Vulcanized polymeric cathode material featuring a polyaniline skeleton for high-rate rechargeability and long-cycle stability lithium-sulfur batteries
Introduction
Lithium (Li)-ion batteries (LIBs) with high rate performance and high capacity are currently considered an attractive power source for a wide variety of applications, such as electric and hybrid-electric vehicles [[1], [2], [3], [4]]. Despite extensive research, the overall energy density of LIBs is still limited by the low capacity of current cathode materials, which ranges from 150 to 270 mAh g−1. Sulfur is a promising cathode material because in addition to its natural abundance, low cost, and environmental friendliness, it has the high theoretical specific capacity of 1675 mAh g−1 [[5], [6], [7], [8]]. However, the low electrical conductivity of sulfur (S) (5 × 10−30 S cm−1), dissolution of polysulfide into electrolytes, and the volume expansion of S during cycling limit the cycle life of the S cathode and its application in rechargeable Li-S batteries [9,10].
In response, substantial efforts have been devoted to mitigating the polysulfide shuttling effect to enhance cyclic stability. For example, new nanostructured materials have been prepared for use with S and various types of carbon, such as porous carbon, carbon nanotubes, carbon hollow spheres, and graphene [[11], [12], [13], [14], [15], [16], [17]]. These strategies in response to the weak interactions between the carbon and LixS along with the physical encapsulation of sulfur inadequately relieving the dissolution of polysulfides, have improved the cycling stability of S-based electrodes [18]. However, these methods, involving mostly physical interactions that do not comprise covalent bonding, have only frail adsorption. In addition, the rate performance of Li-S batteries is still limited, and the discharge rate remains extremely low, ranging from only 0.1C–1C, which is unsatisfactory. Moreover, the sulfur loading of the electrode also quiet low owing to large carbon materials content, which limits the high energy density application of Li-S batteries.
Another promising approach for solving polysulfide shuttling is using polysulfide adsorbent separator [19]. Recent studies, have shown that cation-selective organic membranes, such as sulfonated polymer and Nafion, were able to counteract the shuttle effect and be used as separators for Li-S batteries [[20], [21], [22]]. Nonetheless, these separators still introduced free polysulfides into the battery system, causing concerns for long-term application and increased fabrication cost. Therefore, in another recent study, sulfur was copolymerized to form a polymeric cathode exhibiting satisfactory cyclic stability; unfortunately, it still required physical confinements to enhance the cycle life and increase the rate performance [23].
As response for permanently solving shuttling effect of polysulfides, this work presents the formation of cross-linked networks with numerous covalent bonds between the sulfur and polymer as the cathode material. Under these conditions, the redox reaction between the S and Li mostly occurs with the lower-order polysulfides of the vulcanized polymer. Polyaniline can act not only as an electronically conductive matrix, but also as a framework to bond the polysulfides. Therefore, this polymeric cathode features a reversible capacity of 418 and 230 mAh g−1 at 0.1 and 10 A g−1, respectively; further, specific capacity retention remains at 90% even after 300 cycles. Most interestingly, the polymeric cathode can function in carbonated-electrolytes systems, which differs from typical Li-S batteries. Moreover, this material can be easily prepared and so can be considered a promising candidate for practical use, thereby leading to a dramatic performance improvement in Li-S batteries.
Section snippets
Preparation of S@P and S@h-P composites
The nanocomposites were synthesized through chemical-oxidation polymerization of aniline on the sulfur particles. A 10 mL solution of isopropyl alcohol containing 0.05 g of sulfur particles was stirred before the synthesis, and then 0.05 g of aniline monomers in 10 mL 0.5 M H2SO4 was added to this suspension. The mixtures were subsequently stirred overnight at room temperature, after which a solution of 0.5 M H2SO4 containing the oxidant ammonium peroxodisulphate, or (NH4)2S2O8, was
Characterization of S@h-P polymer/sulfur cathode
Preparation of the S@h-P polymeric cathode is schematically illustrated in Fig. 1. Sulfur powder was mixed thoroughly with aniline monomer solution, after which the polyaniline polymerization process occurred. Subsequently, the sulfur was coated with polyaniline by dispersing the ground sulfur/polyaniline (S@P) powder, followed by calcinations at 300 °C to form the S@h-P polymeric cathode. Because the sulfur is bonded to the polyaniline chain, the polysulfide dose not dissolve into the liquid
Conclusion
A vulcanized polymeric cathode was synthesized by polyaniline and sulfur via heat treatment. Morphological and structural investigations revealed that this novel polymeric cathode was uniform and had no free sulfur particles; further, XRD and DSC results showed no crystal sulfur in these analyses. The S@h-P features excellent cyclic stability with a commercially-available separator without any modifications, which is a great improvement to typical Li-S batteries. Moreover, polyaniline provides
Acknowledgments
The authors would like to thank the Ministry of Science and Technology, Taiwan, for their generous financial support of this research.
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These authors contributed equally to the work.