| With the rapid development of world economy,non-renewable fossil fuels(such as coal,oil,natural gas)is increasingly exhausted,which are accompanied with a series of severe environmental problems.Therefore,exploration and development of high capacity,high energy density,long cycle life,light weight and eco-friendly electrochemical storage systems as well as to effectively take advantage of the natural resources is an inevitable trend during the economy progress.As a new generation of reproducible energy storage system,lithium batteries are widely used in electrical/plug-in hybrid vehicles and modern portable electronic devices due to their high energy density,high operation voltage and no memory effect features.Therefore,the development of energy materials with good physicochemical properties,low-cost,environment friendliness and electrochemical activity is an effective way to improve the electrochemical performances of electrode materials in the energy-related storage system.Natural cellulose substance,one of the inexhuastible biomass resources,is possessed with superior biocompatibility and degradation as well as good flexibility and mechanical strength.As a natural polymers,its unique hierarchical three-dimensional network structures endowing natural cellulose substance with a large specific surface area and porosity.Hence,it can be employed as a structural scaffolds,carbon source and reaction substrate,where the guest materials can be modified onto the surface of the cellulose nanofibers by reactions with the hydroxyl groups through the layer-by-layer self-assembly technique.In this dissertation,a variety of electrode materials with good electrochemical activity are fabricated,alternatively,by employing natural cellulose substance as a carrier for electrode materials would further expand their application in the energy-related fields.The main contents are as follows:1.Nanofibrous silver-nanoparticle/titania/carbon composite:An ultrathin titania gel film was firstly deposited onto each cellulose nanofiber of filter paper by the surface sol-gel process.Then,the as-obtained titania/cellulose composite sheet was carbonized to obtain the nanofibrous titania/carbon composite,and followed by deposition of silver nanoparticles(Ag-NPs)onto the surfaces of titania/carbon nanocomposite fibers which resulted in a new nanofibrous ternary Ag-NP/titania/carbon composite material.When this composite was employed as an anode material for lithium ion batteries,it showed improved electrochemical performances compared with titania,carbon and titania/carbon hybrid counter materials.For such a composite with 22.48 wt%of silver and Ag-NPs with the size of 5-10 nm,the initial discharge capacity was 1323 mA h g-1 at a current density of 100 mA g-1,and it showed a stable capacity of 320 mA h g-1 after 150 charge/discharge cycles.It was found that the electrochemical performances of the anode material got better with the increase of the silver content in the composites.Due to the enhanced electric conductivity caused by the silver nanoparticles,as well as the unique three-dimensional cross-linked network structure of the composite,the anode performances in terms of the capacity,cycling stability and rate capacity are significantly improved.2.Nanofibrous Fe3O4-TiO2-carbon composite:A bioinspired hierarchical nanofibrous Fe3O4-TiO2-carbon composite was fabricated by employing natural cellulose substance as both the scaffold and the carbon source.The titania gel layer pre-coated cellulose nanofibers of commercial filter paper were obtained by the self-assembly technique.Then,the FeOOH nanoparticles were grown uniformly onto the surface of the titania thin-layer precoated cellulose nanofibers,and thereafter,the as-prepared FeOOH-TiO2-cellulose composite was calcined and carbonized in argon atmosphere at 500 ? for 6 h to produce the Fe3O4-TiO2-carbon composite.The resultant composite possesses a hierarchical structure that was faithfully inherited from the initial cellulose substance,which was composed of titania-coated carbon fibers with corncob-like shaped Fe3O4 nanoparticles immobilized on the surfaces.When this composite employed as an anode for LIBs,it showed a first-cycle discharge capacity of 1340 mA h g-1,delivering a stable reversible capacity of ca.565 mA h g-1 after 100 charge-discharge cycles at a current density of 100 mA g-1,which is over the theoretical value.This is much higher than those of the commercial Fe3O4 powder(233 mA h g-1)and the Fe3O4-carbon counter material(324 mA h g-1).It was demonstrated