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Review Article

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

Zechao Zhuang1, Qi Kang2, Dingsheng Wang1( ), Yadong Li1( )
Department of Chemistry, Tsinghua University, Beijing 100084, China
Department of Polymer Science and Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240, China
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Abstract

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.

Keywords

single-atom catalysislithium-sulfur batterypolysulfide conversionshuttle effectatomic-level insight

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. ChemElectroChem 2019, 6, 3570-3589.
Crossref 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.
Crossref Google Scholar
[3]
Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical energy storage for the grid: A battery of choices. Science 2011, 334, 928-935.
Crossref 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.
Crossref Google Scholar
[5]
Larcher, D.; Tarascon, J. M. Towards greener and more sustainable batteries for electrical energy storage. Nat. Chem. 2014, 7, 19-29.
Crossref 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.
Crossref 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. Energy 2016, 1, 16132.
Crossref Google Scholar
[8]
Liu, Y. Y.; Zhu, Y. Y.; Cui, Y. Challenges and opportunities towards fast-charging battery materials. Nat. Energy 2019, 4, 540-550.
Crossref 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.
Crossref Google Scholar
[10]
Yang, Y.; Zheng, G. Y.; Cui, Y. Nanostructured sulfur cathodes. Chem. Soc. Rev. 2013, 42, 3018-3032.
Crossref 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.; Xin, S.; Guo, Y. G.; Wan, L. J. Lithium-sulfur batteries: Electrochemistry, materials, and prospects. Angew. Chem., Int. Ed. 2013, 52, 13186-13200.
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.
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.
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.
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.
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.
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 nonconductive oxides for lithium-sulfur battery design. Nat. Commun. 2016, 7, 11203.
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.
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.; Xiao, 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.; Xie, 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.
Google Scholar
[23]
Zhang, K.; Chen, Z. X.; Ning, R. Q.; Xi, 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. Interfaces 2019, 11, 25147-25154.
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.
Google Scholar
[25]
Wang, A. Q.; Li, J.; Zhang, T. Heterogeneous single-atom catalysis. Nat. Rev. Chem. 2018, 2, 65-81.
Google Scholar
[26]
Xu, Q.; Guo, C.; Tian, S.; Zhang, J.; Chen, W.; Cheong, W.; Gu, L.; Zheng, L.; Xiao, 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, .
Crossref Google Scholar
[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, .
Crossref Google Scholar
[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.
Google Scholar
[29]
Cheng, N. C.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese, A.; Xiao, 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.
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.
Google Scholar
[31]
Xiong, Y.; Dong, J. C.; Huang, Z. Q.; Xin, P. Y.; Chen, W. X.; Wang, Y.; Li, Z.; Jin, Z.; Xing, W.; Zhuang, Z. B. et al. Single-atom Rh/N-doped carbon electrocatalyst for formic acid oxidation. Nat. Nanotechnol., in press, .
Crossref Google Scholar
[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.
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.
Google Scholar
[34]
Huang, X. H.; Xia, 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.
Google Scholar
[35]
Ta, H. Q.; Zhao, L.; Yin, W. J.; Pohl, D.; Rellinghaus, B.; Gemming, T.; Trzebicka, B.; Palisaitis, J.; Jing, G.; Persson, P. O. Å. et al. Single Cr atom catalytic growth of graphene. Nano Res. 2018, 11, 2405-2411.
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.
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.
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.
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.
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, .
Crossref Google Scholar
[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. Science 2016, 352, 797-800.
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.
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.
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.; Xiong, 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. Chem 2020, 6, 725-737.
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.
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, .
Crossref Google Scholar
[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.
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.
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. Interfaces 2018, 10, 19311-19317.
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. A 2020, 8, 3421-3430.
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. Express 2019, 6, 095620.
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 high-performance lithium-polysulfide batteries. Sci. Bull. 2019, 64, 1875-1880.
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.
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.; Xie, 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.
Google Scholar
[58]
Xie, 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.
Google Scholar
[59]
Li, Y. J.; Zhou, P.; Li, H.; Gao, T. T.; Zhou, L.; Zhang, Y. L.; Xiao, N.; Xia, 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 Methods 2020, 4, 1900701.
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.; Xiao, 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.
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.
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 Methods 2019, 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 doping 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. Interfaces 2019, 11, 32008-32014.
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 Nano 2020, 14, 891-898.
Google Scholar
Nano Research
Volume 13 Issue 7,
July 2020
Pages 1856-1866
DOI: 10.1007/s12274-020-2827-4
Cite this article:
Zhuang Z, Kang Q, Wang D, et al. Single-atom catalysis enables long-life, high-energy lithium-sulfur batteries. Nano Research, 2020, 13(7): 1856-1866. https://doi.org/10.1007/s12274-020-2827-4
Topics:
BatteriesCatalysis CatalystsElectrodesElectrolytesLithium batteriesPolysulfides
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Received: 17 March 2020
Revised: 20 April 2020
Accepted: 21 April 2020
Published: 29 April 2020
© Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020
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