New crystal structure of Li₃YCl₆: structural relationship and ionic conductivity for solid-state electrolytes

In the pursuit of safer and more energy-dense all-solid-state Li-ion batteries, solid-state electrolytes (SSEs) have emerged as pivotal components, with halide SSEs distinguished by their excellent electrochemical stability, enhanced Li-ion diffusion, and potential cost-efficiency. These properties depend on the anion elements and the structure of closely packed anion sublattices, such as cubic close-packed (ccp) and hexagonal close-packed (hcp) frameworks. Hence, understanding these key differences is essential because they influence the ion diffusion kinetic properties of various halide SSEs. However, research has predominantly shown that ccp anion sublattices generally exhibit higher ionic conductivities than their hcp counterparts, often overlooking the importance of the structural frameworks. To address this issue, we re-evaluated the assumption that a ccp framework is necessary for high electrochemical performance. Specifically, we utilized the three previously synthesized hcp and a ccp frameworks, all with an identical composition of Li₃YCl₆, to assess their thermodynamic stability, synthesizability, and ionic conductivity through ab initio molecular dynamics simulations. The results revealed that hcp frameworks could be promising candidates for SSEs, challenging the conventional preference for the ccp framework. With this structural insight, we designed a novel hcp framework to predict a new Li₃YCl₆ crystal structure with the highest ionic conductivity (38 mS·cm⁻¹) among the halide frameworks and a superior 2D Li-ion diffusion pathway. This breakthrough underscores the significance of the anion framework geometry in Li-ion diffusion and highlights the importance of precise crystallographic predictions in developing more efficient and cost-effective battery technologies.