Simulation Study On The Transport Mechanism Of Lithium Ions In Poly (Ionic Liquid) Electrolyte

In recent years,poly（ionic liquid）（PIL）have attracted extensive attention as a state-of-the-art electrolyte material.PIL are produced by polymerization of ionic liquid monomers,so they have the excellent properties of both ionic liquids and polymers,such as high thermal stability,low vapor pressure,large electrochemical window of ionic liquids,and polymers Excellent processability,good flexibility and so on.In addition,compared with liquid electrolytes,PIL electrolytes also have the advantages of non-leakage and safety.Due to unique physicochemical properties,PIL have been successfully applied to lithium battery electrolytes.However,PIL electrolytes has the problems of low ionic conductivity and poor ion transfer number,which are difficult to meet the needs of practical battery applications.At present,a series of strategies have been proposed to improve the above problems of PIL,but there is a lack of theoretical understanding of the underlying mechanism,especially the relationship between the coordination structure and the transport behavior of lithium ions in the electrolyte.Therefore,this work performed molecular dynamics simulation method to systematically study the coordination structure and dynamic properties of lithium ions in PIL electrolytes.The structure-effect relationship between the structure of PIL and ion diffusion was revealed from the molecular level,which provided theoretical guidance for the design and optimization of PIL electrolyte.In Chapter 2,we performed molecular dynamics（MD）simulations to investigate the effect of asymmetric anions in PIL electrolytes on the coordination and diffusion of lithium ions in PIL electrolytes.We investigated lithium salt-doped PIL electrolytes,containing the poly（diallyldimethylammonium）polycation,lithium ions and different imide-based anions of bis（trifluoromethyl）sulfonyl imide（TFSI）,fluorosulfonyl-N-（trifluoromethylsulfonyl）imide（FTFSI）and2,2,2-trifluoromethanesulfonyl-N-cyanoformamide（TFSAM）.Our simulation results showed that the lithium ions relative to polycations have stronger interactions with anions via monodentate coordination and the lithium ions,and we found that the ratio of monodentate coordination increases with the asymmetry of the anion structure.In addition,the anion has the highest percentage of co-coordination with the lithium ion and the polycation in the TFSAM system,and this coordination structure will weaken the interaction between the lithium ion and the anion,and promoting the transport of lithium ions in the PIL electrolyte.In addition,the diffusion of lithium ions with neighboring anions is found to result from a combined contribution of vehicular and structural mechanisms,but an increasing contribution of structural mechanism is seen for the PIL electrolyte with asymmetric anions.In this part of the work,our simulation results reveal that asymmetric anions can effectively regulate the coordination and kinetic properties of lithium ions in PIL electrolytes.In Chapter 3,we employed MD simulation to investigate the effect of glucose additive concentration on the coordination structure and dynamic properties of lithium ions in PIL electrolyte.Our simulation results showed that the interaction of lithium ions with glucose molecules and anions is stronger than that with polycations.Meanwhile,we found that there is a competition between glucose molecules and anions in coordinating with lithium ions,and the introduction of glucose additive would reduce the coordination number of lithium ions with anions.In the PIL electrolyte with different concentrations of glucose,the diffusion coefficients of each component follow an order of lithium ion>anion>glucose>polycation,and the higher the concentration of glucose in the system,the faster the diffusion coefficient of lithium ion.Additionally,when the glucose additive concentration was low,the lithium and anion ions tended to form[Li（TFSI）₄]^3-clusters.Upon increasing the glucose concentration,the anion and lithium ions would partially dissociate and eventually form[Li（TFSI）₃]^2-clusters.Furthermore,we also found that the average number of hydrogen bonds between glucose molecules and anions increases with increasing glucose concentration,which weakens the interaction between lithium ions and anions and accelerated lithium ion diffusion.In summary,this thesis reveals the coordination structures,dynamic properties,and hydrogen bonding properties of lithium ions in PIL electrolytes at the molecular level.Our findings is helpful for the scientists to understand the microscopic properties of PIL electrolyte from a theoretical perspective and provide theoretical guidance for the design and optimization of PIL electrolytes.