...lithium−oxygen battery research is facing a lot of challenges. The discharge process involves the reduction of oxygen to superoxide (O₂⁻), the formation of LiO₂ and its disproportionation into Li₂O₂ and O₂; while the charge process is the direct oxidation of Li₂O₂ into O₂. As a result of the asymmetric reaction mechanism, battery charge has a much higher overpotential (∼1−1.5 V) than that of discharge (∼0.3 V), which renders the system with a low round-trip energy efficiency around 60%.

Recently, researchers have also found out the instability of electrolyte and carbon electrode under the high charging potential (>3.5 V), which contributes to the low rechargeability. Different electrocatalysts have been explored to lower the overpotentials. But the necessity of catalysts has been argued, because the catalyst on carbon may not be able to work once its surface is blocked. Moreover, side reactions can be facilitated by the catalysts. On the other hand, the insulating nature of bulk Li₂O₂ (band gap larger than 4 eV, calculated values) can hinder the charge transfer reactions and result in a limited battery capacity.

...Therefore, new chemistry is needed to solve the problems in Li−O₂ batteries....In contrast with LiO₂ and NaO₂, KO₂ is thermodynamically stable and commercially available.

—Ren and Wu

Xiaodi Ren and Yiying Wu at OSU started their investigation by carrying out electrochemical measurements to verify the influence of cations on the redox chemistry of oxygen. They found that the oxygen reduction and oxidation potential gap in the electrolyte with K⁺ is much smaller than that in the electrolyte with Li⁺, implying that a K−O₂ battery may operate at much lower overpotentials than a Li-O₂ battery, and thus with higher roundtrip efficiences.

They then fabricated a K−O₂ battery containing a potassium metal foil, a glassy fiber separator and a porous carbon electrode, with 0.5 M KPF₆ in an ether solvent (1,2- dimethoxyethane (DME) or diglyme) as the electrolyte. For comparison, they built a Li−O₂ battery in a similar manner.

Cyclic voltammetry in the two-electrode battery setup showed the small difference between the onset potentials of oxygen reduction and oxidation for the K-O₂ battery. They determined that the oxidation process can be complete within the potential range where the carbon electrode and the electrolyte are relatively stable.

The smaller discharge overpotential of the K−O₂ battery may result from the better conductivity of KO₂ (>10 S/cm₂, room temperature) than Li₂O₂, they suggested.

More importantly, in the subsequent charging process, the voltage is as low as 2.50− 2.52 V for the K−O₂ battery. The charge overpotential η_chrg of ∼20−40 mV is significantly smaller than the Li−O₂ battery. Moreover, within this small charging potential range, almost 90% of the discharged product can be oxidized. In contrast, for the Li−O₂ battery, only half of the product was able to be removed even when the voltage reaches 4.0 V, where the ether electrolyte and the carbon electrode become unstable. The charge/discharge potential gap of about 50 mV is the lowest one that has ever been reported in metal−oxygen batteries. Compared with the typical Li−O2 battery, which has a potential gap larger than 1 V, our K−O₂ battery can can provide an exceptional round-trip energy efficiency of >95%.

...As a final note, very recently during the preparation of this paper, another group of researchers has published their results about a Na−O₂ battery. [Earlier post.] Although it shares a similar mechanism with our K−O₂ battery, a notable difference is that KO₂ is both kinetically and thermodynamically stable, while NaO₂ is only kinetically stable. This can bring some advantages to our K−O₂ battery. For example, as shown earlier, a KO₂-loaded carbon electrode can be prepared as the artificially discharged cell, which allows us to separately study the charging step and the discharging step. This is helpful for an in-depth understanding of the battery processes. Despite this difference, the promising results from both K−O₂ and NaO₂ batteries indicate the potential of the superoxide batteries.

—Ren and Wu

Resources

• Xiaodi Ren and Yiying Wu (2013) A Low-Overpotential Potassium–Oxygen Battery Based on Potassium Superoxide. Journal of the American Chemical Society doi: 10.1021/ja312059q

• Pascal Hartmann, Conrad L. Bender, Miloš Vračar, Anna Katharina Dürr, Arnd Garsuch, Jürgen Janek & Philipp Adelhelm (2012) A rechargeable room-temperature sodium superoxide (NaO₂) battery. Nature Materials doi: 10.1038/nmat3486

• Ali Eftekhari (2004) Potassium secondary cell based on Prussian blue cathode. Journal of Power Sources, Volume 126, Issues 1–2, Pages 221-228, doi: 10.1016/j.jpowsour.2003.08.007