| Graphite is a difficult-to-replace anode electrode material for lithium batteries,and its electrochemical performance is closely related to the composition of the electrolyte.Because the lithium intercalation potential of graphite is much lower than the reduction potential of each component of the electrolyte,the electrolyte is reduced on the electrode surface to form a Solid Electrolyte Interphase（SEI）.The SEI film is located between the electrode and the electrolyte,connecting the solid-liquid two phases to protect the electrode and achieve long-term stable circulation.The electrolyte additive directly determines the composition and structure of the SEI film through preferential redox reaction in the cycle,and plays a significant role in protecting the electrode,improving the electrolyte,and improving the performance of lithium batteries.However,the properties of SEI films and the mechanism of action of additives are not yet fully understood,and there is a lack of research on the dynamic changes of the electrode interface.In the first part of the thesis,through the theoretical calculation of common additives,we selected the additive ES with high reduction potential to configure the new VC+ES electrolyte.The performance of graphite in the new VC+ES electrolyte was systematically evaluated compared with the electrolyte containing only VC additive.By comparison,we found that the VC+ES electrolyte has peculiar electrochemical behavior.Due to the introduction of ES additives,the SEI film on the graphite surface becomes unstable,and the peculiar electrochemical behavior of the capacity first decreases and then increases during cycling at a rate exceeding 0.5 C.Through EIS in situ analysis,it was found that the SEI film resistance and interfacial transfer resistance changes on the graphite surface were related to the first decrease and then increase behavior of the capacity.Further elucidated by XRD,SEM,EDS,AES,and TOF-SIMS,the electrochemical behavior is caused by the first decomposition and then reformation of the unstable SEI film at high magnification.It is also found in the experiment at high temperature that the capacity of VC+ES electrolyte does not decrease first and then increase,and the capacity and cycle stability of graphite are better than those of VC electrolyte.At the same time,the effect of VC+ES electrolyte on the electrochemical performance of lithium manganate cathode and full cell was also explored.This peculiar electrochemical behavior does not exist in cathode and full cell.The capacity fading caused by graphite exfoliation is a difficult issue for the practical application of graphite anodes in lithium batteries.In the second part of the thesis,we investigate the diazotized grafted phenol layer to solve the problem of poor cycling stability of graphite anodes caused by exfoliation of graphite sheets.Using 4-aminophenol and tert-butyl nitrite as diazotization reagents,the covalent grafting of phenol on the graphite surface was achieved.The feasibility of grafting the phenol layer on the graphite surface was confirmed by XRD,FT-IR,XPS,SEM and HRTEM technical characterization of the samples.The diazotized grafted phenol can form a protective layer on the surface of the graphite sheet,expand the graphite interlayer spacing,increase the specific capacity of the graphite,and finally improve the rate performance of the graphite and significantly improve the cycle stability of the graphite.The mechanism of diazotized grafted phenol on graphite surface and the energy storage mechanism of phenol layer were also proposed.When the molar percentage of 4-aminophenol and graphite is 5%,the electrochemical performance of modified graphite is the best.After 300 cycles at 1 C,the discharge specific capacity of 361 m A h g-1 is still maintained,which is 90.5% relative to the initial cycle,much higher than the 156 m A h g-1 and 54.2% of original graphite. |
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