Superoxide stability for reversible Na-O2 electrochemistry

V S Dilimon,Seok Ju Kang,Juchan Yang,Hee-Dae Lim,Kisuk Kang,Yoon-Gyo Cho,Hyun-Kon Song,Chihyun Hwang

doi:10.1038/s41598-017-17745-9

V S Dilimon, Seok Ju Kang + Show 6 more

Open Access

https://doi.org/10.1038/s41598-017-17745-9

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Abstract

Stabilizing superoxide (O2−) is one of the key issues of sodium-air batteries because the superoxide-based discharge product (NaO2) is more reversibly oxidized to oxygen when compared with peroxide (O22−) and oxide (O2−). Reversibly outstanding performances of sodium-oxygen batteries have been realized with the superoxide discharge product (NaO2) even if sodium peroxide (Na2O2) have been also known as the discharge products. Here we report that the Lewis basicity of anions of sodium salts as well as solvent molecules, both quantitatively represented by donor numbers (DNs), determines the superoxide stability and resultantly the reversibility of sodium-oxygen batteries. A DN map of superoxide stability was presented as a selection guide of salt/solvent pair. Based on sodium triflate (CF3SO3−)/dimethyl sulfoxide (DMSO) as a high-DN-pair electrolyte system, sodium ion oxygen batteries were constructed. Pre-sodiated antimony (Sb) was used as an anode during discharge instead of sodium metal because DMSO is reacted with the metal. The superoxide stability supported by the high DN anion/solvent pair ({{rm{CF}}}_{3}{{rm{SO}}}_{3}–/DMSO) allowed more reversible operation of the sodium ion oxygen batteries.

Highlights

Lithium metal-oxygen (Li-O2) batteries have the highest theoretical specific energy (3450 Wh kg−1) than any other reported battery systems[1,2,3]
Anions possibly affect the stability of Na+-O2− formation, which is a lesson from the McCloskey et al.’s and Lutz et al.’s work showing that stable existence of superoxide was guaranteed by a high donor numbers (DNs) anion even with a low-DN solvent (DME)[4,14]
This study showed for the first time in literature that the nature of discharge products (NaO2 or Na2O2) (E0 (Na2O2)), battery performances and the discharge-charge reversibility greatly depend on the anion part of the sodium salt

Summary

Introduction

Lithium metal-oxygen (Li-O2) batteries have the highest theoretical specific energy (3450 Wh kg−1) than any other reported battery systems[1,2,3]. As in Li-O2 cells, the major solid discharge product of Na-O2 cells has been identified as peroxide salt (Na2O2; Li2O2 in Li-O2 cells)[6,7,8,9,10] before Hartmann et al.’s work[5]. Hartmann et al reported superoxide salt (NaO2) as the solid discharge product of ORR in their Na-O2 cells based on NaSO3CF3 in DEGDME (diethylene glycol dimethyl ether), showing significantly improved performances of Na-O2 batteries[5]. By analyzing previous works on discharge products of Na-O2 electrochemistry, solvents and anions of sodium salts in electrolytes are expected as important factors to determine the final products or superoxide stabilization. It looks clear that the solvents affect the superoxide stability and determining discharge products

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