Effect of Static Jahn-Teller Distortion on the Li⁺ Transport in a Copper Hexacyanoferrate Framework

Prussian blue (PB) and its analogues (PBAs) are potential cathode-active materials for rechargeable lithium-ion batteries. Although a body of research has assessed the performances of various PB/PBA cathodes with an eye to practical use, the underlying Li⁺-transport mechanism is still unclear. Focusing on copper hexacyanoferrate (CuHCF), a PBA that exhibits static Jahn-Teller (JT) distortion, we theoretically investigate how the framework's distortion affects the pathways and energetics of the Li⁺ transport. Density functional theory calculations of a local structure model of CuHCF reveal that the static JT distortion makes the favorable Li⁺-transport pathways quasi-two-dimensional, contrary to an intuitive picture of isotropic Li⁺ diffusion within the regular jungle-gym framework. The pathways are mutually interconnected, thereby creating an almost barrierless transport network. To better understand the distortion-induced transport anisotropy, we visually analyze the framework's electronic structure and noncovalent Li⁺-framework interactions. This study helps deepen the fundamental understanding of intrinsic Li⁺-transport properties of a distorted porous framework.