The Thermal Stability Of Solid Electrolyte Interphase(SEI)

With the development of commercial lithium-ion batteries(LIBs),the safety of batteries is one of the key factors for their large-scale application in electric vehicles,aircraft,and other fields.As one of the dominant modes of LIBs accidents,thermal runaway is closely related to the solid electrolyte interphase(SEI)on the graphite electrode.Therefore,a deep understanding of the thermal stability of SEI and precise regulation of its properties are the prerequisites and important ways to improve the safety of LIBs.In this paper,focusing on the thermal stability of SEI,the composition and structural evolution of SEI during thermal decomposition are systematically studied through a series of in-situ and ex-situ characterization tools,then construct the artificial SEI by various methods and their effects on the rate performance of graphite are evaluated.The thermal decomposition process of SEI film is the initial reaction of the thermal runaway of the battery.Due to the complex composition,structure,and low content of SEI,it’s easily damaged during sample transfer and testing,which brings many difficulties to characterization.Rare work has been done to study the mechanism of the thermal decomposition reaction of SEI before.Therefore,in situ heating photoelectron spectroscopy(XPS)is used to monitor the thermal decomposition process of SEI on the surface of vapor-grown carbon fiber(VGCF).Combined with low-temperature transmission electron microscopy(cryo-TEM),electron energy loss spectroscopy(EELS),gas chromatography(GC),and other characterization methods,the structural evolution and gas-releasing processes of SEI during thermal decomposition are deeply studied.The results show that when heated from room temperature to 200 ℃,the organic components in the SEI are gradually decomposed,releasing H2,CH4,CO2,CO,C2H4,C2H6,and other flammable gases;its thickness is reduced accordingly,from 15.2 nm to 8.1 nm.The SEI after heat treatment is rich in inorganics,which provides a structural model for the design and preparation of SEI with high thermal stability.A Li F-rich artificial SEI is constructed on the surface of graphite by a variety of methods,and its effect on the rate performance of graphite is evaluated.The reversible capacity of the graphite anode at a lower rate(< 2 C)can be improved to a certain extent by heat treatment of the pre-cycled electrode and simple liquid phase coating.The sample treated at 100 ℃ had a discharge specific capacity of 337 m Ah/g at 1 C,which is higher than the original sample with 303 m Ah/g.The 1% Li F-coated sample obtained by the liquid phase coating method had a capacity of 357 m Ah/g at0.5 C than the orginal sample(332 m Ah/g).Because Li F has a low ionic conductivity,too much pure Li F coating(5%)will increase the polarization and reduce the capacity of the battery,only 316 m Ah/g capacity at 0.5 C.These preliminary attempts can provide reference and guidance for future interface regulation.