Bifunctional MnO₂‑Coated Co₃O₄ Hetero-structured Catalysts for Reversible Li‑O₂ Batteries

The structural design and synthesis of effective cathode catalysts are important concerns for achieving rechargeable Li-O₂ batteries. In this study, hexagonal Co₃O₄ nanoplatelets coated with MnO₂ were synthesized as bifunctional catalysts for Li-O₂ batteries. The oxygen reduction reaction catalyst (MnO₂) was closely integrated on the surface of the oxygen evolution reaction catalyst (hexagonal Co₃O₄) so that this hetero-structured catalyst (HSC) hybrid would show bifunctional catalytic activity in Li-O₂ batteries. A facile synthesis route was developed to form a unique HSC structure, with {111} facet-exposed Co₃O₄ decorated with perpendicularly arranged MnO₂ flakes. The catalytic activity of the HSCs was controlled by tuning the ratio of Co to Mn (the ratio of OER to ORR catalysts) in the hybrids. With the optimized Co₃O₄-to-MnO₂ ratio of 5:3, a Li-O₂ cell containing the HSC showed remarkably enhanced electrochemical performance, including discharge capacity, energy efficiency, and especially cycle performance, compared to cells with a monofunctional catalyst and a powder mixture of Co₃O₄ and MnO₂. The results demonstrate the feasibility of reversible Li-O₂ batteries with bifunctional catalyst hybrids

Lithium Superoxide Hydrolysis and Relevance to Li–O₂ Batteries

Fundamental understanding of reactions of lithium peroxides and superoxides is essential for the development of Li–O₂ batteries. In this context, an investigation is reported of the hydrolysis of lithium superoxide, which has recently been synthesized in a Li–O₂ battery. Surprisingly, the hydrolysis of solid LiO₂ is significantly different from that of NaO₂ and KO₂. Unlike KO₂ and NaO₂, the hydrolysis of LiO₂ does not produce H₂O₂. Similarly, the reactivity of Li₂O₂ toward water differs from LiO₂, in that Li₂O₂ results in H₂O₂ as a product. The difference in the LiO₂ reactivity with water is due to the more exothermic nature of the formation of LiOH and O₂ compared with the corresponding reactions of NaO₂ and KO₂. We also show that a titration method used in this study, based on reaction of the discharge product with a Ti­(IV)­OSO₄ solution, provides a useful diagnostic technique to provide information on the composition of a discharge product in a Li–O₂ battery