Highly Deformable, Ion‐Conductive Borohydride‐Substituted Sulfide Electrolyte for Superior Performance at Low Stack Pressure

All-solid-state batteries (ASSBs) are promising next-generation energy storage systems that can replace conventional lithium-ion batteries. Further enhancement in battery performance requires the formation of a stable physical interfacial contact between the active material (AM) in the electrode and the solid electrolyte (SE). However, reducing the resistance at the AM-SE interface remains a key challenge. This study focuses on Li₃PS₄-xLiBH₄ (LPSBH), a sulfide-based SE with an argyrodite structure, synthesized by mechanical milling. Although LPSBH is known for its high ionic conductivity, its mechanical properties are not thoroughly examined. Here, the deformability of LPSBH is evaluated by demonstrating that it can be formed at low pressures to achieve high relative density. A quantitative evaluation of the AM-SE interfacial contact using symmetric cells demonstrates the formation of a good AM-SE interfacial contact within the electrode layer. A 13 mAh-class laminated cell with LPSBH stacked onto the negative electrode achieves 6C charging at 25 °C under a low stacked pressure of 5 MPa, along with significant cycle stability, which retains ≈70% capacity after 1000 cycles under 1C/1C conditions.