Dual ‐ single ‐ atoms of Pt – Co boost sulfur redox kinetics for ultrafast Li – S batteries

Applications of lithium – sulfur (Li – S) batteries are still limited by the sluggish conversion kinetics from


| INTRODUCTION
The growing demand for electric vehicles and portable electronic devices has largely promoted the development of high-energy-density rechargeable batteries.2][3][4][5] Nevertheless, Li-S batteries are still confronted with several issues hindering their practical applications, for instance, the insulating nature of sulfur and its discharge products, 6 the soluble polysulfides (Li 2 S x , 4 ≤ x ≤ 8) shuttle between anode and cathode, namely, shuttle effect, 7 and the sluggish conversion kinetics from liquid-state Li 2 S x to solidstate Li 2 S. 8 These aforementioned drawbacks result in low sulfur utilization, low Coulombic efficiency, and fast capacity decay. 9,10Therefore, to overcome the aforementioned issues, the introduction of active centers with absorption and catalytic capabilities is crucial for the cathode to improve sulfur utilization and accelerate the reversible conversion between liquid-state Li 2 S x and solid-state Li 2 S. 11 Up to now, carbon nanotubes with high surface area and porous structure have been frequently used as cathode materials for Li-S batteries. 12,13However, the weak polarity of carbon materials shows low absorption capacity for Li 2 S x . 14Recently, carbon materials doped with nonmetallic (N, O, S, P, et al.) elements have been shown to significantly enhance the fixation and catalytic sites of Li 2 S x , thus improving the absorption capacity and catalytic activity of carbon materials, 15,16 but it is still not enough to stabilize the capacity of Li-S batteries only by nonmetallic element-doped carbon materials. 12][20][21][22][23][24] However, the introduction of a higher number of nanoparticles in carbon materials not only leads to the aggregation of nanoparticles but also reduces the surface area and conductivity of the carbon host, which results in low sulfur loading and utilization.Until now, most of the reported catalysts were based on the nanocatalysts 25 or nanoclusters, 26,27 where only the outer atoms could be acted as active sites, thereby limiting the catalytic performance of the catalyst.
9][30][31] Among various SACs, cobalt (Co)-based SACs are most often used in Li-S batteries because of their excellent catalytic properties for Li 2 S x . 25,30,31For example, Zhou et al. 11 successfully synthesized Co SAs embedded in nitrogen-doped graphene (SACo@NG) as catalyst cathodes.The result showed that an initial capacity of 1120 mAh g −1 could be obtained at 0.2 C and remained at 675 mAh g −1 after 100 cycles in the SACo@NG electrode.Nevertheless, the capacity was still far away from the theoretical capacity of Li-S batteries, due to poor sulfur utilization. 32By contrast, platinum (Pt)-based SACs have adsorption effects on materials, which can raise the utilization of materials.However, the catalytic properties of the Pt SACs are not strong.For instance, Fang et al. 33 deposited Pt SAs on N-doped carbon frameworks by atomic layer deposition (ALD) method, which exhibited high hydrogen evolution activity with only 19 mV overpotential in 0.5 M H 2 SO 4 and only 10 mA cm −2 in 1.0 M NaOH 46 MV.The near-free state of Pt greatly facilitated the adsorption of H 2 O in alkaline electrolytes and, in turn, resulted in the more optimized H adsorption.
Therefore, to improve the performance of Li-S batteries, different types of SACs should be used in Li-S batteries to improve the catalytic conversion kinetics and simultaneously suppress the shuttle effect.Herein, dual-single atoms SACs (Pt and Co) were successfully deposited on the surface of nitrogen-doped carbon nanotubes (NCNTs) by the ALD method, denoted as Pt&Co@NCNT which was used as a sulfur host.Comparison samples with/without SACs were synthesized in the same way, named Co@NCNT, Pt@NCNT, and NCNT.Benefiting from the catalysisabsorption synergistic effect of dual-site SACs, the conversion kinetics of liquid-state Li 2 S x to solid-state Li 2 S in the S/Pt&Co@NCNT electrode was improved, and the shuttle effect was also suppressed.Therefore, cells assembled with S/Pt&Co@NCNT electrodes exhibited the improved utilization of sulfur and the enhanced stability of cycling performance.Especially, in contrast with other samples, the S/Pt&Co@NCNT delivered the highest initial capacity output of 1460.9 mAh g −1 at a current density of 1.3 mA cm −2 and remained at 1185.5 mAh g −1 after 100 cycles.Moreover, even at a high current density of 3 mA cm −2 cycled 500 cycles, the capacity decay rate was only 0.12% per cycle with a Columbic efficiency of over 97.7%.The X-ray absorption near edge structure (XANES) measurement results verified that the dual-site SACs enhanced the reversible conversion between liquid-state Li 2 S x and solid-state Li 2 S. Density function theory (DFT) calculations also delivered the structure of the S/Pt&Co@NCNT electrode with a higher free energy of Li 2 S x conversion and lower decomposition energy of solid Li 2 S compared with the comparison samples.
