Background
With a theoretical capacity of 1675 mAh g-1 and an energy density of 2600 Wh kg-1, lithium-sulfur batteries (Li-S) are considered strong contenders for next-generation energy storage systems. However, there are still significant technical hurdles to overcome in its large-scale commercialization. One of the most challenging issues is the low conductivity of sulfur and lithium sulfide (Li2S Li2S2), which leads to slow redox kinetics, resulting in insufficient actual capacity and poor cycling stability. Therefore, the search for efficient electrocatalysts remains an important challenge.
Interestingly, a significant charge redistribution can be elicited by the heterocrystalline design, thereby establishing a metastable electronic state, which provides a promising prospect for enhancing the surface structural stability of the catalyst. In addition, through the design of the multi-component system, the adsorption and desorption processes of the reaction sites can be optimized to achieve the ideal electrochemical activity.
Although many heterostructure designs have been implemented, the performance often fails to meet expectations due to the fact that heterogeneous grain boundaries are designed only over a small area and global electronic control cannot be realized. In addition, the previous heterogeneous crystals often led to lattice confusion due to obvious conformational and spatial differences, which hindered the migration of active species. Therefore, optimizing the synergistic effect between multiple components in heterocrystals, maximizing electron transfer, controlling the stability surface of the catalyst surface, and solving the problems of loose interfacial contact and high electron delocalization resistance are still considerable obstacles.
Recently, the team of Associate Professor Zhang Yayun and Professor Long Donghui of East China University of Science and Technology proposed a puzzle catalyst design strategy to stabilize the highly active crystal plane and reduce the relevant reaction energy barrier by in-situ assembly of NBN-NBC colattive nano heterogeneous crystal groups (CNES). The stable and highly active crystal plane promotes the aggregation of polysulfides, and the surrounding surface crystal plane with enhanced activity promotes the sulfide deposition and diffusion of lithium ions, thereby synergistically promoting the continuous and efficient sulfur redox reaction, and improving the durability and energy density of the battery.
In practice, CNES-based flexible packaging batteries achieve a high energy density of 357 Wh kg1. The results were published in the internationally renowned journal "Enhanced electron delocalization within coherent nano-heterocrystal ensembles for optimizing polysulfide conversion in high-energy-density li-s batteries". Advanced Materials. Zhao Zhiqiang and Pan Yukun, master's students of East China University of Science and Technology, are the co-first authors of this paper.
Figure 1Schematic diagram of the preparation of colattice nano heterogeneous crystal group CNES and its mechanism of action in Li-S cells.
Content Description] 1In-situ construction and electron delocalization of colattive nanoheterogeneous interfaces.
NBN-NBC crystals with co-lattice interface were prepared by hydrothermal method, oxidation method and melamine-assisted carbide nitriding method. High-resolution transmission electron microscopy (HRTEM) clearly reveals the formation of high-density coherent nanocrystals. Zooming in to the nanoscale, it was observed that the nanoparticles consist of multiple particles with lattice spacing corresponding to NBN, NBN, NBC, and NBC. Through systematic examination of a number of boundary positions, seamless integration between nanoisocrystalline grains in NBN-NBC was observed.
In addition, the electron diffraction (SAED) pattern of the selected region is consistent with XRD analysis, revealing the formation of NBN-NBC heterocrystals. The lattice fringes of the NBN-NBC junction were observed using a high-resolution spherical aberration electron microscopy coupled with an electron energy loss spectroscopy (EES) system. The stripes on both sides of the border can be observed to be arranged in an asymmetrical manner, with the two substrates connected uninterruptedly. In addition, the electron localization function analysis revealed the delocalization and aggregation of electrons at the NBN-NBC interface, indicating the rearrangement of electron density at the interface and the delocalization distribution of electron from the D orbital on the NBC side. The obtained X-ray absorption near-edge structure (Xanes) and extended X-ray absorption fine structure (ExAFS) spectra elucidate the local structure of the NB site.
2.Trapping behavior of polysulfides, diffusion behavior of lithium ions, and electrochemical redox reaction kinetics.
Combined with the results of visual adsorption test, DFT calculation, XPS test after adsorption, concentration diffusion test and self-discharge test, the strong adsorption effect of NBN-NBC on polysulfides was revealed, which effectively inhibited the shuttle effect of polysulfides. NBN-NBC captures polysulfides primarily through chemical bonding. Deposition dissolution experiments verified the two-way promotion effect of CNES electrocatalysts on the transformation of sulfur species. In combination with In-Situ Raman, Ex-Situ XPS confirmed the reversibility of sulfur redox.
The results of cyclic voltammetry (CV) showed that the NBN-NBC-modified separator exhibited enhanced peak intensity and significant positive and negative shifts of the cathode and anode peaks, indicating that the current exchange was efficient and fast during the LIPS conversion process, which further confirmed that NBN-NBC had higher catalytic activity.
3.DFT calculations reveal catalyst-polysulfide interactions and catalytic mechanisms.
