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

Tuesday, October 13, 2015: 16:10
102-C (Phoenix Convention Center)
N. Ortiz-Vitoriano (CIC Energigune, Massachussets Institute of Technology), C. V. Amanchukwu, D. Kwabi, P. T. Hammond (Massachusetts Institute of Technology), and Y. Shao-Horn (Massachusetts Institute of Technology)
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 Recently, a metal-air battery in which lithium has been replaced by sodium has received increasing attention.2 Although Na-O2 batteries present lower gravimetric energies on a cell basis (1605 or 1108 Wh/kg based on Na2O2 or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/kgLi2O2),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.



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).