Synthesis And Properties Of Porous Silicon-based Anode Materials For Lithium Ion Batteries

Si has been regarded as the most promising anode material for lithium-ion batteries due to its highest theoretical specific capacity,low discharging voltage,and natural abundance.However,the huge volume changes during cycling and its low electronic conductivity hinder the application of Si as an anode by limiting the cycling life,rate performances and initial Coulomb efficiency.In this thesis,in order to solve the above problems of silicon anode materials,porous Si@C composite materials were prepared by nanostructuring and carbon coating methods to improve the cycling stability and rate performances of silicon anode materials.In order to further improve the initial Coulomb efficiency and cycling stability of silicon material,the boron-doped porous silicon material was prepared by carrier-doping and its electrochemical performances were studied.The main contents are as follows:1.In order to improve the cycling stability and rate capability of Si anode materials for Lithium-ion batteries.We used the intrinsic Si wafers with ball milling about size of 2μm as a Si source to mix with Mg powders to produce Mg₂Si at 550°C for 5 h in argon gas atmosphere,and then oxidize at 600°C for 5 h in a muffle furnace to form the mixture of MgO/SiO_x/Si.The outer surface of the mixture was coated with carbon through pyrolysis of acetylene gas and then to obtain pSi@C-1 composite anode materials washed with acid.The above mixture was directly washed with acid to obtain porous silicon（pSi）,and the outer surface of p Si was coated through pyrolysis of acetylene gas,and then obtained pSi@C-2 composite anode materials.The initial discharge capacities of pSi@C-1 and pSi@C-2 composite electrodes at the current density of 0.4 A g^-1 are 2037 mAh g^-1 and 2510 mAh g^-1,respectively,and the initial Coulomb efficiency of p Si@C-1 and pSi@C-2 composite electrodes are 80.8%and76.9%.The reversible specific capacities of the pSi@C-1 and pSi@C-2 composite electrodes are kept at 755 and 1215 mAh g^-1 after 200 cycles at a current density of 2 A g^-1.It is demonstrated the electrode of pSi@C-2 has a good cycling stability.The reversible specific capacities of pSi@C-2 composite electrode at different current densities of 0.4,0.8,1.6,3.2,and 6.4 A g^-1 are 2510,2086,1743,1285,and 758 mAh g^-1,respectively.The electrochemical test shows that the pSi@C-2 composite electrode has better rate performance.2.In order to further improve the initial Coulomb efficiency and cycling stability of Si anode materials for Lithium-ion batteries.B-doped pSi material was prepared through air-oxidation demagnesiation of Mg₂Si from p-type Si wafers.The prepared B-doped pSi material was assembled into half-cell for electrochemical testing.The initial Coulomb efficiency of B-doped pSi electrode is as high as 89%at a current density of 0.4 A g^-1.After 100 cycles,B-doped pSi electrode has a reversible specific capacity of 2232 mAh g^-1.The prepared B-doped pSi electrode shows large special capacity and good rate capability(2900 mAh g^-1 at 0.4 A g^-1,2300 mAh g^-1 at 1 A g^-1 and 1107 mAh g^-1at 6.4 A g^-1).Furthermore,a reversible capacity of 1500 mAh g^-1 can still be kept after 300 cycles at a current density 2 A g^-1.The B-doped pSi material electrode shows large specific capacity,high initial Coulomb efficiency,and an excellent cycling stability.