Density functional theory (DFT) was used to calculate the adsorption and energy barrier advantages of each crystal plane of the colattice nano heterogeneous crystal group structure. Among them, NBC-111 exhibits strong adsorption capacity and can form a droplet-like dense phase composed of soluble LIPS, thereby inducing the transient deposition of non-equilibrium nanocrystalline amorphous Li2S. The conversion from S8 to Li2S6 is considered to be thermodynamically advantageous, and in this regard, each crystal plane has different catalytic effects, and the ΔG of the rate-determining step of NBN-111 and NBN-NBC-111 is significantly lower than that of NBC-111.
On NBC-111, the decomposition energy of Li2S is lower than that of NBN. Compared with pure NBN and NBC, the diffusion barrier of lithium ions on all surfaces of NBN-NBC is lower, which promotes the rapid diffusion kinetics of the interface layer and prevents the aggregation of lithium ions on the catalyst surface.
4.Lithium-sulfur batteries based on CNES modified separators exhibit excellent battery performance.
Batteries using NBN-NBC separators are at 0Excellent performance at 2 C current density. It has an initial capacity of up to 1270 mAh g-1 and still retains 898 after 200 cyclesGood capacity of 2 mAh g-1, significantly better than control samples.
In addition, the NBN-NBC battery has a reversible capacity of 1 mAh g-1 at 1 C current density after initial activation, and after 1000 charge-discharge cycles, it still retains 65High capacity retention rate of 2%. Even in 5At a high rate of 0 C, after 1000 cycles, the capacity decay per revolution is only 00378%。Batteries with NBN-NBC separators have a sulfur load of 49 mg cm-2 and 67 mg cm-2 at 0After 100 cycles of 2 C, the capacity retention rate reached 892% and 757%。
In addition, pouch batteries equipped with NBN-NBC can continuously power the LED devices. In 83At a current density of 75 mA g-1, the capacity of the pouch battery exceeds 1300 mA g-1. The calculated weight-specific energy density (WED) maximum is 357 Wh kg-1 and remains stable after 25 cycles.
Conclusion] In this study, a new catalyst design strategy is proposed to prepare a CNE catalyst with enhanced electron delocalization for the development of high-performance Li-S batteries. Experiments and DFT calculations demonstrate the facilitating effect of lithium polysulfide conversion and lithium diffusion.
The catalytic mechanism was studied by in-situ Raman spectroscopy and in-situ optical microscopy, and it was found that the CNE catalyst achieved fast reaction kinetics and effectively inhibited the migration of polysulfides, thus showing excellent performance. By using NBN-NBC as a separation modifier, Li-S bag batteries achieve high energy density (>300 Wh kg 1) and exhibit excellent flexibility, providing important prospects for high energy density energy storage systems. This work provides an effective strategy for the development of new electrocatalysts with high active sites, and enriches the design perspective of electrocatalysts.
zhiqiang zhao, yukun pan, shan yi, zhe su, hongli chen, yanan huang, bo niu, donghui long, yayun zhang, enhanced electron delocalization within coherent nano-heterocrystal ensembles for optimizing polysulfide conversion in high-energy-density li-s batteries. adv. mater. 2023.
About the Author. 
Associate Professor Zhang Yayun is an associate professor at the School of Chemical Engineering, East China University of Science and Technology. His research interests include organic pollutant control and recycling, novel chemical batteries, and multi-scale theoretical calculations for thermal protection. In the past five years, he has been a writer or corresponding author in ADV mater.、pnas.、angew. chem.、acs nano、acs catal.He has published more than 60 SCI articles in top international journals such as energy and environmental catalysis, which have been cited more than 3,400 times and 4 ESI highly cited articles.
In the past three years, he has presided over scientific research projects such as the National Natural Science Commission, the Shanghai Municipal Science and Technology Commission and the Military Industry Commission, with a cumulative funding of more than 300 yuan. In 2020, he won the Shanghai Young Science and Technology Talent Sailing Program. In 2021, he was awarded the Shanghai Talent Development Fund. Currently, he serves as Environ sci. tech., appl. catal.B, Small, and other internationally renowned journal reviewers, and Chinese Chemical Letters Young Editorial Board Member.
Professor Long Donghui, professor and doctoral supervisor of East China University of Science and Technology, is currently the director of the Shanghai Collaborative Innovation Center for Aerospace Advanced Composites, and the executive deputy director of the Key Laboratory of Special Functional Polymer Materials and Related Technologies of the Ministry of Education. Focusing on the national strategy and the frontier needs of energy and environmental development, with porous structural materials as the research goal, the trinity research of "controllable synthesis-structure construction-engineering application" was carried out for the design and preparation of materials, multi-scale structure control and structure-oriented thermal protection, energy and catalysis.
His main research interests include: (1) theoretical innovation and application of aerospace thermal protection materials, (2) multi-scale calculation and data-driven design of materials, (3) energy storage (lithium-ion batteries, lithium-sulfur batteries, zinc-ion batteries) and environmental catalysis. As the first or corresponding author in Nature Communications, Energy Environ sci.、jacs、angew. chem. int. ed.、adv. funct. mater.He has published more than 200 articles in journals such as Journal of Composite Materials and Aerospace Materials Technology, and has been cited more than 10,000 times by SCI with an H-factor of 54. 26 invention patents and 5 national defense invention patents have been authorized, and two first-class standards have been formulated. He has made leading research achievements in the field of new ablation-resistant resins and integrated composite materials for heat insulation.