In situ X-ray photoelectron spectroscopy investigation of the solid electrolyte interphase in a Li/Li_6.4Ga_0.2La₃Zr₂O₁₂/LiFePO₄ all-solid-state battery

The fundamental understanding of the solid electrolyte/electrode interphase during charge and discharge is promising for the development of all-solid-state batteries. Here, we employ a solid-state Li/Li_6.4Ga_0.2La₃Zr₂O₁₂/LiFePO₄ cell and study in situ the electrolyte/anode and electrolyte/cathode interphase as a function of applied potential under ultra-high vacuum using X-ray photoelectron spectroscopy. An ultra-thin interfacial passivation layer is formed at the Li/Ga-doped LLZO interphase, which inhibits the Ga-LLZO from being further reduced by Li. During charging, the oxidation reaction of Li with Ga-LLZO still occurs, which is evidenced by the broadening of the full width at half maximum (FWHM) and shifting of the Li 1s peak towards lower binding energies and the increment of Li₂O peak intensity in the O 1s range. The Ga-LLZO solid electrolyte/LiFePO₄ cathode interphase was also studied by in situ XPS under UHV. The deintercalation of Li⁺ ion is evidenced by the broadening and shifting of Fe 2p peak during charging. Other components shift by the same values as the applied potential. These results indicate good reversibility of the LiFePO₄/Ga-LLZO interphase upon charge/discharge. The long-term stability of the LiFePO₄/Ga-LLZO interphase was also examined during charging/discharging for 100 cycles. These insights provide a new perspective for understanding of the solid electrolyte interphase, which enables high energy density, long-term stability, and safety for next-generation batteries. [Figure not available: see fulltext.]