Preparation Of Cellulose Paper-based Composite Separator Through The Construction Of 3D-crosslinked Network Coating Layer And Its Lithium Ion Battery Performance

With the development of energy storage lithium-ion batteries,the performance requirements such as energy density,discharge efficiency and charge/discharge voltage to be achieved by the batteries are further improved.Therefore,the battery components are also facing the requirement of performance improvement.As a lithium-ion battery"third electrode"called the separator,the most widely used in commercial is still the traditional polyolefin separators.However,this type of polyolefin separators have been unable to meet fast lithium conductivity,stable performance,security and a series of requirements.Among the various new separators,nonwoven separators stand out for their unique structural advantages and ease of industrial production.However,before being applied to battery separators,nonwoven fabrics need further surface modification due to inherent pore size defects.Among the various modification materials,polymer electrolytes have gained widespread attention for their excellent electrolyte affinity,and their high absorption rate can effectively improve the interfacial compatibility between diaphragm and electrode,thus reducing the polarization problem during battery operation.However,the conventional polymer electrolyte as a coating will collapse structurally during long-term cycling due to the dissolution of electrolyte,and eventually lead to micro-short circuit phenomenon and degradation of cycling performance.Tissue paper is a cellulose-based nonwoven fabric formed by the wet papermaking process,which is environmentally friendly,highly hydrophilic and thermally stable,and has received wide attention in the battery field in recent years.Based on this,this thesis realizes the preparation of paper-based composite diaphragm by the construction of multiple cross-linked network coatings on the surface with paper as the support material,and investigates the physical and chemical properties and battery performance in detail.In the selection of cross-linking components,hyperbranched polyethyleneimine（PEI）is used in this thesis.the good affinity of PEI with electrolyte and internal cavity are expected to improve the electrolyte membrane absorption performance and lithium ion conductivity.more amine groups at the periphery of PEI will facilitate the formation of multiple cross-linking bonds and enhance the structural stability of the coating.The details of the study are as follows:（1）A hybrid cross-linked network electrolyte constructed by polyvinylidene fluoride-hexafluoropropylene（PVDF-HFP）,PEI,and nano-Si O₂was introduced to the surface of paper-based nonwoven fabric to obtain cross-linked composite films CS（5:2）,CS（5:1）,CS（10:1）,CS（20:1）,and CS（1:0）.Among them,PVDF-HFP is the commonly used polymer matrix material,PEI is the cross-linking agent,and nano-Si O₂is used to further improve the thermal stability and flame retardant properties of the coating.Due to the strong polarity of the C-F bond and the nucleophilicity of the amine group,PVDF-HFP and PEI can rapidly react chemically to form a new chemically bonded C=N structure and successfully achieve the construction of hybrid cross-linked electrolytes on the surface of paper-based nonwoven fabrics.It was found that the fracture strength of the cross-linked composite diaphragm was significantly increased to about 9.9 MPa.In addition,the construction of the cross-linked coating significantly improved the charge/discharge performance and cycle stability of the battery compared with the conventional electrolyte coating.（2）A single cross-linked network was constructed with epoxy capped polydimethylsiloxane（PDMSDGE）and hyperbranched PEI and introduced to the surface of paper-based nonwoven fabric to obtain cross-linked composite films 2:1,4:1 and 8:1.Among them,the amine group in PEI and the end-group epoxy of PDMSDGE can undergo ring-opening reaction and form a cross-linked network through a stepwise polymerization mechanism.By changing the cross-linking ratio,the highest tensile strength of the composite film can reach 9.3 MPa and the electrochemical stability window can reach 4.7 V.However,the lithium transfer performance of the electrolyte film is not satisfactory due to the lower surface energy of polydimethylsiloxane to the insufficient abundance of pores inside the coating.（3）A double cross-linked network electrolyte was introduced on the surface of paper-based nonwoven fabric by the reaction between PDMSDGE,PEI and PVDF-HFP to obtain composite film CS（2:1:10）.Among them,a primary network was formed between PDMSDGE and PEI;based on this,a secondary cross-linked network was formed by the reaction between residual amine groups in PEI and PVDF-HFP.It was found that the construction of the double cross-linked network coating could significantly increase the tensile strength of the film to about 19.9 MPa compared with that of the single cross-linked network,while the construction of the double cross-linked network coating could maintain the stable charge/discharge specific capacity of the assembled battery after 200 cycles.