Understanding and Optimizing Ionic Conductivity in Polyborane Solid Electrolytes from Ab Initio molecular Dynamics

Abstract

Polyborane salts based on B12H12 2-, B10H10 2-, CB11H12 - and CB9H10 - demonstrate extraordinary Li and Na superionic conductivity that make them attractive as potential electrolytes in solid-state batteries. The rich chemical and structural diversity of the various cation-anion combinations establishes a diverse design space offering a wide range of superionic transition temperatures and ionic conductivities. Recent synthesis and processing advances have improved ionic conductivity at modest temperatures; however, the origins of these successes are generally poorly understood. Likewise, key 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 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 influence of local 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. In this case, benefits are traceable the specific chemistry of the cation-anion interaction, which acts to create intrinsic frustration that motivates cation mobility. Our computational studies offer new mechanistic understanding and guidance for future optimization of ionic conductivity in emerging polyborane-based solid electrolytes. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.