Construction And Performance Study Of Single-Ion Conducting Polymer/PEO Composite Solid Electrolytes

Over the past few decades,lithium-ion batteries(LIBs)have demonstrated remarkable performance and found widespread application in various industries,catalysing progress in people’s lives and society.However,challenges related to safety,cycle life and energy density have hindered their further use and advancement,prompting the exploration of solid-state lithium metal batteries as the ideal next generation energy storage device.Polyethylene oxide(PEO)possesses desirable properties such as affordability,excellent flexibility and processability,making it an extensively studied and applied matrix for solid polymer electrolytes(SPEs).However,the limited interfacial stability,low ionic conductivity and mechanical strength associated with PEO-based SPEs have limited their applicability in solid-state batteries.To address these issues,this study first focuses on the design and synthesis of single ion conducting polymers,which are subsequently incorporated into PEO matrices to fabricate single ion conducting solid polymer electrolytes(SSPEs).The introduction of cross-linked networks and single ion conducting polymer lithium salts effectively reconciles the conflicting demands between ionic conductivity and mechanical properties within the electrolyte.The modification and optimisation of SSPEs is achieved through approaches such as the introduction of fluorinated functional groups,modification of cross-linking agents and incorporation of inorganic fillers.The primary research objectives and corresponding results are elucidated as follows:(1)A novel design of SSPE was proposed,consisting of the single ion conducting polymer lithium salt PLiVz as the primary ion conducting phase and PEO combined with TMPTA as the mechanical support phase.The influence of the EO side chain length of PLiVz and the amount of TMPTA on the performance of SSPEs was systematically investigated.The experimental results showed that the incorporation of PLiVz significantly reduced the crystallinity of the polymer matrix and increased the tLi+.Remarkably,the electrolyte membrane fabricated with PLiV750 exhibited superior ionic conductivity at room temperature.Conversely,the introduction of TMPTA could improve the mechanical properties of the electrolyte,but at the expense of its electrochemical performance.In particular,the PLiV750/PEO-4%composition showed exceptional overall performance by achieving a harmonious and unified balance between electrochemical and mechanical properties.(2)To further enhance the electrochemical performance of SSPEs,three fluorine-containing lithium salt monomers,namely SSFPSILi,SSTFMOPSILi,and SSTFMPSILi,were synthesized by introducing electron-withdrawing groups(-F,-OCF3,and-CF3)into the structure of lithium salt monomers.The influence of these electron-withdrawing groups on the electrochemical performance of SSPEs was investigated.Through the utilization of frontier molecular orbital theory,density functional theory(DFT)calculations,and experimental data,it was demonstrated that the introduction of electron-withdrawing groups effectively reduces the dissociation energy barrier between Li+and the anions present in the lithium salt monomers.Consequently,the dissociation ability of Li+within the polymer matrix is enhanced.The extent of improvement in electrochemical performance correlates with the increasing electron-withdrawing strength of the groups.Notably,the electrolyte prepared by substituting-CF3 exhibited the most optimal electrochemical performance.(3)In order to mitigate the impact of crosslinking agents on the electrochemical performance of the electrolyte,ethoxylated trimethylolpropane triacrylate(TMPTEA)containing ethylene oxide(EO)segments was utilized as the crosslinking agent.A one-pot synthesis method was employed to prepare the polymer lithium salt,PCF3LiV750T-X,which was then blended with PEO to obtain self-crosslinking single-ion conducting solid polymer electrolytes(PCF3LiV750TX/PEO).Experimental findings revealed that the inclusion of EO segments in the crosslinking agent,TMPTEA,had the ability to decrease the crystallinity of the polymer matrix to a certain extent while enhancing the electrochemical performance.Notably,PCF3LiV750T-6/PEO exhibited the highest ion conductivity among the compositions tested.However,excessive crosslinking and a subsequent decline in electrochemical performance were observed with further increments of the crosslinking agent.Achieving a harmonious balance between electrochemical performance and mechanical properties,the PCF3LiV750T-9/PEO electrolyte film demonstrated optimal characteristics.(4)Composite single-ion conducting solid polymer electrolytes(CSSPEs)were synthesized by incorporating fluorine-modified graphitic carbon nitride(Fg-C3N4).The introduction of F-g-C3N4 resulted in enhanced electrochemical and mechanical performance of the composite electrolyte.The two-dimensional F-gC3N4 sheets effectively facilitated ion transport pathways within the composite electrolyte.Moreover,in comparison to pure g-C3N4,F-g-C3N4 exhibited superior dispersibility,and its surface atoms interacted with functional groups in the polymer lithium salt,thereby promoting further dissociation of the lithium ions.The fully solid-state battery assembled with the F-5/PCF3LiV750T-9/PEO composite electrolyte demonstrated exceptional electrochemical performance.The F-g-C3N4 nanosheets possess notable advantages,including facile synthesis and environmental friendliness,rendering them promising fillers for solid polymer electrolytes.