Metal Site Substitution Regulates Li+ Transport in Li2ZrCl6 Halide Solid Electrolytes: A Comparative Atomistic Study of Zn, Ti, and Ca

Li2ZrCl6 is a cost-effective chloride solid electrolyte for all-solid-state batteries, yet the mechanistic relationship between dopant chemistry and Li+ transport remains insufficiently understood. Building on prior data-driven screening, Zn, Ti, and Ca are examined as representative metal-site substituents using fixed-Li-stoichiometry models to compare substituent-dependent chemical and bonding responses under a common compositional constraint. Combining first-principles calculations, ab initio molecular dynamics, CI-NEB calculations, and COHP analysis, we show that Li+ migration is intrinsically anisotropic: the c axis provides the dominant transport channel, whereas fragmented ab-plane pathways limit long-range percolation. Within the present three-temperature AIMD-Arrhenius protocol, Zn substitution gives the most favorable comparative transport response, with the lowest fitted apparent activation energy and the highest extrapolated 300 K conductivity among the investigated models. This behavior is associated with suppression of the representative ab-plane migration bottleneck and moderate Zn−Cl framework constraints. In contrast, Ti substitution lowers selected local barriers but induces pronounced electronic-structure and local-bonding reconstruction, while these pathway-specific barrier reductions do not translate into an enhanced overall AIMD-derived transport response. These results show that pathway connectivity and framework bonding must be considered together when designing substituted Li2ZrCl6 electrolytes.