How To Improve Electrolyte Wetting Level

1. Why Electrolyte Wetting Determines Battery Performance and Safety

Electrolyte wetting is one of the most critical factors governing lithium-ion battery kinetics, cycle life, and safety. When the electrolyte fully wets the electrode and separator pores, it forms a uniform solid–liquid interface that supports efficient electrochemical reactions and promotes the formation of a stable SEI (Solid Electrolyte Interface) film — which directly enhances cycling stability and cell safety.^[1–3]

Electrolyte wettability is defined by the contact angle (θ) between the electrolyte droplet and the electrode surface: lower θ indicates better wetting. In practice, θ < 30° is typically required for rapid, complete electrode infiltration during cell manufacturing.

When battery electrolyte wetting is incomplete, the consequences cascade across multiple failure modes:

• Unwetted pores extend the lithium-ion transport path, reducing ionic conductivity and degrading rate capability.
• Incomplete infiltration at the separator–electrode interface promotes non-uniform current distribution, accelerating active material stripping and SEI film inhomogeneity.
• Localized dry zones concentrate current density, creating preferential nucleation sites for lithium dendrite growth — a primary cause of internal short circuits.
• In severe cases, localized current concentration causes overheating that initiates thermal runaway, leading to fire or explosion (Figure 1).

Figure 1. Thermal runaway of a battery cell. Poor electrolyte wetting creates dry zones that concentrate current density, causing localized overheating that can escalate to thermal runaway, fire, or explosion.

2. Five Strategies to Improve Electrolyte Wetting

Electrolyte wetting performance can be improved through formulation, electrode design, compaction, cell assembly, and injection process optimization:

2.1 Electrolyte Formulation Optimization

Electrolyte formulation directly controls electrolyte wettability via three interdependent parameters: contact angle (surface affinity), viscosity (flow resistance), and surface tension (capillary driving force). A lower contact angle, lower viscosity, and lower surface tension all favor faster and more complete electrode infiltration.

• Solvent selection: low-viscosity solvents (e.g., DMC, EMC) improve infiltration speed. Mixed solvent systems balance the high dielectric constant needed for lithium salt dissociation against the low viscosity needed for fast wetting — neither property can be optimized in isolation.
• Wetting agents and co-solvents: small additions of surfactant-type additives or co-solvents that increase electrode surface affinity can measurably reduce contact angle without significantly increasing viscosity.
• Additive trade-offs: many additives that improve SEI stability or electrochemical window also increase electrolyte viscosity. Always evaluate the combined effect on wettability and electrochemical performance rather than optimizing either parameter independently.

Figure 2. Electrolyte contact angle on electrode surface: lower contact angle (left) = better electrolyte wettability and faster pore infiltration; higher contact angle (right) = poor wetting and incomplete electrode penetration.

2.2 Electrode Pre-Process Optimization

Electrode microstructure — particle morphology, conductive carbon distribution, binder network, and pore connectivity — determines how readily the electrolyte penetrates into the coating at the microscale:

• Active material morphology: higher specific surface area and appropriate pore size distribution accelerate infiltration. Avoid over-agglomerated morphologies that create isolated pore regions inaccessible to electrolyte.
• Slurry and coating optimization: optimize slurry rheology (viscosity, solid loading), coating thickness, and drying protocol to maximize through-thickness pore connectivity. Non-uniform drying can create binder concentration gradients that block pore networks near the coating surface.

2.3 Compaction Density and Electrode Porosity

Electrode compaction density and porosity are in direct tension with electrolyte wetting performance: higher compaction increases volumetric energy density but reduces porosity and impedes electrolyte infiltration. This trade-off must be explicitly managed:

• Avoid over-compaction: compaction beyond the porosity threshold required for adequate ionic transport severely reduces electrolyte access to inner electrode regions. The weight infiltration method (Section 3.2) provides a direct, quantitative measure of how compaction density affects infiltration rate.
• Optimize calendering force: tune roll press parameters to a compromise that provides low electronic resistance without creating an impermeable dense layer at the electrode surface that blocks electrolyte entry.

2.4 Cell-Level Design and Assembly

Internal cell geometry — winding tension, separator selection, tab positioning, and hot-press parameters — determines the distribution of internal gaps and the available flow paths for electrolyte during filling:

• Winding/stack tightness: control winding or stacking tension to maintain sufficient inter-layer gaps for electrolyte access without causing core deformation or separator wrinkling that blocks flow paths.
• Separator selection: choose separator thickness, porosity, and surface chemistry that are compatible with the electrolyte — hydrophilic separator surfaces reduce contact angle and improve wetting rate.

