Preparation And Performance Study Of PVA-β-CN Composite Soild State Electrolytes

The electrolytes currently used in lithium-ion batteries are mostly liquid electrolytes,which have safety problems such as leakage and explosion.The composite solid electrolyte,which combines the advantages of organic and inorganic solid electrolytes,has excellent safety and high ionic conductivity,and is a promising material for development.In this paper,a PVA-β-CN@LLZTO composite solid-state electrolyte was prepared by introducing an inorganic solid-state electrolyte,lithium-lanthanum-zirconium-tantalum-oxygen（LLZTO）,as a polymer electrolyte matrix,and a correlation model between ionic percolation and viscoelastic percolation was proposed.The correlation model was proposed.Two composite electrolytes with excellent electrochemical performance were also prepared by blending PVA-β-CN with a composite cellulose membrane and LLZTO surface modification.Secondly,the PVA-β-CN/SN composite electrolytes were prepared by doping plastic crystal SN with PVA-β-CN.The work is as follows:1.This paper used PVA-β-CN as a polymer matrix,Li TFSI as a lithium salt,and lithium lanthanum zirconium tantalum oxygen Li_6.5La₃Zr_1.5Ta_0.5O₁₂（LLZTO）（0～28wt%）as inorganic nanoparticles to prepare a composite solid-state electrolyte（PVA-β-CN@LLZTO）for lithium-ion batteries.Firstly,the effects of the lithium salt Li TFSI and the inorganic nanoparticles LLZTO on the structure of the PVA-β-CN@LLZTO composite solid-state electrolyte were investigated.The infrared spectroscopy spectra showed that the absorption peaks of the stretching vibrations of the lithium ion and cyano coupling peaks at 2276 cm^-1 and the positions of the characteristic stretching vibrations of Li TFSI at 1323 cm^-1 and 1242 cm^-1 were significantly shifted,which proved that PVA-β-CN@LLZTO has excellent lithium-ion transport ability.The microscopic cross-sectional structure of the PVA-β-CN@LLZTO composite solid-state electrolyte was observed by scanning electron microscopy,and it was found that the dispersion of inorganic particles LLZTO in the polymer PVA-β-CN showed a trend of first uniform dispersion and then gradual aggregation as the LLZTO content increased,leading to a change pattern of first increasing and then decreasing its ionic conductivity.Whenφ_LLZTO=20 wt%,the ionic conductivity of the PVA-β-CN@LLZTO composite solid electrolyte reaches a maximum value,corresponding to the ionic conductivity overshoot thresholdφ₁.In tests to evaluate the effect of LLZTO content on the viscoelasticity of the composite solid electrolyte,it was found that whenφ_LLZTO≥20 wt%,the frequency dependence of the storage modulus of the PVA-β-CN@LLZTO composite solid electrolyte diminished,corresponding to the modulus over-permeability thresholdφ_G of the system.Whenφ₁≈φ_G,it indicates that the lithium-ion transport percolation network within the composite solid electrolyte corresponds to the viscoelastic percolation network of the filled system,and as the value ofφapproaches 20 wt%,there is a sudden increase in the value of the correction factor K for the filler sensitivity factor,indicating the formation and development of the ion transport network.2.Given the poor electrochemical performance of the composite solid electrolyte PVA-β-CN@LLZTO prepared directly by doping inorganic nanoparticles LLZTO into the polymer matrix PVA-β-CN,two methods were used in this work to improve the electrochemical performance of this composite solid electrolyte system in order to improve its electrochemical performance.（1）A two-layer composite polymer electrolyte with a thickness of approximately 60μm was obtained by coating a PVA-β-CN/LLZTO polymer electrolyte slurry on top of a cellulose membrane skeleton material,and a composite electrolyte was prepared with a commercial electrolyte.At 30°C,the ionic conductivity was 8.4×10^-4 S/cm,the electrochemical window was 5.1 V and the lithium-ion migration number was 0.41.The specific discharge capacity was 134.4 m Ah/g for300 cycles at 0.5 C and 30°C,with a capacity retention rate of 95.6%.（2）A core-shell structured inorganic particle was prepared using dopamine by modifying the surface of the inorganic particle LLZTO.At 60℃,the ionic conductivity was 1.4×10^-4 S/cm,the electrochemical window was 5.4 V,and the lithium-ion migration number was 0.49.At0.1 C,60°C,the specific discharge capacity was 125.7 m Ah/g after 100 cycles,with a capacity retention rate of 92.2%.3.In order to improve the electrochemical performance of the PVA-β-CN polymer electrolyte,a composite solid-state electrolyte with excellent electrochemical performance was prepared using butanedinitrile（SN）as the plasticizer,Li TFSI as the lithium salt and glass fibre membrane as the backbone material.The effects of SN and Li TFSI content on the ionic conductivity of the composite solid-state electrolyte were investigated.When the molar ratio of cyano to lithium ions was M（CN）:M（Li⁺）=1:2and the mass ratio of polymer matrix to plasticiser was m（PVA-β-CN）:m（SN）=1:2,the ionic conductivity of the composite solid electrolyte was 3.6×10^-4 S/cm,the electrochemical window was 4.4 V and the lithium-ion migration number was 5.5 at60°C.Moreover,the composite electrolyte had excellent long cycle performance,with a discharge specific capacity of 130.2 m Ah/g after 200 cycles at a current density of 0.5C and a capacity retention rate of 94.7%.