Study On Structural Design And Properties Of Polycarbonate Based Solid Polymer Electrolyte

Nowadays,with the rapid development of electric vehicles,the safety and energy density of power batteries are required to be higher and higher,and solid-state lithium metal batteries are regarded as the most promising development direction.The development of polymer electrolytes with high ionic conductivity at room temperature,wide electrochemical stability window and high mechanical strength is an important prerequisite for the application of lithium metal batteries.It has always been difficult to combine mechanical strength and ionic conductivity in the structural design of polymer electrolyte.To solve this problem,a biphase electrolyte composed of mechanical phase and ionic conductive phase is designed to balance the contradiction between mechanical strength and ionic conductivity of polymer electrolyte.Then the effect of different functional groups on the migration of lithium ions was systematically studied by theoretical calculations,and the structure of the polymer of the ionic conducting phase was designed.The critical properties of the designed polymer electrolyte were evaluated,and the influence of functional group on the performance of polymer electrolyte was deeply analyzed.First,a biphase polymer electrolyte composed of the ionic conductive phase of poly?ethylene carbonate??PEC?and the mechanical supporting phase of the PVDF-HFP was designed.The interaction between two kinds of polymers and lithium salt was studied,demonstrating that lithium salt was mainly dissolved in PEC to form “polymer in salt” structure with high lithium salt concentration that realizes high conductivity.The coordination between functional groups of polymer and lithium ions was studied by DFT calculation,combined with the experimental results,demonstrating that at the interface between PEC and PVDF-HFP,F atoms participate in the coordination of lithium ions and weaken the coordination between-C=O and lithium ions,improving the ionic conductivity of electrolyte(1.1×10-4 S cm^-1 at 30 ?).LFP/Li batteries assembled with this electrolyte also exhibit superior rate capacity and excellent cycling stability.For the polycarbonate electrolyte as the ionic conduction phase,a large amount of lithium salt needs to be added to realize low glass transition temperature?Tg?,so as to improve the ionic conductivity.The theoretical calculation is used for the structural design of a new type of polymer electrolyte CPEG containing two functional groups of ethylene carbonate?EC?and ethyl oxygen reactive?EO?.Via DFT calculation the coordination binding energy of EC,EO and lithium ion was investigated,which shows that the introduction of EC can weaken the coordination of polymer and lithium ions.Molecular dynamics?MD?simulation was used to compare the occurrence probability of oxygen atoms around lithium ions in the polymer electrolytes with different functional groups,indicating that oxygen atoms exists at varied distance from Li⁺ ions in the CPEG polymer electrolyte to form a continuous conduction path.The synthesized polymer electrolyte has a high conductivity of 1.84×10-4 S cm^-1?30 ??and a wide electrochemical window of 4.75 V?vs.Li⁺/Li?at 20 wt.% Li TFSI.The mechanical strength of the polymer electrolyte was improved by introducing PVDF-HFP to form a composite electrolyte sCPEG.Combining with in-situ interface modification technology,the SPE not only prolongs the cycle life of solid-state LFP/Li battery to 600 cycles?110 m Ah g-1 retained?,but also shows potential application in high energy density NCM523 cathode.It has been considered that EO chain containing many EO units will form a stable coordination structure with lithium ion to restrict its migration,affecting the conductivity of polymer electrolyte.Therefore,in the CPEG,the length of EO chain was designed and studied by theoretical calculations.The coordination structures and diffusion coefficients(D_Li+)of lithium ions in different electrolytes are investigated by theoretical calcutions,showing that with shorter EO chain,the coordination of EO-Li⁺ was weak,resulting in the increase of the D_Li+.Based on the results,polyether homopolymer electrolytes and polyether/carbonate copolymer electrolytes containing different length of EO chains were synthesized.The polymer electrolyte containing short EO chains?CPEG?2??exhibits a higher conductivity of 2.97 ×10-4 S cm^-1?30 ° C?and a higher t_Li+ of 0.78,and a good electrochemical stability.Solid-state lithium battery was prepared by in-situ polymerization,and the interface between the electrode and the polymer electrolyte was improved,accompanied with a simplified battery assembly process.Through the analysis of the cathodes of the 10-cycle solid-state lithium battery and lithium battery using liquid electrolyte,it is found that the polymer electrolyte forms a uniform coating on the surface of the cathode particles,which acts as an artificial CEI to inhibit the side reaction of cathode and electrode cracking caused by volume change.In addition,the performance of the high energy density batteries with NCM523 and riched-lithium manganese oxide?LRO?was evaluated at room temperature using a PC-plasticized polymer electrolyte,demonstrating the potential application of polymer electrolyte in high voltage cathode.