Modification And Thin Film Preparation Of The LiCoO₂ Cathode For Lithium Ion Battery

With the application of lithium-ion batteries（LIBs） extending from traditional 3C electronics to electric vehicles, cathode material with high energy density, high power density, safety and excellent cycling performance is very important. As an original cathode material applied in commercial LIBs, layered LiCoO₂ is typically charged up to 4.2 V vs. Li/Li+ and its corresponding discharge capacity is only around 140 mAh g-1, which is much lower than its theoretical capacity of 274 mAh g-1. Recently, the commercial LiCoO₂-based LIBs can achieve a high voltage up to 4.35 V and deliver an increased capacity. Further increase of the upper cut-off potential will reach a higher capacity. However, the subsequent electrode surface side reactions associated with transition metal dissolution, structural degradation and intensified electrolyte decomposition will result in the poor performance of LiCoO₂ at high voltages. Surface modification is a major strategy to address these issues. An appropriate surface coating can improve the cycling performance and the rate capability of the LiCoO₂ electrodes in the high voltage range. However, traditional coating methods usually focus on the LiCoO₂ powders, noted that most of the coating materials are poor Li+ ion or electronic conductor, so the coating layer on LiCoO₂ powders will block the Li+ ion and electronic transport, which impedes the further improvement of the electrochemical performance.In this work, coating materials were directly coated on as-prepared LiCoO₂ composite electrodes via radio-frequency（RF） magnetron sputtering. It was not only coated on the electrode surface, but also diffused through the whole electrode. The electrochemical performance was improved ascribed to the better Li+ ion and electronic transport pathways supported by the as-prepared electrode, a more stable structure and surface assisted by the coating layer. On the other hand, to meet the demand of the microelectronics products, we prepared all-solid-state thin film lithium ion batteries. Furthermore, mono-like LiCoO₂ thin film with specific crystal orientations was successfully prepared. The main works were summarized as follows:（1） Metal oxides ZnO or Al₂O₃ was coated on as-prepared LiCoO₂ composite electrodes via RF magnetron sputtering, the coating thickness and modification mechanism were investigated. It was found that metal oxides coating can significantly improve the cycling performance and the rate performance of the LiCoO₂ electrodes. The sample with an optimum ZnO coating thickness of 17 nm exhibited an initial discharge capacity of 191 mAh g-1 at 0.2 C and the capacity retention was 83.3% after 150 cycles. It also delivered superior rate performance with a reversible capacity of 106 mAh g-1 at 10 C, whereas the bare LiCoO₂ electrodes showed only 75 mAh g-1. The capacity retention of the optimum Al₂O₃ coating was 83.3% after 150 cycles in the voltage range of 3.0 V – 4.5 V at 0.2 C, much higher than the bare one（46.9%）. The enhanced performance could be attributed to the following aspects. Firstly, the coating layer suppressed the surface side reactions, and inhibited the formation of the surface passivation layer and slower charge transfer resistance growth during cycle. Secondly, formation of a solid solution at the interface between LiCoO₂ and coating layer, this solid solution enhanced the structural stability of LiCoO₂, and increased the Li+ ion diffusion coefficients. Thirdly, the phase transitions of LiCoO₂ was inhibited to some extent and the reversibility of Li+ ion insertion/extraction was enhanced with the existence of the coating layer.（2） In order to further improve the performance of LiCoO₂ electrode, a conductive Al₂O₃-doped ZnO（AZO） layer was directly coated on the LiCoO₂ porous composite electrode by magnetron sputtering. The AZO diffused through the whole electrode, which formed a three-dimensional conductive network, offering a more efficient transport channel for the electron and Li+ ion. Meanwhile, the AZO coating layer combined the advantages of ZnO and Al₂O₃, which was beneficial to further improve the electrode performance. In addition, AZO layer could consume HF and form a metal fluoride on the electrode surface by electrochemical reactions. The newly formed metal fluoride layer would be greatly effective against HF attack during cycling, and could prevent further electrolyte decomposition and Co dissolution. Up to 90% of the initial capacity of the optimum thickness（20 nm） AZO-coated electrode could be retained（173 mAh g-1） after 150 cycles between 3.0 and 4.5 V vs. Li/Li+ at 0.2 C. Meanwhile, the rate performance was remarkably improved with a reversible capacity of 112 mAh g-1 at 12 C.（3） Since the electrode-electrolyte interface is mainly for the Li+ ion transport, a good Li+ ion conductor Li₂CO₃ was choosed as sputtering coating layer. The sputtered Li₂CO₃ layer served as an artificial SEI layer on the LiCoO₂ electrode and could prevent the formation of the primary SEI layer. The Li₂CO₃ coated electrode could achieve over 87% capacity retention after 60 cycles when cycled in the range of 3.0 – 4.5 V at room temperature, delivering an extended discharge capacity of 161 mAh g-1. With further increasing the charge voltage up to 4.7 V vs. Li/Li+, or elevating the operation temperature to 55 °C, the Li₂CO₃ coated electrodes still exhibited an excellent cycling stability.（4） In order to further optimize the interface between LiCoO₂ electrode and electrolyte, a higher lithium ion conductivity and more stable Li₄Ti₅O₁₂ was used as sputtering coating layer. The optimum coating thickness was confirmed rapidly and accurately by the high throughput method. It was found that the Li₄Ti₅O₁₂ coated LiCoO₂ electrode showed a greater Li+ ion diffusion coefficient（3.9×10-11 cm2 s-1） and a better rate performance（reached 113 mAh g-1 at 12 C） compared with the metal oxides and Li₂CO₃ coating. Furthermore, the results showed that the electronic conductive oxide AZO coating was the most effective in cycling performance, while the lithium ion conductive Li₄Ti₅O₁₂ coating was the most effective in high rate performance.（5） On the other hand, a specific crystal preferred orientation of mono-like LiCoO₂ thin films was prepared on stainless steel substrate by RF magnetron sputtering, and the surface morphology characteristics and the electrochemical performance were analyzed. The LiCoO₂ film with the（101） preferred orientation displays a porous and loose surface, with a larger initial discharge capacity, and a poor cycling performance. While the（003） preferred orientation LiCoO₂ film exhibits a compact surface, lower initial discharge capacity but provide a better capacity retention than the（101）-preferred film. Furthermore, an all-solid-state thin film lithium ion battery with typical charge-discharge profiles and cycling stability was prepared.（6） To improve the conductivity and the structure stability of LiCoO₂ thin films, a hybrid film of Sn and LiCoO₂ was achieved via co-sputtering method. The results showed that the electrochemical performances of the LiCoO₂ films were improved by Sn doping. The Sn doped LiCoO₂ film exhibited an discharge capacity of 45.2 mAh cm-2mm-1 after 100 cycles between 3.0 and 4.2 V vs. Li/Li+, with 93.2% of the initial capacity retained, whereas the capacity ratention of the LiCoO₂ film showed only 77.7% under the same condition.