Reduction of Charge and Discharge Polarization By Cobalt Nanoparticles-Embedded Carbon Nanofibers for Lithium-Oxygen Batteries

The battery material community has a persistent consensus that Li–O2 batteries would be the most promising next generation batteries due to its extraordinarily high energy densities; from a practical view point, at least 2~3 times higher than those of conventional lithium ion batteries (LIBs).[1] However, Li–O2 batteries suffer from their chronically low round trip efficiency due to poor reversibility of lithium peroxide, Li2O2, which is main product of discharged Li–O2 batteries. This truly originates from its intrinsically poor electron and ionic conductivity.[2] Therefore, the reduction of charging overpotential is in urgent need for economical progress of Li–O2 batteries. In an effort to achieve high round trip efficiency, recent studies have attracted great attention that the charge behaviors of Li–O2 cells can be manipulated by controlling the phase structure and morphology of Li2O2 formed during discharge. In particular, at a low current density, the large toroidal structured, crystalline Li2O2 was primarily formed. In contrast, the formation of film-like amorphous Li2O2 is favored at high current density. [3,4] Owing to its structural advantages of enhanced ionic and electronic transport[5], the oxidation of less crystalline Li2O2 is more efficient, resulting in significant decrease on charge overpotential. Consequently, the controlling phase and morphology of Li2O2 is an effective strategy for manipulating charge behavior of Li–O2 cells. As a possible approach for controlling Li2O2, we designed an effective electrochemical catalyst of oxygen reduction reactions in Li-O2 batteries, a cobalt nanoparticles (Co NPs) embedded carbon nanofibers (Co-CNFs). As a result, embedded Co NPs leads to the formation of uniform film-like amorphous Li2O2, and thus, the Li–O2 battery employing the Co-CNF provides a round trip efficiency of > 77% and extends cycling stability by more than 6 times in comparison with a CNF electrode. These change of Li2O2 mainly is derived from a charge delocalization originating from the electron transfer from Co NPs to C-atoms. According to HSAB theory, highly concentrated Li+ ions near electrode resulting from the preferable interaction with Co-CNFs could lead to the rapid precipitation of Li2O2 and prevent the crystallization of Li2O2. The findings suggest a novel electrode design strategy of combining inexpensive metal and carbons for modulating the phase of discharge product. References [1] Nat. Mater. 2012, 11, 19-29 [2] J. Phys. Chem. Lett. 2013, 4, 93−99 [3] J. Am. Chem. Soc, 2013, 135, 15364−15372 [4] Energy Environ. Sci., 2013, 6, 1772 [5] Chem. Mater, 2014, 26, 2952−2959

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