Research On Electrolytes Of High Specific Capacity And High Nickel NCM-based Lithium Ion Battery

In recent years,the field of electric vehicles and special fields such as aerospace technology,military operations and deep-sea mining have developed rapidly,which puts forward higher requirements for the energy density,cycle life,cost and working temperature range of lithium-ion batteries（LIBs）.Although Li Ni00.8Co0.1Mn0.1O2（NCM811）cathode material is expected to achieve high energy density,it is difficult for its battery to balance high capacity and excellent long cycle performance.Meanwhile,when the ambient temperature is as low as-40 °C or below,the limited temperature range and poor electrochemical performance of conventional electrolytes limit the application of NCM811-based LIBs at low temperature.To meet the two challenges,NCM811 was selected as the cathode in this theis and a new electrolyte was designed by introducing carboxylic ester-based co-solvents with low melting point and low viscosity.The effect and mechanism of co-solvents on the electrochemical performance of the electrolyte and NCM811-based LIBs were analyzed.In view of the main research work for the thesis,the summary is as follows:（1）In order to answer whether the electrolyte with lower melting point of co-solvent will show better low temperature performance,ethyl propionate（EP）and propyl acetate（PA）with melting point difference of 22 °C were introduced as co-solvents to design electrolytes and the low-temperature performances of Li||NCM811 and artificial graphite（AG）||NCM811 using two electrolytes were compared.In the range of-60 °C – 25 °C,Li||NCM811 and graphite||NCM811 using the two electrolytes showed considerable lowtemperature discharge performance.The results of molecular dynamics simulation analysis showed that the two electrolytes exhibited similar contact ion pair solvent structure and similar coordination number.To sum up,the cosolvent with lower melting point does not necessarily show better low-temperature performance,and the solvation effect of electrolyte is the key to select co-solvents.（2）The effects of EP,PA and ethyl acetate（EA）co-solvents on the performance of1 mol/L Li PF6（EC/EMC,1/1,v/v）and Li||NCM811 were systematically investigated.The co-solvents can significantly improve the ionic conductivity of the electrolyte at low temperature and broaden the application range of temperature.However,the lowtemperature performance of Li||NCM811 at rate of 0.2C showed no improvement accordingly,and the electrolyte with high ionic conductivity did not show prominent lowtemperature performance,indicating that ionic conductivity was not the only factor determining the low-temperature performance.In addition,the capacity retention of Li||NCM811 with 11 EP electrolyte is 88.6% after 200 cycles at 1C.（3）Without changing co-solvent and additive fluoroethylene carbonate（FEC）,this theis investigated the influence of EC/EMC volume ratio change on the performances of both electrolyte and Li||NCM811.New electrolytes were designed by changing the volume ratio of EC/EMC from 1:1 to 3:7.After changing the volume ratio of EC/EMC,the low-temperature performances of Li||NCM811 with 37-V,37EP-FEC,37PA-FEC and37EA-FEC electrolytes were improved,indicating that changing the proportion of EC/EMC is beneficial to improve the low-temperature performance of LIBs.Thanks to the formation of stable interface layer between 37 EP electrolyte and NCM811 electrode,Li||NCM811 showed excellent cycle performance,and the capacity retention was 88.27%after 200 cycles at 1C.