Ethylene carbonate (EC) is blended with dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) or diethyl carbonate (DEC) to combine complementary properties. EC favors dissociation in many lithium salt systems and can participate in film formation in conventional graphite systems. Linear carbonates help adjust fluidity, phase behavior and evaporation. The aim is a liquid suited to the complete battery system, rather than maximizing a single parameter.
EC’s role and the limitations of using it alone
Ethylene carbonate (EC) is a cyclic carbonate. Its dielectric properties and coordination with lithium ions favor salt dissociation in many formulations. In many graphite negative-electrode systems, EC also participates in forming a solid electrolyte interphase (SEI), helping reduce continuing electrolyte decomposition.
However, EC may be solid at ordinary room temperature, and its viscosity when liquid also limits its ability to serve as the entire solvent system. Dissolving salts and participating in film formation do not mean it simultaneously meets requirements for solution preparation, wetting and low-temperature use.
What linear carbonates contribute
Dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are common linear carbonates. Adding them can adjust the viscosity and phase behavior of an EC system and change evaporation rates. Their physical properties differ, so replacing one with another changes more than a name.
Lower viscosity helps liquid enter porous electrodes and separators, but wetting also depends on surface properties, pore structure and processing. Greater fluidity after blending explains part of wetting behavior; it does not guarantee complete wetting.
The three linear carbonates can adjust different aspects of an EC system. The table uses pure-substance characteristics to explain factors that need to be balanced in blending. The behavior of the actual salt-containing liquid also depends on composition and temperature.
| Combination | Main characteristics of the linear component | Factors to balance when blending |
|---|---|---|
| EC/DMC | DMC has low viscosity when liquid and is relatively volatile | Can improve fluidity; evaporative losses need control and crystallization on cooling needs assessment |
| EC/EMC | Pure EMC has a relatively low freezing temperature | Can help adjust the liquid-temperature range; actual crystallization still depends on mixture composition |
| EC/DEC | DEC has a higher atmospheric boiling point than DMC and EMC | Relatively slow evaporation; viscosity and ion transport in the salt-containing liquid still need assessment together |
Compare the pure-substance properties and volatility of DMC, EMC and DEC
Blending is more than averaging physical properties
After blending, the combination of molecules around lithium ions changes with solvent ratios and salt concentration: this is a change in solvation structure. A solvent’s proportion around lithium ions need not match its proportion in the bulk liquid. After salt is added, ion numbers, coordination and interactions jointly affect transport, so mixed-electrolyte conductivity cannot be obtained by simply adding pure-solvent values.
The same applies to phase behavior. Crystallization of EC mixed with other carbonates depends on composition and changes further with lithium salts and additives. Pure EC’s melting point helps explain storage behavior, but is not directly the minimum operating temperature of a complete electrolyte.
Research note: solid–liquid equilibrium experiments on binary carbonates show that crystallization needs to be studied as a function of mixture composition. When a solid phase precipitates on cooling, the remaining liquid’s composition also changes. The study uses non-ideal solution models to describe these phase-equilibrium relationships. A weighted average of two pure-substance melting points therefore cannot replace a mixture’s phase-transition temperature. The original study is indexed by NIST and listed in the references.
Which properties need balancing when ratios change?
Increasing EC may help salt dissolution or film formation in some systems, but may also increase viscosity and change low-temperature crystallization and interfacial reactions. Increasing a linear carbonate may improve flow while changing solvation and evaporation. These effects often occur together; no component should be understood as providing benefits alone.
Ratios need to be defined in relation to lithium salts, additives, electrode materials and operating temperature range. Changes in these conditions also change the required interphase and transport conditions. Low-temperature output, higher voltage and storage at elevated temperature may impose different demands on the same combination.
What to assess in the blended liquid and in the cell
Mixture viscosity, conductivity and crystallization describe the liquid itself. Charge–discharge efficiency, cycling and gas evolution reflect its operation together with the other parts of the cell. Clear salt identity, concentration, temperature and electrode conditions make it possible to assess changes caused by component substitution or ratio adjustments.
Selecting EC–linear carbonate combinations requires balancing fluidity, phase behavior, solvation and interphase formation. First establish the state and transport behavior of the mixture under the target conditions, then use cell results to assess suitability.
References
Lee et al.: lithium-ion solvation exchange in carbonate electrolytes
Mitsui Fine Chemicals: reference physical properties of linear carbonates