Bidirectional catalytic effect of Ni-Co single atoms on redox of sulfur cathode for lithium-sulfur batteries

Bidirectional catalytic effect of Ni-Co single atoms on redox of sulfur cathode for lithium-sulfur batteries | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 3 November 2025 V1 Latest version Share on Bidirectional catalytic effect of Ni-Co single atoms on redox of sulfur cathode for lithium-sulfur batteries Authors : Wei Du , Yanshuang Meng , Dongming Qi , Jiawei Feng , Qiang Xiang , Fuliang Zhu 0000-0001-6737-0135 [email protected] , and Zhaoyang Fan Authors Info & Affiliations https://doi.org/10.22541/au.176215883.38637006/v1 203 views 130 downloads Contents Abstract Introduction Results and Discussion 3. Conclusion Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Lithium-sulfur (Li-S) batteries, as a next-generation energy storage technology, have attracted significant attention due to their theoretically high energy density. However, their practical application is hindered by the serious shuttle effect of lithium polysulfides (LiPSs) and sluggish redox kinetics. Herein, Ni-Co bimetallic single atoms were incorporated into nitrogen-doped carbon hollow spheres (Ni/Co-NC) to simultaneously mitigate the polysulfide shuttle effect and enhance the bidirectional redox kinetics of the sulfur cathode. Ni single atoms facilitate the transformation of Li 2 S, while Co single atoms promote the decomposition of Li 2 S, demonstrating a synergistic catalytic effect that improves the overall electrochemical performance of Li-S batteries. The S@Ni/Co-NC cathode exhibits an initial discharge capacity of 1422.4 mAh g -1 at 0.5 C, along with excellent rate performance (674.4 mAh g −1 at 10 C) and long-term cycling stability, maintaining a low capacity decay rate of 0.068% over 1000 cycles at 1 C and 90% capacity retention after 100 cycle at 10 C. Both theoretical calculation and experimental results confirm that the excellent electrochemical performance of S@Ni/Co-NC is attributed to the synergistic effect of Ni and Co single atoms, which effectively suppress the polysulfide shuttle effect and accelerate the redox kinetics of the sulfur cathode in Li-S batteries. Bidirectional catalytic effect of Ni-Co single atoms on redox of sulfur cathode for lithium-sulfur batteries Wei Du a , Yanshuang Meng* ab , Dongming Qi a , Jiawei Feng a , Qiang Xiang a , Fuliang Zhu* ac , Zhaoyang Fan* d a School of Metallurgy and Environment, Lanzhou University of Technology, Lanzhou 730050, China b School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, AZ 85281, USA c State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou 730050, China d Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China *Corresponding author. E-mail: [email protected] (Yanshuang Meng); [email protected] (Fuliang Zhu); [email protected] (Zhaoyang Fan) Abstract: Lithium-sulfur (Li-S) batteries, as a next-generation energy storage technology, have attracted significant attention due to their theoretically high energy density. However, their practical application is hindered by the serious shuttle effect of lithium polysulfides (LiPSs) and sluggish redox kinetics. Herein, Ni-Co bimetallic single atoms were incorporated into nitrogen-doped carbon hollow spheres (Ni/Co-NC) to simultaneously mitigate the polysulfide shuttle effect and enhance the bidirectional redox kinetics of the sulfur cathode. Ni single atoms facilitate the transformation of Li 2 S, while Co single atoms promote the decomposition of Li 2 S, demonstrating a synergistic catalytic effect that improves the overall electrochemical performance of Li-S batteries. The S@Ni/Co-NC cathode exhibits an initial discharge capacity of 1422.4 mAh g -1 at 0.5 C, along with excellent rate performance (674.4 mAh g −1 at 10 C) and long-term cycling stability, maintaining a