Elsevier

Materials Letters

Volume 264, 1 April 2020, 127312
Materials Letters

A reinforced-concrete-like ultrathick cathode supported by carbonization kapok fibers for high loading lithium sulfur batteries

https://doi.org/10.1016/j.matlet.2020.127312Get rights and content

Highlights

  • Increase the thickness of electrode to achieve high sulfur loading by blade method.

  • Adding carbonization kapok fibers solves weak strength of ultrathick electrode.

  • Percolation network constructed by hollow fibers faciliates Li+ diffusion.

  • Kapok fibers are cheap and processed easily, supplying enough active site.

Abstract

It’s a focus on high sulfur loading electrode for lithium sulfur batteries (LSBs) to improve the practical energy density. Blade casting method is an ideal method due to its efficiency and convenience that means thick electrode was utilized to increase sulfur loading. However, the weak strength and sluggish kinetics hinder to improve the practical energy density. To overcome issues, carbonization kapok fibers (CKFs) were overlapped to form the percolation network like rebar in the reinforced concrete. The cathode (sulfur loading: 8.22 mg cm−2) exhibits the superior initial capacity (1090 mAh g−1 at 0.1 C) and cycling performance (728 mAh g−1 after 80 cycles).

Introduction

Sulfur cathode in lithium sulfur batteries (LSBs) have high theoretical energy density (1675 mAh g−1), safety and cheap. Because of the insulating nature of sulfur and Li2S2/Li2S, carbon materials were added to facilitate the electrochemical utilization of sulfur in the cathode, which reduced the sulfur mass and content in the cathode. So, the advantage of high energy density can’t be sufficiently taken. It’s a tendency to improve sulfur content and loading for the high energy density of LSBs. The blade-casting method is crucial to high sulfur loading LSBs cathode fabrication for practical application [1]. To achieve high sulfur loading, the thickness of the electrode increases made by blade casting method. But there are two serious issues for thick electrodes: weak mechanical stability [2] and poor electrochemical performance. There are some cracks or pinholes in the thick S/GNM2 cathode [3]. And the ions transport distance and resistance heighten in thick electrode [4]. The mass transport polarization would occur accompanied with seriously hamper performance [5]. Here in, this paper reports a rational design of thick electrodes based on blade casting method. The composite of graphene and M2 type (product model, tube diameter is 8–15 nm) carbon nanotube (GNM2) was utilized as the sulfur host. The kapok fibers with the hollow structure by carbonization were utilized as conductive addition to avoid fracture and delamination in the electrode. CKFs can easily form 3D percolation network, facilitating electrons and ions transport in the thick electrode. The method of this paper is simply and low cost, more potential for practical application.

Section snippets

Synthesis of S/GNM2 composite

The S/GNM2 composite was obtained according to the following process: firstly, sulfur and GNM2 (purchased from Time Nano Co Ltd, Chengdu, China) were mixed by hand milling in the different weight ratios. Prepared S/GNM2 composite were sealed in a stainless-steel autoclave filled with argon gas for heat treatment at 155 °C for 24 h.

Fabrication of CKFs

Kapok fibers were washed with ethanol. Then fibers were calcined at 1200 °C for 6 h with heating rate of 2 °C/min under flowing nitrogen gas. After calcination, rod-like

Result and discussion

As seen in Fig. S1, more cracks appeared in the surface of S/GNM2 cathode while no cracks appeared in the surface of S/GNM2/10 wt% CKFs from optical photograph. In SEM images (Fig. 1a-c), CKFs addition in S/GNM2 can reduce cracks or pinholes efficiently. When 10 wt% CKFs in S/GNM2 cathode, there are nearly no cracks in the surface of cathode. In SEM section image (Fig. 1d), CKFs are distributed uniformly in cathode to form the framework while Fig. 1d demonstrates the hollow structure of CKFs

Conclusion

A kind of micro carbon fibers derived from kapok fibers were applied as the addition for advanced high loading LSBs. CKFs network can suppress the formation of cracks on the ultra-thick cathode and facilitate the electrolyte penetration, improving Li+ diffusion while offering enough active site for redistribution of active sulfur to improve sulfur utilization of electrode. LSBs assembled with S/GNM2/CKFs exhibit favorable cycle performance at high sulfur loading and thick electrode condition.

CRediT authorship contribution statement

Jun Ma: Conceptualization, Resources, Data curation, Software, Visualization, Writing - original draft. Xin xiu Yan: Methodology, Writing - review & editing. Huan Zhang: Validation. Mei zhen Qu: Project administration. Zhi kai Wei: Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors acknowledge financial supported by the Western Young Scholars of the Chinese Academy of Sciences and Chengdu science technology office.

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