Electrode/electrolyte Interface in the Li-O2 Battery: Insight from Molecular Dynamics Study

TL;DR

This study uses molecular dynamics simulations to analyze the electrode/electrolyte interface in Li-O2 batteries, revealing how potential and solvent properties influence reaction pathways and passivation, guiding future battery design.

Contribution

First molecular dynamics simulation of Li-O2 cathode interface at realistic potentials, providing insights into solvent effects and reaction kinetics for battery improvement.

Findings

Electrode potential pushes oxygen anions away from the reaction layer.

Passivation mainly caused by lithium superoxide near the electrode.

Lower free energy solvents can suppress passivation and enhance capacity.

Abstract

In this paper for the first time we report the results of molecular dynamics simulation of electrode/electrolyte interface of Li-O2 cathode under potential close to experimental values in 1M dimethyl sulfoxide (DMSO) solution of LiPF6 salt. Electric potential profiles, solvent structuring near the electrode surface and salt ions distributions are presented and discussed here as well as potentials of mean force (PMF) of oxygen and its reduction products. The latter would be of a great use for the future theoretical studies of reaction kinetics as PMF being essentially the work term is a required input for the reaction rate constant estimations. Electrode/electrolyte interface under the realistic potential effectively push oxygen anions out of the reaction layer that makes second reduction of superoxide anion hardly probable. Thus the main cause of the passivation should be the lithium superoxide presence near the electrode surface. The way to suppress the passivation is the shifting of equilibrium O_2^- + Li_+ <-> LiO_2 to the side of separately solvated ions, for example by using solvents resulting in lower free energy of the ions. This conclusion is in agreement with the hypothesis stating that high donor number solvents lead to dominantly solution Li2O2 growth and significantly higher cell discharge capacities.