Understanding the surface properties of Li4Ti5O12 (LTO) for high-performance anode material for lithium-ion batteries

Abstract

The Li4Ti5O12 (LTO) defect spinel is known for its excellent durability of “10 000” cycle counts and a high level of safety as an anode material in lithium-ion batteries However it shows an intrinsic insulating property low energy density and prevalent gassing issues Furthermore the understanding of the surface structure and chemistry of LTO and its solid-electrolyte interphase (SEI) are not fully understood The goal of this thesis is to systematically study the surface properties of LTO and propose approaches that reduce the internal resistances consisting of Li-ion and electron transport This thesis starts with developing an understanding of the inherent bulk properties of LTO Doping is a direct approach to reducing resistance within the electrode by manipulating the electronic conductivity of LTO However doping may induce multiple effects influencing the overall electrochemical kinetics e g changing the size of particles and the ionic and electronic conductivities Here we systematically investigated the phase stability electronic conductivity and electrochemical kinetics of M-doped LTO (M = Na K Mg Ca Sr Al Ga V Cr Mn Fe Co Ni Cu Zn Zr Nb Mo Ta and W) With both ab initio calculations and experiments the mechanism of electron transport within LTO is elucidated the desired type of dopants for improving the electronic conductivity of LTO is clarified and the role of electronic conductivity in the electrochemical kinetics of LTO is revealed These results provide an in-depth understanding of metal-doped LTO and would help the development of a variety of electrode materials By also using a model Cr-doped thin-film electrode we have related the understanding of the enhancements in the bulk property to its surface structure and chemistry possibly revealing the true doping effect As previously mentioned the electrochemical characteristics of LTO at low potentials and the property of the SEI on LTO are not well understood Here we investigate the charge-transfer kinetics of the SEI formed between the model LTO thin-film electrode and the organic electrolytes with distinct solvation ability by AC impedance spectroscopy and their stability by cyclic voltammetry of ferrocene With the SEI film on LTO the Li+ de-solvation was still a rate-determining step but with larger activation energies which showed a strong dependence on the solvation ability of electrolyte The activation energies of de-solvation for the fluoroethylene carbonate + dimethyl carbonate (FEC+DMC)- and ethylene carbonate + diethyl carbonate (EC+DEC)-based systems rose from 35 and 55 to 44 and 67 kJ·mol–1 respectively and that for the propylene carbonate (PC)-based system did not noticeably change at around 67 kJ·mol–1 Also the SEI passivation of LTO was much slower than graphite and the rate strongly depended on the solvation ability of the electrolyte Understanding the surface properties of LTO at low potentials opens the door for higher energy density LTO-based LIBs Surface modifications e g coating and defect engineering play an intriguing role in interfacial electrochemical processes Herein we report a novel synthesis of highly oxygen-deficient “defective-LTO” with a conformal carbon coating of 2-5 nm as an anode material with high-rate performance Lastly defect engineering with doping and carbon coating are practical approaches to improve surface structure and chemistry but fine defect characterization and probing are almost impossible for dilute concentrations In this thesis we also proposed to re-visit the use of Raman spectroscopy as a complementary tool for defect probing and analysis for the metal-ion doped- and carbon-coated LTO

Date of Award	2020
Original language	English
Supervisor	Shih-kang Lin (Supervisor)