Study On Modification Of Silicon-Based Material As The Anode Of Lithium-Ion Battery

Silicon-based materials have received widespread attention as the anode for lithium-ion batteries,due to their extremely high theoretical lithium insertion capacity(Li22Si5,4200 mAh/g)and low voltage platform(<0.5V vs Li/Li+).However,they have the problem of severe volume expansion during the charge-discharge process,which leads to the pulverization of the electrode,thereby resulting in the continuous deterioration of the electrochemical performance of the battery.Additionally,silicon,as a semiconductor material,has poor conductivity,which hinders its development in the field of lithium batteries.In this paper,the recycled industrial photovoltaic silicon powder is introduced as the raw material after pickling with HCl and HF mixed in a certain ratio to remove iron,copper and other metal impurities and unknown oxide layer on the surface.Then,the obtained silicon powder with the virtues of low cost and good crystallinity were modified by surface treatment and ion doping,respectively,to improve its electrochemical performance.The details are listed as follows:(1)The acid-washed silicon powder material was heat-treated at 800? under a nitrogen flow of 150 mL/min.A limited oxidation reaction between the silicon powder material and the residual oxygen produces a thin layer of SiOx of about 2-5 nm on the surface of the silicon powder,which can effectively suppress the volume expansion of the lithium ion charge and discharge process,thanks to the heat treatment The good surface morphology of silicon powder material greatly improves the electrochemical performance of silicon material.After 10 cycles(activation at a current density of 0.02 C for one cycle and then tested at 0.1 C,1C=4200 mAh/g),the reversible charge specific capacity of heat-treated silicon is up to 2351.6 mAh/g,whereas the pristine silicon material has 827.6 mAh/g.In addition,the first coulomb efficiency was increased from 63.1%to 87.9%after surface oxidation.Although the oxide layer coating can suppress the volume expansion of silicon material,its intrinsic conductivity still cannot be improved.Therefore,in this chapter,we mixed the silicon powder with H3PO4,and followed by high-temperature calcination to prepare P-doped,in order to improve the conductivity of silicon material.The optimal ratio of silicon to H3PO4 has also been investigated.The results show that the battery material maintains the highest reversible charge capacity after 10 cycles when the doping ratio is 0.5%.After 10 cycles the charging capacity remains at 1313.2 mAh/g,which is 485.6 mAh/g higher than the original untreated silicon material.It is worth mentioning that additon of 0.7%H3PO4 resulted in the highest first coulomb efficiency of 81.4%.(3)In this chapter,LiPF6 is used as a reaction additive,and it is calcined at 800? to obtain the final product after mixing with silicon powder raw materials.The purpose is to dope the silicon material with P element and coat a layer of LiF on the surface of the material.The coating of LiF helps to form a stable solid electrolyte interface film(SEI film)and reduces side reactions during charging and discharging,and the presence of LiF with molten salt properties can also improve the agglomeration of silicon powder.The experimental results show that when 0.5%LiPF6 is added,the electrochemical performance of the resulting silicon-based material is greatly improved.The first coulombic efficiency of the battery is increased from the initial 63.1%to 70.0%,and after 10 cycles(activation one cycle at a current density of 0.02C and then 0.1C cycle,1C=4200 mAh/g).The charging specific capacity of 2183.1 mAh/g is still maintained,which is 1355.5 mAh/g higher than the original untreated silicon material.