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Single-atom catalysis enables long-life, high-energy lithium-sulfur batteries

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With high energy density and low material cost, lithium-sulfur batteries (LSBs) emerge quite expeditiously as a fascinating energy storage system over the past decade. Broad applications of LSBs ranging from electric vehicles to stationary grid storage seem rather bright in recent literatures. However, there still exist many pressing challenges to be addressed because we do not yet fully understand and control the electrode-electrolyte interface chemistries during battery operation, such as polysulfide shuttling and poor utilization of active sulfur. Single-atom catalysts (SACs) pave new possibilities of tackling the tough issues due to their decent applicability in the atomic-level identification of structure-activity relationships and reaction mechanism, as well as their structural tunability with atomic precision. This review comprehensively summarizes the very recent advances in utilization of highly active SACs for LSBs by stating and discussing the related publications, which involves catalyst synthesis routes, battery performance, catalytic mechanisms, optimization strategies, and promises to achieve long-life, high-energy LSBs. We see that endeavors to employ SACs to modify sulfur cathode have allowed efficient polysulfide conversion and confinement, leading to the minimization of shuttle effect. Parallel efforts are being devoted to extending the scope of SACs to cell separator and lithium metal anode in order to unlock the full potential of LSBs. We also obtain mechanistic insights into battery chemistries and nature of SACs in their strong interactions with polysulfides through advanced in situ characterizations documented. Overall, acceleration in the development of LSBs by introducing SACs is noticeable, and this cutting edge needs more attentions to further promoting the design of better LSBs.

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References

  1. Zhuang, Z. C.; Huang, J. Z.; Li, Y.; Zhou, L.; Mai, L. Q. The holy grail in platinum-free electrocatalytic hydrogen evolution: Molybdenum-based catalysts and recent advances. ChemElectroChem2019, 6, 3570–3589.

    CAS  Google Scholar 

  2. Zhuang, Z. C.; Li, Y.; Huang, J. Z.; Li, Z. L.; Zhao, K. N.; Zhao, Y. L.; Xu, L.; Zhou, L.; Moskaleva, L.; Mai, L. Q. Sisyphus effects in hydrogen electrochemistry on metal silicides enabled by silicene subunit edge. Sci. Bull.2019, 64, 617–624.

    CAS  Google Scholar 

  3. Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical energy storage for the grid: A battery of choices. Science2011, 334, 928–935.

    CAS  Google Scholar 

  4. Choi, J. W.; Aurbach, D. Promise and reality of post-lithium-ion batteries with high energy densities. Nat. Rev. Mater.2016, 1, 16013.

    CAS  Google Scholar 

  5. Larcher, D.; Tarascon, J. M. Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem.2014, 7, 19–29.

    Google Scholar 

  6. Seh, Z. W.; Sun, Y. M.; Zhang, Q. F.; Cui, Y. Designing high-energy lithium-sulfur batteries. Chem. Soc. Rev.2016, 45, 5605–5634.

    CAS  Google Scholar 

  7. Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F. Advances in lithium-sulfur batteries based on multifunctional cathodes and electrolytes. Nat. Energy2016, 1, 16132.

    CAS  Google Scholar 

  8. Liu, Y. Y.; Zhu, Y. Y.; Cui, Y. Challenges and opportunities towards fast-charging battery materials. Nat. Energy2019, 4, 540–550.

    Google Scholar 

  9. Peng, H. J.; Huang, J. Q.; Zhang, Q. A review of flexible lithium-sulfur and analogous alkali metal-chalcogen rechargeable batteries. Chem. Soc. Rev.2017, 46, 5237–5288.

    CAS  Google Scholar 

  10. Yang, Y.; Zheng, G. Y.; Cui, Y. Nanostructured sulfur cathodes. Chem. Soc. Rev.2013, 42, 3018–3032.

    CAS  Google Scholar 

  11. Peng, H. J.; Huang, J. Q.; Cheng, X. B.; Zhang, Q. Review on high-loading and high-energy lithium-sulfur batteries. Adv. Energy Mater.2017, 7, 1700260.

