Research On The 4.6V High-Voltage LiCoO-2 Cathode Material For High Energy Density Lithium-ion Battery

At the charging voltage of 4.2 V?vs.Li/Li⁺?,the capacity of LiCoO₂ is limited to140 mAh/g?just half of its 274 mAh/g theoretical capacity?,which leads to low energy density that can not satisfy the demand for high energy density.Improving the charging voltage of LiCoO₂ can effectively improve its capacity,however,with rapidly worsening of its cycling stability.This study is committed to obtain LiCoO₂ with great cycling performance at high voltage??4.5V vs.Li/Li⁺?.At first,Mg-doping is employed to improve the electrochemical performance.Secondly,Ti is introduced to form Mg-Ti co-doping,which aims to stabilize the crystal structure.And finally,the surface of co-doped sample is modified by AlPO₄-coating for the further enhancement to the cycling stability of LiCoO₂ at high voltage.The crystal structure and surface morphologies of the obtained samples are investigated by XRD and SEM analysis.And the electrochemical properties of the obtained samples are investigated by galvanostatic charge/discharge,electrochemical impedance spectra?EIS?and cyclic voltammogram?CV?.The results are shown below.?1?After 50 cycles at 0.5C in 2.75-4.5 V?vs.Li/Li⁺?voltage range,the capacity retention rate of the LiCo_0.97Mg_0.03O₂ sample synthesized by sol-gel method is 78.6%,and this value of the bare LiCoO₂ synthesized by the same method is only 40.6%.As for solid-reaction method,the capacity retention rate of the LiCo_0.97Mg_0.03O₂ sample reaches up to 90.4%,while this value of the un-doped sample is only 72.4%.This result indicates that doping LiCoO₂ with Mg element can effectively improve its electrochemical performance at 4.5 V?vs.Li/Li⁺?.?2?After 50 cycles at 0.5C in 2.75-4.5 V?vs.Li/Li⁺?voltage range,the capacity retention rate of the LiCo_0.99Mg_0.01O₂,LiCo_0.97Mg_0.03O₂ and LiCo_0.95Mg_0.05O₂ sample synthesized by sol-gel method is 61.4%,78.6%and 83.2%,respectively.By contrast,thisvalueoftheLiCo_0.98Mg_0.01Ti_0.01O₂,LiCo_0.96Mg_0.02Ti_0.02O₂and LiCo_0.94Mg_0.03Ti_0.03O₂ sample synthesized by the same method is 87.3%,91.0%and96.2%,which is obviously superior to the Mg-doped samples.After 50 cycles at 0.5C in2.75-4.6 V?vs.Li/Li⁺?voltage range,the capacity retention rate of the LiCo_0.99Mg_0.01O₂,LiCo_0.97Mg_0.03O₂ and Li Co_0.95Mg_0.05O₂ sample synthesized by solid-reaction method is62.8%,80.2%and 82.3%,respectively.As for the LiCo_0.98Mg_0.01Ti_0.01O₂,LiCo_0.96Mg_0.02Ti_0.02O₂ and LiCo_0.94Mg_0.03Ti_0.03O₂ sample synthesized by solid-reaction method,this value is improved to 85.7%,88.4%and 88.5%.This result indicates that Mg-Ti co-doping can further stabilize the material structure based on Mg-doping.?3?After 100 cycles at 0.5C in 2.75-4.6 V?vs.Li/Li⁺?voltage range,the capacity retention rate of the LiCo_0.98Mg_0.01Ti_0.01O₂ sample synthesized by solid-reaction method is only 74.5%.By coating 0.3 wt.%AlPO₄,the capacity retention rate is improved to90.7%at the same condition,which is remarkably superior to the un-coated sample.After 200 cycles at 2.0C in the same voltage range,the capacity retention rate of the un-coated sample is 62.0%,while this value of coated sample reaches up to 88.0%,which is greatly improved.This result indicates that coating the co-doped sample with AlPO₄ can effectively enhance its cycling stability at high voltage.