A Novel Mesoporous Carbon as a Flexible Conductive Support for Cathode of Li-S Batteries
Authors: Liu Tianshu , Kawase Ayako , Pan Zheng-Ze , Iwamura Shinichiroh , Nishihara Hirotomo 
Published: December 2024

Introduction

Sulfur is a promising cathode material for next generation rechargeable batteries due to its high theoretical capacity, cost-effectiveness, and abundance. However, challenges such as its insulating nature, large volume expansion upon lithiation (up to 80%), and the polysulfide shuttle effect have hindered its practical application. Carbon materials, known for their high electrical conductivity and large surface area, have been explored as the hosts for sulfur. However, the sulfur content in most of the sulfur carbon composites remains below 70 wt%, and the sulfur loading is less than 2 mg cm–2, mainly due to the greater shuttle effect and lower conductivity observed in the electrodes with higher sulfur loading. Our group has developed a mesoporous carbon material named carbon mesosponge (CMS) through a template carbonization method. CMS comprises three-dimensional graphene structures with abundant mesopores, acting as reservoirs for the sulfur volume expansion and facilitating the physical adsorption of lithium polysulfide through van der Waals interaction. Moreover, CMS possesses oxygen functional groups advantageous for chemically trapping lithium polysulfide within the cathode. In this work, sulfur is mostly deposited inside the pores of CMS using a chemical reaction method, demonstrating superior cycling stability compared to the conventional melt-diffusion method where sulfur tends to deposit on the exterior of mesopores.

Experimental

The CMS was synthesized via the template carbonization approach, where chemical vapor deposition (CVD) using CH4 as a carbon source was performed onto a nanoplate MgO. Subsequently, the CMS was obtained as the MgO were removed by HCl treatment. CMS/sulfur composite was chemically prepared by mixing CMS with sodium polysulfide (Na2Sx) solution, followed by formic acid introduction for sulfur deposition. A counterpart CMS/sulfur composite was synthesized via melt diffusion by heating sulfur and CMS powder at 155 ºC for 12 hours. Both composites were heated at 250 ºC for 30 minutes to remove excessive sulfur. For comparison, carbon/sulfur composites were prepared using commercialized carbon materials, including activated carbon MSC30 and graphene oxide (GO), following the same preparation method. Electrochemical performance of the samples were evaluated using a coin cell configuration, with electrolyte of 1 M LiTFSI + 1M LiNO3 + 0.08 M La(NO3)3 in DME/DOL and counter electrode of Li metal.

Results and Discussion

In the TG curves under nitrogen atmosphere (Figure 1a), CMS/sulfur (chemical reaction) exhibits a higher sulfur evaporation onset temperature (279 ºC) compared to CMS/sulfur (melt diffusion) and other carbon/sulfur composites. This suggests that the chemical reaction method mainly confines sulfur inside pores of CMS, rather than depositing it on the outer surface. In contrast, MSC30 and GO have limited pore structures and planar surfaces, respectively, resulting in weaker physical sulfur confinement. Figure 1b displays the cycling stability at 1/10 C, revealing CMS's superior capacity retention over other carbon host materials. CMS/sulfur (chemical reaction) retains 90% capacity at 655 mAh g–1 after 100 cycles, and CMS/sulfur (melt diffusion) retains 82 % with 583 mAh g–1. Using CMS/sulfur (chemical reaction) as a cathode material (active material area: 12 cm2) with a sulfur mass loading of 3.8 mg cm-2, pouch cells were assembled (electrolyte/sulfur ratio: 5 μL mg–1). In Figure 1c, the cell displays an initial discharge capacity at 1414 mAh g–1 (1/40 C), stabilizing at around 1018 mAh g–1 from 2nd cycle to the 10th cycle at 1/10 C, and then gradually stabilizing at 650 mAh g–1. The introduction of CMS shows promise in improving the cycling performance of Li-S batteries.
Figure 1. (a) TG curves of carbon/sulfur composites under nitrogen atmosphere. (b) Cycling performance of different carbon/sulfur composites using coin-cells. (c) Cycling performance of CMS/sulfur (chemical reaction) using a pouch cell.

Figure 1