Unveiling the Advances of 2D Materials for Li/Na-S Batteries Experimentally and Theoretically

Li/Na-sulfur batteries hold practical promise for next-generation batteries because of high energy density and low cost. Significant progress has been made in understanding mechanisms of sulfur redox and metal stripping-plating with a judicious combination of experimental and theoretical approaches. Two-dimensional (2D) nanomaterials offer a suitable model to correlate experimental results with theoretical predictions and, importantly, with which to explore structure-property relationships. Future research effort should focus on establishment of correlations between macroscopic conversion kinetics and electronic structure of the electrode materials with agreed standards and advanced combined experiments and theory. In addition, fabrication of three-dimensional (3D) electrodes from 2D materials might be a promising approach to promote the energy and power densities of the Li/Na-sulfur batteries and other metal-sulfur batteries. Metal-sulfur batteries hold practical promise for next-generation batteries because of high energy density and low cost. Development is impeded at present, however, because of unsatisfied discharge capacity and stability in long cycling. Combination of experimental and theoretical approaches can be used to develop insight into the relationship between electrochemical behavior of sulfur redox and metal stripping-plating and the structural properties of electrode materials. With metal-sulfur batteries, two-dimensional (2D) nanomaterials are a suitable model with which to connect and test experimental results with theoretical predictions and to explore structure-property relationships. Here, through the view of combining experimental and theoretical approaches, we explore sulfur redox conversion on 2D nanomaterials in various reaction stages and critically review crucial factors affecting 2D nanomaterials as artificial solid electrolyte interfaces (SEIs) and host materials in protecting Li and Na metal anodes. We conclude with a focused discussion on promising research orientations for developing high-performance metal-sulfur batteries. Metal-sulfur batteries hold practical promise for next-generation batteries because of high energy density and low cost. Development is impeded at present, however, because of unsatisfied discharge capacity and stability in long cycling. Combination of experimental and theoretical approaches can be used to develop insight into the relationship between electrochemical behavior of sulfur redox and metal stripping-plating and the structural properties of electrode materials. With metal-sulfur batteries, two-dimensional (2D) nanomaterials are a suitable model with which to connect and test experimental results with theoretical predictions and to explore structure-property relationships. Here, through the view of combining experimental and theoretical approaches, we explore sulfur redox conversion on 2D nanomaterials in various reaction stages and critically review crucial factors affecting 2D nanomaterials as artificial solid electrolyte interfaces (SEIs) and host materials in protecting Li and Na metal anodes. We conclude with a focused discussion on promising research orientations for developing high-performance metal-sulfur batteries. 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Advances in lithium-sulfur batteries based on multifunctional cathodes and electrolytes.Nat. Energy. 2016; 1: 16132Crossref Scopus (1084) Google Scholar The adsorption energies to polysulfides and kinetics of particular steps are widely considered as crucial factors impacting reversibility and kinetics of sulfur electrochemical redox on sulfur cathodes: for example, in Li-S batteries, crucial steps such as reduction of soluble lithium polysulfides to insoluble Li2S2/Li2S, Li2S decomposition, and Li2S2 reduction to Li2S. These have been widely studied based on 2D nanomaterial models through combining experimental and theoretical approaches. Among the more widely studied metal-free materials in sulfur cathodes, graphene and hybrids possess several unique properties. These include (1) large theoretical surface area, (2) large modulus of elasticity, and (3) attractive thermal/electrical conductivities. These properties make graphene an ideal sulfur host material.43Wang Z. Dong Y. Li H. 