Colloquium: On the Origin of Parasitic (Electro)Chemistry in Rechargeable Aprotic Metal-Oxygen Batteries

Aprotic metal-oxygen batteries, especially Li-O2 and Na-O2 batteries, could afford theoretical specific energies in excess of 1000 Wh/kg1,2. However, the practical realization of the theoretically possible high specific energies, while not compromising on rechargeability, has been elusive. This is believed to be, at least in part, due to the electronically insulating nature of Li2O2 and NaO2 (or Na2O2), the discharge products of aprotic Li-O2 and Na-O2 batteries, respectively. Further, the elementary step(s) that limit recharge rates and efficiencies in metal-oxygen batteries are unclear.

In this thesis, first, we attempted to develop mechanistic insights into the rate-limiting processes that limit rechargeability in metal-oxygen batteries. We explored the two metal-oxygen battery systems (Li-O2 and Na-O2) using a combination of electrochemical impedance spectroscopy (EIS), Differential Electrochemical Mass Spectroscopy (DEMS), SEM, and chemical titrations with an aim to understand the differences between them in terms of charge overpotential and parasitic chemistry occurring during cell operation. Further, through distribution of relaxation times (DRT) and equivalent circuit model analysis of impedance spectra, we deconvolute the timescales related to various processes occurring in these systems. Contrary to common opinion, we find that parasitic electrochemistry during the discharge step directly correlates with oxidative over potentials required during the charge step. Our works shows that the origin of recharge inefficiencies is the formation of parasitic side products, which possibly passivate the electrode surface during discharge and increase the oxidative over potential during charge. Our findings are further supported by in-operando mass spectroscopy analysis and chemical titrations.

Through a combination of electrochemical experiments with solvents of various acceptor numbers and using electrolyte additives, we establish that solvated singlet oxygen species generated via the disproportionation3,4 of LiO2 to Li2O2 is responsible for the observed parasitic chemistry in Li-O2 batteries during discharge. These parasitic products also limit the final discharge capacity attainable in Li-O2 batteries. In Na-O2 batteries, where the final discharge product is NaO2, we find that discharge capacity is not limited by parasitic product formation.

In experiments with electrolyte solvents of different electron accepting tendencies, we show that solvents with the highest tendency to solvate the superoxide anions show the lowest Li2O2 formation yield. We note that higher solubility of superoxide anions is preferred for the formation of particulate Li2O2 that leads to high discharge capacities5. Our results suggest that there seems to be an unavoidable trade-off between high discharge capacities and high recharge efficiencies in aprotic Li-O2 batteries with Li2O2 as the discharge product. We show that high capacity and high recharge efficiencies could be possible in electrolyte solutions when singlet oxygen quenchers are employed as electrolyte additives. In fact, in electrolyte solutions with high superoxide solubility, we show that both Li2O2 formation yields and discharge capacities can be simultaneously improved using singlet oxygen quenchers. Our findings offer valuable insights into approaches for attaining higher capacity by utilizing superoxide solvating electrolyte solvents while mitigating the adverse effects of parasitic reactions.

Finally, we also touch upon the parasitic gas evolution in conventional Li-ion batteries.  We show that the formation of solid electrolyte interface layer (SEI) on graphite anode contributes to a significant fraction of gas evolution in the first charging cycle. Further, we conclude by positing that singlet oxygen induced parasitic gas evolution might also contribute to inefficiencies in Li-ion batteries. Electrochemically stable electrolyte additives that quench or bind to singlet oxygen might be useful for enhancing recharge efficiencies in Li-ion batteries as well.