Synergy of metal halide doping and a polymeric interface enables improved electrochemical performance of all solid-state Li batteries

Sulfide solid-state electrolytes are promising candidates for all-solid-state lithium metal batteries (ASSLBs) having higher energy density and practical safety due to their high ionic conductivity and favorable mechanical properties. However, their practical integration is hindered by low critical current density (CCD), a narrow electrochemical stability window, and high impedance with electrodes. Herein, we demonstrate that doping lithium phosphorus sulfide (Li₇P₃S₁₁) solid electrolyte with zirconium chloride (ZrCl₄) significantly enhances its electrochemical performance. Unlike previously reported doping strategies, ZrCl₄ doping uniquely introduces dual dopants (Zr⁴⁺ and Cl⁻) into the Li₇P₃S₁₁ matrix. Density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations reveal that the Zr⁴⁺ ions increase dynamic structural flexibility, while Cl⁻ ions create additional Li⁺ vacancies, collectively enhancing structural stability and ionic conductivity beyond the capacity of single-element doping strategies. Optimized doping content of ZrCl₄ improved the CCD of Li₇P₃S₁₁ from 0.55 mA cm⁻² to 1.7 mA cm⁻², while the ionic conductivity improved from 1.8 × 10⁻³ S cm⁻¹ to 3.0 × 10⁻³ S cm⁻¹. Li/Li symmetrical cells with doped electrolyte exhibited improved cycling stability at 0.1 mA cm⁻² compared to the control counterparts. Furthermore, a thin solid polymer electrolyte (SPE) was used at the interface between the cathode and solid electrolyte to enable the stack pressure free operation of full cells. Li/LiFePO₄ (LFP) full cells using doped solid electrolyte (SE) in combination with SPE catholyte demonstrated stable performance compared to undoped SE based cells. The enhanced Li-dendrite suppression and improved electrochemical properties due to doped Li₇P₃S₁₁ and the stack free operation due to the addition of SPE as catholyte will add significant potential for advancing ASSLB technology.