| Lithium-ion batteries are widely used in devices such as smartphones,cameras,intelligent robots,and electric vehicles,due to their advantages such as long cycle life,high energy density,high operating voltage and low self-discharge effect.However,the organic liquid electrolytes commonly used in conventional lithium-ion batteries are prone to leak and burn,which can also easily react with lithium metal to form an interface layer,reducing the coulomb efficiency and causing serious battery capacity attenuation.All-solid-state battery based on solid polymer electrolyte?SPE?is of great significance to solve the potential safety problems of traditional liquid battery.In addition,using lithium metal as anode instead of graphite anode has the advantages of higher specific capacity and longer cycle life.Polycaprolactone is a good matrix material for solid polymer electrolytes because of its high dielectric constant,strong complexation ability with lithium ions,and high oxidation potential.In the thesis,starting with the structural modification of polycaprolactone,two kinds of polymer solid electrolytes with both electrochemical and mechanical properties have been prepared by different structural design and synthesis methods,and have been applied to all-solid-state lithium-ion batteries by characterizing rate and cycle performance.?1?Synthesis of Polycaprolactone?PCL?-Polystyrene?PS?copolymer via the click reaction of azide and alkyne.The introduction of polystyrene can reduce the crystallinity of the polycaprolactone and enhance the mechanical properties of the system.In this paper,a series of hyperbranched solid polymer electrolytes?HB-SPEs?were prepared by controlling the proportion of styrene in hyperbranched copolymer.In the polymer electrolyte system,the ester group in the PCL chain can chelate with lithium ion and ionize to transport lithium ions through segment movement;as a rigid segment,PS can improve the mechanical properties of polymer electrolytes and restrain the growth of lithium dendrite effectively.By characterizing the electrochemical and mechanical properties of polymer electrolytes with different content of polystyrene,the polymer electrolyte?HB-SPE-30?with 30%PS showed the best performance:its ionic conductivity was 1.56×10-5 S·cm-1 at 80°C,the Li/HB-SPE-30/Li symmetric cell can cycle stably for 500 hours at 0.1 m A·cm-2.The Li Feall-solid-state battery assembled with LFP cathode exhibited good electrochemical performance,which can stably cycle for more than 1200 cycles at 80°C and 1 C current density,and its capacity retention rate after 1200 cycles was 76%.?2?The composite comb-like polycaprolactone-based solid polymer electrolyte?CPE?was prepared by in-situ photo-curing polymerization using acrylate-terminated polycaprolactone as monomer and inorganic nano-clay halloysite?HNT?as the filler.In the polymer matrix,the free movement volume of the comb-like branched chain increases,which is beneficial to the conduction of lithium ions in the polymer matrix;the introduction of HNT can not only reduce the crystallinity of the polymer matrix,but also improve the mechanical properties of the electrolyte,moreover,the surface charge of the tubular structure can assist the transportation of lithium ions and improve the conductivity of the composite electrolyte.By comparing the electrochemical performance of CPE,the highest ionic conductivity of CPE doped with 5 wt%HNT at 30°C was 6.62×10-5 S·cm-1.Moreover,the solid-solid interface contact problem between electrolyte and electrode can be effectively solved by in-situ polymerization,and the interfacial compatibility between CPE and lithium metal can be improved.The lithium metal symmetrical battery based on CPE can be cycled stably for 800 hours at a current density of 0.2m A·cm-2,showing excellent lithium stability.The battery assembled with LFP as the positive electrode has a capacity retention rate of 92.1%at 60°C and a current density of 1 C for more than 600 cycles.The battery with high voltage LFMP as the positive electrode can be stably cycled for 115 cycles at 60°C and a current density of 0.2 C,with a capacity retention rate of90.8%.The above results indicate that this composite electrolyte has great potential in high voltage cathode system,which can improve the energy density of solid-state batteries. |
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