Mechanically Balanced Elastomer-Filled 3D LAGP Framework Composite Solid Electrolytes for Stable Lithium Metal Batteries

Abstract Ceramic framework-based composite solid electrolytes (CSEs) are promising for lithium metal batteries (LMBs) because they integrate continuous ion-transport pathways with improved electrode/electrolyte contact. However, the role of the mechanical properties of the soft phase infiltrated into the ceramic framework remains insufficiently understood. Herein, we design elastomer-filled CSEs by infiltrating a fluorinated elastomer electrolyte into a porous natrum super ionic conductor (NASICON)-type Li1.5Al0.5Ge1.5(PO4)3 (LAGP) framework. The crosslinking density of the elastomer phase is controlled by varying the poly(ethylene glycol) diacrylate (PEGDA) content, enabling systematic tuning of toughness, elastic recovery, interfacial adhesion, and compressive stiffness while maintaining comparable ionic conductivity. Although all CSEs exhibit ionic conductivities in the 10–4 S cm–1 range, their Li metal interfacial stability and half-cell performance strongly depend on the mechanical balance of the elastomer phase. Insufficient crosslinking leads to poor toughness and adhesion, whereas excessive crosslinking enhances adhesion but compromises recovery, toughness, and stiffness. The optimized P20 + LAGP exhibits an ionic conductivity of 0.18 mS cm–1, adhesion energy of 44.1 J m–2, and compressive modulus of 0.85 MPa, enabling stable Li plating/stripping over 100 h and 91.4% capacity retention after 100 cycles in NMC811|CSE|Li half cells. In contrast, the other CSEs show inferior cycling stability, with capacity retentions below 80%.