Rare-Earth Ions Regulating Lattice-Softened Bromide Solid Electrolytes for Highly Stable Fast-Charging Solid-State Batteries

All-solid-state lithium-based batteries (ASSLBs) with good safety and high energy density are valuable. To realize stable and high-efficiency ASSLBs, high-performance solid-state electrolytes (SEs) with good processability are necessary. Bromide (Br)-based rare-earth halide SEs (RE-HSEs) exhibit good deformability for large radii of RE and Br ions. Here, the influence of RE ions on the crystalline structure and mechanical properties of Br-based RE-HSEs (RE = Y, Gd, Tb, Ho, or Er) was analyzed in detail, and Li₃GdBr₆ (LGdB) showed the softest lattice and the best deformability due to having the longest RE-Br bond length. Furthermore, LGdB exhibits satisfactory ionic conduction ability (1.4 mS cm^-1), and the assembled ASSLBs exhibit a reversible redox process, excellent fast-charging performance, and superior cycling stability for 6000 cycles at 10 C. This study indicates that the relationship between the RE and deformability of Br-based RE-HSEs is significant for the rational design and improvement of HSEs in ASSLBs.