Design,Fabrication And Electrochemical Properties Of Cathode For Lithium Sulfur Battery

Lithium-sulfur batteries have attracted extensively attention and have been extensively studied due to their low cost,high natural abundance,environmental friendship and high theoretical specific capacity.The lithium-sulfur battery is considered as one of the candidates for next-generation energy storage systems.However,due to the poor cycle performance,low Coulombic efficiency,low active material utilization,low rate performance,self-discharge and many other issues,commercialization of lithium-sulfur batt,eries has been difficult to achieve.The main challenges of the sulfur cathode are described as follows:(1)the conductivity of the active material S and discharge product Li2S is poor;(2)the polysulfide intermediates generated in the charge-discharge cycle are easily dissolved in the organic electrolyte,leading to the "shuttle effect";(3)the volume expansion of S cathode during charging and discharging cycle.Consequently,the dissertation aims at improving or alleviating the inherent problems of sulfur cathode.The main focus is to enhance the conductivity and reduce the dissolution and migration of polysulfides during cycling.Based on the above key factors,novel composite materials are designed and prepared and the electrode structure is fabricated to enhance the performance of lithium-sulfur batteries.The research is divided into the following five parts:(1)The near-spherical S-CNT composites(S-CNT-PEG-NNH)were prepared by simple ball milling and with a solution coating method to improve the electrochemical performance of lithium-sulfur batteries.The results showed that the performance of cathode made by the SCNT-PEG-NNH composite materials was improved.When charged-discharged at the current density of 200 mA g-1,the specific capacity was 974 mAh g-1 at the initial discharge and 575 mAh g-1 was obtained after 200 cycles with a capacity retention ratio of 59.0%for the Li/(SCNT-PEG-NNH)cells.This is because that the spherical conductive frame constructed by CNT ensures a continuous e-/Li+ transport path and accommodate large volume expansion.Most importantly,the pore formed by CNT winding,PEG and polar group of layered nickel hydroxide(NNH)can well inhibit polysulfides dissolution.(2)The monodispersed sulfur nanoparticles(S)-partially reduced graphene oxide(prGO)-PDA(S-prGO-PDA)composite was prepared with a simple method to study the effect of the reduction degree of prGO on the electrochemical performance of sulfur cathode.The results showed that the cycle performance,coulombic efficiency and capacity retention rate of S-prGO-PDA composites were improved when the reduction degree of prGO was well controlled.After 100 cycles at a current density of 200 mA g-1,the specific capacity of the S-prGO-PDA composite was around 650 mAh g-1 with a Coulombic efficiency of 98%.At the same time,this work also proved that control of the reduction degree of graphene oxides can adjust the ability to fix sulfur and the conductivity of the S-prGO-PDA composite,leading to the significantly enhanced electrochemical performances.(3)A sulfur-rich carbon polysulfide polymer was synthesized for the sulfur cathode of lithium-sulfur battery.The chemical-confinement effect was used to inhibit the dissolution and shuttling of polysulfides to improve the electrochemical performance of lithium-sulfur battery.The results revealed that the carbon polysulfide polymer exhibited similar electrochemical activity to Sg,excellent cycling stability and high Coulombic efficiency.This is due to that the carbon polysulfide polymer structure has the following advantages:(1)the carbon polysulfide polymer has a highly uniform structure and a high sulfur content;(2)the strong chemical interaction of sulfur with the carbon framework(C-S bonds)inhibits the shuttle effect;(3)the on electrons of the carbon polysulfide polymer enhance the transfer of electrons and Li+;(4)conductive graphene can provide the paths for fast electron transport and accommodate sulfur volume expansion.(4)A double current collector sulfur cathode(abbreviated as Al-S-VGCF)was constructed to improve the electrochemical performance of lithium-sulfur battery.The results revealed that the double current collector sulfur cathode showed high S utilization and good cycling stability.The reversible specific capacity and area specific capacity of the double collector electrode were up to 650 mAh g-1 and 3 mAh cm-2,respectively,with a good cycling stability even when the loading of S was?5 mg cm-2.The electrochemical performance of the double current collector sulfur cathode is improved because the under current collector Al foil and the upper current collector VGCF layer next to the S active material layer can act as double current collectors to accelerate electron transfer to the S layer,while acting as physical barriers to compromise the volume change of the sulfur cathode during cycling.In addition,the VGCF layer which contains the critical component of PEO works as an effective reservoir to trap the dissolved polysulfides within the cathode region,leading to the suppression of the shuttle effect.(5)Multilayer graphene-supported Ni nanoparticles(G/Ni)were prepared with a simple carbothermal reaction to facilitate polysulfides conversion while ensuring a high surface area to contact polysulfides.The results showed that G/Ni composites could promote the conversion of polysulfides well,that was to say,the specific capacity and Coulombic efficiency of lithium-sulfur batteries were improved.Meanwhile,the in-situ electrochemical Raman spectroscopy,combined with ex-situ XPS and ex-situ XRD techniques,were used to study the process and mechanism of G/Ni composites to promote polysulfides conversion reaction.