Exploring the Electrochemical Stability Window of an All-Solid-State Composite Cathode via a Novel Operando Tender XPS Setup

All-solid-state batteries (ASSBs) have the potential to provide greater energy density than conventional batteries based on liquid electrolytes. Here, an operando ASSB cell setup for tender X-ray photoelectron spectroscopy (XPS) was developed, and the interface of a Ni-rich layered transition metal oxide cathode active material (CAM) and an Li₆PS₅Cl (LPSCl) solid electrolyte (SE) was evaluated during initial charge/discharge cycles. After validating the cell performance against a conventional pouch cell operated at high compression, intermittent galvanostatic cycling was performed, and XPS data were recorded as a function of state of charge (SOC). Upon the initial charge of the cell to ≈3.3 V_Li, the LPSCl appears to decompose into LiCl, Li₃PS₄, and polysulfides, whose amount gradually increases with potential. Upon further charge, at a potential higher than ≈3.8 V_Li, initially, present sulfate and sulfite impurities decompose, and at ≈74% SOC (corresponding to a cathode potential of ≈4.10 V_Li), surface reconstruction of the CAM particles due to lattice oxygen release is detected. In addition, at potentials beyond ≈4.6 V_Li, a decrease of the S 1s counts of the sum of the LPSCl, the thiophosphate, and polysulfide species suggests the formation of elemental sulfur that is lost via sublimation into the vacuum chamber.