Optimization Of Interfacial Film Properties To Inhibit The Destruction Of Mn²⁺Deposited On Anode

With the development of society,people’s demand for various digital products using lithium-ion batteries as energy is increasing day by day.Manganese-containing cathode materials containing manganese transition metal elements(Such as Li Mn₂O₄and Li Ni_0.5Mn_1.5O₄)are widely used because of their abundant resources and environmental friendliness.However,in the process of cycling,the dissolution of positive Mn²⁺will inevitably occur,which will affect the capacity and service life of the battery,resulting in the deterioration of the battery performance.In this thesis,the reduction and decomposition process of Li BOB on the surface of graphite electrode is mainly limited by its soluble products,with the electrolyte system of Li BOB-ethylene carbonate（EC）/diethyl carbonate（DEC）as the research object.Based on this conclusion,by carrying out the method of intermittent discharge in the range of Li BOB decomposition voltage,the soluble reaction products attached to the graphite electrode surface can be diffused into the electrolyte bulk,so that Li BOB can be decomposed more thoroughly,and the formed interfacial film（SEI）can more effectively inhibit the deposition of Mn2+on the anode electrode.In addition,in order to overcome the disadvantage of low ionic conductivity of SEI film formed by Li BOB,sulfur additives were selected and added on this basis to reduce the charge transfer impedance of SEI film.The main research contents and results of this thesis are as follows:（1）The key factors restricting the reduction and decomposition process of Li BOB were explored by multi-sweep cyclic voltammetry（CV）and potential resolved in situ electrochemical impedance（PRIS-EIS）techniques.The results showed that a certain number of soluble products were produced during the reduction of the lithium salt.These products will accumulate on the surface of the graphite electrode,preventing further contact between Li BOB and the electrode,thus preventing its continuous decomposition.Based on this conclusion,in this paper,the diffusion of soluble products in the electrolyte was strengthened by intermittent discharge mode,which weakened its inhibitory effect on Li BOB reduction reaction,so that the generated SEI film has better protection performance.The results showed that the capacity retention rate of Mn²⁺-containing cell was 88.4%and the irreversible capacity loss was only 14.2 m Ah g^-1after 50 cycles when the batch time was 8 h.Scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）and electrochemical impedance（EIS）analysis showed that a smooth and compact protective film with low charge transfer impedance was formed on the electrode surface.The SEI film formed by a certain amount of Li BOB decomposition and composed of boron-containing compounds effectively inhibited the deposition of Mn²⁺on the electrode surface（content only 3.14 ppm）.In contrast,the SEI film formed by the traditional discharge method cannot effectively inhibit the erosion of Mn²⁺,resulting in serious damage to the anode material structure and rough surface morphology.However,the mass transfer impedance of SEI film formed by a certain amount of Li BOB decomposition is too large,which will lead to a significant increase in the loss of capacity of the battery.（2）In this thesis,sulfur additives were added to the electrolyte to reduce the mass transfer impedance by increasing the conductivity of Li⁺in the SEI film with the method of intermittent discharge.The effects of three sulfur additives,butene bisulfite（BS）,1-allyl 1,3-sulfonolactone（PES）and vinyl sulfate（DTD）,in reducing the interfacial impedance were compared.The results show that DTD has excellent film forming performance,and its reduction products can significantly reduce the impedance of SEI film.After the addition of DTD,the capacity retention rate of the battery reaches 90.3%after 50 cycles,the irreversible capacity loss is 5.9 m Ah g^-1,and the surface SEI film is more smooth and compact.The composition analysis of SEI film showed that it contained a certain amount of disulfide and various other sulfur compounds.This is due to the strong electronegativity of sulfur atoms in the center of DTD,which is easy to capture the electrons.In summary,this paper studies the key factors that inhibit the continuous decomposition of Li BOB,and improves the film formation efficiency of the lithium salt through intermittent discharge,so that the SEI film formed by it can more effectively inhibit the deposition of Mn²⁺on the electrode surface.Finally,by introducing DTD into the electrolyte,the SEI film is enriched with sulfur compounds,which further reduces the SEI film impedance and improves the battery performance.This work systematically solves the problem of cell performance attenuation caused by Mn²⁺released during the working process of lithium ion batteries with manganese-based material as the positive electrode,and provides a new idea for inhibiting the deposition of transition metal ions in the electrolyte.