As shown in Figure 1A, the Pt&Co@NCNT host was prepared via a two-step ALD process.First, NCNTs were grown on carbon paper as supported by the surface plasma chemical vapor deposition method. 9Then, Pt SACs were first deposited on the surface of NCNTs by the ALD method.In this process, trimethyl (methylcyclopentadienyl)-platinum (IV) (MeCpPtMe 3 ) was used as a precursor of Pt SACs.After forming Pt single atoms on NCNTs, the Co atoms were selectively deposited on Pt single atoms by using cobaltocene (Co (Cp) 2 ) as a precursor.Further details of the synthesis process of the carriers could be found in the experimental section of the Supporting Information.After the materials were synthesized, the morphology and microstructure of different samples were studied by scanning electron microscopy (SEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM).As displayed in Figures S1-S4, SEM images of X@NCNT (X = Pt, Co, or Pt&Co) at different magnifications were presented, where NCNTs with high aspect ratio, smooth surface, and neat arrangement could be observed.In addition, no obvious nanoparticles or nanoclusters were obtained in the SEM images of Pt@NCNT (Figure S2) and CO@NCNT (Figure S3).Even after the deposition of both Pt and Co types of SACs, the surface of Pt&Co@NCNT remained smooth without any particle aggregation, as shown in Figure S4.The same phenomenon could be observed in the transmission electron microscopy (TEM) images of Figure S5, where NCNTs with a bamboo-like structure and without metal agglomeration could be observed.To further explore the distribution of SACs on NCNTs, higher magnification of TEM and HAADF-STEM on the surface of substrates containing SACs (Pt@NCNT, Co@NCNT, F I G U R E 1 Synthesis and morphological characterization of the hosts.(A) Schematic illustration of Pt&Co@NCNT host preparation.TEM images of (B) Pt@NCNT, (C) Co@NCNT, and (D) Pt&Co@NCNT.HAADF-STEM images on (E) Pt@NCNT, (F) Co@NCNT, and (G) Pt&Co@NCNT.(H) EELS spectra at the location of the white circle in (G).
or Pt&Co@NCNT) were also studied.As shown in Figures 1B-D, structurally similar NCNTs could be found in the TEM images.Meanwhile, as shown in Figures 1E-G, some uniformly distributed bright spots could be observed on the NCNT surface, which was most probably caused by the deposition of SACs.More importantly, the HAADF-STEM results presented the appearance of two brightness spots on the Pt&Co@NCNT surface, which corresponded with the Pt atoms (brighter distribution points circled in red) and the Co atoms (less brightness circled in white).To further verify this conclusion, electron energy loss spectroscopy (EELS) measurements were performed on atoms with lower brightness, and the results were demonstrated in Figure 1H.Signals of Co L 3 and L 2 were detected on the Pt&Co@NCNT surface, which indicated the presence of Co 1 atoms. 34Furthermore, the crystal structure of the carrier surface was studied by X-ray diffraction and exhibited in Figure S6.The results showed that no relevant crystalline phases appeared on the Pt&Co@NCNT surface.Therefore, as combined with the above characterization results, the Pt&Co SACs were successfully deposited on the NCNTs surface with uniform distribution.
