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Reviews 0.85 V (ref.145) and 1.05 V, respectively, indicating that the cation reducibility increases as Zr4+ < Ta5+ < Nb5+. LLZO was computed to be only marginally unstable against Li metal with a driving force for reduction decomposition of 20 meV/atom65 and with possible reduction products that include Zr, La2O3, Li8ZrO6, Zr3O and Li2O (refs64–66). Such a small driving force may not be sufficient to nucle- ate the solid products, which may lead to a kinetically stabilized LLZO/Li interface. The kinetic stability of garnets against Li metal can also be evaluated by con- sidering the Li insertion into the garnet structure during the initial reduction process. DFT calculations predict that the topotactic lithium-insertion voltages of LLZO and LLTaO are −0.95 V and −1.03 V, respectively146, indi- cating that initiating the reduction of LLZO and LLTaO requires a high activation energy. By contrast, the com- puted topotactic lithium insertion voltage for LLNbO is positive (0.07 V), which suggests the facile reduction of LLNbO by Li metal146. Experimental observations at the garnet/Li interfaces agree well with the DFT calculations. Early studies involv- ing contacting a garnet pellet with molten Li and observing the colour change suggested that LLZO and LLTaO may be stable against Li metal25,136,142,147, whereas LLNbO is not stable, likely because of the reduction of the Nb5+ cation148. The impedance of a Li|Li6.25La3Zr1.25Nb0.75O12|Li symmetric cell was observed to increase with time148 and cycle number149; however, that of a symmetric cell using LLZO or Li6.25La3Zr1.25Ta0.75O12 (refs149–151) did not increase. Several studies based on XRD and XPS analy- ses also revealed no detectable structural or oxidation- state change in LLZO and LLTaO upon contacting Li (refs142,147,152,153), confirming the apparent stability of LLZO/Li and LLTaO/Li interfaces. Despite the predicted high kinetic barrier for LLZO reduction146, the reactivity at the LLZO/Li interface can be revealed by elevating the reaction temperature, thereby accelerating the reaction kinetics, or by using advanced characterization techniques that allow mini- mal reactions to be detected. Heating Al-doped LLZO samples in molten Li at 300–350 °C indeed enabled the observation of the chemical coloration of the LLZO sur- face153. In situ STEM characterization of the LLZO/Li interface indicated that Zr4+ was slightly reduced when contacting Li metal, producing a ~6-nm-thick tetragonal LLZO interphase154. DFT calculations have shown that the tetragonal phase is lower in energy than the cubic phase at higher Li concentration154, suggesting that the formation of the tetragonal LLZO layer is caused by Li insertion into the cubic LLZO. The reduction of Zr4+ to one of the predicted decomposition products, Zr3O (ref.66), was confirmed after discharging to 0 V in a Li|liquid electrolyte|LLZO+C half cell; the associated XPS results are presented in fig. 4b (ref.97). Very recently, the effect of dopants (Nb, Ta, Al) in LLZO on its stabil- ity with Li metal was studied155. Similar to the previous findings, the XPS data indicated that Nb5+ is reduced by Li metal, leading to the formation of an MCI and causing a continuous increase of the interfacial imped- ance with time. Some Zr4+ reduction to Zr2+ or Zr0 was also observed in all three doped samples. Other reduc- ible dopants such as Fe3+ in LLZO also lead to strong reduction at the LLZO/Li interface, resulting in the for- mation of a thick (130 μm) tetragonal LLZO interphase and large interfacial resistance156. Chemical mixing at high temperature. The chemi- cal stability of garnets against different cathodes has been investigated using DFT64,65,80,145. The stability of the garnet/cathode interface at elevated temperature is important, because sintering is typically required for oxide SE processing27. The predicted driving force for LLZO reaction with LiCoO2 and LiNi1/3Co1/3Mn1/3O2 at 0 K is extremely low (1 meV/atom) but is higher for LiMn2O4 (63 meV/atom) and LiFePO4 (94 meV/atom)65,67. However, at high temperature, configurational entropy may further favour interdiffusion of elements between the SE and cathode, increasing the interfacial chemical reactivity. Li loss above 1,000 °C (ref.157) and the gen- erally more reducing environment at high tempera- ture80,158 may also shift the system to off-stoichiometry and induce instability of garnets. For instance, the decomposition products La2Zr2O7 and La2O3 have been observed in LLZO thin films sintered at 1,090 °C and 1,100 °C (ref.159). These two products are also predicted to form when charged Li0.5CoO2 is brought in contact with LLZO or when LLZO is oxidized at high voltage65, indicating that their formation is driven by the loss of Li from LLZO. The results of several experimental characterization studies of the LLZO/cathode interface at high temper- ature are consistent with the thermodynamic predic- tions. XRD analysis showed that LiMn2O4 and LiFePO4 react strongly with LLZTO at 500 °C, whereas LiCoO2 and NCM only showed evidence of a slight reaction with LLZTO to form LaCoO3 at 700 °C, as detected by XRD and Raman measurements160. Similar results were obtained for a garnet Li6BaLa2Ta2O12 with other oxide cathodes161. Furthermore, no evidence of chemical reac- tion between LiCoO2 and LLZTO was observed during sintering using Raman analysis162. However, conflicting results have been reported. Decomposition products such as La2CoO4 (ref.79), La2Li0.5Co0.5O4 (ref.163), La2Zr2O7 (ref.164) and tetragonal LLZO128 have been observed in different studies of the LLZO/LiCoO2 interface. The formation of tetragonal LLZO was explained by the observed Al diffusion from LLZO to LiCoO2 during sintering, which destabilized the cubic LLZO phase128. It was also shown that the interdiffusion of La and Co already occurs at 400 °C to form Co3O4 (ref.81). Note that the reduced transition-metal cation Co2+ is present in both La2CoO4 and Co3O4, as expected from the reduc- ing environment at high temperature80. The reactivity between LLZTO and spinel cathodes was investigated using first-principles calculations and experimental characterization80. In fig. 4c, the calculated reaction energy at 800 °C is plotted as a function of the mixing fraction of LLZTO in the cathode/SE mixture. The minimum reaction energy ranges between −60 and −30 meV/atom, indicating a mild driving force for the decomposition at the garnet/spinel cathode interface at high temperature. The chemical reactivity was verified by XRD analysis, with the detection of reaction products including La2O3, La2Zr2O7, NiO, Li2MnO3 and LaMnO3, www.nature.com/natrevmatsPDF Image | Understanding interface stability in solid-state batteries
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