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.
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