EC vs. PC: Why do two high-dielectric solvents behave differently with graphite anodes?

EC (ethylene carbonate) and PC (propylene carbonate) are both high-dielectric cyclic carbonates commonly used in lithium-ion battery electrolytes, where they help promote lithium-salt dissociation and solvation [1]. Yet they are not simply interchangeable in graphite-anode systems. EC is more conducive to forming a relatively stable solid electrolyte interphase (SEI), whereas PC, despite its low melting point and broad liquid-temperature range, can co-intercalate into graphite together with solvated lithium ions in some systems. This can cause graphite-layer expansion, exfoliation, and capacity loss. A meaningful comparison of EC and PC therefore needs to consider both their reduction behavior and SEI formation at the graphite surface, as well as their effects on electrolyte viscosity, ionic conductivity, electrode wetting, and low-temperature fluidity.

The first distinction lies at the graphite interface. Graphite operates at a low potential, so the electrolyte undergoes reduction during initial charging and forms an SEI [2]. EC tends to form a protective interphase that can suppress continued electrolyte decomposition, although the quality of this SEI still depends on the lithium salt, additive package, graphite surface, and formation conditions. By contrast, when interfacial protection is insufficient or the solvation structure is unfavorable, PC can enter the graphite galleries together with solvated Li⁺, leading to solvent co-intercalation, graphite expansion and exfoliation, and subsequent capacity loss.

After comparing interfacial behavior on graphite, the next step is to consider the bulk properties of the electrolyte. EC and PC influence viscosity, lithium-ion transport, electrode wetting, and fluidity across different temperatures, and these differences make the two solvents better suited to different formulation objectives. When comparing EC and PC, both graphite-interface compatibility and bulk electrolyte transport properties should therefore be evaluated.

PC has a melting point of about -49 °C and remains liquid over a broad temperature range; its relatively high boiling point can also help widen the usable temperature window. EC supports lithium-salt dissolution and solvation but melts at about 36 °C, so it is normally blended with DMC, EMC, or DEC to improve fluidity. In practical terms, PC is stronger in low-temperature liquid-range performance, whereas EC is better established for lithium-salt solvation and compatibility with conventional graphite interfaces.

PC's low-temperature fluidity and wide liquid range are useful bulk-electrolyte properties, but they do not mean that PC is inherently compatible with graphite. Whether solvent co-intercalation occurs and whether a stable SEI can form also depend on the lithium salt and additives, salt concentration, graphite surface condition, and formation protocol. A high dielectric constant mainly assists salt dissociation and charge screening; it does not by itself determine surface reduction reactions, desolvation, or SEI structure [1][3]. PC also contains one additional methyl group relative to EC, which changes its solvation environment and interfacial reaction pathways. Accordingly, when assessing whether PC is suitable for a conventional graphite system, low-temperature properties and dielectric behavior are not enough; co-intercalation risk and SEI stability should be verified in the complete electrolyte formulation under the intended formation conditions.

This leads directly to formulation design. The next question is whether the lithium salt, additives, and solvent ratios can promote a more stable SEI while reducing the likelihood that PC co-intercalates into graphite with solvated lithium ions. Film-forming additives such as VC and FEC can participate in interphase formation, while certain salts and high-concentration electrolytes may alter the solvation structure and thereby mitigate co-intercalation to some extent. However, the effect cannot be predicted from the additive name alone. Gas generation during formation, interfacial resistance, cycle retention, and post-storage performance still need to be confirmed. For a conventional graphite system in which PC is being considered as a major replacement for EC, the formulation should first be tested with the intended lithium salt, additive package, cathode, and anode materials, and the decision should be based on verified interfacial stability, gas generation, and cycling behavior.

Once the initial formulation direction is clear, application-level selection can follow. Conventional graphite systems commonly use EC as a base solvent and use linear carbonates to tune viscosity and low-temperature performance. If the cell must operate at low temperature, or if low-temperature performance is a primary design target, PC can be included in comparative screening. When the cathode is high-nickel layered oxide or high-voltage lithium cobalt oxide, the effects of PC and its decomposition products on the cathode interface and high-voltage stability should also be evaluated. Because an anode half-cell cannot represent compatibility at both electrodes, the final judgment should be made in a full cell using the intended materials, process, and formation protocol.

Once the application boundary is defined, procurement criteria become clearer. Both EC and PC should be evaluated for water content, acidity, metal impurities, and batch-to-batch consistency. EC also requires attention to melting point and crystallization risk, while PC should be checked for compatibility with the target formulation. When switching suppliers or qualifying a second source, viscosity, ionic conductivity, wetting time, gas generation, impedance, and cycle retention should be compared using the same formulation so that material differences can be translated into quality-stability and total-manufacturing-cost implications.

In summary, EC and PC should be selected by considering both bulk-electrolyte properties and electrode-interface behavior rather than by trying to identify an absolute winner outside the context of a complete formulation and cell system. EC is usually the baseline choice for conventional graphite electrolytes because of its well-established role in graphite SEI formation. PC offers a lower melting point and broader liquid-temperature range, which creates formulation options for low-temperature or specialized solvation systems, but its co-intercalation risk and effects on gas generation and interfacial stability must be verified in the target graphite system. The appropriate choice—EC, PC, or a combination of both—should therefore be determined from the cathode and anode materials, operating voltage, temperature range, lifetime targets, and the corresponding solvent ratios and formulation design.