Construction Of Carbon/Metal Compound Heterostructures And Their Applications In Lithium Sulfur Batteries

Lithium-sulfur batteries(Li-S batteries)with high theoretical energy density are one of the most promising candidates for next generation high-energy and high-efficiency electrochemical energy storage systems.Although the development of lithium-sulfur battery has made great progress in recent years,the commercial application of Li-S batteries still confronts the following challenges:(1)Poor electronic and ionic conductivity of sulfur and its reduction products Li₂S leads to slow electrochemical reaction kinetics;(2)Large volume change of sulfur cathode during charging and discharging results in damage of the electrode structure;(3)The high solubility of the intermediate product lithium polysulfide(Li PS)in the electrolyte produces the?shuttle effect?,causing the decay of the capacity.In order to produce the Li-S batteries with high energy density,low cost and prospects towards commercialization,we selected low-cost raw materials to prepare the composite of metal compounds supported on carbon materials.Carbothermal reduction of metal sulfate,water-soluble salt template method,which are available in large scale production,combining with spray drying and chemical vapor deposition(CVD)method were used to prepare the composite.The anchoring effect,catalytic mechanism,and electrochemical performance of the composite used as the active material carrier were studied.The main research contents and results are as follows:Co₉S₈/CoO core-shell heterostructures were prepared by carbothermal reduction method using glucose and CoSO₄ as raw materials.Electrochemical characterization and density functional theory calculations show that the Co₉S₈/CoO heterostructures can capture Li PSs effectively and exhibit significant electrocatalysis function.The outer CoO has a strong anchoring effect on Li PS,and the internal Co₉S₈ possesses a stable catalytic surface,especially for the selective catalysis of the slow kinetic step of Li₂S₂?Li₂S.The heterointerface formed between CoO and Co₉S₈ enhances the capture and electrocatalysis to polysulfides.The Li-S cells with the Co₉S₈/CoO-G coated separator exhibit a Coulombic efficiency(CE)of approximately 100%and deliver a specific capacity of 925 m Ah g^-1 with a fade rate of 0.049%per cycle over1000 cycles with a high S loading(2.5 mg cm^-2).By using citric acid and CoSO₄ as raw materials,Na Cl as the template,the Co₂N/CoO heterostructures supported on N,S-codoped three-dimensional carbon network(3DC)(N,S-3DC-Co₂N/CoO)were obtained via spray drying,calcination in Ar,and heat treatment in ammonia atmosphere sequentially.It was found that N,S-3DC-Co₂N/CoO had a significant catalytic effect on the reaction of Li₂S₄?Li₂S.The catalysis effect originates from the heterostructure consisting of CoO outer layer and Co₂N inner layer,which can anchor the polysulfides simultaneously.Microscopic analysis shows that Co₂N/CoO was embedded in the pores left on the surface of 3DC after the reduction of the CoSO₄ particles,and the remaining space was used to fill sulfur.The special structure achieves sufficient contact between sulfur and Co₂N/CoO to further improve the utilization of active materials,thus obtaining a higher capacity.The initial discharging capacity of the S@N,S-3DC-Co₂N/CoO electrode is 1135m Ah g^-1 with a decay rate of 0.051%per cycle over 1000 cycles at 1C.The composite of Ni particles supported on 3DC was prepared by spray drying and calcination using glucose and NiNO₃ as raw materials with Na Cl as template.Carbon nanotubes(CNTs)were grown on the surface of 3DC by in-situ CVD to obtain the 3DC-CNTs-Ni composite.As a current collector for Li metal deposition,the tip discharge effect of CNTs guides the deposition of Li,so that the Li metal grows into sheets,avoiding the formation of Li dendrites.The 3DC-CNTs-Ni current collector can stably plating/stripping with a cycling capacity of 4 m Ah cm^-2 of Li over500 h,and its CE is maintained above 98.5%.A 3DC-CNTs-NiS₂ hierarchical composite is obtained by sulfuration of 3DC-CNTs-Ni composite.The3DC-CNTs-NiS₂ hierarchical composite exhibits high capacity,excellent rate performance,and stable cycling performance.With high S loading(6.5 mg cm^-2,the S content in the whole electrode is 76.5 wt.%),the S@3DC-CNTs-NiS₂ delivers a high area capacity of 4.8 m Ah cm^-2.The soft-packed full battery using S@3DC-CNTs-NiS₂ cathode and Li@3DC-CNTs-Ni anode delivers a high initial capacity of 1200 m Ah g^-1 with the capacity retention of 87%after 50 cycles.Lithium sulfide@porous carbon composites(Li₂S@PC)were obtained by one-step calcination using glucose and Li₂S as raw materials.Graphite-Li₂S@PC full cell was assembled using Li₂S@PC as the cathode,graphite as the anode,and the DOL/DME solvent with LiNO₃ additive as the electrolyte.The full cell exhibits stable cycling performance at 0.5 C over 100 cycles.Li₂S/Co₉S₈@PC composite was in-situ prepared by carbothermal reduction with addition of CoSO₄in raw materials.The Graphite-Li₂S/Co₉S₈@PC full cell with Li₂S/Co₉S₈@PC cathode and prelithiated graphite anode delivers an initial discharge capacity of 445 m Ah g^-1 at 0.5C,which is39%higher than that of Graphite-Li₂S@PC full battery.The CE of the Graphite-Li₂S@PC full battery is 99.5%,which is much higher than that of the Graphite-Li₂S@PC full cell(95%).It is indicated that the adsorption of polysulfide by Co₉S₈ inhibits the shuttle effect,and the catalytic effect increases the utilization of active materials.In summary,based on simple carbothermal reduction reactions,the metal compounds/carbon composites were prepared.The anchoring and catalytic functions of Li PS on the composite were improved through structural optimization design.Combining with the optimized metal Li anode and graphite anode,Li-S full cells with high-sulfur load and high-capacity were realized.