To demonstrate the excellent flexibility of the fabricated substrates, flexibility tests were explored on different samples.As shown in Figure S7, all of the samples were flexible and had excellent mechanical properties, which could provide some ductility to the cathode during cycling.Therefore, the volume expansion of the electrodes during the charging/discharging process could be alleviated.Moreover, the superior flexibility could promote the uniform dispersion of sulfur elements during the subsequent sulfur impregnation process.In addition to the flexibility, the excellent adsorption effect for Li 2 S x was essential to effectively suppressing the shuttle effect and improving the cycling stability of the cells.Therefore, the chemical interaction between Li 2 S x and the host was studied by adsorption experiments with Li 2 S x solutions.Here, Li 2 S 6 solution was used as dissolved Li 2 S x , and NCNTs powders with or without SACs were used as adsorbents.The detailed preparation of Li 2 S 6 solutions can be found in the Supporting Information.As presented in Figure S8, after 10 h of standing, the solutions adsorbed using pure NCNTs powder showed no obvious color change, while the solutions adsorbed with Pt@NCNT, Co@NCNT, and Pt&Co@NCNT powders showed discoloration, especially with Pt&Co@NCNT powders.This intuitive fading change phenomenon effectively demonstrated the excellent chemical effect of Pt&Co@NCNT on Li 2 S x .In other words, the structure of dual-site SACs favored the adsorption of the polysulfides.
To further verify the effect of dual-site SACs, the electrochemical performance of X@NCNT (X = Co, Pt, or Pt&Co) impregnated with sulfur was tested and the electrode was named S/X@NCNT (X = Co, Pt, or Pt&Co) presented in Figure 2. First, the cyclic voltammogram (CV) curves for different cathodes were shown in Figure 2A.A typical curve with two sharp cathodic peaks and one anodic peak was observed over the selected voltage range, which was due to the two-step reduction of solid S 8 molecules conversion to insoluble Li 2 S 2 /Li 2 S and reversible oxidation of Li 2 S 2 /Li 2 S to sulfur. 35It should be noted that the S/Pt&Co@NCNT electrode not only exhibited sharper anodic and cathodic peaks but also delivered a potential difference between the anodic peak and cathodic peaks decreasing around 2.0 V (the overpotential of the S/NCNT, S/Pt@NCNT, S/ Co@NCNT, and S/Pt&Co@NCNT were about 0.48, 0.43, 0.43, and 0.39 V, respectively).This result suggested the improvement of the electrochemical reaction kinetics by Pt and Co SACs codeposition.The same phenomenon could be further observed in electrochemical impedance spectroscopy (EIS) analysis.As presented in Figure S9, although the S/Pt&Co@NCNT electrode (4.25 Ω) exhibited a larger ohmic resistance (R s ) compared to S/NCNT (2.961 Ω), it was worth noting that the smallest charge transfer resistance (R ct ) was exhibited in the S/ Pt&Co@NCNT electrode (26.61 Ω), which was a parameter closely related to the chemical reactivity.However, the S/NCNT, S/Pt@NCNT, and S/Co@NCNT electrodes were 59.87, 43.83, and 28.3 Ω, respectively.These results indicated that the electrochemical reaction kinetics were fast with Pt and Co SACs loading.Therefore, combining the results of CV and EIS, the chemical reaction kinetics were improved by the dual-site Pt&Co SACs that could catalyze electrochemical reactions and improve the electrochemical activity of charging or discharging and even the whole reaction process.
