Battery Wetting and Electrolyte Spreading

Electrolyte Wetting and Electrolyte Additive Selection for Lithium-Ion Batteries: Optimization of Electrolyte Filling and Wetting on the Separator and Positive Electrode in Battery Production

Electrolyte filling and wetting is one of the slowest steps in battery production, and a cell that is not fully wet before formation can plate lithium. Measure electrolyte surface tension and contact angle on separators and electrodes to rank each electrolyte additive, separator and electrode treatment before you build cells.

Who this is for: Battery R&D teams, electrolyte and separator suppliers, and cell production engineers working on lithium-ion batteries.

Where it fits: Electrolyte design and additive screening, separator and electrode lot release, and filling process development.

What it does not do: It does not measure ionic conductivity, SEI quality or cell performance, and it does not replace cell builds.

Method

Pendant drop surface tension and sessile drop contact angle

Standard

No battery specific wetting standard; general sessile drop method

Setup

About 2 minutes

Book a 15 minute demo Request a quote

Rows of cylindrical lithium-ion battery cells on a production carousel

Written by

Gurdeep Singh Saini

Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.

COO at Droplet Lab

Why electrolyte wetting matters in battery manufacturing

10 to 14 days

for the standard wetting and formation process in lithium-ion cell production, one of the main targets for cutting processing cost.

Wood, Li and Daniel, Journal of Power Sources 275:234 (2015), ORNL

1.5 h vs under 40%

electrolyte reached every surface in 1.5 hours with optimised filling settings; with poor settings under 40% of the area was wet in the same time.

Günter et al., Journal of Power Sources 517:230668 (2022)

46.2° to 15.2°

contact angle of a LiPF6 carbonate electrolyte on an uncoated PE separator versus an Al2O3 coated one, measured by sessile drop.

Hofmann et al., International Journal of Molecular Sciences 16:20258 (2015)

Sources: D. L. Wood III, J. Li, C. Daniel, Prospects for reducing the processing cost of lithium ion batteries, Journal of Power Sources 275:234 to 242, 2015; F. J. Günter et al., Influence of pressure and temperature on the electrolyte filling of lithium-ion cells, Journal of Power Sources 517:230668, 2022; A. Hofmann et al., Interaction of high flash point electrolytes and PE based separators for Li-ion batteries, International Journal of Molecular Sciences 16:20258 to 20276, 2015. Figures come from these sources, not from Droplet Lab measurements.

Quick reference

What this electrolyte wetting check does and what it does not

A quick reference for battery teams checking fit before reading further.

Evidence Box

Problem this solves

Slow or incomplete electrolyte wetting of separators and electrodes, and choosing additives and materials that wet faster.

Standards

There is no battery specific wetting standard we know of. Contact angle follows the general sessile drop method, and surface tension the pendant drop method.

Peer reviewed method

The Dropometer drop shape method was validated against a KRUSS DSA100E in two peer reviewed papers, in Review of Scientific Instruments and Colloids and Surfaces A.

Customer case study

A North American battery materials R&D team uses the Dropometer to see how ionic liquid and aqueous electrolytes wet treated cathode and anode materials.

Published method fit

Hofmann et al. measured electrolyte surface tension by pendant drop and separator wetting by sessile drop, the same two methods the Dropometer runs.

Honest limit

Wetting readings rank materials. Conductivity, SEI and cell performance still need electrochemical tests and cell builds.

Who this is for

What electrolyte wetting question are you trying to answer?

Pick the card closest to your role. Each one points to the part of this page that answers it.

Electrolyte R&D

You are choosing additive quantities and solvent ratios. Surface tension and contact angle on your separator and electrodes show the effect of additives on wetting before you commit to cell builds and formation.

A film-forming additive chosen for SEI may not help wetting; measure both effects.

Separator or electrode supplier

Customers report slow filling with some lots. A contact angle and absorption check on each lot catches finish, coating or calendering changes before material ships.

Release lots against a reference lot measured the same day with the same electrolyte.

