Preparation And Electrochemical Properties Of Carbon Based Nanostructured Materials Applied In Energy Storage Devices

Lithium ion batteries possess high energy densities than those of the conventional storage batteries.In comparison of commercial graphite anode materials,their lower power densities and poor cycling performance are critical challenges for their applications.The energy and power densities would depend on electrode’s structural stability and properties.In this thesis,I mainly focused on the preparation of heteroatom doped,electrical conductive and porous nanostructured carbon layer modified tin oxide（Sn O₂）,silicon nanoparticles and layered transition metal sulfides with high storage capacity via polymer precursor pyrolysis under high temperature to improve power densities and cycling performance.And the possible application in anode materials of lithium ion cell batteries was also explored.It mainly includes.The first aspect of this thesis is to explore the honeycomb-like heteroatom doped flexible carbon network on tin oxide（Sn O₂）anode materials.We proposed a novel yet simple strategy to cushion the mechanical strain of Sn O₂ nanoparticles（NPs）via heteroatom doped ultra-fine honeycomb-like carbon layer for high performing lithium-ion batteries（LIBs）.The Sn O₂ NPs were coated with a heteroatom doped carbon layer by phenolic resin and 2,5-dimercapto-1,3,4-thiadiazol（sulphur and nitrogen）followed by reduction and carbonization.The carbonization of polymeric resin led to thin,porous and dual doped carbon film on the surface of Sn O₂ NPs（denoted as Sn O₂@S,N-C）.The porous nature of carbon layer provided easy access of electrolyte to the Sn O₂ and thin coating offer flexibility for reversible volume expansion while dual doping deliver active sites and more defects in carbon film for reversible adsorption of lithium ions.The designed Sn O₂@S,N-C and Sn O₂@C anode material demonstrated excellent Li storage capacity(840 m Ah g^-1,640 m Ah g^-1at 0.1 A g^-1（≥99%coulombic efficiency up to 750 cycles）.The first-principle density functional theory calculations further suggested favorable binding energy of lithium ions.The second aspect of this thesis is the assessment of electrochemical behaviors of silicon@carbon（Si@C）anode materials.Silicon nanoparticles have great potential applications in lithium-ion batteries.However,huge mechanical strain,capacity retention and unstable solid electrolyte interface formation weaken their applications in industry.To overcome the above challenges,a novel facile route was adopted to design nanostructured silicon core carbon shell composites,where the carbonization of phenolic resins led to a uniform porous thin interfacial layer of carbon.The phenolic resin precursor endowed mesoporous morphology with the carbon layer due to the carbonization of aromatic carbon,methylene linkages and hydroxyl groups.The mesoporous conductive carbon helped effectively to control the mechanical strain of silicon nanoparticles which maintained the integrity of Si@C nanocomposites and provided effective channels to easy access of electrolyte and short lithium ions transport.This novel Si@C anode offered a stable specific capacity of～868 m Ah g^-1 at 0.1 Ag^-1（up to 500 cycles with≥99%columbic efficiency）.In the third part,the engaging tailored capacity of layered WS₂ via sulfur bonding coupled with polyetherimide as anode materials was presented.Layered transition metal sulfides have drawn great research attention due to their excellent properties for energy storage and conversion devices.However,there is a need to innovate effective approaches to enhance the performance of metal sulfides for modern energy storage devices.Herein,a rational surface engineering strategy was adapted to design tungsten disulfide nanosheets coated polyetherimide（WS₂@PEI）nanocomposite by facile sol-gel method combined with subsequent pyrolysis.The WS₂@NC nanocomposite exhibits excellent lithium-ion storage properties as compared to pristine WS₂ nanosheets.The origin of enhanced performance of WS₂@NC nanocomposite is carbon-sulphur（C-S）bonding with WS₂,offered by thermally stable polyetherimide（N-C）.The increased strength of charging was subjected to C-S and N-C bindings,which significantly enhanced the active sites and defects in WS₂@NC nanocomposite framework.The thin porous carbon layer provided elasticity to control the volume expansion during the lithium ions（Li⁺）insertion/deinsertion,while nitrogen and sulphur components offered more active sites and defects in the nanocomposite for reversible adsorption of Li⁺ions.Thus,the WS₂@NC nanocomposite could deliver a high reversible specific capacity(712 m Ah g^-1at 0.1 A g^-1 after 100 cycles and 417.6 m Ah g^-1 at 0.8 A g^-1with 99%average coulombic efficiency).In the fourth part,the engineered hierarchical heteroatom doped porous biochar from biomass waste for energy storage and environmental protection was tried.Motivated by hetero-atom doping,there is a recent trend of biomass waste material’s applications in energy storage and environmental protection purposes.The first time,we explored phosphorus,nitrogen,and sulfur decorated biomass-derived porous carbon biochar（PNS@BC）via a one-step hydrothermal method followed by pyrolysis at 700°C to increase the conductivity and the porosity.In PNS@BC material the sp² bonded carbon atom was substituted by P,N and S atoms which changed the structural credibility of an amorphous carbon ring.The dopants enhanced extrinsic defects and active binding sites for Li⁺ions insertion.It’s a cost-effective and template-free method.Turning a profit as of the assets of active sites,defects and improved conductivity of the PNS@BC enhanced the outstanding discharge capacity of 1073m Ah g^-1(0.1A g^-1)and 523 m Ah g^-1 capacity retention up to 3000 cycles(0.6 A g^-1),proved to be long-term reliability in the coin.Remarkably,based on decoration with hetero-atom doped atoms,our designed ultrafast lithium-ion battery（PNS-BC/Li）anode achieved this high capacity.