Non-Aqueous Rechargeable Rb-O2 Batteries
Authors: Fujimoto Ryusei , Tatara Ryoichi , Igarashi Daisuke , Hosaka Tomooki , Komaba Shinichi 
Published: February 2024

Li-O2 batteries have attracted increasing attention because their theoretical energy density exceeds that of current lithium-ion batteries.[1] Generally, the positive electrode reaction in nonaqueous alkali metal-O2 batteries proceeds through oxygen reduction and corresponding reoxidation, forming alkali metal superoxide (MO2) as the discharge product. It is known that the stability of the superoxide (MO2) depends on the relationship between the ionic radius of the alkali metal ion (M+) and the superoxide ion (O2
−).[2] For example, LiO2 consists of small Li+ (ionic radius in octahedral coordination: 0.76 Å) and relatively large O2
− (1.49 Å), resulting in its short lifetime and rapid disproportionation reaction; 2LiO2 → Li2O2+O2. As the two-electron reduction product, Li2O2, exhibits a large overpotential to re-oxidize it, Li-O2 cells are known to have low reversibility.[1] On the other hand, NaO2 is relatively stable because the ionic radius of Na+ (1.02 Å) is relatively large compared to that of Li+. Thus, Na-O2 battery is also promising due to the low charge overpotential of the one-electron reduction product, NaO2. However, NaO2 is highly reactive with a trace amount of moisture, CO2, and electrolyte components, partially forming Na2O2 and/or its hydrates.[3] Further stabilization was observed by using K+ (1.38 Å) to form KO2, thus K-O2 cell is known to have high stability and reversibility as well as low overpotential.[4] Following these trends, in this study, we focused on Rb+ (1.52 Å), which has a similar ionic radius to O2
− (1.49 Å), and investigated the electrochemistry of oxygen reduction and evolution reaction in Rb-containing non-aqueous electrolytes.
The working electrode is prepared by mixing Ketjen black (KB) and polytetrafluoroethylene (PTFE) in a weight ratio of 80:20, followed by coating it on carbon paper. The counter electrode consists of activated carbon, KB, and PTFE mixed with an 80:10:10 weight ratio. Ag+/Ag reference electrode and rubidium bis(trifluoromethanesulfonyl)amide (RbTFSA)/ triglyme (G3) electrolytes were also used to fabricate three-electrode O2-cell. The cells were first assembled in an Ar-filled glove box, followed by purging dry-O2 gas to supply an adequate amount of O2 in the cell. Note that Rb metal was not used for safety reasons.
Figure (a) presents the charge-discharge curves obtained with 1 mol/dm3 RbTFSA/G3 electrolyte under a dry-O2 and dry-Ar atmosphere. While no capacity was observed under the Ar atmosphere, distinct plateaus appeared at 2.57 and 2.67 V (vs. Rb+/Rb) at discharge and charge sequence, respectively, under the O2 atmosphere. A discharge capacity of ~0.6 mAh cm−2 and a relatively small overpotential of 100 mV were achieved, as well as high Coulombic efficiency at the initial cycle of 71 %. Figure (b) shows XRD patterns of pristine and fully discharged electrodes, where all of the new peaks that appeared through discharge reaction can be assignable to RbO2. Therefore, the discharge product of the Rb-O2 cell is confirmed to be RbO2, similar to Na-O2 and K-O2 cells. The reaction mechanism, reversibility, and stability of the O2/RbO2 electrode will be presented and discussed.
Reference:
[1] P. Bruce, J-M. Tarascon, et al., Nat. Mater., 11, 19-29 (2012).
[2] L. Qin, Y. Wu, et al., J. Am. Chem Soc., 142, 11629 (2020).
[3] P. Hartmann, P. Adelhelm, et al., Nat. Mater., 12, 228 (2013).
[4] X. Ren, Y. Wu, J. Am. Chem. Soc., 2, 2923 (2013).

Figure 1