Preparation And Electrochemical Properties Of SiOC Composites Materials Based On Silsesquioxane

Lithium-ion batteries are extensive application in portable devices and new energy vehicles,which is due to the advantages of environmental protection,high energy density and long cycle life.As a significant component of lithium-ion batteries,the current commercial graphite anode material limits the development of lithium-ion batteries as a result of inferior theoretical capacity.Hence,it is urgent to exploit an anode material possess high energy density and long cycle life.Silicon-based anode materials have high theoretical specific capacity.However,serious volume effect will occur during cyclic process,cause the rapid attenuation of battery property.In order to resolve the volume expansion of the silicon-based anode material during the cycle.In this paper,N-doped SiOC composites possess carbon coated structure were prepared by in-situ polymerization of sesquisiloxane and 1-vinyl imidazole.Utilizes the nanometer size and inorganic silicone skeleton of sesquisiloxane to effectively mitigate the volumetric effect of silicon oxide.The structure and morphology of SiOC composites were characterized by TEM,SEM,XRD,XPS and Raman spectroscopy.To investigate the effects of temperature and oxygen content on the electrochemical properties of the anode materials,the N-doped SiOC composites with different oxygen contents were tested by galvanostatic charging and discharging,full-life cycle,cyclic voltammetry and AC impedance at 30°C and60°C,respectively.According to the morphology,structure and impedance changes of the electrode material in different stages of the full-life cycle,the electrochemical decay mechanism of the composite materials was studied.The main results are as follows:（1）For explore the effect of oxygen content on the electrochemical properties of the composites,N-doped SiOC composites with different oxygen content of 1%,5%,10%and 15%were obtained by changing the amount of magnesium powder during magnesium thermal reduction.With the increase of oxygen content in the composite,the first discharge specific capacity decreased,but the cycling stability increased.The first charge and discharge specific capacities of N-doped SiOC composites with oxygen content of 5%at 60℃is 1528.7 m Ah·g^-1 and 2103.1m Ah·g^-1,respectively,with the initial coulomb efficiency of the first cycle is as high as 72.7%.The reversible specific capacity remains stable at 1500 m Ah·g^-1 after 750cycles.It is found that the SiOC anode material has excellent cycle performance and long cycle life at high current.（2）During the whole life cycle of this SiOC composite,the performance is constantly attenuated after the first high discharge capacity,and drops to the lowest point at the 50th cycle,and the specific capacity sustained increase to 1657 m Ah·g^-1,in the final phase,the discharge specific capacity drops to 388 m Ah·g^-1 after 1000cycles.（3）The essence of the attenuation of the electrochemical properties of the system is:the solid electrolyte film（SEI film）will be formed during the first cycle,and the process will consume a large amount of lithium ions,with a lowest coulomb efficiency,due to the dense carbon layer and the lower activity of the silicon oxide,resulting in further decay of the specific capacity,the silicon oxide in the composite material is continuously activated with the performance of the battery increased during charging and discharging,which causes the embedded amount of lithium ions increased and the carbon coated layer can suppress the volume effect of the silicon oxide.At the end of cycling,the continuous expansion of the silica oxide causes the powdering off of the active material,leads to a dramatic decrease in the performance of the battery.