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Zhang and co-workers highlighted that sulfur cathodes with N-doped graphene (NG) were effective for boosting polysulfide adsorption. Ab initio molecular dynamics (AIMD) computations implied that the polysulfide adsorption energies of the NG are greater than those for pristine graphene. Importantly, dominant pyridinic N and pyrrolic N in NG can form SxLi-N interactions with lithium polysulfides via N lone-pair electrons.52Qiu Y. Li W. Zhao W. Li G. Hou Y. Liu M. Zhou L. Ye F. Li H. Wei Z. High-rate, ultralong cycle-life lithium/sulfur batteries enabled by nitrogen-doped graphene.Nano Lett. 2014; 14: 4821-4827Crossref PubMed Scopus (309) Google Scholar Li and co-workers reported a fundamental study on polysulfide adsorption origin on NG. Through systematic DFT computations on behavior of lithium polysulfide adsorption on NG with various N dopants, they showed that only NG with pyridinic N dopants interacts with the polysulfides.53Yin L.-C. Liang J. Zhou G.-M. Li F. Saito R. Cheng H.-M. 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Commun. 2015; 6: 8760Crossref PubMed Scopus (543) Google Scholar Recently, metal-free 2D nanomaterials have been utilized as a substrate to support low-concentration metal single atoms that exhibit boosted performance in sulfur cathodes. For example, [email protected] was reported to have a lower barrier for Li2S decomposition than that for NG. The Ni-N-C moieties therefore facilitated formation and decomposition of Li2S in the discharge and charge process. Moreover, the low decomposition energy of Li2S on [email protected] contributes to enhanced conversion kinetics of polysulfides through Sx2−⋅⋅⋅Ni-N bonding. These properties resulted in significantly boosted rate performance and cycling stability in the Li-S batteries (Figures 3D–3F).57Zhang L. Liu D. Muhammad Z. Wan F. Xie W. Wang Y. Song L. Niu Z. Chen J. Single nickel atoms on nitrogen-doped graphene enabling enhanced kinetics of lithium-sulfur batteries.Adv. Mater. 2019; 31: 1903955Crossref PubMed Scopus (213) Google Scholar Another example can be found with Co single-atom-decorated NG at a high S mass loading (6.0 mg cm−2), in which [email protected] sulfur cathodes delivered a significant aerial capacity of 5.1 mA h cm−2 at 0.2 C together with a low capacity decay of 0.029% per cycle.59Du Z. Chen X. Hu W. Chuang C. Xie S. Hu A. Yan W. Kong X. Wu X. Ji H. Cobalt in nitrogen-doped graphene as single-atom catalyst for high-sulphur content lithium-sulphur batteries.J. Am. Chem. Soc. 2019; 141: 3977-3985Crossref PubMed Scopus (0) Google Scholar Other metal-free 2D nanomaterials have been widely studied in sulfur cathodes. For example, graphitic carbon nitride (g-C3N4) is a 2D crystal with a layered structure with van der Waals interaction between the layers. Importantly, plentiful pyridinic nitrogen species in g-C3N4 is considered crucial to improved polysulfide chemical adsorption. A seminal report was the synthesis of oxygenated carbon nitride for Li-S batteries: enriched nitrogen and micropores and mesopores in the material resulted in high sulfur utilization and stable long-term cycle life.60Liu J. Li W. Duan L. Li X. Ji L. Geng Z. Huang K. Lu L. Zhou L. Liu Z. A graphene-like oxygenated carbon nitride material for improved cycle-life lithium/sulfur batteries.Nano Lett. 2015; 15: 5137-5142Crossref PubMed Scopus (283) Google Scholar Quan et al. demonstrated electron transfer from electron-rich pyridinic Nδ− groups to Li+ in the polysulfides using Li 1s X-ray photoelectron spectroscopy (XPS) (Figure 3G). DFT computations implied that the Li2S2 is adsorbed via Li-N bonding, illustrated by a shortened bond length of 2.06 Å, in comparison with the average length of 2.14 Å. They pointed out that 53.5 atom % N concentration is sufficient to absorb all polysulfides of a sulfur cathode containing 75 wt % sulfur, and resultant sulfur cathodes of 0.04% capacity fade per cycle under 0.5 C in 1,500 cycles (Figures 3H and 3I).58Pang Q. Nazar L.F. Long-life and high-areal-capacity Li-S batteries enabled by a light-weight polar host with intrinsic polysulfide adsorption.ACS Nano. 2016; 10: 4111-4118Crossref PubMed Scopus (291) Google Scholar Additionally, g-C3N4 hybrids such as hybrid of graphene and g-C3N4 were also investigated as host materials to promote electrochemical properties of sulfur cathodes.61Liang J. Yin L. Tang X. Yang H. Yan W. Song L. Cheng H.-M. Li F. Kinetically enhanced electrochemical redox of polysulfides on polymeric carbon nitrides for improved lith