in the electroactive materials such as LiFePO4 in which the diffusion channels are along a specific direction.8 In these cases, preparing thin sheets of the electroactive material with the preferred crystal orientation can expose the open channels at the solid/electrolyte interface where the solid-state diffusion is commenced, and thus high rate capability can be achieved.9 In the complicated structure of the whole electrode composed of the electroactive material, binder, and other additives, which are closely packed, the accessibility of these channels is substantially controlled by the material morphology. Effect of Morphology. The electrode material structure has a huge impact on the rate capability (if not the most important factor). The reason is that the material porosity provides an opportunity for the Li ions to diffuse quickly within the electrode to reach the particle/electrolyte interface, and thus the slow solid-state diffusion is minimized. However, it is not simply a matter of the material porosity but also how the pores are interconnected and if the corresponding interconnections provide shortcuts for the Li diffusion. It also should be taken into account that reaching the solid electroactive material is not the only controllable factor but also the lattice direction which is exposed to the electrolyte. For instance, in layered electrode materials, the Li-ions should reach the interlayer direction rather than perpendicular to the basal plane. Chen and co-workers have recently proposed a rational overview for designing nanostructured electroactive materials in favor of fast battery performance.10 The impact of the morphology can be well understood in a comparative study of ordered mesoporous carbons (OMCs),11 which provide similar mesopores for the diffusion within the electrolyte.11 Figure 2 compares the rate capability of two OMCs with the channel-like pore (OMC-CP) and interconnected pore (OMC-IP) prepared by different hard templates. Owing to the identical synthesis route followed, both OMCs have similar microcrystalline structure and surface chemistry.12 The first interesting point is that the high-level porosity has caused achieving a specific capacity higher than that of graphite (i.e., 372 mAh g−1)6,13−15 due to the 2802


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Figure 3. Galvanostatic lithiation of (a) commercial and (c) flower-like Li4Ti5O12 electrodes, and the corresponding delithiation profiles (b and d, respectively). Reproduced with permission.136 Copyright 2015, Wiley-VCH.

to participate in the electrochemical reaction with minimum energy. However, this general statement strongly depends on the initial battery performance, as can be negligible for the materials originally delivering flat plateau at slow rates. Although many reports in the literature showing the transformation to pseudocapacitive behavior upon increasing the charge/discharge rate or cycling due to some structural changes (see for example, 21−23), Figure 3 shows the battery performance of Li4Ti5O12 remains intact by cycling, though a significant portion of the capacity is fading. Although some redox sites are not electrochemically accessible, this has no impact on the other active sites. Of course, it is merely a statistical statement, as the Li diffusion pathways are subject to change. On the other hand, the battery performance inclines to a pseudocapacitive behavior, but considering the potential window, it is still excellent battery performance. In conclusion, because of the natural tendency of the battery performance toward pseudocapacitive behavior, high-rate electrode material should deliver an ideally flat plateau at low rates to survive this essential performance transformation at high rates.

behavior is mostly dominated by surface and subsurface reactions. Note that reducing the ratio of solid-state diffusion to the surface reaction by reducing the particle size does not induce a pseudocapacitive behavior for a battery material, as the main requirement is the existence of a set of different redox sites.19 On the other hand, a pseudocapacitive behavior is not limited to the surface reaction and bulk solid also significantly participate in the electrochemical reaction, as can be judged by the high values reported for the specific capacitance of novel pseudocapacitors, which are comparable with the specific capacities of similar battery materials. The importance of pseudocapacitance in batteries is during the high rate charge/discharge where the battery performance tends toward a pseudocapacitive behavior. As a matter of fact, many high rate battery materials introduced in the literature are indeed pseudocapacitors, as can be judged by the diagonal charge/discharge curves instead of the characteristic flat plateaus of the batteries. Linking these materials can provide a better opportunity for designing high rate LIBs and pseudocapacitors. Some authors have attributed the high rate capability of novel battery materials to the associated pseudocapacitive behavior.20 In general, differentiating the mechanisms of the battery performance and pseudocapacitive behavior at an atomic scale can assist us in better designing the high rate batteries. In a rational sense, battery performance tends to a pseudocapacitive behavior by increasing the charge/discharge rate, as the slow reacting redox sites do not have enough time

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ELECTRODE MATERIALS For designing the high-rate batteries, it is necessary to have both cathode and anode materials with high rate capabilities. Although there are considerable overlaps between the anode and cathode materials, the mechanisms of the lithiation/ delithiation are substantially different. On the other hand, the problems associated with the electrode performance are 2803