low capacity decay rate of 0.068% over 1000 cycles at 1 C and 90% capacity retention after 100 cycle at 10 C. Both theoretical calculation and experimental results confirm that the excellent electrochemical performance of S@Ni/Co-NC is attributed to the synergistic effect of Ni and Co single atoms, which effectively suppress the polysulfide shuttle effect and accelerate the redox kinetics of the sulfur cathode in Li-S batteries. Keywords: Lithium-sulfur battery; Dual single-atom catalysts; Polysulfide transformation; Carbon hollow sphere; Bi-directional Redox Introduction With the rapid development of new energy vehicles and energy storage industry, the demand for high-energy-density storage devices is steadily increasing 1-3 . Among emerging battery technologies, lithium-sulfur (Li-S) batteries have attracted significant attention due to their high theoretical specific capacity (about 2600 Wh kg -1 ), positioning them as promising candidates for next-generation energy storage systems 4-7 . However, several challenges hinder their practical application. In Li-S batteries, elemental sulfur serves as the cathode material, undergoing a redox reaction to provide a high specific capacity (1675 mAh g -1 ) and a high energy density. However, the slow redox kinetics of sulfurinduces severe polarization, resulting in low discharge capacity and poor rate performance. In addition, the shuttle effect of intermediate lithium polysulfides (LiPSs) significantly deteriorates battery performance, limiting the long-term cycling stability of Li-S batteries 8-11 . Researchers have explored various host materials for the sulfur cathode in order to resolve these issues. Recently, introducing electrocatalysts into the host structure has emerged as an effective solution to promote the sulfur redox kinetics and mitigate the polysulfide shuttle effect . Reported electrocatalysts include metal oxides, nitrides, sulfides and metal single atoms 12-14 . Among them, single-atom catalysts have gained attention, due to their unique electronic structure, high atomic utilization and superior catalytic activity, which contribute to enhanced polysulfide adsorption and accelerated redox kinetics in Li-S batteries 15, 16 . The charge/discharge process of the sulfur cathode follows a reversible redox mechanism, involving the formation and decomposition of Li 2 S. Conventional single-atom catalysts typically promote either the oxidation or reduction process. Introducing a second metal center to form a bimetallic single-atomic catalyst could enable the simulataneous enhancement of both oxidation and reduction due to synergistic effect. For example, Sun et al. 17 developed Fe-Co bimetallic SACs supported on hollow carbon spheres, achieving efficient polysulfides conversion and Li 2 S decomposition. Similarly, Song et al. 18 incorporated Fe-Co bimetallic SACs into three-dimensional dodecahedral ZIF-8-derived material, effectively inhibiting the shuttle effect through the synergistic catalysis of Fe and Co atoms. In this study, we synthesized Ni-Co bimetallic single-atom catalyst supported on nitrogen-doped carbon hollow spheres (Ni/Co-NC) using a two-step solvent impregnation method. When used as the cathode host material in Li-S batteries, the hollow carbon spheres provide larger cavities, mitigating the volume expansion issue of the sulfur cathode while maximizing the utilization of Ni-Co bimetallic single atoms. Electrochemical analysis confirms the synergistic effect of Ni-Co, where Ni atom facilitate Li 2 S nucleation in the sulfur reduction process, while Co atom accelerates Li 2 S decomposition in the oxidation process, thereby significantly improving the redox kinetics of the sulfur cathode. Furthermore, adsorption experiments and theoretical calculations demonstrate that Ni/Co-NC exhibits superior polysulfide adsorption