Full Length ArticleFreestanding porous sulfurized polyacrylonitrile fiber as a cathode material for advanced 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.
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).
References (36)
- et al.
Nanostructured nitrogen-doped mesoporous carbon derived from polyacrylonitrile for advanced lithium sulfur batteries
Appl. Surf. Sci.
(2016) - et al.
A sulfur-carbon composite for lithium/sulfur battery based on activated vapor-grown carbon fiber
Solid State Ionics
(2013) - et al.
Enhanced performance of lithium sulfur battery with polypyrrole warped mesoporous carbon/sulfur composit
J. Power Sources
(2014) - et al.
Polymer lithium cells with sulfur composites as cathode materials
Electrochim. Acta
(2003) Liquid electrolyte lithium/sulfur battery: Fundamental chemistry, problems, and solutions
J. Power Sources
(2013)- et al.
Cycleability of sulfurized polyacrylonitrile cathode in carbonate electrolyte containing lithium metasilicate
J. Power Sources
(2015) - et al.
Porous spherical polyacrylonitrile-carbon nanocomposite with high loading of sulfur for lithium-sulfur batteries
J. Power Sources
(2016) - et al.
Lithium storage in conductive sulfur-containing polymers
J. Electroanal. Chem.
(2004) - et al.
Kinetic investigation of sulfurized polyacrylonitrile cathode material by electrochemical impedance spectroscopy
Electrochim. Acta
(2011) - et al.
High performance freestanding composite cathode for lithium-sulfur batteries
Electrochim. Acta
(2016)
Issues and challenges facing rechargeable lithium batteries
Nature
Building better batteries
Nature
Balancing surface adsorption and diffusion of lithium-polysulfides on nonconductive oxides for lithium-sulfur battery design
Nat. Commun.
Porous hollow carbon@sulfur composites for high-power lithium-sulfur batteries
Angew. Chem. Int. Ed.
Sulphur-TiO2 yolk-shell nanoarchitecture with internal void space for long-cycle lithium-sulphur batteries
Nat. Commun.
Nitrogen-doped mesoporous carbon: A top-down strategy to promote sulfur immobilization for lithium-sulfur batteries
ChemSusChem
Sulfur encapsulated porous rattle-type carbon sphere as cathode material for lithium sulfur batteries
J. Nanosci. Nanotechnol.
Sulfur quantum dots wrapped by conductive polymer shell with internal void spaces for high-performance lithium-suflur batteries
J. Mater. Chem. A
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