    Google Scholar 

  12. Yin, Y. X.; **n, S.; Guo, Y. G.; Wan, L. J. Lithium-sulfur batteries: Electrochemistry, materials, and prospects. Angew. Chem., Int. Ed.2013, 52, 13186–13200.

    CAS  Google Scholar 

  13. Ji, X. L.; Lee, K. T.; Nazar, L. F. A highly ordered nanostructured carbon-sulphur cathode for lithium-sulphur batteries. Nat. Mater.2009, 8, 500–506.

    CAS  Google Scholar 

  14. Wang, H. L.; Yang, Y.; Liang, Y. Y.; Robinson, J.; Li, Y. G.; Jackson, A.; Cui, Y.; Dai, H. J. Graphene-wrapped sulfur particles as a rechargeable lithium-sulfur battery cathode material with high capacity and cycling stability. Nano Lett.2011, 11, 2644–2647.

    CAS  Google Scholar 

  15. Tang, C.; Li, B. Q.; Zhang, Q.; Zhu, L.; Wang, H. F.; Shi, J. L.; Wei, F. CaO-templated growth of hierarchical porous graphene for high-power lithium-sulfur battery applications. Adv. Funct. Mater.2016, 26, 577–585.

    CAS  Google Scholar 

  16. Song, J. X.; Gordin, M. L.; Xu, T.; Chen, S. R.; Yu, Z. X.; Sohn, H.; Lu, J.; Ren, Y.; Duan, Y. H.; Wang, D. H. Strong lithium polysulfide chemisorption on electroactive sites of nitrogen-doped carbon composites for high-performance lithium-sulfur battery cathodes. Angew. Chem., Int. Ed.2015, 54, 4325–4329.

    CAS  Google Scholar 

  17. Tang, C.; Zhang, Q.; Zhao, M. Q.; Huang, J. Q.; Cheng, X. B.; Tian, G. L.; Peng, H. J.; Wei, F. Nitrogen-doped aligned carbon nanotube/graphene sandwiches: Facile catalytic growth on bifunctional natural catalysts and their applications as scaffolds for high-rate lithium-sulfur batteries. Adv. Mater.2014, 26, 6100–6105.

    CAS  Google Scholar 

  18. Tao, X. Y.; Wang, J. G.; Liu, C.; Wang, H. T.; Yao, H. B.; Zheng, G. Y.; Seh, Z. W.; Cai, Q. X.; Li, W. Y.; Zhou, G. M. et al. Balancing surface adsorption and diffusion of lithium-polysulfides on noncon-ductive oxides for lithium-sulfur battery design. Nat. Commun.2016, 7, 11203.

    CAS  Google Scholar 

  19. Yuan, Z.; Peng, H. J.; Hou, T. Z.; Huang, J. Q.; Chen, C. M.; Wang, D. W.; Cheng, X. B.; Wei, F.; Zhang, Q. Powering lithium-sulfur battery performance by propelling polysulfide redox at sulfiphilic hosts. Nano Lett.2016, 16, 519–527.

    CAS  Google Scholar 

  20. Sun, Z. H.; Zhang, J. Q.; Yin, L. C.; Hu, G. J.; Fang, R. P.; Cheng, H. M.; Li, F. Conductive porous vanadium nitride/graphene composite as chemical anchor of polysulfides for lithium-sulfur batteries. Nat. Commun.2017, 8, 14627.

    Google Scholar 

  21. Wang, J.; Jia, L. J.; Zhong, J.; **ao, Q. B.; Wang, C.; Zang, K. T.; Liu, H. T.; Zheng, H. C.; Luo, J.; Yang, J. et al. Single-atom catalyst boosts electrochemical conversion reactions in batteries. Energy Storage Mater.2019, 18, 246–252.

    Google Scholar 

  22. Du, Z. Z.; Chen, X. J.; Hu, W.; Chuang, C. H.; **e, S.; Hu, A. J.; Yan, W. S.; Kong, X. H.; Wu, X. J.; Ji, H. X. et al. Cobalt in nitrogen-doped graphene as single-atom catalyst for high-sulfur content lithium-sulfur batteries. J. Am. Soc. Chem.2019, 141, 3977–3985.