capacity, effectively suppressing the shuttle effect in Li-S batteries. As a result, the S@Ni/Co-NC cathode delivers excellent electrochemical performance, achieving an initial capacity of 1267.1 mAh g -1 , retaining 795.1 mAh g -1 after 100 cycles at 2 C, and demonstrating outstnding rate performance with a capacity of 674.4 mAh g −1 at 10 C. These results highlight the practical potential of bimetallic single-atom catalysts for the development of high-performance Li-S batteries. Results and Discussion Fig. 1 Schematic diagram for S@Ni/Co-NC synthesis process. The synthesis process of S@Ni/Co-NC is illustrated in Fig. 1 . First, silica particles were synthesized through the hydrolysis of ethyl orthosilicate. Subsequently, a solution of dopamine hydrochloride was added and polymerized to form SiO 2 @PDA. SiO 2 @Co-PDA was prepared by adsorping cobalt ions onto SiO 2 @PDA. The synthesized SiO 2 @Co-PDA was dispersed in an n-hexane solution using ultrasound for 1 hour, followed by adding a nickel nitrate solution. The cobalt ions act as the chemisorption center to adsorb polar nickel ions. The final product, Ni-Co bimetallic single atoms loaded onto nitrogen-doped carbon hollow spheres (Ni/Co-NC), was obtained after carbonization and chemical etching. The preparation process of other samples is provided in the supporting information. The morphology and structure of the samples were analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images of Ni/Co-NC, Co-NC, Ni-NC and NC are shown in Fig. 2 a~d , respectively. The results indicate that the morphology and structure of the carbon hollow spheres were not changed by the introduction of the metallic single atoms. Fig. 2 e~f show that Ni/Co-NC consists of hollow spheres with a diameter of approximately 220 nm and a wall thickness of 22 nm. The TEM image of S@Ni/Co-NC ( Fig. S1 ) reveals a yolk-shell structure, confirming the successful loading of sulfur into the carbon hollow spheres. Thermogravimetric (TG) analysis results ( Fig. S2 ) indicates that S@Ni/Co-NC has a sulfur loading of up to 77%. Fig. 2 (a~d) SEM images of Ni/Co-NC, Co-NC, Ni-NC and NC; (e~f) TEM image of Ni/Co-NC; (g) HAADF-STEM image of Ni/Co-NC; (h~l) Elemental mapping images of Ni/Co-NC. As shown in Fig. 2 g , HADDF STEM was used to examine the distribution of Ni and Co single atoms in the Ni/Co-NC host material. No obvious Ni or Co nanoparticles were observed, but some bright spots were identified, indicating that Ni and Co were atomically dispersed in the carbon matrix. The element mapping analysis ( Fig. 2 h~l ) further confirmed the uniform distribution of Ni and Co atoms throughout the hollow sphere structure, demonstrating the successful dispersion of metal single atoms in the Ni/Co-NC host material. Fig. 3 (a) XRD and (b) Raman spectra of Ni/Co-NC, Ni-NC, Co-NC and NC; (c) XPS spectrum of Ni/Co-NC; (d) N1s, (e) Ni2p and (f) Co2p spectrum of Ni/Co-NC X-ray diffraction (XRD) analysis was carried out to further characterize the crystal structure of Ni/Co-NC, as shown in Fig. 3a . The broad peaks at 26° and 44° correspond to the characteristic (002) and (100) diffraction peaks of graphitized carbon materials, respectively. No other distinct peaks were observed, indicating that the Ni-Co metal atoms were highly dispersed in the carbon matrix without aggregating into nanoparticles 17 . In the Raman spectrum ( Fig. 3b ), the peaks at 1312 and 1566 cm - corresponds to the D and G bands of graphene, respectively. Their relative intensity ratio (I D /I G ) of Ni/Co-NC is higher than that of other samples, which may be due to the introduction of Ni-Co metal single atoms 19 . The chemical composition of Ni/Co-NC was verified by XPS analysis. In Fig. 3d , the N1s spectrum exhibits five peaks at 398.60, 399.89, 400.77, 401.32 and 402.67 eV, corresponding to