Cell production engineer

Wetting time sets how long cells wait before formation. Knowing which materials wet fastest helps you set filling and wetting process windows and avoid lithium plating from incomplete wetting.

Lab drops do not replace in cell wetting checks; use them to choose materials and spot lot changes.

Fit check

Is this the right test for your electrolyte wetting problem?

It helps when the question is how well a liquid electrolyte wets a material. It helps less for in cell distribution and electrochemistry.

Good fit if

You compare electrolyte additives, solvents or salt concentrations by wetting, for lithium-ion or lithium metal batteries.

You qualify separators, coatings or surface treatments.

Electrode calendering or binder changes affect electrolyte infiltration.

You need a release check for separator or electrode lots.

You work with ionic liquid or aqueous electrolytes that attack metal needles.

Less relevant if

You need to see electrolyte distribution in lithium-ion batteries after filling.

You need ionic conductivity, transport or SEI data.

Your electrolyte cannot leave a glovebox and there is no space for an instrument inside.

You work only with a solid or polymer electrolyte with no liquid phase.

Summary

How to optimise electrolyte wetting and additive selection

The answer in under a minute.

Measure the battery electrolyte's surface tension by pendant drop, then its contact angle and absorption time on your separator and electrodes. Lower surface tension and lower contact angle usually mean faster wetting. Use the readings to choose additive quantities and materials, then confirm wetting time in cells before formation.

The Dropometer contact angle and surface tension kit runs both checks with disposable plastic needles; our contact angle measurement guide and surface tension measurement guide explain the methods. A North American battery materials R&D team uses it on ionic liquid and aqueous electrolytes. Our wettability glossary entry and transportation guide cover the wider context.

Expert Quote

Expert perspective on additive screening

“

Surface tension versus concentration is one of the few measurements that lets you watch a surfactant reorganise an interface in real time; read the curve, not just the endpoint.

The problem

Why filling liquid electrolytes in lithium-ion batteries is slow

In battery manufacturing, the liquid electrolyte must fill every pore of the separator and the positive and negative electrodes before formation, especially in lithium-ion batteries using a carbonate-based electrolyte. The wetting process is slow: Wood and colleagues at ORNL describe a standard 10 to 14 day wetting and formation process. How fast it goes depends on the electrolyte composition, electrolyte viscosity and surface tension, and on how the electrode material and separator surfaces wet. Process settings matter too: in one neutron imaging study, a visualization of the electrolyte filling, optimised pressure and temperature wet a large cell in 1.5 hours while a poor process wet under 40% of the area in the same time. Incomplete wetting has a cost. Hagemeister and colleagues found strong lithium plating in pouch cells charged before they were fully wet, and concluded that about 98% wetting is needed before formation. Each electrolyte additive, separator coating or calendering change shifts this behavior. Contact angle and surface tension measurements show the shift before you build cells.

Long wetting times before formation can start.

Lithium plating or uneven SEI after formation.

Filling time changes between separator or electrode lots.

A new additive improves cycle life but slows wetting.

Batteries at low temperatures fill more slowly.

Troubleshooting

Why electrolyte wetting is slow and what to check

Why:

Polyolefin separators have low surface energy, so carbonate electrolytes sit at a high contact angle and enter the pores slowly.

How to detect:

High contact angle and long absorption time on the separator against a coated or treated reference.

Corrective action:

Compare ceramic coated or surface treated separators, or a wetting additive, to improve the wetting. Our guide to reproducible contact angle measurement covers fair comparisons.

Why:

Higher lithium salt concentration and some electrolyte solvent choices raise viscosity and slow wetting; the stability of the electrolyte limits how far you can change it.

How to detect:

Surface tension changes with composition, and absorption is slower even when contact angle is similar.

Corrective action:

Screen solvent ratios and salt levels on the same samples, and record viscosity alongside surface tension.

Why:

Film-forming additives such as VC are chosen for the solid electrolyte interphase, not for wetting; some barely change surface tension.

