| Since SONY first commercialized lithium-ion batteries in 1991,they have been used extensively in communications,electronics,new energy vehicles and energy storage.Lithium ion battery diaphragms as the main component of lithium-ion batteries,have a direct impact on the performance,security,service life and cost of lithium ion batteries.Therefore,to get the cheap lithium battery diaphragms of high ionic conductivity,excellent electrochemical performance and safety performance is of great significance to the improvement of the performances of the batteries.In this thesis,PVDF?polyvinylidene fluoride?,TPU?thermoplastic polyurethane?,PMMA?polymethyl methacrylate?were used as substrate to prepare the polymer composite membranes with different concentrations,and then find the optimal concentration of the films.Finally,adding the SiO2 inorganic nanoparticles to modify the original polymer composite membranes and study the effects on the physical and electrochemical properties of the composite membranes.The main research content includes the following three aspects:First,PVDF,TPU and PMMA were selected as the matrix materials of GPEs.The PVDF/PMMA,TPU/PMMA and PVDF/TPU/PMMA matrix films were prepared by high-voltage electrostatic spinning method.The membranes were dipped into 1M LiClO4-EC/PC electrolyte,activated for 1h,and then were taken out to prepare the button batteries.Comparing the morphology,pyrolysis behaviors,liquid absorption rate,porosity,mechanical properties and other properties of the composite films,it was found that the ternary PVDF/TPU/PMMA matrix film has slender and uniform fiber structure,without microphase separation,and owns the highest liquid absorption rate,the best thermal stability and mechanical properties.Through the electrochemical performance study by the electrochemical workstation and battery cycle test system,it was found that the ionic conductivity of the ternary PVDF/TPU/PMMA matrix membrane can be up to 5.6×10-3 S cm-1,and its electrochemical stability window was5.56 V.The initial discharge capacity of the battery with the PVDF/TPU/PMMA GPE was 164.8 mAh g-1.Afte 50 cycles,the discharge capacity is 97.9%.The comprehensive performance is much better than that of the binary PVDF/PMMA and TPU/PMMA matrix film,which indicates that the ternary PVDF/TPU/PMMA electrospun film is very suitable as the lithium ion battery electrolyte.Secondly,a series of PVDF/TPU/PMMA polymer solutions with different concentrations were electrospun into films,and all the membranes were soaked in 1M LiClO4-EC/PC electrolyte to be activated,and the GPEs with different concentrations were obtained.At room temperature,when the polymer solution concentration was 9%,the liquid absorption rate of the electrospun PVDF/TPU/PMMA fiber membrane reached 288%.Its ionic conductivity was up to 5.6×10-3 S cm-1.Under the 0.1 C rate,the initial charge and discharge capacities were 164.8 mAh g-1 and 161.6 mAh g-1 of the button battery with the 9%PVDF/TPU/PMMA based GPE.The first charge-discharge efficiency was 98.1%.After 50 cycles,we found that the capacity attenuation of the button battery with the 9%PVDF/TPU/PMMA based GPE was also minimal.However,the physical and electrochemical properties of electrospun films with other concentrations are relatively poor.Considering all the results,the best concentration ratio of PVDF/TPU/PMMA polymer electrospun membranes was 9%.Finally,SiO2 nanoparticles were added into the polymer solution with a mass fraction of 9%,and the mass of SiO2 nanoparticles was changed to study the effects on the performances of the polymer separator.We can see that from the SEM figures,when mixing the SiO2 particles,the surface of polymer membranes was coated with a layer of SiO2,making the higher liquid absorption rate and ionic conductivity.When the mass fraction of SiO2 was 2%,the ionic conductivity of the corresponding GPE was up to8.0×10-33 S cm-1.The GPE with 2%SiO2 owned the best performance under the low rate?0.1 C?.The initial charge and discharge capacities of the battery containing this GPE were 165.8 mAh g-1 and 163.9 mAh g-1,respectively.And the electrochemical stability window of this GPE was up to 5.86 V.In conclusion,the PVDF/TPU/PMMA based electrospun film with the SiO2 doping mass of 2%has the best comprehensive performance. |
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