Why Does Dimethyl Carbonate Dissociate Li Salt Better Than Other Linear Carbonates? Critical Role of Polar Conformers

Abstract

The marked difference in the ionic conductivities of linear carbonate (LC) electrolyte solutions despite their similar viscosities and permittivities is a long-standing puzzle. This study unraveled the critical impact of solvent conformational isomerism on salt dissociation in 0.1–3.0 M LiPF6 dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) solutions using Raman and dielectric relaxation spectroscopies. The extent of salt dissociation in the LC solutions, which decreased in the order DMC > EMC > DEC, is closely related to the fraction of polar cis–trans LC conformers, as this conformer participates in Li ion solvation more readily than the nonpolar cis–cis counterpart. Our first-principles calculations corroborated that the cis–trans conformer facilitates free ion formation more than the cis–cis conformer, and the extent of this effect decreased in the order DMC > EMC > DEC. This study provides an avenue for the design of highly conductive electrolytes by exploiting the conformational isomerism of solvent molecules.

Controlling the Phase Separation of Dimethyl Carbonate Solvents Using a Dual-Cation System: Applications in High-Power Lithium Ion-Based Hybrid Capacitors

“…In addition, DMC solvents exhibit a higher oxidation resistance (5.0 V vs Li/Li + ) compared to other high dielectric solvents such as PC and EC (4.8 V vs Li/Li + ) on the Pt electrode surface, which agrees with the order of calculated highest occupied molecular orbital (HOMO) energies . Thus, several studies have focused on the development of electrolytes based on DMC and BF 4 anion-based salts for achieving a high power density and long stability. ,− However, the aggregation of LiBF 4 in DMC (forming a contact–ion pair) results in a lower ionic conductivity (0.5 mS cm –1 , at 1 M LiBF 4 ), which is 1/15 of that of LiPF 6 (7.5 mS cm –1 , at 1 M LiPF 6 ) . Two approaches have been proposed to overcome the issue; (i) increasing the salt concentration to increase the number of Li + carriers (4–8 M) and (ii) the addition of a high dielectric constant solvent (such as EC) for improving the dissociation of ions .…”

“…Among low viscous solvents, DMC is a commercially used solvent having a lower viscosity (0.58 cP) than other linear carbonates [diethyl carbonate (DEC, 0.75 cP) and ethyl-methyl carbonate (EMC, 0.65 cP)]. , The low polarizability of DMC allows for a low activation energy (38–40 kJ mol –1 ) of the charge-transfer resistance both for the graphite anode and the LTO anode compared to that for propylene carbonate (PC) or the mixture of EC + DMC (54–64 kJ mol –1 ), − resulting in a high rate capability . In addition, DMC solvents exhibit a higher oxidation resistance (5.0 V vs Li/Li + ) compared to other high dielectric solvents such as PC and EC (4.8 V vs Li/Li + ) on the Pt electrode surface, which agrees with the order of calculated highest occupied molecular orbital (HOMO) energies .…”

Solvation Structure around Li+ Ions in Organic Carbonate Electrolytes: Spacer-Free Thin Cell IR Spectroscopy

“…A variety of analytical and computational methods, including FTIR spectroscopy, − Raman spectroscopy, − electrospray ionization mass spectrometry, ,, NMR spectroscopy, ,,, internally referenced DOSY, − ultrafast IR spectroscopy, − ,,− density functional theory (DFT) calculation, − ,, and molecular dynamics simulation − have been used. Still, vibrational spectroscopy such as FTIR and Raman spectroscopy has advantages in studying the solvation structure of ions in solutions because they could provide information on the effective concentrations or numbers of solvent molecules directly interacting with ions when the vibrational frequencies of coordinating and non-coordinating solvent molecules around each ion are different from each other.…”

Equilibrium and Dynamical Characteristics of the Solvation Associated with the Li+/Li Redox Couple at the Ethylene Carbonate/Graphene Interface

“…In recent times, there has been a growing interest in analyzing the characteristics of electrolyte solutions comprising lithium salts dissolved in different cyclic and noncyclic carbonates from both experimental − and theoretical perspectives. − This interest was mainly spurred by the convenient applicability of these solutions as conducting organic electrolytes in Li-ion batteries. − The choice of these media represents one of the key features in the successful design of these kinds of devices along with appropriate selections of the electrode materials. Commonly used arrangements combine lithium/cobalt oxide cathodes with graphite composite anodes, allowing Li + intercalation, complemented by solid protective films that prevent the decomposition of the different elements of the cells .…”