Study On The Impact Mechanism Of Solvation Effect On Interface Of Carbonate-Based Electrolytes/Electrodes

In recent years,due to the increasing demand for energy storage and portable electronic devices,it has become increasingly urgent to improve the performance of lithium-ion batteries（LIBs）.However,despite being a key component of the battery,electrolyte design is still in the“trial-and-error”stage.Furthermore,an effective correlation model between Li⁺-solvation in the electrolyte,electrode/electrolyte interface film properties,and battery performance has yet to be established.Thus,this thesis aimed to study the solvent competitive reaction in double carbonate-based electrolytes,clarify the effects of formation and evolution of the interface film,and reveal the impact of Li⁺-solvation on the electrode/electrolyte interface film.Finally,a proposed solvation effect of the high concentration electrolyte strategy to improve the cycle stability of single solvent carbonate-based electrolytes at high temperatures and voltages.The main research contents and results are listed as follows:（1）By analyzing the differences in Li⁺behavior in a carbonate-based electrolyte of lithium hexafluorophosphate（LiPF₆）dissolved in ethylene carbonate（EC）,dimethyl carbonate（DMC）,and EC+DMC,this chapter illuminated the impact of Li⁺-solvation structures on the formation and evolution of solid electrolyte interface（SEI）film and battery performance.The Li||C half-cell assembled with 1.0 M LiPF₆-DMC（single solvent）exhibited poor cycling performance,with its discharge specific capacity(70.9 mAh g^-1)after the 500th cycle being much lower than that of 1.0 M LiPF₆-EC/DMC（1:1）（double solvents）(303.6 mAh g^-1).Cyclic voltammetry,potential-resolved in situ impedance tests,and theory calculations indicated that Li⁺-DMC was preferentially reduced and formed an initial SEI film.Due to the stronger solvation effect between EC and Li⁺,the proportion of Li⁺-DMC at the electrode/electrolyte interface decreased.In other words,EC effectively inhibited the excessive decomposition of DMC in single solvent carbonate-based electrolytes by affecting the solvation migration and reduction reaction process of Li⁺-DMC,which is beneficial for improving the cycling stability of Li||C half-cells.（2）Based on the research conclusions above,the electrochemical performance of single solvent carbonate-based electrolyte under extreme conditions（55℃,5.0 V）has been further improved by combining an HCEs strategy.It was found that the discharge specific capacity and capacity retention rate of the lithium nickel manganese oxide(Li Ni_0.5Mn_1.5O₄,LNMO)||Li half-cell assembled with 5.5 M LiPF₆-DMC(105.7 mAh g^-1 and 92%)after 200 cycles were much higher than those of 1.0 M LiPF₆-DMC(26.82 mAh g^-1 and 1.6%).By increasing the concentration of lithium salt to 5.5 mol L^-1,more PF₆^-are introduced into the Li⁺solvation structure,promoting the construction of an anion-derived cathode electrolyte interphase（CEI）film enriched with lithium fluoride（Li F）and Li_xPO_yF_z.Additionally,the results indicated that a soluble product interface enriched with PO₂F₂^-anions was formed on the surface of the electrode with highly concentration electrolyte.This inert interface exhibited reactive inertness towards hydrogen fluoride（HF）,effectively suppressing HF corrosion on the LNMO electrode and the dissolution of transition metal ions.In summary,this thesis has studied the influence of solvation effects in carbonate-based electrolytes on Li⁺transport behavior and proposed the effect mechanism of solvent competition on the evolution of electrode/electrolyte interface film and battery performance.The newly proposed strategy to improve the high-temperature tolerance of LIBs through an inert interface film derived from soluble products is helpful in analyzing the mechanism of the solvation effect and promoting the development of high energy density LIBs.