Sputter-coating Lithium Cobalt Oxide As Cathode For Lithium Ion Batteries

Currently, increasingly serious energy crisis and deterioration of human environment are forcing the world to search for renewable energy to replace fossil fuels, where and one of the key problems is how to store the electricity generated from the renewable energy. Chemical energy is one of the most efficient ways for energy storage. Lithium-ion battery, the electrochemical energy storage battery, dominates the market of consumer electronics and electric vehicle, in consideration of its numerous advantages: high efficiency, high-energy density, and so on. Layered Li Co O₂, the earliest commercial cathode material, occupies a large market share of cathode material for its outstanding electrochemical performance under the low cut-off potentials（4.2 V vs. Li/Li+）. To satisfy more demanding requirements for higher energy density and power density, increasing the cut-off charging potential of Li Co O₂ is a popular way. However, fast damping of capacity and poor high-temperature performance prevents its practical application.In this thesis, a radio frequency magnetron sputter technique is adopted to realize the overall coating of Li Co O₂ electrodes for improved performance. The material characterization and electrochemical performance of Li Co O₂ electrodes were systematically done. The research content is listed as follows:（1） The lithium phosphate was used as sputter target material for its good chemical stability and favorable ionic conductivity, and an amorphous lithium phosphate coating layer was successfully deposited on the surface of Li Co O₂ electrodes for the first time. The influence of the lithium phosphate film on the surface morphology, crystal structure, and electrochemical performance of Li Co O₂ electrodes was studied. Results from SEM and XRD confirmed that the surface morphology of electrodes was changed after the appearance of the lithium phosphate coating layer, but the crystal structure of Li Co O₂ was not unchanged. Electrochemical results indicated the cycling stability and rate performance of electrodes were tremendously improved at both room temperature and 50 ℃ after lithium phosphate coating, and the 100 th discharged capacity and capacity retention of the electrode with a coating layer thickness of 60 nm reached respectively 140 m Ahg-1 and 78.2 % at 50 ℃, approach to the performance at room temperature（146 m Ah g-1, 79.3 %）, when the cut-off charging potential was 4.5 V and the current rate was 1 C. Meanwhile, during the rate test at 50 ℃, the discharged capacity achieved 137 m Ahg-1 at a rate of 12 C, higher than that of the room temperature（127 m Ah g-1）.（2） Li Co O₂ electrodes were coated by sputtering titanium dioxide, and the difference in surface morphology, crystal structure, and electrochemical performance of uncoated and coated electrodes was also studied. Results showed that the titanium dioxide coating layer on electrode surface was a continuous uniform thin film composed of nano particles, and that the cycleability at room temperature was good for Li Co O₂ electrodes coated with titanium dioxide. The discharged capacity and capacity retention of electrodes with 5 min sputtering could respectively reach 154 m Ahg-1 and 84 % after 100 cycles at room temperature with a potential window of 3.0-4.5 V and a rate of 1 C, which was higher than those of electrodes with a lithium phosphate coating layer. However, the cycleability at 50 ℃ was obviously unimproved.（3） The titanium dioxide with oxygen vacancies(Ti O_1.8) was investigated as sputter target material. Meanwhile, the electrochemical performance of coated electrodes was also discussed. Results showed that at room temperature the cycleability of coated Li Co O₂ electrodes was drastically improved, and the electrode with an optimal coating time of 5 min obtained a discharged capacity of 160 m Ahg-1 and a corresponding capacity retention of 86 % after 100 cycles at room temperature with a cut-off charging potential of 4.5 V and a rate of 1 C, better than the electrodes coated by titanium dioxide without oxygen vacancies. Besides, the discharged capacity was 109 m Ahg-1 at the rate of 10 C, which was superior to both the bare electrode and electrodes coated by titanium dioxide without oxygen vacancies. However, the cycleability of coated electrodes was not obviously improved when the cut-off charging potential was further elevated to 4.7 V.