Study On The Preparation Of Transition Metal Sulfide/carbon Composite And Its Lithium Storage

As a common chemistry power source,lithium-ion batteries(LIBs)have attracted great attention due to its high energy density,long cycle life,and excellent safety and nontoxic.Rencently,Promoted by energy problem and surported by national policy,LIBs were applied from in mobile phone,laptop and digital camera to electric bicycle,electromobile,industrial power system,aerospace field as well as grid-scale energy storage systems.High-energy-density LIBs can be gained by employing electrode materials with higher specific capacities.Graphite,the dominant anode material for current LIBs,has become one of the limiting factors for achieving high-energy-density batteries because of its low theoretical capacity of 372 mA h g-1.Therefore,it is highly desired to design and synthesize the anode materials with high specific capacity,high rate capability as well as long cycle life.Previous reports demonstrate that transition metal sulfides are promising candidates for lithium-ion battery anodes owing to their high theoretical capacities.However,they usually suffer from poor cycling stability because of the low electronic conductivity and the substabtial volume change during lithium insertion/extraction process.In this thesis,we have prepared the Co3S4-PNS/GS,WS2@C/RGO,and Fe7S8@CNF composites and systematically investigated their lithium storage properties.First,we have synthesized a Co3S4 porous nanosheet/graphene sheet(denoted as Co3S4-PNS/GS)composite with unique sandwichlike architecture through freeze-drying and subsequent hydrazine treatment.The as-prepared composite benefits from the synergistic effect of its two components.In specific,the encapsulated Co3S4 porous nanosheets effectively promote lithium ion transport.Besides,the excellent electrical and mechanical properties of graphene sheets can not only improve the electrode conductivity but also well buffer the large volume changes of Co3S4 during cycling.As a result,the Co3S4-PNS/GS composite manifests enhanced cycling stability and rate performance compared with that of Co3S4-PNS.Second,we have developed a double carbon coating for the WS2 anode through a self-assembly process between oleylamine(OLA)-coated WS2 nanosheets and graphene oxide and subsequent pyrolysis treatment.The self-assembled duplicate carbon coating consists of an OLA-derived surface carbon layer and an external electronically conductive and flexible RGO shell,both of which can not only effectively avoid direct contact between WS2 nanosheets and the electrolyte but also impede the aggregation and accommodate the large volume changes of WS2 nanosheets during cycling.When measured as an anode material for LIBs,the double carbon-coated WS2 nanosheets exhibit superior lithium storage properties in terms of high capacity,stable cyclability,and excellent rate capability.Last,we have demonstrated the synthesis of Fe7S8 nanoparticles embedded in carbon nanofibers(denoted as Fe7S8@CNF)through electrospinning technique and subsequent high-temperature-vulcanization method by using iron acetylacetonate and sulfur powder as iron and sulfur sources,respectively.The strong chemical bonding between Fe7Ss nanoparticles and the CNF matrix can not only effectively prevent the nanoparticle aggreagation but also significantly boost the transport of lithium ions and electrons.As an anode material for LIBs,the as-prepared Fe7S8@CNF composite shows high specific capacity,excellent cycling performance,and superior rate capability.