Revealing Dynamic Evolution of the Anode‐Electrolyte Interphase in All‐Solid‐State Batteries with Excellent Cyclability

Abstract All‐solid‐state‐batteries (ASSBs) based on a halide solid electrolyte (SE) and a lithium‐metal based anode typically have poor cyclability without a buffer layer (such as Li 3 PS 4 or Li 6 PS 5 Cl) to prevent the degradation reactions. Here excellent cycling stability of ASSB consisting of an uncoated single‐crystal LiNi 0.8 Co 0.1 Mn 0.1 O 2 cathode and a Li 3 YCl 6 (LYC) SE separator in direct contact with a Li‐In anode are demonstrated. Through a combination of electrochemical measurements, synchrotron micro‐X‐ray absorption and diffraction analyses, and density functional theory calculations, reveal for the first time that along with the standard Li + transport during charge/discharge, indium in the Li‐In anode also participates in the redox reactions. The in‐situ generated In 3+ preferentially occupies the vacant Li sites in the trigonal LYC lattice, leading to the formation, growth, and eventual stabilization of an anode‐electrolyte interphase (AEI) layer consisting of an In‐doped Li 3‐ x In x YCl 6 ( x = ≈0.2) phase. It is discussed how the presence of such an AEI layer prevents LYC decomposition and suppresses dendrite formation and propagation, enabling stable cycling of ASSB with ≈90% capacity retention over 1000 cycles. This work sheds light on the dynamic evolution of the halide SE and alloy anode interphase, and opens new avenues in the future design of long‐lasting high‐energy all‐solid‐state‐batteries.