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be achieved at higher discharge rates. As a result, a specific capacity of 90 mAh g−1 was obtained at 50C. Li et al. synthesized large sheets of LiNi1/3Co1/3Mn1/3O2 by utilizing carbon nanotubes as a sort of scaffold.30 The resulting cathode could deliver a specific capacity of 137.7 mAh g−1 at 20C. They ascribed this performance to the uniform porosity formed throughout the sheet, which made the sheet accessible by the Li+ diffusion in perpendicular directions toward the sheets. The family of xLiMO2·(1−x)Li2MnO3 (M = Ni, Co, Mn, etc.) has been recently considered as a promising potential because of the high theoretical capacity at the level of 250 mAh g − 1 . 3 1 − 3 5 0.5LiNi 0 . 5 Mn 0 . 5 O 2 ·0.5Li 2 MnO 3 , a.k.a., Li(Li0.2Ni0.2Mn0.6)O2 is one of the best choices because of its inexpensive composition and excellent electrochemical performance.36−38 Zhang et al. employed a two-step hydrothermal synthesis for the formation of carbon sphere templates that helped the growth of the electroactive material while providing the electrical conductivity required for the charge transfer within the cathode.39 With this architecture, they were able to achieve a specific capacity of 182.7 mAh g−1 at 10C. Li et al. designed a hierarchical nano/microspherical architecture for Li1.2Ni0.2Mn0.6O2, and employed polyvinylpyrrolidone (PVP) as a structure-directing agent during a hydrothermal synthesis to activate the Li2MnO3 component.40 Although the cathode was able to deliver a capacity of 283 mAh g−1 at 0.1C, the capacity dropped to 123 mAh g−1 by increasing the discharge rate to 10C. LiFePO4. Lithium iron phosphate is the flagship of the olivine family of lithium transition metal phosphates, which attracted considerable attention since the first report of electrochemical performance.8 At first glance, LiFePO4 seems to be a less practical cathode material in comparison with conventional candidates due to its poor electrical conductivity and slow Li diffusion coefficient. Therefore, considerable battery performance of LiFePO4 can only be achieved whether at an elevated temperature41 or in a composite with carbon conductive agents.8 At higher temperatures, the diffusion is obviously faster; but, the capacity fading is consequently more severe due to the material dissolution within the electrolyte. As a result, LiFePO4/C is indeed the cathode material, which is considered for the practical development.42−49 Recent reports on LiFePO4/C cathode materials show high capacity and rate capability.8 In most cases, the specific capacity achieved is close to the theoretical specific capacity of LiFePO4 (i.e., 170 mAh g−1). On the other hand, the rate capability is comparable with that of LiCoO2 if not better. The best design for the preparation of LiFePO4/C nanocomposite is to uniformly cover the LiFePO4 particles with a thin layer of carbon. In this form, the electrical conductivity between the particles is improved while the carbon coating protects the electroactive material in participating in side-reactions.50−54 However, achieving a practical uniformity of the carbon coating is not easy, and the permeability of the carbon layer is the key favor in the electrochemical performance as it should not block the Li diffusion from the electrolyte to the electroactive particle. Zhang et al. designed a macro−mesoporous hierarchical architecture for the LiFePO4/C to achieve a specific capacity of 111 mAh g−1 at 10C.55 However, the only factor is not the electrochemical accessibility of the particles or electrical conductivity between them. Another important factor is the charge transfer between the composite components as a result of chemical interactions. Feng and Wang doped the carbon

different at positive potentials and potentials close to that of Li/ Li+. Table 1 summarizes common electrode materials, which have shown high rate capability in the LIB performance. The cathode materials are categorized on the top and the anode materials on the bottom of the table. However, the purpose is to highlight the capability for fast lithiation/delithiation process regardless of the redox potential. Among these high-rate electrode materials, only a few have shown the capability of charging/discharging at extremely high rates (within a minute or two). Cathode Materials. Cathode materials are typically based on an intercalation mechanism in which the Li intercalation/ deintercalation results in the reversible change of the oxidation valence of a central transition metal such as Co, Mn, Ni, etc. Almost no considerable cathode material is based on a conversion mechanism. Se can provide a conversion-based redox with Li as a cathode material, but the corresponding system is considered as a lithium−selenium battery.24 Layered Lithium Metal Oxides. Layered metal oxides are among the oldest candidates for the LIBs. The simplicity and plain 2D horizontal diffusion channels make them potential cathode materials, but this also leads to the disadvantage of feeble lattice structure resulting in poor cyclability. The first choices for replacing the lithium cobalt oxide are similar manganese or nickel oxides because Mn and Ni are cheaper and less toxic than Co. As a combination, LiNi0.5Mn0.5O2 has been long examined as a potential cathode material because the presence of different metal atoms having various sizes and charge densities can improve the battery performance (i.e., a variation of doping). Li et al. synthesized hollow microspheres of LiNi0.5Mn0.5O2 by a solid-state route and reported excellent battery performance with a specific capacity of 152 mAh g−1 at 10C.25 They attributed this performance to the nano/micro hierarchical architecture, as the solid-state diffusion lengths are reduced by increasing the contact area between the electrode and the electrolyte. As an alternative to the common cathode material of LiCoO2, a mixed metal oxide can reduce the Co content, which is toxic and expensive. LiNi1/3Co1/3Mn1/3O2 has been a promising candidate for a long time. Having a typical rhombohedral structure, the valences of the transition metals are as Ni 2+ , Co 3+ , and Mn 4+ . The Li intercalation/ deintercalation is based on the redox systems of Ni2+/Ni4+ and Co3+/Co4+ when the valence of Mn4+ remains intact.26 The permanent presence of the Mn4+ stabilizes the lattice structure while avoiding the common Jahn−Teller effect associated with the Mn-based cathode materials. Many researchers attempted to overcome the common problems of this promising cathode material by improving its rate capability and cyclability. A general approach is to reduce the particle sizes, as the solidstate diffusion lengths are reduced but the interfacial resistance between the individual particles is not in favor of better battery performance. As an alternative, microspheres with hierarchical nanostructures of LiNi1/3Co1/3Mn1/3O2 showed promising performance in the absence of interfacial resistance usually forming as a result of mechanical packing of nanoparticles.27,28 Chen et al. used a carbon gel combustion process to prepare highly crystalline LiNi1/3Co1/3Mn1/3O2 nanoparticles, as the vigorous flow of gas generated during the pyrolysis restricts the growth mechanism in favor of the formation of small separated nanoparticles.29 Having individually separated nanoparticles, the solid-state diffusion can be completed within a shorter period of time, and thus acceptable battery performance should 2804


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Figure 4. TEM (a,c,e,g,i), HRTEM images (b,d,f,h,j), and their corresponding FFT of a series of LiFePO4/C materials (marked as H, C-1, C-2, C-3, and G-15). LiFePO4 is rod-like and one short axis is along (010), favoring the Li+ diffusion. Note different thicknesses of carbon layers and carbon distribution. (k) Rate performance of samples. Reproduced with permission.60 Copyright 2015, American Chemical Society.

