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

Abstract

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/gcarbon) 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. Figure 1