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Optical Microscopy Reveals the Ambient Sodium–Sulfur Discharge Mechanism

2020; American Chemical Society; Volume: 9; Issue: 1 Linguagem: Inglês

10.1021/acssuschemeng.0c07180

2168-0485

Rachel Carter, Addison NewRingeisen, Daniel Reed, Robert W. Atkinson, Partha P. Mukherjee, Corey T. Love,

Advanced Battery Technologies Research

With growing demand for energy storage, there is renewed interest in ambient sodium–sulfur batteries, which boast raw material costs below $1/kWh owing to the natural abundance and high theoretical energy density of the pairing. As with lithium, sodium electrochemically reacts with sulfur in ether-based electrolytes, and the intermediate discharge products (polysulfides) dissolve in the battery electrolyte. These polysulfide intermediates have distinct colors, from red-brown to yellow. Additionally, when the solvent permits chemical reordering, the S3•– radical is detected with a blue hue. Radicalization hinders the electrochemical reaction by altering charge balance. Since the reaction intermediates exist with distinct colors, their evolution can be identified during electrochemical discharge with an in-situ optical cell. Optical analysis facilitates detection and characterization of intermediate products across a broader concentration range that is not accessed by more complex in-situ UV–vis spectroscopy. We demonstrate the utility of in-situ optical microscopy for comparing the ambient discharge mechanism in electrolytes from the glyme family. These chain-like solvents, from monoglyme (G1) to tetraglyme (G4), have a greater stabilizing effect on sodium electroplating than for lithium, warranting their investigation at the sulfur cathode. Both the in-situ experiment and stoichiometric solutions reveal that G1 results in the lowest polysulfide solubility and the least sulfur radicalization, while G4 has the greatest. G2 falls between them. Image analysis of the electrolyte between the sulfur working electrode and sodium counter allow for the red, green, and blue image pixilation (RGB) and image brightness to be assessed. With this analysis, we can assign the evolution of particular polysulfides to discharge voltage features.