Johnson et al. investigated the influence of V doping on the battery performance of LiFePO4/C.61 The optimum values for the best capacity and rate capability were noticeably different. This is worth considering that the mechanism for enhancing the capacity can be different from that of rate capability, though they are usually similar. In addition to carbon coating, substituting Fe with similar transition metal can significantly improve the electrical conductivity of LiFePO4 and subsequently its battery performance. Mn is a good choice for this approach, as LiMnPO4 itself is a promising candidate as a LIB cathode material, though its performance is affected by the Jahn−Teller effect (similar to other Mn-based cathode materials such as LiMn2O4).62,63 Mi et al. synthesized LiFe0.6Mn0.4PO4 microspheres and loaded carbon nanotubes.64 The resulting cathode could deliver a capacity of 114 mAh g−1 at 20C and 64 mAh g−1 at 50C. Zou et al. coated the same material by 2 wt % carbon during a hydrothermal synthesis, and the capacity was noticeably better to be 128 mAh g−1 at 20C.65 Similar performance has been reported for LiMn0.5Fe0.5PO4.66 It should be taken into account that Fe and Mn redox couples are significantly different and forming a mixed-metal compound is accompanied by sacrificing the well-defined redox system of LiFePO4 delivering flat plateaus during the charging/

layer with boron to make it more electrochemically permeable.56 This is indeed an important point as the regular sp2 hybrid of carbon is electrochemically inactive, and it is necessary to alter the sp2 hybrid to facilitate the charge transfer.57,58 The [email protected] cathode delivered a specific capacity of 126.8 mAh g−1 at 10C. In general, doping of the carbon coating with various elements can improve the charge and ion transport due to the electron and hole-type carries provided by the dopants.59 Tian et al. synthesized the LiFePO4/C in a controllable CVD route by employing glucose as the carbon precursor.60 Quite impressively, the rate capability was substantially improved as the cathode was able to deliver a considerable capacity by discharging at 250C (14.4s). At the discharge rate of 150C, the specific capacity was fairly over 100 mAh g−1, which is comparable with many low-rate cathode materials. Figure 4 shows the microscopic images of the different LiFePO4/C samples prepared by CVD. Although all samples deliver reasonable performances, there is a dependency of the battery performance on the particle shape and thickness of the carbon coating. Doping can alter the electronic structure of the LiFePO4 lattice in favor of better electrical and ionic conductivity. This can improve both the practical capacity and the rate capability. 2805


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Figure 5. CV curves of a series of LiMnxFe1−xPO4 electrodes at a scan rate of 0.1 mV s−1. Reproduced with permission.66 Copyright 2016, Royal Society of Chemistry.

alternative, NaTi2(PO4)3 can provide a wider diffusion channels because of the larger size of Na atoms.74,75 Neither LiTi2(PO4)3 nor NaTi2(PO4)3 can deliver a noticeably high capacity as compared with similar counterparts, but the rate capability is considerable. The capacity fading of a NaTi2(PO4)3 electrode is less than 40% when increasing the discharge rate from 2.57C to 100C.76 Of course, this performance is much better for the Na intercalation/deintercalation in a sodium-ion battery. This excellent rate capability can provide a useful insight into the solid-state chemistry of high rate LIBs. Vanadium Pentoxide. V2O5 has a theoretical capacity of 294 mAh g−1 for the intercalation of 2 Li-ions,77 but in practice, only one Li-ion is reversibly intercalated/deintercalated to achieve a specific capacity of about 150 mAh g−1, which is comparable with other candidates. The inner porosity has a considerable impact on the battery performance of V2O5. 3D interconnected nanonetworks showed about 40% better performance at high discharge rates (e.g., 2 A g−1) whereas the specific capacity was at the level of conventional V2O5 at low discharge rates such as 0.1 A g−1.78 Zhang et al. designed a special architecture to form hollow microclew of V2O5.79 The cathode delivered a specific capacity of 95 mAh g−1 at a high discharge rate of 65C. Vanadium Disulfide. Transition metal dichalcogenides (TMDCs) are usually considered as potential anode materials due to their low redox potentials, but VS2 has a positive enough redox potential to be examined as a cathode material. Like other TMDCs, VS2 has a layered structure in which the Li storage capacity depends on the interlayer diffusion of the Li+, which is considered as a slow solid-state diffusion. Exfoliating the VS2 layers can facilitate the diffusion but the Li-ions should

