The Rechargeable Aprotic Lithium-oxygen Battery
Authors: Holc Conrad , Pateman Alexander , Gao Xiangwen , Chen Yuhui , Bruce Peter G. , Johnson Lee R 
Published: February 2020

Interest in the rechargeable Li-O2 battery is driven by its high theoretical specific energy (3500 Whkg-1).1-2 However, a number of challenges face the realization of practical devices.3-10 Overcoming these challenges requires an understanding of the reactions and processes in the cell, especially at the electrodes. Our focus has been on the reaction at the positive electrode:
O2 + 2e- + 2Li+ = Li2O2

This simple reaction belies the complexity of the problems during charge and discharge. Li2O2, is an insulating solid, if it grows on the electrode surface it can only do so to a thickness of approx. 6 to 7 nm. The resulting passivation leads to low rates and low capacities.11 If Li2O2 grows from solution, passivation is avoided, leading to high rates and capacities.12 The solvent donor number is an important factor in controlling which pathway, surface film or solution growth, occurs. Low donor number ethers promote surface films while high donor numbers result in solution growth. Unfortunately, high donor number solvents are more susceptible to decomposition by the reactive O2
- nucleophile (the intermediate in the O2/Li2O2 reaction is LiO2).13 We show that using redox mediator molecules to shuttle electrons between the electrode surface and solution, Li2O2 can be formed and decomposed in solution even in low donor number solvent like ethers.14 As a result, rates and capacities of several mA and mAh cm-2 respectively are observed. The impact of the mediators on solvent and electrode stability will be discussed in the context of the mechanism of Li2O2 formation and decomposition. Ultimately, the Li-O2 cell must operate in air. The effect of H2O in the gas stream and hence in the electrolyte solution on the mechanism of O2/Li2O2 and the effect on cell performance are important. The influence of H2O on the O2 reduction mechanism will be considered.
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