Despite the high theoretical capacity (1675 mAh g-1), cost eﬀectiveness, natural abundance, and environmental friendliness of elemental sulfur, the commercialization of lithium-sulfur (Li–S) batteries is seriously restricted by their low sulfur loading and utilization, sluggish reaction kinetics, and poor cycling stability. So far, appropriate active adsorption and catalytic centers have been introduced to enhance sulfur utilization and accelerate the reversible conversion between lithium polysulfides (LiPSs) and Li2S. Single-atom metal catalysts (SACs) comprising monodispersed metal atoms on appropriate substrates have a theoretical 100% atom utilization eﬃciency, and therefore have a much higher activity than conventional bulk metal and nanoparticle catalysts. Various SACs have been introduced into Li-S batteries to improve their electrochemical performance. However, the lack of a fundamental understanding of the catalysis mechanism and the material properties that govern catalytic activity has hindered the selection and rational design of SACs for Li-S batteries.
Recently, the team led by Prof. Hui-Ming Cheng and Associate Prof. Guangmin Zhou proposed that the d-p orbital hybridization between SAC and sulfur species can be used as the descriptor to develop highly active SACs for advanced Li-S batteries. They found that when SACs adsorb Li-S intermediates, the hybridization between d orbitals from SACs and p orbitals from sulfur will modulate the adsorbate's electronic structure and activation process. After calculating the electron filling state of nine kinds of SACs from Sc to Cu, they found that low atomic number transition metals, like Ti, have fewer filled anti-bonding states and more effective d-p orbital hybridization, which bind LiPSs/Li2S and lower the energy barrier during LiPS reduction and Li2S oxidation. A series of single-atom metal catalysts (Mn, Cu, Cr, Ti) embedded in three-dimensional electrodes were then prepared by a controllable nitrogen coordination approach, preventing aggregation during the following electrode fabrication. Among them, the single-atom Ti-embedded electrode has the lowest electrochemical barrier to LiPSs reduction/Li2S oxidation and highest catalytic activity, matching well with the theoretical calculations.
Figure 1. d-p orbital hybridization between SAC and sulfur species guides developing highly active SACs for Li-S batteries
By virtue of the highly active catalytic center of single-atom Ti on the conductive transport network, an area capacity of 8.2 mAh cm-2 was realized with a low catalyst loading (1 wt.%) and a high area-sulfur loading (8 mg cm−2), better than state-of-the-art LiNixCoyMn1-x-yO2-based cathodes (~4 mAh cm-2) and many reported S cathodes. Meanwhile, the open-circuit of the assembled soft-pack battery shows no noticeable change even after 500 bending cycles. The superior mechanical stability indicates that these three-dimensional electrodes are promising for fabricating bendable/foldable Li-S batteries for flexible electronics. In summary, the proposed descriptor, d-p orbital hybridization between SAC and sulfur species, may provide valuable insight into developing highly active SACs for advanced Li-S batteries and is expected to be further extended to other materials and reactions.
Figure 2. The electrochemical performance of SAC
Figure 3. The kinetic analysis and flexibility test of SATi.
These results have recently been published in the journal Advanced Materials in a paper titled "Engineering d-p Orbital Hybridization in Single-Atom Metal-Embedded Three-Dimensional Electrodes for Li–S Batteries" that was featured on the back cover of the journal. The corresponding authors are Prof. Hui-Ming Cheng, Associate Prof. Guangmin Zhou, Dr. Shiyong Zhao from Curtin University, and Prof. Qianfan Zhang from Beihang University. The first author is Zhiyuan Han. Dr. Shiyong Zhao and and Jiewen Xiao are the co-first authors. The authors of this paper also include Prof. Wei Lv, Dr. Xiongwei Zhong, and Dr. Jinzhi Sheng.
Based on their previous research, the team also recently published a review in Advanced Materials, guiding the design and fabrication of atomically dispersed metal catalysts. This review analyzed the working principles and the major factors that limit the dynamics of reversible conversion of rechargeable batteries based on conversion reactions. Then the relationship between atomic structure and intrinsic activity was revealed. A summary and outlook on the development of bifunctional graphene-like carbon-supported atomically dispersed metal catalysts for next-generation batteries are also given. This review provides valuable insight for developing next-generation rechargeable batteries with high energy efficiency and density.
Figure 4. Graphene-supported atomically dispersed metals as bifunctional catalysts for batteries based on conversion reactions
Figure 5. The relationship between atomic structure and intrinsic activity of graphene-supported atomically dispersed metals catalysts
Figure 6. Summary and outlook of graphene-supported atomically dispersed metals bifunctional catalysts for batteries based on conversion reactions
The review was published in the journal Advanced Materials in a paper titled "Graphene-Supported Atomically Dispersed Metals as Bifunctional Catalysts for Next-Generation Batteries Based on Conversion Reactions." The corresponding authors are Prof. Hui-Ming Cheng and Associate Prof. Guangmin Zhou. The first author is Dr. Biao Chen. The authors of this paper also include Prof. Naiqin Zhao and Dr. Xiongwei Zhong.
These research projects were supported by the National Key R & D Program, National Natural Science Foundation of China, China Postdoctoral Science Foundation, Shenzhen Geim Graphene Center, and the Overseas Research Cooperation Fund of Tsinghua SIGS.
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Written by Zhiyuan Han & Biao Chen
Edited by Alena Shish & Yuan Yang