discharging. Mn redox couple occurs at a slightly higher potential, which is more favorable, but the shape is not as welldefined as that of LiFePO4 (Figure 5). As a result, the inbetween olivine mixed metal phosphate delivers two-plateau electrochemical performance. The same phenomenon occurs when designing mixed metal compounds for the cathode or anode. Li3V2(PO3)3. In comparison with olivine-type lithium metal phosphates, NASICON-type Li3V2(PO4)3 has some noticeable advantages: its theoretical capacity is slightly higher (i.e., 197 mAh g−1) and have a better ionic conductivity. However, the poor electronic conductivity is still a major issue. Liu et al. prepared a nanocomposite of Li3V2(PO4)3/graphene, which could deliver a specific capacity of 82 mAh g−1 at 50C. Xiong et al. reported that only 10% capacity fading occurs in the Li3V2(PO4)3/C nanocomposite when increasing the rate from 0.5C to 10C.67 Owing to the fact that substituting some carbon atoms by a similar element can delocalize the charge distribution over the graphene sp2 carbon atoms,58 Ren et al. employed a nitrogendoped graphene to improve the rate capability of Li3V2(PO4)3, but the results were similar with a specific capacity of 86.9 mAh g−1 at 40C.68 Of course, the cyclability was quite good, as the electrode maintained this value over 800 cycles. Similar results have also been reported by other authors for improving the rate capability of Li3V2(PO4)3 by N-doped graphene.69 Nevertheless, the rate capability was still limited by the solid-state diffusion. Similar to lithium vanadium phosphate, NASICON-type LiTi2(PO4)3 is a promising candidate, though its solid-state chemistry of Li intercalation is still subject to debate.70−73 As an 2806


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Figure 6. FESEM images of the Fe3O4 electrodes on Cu foil and after 0, 150, and 300 discharge/charge cycles; and the corresponding galvanostatic profiles for (A−C) Fe3O4 octahedrons, (D−F) Fe3O4 cuboctahedrons, (G−I) Fe3O4 cubes. Reproduced with permission.89 Copyright 2016, Elsevier.

performance LIBs. Although numerous high capacity anode materials are now promising candidates, carbon-based materials are still under heavy investigation. In this regard, the primary focus is on improving the rate capability. As pointed out earlier about OMC, novel carbon nanostructures can provide an opportunity for achieving high capacities but in most cases, they are at the same level of the graphite capacity. Graphene as an attractive material has also been examined as a potential anode material, but the Coulombic efficiency is poor even at slow rates. Nevertheless, this is not a general behavior, as various types of architectures can be built by graphene building blocks. It should be taken into account that the Li intercalation occurs between the graphene layers in the graphite structure rather than intercalation into a 3D lattice. Therefore, the Li storage capacity of graphene is directly related to its architecture at the atomic scale. Wu et al. utilized doped graphene sheets, which could have a high specific capacity of 1040 mAh g−1 at slow rates (e.g., about 0.25C).81 Altering the regular sp2 hybrid of the carbon atoms in the basal plane by dopants induces a chemical reactivity to facilitate the Li storage on the carbon atoms rather than between the sheets.58 This is the reason for the extremely high specific capacity reported. Owing to the minimum role of solidstate diffusion within the material lattice, the Li reaction at the graphene surface can be conducted fast. As a result, the capacities for the N- and B-doped graphene samples were 199 and 235 mAh g−1 at an extremely high charge/discharge rate of 120C. Mukherjee et al. synthesized a photochemically reduced graphene, which could deliver a specific capacity of about 100 mAh g−1 at 100C.82 This suggests that carbonaceous materials have the possibility of charging/discharging at high rates. Kakunuri. prepared fractal-like carbon nanoparticles from candle soot.83 At the 10C rate, the initial specific capacity was

be stored on the basal planes. Immobilizing VS2 on functionalized graphene can provide a new architecture for the Li storage. Whereas the Li ions have enough time to complete the solid-state diffusion, no difference can be observed between the battery performance of VS2 and VS2/ graphene nanocomposites, but a noticeable difference appears at discharge rates above 0.4C reaching to 20% at 20C.80 In any case, even the pristine VS2 delivers a specific capacity over 100 mAh g−1 at 10C, which is better than the performance of many available cathode materials. Anode Materials. The conventional graphite anode is based on an intercalation mechanism, but it is different from the common intercalation mechanism of the cathode materials. Graphite intercalation compounds (GICs) have been wellknown materials for decades, long before the birth of LIBs. Instead of incorporation into the lattice structure, Li ions are accommodated between the graphene layers in the graphite structure via van der Waals forces. There are also a few intercalation anode materials such as Li4Ti5O12 as analogs to the conventional cathode materials. These intercalation anode materials follow the same mechanism similar to the cathode materials but with a redox system associated with a central transition metal at much lower potentials, though, still significantly higher than that of Li/Li+. The most common anode materials with the capability of delivering a potential close to that of Li/Li+ are based on conversion mechanism because at such reductive potentials, the reactive materials are usually in the elemental form. Various classes of anode materials have been recently categorized based on the operating potential,2 and the present focus is merely on the rate capability rather than the anode potential. Carbon. The dominant commercial anode material of LIBs is still graphite, though it has severe limitations for high2807