    CAS  Google Scholar 

  23. Zhang, K.; Chen, Z. X.; Ning, R. Q.; **, S. B.; Tang, W.; Du, Y. H.; Liu, C. B.; Ren, Z. Y.; Chi, X.; Bai, M. H. et al. Single-atom coated separator for robust lithium-sulfur batteries. ACS Appl. Mater. Interfaces2019, 11, 25147–25154.

    CAS  Google Scholar 

  24. Qiao, B. T.; Wang, A. Q.; Yang, X. F.; Allard, L. F.; Jiang, Z.; Cui, Y. T.; Liu, J. Y.; Li, J.; Zhang, T. Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat. Chem.2011, 3, 634–641.

    CAS  Google Scholar 

  25. Wang, A. Q.; Li, J.; Zhang, T. Heterogeneous single-atom catalysis. Nat. Rev. Chem.2018, 2, 65–81.

    CAS  Google Scholar 

  26. Xu, Q.; Guo, C.; Tian, S.; Zhang, J.; Chen, W.; Cheong, W.; Gu, L.; Zheng, L.; **ao, J.; Liu, Q. et al. Coordination structure dominated performance of single-atomic Pt catalyst for anti-Markovnikov hydroboration of alkenes. Sci. China Mater., in press, DOI: https://doi.org/10.1007/s40843-020-1334-6.

  27. Li, X. Y.; Rong, H. P.; Zhang, J. T.; Wang, D. S.; Li, Y. D. Modulating the local coordination environment of single-atom catalysts for enhanced catalytic performance. Nano Res., in press, DOI: https://doi.org/10.1007/s12274-020-2755-3.

  28. Chen, Y. J.; Ji, S. F.; Wang, Y. G.; Dong, J. C.; Chen, W. X.; Li, Z.; Shen, R. A.; Zheng, L. R.; Zhuang, Z. B.; Wang, D. S. et al. Isolated single iron atoms anchored on N-doped porous carbon as an efficient electrocatalyst for the oxygen reduction reaction. Angew. Chem., Int. Ed.2017, 56, 6937–6941.

    CAS  Google Scholar 

  29. Cheng, N. C.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese, A.; **ao, B. W.; Li, R. Y.; Sham, T. K.; Liu, L. M. et al. Platinum single-atom and cluster catalysis of the hydrogen evolution reaction. Nat. Commun.2016, 7, 13638.

    CAS  Google Scholar 

  30. Yang, H. P.; Lin, Q.; Zhang, C.; Yu, X. Y.; Cheng, Z.; Li, G. D.; Hu, Q.; Ren, X. Z.; Zhang, Q. L.; Liu, J. H. et al. Carbon dioxide electroreduction on single-atom nickel decorated carbon membranes with industry compatible current densities. Nat. Commun.2020, 11, 593.

    CAS  Google Scholar 

  31. **ong, Y.; Dong, J. C.; Huang, Z. Q.; **n, P. Y.; Chen, W. X.; Wang, Y.; Li, Z.; **, Z.; **ng, W.; Zhuang, Z. B. et al. Single-atom Rh/N-doped carbon electrocatalyst for formic acid oxidation. Nat. Nanotechnol., in press, DOI: https://doi.org/10.1038/s41565-020-0665-x.

  32. Long, B.; Tang, Y.; Li, J. New mechanistic pathways for CO oxidation catalyzed by single-atom catalysts: Supported and doped Au1/ThO2. Nano Res.2016, 9, 3868–3880.

    CAS  Google Scholar 

  33. Liang, J. X.; Yu, Q.; Yang, X. F.; Zhang, T.; Li, J. A systematic theoretical study on FeOx-supported single-atom catalysts: M1/FeOx for CO oxidation. Nano Res.2018, 11, 1599–1611.

    CAS  Google Scholar 

  34. Huang, X. H.; **a, Y. J.; Cao, Y. J.; Zheng, X. S.; Pan, H. B.; Zhu, J. F.; Ma, C.; Wang, H. W.; Li, J. J.; You, R. et al. Enhancing both selectivity and coking-resistance of a single-atom Pd1/C3N4 catalyst for acetylene hydrogenation. Nano Res.2017, 10, 1302–1312.

