Synthesis And Electrochemical Performances Of Transition Metal Sulfide Nanocomposite Cathodes For All-Solid-State Lithium Batteries

Commercialized lithium-ion batteries using organic liquid electrolyte and porous separator have leakage,combustion and explosion safety issues.And traditional lithium transition metal oxides and graphite electrodes have low theoretical specific capacities,which couldn't meet the demand of the higher energy density.All-solid-state lithium batteries using nonflammable inorganic solid electrolyte can completely overcome these safety issues in lithium-ion batteries.Moreover,metallic lithium with low electrode potential and high theoretical specific capacity can be used as anode in the all-solid-state lithium batteries to improve the operating voltage and energy density.Therefore,all-solid-state lithium batteries have been considered as one of the most promising energy storage systems with high safety and energy density.Transition metal sulfide electrodes are expected to achieve high energy density in all-solid-state lithium batteries because of their high theoretical specific capacities and moderate operating voltages.Moreover,the interfacial compatibility and stability could be greatly improved due to the similar chemical composition and chemical potential between transition metal sulfide electrodes and sulfide-based solid electrolytes.In this thesis,unique morphologies and structures of transition metal sulfide nanocomposites are reasonably designed and synthesized to improve the electronic/ionic conductivities and structural stabilities.In addition,their electrochemical performances in all-solid-state lithium batteries are systematically studied.The main contents are as follows:1.The synthesis and electrochemical performances of Fe₃S₄@Li₇P₃S₁₁nanocomposite cathodeThe Fe₃S₄ nanosheets were successfully synthesized by a facile PVA-assisted precipitation method.Ultrathin 2D nanosheets can not only shorten the Li⁺diffusion path and enhance the charge transport kinetics,also can increase the contact area between active materials and solid electrolytes.To further improve the interfacial contact and lower charge transfer resistance,Li₇P₃S₁₁ solid electrolytes nanoparticles were in-situ deposited on the surface of Fe₃S₄ nanosheets by liquid phase method.Due to the intimate interfacial contact and favorable structural stability,Fe₃S₄@Li₇P₃S₁₁ nanocomposite cathode delievered reversible capacity of 1001 mAh g^-1 at 0.1 A g^-1 after 200 cycles.2.The synthesis and electrochemical performances of NiS-CNTs nanocomposite cathodeBulk-NiS and NiS-CNTs nanocomposites were successfully prepared by hydrothermal sulfuration method.Ultrasmall NiS nanoparticles were distributed evenly on the surface of CNTs.The unique nanostructures can not only shorten the charge transfer distance and increase the contact area,also can construct conductive network to enhance the conductivities of composite electrodes.In addition,CNTs can serve as buffer materials to alleviate volume change and local stress/strain during charge-discharge process.The discharge capacity of NiS-CNTs nanocomposite cathode in all-solid-state lithium batteries maintained at 170 mAh g^-1 under 1.0 A g^-1after 150 cycles.In addition,the convesion reaction mechanism of NiS in all-solid-state lithium batteries was revealed by ex-situ XRD technique.3.The synthesis and electrochemical performances of rGO-MoS₃ nanocomposite cathodeAmorphous MoS₃ nanoparticles and rGO-MoS₃ nanocomposites were synthesized by acid precipitation method.Ultrafine MoS₃ nanoparticles deposited evenly on the surface of 2D rGO nanosheets.Unique nanostructures can not only shorten the Li⁺diffusion path and improve the electrochemical reaction kinetics,but also can increase the contact area between active materials and solid electrolytes to improve discharge specific capacities.Moreover,the presence of rGO nanosheets could improve the electronic conductivity and structural stability of composite electrodes.Compared with lithium-ion batteries,all-solid-state lithium batteries using amorphous MoS₃ exhibit an anionic redox driven chemistry.Due to the reduction of volume expansion and polysulfides shuttle effect,rGO-MoS₃ nanocomposite cathode exhibit high discharge capacity of 414 mAh g^-1 at 1.0 A g^-1 after 500 cycles.4.The synthesis and electrochemical performances of rGO-VS₄@Li₇P₃S₁₁ nanocomposite cathodeThe rGO-VS₄ nanocomposites were fabricated by one-pot hydrothermal method.The 1D chain-like VS₄ nanorods can provide large open channel for Li⁺transport and storage,and the weak interchain interaction also can enhance the charge transfer kinetics.Besides,2D rGO nanosheets can improve the electronic conductivity and structure stability.To further improve the ionic conductivity and interfacial contact,Li₇P₃S₁₁ solid electrolyte nanoparticles were in-situ deposited on the surface of rGO-VS₄ nanocomposites by liquid phase method.Due to the multichannel continuous electronic/ionic conductive network and improved structure stability,10%rGO-VS₄@Li₇P₃S₁₁ nanocomposite cathode as alternative materials for elemental sulfur cathodes in all-solid-state lithium batteries show 611 mAh g^-1 at 1.0 A g^-1 after100 cycles.After the first irreversible conversion reaction,VS₄ undergoes a reversible Li-S reaction process in all-solid-state lithium batteries.In this thesis,the electrochemical performances of transition metal sulfides?Fe₃S₄,NiS?based on conversion reactions and transition metal polysulfides?MoS₃,VS₄?based on anionic redox-driven chemistry were investigated in all-solid-state lithium batteries.The conversion reaction electrode materials exhibit low discharge specific capacities and operating voltages.Moreover,the large volume change would cause poor cycle stability in all-solid-state lithium batteries.By comparision,the energy density of the all-solid-state lithium batteries can be improved by using transition metal polysulfides as active materials owing to their high reversible specific capacities and operating voltages.This thesis provides promising research directions of cathode materials for high-energy all-solid-state lithium batteries.