Sodium Storage and Pseudocapacitive Charge in Textured Li4Ti5O12 Thin Films

Abstract

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