How Electrolyte Solvent and Lithium Salts Affect Battery Electrolyte Viscosity and Wettability

1. Background: Electrolyte Viscosity, Battery Solvents, and Cell Performance

Electrolytes for lithium-ion batteries are organic solvent mixtures that dissolve lithium salts to form conductive ions. Electrolyte solvent selection and salt concentration directly determine ionic conductivity, battery electrolyte viscosity, wettability, and ultimately cell performance. Figure 1 provides an overview of electrolyte research scope and cost structure: solvents represent approximately 85% of electrolyte mass and ~30% of electrolyte cost; the electrolyte itself accounts for roughly 6–8% of a power battery’s total cost. Since power batteries represent ~40% of a new energy vehicle’s cost, solvent choices have measurable downstream economic consequences.

Current electrolyte systems use a “mixed solvent” approach in which ~95% of the solvent volume is carbonate-based. Cyclic carbonates—ethylene carbonate (EC) and propylene carbonate (PC)—provide high dielectric constants for lithium salt dissociation. Chain carbonates—dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC)—contribute low viscosity to improve ionic mobility and electrode wetting. Lithium hexafluorophosphate (LiPF₆) is the dominant lithium salt in commercial cells, but its dissolution substantially increases salt battery viscosity relative to pure solvents, creating a formulation trade-off between ionic conductivity and wettability.

Figure 1. Overview of electrolyte research scope and cost breakdown — solvents represent ~85% of electrolyte mass and ~30% of electrolyte cost in lithium-ion batteries

This study uses the IEST Electrolyte Wetting System (EWS1100) to quantify how different battery solvents and lithium salt concentrations affect capillary wettability at the electrode level, providing a reproducible screening method for electrolyte formulation and cell filling process development.

2. Objective and Test Overview

This work evaluates the effect of salt battery viscosity and battery electrolyte viscosity on electrode wetting using a capillary infiltration method. Four pure solvents (PC, DEC, DMC, EMC) and four electrolyte formulas (L01–L04, each containing 1 M LiPF₆) are compared on identical negative electrode sheets at controlled compaction density. The objective is to provide a reproducible test solution for screening solvent/salt systems and to guide electrolyte formulation and cell filling process optimization.

3. Experimental Equipment and Testing Methods

3.1 Instrument

Testing was performed with the IEST Electrolyte Wetting Testing System (EWS1100). The EWS1100 automates capillary suction, monitors liquid level via machine vision, and records dynamic infiltration volume in real time—eliminating the operator variability inherent in manual contact-angle or weight-gain methods.

Figure 2. IEST EWS1100 Electrolyte Wetting Testing System — automated capillary level monitoring with machine vision for real-time electrolyte wetting quantification

3.2 Sample Preparation

• Solvents tested: PC, DEC, DMC, EMC (physical properties summarized in Table 1).

• Electrolytes: four solvent blends (L01–L04) each containing 1 M LiPF₆ (formulations in Table 3).

• Electrode: same anode electrode sheet, with controlled compaction density.

3.3 Test Procedure (Automated)

1. Sample pretreatment and standardized fixation.

2. Software parameter setup.

3. Automatic capillary suction and capillary pressure test.

4. Real-time visual monitoring of capillary liquid level.

5. Data collection, normalization, and export.

3.4 Test Principle

The EWS1100 can quantitatively evaluate the difference in infiltration of electrolyte between different positive and negative electrode plates and separators, providing an effective method for electrolyte infiltration evaluation. Figure 3 is a schematic diagram of the test principle of the capillary infiltration method. The capillary glass tube is in contact with the surface of the pole piece, and electrolyte is injected into the capillary. As the electrolyte continues to infiltrate the coating, the capillary liquid level continues to decrease. The visual recognition system records the liquid level height of the capillary in real time. The dynamic evolution process of the liquid level is the real-time process of electrolyte infiltration. The height change is the amount of electrolyte infiltration.

Figure 3. Capillary infiltration test principle — the EWS1100 tracks liquid level drop in a 10 µL capillary as electrolyte infiltrates the electrode surface; faster drop indicates lower viscosity and better wettability

4. Results — How Battery Solvent Viscosity Governs Electrode Wetting

Lithium-ion battery solvents generally use organic solvents with high dielectric constant and small viscosity. The higher the dielectric constant, the easier it is for lithium salts to dissolve and dissociate; the lower the viscosity, the faster the ions move. However, solvents with high dielectric constants usually have high viscosity, and solvents with low viscosity have low dielectric constants. Therefore, in practical applications, multiple solvents are usually mixed to obtain the optimal electrolyte system. Compared with the study of mixed systems, the performance evaluation of a single system is the basis for systematic research and development of electrolytes. In this experiment, four solvents with different properties (for example, Table 1 shows the physical property indicators of the four solvents) were first selected to conduct capillary infiltration characterization at the pole piece level to evaluate the infiltration differences of different solvents under this method.

Using the EWS1100 with a 10 µL capillary, normalized infiltration data were recorded for each pure solvent on the same negative electrode sheet. Table 2 and Figure 4 present the results.