pyridine-N, metal-N, pyrrole-N, graphitic-N and oxide-N, respectively, indicating the formation of Ni-N and Co-N chemical bonds. In the Ni2P spectrum ( Fig. 3e ), the peaks at 855.29 and 872.85 eV are attributed to the Ni 2+ 2p 3/2 and Ni 2+ 2p 1/2 signals, respectively, while the peaks at 861.16 and 879.20 eV correspond to satellite signals. Similarly, in the Co 2p spectrum ( Fig. 3f) , the peaks at 779.92 and 795.70eV represent the Co 2+ 2p 3/2 and Co 2+ 2p 1/2 species. These results show that, due to the coordination of N atoms, the single Ni and Co atoms in Ni/Co-NC carry a partial positive charge with valence states between 0 and +2. Additionally, the C 1s spectrum, as shown in Fig. S3 , displays three peaks corresponding to C-C, C-O-C and C-N bonds. These results confirm that the Ni and Co atoms were successfully incorporated into the nitrogen-doped carbon hollow spheres via M-N x functional groups 19 . Fig. 4 (a) The cyclic performance of S@Ni/Co-NC, S@Ni-NC, S@Co-NC and S@NC at 0.5 C ; (b) Rate performances ; (c) 2 C, (d) S@Ni/Co-NC cyclic performance at 1 C Lithium-sulfur batteries were assembled using different host materials. Fig. 4a shows the cycling performances of S@Ni/Co-NC, S@Ni-NC, S@Co-NC and S@NC at 0.5 C. The S@Ni/Co-NC electrode exhibited an initial discharge capacity of 1422.4 mAh g −1 , retaining 728.7 mAh g −1 after 100 cycles, demonstrating an excellent capacity retention. Fig. 4b presents the rate performances of the same electrodes. The specific capacities of S@Ni/Co-NC were 1614.5, 1135.1, 903.3, 786.1 and 674.4 mAh g −1 at 0.2 C, 0.5 C, 2 C, 5 C and 10 C, respectively. Notably, the S@Ni/Co-NC electrode exhibited minimal capacity change when the charge rate varied from 0.2 C to 10 C. In addition, the capacity recovered to 1070.7 mAh g −1 when the rate was returned to 0.2C, highlighting excellent rate performance and reversible capacity. Fig. S4a~d display the charge/discharge profiles of S@Ni/Co-NC, S@Ni-NC, S@Co-NC and S@NC under different charge/discharge rates from 0.2 to 10 C. The S@Ni/Co-NC electrode exhibited a higher capacity than other samples, indicating that the Ni-Co bimetallic single-atom catalyst (SAC) enhance sulfur redox reaction kinetics more effectively than Ni or Co SACs. The S@Ni/Co-NC electrode had an initial capacity of 1267.1 mAh g -1 at 2 C, retaining 795.1 mAh g -1 after 100 cycles, significantly outperforming other materials ( Fig. 4c ). It also maintained a capacity of 464.3 mAh g -1 at 1 C after 1000 cycles, with a low capacity decay rate of 0.068% per cycle ( Fig. 4d ). Even at a high rate of 10 C ( Fig. S5 ), the S@Ni/Co-NC demonstrated excellent cycling stability, with a 90% capacity retention after 100 cycles. All these results confirm that the Ni-Co bimetallic single atoms significantly catalyze the sulfur redox reaction, thereby improving the electrochemical performances of the lithium-sulfur batteries. Fig. 5 (a~d) CV curves at different scan rates of the S@Ni/Co-NC, S@Ni-NC, S@Co-NC and S@NC The diffusion coefficient of lithium ions (D Li + ) was analyzed to evaluate the catalytic effect of single atoms on the conversion reaction of lithium polysulfides (LiPSs). Cyclic voltammetry (CV) analysis was conducted to assess the lithium ion diffusion rates (D Li ). Fig. 5 a~d shows the CV curves of S@Ni/Co-NC, S@Ni-NC, S@Co-NC and S@NC in the scan rate range from 0.1 to 0.5 mV s −1 , which were used to determine D Li via the Randles Sevcik equation 20 ： \begin{equation} I_{p}=2.687\times 10^{5}n^{1.5}AD_{\text{Li}}^{0.5}\upsilon^{0.5}c_{\text{Li}}\nonumber \\ \end{equation} where I p represents the peak current, n is the number of charge transfer electrons, A is the electrode area, c Li is the Li + concentration, and ν is the scan rate. When all other parameters remain constant, the slope of the I p vs. ν 0.5 curve is proportional to D Li , indicating