Study On Free-standing Carbon/Silicon Suboxide Anode Materials For Lithium Ion Batteries

Lithium ion batteries have been widely used due to their high energy density,no memory effect,long cycle life and many other advantages.At present,the anode materials of lithium ion batteries in the market are mainly graphite materials,but the lithium storage capacity of high quality graphite materials can reach about 360mAh g^-1,which is very close to the theoretical value of 372mAh g^-1,it is difficult to further improve.C/SiO_x composite anode material with excellent electrochemical performance was prepared by adding SiO_x material with high lithium storage capacity into carbon substrate,which is considered to be the ideal choice for the next generation of anode materials.Most of the excellent C/SiO_x composites are prepared by nano-sized silicon-based particles as silicon source,but the silicon-based materials with small particle size are difficult to disperse uniformly in the carbon substrate,which prone to agglomeration phenomenon,and the cost is high.At the same time,the rising price of copper has increased the anode cost of lithium ion batteries.In the face of the above problems,this paper uses tetraethyl orthosilicate（TEOS）as the liquid silicon source,polyvinylpyrrolidone（PVP）as the carbon source,homogeneously mixed in the liquid phase,and then processed through the electrospinning technology and heat treatment process,SiO_x is introduced into the carbon fiber to prepare free-standing carbon/silicon suboxide composite material（C/SiO_x）.In order to solve the problem that the addition of SiO_x reduces the overall conductivity of the free-standing material,the free-standing nitrogen-doped carbon/silicon suboxide composites（N-doped C/SiO_x）with better electrochemical performance were further prepared by doping nitrogen into carbon fibers.The main research contents and conclusions of this paper are as follows:（1）The precursor solution was prepared by mixing tetraethoxysilane（TEOS）as the silicon source and polyvinylpyrrolidone（PVP）as the carbon source in the liquid phase,the free-standing C/SiO_x material was obtained by electrospinning technology,pre-oxidation treatment and carbonization treatment.Through experimental investigation,when the precursor ratio is PVP:TEOS=1:1,and the pre-oxidation conditions is kept at 240°C for 3h,and the high-temperature carbonization process is kept at 1100°C for 1h,free-standing C/SiO_x material has a stable three-dimensional network structure and good electrochemical performance.In the cyclic performance test,free-standing C/SiO_x material has a reversible capacity of 425.6 mAh g^-1 after 200cycles at a rate of 0.5C(1C=1.2A g^-1).In the rate performance test,the capacity at the rate of 0.1C,0.2C,0.5C,1.0C,2.0C,0.1C is 588.2mAh g^-1,517.4mAh g^-1,431.4mAh g^-1,359mAh g^-1,290.6mAh g^-1,547mAh g^-1 respectively,and the capacity recovery rate is93%.（2）Based on the free-standing C/SiO_x material,the free-standing N-doped C/SiO_xwas prepared by high temperature diffusion with melamine as the nitrogen source.The study found that the optimal nitrogen doped process is the ratio of melamine to PVP/TEOS film at 1:1,and the optimal holding time was 1h at 1100°C.The doping of N element improves the electronic conductivity of the free-standing electrode material and reduces the electrochemical reaction resistance,so that the free-standing N-doped C/SiO_x material exhibits good electrochemical performance.After 200 cycles at 0.5C,there is still a discharge capacity of 487.7mAh g^-1.The discharge capacities at 0.1C,0.2C,0.5C,1.0C,2.0C,0.1C are 613.1mAh g^-1,559.1mAh g^-1,486.9mAh g^-1,423.4mAh g^-1,361.4mAh g^-1,588.4mAh g^-1respectively.The capacity recovery rate is 96 %.