Design Of Localized High Concentration Electrolyte And Application For Lithium Metal Batteries

Lithium metal batteries have received extensive attention due to their high energy density,but the anode suffers from dendrite growth and poor interfacial stability,which affects the practical application of lithium metal batteries.Constructing a high concentration electrolyte（HCE）with unique physicochemical properties and solvated structure is one of the effective ways to suppress the growth of lithium dendrites and improve the stability of the anode interface,thereby enhancing the performance of lithium metal batteries.However,HCE has the problems of high viscosity and poor wettability.Adding a diluent to form localized high concentration electrolyte（LHCE）can not only retain the advantages of HCE,but also overcome its shortcomings of high viscosity and poor wettability,and effectively improve the battery performance.Among them,the molecular structure and the amount of the diluent are the main factors affecting the physicochemical properties of electrolytes,and the systematic study of its influence is the basis for optimizing the design of the LHCE system and improving the performance of lithium metal batteries.In this thesis,different fluorinated ethers were selected as diluents to design a variety of LHCE systems.The influence of the molecular structure and the amount of additive on the physicochemical properties of electrolytes were systematically studied.The interface stability and the performance improvement effect on lithium metal batteries provide a certain theoretical basis for the design of LHCE and promote its application in lithium metal batteries.The main works are as follows:（1）The distribution of the diluent and the solvation structure formed in the LHCE system were studied.Analytical testing combined with molecular simulations showed that the low polarity and limited mutual solubility with the electrolyte caused the diluent to form a"micelle-like"structure in the LHCE system,wrapping around the HCE.The structure improves the physicochemical properties of the electrolyte on the basis of retaining the high concentration characteristics of HCE and the solvation structure（aggregates）.（2）The influence of the molecular structure and the amount of additive on the physicochemical properties of the electrolyte were studied.Studies have shown that the semi-fluorine-substituted T5202（1,1,2,2-tetrafluoroethyl ether）has the best effect on improving the wettability of the electrolyte and maintaining the solvated structure;As the amount of diluent increases,the wettability of the LHCE system was improved,the viscosity was reduced,and the flame retardancy was improved,and the electrical conductivity firstly increased and then decreased.The system had the highest electrical conductivity(up to 7.23 m S·cm^-1)at the addition of 40%.（3）The modification effect of different LHCE systems on the stability of lithium metal anode was studied.The results show that under the addition of 40%,the polarization voltage always remains low after stabilization,the resistance of solid-phase diffusion and charge transfer is the lowest,and the average coulombic efficiency of the battery can reach 97.39%.This is mainly because the improved wettability and electrical conductivity of the LHCE system during the charge-discharge process make the metallic lithium tend to be modularized and planarly deposited,which is beneficial to achieving higher interfacial stability.（4）The modification effect of different LHCE systems on the stability of Li Co O₂studied.The results show that in the T5202 system（40%addition）,the Li Co O₂ has a capacity of 148m Ah·g^-1 at 1C rate,the capacity retention rate keeps 66.9%after 100cycles,and the interlaminar structure of Li Co O₂ still remains after 300 cycles.Applied to pouch cells（design capacity of 70 m Ah）,the LHCE system with T5202 as diluent performed the best cycle stability,with the capacity retention of 99.5%after 20 cycles.