Tin-based Nanocomposite Materials With Hierarchical Fibrous Structure:Fabrication And Addlication In Lithium-ion Batteries

The development of human society depends on the supply of energy.With the growing demand for energy,the fossil fuels have been immoderately consumed,leading to serious environment problems.Hence,people focus on development of clean and high efficiency energy.During various energy storage devices,lithium-ion battery has received extensive attention due to its high working voltage,high capacity,high security,long cycle life,and portability.Nowadays,with the increasing demand for energy strorage devices,commercialized lithium battery using graphitic carbon as anode materials can no longer meet people's requirements.Therefore,researchers begin to develop alternative anode materials with higher energy densities and security.Among these alternatives,tin-based anode materials became one of the highlitghts in scientific research for its high theoretical capacity,high security and low cost.However,during the lithium alloying/de-alloying process,tin-based materials with relatively low conductivity undergo drastic volume change which leads to crumbling and cracking of the electrodes,resulting in abrupt capacity fading and poor rate performance of the anodes.The factors mentioned above become obstacles to its commercial application.To overcome the above-mentioned problems,various specific nanoscale structural tin-based materials with their composite materials were fabricated.As one of the most abundant natural resources,natural cellulose substance is an evioronmentaly friendly polymeric material which pocesses many advantages such as good bio-capatibility,bio-degradability,wide availability and low cost.Moreover,natural cellulose substances holds unique porous fibrous structure,flexibility and strong mechanical property.Herein,taking advantages of the 3D hierarchical porous structure of natural cellulose substances,tin-based materials endowed with unique nanostructure was prepared by means of surface sol-gel process.The details are described as follows.1.Tin oxide/carbon composite material:tin oxide gel layers was first deposited on the surface of natural cellulose substances,followed by carbonization under argon atmosphere to give tin oxide/carbon composite material.The as-obtained material replicate the hierarchical fibrous structure of the original substances.Each of the microfiber is composed of assemblies of nanofibers with fine nanoparticles anchored on the surfaces.The tin oxide coating layer is approximately 15 nm composed of fine tin oxide nanocrystallites with sizes of about 6 nm.The material possess a surface area of 364.2 m² g^-1.As being employed as an anode material for lithium-ion batteries,the porous structures,small SnO₂ crystallite sizes,and the carbon buffering matrix possessed by the nanocomposite facilitate the electrode-electrolyte contact,promote the electron transfer and Li+ diffusion,and relieve the severe volume change and aggregation of the active particles during the charge/discharge cycles.Hence,the nanocomposite showed high reversible capacity,significant cycling stability,and rate capability that are superior to the nanotubular tin oxide.For the tin oxide/carbon composite material,a capacity of 623 mAh g^-1 was delivered after 120 cycles at 0.2 C.Further coating of the SnO₂/carbon nanofibers with an additional carbon layer resulted in an improved cycling stability and rate performance.2.Polypyrrole-coated tin oxide nanotubular material:calcination of ultra thin tin oxide gel film pre-coated cellulose nanofibers of natural cellulose substance yielded tin oxide nanotubular material.The as-obtained material was uniformly coated with polypyrrole via an in-situ polymerization method.The thickness of this polymer layer was about 20 nm.The resultant nanocomposite material faithfully retains the initial cellulose substance.When employed as anode material for lithium-ion batteries,the composite material showed excellent cycling performance.It showed an initial Coulumbic efficiency of 61.8%.After 120 cycles,discharge capacity of SnO₂/PPy remains 680 mAh g^-1.While the initial Coulumbic efficiency of tin oxide nanotubular material is on 49.9%and the discharge capacity after 120 cycles is 200 mAh g^-1.This could be ascribed to PPy that act as a buffering material and helps the electrode stay intact.3.Sb-or Ti-doped tin oxide nanotubular material:Ultrathin SnxSbi-xO₂ or SnyTi1-yO₂ gel layers were first deposited on cellulose fibers through surface sol-gel process using Sn?OiPr?4,Sb?OC2Hs?3 or Ti?OC4H9?4 as precursor,followed by calcination in air at 500? to give Sb-or Ti-doped SnO₂ nanotubular material.The resultant nanotubular materials all faithfully retains the initial cellulose substance with tin oxide nanocrystallites smaller than 10 nm.The materials were employed as anode material for lithium-ion batteries.It is found that the sample with a proper doping ratio?ATO-0.1225,TTO-0.3?endowed with enhancement of electrochemical performance compared to bare SnO₂ due to the improvement of conductivity.After 200 cycles,discharge capacity of ATO-0.1225 remains 325 mAh g^-1,and the value is 334 mAh g^-1 for TTO-0.3.But excessive doping and insufficient doping all result in unsatisfactory electrochemical property.4.Tin sulfide/carbon nanocomposite material:Tin oxide gel layer was first deposited on the surface of cellulose nanofibers.Tin sulfide nanoflakes was then grown on the as-prepared tin oxide gel/cellulose composite through hydrothermal process followed by calcination and carbonization in Ar atmosphere.The resultant nanocomposite material is composed of intervening carbon nanofibers on which anchored a thin layer of tin sulfide nanoflakes??50 nm in diameter?.The material possess a surface area of 106 m² g^-1 with an average pore size of 50nm.The carbon buffering matrix possessed by the nanocomposite facilitate the electrode-electrolyte contact,promote the electron transfer and Li+ diffusion,and relieve the severe volume change and aggregation of the active particles during the charge/discharge cycles.As being employed as anode material for lithium-ion batteries,the nanocomposite showed superior cycling performance.After discharging/charging for 70 cycles,tin sulfide/carbon nanocomposite display a reversible capacity of 612 mAh g^-1.