| The development of efficient and clean energy storage and conversion systems is an important way to solve the problems of environmental pollution and energy crisis.Lithium ion batteries(LIB)have been intensively studied as a high density energy storage source,and widely applied in electric and vehicle electric field.Commercial liquid electrolyte obtained by dissolving lithium salt into carbonate solvents used in LIBs generally show high ionic conductivity,while suffering safety issues such as leakage and inflame due to the low flash point of the nonaqueous solvents.All solid-state lithium-ion lithium batteries are considered to be an effective way to solve these problems.All solid-state electrolyte materials mainly include inorganic solid electrolytes(ISE),solid polymer electrolytes(SPE)and organic-inorganic hybrid composite electrolytes(CPE).Since the lithium ion transference number of the ISE is almost unity,their ionic conductivity is almost comparable to that of organic liquid electrolyte.However,in spite of the presence of highly ion conductive ISE,there are still many issues that limit the practical application at the present stage,such as the large interface impedance between electrode and ISE and the difficulty of processing.SPE possess several advantages including no leakage of liquids,low flammability,good flexibility,safety and stable contact between the electrode and electrolyte.Compared with that of ISE,SPE show better processibility and flexibility.These properties make SPE have strong adhesion on the surface of electrodes,and thus decrease the interface impedance between electrolytes and electrodes.However,the conductivity of SPE is low at room temperature(10-6?10-5 S cm-1).The conductivity of SPE can be improved by introducing functional groups onto the polymers that promoting the migration of lithium ion,and adding plasticizers into the SPE.In this thesis,polyvinylidene fluoride(PVDF)is used as the matrix to prepare three different polymer electrolytes.The specific work is as follows:(1)Chlorosulfonic acid,PVDF and Li OH were used to prepare sulfonated polyvinylidene fluoride(SPVDF)and polyvinylidene fluoride sulfonate lithium(SPVDFLi).A series of single-ion polymer electrolytes(SIPEx)were prepared by a solvent casting method from PVDF and SPVDFLi mixture solution.To further enhance the ion conductivity of electrolyte,PVDF/SPVDFLi/Li TFSI composite electrolytes(SPVDFLi/Li TFSI-y)obtained by adding various content of Li TFSI into the SIPE.The ion conductivity of SIPEs increased by increasing the content of SPVDFLi,and the ion conductivity of PVDF/SPVDFLi/Li TFSI composite electrolytes further increased by increasing the content of Li TFSI,SPVDFLi/Li TFSI-40 showed the conductivity of 1.41×10-4 S cm-1 at 25?,the steady voltage of up to 4.84V,and the capacity retention rate of is 99.1%after 50 cycles at 0.2 C.The excellent of the lithium-ion battery assembled with SPVDFLi/Li TFSI-40 due to its high conductivity,high lithium-ion migration number and the high efficiency in inhibiting the growth of lithium dendrites of SPVDFLi.These results indicated that the PVDF/SPVDFLi/Li TFSI composite electrolytes are expected to be used to prepare high-performance solid-state lithium-ion batteries.(2)In order to further increase the energy density of LIBs,the imidazlium-based ionic liquid([Bmim][NTf2])was synthesized and added into the double salt polymer electrolyte(SPVDFLi/Li TFSI-y)to obtain ionic liquid-based composite polymer electrolyte IL-z(z represents the mass fraction of ionic liquid).The composite polymer electrolyte with 50 wt%ionic liuiqd(IL-50)showed a conductivity of 5.9×10-4 S cm-1at room temperature,and the battery has an optimal ratio of charge and discharge at a rate of 0.2 C.The specific discharge capacity of the polymer electrolyte reaches 154 mAh g-1 after 70cycles.The battery capacity retention rate of Li Fe PO4/IL-z/Li is above 99%.(3)In order to further improve the charge and discharge performance of the battery at high rates,a copolymer of polyethylene glycol methacrylate(PEGMA)and acrylonitrile(AN)was prepared by free radical polymerization,and the copolymer was mixed with PVDF to obtain polymerization Material electrolyte P(PEGMA-AN)-t(t represents the mass fraction of P(PEGMA-AN)).As the proportion of P(PEGMA-AN)increases,the electrochemical properties of the polymer electrolyte are improved.The optimal ratio of polymer electrolyte P(PEGMA-AN)-60 has an ionic conductivity of 7.31×10-4 S cm-1 at room temperature,and a conductivity of2.1×10-3 S cm-1at 60°C.The stable voltage is 4.82 V,and the specific capacity can reach 168 mAh g-1 at a rate of 1 C,and the capacity hardly decreases after 100 cycles. |
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