|Operating principle of oxygen rocking batteries using CLFO. Hibino et al. Click to enlarge.|
Researchers at the University of Tokyo are exploring the development of novel lithium-free “oxygen rocking” rechargeable aqueous batteries for large-scale energy storage. They describe the principle and their initial work in a paper published in Scientific Reports, an open access, primary research publication from the publishers of Nature.
As Hibino et al. describe in their paper, in the new batteries oxygen shuttles between the cathode and anode composed of iron-based perovskite-related oxides Ca0.5La0.5FeOz (CLFO) (2.5 ≤ z ≤ 2.75 and 2.75 ≤ z ≤ 3.0). Compound Ca0.5La0.5FeOz can undergo two kinds of reduction and reoxidation of Fe4+/Fe3+ and Fe3+/Fe2+ that are accompanied by reversible and repeatable topotactic (transformation within a crystal lattice) oxygen extraction and reinsertion during discharge and charge processes.
Lithium ion batteries (LIBs) based on rocking lithium between cathode and anode electrodes are used in a wide range of applications from portable electronics to vehicle propulsion. However, there are cost and resource problems since most LIBs use not only expensive lithium but also less abundant metals such as cobalt. In addition, use of flammable organic electrolyte solutions may cause a safety problem. Those problems are critical especially for large-scale batteries used for electric vehicles and electric grids.
...It is well known that some kinds of perovskite-type oxides of A1−xSrxCoO3 (A = La or Nd) show good oxygen diffusivity and the oxygen atoms of such compounds can be extracted and reinserted topotactically by electrochemical reduction and reoxidation in an aqueous alkaline solution...we have reached an idea that compounds A1−xLaxFeOz (A = Ca, Sr) can work as not only a cathode but also an anode utilizing different oxygen composition ranges of the Fe3+/Fe4+ and Fe2+/Fe3+ couples, respectively.—Hibino et al.
The team prepared CLFO samples (z = 2.827 − 2.618) by electrochemical reduction of Ca0.5La0.5FeO2.863 with three electrode glass cells. Electrochemical measurements were performed using a three electrode beaker cell with a reference electrode (Hg/HgO) and an electrolyte solution (1 M NaOH aq). A platinum mesh was used as a counter electrode. A mixture of CLFO, conducting additive, and binder (PTFE powder) was ground, pressed on a current collector, and used as a working electrode.
The oxidation or reduction at a constant current ranging from 1.40 to 28.0 mA g−1 was performed to investigate electrochemical behavior. The reversibility and repeatability were investigated by the redox tests at a constant current of 5.60 mA g−1 in the potential ranges 0.5 − −0.5 V and −0.4 − −1.1 V.
Among the findings were that the maximum capacities of the reduction and reoxidation processes in the potential region around −0.8 V were 20 and 15 mA h g−1, respectively; much smaller than 70.2 mA h g−1 estimated. This difference in capacities does not result from side reactions, the team suggested, but from the slow diffusion of oxygen, which would be overcome by fabricating a well-designed nano-structured electrode.
In conclusion, the electrochemical reduction of Fe4+ to Fe3+ and Fe3+ to Fe2+ and their reoxidation in CLFO proceed reversibly and repeatedly via extraction and reinsertion of oxygen in two potential regions around 0 V and − 0.8 V. This shows that CLFO can serve as both cathode and anode materials in an oxygen rocking battery. The CLFO is, however, one of model electrode materials of the oxygen rocking batteries, and the developments of more suitable materials for each of a cathode and an anode will enable the new practicable oxygen rocking batteries.—Hibino et al.
Mitsuhiro Hibino, Takeshi Kimura, Yosuke Suga, Tetsuichi Kudo & Noritaka Mizuno (2012) Oxygen rocking aqueous batteries utilizing reversible topotactic oxygen insertion/extraction in iron-based perovskite oxides Ca1–xLaxFeO3−δ. Scientific Reports 2, Article number: 601 doi: 10.1038/srep00601