| Nowadays,lithium-ion batteries have occupied the portable electronic product market,and are gradually extended to high-power systems such as electric vehicles and large-scale energy storage systems,due to the fascinating features including high specific energy,excellent cycling performance and environmental benignity etc.However,the conventional carbon-based anode materials for commercial lithium-ion batteries cannot meet the current demands for high energy and power densities because of the low theoretical capacity.Thus,developing next-generation anode materials with higher energy density and better cyclic and rate capabilities have attracted more and more attention.Among the many alternative anode materials,tin-based anode materials are regarded as an appealing candidate considering its high theoretical capacity,safe working,low cost and nontoxicity.However,the large volume change during lithium insertion process,the low conductivity and the poor cycling stability of the tin-based anode materials limit their practical applications for high power output.Many approaches have been developed to circumvent the above-mentioned obstacles.At present,constructing nanostructures and compositing with other active/inactive materials are the effective strategies to improve the electrochemical performance of tin-based anode materials.Natural cellulose has the advantages of abundant resources,good biocompatibility and renewability,and has been widely used in industry production and daily life.Natural cellulose has a unique three-dimensional network structure,excellent flexibility and mechanical strength.Meanwhile,its surface has a large amount of hydroxyl groups,so guest substance can form a gel film on its surface through hydrogen bonding or covalent bond.The corresponding functional nanostructured materials are obtained by selectively removing the original cellulose template or using it as the scaffold and carbon source.Herein,to improve the stability and the reversibility of the tin-based anodic materials,a series of tin-based nanocomposites were fabricated by employing natural cellulose substance as structural template or carbon scaffold,which exhibited improved electrochemical performance as the anode materials for lithium-ion batteries.The main research contents are described as follows:1.Tin-nanoparticle/carbon-nanofiber composite material:ultrathin tin-oxide gel films were first deposited to coat each cellulose nanofiber of the filter paper by the surface sol-gel process to obtain tin-oxide/filter-paper hybrid,which was thereafter calcined and carbonized in argon atmosphere to yield the nanofibrous tin-oxide/carbon composite.The surface of the tin-oxide/carbon composite fiber was further coated with ultrathin carbon layer by a glucose hydrothermal process to give the nanofibrous carbon/tin-oxide/carbon hybrid,which was reduced by hydrogen gas to obtain the tin-nanoparticle/carbon-nanofiber composite.In order to adjust the tin content in the final nanocomposite,the depositing cycles of the tin-oxide gel film were repeated 5,10,and 15 times,respectively.The nanocomposite with 16 wt%tin content was composed of fine metallic tin nanocrystallites with sizes of 20-50 nm that were uniformly immobilized on the surface of the cellulose-derived carbon nanofibers.When employed as an anodic material for lithium-ion batteries,because of its unique three-dimensionally hierarchical porous structures and the buffering effect of the carbon matrix,it showed a stable discharge capacity of ca.430 mAh g-1 after 200 charge/discharge cycles at a current density of 100 mA g-1.In addition,its structural stability upon extensive charge/discharge cycling processes is outstanding.2.Ag-nanoparticle/tin-oxide/carbon ternary nanocomposite material:The nanofibrous tin-oxide/carbon was achieved by calcination/carbonization of the as-deposited SnO2-gel/cellulose hybrid in an argon atmosphere.The Ag nanoparticles with the diameter of about 15-20 nm were loaded on the surface of the tin-oxide/carbon nanofiber via photocatalytic reduction reaction by using silver nitrate as the source of siver to obtain the Ag-nanoparticle/tin-oxide/carbon ternary nanocomposite.The Ag content in the nanocomposite could be controlled by the concentration and temperature of the silver nitrate solutions.When employed as anode materials for lithium-ion batteries,the Ag-nanoparticle/tin-oxide/carbon composite showed enhanced electrochemical performances compared with the hybrid tin-oxide/carbon material.Its good electrochemical performances are attributed to the unique three-dimensional buffering network structures of the carbon fiber scaffold,the enhanced electric conductivity derived from the presence of Ag nanoparticles,as well as the synergistic effect of SnO2 and carbon.In the eletrochemical examinations,the Ag-nanoparticle/tin-oxide/carbon composite with the highest Ag content of 24.5 wt%showed a superior performance with a stable specific capacity of 690 mAh g-1 after 120 charge/discharge cycles at a current density of 100 mA g-1.3.Cu-nanoparticle/tin-oxide/carbon and Ni-nanoparticle/tin-oxide/carbon nanocomposite materials:A surface sol-gel process was carried out to coat the nanofibers of the cellulose substance with the ultrathin tin-oxide gel layers to obtain the tin-oxide/cellulose hybrid,which was then calcined and carbonized in argon atmosphere to give the nanofibrous tin-oxide/carbon composite.The resulted nanofibrous tin-oxide/carbon composite was further uniformly decorated with the metallic Cu or Ni nanoparticles via a facile chemical reduction process using copper nitrate or nickle nitrate as the precursor to give the Cu-nanoparticle/tin-oxide/carbon or Ni-nanoparticle/tin-oxide/carbon naocomposite,respectively.The content of the metal nanoparticles in the nanocomposites were regulated by varying the concentration of the precursor solution.As the anode materials for lithium-ion batteries,the three-dimensional porous structure of the nanocomposite inherited from the initial cellulose substance effectively buffered the large volume change during the cycling processes;moreover,the uniformly decorated metal nanoparticles significantly facilitated the electron transport,as well as reversibly promoted catalytic decomposition of Li2O during delithiation process to enhance the reversible capacity of the electrode.Therefore,the nanocomposites exhibited improved electrochemical performance.4.Ag-nanoparticle/manganese-dioxide/tin-oxide nanotubular composite material:A layer-by-layer self-assemble method combined with subsequent carbonization process was first adopted to fabricate the nanofibrous tin-oxide/carbon composite,and MnO2 nanosheets were then grown uniformly onto the surface of which by a simple hydrothermal approach using potassium permanganate as the precursor.The as-prepared manganese-dioxide/tin-oxide/carbon material was calcined in air to produce the nanotubular manganese-dioxide/tin-oxide composite,which further uniformly decorated with the metallic Ag nanoparticles via a silver mirror reaction to yield the Ag-nanoparticle/manganese-dioxide/tin-oxide nanotubular composite.As the anode material of lithium-ion battery,the Ag-nanoparticle/manganese-dioxide/tin-oxide nanotubular composite showed superior cycling and rate capacities due to its three-dimensional hierarchical structure inherited from the initial cellulose substance,the good conductivity of the Ag nanoparticles and the synergistic effect of the manganese-dioxide and tin-oxide.The composite prepared through a 2-min-silver mirror reaction exhibited excellent cycling stability with a high capacity of 387 mAh g-1 after 200 cycles at a high current density of 500 mA g-1.Tin-based materials,especially those with specific nanostructures,have been the research hotspot of anode materials for lithium-ion battery.In this thesis,a series of tin-based nanocomposites were fabricated by self-assembled biomimetic synthesis strategy using natural cellulose substance as structural template or carbon scaffold,which have both the excellent electrochemical properties of tin-based materials and the unique structural characteristics of natural cellulose substance.Thanks to the unique three-dimensional network structure of the cellulose template,the tin-based nanocomposites exhibited enhanced structural stability,good electrical conductivity and improved electrochemical performance when used as anode materials for lithium ion batteries.This work demonstrates that self-assembled biomimetic synthesis technique is an effective method for the design and preparation of functional nanomaterials,and has considerable prospects in the synthesis of advanced energy-related materials. |
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