Figure 3. Battery cell cross-sectional structure: (a) overall assembly; (b) internal gap distribution before hot pressing; (c) after hot pressing — showing how hot-press parameters modify internal channel geometry and affect electrolyte infiltration pathways.

2.5 Electrolyte Injection Process Optimization

The liquid injection process is one of the most directly controllable variables for improving electrolyte wetting in cell manufacturing:

• Vacuum injection: injecting electrolyte under vacuum removes trapped gas from electrode pores and separator channels before filling — eliminating the pneumatic resistance that slows infiltration under atmospheric conditions. Vacuum filling enables the electrolyte to contact the electrode surface directly without displacing trapped air, reducing total wetting time and improving uniformity across the cell cross-section.
• Injection temperature: higher injection temperature reduces electrolyte viscosity, lowering the capillary resistance to pore penetration and increasing infiltration rate. Standard practice is warm filling followed by controlled high-temperature soaking — elevated temperature during the soak period drives the electrolyte into smaller internal pores and electrode particle surfaces, maximizing the contact area between electrolyte and active material before formation cycling.
• Multi-step injection: dividing the total electrolyte volume into sequential injection steps with intermediate soaking periods allows the electrolyte from the first injection to fully redistribute before the second injection fills remaining unfilled regions.

3. Quantitative Electrolyte Wetting Testing: Three Methods Compared

Quantitative electrolyte wetting testing is essential for objectively ranking electrolyte formulations, electrode designs, and injection processes before committing to cell prototyping. IEST has developed the EWS Electrolyte Wetting System, which implements all three standardized wetting measurement methods in a single platform.

Table 1. Comparison of electrolyte wetting test methods — capillary, weight, and height infiltration

Method	Principle	Primary Output	Best For
Capillary infiltration	Capillary tube contacts electrode surface; liquid level drop rate = infiltration rate	Infiltration volume vs. time; comparison of electrolyte formulations on same electrode	Rapid ranking of electrolyte formulations; detecting coating surface wettability differences
Weight infiltration	Electrode or bare cell suspended under balance; weight increase = cumulative electrolyte uptake	Infiltration mass vs. time; K value (Washburn slope); total uptake capacity	Effect of compaction density; quantifying uptake in bare cells; comparing electrode porosity
Height infiltration	Vertical electrode in contact with electrolyte; optical system tracks rising infiltration front height	Infiltration height vs. time; wicking rate along electrode thickness direction	Comparing cathode/anode electrode materials; detecting anisotropic wetting behavior

3.1 Electrode Electrolyte Capillary Wetting System

Figure 4a shows the schematic principle of the capillary infiltration method. The electrolyte is injected into the capillary tube, and after the capillary glass tube is vertically contacted with the surface of the pole piece, the capillary liquid level decreases as the electrolyte continues to infiltrate the coating. The visual recognition system records the capillary liquid level height in real time, and the dynamic evolution of the liquid level height is the electrolyte infiltration process in real time, and the amount of height change is the amount of electrolyte infiltration. As shown in Fig. 4b, the values of electrolyte liquid level decrease in 50s and 100s for sample 1 are significantly larger than those for sample 2, which indicates that the electrolyte has a better infiltration ability in sample 1.

Figure 4. Capillary infiltration method for electrolyte wetting testing: (a) schematic — capillary tube contacts electrode surface; liquid level drop rate = electrolyte infiltration rate; (b) infiltration curves for two cathode electrodes

3.2 Weight Infiltration Method

The weight infiltration method directly measures electrolyte uptake by mass — ideal for quantifying how compaction density affects battery electrolyte level and infiltration capacity. The electrode or bare cell is suspended beneath a precision balance and partially immersed in electrolyte. As the electrolyte infiltrates the porous structure, the measured weight increases over time. The Washburn infiltration constant K (slope of the weight²-vs.-time curve) characterizes the infiltration kinetics.

Comparison of two electrode samples with different compaction densities confirms a direct relationship: higher compaction density → lower K value → slower and less complete electrolyte infiltration. Sample A (lower compaction density) shows a larger K value than Sample B (higher compaction density), quantitatively demonstrating that over-calendering impairs wettability.

Figure 5. Weight infiltration method for electrolyte wetting testing: (a) schematic — electrode or bare cell suspended under balance; weight increase = cumulative electrolyte uptake; (b) infiltration curves for electrodes with different compaction densities — lower compaction density (Sample A) gives higher K value, confirming that higher compaction density reduces electrolyte wettability.

