Due to the resource limitation of lithium, electrochemical energy storage with an anodic chemistry of sodium is receiving more attention.¹ Recently, the rapid progress in the R&D of lithium-sulfur (Li-S) batteries has triggered the interest to couple the sodium anode with the high-capacity sulfur cathode and to operate the sodium-sulfur batteries at ambient temperatures.^2-5

According to our preliminary research and a few related studies in the literature,^{6, 7} the charge-discharge of a room-temperature sodium-sulfur (RT Na-S) battery follows similar electrochemical processes to its analogous Li-S battery system with a series of soluble sodium-polysulfide (Na-PS) species involved. Such unique charge-discharge characteristics provide the possibilities/advantages of development of RT Na-S batteries in various ways.

Herein we present three strategies for fabricating the RT Na-S batteries: (1) in a full-charge state with elemental-sulfur cathode, (2) in a full discharge state with sodium-sulfide (Na₂S) cathode, and (3) in an intermediate charge/discharge state with a dissolved sodium-polysulfide (Na-PS) cathode. Strategy 1 demonstrates a conventional RT Na-S battery which requires the use of sodium-metal anode. Assembling the RT Na-S batteries with strategy 2 allows the use of sodium-metal-free anodes (such as carbon-based, silicon-based, or metal oxide-based anodes that are facile for Na-intercalation), which can eliminate the safety concerns of handling Na metal. However, the first charge of this battery needs to initially overcome an energy barricade for the oxidation of the large-crystalline Na₂S to the Na-polysulfides. As such, it usually takes much longer time and a larger overpotential than expected to complete the first charge. Strategy 3 provides a facile way to homogeneously disperse the active sulfur material into the conductive cathode matrix with a liquid-phase active cathode material. It also provides an advantage of using a partially-intercalated, non-sodium-metal anode for fabrication of the RT Na-S batteries. The cell assembled with the Na-PS cathode can be either first discharged or first charged. There is not a first-charge barrier as that shown for the Na₂S-cathode case.

Like the Li-S system, due to the involvement of the soluble Na-polysulfide species, development of the RT Na-S batteries is also facing a critical challenge as recognized as “polysulfide-shuttle” behavior.⁸ In addition to the various strategies described above, a few strategic approaches towards the suppression of the Na-polysulfide shuttle will also be presented based on our previous and ongoing relevant research, including the advanced cathode matrices, structural cell configuration design, and integration of the alternative electrolytes.

References

 

1.    V. Palomares, P. Serras, I. Villaluenga, K. B. Hueso, J. Carretero-Gonzalez and T. Rojo, Energ Environ Sci, 2012, 5, 5884-5901.

2.    C. W. Park, J. H. Ahn, H. S. Ryu, K. W. Kim and H. J. Ahn, Electrochem Solid St, 2006, 9, A123-A125.

3.    T. H. Hwang, D. S. Jung, J. S. Kim, B. G. Kim and J. W. Choi, Nano Lett, 2013, 13, 4532-4538.

4.    D. J. Lee, J. W. Park, I. Hasa, Y. K. Sun, B. Scrosati and J. Hassoun, J Mater Chem A, 2013, 1, 5256-5261.

5.    A. Manthiram and X. W. Yu, Small, 2015, 11, 2108-2114.

6.    X. W. Yu and A. Manthiram, Chemelectrochem, 2014, 1, 1275-1280.

7.    H. Ryu, T. Kim, K. Kim, J. H. Ahn, T. Nam, G. Wang and H. J. Ahn, J Power Sources, 2011, 196, 5186-5190.

8.    I. Bauer, M. Kohl, H. Althues and S. Kaskel, Chem Commun, 2014, 50, 3208-3210.