To further investigate the catalytic and adsorbent effects of Pt and Co SACs co-deposition, the Li-S batteries were assembled with S/X@NCNT (X = Pt, Co, Pt&Co) electrode and cycled at a current density of 1.3 mA cm −2 .The result was shown in Figure 2B.Figures S10 and S11 displayed the corresponding charge-discharge profiles of different electrodes.There was a capacity decay from the first cycle to the fifth cycle.This is mainly due to the irreversible capacity loss in the first few cycles.During the following charging, the discharge products cannot be completely oxidized back to sulfur.In most cases, the final charge products are longorder polysulfides such as Li 2 S 8 and Li 2 S 6 , which can output less capacity compared with sulfur in the following discharge steps.In addition, during the charge/discharge process, some polysulfides will be remained in the electrolyte and cannot be utilized, which results in capacity decay.It could be found that the S/Pt&Co@NCNT electrode exhibited the highest initial capacity output of 1460.9 mAh g −1 at a current density of 1.3 mA cm −2 .However, the capacity of the S/NCNT, S/ Pt@NCNT, and S/Co@NCNT electrodes were only 1216.7, 1421.7, and 1328.1 mAh g −1 , respectively.Moreover, even when cycled for 100 cycles, the S/Pt&Co@NCNT electrode still maintained a discharge capacity of 1186.5 mAh g −1 with above 99.7% of the Columbic efficiency, which was higher than those of other electrodes (S/NCNT, S/Pt@NCNT, and S/Co@NCNT were 595.4,853.6, and 927.5 mAh g −1 , respectively).In this process, the increase of the capacity was mainly attributed to the catalytic effect of the Co SAs on the S/Pt&Co@NCNT electrode, which accelerated the conversion kinetics of Li 2 S x during the charge/discharge processes.Meanwhile, the superior adsorption effect of Pt SCAs to Li 2 S x could suppress the shuttle effect and enhance sulfur utilization.The C-rate performance of the S/X@NCNT (X = Co, Pt, or Pt&Co) electrode was further measured at various current densities from 1.3 to 12.7 mA cm −2 , and it was illustrated in Figure 2G.Obviously, the S/Pt&Co@NCNT electrode delivered the highest average discharge capacities at all current densities (the capacity of 1285.5, 1120.4,1091.2, 1053.0,1012.7,931.9, and 822.1 mAh g −1 were delivered at the current densities of 1.3, 2.5, 3.8, 5.1, 6.4 9.5, and 12.7 mA cm −2 , respectively).However, when current densities increased from 1.3 to 12.7 mA cm −2 , the capacity of S/NCNT were only 926.2, 824.2, 680.8, 719.7, 602.3, 109.9, and 20.8 mAh g −1 , respectively.It was worth noting that even at a high current density of 12.7 mA cm −2 , a high average capacity of 822.1 mAh g −1 was maintained in the S/Pt&Co@NCNT electrode, while those of the S/NCNT, S/Pt@NCNT, and S/Co@NCNT were only about 20.8, 140.2, and 251.3 mAh g −1 at 12.7 mA cm −2 , respectively.Moreover, when the current density returned to 1.3 mA cm −2 , the S/Pt&Co@NCNT electrode was recovered to a high capacity of 1163.3 mAh g −1 , while the capacity of the S/NCNT, S/Pt@NCNT, and S/Co@NCNT electrode were only 833.5, 964.0, and 865.6 mAh g −1 , respectively.The improved C-rate stability and capacity reversibility further highlighted the advantages of dual-site SACs in improving the utilization of sulfur and cycling stability through catalytic-adsorption synergy.In addition, the small change of the overpotential was represented by faster conversion kinetics of sulfur species and the reduction of cell polarization on different substrates. 4,17According to previous reports, the overpotential of the S/X@NCNT (X = Co, Pt, or Pt&Co) electrode was determined based on the voltage difference of the median of the second discharge plateau. 36The corresponding charge/discharge curves at different current densities were shown in Figures 2D and S12.Noteworthy, when the current density increased to 12.7 mA cm −2 , the discharge plateaus of the three comparison samples (S/NCNT, S/Pt@NCNT, and S/Co@NCNT) disappeared, so the median voltage of the second plateau of the discharge could not be calculated.Figure 2E displayed the change in overpotential for current densities ranging from 1.3 to 9.5 mA cm −2 .The overpotentials of all cathodes were determined by the voltage difference based on the median value of the second discharge plateau at different current densities.The electrochemical kinetics of sulfur species on different substrates can be reflected by the change of overpotential.It was obvious that when the current density increased to 9.5 mA cm −2 , the overpotential of S/ Pt&Co@NCNT (∼511.2mV) was smaller than those of S/ NCNT (∼941.8mV), S/Pt@NCNT (∼645.8mV), and S/ Co@NCNT (∼624.7 mV) which showed the overpotential decrease when the current density increase, indicating faster conversion kinetics of sulfur species and smallest polarization of cell on S/Pt&Co@NCNT electrode at a high current density.Combined with the above analysis, the decrease of the polarization of Pt and Co SACs co-deposition electrode was not only attributed to the introduction of Co SACs increasing the electrochemical kinetics energy but also benefited from the participation of Pt SACs facilitating the selective deposition of discharge products.Both played an important role in improving the stability of batteries at a high current density.
