DMC, DEC, EMC, EC and PC are all organic carbonates, but they are not different grades of the same solvent. Their different molecular structures produce differences in solvency, fluidity, crystallization temperature and evaporation behavior. These differences explain why some are suited to blending with other solvents and why some crystallize at room temperature.
Start with names and structures
Dimethyl carbonate (DMC), diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) are linear carbonates. DMC has a methyl group at each end, DEC an ethyl group at each end, and EMC one methyl and one ethyl group. Linear distinguishes them from cyclic structures; it does not mean that the molecule forms a straight line in space.
Ethylene carbonate (EC) and propylene carbonate (PC) are cyclic carbonates, with the carbonate group contained in a ring. PC can be viewed as an EC ring with one hydrogen replaced by a methyl group. This seemingly small structural difference changes intermolecular interactions and crystal packing, affecting physical properties.
For example, DMC and EMC are different carbonates, so switching between them changes the main composition. Industrial-grade DMC and battery-grade DMC are different grades of the same carbonate, with differences mainly in water and impurity controls and packaging requirements. The material name identifies the compound; the grade and specification describe the uses it is suited to.
Main differences between the five carbonates
| Name and abbreviation | Structure | Main physical characteristics | Why the difference matters |
|---|---|---|---|
| Dimethyl carbonate DMC | Linear, with methyl groups at both ends | Low viscosity when liquid; relatively volatile among the three linear carbonates | Can improve flow, but evaporative losses and ignition risks require control |
| Diethyl carbonate DEC | Linear, with ethyl groups at both ends | Higher atmospheric boiling point than DMC; usually evaporates more slowly at the same temperature | Can adjust evaporation rate, but still requires fire-risk management |
| Ethyl methyl carbonate EMC | Linear, with asymmetric ends | Lower pure-substance freezing temperature than DMC; volatility generally between DMC and DEC | Can adjust the liquid range of a mixture, but does not directly predict low-temperature battery performance |
| Ethylene carbonate EC | Cyclic | High dielectric constant; may be solid at ordinary room temperature | Promotes dissociation in many lithium salt systems; phase state matters for liquid use |
| Propylene carbonate PC | Methyl-substituted cyclic structure | High dielectric constant; more readily remains liquid at low temperatures than EC | Attractive liquid-temperature range, but graphite compatibility depends on conditions |
The table compares common characteristics of pure substances. Viscosity and volatility should be compared at the same temperature. Solid EC cannot be compared directly with liquid solvents for viscosity. Mixing and salt dissolution change physical properties further.
Why salt dissolution and fluidity must be considered together
A solvent’s dielectric properties affect electrostatic interactions between positive and negative ions. Coordination of solvent molecules with lithium ions concerns their local bonding environment. These are related but distinct concepts. EC and PC favor lithium salt dissociation in many electrolytes, but dissolving a salt does not mean its ions move quickly.
Viscosity describes a liquid’s internal resistance to flow. DMC, EMC and DEC are often combined with EC to adjust fluidity and electrode wetting. The result depends on composition and temperature, rather than being a simple average of pure-solvent values.
Why polarity alone does not make EC and PC interchangeable
In many conventional graphite negative-electrode systems, EC participates in forming a solid electrolyte interphase (SEI) that reduces continuing electrolyte decomposition. PC carries a risk of co-intercalating with lithium ions between graphite layers and damaging the structure. Their different interfacial behavior means that polarity alone is not a basis for substitution; suitability also depends on lithium salts, additives and electrode conditions.
Learn more about PC–graphite compatibility and interfacial protection
Preserve comparison conditions when reading physical property data
Melting point helps establish phase state, boiling point describes the boiling temperature at a given pressure, and flash point concerns whether vapor can be ignited. They answer different questions. A higher boiling point and slower evaporation do not mean a material cannot burn.
Pure-solvent data help explain material differences, tests on salt-containing electrolytes describe formulation properties, and cell tests reflect performance in a particular battery. Before comparing data, establish what material or system they describe and under what conditions.
For melting point, the reference value for DMC is 4.65°C and that for EC is about 36°C. They may therefore have different phase states in an ordinary environment around 20°C.
Data note: the DMC value comes from Mitsui Fine Chemicals’ general physical properties, and the EC value from TCI’s reference property data. Both are third-party reference data for pure substances, not Lixing delivery specifications.
Continue reading: an approach to selecting electrolyte solvents
References
Mitsui Fine Chemicals: physical properties and uses of carbonate products
TCI: ethylene carbonate physical property data
Pan et al.: LiTFSI/PC on binder-free graphite electrodes (2016)