Designing High‐Ionic‐Conductivity and Air‐Stable Composite Sulfide Electrolytes via Polymer‐in‐Salt Binder with Surface Engineering for All‐Solid‐State Lithium Batteries

Abstract Composite sulfide electrolytes (CSEs), composed of a sulfide electrolyte matrix and a small amount of polymer binder, are promising for all‐solid‐state lithium batteries (ASSLBs) owing to their potential to achieve both high ionic conductivity and mechanical robustness. However, current polymer‐based binders are not well ionically conductive and generally have non‐/low‐polarity, leading to a dramatic decrease in ionic conductivity and weak interparticle bonding; furthermore, CSEs are still unstable in air. Herein, a unique polymer‐in‐salt binder is proposed, further engineered with an ion‐conducting hydrophobic layer (IHL) on surface to design hydrophobic CSE (HCSE) to address the above challenges. Specifically, poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HFP)‐based polymer‐in‐salt binder is introduced into Li ₆ PS ₅ Cl to bridge interparticle ion transport via facile wet process, constructing a continuous fast‐ion transport network and achieving an ionic conductivity exceeding 10 ⁻³ S cm ⁻¹ ; while the F‐containing groups in PVDF‐HFP provide strong bonding, enabling good mechanical properties with a film thickness of only 57 µm. Besides, IHL with low surface energy ensures HCSE to remain air‐stable. The LiNi _0.8 Mn _0.1 Co _0.1 O ₂ /Li‐In full cell delivers high capacity and remains stable over 100 cycles, while corresponding pouch cell with no extra pressure shows excellent operation safety and reliability. This work presents a new avenue for developing high‐performing CSE‐based ASSLBs.