Research On Preparation And Electrochemical Attenuation Mechanism Of Si@C Composite Derived On Silsesquioxane

As an important part of new energy storage devices,the research of electrode materials is very important.It is of great significance for the further application and development of energy storage devices to find precursors with their own structural advantages.As an excellent precursor,the general formula of silsesquioxane is R8Si8O12,R is an arbitrary organic group,so the required group-doped composites can be prepared by polymer modification.On the other hand,because of its polyhedral core-shell and nano-size structure,introducing the material as a silicon source into the composite system can not only improve the pore structure of the system,but also improve the formation of coated composite structure.The precursor material has considerable prospects in the field of lithium-ion battery industry.Si@C composites were prepared by vinyl copolymerization of octavinyl silsesquioxane?POSS?and styrene.The electrochemical properties of the composites as anode materials for lithium ion batteries were tested,including cyclic stability,AC impedance and rate performance.The trend of electrochemical performance change during the whole life of lithium-ion batteries was analyzed in order to explore the micro-change of material structure in different charging and discharging processes,and to explain the mechanism of electrochemical attenuation.The results of the above studies are as follows:?1?Si@C composites are used as anode materials for lithium-ion batteries.With the increase of silicon content,the first cycle discharge capacity of the composites increases and the cycle stability decreases.The specific discharge capacity of the composite anode material with 14.43%silicon content is 1500 mAhg^-1 in the first cycle.After 550 charging and discharging cycles,the discharge capacity returns to1430 mAhg^-1 again.?2?In the whole life cycle of lithium-ion batteries,the coulomb efficiency decreases after the first high specific capacity,following that specific capacity increase continuously.When the specific capacity rises close to the first cycle,the continuous capacity of lithium-ion batteries decreases until it reaches 400 mAhg^-1?as the end of battery life?.?3?The essence of the electrochemical attenuation change is that the formation of early solid electrolyte film?SEI?prevents the embedded lithium ions from completely detaching,resulting in low coulomb efficiency after the first high specific capacity;with the continuous charge-discharge process,the activation of silicon in the composite system increases the active sites of lithium intercalation,and the carbon coated by the outer layer is continuously embedded in the silicon polyhedron junction.As a result,the volume effect of silicon is buffered and the lithium intercalation capacity is increased.When the battery is subsequently charged and discharged,the powdering of silicon in the material system and the separation of silicon and carbon make the material structure collapse,leading to the continuous and rapid decline of battery performance until the end of life.