Disorder, Frustration, and Correlation in Polyborane Solid Electrolytes from First-principles Computations

Polyborane salts based on B$_{12}$ H$_{12}^{2–}$ , B$_{10}$ H$_{10}^{2–}$ , and their carboborane counterparts CB$_{11}$ H$_{12–}$ and CB$_{9}$ H$_{10–}$ demonstrate extraordinary Li and Na superionic conductivity that make them attractive as electrolytes in all-solid- state batteries. Their rich chemical and structural diversity creates a versatile design space that could be used to optimize materials with even higher conductivity at lower temperatures; however, many mechanistic details remain enigmatic, including reasons why certain modifications lead to improved performance. Here, we use 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. We introduce computational “experiments” that systematically vary factors such as stoichiometry, strain, alloy composition, and crystal structure. Data from these simulations are then analyzed using a suite of conventional and novel tools in a high-throughput fashion. Our findings point to the importance of highly correlated motion and dynamical fluctuations in the broader structural environment. They also reveal the universal importance of frustration, which lowers the barrier for ionic mobility and motivates the order-disorder transition to a superionic state.