Higher, Stronger, Better A Review of 5 Volt Cathode Materials for Advanced Lithium-Ion Batteries

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1 Higher, Stronger, Better A Review of 5 Volt Cathode Materials for Advanced Lithium-Ion Batteries Alexander Kraytsberg and Yair Ein-Eli * The ever-increasing demand for high-performing, economical, and safe power storage for portable electronics and electric vehicles stimulates R&D in the field of chemical power sources. In the past two decades, lithium-ion technology has proven itself a most robust technology, which delivers high energy and power capabilities. At the same time, current technology requires that the energy and power capabilities of Li-ion batteries be beefed up beyond the existing state of the art. Increasing the battery voltage is one of the ways to improve battery energy density; in Li-ion cells, the objective of current research is to develop a 5-volt cell, and at the same time to maintain high specific charge capacity, excellent cycling, and safety. Since current anode materials possess working potentials fairly close to the potential of a lithium metal, the focus is on the development of cathode materials. This work reviews and analyzes the current state of the art, achievements, and challenges in the field of high-voltage cathode materials for Li-ion cells. Some suggestions regarding possible approaches for future development in the field are also presented. 1. Introduction The most essential parameters in chemical energy storage devices (batteries) are specific energy, energy density (in both cases, the larger the better), cost (the lower the better), and safety. The cell specific energy and energy density depend, first of all, on the cell chemistry, being reflected in its potential and charge capacity values. From this standpoint, Li-based cells hold much promise because Li metal is the most electropositive (E 0 = 3.04 V vs. standard hydrogen electrode) and light ( ρ = 0.53 g cm 3 ) material. However, employing Li metal in a secondary cell is challenging, since the possibility of dendrite growth poses risks of anode-cathode shorting (followed by the instant release of all stored energy). In the 1970s 1980s, the concept of a Li-ion cell ( rocking chair battery ) was demonstrated; [ 1 4 ] this concept was based on the substitution of a Li metal anode with Li-ion intercalation compounds. The lithium is in an almost atomic state in a carbonaceous anode material, and it is in an almost Li + -state Dr. A. Kraytsberg, Prof. Y. Ein-Eli Department of Materials Engineering Technion-Israel Institute of Technology Haifa 32000, Israel eineli@tx.technion.ac.il DOI: /aenm inside the cathode material, being oxidized by a transition metal redox couple. Whereas lithium mobility in the carbon anode is sufficiently high, the development of cathode materials with substantial Li + - mobility turned out to be an issue of prime importance. Such a material was first presented by Whittingham, [ 1 ] who employed a TiS 2 -based cathode material in a cell with a metallic Li anode. The structure of TiS 2 comprises layers of hexagonal closepacked octahedral atomic groups, formed by a layer of titanium atoms between two layers of sulfur atoms, [ 5 ] thus allowing insertion of Li + into the layered gap. Upon discharge, Li + ions occupy the vacant octahedral sites between the layers; the charge balance is maintained by electron current via the external circuit, converting Ti 4 + into Ti 3 +. A reverse process occurs on charging, maintaining the pristine TiS 2 structure. This work promoted research on other sulfides and chalcogenides during the 1970s and 1980s; however, cells employing such cathodes exhibited insufficient voltages of V cell < 2.5 V. In the beginning of the 1980s Goodenough et al. started working with oxide cathode material LiCoO 2 ; this layered oxide, having the structure similar to the structure of LiTiS 2, demonstrates V cell > 4 V. [ 2 ] The approach paved the way to safe Li-ion cells but required the development of practical anode/cathode materials, which remain undamaged over numerous Li + insertion/extraction cycles; also, the development of adequate nonaqueous electrolytes [ 6, 7 ] was needed. In the early 1990s, Sony succeeded in the commercialization of the first rechargeable Li-ion cell based on a carbon anode (petroleum coke) and a LiCoO 2 cathode; the cell demonstrated an open circuit voltage of over 3.6 V and an energy density of 150 Wh kg 1. [ 8, 9 ] Since then, Li-ion batteries have been recognized as high energy and high operation voltage, rechargeable power sources, [ 10 ] outperforming other available battery systems in terms of energy density, design flexibility, cycle life, and low self-discharge rate. These features make them the ideal choice for mobile electronic devices and also an appealing option for hybrid and electric vehicle energy storage. Current R&D in this field is focused on developing high voltage cathode materials with a high charge capacity and cycling capability; substantial efforts are also being applied towards the development of organic electrolytes with a broad voltage window and high conductivity. In addition, the issues of 922 wileyonlinelibrary.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 the Li-ion material cost and their environmental compatibility now receive much attention. 2. Rechargeable Li-Ion Batteries: The Concept 2.1. Li-Ion Battery Basics The operating principle of a Li-ion battery is illustrated in Figure 1. The cell consists of an anode, a cathode, an electrolyte and a separator. Lithium ions reversibly intercalate and de-intercalate into/from the anode and cathode materials on operation. The materials consist of a host material with Li + -ions accessible to inter-atomic sites. Lithium ion intercalation/deintercalation causes a change in the charge distribution inside the host material skeleton and an overall change in the material charge which, in turn, causes electron flow in the external circuit. Figure 1 is a schematic representation of Li-ion cell with a carbon-based anode and a metal-oxide based cathode. The thermodynamic value of a Li-ion cell voltage (which in the absence of corrosion reactions is equal to the open circuit potential, V OC ) is determined by the difference in the chemical potentials of Li into the cathode μcath Li and the anode μan Li : V cell = μcath Li μli an F (1) Here F stands for Faraday constant Design Considerations In order to fabricate a Li-ion cell with high volumetric and/or gravimetric energy density, a long cycle life, and a safe operation, it has to exhibit low energy losses in course of charge/discharge cycle, and also a high