Polyborane salts based on B12H122-, B10H102-, and their carboborane counterparts CB11H12- and CB9H10- demonstrate extraordinary Li and Na superionic conductivity (~0.01 S cm-1) that make them attractive as potential electrolytes in solid-state batteries. The transition temperature to the superionic phase and the resulting ionic conductivity depend on the cation-anion combinations, whose rich chemical and structural diversity creates a versatile design space. Despite advances that have driven the superionic transition to lower temperatures and improved ionic conductivity, the origins of these successes are generally poorly understood. Likewise, questions remain regarding the fundamental mechanisms that drive ionic conductivity, which has impeded the adoption of a more rational design approach. We have performed extensive ab initio molecular dynamics simulations within an automated high-throughput scheme to broadly explore the dependence of ionic conductivity on cation/anion pair combinations for Li and Na polyborane salts. Additional simulations are used to probe the additional influence of local perturbations, including modifications to chemistry, stoichiometry, and composition. Carbon doping, anion alloying, and cation off-stoichiometry are found to be favorable because they introduce intrinsic disorder, which facilitates local deviations from the expected cation population. Anion reorientations are also discovered to be critical for conduction, with benefits associated with lattice expansion traceable to the facilitation of anion rotation at larger volumes. Our computational studies offer new mechanistic understanding and guidance for future optimization of ionic conductivity in emerging polyborane-based solid electrolytes.

Mechanisms and motivations for superionic conductivity in polyborane solid electrolytes from ab initio molecular dynamics

253rd National Meeting (2017)

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