Influence of Ammonium Salts on Discharge and Charge of Li–O2 Batteries

Li–air (O2) batteries are promising because of their high theoretical energy density. However, these batteries are plagued with numerous challenges, one of which involves modulating the battery discharge process between a solution or surface-driven formation of the desired lithium peroxide (Li2O2) discharge product, and the oxidation of Li2O2 below 4 V (vs Li/Li+). In this work, we show that tetrabutylammonium (TBA) salts dissolved in ether or dimethyl sulfoxide (with no lithium salt present) can be used as a Li–O2 electrolyte with a lithium metal anode to support Li2O2 formation, lead to >500 mV reduction in charging overpotentials at low current rates as compared to that with lithium salt, and support the oxidation of Li2O2 during charge. Furthermore, on the basis of results from several spectroscopic techniques, we propose a mechanism that involves electrochemical-induced transformation of TBA to tributylamine at ∼3.55 V, and the formation of a tributylamine oxide intermediate in the presence of O2 or Li2O2 that is responsible for Li2O2 oxidation during charging. This mechanism can also be translated to other ionic liquid-based Li–O2 batteries where significantly low charging potentials are observed. This work showcases an additive that can be used for Li–O2 batteries to allow for finer control of the discharge process, and the ability of amine oxides to oxidize Li2O2.