power performance, needed for some applications. Finally, the cell must be environmentally friendly and inexpensive. The volumetric and/or high gravimetric energy capacity is governed by the relation E stored = Qcell V cell (here Q cell stands for the cell charge capacity); the relation suggests that enhancement in Li-ion cell voltage is the appropriate approach to increasing the cell energy (the charge capacity should not be Alexander Kraytsberg graduated from the Moscow Institute of Physics and Technology in 1972 and joined the Russian Academy of Science to study the electrochemistry of oxides, semiconductors, and photoelectrochemistry. During he worked for Tracer Technologies Inc. (Somerville, MA, USA) conducting Li-ion battery-related research. During he was a Chief Scientist of Hi-Cell Ltd. (Israel) and studied DMFC technology. In 2009 he joined the Department of Materials Engineering at the Technion. Currently his work is focused on the electrochemistry of oxides, non-aqueous electrochemistry, and materials for power storage. Yair Ein-Eli After graduating from Bar-ilan University in 1995, Prof. Ein-Eli was a post doctoral fellow at Covalent Associates Inc located at Woburn MA, USA ( ), where he eventually headed the Li-ion research group until He then proceeded and joined Electric Fuel Ltd. and was appointed Director of Research and Battery Technology. In 2001 he joined the Department of Materials Engineering at the Technion. Current research interests involved materials for batteries, solar processing and corrosion inhibitors studies. compromised). / Since the carbonaceous anode potential (vs. Li/Li + ) V anode Li Li + 0, the / cell potential, V cell, is dictated by the cathode potential V cathode Li Li +, i.e., by the type of cathode material employed Electrolyte Requirements Figure 1. Schematics of a typical Li-ion cell. Cell potential, V cell is not governed by cell electrolyte, but the employment of inadequate electrolyte compromises cell performance: the electrolyte should not experience oxidation/ reduction at the electrode surface in the course of operation. If the electrolyte solution comprises solvent, S, and ion species S{A + }{B}, the thermodynamic conditions of electrolyte stability are that the potentials of the electrolyte redox reactions should be in the cell voltage window, as shown in Figure 2. The necessary conditions for these potentials are given by Equation 2 : 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 923

3 Figure 2. Electrolyte/electrode interface energy diagram in a Li-ion cell at V OC. Electrolyte stability window (W el ) = V red S{A + }{B } V ox S{A + }{B } > V cell (2) Here V red S{A + }{B } is the potential of the solvent-ion species reduction and V ox is the potential of the solvent-ion species reduction. S{A + }{B } It is convenient to infer V red S{A + }{B } and V ox S{A + }{B } values by assuming that the potential for an oxidation reaction ( E ox ) correlates with the energy of the highest occupied molecular orbital ( E HOMO ) of the oxidizing species and the potential for a reduction reaction ( E red ) correlates with the energy of the lowest unoccupied molecular orbital ( E LUMO ) of the reducing species: E LUMO = E red E sol r + const 1 (3) E HOMO = E ox E sol o + const 2 (4) Here ΔE sol r and ΔE sol o are the differences between the solvation energies of the pristine molecules and their ionizing forms, respectively. Equation 2 and Equation 4 include the solvation energies, which are different for the same redox couple for different solvents and also different for different couples in the same solvent. [ 12 ] Practically, Equation 2 and Equation 4 are quasi-linear (see Figure 3 ) [ 13 ] and thus, it is possible to use theoretically calculated E LUMO and E HOMO as guidelines. The computational results fit the estimation of the redox potentials of candidate electrolytes but not for an accurate prediction of E red and E ox [ (and W el ) 13, 14 ] and, therefore, more emphasis is to be put on experimental results for V red S{A + }{B } and V ox S{A + }{B }. There is some uncertainty in an experimental determination of these values since electrochemical decomposition is usually a complicated process, which is determined by both thermodynamic as well as kinetic factors. Thus, electrochemical stability data reported for an electrolyte may depend on the conditions under which they are obtained; as a result, the literature data on W el are not always in good agreement with each other. Figure 3. Correlation of calculated HOMO energies of organic solvents with the experimental oxidation potentials; all of the electrolytes contain 0.5 M Et 4 NBF 4 as a supporting electrolyte salt. The solvents belong to the following classes: carbonates ( ); lactones ( ); nitriles ( ); formamides ( ); oxazolidinones ( Δ ); nitroalkanes ( ); sulfur-containing compounds ( ). Reproduced with permission. [ 13 ] Copyright 2002, Elsevier. The detailed consideration of the measurement approaches and of the reported values for V red S{A + }{B } and Vox S{A + }{B } for various electrolytes are out of the scope of this review. It is enough to mention that currently the highest reported values of W el are slightly higher than 5 V (e.g., ethylmethoxyethoxyethyl sulfone + 1 M Li[bis(trifluoromethanesulfonyl)imide]) [ 15 ] and, therefore, at present, the development of cathode materials with V cathode Li Li + 5 V certainly makes sense and is most / appealing Anode and Cathode Material Requirements Cell potential, Vcell is determining by the difference between μcath Li+ and μli+ an, and since μan Li+ > μli metal (preventing Liplating), V cell may be enhanced only by increasing μcath Li+. Since the most common carbon-based anodes weigh on average 2.5 times less than the oxide cathodes (cathode contributes 50% of the total cell weight), [ 16 ] research in the field of cathode materials promises more gain regarding cell energy density. Achieving a good cell cycle life requires that electrode host compounds demonstrate a fairly good structural stability and minimal volume change over the entire operational Liinsertion/extraction voltage range. Since Ohmic losses compromise cell efficiency and power density, electrode materials need to maintain good electronic and Li + -ion conductivity; this problem is related mostly to cathode materials, though, because carbon has high electronic and Li + conductivity. 924 wileyonlinelibrary.