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Reviews This observation agrees well with DFT results predicting Co3+ reduction to Co2+ and N3− oxidation to N2 at this interface, with other possible products including CoN, Li3PO4 and Li2O (ref.65). DFT can also capture inter- facial reactions under battery-cycling conditions. Using STEM with electron energy loss spectroscopy, a dis- ordered interfacial layer in the pristine LiCoO2/LiPON interface was identified, from which CoO evolved after battery cycling177. Indeed, CoO formation was predicted by DFT in the reaction between half-charged LiCoO2 and LiPON65. Perovskites Perovskite-type lithium lanthanum titanate Li3xLa2/3−x ◻1/3−2xTiO3 (0 < x < 0.16) (LLT) and structurally related materials28 exhibit high bulk Li-ion conductivity up to ~10−3 S cm−1 at room temperature178. However, the use of LLT as an SE in SSBs is not desirable as it has been observed to form an MCI in contact with Li metal due to the reduction of Ti4+ (ref.28). This phenomenon is con- sistent with the DFT prediction that LLT decomposes against Li metal into La2O3, Li2O and metallic Ti6O (ref.65). The reduction stability of LLT has been investigated experimentally by intercalating Li into LLT. The Li inter- calation voltage was determined to be 1.8 V using CV179 and 1.5 V using galvanostatic discharging180,181, both of which are close to the predicted reduction limit of LLT (1.75 V)65,66. X-ray absorption spectroscopy analy- sis of a Li-inserted LLT sample revealed the reduction of Ti from 4+ to 3+, with the La3+ valency remaining unchanged, as predicted182,183. In situ XPS measurements on the LLT/Li interface confirmed the presence of Ti3+, Ti2+ and Ti metal63. On the high-voltage side, LLT is predicted to be stable up to 3.71 V and form O2, TiO2 and La2Ti2O7 at higher voltages65, indicating that LLT may be paired with low-voltage cathodes such as LiFePO4. Recently, a Li|LLT|LiFePO4 solid-state cell was cycled between 2.8 and 4.0 V, with polyethylene oxide used as the catholyte and also buffer layer between Li and LLT182. The observed high coulombic efficiency after the first five cycles suggests that LLT oxidation, if occurring, is self-limiting. A negligible driving force for chemical mixing of LLT with LiCoO2 (0.5 meV/atom) to form Co3O4, La2Ti2O7, Li2TiO3 and Li0.5CoO2 is predicted by DFT calcula- tions65. Indeed, high-resolution TEM analysis revealed that a sharp LLT/LiCoO2 interface is formed using pulsed-laser deposition without the formation of any intermediate phases184. At elevated temperatures, it was also demonstrated, using XRD, that LLT is chemically stable with LiMn2O4 up to 800 °C and stable with LiCoO2 up to 700 °C, although β-LLT was observed in the lat- ter case at a higher temperature185. The decomposition products at the LLT/LiCoO3 interface at 700 °C were further characterized, detecting the formation of Co3O4 and La2Ti7O2 (ref.186), which agrees well with the DFT prediction65. By contrast, LiNiO2 was observed to react strongly with LLT to form NiO and La2Ti2O7 at 500 °C, a lower temperature than the reaction-onset temperature of700°CforLiCoO2 (ref.185).DFTcalculationsverified that LLT has a higher reaction driving force with LiNiO2 (17 meV/atom) than with LiCoO2(0.5 meV/atom), and the observed NiO and La2Ti2O7 were also predicted to be present at the LiNiO2/LLT interface. Antiperovskites Li-rich antiperovskites are a class of recently discovered ionic conductors with the basic formula Li3XY, where X and Y are divalent (for example, O2−) and monovalent (for example, Cl−) anions, respectively. The reported ionic conductivities of antiperovskites range widely from 10−7 to 10−3 S cm−1 (refs29,124,187,188). The most unique feature of antiperovskites is the absence of non-Li cations in the composition, which, in principle, leads to an absolute reduction stability at 0 V, as no element can be further reduced by Li metal189. However, the self-decomposition of metastable Li3OCl and Li3OBr into Li2O and LiCl or LiBr is still possi- ble189,190. The Li3OCl/Li interface was investigated by cycling a Li|Li3OCl thin film|Li symmetric cell187. The voltage of the symmetric cell increased in the first three cycles and then stabilized in subsequent cycles, indicating the apparent stability of Li3OCl with Li metal. The origin of the initial increase in the cell voltage remains unclear and might be linked to Li3OCl self-decomposition. On the other hand, the lack of non-Li cations in the antiperovskites, which can covalently lower the energy of the anion electron states191, also limits their oxidation stability to below 3 V. DFT calculations pre- dicted the onset of oxidation of Li3OCl at 3 V (ref.64) or 2.55 V (ref.189) to form products including ClO3, LiClO3, LiClO4, Li2O2 and LiCl. Because these reaction products are electronic insulators, an SEI is expected to form at high voltage and may prevent further SE oxidation. Electrochemical stability windows estimated from CV measurements indicate an oxidation stability of 8 V for the stoichiometric and Ba-doped Li3OCl (ref.192) and even above 9 V for Li2(OH)0.9F0.1Cl and Li2OHBr (ref.193). These high voltages clearly cannot represent the intrin- sic stability of these conductors and are more likely an indication of the passivation by the SEI formation at the SE/inert-electrode interface at high voltage. When pair- ing Li3OCl with a LiCoO2 cathode and a graphite anode in a thin-film battery, the coulombic efficiency in the first cycle is 83% and increases to approximately 95% in subsequent cycles187. Because the computed driving force for chemical mixing between LiCoO2 and Li3OCl is negligible (7 meV/atom), this phenomenon likely orig- inates from the Li3OCl oxidation and passivation at high voltage. However, similar to the Li3OCl/Li interface187, there is no direct experimental evidence of the interfa- cial passivation of Li3OCl at high voltage in the litera- ture, and more careful measurements of the passivation layer and its growth are needed. NASICONs The general formula Li1+xAxM2−x(PO4)3, where A is a tri- valent cation (such as Al3+, La3+, In3+ or Cr3+) and M is a tetravalent cation (such as Ti4+, Ge4+, Hf4+, Zr4+ or Sn4+), represents a class of ionic conductors with the NASICON structure194. Two representative compounds in this class, Li1+xAlxGe2−x(PO4)3 (LAGP) and Li1+xAlxTi2−x(PO4)3 (LATP), have been studied extensively because of their high ionic conductivity (>10−4 S cm−1)14,195,196. www.nature.com/natrevmatsPDF Image | Understanding interface stability in solid-state batteries
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