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