Li-O₂ batteries offer the highest energy density among rechargeable batteries. This superb energy density makes the system one of the most attractive energy storage devices for electric vehicles (EV), energy storage systems (ESS), and other applications1 where high energy density is the most important figure of merit. There remain many challenges, however, for the practical application of Li-O₂ batteries including low energy efficiency caused by the large energy loss during Li-O₂ electrochemical cycles. The widely-accepted theoretical reaction of Li-O₂ batteries is 2Li⁺ + O₂ + 2e^- ↔ Li₂O₂. The forward direction forming Li₂O₂ represents the discharge process and the reverse reaction of Li₂O₂ decomposition the charge process; however, many parasitic reactions have been reported, especially involving electrolyte and carbon materials.2 These undesired side reactions significantly reduce the round-trip efficiency of Li-O₂ batteries. Carbon in particular, the most common material for air electrodes because of its high surface area and good electrical conductivity, may react to Li₂CO₃ in Li-O₂ cells that impede the reversible Li-O₂ reaction.2

Some of the alternative cathode materials suggested recently include TiC,3 indium tin oxide (ITO).4 Bruce and his co-workers3 ascribed the stability of TiC cathodes to the surface TiO₂ layer on TiC. The TiO₂ surface layer was stable from the attack of oxygen radicals during cycling. Recently, TiO₂ nano tubes have been explored as a support for electrocatalysts.5 The resulting cells showed enhanced cycle life due to the higher stability of TiO₂ when compared with carbon materials. The Pt or RuO₂ catalyst-loaded TiO₂ nanotube exhibited an enhanced cyclability with low charge potentials, especially under high current density. Despite the high specific capacity in their research, however, the actual capacity of Li-O₂ batteries was not sufficient for practical application, since the specific capacity was calculated based on the catalyst loading mass, which is only 0.1 mg on a 15 mm support. Li-O₂ batteries must be highly reversible with a high cell capacity for high energy applications.

Here, we synthesized RuO₂/mesoporous TiO₂ composites, abbreviated as RuO₂/mTiO₂, and used as a cathode material for Li-O₂ batteries. A carbon-free electrode of RuO₂/mTiO₂ showed 4 mAh of cell capacity with alow charge potential of under 4 V.

References

1. J. Lu, L. Li, J.-B. Park, Y.-K. Sun, F. Wu, K. Amine, Chem. Rev. 114, 2014, 5611-5640.

2. B. D. McCloskey, A. Speidel, R. Scheffler, D. C. Miller, V. Viswanathan, J. S. Hummelshøj, J. K. Nørskov, A. C. Luntz, J. Phys. Chem. Lett. 3, 2012, 997-1001.

3. M. M. Ottakam Thotiyl, S. A. Freunberger, Z. Peng, Y. Chen, Z. Liu, P. G. Bruce, Nat. Mater. 12, 2013, 1050-1056.

4. F. Li, D. M. Tang, Y. Chen, D. Golberg, H. Kitaura, T. Zhang, A. Yamada, H. Zhou, Nano Lett. 13, 2013, 4702-4707..

5. G. Zhao, F. Mo, B. Wang, L. Zhang, K. Sun, Chem. Mater. 2014, 26, 2551-2556.