3.3 Electrolyte height infiltration system

The height infiltration method captures the directional wicking behavior of electrolyte along the electrode thickness direction — particularly useful for detecting differences in electrode material and coating structure that are not visible in capillary or weight measurements. The electrode strip is placed vertically with its bottom edge in contact with the electrolyte. Electrolyte wicks upward through capillary action, and the rising infiltration front height is tracked continuously by a high-precision optical imaging system. Figure 6b shows the measurement results of the height method for different negative electrodes, from which the height method can also distinguish the difference in electrolyte infiltration of different electrodes.

Figure 6. Height infiltration method for electrolyte wetting testing: (a) schematic — vertical electrode wicks electrolyte upward; optical system records rising infiltration front; (b) height curves for different cathode electrodes — height method resolves wettability differences between electrode materials that may be indistinguishable by other methods.

4. Practical Recommendations for R&D and Production

• Screen electrolyte candidates early: use capillary infiltration for rapid formulation ranking before cell assembly; follow with weight infiltration for quantitative uptake data. Identifying the best-wetting electrolyte candidate before prototyping eliminates a major source of cell performance variability.
• Measure both viscosity and contact angle: viscosity typically dominates capillary wetting rate, but contact angle determines whether the electrolyte will spread across the electrode surface at all. Both must be within acceptable limits; optimizing only one can give a misleading picture of expected wetting performance.
• Implement vacuum filling and controlled temperature in production: vacuum electrolyte injection removes trapped gas from electrode pores, and warm filling temperature reduces viscosity during injection — both measurably improve infiltration completeness and reduce the required soaking time before formation cycling.
• Quantify the compaction density–wettability trade-off: use weight infiltration testing at multiple compaction density targets to identify the specific calendering pressure at which porosity becomes limiting for electrolyte infiltration — then set this as the maximum calendering specification.
• Verify wetting by EIS after filling: measure electrochemical impedance spectroscopy (EIS) after the electrolyte soak period to confirm that interfacial resistance is within target range. Elevated charge-transfer or Warburg impedance after filling is a direct indicator of incomplete electrolyte wetting in the finished cell.

5. Summary

Reliable electrolyte wetting is essential for high-performance, durable, and safe lithium-ion cells. Poor electrolyte wettability — manifested as high contact angle, slow infiltration, or incomplete pore penetration — increases internal resistance, promotes lithium dendrite growth, accelerates aging, and in severe cases triggers thermal runaway. IEST’s EWS Electrolyte Wetting System quantifies wetting performance at both electrode and cell level using three complementary methods: capillary infiltration (rapid formulation screening), weight infiltration (quantitative uptake and compaction density effects), and height infiltration (directional wicking characterization). Combined with optimized electrolyte formulation, controlled compaction density, and vacuum injection processes, systematic wetting testing enables R&D and production teams to ensure consistent battery electrolyte level and superior cell performance across all cell formats.

Key technical relationships: contact angle (θ) between electrolyte and electrode should be <30° for rapid complete infiltration; higher compaction density directly reduces infiltration rate (lower Washburn K value) by closing electrode pores; vacuum electrolyte injection reduces wetting time by eliminating gas-phase resistance to pore penetration; and elevated injection temperature reduces electrolyte viscosity, increasing capillary flow rate proportionally to 1/η. The three IEST EWS measurement methods — capillary, weight, and height infiltration — are complementary: each resolves a different aspect of wetting behavior and together provide a complete characterization of electrolyte wettability at the material, electrode, and cell assembly level.

6. References

[1] Zheng Honghe et al. eds. Electrolytes for lithium ion batteries. Beijing: Chemical Industry Press, 2007.01.

[2] Wang B L, Wang J P, Zhang L, et al. Adsorptive Shield Derived Cathode Electrolyte Interphase Formation with Impregnation on LiNi_0.8Mn_0.1Co_0.1O₂ Cathode: A Mechanism-Guiding-Experiment Study[J]. ACS Applied Materials & Interfaces, 2024:16, 38, 50747-50756.

[3] Zhang Shuanghu. Progress in Electrolyte Wetting for Lithium Ion Battery [J]. Chemistry World,2021,62(03):129-136.

[4] Yao N, Yu L, Fu Z H, et al. Probing the origin of viscosity of liquid electrolytes for lithium batteries[J]. Angewandte Chemie International Edition，2023：e202305331.

7. FAQ: Electrolyte Wetting in Lithium-Ion Batteries