(560cr) Mechanistic Insights into Solution Phase Oxygen Reduction Reactions and the Effect of Metal Cation Dopants

Non-aqueous metal-O₂ batteries with high theoretical gravimetric
energy storage density stands to be one of the potential future battery
technologies to replace current Li-ion batteries. ^1-4This has led to renewed interest into fundamental studies of
oxygen reduction reaction (ORR) in non-aqueous electrolytes because they are
the central electrochemical processes involved at the metal-air cathodes.
Fundamental insights into these reactions are critical to further improvement
and eventual commercialization of these technology prototypes.

During discharge, the main
fundamental reaction involved is ORR which leads to the formation of discharge
products. The discharge products can grow either through a surface mediated
mechanism ⁵ that produces the discharge product directly on the
electrode or a solution mediated mechanism ^6-7 which leads to the
precipitation of nanoparticles. In this talk, we show that the formation and
solvation of O₂^- at low
discharge overpotentials (shown in Fig. 1a) open up pathways to thermochemical ORR in the solution
phase. This leads preferentially to the formation of solid Li₂O₂
vs. solid NaO₂ and KO₂
at low overpotentials. We propose, based on
theoretical calculations, the reaction pathways for the formation of alkali
superoxide and peroxide in several solvents that explain the observed product
selectivity.

Figure
1: (a) Schematic of surface and solution mediated discharge mechanism governed
by discharge overpotential. (b) Schematic of metal
cation doping in Li₂O₂ bulk and
adsorption on Li₂O₂(0001)  surface.

The electrolyte solution for such
batteries contain metal cations through added salts. The effect of of these
metal cations (dopants) can have morphological effects on the final discharge
product through formation of localized domain microstructures as shown in Fig. 1b. This translates to microstructures with different
lattice types than the thermodynamic minimum 12.0pt;mso-bidi-font-family:" times new roman>crystalline P6₃/mmc Föppl
structure of Li₂O₂as
the discharge product. The formation of these microstructures can aid in
the overall OER process. Such kind of incorporation
of foreign particles in the form of metal cations into the discharge product
has already been shown to be successful experimentally.⁸  Using evolutionary
algorithms coupled with DFT we explore the
possibility of thermodynamic formation of a new phase of bulk solid spurred by
the incorporation of high levels of metal cation dopants and its consequential
effects on the electronic properties of the resultant bulk solid. We show that
these microstructures can aid in improving electron conduction and hence reduce
the high overpotential for OER
which the bulk discharge product is associated with in Li-O₂
electrochemistry.⁹ We consider a only a
few commonly used elements with known electrocatalytic
properties for presenting a general approach to doping. We also present a
theoretical study for the adsorption of metal cations on the Li₂O₂(0001) surface with and without Li/O vacancy defects. We
present an extended thermodynamic formulation for the adsorption of these
cations to conclude the required conditions that would facilitate the facile
adsorption of the concerned adsorbates.   

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