Preparation And Electrochemical Performances Of Si-based Nanocomposite Films As Anode Materials For Lithium-ion Batteries

Lithium ion battery has become one of the primary choices of the green batteries in the 21th century due to its properties of high voltage,high energy density,no memory and non-pollution.At present,carbonaceous is the major anode active material in commercial lithium ion rechargeable batteries,but it cannnot meet the demand of the next generation due to its low thoretical capacity(372 mAh g^-1).So,attempts at developing new noncarbon negative_materials are necessary.In this present work,in order to improve the electrochemical cyclability and stability,by introducing nanostructered barfer matrix,several Si-based nanocomposites were fabricated,and their microstructures and electrochemical properties were investigated.With addition of poly(vinylpyrrolidone)(PVP),a crack free Si/TiC nanocomposite film was prepared by a sol-gel coating method.As a less active matrix in the composite anode,TiC could effectiyely alleviate the volume change caused by the reaction of Li⁺ with Si and guarantee the contact between Si particles.On the other hand,lithium ions in electrolyte could facilely fill in the interspace between these small particles to reach the surface of active Si promptly.So,the Si/YiC composite showed reversible lithium storage capacities of about 1000 and 1300 mAh g^-1at different current densities of 160 and 80 mA g^-1even after 80 cycles.Using two-parallel diffusion path model,the Nyquist plots of the composite anode were fitted well and their reactive mechanisms were also interpreted theoretically.The chemical diffusion coefficient of the Si/YiC nanocomposite was calculated on the order of 10^-12cm² s^-1.From the varing trend of D_Liobserved at different electrode potentials,it was concluded that the minima of D_Li,which corresponded to the CV peaks,appeared at the halfway point of the intercalation process with its related phase transition.With poly block copolymer(EO₂₀PO₇₀EO₂₀)as a structure-directing agent,polyvinylpyrrolidone (PVP)as film-forming assistant,a mesoporous Si/TiO₂ nanocomposite film was fabricated by a sol-gel coating method.The mesoporous TiO₂ possessed channels in all directions,not only improved the mobility of Li⁺,abbreviated the diffusion distance,but also suppressed the expansion caused by Li-Si alloying,thus assured the cyclability and stability of the composite electrode.The Si/TiO₂ composite exhibited a reversible charge/discharge capacity of about 1500 mAh g^-1after 50 cycles.The Li diffusion coefficient in Si/TiO₂ composite was also improved because of the mesostructure,and it was estimated as 10^-12 -10^-13cm² s^-1.With addition of poly(vinylpyrrolidone)(PVP)as film-forming assistant,Si/LiTi₂O₄ nanocomposite film was synthesized by a sol-gel method.The nanosized Si particles were distributed homogeneously in the LiTi₂O₄ matrix.The poly pore structure of Si/LiTi₂O₄ composite provided channels to facile the mobility of lithium ions and also provided spaces to alleviate the expansion caused by the Li-Si alloying.As a results,the Si/LiTi₂O₄ nanocomposite exhibited a reversible lithium storage capacity of about 1100 mAh g^-1,and the average discharge capacity fading from 2nd to 50th cycle was 1.7 mAh g^-1(0.15%), suggesting its promising nature in anode materials for lithium ion batteries.A two-parallel diffusion paths model fitted well with the impedance data of porous Si/LiTi₂O₄ composite. The lithium ion diffusivity was on the order of 10^-13cm² s^-1based on the semi-infinite diffusion model.