Rotating-disk electrode analysis of the oxidation behavior of dissolved Li₂O₂ in Li-O₂ batteries

The development of the rechargeable Li-O₂ battery (LOB) has encountered several bottlenecks till date. One of the biggest challenges is to lower the oxidation potential of Li₂O₂, which is the insulating and insoluble discharge product. A possible solution to this problem is to use high acceptor number (AN) or donor number (DN) solvents to increase the solubility of Li₂O₂, so that the dissolved Li₂O₂ can diffuse to the cathode surface and get oxidized at a relatively low potential. Herein, we explored the efficiency and side-reactions in the LOB charge process with different Li₂O₂ soluble electrolytes. The relationship between the solubility of Li₂O₂ and charging rate was analyzed quantitatively with ultraviolet-visible (UV-Vis) spectroscopy and rotating disk electrode experiments. As a result, electrolytes with high AN usually have higher solubility for Li₂O₂ than electrolytes with high DN, and thus exhibit higher Li₂O₂ oxidation rates. Nevertheless, higher Li₂O₂ solubility in high AN electrolytes also induces more severe side reactions and easily passivates the electrode surface. The trade-off between charging reaction rate and electrolyte stability is a key issue to be considered when designing high performance LOB electrolytes.