Optimizations And Applications Of Li-excess Layered Cathode Materials For Lithium-ion Batteries

Lithium ion batteries have the highest energy density in the all practical current batteries and are considered to be the most promising secondary batteries.The electrochemical performance of lithium-ion batteries depends on the cathode materials largely.So far,in all reported cathode materials,Li-excess layered oxides with the formula of x Li₂MnO₃??1-x?Li MO2?M = Mn,Ni,or Co?have become a most promising class of cathode materials for next generation lithium ion batteries because of their considerable capacities,over 250 m A?h?g-1.However,poor cycling performance of these materials at room temperature and high temperature greatly hinders their practical applications,as electrodes suffer from serious interfacial reactions and erosion behavior in the existing electrochemical system.Therefore,in order to solve the above problems,we optimized the electrolyte system of the Li-excess layered cathode material Li_1.18Co_0.15Ni_0.15Mn_0.52O₂,and improved the interfacial stability and electrochemical performance of the material at room temperature and high temperature.The research results are as follows:Firstly,we prepared different proportions of Li ODFB/Li PF₆ double salt electrolytes.Constant current charge and discharge test showed that 0.2 mol·L-1 Li ODFB/0.8 mol·L-1 Li PF₆ double salt electrolyte significantly improved the specific capacity of materials and cycle stability at room temperature and high temperature.The material with 1 mol·L-1 Li PF₆ electrolyte showed a discharge capacity of 143 m A?h?g-1 after 200 cycles,resulting in capacity retention of 69.4%.In comparison,the material with the dual-salt electrolyte showed a discharge capacity of 200 m A?h?g-1 after 200 cycles,resulting in capacity retention of 87%.When the temperature was raised to 55 ?,the capacity of the material with 1 mol·L-1 Li PF₆ electrolyte decreased rapidly after 60 cycles.While the material with the dual-salt electrolyte showed a discharge capacity of 231 m A?h?g-1 after 100 cycles,resulting in capacity retention of 92%.The dual-salt electrolyte has better electrochemical stability and can generate a thinner and more stable SEI film on the surface of the material,so that the material has a smaller charge transfer resistance,which effectively prevent the corrosion of HF on the material,inhibit the dissolution of Mn ions,and improve the thermal stability of the material.Furthermore,we used PVDF-HFP as the polymer matrix,then a conventional 1 mol L-1 Li PF₆ liquid electrolyte was prepared into the gel polymer electrolyte.The SEM of the electrode shows that the contact between the gel polymer electrolyte and the electrode material is close.Electrochemical test shows that gel polymer electrolyte significantly improved the cycle stability of materials at room temperature and high temperature.When the temperature was raised to 70?,the material with 1 mol·L-1 Li PF₆ electrolyte has been unable to work normally.In comparison,the material with gel polymer electrolyte showed an initial discharge capacity of 295 m Ah g-1 at 0.5C,resulting in capacity retention of 84.1% after 70 cycles.TEM and EDS tests show that the gel polymer electrolyte has less corrosion to the material,enhances stability of the interface,and can effectively inhibit the dissolution of transition metal ions.In addition,DSC test shows that the material using the gel polymer electrolyte also had more excellent thermal stability.In this dissertation,the interfacial properties,electrochemical properties and thermal stability of the Li_1.18Co_0.15Ni_0.15Mn_0.52O₂ at room temperature and high temperature were successfully improved by optimizing the electrolytic system of the material,and then its mechanism was analyzed,which provides some experimental and theoretical basis for promoting the practical application of Li-excess layered cathode materials.