Study On Electrochemical Properties And Modification Mechanism Of Surface Modified Lithium Titanate Thin Film Anode With Aluminum Doped Zinc Oxide

As people pay more and more attention to energy and environmental issues,renewable clean energy has gathered the attention of researchers.Among them,lithium-ion batteries（LIBs）have become the focus of research due to their lightness,safety,no memory effect,and long cycle life.Lithium titanate（LTO）,as a substitute for carbon-based anode materials in lithium-ion batteries,has good cycle stability and safety,but its theoretical capacity is low(only 175 m Ah g^-1),and the electronic conductivity and ion conductivity are also low,which limits its commercial application.Therefore,there have been studies using aluminum-doped zinc oxide（AZO）to coat the surface of LTO particles and expand the voltage window of the charge/discharge process to improve electrochemical performance.AZO is a transparent conductive oxide material.Its conductivity is related to the doping amount and crystal phase of alumina（Al₂O₃）.These factors and the thickness of the AZO coating layer will affect the electrochemical performance of LTO.In this thesis,different AZO coated LTO film samples were prepared by magnetron sputtering to determine the most suitable AZO coating layer,and then to explore the mechanism of its modification and improvement.Surface decoration is an effective and widely used method in lithium-ion batteries both for cathode/anode materials,while the underline fundamentals for performance enhancement mechanisms may vary in different material system as well as their operation conditions.Understanding the key role of this nano coating/decoration layer is vital important for further improving battery electrochemical performance.In this study,we prepared LTO polycrystalline thin-film electrodes with and without an AZO surface decoration layer as the ideal model.The test results show that the conductivity of the AZO film prepared with the doping amount of 2 wt%Al₂O₃and the preparation temperature of 250℃is the best.Electrochemical characterizations show that an optimized coating layer can dramatically improve the cycle/rate capacity and accelerate the charge transfer processes.Among them,the AZO modified layer with the sputtering time of 60 s could significantly improve the electrochemical performance of the LTO thin-film electrodes.After300 cycles at room temperature and 1 C,the specific capacity could be increased from～160 m Ah g^-1to～200 m Ah g^-1.After 500 cycles at 60℃and 5 C,the specific capacity could be increased from 120.3 m Ah g^-1 to 200 m Ah g^-1.Not only that,it shown the best performance at high rates.XPS and operando AFM characterizations confirmed that the AZO coating layer was electrochemically converted into a protective layer with Li-Zn alloy and Li₂O conductive matrix,protecting the electrode surface from further electrolyte decomposition and therefore resulting in a more stable SEI layer.The increased surface reduction capability of AZO coated LTO to the electrolyte was suggested to be the critical parameter for the‘self-sacrificing’process of AZO coating layer.Moreover,it is also found that this artificial SEI layer in LTO electrode can facilitate the rapid lithium-ions redistribution during the Li₄Ti₅O₁₂（?）Li₇Ti₅O₁₂phase separation,thereby inhibiting the orderly collective phase-separation behavior in the LTO polycrystalline thin-film electrode.