Elsevier

Applied Surface Science

Volume 472, 1 April 2019, Pages 135-142
Applied Surface Science

Full Length Article
Freestanding porous sulfurized polyacrylonitrile fiber as a cathode material for advanced lithium sulfur batteries

https://doi.org/10.1016/j.apsusc.2018.03.062Get rights and content

Highlights

  • Freestanding porous sulfurized polyacrylonitrile/VGCF composite was synthesized via a facile electrospinning.

  • Highly porous structure can effectively improve ionic transfer.

  • The high conductivity of VGCF can improve the electrical conductivity of the composite.

Abstract

A freestanding porous sulfurized polyacrylonitrile/vapor grown carbon fiber (SVF) composite was prepared as cathode material for high-performance lithium sulfur batteries by a facile electrospinning technique. The synthesized composite possessed high sulfur utilization, high Coulombic efficiency, and excellent cycling stability with the property of flexibility, essential to the development of flexible batteries. The capacity retentions of the SVF cell were 903 mAh g−1 after 150 cycles at 1 C and 600 mAh g−1 after 300 cycles at 2 C. At a high rate of 4 C, the SVF composite showed reasonable capacity retention. The superior performance of SVF composite was attributed to the highly porous structure, which effectively improved the wettability, accessibility, and absorption of electrolyte to facilitate rapid ion transfer in the cell. Vapor-grown carbon fibers embedded inside SVF as a carbon material notably enhanced the electrical conductivity of the cell, guaranteeing the electrochemical performance at high C-rates. The freestanding porous SVF fiber composite is a promising cathode material for advanced flexible lithium sulfur batteries.

Graphical abstract

Freestanding porous sulfurized polyacrylonitrile/vapor grown carbon fiber (SVF) composite was prepared through a facile electrospinning process followed by sulfurization, as cathode material for high-performance flexible lithium sulfur batteries. The highly porous structure, high electrical conductivity and flexible property of the composite could greatly improve electrochemical performances.

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Introduction

Advanced electrical energy conversion and storage systems are required to fulfill the increasing energy requirements of modern portable electronic devices and electric vehicles [1], [2]. Conventional lithium-ion batteries (LIBs) not only suffer from limited energy density but are also highly expensive and toxic. Lithium sulfur (Li-S) batteries are an attractive candidate for next generation power sources due to their high theoretical capacity (1675 mAh g−1) and high energy density (2600 Wh kg−1), which is a six-fold increase over that of conventional LIBs [3]. The natural abundance of sulfur and environmental favorability are also substantial advantages. However, the complex electrochemical system of Li-S batteries with the insulating nature of sulfur, dissolved lithium polysulfide intermediates (Li2Sn, 4 ≤ n ≤ 8), and electrode volume expansion during the lithiation and de-lithiation processes has hindered commercialization. These issues result in low utilization of the active material, severe capacity fading, and safety problems [4], [5].

Various strategies have been applied to overcome the cathode limitations to improve the electrochemical performance of Li-S batteries, such as encapsulating sulfur into the porous carbon matrix [6], [7], [8], [9] and wrapping with a conductive polymer [10], [11], [12]. These strategies improve the utilization of the active material, reduce polysulfide dissolution and provide a buffer for the deformation of electrodes; however, the issues of active material loss and associated capacity fading remain unresolved due to physical interaction limitations.

A new cathode active material for rechargeable Li-S batteries was introduced by Wang et al. [13], [14], [15], where S atoms bind to a polyacrylonitrile (PAN) matrix during the pyrolysis process. The reaction mechanism of sulfurized polyacrylonitrile (SPAN) is different from that of a traditional Li-S battery. Li-SPAN chemistry is a solid-to-solid single-phase reaction while Li-S chemistry is a solid–liquid-solid reaction [16], [17]. The Li-SPAN cell features high sulfur utilization, high Coulombic efficiency, and excellent cycling stability, because the polysulfide generation during the charge and discharge processes can be avoided [18], [19].

However, the limited sulfur content of the composite (typically less than 50 wt%) hinders the development of SPAN composite [20], [21], [22]. A hybrid system consisting of both SPAN and elemental S was investigated by Yin et al. [23] to solve this problem. Short-term cyclability was demonstrated, probably because high sulfur utilization and cycle stability are very difficult to maintain over long cycles due to the difference in electrochemical mechanisms between Li-SPAN and Li-S cells. Enhancing the sulfur loading per electrode remains challenging because the traditional method of electrode fabrication involves slurry casting comprising the active material, conducting additive, and binding agent on the Al foil current collector. The collective weight of an Al foil current collector (∼5.0 mg cm−2) and binding agent dominate the overall electrode weight, leading to the low mass loading of S on the whole cathode electrode and reducing the available capacity per unit mass of the electrode [24], [25], [26], [27]. Therefore, a new strategy was proposed to improve the loading mass of S in the overall electrode and expand its applications due to its flexibility and ductility.

We prepared a freestanding porous sulfurized polyacrylonitrile/vapor grown carbon fiber (SVF) composite through a facile electrospinning process followed by sulfurization, as the cathode material for high-performance Li-S batteries. The SVF fiber with a highly porous structure effectively improved the wettability, accessibility, and absorption of the electrolyte to facilitate rapid ion transfer in the cell. Vapor-grown carbon fibers (VGCFs) with extraordinary electronic conduction were embedded inside the SVF composite to ensure high composite conductivity, improve the electrochemical performance at high C-rates, and overcome the conductivity limitations of SPAN composites at high C-rates [28]. The freestanding SVF fiber can be used directly as the cathode without a current collector and binder, which greatly enhances the active material loading of the whole electrode. The freestanding porous SVF fiber cathode material demonstrated excellent electrochemical performance and cycling stability, and is a promising cathode material for advanced Li-S batteries.

Section snippets

Materials

Polyacrylonitrile (PAN, average MW 150,000; Sigma-Aldrich), polystyrene (PS; Yakuri Pure Chemicals Co., Ltd.), vapor grown carbon fibers (VGCFs; Showa Denko K.K.), N,N-Dimethylformamide (DMF, 99.0%; Samchun Pure Chemical Co., Ltd.), N-methylpyrrolidone (NMP, 99.5%; Samchun Pure Chemical Co., Ltd.), carbon disulfide (CS2; High Purity Chemicals), and sulfur (S, 99.5%; Sigma-Aldrich) were used as received.

Preparation of SVF and SP composites

The electrospinning solution was prepared by dissolving PAN (3 g) and PS (1 g) in DMF (30 g)

Results and discussion

A facile and scalable approach was developed to synthesize the freestanding porous SVF cathode material as shown in Fig. 1. VGCFs were distributed uniformly in the mixed solution of PAN and PS to enhance the electrical conductivity and ensure full utilization of the active material at high C-rates. PS formed a microemulsion in the solution, which was stretched during the electrospinning process [29]. When the electrospun PAN/PS/VGCF fibers were sulfurized at a high temperature, the

Conclusions

A freestanding porous sulfurized polyacrylonitrile/vapor grown carbon fiber (SVF) composite was developed as a cathode active material to increase the mass loading of sulfur of the overall electrode and provide a new cathode material for flexible Li-S batteries. The binder/current collector-free SVF electrode combined the advantages of sulfurized polyacrylonitrile with excellent physical structure and conductivity. The highly porous structure of the composite enhanced the wettability,

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (No. NRF-2017R1A4A1015711).

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