Because of the growing needs for secondary batteries with greater energy density in various applications including electric vehicle, rechargeable non-aqueous lithium–oxygen (Li-O₂) battery is receiving a great attention owing to its high theoretical energy density of 3505 Wh kg^-1 which far exceeds that of conventional lithium ion battery (LIB) [1, 2]. However, due to the insulating nature of main discharge product, lithium peroxide (Li₂O₂) the oxygen-evolving reaction (OER) during charge requires a much higher overpotential, which is the one of the main challenges to practical applications [2, 3]. Incomplete decomposition of Li₂O₂ at the cathode causes excessive accumulation of solid discharge products on cathode surface during cycling, resulting in cell failure by pore clogging. As a consequence, Li-O₂batteries usually present poor round-trip efficiencies and severe capacity decays. To alleviate this problem, redox mediators (RMs) as soluble and mobile catalyst in aprotic media, have been intensively studied to address the overvoltage problem [3, 4]. In spite of such a smart strategy, self-discharge of redox mediator is inevitable in current battery configurations due to its chemical reduction at Li metal anode, as shown in Figure 1.

In this presentation, we suggest a synergic combination of soluble redox mediator and protected Li metal electrode for preventing the self-discharge of redox mediator and realize the concept by exploiting a redox mediator of 2,2,6,6,-tegramethylpiperidinyl-1-oxyl (TEMPO) and a composite protective layer (CPL) of Al₂O₃/PVdF-HFP composite. First, from the electrochemical characterization in conjunction with SEM and XPS analyses, the effectiveness of the highly reversible TEMPO/TEMPO⁺ redox couple as a RM was verified. More importantly, this presentation provides a detrimental effect of TEMPO on the interfacial stability of Li metal electrode by monitoring the battery cycling under Ar atmosphere, and thus, the necessity of Li metal anode protection was suggested in terms of the reversibility of RMs in the non-aqueous media. The CPL coated on Li metal electrode could suppress the reaction of TEMPO at Li metal electrode and maintain the redox mediated Li₂O₂ oxidation over repeated cycles, opening a new possibility for a robust use of RM for Li-O₂batteries.

Figure 1. Schematic illustration of (a) the self-discharge of redox mediator in Li-O₂ battery and (b) the CPL-coated Li electrode which prevent the reaction between redox mediator and Li metal electrode.

References

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

[2] G. Girishkumar, B. McCloskey, A. C. Luntz, S. Swanson, W. Wilcke J. Phys. Chem. Lett. 2010, 1, 2193-2203.

[3] Y. Chen, S. A. Freunberger, Z. Peng, O. Fontaine, P. G. Bruce Nature Chem. 2013, 5, 489-494.

[4] B. J. Bergner, A. Schürmann, K. Peppler, A. Garsuch, J. Janek J. Am. Chem. Soc. 2014, 136, 15054-15064.