Density Functional Theory Study of the Role of Anions on the Oxidative Decomposition Reaction of Propylene Carbonate

Abstract

The oxidative decomposition mechanism of the lithium battery electrolyte solvent propylene carbonate (PC) with and without PF(6)(-) and ClO(4)(-) anions has been investigated using the density functional theory at the B3LYP/6-311++G(d) level. Calculations were performed in the gas phase (dielectric constant ε = 1) and employing the polarized continuum model with a dielectric constant ε = 20.5 to implicitly account for solvent effects. It has been found that the presence of PF(6)(-) and ClO(4)(-) anions significantly reduces PC oxidation stability, stabilizes the PC-anion oxidation decomposition products, and changes the order of the oxidation decomposition paths. The primary oxidative decomposition products of PC-PF(6)(-) and PC-ClO(4)(-) were CO(2) and acetone radical. Formation of HF and PF(5) was observed upon the initial step of PC-PF(6)(-) oxidation while HClO(4) formed during initial oxidation of PC-ClO(4)(-). The products from the less likely reaction paths included propanal, a polymer with fluorine and fluoro-alkanols for PC-PF(6)(-) decomposition, while acetic acid, carboxylic acid anhydrides, and Cl(-) were found among the decomposition products of PC-ClO(4)(-). The decomposition pathways with the lowest barrier for the oxidized PC-PF(6)(-) and PC-ClO(4)(-) complexes did not result in the incorporation of the fluorine from PF(6)(-) or ClO(4)(-) into the most probable reaction products despite anions and HF being involved in the decomposition mechanism; however, the pathway with the second lowest barrier for the PC-PF(6)(-) oxidative ring-opening resulted in a formation of fluoro-organic compounds, suggesting that these toxic compounds could form at elevated temperatures under oxidizing conditions.

Molecular Modeling of Electrolytes

“…For example, oxidation of carbonate solvents such as EC, PC, and DMC complexed with anions such as PF 6 − , BF 4 − , and ClO 4 − resulted in an anion nucleophilic attack on the solvent upon oxidation (electron removal) and deprotonation of the carbonates [34,35] that significantly reduced the oxidation stability of these solvent-anion complexes compared to values obtained for isolated solvents. Xing et al [36] reported that the oxidation potentials of EC 2 and EC 4 clusters were significantly lower than the intrinsic oxidation potential of an isolated EC and even lower than the oxidation potential of the EC-BF 4 − complex.…”

“…The majority of the studies considered isolated solvents or anions that do not explicitly interact with the electrode; in situations where there is no specific interaction such as hydrogen bonding with other electrolyte components; and where the solvent effect is included via the PCM. A few studies went beyond to include the condensed-phase interactions present in ionic liquids [32], cation-anion interactions [33], or solvent-anion interactions [16,[34][35][36][37]. The most successful were DFT predictions of shuttle redox potentials [26,38], which reported a close agreement with the experimental data showing a root mean square deviation of 0.08 V. Similarly, Wang, Buhrmester, and Dahn [38] reported excellent agreement between the calculated values for 17 redox shuttle additives, with a root mean square deviation between the calculated and measured oxidation potentials of 0.15 V and a maximum deviation of 0.25 V, indicating that DFT calculations at B3LYP/6-31G(d,p) could be effectively used for screening redox additives.…”

“…For example, the intrinsic oxidation potentials of common electrolyte solvents or anions obtained from DFT and higher level correlated methods were significantly higher than experimentally determined values from LSV experiments, especially when LSV measurements were extrapolated to low scan rates [16,[34][35][36]. A step toward reconciling QC predictions and LSV measurements was made in recent DFT studies [16,[34][35][36], which focused on the oxidation stability of solvent-anion and solvent-solvent clusters. These studies showed that the oxidation stability of solvent-solvent and solvent-anion clusters is lower than stability of the isolated solvents and anions that comprise these clusters.…”

Review on modeling of the anode solid electrolyte interphase (SEI) for lithium-ion batteries

“…Likewise, the electrolyte is expected to be stable if the highest occupied molecular orbital (HOMO) of the electrolyte is lower than the cathode Fermi energy level. The density functional theory (DFT) computed electrochemical windows of the common electrolyte components [11][12][13][14][15] have been summarized in the past. 16 However, Fig.…”

First-Principles Modeling of the Initial Stages of Organic Solvent Decomposition on LixMn2O4(100) Surfaces

“…2e) to form volatile organic products. 65 This may expose reactive sites on the oxide surface, allowing other EC molecules to react and potentially deposit more protons. This will be considered in the future.…”