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ance.92,93 Thermal treatment obviously enhances the crystallinity and improves the battery performance, but a nanostructure increases the number of surface active sites to conduct the hydrogen storage via a pseudocapacitive mechanism.94 Jin et al. achieved a composition of Li0.61TiO2, and the electrode could reach a capacity of 105 mAh g−1 at 20C.94 In this case, the diffusion channels within the lattice structure are the key factors in controlling the battery performance and rate capability. Xiu et al. employed a metal−organic framework (MOF) as the precursor for the synthesis of TiO2 because the MOF leaves a porosity at the lattice scale.95 The result was considerable, as the TiO2 anode could deliver a specific capacity of 127 mAh g−1 at 10C after 1100 cycles. Although this capacity is not high in comparison with the common anode materials, this performance is quite good as an intercalation material with a rigid lattice skeleton. Regardless of the cell voltage, which is suitable for an anode, the performance of TiO2 electrode is indeed similar to those of intercalation cathode materials. The effective impact of doping for improving the battery performance of various electroactive materials was discussed above, but in the case of TiO2, even self-doping with Ti3+ can have a significant effect on the enhancement of the rate capability. Although the capacities of TiO2 and self-doped TiO2 (a.k.a., dark TiO2) are almost the same at low discharge rates, the capacity of Ti3+-doped TiO2 is 2 times higher than that of the original TiO2 at 20 C, i.e., 112 mAh g−1.96 At higher rates where the original TiO2 does not deliver any noticeable capacity, the self-doped TiO2 still can be a practical electrode by delivering 94 and 77 mAh g−1 at 50C and 80C, respectively. This high rate capability of the self-doped TiO2 is attributed to the enhanced electrical conductivity mediated by the Ti3+ ions. Altering the TiO2 lattice structure by an additional element can also be beneficial to improve the battery performance and rate capability. Li et al. reported a specific capacity of 120 mAh g−1 at 20C for TiOF2 nanocubes.97 Of course, because the redox potential of TiOF2 is slightly higher than that of TiO2, there is a possibility for employing TiOF2 as a cathode material. In any case, our focus is on the rate capability for fast charging/ discharging by accommodating/releasing the Li ions. Hong et al. introduced Li2ZnTiO3O8 spinel as a new member of this family.98 Creating pores within Li2ZnTiO3O8 nanoflakes improved the rate capability to achieve a specific capacity of about 100 mAh g−1 at 2 A g−1.99 Although the initial capacity of a similar Li2CoTiO3O8 was smaller, it was subject to gradual increase in the course of cycling. This suggests that this family of lithium titanium oxide spinels has a considerable potential for LIBs with high rate capability. A La2O3-coated Li2ZnTi3O8 showed a specific capacity of 188.6 mAh g−1 at 2 A g−1 (about 10C).100 Li et al. demonstrated that not only the electrode/electrolyte interface and the interfacial structure within the electrode (between the electroactive particles) are of utmost importance in the battery performance, but also the current collector/ electroactive material also plays a crucial role in providing the required network for the electron transport.101 They mediated the electron transport between a Li2ZnTi3O8 film and Cu current collector by graphene and Au nanoparticles. Li4Ti5O12. Lithium titanium oxide spinel is one of the most promising cathode materials because of its fast Li intercalation/ deintercalation kinetics occurring at a relatively constant potential. However, its intrinsic low electrical conductivity has limited its rate capability. The synthesis route has a huge impact on the battery performance of Li4Ti5O12. The conventional

still higher than the theoretical capacity of graphene but rapidly fading during the first 100 cycles. However, a specific capacity of about 150 mAh g−1 could be maintained in a long-term cycling. All these reports indicate that low-cost carbon anode materials with high rate capability can be considered as an alternative to graphite by a subtle design of the Li diffusion channels and increase of the reactive sites for the corresponding Li redox. Fe3O4. Owing to a high theoretical capacity of 924 mAh g−1 and low cost, Fe3O4 has been named as a promising candidate as a LIB anode material.84−86 The redox system for the Li storage is based on the following reaction: Fe3O4 + 8Li+ + 8e− ↔ 3Fe + 4Li 2O

(1)

Because of the different structure of the anode in the charged and discharged forms, it is evident that the electrode undergoes a massive volume change in the course of charging/discharging. Various high surface area carbonaceous nanomaterials have been employed to build Fe3O4 nanocomposites in which there is enough space for accommodating the intercalating Li-ions while improving the electrical conductivity. Yu et al. embedded the Fe3O4 particles within a 3D microcarbonaceous matrix made of cellulose acetate. With a total 5.1 wt % carbon, the 3D Fe3O4/C anode delivered a capacity of 790 mAh g−1 at a 2C rate after 1000 cycles.87 With a similar architecture utilizing graphene aerogel, He et al. reported a specific capacity of 634 mAh g−1 at 7C, and the electrode could fairly maintain its capacity over long-term cycling over 1000 cycles.88 They attributed this excellent performance to the role of the graphene aerogel employed because similar electrodes prepared by other carbonaceous materials could not deliver any noticeable capacity with a discharge rate faster than 5C. The morphology of the individual particles plays a crucial role in the battery performance, because not only the particle interface with the electrolyte is a barrier for the Li diffusion but also this shapes the overall connections of the particles within the electrode material. Ding et al. synthesized Fe3O4 with several geometrical shapes and compared their battery performance.89 Figure 6 shows three Fe3O4 with different geometrical shapes. In addition to the different specific capacities recorded, the interesting point is the morphological change upon cycling. Although the initial electrodes are similar (despite the differences in the geometrical shape of the individual particles), the structural changes are massively different. This suggests that the interfacial connections of the individual particles have an enormous impact on the solid-state diffusion. In some cases, the particles tend to merge into each other to form a concrete film. Ti-Based Intercalation Materials. Titanium oxide has been one of the oldest candidates for the LIB anode delivering a solid battery performance comparable with the cathode intercalation materials. On the contrary to other common anode materials, it does not undergo a chemical transformation, and the Li-ions are literally intercalated into the TiO2 lattice structure. The theoretical capacity of TiO2 is 336 mAh g−1 according to the following reaction:90 TiO2 + x Li + x e− ↔ LixTiO2 (x ≤ 1)

(2)

In practice, only half of the lithium can be intercalated to achieve a specific capacity of 168 mAh g−1.91 Further intercalation of Li ions is normally conducted through a pseudocapacitive mechanism rather than battery perform2808