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Equations 1 and 2 and a final degradation of the cathode material following Equation 5 is expected (more elaborated discussions of this approach may be found in the literature [ 19, 20 ] ): 2 (O 2 ) 2 2O 2 + O 2 (5) This approach suggests that V cath x 0 may depend on fine details of the oxide cathode electronic structure. Indeed, a removal of Li + from Li x M y O z can be considered as the addition of a hole. The hole may be located mostly in oxygen (which favors oxide decomposition) or it may be distributed over both the metal and oxygen. For example, Li x CoO 2 starts to decompose at x < 0.5 [ 17 ] whereas Li x NiO 2 may be completely de-lithiated (down to NiO 2 ) without any decomposition. [ 21 ] At the present, ab-initio quantum mechanics calculations are not able to provide an accurate explanation of the hole distribution upon de-lithiation [ 22 ] and, therefore, only experimental data on V cath x 0 may be considered reliable. Figure 4. Schematic representation of a band structure of a Li-insertion material; slightly oxidized redox couple as it passes through the top of the p an : (a) itinerant vs. polaronic character of holes states of couple on the approach to the top of p an, (b) pinned couple with predominantly p an holes states. Adapted with permission. [ 19 ] Copyright 2009, Elsevier. 3. Lithium Transition Metal Oxide-Based Cathodes 3.1. Decomposition Limit of the Cathode Material The potential-determining reaction of a metal-oxide cathode may be represented as δli + + Li xm y O z DLi x + δ M y O z, and the potential μ cath x depends on the degree of lithiation, x. At the same time, there is a possibility that an oxide is involved in a parallel reaction of decomposition: V cath x 0. At a certain value of V cath x 0, i.e., at lithiation x 0, starting from V cath x 0 the cathode experiences decomposition instead of charging; the decomposition onset potential V cath x 0 is an intrinsic cathode material parameter. It is reported that, in the course of de-lithiation of Li 1 x CoO 2 where x > 0.5, electrons are transferred from the oxygen band p [ an, and O 2 evolves. 17 ] Oxidative decomposition of the metal oxide may be discussed based on the concept of pinning of a transition metal redox energy level. [ 18, 19 ] Li-insertion compounds may be considered semiconductors with a conductive band ( CB ) formed by transition metal d-orbitals ( d met ) and a valence band ( VB ) formed by oxygen (or sulfur, etc.) p-orbitals ( p an ); in a real material the metal anion bonding is not 100% ionic, thus CB is formed by hybrid orbitals ( d met p an ), with a dominant input of d met, and VB is formed with a dominant input of p an (see Figure 4 ). As the material oxidation increases (i.e., as the charge carrier concentration changes), the redox level along with the Fermi level E F moves toward VB and the p an share in the hybridization increases. Finally, the transition metal redox couple-related band reaches the ceiling of VB (Figure 4 b), and E F pinning takes place. Starting from this point, if further Li + de-intercalation follows, electrons tend to be taken from the ceiling of VB ; i.e., holes appear in VB, forming di-anionic states like (O 2 ) 2, followed by a disproportionation of these states according to 3.2. Major Types of Compounds Employed As outlined above, electrode materials for Li-ion cell cathode should maintain a good Li + -ion conductivity; currently, three types of metal oxides are used as cathode materials in lithiumion batteries; [ 23 ] Layered oxides with the α-nafeo2-type structure Oxides with a spinel structure, with (Fd 3m) symmetry Poly-anion oxides with the olivine and olivine-related structures, with (Pnma) symmetry These types of the materials are considered and discussed in the following sections, focusing on the development of high voltage cathodes Layered Oxides These oxides with the general formula of LiMO 2 form a distorted rock-salt ( α-nafeo 2 -type) crystal structure. All known structures are derived from anionic stacking: 03, P3, 02, and P2. Here the letters O and P stand for the Li site (octahedral or prismatic) and the number indicates the amount of layers contained in the elementary crystallographic cell; the <MO 2 > -layers are built up of edge-sharing <MO 6 > octahedra ( Figure 5 ). These layers are not deconstructed Figure 5. Schematic representation of a layered LiMO 2 structure (M stands for Co, Ni, or Mn). Adapted with permission. [ 24 ] Copyright 2007, Elsevier WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 925

5 during Li + intercalation/de-intercalation. The oxygen ions form a cubic-close-packed array along the C axis and the Li + and M 3 + ions alternately occupy the close-packed (111) interstitial octahedral-site planes. Two-dimensional Li + ion diffusion in the space between <MO 2 > layers occurs since the edge-shared octahedral <LiO 6 > arrangement allows ion movement from one (vacant) octahedralsite plane to another via a tetrahedral site. The edge sharing of <MO 6 > octahedra maintains a direct <M M > interaction, responsible for the material s electronic conductivity. [ 24 ] Thus, layered metal oxides are the most commonly used cathode materials. [ 25 ] LiCoO2 Li x CoO 2 [ 25 ] is the most popular cathode material with fair electrical conductivity and Li + mobility. Consistent with the high redox value of the Co 4 + /Co 3 + couple, its operational voltage is around 4 V. [ 2, 3, 25 ] A capacity of 274 mah/g corresponds to a full Li + extraction, producing CoO 2, which generally is possible. [ 26 ] However, a reversible extraction/insertion of Li + is practically limited to x = 0.5, yielding a capacity of 140 mah/g; the material demonstrates hundreds of cycles within this range, but deintercalation of lithium below x = 0.5 results in a substantial capacity fade, which is attributed to material decomposition. It manifests as oxygen evolution, [ 17 ] cobalt dissolution, and a serious lattice shrinkage: on full Li loss, inter- <CoO 2 >-layer distance shrinks from down to nm, [ 26 ] which is large enough to cause material crumbling. Major material drawbacks are the high cost of cobalt and its environmental hazard LiNiO2 Nickel is fairly inexpensive and is also a fairly environmentally friendly element. Thus, it is not a surprise that LiNiO 2 (its structure is a slightly Jahn-Teller distorted version of α-nafeo 2 ) had attracted much of an attention. It has a similar operation potential (somewhat lower than LiCoO 2, though, namely, 4.1 V for Li ½ CoO 2 vs V for Li ½ NiO 2 ) [ 27, 28 ] but develops higher stability on de-lithiation than LiCoO 2 ; it may be even de-lithiated to form NiO 2 [ 29 ] and thus, it may be cycled up to 200 mah/g. [ 28 ] The most interesting feature is that the NiO 2 Li + -intercalation voltage is 4.8 V, and electrons are extracted from the e g band. Since the e g band lies well above the O2p band, lattice oxygen is not