(606d) Operando Observation of Structural Evolution in Sulfur-Based All Solid-State Lithium Batteries

All solid-state lithium-ion batteries (ASSBs) offer higher power and energy density than traditional lithium-ion batteries and are inherently safer due to the replacement of flammable organic liquid electrolytes with solid-state electrolytes (SSEs).Due to their high room temperature ionic conductivity and malleability easily allowing for good contact at the electrodes, sulfide-based SSEs are promising candidates for ASSBs. However, thiophosphate based SSEs face two main challenges for commercialization: the narrow voltage window where the electrolyte is electrochemically stable, and its reactivity with NMC and Li metal active materials. Pairing sulfide SSEs with compatible cathodes is crucial to designing high energy dense ASSBs.

An FeS₂ metal sulfide cathode offers a high theoretical capacity (894 mAh/g) and excellent compatibility without additional interface engineering. Due to the solid confinement in the ASSBs, the polysulfide shuttle effects and Fe⁰ aggregation seen in liquid electrolyte systems are mitigated. ASSBs based on Li_6.6Ge_0.6Sb_0.4S₅I (LGSSI) and FeS₂ exhibit an initial capacity of 715 mAh/g at C/10 and are stable for 220 cycles with a capacity retention of 84.5% at room temperature. The structural stabilities of Li_6.6Ge_0.6Sb_0.4S₅I (LGSSI) during cycling are characterized by operando energy dispersive X-ray diffraction (EDXRD), which allows rapid collection of spatially-resolved structural data without redesigning or disassembling the sealed cells and risking contamination by air (Figure 1a). The electrochemical stability is assessed, and an operating voltage window from 0.7-2.4 V (vs. In–Li) is confirmed. Initial EDXRD of a carbon/In-Li half cell is shown in Figure 1b, with the primary reflection of LGSSI observed near a photon energy of 86 keV. During a negative potential sweep (shown in Figure 1c) LGSSI reflections located in the carbon cathode shifted to lower energy, indicating a locally higher d-spacing. This demonstrated that contact with conductive carbon caused a structural instability in the LGSSI.