Electrolyte Solvent Ratios

Commercial lithium-ion electrolytes are never a single solvent. They are blends of ethylene carbonate (EC) with one or more linear carbonates, typically dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and/or diethyl carbonate (DEC). The ratio between them is one of the more consequential, and least visible, design decisions in a cell. Get it wrong and a cell that looks fine on a datasheet at 25°C can lose most of its power at -20°C, or fade rapidly if it spends its life in a hot vehicle pack.

Why Blend At All

EC has a high dielectric constant, which helps dissociate the lithium salt (almost always LiPF₆) into free ions, and among the common carbonate solvents it forms the most stable solid-electrolyte interphase (SEI) on a graphite anode [1]. The problem is that pure EC melts at around 36°C, so a cell built on EC alone would be solid at room temperature. Every commercial electrolyte therefore dilutes EC with one or more linear carbonates that stay liquid across a much wider range and bring the overall viscosity down, trading away some of EC’s SEI-forming strength to do it.

The Linear Carbonates: DMC, EMC and DEC

DMC (dimethyl carbonate) – the lowest-viscosity and lowest-cost of the three linear carbonates, but also the lowest-boiling, which leaves it most prone to decomposition and gassing as temperature rises.
EMC (ethyl methyl carbonate) – an asymmetric molecule that sits between DMC and DEC: usefully low viscosity combined with a wider liquid range than DMC. That balance is a large part of why EMC has become the default linear carbonate in most modern commercial cells.
DEC (diethyl carbonate) – the most viscous of the three, but the most stable at elevated temperature and the least prone to gas-generating decomposition.

Cold-Temperature Performance

At low temperature, the factor limiting both power and usable capacity is ionic conductivity, which falls as viscosity rises. Because EC is the most viscous, highest-melting component of the blend, cold-optimised formulations cut EC content and lean harder on the linear carbonates instead. Work at NASA’s Jet Propulsion Laboratory on quaternary EC:DEC:DMC:EMC blends, in ratios around 1:1:1:2 to 1:1:1:3 by volume, demonstrated usable cell performance down to roughly -40°C in prototype hardware — well beyond what EC-heavy, room-temperature-optimised electrolytes can manage [2]. The catch is that very low-EC electrolytes can leave the SEI under-formed unless the formulation is paired with film-forming additives (see Electrolyte Additives), which is why aggressive EC reduction is rarely done in isolation.

High-Temperature and Cycle-Life Behaviour

The same EC reduction that helps cold-temperature conductivity tends to work in the opposite direction at high temperature and over long-term cycling. EC content that is too low undercuts the SEI, but EC that is too high brings its own high-temperature penalty. Comparative data in the patent literature shows cells built with EC held to around 20% by volume or less retaining substantially more capacity after high-temperature cycling than comparison cells built at 25–30% EC, where fade was markedly worse [3]. Separately, DEC-rich blends tend to outperform DMC-rich blends at elevated temperature, since DMC’s lower boiling point and weaker thermal stability make it more prone to decomposition and gas generation as the cell heats up.

Power and Rate Capability

Rate capability tracks the same viscosity-conductivity relationship as cold-temperature performance, just without the added constraint of the electrolyte needing to stay liquid. Linear-carbonate-rich blends, more DMC or EMC relative to EC, lower bulk viscosity and raise ionic conductivity, supporting higher continuous and pulse current. It’s why high-power cell designs (power tools, HEV) typically sit at lower EC fractions than energy-optimised cells for the same chemistry, again usually leaning on additives to protect an SEI that a low-EC blend would otherwise struggle to form on its own.

Reading a Datasheet Ratio

When a cell or electrolyte datasheet quotes something like “EC:EMC 3:7” or “EC:DMC:EMC 1:1:1”, that ratio is rarely arbitrary, it reflects where the designer chose to sit on the cold-performance / high-temperature-stability / power triangle above.

As a rough rule of thumb:

EC fractions above roughly 30% by volume favour high-temperature cycle life and SEI stability at the expense of low-temperature power
EC fractions below roughly 20% favour cold-temperature and rate performance but need supporting additives to hold onto cycle life
choice of DMC, EMC or DEC as the co-solvent shifts the same trade-off again, between cost (DMC), balance (EMC) and high-temperature stability (DEC)

References

Xu, K. “Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries.” Chemical Reviews, 2004, 104(10), 4303–4418. https://doi.org/10.1021/cr030203g
Smart, M.C., Ratnakumar, B.V., et al. “Improved low-temperature performance of lithium-ion cells with quaternary carbonate-based electrolytes.” Journal of Power Sources, 2003, 119–121, 349–358.
US Patent 9,088,036, “Rechargeable lithium battery” – comparative ethylene carbonate content vs. high-temperature cycle-life data. patents.google.com/patent/US9088036