Spatio-Chemical Deconvolution of the LiNi0.6Co0.2Mn0.2O2/Li6PS5Cl Interphase Layer in All-Solid-State Batteries Using Combined X-ray Spectroscopic Methods

The (electro)chemical degradation at the interface between Li6PS5Cl (LPSC) and LiNi0.6Co0.2Mn0.2O2 (NCM622) is systematically investigated using nondestructive synchrotron X-ray absorption spectroscopy and X-ray photoemission electron microscopy. These measurements were surface chemical depth profiling (from 2 to several hundred nanometers) and high-resolution elemental imaging of both LPSC and NCM622 particles. This analysis was complemented by galvanostatic cycling, impedance spectroscopy, and operando cell pressure characterization. Several correlations between interphase evolution and cell electrochemical performance are clarified, while some inconsistencies are rationalized and discussed. First, the intrinsic LPSC electrochemical oxidation mechanisms were studied using an LPSC:C65 working electrode (WE). The results showed that increased cell resistance during the first charge stemmed from polysulfide byproducts and particle contact loss due to LPSC volume shrinkage at the interface. Second, when using an NCM622:LPSC WE, species, such as SO3 2-, SO4 2-, and PO4 3-, were detected on both LPSC and NCM622 particles, while electrochemically inactive reduced transition metals were observed only at NCM622 surfaces. These species, initially present at open-circuit potential, increased after the first charge due to the chemical reactions between LPSC and NCM622 surface lattice oxygen. The estimated interphase thickness on the LPSC and NCM622 surfaces over the cycling remains below similar to 3 nm. Our findings highlight that the formation of an electrochemically inactive NCM622 surface is a primary cause of impedance rise during the first charge, along with the formation of LPSC byproducts and contact loss. However, the continuous increase in cell resistance could not be attributed to further interphase growth after the first charge. We hypothesize that this may result from slow and progressive LPSC polymerization reactions (e.g., Li2P2S6 and P2S5) and structural changes at the NCM622 surface.