  Electrochemical degradation of solid electrolytes is a major roadblock in the
development of solid-state batteries, and the formed solid-solid interphase
(SSI) plays a key role in the performance of solid-state batteries. In this
study, by combining experimental X-ray absorption spectroscopy (XAS)
measurements, first-principles simulations, and unsupervised machine learning,
we have unraveled the atomic-scale oxidative degradation mechanisms of sulfide
electrolytes at the interface using the baseline Li3PS4 (LPS) electrolyte as a
model system. The degradation begins with a decrease of Li neighbor affinity to
S atoms upon initial delithiation, followed by the formation of S-S bonds as
the PS4 tetrahedron deforms. After the first delithiation cycle, the PS4 motifs
become strongly distorted and PS3 motifs start to form. Spectral fingerprints
of the local structural evolution are identified, which correspond to the main
peak broadening and the peak shifting to a higher energy by about 2.5 eV in P
K-edge XAS and a new peak emerging at 2473 eV in S K-edge XAS during
delithiation. The spectral fingerprints serve as a proxy for the
electrochemical stability of phosphorus sulfide solid electrolytes beyond LPS,
as demonstrated in argyrodite Li6PS5Cl. We observed that the strong distortion
and destruction of PS4 tetrahedra and the formation of S-S bonds are correlated
with an increased interfacial impedance. To the best of our knowledge, this
study showcases the first atomic-scale insights into the oxidative degradation
mechanism of the LPS electrolyte, which can provide guidance for controlling
macroscopic reactions through microstructural engineering and, more generally,
can advance the rational design of sulfide electrolytes.
