Glass-Ceramic Lithium Thiophosphate Electrolytes with Enhanced Conductivity and (Chemo)mechanical Properties for All-Solid-State Batteries

Solid-state batteries (SSBs) based on inorganic solid electrolytes (SEs) possibly offer enhanced energy and power densities, along with increased safety, compared to state-of-the-art rechargeable batteries using liquid organic electrolytes. However, the stiffness and brittle nature of inorganic SEs can complicate cell fabrication and lead to the (chemo)mechanical failure of SSBs during operation. In the past, the design of SEs has mainly focused on optimizing the ionic conductivity and (electro)chemical stability. However, to mitigate detrimental (chemo)mechanical degradation in SSBs, due to electrode volume and morphology changes upon charge and discharge, the mechanical properties of SEs also need to be considered in their development. In this regard, glass-ceramic SEs offer a reduced hardness but often suffer from rather low ionic conductivities. Herein we systematically investigate the effect of LiI additive and annealing temperature on phase composition and charge-transport properties of a series of SEs with the general composition of 4.25Li2S–0.75P2S5–1.5SiS2–xLiI (0 ≤ x ≤ 2). We demonstrate that the glass-ceramic material (LPSI-GC) with x(LiI) = 1.25 achieves a high room-temperature ionic conductivity of 4.38 mS cm–1 and further exhibits favorable mechanical properties owing to the combination of crystalline t-Li10.5P1.5Si1.5S12 and I-rich amorphous phases. When implemented in SSBs together with a layered Ni-rich oxide cathode material, the LPSI-GC SE enables stable cycling for over 100 cycles, although (electro)chemical decomposition, detected by X-ray photoelectron spectroscopy, is evident. Collectively, our results demonstrate that glass-ceramic SEs allow for simultaneous optimization of the ionic conductivity and mechanical properties, thus enabling long-term SSB operation.