Triple-phase boundary instability as a key degradation factor in sulfide|(oxy)halide dual-electrolyte solid-state batteries

Dual-electrolyte solid-state batteries (SSBs) that combine sulfide separators with an (oxy)halide in the positive electrode offer a promising configuration. However, most cell designs rely on additional (oxy)halide interlayers between the separator and the positive electrode. This leaves the intrinsic reactivity at the sulfide-separator|(oxy)halide positive electrode interface largely unexplored. Here, we systematically investigate this interface using Li$_6$PS$_5$Cl and six (oxy)halide solid electrolytes across three cell configurations. We show that the triple-phase boundary between the active material and two solid electrolytes is intrinsically detrimental, leading to rapid performance decay regardless of the (oxy)halide chemistry. High-capacity retention is only achieved when this boundary is avoided. Ex situ and operando X-ray photoelectron spectroscopy (XPS) reveal the formation of localized degradation products, including metal sulfides and elemental sulfur/oxidized sulfide compounds, alongside sulfur gas evolution at ∼4.3 V vs. Li⁺/Li. These findings identify triple-phase boundary instability as a key degradation mechanism and provide design guidelines for stable dual-electrolyte architectures.