Preparation And Electrochemical Behavior Of Metal Phosphide Cathode Materials For Lithium Sulfur Batteries

Lithium-sulfur batteries have the excellent theoretical capacity(2600 Wh kg^-1)and theoretical specific capacity(1675 m Ah g^-1),which is considered to be one of the most promising secondary devices.Lithium-sulfur batteries have been extensively investigated by researchers over the past decades.However,the intermediate polysulfide is easily soluble and shuttles back and forth in the electrolyte,causing rapid capacity decay due to the loss of sulfur and delaying the progress of research on lithium-sulfur batteries.To address these issues,researchers have focused on transition metal compounds with intrinsic polarity in the hope of reducing the shuttle effect through chemisorption and catalytic polysulfide conversion.In particular,metal phosphides with high electrical conductivity and excellent electrocatalytic ability are one of the most promising catalytic materials for lithium-sulfur batteries.However,compared to pure carbon electrodes,the electrical conductivity of metal phosphides is still unsatisfactory,which hinders the effective transport of ions and electrons,resulting in electrode materials that cannot fully achieve their electrochemical performance.Therefore,compounding metal phosphides with carbon materials that have good electrical conductivity and a large specific surface area improves the electronic conductivity of the whole electrode and the electron and ion transport kinetics.Graphene outstands all carbon materials in particular,meeting all of these advantages.Metal phosphides loaded on graphene were chosen to be used as a sulfur substrate for lithium-sulfur batteries.The monometallic phosphides（Co P/rGO,Ni₂P/rGO）and bimetallic nickel-cobalt phosphides（Ni Co₂P_X/rGO）were successfully prepared by anion-exchange and phosphating process.The structural characterization was investigated by SEM,TEM and XRD,and the electrochemical behavior and catalytic processes of the three metal phosphide composites were showed.The reasons for the better catalytic and electrochemical properties of the bimetallic phosphides than the monometallic phosphides were analyzed on the basis of the experiments.The specific research results are summarised as follows:（1）Co P/rGO was obtained by multiple solvothermal and phosphating processes with graphene as the substrate.The Co P/rGO/S composite was obtained by melt-diffusion method with sulfur for electrochemical testing.Co P/rGO/S composite,a good catalytic material,exhibits good electrochemical ability and catalytic performance as the cathode,which effectively promotes the conversion reaction between polysulfide and Li₂S₂/Li₂S.These excellent properties can be attributed to the rich redox sites of Co P/rGO and the synergistic effect between the components,which can effectively facilitate the rapid conversion of polysulfides.In addition,the unique yolk-shell structure allows for more efficient contact between the polysulfide and the catalytic active sites,providing sufficient space to increase the sulfur loading and buffer the large volume changes during cycling.（2）Ni₂P/rGO/S composites were prepared by the solvothermal,phosphating and melt-diffusion methods.Detailed structural description,electrochemical characterization,visualization experiments and kinetic investigation of Ni₂P/rGO revealed that Ni₂P/rGO with high specific surface area,abundant mesopores and high electrical conductivity provided a large number of polysulfide adsorption catalytic sites and fast ion/electron channels,which enhances the redox kinetics and effectively mitigates the shuttle effect.（3）Ni Co₂P_X/rGO/S composites were prepared by the solvothermal,phosphating and melt-diffusion methods.The batteries assembled with Ni Co₂P_X/rGO/S electrode manifest excellent electrochemical performance(initial discharge capacity of 1238.7 m Ah g^-1 at 0.1 C)and good catalytic properties.The excellent performance is attributed to the fact that the addition of Ni to the Co P effectively modulates the three-dimensional electronic structure,improving its kinetics and obtaining better catalytic performance.Ni Co₂P_X/rGO with excellent specific surface area provides enough active sites to rapidly trap polysulfides in the electrolyte.Hence,the interconversion between the polysulfides and the insoluble Li₂S₂/Li₂S is catalyzed while capturing the polysulfide,allowing the active substance sulfur to be fully utilized.The research in this thesis has important implications for the development of metal phosphides as the sulfur cathode.