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a Fully lithiated cathodes SEs NCM LCO LMO LFPO LPSCI LGPS LPS LLZO 0 –100 –200 –300 –400 –500 –600 Reviews b Oxides Good chemical stability • Same anion chemistry • Strong hybridization Sulfides Good chemical stability • Similar cation chemistry • Strong hybridization Poor chemical stability • Anion exchange • Li3PO4 formation Polyanionic oxides Li2ZrO3 LiNbO3 LiTaO3 Non- polyanionic oxides Polyanionic oxides SEs LiH2PO4 LTi2(PO4)3 LiBa(B3O5)3 LiPO3 LiLa(PO3)4 LiCs(PO3)2 LPSCI LGPS LPS LLZO Nature reviews | Materials Fig. 5 | Polyanionic oxides as a bridge between oxides and sulfides for good chemical compatibility. a | Reaction energies at fully lithiated cathode/solid electrolyte (SE), fully lithiated cathode/coating and coating/SE interfaces. b | Pair-wise chemical compatibility between oxides, sulfides and polyanionic oxides. The red-shaded box indicates high reactivity (>100 meV/atom) and the green-shaded boxes indicate low reactivity (<100 meV/atom). LCO, LiCoO2; LFPO, LiFePO4; LGPS, Li10GeP2S12; LLZO, Li7La3Zr2O12; LMO, LiMn2O4; LPS, Li3PS4; LPSCl, Li6PS5Cl; NCM, LiNi1/3Co1/3Mn1/3O2. Panel a is reproduced with permission from ref.67, Elsevier. LiPON/Li interface. Consistent with these predictions, Li3P and Li2O have been detected at the Li3PO4/Li inter- face by XPS222. In the exploration of other anion chemis- tries for stabilizing the SE against reduction by Li metal, nitrides were found to have the lowest calculated reduc- tion limits compared with other anion chemistries, mak- ing nitride chemistry attractive for SE protection on the anode side224. Indeed, BN was recently reported to pro- tect the LATP/Li interface225, and a Li3BN2 glass electro- lyte has shown good stability with Li metal, as indicated by the stable cycling profile of a Li symmetric cell226. Considerations on interface stability Trade-offs An ideal SE should exhibit high ionic conductivity and interfacial compatibility with both the anode and cathode. In fig. 6, we show the oxidation and reduction limits and room-temperature ionic conductivity for various SE categories. The desired combination of ionic conductivity and electrochemical stability is located at thetop-rightcorner(oxidationlimit=5V,reduction limit = 0 V, ionic conductivity = 10 mS cm−1), which has yet to be achieved by any SE. Many strategies have been employed to enhance the ionic conductivity or stability of SEs by tuning their composition. However, as illustrated in fig. 7, they often result in trade-offs between the ionic conductiv- ity, oxidation and reduction stability, which prevent the discovery of an ideal SE. For example, the strategy for achieving good ionic conductivity can negatively affect the oxidation stability. Room-temperature Li-ion con- ductivity above 10 mS cm−1 has only been observed in sulfide SEs with the highly polarizable S2− anion, which is excellent at shielding the interactions of Li ions with the host structure or with other Li ions. However, the loosely bonded electrons of S2− are also associated with a low electron affinity and subject to facile electron extraction at high voltage, resulting in an oxidation limit below 2.5 V. By contrast, oxide SEs typically have oxidation limits greater than 3 V (fig. 6), but the use of O2− comes at the cost of ionic conductivity at least one order of magnitude lower than that of sulfides because of the reduced shielding effect in oxides34,227. This trade-off between the ionic conductivity and oxidation stability in oxides and sulfides has also been investigated from a lat- tice dynamics perspective228. Switching the anion chem- istry from O and S to halogens such as F− and Cl− can make it more difficult to oxidize the anion. In addition, the monovalent anions can reduce the bare electrostatic interaction of Li ions with the anion lattice, but these halogen anions also have a small polarizability, limiting the shielding effect and making the overall effect on the ion mobility unclear229. The competition between these two effects depends on the specific structure of the mate- rial. There were few halide superionic conductors before the recently reported Li3YCl6 (ref.230), Li3YBr6 (ref.230) and Li3InCl6 (ref.231). Whether this lack of good halide con- ductors is intrinsic or a result of the fact that they may be difficult to synthesize is not yet clear. Hybridizing the anion states may be a viable way to overcome the trade-off between oxidation stability and ionic conductivity. As we discussed, the hybridization between P (or B) with O in polyanionic coatings lowers the O electron states and increases the oxidation stability compared with those of oxide coatings67. This hybridiza- tion effect is also seen in fig. 6, where NASICON conduc- tors containing the PO4 group (dark blue) exhibit higher oxidation limits than other oxide SEs, such as perovskites (brown) and garnets (green). Hybridization may also con- tribute to the increased ionic conductivity of SEs. Upon substituting Sn with Ge and then with Si in Li10MP2S12 (M=Sn, Ge, Si), the increased hybridization between the M and S elements pulls the electron density away from the Li-ion diffusion channel34,232. This effect reduces the Reaction energy (meV/atom)PDF Image | Understanding interface stability in solid-state batteries
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