Correlating Electrochemical Behavior with Morphological and Compositional Changes in Sulfide Solid Electrolyte All-Solid-State Batteries after Charge/Discharge Cycles

Morphological and compositional changes occurring inside an all-solid-state battery of NCA/Li6PS5Cl/graphite after a charge/discharge cycle test were analyzed in detail, combining the interpretation of electrochemical performance measurements and various instrumental analyses. The capacity of the cell faded, and the internal resistance increased after charge/discharge cycles. Also, disassembly analyses show the following features. First, delamination between NCA active materials and the solid electrolyte inside the positive electrode is observed by scanning electron microscopy (SEM), which shows an increase in lithium-ion-transfer resistance at the interface. Second, the chemical structural changes of sulfide solid electrolyte observed around the positive electrode include the generation of the oxygen-substituted and/or chloride-free PS4 tetrahedra and the generation of the S–S bond. The chemical structural changes may have caused the delamination inside the positive electrode layer observed by SEM. Third, similar structural changes are observed inside the sulfide solid electrolyte layer, which proceeded through the grain boundary of the solid electrolyte. Those chemical structural changes resulted in a decrease in lithium-ion mobility, which was confirmed by AC impedance measurement and 7Li and 31P NMR analyses.