Ultra‐High Ionic Conductivity Sulfide‐Based Composite Membranes Paired With Stable Interfacial Layer Enables High‐Energy‐Density All‐Solid‐State Batteries

ABSTRACT Intrinsic properties of sulfide solid‐state electrolytes (SEs) are crucial for achieving all‐solid‐state batteries (ASSBs) with high safety and high energy density, yet it is challenging to fabricate high‐performance sulfide SE films in a scalable approach. Furthermore, the inevitable side reactions between sulfide SE and lithium metal anodes exacerbate interfacial incompatibility, causing degradation in the cyclic stability. Here, a high‐performance sulfide SE (Li 5.4 PS 4.4 Cl 0.8 Br 0.8 ) was selected as the base phase to compensate for the loss of electrochemical performance during the dry film‐forming process. It was then combined with Polytetrafluoroethylene binder to prepare a high‐ionic‐conductivity composite membrane (HICCM). To mitigate side reactions between HICCM and the anode, a LiF‐rich interfacial layer forms in situ on the lithium metal surface, and the assembled symmetric cell achieves a high critical current density of 1.3 mA cm −2 and stable cycling for 2000 h (0.2 mA cm −2 ). For electrochemical performance, HICCM exhibits fast ion transport kinetics and electrical insulation, allowing the ASSB configured as FeS 2 //HICCM//LiF‐Li to achieve an energy density of 416.5 Wh kg −1 at a high loading of 3.4 mAh cm −2 . This work demonstrates that by combining HICCM with a stable interface design, ASSBs can achieve longevity and high cell‐level energy density.