Preparation And Electrochemical Performance Of V-N-C Cathode Materials For Li-S Battery

Among the promising candidates for next-generation energy storage systems,Lithium-sulfur（Li-S）batteries attract worldwide attention for their high theoretical specific density(1675 m Ah g^-1),high theoretical energy density(2600 Wh kg^-1),abundant resources,low cost,and non-toxicity.However,the insulating nature of S and Li sulfides（discharge products Li₂S or Li₂S₂）,the large volume change during charge/discharge,and the"shuttle"effect caused by the dissolution of Li polysulfides（Li PS）hindered the practical applications of Li-S batteries.Especially the"shuttle"effect,which will lead to irreversible loss of the effective capacity of the Li-S battery and impair the battery life.Previous studies have been focused on the physical confinement and/or chemical binding of Li PS to restrain the shuttling effect,but the slow redox reaction kinetics of Li PS still limits the performance of Li-S batteries.In this thesis,based on different metal-polymer interactions,two kinds of vanadium-nitrogen-carbon（V-N-C）three-dimensional hierarchical porous materials with excellent activity for the Li PS conversion were prepared.The developed V-N-C cathode materials of lithium-sulfur batteries can improve the redox kinetics of Li PS conversion and the resulting battery performance.The main research contents and experimental results are described as follows:（1）Preparation of V-N-C cathode materials based on the hydrogen bond/electrostatic interactions of metal-polymer:we have synthesized atomically-dispersed three-dimensional hierarchical porous V-N-C cathode materials with high catalytic activity for Li PS conversion for the first time via freeze-drying and high-temperature pyrolysis methods.During the synthesis,natural polymer chitosan is used as the carbon source and dispersant,ammonium metavanadate（NH₄VO₃）is used as the metal source,and cyanamide is used as the nitrogen source.The protonated-NH₂ group in chitosan and the VO₃^-ion in NH₄VO₃ are combined through electrostatic and hydrogen bonding interactions to form a metal-organic polymer supramolecular framework,which can anchor and protect individual V atoms from agglomeration.During the discharge/charge processes,the V-N-C cathode with unique V-N_x/C active sites can be used as an effective catalyst to accelerate the redox kinetics of Li PS,and the hierarchical porous carbon structure is favorable for the diffusion of Li⁺/e^-and inhibit the shuttle of Li PS.As a result,the S/V-N-C composite delivers high specific capacities of 1111.2 m Ah g^-1 and 573.6 m Ah g^-1 at 0.5C and 5C,respectively,and still maintains the specific capacity of 392.6 m Ah g^-1 after 500 cycles at 1C with a low capacity decay rate of 0.087%per cycle.The rate performance of the developed V-N-C cathode is superior to that of M-N-C and carbon/metal compound composites for Li-S batteries reported in the literatures.（2）Preparation of c-V-N-C cathode materials based on the coordination interaction of metal-polymer:using natural polymer sodium alginate（SA）as the carbon source,VOSO₄ as the metal source,and cyanamide as the nitrogen source,we have obtained c-V-N-C cathode materials based on the coordination interaction of V-SA.The oxygen containing groups such as-COOH in SA and the VO₂⁺in VOSO₄ are complexed by coordination bonds to form a metal-organic polymer hydrogel.Compared to the former V-N-C,the c-V-N-C cathode based on metal-polymer complexation exhibits larger specific surface area and pore volume,leading to a better cycle performance in Li-S battery.The S/c-V-N-C composite delivers a high specific capacity of 1123.4 m Ah g^-1at 0.5C and maintains 573.3 m Ah g^-1 at 5C.It can still maintain a high capacity of 683.8m Ah g^-1with a low capacity decay rate of 0.087%per cycle after 300 cycles at 1C,which is much higher than V-N-C cathode materials based on hydrogen bond/electrostatic interactions of metal-polymer(with a specific discharge capacity of513.0 m Ah g^-1 after 300 cycles at 1C).