Design And Synthesis Of Carbon-Based Electrocatalysts For Lithium-Sulfur Batteries

There has been a growing interest in renewable and sustainable energy sources such as wind,solar,and hydropower.Because renewable energy sources like solar and wind power are intermittent,batteries are essential for storing the excess energy generated during periods of high production and releasing it when demand is high or renewable energy sources are not producing enough electricity.As a promising candidate for a futuristic energy storage system,lithium-sulfur batteries（Li-S）are worth exploring because they have a low cost,little environmental impact,and a fascinatingly high theoretical energy density(2600Wh kg^-1).Intensive research has been focused on this system,and significant improvement has been achieved.However,the application of Li-S batteries suffers from some issues that impede their commercial applicability.The Li-S battery’s performance is determined by the kinetics of the conversion process between the various sulfur species（sulfur,LiPS,Li₂S₂/Li₂S）.Previous research has shown that electrocatalysis may greatly impact the kinetics of Li-S redox processes,leading to improved Li-S battery performance.Nevertheless,a fundamental breakthrough is required to eliminate the shuttle effect in Li-S batteries.Therefore,alleviating the inherent "shuttle effect" has become a hot research topic in Li-S batteries.In this respect,building a sulfur host with a strong affinity towards LiPS and a high catalytic capacity is still essential.Such a host not only has to be able to confine LiPS chemically,but it also needs to be able to enhance the kinetics of the LiPS redox process.To address the challenges facing Li-S batteries,this thesis aims to enhance the cathode materials.Our approach involves exploring different matrices to confine or accelerate sulfur species.In particular,we have investigated the use of metal-free multiwalled carbon nanotubes（MWCNTs）with varying oxygen defect content and types,including high oxygen defect（H-CNT）,medium oxygen defect（M-CNT）,and low oxygen defect（L-CNT）,as cathode materials for Li-S batteries.In addition,we designed a new composite architecture with iron atoms（Fe-As）and iron nanoparticles（Fe-NPs）incorporated into nitrogen-doped carbon nanotubes（Fe-NCNT）produced through pyrolysis of Fe-ZIF-8 for facilitating sulfur reduction reactions and accelerating LiPS conversion.The main contents are shown as follows:1.We used metal-free electrocatalysts carbon nanotubes,oxygen defects on the commercial carbon nanotubes,as cathode materials for Li-S batteries.According to the XPS results,M-CNT has the highest C-O content,which induces high catalytic activity toward LiPS conversion.The M-CNT provides a conductive porous architecture for facilitating lithium-ion,electron transport,and entrapment of LiPS,thus restraining the shuttle effect.The oxygen-containing defects（ODs）on M-CNT not only provide active sites that enhance LiPS conversion but also improve the electrical conductivity of the carbon matrix.Therefore,the existence of the oxygen functional group（C-O）on the surface of the M-CNT plays a vital role in enhancing the catalytic capability.The M-CNT/S composite exhibited a high rate performance at 4 C with capacity retention of 77%,demonstrating long cycle stability having 448.1 mAh g^-1 remaining up to 600 cycles at 1 C.The increased electrochemical performances of M-CNT/S cathodes may be attributed to the C-O bonds present on the surface of M-CNT.Our results will open the door to new applications of surface-oxidized nanomaterials for catalyzing sulfur reduction reactions（SRR）in Li-S batteries.2.The hybrid architecture of Fe-As and Fe-NPs co-embedded within N-doped carbon nanotubes CNTs（Fe-NCNT）as a catalyst and sulfur host for high-performance LiS batteries was successfully prepared through pyrolysis process of ZIF-8 with iron nanocarbonyl（Fe₂（CO₃）₉）.Iron nancarbonyl plays a significant bifunctional role,facilitating CNTs growth during the pyrolysis process and as a source for catalyst active sites of Fe-As and Fe-NPs.As informed by experiment results,the homogeneous dispersion of Fe-As on Fe-NPs synergistically provides efficient active sites to facilitate the diffusion,strengthen the affinities,and promote the conversion reactions for polysulfides leading to an increase in the chemisorption of polysulfides and enhanced the redox reaction kinetics during charge/discharge process of Li-S battery.In addition,the N-doped carbon nanotube NCNT not only improved the electrical conductivity of the cathode but also offered practical Li+transport pathways and exposed the Fe species sites for catalyzing polysulfide conversion.The Fe-NCNT/S composite demonstrated a remarkable initial specific capacity(1502.6 mAh g^-1)and reversible capacity(597.8 mAh g^-1)after 500 cycles at 1 C,with capacity decay as small as 0.069%per cycle.Additionally,the FeNCNT/S electrode with mass sulfur loading of 2.3 mg cm^-2 also showed a high reversible capacity of 611.8 mAh g^-1 at 0.5 C after 200 cycles.