Adaptive Stress Response in 2D Graphene@Se Composite toward Ultra‐Stable All‐Solid‐State Lithium‐Selenium Batteries

All-solid-state lithium-selenium batteries (ASSLSeBs) offer high energy density and improved safety for next-generation energy storage. Still, selenium cathodes suffer from large volume changes during cycling, leading to mechanical stress and rapid capacity fade. To address this, a stress-adaptive 2D graphene@Se composite cathode is developed, where small Se nanoparticles are anchored onto acid-treated expanded graphite (AcEG) to enhance charge transport and alleviate stress. Mechanical characterization confirms that the composite effectively mitigates Li-ion-induced strain. As a result, ASSLSeBs with this cathode achieve exceptional cycling stability with ultrahigh capacity retention after 4000 cycles at 2 C and stable performance for over 400 cycles even under high active-material loading. Furthermore, an all-solid-state Li-Se pouch cell with a record energy density of 376.8 Wh kg⁻¹ is demonstrated, the highest reported for ASSLSeBs. This work presents a strategy for designing stress-adaptive cathodes, enabling ultra-stable ASSLSeBs for practical applications.