Revealing the reaction mechanisms of Li-O₂ batteries using environmental transmission electron microscopy

The performances of a Li-O₂ battery depend on a complex interplay between the reaction mechanism at the cathode, the chemical structure and the morphology of the reaction products, and their spatial and temporal evolution; all parameters that, in turn, are dependent on the choice of the electrolyte. In an aprotic cell, for example, the discharge product, Li₂O_2, forms through a combination of solution and surface chemistries that results in the formation of a baffling toroidal morphology. In a solid electrolyte, neither the reaction mechanism at the cathode nor the nature of the reaction product is known. Here we report the full-cycle reaction pathway for Li-O₂ batteries and show how this correlates with the morphology of the reaction products. Using aberration-corrected environmental transmission electron microscopy (TEM) under an oxygen environment, we image the product morphology evolution on a carbon nanotube (CNT) cathode of a working solid-state Li-O₂ nanobattery and correlate these features with the electrochemical reaction at the electrode. We find that the oxygen-reduction reaction (ORR) on CNTs initially produces LiO₂, which subsequently disproportionates into Li₂O₂ and O₂. The release of O₂ creates a hollow nanostructure with Li₂O outer-shell and Li₂O₂ inner-shell surfaces. Our findings show that, in general, the way the released O₂ is accommodated is linked to lithium-ion diffusion and electron-transport paths across both spatial and temporal scales; in turn, this interplay governs the morphology of the discharging/charging products in Li-O₂ cells.