Ultrathin Al-Doped ZnO Films Coated Li4Ti5O12 As an Anode Material with Excellent Cycling Stability and Rate Capability for Lithium-Ion Batteries

Abstract

Lithium titanate oxide (Li4Ti5O12, LTO) is a lithium ion intercalated compound, which was identified as a promising anode material for LIBs with a theoretical capacity of 175 mAh g-1 within a typical potential range of 1.0 V–3.0 V. Extending the end voltage from 1.0 V to 0.01 V is an effective way to improve the electrochemical performance of LTO; the tetrahedral (8a) sites of LTO would be available for lithium ion storage to have an “extra” reversible capacity, when the end potential is lower than 0.6 V. However, this approach also leads to some other problems: (1) a lower end potential will cause a more intensive decomposition of electrolyte to form the solid electrolyte interface (SEI), which means that the initial irreversible capacity loss will increase, and (2) the change in lattice dimension during the lithium ion insertion/extraction will be increased and thus the structure stability of LTO will become worse. In this study, an Al-doped ZnO (AZO) film with various thicknesses and compositions were deposited on LTO particles by atomic layer deposition (ALD) method with different deposition temperatures and different ratios of the number of Al2O3 ALD to that of ZnO ALD. The electrochemical characterization results indicated that the coating of AZO significantly improved the electrochemical performance of LTO between 0.1 V and 3.0 V with a proper coating thickness. After 250 cycles of charge/discharge at a 1 C rate, the capacity of the optimum AZO-coated LTO sample decreased from 203 to 190 mAh g-1 with a capacity retention of ~94% at room temperature and from 224 to 216 mAh g-1 with a capacity retention of ~96% at 55 ℃. The AZO coating layer with an appropriate thickness not only increased the capacity of LTO by enhancing conductivity, but also assisted the LTO electrodes to form a beneficial interface layer to protect the LTO from the continuous attack by harmful components in the electrolyte, especially under extreme conditions, such as high temperature and high current rates.