Study On Film-forming Electrolyte Additives And Gel Polymer Electrolytes Of Lithium Ion Batteries

Lithium ion batteries, a kind of high energy density secondary batteries, have been developed rapidlysince the1990s and occupied most of the market of portable electronic device. With the breakthrough oflithium ion secondary batteries technology, higher energy density become the goal of next generation oflithium ion batteries and interests entrepreneurs and scientists in electric vehicles.Spinel material LiNi0.5Mn1.5O4(LNMO) has the advantages of high operating voltage, high energydensity, and environmental friendliness. It has been considered to be potential cathode material forelectric vehicles (EVs). However, the serious oxidative decomposition of the conventional carbonate/LiPF6electrolyte under high-voltage and the dissolution of Mn limit its application in the electric vehicles. Tosolve the above problems, this paper presents two methods to improve the cycle performance of lithium-ionbatteries, which are film-forming additives and gel polymer electrolytes. The main contents are as follows:1. An overview of compositions of electrolytes, current situation of film-forming additives forcathodes, preparation methods and classification of gel polymer electrolytes was given. After that, theresearch direction and content of this paper were determined.2. Thiophene was selected as an electrolyte additive to improve the cycling performance of LNMO//Libattery. Theoretical calculations and LSV tests show that the oxidative polymerization potential ofthiophene is at4.3V, which is lower than that of common solvents. It is oxidized firstly and forms apolymer film on the surface of positive electrode. Using thiophene as an electrolyte additive cansignificantly improve the cycling performance of LNMO//Li battery at room temperature.With0.05%thiophene addition into the electrolyte (EC/DMC/EMC with1M LiPF6), the capacity retention increasesfrom89.2%to93.9%after200cycles at1C rate, while, it is almost no use at elevated temperature. Thepresence of polymer film of thiophene was confirmed by transmission electron microscope(TEM).Theeffects of thiophene were investigated by scanning electron microscopy (SEM), electrochemical impedancespectroscopy (EIS),differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS).Theresults demonstrate that the polymer film can effectively inhibit electrolyte oxidative decomposition,reducethe increase of internal resistance of the battery and improve the thermal stability of the battery at fully charged state.3. Diphenyl ether (DPE) was selected as an electrolyte additive to improve the cycling performance ofLNMO//Li battery. Theoretical calculations and LSV tests show that the oxidative polymerization potentialof DPE is at4.5V, which is lower than that of common solvents. It is oxidized firstly and forms aconductive polymer film on the surface of positive electrode. Using DPE as an electrolyte additive cansignificantly improve the cycling performance of LNMO//Li battery especially at elevated temperature.With0.1%DPE addition into the electrolyte (EC/DMC/EMC with1M LiPF6), the capacity retentionincreases from41.2%to90.5%after100cycles at elevated temperature. It was indicated that DPE canform a uniform polymer film having a thickness of18nm on the surface of positive material bytransmission electron microscope(TEM).The effects of TPN were investigated by scanning electronmicroscopy (SEM), electrochemical impedance spectroscopy (EIS), differential scanning calorimetry(DSC), X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR). The results demonstratethat the main reasons of capacity fading are electrolyte oxidative decomposition and degradation of thespinel structure of LNMO at elevated temperature and high voltage. The polymer film can effectivelyinhibit electrolyte oxidative decomposition, avoid degradation of the spinel structure, reduce the increase ofinternal resistance of the battery and improve the thermal stability of the battery at fully charged state.4. The nano-SiO2/PVDF/PMMA composite gel polymer electrolyte was prepared by casting technique.The mass ratio of PVDF and PMMA, type of plasticizer, molar concentration of LiClO4and content ofSiO2were optimized. The optimized ratio of PVDF and PMMA is3:1, and the type of plasticizer is EC+PC.The molar concentration of LiClO4is1mol/L, and the content of SiO2is8wt%. TG tests show that thepolymer electrolyte prepared under optimal conditions is in a substantially thermal steady state before150C. The prepared polymer electrolyte showed a high ionic conductivity about5.54mS/cm at roomtemperature. The electrochemical stability is at5.2V versus Li/Li+, and Li+transference number is0.55.Cycle performance tests show that the initial discharge capacity of batteries with GPE is slightly higherthan batteries using liquid electrolyte and the capacity retention increases from75.4%to80.9%after35cycles at elevated temperature.