Disulfide-Assisted Organic Polysulfide Cathode Design Enables Improved Kinetics in Lithium-Sulfur Batteries

Abstract

Lithium-sulfur batteries (LSBs) is fundamentally limited by the "shuttle effect" and poor kinetics. To address these challenges, this study proposes an approach through developing a novel organic polysulfide composite cathode with high sulfur loading. By implementing a radical reaction between elemental sulfur and a disulfide of tetramethylthiuram disulfide (TMTD), linear organic polysulfides (TMTD-S) containing over 70 wt% sulfur are successfully synthesized. This kind of material features a covalently bonded R-Sn-R (R=C₂H₆N(S)) backbone. Further compounding with the conductive carbon (ECP600JD) and integrating into a paper-based electrode help to improve the electrode's conductivity and optimized ion transport pathways. The obtained TMTD-24S@ECP600JD cathode demonstrates a capacity retention rate of 79.1% after 250 cycles at 0.2C, far superior to traditional S@ECP600JD materials (14.1%). By increasing the sulfur content in TMTD, higher sulfur-content linear organic polysulfides are also obtained. Among them, the TMTD-54S@ECP600JD with 88 wt% sulfur content exhibits the best electrochemical performance and the highest lithium-ion diffusion coefficient, delivering an initial discharge capacity of 941 mAh g^-1 at 0.2C, with a capacity retention rate of 82.1% after 200 cycles. Even at a high rate of 2C, it still maintained a high specific capacity of 638.3 mAh g^-1, making it a potential material for high-performance Li-S batteries.

Keywords: high‐sulfur loading; kinetics; lithium‐sulfur battery; organic sulfides; paper‐based electrode; tetramethylthiuram disulfide.

Figure 1

a) The reaction diagram of TMTD‐S synthesis. b) The image of color evolution during sample preparation. c–e) SEM images of TMTD‐S. f) The particle size distribution map of TMTD‐S@ECP600JD. g) SEM image of TMTD‐24S. h) The corresponding EDS spectra of C, S, and N elements.

Figure 2

a–d) TEM images of TMTD‐24S at high resolution. e) XRD patterns of S, TMTD, TMTD‐24S and TMTD‐24S@ECP600JD. f) FT‐IR spectra of S, TMTD and TMTD‐24S. g) XPS full spectra of TMTD‐24S. h) S 2p XPS spectra of TMTD‐24S. i) C 1s XPS spectra of TMTD‐24S.

Figure 3

a) A schematic diagram of the redox reaction of TMTD during charging and discharging. b) EIS curves of symmetrical batteries. c) CV curves of symmetrical batteries at 1 mV s⁻¹ scan rate. d) CV curves at 0.1 mV s⁻¹ scan rate of S@ECP600JD and TMTD‐24S@ECP600JD and e) EIS curves. f) CV curves of TMTD‐24S@ECP600JD at scan rates from 0.1 to 0.5 mV s⁻¹. CV peak current of g) peak C versus the square root of the scan rate.

Figure 4

Li₂S nucleation curves of a) S and b) TMTD‐24S on sulfur‐free paper. Li₂S dissolution curves of c) S and d) TMTD‐24S. Corresponding electrode morphologies after Li₂S nucleation on sulfur‐free paper of e) S and f) TMTD‐24S. Corresponding electrode morphologies after Li₂S dissolution on sulfur‐free paper of g) S and h) TMTD‐24S.

Figure 5

a) The cycle performance and coulombic efficiency curves of S@ECP600JD and TMTD‐24S@ECP600JD at 0.2 C. b) The charge‐discharge curves of the first and last cycles. c) The charge‐discharge curves of TMTD‐24S@ECP600JD at different cycles. SEM images of d) S@ECP600JD and e) TMTD‐24S@ECP600JD electrode before and after cycling. f) SEM images of TMTD‐24S@ECP600JD battery separator after cycling. g) Schematic illustration of TMTD‐S reduction mechanism and its complexation with Li₂S₆.

Figure 6

TMTD‐S a) XRD patterns with different sulfur contents, b) C 1s XPS spectra and c) S 2p XPS spectra. d) GITT curves and partial GITT curves of TMTD‐54S@ECP600JD electrodes. e) Lithium ion diffusion coefficient D curves of TMTD‐S@ECP600JD with different sulfur contents. f) Internal resistance curves.

Figure 7

a) The cycle performance of Li‐S batteries assembled with S@ECP600JD, TMTD‐24S@ECP600JD, TMTD‐36S@ECP600JD, TMTD‐54S@ECP600JD, and TMTD‐108S@ECP600JD at 0.2 C. b) The charge‐discharge curves of the first cycle. c) The demarcation point of the high discharge stage (Q _H) and the low discharge stage (Q _L) at 0.2 C and d) the discharge specific capacity. e) The rate performance of different Li‐S batteries. The charge‐discharge curves of f) S@ECP600JD and g) TMTD‐54S@ECP600JD at different currents. h) The cycle performance of TMTD‐54S@ECP600JD and S@ECP600JD at 1C.