Syntheses And Electrochemical Properties Of Si-based Porous Electrode Materials

Silicon is one of promising candidates for next generation anode of Li-ion batteries(LIB)owing to its high capacity and abundant natural resources.However,extremely large volume change(-300%)during charge/discharge process easily results in the bulk pulverization and poor cyclic stability.For these reasons,the porous silicon-based materials will be investigated to enhance the electrochemical properties in this work.The mesoporous Si was synthesized via a magnesiothermic reduction method using SBA-15 as a precursor.The measurment results by XRD,SEM,TEM and BET show that the as-synthesized Si nearly preserves the mesoporous structure of precursor and the average pores size of?15 nm.In order to improve the electrochemical performance of product Si,tabletting treatment was introduced to the magnesiothermic reduction method to prepare macroporous silicon.By tabletting treatment,the temperature of the reaction was reduced to 430?435 ?,which is much lower than the reaction temperature of sample without tabletting treatment.SEM,TEM and BET measurements exhibit a uniform macroporous structure and pore size within the range of 60?200 nm,which differs from the product without pretreatment.The charge/discharge test show that such macroporous Si maintains the capacity of 1110 mAh g-1 after 100 cycles,5 times higher than the mesoporous one prepared without tabletting.Moreover,the initial Coulomb efficiency of macroporous Si was lifted to 86.5%in comparison with the mesoporous one.The stacking density of MgO/SBA-15 composites sample with tabletting pretreatment under 10 MPa is 1.602 g cm-3,which is 13.6 times higher than that of without tabletting(0.118 g cm-3).A large amount of heat released in magnesiothermic reduction reaction was more concentrated when the stacking density was increased.The high heat concentration would lead to a much higher sample temperature than the set temperature.So the reaction can be easily triggered for tablet samples.Therefore,the onset temperature would become much lower than that of samples without tabletting.Moreover,high heat concentration would result in the collapses the architecture of precursor and growth of Si grains.For sample with tabletting pretreatment,the coefficient of thermal conductivity was significantly enhanced,which persuades a more homogenous heat transferring and exchange in the reagents,facilitating the formation of uniform macroporous Si.Biosilica from rice husks was substituted for SBA-15 as a precursor to synthesize macroporous silicon via modified magnesiothermic reduction.The structures and electochemical performances of the product were similar with that employing SBA-15 as a precursor.It implies that porous Si prepared by the modified magnesiothermic reduction method rarely depends on the structure of the precursor.Inspired by the MgO/Si composite,the magnesiothermic reduction product,we developed a scalable synthesis method of Si@porous-C in this work using nano-MgO as the pore-former.The measurment results display that the porous silicon-carbon material has a hierarchical porous structure and shows improved cyclic stability and high-rate performance.