Novel Long Cycle Life Room Temperature Sodium-Sulfur Batteries

Sulfur is natural abundant, cheap, non-toxic and has about seven times the electrochemical capacity of standard Li-ion battery cathode active materials (1672 compared to 250 mAh g-1), considering the conversion to S2-.[1] Due to likewise low costs and high reactivity towards sulfur, sodium poses as an optimal candidate for the cell reaction with sulfur.
Since the 1980s sodium sulfur batteries are used as commercial stationary energy storage systems. They consist of a sodium negative electrode, a sulfur positive electrode and a β''-alumina solid electrolyte in between.[2] Due to low ion conductivity of the materials, this battery type has to be heated to temperatures above 300 °C, resulting in safety challenges related to molten sodium and to increased operation costs. To take advantage of the low cost nature of sodium and sulfur, while avoiding its downsides, sodium-sulfur batteries driven at room temperature are aspects of today’s research. One of the largest differences is the use of liquid electrolytes in room temperature cells, in order to ensure sufficient ion conductivity.
However, first reports on room-temperature Na-S-cells indicate significant challenges related to a parasitic reaction of dissolved sodium polysulfide species limiting the coulomb efficiency, the specific capacity and cycle life of this system (so-called polysulfide shuttle).[3]

In this work, electrolyte composition, sulfur/carbon composite cathodes as well as anode materials were tailored to result in room temperature sodium-sulfur batteries with significantly reduced polysulfide shuttle. Electrolyte additives were identified being able to enhance the cell efficiency and increase the sulfur utilization through a redox mediation mechanism. This cell chemistry enables discharge capacities of 1000 mAh g-1 as well as improved capacity retentions exceeding values from recent literature.[3+4] Our results demonstrate the feasibility of stable and low cost sodium-sulfur batteries at room temperature enduring over 1000 reversible charge / discharge cycles.

References:
[1] X. Ji, L. F. Nazar, Nat. Mater., 2009, 8, 500–506.
[2] Z. Wen, Z. Lin, Solid State Ionics, 2008, 179, 1697–1701.
[3] H. Ryu, H.J. Ahn, J. Power Sources, 2011, 196, 5186–5190.
[4] I. Bauer, M. Kohl, H. Althues, S. Kaskel, Chem. Comm., 2014, 50, 3208-3210.