Probing Lanthanide Dynamics and Structural Symmetry in Halide Solid Electrolytes

The search for appropriate solid electrolytes is central to the development of all-solid-state batteries (ASSBs), in which halide solid-state-electrolytes (HSSEs) have recently emerged as promising candidates. HSSEs exhibit a variety of structural chemistries such as cubic spinel (Fd3– m) and monoclinic (C2/m) with cubic close packing (ccp) anion arrangements. The trigonal (P3– m1) and orthorhombic (Pnma) have hexagonal close packing (hcp) arrangements, while the UCl3-type structure (P63/m) with tricapped trigonal prisms offers a unique framework lattice arrangement. Among key design parameters, the choice of M3+ cations such as Y, Sc, In, and especially lanthanides critically influences structural stability, Li-ion transport, and cost. In particular, lanthanide cations present unique chemical behavior due to the lanthanide contraction, which remains underexplored in Li3MCl6. Here, density functional theory and ab initio molecular dynamics simulations are performed to investigate the effects of M3+ cation selection, covering the entire lanthanide series, on the structural, electrochemical, and transport properties of Li3MCl6 phases. Our results reveal that the lanthanide contraction trend is not strictly followed, with certain cations (Ce, Eu, Gd, Yb) deviating from the ideal trend despite nominal 3+ valency. These deviations can arise from electronic structure effects such as 4f electron localization, mixed valence tendencies, and local structural distortions, which lead to reduced coordination numbers. The polarizability of halide anions (X = Cl, Br, I) also influences reduction potential and electrochemical stability. Among studied phases, the monoclinic C2/m structure with intrinsic Li vacancies shows superior thermodynamic stability, while the UCl3-type LiSm0.83Nd0.83Cl6 phase demonstrates high ionic conductivity of 8 mS cm−1 at room temperature with an activation energy of 0.26 eV. These theoretical insights provide fundamental understanding for designing HSSEs for next-generation ASSBs.