The Pore Size Effect of Ordered Mesoporous Carbon in Li-O2 Batteries

Among the rechargeable battery devices investigated to date, non-aqueous Li-O₂ cells remain the most attractive alternative system largely due to its high energy density.[1,2] However, several lingering issues about the Li-O₂ cell still impede its practical application.[3] Among the many problems, to decrease the charging potential, namely oxygen evolution reaction (OER) potential, porous carbon, metal (oxide) catalyst and their composites have been employed as oxygen electrode materials. Because carbon materials have high surface area and large pore volume for Li₂O₂ deposition, they are used for cathode materials of Li-O₂ cells.

The carbon materials helped to increase capacity, but failed to decrease the OER potential due to the low electronic conductivity of Li₂O₂. To increase the capacity and decrease the OER potential simultaneously, several metal (oxide) catalyst and carbon composite material was used for the cathode of Li-O₂ cells. By using a catalyst, it was possible to lower the charging potential under 4 V corresponding overpotential of 1.1 V.[4]

In spite of this, to the best of our knowledge, there has been no attempt to decrease the charging potential of Li-O₂ cells by changing the morphology of the carbon materials. Furthermore, there has not been an approach that solves the low rate capability due to the low electrical conductivity of Li₂O₂.

Among the various carbon materials, ordered mesoporous carbon with pore sizes ranging from 2 to 50 nm can be a potential candidate electrode material because of the high surface area, various pore sizes and structures.[5] Other authors[6] have already applied the mesoporous carbon as an oxygen electrode for Li-O₂ cells, but it was difficult to directly observe the lithium peroxide by transmission electron microscopy (TEM) after discharge due to the various pore sizes and disordered pore distributions of mesoporous carbon.

In this work, we used two different highly ordered mesoporous carbons with a pore size of 6 nm (OMC-6) and 17 nm (OMC-17) for an oxygen electrode without catalyst. By using the OMC material as an oxygen electrode, it was demonstrated that the charge potential can be lowered while improving the rate capability compared with super P carbon. The morphology of the discharge product that formed on the OMC material was found to be different from that of super P.

References

[1] Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J.-M. Nat. Mater. 2012, 11, 19−29.

[2] Scrosati, B.; Hassoun, J.; Sun, Y.-K. Energ. Environ. Sci. 2011, 4, 3287−3295.

[3] Shao, Y.; Ding, F.; Xiao, J.; Zhang, J.; Xu, W.; Park, S.; Zhang, J.-G.; Wang, Y.; Liu, J. Adv. Funct. Mater. 2013, 23, 987−1004.

[4] Wang, L.; Zhao, X.; Lu, Y.; Xu, M.; Zhang, D.; Ruoff, R. S.; Stevenson, K. J.; Goodenough, J. B. J. Electrochem. Soc. 2011, 158 (12), A1379−A1382.

[5] Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710−712.

[6] Shitta-Bey, G. O.; Mirzaeian, M.; Hall, P. J. J. Electrochem. Soc. 2012, 159 (3), A315−A320.