    CAS  Google Scholar 

  35. Ta, H. Q.; Zhao, L.; Yin, W. J.; Pohl, D.; Rellinghaus, B.; Gemming, T.; Trzebicka, B.; Palisaitis, J.; **g, G.; Persson, P. O. Å. et al. Single Cr atom catalytic growth of graphene. Nano Res.2018, 11, 2405–2411.

    CAS  Google Scholar 

  36. Zai, H. C.; Zhao, Y. Z.; Chen, S. Y.; Ge, L.; Chen, C. F.; Chen, Q.; Li, Y. J. Heterogeneously supported pseudo-single atom Pt as sustainable hydrosilylation catalyst. Nano Res.2018, 11, 2544–2552.

    CAS  Google Scholar 

  37. Xu, Y. S.; Zhu, L. P.; Cui, X. X.; Zhao, M. Y.; Li, Y. L.; Chen, L. L.; Jiang, W. C.; Jiang, T.; Yang, S. G.; Wang, Y. Graphitizing N-doped mesoporous carbon nanospheres via facile single atom iron growth for highly efficient oxygen reduction reaction. Nano Res.2020, 13, 752–758.

    CAS  Google Scholar 

  38. Sun, T. T.; Xu, L. B.; Wang, D. S.; Li, Y. D. Metal organic frameworks derived single atom catalysts for electrocatalytic energy conversion. Nano Res.2019, 12, 2067–2080.

    CAS  Google Scholar 

  39. Fu, N. H.; Liang, X.; Li, Z.; Chen, W. X.; Wang, Y.; Zheng, L. R.; Zhang, Q. H.; Chen, C.; Wang, D. S.; Peng, Q. et al. Fabricating Pd isolated single atom sites on C3N4/rGO for heterogenization of homogeneous catalysis. Nano Res.2020, 13, 947–951.

    CAS  Google Scholar 

  40. Ji, S. F.; Chen, Y. J.; Wang, X. L.; Zhang, Z. D.; Wang, D. S.; Li, Y. D. Chemical synthesis of single atomic site catalysts. Chem. Rev., in press, DOI: https://doi.org/10.1021/acs.chemrev.9b00818.

  41. Liu, P. X.; Zhao, Y.; Qin, R. X.; Mo, S. G.; Chen, G. X.; Gu, L.; Chevrier, D. M.; Zhang, P.; Guo, Q.; Zang, D. D. et al. Photochemical route for synthesizing atomically dispersed palladium catalysts. Science2016, 352, 797–800.

    CAS  Google Scholar 

  42. Wan, J. W.; Chen, W. X.; Jia, C. Y.; Zheng, L. R.; Dong, J. C.; Zheng, X. S.; Wang, Y.; Yan, W. S.; Chen, C.; Peng, Q. et al. Defect effects on TiO2 nanosheets: Stabilizing single atomic site Au and promoting catalytic properties. Adv. Mater.2018, 30, 1705369.

    Google Scholar 

  43. Chen, Y. J.; Ji, S. F.; Sun, W. M.; Chen, W. X.; Dong, J. C.; Wen, J. F.; Zhang, J.; Li, Z.; Zheng, L. R.; Chen, C. et al. Discovering partially charged single-atom Pt for enhanced anti-Markovnikov alkene hydrosilylation. J. Am. Chem. Soc.2018, 140, 7407–7410.

    CAS  Google Scholar 

  44. Chen, Y. J.; Ji, S. F.; Sun, W. M.; Lei, Y. P.; Wang, Q. C.; Li, A.; Chen, W. X.; Zhou, G.; Zhang, Z. D.; Wang, Y. et al. Engineering the atomic interface with single platinum atoms for enhanced photocatalytic hydrogen production. Angew. Chem., Int. Ed.2020, 59, 1295–1301.

    CAS  Google Scholar 

  45. Tang, Y.; Asokan, C.; Xu, M. J.; Graham, G. W.; Pan, X. Q.; Christopher, P.; Li, J.; Sautet, P. Rh single atoms on TiO2 dynamically respond to reaction conditions by adapting their site. Nat. Commun.2019, 10, 4488.

