1. Introduction

Lithium-sulfur (Li-S) batteries are rechargeable devices assembled with a sulfur cathode and a lithium metal anode. Li-S batteries have twice the volumetric energy density and 5 times the gravimetric energy density of lithium-ion batteries (LIB). Hence, Li-S batteries are expected to be applied to stationary power sources and EV vehicles [1]. However, Li-S batteries have the following issues:

・Sulfur and the final discharge product (Li₂S) are insulators.

・In the discharge process, sulfur expands up to 1.8 times, so the structure of batteries is unstable.

・Intermediate products (Li₂S_x (x = 4 – 8)) dissolve in an electrolyte; Li₂S_x (x = 4 – 8) diffused to an anode to provide an insulating layer at the anode surface.

・In the charge process, Li₂S_x (x = 4 – 8) causes redox shuttling.

As a result, Li-S batteries cannot charge and discharge stably. In order to deal with this problem, Nazar et al. reported the method that sulfur is confined in porous carbon [2]. This approach provides a cathode realizing good electronic conductivity, restriction of sulfur expansion, and suppression of Li₂S_x (x = 4 – 8) dissolution. Although these improved characteristics allow Li-S batteries to operate, The discharge capacity of Li-S batteries is still not high enough and this needs to be addressed.

In our previous study, we reported that oxidation treatment to microporous carbon (MC) with HNO₃ improves Li-S batteries' discharge capacity [3]. Moreover, we clarified that the discharge capacity of Li-S batteries has an approximate proportional relation with the amount of oxygen-containing functional groups on the MC surface [4].

This work attempts to elucidate the mechanism of improved Li-S battery performance by oxidation treatment to MC. Our report would lead to the proposal of a novel strategy to improve the performance of Li-S batteries.

2. Method

2.1 Preparation of Oxidized MC-Sulfur composite (Ox MC-S)

MC was added into 69 wt.% HNO₃ and refluxed at 120ºC for 2 h. By vacuum filtration and washing with deionized water, Ox MC was obtained. Ox MC dried in vacuum at 80ºC overnight was mixed with sulfur at a weight ratio of Ox MC: S = 48: 52. The mixture was thermally annealed at 155ºC for 5 h (Ox MC-S). Untreated MC was also composited with sulfur by the same method (MC-S).

2.2 Assembling of Cells

Each MC-S cathode was prepared by mixing the MC-S, acetylene black, carboxymethyl cellulose, and styrene butadiene rubber at a respective weight ratio of 89: 5: 3: 3 and coating the resulting aqueous slurry on an Al foil current collector. The cells with the MC-S electrode and Li metal foil as an anode were assembled in a glove box filled with Ar. Lithium bis(trifluorosulfonyl)imide (LiTFSI): tetraglyme (G4): hydrofluoroether (HFE) = 10: 8: 40 (by mol) was used as the electrolyte.

2.3 Electrochemical Impedance Spectroscopy (EIS)

To elucidate the effect of oxidation treatment on the internal resistance of Li-S batteries, EIS was carried out at various potentials (Discharge 2.0 – 1.0 V and Charge 1.0 – 3.0 V). The obtained Nyquist plots were used for the evaluation of solid electrolyte interphase (SEI) resistance (Rsei), charge-transfer resistance (Rct), and Warburg impedance (Rw). Rw was investigated with the calculation of Warburg coefficient (σ).

3. Major results and conclusion

Since oxidation treatment to MC significantly increased the discharge capacity of Li-S batteries [3][4], it was expected that oxidation treatment would lower the internal resistance of Li-S batteries. EIS of MC-S and Ox MC-S at various potentials showed that oxidation treatment reduced Rsei by an average of 12.2 Ω. This indicates that the SEI thickness was reduced, or the SEI was composed of highly ion-conductive components by the oxidation treatment. Rct decreased only at lower potentials, and the Warburg coefficient decreased except at the end of charge and discharge potential. These results suggest that the oxidation treatment decreases overall resistance, but especially SEI resistance and Warburg impedance, which may improve the discharge capacity of Li-S batteries.

We will also report the activation energy of Rsei and Rct and mechanism analysis of decreasing Rsei by oxidation to MC. This work was supported by “Advanced Low Carbon Technology Research and Development Program, Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING [JPMJAL1301])” from JST.

[1] Y. Guo et al., Angew. Chem. Int. Ed., 52 (2013) 13186.

[2] X. Ji et al., Nat. Mater., 8 (2009) 500.

[3] S. Okabe et al., Electrochemistry, 85 (2017) 671.

[4] L. Yoshida et al., ECS 238th PRiME Meeting Abstracts (2020).