Sodium Storage and Pseudocapacitive Charge in Textured Li4Ti5O12 Thin Films

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 IC is achievable.