Synthesis Of Single Lithium Ion Gel Polymer Electrolyte And Its Application In Lithium Metal Batteries

With the improvement of human living standards,the demand for energy is getting higher and higher.Because of its wide range of applications,the importance of lithium-ion batteries is self-evident.However,the low specific capacity has increasingly become a bottleneck limiting its development.Lithium metal batteries can theoretically greatly increase the specific capacity,but lithium dendrite growth can cause the battery to be unstable and unsafe.Therefore,designing an electrolyte that can suppress the growth of lithium dendrites is of great significance for optimizing lithium metal batteries.In order to inhibit the dendrite growth of lithium metal batteries,a single lithium ion gel polymer electrolyte was prepared by in situ cross-linking method.The physical and electrochemical properties of polymer electrolytes with different crosslinking degrees were analyzed.The optimal cross-linking ratio was revealed,and the mechanism of the gel electrolyte was revealed.The main research contents and conclusions are as follows:1.A series of lithium monoionic monomers with strong polar groups were synthesized,including Lithium(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide(LiSTFSI)and Lithium 4-styrene sulfonyl(phenylsulfonyl)imide(LiSSPSI)The structures of these two monomers were detected by 1H NMR and FT-IR.2.Through in situ cross-linking,single lithium ion polymer membranes(SLIMs),containing different proportions of cross-linking agents and STFSI-pendants were successfully synthesized,using AIBN as an initiator.SLIMs were immersed in electrolyte(1.0 M LiTFSI,0.1 M LiNO3 in DME:DOL=1:1 vol%).After absorption and saturation,single lithium ion gel polymer electrolytes(SLIGPEs)was obtained.The surface and cross-section morphology of the membrane were analyzed by SEM,indicating that the electrolyte membrane has a uniform and stable structure.So it can form good contact with the electrode interface.SLIMs with different crosslinking degrees were analyzed by FTIR,and thermal decomposition temperatures of SLIMs were obtained by TGA.Based on the results of TGA and CRD of SLIMs,the graft contents of STFSI-groups in the synthesized polymer film were calculated,and the results obtained were basically consistent with the mass fraction of the feed.Through DSC analysis,its glass transition temperature(Tg)was tested,and it was proved that it remained amorphous within the temperature test range.The XRD data of the SLIMs polymer film also showed that the polymer film was not crystalline,proving that such amorphous SLIMs can promote the conduction of lithium ions in the electrolyte.3.The hydrophilicity of SLIMs can affect the amount of liquid electrolyte absorption,and thus affect the ionic conductivity of gel electrolytes.Therefore,its contact angles were tested.The analysis found that the contact angles of SLIMs were all less than 90°,which proved that they were all hydrophilic.The liquid absorption capacity of the SLIMs was intuitively characterized through the test of the liquid absorption capacity of the electrolyte,which provided the basis for the immersion time required for the preparation of the gel polymer electrolytes.Through tensile test and dynamic thermomechanical analysis,the excellent mechanical properties of SLIMs and the structural thermal stability of SLIGPEs were verified respectively.4.The conductivity of SLIGPEs at 20-90? was characterized.The results show that the change of conductivity with temperature is consistent with the Arrhenius equation,indicating that the electrolyte has higher structural stability in this temperature range.The lithium ion transference numbers of SLIGPEs were measured by chronoamperometry.It was found that the lithium ion transference numbers of SLIGPEs were higher than those of traditional dual-ion liquid electrolytes(0.3-0.4).The repulsive force of the STFSI-pendants with free anions was verified.Its wide electrochemical stability window and long-term stability were characterized by linear scanning voltammetry and aging impedance tests,respectively.5.Charge/discharge tests of symmetrical lithium half-cells at room temperature and different current densities showed its dynamic interface stability and cycle stability.It was found that SLIGPE-2 has a relatively small polarization potential and longer cycle stability,which exhibits better cycle reversibility and stability than SLIGPE-0.With the increase of current density,the results remained unchanged,and when the current density is 0.1 mA·cm-2,SLIGPE-0 is short-circuited due to the high polarization potential,while SLIGPE-2 can still cycle normally for more than 600 hours.It further illustrates the excellent performance in the presence of STFSI-pendants.Charge/discharge capacity and rate performance of LiFePO4/SLIGPEs/Li were characterized by full-cell tests.Compared with SLIGPE-0,SLIGPE-2 showed higher cycle stability,charge-discharge specific capacity,and rate performance.SEM was used to characterize the lithium metal anode in LiFePO4/SLIGPEs/Li after 150 cycles at 0.1 C.It was found that there was no obvious lithium dendrite growth on the surface of the lithium metal anode in SLIGPE-2 batteries.On the contrary,there were obvious lithium dendrites on the lithium metal anodes in SLIGPE-0 batteries,which verified that the electrolyte further inhibited the growth of lithium dendrites.Combining DFT calculations,the interaction of the STFSI-pendants with free anions and cations in the electrolyte was verified.