that the thin titania precoating layer(thickness ca.10 nm)is necessary for the high content loading of the Fe3O4 nanoparticles onto the carbon nanofibers.Owing to the unique three-dimensional porous network structure of the carbon-fiber scaffold,together with the ultrathin outer carbon-coating layer,the composite showed significantly improved cycling stability and rate capability.3.Nanofibrous MnO2/TiO2/carbon composite:A bio-inspired ternary nanofibrous MnO2/TiO2/carbon composite was fabricated by combination of the surface sol-gel method and hydrothermal processes.Mesoporous MnO2 nanosheets densely grew on the titania gel film pre-coated carbon nanofibers,which gives a hierarchical MnO2/TiO2/carbon nanoarchitecture and shows superior electrochemical performances when employed as anodic materials for lithium-ion batteries.The MnO2/TiO2/carbon with a MnO2 content of 47.28 wt%delivers a specific discharge capacity of 677 mA h g-1 after 130 discharge/charge cycles at a current density of 100 mA g-1,corresponding to a contributed percentage as high as 95.1%.It was proved that the ultrathin titania precoating layer(thickness ca.2 nm)acts as a crucial interlayer for the contribution of higher content loading of well-organized MnO2 nanosheets onto the surface of titania/carbon nanofibers.Due to the improved electric conductivity caused by the internal interconnected carbon nanofibers and the titania coating layer with extremely low volume change as well as the higher theoretical specific capacity of the MnO2,the MnO2/TiO2/carbon electrodes displayed superior cycling stability and reversible rate capability during Li+ insertion/extraction processes.4.Hierarchical porous carbon/sulfur composite:A hierarchical activated carbon nanofiber(ACF)with a large specific surface area was fabricated via the alkali activation treatment by using the natural cellulose substance as sturctural scaffold and carrier,and the obtained ACFs material was further impregnated with sulfur as cathodes for lithium-sulfur(Li-S)batteries.The specific surface area and pore volume of the activated nanofibers(ACFs)measured was increased from 83.66 m2 g-1 and 0.140 cm3 g-1(carbon nanofiber pre-carbonized at 450 ?)to 1480.18 m2 g-1 and 0.733 cm3 g-1,respectively.The resultant ACFs impregnated sublimed sulfur with a content of 49.4 wt%which designated as A-C-S-2 composite was exhibited an initial discharge capacity of 1393 mA h g-1 and stabilized at 520 mA h g-1 after 100 cycles.A good rate capability with a reversible capacity of 789 mA h g-1 was obtained after high rate charge/discharge cycles.The improvement of cycling stability and rate capability of the A-C-S-2 material can be attributed to the unique three-dimensional conductive network structures of ACFs scaffolds with high surface area and large pore volume,which can effectively facilitate the Li+/e-transport and accommodate the volume expansion from the formation of sulfur to Li2S process in Li+ insertion process.Hence,the A-C-S-2 cathode showed excellent cycling stability and superior reversible rate capability.Herein,a variety of nanofibrous titania-based/carbon composites and a hierarchical porous carbon/sulfur nanocomposite were fabricated by employing natural cellulose substances(laboratory quantitative filter paper)as the structural scaffolds.These materials can be either used as anodes for lithium-ion batteries(LIBs)or cathodes for lithium-sulfur(Li-S)batteries,and their electrochemical performances are also investigated.It is revealed that the unique three-dimensional network structures with large specific surface area of the initial cellulose substances are faithfully maintained by the resultant nanocomposites,It is beneficial for the effect contact of the electrode/electrolyte and provide with more active sites as well as facilitate the charge transfer and shorten the diffusion path of lithium ion during the Li+ charge/discharge processes.In addition,these materials acts as a conductive matrix to buffer the large volume expansion of the active materials and maintain the structural integrity of the electrodes during the repeated Li/Li+ processes.Hence,the cycling stability and the rate capability of the composites are significantly improved.This work demonstrates that the biomimetic synthesis method based on natural cellulose substances could provide an unique pathway for the design and fabrication of energy-related functional nanomaterials with sophisticated structures and specific physicochemcial properties. |
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