472346 Controlling the Li-Air (O2) Discharge Process with a Gel Polymer Electrolyte
Li–O2 chemistry is governed by the reduction of oxygen during discharge and the oxidation of the reduced oxygen discharge product during charge. In conventional Li–O2 cells, non-aqueous liquid electrolytes are often used. The oxygen reduction chemistry is dominated by a 2 mol e−/mol O2 (peroxide) process that some have attributed to be partially responsible for the sluggish reduction and oxidation kinetics, limited current rate, and poor capacity retention of Li–O2 batteries. Battery chemistries such as Na–O2 and K–O2 that utilize a 1 mol e−/mol O2 chemistry have been shown to support higher current rates and better energy efficiencies.
In this work, we incorporate ionic liquids in a polymeric matrix and show that controlling the lithium/ionic liquid molar ratio in the gel polymer electrolyte can allow for a 1 mol e−/mol O2 reduction process in a Li–O2 battery. Ionic liquid cations has been shown to support a 1 mol e−/mol O2 process using cyclic voltammetry, but not in actual Li–O2 cells, where a 2 mol e−/mol O2 process (and Li2O2) is observed. Furthermore, we use multiple spectroscopic tools to confirm for the first time the formation of a solid ionic liquid-superoxide discharge product. Knowledge gained from this work should spur development of newer and more stable ionic liquids and polymers that can allow for better long-term Li–O2 cycling. In addition, the mechanism observed here could prove vital for other battery chemistries such as metal-air and metal-sulfur where controlling intermediate solubility is paramount.