Chemical and Electrochemical Differences in Nonaqueous Li–O₂ and Na–O₂ Batteries

Abstract

We present a comparative study of nonaqueous Li-O2 and Na-O2 batteries employing an ether-based electrolyte. The most intriguing difference between the two batteries is their respective galvanostatic charging overpotentials: a Na-O2 battery exhibits a low overpotential throughout most of its charge, whereas a Li-O2 battery has a low initial overpotential that continuously increases to very high voltages by the end of charge. However, we find that the inherent kinetic Li and Na-O2 overpotentials, as measured on a flat glassy carbon electrode in a bulk electrolysis cell, are similar. Measurement of each batteries' desired product yield, YNaO2 and YLi2O2, during discharge and rechargeability by differential electrochemical mass spectrometry (DEMS) indicates that less chemical and electrochemical decomposition occurs in a Na-O2 battery during the first Galvanostatic discharge-charge cycle. We therefore postulate that reactivity differences (Li2O2 being more reactive than NaO2) between the major discharge products lead to the observed charge overpotential difference between each battery.

Singlet Oxygen during Cycling of the Aprotic Sodium–O2 Battery

“…X‐ray diffraction confirms that the cube‐shaped discharge product is NaO 2 (Figure S3, S4) in accord with many previous reports on similar cells 1b,1c, 2a,2b, 3a. After discharge or recharge to 2.8 and 3.65 V, 4.1, 4.3, and 7.2 %, respectively, of the DMA at the sample points ②, ③, and ④ was converted into DMA‐O 2 (Figure 1 b).…”

“…Previously it was reported that most parasitic chemistry occurs during discharge and less on charge 2a, 3a. Charging above around 3.5 V caused the number of O 2 per e − evolved to deviate more significantly from one than at lower voltages.…”

“…During discharge, this ratio is typically at the ideal value of one despite approximately 5 % of the expected NaO 2 being missing and replaced by side products, such as Na 2 CO 3 , Na acetate, and Na formate. During subsequent resting and charge, more of these side products form and typically the e − /O 2 ratio deviates by several percent from one 2a, 3c, 4b, 5. Although less side products form than in the Li–O 2 cell, cyclability is similarly poor: restricted capacity can often be maintained for up to hundreds of cycles albeit at the expense of true energy, but at full discharge cells fail within some 10 cycles and capacity fading becomes significantly worse with rising charge cut‐off voltage 1c, 2a,2c,2d, 3a,3d, 6…”

Solvent‐Mediated Control of the Electrochemical Discharge Products of Non‐Aqueous Sodium–Oxygen Electrochemistry

“…The suggested mechanism defined the role of water as an active part of surface intermediates, through hydrogen bonding to LiO 2 , which promotes the formation of Li 2 O 2 and the resulting partial dissociation of H 2 O to HO 2 and OH − 13. Superior discharge capacity has been demonstrated with benzoic and acetic acid in Na‐O 2 , along with phenol and ethanol in Li‐O 2 , providing additional evidence for this phenomenon 6a,6b…”

High capacity surface route discharge at the potassium-O2 electrode

“…The voltage gap between discharge and charge is 100 mV for Na-O2 and 50 mV for K-O2, compared to 1 V for Li-O2. [8][9][10] This variation is attributed to the different type of discharge product, superoxide in Na-O2/K-O2 versus peroxide in Li-O2. Recently the chemistry at the cathode in the aprotic Li-O2 battery has been established and two competitive routes have been demonstrated on discharge, the solution route and the surface route.…”