An Electrochemical Study on the Cathode of the Intermediate Temperature Tubular Sodium-Sulfur (NaS) Battery

Abstract

The development of low-cost energy storage schemes is imminent in light of the ever-growing demand of electricity. Sodium-sulfur (NaS) batteries offer low-cost technology for energy storage applications due to the intrinsically high capacities of elemental sodium and sulfur as well as their abundant resources. Operating this battery technology on the intermediate range (130-200°C) can lead to lower material costs, mitigate thermal management and safety issues and enhance cycle life. Herein, an electrochemical study (in the form of different carbonaceous materials) on the cathode of the IT NaS cell is performed at 150°C and a concentration range of 1.5 to 3 M sodium pentasulfide dissolved in tetraglyme, showing a robust long term performance (42 days of continuous cycling) with a volumetric energy density of 83 Wh L-1. Most importantly, the cell was eligible for a tenfold volume scale-up considerably enhancing its capacity (790 mAh) but in the same time somewhat hindered by mass transport, especially during the end of the discharge process as manifested by electrochemical impedance spectroscopy.

Operational Strategies to Improve the Performance and Long‐Term Cyclability of Intermediate Temperature Sodium‐Sulfur Batteries

“…[16] The volumetric capacity (mAh/mlcatholyte) of the scaled-up cell did not scale up uniformly with the larger catholyte volume. [14] But interestingly, the scale-up cell exhibited a comparatively lower internal resistance at the initial stages of cycling compared to the lab-scale cell. Since the cell casing material, BASE tube, and catholyte composition (2.5 M Na 2 S 5 in TEGDME) were the same, we attribute this discrepancy to the different configurations of the cells.…”

“…[12] With the above in mind, intermediate temperature (IT) NaS batteries operable at 150°C aspire to combine the best of both HT and RT NaS technologies. [13][14][15][16][17] The IT-NaS adopted an analogous tubular configuration to the HT-NaS with the BASE solid separator in-between central anode and outer cathode. [18] The commercialized HT-NaS and Zebra battery technologies prefer the central sodium anode to be encapsulated by tubular BASE than the stacked planner cell designed with flat plate BASE, owing to safety reasons and simplified manufacturing process.…”

“…[19] To reduce the operating temperature, the cathodic side of IT-NaS was altered by presolvating the sulfur/polysulfide mixture in an organic solvent analogous to the one used in RT-NaS's electrolytes. [13,14,16] The selection of suitable organic solvent and co-solvent with adequate solubility for sulfur/polysulfide is of paramount importance, together with thermal, chemical, and electrochemical stability. [17,20] More precisely, having the BASE within the interface of anode-electrolyte-cathode mitigates polysulfide shuttling and dendrite growth; meanwhile, the operating temperature of IT-NaS improves the BASE's Na + conductivity (BASE's Na + conductivity at 100 and 200°C are 16.7 and 100 mS/cm, respectively [11] ).…”

Remedies for Polysulfide Dissolution in Room‐Temperature Sodium–Sulfur Batteries

“…Accordingly, there are three types of NaS batteries with different operating temperature regions, including HT (270–350 °C), intermediate temperature (IT, 130–180 °C), and room temperature (RT), which have been developed with different cell configurations . As illustrated in Figure b, HT‐NaS and IT‐NaS batteries are typically constructed in a tubular design by using metal cases, which contain central molten Na as the cathode, which is confined in a beta‐alumina solid electrolyte (BASE) tube with a molten S cathode surrounding the tube due to the high operation temperature . NGK Insulator, Inc. commercialized the HT‐NaS system in Japan in 2002, storing megawatt‐size energy for utility‐based load‐leveling and peak‐shaving applications 1a,9.…”

“…a) Phase diagram of Na 2 S‐S for high, intermediate, and room temperatures (HT, IT, and RT); b) Schematic representation and operation of tubular HT‐NaS and IT‐NaS batteries. Reproduced with permission . Copyright 2019, The Electrochemical Society.…”