Energy Storage Characteristics Research And Surface Modification Of High Voltage Lithium Cobalt Oxide

With the increasingly demand of consumer electronic products and electric vehicles,it is necessary to develop energy storage devices with higher energy density and longer service life to meet human needs.LiCoO₂ as the main cathode material of small electronic device has be limited further application,because its actual specific capacity is lower than the theoretical value.C urrently,LiCoO₂ posses a specific capacity of 140mAh/g at cut-off voltage of 4.2V（vs.Li/Li⁺）.Increasing the charge cut-off voltage of LiCoO₂ can increase its specific capacity.However,the structure of LiCoO₂ inherently will be unstable due to its large number of de-intercalated Li⁺,and at high cut-off voltage the electrolyte will decompose quickly.The surface coating of metal oxide can effectively improve the performance of LiCoO ₂ at high voltage.Compareed with the previous reports,this thesis has demonstrated a simple but effective coating method with advanced performance.The following issues have been addressed:（1）Using traditional wet chemical methods,Al₂O₃,ZnO and Al₂O₃/ZnO co-coating were achieved on LiCoO₂.The cycle performance of coated LiCoO₂ is improved.Coated with Al₂O₃,exhibite a capacity retention rate of 75%after 500 cycles in the voltage rang of 3.0-4.5V.Al₂O₃/ZnO co-coated sample with the optimized molar ratio of Al:Zn:Co=0.3:2.7:100,shows the best performance,which the capacity retention rate is79%at 500^{t h} cycle between the voltage window of 3.0-4.5V.（2）The traditional co-precipitation coating methods was modified in which,the hydrolysis of Al³⁺in aqueous solvent leads to coat the surface of LiCoO₂ with a uniform layer of Al₂O₃.Further thermal treating can generated a mixed coating layer consisting of Al₂O₃ and LiAl_x Co_1-x-x O₂ according to the analysis results of the morphology and composition.The cycle stability after coating is greatly improved.The capacity retention rate after cycling for 1,000 times at 3.0-4.5 V is 72%.And the actual specific capacity obtained at high cut-off voltage is higher than that at 4.2 V,which proves that the surface-coated LiCoO₂ has practical economic significance.There are several reasons for the improvement of performance:1)LiAl_x Co_1-x-x O₂ solid solution layer can stabilize the LiCoO₂ lattice structure;2)The surface coating of Al₂O₃ can well prevent electrolyte from corroding the electrode material,and restricting Co dissolution;3)Al₂O₃ coating layer will change the surface activity of the electrode,slow down the formation of SEI film,and dpress electrolyte decomposition on the electrode surface.（3）In order to explore the electrochemical performance of LiCoO ₂ at higher voltage（4.6,4.7V）,the cycling performance of bare LiCoO ₂ and coated LiCoO₂ have been tested at 4.6V and 4.7V.At higher voltages,the structure of LiCoO ₂ degenerates faster,and irreversible phase transition occurs near 4.55V.The surface coating solely has the limitation to improve the performance.Since the doping can stabilize the lattice structure,so I try to coate the doped LiCoO₂.The performance of doped-coated LiCoO₂has greatly improved,and the capacity retention rate at the 500^{t h} cycles between the voltage window of 3.0-4.6 V is 72%.It is the best reported performance.The doping and coating can improve the performance of LiCoO₂ under high voltage,and the treating process is simple,low cost and easy to be implemented.This method is expected to be applied in commercial production soon.