Preparation,Interface Modification And Electrochemical Performance Of PPC/LLZTO Composite Solid-state Electrolyte Membrane

Current lithium-ion batteries use liquid electrolytes with problems such as low volume energy density and poor safety.Solid-state lithium batteries use solid electrolytes,which can greatly improve the energy density and safety of the battery,and are considered to be one of the most promising energy storage technologies.As the most critical material for solid-state lithium batteries,solid-state electrolytes’ionic conductivity and interface compatibility with electrodes determine the electrochemical performance of solid-state lithium batteries.The organic/inorganic composite solid electrolyte combines the advantages of both organic and inorganic solid electrolytes and is expected to become the electrolyte material of choice for solid-state lithium batteries in the future.However,interface problems have always been the key issues indering the promotion and application of solid electrolytes,including poor interface contact between the electrolyte and the cathode and the anode,chemical instability,and lithium dendrite generation,which will reduce battery performance.Aiming at the above interface problems,this paper proposes a poly carbonate/Ta-doped Li₇La₃Zr₂O₁₂（PPC/LLZTO）organic-inorganic composite solid electrolyte as a research system,which improves the rate and cycle performance of the battery through electrode/electrolyte interface modification and the integration.The main contents and results are as follows:（1）A PPC/LLZTO composite solid electrolyte membrane（CEM）was prepared by a doctor blade method.The effects of wet ball milling process and ultrasonic crushing and dispersing process on the morphology and particle size of LLZTO powder were studied,and the average particle size after refinement could reach 370 nm;the process parameters such as viscosity of electrolyte solution,coating thickness and heating temperature were explored for electrolyte performance impact.The study found that:under the conditions of a solution viscosity of 0.25 g/mL,an upper coat thickness of 200μm,an overall coat thickness of 400μm,heating at 90°C for 30 min,and a non-woven cellulose film was used as a supporting skeleton,the best membrane was obtained.The thickness is 67±2μm and the uniformity of the membrane structure is good.The ion transfer number reaches 0.79 when the filler content reached 20 wt%.The room-temperature ion conductivity is 1.57×10^-44 S/cm,the potential window is close to 4.7 V,and the mechanical properties reach 15 MPa.（2）Graphite coating and electrolyte coating were respectively implemented on the surface of the PPC/LLZTO composite solid electrolyte membrane and the cathode to improve the interfacial compatibility of the electrolyte with the Li anode and the cathode.The results show that the electrolyte coating can fill the gaps between the cathode particles,improve the contact between the cathode and the electrolyte,reduce the interface impedance,and reduce the cell impedance of CEM（20%）from the original 233Ωto 170Ω.The graphite coating can significantly improve the contact between the lithium（Li）anode and the electrolyte.The impedance of the CEM battery modified by the graphite coating is 70Ω,lower than that of the CEM（20%）battery without the coating.The graphite coating can also suppress the growth of lithium dendrites and the decomposition of PPC to a certain extent by alleviating the reaction between PPC and Li metal,thus improves the coulomb efficiency of the battery.At the same time,the synergy between the two coatings significantly improves the rate performance of solid-state batteries.（3）An integrated structure of NCM622 electrode,composite solid electrolyte,and graphite coating was prepared.The research results show that the solid-state battery composed of an integrated structure exhibits excellent cycle stability performance.Its first-cycle discharge specific capacity is 162 mAh/g.After 200 cycles,its discharge specific capacity remains at100 mAh/g.The interface buffer layer between the electrode and the electrolyte in the integrated structure significantly reduces the interface resistance.The graphite coating eases the reaction between PPC and metal Li,inhibits lithium dendrite growth,and improves the cycling performance of the battery.The battery also exhibits excellent cycling performance at higher loads,and has been successfully applied to soft-pack batteries.