| dc.description.abstract | Aprotic metal-oxygen batteries, especially Li-O2 and Na-O2 batteries, could afford theoretical specific
energies in excess of 1000 Wh/kg. 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 (LiO2 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
disproportionation 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 capacities. 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 | en_US |
