Design And Electrocatalytic Performance Research Of Graphene-based Single Atom Catalysts In Sulfur Cathodes

With the rapid development of electric vehicles and portable electronic devices,people have put forward higher demands for energy storage systems.At present,the traditional lithium-ion batteries have reached its theoretical specific energy density through considerable efforts of researchers,and it is difficult to improve further.Therefore,the development need of efficient,safe,and reliable energy storage systems is urgent.Lithium-sulfur（Li-S）batteries have attracted much attention of researchers due to their high theoretical specific capacity(1675 m A h g^-1)and energy density(2600 Wh kg^-1).In addition,the active material of sulfur takes the advantages of high abundance,low cost,and environmental friendliness.Therefore,Li–S batteries are regarded as the most promising alternatives to lithium-ion batteries in the next-generation energy storage devices.However,Li-S batteries are still limited by several problems,such as the low conductivity of sulfur and Li₂S,volume changes during the charge/discharge process,“shuttle effect”of polysulfides,and the sluggish cathode kinetics,which result in the low utilization of sulfur and poor stability of long-term cycling.These issues severely restrict the practical application of Li-S batteries.In recent years,these limitations have been alleviated by utilizing electrocatalysts,which can effectively restrain the"shuttle effect"and boost the conversion of polysulfide.Since only the exposed surfaces and edges of these electrocatalysts are active,the sulfur host materials typically require a relatively high catalyst loading（often>20 wt%）.By contrast,single-atom catalysts（SACs）can effectively reduce the metal content in the material while maintaining efficient catalytic effect.In this paper,a series of sulfur hosts were efficiently constructed by the design of SACs.These materials have significantly enhanced the electrochemical performance of Li-S batteries and were expected to be applied in commercial Li-S batteries.The research contents of this paper are summarized as follows:（1）The main difficulty in the preparation of SACs is that the single atoms are thermodynamically unstable and prone to form clusters or nanoparticles particularly at elevated temperatures,because of their high surface free energy.In order to avoid this result,the metal atoms were loaded onto the surface of graphene via theπ-πinteraction and electrostatic attraction between metalloporphyrin complexes and graphene oxide,and the Co-N bond in the porphyrin complex effectively prevents the formation of Co-Cobonds.The aggregated form of CoSACs（Co-NG（800））was obtained by annealing at 800℃.Although the distance between adjacent Coatoms were within the van der Waals radii of two Coatoms,Co–Cobonds were not detected by the X-ray absorption fine structure（XAFS）.Therefore,each Coatom in the aggregates can still function as a SAC.X-ray absorption near-edge structure（XANES）spectrum illustrate that the catalytic effect of Co-NG（800）composites on electrochemical reactions in lithium-sulfur batteries was attributed to the sustained and stable strong interaction between Coand sulfur.Electrocatalytic testing proved that CoSACs can effectively promote the conversion kinetics between sulfur species,increase the deposition of Li₂S and the diffusion coefficients of lithium ions.The Li?S batteries prepared from S@Co-NG（800）showed outstanding performance,including a high rate capacity(648 m A h g^?1at 6 C)and excellent cycling stability(505 m A h g^-1at 0.5 C after 600 cycles,corresponding to a decay of only 0.08%per cycle).Moreover,under the condition of high sulfur loading,Li?S cells prepared from S@Co-NG（800）still exhibit excellent performance.When the sulfur load is 11.8 mg cm^-2,the area specific capacity of the Li-S battery can reach12.52 m A h cm^-2,which is much higher than the current requirements for the commercialization of lithium-ion batteries(4 m A h cm^-2).（2）The morphology of CoSACs was regulated by different annealing temperatures.The Coatoms exist in an aggregated form（Co-NG（800））when annealed at 800°C.While the annealing temperature was reduced to 400°C,Coatoms exist in isolated form（Co-NG（400））.Electrochemical tests demonstrated that the catalytic performance of aggregated form of SACs in Li-S batteries is superior than that of isolated form of SACs.The Li-S batteries prepared with Co-NG（800）composites showed better rate capabilities,higher specific capacity and more stable cycle performance than the batteries prepared with Co-NG（400）composites.To further clarify the electrocatalytic effect of the single-atom Coaggregates,a series of DFT theoretical calculations were performed.Three SACs models（isolated form Co-NG-1.37,aggregated form Co-NG-0.40 and Co-NG-0.32）are designed according to the high-angle annular dark field-scanning transmission electron microscopy（HAADF-STEM）and XAFS results.The binding energy（E_b）between the three models and different polysulfides as well as the changes of Gibbs free energy（（35）G）of each reaction step during the discharge of Li-S batteries on the surface of these three models were calculated,which explained the superior catalytic ability of the aggregated form of SACs to the redox reactions of sulfur cathode.（3）The water-soluble nickel porphyrin was combined with graphene oxide using the same mathod,and then thermally annealed at 800℃ to obtain Ni SACs（Ni-NG）with an isolated form,which may be responsible for the higher thermal stability of nickel porphyrin complexes than cobalt porphyrin complexes.Due to the low nickel content,the conductive network of graphene is well-maintained,and the conductivity of Ni-NG composites even exceeds the commonly used conductive carbon black.The excellent conductivity of Ni-NG composites realizes the assembly of batteries without conductive agent,which effectively improved the sulfur content in the sulfur cathode.Electrochemical tests showed that Ni SACs provided fast transfer channels for lithium ions and electrons as well as enhanced the conversion rate of lithium polysulfide during the charge/discharge process of Li-S batteries.The Li-S batteries prepared from S@Ni-NG exhibit ultra-high specific capacity(1417m A h g^-1at 0.1 C and 582 m A h g^-1at 6 C)and excellent cycling stability(the specific capacity remained above 500 m A h g^-1after 800 cycles at 0.5 C,corresponding to a decay of only 0.058%per cycle).Due to the strong suppression of“shuttle effect”by Ni SACs,Q₂/Q₁values of the Li-S batteries prepared from the S@Ni-NG are closer to the theoretical value.