Multiple Strategies for Development of Ambient-Temperature Sodium-Sulfur (Na-S) Batteries
Authors: Yu Xingwen , Manthiram Arumugam 
Published: February 2020

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.

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