Abstract:

Objective With its high theoretical specific capacity and high energy density, lithium-sulfur batteries stand out from many energy storage systems. However, the shuttle effect of polysulfides and the slow redox reaction kinetics are serious challenges in the development of lithium-sulfur batteries. Therefore, the adsorption limiting polysulfide and the catalytic conversion of polysulfide have become the research focus of lithium sulfur batteries. Functional interlayers can be endowed with high electrical conductivity, strong adsorption and catalytic capabilities, so interlayer insertion is a simple and effective strategy. The electrospun carbon nanofiber network has a three-dimensional porous structure, which can effectively intercept polysulfide and promote electron conduction inside the battery. In order to enhance the interaction between the material and polysulfide and catalyze its transformation, the introduction of functional material MXene is to be explored in this research.

Method Using polyacrylonitrile (PAN) as carbon source, 1, 10-Philolin as nitrogen source, polymethyl methacrylate (PMMA) as pore-making agent, cobalt acetate tetrahydrate as cobalt source and MXene as functional material, electrostatic spinning nanofibers were prepared. After pre-oxidation and high temperature carbonization under specific conditions, flexible self-supporting carbon nanofiber interlayer (MX-Co/N-PCNFs) embedded with cobalt single atom was obtained.

Results The morphology, structure, and impact of MXene addition on the adsorption of polysulfides and electrochemical performance were studied. After 180 degrees of twisting and bending, the surface of the sample showed no cracks and damage, and demonstrated excellent flexibility. The test results show that the total XPS spectrum analysis and peak fitting confirmed the introduction of MXene and the doping of cobalt nitrogen, and that the high degree of graphitization of the sample could be seen through the analysis of Raman spectrum. The specific surface area of MX-Co/N-PCNFs was 257.5 m²/g when the concentration of MXene dispersion was 90 mg/mL and the carbonization temperature was 800 ℃. The dynamic and static adsorption of sulfur proved that MX-Co/N-PCNFs intermediate layer achieved both physical and chemical adsorption to limit the shuttle effect of polysulfide, and also showed that it had the best catalytic activity. By testing the cyclic voltammetry curve (CV), the potential difference between the oxidation peak and the reduction peak of the MX-Co/N-PCNFs intermediate layer was 0.27 V respectively, which proved that it had a small polarization and a strong redox reaction kinetics. It is used as a polar plate and assembled into a symmetrical battery to characterize the liquid-liquid conversion process between liquid sulfur species. At a sweep speed of 50 mV/s, MX-Co/N-PCNFs based symmetric cells exhibit excellent REDOX current response among the three, indicating significant catalytic performance in liquid-liquid conversion processes. The electrochemical impedance spectroscopy results showed that the introduction of MXene nanosheets further promoted the conductivity and wettability of the middle layer of cobalt-nitrogen doped carbon nanofibers, resulting a very small interface impedance and charge transfer impedance. The battery assembled with MX-Co/N-PCNFs as the interlayer showed a capacity of 971.5 mA·h/g after 100 cycles at 0.2 C. The capacity decay was only 0.063% after 400 cycles at 1 C, and the capacity was still 853 mA·h/g after 50 cycles at 0.5 C with high sulfur loading (4 mg/cm²), which was better than that of the interlayer without the addition of MXene nanosheets.

Conclusion MXene was added to the middle layer of nitrogen-doped carbon nanofibers embedded with cobalt single atom. Through electrochemical comparison, it is confirmed that MXene could improve the electrochemical and electrocatalytic properties of porous electrostatic spinning carbon nanofibers, promote the adsorption and catalytic conversion of polysulfide in lithium-sulfur batteries, and improve the energy density of the positive electrode. The inclusion of non-active materials in the binder is avoided, and the influence on the energy density of the lithium-sulfur battery is reduced.

Key words: electrospinning, carbon nanofiber, interlayer, MXene, lithium-sulfur battery