that the peak current (I p ) is linearly propotional to the square root of the scan rate (ν 0.5 ). The calculated results ( Fig. S6 a~d )show that S@Ni/Co-NC exhibits a higher slope than the other three cathode materials, indicating a faster lithium-ion diffusion rate and superior redox kinetics compared to S@Ni-NC, S@Co-NC, and S@NC 21, 22 . Fig. 6 (a~d) CV curves of S@Ni/Co-NC, S@Ni-NC,S@Co-NC and S@NC Fig. 6a-d show the CV curves of S@Ni/Co-NC, S@Ni-NC, S@Co-NC and S@NC. Fig.6 a presents the CV curves of S@Ni/Co-NC, which features two obvious reduction peaks at 2.313V and 2.042V. These peaks correspond to the stepwise reduction of sulfur to lithium polysulfides (LiPSs) (2.313 V) and further reduction to Li 2 S 2 and Li 2 S (2.042 V). The oxidation peak at 2.326V represents the reverse process, where Li 2 S 2 and Li 2 S are oxidized back to sulfur. The voltage difference between the oxidation peak and the two reduction peaks of S@Ni/Co-NC is 0.013V and 0.284V. Comparatively, the voltage differences of the other three samples are higher, indicating greater polarization. These results confirm that S@Ni/Co-NC exhibits superior redox reaction kinetics, facilitating faster charge transfer and improved sulfur conversion efficiency 23 . The sulfur cathode kinetics with different host materials were further evaluated using the corresponding Tafel curve derived from the CV analysis. During the reduction process ( Fig. S7a ), S@Ni/Co-NC exhibited the lowest Tafel slope (40 mV dec -1 ), significantely lower than S@Ni-NC (62 mV dec -1 ), S@Co-NC (67 mV dec -1 ) and S@NC (86 mV dec -1 ). Similarly, during the oxidation process ( Fig. S7b ), S@Ni/Co-NC maintained the lowest Tafel slope (124 mV dec -1 ) compared to S@Ni-NC (135 mV dec -1 ), S@Co-NC (133 mV dec -1 ) and S@NC (156 mV dec -1 ). The smallest Tafel slope confirms that the Ni/Co-NC exhihibts excellent catalytic activity, effectly enhancing the redox kinetics of the sulfur cathode 17 . Fig. S7a further reveal that S@Ni-NC has a lower Tafel slope than S@Co-NC in the reduction process, suggesting that Ni single atoms more effectively promote Li 2 S formation. Conversely, during the oxidation process, S@Co-NC has a lower Tafel slope than S@Ni-NC ( Fig. S7b ), indicating that the Co single atoms more efficiently facilitate the decomposition of Li 2 S. These findings demonstrate that the Ni/Co-NC host material exhibits a bidirectional catalytic effect on the sulfur redox reaction in lithium-sulfur batteries, promoting both Li₂S formation and decomposition for enhanced electrochemical performance. Fig. 7 Bidirectional catalytic performance test of Ni/Co-NC, Ni-NC, Co-NC. (a~c) nucleation curves of Li 2 S; (d~f) decomposition curves of Li 2 S The bidirectional catalytic effect of Ni/Co-NC on the sulfur cathode redox was further verified through Li 2 S nucleation and decomposition tests. As shown in Fig. 7a-c and Fig. S8a , the calculated Li 2 S nucleation capacity of Ni/Co-NC reached 1004.61 mAh g -1 , significantely higher than that of Ni-NC (403.78 mAh g -1 ), Co-NC (333.67 mAh g -1 ) and NC (281.11 mAh g -1 ). This result demonstrates that Ni/Co-NC host material effectively enhances the nucleation dynamics of Li 2 S, promoting the sulfur reduction process. Notably, Ni-NC exhibits a higher nucleation capacity than Co-NC, confirming that the Ni single atoms are more favorablefor Li 2 S nucleation compared to Co single atoms. For comparison, Fig. 7d-f and Fig. S8b present the results of the Li 2 S decomposition test. The calculated decomposition capacity of Ni/Co-NC (2712.24 mAh g -1 ) exceeds that of Ni-NC (1344.84 mAh g -1 ), Co-NC (1686.85 mAh g -1 ) and NC (1222.58 mAh g -1 ). This indicates that Ni/Co-NC exhibits superior catalytic activity for Li 2 S decomposition. Morover, Co-NC demonstrated a higher decomposition capacity than Ni-NC, suggesting