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graphene structure and casting architecture. For instance, Xue et al. reported a specific capacity of 126 mAh g−1 for Li4Ti5O12/ graphene composite at 20C.104 As an alternative method, it is possible to form Li4Ti5O12 nano-objects such as nanotubes, nanowires, nanosheets, and so forth on a titanium substrate. The resulting electrode can have a high rate capability to deliver a specific capacity of over 100 mAh g−1 at 100C.105−108 The mesoporous structure can provide an appropriate architecture for supplying the electroactive species at the electroactive material/electrolyte interface. Because the electrolyte fills the mesopores and the Li+ diffusion within the electrolyte is much faster than the solid-state diffusion, Li ions can always be ready for entering into the solid materials at the corresponding interface. However, this hypothetical condition is for the ordered mesopores directly exposed to the electrolyte. In practice, mesopores are within the particles, which are closely packed. As a result, not all the mesopores are electrochemically accessible or in the right direction. Hui et al. prepared mesoporous Li4Ti5O12 nanoparticles by a microwave-assisted hydrothermal route.109 The capacity was 123 mAh g−1 at 40C, and the anode could deliver at least 90 mAh g−1 during 1 min charging/discharging. Li et al. reported that a sponge-like nanostructured Li4Ti5O12 electrode could deliver a specific capacity of 161.2 mAh g−1.110 In fact, there was only negligible capacity fading when increasing the rate from 1C to 20C. Unfortunately, the battery performance was not reported for higher rates, but it seems that the electrode could deliver an acceptable capacity during a fewsecond charging/discharging. In comparison with other morphologies, the authors concluded that this excellent performance is because of the nanoparticles in the spongelike structure, which are electrochemically accessible to shorten the solid-state diffusion length. Because a rate-determining step in the Li intercalation/ deintercalation into Li4Ti5O12 spinel is the Li migration between the 16c octahedral and 8c tetrahedral sites within the lattice,111 Cheng et al. synthesized Li4−xTi5O12 by an ionexchange approach.112 They reported that when x < 3, the Lideficient Li4−xTi5O12 still preserves its spinel structure, then the electrochemical intercalation can result in a better reversible intercalation/deintercalation. As a result, the electroactive material could deliver a specific capacity of 148 mAh g−1 at 20C. Copper-doped Li4Ti5O12 electrode exhibited a capacity of 144 mAh g−1 at 30C, though increasing the Cu dopant (over 0.05 in the stoichiometric formula) significantly decreased the specific capacity.113 However, doping is not limited to the

solid-state reactions result in the agglomeration of the smaller particles, and thus the rate capability will be poor. Obviously, a more electrochemically accessible morphology shortens the solid-state diffusion lengths and can improve the rate capability. Cao et al. showed that mesoporous hollow microspheres of Li4Ti5O12 could have a good rate capability up to 20C where the capacity gradually decreases by increasing the charge/ discharge rate.102 The interesting point is that the capacity suddenly decreases when increasing the rate from 20C to 40C. This is indeed the rate capability limit in which the applied rate is not simply a factor of speed, as the mechanism of the battery performance is somehow changing. This change in the mechanism refers to the dominance of the diffusion processes at different boundaries. The common approach is to utilize conductive agents such as carbon. Moreover, the presence of carbon conductive agent can reduce the difference between the charge and discharge potentials (Figure 7). This difference which is characterized

Figure 7. Voltage difference (ΔE) between charge plateau and discharge plateau of pristine Li4Ti5O12 and Li4Ti5O12 nanosheet/CNT composites: (a) 0, (b) 4, (c) 7, and (d) 10 wt % CNTs. Reproduced with permission.135 Copyright 2015, Elsevier.

by peak-to-peak separation in cyclic voltammetry is indeed the overpotentials in respect with the redox formal potential. Compositing Li4Ti5O12 with graphene significantly improves the capacity and rate capability, but even with the optimum ratio of graphene in a comparative study, the specific capacity was below 120 mAh g−1 at 10C.103 Because graphene is not a standard material, the performance is highly dependent on the

Figure 8. Electrochemical performance and in situ EIS study of the binder-free Li4Ti5O12/RGO samples. (a) Rate performance of the RGO, Li4Ti5O12, Li4Ti5O12/RGO hybrids; (b) discharge curves at different current rates from C/5 to 90C; (c) the first 10 consecutive cycles of CV curves at a scan rate of 0.1 mV s−1. Reproduced with permission.115 Copyright 2016, Wiley-VCH. 2809


