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

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 LiPSCl to bridge interparticle ion transport via facile wet process, constructing a continuous fast-ion transport network and achieving an ionic conductivity exceeding 10 S cm; 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 LiNiMnCoO/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.