displaced even at a high degree of de-lithiation ( Figure 6 ). [ 21 ] Figure 6. Energy diagrams of Li1 xcoo2 and Li 1 x NiO 2. Adapted with permission. [ 21 ] Copyright 2001, American Chemical Society. Figure 7. Cycle characteristics of LiNiO 2 under a constant-charge capacity, 1.0 ma cm 2, 1 M LiCIO 4 in a solution of propylene carbonate and dimethoxyethane. Adapted with permission. [ 28 ] Copyright 1995, Elsevier. However, LiNiO 2 employment meets some problems (and hence, challenges) for a practical Li-ion cell. The first problem is associated with the preparation of well-ordered LiNiO 2. In course of the preparation, Ni 2 + /Li + -containing precursors must form a new compound so that Ni 2 + is oxidized and both elements have to settle in their new places to form a LiNiO 2 compound with an α-nafeo2 structure. Nickel ions are only sufficiently itinerant in the Ni 2 + /Li + -oxide precursor mix at temperatures above 600 C, [ 30 ] whereas Ni + 3 -ions are not stable at high temperatures. As a result, up to now experimentalists have failed to avoid the presence of divalent nickel ions, and half of them occupy the lithium sites between <NiO 2 > layer space. [ 31 ] Being larger than Li + (0.83 Å vs Å), [ 32 ] Ni + 2 substantially reduces Li + mobility and thus degrades the electrode power capability of the material; in fact, the Li + -diffusion coefficient in commonly prepared LiNiO 2 is several times less than that in LiCoO 2. [ 33 ] The second problem is that LiNiO 2 actually has several crystal modifications, the most electrochemically favorable being LiNiO 2 (R 3m, α-nafeo2 -type phase), with a layered structure, and (among others) LiNiO 2 (Fm3m), with a rock salt-type structure; the lattice parameters of both modifications are very close and thus the modifications coexist, but the latter structure is electrochemically inactive. As a result, commonly prepared LiNiO 2 (R 3 m) is contaminated with electrochemically inactive rock-salt domains. Numerous synthetic routes have been offered to bypass these obstacles, [ 31, 34 ] though an adequate solution has not yet been found. The third problem is that, upon de-lithiation, Li 1 x NiO 2 exhibits several successive phase transformations accompanied by substantial volume changes; this seriously degrades the material integrity and compromises the cycling ability. The result is that Li 1 x NiO2 may be cycled only in a considerably narrow interval of x (Figure 7 ) [ 28 ] and, therefore, the high-voltage capability of the material cannot be exploited. An additional problem is the low thermal stability of the low-li 926 wileyonlinelibrary.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

6 gas formation during cell operation, persist. [ 24, 37, 38 ] Additives such as Al, Mg, Ca and Ba have also been employed. The introduction of these ions results in the elimination of phase transformations and, thus, diminishes material volume changes during cycling. The thermal stability of LiNiO 2 and LiNi 1 y Co y O 2 may also be enhanced by Al, Mg, and Mn doping. [ 24, ] The bottom line is, though, that all these LiNiO 2 -based doped materials generally show lower operating voltages and lower cycling capabilities than those of LiCoO LiMnO2 Figure 8. Thermogravimetric analysis-based data on Li0.3NiO 2, Li 0.4 CoO 2, and λ-mno2 ; oxygen release onset temperature t onset is presented as a function of the heating rate V heat, i.e., as t onset (V heat ); tonset is determined by t onset (V heat ) extrapolation to V heat = 0. Adapted with permission. [ 36 ] Copyright 1994, Elsevier. Li x NiO 2 (x 1), which contains unstable Ni 4 + ions. Ni + 4 may be reduced back to Ni 3 + at elevated temperatures; the process Ni 3 + Ni +4 results in a change to the lattice structure (specifically, α-nafeo2 -type phase transfers into a pseudo-spinel phase and then into a highly disordered R3hm phase), which is accompanied by oxygen loss, as shown in Equation 6; the smaller the value of x, the lower the Li x NiO 2 -phase temperature stability: [ 35 ] Li 0.30 Ni 1.02 O O 2 + Li 0.30 Ni 1.02 O 1.76 (6) This possibility of oxygen release seriously compromises cell safety; its onset temperature value t onset is crucial: the loss of oxygen will not compromise battery safety if the t onset is higher than the practical operation temperature. It has been demonstrated ( Figure 8 ) that this Li 0.3 NiO 2 tonset is just above 100 C (though the decomposition is fairly slow up to 150 C). [ 36 ] Numerous attempts to improve the properties of Ni-based materials have been, up to now, mostly focused on substituting some other ion for nickel; the first candidate is (as expected) cobalt. The LiNi 1 y Co y O 2 compounds and adequate synthetic routes to them have been extensively studied; the presence of Co instead of Ni can stabilize the layered structure and suppress cation disorder, enabling a reversible capacity close to 180 mah/g; for the optimal formulation, y should be within the interval Cobalt also suppresses the phase transitions associated with Li + extraction from LiNiO 2. Whereas low cost and fair capacity makes LiNi 1 y Co y O 2 compounds attractive candidates for a cathode material, some problems are yet unsolved, namely, capacity fade during cycling, which is due to the migration of Ni 3 + ions from the Ni planes to the Li planes still being too high (LiCoO 2 has better cyclability). The operating voltages lie in the range between those of LiNiO 2 and LiCoO 2 materials, and safety issues, which are related to the possibility of oxygen Thia material looks quite attractive, since Mn is inexpensive and more environmentally friendly than Co or Ni. The stable phase of LiMnO 2 has an orthorhombic structure, [ 46 ] and layered LiMnO 2 is metastable; layered LiMnO 2 provides easy Li + -diffusion paths, while also exhibiting a fairly smooth intercalation/deintercalation voltage profile. [ 11 ] However, although most Li ions can be removed from the structure on charging (providing a capacity of 285 mah/g), the following Li + insertion does not restore the layered LiMnO 2. On extraction of half the Li from the layered LiMnO 2, manganese ions penetrate the interlayer space, and this results in the formation of crystal regions with a spinel structure. [ 11, 47, 48 ] This phase transformation is irreversible and compromises the cathode cycle life. The operation voltages of the layered LiMnO 2 areall found to be lower than 4.1 volts, [ 11 ] which is consistent with quantum-mechanical calculations. [ 49 ] Attempts to stabilize layered LiMnO 2 by doping the compound with elements such as Co, Ni, Cr, etc., are the subject of several experimental and theoretical studies. [ 48, ] It was reported that Li(Ni 