In-Depth Analysis of the Origin of Enhanced Ionic Conductivity of Halide-Based Solid-State Electrolyte by Anion Site Substitution

Abstract

Halide-based solid-state electrolytes (HSEs) offer higher anodic stability than sulfide solid electrolytes with Li2ZrCl6 (LZC) standing out due to its low cost and elemental abundance. However, its limited ionic conductivity has inhibited wider application up to now. In this context, S-doped Li2ZrCl6 (LZCS) electrolytes that reach a conductivity of up to 0.64 mS cm−1 (3 times higher than pristine LZC, 0.21 mS cm−1) with good electrochemical stability have been synthesized and studied. X-ray and neutron diffraction reveal the coexistence of monoclinic and trigonal phases in LZCS, which is the probable reason for enhanced ionic conductivity. The monoclinic phase with antisite disorder leads to 2D diffusion pathway, which is confirmed by density functional theory simulations. Stripping/plating results show that Li2ZrCl5.6S0.2 exhibits the smallest polarization over 600 h at 0.1 mA cm−2. Full cell tests with LiNi0.6Mn0.2Co0.2O2 (NMC622) positive electrode further demonstrate a better capacity retention of 96.3% for Li2ZrCl5.6S0.2 (LZCS02) in comparison to that of Li2ZrCl6 (68.5%).Focused ion beam (FIB) line scans show less nickel and oxygen diffusion at the LZCS02/NMC622 interface. In this study, an in-depth analysis of the structure and ionic conductivity relationship in S-doped Li2ZrCl6 HSEs is provided, opening a new approach for designing highly performing HSEs.