Research On Low Temperature Performance Of Lithium Ion Batteries Based On DTA/FEC Additives

The performance of lithium-ion batteries will rapidly deteriorate under low temperature conditions,which is one of the main obstacles in the field of precision in aviation and military,where lithium-ion batteries cannot be industrialized.As the temperature drops below zero degrees,utilization and battery voltage drop rapidly.This is mainly related to the following factors:?i?the ionic conductivity of the electrolyte decreases and the properties of the solid-liquid interface passivation layer?SEI film?deteriorate;?ii?the diffusion rate of lithium ions in the electrode decreases;?iii?The charge transfer impedance increases.Based on the market's growing demand for low-temperature performance of lithium-ion batteries and the low-temperature performance of lithium-ion batteries can not meet the market demand,research on the low-temperature performance of lithium-ion batteries has attracted widespread attention.The use of suitable electrolyte additives is one of the effective ways to improve the performance of lithium-ion batteries at severe temperatures.The study used electrolyte ionic conductivity test,constant current charge and discharge test?GC?,cyclic voltammetry?CV?,scanning electron microscopy?SEM?,electrochemical impedance spectroscopy?EIS?and X-ray photoelectron spectroscopy.?XPS?and other means to explore the additives N,N-dimethyl trifluoroacetamide?DTA?and fluoroethylene carbonate?FEC?on the graphite anode and ternary LiNi_0.6Co_0.2Mn_0.2O cathode at different temperatures The effect of chemical properties.The main content and conclusions of the study are as follows:?1?The electrolyte with 2%DTA improved the electrochemical performance and electrolyte ionic conductivity of the graphite electrode at 20?and-20?.At-20?,1mol/L LiPF₆/EC:DEC:EMC?2:1:4?has an ionic conductivity of 0.52 S/m and 0.45S/m in the presence of 2%DTA and without DTA,respectively.At a normal temperature of 20?,the first charge capacity of the graphite electrode in the electrolyte containing 2%DTA and without DTA was 370.80 mAh g^-11 and 325.10mAh g^-1,respectively,and the coulombic efficiencies were 87.41%and 83.37%,respectively.After 100 cycles,the capacity retention rate was 93.9%and 92.12%.At a low temperature of-20?,the first charge capacity of the graphite electrode in the electrolyte containing 2%DTA and without DTA was 114.6 mAh g^-11 and 72.7 mAh g^-1.After 100 cycles,the capacity retention rates were 88.7%and 94.8%,respectively,compared to the first cycle at-20?.The capacity retention ratio was 29.67%and19.54,respectively.The CV test results indicate that DTA inhibits the production of lithium carbonate.The results of SEM test show that DTA can make the solid-liquid interface passivation layer?SEI?formed on the surface of graphite anode more dense and smooth.XPS test and EIS test results show that DTA enriches the content of LiF on the solid-liquid interface passivation layer?SEI?.The resistance of lithium ions to intercalation and extraction in the SEI film becomes small.In summary,DTA improved the electrochemical performance of graphite anodes at different temperatures to some extent.?2?Further applying DTA to the ternary LiNi_0.6Co_0.2Mn_0.2O electrode.At room temperature 20?,the first discharge capacity of LiNi_0.6Co_0.2Mn_0.2O electrode in 2%DTA and without DTA electrolyte was 172.61 mAh g^-11 and 157.79 mAh g^-1,respectively,and the coulombic efficiency was 90.39%and 88.06%.After 100 cycles,the capacity retention rates were 95.06%and 88.7%.The initial discharge capacity of the NCM622 electrode in the electrolyte with 2%and without DTA at low temperature-20?was 126.59 mAh g^-11 and 117.32 mAh g^-1.The first week of Coulomb efficiency was 75.8%and 74.9%,respectively.After 100 cycles,the capacity retention rates were 89.8%and 84.2%,respectively,compared to the first cycle of-20?.The results of EIS,CV and XPS test results show that DTA can form a solid-liquid interface passivation layer?SEI film?with better performance on the surface of NCM622 electrode,enriching the content of LiF and inhibiting the formation of lithium carbonate.Both the resistance and charge transfer resistance of the embedded and extracted in the SEI film become small.In summary,DTA improved the electrochemical performance of ternary LiNi_0.6Co_0.2Mn_0.2O electrode at different temperatures.?3?The electrolyte with 1%FEC added improved the ionic conductivity and electrochemical performance of the ternary LiNi_0.6Co_0.2Mn_0.2O electrode at 20?and-20?.At-20?,1 mol/L LiPF₆/EC:DEC:EMC?2:1:4?in the 1%FEC and FEC-free ionic conductivity were 0.56 S/m and 0.45 S/m.At the normal temperature of 20?,the first discharge capacity of LiNi_0.6Co_0.2Mn_0.2O electrode in 1%FEC and without FEC electrolyte was 175.4 mAh g^-11 and 157.79 mAh g^-1,respectively,and the coulombic efficiency was 92.6%and 88.06%.After 80 cycles,the capacity retention rate was 93.7%and 92.6%.At-20?,the first discharge capacity of the 622 electrode in the electrolyte with 1%FEC and without FEC was 134.59mAh g^-11 and 117.32 mAh g^-1.The first week of Coulomb efficiency was 78.9%and 74.9%,respectively.After80 cycles,the discharge capacity was attenuated to 120.6 mAh g^-11 and 101.3 mAh g^-1,and the capacity retention rates were 89.6%and 86.3%,respectively,compared to the first cycle at-20?.The NCM622 electrode with FEC added at-20?has better rate performance.The results of EIS,CV and XPS test results show that FEC can form a solid-liquid interface passivation layer?SEI film?with better performance on the surface of NCM622 electrode,enriching its LiF content,inhibiting the formation of lithium carbonate and excessive electrode sheet.The dissolution of the metal reduces the resistance of lithium-ions to intercalate and escape in the SEI film.In summary,FEC improved the electrochemical performance of ternary LiNi_0.6Co_0.2Mn_0.2O electrode at different temperatures.