Electrolyte Effects on Chemical Stability of NaO2 in Na-O2 Batteries

Rechargeable metal-air (oxygen) batteries are receiving intense interest as possible alternatives to lithium-ion batteries, because of their potential to provide higher gravimetric energies. ¹ While much attention has been focused on aprotic Li-O₂ batteries, substantial challenges must be addressed before widespread commercialization is possible. Recently, a metal-air battery in which lithium has been replaced by sodium has received increasing attention.² Although Na-O₂ batteries present lower gravimetric energies on a cell basis (1605 or 1108 Wh/kg based on Na₂O₂ or NaO₂ discharge products, respectively)² than Li-O₂ batteries (3505 Wh/kg_Li2O2),¹ much lower charge overpotentials (~100 mV) than those in typical Li-O₂ batteries (~1000 mV) have been reported based on reversible sodium superoxide (NaO₂) formation . Unlike Li-O₂ batteries, for which Li₂O₂ is the only discharge product, NaO₂, sodium peroxide (Na₂O₂) and sodium peroxide dihydrate (Na₂O₂ · 2H₂O), or a mixture, have been identified in Na-O₂ batteries in ether-based electrolytes using a range of carbon electrode types.

Here we show NaO₂ as the only discharge product from Na-O₂ cells with carbon nanotubes in 1,2-dimethoxyethane from X-ray diffraction and Raman spectroscopy. Immediately after discharge, no Na₂O₂ · 2H₂O was found in the discharged electrode, even with H₂O present in the electrolyte up to 6000 ppm. However, Na₂O₂ · 2H₂O was detected upon ageing the electrode in the electrolyte, and the amount of Na₂O₂ · 2H₂O was seen to increase with increasing rest time in the electrolyte. More importantly, the reversibility of the reaction is affected by the presence of Na₂O₂ · 2H₂O, where the cells charged immediately after discharge show a reversible reaction , and the cells charged after resting the electrode in the electrolyte for several hours  showed poor reversibility.

The rechargeable nature of a Na-O₂ cell is governed by the formation and oxidation of NaO₂, making it vital to study the stability of the electrolyte in the presence of NaO₂, and also to prevent the formation of irreversible side products.

 

REFERENCES

1. D. G. Kwabi, N. Ortiz-Vitoriano, S. A. Freunberger, Y. Chen, N. Imanishi, P. G. Bruce, Y. Shao-Horn, MRS Bull. 39, 443 (2014).

2. P. Hartmann, C. L. Bender, M. Vračar, A. K. Dürr, A. Garsuch, J. Janek, P. Adelhelm, Nat. Mater. 12, 228 (2013).