Tuning Li₂O₂ Formation Routes by Facet Engineering of MnO₂ Cathode Catalysts

In lithium-oxygen batteries, the solubility of LiO₂ intermediates in the electrolyte regulates the formation routes of the Li₂O₂ discharge product. High-donor-number electrolytes with a high solubility of LiO₂ tend to promote the formation of Li₂O₂ large particles following the solution route, which eventually benefits the cell capacity and cycle life. Here, we propose that facet engineering of cathode catalysts could be another direction in tuning the formation routes of Li₂O₂. In this work, β-MnO₂ crystals with high occupancies of {111} or {100} facets were adopted as cathode catalysts in Li-O₂ batteries with a tetra(ethylene)glycol dimethyl ether electrolyte. The {111}-dominated β-MnO₂ catalyzed the formation of the Li₂O₂ discharge product into large toroids following the solution routes, while {100}-dominated β-MnO₂ facilitated the formation of Li₂O₂ thin films through the surface routes. Further computational studies indicate that the different formation routes of Li₂O₂ could be related to different adsorption energies of LiO₂ on the two facets of β-MnO₂. Our results demonstrate that facet engineering of cathode catalysts could be a new way to tune the formation route of Li₂O₂ in a low-donor-number electrolyte. We anticipate that this new finding would offer more choices for the design of lithium-oxygen batteries with high capacities and ultimately a long cycle life.