Theoretical Studies On The Structure And Electrochemical Properties Of Poly(Ethylene Carbonate) Based Solid Electrolyte

Traditional lithium ion batteries(LIBs)use liquid organic solvents as electrolytes,which have serious safety risks.Solid polymer electrolyte(SPE)has sufficient mechanical properties,excellent processability and outstanding safety performance,and is one of the most promising electrolyte materials.Among them,the polycarbonate-based electrolyte has attracted widespread attention of researchers,because it contains a strong polar carbonate group that can increase the dielectric constant,and has an amorphous structure at room temperature.However,the ionic conductivity and high voltage stability of the electrolyte are still the main problems to be faced in the development of high energy density batteries.The study of the coordination environment of lithium ions in the electrolyte and the electrochemical stability of the electrolyte by quantum chemistry method is helpful to provide theoretical guidance for the design of the next generation of new solid polymer electrolyte systems.Polyethylene carbonate(PEC)is a typical polycarbonate-based polymer electrolyte,which has made great progress in conductivity and electrochemical stability.In this paper,the density functional theory is adopted to optimize the structures of the complexes formed by Li⁺ion and PEC,to understand the composition of the first coordination shell and analyze the stability of different complex structures,as well as the influence of the solvation effects and the presence of different salt anions(BF₄^-,PF₆^-,Cl O₄^-and TFSI^-)on the structure and stability of the complexes.Furthermore,the redox potentials of a series of polymer molecules are calculated to analyze the influence of polar groups,alkyl chain length and structure,substituent groups in the repeating unit of the polymer and salt molecules on the redox potential of the polymer.The results are shown as follows:(1)By calculating the binding energy of Li⁺-PEC,Li⁺-O=C distances and Mulliken charges on Li⁺show that,with the increase of C=O groups of PEC molecules involved in the coordination,the coordination structure formed with Li⁺ion is more stable,and the coordination shell of the Li⁺is most stable when it is occupied by four carbonyl oxygen atoms.PCM simulation displays that the structure change of the complexes is no significant in the different solvent environment,and with the increase of the solvent dielectric constant,the absolute value of binding energy is reduced.The high charges on the Li⁺ion and the low absolute value in binding energy of[Li(PEC)][TFSI]complex can be a reasonable reason for high transference number of PEC-Li TFSI electrolytes.(2)According to the analysis of the redox potentials of various polymer systems,it is found that the polymeric molecules with carbonate group display the strong redox stability than those with ketone group and ether group.The length increasing or the configuration changing of the alkyl chain slightly affect the EW of the electrolyte.The results also indicate that the width of the EW is tuned by the substitution of the functional group.The oxidation stability of PEC can be enhanced by the substitution of the electron-withdrawing groups-F and-CF₃,and further increases with increasing number of electron-withdrawing groups.When the substituents are the electron-donating groups,the oxidation potential of PEC is reduced,but it has little effect on the reduction stability.In addition,the corresponding HOMO/LUMO energies of these PEC derivatives are consistent with the variation of the redox potentials.However,the substitution of-CN group makes the structure of the PEC molecule unstable during the redox process,with ring formation or chemical bond breaking,resulting in a significant decrease in oxidation stability and reduction stability of the PEC.The presence of Li BF₄,Li TFSI,Li Cl O₄ and Li PF₆is found to significantly reduce the width of EW.All these findings would provide helpful insight into the structural properties and redox chemistry of PEC,enrich the information of polycarbonate in electrolyte systems.