FeS₂/WS₂ Based Cathodes For All-Solid-State Lithium Batteries:Synthesis And Electrochemical Performances

Currently,lithium-ion batteries employing flammable organic liquid electrolyte and porous separator suffer from serious safety issues.Also,their electrode materials based on traditional transition metal oxides and graphite have low theoretical specific capacities,which are far from meeting the demand for high energy density batteries.All-solid-state lithium batteries employing inorganic solid electrolytes in place of flammable liquid electrolytes can ultimately solve these safety concerns in lithium-ion batteries.Besides,lithium metal anode has a higher theoretical specific capacity and lower operating potential than that of graphite anode,which are beneficial for energy density and operating voltage of batteries.Therefore,all-solid-state lithium batteries have been recognized as promising energy storage systems with high energy density and safety.Transition metal disulfide electrodes have high theoretical specific capacities as well as moderate operating voltages,making them novel candidates for high energy density all-solid-state lithium batteries.Moreover,the interface compatibility and stability between transition metal disulfides and sulfide solid electrolytes can be enhanced because of their similar chemical potentials and compositions.However,the electrochemical performance of transition metal dichalcogenide cathodes is severely limited by not only low electronic/ionic conductivity but also their large volume change during conversion reaction process.In this thesis,to improve the electronic/ionic conductivities and structural stability,nanostructured transition metal disulfides are designed and synthesized via different methods.Meanwhile,the morphologies and structures of the as-synthesized transition metal disulfides are investigated.In addition,their electrochemical performances are systematically evaluated in all-solid-state lithium batteries with sulfide solid electrolytes.The main contents are as follow:1.Preparation and electrochemical performance of FeS₂@S composite cathode.FeS₂ is employed as host material for sulfur due to its low cost,non-toxic,superior electronic/ionic conduction and higher theoretical specific capacity than other transition metal disulfides.However,the electrochemical performance of FeS₂-sulfur cathode in all-solid-state cells is limited by poor interfacial contact between FeS₂ and sulfur.Therefore,to improve the interface contact,the FeS₂@S composite were successfully synthesized by a facile liquid-phase method.FeS₂@S composite with uniform distribution of micro-sized spheres consisting of tightly aggregated nanoparticles were obtained.FeS₂@S microsphere cathode as alternative elemental sulfur cathodes in all-solid-state lithium-sulfur batteries at room temperature retained a capacity of 363.6 mAh g^-1 corresponding to the normalized capacity of 430.7 mAh g^-1 for sulfur at 1000m A g^-1 after 200 cycles.The excellent performance could be reasonably attributed to the stable FeS₂ host for sulfur,enabling high capacity contribution of sulfur as well as efficient contact between FeS₂@S and high ionic conductive Li₁₀GeP₂S₁₂ solid electrolyte resulting in the formation of stable interface with smooth fast electron and ion conductions across the composite positive electrode and sulfide solid electrolyte.2.Preparation and electrochemical performance of FeS₂@Li₇P₃S₁₁ nanocomposite cathodeThe electrochemical performance of FeS₂ cathode in all-solid-state lithium batteries is severely impaired by strain/stress concentration which may create electrode pulverization as well as sluggish of electrochemical reaction products.Therefore,nanostructured design of materials is believed to mitigate the volume change of FeS₂ electrode during continuous cycling process.The FeS₂ nanosheets were successfully synthesized by simple hydrothermal method.Li₇P₃S₁₁ solid electrolytes nanoparticles were anchored on the surface of FeS₂ nanosheets via in situ liquid-phase method followed by annealing treatment to improve the contact interface and lower the charge transfer resistance.FeS₂@Li₇P₃S₁₁nanocomposite cathode exhibited a reversible discharge capacity of 613.4 mAh g^-1 after 180 cycles,at a high current density of 1.0 A g^-1.The excellent rate capability and cyclic performance are mainly attributed to the formation of intimate interfacial contacts between electrodes and solid electrolytes.3.Preparation and electrochemical performance of WS₂@Li₇P₃S₁₁ nanocomposite cathodeTwo-dimensional layered WS₂ was selected as active cathode material owing to its high electric conductivity and layered structure which are separated by a weak Van der Waals force,thus,facilitate Li-ion to effectively intercalate/deintercalate between them.WS₂ has also a high theoretical specific capacity.However,the poor ionic conductivity and severe agglomeration and pulverization of electrode particles rising from the large volume change during lithiation/delithiation processes lead to its poor electrochemical performance.Therefore,a novel strategy is highly required to synthesize WS₂-based cathode with high electrochemical performance.WS₂ nanosheets were successfully synthesized by solution phase route with an annealing treatment since it can provide more extra sites for Li⁺transport and storage compare to others.To further enhance the interfacial contact and ionic conductivity,Li₇P₃S₁₁ solid electrolytes nanoparticles were successfully in-situ coated on the surface of WS₂ nanosheets through liquid-phase method.The assembled Li/75%Li₂S-24%P₂S₅-1%P₂O₅/Li₁₀GeP₂S₁₂/WS₂-Li₇P₃S₁₁all-solid-state batteries delivered a high discharge capacity of 486.8 mAh g^-1 at a current density of 100 mA g^-1 after 50 cycles.It maintained also a discharge capacity of 431.3 mAh g^-1 after 100 cycles at a current density of 500 mA g^-1.It shows that the special designed WS₂-Li₇P₃S₁₁ nanocomposite cathode can strongly suppress the strain/stress at the electrode/electrolyte interface which facilitates the interfacial ion transfer dramatically during cycling.In this thesis,the electrochemical performances of transition metal disulfides(FeS₂,and layered WS₂)were investigated as cathode materials in all-solid-state lithium batteries with sulfur-based solid electrolytes.Transition metal disulfide active materials can improve the energy density of all-solid-state lithium batteries due to their high reversible specific capacities and operating voltages.This thesis,would open up the doors for designing and exploring the electrochemical performances of electrode materials for all-solid-state lithium ion batteries with high energy density.