Design principles for grain boundaries in solid-state lithium-ion conductors

Lithium dendrite formation and insufficient ionic conductivity remain primary concerns for the utilization of solid-state batteries. Given that the interpretation of experimental results for polycrystalline solid electrolytes can be difficult, computational techniques are invaluable for providing insight at the atomic scale. Here, first-principles calculations are carried out on representative grain boundaries in four important solid electrolytes, namely, an anti-perovskite oxide, Li3OCl, and its hydrated counterpart, Li2OHCl, a thiophosphate, Li3PS4, and a halide, Li3InCl6, to develop the first generally applicable design principles for grain boundaries in solid electrolytes for solid-state batteries. The significantly different impacts that grain boundaries have on electronic structure and transport, ion conductivity and correlated ion dynamics are demonstrated. The results show that even when grain boundaries do not significantly impact ionic conductivity, they can still strongly perturb the electronic structure and contribute to potential lithium dendrite propagation. It is also illustrated, for the first time, how correlated motion, including the so-called paddle-wheel mechanism, can vary substantially at grain boundaries. These findings reveal the dramatically different behavior of solid electrolytes at the microscale compared to the bulk and its potential consequences and benefits for the design of solid-state batteries. These design principles are expected to aid the synthesis and engineering of solid electrolytes at the microscale for preventing dendrite propagation and accelerating ion transport.