Preparation And Properties Of Lithium Selenium Battery Cathode Materials Based On Microporous Carbon Spheres/Selenium Complex

As a new energy source that directly converts chemical energy into electrical energy,lithium-ion battery has the characteristics of stable performance,simple structure and good portability.But the traditional lithium-ion batteries have low energy density that cannot meet the growing energy demands of people.Especially with the appearance of new energy car,the higher level demands are asked for batteries with the performance of capacity and cycle life.And as a new type of lithium-ion secondary battery developed in recent years,lithium-selenium battery has a large density of 4.81 g/cm3 and a theoretical volumetric capacity of 3255 mAh/g.So it has broad application prospects in the filed of mobile communications,electric vehicles and aerospace industry.The most prominent problem of Li-Se battery is bad cycle stability.Since selenium particles form the polyselenide in the charged and discharged cycle of the battery,it is easily dissolved in the electrolyte to cause "shuttle effect",which lead to the loss of active material and low coulombic efficiency when the active material was dissolved and shuttled through the electrolyte to the negative electrode.It seriously restricts the promotion and application of Li-Se batteries with the easy expansion of the selenium positive electrode during the cycling as well as the low ion transmission rate and conductivity.In order to solve the above problems,selenium is coated by prepared microporous carbon spheres in this paper,which the excellent electronic conductivity and ion transmission rate of the carbon spheres are utilized to improve the electrochemical performance of the carbon selenium complex.The selenium particles are fixed in the microporous pores of the carbon sphere,that can not only limit the reaction area of selenium in the cell cycle,but also reduce the possibility of polyselenide being dissolved in the electrolyte to inhibit the shuttle effect.At the same time,it can also utilize the porous structure of the carbon sphere to remit the volume expansion of the selenium particles during charging and discharging of the battery,and then keep the structure stabilization.Therefore,the purpose in this paper is to find the optimal synthesis conditions for the preparation of microporous carbon spheres coated with selenium and the most suitable choice of carbon source materials.In this paper,sucrose was used as the carbon source,and the concentration gradient experiment was carried out about the oxidant of sulfuric acid which reduced sucrose.Three kinds of microporous carbon spheres with different carbon particle sizes were prepared by KOH activator under three concentrations of 3 mol/L,6 mol/L and 9 mol/L sulfuric acid solutions(the carbon spheres named as 3M-MPC,6M-MPC,9M-MPC).With the increasing concentration of sulfuric acid,the particle size of the prepared carbon sphere are smaller,as well as the specific surface area,pore volume and pore size are larger.And when compounded with the same quality selenium powder,the carbon spheres 3M-MPC have the lowest selenium storage capacity and the worst load effect.The others have better loading effect and higher selenium storage.After the composite materials were fabricated to positive electrode,the composite cathode materials of the carbon spheres 6M-MPC and the selenium had the best electrochemical performance in the carbonate electrolyte.And the specific capacity was 565 mAh/g at a current density of 0.2 C(1 C=675 mA/g)in 100 cycles.After 500 cycles at 1 C,the specific capacity is still the highest,reaching to 527 mAh/g,showing excellent cycling stability.With the optimal concentration of the sulfuric acid solution being determined,the microporous carbon spheres were prepared by using glucose which is the hydrolysate of sucrose as carbon source.After SEM observation and specific surface area test,the microporous carbon sphere prepared by using glucose has smaller particle size,larger specific surface area,pore volume and pore diameter than the carbon sphere prepared by sucrose under the same synthesis conditions.And the carbon sphere prepared by glucose has better composite effect to the selenium particles and higher selenium storage.In the electrochemical performance experiment,the composite in the carbonate electrolyte is significantly better than that in the ether electrolyte.The specific capacity in the carbonate electrolyte at 0.2 C is 590 mAh/g,higher than the 530 mAh/g in the ether electrolyte,and it is also higher than the specific capacity of the sucrose cathode material.It also shows the same consequence at 1 C and 10 C after a long period of cycling.It demonstrates the excellent performance of the Li-Se battery in the carbonate electrolyte and the uniqueness of glucose as a carbon-based material.