1/3 Mn 1/3 Co1/3 )O 2 can operate at voltage of 4.5 V with a capacity of around 200 mah/g; the cyclability was not reported. [ 53 ] Nevertheless, these studies have only a limited success and the cyclability is still an issue. More important, from the point of the subject of the present review, the working voltages of modified LiMnO 2 are still below 4.5 V, and the bulk of their capacity is associated with voltages even below 4 V On the Chance to Develop a Layered Oxide Material with an Elevated Voltage Whereas current ab-initio quantum-mechanical computations cannot deliver the exact values of V cath x, these computations are useful for material pre-screening before their synthesis and investigation. The computations also are quite helpful in revealing the detailed mechanism of the lithium intercalation process. [ 22, 54, 55 ] Particularly, it was revealed that the intercalation voltage is governed not only by the redox potential of the transition-metal ion, which changes its valency upon Li + intercalation; the lattice oxygen also may be involved in the electron exchange ( Table 1 ).[ 55 ] It was suggested that this increased oxygen participation correlates with increased V cath x. [ 54 ] This approach implies that it may be possible to preserve the electrochemical activity and deliver higher V cath substituting a part (or all) of the oxide x transition metal with non-transition-metal ions. Since oxygen participation in charge transfer is highest in the case of aluminum, it is expected that the calculated V cath 0 for LiAlO 2 will be as high as 5.4 V. [ 54 ] Pure LiAlO2 is not electronic conductor but the mixed oxides Li(Al x Co y )O 2 have the α-nafeo WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 927

7 Ta b l e 1. Calculated electron share for oxygen and metal on Li + intercalation into α-nafeo2 -structured layered oxides. Adapted with permission. [ 55 ] Copyright 1997, American Physical Society. Bond Ti-O V-O Mn-O Co-O Ni-O Cu-O Zn-O Al-O Oxygen Metal structure and are semiconductors, and calculated values V cath 0 for such oxides are higher than for LiCoO 2. The experimental results are ambiguous on Al-substituted layered oxides, though, and seemingly depend on the material morphology and preparation methods. Specifically, whereas the results of Jang et al. [ 56 ] ( Figure 9 ) support these theoretical predictions, other works do not confirm that Al-doping causes Li + -intercalation voltage enhancement. [ 57, 58 ] This approach paves the way to the development of new high-voltage layered oxides Spinel-Type Oxide Materials Spinel oxides with the general formula LiM 2 O 4 comprise octahedral-coordinated M-cations and Li-cations in tetrahedral positions of a cubic-closed-packed O 2 lattice. The M cations are forming a 3D-[M 2 ]O 4 framework in which the interstitial space is formed by edge-sharing octahedral sites that share faces with the tetrahedral Li-containing sites, as shown in Figure 10. The framework remains stable while Li + ions reversibly move between the tetrahedral sites, and good Li + conductivity is imparted by an edge-shared <MO 6 > -octahedral arrangement with a direct <M M > interaction: interconnected tetrahedral sites provide 3D paths for Li + diffusion through the spinel framework, which makes spinel-type oxides promising materials for Li-ion cell cathodes. [ 4 ] Ab-initio quantum-mechanical calculations hint that spinel materials may have a higher voltage at low Li + content compared with layered materials. The calculated voltage of a virtual spinel modification of LiCoO 2 is substantially higher that the voltage of layered LiCoO 2 [ 59 ] (Figure 11 ); it was suggested that spinel Figure 10. LiMn 2 O 4 spinel structure representation; LiMn 2 O4 shown being composed of dark gray MnO 6 octahedra and lithium ions (light gray balls) occupying interconnected tetrahedral positions. Adapted with permission. [ 24 ] Copyright 2007, Elsevier. Figure 9. Voltage curves of Li1 xal y Co 1 y O 2 in charging up to x = 0.4, followed by discharging to x = 0.2, 0.4 ma cm 2 ; (a) y = 0, (b) y = Adapted with permission. [ 56 ] Copyright 1999, The Electrochemical Society. Figure 11. Comparison of calculated voltage intercalation curves for virtual spinel Li x CoO2 (dashed line) and actual layered Li x CoO2 (solid line) at 300 K. Adapted with permission. [ 59 ] Copyright 1999, American Physical Society. 928 wileyonlinelibrary.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

8 Figure 12. Open-circuit voltage curve of spinel-related Li x Mn at 30 C. Adapted with permission. [ 64 ] Copyright 1990, The Electrochemical Society. LiCoO 2 can be obtained experimentally but its intercalation potential was found to be substantially lower than the calculated value. [ 60 ] Figure 13. Variation in the Mn-O COOP as a function of the substituting element; the substitution corresponds to 25% of the entire manganese atoms. Adapted with permission. [ 68 ] Copyright 2000, The Electrochemical Society LiMn2O 4 It is expected that spinel LiMn 2 O 4 attracts a large amount of attention, as the combination of the spinel 3D lattice with chemical stability of the Mn + 3 /Mn + 4 couple promises good safety and high power capability for a cathode material. Manganese is also a fairly cost-effective material. [ 61 ] It surfaced, though, that LiMn 2 O 4 is a problematic cathode material because of its high capacity fade, especially at temperatures over 50 C [ 62 ] (the material loses Mn ions in the course of cycling), which makes its implementation impractical. The material may also undergo a phase transition in the course of Li de-intercalation, which compromises cycle life [ 63 ] and results in the existence of a voltage step, and even the maximal de-lithiation voltage is just a bit higher than 4 V ( Figure 12 ). [ 64 ] Therefore, a pure LiMn 2 O4 may hardly be considered a high-voltage cathode material. Spinel-type oxides LiM 2 O 4 with M = Ti, V, Co also demonstrate the operational voltages below 4.0 V. [ 65 ] LiMn2O 4 -Based High-Voltage Spinel Compounds In pure LiMn 2 O 4, manganese ions can be partially replaced by other metal ions; computations have revealed that the introduction of alien cations doesn t compromise the spinel structure [ 66 ] but modifies the electronic properties of the material. Particularly, such substitution changes the crystal orbital overlap population (COOP), [ 67 ] as demonstrated in Figure 13, [ 68 ] and also delays the onset of Jahn Teller distortions of the [Mn 3 + O 6 ] octahedra during discharge; [ ] the latter process provokes a structural transition accompanied by a volume increase, [ 72 ] which triggers cathode material crumbling. Since doping suppresses such