that Co single atoms are more effective at catalyzing Li 2 S decomposition. These findings confirm that the Ni/Co-NC serves as an efficient bidirectional catalyst, promoting both Li₂S nucleation and decomposition. This dual functionality enhances the overall redox kinetics of the sulfur cathode, contributing to improved electrochemical performance in lithium-sulfur batteries 17, 19, 24 . Fig. S9a shows the CV curves of symmetrical batteries with Ni/Co-NC, NI-NC, Co-NC and NC as host materials. Among them, Ni/Co-NC exhibited the highest current response, indicating a significant enhancement in LiPSs redox kinetics during the charge/discharge process 25-27 . Fig. S9b shows the electrochemical impedance spectroscopy (EIS) results for Li-S batteries with Ni/Co-NC, NI-NC, Co-NC and NC as host materials. The charge transfer resistance (R ct ) of S@Ni/Co-NC is significantly lower than that of the other samples, further confirming that the Ni/Co-NC has superior catalytic activity in facilitating both the oxidation and reduction reactions of sulfur cathode 28, 29 . Ni/Co-NC’s strong lithium polysulfide (LiPSs) adsorption capability is an essential aspect of its catalytic function, as it anchors LiPSs at the cathode, suppresses the polysulfide shuttle effect, and facilitates redox kinetics, leading to improved sulfur conversion and cathode stability. The adsorption test results are shown in Fig. 8a-b depict the color change of Li 2 S 6 solutions after 6 hours of exposure to different adsorbents. The most pronounced color change was observed in the solution containing Ni/Co-NC, indicating that Ni/Co-NC has the highest LiPSs adsorption capacity compared to other host materials 30, 31 . Fig. 8 Lithium polysulfide adsorption experiment (a) 0.5h; (b) 6h; (c) Optimized structure of Ni/Co-NC; (d~g) Charge density of Ni/Co-NC, Ni-NC, Co-NC and NC; (h~j) Two-dimensional cross-section diagram of Ni/Co-NC, Ni-NC and Co-NC; (k) Binding energy of lithium sulfides adsorbed on Ni/Co-NC, Ni-NC, Co-NC and NC. The adsorption behavior of different host materials (Ni/Co-NC, Ni-NC, Co-NC and NC) was further analyzed through theoretical calculation. Fig. 8 c and Fig. S10 present the optimized structures of the four host materials. As shown in Fig. 8d-g , the simulated charge density difference reveals that Ni/Co-NC exhibits the strongest adsorption ability for Li 2 S 6 . Additionally, Fig. 8h-j and Fig. S11 display the two-dimensional cross- sectional diagrams of charge density differences for Ni/Co-NC, Ni-NC, Co-NC and NC . The charge density in the M-S (metal-sulfur) interatomic region of Ni/Co-NC is significantly higher than in other three host materials, which explains the excellent adsorption ability of Ni/Co-NC for Li 2 S 6 32-34 . Fig. S12-15 shows the top view of structural optimization of sulfides (S 8 , Li 2 S 8 , Li 2 S 6 , Li 2 S 4 , Li 2 S 2 , Li 2 S) on Ni/Co-NC, Ni-NC, Co-NC and NC substrates. Fig. 8k presents the calculated adsorption binding energy of these sulfides on different host materials. The calculation results demonstrate that Ni/Co-NC exhibits the highest adsorption capacity, which is crucial for trapping polysulfide (LiPSs) during charge/discharge process, reducing the shuttle effect, and improving cycle stability of the Li-S batteries 35 . These findings are consistent with previous research results and further confirms the key role of Ni-Co bimetallic single atoms in improving lithium-sulfur battery performance. Fig. S16a shows the decomposition energy barriers of Li 2 S on Ni/Co-NC, Ni-NC, Co-NC and NC catalysts. The decomposition energy barrier of Li 2 S on Ni/Co-NC (0.05 eV) is much lower than that of other materials, such as Ni-NC (0.11), Co-NC (0.9), and NC (0.19). These results indicate that Ni/Co-NC has high catalytic activity for reversible polysulfide conversion. By comparing