Study On The Surface Coating And Modification Mechanism Of High Voltage LiCoO₂ Cathode Material

As the earliest commercialized cathode material,LiCoO₂has a theoretical specific capacity of 274 m Ah g^-1.However,in the current commercial application,the LiCoO₂can only provide a stable discharge capacity of about 140 m Ah g^-1,so there is still much room for improvement.Although increasing the working voltage can further tap its discharge capacity,a series of irreversible structural phase transitions and surface side reactions will seriously affect the electrochemical performance of LiCoO₂and shorten its service life under high voltage working conditions,and surface coating is an effective means to solve the above problems.In this paper,the LiCoO₂particles were modified by simple wet chemical method and high-temperature solid-state sintering,which effectively improved the electrochemical performance of LiCoO₂at high cut-off voltage of 4.5 V and above.The specific research contents are as follows:（1）The Al（PO₃）₃-Li PO₃composite coating layer was constructed on the surface of LiCoO₂by chemical method and high-temperature sintering,which stabilized the surface structure of LiCoO₂and optimized the migration environment of Li⁺by using the two substances.The study showed that the sample with 2wt%coating amount still had 90.0%capacity retention(181.5 m Ah g^-1)after 100 cycles in the voltage range of3.0-4.5 V.Even in the range of 3.0-4.6 V at 0.5 C,the capacity retention was 91.8%(180.4 m Ah g^-1)after 100 cycles,showing excellent high voltage cycling stability.（2）To further explore the LiCoO₂cathode material with better performance,the Ti F₄was used as the coating material to pretreat the LiCoO₂surface,and the chemically stable Ti O₂-Li F composite cladding layer was prepared on the LiCoO₂surface after high-temperature annealing,and the surface layer was doped at the same time.The test results show that in the range of 3.0-4.5 V,the capacity retention rate of the sample with1wt%coating amount after 110 cycles at 0.2 C is as high as 97.4%,and the discharge capacity at 5 C was 151.5 m Ah g^-1.Even in the range of 3.0-4.6 V and at 0.5 C,the capacity retention rate of the sample after 100 cycles is as high as 88.6%,showing a good high-pressure cycle stability.（3）To further explore the LiCoO₂cathode material that can be stably cycled at high voltages of 3-4.7 V,the LiCoO₂was modified by KAl F₄coating,and the Li F,Li Al_xCo_1-xO₂and KF phases were derived on the surface of LiCoO₂during high-temperature calcination,which greatly stabilized the surface structure of LiCoO₂,inhibited the generation of harmful phase transformation and improved the stability of LiCoO₂surface structure.The initial discharge capacity of the sample with 1wt%coating was up to 216.3 m Ah g^-1at a high voltage of 4.7 V.After 200 cycles,the sample still had 60%capacity retention,and the cycling stability at high voltage was greatly improved.