structural transitions, [ 73 ] mixed ternary Li( A M) α Mn 2 α O 4 (and quaternary Li( A M) α ( B M)β Mn 2- α - β O 4 ) spinels (here B M and A M are di- and/or trivalent metal cations) may have better cyclability and, particularly, higher operating voltages than LiMn 2 O 4. [ ] Indeed, the Mn ion has a formal oxidation state of [Mn ] in spinel, and the substitution of Mn with X M metals, whose oxidation state is less than [ X M +3.5 ], increases the average oxidation state of the remaining manganese. It is also known that the higher the average oxidation state of Mn in the material, the higher the material s cyclability; [ ] this is particularly true for near-surface Mn. [ 80 ] Increasing the average Mn ion valency (which accompanies doping) not only suppresses Jahn Teller distortions and phase transitions, but also may decrease Mn dissolution. [ 80, 81 ] It was found that Mn + 2 dissolution does not always account for the major part of the cathode material degradation. Specifically, it was estimated that Mn dissolution may account for less than 30% of capacity loss. [ 77 ] The reduction of Mn dissolution is important for preventing the whole Li-ion cell degradation. The associated problem with the anode is that the dissolved Mn cations are deposited on the graphite anode, and this deposited Mn causes anode capacity loss. [ 82, 83 ] The problem is that the cathode material particles dissolve, resulting in loss of contact between active material particles and conductive additives and, thus, the demise of cathode conductivity. [ 84 ] Concluding the overview of the Mn-spinel cathode material dissolution issue, it also worth outlining that dissolution depends not only on the material features but also strongly on the electrolyte content. [ 85, 86 ] At the same time, lower-valence substitution decreases the number of Li + ions that can be extracted from the spinel 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 929

9 Experimentally, a substantial number of substituted Mnspinels Li( A M) α ( B M) β Mn 2- α- β O 4, which operate over 4.5 V, have been identified. Among these compounds are those substituted with Ti (LiNi 0.5 Ti α Mn1.5- α O 4, [ ] (LiTiα Mn 2- α O 4 doesn t demonstrate a high-voltage plateau)), [ 94, 95 ] with Cr (LiCr α Mn 2- α O [ 74 76, ] 4 and LiMn 1.5 α Cr 2 α Ni 0.5 α O 4 ), [ ] with Fe, [ 75, 76, 101, 102 ] with Co, [ 74 76, 103, 104 ] with Ni, [ 74 76, ] with Co and Ni, [ 98, 112, 113 ] with Fe and Ni, [ 106, 114 ] with Co, Ni, and Fe (LiNi z Co x Fe y Mn 2-x-y-z O 4 ), [ 115 ] with Cu, [ 75, 76, ] with Cu and Ni, [ 74, 106 ] with Zn, [ 74, 119 ] with Zr, [ 100 ] with Mg, [ 120 ] with Mg and Ni, [ 106, 121 ] and with Al. [ 74, 106, 100 ] 3.5. High Voltage Capacity: Spinel-type Oxides vs. Layered Oxides Figure 14. Calculated Li + -diffusion barriers in doped LiMn 2 O 4 (LiM 0.5 Mn 1.5 O 4 ); here M stands for Cr, Fe, Co, Ni, Cu, Mn; - one Mn-ion is substituted in 16c site surrounding six Mn- ring, three Mn-ions are substituted in 16c site - surrounding six Mn-ions ring, - non-doped LiMn 2 O 4. Adapted with permission. [ 90 ] Copyright 2011, American Chemical Society. Datum on LiCoO 2 acquired and replotted. [ 89 ] structure before all Mn ions are in the oxidation state [Mn + 4 ], which results in capacity reduction. Thus, increasing operational voltage and cyclability via manganese substitution is inevitably a capacity trade-off. The work in this field is also supported by calculations of Li -mobility (cathode materials with low Li + conductivity are not practical); the calculations hint that Mn substitution may preserve Li + mobility in the doped spinels. Indeed, the spinels belong to the F d 3m space group with oxygen ions in 32e sites forming a close-packed fcc lattice. In manganese spinel, LiMn 2 O 4, Mn ions are located in the 16d octahedral sites, while Li ions occupy the 8a tetrahedral sites, and the 16c octahedral sites are empty. Lithium ion diffusion occurs via hopping between adjacent 8a sites through the intermediate 16c sites; these 16c sites are actually surrounded by six Mn ions forming a kind of Mn fence, or Mn ring. [ 87 ] Thus, Li + mobility and hence ionic conductivity (which determines material power performance and energy losses) are functions of the barrier between two adjacent 8a sites. In connection with this, the higher Li + mobility and ionic conductivity of the layered oxides (if compared with spinel-type oxides) [ 88 ] stems from the fact that in layered oxides this barrier is substantially lower. [ 89 ] Figure 14 demonstrates that this hopping barrier often doesn t substantially alter Mn-spinel doping; moreover, some dopants may even increase the ionic conductivity of the material. [ 90 ] Up to now, most of the research in the field of spinel-type materials has been focused on mixed manganese-oxide based materials with a general formula of Li x M y Mn2 yo 4 (M = Ni, Co, Fe, Cr, etc.). Among these materials, there are a substantial number of cathodes with the highest de-lithiation potential, over 4.8 V and even over 5 V (e.g., see Table 2 ),[ 66 ] whereas the popular layered oxides have substantially lower maximal operational voltages of 3.95 V for Li ½ NiO [ 28 ] 2 and 4.1 V for Li ½ CoO2. [ 27 ] The issue is, however, that the discharge curves of most of these materials comprise two steps: in plain terms, the low voltage step is related to the oxidation of Mn ions, and the high voltage step is related to the oxidation of the other metal in the Li x M y Mn2 yo 4 spinels. Thus, the high-voltage step occupies only part of the lithiation curve and the average discharge voltage is not too high, despite the impressive 5 V height of the voltage plateau. The 5 V-step increases the overall energy of Li x Co0.4Mn 1.6 O 4 only by 6%: this behavior is demonstrated by most spinels of Li-Mn-M-O systems with M = Co, Cr, Co, Fe ( Figure 15 ; and references in Table 2 ). [ 75, 128 ] The only known exception (and one of the most studied spinel materials) is LiNi 0.5 Mn 1.5 O 4, which, being properly prepared, may offer a one-step discharge curve. [ 105, 111, 129, 130 ] Another challenge is that most of the spinel-type oxides develop two-phase behavior; judging from the results of abinitio computations, [ 59 ] existence of this two-phase region is not determined by a specific component (e.g., manganese) but is a feature of the spinel structure. The computations indicate that not only real spinel cathode materials, but also a simulated nonexistent spinel-type single phase Li x CoO 2, break down into two phases upon lithiation, forming a two-phase region which results in a step-wise charge curve ( Figure 16 ). This is an unfavorable situation since the coexistence of two phases in a substantially wide interval of lithium content results in phase bordering and phase interface movement through cathode material grains; this is exactly the process which forms intergrain stresses and is detrimental for the preservation of grain integrity. At the same time, layered oxides may keep the same structure over the course of lithiation and thus avoid phase border shifts through material grains, and therefore it is not a surprise that up to now layered oxides develop better cycle life then spinels (e.g., see data on spinel-type cathodes). [ 106 ] While considering safety issues, the most employed layered material (LiCoO 2 ) has developed decomposition with oxygen 930 wileyonlinelibrary.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

