Oxygen Vacancies in Bismuth Tantalum Oxide to Anchor Polysulfide and Accelerate the Sulfur Evolution Reaction in Lithium-Sulfur Batteries

Abstract

The shuttling effect of soluble lithium polysulfides (LiPSs) and the sluggish conversion kinetics of polysulfides into insoluble Li₂S₂/Li₂S severely hinders the practical application of Li-S batteries. Advanced catalysts can capture and accelerate the liquid-solid conversion of polysulfides. Herein, we try to make use of bismuth tantalum oxide with oxygen vacancies as an electrocatalyst to catalyze the conversion of LiPSs by reducing the sulfur reduction reaction (SRR) nucleation energy barrier. Oxygen vacancies in Bi₄TaO₇ nanoparticles alter the electron band structure to improve instinct electronic conductivity and catalytic activity. In addition, the defective surface could provide unsaturated bonds around the vacancies to enhance the chemisorption capability with LiPSs. Hence, a multidimensional carbon (super P/CNT/Graphene) standing sulfur cathode is prepared by coating oxygen vacancies Bi₄TaO_7-x nanoparticles, in which the multidimensional carbon (MC) with micropores structure can host sulfur and provide a fast electron/ion pathway, while the outer-coated oxygen vacancies with Bi₄TaO_7-x with improved electronic conductivity and strong affinities for polysulfides can work as an adsorptive and conductive protective layer to achieve the physical restriction and chemical immobilization of lithium polysulfides as well as speed up their catalytic conversion. Benefiting from the synergistic effects of different components, the S/C@Bi₃TaO_7-x coin cell cathode shows superior cycling and rate performance. Even under a high level of sulfur loading of 9.6 mg cm^-2, a relatively high initial areal capacity of 10.20 mAh cm^-2 and a specific energy density of 300 Wh kg^-1 are achieved with a low electrolyte/sulfur ratio of 3.3 µL mg^-1. Combined with experimental results and theoretical calculations, the mechanism by which the Bi₄TaO₇ with oxygen vacancies promotes the kinetics of polysulfide conversion reactions has been revealed. The design of the multiple confined cathode structure provides physical and chemical adsorption, fast charge transfer, and catalytic conversion for polysulfides.

Keywords: electrochemical performance; high areal mass loading; lithium–sulfur battery; oxygen vacancies.

Figure 1

Schematic illustration of the synthesis of S/C@Bi₃TaO_7−x.

Figure 2

(a) SEM images. (b) TEM images. (c) FFT pattern image of Bi₃TaO_7−x in the selected area. (d–f) Corresponding EDS elemental mappings of Bi₃TaO_7−x.

Figure 3

(a) XRD patterns of the Bi₃TaO_7−x and Bi₃TaO₇. (b) EPR spectra of the Bi₃TaO_7−x and Bi₃TaO₇. (c) Raman spectra of the Bi₃TaO_7−x and Bi₃TaO₇. (d) High-resolution O1s XPS spectra of the Bi₃TaO_7−x. (e) UV–vis spectra of Li₂S₆ solution with Bi₃TaO_7−x and Bi₃TaO₇ after treatment for 12 h. Insert image corresponds to the optical photograph of the above solution.

Figure 4

(a) Valence band XPS spectra, (b) Kubelka–Munk plot, and (c) band diagram of Bi₃TaO_7−x and Bi₃TaO₇. (d) Electrical conductivity of Bi₃TaO_7−x and Bi₃TaO₇.

Figure 5

(a) Cyclic voltammetry curves for Li₂S₆ symmetric cells employing Bi₃TaO_7−x and Bi₃TaO₇. (b) Tafel plots of Li₂S₆ symmetric cells. (c) EIS spectra of Li₂S₆ symmetric cells. (d–f) Precipitation profiles of Li₂S.

Figure 6

(a) CV profiles of coin cells at a scan rate of 0.1 mVs⁻¹ employing Bi₃TaO_7−x, Bi₃TaO₇, and MC electrodes. (b) Charge–discharge profiles at 0.1 C of various sulfur electrodes. (c) Rate performance of different sulfur electrodes. (d) Long cycling performances of Bi₃TaO_7−x, Bi₃TaO₇, and MC electrodes at 0.5 C. (e) Cycle performance of the Bi₃TaO_7−x-based pouch cell at 0.1 C.

Figure 7

(a) The section of charge density difference of Bi₃TaO₇ and Bi₃TaO_7−x. (b,c) The calculated DOS for Bi₃TaO₇ and Bi₃TaO_7−x. (d) The binding energies between three samples and different LiPSs. (e) The Gibbs free energy from S₈ to Li₂S on surface Bi₃TaO_7−x, Bi₃TaO₇, and MC.