Sodium\nStorage and Pseudocapacitive Charge in Textured\nLi₄Ti₅O₁₂ Thin Films

Abstract

Phase\ntransformation reactions including alloying or conversion\nones have often been utilized recently to improve the capacity performance\nof Na-ion battery anodes. However, they tend to induce larger volume\nchange and more sluggish Na-ion transport at multiphase solid interfaces\nthan for Li-ion batteries, leading to inefficiency of mixed conductive\nnetworks and thus degradation of reversibility, polarization, or rate\nperformance. In this work, we use a structurally stable Li₄Ti₅O₁₂ spinel thin film as insertion-type model\nmaterial to investigate its intrinsic Na-ion transport kinetics and\ncoupled pseudocapacitive charging. It is found that the latter effect\nis remarkably activated by the nanocrystalline microstructure full\nof defect-rich surface, which can simultaneously promote Na-ion and\nelectron accessibility to the surface/subsurface. It is proposed that\nthe extra pseudocapacitive charge storage is a potential solution\nto the high-capacity and high-rate insertion anodes without trade-off\nof serious phase transformation or structural collapse. Therefore,\na highly reversible charge capacity of 225 mAh g^–1 (exceeding the theoretical value 175 mAh g^–1 based\non insertion reaction) at 1C is achievable.