10 Ta b l e 2. Electrochemical data for high-voltage lithium cathode materials, Li 2 MMn 3 O 8 and LiMMnO 4 based on spinel-structure oxides. Adapted with permission. [ 66 ] Copyright 2007, Elsevier. Material Mid-discharge voltage [V] plateau >4.5 V Redox couple b) at plateau >4.5 V Plateau centered at 4.0 V [mah g 1 ] Plateau >4.5 V [mah g 1 ] V range [V] Reference Li 2 CrMn3O Cr 3 + / [122,123] LiCrMnO Cr 3 + /4 + 0 c) [122,123] Li 2 FeMn 3O Fe 3 + / [124] Li 2 CoMn3O Co 3 + / [104,66] LiCoMnO Co 3 + / [125] Li 2 NiMn 3 O Ni 2 + / [105,126,127] Li 2.02 Cu 0.64 Mn 3.34 O a) Cu 2 + / [117] a) b) Composition of the spinel component at the nominal composition Li 2 CuMn 3 O 8 ; all redox couples are located in octahedral sites; c) an inclined single plateau ranging V was observed. Figure 15. Comparison of charge (100 mca/cm2 ) curves for Li/Li + /Li y M x Mn 2-x O 4 (M = Cr, Fe, Co, Ni and Cu) cells. Adapted with permission. [ 75 ] Copyright 2004, Elsevier. evolution starting from T dec = 230 C [ 131 ] (naturally, reaction with the electrolyte may start at 190 C [ 132 ] or even as low as at 155 C). [ 133, 134 ] The introduction of doping and alloying elements may drastically shift up the decomposition onset temperature T dec [ 135, 136 ] (e.g., T dec 300 C for LiNi Co 0.55 Mn O 2 ) [ 136 ] and also avoid high-temperature reactions of the material with the electrolyte. [ 137 ] Aluminum (a high-voltage dopant for layered oxides) may enhance the thermal stability of layered cathode compounds or, at least, will not compromise it. [ 35, 58, ] There is also the option to coat the grains of the base cathode material with a thin layer of thermally stable Li + -conducting compounds; this approach may increase T dec by up to 50 C compared with the uncoated material. [ ] One more option is to introduce fluorine; e.g., T dec is 20 C higher for Li[Ni 1/3 Co 1/3 Mn (1/3-0.04) Mg 0.04 ]O 1.92 F 0.04 than for Li[Ni 1/3 Co 1/3 Mn (1/3-0.04) Mg 0.04 ]O 2. [ 145 ] Spinel-type materials are thermally more stable; e.g., LiMn 2 O4 decomposes with oxygen evolution starting at about 600 C; [ 146 ] Li x Mn2O 4 with x 1 decomposes at lower temperatures, though, and the reaction with the electrolyte also takes place at lower temperatures. [ 147 ] Nickel doping decreases the decomposition temperature, shifting it as low as 240 C. [ 148 ] This is particularly true for the spinel high-voltage champion LiNi 0.5 Mn 1.5 O 4 [ 149 ] but its thermal stability may be substantially increased by fluorination [ 148, 150 ] and also Cr doping. [ 151 ] Summing up, it might be concluded that spinel compounds look more promising as candidates for commercial high voltage cathode materials (compared to layered oxides) from the points of view of intercalation potentials, rate capability, energy density, safety, and environmental compatibility. Considering further improvements of spinel-based materials, the major field of activity is increasing its cycle life. [ 152 ] Recently it was demonstrated that Ru-doped LiNi 0.5 Mn 1.5 O 4 materials have an excellent cycle life and power capability, [ 153 ] and don t develop different immiscible phases. It is now commonly accepted that a material s particle size plays an important role in electrochemical performance [ 154, 155 ] and specifically in determining the charge/discharge voltage of the cathode materials. [ 156 ] There have been some encouraging results on grain size-related properties of spinel oxide cathodes ( Figure 17 ).[ 153, 157 ] 3.6. Poly-Anion Compounds Poly-anion cathode materials have attracted quite a large amount of attention following the pioneering works of 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 931

11 Figure 16. Calculated phase diagram of simulated spinel Li x CoO 2 (Co ions remain in the 16d sites). The full line bounds the region with two immiscible phases (between x = 0.5 and x = 1.0), the dashed line bounds the region where a second-order transition (disordered phase) (ordered phase) takes place (centered on x = ¼); in the ordered phase, Li + occupy every second [8a] site while other tetrahedral and octahedral sites are empty. Adapted with permission. [ 59 ] Copyright 1999, American Physical Society. Goodenough et al. [ 158, 159 ] These materials. include compounds with a NASICON-type crystal lattice Li x M 2 ( Ẋ O 4 ) 3 (here Ṁ is Ni, Co, Mn, Fe, Ti or V and X. is S, P, As, Mo or W), an olivine-type crystal lattice Li MẌ x O 4 (here M is Fe, Co, Mn or Ni and Ẍ is P, Mo, W or S), [ 160 ] and poly-anion materials with tavoriterelated structures (tavorite, triplite, maxwellite,. sillimanite), with the general formula LiM M 1 δ δ (ZO 4 )X 1 α X α, where at least one of Ṁ or M is a metal with several possible oxidation states, Z is commonly phosphorus or sulfur, and X and Ẋ are oxygen, a hydroxyl group, or a halogen (commonly fluorine). [ ] All these are materials with open 3D frameworks, which are available for Li migration. These poly-anion compounds have been extensively investigated in the last decade; their cyclability, safety (the materials are less prone to decomposition and releasing oxygen at elevated temperatures than corresponding i.e., with the same transition metal layered oxides and spinels), [ 134, 165 ] environmental compatibility, and potentially low production cost make them the most promising electrode material in prospective Li-ion batteries. NASICON frameworks are formed by Ṁ. 2 ( X O 4 ) 3 ; in these frameworks all [ Ṁ O 6 ]-octahedral corners are common with Ẋ O 4 -tetrahedral corners, and all Ẋ O 4 -tetrahedral corners are common with [ Ṁ O 6 ]-octahedral corners; materials with such frameworks are known to be available for topotactic insertion/extraction of alkaline ions, [ 166 ] and specifically Li + ions, due to the availability of the open 3D framework for easy Li + movement along conducting channels. [ 167 ] This feature makes NASICONs promising materials for rechargeable lithium batteries. [ 168 ] The conduction pathways, which NASICON frameworks offer for Li + motion, are commonly considered 1D paths along the C-axis. [ 160, 169 ] The conducting channels are not really straight, linear pathways, but zigzag, wavy pathways. [ 170, 171 ] The LiNi 0.5 Mn 1.5 O C, 300nm 1C, mcm 5C, 300nm 5C, mcm 10C, 300 nm 10C, mcm Volts vs. Li/Li mah g-1 Figure 17. Discharge curves of LiMn1.5 Ni 0.5 O 4 cathodes at various C-rates and grain sizes. Adapted with permission. [ 153 ] Copyright 2011, American Chemical Society. 932 wileyonlinelibrary.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

