Fabrication And Performance Of Ionic Liquid Gel Polymer Electrolytes For Lithium Metal Batteries

With the development of electric vehicles,the safety of lithium secondary batteries with high energy density have become a research hotspot.Compared with lithium-ion batteries,lithium metal batteries show a higher energy density due to the advantages of lithium metal.However,the use of commercial liquid electrolytes causes severe safety issues.Solid-state lithium metal batteries are considered as a promising solution to solve these issues.In this thesis,an ionic liquid gel polymer electrolyte(ILGPE)was prepared,which had a high ionic conductivity.The interfacial stability of the ILGPE/electrode was discussed.To further improve the mechanical properties of ILGPE,TiO₂ was added to ILGPE,and the effect of these particles in ILGPE was studied.The electrochemical stability window and room temperature ionic conductivity of1M Li TFSI-N-methyl-N-methoxycarbonylpiperidinium bis(trifluoromethanesulfonyl)-imide(MMOCPPTFSI)and 1M Li TFSI N-methyl-N-propyl-piperidinium bis(trifluoro-methyl-sulfonyl)amine(PP13TFSI)electrolytes were studied by linear scanning voltammetry(LSV)and a conductivity meter.The results illustrated that PP13TFSI electrolytes exhibited higher ionic conductivity and a wider electrochemical stability window.Therefore,PP13TFSI was selected as a plasticizer for the preparation of ILGPE.ILGPE was fabricated by a solution casting method with various weight ratios of poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP),Li TFSI,and PP13TFSI.A thorough compositional optimization showed that ionic conductivity of ILGPE increased with the increase of PP13TFSI and Li TFSI,reaching maxima of 1.3 m S cm^-1 at23°C when the ratio of PVDF-HFP/Li TFSI/PP13TFSI is 1/1/3.The optimized ILGPE showed excellent interfacial stability against lithium metal,as studied by the stable interfacial resistance of the Li/Li symmetrical cells upon long-term storage.The interfacial resistance of optimized ILGPE is 3.5 times that of GPE,which was composed of PVDF-HFP and Li TFSI.The performance of LiFePO₄/Li battery was studied,and the battery could deliver a discharge capacity of 152.2 mAh g^-1,which was 89.5%of the theoretical capacity of LiFePO₄.The capacity retention was 95.0%after 200 cycles with a current density of C/5 at 23°C.The electrode/electrolytes interphases formed primarily during the initial 15 cycles,which probably accounted for the capacity increase.The ILGPE with optimized composition was selected as the electrolyte and separator for lithium metal batteries.By EIS tests,the bulk resistance,interfacial resistance,and charge transfer resistance were investigated during the battery cycling using commercial electrolytes PL57(LE)and 1M Li TFSI-PP13TFSI(IL)to wet the interface of cathode/ILGPE.The results indicated that the bulk resistance of LFP/LE-ILGPE/Li battery and LFP/IL-ILGPE/Li battery was 18±3 W and 70±20 W,respectively.The bulk resistance exhibited a neglected change during battery cycling.The interfacial resistance and charge transfer resistance of LFP/LE-ILGPE/Li battery increased by 50 times and825 times,while the interfacial resistance and charge transfer resistance of LFP/IL-ILGPE/Li battery were increased by less than 20 times,indicating these resistance increase could cause the battery capacity fading,even short the battery cycling life.Furthermore,the interfacial components were investigated by X-ray photoelectron spectroscopy.The results illustrated that LE was more easily decomposed than IL during cycling,which led to an increase in the interfacial resistance and charge transfer resistance.The interfacial resistance was decreased by at least 4 times by LE and IL wetting the cathode/ILGPE interface.The interfacial resistance of the LFP/ILGPE/Li battery increased by 1.5 times after 35 cycles.Additionally,the lithium deposition was investigated in Cu/Li batteries,and the results indicated that LE would change the growth mode of deposited lithium,increase the growth probability of lithium dendrites,and shorten the lifespan.To improve the mechanical property and ionic conductivity,ILGPE was combined with the inorganic particles to prepare composite polymer electrolytes.TiO₂ was selected as the filler of ILGPE by analyzing the ionic conductivity and mechanical properties of the variety of inorganic particles.When the content of TiO₂ is 5wt%,and ionic conductivity was 1.51 m S cm^-1(30°C)by investigating the effect of TiO₂ content on the ionic conductivity of ILGPE.By the nanoindentation test,the mechanical property of TiO₂-ILGPE was investigated.The loss modulus increased with the content of TiO₂,indicating the increase of the viscosity and ion-solvation of ILGPE.The performance of LFP/Li and Li Ni_0.6Mn_0.2Co_0.2O₂/Li batteries showed that the addition of TiO₂ stabilized the battery capacity,improved the Coulombic efficiency,and prolonged the battery cycle life.After 200 cycles,the capacity retention of LFP/5wt%TiO₂-ILGPE/Li battery was83.2%,while the retention of LFP/ILGPE/Li battery was 75.6%at 1C and 60?.The deposition behavior of metallic lithium on copper foil was studied.The batteries with TiO₂-ILGPE can effectively form a densely deposited lithium layer,reduce the growth of lithium whiskers/lithium dendrites,decrease the volume expansion of lithium anodes,and improve the safety of lithium metal batteries.