Recently, the research groups of Professors Sun Kening and Professor Wang Zhenhua from the Beijing Key Laboratory of Chemical Power Sources and Green Catalysis at Beijing Institute of Technology have made important progress in regulating the electrocatalytic interface of lithium-sulfur batteries. Relevant results were published in the top international journal "Nano Letters" (impact factor 10.8). Beijing Institute of Technology is the first corresponding unit, Professor Wang Zhenhua is the corresponding author, and Professor Sun Kening of Beijing Institute of Technology and Professor Wang Guoxiu of the Center for Clean Energy Technology, University of Technology Sydney are joint correspondent authors.

Lithium-sulfur batteries are considered to have the potential to become candidates for the next generation of energy storage systems due to their high theoretical specific capacity (1675 mAh g-1) and theoretical energy density (2600 Wh kg-1). However, the shuttle effect and sluggish conversion kinetics of the intermediate lithium polysulfide have become major issues hindering the development of lithium-sulfur batteries. Inhibiting polysulfide shuttle and accelerating conversion by introducing polar sulfur carriers or catalysts is one of the important directions in current research on lithium-sulfur batteries. In lithium-sulfur batteries, the reaction is mainly a heterogeneous reaction, which only occurs at a limited site, that is, the interface where the electrolyte, catalyst and conductive material contact, called triple-phase interfaces (TPI). Therefore, how to expand TPI in lithium-sulfur batteries to increase the catalytic effect and improve the utilization rate of active materials is very critical for the application of lithium-sulfur batteries. The research team proposed a TPI engineering method containing layered double perovskite PrBaCo2O5+δ (PBCO) with excellent conductivity (453 S cm-1) as an electrocatalyst to promote the conversion of polysulfides. The results show that under this strategy, PBCO has excellent conductivity and oxygen-rich vacancies, which can effectively extend TPI to its entire surface. DFT calculations and in situ Raman spectroscopy demonstrated the electrocatalytic effect of PBCO, indicating the key role of this electrocatalyst in enhancing conductivity. The PBCO-based Li−S battery exhibited a reversible capacity of 612 mAh g−1 after 500 cycles at 1.0 C, with a capacity fading rate of 0.067% per cycle. The research results are titled "Engineering Triple-Phase Interfaces Enabled by Layered Double Perovskite Oxide for Boosting Polysulfide Redox Conversion", with doctoral student Bai Zhe as the first author.

Figure 1. Schematic diagram of the catalytic process of polysulfides on the surface of traditional oxides and PBCO.

The study demonstrated the chemical interaction between PBCO and Li2S6 through visual Li2S6 adsorption experiments and XPS. At the same time, DFT calculations also show that PBCO exhibits strong adsorption of polysulfides. The S-S bond length of polysulfides greatly increases after adsorption, which is beneficial to the subsequent cleavage of S-S bonds and the reduction of polysulfides. Secondly, the role of oxygen vacancies in PBCO was further revealed. Compared with oxygen vacancies in the bulk phase, surface oxygen vacancies have been shown to be easier to generate and exhibit stronger adsorption of polysulfides. The calculation of Gibbs free energy also shows that the presence of PBCO achieves a lower reaction energy barrier, and the rich TPI of PBCO-based lithium-sulfur batteries can effectively enhance the conversion of polysulfides.

Figure 2. Effect of PBCO on polysulfide anchoring and catalytic conversion

The electrochemical performance was evaluated by assembling coin cells. Cyclic voltammetry (CV) tests were performed to reveal the transformation of sulfur species during charge and discharge, and PBCO exhibited lower voltage hysteresis and better electrochemical activity. The CV-derived Tafel slope and relative activation energy also indicate that PBCO is more favorable for polysulfide conversion. In addition, Li2S deposition and dissolution experiments, symmetric Li2S6 batteries, and activation energy measurements demonstrated the improvement mechanism of PBCO for the catalytic conversion of polysulfides. At the same time, the in-situ Raman spectroscopy results intuitively revealed the real-time changes of polysulfides in the electrolyte during the entire process, indicating that PBCO alleviated the shuttle effect and accelerated the redox transformation of polysulfides.

Figure 3. Electrochemical characterization of polysulfide-related redox kinetics

In order to verify the role of the excellent conductivity of the electrocatalyst in polysulfide conversion, we designed SmBaCo2O5+δ (SBCO) layered double perovskite with the same crystal structure, similar specific surface area and polysulfide adsorption strength as PBCO. The room temperature electronic conductivity of SBCO is 23.84 S cm-1. Part of the reason why PBCO has a higher conductivity than SBCO is that the ion radius of Pr (III) ions is greater than that of Sm (III) ions, so the influence of Sm atomic nuclei on the O electron cloud is stronger than that of Pr. The overlap between Co 3d and O 2p electron clouds in PBCO is greater than that in SBCO, so the electron transfer of Co3+- O − Co4+in PBCO is better than that in SBCO. Through Li2S deposition experiments, PBCO exhibited faster and higher peak current than SBCO, indicating that compared with the poor conductivity of SBCO, the excellent conductivity of PBCO enriches TPI and facilitates the conversion of polysulfides into Li2S. Since PBCO and SBCO are relatively close in terms of morphology, polysulfide binding energy, and d-band center, and have large differences in conductivity, PBCO is more conducive to improving the liquid-to-solid conversion process of polysulfides. This study reveals the role of abundant TPI in the polysulfide redox process and provides new insights into the design of new catalysts for high-performance Li−S batteries.

Figure 4. Comparison of PBCO and SBCO in theoretical calculations and experiments on polysulfide adsorption and redox conversion.

Full text link: https://doi.org/10.1021/acs.nanolett.3c00566