Considering the long-term cycling stability of the S/ Pt&Co@NCNT electrode at a high current density, the cycling performance of the S/Pt&Co@NCNT electrode at a current density of 3 mA cm −2 was also investigated (Figure 2F).Corresponding charge-discharge curves were shown in Figure S13.The results delivered that for the cell assembled with the S/Pt&Co@NCNT electrode, a high initial capacity of 1332.6 mAh g −1 could be obtained at a current density of 3 mA cm −2 .After one cycle of rapid capacity decay, a reversible capacity with 1053.6 mAh g −1 was maintained.Even after 500 cycles, a reversible capacity of 507.6 mAh g −1 was still maintained in S/Pt&Co@NCNT, with a low-capacity decay rate of 0.12% per cycle, demonstrating that the dual-sites SACs electrode had superior cycling stability at a high current density.In addition, compared with previous reports of SACs works in Li-S batteries, our work exhibited the highest initial capacity output and excellent cycling stability, as exhibited in Figure 2G and Table S1.The improved stability and capacity of the S/Pt&Co@NCNT electrode further confirmed the benefits of Pt&Co SACs co-deposition by synergistically enhancing catalytic electrochemical kinetics and improving the chemical interaction between Li 2 S x and the hosts.
Moreover, during the discharge process, the conversion from liquid-state Li 2 S x to solid-state Li 2 S can provide about 50% capacity of the cell. 37,38However, the sluggish conversion kinetics of Li 2 S x to Li 2 S, and the inhomogeneous nucleation deposition of Li 2 S were the main reasons for limiting the capacity output. 17Therefore, it was essential to increase the capacity output and maintenance of Li-S batteries by improving the liquid-solid phase conversion kinetics between Li 2 S x and Li 2 S and simultaneously inducing a uniform deposition of Li 2 S. 39 In this work, Li 2 S deposition experiments and corresponding SEM images of cycled electrodes were also investigated to analyze the enhanced conversion kinetics and the induced Li 2 S selective deposition of the S/Pt&Co@NCNT electrode (Figure 3).Before SEM testing, the cycled batteries were disassembled in an argon-filled glove box, and the cycled electrodes were obtained.Then, the electrodes were cleaned with the 1,2dimethoxymethane solution to remove excess Li 2 S x . 40inally, SEM measurements were performed.As presented in Figure 3D, the S/Pt&Co@NCNT electrode exhibited the highest current response for Li 2 S deposition which meant the conversion of Li 2 S x to Li 2 S on the S/Pt&Co@NCNT was faster.Moreover, as shown in the SEM images at different magnifications, the S/Pt&Co@NCNT electrode surface (Figures 3H,I) showed more dispersed and homogeneous Li 2 S deposition sites after cycling, which also implied the presence of more anchor localization sites for Li 2 S on the S/ Pt&Co@NCNT electrode surface.However, the S/NCNT (Figures 3A,E,I), S/Pt@NCNT (Figures 3B,F,J), and S/ Co@NCNT (Figures 3C,G,K) not only showed lower deposition current but also had aggregated deposition locations on their surface.In other words, the formation and deposition of Li 2 S were more rapid and uniform on S/ Pt&Co@NCNT electrode, while it was more sluggish and inhomogeneous on S/NCNT, S/Pt@NCNT, and S/ Co@NCNT electrodes.The Li 2 S nucleation experiments showed that Li 2 S achieved rapid and uniform nucleation on the dual-site SACs electrodes (Figure 3M), which was mainly achieved by enhancing the catalytic conversion kinetics from liquid phase Li 2 S x to the solid phase Li 2 S and inducing the uniform deposition of Li 2 S.
