Study On Design,preparation And Performance Of High Load Porous Composite Sulfur Anode

The energy density of lithium ion batteries based on intercalation reaction is much higher than that of lead-acid,nickel-cadmium,nickel-hydride secondary batteries,and also has great advantages in terms of cost,cycle life and environmental safety.Therefore,lithium ion batteries have get widely used in digital products and electric vehicles.However,lithium ion batteries are still difficult to meet the high-energy-density demand of marcket,and the real energy density of lithium-ion batteries is close to their theoretical limits.Now that sulfur based on conversion reaction has ultra-high specific capacity,and is abundant in nature,low-cost and environmentally friendly,lithium-sulfur batteries have become one of the hotspots in the research of new-generation energy storage devices.However,lithium-sulfur batteries have several serious problems,prevent from achieving large-scale application.For example,1)the ionic and electronic conductivity of sulfur and its discharge products are very low,resulting in its low utilization;2)The conversion between sulfur and lithium sulfideresults in huge volume changes?about 80%?,making it easy for sulfur to detach from the electrode sheet;3)Lithium polysulfides,the intermediates formed in cycle process,are soluble in electrolytes and migrate to anode side,leading to severe shuttle effects and undesarible side reactions with lithium metal.In our work,several new methods were proposed to optimize the cathode of lithium-sulfur batteries and improve the electrochemical performance under high load.?1?Design a porous sulfur cathode with high sulfur load,prepared by freeze-drying method.The traditional sulfur-carbon composite electrode shrinks violently,cracks and even falls off from Al foil during the drying process due to the evaporation of the solvent.It was proved that there is a lot of three-dimensional porous structures in the electrode after the freeze-drying process by SEM,which can alleviate the huge volume change of sulfur electrode,and promote the electrolyte infiltrating the electrode.Polar water-based binder was adapted to trap polysulfide ions,suppressing the shuttle effect.As a result,the electrochemical performance of as-prepared porous electrode was improved greatly.SP-S@KB electrode,with the sulfur load of 18 mg cm^-2,still had an initial specific capacity of 845.7 m Ah g^-1 at 0.05 C.But after 45 cycles,the capacity only maintained at 499.7 m Ah g^-1,the capacity retention was only 59.1%.The cyclic stability of the electrode needs to be further improved,but the three-dimensional porous structure has become the basis for further optimization.?2?Design of an oxygen-deficient ferric oxide electrocatalyst was proposed to accelerate the conversion of soluble polysulfides to insoluble end products and inhibit the diffusion of polysulfides.The results of theoretical calcularitionn and symmetric cells indicated that oxygen deficiencies improved the catalytic activity and adsorption ability to polysulfides.It was further proved that the oxygen-deficient iron oxide remained stable during the charge and discharge process by in-situ XRD.As a result,when introducing a samll amount of Fe₂O_3-x,the sulfur electrode exhibited excellent electrochemical performance.At an ultra high rate of 4 C,after a stable cycle of 500cycles,the capacity can still be maintained at 512 m Ah g^-1.And Fe₂O_3-x significantly reduced the overpotential,improved the utilization of sulfur and cycle stability.In addition,a porouselectrode with a sulfur areal load of up to 12.73 mg cm^-2 was prepared by freeze-drying method.It has a initial discharge specific capacity of 12.24 m Ah cm^-2 at 0.05 C and achieved a stable cycle of 60 cycles.?3?Tiblack composite?Ti?materials were designed to solve the problem that the combination of sulfur and other inert additives result in the decrease of energy density.TiS₂ is a promising alternative of inert additives.On one hand,TiS₂ can provide additional specific capacity.On the other hand,TiS₂ has the similar function with inert addictives,such as good ionic/electronic conductivity and strong adsorption to polysulfides.At low sulfur loading,Tielectrode exhibited a high rate performance of 766.1 m Ah g^-1 at 5 C and a long stable cycle life of 300cycles at 1 C.However,under high sulfur loading,Tielectrode performed poorly.SP-TiKB electrode prepared by the freeze-drying method with a sulfur loading of 5.50 mg cm^-2 exhibited a specific discharge capacity of 800 m Ah g^-1 at 0.1C,and only stably run for 30 cycles.The introduction of TiS₂ into the sulfur may affect the viscosity of the slurry,thus affecting the pores and their porous skeleton,resulting in the failure of the electrode to achieve the desired effect.