Specific Surface Area Control And Electrochemical Performance Research Of Nano-silicate@C

The specific surface area of Fe₂SiO₄/C and Li₂FeSiO₄/C nanocomposites was successfully controlled by solid state reactions and different SiO₂ using as silicon resource.Ultrafine porous Fe₂SiO₄/C composites（FS221/C,FS263/C and FS325/C）are successfully fabricated by solid state reactions from the mixture of ammonium citrate,ferrous oxalate dehydrate and nano porous SiO₂ with different specific surface area(221,263 and 325 m²·g^-1).XRD results proved thatα-Fe₂SiO₄ phase are formed in the three samples.SEM indicates that all the three samples are composites made up of nanoparticles of 30-70 nm and submicro particles of 200-400 nm.Pore structure analysis indicates that the external surface area of the ultrafine porous Fe₂SiO₄/C composites can be controlled by tuning the surface area of nano porous SiO₂ and the external surface area of FS325/C is the largest among the three samples.These unique features make FS325/C possess the highest reversible lithium storage capacity,best rate performance,longest cycling life and highest lithium ion diffusion coefficient among the three samples.Mesoporous Li₂FeSiO₄/C nanocomposites（LFS-FNS and LFS-NS）were prepared from fumed nano silica（FNS）and nano silica（NS）through facile solid-state reactions,respectively.XRD analysis indicates that the crystalline structures of LFS-FNS and LFS-NS are indexed to monoclinic Li₂FeSiO₄ of P2₁.SEM results prove that the particle size of LFS-FNS and FNS（25-40 nm）is smaller than that of LFS-NS and NS,revealing the particle size of Li₂FeSiO₄/C nanocomposites can be tuned by choosing different silica.TEM further indicates Li₂FeSiO₄ nanoparticles are uniformly dispersed in the amorphous carbon networking of LFS-FNS.BET analysis indicates the external surface areas of LFS-FNS as well as LFS-NS are 51.4 and 36.1 m²·g^-1,indicating the pore properties of mesoporous Li₂FeSiO₄/C nanocomposites can be controlled by using different silica as silicon resource.The reduced particle size and high external surface area shorten the lithium-ion diffusion path and make LFS-FNS possess excellent electrochemical performance.The discharge capacity of LFS-FNS is as high as 172mAh·g^-1 at 0.1 C.Hierarchical mesoporous Li₂FeSiO₄/C sheaf-rods（HMLFS-SR）and Li₂Fe SiO₄/C nanocomposites（LFS）are successfully fabricated by an improved solid state reaction from SBA-15 and nano SiO₂,respectively.XRD proves that HMLFS-SR and LFS both contain Li₂FeSiO₄ crystals and tiny Fe₃O₄ impurities.SEM indicates that HMLFS-SR are hierarchical sheaf rods constituted by nanoparticle.EDS proves that Fe,Si,O and C elements are uniform distributed through the whole Li₂FeSiO₄/C sheaf rods.Nitrogen adsorption/desorption analysis indicates that HMLFS-SR are typical mesoporous materials with an external surface area of 47 m2·g^-1,which is much higher than that of LFS.HMLFS-SR deliver a high discharge capacity of 212.2 mAh·g^-1 at 0.1 C.In addition,HMLFS-SR also have good cycling performance,low charge transfer resistance and large lithium ion diffusion coefficient.