To further understand the detailed catalytic mechanism and interaction between dual-sites SACs and Li 2 S x , the Xray absorption near edge structure measurements were conducted for S/NCNT and S/Pt&Co@NCNT electrodes at different charge/discharge stages.As exhibited in Figures 4A,B, the signals for the S K-edge (1s) of S/ Pt&Co@NCNT and S/NCNT could be detected.Before discharging, a peak at 2472.0 eV appeared for both electrodes, which was attributed to the transition of elemental sulfur from S 1s to the S-S Π* state. 41,42With the increase of the discharge degree until 2.1 V, a characteristic peak of Li 2 S x appeared at 2470.5 eV, and meanwhile, the characteristic peaks of LiTFSI appeared at 2484.7 and 2479.8 eV. 42More importantly, the signal intensity of the Li 2 S x characteristic peak at 2470.0 eV on the S/ Pt&Co@NCNT electrode was significantly higher than that on the S/NCNT electrode, suggesting that more elemental sulfur was rapidly converted to Li 2 S x on the S/ Pt&Co@NCNT electrode.Subsequently, at the depth of 1.7 V, with the weakening of Li 2 S x at 2470 eV and the disappearance of S-S Π* at 2472 eV, new features appeared at 2472.6 and 2475.6 eV in the S K-edge XANES of S/ Pt&Co@NCNT.This indicated the final discharge products Li 2 S formation, and the complete conversion of Li 2 S x . 43,44It should be noted that the peaks belonging to Li 2 S were not detected in the S K-edge XANES of S/NCNT, indicating incomplete conversion of Li 2 S x in S/NCNT (as displayed in Figure 4D), which directly resulted in low S utilization and low capacity output of the cell.In other words, with the help of multiatomic catalysts, the electrochemical kinetics of the interconversion of Li 2 S x to Li 2 S was improved and the conversion became more rapid and complete (as illustrated in Figure 4C), which achieved high utilization of sulfur and high-capacity output.More importantly, at the end of the charging process, compared with the incomplete conversion of S/NCNT (the characteristic peak of Li 2 S x still exists), the S K-edge XANES of S/Pt&Co@NCNT showed that the feature of S-S Π* reappeared at 2472.0 eV, and that the peaks corresponding to Li 2 S x and Li 2 S at 2472.6 and 2475.6 eV disappeared.This indicated that the Li 2 S could be oxidized back to elemental sulfur during the charging process, which was important to maintaining high-capacity output during cycling.
To fully understand the reasons for X@NCNT (X = Co, Pt, or Pt&Co) cathodes in improving charge/ discharge reaction kinetics, the first-principles calculations were performed in our work, where the different possible reactions on X@NCNT (X = Co, Pt, or Pt&Co) were investigated.As illustrated in Figure S14, three kinds of models of the N-doped graphene with and without X atoms (X = Co, Pt, or Pt&Co) were considered in our simulation and the second pyridine-nitrogen NCNT (Figure S14A) was selected for calculations.Afterward, the reduction pathways of S in the S/NCNT, S/Co@NCNT, S/Pt@NCNT, and S/Pt&Co@NCNT cathodes were studied.In this part, we considered the overall reaction based on the reversible formation of Li 2 S from S 8 and Li.As shown in Figures 5C-F, during the discharge process, the first step was Li 2 S 8 formation, which resulted from the double reduction of S 8 with two Li + .Subsequently, Li 2 S 8 was further reduced and disproportionated, and three soluble intermediate products Li 2 S x , namely, Li 2 S 6 , Li 2 S 4 , and Li 2 S 2 , were gradually formed.Finally, Li 2 S was formed as the final product.The Gibbs free energy was calculated for the above reactions on S/NCNT, S/Co@NCNT, S/Pt@NCNT, and S/Pt&Co@NCNT electrodes.Figure 5A exhibited the intermediates and their Gibbs free energy curves during the discharge process.It could be observed that after the spontaneous exothermic transformation from S 8 to Li 2 S 8 , the four subsequent steps to form Li 2 S 6 , Li 2 S 4 , Li 2 S 2 , and Li 2 S were either endothermic or nearly thermoneutral.Among them, the Gibbs free energy of Li 2 S formation from Li 2 S 2 was the largest positive, indicating that this was the rate-limiting step in the entire discharge process.These values were 0.98 eV on NCNT, 0.82 eV on Pt@NCNT, 0.77 eV on Co@NCNT, and 0.55 eV on Pt&Co@NCNT.The lower Gibbs free energy for the reduction of Li 2 S 2 was obtained on Pt&Co@NCNT, indicating that the reduction of S on Pt&Co@NCNT was thermodynamically more favorable.During the charging process, the Li 2 S decomposition was the first step.To further reveal the nature of this sulfur-producing reaction, the climbing-image nudged elastic band method was used to evaluate the delithiation reaction kinetics on these surfaces, by calculating the decomposition energy and barrier of Li 2 S. The dissociation of Li 2 S yielded LiS and a single Li-ion.The energy profiles for the decomposition processes on NCNT, Co@NCNT, Pt@NCNT, and Pt&Co@NCNT surfaces were shown in Figure 5B.Likewise, the Li 2 S decomposition energy barriers calculated on NCNT, Pt@NCNT, Co@NCNT, and Pt&Co@NCNT were 2.72, 2.06, 1.30, and 1.08 eV, respectively, clearly affirming that the Pt&Co@NCNT structure accelerated the phase transformation of Li 2 S and improved sulfur utilization in Li-S batteries.It was critical to catalyze the oxidation of Li 2 S on Pt&Co@NCNT to reduce the energy barrier and largely contributed to enhancing battery performance.

