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Reviews 4.0 3.0 2.0 1.0 0.0 Ideal Sulfides Garnets LiPON Perovskites Antiperovskites NASICON Halides Li3N 5 4 3 2 1 0 Fig. 6 | electrochemical stability windows of common solid electrolytes. Each solid electrolyte is represented by a circle with area proportional to the order of magnitude of its ionic conductivity (in μS cm−1). The stability window is the vertical distance from the centre of the circle to the diagonal line, as illustrated by the arrow. The dashed lines are the contours of the width of the stability window. LiPON, lithium phosphorus oxynitride; NASICON, Na superionic conductor. electrostatic interaction between Li ions and the host structure, leading to a lower Li-ion migration barrier34,232. Metals and metalloids make up over 70% of the peri- odic table. Their introduction into SEs has resulted in a structural diversity that has greatly enlarged the param- eter space for the optimization of ionic conductivity. Indeed, the best sulfide and oxide conductors such as LGPS, Li9.54Si1.74P1.44S11.7Cl0.3, garnets and NASICONs all contain at least one metal or metalloid element(s). However, these cations are often reduced against Li metal, creating an MCI at the SE/Li interface. To miti- gate this issue, metal or metalloid cations that are more difficult to reduce (such as Ca2+ or La3+) can be used or the content of non-metal cations such as P5+ and H+ that can form a passivating SEI can be increased, as observed in the hydration of Na3SbS4 (ref.114). Anion chemistry can also affect the reduction stability of metal cations224,233. With the same cation, the reduction stability follows the trend fluorides < sulfides < oxides < nitrides. Table 1 summarizes the anion-dependent stability of various cations against Li metal based on computational and experimental data. The table also shows whether an SEI or MCI interphase is expected to form when the cation is reduced by Li metal. This table can serve as a reference for selecting dopants or designing the composition of new SEs and anode coatings. The nitrogen anion stabi- lizes numerous cations (such as Al3+) against reduction by Li metal that would otherwise be reducible with other anion chemistries224. However, these nitrides suffer from a low intrinsic oxidation limit typically below 2 V, making them difficult to pair with high-voltage cathodes64,233. Completely avoiding the use of reducible cations leads to absolute reduction stability against Li metal, which is the case for the nitride conductor Li3N and anti- perovskite conductors Li3OCl and Li3OBr0.5Cl0.5 (fig. 6). However, the lack of any covalent bonding with anions leads to an oxidation limit below 3 V. For antiperovskites, decomposition products such as LiCl and LiClO4 may passivate the SE/cathode interface, as indicated by the measured wide voltage stability window192. For Li3N, oxidation decomposition likely leads to continuous gas formation and SE consumption. It has been shown that increasing the Li content shifts the electrochemical stability window down towards 0 V, as observed in the Li–Si–O system65,67, directly leading to a trade-off between the oxidation and reduction stabil- ity. The decrease of oxidation stability with increasing Li content can be viewed as a result of the weakened cova- lency of the anions, as they are increasingly interacting with Li. Increasing the Li content also typically benefits the ionic conductivity of an SE. This trend was observed in a statistical learning study of the ionic conductivity of crystalline SEs234 and experimentally demonstrated in garnets27 and in the glass systems Li2O–B2O3 and Li2S– P2S5 (ref.235). Hybridization, by contrast, can extend the stability window on both the oxidation and reduction limits by lowering the bonding-state energy and elevat- ing the antibonding-state energy. The increase of the oxidation limit by hybridization was discussed above in the comparison between NASICON SEs and other oxide SEs. The hybridization effect on the reduction limit can be demonstrated by comparing the reduction limit of Li3PS4 (1.69 V) with that of Li3PO4 (0.71 V). P–O bond- ing in Li3PO4 has a higher degree of hybridization than P–S bonding in Li3PS4, as indicated by their large bond-energy difference (596.6 kJ/mol for P–O versus 346 kJ/mol for P–S)106. Pitfalls of CV measurements Commercialized solid-state cells must provide consistent operation over thousands of cycles and excellent cou- lombic efficiency, thereby requiring the minimization of interfacial reactions after an initial passivation, if any. Thus, careful studies on the degradation behaviours and mechanisms at the electrode/SE interface are needed. In this context, it is important to note that CV, a conven- tional method that has been widely used to estimate the voltage stability of liquid electrolytes, can lead to an overestimated stability window of the SE if the data are not interpreted carefully. Several stability windows determined from CV measurements are unphysically wide and have often been corrected by more careful follow-up studies. For instance, the claims of stability windows of 0–5 V for LGPS32, 0–9 V for LLZO26 and 0–8 V for Ba-doped Li3OCl (ref.192) defy basic chemistry. These oxidation limits are significantly higher than the thermodynamically predicted values and cannot be sim- ply justified by kinetic stabilization. As noted in several studies50,97,233, the CV method only reliably detects the presence of either a non-passivating reaction forming an MCI that continues to grow (as in the reduction of LLT)179 or a passivating decomposition reaction with a large enough reaction region. The absence of notice- able current at high voltages during a CV sweep is often taken as evidence of the wide voltage window of an SE when, in reality, a passivation layer could have formed or the reaction area may be restricted by the limited 543210 Reduction limit (V) www.nature.com/natrevmats Oxidation limit (V)PDF Image | Understanding interface stability in solid-state batteries
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