12 Figure 18. LiFePO4 (olivine structure) representation; the oxide shown being comprised of dark gray FeO 6 octahedra and light gray PO 4 tetrahedra lithium ions are shown as light gray balls occupying octahedral sites, which form linear chains of edge-shared octahedra; Reproduced with permission. [ 24 ] Copyright 2007, Elsevier. pathway topology depends on the morphology of the material. Specifically, Li + hopping between pathways is possible in the presence of Ẋ O [ 4-tetrahedral disorder, 172 ] which actually turns conductivity lines into conductivity branches; it may be concluded that whereas Li + motion is strongly anisotropic it is not purely.. a 1D process and has a quasi-2d character. NASICON M 2 ( X O 4 ) 3 frameworks are versatile from the chemical point of view: their structure could accommodate a variety of transition metal cations with a set of redox potentials, and various X. O 4 anions. The Li MẌ x O 4 [ M = Fe, Co, Mn or Ni and Ẍ = P, Mo, W or S] olivine structure has Li and M atoms in the octahedral sites and Ẍ atoms in the tetrahedral sites of a hexagonal-close-packed oxygen array. With Li in a continuous chain of edge-shared octahedra on alternate planes, a reversible topotactic insertion/ extraction of lithium from/to these chains is also possible and appears to be analogous to the extraction or insertion of lithium in NASICON-type materials. [ 160, 173 ] Similarly to NASICON, Li + motion is predominantly exercised through 1D wavy pathways along a [010] crystallographic axis [ 174, 175 ] (this finding, which looks natural from the schematic structure image of the olivinetype material, is shown in Figure 18 ); a careful consideration of Li + dynamics in the olivine lattice also reveals that there is Li + hopping between different [010] pathways as well. [ 176 ] However, the diffusion coefficient for the displacement along [010] pathways is two orders of magnitude higher than for the displacement between these pathways. [ 177 ] Tavorite-family poly-anion materials crystallize in triclinic (tavorite, triplite, maxwellite) and orthorhombic (sillimanite) structures; the family is named after the mineral tavorite (LiFe- PO 4 OH). Comprising isostructural lithium fluoro(hydroxyl) phosphates/sulfates with a variable F /OH content ratio, the tavorite structure constitutes linear chains of corner-sharing MO 4 F 2 octahedra propagating along the C axis interconnected by corner-sharing SO 4 or PO 4 tetrahedra along the ā and b axes, and this design forms a 3D network of Li-conducting tunnels. [ 178, 179 ] Other materials of the tavorite-family are variations of this basic 3D framework. By now, the lithium transport mechanism is under dispute concerning the specific mobility along different space directions in the tavorite-type lattice. Calculations [ 180 ] demonstrate that hopping activation energies (and thus, the mobility) along all three [100], [010], and [111] directions are quite similar in the case of LiFeSO 4 F (resulting in 3D lithium conductivity), whereas the work by the Ceder group [ 161 ] predicts that this energy is substantially lower along the [111] axis than along other axes, which results in 1D conductivity. Apparently, as far as the implementation in Li-ion cells with a long cycle life is concerned, a complete substitution of fluorine for hydroxyl is advantageous since it avoids the undesirable irreversible electrochemical reaction of the hydroxyl group with Li. Considering their electronic structure, poly-anion cathode materials are a promising class of compounds in terms of high operational voltage. Strong covalent bonding within the poly-anion reduces the covalent bonding to the iron ion (inductive effect), [ 167, 181, 182 ] which, in turn, lowers the redox energies of M + (n) / M + (n +1) and Ṁ. ( +n) / M + (n +1). The higher the covalent component of the bonding within the poly-anion, the lower the position of the metal cation redox level is against the Li/ Li + energy level. [ 158 ] The average discharge voltage is 2.98 V for LiFeO 2 cathode, [ 183 ] whereas the discharge voltage is 3.5 V for LiFePO [ 4 cathodes; 184 ] the same effect is demonstrated by the cobalt-based redox couple: the discharge voltage is 4.1 V for a LiCoO [ 2 cathode but it is 4.8 V for a LiCoPO 4 cathode. 185 ] This inductive effect assumes that intercalation/de-intercalation voltages may be tuned by changing poly-anions. Particularly, ab-initio calculations suggest that in the case of materials such as LiCoXO 4 (X = P; As; Sb), Li 2 VOXO 4 (X = P; As; Si; Ge) and Li 2 MXO 4 (M = Mn, Fe, Co, Ni and X = P, Si, Ge) the intercalation/de-intercalation voltages change in the following order: V GeO4 < V Si O4 < V SbO4 < V As O4 < V PO4 ; generally, this sequence is confirmed experimentally. This order is observed for vanadium-based materials, [ 186 ] and in the case of iron based materials V discharge Si O V, [ 184 ] while on cobalt, experimental results show that V discharge Si O 4 is between 4.1 V [ 188 ] and 4.25 V, [ 189 ] V discharge As O 4 is 4.6 V, [ 190 ] and V discharge PO 4 is 2.75 V [ 187 ] and V discharge PO V. [ 185 ] Considering poly-anion cathode materials, it is well to bear in mind that poly-anions bring some inactive mass, which compromises the specific charge capacity. Atomic weights of Si (at. wt. 28), S (at. wt. 32) and P (at. wt. 32) are close enough and all these atoms are substantially lighter than Ge (at. wt. 73) and As (at. wt. 75). Thus, the implementation of the PO 4 anion looks favorable from the point of view of high voltage as well as high specific capacity. Since boron (at. wt. 11) is substantially lighter than all the above elements, and the structure of LiMBO 3 (M = Fe, Co, Mn) offers easy paths for Li + (the structure is built by MO 5 trigonal bipyramids, and BO 3 planes form the 3D M(BO 3 ) open framework), boron-based poly-anion compounds have attracted the attention of theoreticians [ 191, 192 ] and experimentalists. Unfortunately calculations have revealed that values of 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 933