    Google Scholar 

  46. Zhang, J.; Wang, Z. Y.; Chen, W. X.; **ong, Y.; Cheong, W. C.; Zheng, L. R.; Yan, W. S.; Gu, L.; Chen, C.; Peng, Q. et al. Tuning polarity of Cu-O bond in heterogeneous Cu catalyst to promote additive-free hydroboration of alkynes. Chem2020, 6, 725–737.

    CAS  Google Scholar 

  47. Park, J.; Lee, S.; Kim, H. E.; Cho, A.; Kim, S.; Ye, Y. J.; Han, J. W.; Lee, H.; Jang, J. H.; Lee, J. Investigation of the support effect in atomically dispersed Pt on WO3−x for utilization of Pt in the hydrogen evolution reaction. Angew. Chem., Int. Ed.2019, 58, 16038–16042.

    CAS  Google Scholar 

  48. Ji, S. F.; Qu, Y.; Wang, T.; Chen, Y. J.; Wang, G. F.; Li, X.; Dong, J. C.; Chen, Q. Y.; Zhang, W. Y.; Zhang, Z. D. et al. Rare-earth single erbium atoms for enhanced photocatalytic CO2 reduction. Angew. Chem., Int. Ed., in press, DOI: https://doi.org/10.1002/anie.202003623.

  49. Ye, X. X.; Wang, H. W.; Lin, Y.; Liu, X. Y.; Cao, L. N.; Gu, J.; Lu, J. L. Insight of the stability and activity of platinum single atoms on ceria. Nano Res.2019, 12, 1401–1409.

    CAS  Google Scholar 

  50. Zhou, X.; Shen, Q.; Yuan, K. D.; Yang, W. S.; Chen, Q. W.; Geng, Z. H.; Zhang, J. L.; Shao, X.; Chen, W.; Xu, G. Q. et al. Unraveling charge state of supported Au single-atoms during CO oxidation. J. Am. Chem. Soc.2018, 140, 554–557.

    CAS  Google Scholar 

  51. Liu, Z. Z.; Zhou, L.; Ge, Q.; Chen, R. J.; Ni, M.; Utetiwabo, W.; Zhang, X. L.; Yang, W. Atomic iron catalysis of polysulfide conversion in lithium-sulfur batteries. ACS Appl. Mater. Interfaces2018, 10, 19311–19317.

    CAS  Google Scholar 

  52. Wang, C. G.; Song, H. W.; Yu, C. C.; Ullah, Z.; Guan, Z. X.; Chu, R. R.; Zhang, Y. F.; Zhao, L. Y.; Li, Q.; Liu, L. W. Iron single-atom catalyst anchored on nitrogen-rich MOF-derived carbon nanocage to accelerate polysulfide redox conversion for lithium sulfur batteries. J. Mater. Chem. A2020, 8, 3421–3430.

    CAS  Google Scholar 

  53. Zeng, Q. W.; Hu, R. M.; Chen, Z. B.; Shang, J. X. Single-atom Fe and N co-doped graphene for lithium-sulfur batteries: A density functional theory study. Mater. Res. Express2019, 6, 095620.

    CAS  Google Scholar 

  54. Wu, J. L.; Chen, J. M.; Huang, Y.; Feng, K.; Deng, J.; Huang, W.; Wu, Y. L.; Zhong, J.; Li, Y. G. Cobalt atoms dispersed on hierarchical carbon nitride support as the cathode electrocatalyst for highperformance lithium-polysulfide batteries. Sci. Bull.2019, 64, 1875–1880.

    CAS  Google Scholar 

  55. Zhou, G. M.; Zhao, S. Y.; Wang, T. S.; Yang, S. Z.; Johannessen, B.; Chen, H.; Liu, C. W.; Ye, Y. S.; Wu, Y. C.; Peng, Y. C. et al. Theoretical calculation guided design of single-atom catalysts toward fast kinetic and long-life Li-S batteries. Nano Lett.2020, 20, 1252–1261.

