Study On The Properties Of Flame Retardant Electrolytes For Lithium-ion Batteries

Energy shortage and environmental protection issues have led to the application of traditional energy sources towards new energy sources.With the advantages of high power,high energy,long cycle life and environmental friendliness,lithium-ion batteries have become the preferred choice among current power sources.The safety of Li-ion batteries restricts the full promotion of electric vehicles.The high flammability of the electrolyte is a key factor in the safety problems of the battery,so finding a suitable flame retardant electrolyte can improve the overall safety performance of the battery.Different levels of ethoxy pentafluorocyclotriphosphazene nitrile（PFPN）were added to1 mol/L LiPF₆EC:DEC=1:1（vol）electrolyte respectively,and combustion tests revealed that the flammability resistance of the electrolyte increased as the PFPN content increased.The electrolyte with 3%flame retardant no longer spreads when ignited,and the electrolyte with 5%flame retardant no longer burns.Considering commercial applications,the addition of 5%PFPN flame retardant is the most suitable dosage.Soft pack batteries were assembled with LiNi_0.5Co_0.2Mn_0.3O₂as the positive electrode material and graphite as the negative electrode material,1 mol/L LiPF₆EC:DEC=1:1（vol）as the base electrolyte,5%PFPN flame retardant additive was added to the base electrolyte,and the performance of LiPF₆and LiFSI was compared using different electrolyte lithium salts.The results showed that the electrolyte system with 5%PFPN flame retardant additive and LiFSI as the main salt outperformed the other electrolyte systems.At room temperature,the discharge capacity was 1570.4 m Ah after 400 charge/discharge cycles,and its cycle efficiency was 97.43%.Under high temperature tests（60°C and 80°C）,the discharge capacity was1642.1 m Ah and 1539.1 m Ah respectively,with a capacity recovery rate of 99.12%and92.99%respectively.VC was added to the above excellent electrolyte system and the effect of different VC concentrations on the electrochemical performance of Li-ion batteries was investigated.The optimum VC content of the electrolyte at room temperature was 1%,and the discharge capacity was 1563.9 m Ah after 375 charge/discharge cycles,with a capacity retention rate at97.43%.The better VC content at high temperature（60°C and 80°C）tests was 1%,with discharge capacities of 1639.0 m Ah and 1563.9 m Ah and capacity recovery rates of 95.07%and 99.87%respectively.LiFSI was used as the main salt and PFPN as the flame retardant additive,and the organic solvents were EC:DEC=3:7（vol）,EC:DMC=1:1（vol）,EC:DMC:EMC=1:1:1（vol）and EC:DMC:DEC=1:1:1（vol）respectively to form four new electrolyte systems.The electrolyte system EC:DEC=3:7（vol）has a better electrochemical performance than the other electrolyte systems at room temperature,with a discharge capacity of 1641.5 m Ah and a capacity retention rate of 99.20%after 275 cycles of charging and discharging.EC:DEC=3:7（vol）was the better electrolyte system at high temperatures（60°C and 80°C）with a discharge capacity of 1653.9 m Ah and 1606.6 m Ah and a capacity recovery rate of 99.24%and 95.93%,respectively.