Research On Electrode Modification And Functional Electrolytes For High-Voltage LiCoO₂ Batteries

With the development of 3G/4G mobile phones and pads,batteries must support longer operation time,which means the longer endurance of lithium-ion batteries.Therefore,improvement of the specific energy densities of lithium ion batteries has become more and more important.Owing to its advantages of high energy density,good cycle and rate performance,LiCoO₂ has been dominating the cathode material market for the electronic devices.At present,the specific energy density of the lithium ion battery can be improved by increasing the charging cut-off voltage of the LiCoO₂ cathode material.When the charging cut-off voltage is increased from 4.2V to 4.5V,the specific energy density will be improved from 137mAhg^-1 to 193 m Ahg^-1.Therefore,the development of 4.5V high voltage LiCoO₂ battery has important practical significance.However,when the charge cut-off voltage is higher than 4.2V,on the one hand,the structure of the electrode material changes;on the other hand,the traditional electrolyte can undergo severe oxidative decomposition at high voltages;resulting in rapid decline of lithium-ion battery capacity.In this dissertation,following researches on electrode modification and functional electrolytes for high-voltage LiCoO₂ batteries are mainly investigated:Firstly,LiCoO₂ was modified by MgAl₂O₄ by a liquid phase method,and similarly the Al₂O₃ and MgO modified LiCoO₂ samples were prepared for comparison.The effect of MgAl₂O₄ modification on the physical and electrochemical properties of LiCoO₂ was studied.The results show that MgAl₂O₄-modified LiCoO₂ cathode material has the best electrochemical performance,especially the cycle performance.1%MgAl₂O₄-modified LiCoO₂ still has a capacity retention of more than 96% after 70 cycles in 3-4.5V at 1C,while the pristine LiCoO₂ has a capacity retention of only 42%.Secondly,1H,1H,5H-Perfluoropentyl-1,1,2,2-tetrafluoroethylether?F-EAE?was used as an electrolyte co-solvent and then a new type of high-voltage electrolyte was prepared.1M of LiPF6/EC-DEC?1:3,w/w?was used as the baseline electrolyte for comparison.The effects of F-EAE as electrolyte co-solvent on the electrochemical properties of LiCoO₂ and graphite were studied.The results show that the electrochemical performance of LiCoO₂ and graphite can be improved obviously when the amount of F-EAE in the electrolyte is 10%.The specific capacity of LiCoO₂ is166mAhg^-1 after 100 cycles in 3-4.5V at 1C,with capacity retention of 88%.While thespecific capacity of LiCoO₂ in the baseline electrolyte is substantially decayed to0mAhg^-1.The initial charge and discharge capacity of graphite at 0.05 C is 324.9mAhg^-1and 386.6mAhg^-1,better than 321mAhg^-1 and 436.8mAhg^-1 in the baseline electrolyte.Finally,1M Li PF6 dissolved in a mixture of ethylene carbonate?EC?,dimethyl carbonate?DMC?and diethyl carbonate?DEC??5:3:8,w/w/w?was selected as a baseline electrolyte.Succinonitrile?SN?and fluoroethylene carbonate?FEC?were selected as electrolyte additives to prepare high-voltage electrolytes.The effects of SN and FEC as electrolyte additives on the electrochemical properties of LiCoO₂ were studied.The results show that electrolyte using 2%FEC-1%SN compound additive has the best comprehensive performance,and can significantly improve the cycling stability of LiCoO₂.The specific capacity of LiCoO₂ is 173mAhg^-1after 100 cycles in 3-4.5V at0.5C with a capacity retention of 87.7%.While the specific capacity of LiCoO₂ in the baseline electrolyte is 130.2mAhg^-1,the capacity retention rate is 72.4%.