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Flexible Carbon Nanotubes-Graphene/Sulfur Composite Film: FreeStanding Cathode for High-Performance Lithium/Sulfur Batteries Yan Chen, Songtao Lu, Xiaohong Wu, and Jie Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02596 • Publication Date (Web): 27 Apr 2015 Downloaded from http://pubs.acs.org on May 3, 2015
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The Journal of Physical Chemistry
Flexible Carbon Nanotubes-Graphene/Sulfur Composite Film: Free-Standing Cathode for High-Performance Lithium/Sulfur Batteries Yan Chen, Songtao Lu, Xiaohong Wu,*, Jie Liu*, †
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Department of Chemistry, Harbin Institute of Technology, Harbin, 150001, P. R. China.;
‡Department of Chemistry, Duke University, Durham, NC 27708, USA.
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Flexible Carbon Nanotubes-Graphene/Sulfur Composite Film: Free-Standing Cathode for High-Performance Lithium/Sulfur Batteries ABSTRACT: Flexible lithium batteries with high energy density have received tremendous interest recently due to their potential applications in flexible electronic devices. Herein, we report a novel method to fabricate highly flexible and robust carbon nanotubes-graphene/sulfur (CNTs-RGO/S) composite film as free-standing cathode for flexible Li/S batteries with increased capacity and significantly improved rate capability. The free-standing CNTs-RGO/S cathode was able to deliver a peak capacity of 911.5 mAh g-1sulfur (~483 mAh g-1electrode) and maintain 771.8 mAh g-1sulfur after 100 charge−discharge cycles at 0.2C, indicating a capacity retention of 84.7%, which were both higher than the cathodes assembled without CNTs. Even after 100 cycles, the cathode showed a high tensile strength of 62.3 MPa. More importantly, the rate capability was improved by CNTs introducing. The CNTs-RGO/S cathode exhibited impressive capacities of 613.1 mAh g-1sulfur at 1C with a capacity recuperability of ~ 94% as the current returned to 0.2C. These results demonstrate that the well-designed nanocomposites are of great potential as the cathode for flexible lithium sulfur (Li/S) batteries. Such improved electrochemical properties could be attributed to the unique coaxial architecture of the nanocomposite, in which the evenly dispersed CNTs enable electrodes with improved electrical conductivity and mechanical property, better ability to avoid the aggregation and ensure the dispersive distribution of the sulfur species during charge/discharge process. 2
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The Journal of Physical Chemistry
1. INTRODUCTION The rapid development of flexible electronic devices, such as flexible liquid crystalline displays (LCDs), organic light-emitting diodes (OLEDs), wearable computers, have stimulated intense research on thin, flexible lithium batteries (LBs) due to their high energy and power densities as well as low maintenance cost.1-5 However, their research is still in the nascent stage and the large-scale use is limited, mainly because of the low capability to fabricate flexible electrodes. In this context, the primary components of flexible LBs are the electrodes fabricated by coating with slurry-typed mixtures of nanostructured active materials on a metal strips as current collectors, which are mainly aluminium (5 mg/cm2) and copper (10 mg/cm2).6 In recent years, fabricating flexible electrodes by depositing electroactive material directly on flexible and conductive substrates, such as CNT buckypaper,7 graphene paper8,9 and carbon cloth,10 etc., have shown advanced performance for flexible LBs due to their high conductivity and flexibility. For instance, flexible cathodes consisting of networks of flexible CNTs and nanosized LiMn2O4 or Ni-based cathode materials (LiNi0.6Co0.2Mn0.2O2),2,11-14 have shown improved performance. However, the performance of these batteries will be limited by the low capacity of the cathode materials. Hence, much considerable research attempt has been made to explore new electrode materials with high energy and high power density. Especially, among these materials, sulfur has a high theoretical capacity (1675 mAh g−1) and a high specific energy of 2580 Wh/kg, assuming complete reaction between Li and S.15-17 Other advantages of sulfur include its extremely low cost (about $150 per ton), nontoxicity, and widespread availability. Consequently, it is anticipated that sulfur based lithium batteries are of particular promise for next-generation energy 3
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storage systems. However, the practical application of sulfur as cathode is hampered by its poor cycle performance, resulting from the inherent poor electrical conductivity of sulfur (5 × 10−30 S cm−1), the formation of soluble polysulfide Li2Sx (2
