Effect of Solvents on the Behavior of Lithium and Superoxide Ions in Lithium–Oxygen Battery Electrolytes

Abstract

The molecular life of intermediates, namely, O and Li , produced during the discharge of aprotic Li-O batteries was investigated by molecular dynamics simulation. This work is of potential interest in the development of new electrolytes for Li-air batteries. We present the results on the structure and stability of the Li and O solvation shells and the thermodynamics and kinetics of the ion-association reaction in solvents such as dimethyl sulfoxide (DMSO), dimethoxyethane (DME), and acetonitrile (ACN). The residence time of solvent molecules in the Li solvation shell increases with the solvent donor number and is 100 times larger in DMSO than in ACN. In DMSO and DME, the Li ion diffuses with its solvation shell as a whole. On the contrary, in ACN it diffuses as a "bare" ion because of weak solvation. The rate constant for the association of the lithium ion with the superoxide anion in DMSO is two orders of magnitude slower than that in ACN due to fact that the free-energy barrier is 2.5 times larger in DMSO than in ACN. In addition, we show that despite the strong dependence of the Li shell stability on donor number, the rate of association does not necessarily correlate with this solvent property.

Effect of Cation Size on Solvation and Association with Superoxide Anion in Aprotic Solvents

“…The common feature of these interpretations is postulating the relation between the solvation strength and the solvent donor number. It was previously assumed that the larger DN, the stronger the Li + ‐solvent bond and this is in line with our previous simulations . At the same time, our present results provide new insight that in DMSO the stability of the Li + solvation shell is determined not only by the solvent properties, but also by a suitable size of Li + , which matches the maximum of the solvent shell stability (Figure a).…”

“…Thus, a clear correlation between the height of the barrier and the stability of the cation solvation shell in DMSO is observed (Table ). According to our previous work, the first coordination sphere of O 2 − anion has low stability in both DMSO and ACN (probably because of the large anion size). Therefore, it does not affect the kinetics of the ion‐association reaction.…”

“…It is also important to note that realistic modeling of an ensemble of O 2 − and Li + ions is hardly possible in the frame of non‐reactive molecular dynamics, since clustering of ions is accompanied by the chemical reaction with the formation of the new phase Li 2 O 2 and O 2 (disproportionation reaction) . On the contrary, the interaction of a single Li + ion with a single O 2 − ion in solvent can be properly described by Coulomb and Lennard‐Jones potentials only …”

Alkali‐Oxygen Batteries Based on Reversible Superoxide Chemistry

“…An important characteristic for the electrolyte is its ability to dissolve the superoxide product. It has been shown that high‐donor solvents, such as DMSO, increase Li + solvation and increase solubility of LiO 2 . This is important for maximizing discharge capacity because it facilitates crystal formation, which is a solution‐mediated process …”

“…It has been shown that high-donor solvents, such as DMSO, increase Li + solvation andi ncrease solubility of LiO 2 . [54,55] This is important for maximizingd ischarge capacity because it facilitates crystal formation, which is as olutionmediated process. [39] Directly relatedt ot he stabilityo ft he anode, is the stability of the electrolyte during operation of the superoxide battery.…”