Basic Research On Chemical Stability Of NASICON Solid Electrolyte And Its Application In Sodium Metal Batterie

The Na super ionic conductor（NASICON）has the advantages of isotropy,strong three-dimensional framework,wide electrochemical stability window,good thermal stability and low cost,and is widely used in selective ion membranes,seawater batteries,sodium metal batteries and air sensors and other fields.However,the chemical stability mechanism of NASICON solid electrolytes in acid/alkaline solutions,seawater,liquid electrolytes,humid air and other environments is unknown,and its corrosion decomposition during use leads to limited device performance and service life.In addition,the application of NASICON solid electrolyte in quasi-solid sodium metal batteries/all-solid sodium metal batteries still faces the following problems to be solved:large electrode-solid electrolyte interface impedance,sodium dendrite growth,interface stability and poor compatibility.In order to solve the basic scientific problems existing in the application of NASICON solid electrolyte,the chemical stability of NASICON solid electrolyte in different solutions was studied,and its application scenarios and conditions were determined.Then,a new composite solid electrolyte was prepared based on NASICON solid electrolyte.PEG modification and cathode-composite solid electrolyte integration improved the anode and cathode interface stability respectively.The research contents and conclusions of this thesis are as follows:（1）The effects of the amount of Na₃PO₄·12H₂O and the particle size of the precursor on the phase and morphology of the NASICON solid electrolyte were studied.The ionic conductivity of the prepared NASICON solid electrolyte at room temperature was as high as 1.6×10^-3 S cm^-1.By means of material characterization and electrochemical analysis,it was determined that H₃O⁺and Na⁺proton exchange,grain refinement,and surface crack growth were the main reasons for the increase of bulk impedance,grain boundary impedance,and cracked surface impedance in the corrosion process of NASICON solid electrolyte,respectively.The corrosion mechanism of NASICON solid electrolytes in different solutions was revealed,and the microscopic strain accumulation caused by H₃O⁺occupying Na⁺sites was proposed to be the main driving factor for the corrosion process of NASICON solid electrolytes.In addition,it was found that NASICON solid electrolyte has good chemical stability in high Na⁺concentration/low H₃O⁺concentration solution and ester electrolyte,which can provide guidance for the selection of solvent and interface wetting agent for preparation of composite solid electrolyte by solution pouring method.（2）A new type of composite solid electrolyte with polyethylene glycol（PEG）and poly（Vinylidene fluoride-Co-hexafluoropropylene）（PVDF-HFP）as polymer matrix and NASICON solid electrolyte as active filler was prepared by solution pouring method,which was infiltrated in ester liquid electrolyte to synthesize quasi-solid electrolyte.PEG crosslinked with PVDF-HFP,which enhanced the coordination strength of Na⁺and solvent molecules,inhibited the volatilization of solvent molecules,and enhanced the transport stability of Na⁺at the electrode-quasi-solid electrolyte interface.In addition,the"sodiophilic"-OH functional group of PEG reduces the content of"sodiophobic"-CF₃ terminal group on the surface of the quasi-solid electrolyte,improves the affinity between the quasi-solid electrolyte and sodium metal,effectively inhibits the growth of sodium dendrites.The quasi-solid-state sodium metal battery has an initial discharge specific capacity of 100.3 m Ah g^-1 at room temperature and 1C rate,and a capacity retention rate of 94.73%after 200 stable cycles.（3）NASICON solid electrolyte was used as the active filler and fluoroethylene carbonate（FEC）as the modifier.Using solvent-free UV curing technology to induce poly（ethyleneglycol）dimethacrylate（PEGDMA）monomer to polymerize on Na₃V₂（PO₄）₃ cathode electrode plate,the cathode-composite solid electrolyte integrated material was quickly synthesized.The FEC modifier is reduced during the charge discharge cycle,and a Na F-rich solid electrolyte interphase（SEI）is constructed in-situ at the interface between the metal sodium cathode electrode and the composite solid electrolyte.The electronically insulating passivation layer inhibits interfacial side reactions and promotes the uniform deposition of sodium metal anode.The first specific discharge capacity of the all-solid sodium metal battery assembled by cathode-composite solid electrolyte integrated material is as high as 104.0 m Ah g^–1 at room temperature and 1C rate,and the capacity retention rate of 500 stable cycles is 86.1%.