Fabrication And Lithium Storage Performance Of Porous Silicon Materials

As anode material of lithium ion batteries,silicon has a Li-storage capacity of4200 mAh g^-1,as well as low insertion potential and rich reserves,which makes Si be viewed as a best material for increasing the energy density of lithium ion batteries.However,the volume of Si will expand 3 times,which will produce a huge force in Si,leading to the pulverization of Si during the charge and discharge process,thus the electrochemical performance will deteriorate.With the aim to accommodate the force in Si,this thesis aims to study the fabrication and decoration of porous Si,and the enhancing mechanism of the electrochemical performance.The main research contents are as follows:1.Porous Si is prepared through a route of Si-Al alloy etched by HCl,and porous Si spheres construct by many Si fibers with the diameter about 50 nm.A layer of carbon is formated on the surface of porous Si by pyrolyzing toluene at 700 ^oC under nitrogen atmosphere,forming porous Si/C composite.Si/C displays an enhanced performance in comparison with pure porous Si.At 200 mA g^-1,the discharge capacities of Si and Si/C in the first cycle are 2374.8 and 2067.4 mAh g^-1,respectively.At the 100^th cycle,the discharge capacities of Si and Si/C are 1402 and585.3 mAh g^-1,respectively.At 3200 mA g^-1,the discharge capacity of Si is 505.1mAh g^-1,but that for pure porous Si is only 47.1 mAh g^-1.The improvement for cycling stability and capability is due to the pore with the ability to buffer the volume variation,and the coating carbon with the ability to inhibit the melting of Si particles during the process of volume expanding as well as increase the electron conductivity.2.Nitrogen doped carbon?NDC?in-situ deposite on the surface of porous Si by carbonizing melamine at high temperature under nitrogen atmosphere,forming Si/NDC composite.NDC is well in Li⁺diffusing and electron conductivity,which benefit to improve the performance of lithium storage of porous Si.At 200 mA g^-1,the discharge capacities of Si/NDC in the first cycle and the 100^th cycle are 2181.2and 499.6 mAh g^-1,respectively.At 3200 mA g^-1,the discharge capacity can be 499.6mAh g^-1.The theory calculation and XPS analysis shows Si has an interaction with O and N of NDC,which helps to transfer electron in the interface between Si and NDC,leading to a increasing in electron conductivity,thus performance of lithium storage of Si will be improved greatly.3.Aniline is used to in-situ polymerized on the surface of porous Si,forming silicon/polyaniline?Si/PANI?composite.The soft structure of PANI benefits to stabilize the porous structure of Si,which will make a great improvement in cycling and capability.At 200 mA g^-1,Si/PANI can discharge 1828 mAh g^-1 in the first cycle,and which also can keep at 929.7 mAh g^-1 at the 100^th cycle.At 5000 mA g^-1,the discharge capacity can be 421.1 mAh g^-1.In Si/PANI composite,PANI can separate Si and electrolyte to stabilize the SEI film,which is another reason for the improvement of lithium storage of Si.