Supercritical Carbon Dioxide Deposition Of Composite Cathode Materials For Lithium-Sulfur Battery

Owing to their high theoretical specific capacity(1,675 mAh g^-1),high theoretical energy density(2,600 Wh kg^-1),and environmentally friendly and naturally abundant raw material sulfur,lithium-sulfur?Li-S?batteries has been considered as an valuable asset for the research&development of next-generation secondary batteries.However,unlike the simple ion deintercalation process of lithium ion batteries,the electrode reaction of Li-S batteries involves a multi-step and multi-phase complex process.The long-chain lithium polysulfide produced during the reaction process can be easily dissolved in the organic electrolyte to cause a“shuttle effect”and hence irreversible attenuation of battery capacity,which is the main root cause limiting the commercialization of Li-S batteries.At present,effort to overcome the“shuttle effect”of Li-S batteries mainly focuses on the development of cathode materials,using uniquely designed structure to confine the dissolution of polysulfides and doping with polar materials to enhance the adsorption to polysulfide.Since the sulfur host materials are mostly porous and have complicate surface features,it remains a difficulty to uniformly deposit sulfur or other active substances onto the host materials.This research mainly aims at the structure design and synthetic method of the cathode material.On one hand,the strong chemical adsorption of metal oxides to polysulfides was combined with the unique structure and excellent conductivity of porous carbon to alleviate the"shuttle effect"in Li-S batteries.On the other hand,supercritical carbon dioxide?scCO2?with excellent diffusivity,permeability and wettability was used to uniformly deposit highly dispersed metal oxides and sulfur on conductive porous carbon to improve the utilization of active materials.The main contents of the research are summarized as follows:?1?A conductive material as sulfur host in cathode of Li-S batteries,N-doped nanosheet-like porous carbon?NNPC?was synthesized using orange peel as carbon source.NNPC/S-M composite material was prepared according to the combination of two reported conventional methods,impregnation and melt permeation,and NNPC/S-CO₂ composite material was prepared by scCO₂ deposition of sulfur.The composition,structure and electrochemical properties of the two materials were tested.The results show that the scCO₂ deposition method is more favorable for sulfur to penetrate into the high aspect ratio pores of NNPC whilst maintaining the integrity of porous structure.Compared to NNPC/S-M,NNPC/S-CO₂ exhibits higher sulfur loading.In testing Li-S batteries,the initial discharge specific capacity of NNPC/S-CO₂ is 1065 mAh g^-1 at 0.2 C,higher than NNPC/S-M.?2?A nano-sheet porous carbon?NSPC?was synthesized using corn flour as carbon source.NSPC/CeO₂ composites were prepared by scCO₂ assisted deposition of CeO₂ on NSPC,and NSPC/CeO₂/S composites were fabricated by scCO₂deposition of sulfur on NSPC/CeO₂.For comparison,a NSPC/S composite was also synthesized similarly.Structural characterization of NSPC/CeO2 shows that CeO2nanoparticles are uniformly dispersed on NSPC,and the particle size is only about 3nm,indicating that scCO₂ deposition is beneficial to alleviate the agglomeration of metal oxide nanoparticles and control the size of the nanoparticles.The electrochemical performance test was used to study the effect of CeO₂ in the electrode reaction process.The initial discharge capacity of NSPC/CeO₂/S is 1538 mAh g^-1 at0.1 C,and decreases to 929 mAh g^-1 after 100 cycles.Compared with NSPC/CeO₂/S cathode,the NSPC/S cathode delivers a lower initial discharge capacity of 1194 mAh g^-1 and decreases to 482 mAh g^-1 after 100 cycles.The results show that both the cycling and rate performance of NSPC/CeO₂/S are significantly better than NSPC/S,which is mainly attributed to the excellent catalytic activity and rich active sites of CeO₂.?3?A flower cluster-like porous carbon?FPC?material was synthesized by using sucrose as carbon source.FPC/ZrO₂/S was prepared as a cathode material for Li-S batteries by scCO₂ assisted deposition.Morphological and structural characterization of the synthesized materials was carried out to study the preparation and dispersion of ZrO₂ nanoparticles.The effect of ZrO₂ on the electrochemical performance of the Li-S battery was investigated by comparing FPC/ZrO₂/S with FPC/S.The initial discharge specific capacities of FPC/ZrO₂/S and FPC/S are 1058.9 mAh g^-1 and 930.8mAh g^-1,respectively.After 100 cycles,the specific capacities of two composite electrodes decreases to 760.8 mAh g^-1 and 534.2 mAh g^-1,respectively.What's more,at 1 C,the capacity retention of FPC/ZrO₂/S is 93.4%after 100 cycles,corresponding to an average capacity loss of around 0.06%per cycle.Electrochemical performance tests prove that the incorporation of ZrO₂ nanoparticles is beneficial to improve the cycling performance and alleviate the"shuttle effect"of Li-S batteries.