Development And Application Of New High-safety Electrolyte For Lithium Batteries

In recent years,lithium batteries are widely used in consumer electronics and new energy vehicles,which put forward higher requirements for specific energy and safety performance of lithium batteries.At present,liquid electrolyte is widely used in traditional commercial lithium batteries,which often faces many safety problems such as flammable electrolyte,easy leakage and so on.Based on this situation,on the one hand,from the perspective of functional additives,we try to develop new electrolyte additives to improve the performance of the electrolyte and further improve the safety of lithium batteries.The solid-state electrolyte that is the vital component of solid-state lithium battery can replace liquid electrolyte to realize lithium conduction between positive and negative electrodes,thus improving the safety and energy of lithium battery.Among them,organic-inorganic composite solid-state electrolyte has become one of the current research hotspots due to its high ionic conductivity,high stability and good interface contact.Polyvinylidene fluoride(PVDF)is often used as polymer matrix of composite solid electrolyte due to its good electrochemical stability,high solubility in dispersion medium and high thermal stability.On the other hand,we try to develop a new PVDF based composite solid electrolyte,in which the inorganic filler is a new type of lithium ion conductor.The main results are as follows:1.Phenyl trifluoromethane sulfonate(PTM)is demonstrated as the novel electrolyte additive to enhance the electrochemical behavior of LiNi0.6Co0.2Mn0.2O2/graphite cells at 25℃,-20℃ and 45℃.The cells with 1.0 wt.%PTM-containing electrolyte deliver 80.6%capacity retention after 350 cycles at 25℃,73.4%after 100 cycles at-20℃,and 82.6%after 300 cycles at 45℃,which are significantly higher than that in standard electrolyte(STD),corresponding to 36.4%at 25℃,40.3%at-20℃,and failure at 45℃,respectively.According to density functional theory(DFT)simulations and quantitative calculations,the PTM preferentially reduces on the anode and oxidizes on the cathode to participate in the formation of solid electrolyte interphase(SEI)films.In addition,the scanning electron microscopy(SEM),X-ray photoelectron spectroscopy(XPS)and other analysis suggest that the formed SEI films are thinner and more stable for the electrolyte containing PTM,and the formed SEI films effectively control the decomposition of carbonate-based electrolytes and greatly decrease the increased resistance during the cycle.2.A new solid electrolyte of NASICON-type Li3Zr2Si2PO12(LZSP)was prepared by the sol-gel method,and then a new type of composite polymer electrolyte membrane containing LZSP and Poly(vinylidene fluoride)(PVDF)was synthesized by slurry-casting method.The composite polymer electrolyte membrane presented much higher ionic conductivity of 5.66×10-5 S·cm-1 at 25℃ and stronger electrochemical stability compared to the one without LZSP.Working at the current density of 0.1C between 2.5 and 4.2 V at 25℃.The LFP|PVDF/LZSP-CPE|Li battery delivered the high capacity retention of 92.26%and high coulombic efficiency of 98%after 50 cycles.3.A new type of lithium ion conductor with NASICON structure was prepared by ion-exchange method,which was combined with PVDF to form a new type of composite solid electrolyte.it was found that the LZSP/PVDF-CPE has wide electrochemical window(>5V)to match most voltage cathode materials and excellent lithium ion conductivity(2.28×10-4 S.cm-1 at 25 ℃).Furthermore,LZSP/PVDF-CPE not only has sufficient flexibility but also outstanding mechanical strength and stability to lithium.For the cycle in the 3.0-4.3 V,the battery delivers the high capacity retention of 88.5%(1st:170.5 mAh·g-1 and 100th:150.8 mAh·g-l)and high coulombic efficiency of 97.0%after 100 cycles,indicating that PVDF/LZSP-CPE can well realize the transmission of Li+and inhibit the growth of lithium dendrites to maintain the stable cycle of the battery.To evaluate the high voltage performance of LiNi0.5Co0.2Mn0.3O2 |PVDF/LZSP-CPE|Li battery to o achieve higher capacity,the cut-off voltage is adjusted to 4.5V.During the 60 high-voltage cycles,the battery capacity is more and delivers the high capacity retention of 83.4%(1st:204.3 mAh·g-1 and 60th:170.3 mAh·g-1)and coulombic efficiency of 93.4%.The composite polymer electrolyte shows extremely good ionic conductivity,chemical and electrochemical stability,and can be used in LiNi0.5Co0.2Mn0.3O2|PVDF/LZSP-CPE|Li battery system.