Solvent Decompositions and Physical Properties of Decomposition Compounds in Li-Ion Battery Electrolytes Studied by DFT Calculations and Molecular Dynamics Simulations

Abstract

The density functional theory (DFT) calculations have been performed for the reduction decompositions of solvents widely used in Li-ion secondary battery electrolytes, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonates (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC), including a typical electrolyte additive, vinylene carbonate (VC), at the level of B3LYP/6-311+G(2d,p), both in the gas phase and solution using the polarizable conductor calculation model. In the gas phase, the first electron reduction for the cyclic carbonates and for the linear carbonates is found to be exothermic and endothermic, respectively, while the second electron reduction is endothermic for all the compounds examined. On the contrary, in solution both first and second electron reductions are exothermic for all the compounds. Among the solvents and the additive examined, the likelihood of undergoing the first electron reduction in solution was found in the order of EC > PC > VC > DMC > EMC > DEC with EC being the most likely reduced. VC, on the other hand, is most likely to undergo the second electron reduction among the compounds, in the order of VC > EC > PC. Based on the results, the experimentally demonstrated effectiveness of VC as an excellent electrolyte additive was discussed. The bulk thermodynamic properties of two dilithium alkylene glycol dicarbonates, dilithium ethylene glycol dicarbonate (Li-EDC) and dilithium 1,2-propylene glycol dicarbonate (Li-PDC), as the major component of solid-electrolyte interface (SEI) films were also examined through molecular dynamics (MD) simulations in order to understand the stability of the SEI film. It was found that film produced from a decomposition of EC, modeled by Li-EDC, has a higher density, more cohesive energy, and less solubility to the solvent than the film produced from decomposition of PC, Li-PDC. Further, MD simulations of the interface between the decomposition compound and graphite suggested that Li-EDC has more favorable interactions with the graphite surface than Li-PDC. The difference in the SEI film stability and the behavior of Li-ion battery cycling among the solvents were discussed in terms of the molecular structures.

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

“…60,65 This was also confirmed by electron paramagnetic resonance spectroscopy measurement. 19,[74][75][76][77] and some key liquid properties (such as density, cohesive energy, solubility in the solvent) of SEI components, which will be discussed in the section "Correlation of SEI properties with battery performance, starting from known components".…”

“…First, every solvent and salt species will decompose at its unique voltage. Tasaki 74 found that the order for the solvent molecule to undergo the first electron reduction is EC > PC > VC > DMC > EMC > DEC, with EC being the most likely to be reduced. VC, on the other hand, is most likely to undergo the second electron reduction, followed by EC and PC, as VC > EC > PC.…”

“…VC, on the other hand, is most likely to undergo the second electron reduction, followed by EC and PC, as VC > EC > PC. 74 The reduction voltage is sensitive to the nearby Li + . When the Li + -solvent complexes in implicit solvent are considered, the first electron reduction order changed to VC > PC~EC~DMC (cis-trans) > DMC (cis-cis).…”

Electrode Surface Composition of Dual-Intercalation, All-Graphite Batteries

“…Furthermore, although extensive studies of solid/electrolyte (SEI) formation and stability on graphite anodes, as well as reactions involving LiCoO 2 cathodes, have been completed, SEI formation on graphite cathodes due to anion intercalation has not been examined. Since the intercalation process strongly depends on processes at the electrode-electrolyte interface [17], heterogeneity and composition differences on electrode surfaces are expected to significantly influence capacity and cycling efficiencies. Certain functional groups can be introduced onto carbon surfaces via high-temperature treatments, air oxidation, plasma and gas treatment, and wet chemistry reactions [18,19].…”

Computational chemistry for graphene-based energy applications: progress and challenges

“…94,105 Classical MD simulations have also been used to investigate the structure of carbonate based electrolytes doped with lithium salts, most commonly LiPF 6 , at graphitic interfaces. 97,[109][110][111][112] Vatamanu et al found that the composition of the electrolyte interfacial layer depended strongly on the electrode potential. The amount of EC and DMC at the interface increased as the electrode charge was increased, for both positively and negatively charged electrodes.…”