How to detect:

Surface tension and contact angle stay close to the base electrolyte at the planned concentration.

Corrective action:

Measure the effect of additives at several concentrations, and pair a film-forming additive with a wetting aid if needed.

Why:

Heavy calendering, binder at the surface or a dense coating slow electrolyte infiltration into the battery electrode. Graphite anodes for lithium-ion batteries usually wet much faster than NMC cathodes (Davoodabadi et al., 2019), so a slow graphite electrode is a warning sign.

How to detect:

Long absorption time on the electrode surface after a calendering change. Our contact angle reference values give a sense check for common materials.

Corrective action:

Compare calendering settings and check porosity; track absorption time by electrode batch.

Why:

Reactions between the electrolyte and water turn LiPF6 into HF and change the electrolyte components, and residues on separators change wetting.

How to detect:

Readings drift through the day or differ between storage conditions.

Corrective action:

Measure in a controlled atmosphere, keep vials sealed, and use a fresh needle for each liquid.

Why:

Viscosity rises at low temperature, so wetting slows.

How to detect:

Absorption time rises when samples and electrolyte are measured cooler.

Corrective action:

Set filling temperature from your own wetting data; published work shows wetting is fastest at elevated temperature.

Not sure what is slowing your wetting?

Send a separator or electrode lot that wets well and one that does not, and we will compare them on a call.

Book a 15 minute demo

Quality records

What an electrolyte wetting record contains

Each test produces a record you can file with the electrolyte, separator or electrode lot. These records can feed into your existing quality records under IATF 16949, ISO 9001 or your battery technology development process.

Materials

Electrolyte and electrode materials: electrolyte, additive and concentration, separator type and lot, electrode type and batch.

Conditions

Atmosphere and moisture level, temperature, and sample storage before the test.

Method settings

Needle type, drop age, drop volume, contact angle read time and spots per sample.

Readings

Surface tension, contact angle at the read time, and absorption time.

Decision

Release, shortlist, retest or hold, with the operator and date.

Drop images

Automatic edge and baseline detection gives the same reading from the same image, so a reviewer can recheck any result.

Measurements

What to measure for electrolyte wetting

Liquid

Electrolyte surface tension by pendant drop

Why it matters: Lower surface tension helps the electrolyte spread and enter small pores.

How to interpret: Compare additives and solvent ratios with the base electrolyte. Hofmann et al. measured 29.4 mN/m for LiPF6 in EC/DMC.

When it is not enough: It says nothing about the solid; measure contact angle too.

Separator

Contact angle at a fixed read time

Why it matters: It shows the wetting behavior of the electrolyte on the separator before it soaks in.

How to interpret: Read at the same time after the drop lands for every sample. Set gates from your own reference lot.

When it is not enough: On very porous samples the drop soaks in too fast for a stable angle.

Rate

Absorption time into separator or electrode

Why it matters: Porous electrodes absorb drops, so the wetting rate is a better measure than angle.

How to interpret: Shorter is faster wetting; compare at the same drop volume and use one criterion for the end point.

When it is not enough: It is a surface test, not the full electrode and electrolyte system in a cell.

Additive

Surface tension against additive concentration

Why it matters: It shows where more additive stops helping.

How to interpret: Measure several concentrations of the additives and use the curve for the selection of electrolyte additive quantities, not a single point.

When it is not enough: It does not show the additive's effect on SEI or gas.

Validation

Validated measurement approach for electrolyte wetting

How the Dropometer itself has been validated, and where customers have used it.

Peer reviewed method

The Dropometer drop shape method is published in Review of Scientific Instruments and Colloids and Surfaces A. Contact angle was validated against a KRUSS DSA100E, with a published accuracy of 0.35°.

See the validation papers

Customer evidence in battery R&D

A North American battery materials R&D team needed to see how ionic liquid and aqueous electrolytes wet cathode and anode materials after surface treatment. Some of its electrolytes attack metal, so Droplet Lab set up an automatic dropper with disposable plastic needles for contact angle, surface tension and surface energy.

