First Author: Zhao Haorui
Corresponding Authors: Sun Jingyu, Song Yingze, Zhu Daming, Yang Wenqiang
Correspondence Affiliations: Soochow University, Southwest University of Science and Technology, Shanghai Advanced Research Institute, Chinese Academy of Sciences, University of South Carolina
【Background】
Due to their high energy density (2600 Wh·kg–1), lithium-sulfur batteries have become a new electrochemical energy storage system that has attracted much attention. However, in the process of practical application, lithium-sulfur batteries face key technical problems such as shuttle effect and slow redox kinetics, resulting in their initial capacity and battery cycle stability cannot meet the requirements of commercial batteries. The development of highly active catalyst materials can optimize the sulfur conversion reaction process, thereby improving the discharge capacity and cycle life of lithium-sulfur batteries. According to previous studies, the activity of a catalyst is closely related to its size. When the size of the catalyst decreases from the bulk to the cluster and single-atom level, the activity of the catalyst increases significantly. Among them, single-atom catalysts have become a hot topic in academic research due to their close to 100% atom utilization and tunable coordination environment. However, single-atom catalysts are also affected by the preparation method, and it is difficult to exhibit high metal atomic loading, which limits their catalytic activity for the electrochemical antisystem of lithium-sulfur batteries.
【Key issues to be solved】
The low atomic load of metals limits the activity of single-atom catalysts for the electrochemical reaction of the entire lithium-sulfur battery system, so it cannot effectively solve the problems of low battery capacity and rapid decay caused by slow kinetics and shuttle effect.
[Analysis of Research Ideas]
Due to the plateau utilization rate and the adjustable coordination environment, the single-atom catalyst can ensure the efficiency of metal atom anchoring of lithium polysulfide and promote the nucleation and decomposition of Li2S, but the low loading of metal atoms may lead to insufficient number of active sites to optimize the overall electrochemical reaction of the electrode. In this paper, the design idea of dual-active site catalyst is proposed, and a high-activity Co–N–P catalyst for lithium-sulfur chemistry is prepared. Through the effective integration of Co single atom and metal compound Co2P, the effective synergy of high metal atom utilization, ideal coordination environment, and high active center load was realized, which effectively catalyzed the polysulfur conversion reaction, thereby improving the sulfur utilization efficiency and prolonging the battery cycle life.
【Graphic Introduction】
Figure 1. a) Schematic illustrating the fabrication process of Co–N–P. b) SEM image of Co–N–P. c) Magnified HAADF-STEM image of Co–N–P. showing the presence of Co single atoms and Co2P nanoparticles. d) STEM image of Co–N–P. e) Elemental mapping images of Co–N–P. f) XRD patterns of Co–N–P, CoPN and PNC. High-resolution XPS g) Co 2p, h) N 1s profiles of Co–N–P and CoPN.
Essentials. By introducing triphenylphosphine as a phosphorus source and adjusting the ratio of zinc and cobalt ions in MOF nanocages, the catalyst target system with Co2P nanoparticles and P,N coordination Co single atoms coexisted was successfully prepared. In order to conduct a comparative study, two catalyst systems were designed: CoPN, a catalyst with P,N coordination Co single atom obtained by increasing the ratio of zinc ions to cobalt ions, and a system without adding cobalt metal precursors, with only P,N-doped carbon support PNCs. The characterization results of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the metal monoatoms and metal compound nanoparticles in the target system existed on the surface of the carrier. X-ray diffraction (XRD) proves the existence of Co2P. X-ray photoelectron spectroscopy (XPS) showed that there was chemical bonding between Co and P, N, indicating that there was coordination between the Co metal center and P and N atoms.
Figure 2. a) Co K-edge XANES spectra of Co–N–P and the control samples. b) k3-weight FT-EXAFS spectra of Co–N–P and the control samples. c) The k space EXAFS fitting curve of Co–N–P at Co K-edge. d) k3-weight FT-EXAFS fitting curve of Co–N–P at Co K-edge. e) WT Co K-edge EXAFS profiles of Co–N–P, Co3O4, and CoPc.
Essentials. X-ray absorption spectroscopy (XAS) shows the electronic valence state and coordination environment of the Co–N–P system. X-ray absorption near-edge structure spectroscopy (XANES) can obtain information about the oxidation state and symmetry distribution of target compounds. The experimental results show that the near-edge absorption line of Co–N–P is located between Co foil and CoO, which is closer to Co3O4, indicating that cobalt has a positive valence state. At the same time, characteristic peaks were observed at the anterior edge range (L1) of around 7715 eV, indicating that the structure has planar symmetry. However, there is no obvious fluctuation in Co–N–P in this range, which indicates that the coordination of phosphorus atoms destroys the planar geometry and makes the target compound asymmetrical, thereby activating the cobalt metal atomic center. The anterior edge range (L2) around 7710 eV, which corresponds to the 1s →3d transition, shows a distinct bending feature that differs from the D4h symmetry of phthalocyanine cobalt (CoPc). Extended analysis (EXAFS) can be used to identify information such as the coordination environment and coordination bond length of single-atom catalysts. From the spectral results, the position of the Fourier transform peak is in good agreement with the Co–N and Co–P paths, confirming the coordination environment formed by N and P atoms. Further fitting determined that the distances of the Co–N and Co–P bonds were 2.02 and 2.21 Å, respectively. Compared with the reference system, the Co–N bond distance is shortened and the Co–P bond distance is enlarged, which further confirms the distortion of the Co electronic structure caused by charge transfer. In addition, the wavelet transform (WT) on the spatial distribution of k further verifies the existence of cobalt active sites in the form of single atoms in the Co–N–P system, which is significantly different from the Co3O4 and CoPc reference systems.
