Stability of NaO₂ and LiO₂ discharge products in metal-air batteries: density functional theory study

Materials Modelling Centre, School of Physical and Mineral Sciences, University of Limpopo, Private Bag x1106, Sovenga 0727, South Africa

^* Corresponding author: brianramogayana@gmail.com

Abstract

Rechargeable metal-air batteries are considered the next generation energy storage devices with the highest energy density of ~200 Wh/kg (which is much higher than the Li-ion batteries with ~150 Wh/kg). However, their practical applications are widely affected by the formation of unstable discharge products (like Na/LiO₂, Na/Li₂O, and Na/LiO) which react with electrolyte or the porous electrode material leading to either fire or explosion. Here, we explore the stability of the bulk pyrite and marcasite lithium/sodium superoxide (Li/NaO₂) discharge products produced in Li and Na-air batteries using density functional theory calculations. The calculated lattice and heats of formations are consistent with available reported data. The electronic properties of show the Pnmm LiO₂ as the most stable structure with the least density of states at the Fermi. The vibrational properties also show no imaginary vibrations in all directions for Pnmm LiO₂. This finding can help direct and give an insight into the stability of major discharge products and give research direction towards controlling the formation of desired M-O discharge products in the batteries.