Performance Study Of G-C₃N₄/PVDF-based Composite Solid State Sodium Metal Polymer Electrolytes

Solid-state sodium batteries possess several merits such as the high safety of these batteries,the ability to achieve lightweight battery systems,and the ability to adapt high-energy density positive and negative electrodes,which lead to significantly increased energy density.Due to the abovementioned advantages,the research on solid-state sodium batteries attracted tremendous attention.The performance of solid-state sodium batteries is mainly determined by one of their important components,the solid electrolyte.Polymer-based solid electrolyte has the advantages of good shape versatility,excellent flexibility,lightweight and low processing cost.However,they have low room-temperature ionic conductivity and unstable interfacial properties,which brings great challenges towards commercialization.In this work,we prepared Poly(vinylidene fluoride-cohexafluoropropylene)(PVDF-HFP)-based composite polymer solid-state electrolyte via inorganic nano-fillers incorporation,to improve the mechanical properties as well as the electrochemical properties.It can also inhibit the growth of sodium dendrites and optimize interfacial stability.To achieve the abovementioned purpose,the thesis can be broadly divided into the following sections:(1)Graphitic carbon nitride(g-C3N4)nanofiller was firstly prepared by using dicyandiamide as raw material.g-C3N4 is a two-dimensional material,mainly prepared by thermal oxidation etching.Fourier transform infrared spectroscopy(FTIR),X-ray diffraction spectroscopy(XRD)and atomic force microscopy(AFM)were used to study the structure change and crystallinity.Scanning electron microscope(SEM)showed that g-C3N4 prepared by dicyandiamide had excellent dimensions.Then g-C3N4 was combined with PVDF-HFP and sodium perchlorate(NaClO4),to prepare the composite solid polymer electrolyte(PVDFHFP/NaClO4/g-C3N4)with excellent comprehensive performance.A solid polymer electrolyte without nanofiller(PVDF-HFP/NaClO4)was prepared by the same procedure for comparison.In this work,the effects of the introduction of gC3N4 nanofillers on the mechanical properties and electrochemical properties of the composite solid polymer electrolyte(CSPE),as well as the performance of the all-solid-state sodium battery based on the CSPE,were studied.Through rate capability and cycle performance tests,it was concluded that the composite solid polymer electrolyte has better electrochemical performance.(2)D-xF-C3N4 filler was obtained by doping different metal fluorides with dicyandiamine in a certain proportion.Here,the effects of different F-doped C3N4 on the performance of CSPEs and the corresponding electrochemical performance of all-solid-state sodium batteries were investigated.This part is roughly carried out from the following aspects:firstly,the absorption peak corresponding to the C-F bond at 2300 cm-1 was observed by the Fourier transform infrared spectrometer(FTIR)test,which indicated that the D-xF-C3N4 nanosheet was successfully prepared.Then,CSPEs based on four different F-doped g-C3N4 nanofillers were assembled into battery for testing,delivered a specific capacity close to the theoretical value of the positive electrode material Na3V2(PO4)3(NVP)(117.6 mAh g-1).In addition,when the current density lowered to the initial value,the specific capacity of the cell was almost completely recovered to 110mAh g-1,indicating its high stability and excellent rate performance over a wide range of current densities.The abovementioned results indicated the excellent electrochemical performance of the F-doped g-C3N4 based composite solid-state polymer electrolyte.In this thesis,the composite solid polymer electrolyte with excellent comprehensive performance was prepared.The addition of nano-fillers significantly reduced the crystallinity of the polymer matrix.In CSPEs,g-C3N4 nanosheets provide an effective interface between positive and negative materials and polymers,and promote the dissociation of Na+ through the interaction of surface N atoms with sodium salts.Therefore,the mechanical properties and ionic conductivity of CSPEs are improved.Owing to the favorable mechanical and electrochemical properties,the as-prepared CSPEs are suitable for all solid-state sodium batteries.Compared with the battery assembled with PVDF-HFP/NaClO4 composite electrolyte without g-C3N4 nanofillers,the battery assembled with PVDF-HFP/g-C3N4/NaClO4 composite electrolyte has better cycle performance and higher safety performance.In addition,F-doped carbon nitride nanofillers(DxF-C3N4)further improved the performance of the composite solid polymer electrolyte.The addition of new fillers in this paper opens up new ideas for the research of composite electrolytes,making solid-state batteries practical for largescale energy storage,and becoming the development direction of next-generation rechargeable solid-state batteries.