Mesoporous Carbon Cathode in a Li-O2 Battery: Effect on Li2O2 Product Morphology and Recharge Potential

One of the major challenges of lithium-oxygen (Li-O₂) battery is high recharging overpotential, which is responsible for low stability of both non-aqueous electrolyte and carbon electrode, leading to serious side reactions. The fundamental reason for the high overpotential is very poor conductivity of solid Li₂O₂ as a discharge product, which requires further energy to be electrochemically decomposed (Li₂O₂→2Li⁺ + O₂ + 2e^–). Interestingly, the crystallinity and morphology of the Li₂O₂ profoundly influence on its conductivity, thus affecting the charging overpotential.^1,2 The amorphous Li₂O₂ has much higher ionic and electronic conductivities compared to the crystalline Li₂O₂.¹ In addition, the small size of Li₂O₂ with large contact area to the carbon electrode can aid in its smooth decomposition at low charging potential.² Such type of Li₂O₂, which has been clearly observed in the presence of some solid-state metal/metal oxide promoters,^3,4 is possibly attributable to the high O₂ adsorption affinity of the promoters. Unlike these reports, we herein present the unique structure of Li₂O₂ using a promoter-free mesoporous carbon electrode, which greatly lowers the recharge potential. The mesoporous carbon of CMK-3 consisting of cylindrical-shaped pores with a pore diameter of ~3.5 nm was employed to the cathode (positive electrode) in the Li-O₂ cell using 0.5 M LiTFSI in TEGDME. Strikingly, the CMK-3 based Li-O₂ cell shows a very low recharge potential below 3.5 V until ~75% of recharge process. The pivotal reason for the decreased overpotential may be associated with an amorphous and flake-shaped Li₂O₂ vertically formed on the surface of CMK-3 during discharge, which are almost completely decomposed at the low recharge potential. This result is very distinguished from other carbon electrodes such as ketjen black (KB) carbon particle, multi-walled carbon nanotube (MWCNT) and LPC-80 consisting of spherical pores with a diameter of ~80 nm, where higher recharge potentials shown can be correlated with typical structure of spherical and thick Li₂O₂ particles. We propose that the surface structure of mesoporous carbon electrode is key to implant the Li₂O₂ flake while the effect of cylindrical pores within the CMK-3 may be negligible when considering low capacity (~1100 mAh/g_carbon) associated with a high surface area (~1500 m²/g). A detailed morphological and electrochemical analysis will be discussed in the presentation.

References

(1)  Tian, F.; Radin, M. D.; Siegel, D. J. Chem. Mater. 2014, 26, 2952-2959.

(2)  Dunst, A.; Epp, V.; Hanzu, I.; Freunberger, S.; Wilkening, M. Energy Environ. Sci. 2014, 7, 2739-2752.

(3)  Yilmaz, E.; Yogi, C.; Yamanaka, K.; Ohta, T.; Byon, H. R. Nano Lett. 2013, 13, 4679-4684.

(4)  Xu, J. J.; Wang, Z. L.; Xu, D.; Zhang, L. L.; Zhang, X. B. Nat. Commun. 2013, 4, 2438-2447.