Structural Design Of Metal Doped Porous Carbon Substrates And The Electrochemical Performances In Lithium Sulfur Batteries

Lithium sulfur batteries are considered to be promising as one of the next high energy density storage system,due to their high specific capacity(1675 m Ah g^-1)and energy density(2600 Wh kg^-1),which are much higher than conventional lithium ion batteries.However,the intrinsic deficiencies of sulfur cathode impede their further commercialization.The main course include the low conductivity nature(5×10^-30 S cm^-1)and expansion effect of sulfur,polysulfide shuttle and the sluggish reaction,those would cause a low sulfur utilization and a sever capacity degradation while cycling.How to inhibit polysulfide shuttle,in the meanwhile promoting the conductivity of cathode,resolve the expansion and improve the reaction kinetics of cathode reaction is the key issue.Over decades of developments,studies have explored a solution that to load sulfur onto porous conductive carbon matrix and found that it is conducive to realize a superior sulfur utilization and cycling performance.On the one hand,conductive carbon can enhance the conductivity of the electrode and porous characteristics also conduce to suppress sulfur expansion.On another,by modifying the carbon surface,the lithium polysulfide can be anchored by the functional groups thus inhibiting polysulfide shuttle.In this article,we started with cathode structure design,and construct a specific carbon structure with N doping and suitable catalysts.The concrete content are as follows:（1）By synthesizing polystyrene as template,dopamine and FeCl₃ were used as carbon source and dopant,respectively.After dopamine was polymerized upon polystyrene under certain condition,a carbon material with certain structure was directly obtained through annealing treatment.The influence of Fe³⁺intake on carbon structure and the chemical properties was further discussed.After sulfur/carbon composites were synthesized by melt diffusion method.The electrochemical performances of lithium-sulfur battery was studied.The research results show that with FeCl₃ intake,a hollow carbon sphere structure can be formed,while without FeCl₃,it would be a bowl-like structure（NC）.After annealing process,Fe₃O₄ nanoparticles were embedded on the surface of the hollow carbon sphere（Fe₃O₄-NC）.Further characterization showed that Fe₃O₄-NC has a larger specific surface area than NC,which are 559 m²g^-1 and 190 m²g^-1,respectively.By loading 60 wt%of sulfur and assembling lithium-sulfur cells,S/Fe₃O₄-NC exhibits better electrochemical performance than S/NC,and a reversible capacity of 1264m Ah g^-1 at 0.1 A g^-1 is also achieved.At 1 A g^-1,after 500 cycles,530 m Ah g^-1 is still maintained.Further electrochemical characterizations also prove that Fe₃O₄-NC has better kinetic characteristics.（2）Chitosan and ammonium molybdate are used as carbon source and dopants,respectively.Silica microspheres are synthesized by traditional st?ber method and used as hard templates.After certain treatments,a homogeneous mixture is formed.After annealing and washing template by HF,a specific carbon structure is obtained.The carbon structure and the corresponding chemical properties are compared with or without ammonium molybdate dopants.After loading sulfur and assembling lithium sulfur cells,the electrochemical performances are studied.The results show that both the two carbon materials are porous honeycomb structures,and the doping element Mo is uniformly distributed in the carbon network in the form of Mo₂C nanodots on the carbon surface.The specific surface areas of Mo₂C@PCN and PCN are 632 and 363 m²g^-1,respectively.When sulfur is loaded with 70%sulfur content,Mo₂C@PCN exhibits higher electrochemical performances than PCN,which enables a high reversible discharge capacity of 1188 m Ah g^-1 at 0.2 A g^-1 and a high stable performance of 80%capacity retention over 600 cycles.Further electrochemical characterization also verified the catalytic effect of Mo₂C to polysulfide.In a word,we integrate physical confinement,chemical adsorption and electrochemical catalysis to restrain shuttle effect and thus a good cycling performances for lithium sulfur batteries are achieved.