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Reinforced conductive confinement of sulfur for robust and high performance Lithium-sulfur batteries Chao Lai, Zhenzhen Wu, Xingxing Gu, Chao Wang, Kai Xi, Ramachandran Vasant Kumar, and Shanqing Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07978 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 19, 2015
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ACS Applied Materials & Interfaces
Reinforced conductive confinement of sulfur for robust and high performance Lithium sulfur batteries Chao Laia,b, Zhenzhen Wua, Xingxing Gub, Chao Wang*a, Kai Xi*c, R. Vasant Kumarc, Shanqing Zhang*b *a
School of Chemistry and Chemical Engineering, and Jiangsu Key Laboratory of Green
Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou, Jiangsu 221116, China. *b
Center for Clean Environment and Energy, Environmental Futures Research Institute, Griffith
School of Environment, Griffith University, Gold Coast Campus, QLD 4222, Australia. . c
Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB3
0FS, UK. .
*Corresponding author. E-mail:
[email protected] (Chao Wang);
[email protected] (Shanqing Zhang);
[email protected] (Kai Xi).
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Abstract Sulfur is an attractive cathode material in energy storage devices due to its high theoretical capacity of 1672 mAh g-1. However, practical application of lithium sulfur (Li-S) batteries can be achieved only when the major barriers, including the shuttling effect of polysulfides (Li2Sx, x=3~8), the significant volume change (~80%) and the resultant rapid deterioration of electrodes, are tackled. Here, we propose an “inside-out” synthesis strategy by mimicking the fruit pomegranate structure to achieve conductive confinement of sulfur to address these issues. In the proposed pomegranate-like structure, sulfur and carbon nanotubes composite is encapsulated by the in-situ formed amorphous carbon network, which allows the regeneration of electroactive material sulfur and the confinement of the sulfur as well as the lithium polysulfide within the electrical conductive carbon network. Consequently, a highly robust sulfur cathode is obtained, delivering remarkable performance in a Li-S battery. The obtained composite cathode shows a reversible capacity of 691 mAh g-1 after 200 cycles with impressive cycle stability at the current density of 1600 mA g-1.
Keywords: Lithium sulfur batteries; space confining; cathode; high capacity retention; carbon nanotubes; amorphous carbon.
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1. Introduction Lithium sulfur (Li-S) batteries have been well-recognized as one of the most promising options for electric vehicles （EVs） for realizing long single-charge travel distance owing to their impressive high theoretical energy density of 2500 Wh kg-1, which is several times higher than current lithium-ion batteries of ca. 130-220 Wh kg-1.1,2 Despite enthusiastic studies over past decade, Li-S batteries are not yet commercialized due to three major barriers.1,2 The first problem is the insulating nature of element sulfur, the discharge product Li2S2 and Li2S, which results in the low utilization of the active material sulfur. The second issue is the dissolution of the polysulfide intermediate products formed during the discharge/charge process. The generated higher-order polysulfides can be dissolved in the electrolyte and diffuse to anode and then chemically react with the lithium, forming lower-order polysulfides. These reduced products can also diffuse back to the sulfur cathode, and this cyclic process is known as the “shuttle effect”.3,4 Such a parasitic process results in poor cycle life, low coulombic efficiency and rapid decline of the battery life. The third problem is the substantial volume expansion (up to 80%) of sulfur during charge/discharge processes, which destroys the integrity of the electrode and leads to the loss of electrical contact and even the failure of the cell.1,2 To address these issues, various strategies have been developed, focusing on the modification of sulfur cathodes, electrolytes, binder and separators.1,2,5-18 Among these proposed methods, the development of sulfur/porous carbon composite and surface coating of sulfur-based composites are most promising and frequently investigations. Significant improvements in electrochemical performance and extension of battery cycling life have been reported.5-12 The main idea of these methods is to restrict the dissolution of polysulfide intermediates and simultaneously enhance the
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conductivity of the entire cathode. However, there is a lot of room for improvement. In fact, capacity decay still can be observed, especially in the initial several cycles. As an example, a yolk-shell sulfur-TiO2 nanocomposite has a 20% polysulfides loss into the electrolyte after 1000 cycles.10,15 Furthermore, the consistency of the electrodes, the costs of materials and synthesis process shall be taken into account in the manufacturing practice. The preparation of some carbon nanomaterials with small nanopores (
