Detection of a Cobalt-Containing Interphase at the Li₆PS₅Cl-NMC111 Interface by In Situ XANES and EIS

Sulfide electrolyte all-solid-state lithium batteries (ASLBs) with uncoated Li-NixMnyCo(1-x-y)O(2) (NMC) cathodes suffer from a large capacity loss during initial cycling and an increase in cell impedance. Decomposition reactions are known to occur at the Li(6)PS5Cl-NMC111 interface due to incompatibility between the two materials. If a stabilizing coating is applied to the NMC, it delivers full capacity during initial charge. However, the loss in capacity during discharge still occurs. The interface was studied by mu XANES and through EIS analysis. A chemically-formed interphase was detected by mu XANES, evident from reduction of Co at an uncoated NMC particle surface. This interphase was produced by decomposition at rest. To study the effect of the interphase on electrochemically active surface area, piecewise in situ EIS was performed and the data was modeled using a transmission line model (TLM). The charge transfer resistance RCT was used to estimate the volume specific active surface area (aact). The median value for aact was 296 cm(-1), a factor of 7.5 lower than the theoretical value of 2216 cm(-1). This provided evidence of a lower electrochemically active surface area in the ASLB.