The Study On The Ionic Liquid Electrolyte Of Resistant To High Voltage For Lithium Ion Battery

The high working voltage of the cathode material LiNi_0.5Mn_1.5O₄ is 4.7V.LiNi_0.5Mn_1.5O₄ has strong cycle stability,high energy density,and environmental friendliness,which gives it a broad prospect for Li-ion battery.The stability of the traditional carbonate-based electrolyte drops sharply above 4.5V.Therefore,the development of electrolytes with high voltage resistance,and good compatibility with LiNi_0.5Mn_1.5O₄ materials has been one of the research hotspots.The ionic liquid is a room temperature molten salt,with wide electrochemical window and high operating voltage.Ionic liquid exhibited excellent compatibility with the positive electrode material.In this paper,firstly,the cyclic voltammetry（CV）,galvanostatic charge and discharge,electrochemical impedance（EIS）,scanning electron microscope（SEM）and other methods were tested to research the effect of 1 mol/L LiPF₆^<sub>EC/DMC/BMITFSI electrolyte on LiNi_0.5Mn_1.5O₄/Li battery performance and material structure.By comparing the conductivity of the electrolyte and the overall performance of the battery,the battery with 1 mol/L LiPF₆^<sub>EC/DMC/20vol%BMITFSI showed good cycling performance at the voltage of 3V-5V.The electrolyte could produce a SEI with low-impedance,good conductivity and strong stability on the surface of the LiNi_0.5Mn_1.5O₄ positive electrode,which not only provided a good environment for the migration of Li⁺,but also protected the structural integrity of the electrode material.The results showed that the maximum discharge specific capacity of LiNi_0.5Mn_1.5O₄/Li battery was 127.21mAh·g^-1and the coulomb efficiency was 83.86%at room temperature and low rate of 0.2C;after 50 cycles,the coulomb efficiency was 95%,and the discharge specific capacity was still 120mAh·g^-1,which was 33.3%higher than the battery of 1mol/L LiPF₆^<sub>EC/DMC electrolyte.The average discharge specific capacity at the high rate of 5C was 110mAh·g^-1,which was as twice as the battery of 1mol/L LiPF₆^<sub>EC/DMC electrolyte.Secondly,functionalized ionic liquid PCNMIMTFSI containing-CN was synthesized,meanwhile,the performance of LiNi_0.5Mn_1.5O₄/Li battery in 1 mol/L LiPF₆^<sub>EC/DMC/PCNMIMTFSI electrolyte was explored.The comprehensive performance of electrolyte 1mol/L LiPF₆^<sub>EC/DMC/15vol%PCNMIMTFSI was outstanding.At room temperature and0.2C rate,the battery’s maximum discharge capacity was 132.17mAh·g^-1;the discharge capacity was 125mAh·g^-11 and the coulomb efficiency was 98%after 50 cycles.It was confirmed that the addition of functional group-CN could further increase the high pressure resistance of the electrolyte.The battery’s low rate discharge specific capacity increased,and a stable SEI film could be rapidly generated on the positive electrode surface to enhance the cycle life of the battery.The average discharge specific capacity of the 5C rate was 110 mAh·g^-11 which was not much different with BMITFSI.The paper also found that the electrolyte 1 mol/L LiPF₆^<sub>EC/DMC/15vol%PCNMIMTFSI applied to Li/C batteries had low specific capacity and poor cycle stability.However,the Li/C battery with 0.8 mol/L LiPF₆+0.2 mol/L LiODFB^<sub>EC/DMC/15vol%PCNMIMTFSI electrolyte showed good cycle performance.The main reason was that PCNMIMTFSI continuously occurred reductive reaction on the graphite surface,and LiODFB could generate a stable SEI film on the graphite anode in advance,inhibiting the reduction of PCNMIMTFSI,and improving the stability of the electrolyte.