Exploring Solvent Stability against Nucleophilic Attack by Solvated LiO₂⁻in an Aprotic Li-O₂Battery

Abstract

Solvent degradation due to reactivity with various oxygen species is one of the most important issues in aprotic Li-O 2 batteries. Recently, a more complete mechanism for discharge in an aprotic Li-air battery has been proposed, which accounts for the formation of solvated peroxides by disproportionation. In the present work, nucleophilic attacks by one of these solvated peroxides, LiO 2 − (solv) on some commonly used solvents in aprotic Li-air batteries, including acetonitrile (MeCN), 1-methyl-2-pyrrolidone (NMP), dimethoxy ethane (DME), and dimethyl sulfoxide (DMSO) have been explored by calculating the reaction and activation free energies using density functional theory (DFT) Active research 1-17 on the aprotic Li-air battery has been drawn by its high theoretical energy and power storage capacities of 11,000 Wh/kg and 3800 mAh/g, respectively.18,19 These values double those of the most advanced lithium ion batteries and close to those of gasoline. However, many challenges 20 must be overcome before a commercially viable battery can be produced. One critical issue is the degradation of solvents during the charge and discharge cycles of the battery. Common solvents in Li-air batteries such as propylene carbonate (PC), ethylene carbonate (EC), and dimethyl carbonate (DMC) have long been shown to degrade during discharge and fail to produce lithium peroxide (Li 2 O 2 ), the desired discharge product in a twoelectron process 21-23 described byBryantsev et al. [24][25][26] attributed this failure to the reactions between these solvents and the superoxide radical ion (O 2 − ) with the latter formed inevitably during discharge in the one-electron reduction of oxygen shown in Equation 2 as

A Review of Lithium-Air Battery Modeling Studies

“…Motivated by the stability of undissociated solvated peroxide LiO2 − , Chau et al [64] investigated nucleophilic attacks by LiO2 − on some of the popular organic electrolytes that include acetonitrile, 1methyl-2 pyrrolidone, dimethoxyethane and DMSO. The study employed DFT to calculate activation energies for these reactions, as shown in Figure 38, and these electrolytes are susceptible to chemical instability in presence of LiO2 − .…”

“…Schematic diagram of the reaction profile for the nucleophilic attack by LiO 2 − (solv) on −CH 2 − of DME (going right) and DME with one hydrogen replaced with fluorine (going left) togetherwith the structures of the reaction complex, transition state complex, and product. Republished from Chau et al[64] with permission of The Electrochemical Society conveyed through Copyright Clearance Center, Inc.…”

Redox mediators for high-performance lithium–oxygen batteries

“…Therefore, RMs, especially organic RMs with C–H bonds next to O or N atoms, may be prone to being attacked by the aggressive oxygen species in Li–O 2 batteries [ 83 ]. In the presence of Li + ions, the nucleophilic attack is further exacerbated, which will trigger aggravated parasitic reactions that jeopardize the cycle life of batteries [ 84 , 85 ]. Accordingly, screening out stable RMs has extraordinary significance for the development of Li–O 2 batteries.…”

Li+-ligand binding energies and the effect of ligand fluorination on the binding energies

“…However for lithium-air batteries, no electrolyte has been found that is completely stable for the reactive species at the cathode. The work to date suggests 3,4 that the abstraction of H or H + is responsible for the electrolyte decomposition. It has been shown 5 that replacing the hydrogen atoms in 1,2-dimethoxyethane (DME) with methyl groups changes some of the chemistry that is occurring, showing a possible stabilization of the electrolyte.…”