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push the layers to find a space in-between. As a matter of fact, vigorous intercalation of Li compounds is used as the liquid phase exfoliation method for the preparation of MoS2 monolayers. The battery performance of few-layer sheets is also better because they can fairly buffer the volume change induced as a result of the Li intercalation, but the van der Waals interaction causes them to restack together; therefore, a confinement strategy is employed to avoid this restacking.122 Sun et al. were able to obtain a specific capacity of 563 mAh g−1 for a MoS2−C hybrid sheath at 10 A g−1.123 Owing to the fact that the electrochemical performance of MoSe2 is better than that of MoS2 due to the higher electrical conductivity of diselenides,124 other transition metal dichalcogenides might have better rate capability, due the available reports are mostly limited to those of MoS2 because of its popularity. Sb Alloys. Various alloys have been examined as potential anode materials. Although they provide a relatively negative potential with a high specific capacity, they all suffer from poor cyclability and rate capability due to the lack of empty spaces within the lattice for a facile solid-state diffusion. Instead, Liions should push the host atoms to open the accommodation space, and this is accompanied by severe volume changes and slow diffusion. In the case of Bi−Sb alloys, Zhao and Manthiram showed that the lattice composition has a tricky role in determining the rate capability for the Li storage.125 Although the capacity of Bi0.36Sb0.64-C is higher than that of Bi0.57Sb0.43, the practical capacity of the latter remains intact when increasing the charge/discharge rate from 0.1 A g−1 to 1 A g−1, whereas the capacity of the former almost vanishes. This is just a slight change in the composition with no major change in the lattice parameters, but the impact on the rate capability is enormous. Phosphorus. Li3P has a theoretical capacity of 2595 mAh g−1, and thus is considered as an attractive anode material for LIBs.126−133 However, this theoretical capacity cannot be achieved due to the insulating nature of phosphorus. The volume change is also massive as well as the capacity (over 300%).131 Black phosphorus has noticeable advantages such as a high electrical conductivity in order of 102 S cm−1.131 A porous structure of black phosphorus nanosheets provided an opportunity for approaching the theoretical capacity.134 However, the rate capability is not comparatively good as the specific capacity of about 2000 mAh g−1 achieved at 0.5 A g−1 is reduced to about 700 mAh g−1 by increasing the discharge rate to 20 A g−1. Owing to the high specific capacity of black phosphorus, the specific capacity achieve at this high rate of discharging is really considerable.

transition metals to replace the titanium. Guo et al. synthesized nitrogen-doped Li4Ti5O12 by the thermal decomposition of melamine, and a thin layer of TiN covered the Li4Ti5O12.114 The resulting electrode exhibited a capacity of 74 mAh g−1 at 100C. Although direct growth of the electroactive material onto a metallic substrate is of practical interest for eliminating the role of current collector and binder (despite the fact that Ti is an expensive current collector), this method cannot be easily scaled up or used for customized cell designs such a flexible cells. Tang et al. fabricated a conductive gel out of Li4Ti5O12 hydrogel and graphene oxide colloids.115 They employed this conductive gel to spray the electroactive material on the current collector, and the anode delivered almost 150 and 130 mAh g−1 at high discharge rates of 30C and 90C, respectively. Figure 8 indicates that there are two redox couples responsible for the Li intercalation/deintercalation, but the behaviors at high rates are different during the charge and discharge. Although the lithiation pathway is similar, characterized by a slight decrease in the operating voltage at low capacities, the delithiation process substantially changes at higher rates, as the second plateau disappears. Similar values have also been reported for the peapod Li4Ti5O12 nanoparticles (148 and 125 mAh g−1 at 30C and 90C).116 Nickel Oxide. NiO is one of the attractive electroactive materials with a broad range of applications in various electrochemical systems. Moreover, because of the growing interest in nickel foam substrate for the fabrication of various porous electrode, NiO has been extensively studied. A high theoretical capacity of 718 mAh g−1 has also named NiO as a promising candidate for LIBs, which is supported by its low cost, nontoxicity, and natural abundance. Depositing NiO with a 3D architecture directly on the current collectors can fabricate an anode approaching its theoretical capacity by delivering 700 mAh g−1.117 This also provides an excellent opportunity for the preparation of binder-free electrodes. NiO nanocone arrays were formed on a nickel foam substrate via a hydrothermal route.118 The anode could deliver a specific capacity of 606 mAh g−1 at 10C. Of course, the mechanism of the Li intercalation should be different from the conventional ones, as a capacity of 969 mAh g−1 at 0.5,118 1058 at 0.4C,119 which are significantly higher than the theoretical capacity, have also been reported. Li3VO4. High Li storage capacity of Li3VO4 with a theoretical specific capacity of 394 mAh g−1 has made it a promising candidate for the LIBs. Compositing Li3VO4 with graphite can increase the practical specific capacity 2−3 times.120 Although the graphite separates the Li3VO4 particles and improves the electrical conductivity between the particles, the performance at high rates is still problematic due to the agglomeration of the particles creating interfacial resistance within the agglomerated material. Jin et al. placed the Li3VO4 particles within the 3D graphene networks to uniformly control the distribution of the electroactive material within the overall matrix.121 The resulting cathode delivered a specific capacity of 247 mAh g−1 at 10 C (4 A g−1). MoS2. MoS2 is the flagship of the transition metal dichalcogenides, which has been widely considered for various electrochemical applications including anode of LIBs. The layered structure of MoS2 is similar to that of graphene, and every sheet is three-atom thick in the form of S−Mo−S sandwich. Because these sheets are attached to each other by weak van der Waals interactions, intercalating Li ions can easy

4. PERSPECTIVE High rate capability of lithium batteries is a serious market demand, and thus, a growing interest in this area of research. To this aim, the Li solid-state diffusion should be minimized to speed up the charging/discharging process. However, the solidstate diffusion in crystalline structure provides a steady battery performance as typically characterized by flat plateaus in charge/discharge profiles. In this case, the battery can deliver a constant voltage, which is a major requirement for the electronic devices. Hence, the improvement in the rate capability should not be accompanied by a sacrifice of the well-defined battery performance. In general, battery performance shifts to a pseudocapacitive behavior. Although supercapacitors are promising power sources for the energy storage, their performance in the electronic devices is different. 2810


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ACS Sustainable Chemistry & Engineering Biography

