Electrolyte-controlled discharge product distribution of Na–O2 batteries: a combined computational and experimental study

Abstract

Tuning the composition of discharge products is an important strategy to reduce charge potential, suppress side reactions, and improve the reversibility of metal–oxygen batteries. In the present study, first-principles calculations and experimental confirmation were performed to unravel the influence of O₂ pressure, particle size, and electrolyte on the composition of charge products in Na–O₂ batteries. The electrolytes with medium and high donor numbers (>12.5) are favorable for the formation of sole NaO₂, while those with low donor numbers (<12.5) may permit the formation of Na₂O₂ by disproportionation reactions. Our comparative experiments under different electrolytes confirmed the calculation prediction. Our calculations indicated that O₂ pressure and particle size hardly affect discharge products. On the electrode, only one-electron-transfer electrochemical reaction to form NaO₂ takes place, whereas two-electron-transfer electrochemical and chemical reactions to form Na₂O₂ and Na₃O₄ are prevented in thermodynamics. The present study explains why metastable NaO₂ was identified as a sole discharge product in many experiments, while thermodynamically more stable Na₂O₂ was not observed. Therefore, to achieve low overpotential, a high-donor-number electrolyte should be applied in the discharge processes of Na–O₂ batteries.