Figure 3. CV profiles of symmetric cells of Co–N–P, CoPN and PNC at a) 0.5 mV s–1. b) 50 mV s–1. c) CV profiles of the LSBs with Co–N–P, CoPN and PNC modified separators at 0.05 mV s–1. d) EIS profiles of the LSBs inserted with Co–N–P, CoPN and PNC modified separators. e) Tafel plots for the reduction process of long-chain LiPSs to Li2S2/Li2S based on Co–N–P, CoPN and PNC modified separators. f) Tafel plots for the oxidation process of Li2S to S8 based on Co–N–P, CoPN and PNC modified separators. g–i) Potentiostatic discharge profiles at 2.05 V for the Co–N–P, CoPN and PNC as the catalysts.
Essentials. In order to verify the effective improvement of the sulfur redox kinetic process of Co–N–P, the authors performed the assembly and cyclic voltammetry tests of symmetrical cells. At a sweep rate of 50 mV/s, Co–N–P exhibited the highest current response, and at a sweep rate of 0.5 mV/s, Co–N–P had higher catalytic activity due to the synergy of Co monoatoms and Co2P nanoparticles. In addition, cyclic voltammetry tests were carried out on the whole battery. The results show that at a sweep speed of 0.05 mV/s, the battery optimized by Co–N–P exhibits a higher current response and a smaller polarization voltage. The Tafel slope was obtained by linear fitting, and the results showed that the optimized battery with Co–N–P@PP showed a smaller Tafel slope value in both the reduction and oxidation processes, which further confirmed that the Co–N–P system had higher electrocatalytic activity. These results indicate that the design of two-site synergy can achieve comprehensive control of atom utilization efficiency, active center loading, coordination environment, and conductivity, so that the catalyst can obtain high catalytic activity.
Figure 4. a) Cycling performance of the LSBs inserted with Co–N–P, CoPN and PNC modified separators at 0.2 C. b) The related GCD profiles. c) Comparison of the QH and QL capacities at 0.2 C of the LSBs inserted with Co–N–P, CoPN and PNC modified separators. d) Rate performance of the LSBs with different modified separators. e) Discharge/charge voltage profiles of the LSBs inserted with Co–N–P modified separator at different rates. f) Areal capacities of the LSBs inserted with Co–N–P modified separator obtained at 0.1 and 0.2 C with the sulfur loadings of 3.6 and 3.8 mg cm−2, respectively. g) Cycling performance of the LSBs inserted with various modified separators at 1.0 C.
Essentials. After assembling and testing the separators coated with the three catalysts, it was found that the target system showed a higher initial capacity at 0.2 C rate, and the GCD curve showed a smaller polarization voltage, which was consistent with the results of the CV curve. In addition, at 1.0 C, after 500 cycles, the battery capacity attenuation rate of the target cell is much lower than that of the comparison system. In addition, at sulfur loads of 3.6 and 3.8 mg cm–2, cycling at 0.1 and 0.2 C, the cell exhibited a higher initial area capacity (3 and 3.25 mAh cm–2). These results show that the target catalyst has good functional performance in lithium-sulfur batteries.
Figure 5. Top views and side views of the optimal configurations of (a) Co–N–P and (b) CoPN. (c) Adsorption energies of sulfur species on Co–N–P and CoPN. The decomposition energy barriers of Li2S on (d) CoPN and (e) Co–N–P. Insert: the initial, transition and final configurations. The pink, blue, orange, purple and yellow balls represent Co, N, P, Li and S atoms, respectively.
Essentials. The theoretical results show that the adsorption energy of the Co–N–P system for Li2S6, Li2S4, Li2S2 and Li2S is higher than that on the CoPN, which proves that the two-site synergy can enhance the adsorption capacity of the catalyst for lithium polysulfide. In this case, the decomposition energy barriers of Li2S on Co–N–P and CoPN were further obtained to be 0.14 and 1.42 eV, respectively, which proved that the synergistic energy of the two sites promoted the decomposition reaction kinetics of Li2S.
【Significance Analysis】
1. By introducing different types of active centers and constructing dual active sites, it can not only increase the number of active sites, but also combine the performance advantages of component materials to improve the activity of the catalyst.
2. The innovative design of the dual-activity center in this paper provides a research perspective for the development of lithium-sulfur battery catalysts.
【Original link】
Cooperative Co Single Atoms and Co2P Nanoparticles as Catalytic Tandem for Boosting Redox Kinetics in Li–S Batteries
https://doi.org/10.1016/j.mtener.2024.101504
Note: Most of the articles reproduced on this site are collected on the Internet, and the copyright of the article belongs to the original author and the original source. The views in the article are only for sharing and exchange, if it involves copyright and other issues, please let us know, and I will deal with it in a timely manner.