Electrolyte-controlled discharge product distribution of Na–O₂batteries: 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 O pressure, particle size, and electrolyte on the composition of charge products in Na-O batteries. The electrolytes with medium and high donor numbers (>12.5) are favorable for the formation of sole NaO, while those with low donor numbers (<12.5) may permit the formation of NaO by disproportionation reactions. Our comparative experiments under different electrolytes confirmed the calculation prediction. Our calculations indicated that O pressure and particle size hardly affect discharge products. On the electrode, only one-electron-transfer electrochemical reaction to form NaO takes place, whereas two-electron-transfer electrochemical and chemical reactions to form NaO and NaO are prevented in thermodynamics. The present study explains why metastable NaO was identified as a sole discharge product in many experiments, while thermodynamically more stable NaO was not observed. Therefore, to achieve low overpotential, a high-donor-number electrolyte should be applied in the discharge processes of Na-O batteries.

Tailoring Sodium Anodes for Stable Sodium–Oxygen Batteries

“…Many research groups have intended to design available cathodes and electrolytes for Na-O 2 battery. [4,[22][23][24] However, little attention was paid to the problematic Na anode.…”

Growth and dissolution of NaO2 in an ether-based electrolyte as the discharge product in the Na–O2 cell

“…7 The sudden death of the cell chemistry is attributed to the formation of a passivation film between the cubic structures at the end of the discharge process, where the thickness is discharge rate dependent. 8 Although many factors based on NaO 2 growth and morphology have been reported including rate, 2,8,9 impurity concentration, 7,10 and solvent dependence, [11][12][13][14][15][16][17][18] a unified model of the fundamental control parameters of the discharge process remains unclear, with different conclusions operating under similar conditions being made. Although there has been an extensive review carried out by Bender et al 1 that ascertains the differences and similarities, Bi et al 19 analysed the cell setup concluding that glass cell setups afford different discharge products (mainly Na 2 O 2 Á2H 2 O) in comparison to stainless steel cell setups (NaO 2 only).…”

“…The current increases at 4.0 V, highlighting further oxidation of the remaining reaction products. 12,16,24 The sequential description of the AFM images collected around the CV curve begins at the OCP (2.8 V) where the small striations of the freshly cleaved HOPG surface are observed that follow the step edges of the stacked graphene sheets, and the height profiles show that the step edges are 0.8 nm in height ( Fig. 1b and Fig.…”

Electrolytes for Na-O2 Batteries: Towards a Rational Design

“…In fact, it has been demonstrated, by computational and experimental studies, that the nature of the electrolyte is able to control the distribution and morphology of the discharge products. Through thermodynamic calculations and the prediction of crystalline structures, it has been possible to establish the important role of the electrolyte [45]. More specifically, in the design of the correct electrolyte it is of vital importance to analyze the donor number (DN) associated with the electrolyte.…”

“…Even so, most of the published studies analyze the stability of the species in the electrolyte as a function of the DN of the solvent. Wang et al determined that a high DN (> 12.5, which is the situation of the glymes commonly used as solvents) guides the formation of NaO 2 , while a DN below that limit can favor the NaO 2 -Na 2 O 2 mixture [45].…”