    CAS  Google Scholar 

  56. Su, Y. S.; Manthiram, A. Lithium-sulphur batteries with a microporous carbon paper as a bifunctional interlayer. Nat. Commun.2012, 3, 1166.

    Google Scholar 

  57. Zhang, L. L.; Liu, D. B.; Muhammad, Z.; Wan, F.; **e, W.; Wang, Y. J.; Song, L.; Niu, Z. Q.; Chen, J. Single nickel atoms on nitrogen-doped graphene enabling enhanced kinetics of lithium-sulfur batteries. Adv. Mater.2019, 31, 1903955.

    CAS  Google Scholar 

  58. **e, J.; Li, B. Q.; Peng, H. J.; Song, Y. W.; Zhao, M.; Chen, X.; Zhang, Q.; Huang, J. Q. Implanting atomic cobalt within mesoporous carbon toward highly stable lithium-sulfur batteries. Adv. Mater.2019, 31, 1903813.

    CAS  Google Scholar 

  59. Li, Y. J.; Zhou, P.; Li, H.; Gao, T. T.; Zhou, L.; Zhang, Y. L.; **ao, N.; **a, Z. H.; Wang, L.; Zhang, Q. H. et al. A freestanding flexible single-atom cobalt-based multifunctional interlayer toward reversible and durable lithium-sulfur batteries. Small Methods2020, 4, 1900701.

    CAS  Google Scholar 

  60. Li, Y. J.; Lin, S. Y.; Wang, D. D.; Gao, T. T.; Song, J. W.; Zhou, P.; Xu, Z. K.; Yang, Z. H.; **ao, N.; Guo, S. J. Single atom array mimic on ultrathin MOF nanosheets boosts the safety and life of lithium-sulfur batteries. Adv. Mater.2020, 32, 1906722.

    CAS  Google Scholar 

  61. Shi, Z. P.; Wang, L.; Xu, H. F.; Wei, J. Q.; Yue, H. Y.; Dong, H. Y.; Yin, Y. H.; Yang, S. T. A soluble single atom catalyst promotes lithium polysulfide conversion in lithium sulfur batteries. Chem. Commun.2019, 55, 12056–12059.

    CAS  Google Scholar 

  62. Liu, H.; Chen, X.; Cheng, X. B.; Li, B. Q.; Zhang, R.; Wang, B.; Chen, X.; Zhang, Q. Uniform lithium nucleation guided by atomically dispersed lithiophilic CoNx sites for safe lithium metal batteries. Small Methods2019, 3, 1800354.

    Google Scholar 

  63. Zhai, P. B.; Wang, T. S.; Yang, W. W.; Cui, S. Q.; Zhang, P.; Nie, A. M.; Zhang, Q. F.; Gong, Y. J. Uniform lithium deposition assisted by single-atom do** toward high-performance lithium metal anodes. Adv. Energy Mater.2019, 9, 1804019.

    Google Scholar 

  64. Sun, Y. W.; Zhou, J. Q.; Ji, H. Q.; Liu, J.; Qian, T.; Yan, C. L. Single-atom iron as lithiophilic site to minimize lithium nucleation overpotential for stable lithium metal full battery. ACS Appl. Mater. Interfaces2019, 11, 32008–32014.

    CAS  Google Scholar 

  65. Gu, J. N.; Zhu, Q.; Shi, Y. Z.; Chen, H.; Zhang, D.; Du, Z. G.; Yang, S. B. Single zinc atoms immobilized on MXene (Ti3C2Clx) layers toward dendrite-free lithium metal anodes. ACS Nano2020, 14, 891–898.

    Google Scholar 

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Acknowledgements

This work was supported by the National Key R&D Program of China (No. 2018YFA0702003), the National Natural Science Foundation of China (Nos. 21890383, 21671117, 21871159), and the China Postdoctoral Science Foundation (No. 2019M660607). Z. C. Z. acknowledges support from the Shuimu Tsinghua Scholar Program.

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Zhuang, Z., Kang, Q., Wang, D. et al. Single-atom catalysis enables long-life, high-energy lithium-sulfur batteries. Nano Res. 13, 1856–1866 (2020). https://doi.org/10.1007/s12274-020-2827-4

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