CHEMICAL SCIENCE

The p-charge density descriptor is used to describe the electrocatalytic sulfur reduction reactivity


On February 2, 2023, the team of Professor Wan Ying of Shanghai Normal University and the team of Associate Professor Lu Wei of Tsinghua University Shenzhen International Graduate School published a research paper entitled “Optimizing the p charge of S in p-block metal sulfides for sulfur reduction electrocatalysis” in the journal Nature Catalysis.

This study confirmed that the p-charge density of sulfur in metal sulfides in the p region has a linear relationship with the apparent activation energy and adsorption activation entropy of the sulfur reduction reaction decisive step (Li2Sn→ Li2S/Li2S2), which can be used as a descriptive factor to characterize the activity of sulfur reduction reaction catalysts, and point out the direction for the screening and design of catalytic materials for lithium-sulfur batteries.

The corresponding authors of the paper are Wan Ying and Lv Wei; The first authors are Hua Wuxing, Shang Tongxin, Li Huan, and Sun Yafei.

The construction of an efficient sulfur reduction reaction (SRR) electrocatalyst to increase the reaction rate of polysulfide can effectively improve the utilization rate of sulfur active substances and inhibit the shuttle effect during battery charging and discharging. Therefore, the study of the catalytic reaction kinetics of sulfur reduction reaction is the key to the development of sulfur-based high specific energy batteries. However, based on the correlation between catalyst electronic structure and SRR activity, relatively few reports have been reported on the theory and method of reverse on-demand design of SRR electrocatalytic materials, and it is of great significance to develop high-performance catalysts by proposing an experimentally measurable electronic structure parameter to quantitatively describe the relationship between catalytic activity and battery performance.

By adjusting the specific electronic structure of the catalyst, the activation energy of the soluble lithium polysulfide reaction is changed, and the catalytic activity descriptor factor is obtained, which can be used to design SRR electrocatalytic materials. Recently, Professor Wan Ying’s team and Associate Professor Lv Wei’s team based on the d charge density descriptor (Nat. Commun. 2022, 13, 2754; Nat. Commun. 2020, 11, 4600; Nat. Commun. 2019, 10, 1428; It is proposed that the p-charge density of sulfur in metal sulfides (p-MS) in the p-region can be used to describe the activity of SRR reaction catalysts. The authors correlated the p charge density of sulfur in metal sulfides with the apparent activation energy and adsorption activation entropy of the SRR reaction decisive step (Li2Sn→ Li2S/Li2S2), and found that the catalyst (Bi2S3) with the largest increase in sulfur p electron number (p charge density) made the SRR reaction have the lowest apparent activation energy and the largest adsorption activation entropy. The lithium-sulfur battery containing Bi2S3 catalyst has a capacity retention rate of more than 85% after 500 cycles at 5C. When the sulfur loading was increased to 17.6 mg cm-2, a surface capacity of up to ~21.9 mAh cm-2 was obtained.

Based on the typical galvanostatic charge-discharge curve and cyclic voltammetry curve, the authors simplify the sulfur reduction reaction on several sulfide catalysts into two turnkey processes, high-potential platform reaction and low-potential platform reaction, and analyze the SRR kinetics on different p-MS catalysts. SRR apparent activation energy increased sequentially (0.14 – 0.32 eV) in the order of Bi2S3, In2S3, Ga2S3, Sb2S3 and SnS, and the battery performance test showed that with the decrease of Ea, the polysulfur shuttle coefficient (ks) of the corresponding battery decreased significantly (0.012 – 0.133 h-1), the specific capacity of the battery under each discharge rate was significantly improved, and the performance of the battery containing p-MS catalyst was much higher than that of the pure rGO carrier.

Figure 1: SRR kinetic analysis on different p-MS electrocatalysts.

Figure 2: Quasi-in in situ XPS analysis reaction intermediates.

Figure 3: In situ Raman spectroscopy test.

Figure 4: Electronic structure of different p-MS catalysts.

Combined with in situ characterization analysis, it is found that the battery containing Bi2S3 generates a large amount of polysulfate during the charging and discharging process, which effectively accelerates the conversion of polysulfur, inhibits the large change of polysulfur concentration during the charge-discharge reaction, and helps alleviate the problem of polysulfur shuttle and loss. Based on the electronic structure analysis, the authors speculate that, similar to the hydrodesulfurization reaction, when the polysulfur molecules are adsorbed on the sulfur vacancies or metal vacancies on the surface of the metal sulfide catalyst, a continuous exchange of sulfur atoms occurs between the surface and the subsurface of the sulfide, thereby promoting the formation of polysulfate intermediates and reducing the apparent activation energy of the SRR reaction. In different p-MS, the hybridization of the s(p) orbital of the metal and the 2p orbital of S leads to the change of S electron structure, and the Bi2S3 surface/subsurface with the largest increase in p electron number (p charge density) of S is more prone to continuous exchange of sulfur atoms, thus presenting the smallest apparent activation energy of SRR reaction. It was found that the p charge density of S in metal sulfides in the p region was linearly correlated with the apparent activation energy and adsorption activation entropy of SRR. Therefore, the authors propose that the p charge density of S in metal sulfides is an appropriate descriptive factor that can be used to characterize SRR reactivity.

Figure 5: Descriptive factor: linear relationship between the pcharge density of sulfur in p-MS and the apparent activation energy (Ea) and adsorption activation entropy (ΔS0*) of sulfur reduction reactions.

Figure 6: Battery performance optimization with the addition of Bi2S3 catalyst. (a) Specific capacity of the battery at 0.2−5C rate; constant current charge-discharge curve at (b, c) 0.2(b) and 3C(c); long cycle performance of the battery under (d, e)1(d) and 5C(e); (f) Rate performance and cycle performance at 0.1C of high-sulfur loaded battery (17.6 mg cm-2) at 0.02−0.5C.

Based on the above understanding of the descriptive factors of sulfur reduction reactivity, the authors chose Bi2S3 as a catalyst material for practical lithium-sulfur batteries. By compounding with graphene to improve the conductivity of the catalytic material, the obtained Bi2S3-based catalytic material still has a capacity retention rate of 93.6% at 1C for 400 cycles, and only 0.03% for 500 cycles at a large magnification of 5C. When the sulfur surface load of the battery cathode is increased to 17.6 mg cm-2 and the electrolyte/sulfur (E/S) is reduced to 7.5 μL mg-1, the assembled lithium-sulfur battery can still obtain an ultra-high surface capacity of 21.9 mAh cm-2, with ultra-high sulfur utilization.

In this paper, it is proposed and experimentally confirmed that the p-charge density of sulfur in metal sulfides in the p-region can be used to describe the activity of SRR reaction catalysts, and the p-charge density of sulfur is related to the apparent activation energy and adsorption activation entropy of the SRR reaction decisive step (Li2Sn→ Li2S/Li2S2), revealing the influence of the p-charge density of sulfur in metal sulfides in the p-region on the sulfur reduction reaction activity. The catalytic material of lithium-sulfur battery constructed on this basis makes the lithium-sulfur battery cycle 500 cycles stably at 5C rate, and the capacity single-turn attenuation rate is only 0.03%, which provides a new idea for the rational design of lithium-sulfur battery catalytic materials and the development of practical technology for high-specific energy lithium-sulfur batteries.

The research work was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Shanghai Municipal Education Commission. (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41929-023-00912-9



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