Research Of Preparation And Performance Of PVDF-HFP-composite Solid-state Electrolyte Filled With 3Li₂S-2MoS₂,LLZO

As the core part of solid-state lithium-ion batteries,solid-state electrolytes（SEs）have a decisive effect on the comprehensive performance of the battery.Currently,SEs still have deficiencies,such as low ionic conductivity and poor interface compatibility with electrode materials,which seriously affect the electrochemical performance of solid-state batteries.3Li₂S-2Mo S₂（LMS）and Li₇La₃Zr₂O₁₂（LLZO）were used as research objects in this thesis.The composite solid electrolytes（CSEs）were prepared by the matrix and lithium salt of polyvinylidene fluoride co-hexafluoropropylene（PVDF-HFP）and lithium difluorooxalate（Li ODFB）,with LMS and LLZO as filler,respectively.The influence of preparation technology,added amount,physical and chemical properties of fillers on the microstructure,morphology and electrochemical performance of CSEs were studied.The main research results are as follows:（1）Li₂S was used to prepared by lithium and sulfur via planetary ball milling and quenching,and the parameters were optimized.The pure phase Li₂S was obtained by quenching at 500°C for 2 h after ball milling for 60h,at the molar ratio of lithium and sulfur is 2:1.05.Using Mo S₂ and prepared Li₂S as raw materials,LMS filler was obtained by combining planetary ball milling and heat treatment.The results indicate that LMS is a composite of Li₂S and Mo S₂ and contains a little impurity of Li Mo S₂ and Li₃Mo₆S₈.LMS holds active Li⁺,numerous lattice defects,and ion transport channels.（2）Utilizing different carboxylic acids as ligands,lanthanum zirconium metal framework compounds（La-Zr-MOFs）with stable coordination structure,high thermal stability and specific surface area were prepared by hydrothermal method.The relationship between the structure of organic ligands,the properties and formation mechanism of MOFs were studied.LLZO was prepared by the template of PTA-La-Zr-MOFs via high-temperature calcination,and formation mechanism of LLZO prepared by precursor via calcination was researched.The effect of different lithium compounds,solvent,amount of lithium compounds and calcination temperature on the material properties was studied,and process parameters were optimized.The optimization LLZO has a cubic phase structure and a large specific surface area(158.44 m² g^-1).（3）Using PVDF-HFP,Li ODFB as matrix and lithium salt,LMS as fillers,the composite solid electrolytes CSE-LMS and CSE-LLZO were prepared by solution casting method.The prepared CSE-LMS has high thermal stability（420℃）,wide electrochemical window of 4.8 V,（vs.Li⁺/Li）,and ionic conductivity of 2.51×10^-4 S cm^-1,（60℃）;LiCoO₂/CSE-LMS/Li battery shows an initial capacity of 145.79 m Ah g^-1 at 0.5C（55℃）and a capacity retention rate of 86.22%after 200 cycles.Using LLZO synthesized by different methods as fillers,PVDF-HFP,Li ODFB as matrix and lithium salt,the CSE-LLZOs were prepared by solution casting method.The prepared CSE-LLZO after optimization（PLLP）has also high ionic conductivity(4.35×10^-4 S cm^-1,60℃),wide electrochemical window（4.83 V,vs.Li⁺/Li）,stable interface with lithium,and Li/PPLP/Li cell can work stably for 1000 h at 0.5 m A cm^-2.Based on the analysis of cycled Li/PPLP/Li battery,the result manifests thin and uniform Li F was formed on interface of PLLP and lithium metal,which reduces the interface impedance.PLLP has higher ionic conductivity and interfacial stability compared with CSE-LMS.It is attributed to that LLZO-P has a larger specific surface area and a perfect lithium ion transport channel,which is more conducive to the transport of lithium ions in PLLP.（4）LiFePO₄/PLLP/Li and LiCoO₂/PLLP/Li solid-state batteries were assembled by using LiFePO₄,LiCoO₂,lithium film and PLLP,and the electrochemical performance of the battery was studied.Capacity of LiFePO₄/PLLP/Li and LiCoO₂/PLLP/Li batteries maintained more than80%of the initial discharge capacity after 500 cycles and 227 cycles at 0.5C.The LiCoO₂/PLLP/Li battery has better cycling stability than LiCoO₂/CSE-LMS/Li battery,which ascribed the higher cycling stability to the higher ionic conductivity and interfacial stability of PLLP/Li,which promotes the rapid transport of Li ions inside the battery.Interface analysis of electrolyte and electrode materials in the cycled batteries demonstrated that the main components of interface are Li F,Li₂CO₃.The lithium-containing layer enhances interface contact,reduces the energy barrier of lithium ion migration at the electrode/electrolyte interface,promotes lithium ion transmission and inhibits the growth of lithium dendrites,thus constructing a lithium ion transmission channel between the solid electrolyte and the electrode,which improves the cycle stability of the solid battery.