Rotating-disk electrode analysis of the oxidation behavior of dissolved Li 2 O 2 in Li-O 2 batteries.

The development of the rechargeable Li-O 2 battery (LOB) has encountered several bottlenecks till date. One of the biggest challenges is to lower the oxidation potential of Li 2 O 2 , 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 Li 2 O 2 , so that the dissolved Li 2 O 2 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 Li 2 O 2 soluble electrolytes. The relationship between the solubility of Li 2 O 2 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 Li 2 O 2 than electrolytes with high DN, and thus exhibit higher Li 2 O 2 oxidation rates. Nevertheless, higher Li 2 O 2 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.