Mechanistic Insights into the Challenges of Cycling a Non-aqueous Na-O2 Battery

TL;DR

This paper investigates the causes of poor cycling performance in non-aqueous Na-O2 batteries, identifying side reactions and electrode surface changes as key issues affecting longevity.

Contribution

The study provides mechanistic insights into the chemical and physical factors limiting the cycle life of Na-O2 batteries, using advanced characterization techniques.

Findings

Side reactions from NaO2 cause electrode surface coverage and hinder electron transfer.

Electrode surface and porosity change during cycling, affecting ion diffusion.

Inhomogeneous reactions across electrodes lead to cell failure.

Abstract

Superoxide-based non-aqueous metal oxygen batteries have received considerable research attention, as they exhibit high energy densities and round-trip efficiencies. The cycling performance, however, is still poor. Here we study the cycling characteristic of a Na-O2 battery using solid-state nuclear magnetic resonance, Raman spectroscopy and scanning electron microscopy. We found that the poor cycling performance is primarily caused by the considerable side reactions stemming from the chemical aggressiveness of NaO2 both as a solid phase and dissolved species in the electrolyte. The side reaction products cover electrode surfaces and hinder electron transfer across the electrode-electrolyte interface, being a major reason for cell failure. In addition, the available electrode surface and porosity change considerably during cell discharging and charging, affecting the diffusion of soluble species (superoxide and water) and resulting in inhomogeneous reactions across the electrode. This study provides insights into the challenges associated with achieving long-lived superoxide based metal-O2 batteries.