Controlling the Li-Air (O2) Discharge Process with a Gel Polymer Electrolyte

The 2015 climate change conference in Paris espoused the need to stem the rise in global temperatures to below 2 °C. Therefore, there is a need to shift from fossil fuel use in electricity generation and transportation – industries responsible for the bulk of carbon dioxide (CO₂) emissions. Alternative energy conversion sources such as solar and wind are promising, but their intermittent nature affects long-term adoption. To complement these new conversion technologies, energy storage media such as batteries and fuel cells among others have been explored. The compact nature of batteries make them attractive over other energy storage media. Of commercially available batteries, lithium-ion batteries are the most energy-dense, and are used in electric vehicles and portable electronic devices. However, Li-ion batteries suffer from high cost limiting their use for stationary grid applications, and their energy densities are not high enough to allow for electric vehicles to compete with gasoline-powered cars in terms of mileage and cost. To address these limitations of Li-ion, other battery chemistries such as metal-air and metal-sulfur are being investigated. Lithium-air (O₂) batteries have the highest theoretical gravimetric energy density and have garnered tremendous research interest.

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.