Computational Studies of Solubilities of LiO₂and Li₂O₂in Aprotic Solvents

Abstract

Knowledge of the solubilities of Li 2 O 2 and LiO 2 in aprotic solvents is important for insight into the discharge and charge processes of Li-O 2 batteries, but these quantities are not well known. In this contribution, the solvation free energies of molecular LiO 2 and Li 2 O 2 in various organic solvents were calculated using various explicit and implicit solvent models, as well as ab initio molecular dynamics (AIMD) methods. The solvation energies from these calculations along with calculated lattice energies of Li 2 O 2 and LiO 2 were used to determine the solubility of bulk LiO 2 and Li 2 O 2 . The computed solubility of LiO 2 (1.8 × 10 −2 M) is about 15 orders higher than that of Li 2 O 2 (2.0 × 10 −17 M) due to a much less negative lattice energy of bulk LiO 2 compared to that of Li 2 O 2 . The difference in solubilities between LiO 2 and Li 2 O 2 likely will affect the nucleation and growth mechanisms and resulting morphologies of the products formed during battery discharge, influencing the performance of the battery cell. Lithium-O 2 battery technology has received much research interest due to its high theoretical gravimetric energy density compared to conventional Li-ion batteries.1 The system uses oxidation of lithium at the anode and reduction of oxygen at the cathode to induce a current flow. Depending on the reaction kinetics of different discharge mechanisms, the discharge products are generally composed of Li 2 O 2 with a low solubility and in some cases different ratios of LiO 2 and Li 2 O 2 .2-6 Much of research focus has been on the cathode. One of the challenges of Li-O 2 batteries is the incomplete utilization of the active materials due to the insulating nature of the discharge products and the clogging the porous carbon cathode. The formation of these products may involve a nucleation and growth mechanism from the solution phase.2,7-13 Nazar reported a Li 2 O 2 formation mechanism by disproportionation reaction of a limited amount of LiO 2 in solution.2 A through-solution mechanism was also proposed involving heterogeneous nucleation from a supersaturated solution of LiO 2 . 10There is other evidence that LiO 2 is soluble in the electrolyte prior to deposition on the surface based on a quartz microbalance study.14 Results from several research groups suggest that the composition and morphology of the discharge products have a significant effect on charging overpotential and rechargeability of the Li-O 2 battery. 10,15,16 Therefore, understanding the chemistry and growth mechanism of discharge products in the cell is crucial for improving Li-O 2 battery. The study and modeling of nucleation and crystal growth requires knowledge of bulk solubility. 17,18 However, there are limited experimental solubility measurements reported in the literature on discharge products LiO 2 and Li 2 O 2 . Solubility also highly depends on the size and morphologies of the solid as well as impurities so the values will depend on the experimental conditions. 24 The reason for the wide range of values for the s...

Kinetically Stable Oxide Overlayers on Mo3P Nanoparticles Enabling Lithium–Air Batteries with Low Overpotentials and Long Cycle Life

“…During discharge, the catalytic Li 2 O 2 production takes place on the MoO monolayer (oxidized top layer of Mo 3 P) with a DFT‐derived overpotential of 0.06 V. The Li 2 O 2 units can then transfer (via ionic liquid) to agglomerated Li 2 O 2 particles, also confirmed by SEM image shown in Figure 3a–f, (Section 19, Supporting Information), leaving a clean MoO layer behind for further catalysis of the Li 2 O 2 production. [ 79,80,63 ] Our DFT‐derived and experimental discharge overpotentials on Mo 3 P are smaller than the values calculated on the bulk Li 2 O 2 surface (0.33 V on the thermodynamically stable majority facet (0001) and 0.12 V on kink sites), [ 49 ] confirming the viewpoint that discharge takes place via catalytic MoO monolayer. For the charging process on the other hand, there can be two possible contributions: 1) dissolution of Li 2 O 2 from the MoO monolayer, 2) dissolution of Li 2 O 2 from the surface of the agglomerated Li 2 O 2 particles formed during the discharge.…”

“…Such a monolayer, MoO, has enough kinetic stability due to strong covalent and charge transfer interactions with the molybdenum phosphide layer beneath, as depicted in Figure 5b, making it possible for this monolayer to retain its epitaxial form during the course of the discharge. Additionally, the interaction of the deposited Li 2 O 2 units with the surface is still in a range that, with the help of the ionic liquid solvent, can dissolve into the solvent, [ 63,82–84 ] with only finite points of contact with the Mo 3 P substrate, allowing the (oxidized) Mo 3 P surface to continue catalyzing the Li 2 O 2 production reaction as also observed in our SEM images shown in Figure 3a–f.…”

“…However, the observed sharp increase of charge potentials in the first 100 cycles (Figure 2b) might be correlated to the saturation of electrolyte with discharge product (Li 2 O 2 and LiO 2 ) that affects mass transports and the ionic diffusion properties of the electrolyte as we did not observed any degradation of the cell components, such as, cathode, anode and the electrolyte over 1000 cycles. [ 60–64 ]…”

Exclusive solution discharge in Li-O2 batteries

“…4k) will be more likely than homogenous nucleation in solution. Relatively monodisperse particles at low rates, and the absence of Ostwald ripening (considering the low Li2O2 solubility 64 ) suggest that primary nucleation takes place mainly at early stages of discharge, secondary nucleation and growth at all other stages of discharge. LiO2 supersaturation will be required to form primary nuclei, but once the nuclei exists, LiO2 concentration and primary nucleation rate drop, causing growth to dominate further on 30,65 .…”

Relating Catalysis between Fuel Cell and Metal-Air Batteries

“…The functionality of catalysis for the subsequent reactions changes significantly for aprotic media LABs. The initial intermediate LiO 2 is relatively more soluble compared with Li 2 O 2 in aprotic electrolytes, 85 leading to its dissolution and further subsequent electrochemical reduction or chemical disproportionation to Li 2 O 2 . 81,82,[86][87][88][89] The catalyst for this process can be argued as an electrolyte component that has a high donor number, 87 which can stabilize and prolong the lifetime of LiO 2 allowing it to diffuse, reduce, and deposit in a nonpassivating manner.…”