Therefore, it is still vitally important for a battery to deliver a constant voltage. Because the deviation from the battery performance to pseudocapacitive behavior at high charge/ discharge rates is inevitable, it is recommended to focus on the electrode materials delivering ideal battery performance characterized by a flat plateau at low charge/discharge rate. Otherwise, the deviation from the battery performance should be severe enough to transform the battery into a supercapacitor. This paper reviewed common strategies employed during the last years for improving the rate capabilities of various electrode materials (both anode and cathode). Because most of these electroactive materials are similar and undergo similar mechanisms during the Li intercalation/deintercalation, these strategies can be simply applied to other materials too. In addition to intercalating material, conversion-based materials, which undergo structural change upon reaction with Li, are promising anode materials. These materials have an intrinsic potential for high rate capability, but the main issue is poor cyclability in the absence of a rigid structure. Therefore, the rate capability cannot be improved without improving the cyclability first. Therefore, this class of anode materials is indeed at an early stage for planning high rate lithium batteries. For subtly designing the future high-rate electrode materials, it is necessary to picture the entire diffusion pathway from the bulk solution to the Li redox sites. This should be conducted at different scales: (i) crystal structure and orientation, (ii) the nanoscale morphology where the surface unit cells and the diffusion channels are exposed at the electrode/electrolyte interface, and (iii) the macroscale morphology where the electroactive particles are building the whole electrode. Most works simply pay attention to one of these scales such as controlling the material crystallinity, preparing different nanostructured materials, or coating the particles with carbon conductive agents, respectively. Combining these approaches can be helpful but it is not the ultimate solution because the purpose is to link the diffusion pathways at different scales to pave the path for the fastest possible diffusion to maximize the rate performance. In fact, preparation of the so-called ultrafast electrode materials to meet the market demand needs a profound design of the material architecture at different levels, and this design substantially differs from case to case. The significant difference is evident when comparing the intercalation and conversion materials. However, the crucial differences in the diffusion directions and the lattice growth make even the intercalation materials significantly different. In the case of the conversion materials, this complexity is even more severe, as the material architecture at all levels is always subject to change in the course of charging/discharging.

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Ali Eftekhari is a professor of chemistry temporarily with the University of Ulster and Queen’s University Belfast. He has worked in different capacities in various universities across 4 continents and founded two internationally recognized academic schools from scratch, which were considered as exceptional success stories by major media such as British newspaper, The Guardian. His research interest is focused on the material design for electrochemical systems and has worked on energy storage systems for 20 years. He is the principal author of over 100 papers published in leading scholarly journals and has supervised over 110 postdoctoral researchers and graduate students. He is the President of the American Nano Society.

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REFERENCES

(1) Eftekhari, A.; Jian, Z.; Ji, X. Potassium Secondary Batteries. ACS Appl. Mater. Interfaces 2017, 9, 4404−4419. (2) Eftekhari, A. Low Voltage Anode Materials for Lithium-Ion Batteries. Energy Storage Mater. 2017, 7, 157−180. (3) Zhou, R.; Fan, R.; Tian, Z.; Zhou, Y.; Guo, H.; Kou, L.; Zhang, D. Preparation and Characterization of Core−Shell Structure Si/C Composite with Multiple Carbon Phases As Anode Materials for Lithium Ion Batteries. J. Alloys Compd. 2016, 658, 91−97. (4) Chen, D.; Ji, G.; Ma, Y.; Lee, J. Y.; Lu, J. Graphene-Encapsulated Hollow Fe3O4 Nanoparticle Aggregates As a High-Performance Anode Material for Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2011, 3, 3078−3083. (5) Luo, J.; Xia, X.; Luo, Y.; Guan, C.; Liu, J.; Qi, X.; Ng, C. F.; Yu, T.; Zhang, H.; Fan, H. J. Rationally Designed Hierarchical TiO2@ Fe2O3 Hollow Nanostructures for Improved Lithium Ion Storage. Adv. Energy Mater. 2013, 3, 737−743. (6) Kaskhedikar, N. A.; Maier, J. Lithium Storage in Carbon Nanostructures. Adv. Mater. 2009, 21, 2664−2680. (7) Eftekhari, A. Ordered Mesoporous Materials for Lithium-Ion Batteries. Microporous Mesoporous Mater. 2017, 243, 355−369. (8) Eftekhari, A. LiFePO4/C Nanocomposites for Lithium-Ion Batteries. J. Power Sources 2017, 343, 395−411. (9) Zhao, Y.; Peng, L.; Liu, B.; Yu, G. Single-Crystalline LiFePO4 Nanosheets for High-Rate Li-Ion Batteries. Nano Lett. 2014, 14, 2849−2853. (10) Tang, Y.; Zhang, Y.; Li, W.; Ma, B.; Chen, X. Rational Material Design for Ultrafast Rechargeable Lithium-Ion Batteries. Chem. Soc. Rev. 2015, 44, 5926−5940. (11) Eftekhari, A.; Fan, Z. Ordered Mesoporous Carbon and Its Applications for Electrochemical Energy Storage and Conversion. Mater. Chem. Front. 2017, DOI: 10.1039/C6QM00298F. (12) Liang, Y.; Chen, L.; Cai, L.; Liu, H.; Fu, R.; Zhang, M.; Wu, D. Strong Contribution of Pore Morphology to the High-Rate Electrochemical Performance of Lithium-Ion Batteries. Chem. Commun. 2016, 52, 803−806. (13) Qie, L.; Chen, W.; Wang, Z.; Shao, Q.; Li, X.; Yuan, L.; Hu, X.; Zhang, W.; Huang, Y. Nitrogen-Doped Porous Carbon Nanofiber

AUTHOR INFORMATION

Corresponding Author

*A. Eftekhari. Email: [email protected]. ORCID

Ali Eftekhari: 0000-0003-3568-4812 Notes

The author declares no competing financial interest. 2811


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