Ionic Elastomer Interface for Low‐Pressure and Durable All‐Solid‐State Batteries

ABSTRACT All‐solid‐state batteries (ASSBs) hold significant promise as next‐generation energy storage systems due to their high energy density and intrinsic safety. However, their practical deployment is impeded by the need for high external pressure (typically tens of megapascals) to maintain solid–solid interfacial contact and ensure long‐term cycling stability. Here, we report an ionic elastomer specifically designed to enable stable ASSB operation under substantially reduced stack pressure. The elastomer combines a mechanically flexible polymer matrix with an ionically conductive phase, delivering high room‐temperature ionic conductivity (0.3 mS cm −1 ), a low elastic modulus (29.8 MPa), excellent thermal stability (≥400°C), and strong chemical compatibility with sulfide‐based solid‐state electrolytes (SSEs). When integrated with sulfide SSEs, the composite exhibits a remarkable room‐temperature ionic conductivity of 5.23 mS cm −1 . Incorporation of this composite into ASSBs with high‐nickel cathodes (Ni ≥ 90%) yields an initial capacity of 200 mAh g −1 at 0.05C and outstanding cycling stability over 700 cycles at 1C under a low stack pressure of just 5 MPa. This ionic‐elastomer strategy mitigates electrochemical‐mechanical degradation, eliminates the high‐pressure requirement, and offers a scalable pathway toward practical, durable ASSBs technologies.