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 Li₂ZrCl$_ {5.6}$S$_ {0.2}$ exhibits the smallest polarization over 600 h at 0.1 mA cm$^{-2}$. Full cell tests with LiNi$_{0.6}$Mn$_{0.2}$Co$_{0.2}$O₂ (NMC622) positive electrode further demonstrate a better capacity retention of 96.3% for Li₂ZrCl$_ {5.6}$S$_ {0.2}$ (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.