Deciphering the critical degradation factors of solid composite electrodes with halide electrolytes: Interfacial reaction versus ionic transport

Recently, halide-type Li+ conductors have been revisited for their use in all-solid-state batteries (ASSBs) owing to their stability at high potentials. However, the realization of ASSBs is hindered by the fast performance decay of composite cathodes. From a comparative study using halide and sulfide solid electrolytes (SEs), herein, we reveal the critical degradation factors of halide-SE-based cathodes, which are different from the conventional findings of sulfide-SE-based cathodes. By using impedance decoupling combined with scanning spreading resistance microscopy and force spectroscopy, we elucidate the mechanisms behind the SE-dependent degradation of single-particle LiNi0.8Co0.1Mn0.1O2 (NCM) composite cathodes. Impedance analyses show that NCM-Li6PS5Cl (LPSCl) and NCM-Li3InCl6 (LIC) exhibit considerable increase in interfacial impedance and Li+-transport impedance, respectively, upon cycling. Based on the combined experimental and computational study of microscopic interfacial and mechanical properties, we demontrate that the degradation of NCM-LPSCl originates primarily from the formation of resistive interphases, while the crucial degradation factor of NCM-LIC is the cracking-induced mechanical deformation of the LIC under pressure. Finite element analysis results further reveal how the deformation behavior of the SE materials influences the formation and propagation of cracks in composite cathodes during cycling. This study provides insights into the design of materials and electrodes for ASSBs with high power capabilities and long cycle lifetimes.