Lithium/sulfur (Li/S) cells are receiving significant attention as an alternative power source for zero emission vehicles and advanced electronic devices due to the very high theoretical specific capacity (1675 mA·h/g) of the sulfur cathode.
Li/S cells may supplant current Li-ion batteries that are not able to meet the ever-increasing demands of advanced technologies and the need for lower cost. For example, the energy-storage capacity of batteries must be dramatically improved to increase the driving range of current electric vehicles. For the development of advanced electric vehicles that can provide ∼300 mi range, the battery should provide a cell-level specific energy of 350−400 W·h/kg. This would require almost double the specific energy (∼200 W·h/kg) of current lithium-ion batteries. In addition, the cycle life must be improved to more than 1000 cycles, preferably up to 1500 cycles, and a rate performance greater than 2C would be necessary to provide a peak power of ∼600 W/kg or higher.
In earlier Li/S cells, capacity faded rapidly due to sulfur loss from the cathode by formation of soluble polysulfides in the electrolyte during the charge/discharge cycle. However, the poor cycle life and rate capability have remained a grand challenge, preventing the practical application of this attractive technology.
Now, a Li/S cell employing a cetyltrimethyl ammonium bromide (CTAB)-modified sulfur-graphene oxide (S−GO) nano-composite cathode can be discharged at rates as high as 6C (1C =1.675 A/g of sulfur) and charged at rates as high as 3C while still maintaining high specific capacity (∼800 mA·h/g of sulfur at 6C), with a long cycle life exceeding 1500 cycles and an extremely low decay rate (0.039% per cycle), perhaps the best performance demonstrated so far for a Li/S cell. The initial estimated cell-level specific energy of this cell was ∼500 W·h/kg, which is much higher than that of current Li-ion cells (∼200 W·h/kg). Even after 1500 cycles, it demonstrated a very high specific capacity (∼740 mA·h/g of sulfur), which corresponds to ∼414 mA·h/g of electrode: still higher than state-of-the-art Li-ion cells. Moreover, these Li/S cells with lithium metal electrodes can be cycled with an excellent Coulombic efficiency of 96.3% after 1500 cycles, which was enabled by a new formulation of the ionic liquid-based electrolyte. The performance suggests that Li/S cells may already be suitable for high-power applications such as power tools. Li/S cells may now provide a substantial opportunity for the development of zero-emission vehicles with a driving range similar to that of gasoline vehicles.
In addition, a flexible adhesive, or binder, holds the cathode materials in place despite the electromechanical expansion and contraction that occurs during the charge/discharge cycle. Finally, the Berkeley Lab cell employs an improved ionic liquid electrolyte that further reduces polysulfide formation and blocks trace amounts from binding to the lithium metal anodes of the Li/S cells. The combination of features in the Berkeley Lab cell provides an unprecedented level of performance and reliability for Li/S cells.
In summary, the Berkeley Researchers have developed a long-life, high-rate Li/S cell with a high specific energy through a multifaceted approach by uniquely combining CTAB-modified S−GO nanocomposite with an elastomeric SBR/CMC binder and an ionic liquid-based novel electrolyte containing LiNO3additive. These Li/S cells exhibited a very high initial discharge capacity of 1440 mA·h/g of sulfur at 0.2C with excellent rate capability of up to 6C for discharge and 3C for charge while still maintaining high specific capacity (e.g., ∼800 mA·h/g of sulfur at 6C). They have further demonstrated cycling performance up to 1500 cycles with extremely low decay rate of 0.039% per cycle, which is one of the best performances reported to the best of their knowledge.
With the estimated high specific energy, long cycle life, and excellent rate capability demonstrated in this work, the Li/S cell seems to be a promising candidate to challenge the dominant position of the current Li-ion cells.
This work was supported by University of California, Office of The President, UC Proof of Concept award No. 12PC247581, and by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract No. DE-AC02- 05CH11231.
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