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

The performances of a Li-O-2 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(1-4); all parameters that, in turn, are dependent on the choice of the electrolyte(5-8). In an aprotic cell, for example, the discharge product, Li2O2, forms through a combination of solution and surface chemistries(9-11) that results in the formation of a baffling toroidal morphology(12-15). 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-O-2 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-O-2 nanobattery(16) and correlate these features with the electrochemical reaction at the electrode. We find that the oxygen-reduction reaction (ORR) on CNTs initially produces Li-O-2, which subsequently disproportionates into Li2O2 and O-2. The release of O-2 creates a hollow nanostructure with Li2O outer-shell and Li2O2 inner-shell surfaces. Our findings show that, in general, the way the released O-2 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-O-2 cells.