Decoupling first-cycle capacity loss mechanisms in sulfide solid-state batteries†

Solid-state batteries (SSBs) promise more energy-dense storage than liquid electrolyte lithium-ion batteries (LIBs). However, first-cycle capacity loss is higher in SSBs than in LIBs due to interfacial reactions. The chemical evolution of key interfaces in SSBs has been extensively characterized. Electrochemically, however, we lack a versatile strategy for quantifying the reversibility of solid electrolyte (SE) redox for established and next-generation SSB electrolytes. In this work, we perform tailored electrochemical tests and operando X-ray diffraction to disentangle reversible and irreversible sources of capacity loss in positive electrodes composed of Li₆PS₅Cl SE, Li(Ni_0.5Mn_0.3Co_0.2)O₂ (NMC), and carbon conductive additives. We leverage an atypically low voltage cutoff (2.0 V vs. Li/Li⁺) to quantify the reversibility of SE redox. Using slow (5.5 mA g_NMC⁻¹) cycling paired with >100 h low-voltage holds, our cells achieve a surprising 96.2% first-cycle coulombic efficiency, which is higher than previously reported (mean: 72%, maximum: 91.6% across surveyed literature). We clarify that sluggish NMC relithiation kinetics have been historically mistaken for permanently irreversible capacity loss. Through systematic decoupling of loss mechanisms, we uncover the unexpected reversibility of SE redox and isolate the major contributors to capacity loss, outlining a strategy for accurate assessment of next-generation SE materials and interface modifications.