Interfacial degradation of the NMC/Li₆PS₅Cl composite cathode in all-solid-state batteries

Mitigating interfacial reactions in composite cathode materials remains a key challenge for high-performance all-solid-state batteries (ASSBs), particularly those employing argyrodite-based electrolytes coupled with high-voltage cathodes. Here we study the interfacial chemical reactivity of a Li_1−xNi_0.5Mn_0.3Co_0.2O₂ (NMC532) and Li₆PS₅Cl (LPSC) composite using galvanostatic electrochemical impedance spectroscopy, complemented by surface and bulk-sensitive X-ray techniques. By decoupling the time-dependent interfacial degradation from the overall degradation, we show that the redox reaction between NMC532 (oxidant) and LPSC (reductant) is the main origin of degradation. The chemical reaction products share chemical similarities with the ones after the electrochemical reaction. The analysis shows the oxidation activity of transition metals in the cathode composite is Ni^4+/3+ > Mn⁴⁺/Co³⁺ > Ni^2+/3+. Molecular-dynamics simulations reveal that the reaction is driven by the Li⁺ diffusion from LPSC to the delithiated-NMC532, leading to the LPSC's structural decomposition and collapse.