Figure 4. Capillary wetting curves for PC, DEC, EMC, and DMC on identical negative electrode sheets — DMC achieves complete infiltration in ~20 s; rank PC < DEC < EMC < DMC mirrors the inverse viscosity relationship

DMC completed full infiltration of the 10 µL capillary in approximately 20 seconds. Both 10 s and 20 s infiltration volumes confirm the rank PC < DEC < EMC < DMC, directly reflecting each solvent’s viscosity. This confirms that, for pure-solvent infiltration, battery solvent viscosity is the dominant factor controlling capillary wettability at the electrode level—outweighing the dielectric constant effect, which governs salt dissociation rather than wetting kinetics.

Practical implication: Lowering electrolyte viscosity—by choosing lower-viscosity battery solvents or by raising injection temperature during cell filling—improves infiltration speed and uniformity, reducing the risk of dry zones and non-uniform wetting that harm cycle life.

5. Results — Effect of Lithium Salt on Electrolyte Viscosity and Wetting

The electrolyte lithium salt is the basis for the conduction of lithium ions. A suitable lithium salt must have good thermal stability and not easy to decompose, high ion conductivity, good chemical and electrochemical stability, and low cost; Although there are many types of lithium salts, those suitable for lithium-ion batteries are very limited. Currently, lithium salts commonly used in laboratories and industrial production generally choose lithium salts with larger anion radius and stable oxidation and reduction, among them, lithium hexafluorophosphate (LiPF₆) is currently the most used lithium salt in lithium-ion batteries. This experiment combines different solvents to carry out electrolyte ratios with a fixed lithium salt concentration, and prepares electrolytes with four ratios as shown in Table 3. The main components are based on PC, EMC, DMC and DEC with the addition of 1M LiPF₆ , combined with these four electrolytes, the wettability test under the capillary principle was performed.

Figure 5. Capillary wetting curves for L01–L04 (1 M LiPF₆) — salt addition raises viscosity; rank: L01 < L04 < L02 < L03

For example, Table 4 shows the comparison results of the infiltration amount data of four electrolyte systems based on different solvent ratios. Figure 5 shows the comparison curves of capillary infiltration of different electrolytes. From the curve slope, there are obvious differences in the infiltration conditions of the four electrolytes; From the infiltration amount table, the infiltration trend L01<L04<L02<L03 can be further clarified. This order mirrors the pure solvent rank but with uniformly slower kinetics after salt addition. Thus, salt battery viscosity (viscosity after lithium salt dissolution) is a key metric when choosing practical electrolyte formulas. Comparing DMC with L03 with 1M lithium salt added to it, DMC completed all the initial liquid infiltration in about 20 seconds, while L03 only completed 34.8% of the initial liquid volume in 50 seconds (infiltration volume in 50 seconds/initial liquid level height) ); this result mainly considers that the addition of lithium salt to the solvent increases the viscosity of the liquid, and the change in viscosity directly leads to the decrease in the wettability of the electrolyte.

6. Discussion — Why Electrolyte Viscosity Matters for Cell Manufacturing

• Filling efficiency and homogeneity: Higher battery electrolyte viscosity slows capillary infiltration during cell filling, increasing the risk of dry spots, gas entrapment, and non-uniform wetting. These defects reduce capacity, increase impedance, and accelerate degradation over the cell’s service life.

• Ionic transport: While low viscosity improves ion mobility and wetting kinetics, the dielectric constant of the solvent mixture must still be sufficient to ensure complete LiPF₆ dissociation. This creates the fundamental compromise that drives mixed-solvent formulation design.

• Temperature dependence: Raising filling temperature reduces electrolyte viscosity and accelerates infiltration—a standard practice in large-scale cell manufacturing. However, the usable temperature window is constrained by solvent volatility, material compatibility, and process safety.

• Formulation trade-offs: Electrolyte additives (e.g., SEI-forming agents, flame retardants) and higher LiPF₆ concentration generally increase viscosity. An optimized balance between electrochemical stability and wettability must be established for each specific cell chemistry and filling process.

7. Practical Recommendations for Electrolyte Development and Cell Filling

• Measure salt battery viscosity directly — determine viscosity of each candidate electrolyte (including all additives and at target filling temperature) rather than estimating from pure-solvent data.
• Prefer lower-viscosity solvent blends compatible with required dielectric properties; chain carbonates (DMC, EMC) combined with cyclic carbonates (EC) provide practical viscosity/dielectric balance.
• Control filling temperature within safe operating limits to reduce viscosity during injection and improve electrolyte wetting uniformity.
• Screen electrolyte formulas early using capillary infiltration testing (e.g., EWS1100) before committing to cell prototyping — capillary rank order correlates directly with filling efficiency.
• Minimize additive loading to avoid excessive viscosity increases; compensate via solvent ratio adjustment if required dielectric properties are maintained.

8. Conclusion

Electrolyte solvent selection and lithium salt concentration jointly control battery electrolyte viscosity, which directly governs capillary wettability at the electrode level. Capillary infiltration tests with the EWS1100 show that (1) lower-viscosity solvents wet electrodes faster (pure DMC completes infiltration in ~20 s vs. much longer for PC), and (2) adding 1 M LiPF₆ substantially increases viscosity and reduces wettability across all four electrolyte formulas tested. For reliable cell manufacturing and peak performance, electrolyte developers should explicitly measure salt battery viscosity, use capillary wetting tests for formulation screening, and optimize solvent/salt ratios with the cell filling temperature and process window in mind.

9. References

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