the Gibbs free energy of the Li 2 S conversion reaction ( Fig. S16b ), the high catalytic activity of Ni/Co for polysulfide conversion was theoretically discussed. The Gibbs free energy (ΔG) values of different conversion steps of polysulfides on NC and Co-NC catalysts are given in the figure, indicating that the ΔG values of the Ni/Co-NC catalyst are all lower than those of the other three catalysts, suggesting that the Ni/Co-NC has better catalytic activity, effectively reducing the conversion barrier from Li 2 S 8 to Li 2 S 2 /Li 2 S, and facilitating the conversion of LiPS molecules into the final discharge products. Furthermore, in the calculation of decomposition energy barrier and Gibbs free energy, the decomposition energy barrier of Co-NC is lower than that of Ni-NC, and the ΔG value of Ni-NC is less than that of Co-NC. This also indicates that the Ni monomer is more conducive to Li 2 S nucleation than the Co monomer, and the Co monomer is more effective in catalyzing the decomposition of Li 2 S. It confirms that Ni/Co-NC, as an effective bidirectional catalyst, promotes the nucleation and decomposition of Li 2 S. 3. Conclusion This study demonstrates that Ni/Co bimetallic single-atom catalysts significantly enhance the electrochemical performance of Li-S batteries by simultaneously accelerating redox kinetics and suppressing the polysulfide shuttle effect. Through electrochemical analysis and theoretical calculation, it is confirmed that Ni single atoms facilitate Li 2 S nucleation, while Co single atoms promote Li 2 S dissolution . leading to a synergistic catalytic that optimizes sulfur conversion reactions. Additionally, the strong polysulfide adsorption capability of Ni/Co-NC further stabilizes the cathode environment, ensuring long-term cycling stability. The S@Ni/Co-NC cathode exhibits a remarkable capacity retention of 464.3 mAh g -1 after 1000 cycles at 1 C, with an exceptionally low capacity decay rate of 0.068% per cycle. Furthermore, its high-rate capabilities of 674.4 mAh g −1 at 10 C and 90% retention after 100 cycles highlight the effectiveness of this bimetallic catalyst in practical Li-S batteries. Credit authorship contribution statement Yanshuang Meng, Fuliang Zhu, and Zhaoyang Fan guided all the experimental design and led the manuscript preparation and revision work. Wei Du did most of the experiments and data analysis. Dongming, Jiawei Feng and Qiang Xiang conducted some tests. All of the authors have approved the final version of the manuscript. Data availability Data will be made available on request. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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Keywords bi-directional redox carbon hollow sphere dual single-atom catalysts lithium-sulfur battery polysulfide transformation Authors Affiliations Wei Du Lanzhou University of Technology View all articles by this author Yanshuang Meng Lanzhou University of Technology View all articles by this author Dongming Qi Lanzhou University of Technology View all articles by this author Jiawei Feng Lanzhou University of Technology View all articles by this author Qiang Xiang Lanzhou University of Technology View all articles by this author Fuliang Zhu 0000-0001-6737-0135 [email protected] Lanzhou University of Technology View all articles by this author Zhaoyang Fan Nankai University View all articles by this author Metrics & Citations Metrics Article Usage 203 views 130 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Wei Du, Yanshuang Meng, Dongming Qi, et al. Bidirectional catalytic effect of Ni-Co single atoms on redox of sulfur cathode for lithium-sulfur batteries. Authorea . 03 November 2025. DOI: https://doi.org/10.22541/au.176215883.38637006/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu . 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