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High-Energy, Long-Life Lithium-Sulfur Batteries with a Surface-Coated Separator
With increasing demands for high-energy-density rechargeable batteries for transportation and stationary storage applications, lithium-sulfur batteries have become appealing as sulfur exhibits a high theoretical capacity of 1672 mAh g-1. Over the past few years, researchers have explored composite sulfur cathodes and almost conquered the persistent drawbacks: (i) low sulfur utilization and (ii) fast capacity fade and short cycle life.1 Recently, the innovations in cell configurations (porous current collectors, interlayers, and surface-coated separators) have evidenced enhancement in cathode conductivity and suppression of polysulfide diffusion.2-5
To avoid the extra weight introduced by free-standing interlayers, we present here the use of various surface-coated separators for enhancing the electrochemical performance of Li-S cells employing pure sulfur cathodes.2-4 The surface-coated separator possesses a conductive/porous carbon coating on the commercial separator facing the pure sulfur cathode and functions as (i) an upper current collector for facilitating sulfur utilization and (ii) a polysulfide trap for suppressing polysulfide diffusion. The commercial separator in contact with the Li-metal anode serves as an electrically insulating membrane. This robust surface-coated membrane supports the coating layer and enhances its flexibility and mechanical strength. Therefore, cells utilizing various surface-coated separators achieve high discharge capacity with excellent cyclability. In addition to the improved electrochemical performance, the surface-coated separators solve the problems of low sulfur content by utilizing common laboratory supplies and simple processing techniques, allowing it to be easily translated into industrial processes.
Experimental
The surface-coated separators were prepared by coating conductive carbon black (Super P, TIMCAL), multiwall carbon nanotubes (MWCNT, Nanolab), microporous carbon (MPC, Black Pearls 2000, CABOT), or polyethylene glycol (PEG, Aldrich)/MPC on one side of a commercial separator (Celgard). The Super P carbon, MPC, and the PEG/MPC slurries were prepared by mixing, respectively, carbon and polymer/carbon mixtures with isopropyl alcohol, followed by coating onto the Celgard separator by a tape casting method. The MWCNT-coated separator was prepared by vacuum-filtering a uniform MWCNT suspension through a commercial Celgard separator. The resulting surface-coated separators were dried and cut into circular disks.
The pure sulfur cathode was fabricated by coating the active material slurry onto an Al foil current collector by the tape casting method and dried in an air-oven. The active material slurry contained precipitated sulfur (60–70 wt. %), carbon black, and polyvinylidene fluoride binder. The CR2032-type coin cells were assembled with the pure sulfur cathodes, various surface-coated separators (with the coating layer facing the pure sulfur cathode), lithium foil anode (Aldrich), nickel foam spacers, and the electrolyte.
Results and Discussion
Figure 1 illustrates the cell configuration of a Li-S cell employing the surface-coated separator (with the PEG/MPC coating). The PEG/MPC coating side of the composite separator faces the sulfur cathode as the “polysulfide trap” for intercepting the migrating polysulfides before they diffuse to the separator. It also works as an upper current collector to facilitate electron transport for enhancing the electrochemical utilization of sulfur and for reactivating the trapped active material.
The long-term cycling performance of the Li-S cells utilizing a composite cathode or employing various surface-coated separators is summarized in Figure 2. The PEG/MPC-coated separator provides pure sulfur cathodes with the highest initial discharge capacity of 1307 mAh g-1 and the highest reversible capacity approaching 600 mAh g-1 after 500 cycles as compared to other cell configurations. The capacity fading of the cell employing the PEG/MPC-coated separator is only 0.11 % per cycle. Such long lifespan and low capacity fading conclude that the conductive/porous carbon-coated separator facilitates high electrochemical reversibility with a pure sulfur cathode during long-term cycling.
In summary, the surface-coated separator is a practical solution for utilizing pure sulfur cathodes in Li-S cells that exhibit a high discharge capacity, a long lifespan, and high electrochemical reversibility. Moreover, the cells with the lightweight surface-coated separator can utilize the readily-prepared pure sulfur cathodes, making this composite separator an advanced material for narrowing the gap between scientific research and commercial feasibility.
FIGURE CAPTIONS
Figure 1. Schematic configuration of a Li–S cell with a surface-coated separator.
Figure 2. Long-term cycle life testing of Li-S cells utilizing a composite cathode or employing various surface-coated separators (MPC coating and PEG/MPC coating).
REFERENCEs
1. A. Manthiram, Y.-Z. Fu, S.-H. Chung, C. Zu, Y.-S. Su, Chem. Rev. DOI: 10.1021/cr500062v (2014).
2. S.-H. Chung and A. Manthiram, Adv. Funct. Mater., 24, 5299 (2014).
3. S.-H. Chung and A. Manthiram, J. Phys. Chem. Lett., 5, 1978 (2014).
4. S.-H. Chung and A. Manthiram, Adv. Mater, 26, 7352 (2014).
5. G. Zhou, S. Pei, L. Li, D.-W. Wang, S. Wang, K. Huang, L.-C. Yin, F. Li, H.-M. Cheng, Adv. Mater., 26, 625 (2014).