Study On Fast-Charging Electrolyte By Multiple Additives Strategy For Graphite/LiFePO₄ Pouch Batteries

Lithium ion batteries(LIBs)have been widely used in new energy vehicles.Among many systems,graphite/lithium iron phosphate(graphite/LiFePO4,Gr/LFP)batteries have been widely favored by the market for the long cycle life,super safety and lower cost.However,fastcharging of Gr/LFP batteries presents significant challenges.The key to improve the fastcharging performance is:(1)high ionic and electronic conductivity of the electrodes;(2)low impedance of electrode/electrolyte interface;(3)high ionic conductivity of the electrolyte;(4)stable electrode/electrolyte interface.In the industry,modification or optimization of electrode materials is commonly used to ensure(1),and for(2),(3)and(4),electrolyte additive technology plays an important role.In this research,the effect of 1,3,2-Dioxathiolane 2,2-Dioxide(DTD)additive content(1%-8%,wt.%)on Gr/LFP pouch cells was systematically studied.The performances of the battery at low-temperature,fast-charging and high-temperature were studied by using chemical testing and characterization techniques such as charge-discharge,linear scanning,electrochemical impedance spectroscopy(EIS),scanning electron microscopy(SEM)and Xray energy dispersive spectroscopy(EDS).The results show that low concentration of DTD(13%)in electrolytes are more compatible with Gr/LFP pouch cells,characterized by slow fading in low-temperature performance,fast-charging cyclability and high-temperature stability,due to its lower overpotential and impedance interface.The 3%DTD-based battery,in particular,could maintain 91%capacity after 300 cycles at 0℃ 80%capacity after 800 cycles at 2C/2C fast-charging,and higher cut-off capacity after 1200 cycles at 45℃.When DTD combined with lithium difluorophosphorate(DFP)was applied to Gr/LFP pouch cells,the combination of 2%DTD and 1%DFP(DTD+DFP)exhibits best synergistic effects at low-temperature and fast-charging conditions,which can be attributed to the SEI film that can inhibit the growth of impedance and overpotential during cycles,greatly improving the cells cyclability.The cut-off capacity retention rate is still not reached after 2400 cycles at 2C/2C fast-charging.Meanwhile,the cells with DTD+DFP electrolyte also show the best rate performance,which get 87.50%recovery capacity even after 5 cycles at 5C/5C chargedischarge.The EIS,cyclic voltammetry,and specific capacity tests of the reassembled cointype cells showed that DTD+DFP mainly protected the electrochemical behavior of graphite,maintaining a 90.2%graphite capacity after 2C/2C fast-charging.Scanning electron microscopy(SEM)and X-ray photoelectron spectrum(XPS)were used to further analyze the graphite surface.It was found that the robust and low-impedance SEI film formed by DTD+DFP contained appropriate content of ROSO2Li,Li2SO4,LiF and recrystallized LiPO2F2.Finally,the formation mechanism of the SEI film was described as an equeation:DTD+LiPO2F2+ne→ROSO2Li+Li2SO4↓+Li3PO4↓+LiF ↓+LiPO2F2↓(recrystalized).