Regulating Chemical Bonds in Halide Frameworks for Lithium Superionic Conductors

Developing solid-state electrolytes (SSEs) is a critical task for advancing all-solid-state batteries (ASSBs) that promise a high energy density and improved safety. The dominant strategy in engineering advanced SSEs has been substitutional doping, where foreign atoms are introduced into the atomic lattice of a host material to enhance ionic conduction. This enhancement is typically attributed to optimized charge carriers' concentration or lattice structure alterations. In this study, we extend the concept of substitutional doping to explore its effects on chemical bond modulation and the resulting impact on ionic conduction in halide SSEs. As a case of study, we demonstrate that cation dopants with high charge density indices (e.g., Al³⁺ and Fe³⁺) can increase the covalency of metal-halide (M-X) bonds and induce the local asymmetric field of force, resulting in higher site energy and lower migration barriers, which significantly enhance the ionic conduction in halide frameworks. Specifically, we developed a series of halide SSEs with ionic conductivities exceeding the benchmark value of 1 mS cm^-1 at room temperature. Detailed investigations, including neutron powder diffraction, pair distribution function analysis, and first-principles calculations, are performed to gain an insight into the mechanisms behind this adjustment. Furthermore, these materials exhibit enhanced deformability due to increased covalency of the metal halide framework, enabling high-performance ASSB prototypes operatable at low stacking pressures (<10 MPa). These advancements deepen our understanding of superionic conduction in halide SSEs and mark an important step toward the practical application of ASSBs in the future.