| CONCLUSION
In summary, we successfully deposited dual-single atoms of Pt-Co on NCNTs by the ALD method.The presence of Pt SACs improved the interaction with Li 2 S x , thus immobilizing the sulfur species on the host surface and suppressing the shuttle effect.Meanwhile, the Co SACs on Pt&Co@NCNT could serve as an effective catalyst to enhance the conversion F I G U R E 5 DFT calculations of different carriers.(A) Energy profiles for the reduction of LiS x on NCNT, Pt@NCNT, Co@NCNT, and Pt&Co-NCNT substrates.(B) Energy profiles of the decomposition of Li 2 S cluster on NCNT, Pt@NCNT, Co@NCNT, and Pt&Co@NCNT substrates.Schematic view of LiS x on (C) NCNT, (D) Pt@NCNT, (E) Co@NCNT, and (F) Pt&Co@NCNT.The brown, gray, green, yellow, blue, and pink spheres represent the C, N, Li, S, Pt, and Co atoms, respectively.kinetics of liquid-state Li 2 S x to solid-state Li 2 S, thereby enhancing the utilization of sulfur and increasing the specific capacity of the cell.In addition, the results from XANES and DFT calculations showed that the conversion reaction between Li 2 S x and Li 2 S was facilitated, the free energy of sulfur species was enhanced, and the oxidation bonding energy of Li 2 S was reduced.As a result, a high initial specific discharge capacity of 1460.9 mAh g −1 was obtained at a current density of 1.3 mA cm −2 in the S/Pt&Co@NCNT electrode.Furthermore, even after 500 cycles at a high current density of 3 mA cm −2 , the S/Pt&Co@NCNT electrode exhibited a low-capacity decay rate of 0.12% per cycle at 3 mA cm −2 .This work provides a rational design idea for further improving the cycling stability of Li-S batteries at high current densities through synergistic catalytic adsorption, thus facilitating the application of Li-S batteries at high current densities.

AUTHOR CONTRIBUTIONS
Xuejie Gao conceived the idea and designed the experiments.Hanyan Wu did the experiment and wrote the manuscript.Xueliang Sun and Runcang Sun as supervisors gave guidance and discussion to the project.Ming Jiang helped with the density functional theory data.Xinyang Chen, Junjie Li, Lei Zhang, Weihan Li, and Yang Zhao discussed the experiments and gave some comments on this manuscript's writing.All authors discussed the results and commented on the manuscript.

F I G U R E 2
Electrochemical performances of Li-S batteries with different electrodes.(A) The cyclic voltammetry profiles.(B) Cycle performance of Li-S batteries at a current density of 1.3 mA cm −2 .(C) C-rate performance at various current density.(D) The discharge/ charge profiles of S/Pt&Co@NCNT at various current density.(E) The overpotential evolution at various current density from 1.3 to 9. 5 mA cm −2 .(F) Long-term cycling performance of S/Pt&Co@NCNT electrode at 3 mA cm −2 .(G) Single-atom catalysts comparison data.

F
I G U R E 4 K-edge XANES and schematic diagram of conversion of sulfur species on different carriers.The S K-edge XANES at different discharge/charge states of (A) S/Pt&Co@NCNT and (B) S/NCNT electrode.Schematic illustrations of S species conversion on (C) S/Pt&Co@NCNT and (D) S/NCNT electrode.