Study On Modification And Properties Of Polymer Electrolyte For Lithium Ion Batteries

With the development of green new energy industry,the requirements for the safety and energy density of secondary energy storage batteries are higher and higher.At present,the batteries widely used in the market are mainly liquid electrolytes,which are easy to explode and burn in charging or high temperature environment,and there are great potential safety hazards.Therefore,the research on polymer electrolyte has become one of the research hotspots of lithium battery.Polymer electrolyte has high safety and high energy density,which is expected to solve the current problem of safe application of lithium battery.A single polymer can not meet the working requirements of lithium battery in some aspects,so it needs to be modified to make the prepared polymer electrolyte have excellent battery cycle performance.The traditional method of preparing solid electrolyte consumes a lot of organic solvents and takes a long time to form.The supercritical CO₂（ScCO₂）phase separation method is adopted.ScCO₂is used as the anti solvent to dissolve the organic solvent in the system,so as to realize the phase separation of polymer organic porous membrane and the formation of pores.At the same time,the forming and drying process of electrolyte can be solved in one step by ScCO₂ method,which greatly reduces the preparation time.In this paper,the polymer electrolyte modified by blending and the polymer electrolyte added with inorganic filler were prepared by ScCO₂ phase separation method.The effects of experimental temperature,pressure,blending amount and the amount of inorganic nano filler on the properties of polymer electrolyte were investigated.The main work and conclusions include the following contents:Polyvinylidene fluoride hexafluoropropylene P（VDF-HFP）polymer was selected as the matrix and dissolved in N,n dimethylformamide（DMF）.The effects of supercritical temperature and pressure on the porosity,liquid absorption,micro morphology and ionic conductivity of the prepared electrolyte were investigated.The optimum experimental temperature is 35℃and the experimental pressure is 23 MPa.The effects of polymer electrolytes with different mass fraction of PEO on their physical and electrochemical properties were investigated.When the PEO content is 15 wt%,the crystallinity of the blend polymer decreases,the porosity is 75.65%,and the maximum liquid absorption is 460.11%.At this time,the maximum ionic conductivity is 5.13 m S/cm.The blend polymer electrolyte containing 15%PEO was assembled into a complete battery,and the rate cycle performance was tested at the rate of 0.1 C,0.2 C,0.5 C,1 C,2 C and 0.1 C.compared with the single polymer P（VDF-HFP）electrolyte,the capacity reduction rate of the blend polymer electrolyte was small,and it recovered to the rate cycle of 0.1 C after the high rate cycle,and the discharge capacity could still reach 150.7 m Ah/g,It shows that the electrolyte modified by blending has good high rate cycling performance.After 70 cycles of charge and discharge at 1 C rate,the capacity retention rate is 95.6%,which has good charge and discharge performance.The organic modification of montmorillonite（MMT）can increase its layer spacing,and the modified MMT is easier to disperse in the electrolyte.After Cetyltrimethylammonium bromide（CTAB）modified MMT,the pore size of the modified MMT becomes larger,the specific surface area decreases and the layer spacing increases.Different mass fractions of MMT were added to P（VDF-HFP）and PEO blend polymer electrolyte to prepare electrolyte.The effect of montmorillonite on the properties of polymer electrolyte was investigated.When the mass fraction of MMT is 2 wt%,the number of pores is the largest.At this time,the maximum porosity is 82.96%and the liquid absorption rate is 482.91%.However,when the content of MMT exceeds 2 wt%,MMT particles appear agglomeration,so it is determined that the optimal addition amount of inorganic filler is 2 wt%.The maximum conductivity is 6.47m S/cm,and the electrochemical stability window is 5.5 V.The charge discharge capacity of the first cycle battery is 161.4 m Ah/g and 153.9 m Ah/g,and the capacity efficiency of the first cycle is 95.4%.At 1 C rate,compared with 100 cycles of commercial PP diaphragm circulation,the initially stable capacity is 130.2 m Ah/g,which is reduced to 126.6 m Ah/g,and the capacity retention rate is 97.2%.The discharge capacity has always been higher than that of commercial PP diaphragm,indicating that the electrolyte modified by MMT has good cycle performance.