Read the case study

QC Protocol

How to screen electrolyte additives and separators by wetting

Six steps for one electrolyte system and cell design. Repeat when a material or additive changes.

1

Plan safe handling

Decide where the electrolyte can be handled: a glovebox, a dry room, or a fume hood for less sensitive electrolytes. Keep vials sealed.

2

Lock the method

Fix needle type, drop age, drop volume, contact angle read time and spots per sample, with a fresh needle for each liquid.

3

Baseline the base electrolyte

Measure its surface tension, and its contact angle and absorption time on reference separator, positive electrode and negative electrode samples.

4

Screen additives and materials

Measure various electrolyte formulations and each additive at several concentrations, and each separator or electrode option, on the same day.

5

Confirm in cells

Turn the lab ranking into a recommendation, build test cells with the best options and compare wetting time and formation results with the lab readings.

6

Release lots and trend

Set gates from that comparison, release separator and electrode lots against them, and trend by supplier.

We completed our gage R&R study on the unit and it performed very well.

Brandon Barbee

Corporate Quality Engineer, Zeus Industries, Polymer Manufacturing

Download the electrolyte wetting and additive screening SOP

A two page SOP you can adapt for your electrolyte, separators and electrodes, with the method lock, gate setting, safety notes, a troubleshooting table and a record sheet.

Contact angle measurement with the Dropometer

Example output

Example electrolyte additive and separator screening log

Example data only. Your values and gates will differ and must come from your own references.

How to read this log

Example gates: contact angle at 10 s at or below 35° and absorption at or below 60 s on the separator. Additive A lowered surface tension and wetting time, so it is shortlisted. Additive B made no difference. A new PE lot wet worse than the reference, so it is held.

Sessile drop contact angle measurement: DI Water on Teflon surface, left contact angle 44.9°, right 45.7°

Example electrolyte additive and separator screening log

Electrolyte	Separator	Surface tension (mN/m)	Contact angle at 10 s (°)	Absorption (s)	Result
Base electrolyte	PE, reference lot	29.5	46	120	Reference
Base + additive A 0.5 wt%	PE, reference lot	26.8	31	45	Shortlist
Base + additive B 1 wt%	PE, reference lot	29.2	45	115	No gain
Base electrolyte	Al2O3 coated PE	29.5	16	20	Shortlist
Base electrolyte	PE, new lot	29.5	58	210	Hold

Example data, not customer data. LiPF6 carbonate base electrolyte, measured in a dry atmosphere at 25 °C; ranges are in line with Hofmann et al. (2015).

Signal order

The order to check signals when wetting slows

Check these in order. Each one rules out a cause before you look at the next.

1

Electrolyte surface tension

Shows whether the liquid itself changed.

2

Separator contact angle

Shows whether the separator lot or coating changed.

3

Electrode absorption time

Shows whether calendering or binder changed the electrode surface.

4

Atmosphere and storage

Moisture and storage time for liquids and samples.

5

Filling process settings

Dosing pressure, wetting pressure, temperature and rest time in the cell.

FAQ

Common questions about electrolyte wetting and additives

Questions battery R&D and production teams ask before adding contact angle and surface tension.

Why does electrolyte wetting take so long?

The electrolyte has to fill fine pores in the separator and both electrodes, and carbonate electrolytes wet polyolefin separators poorly. Wetting speed depends on surface tension, viscosity, pore structure and process settings. Studies on the wetting of cells show it is slow: Wood and colleagues describe a standard 10 to 14 day wetting and formation process.

How do electrolyte additives for lithium-ion batteries affect wetting?

The use of electrolyte additives for wetting is simple in principle: wetting additives and surfactants lower surface tension and contact angle, a wetting enhancement that speeds infiltration without hurting the performance of the electrolyte if chosen well. Adding VC to the electrolyte, a film-forming additive, is done for the SEI and may change wetting little. Measure surface tension and contact angle at several concentrations to see the effect of additives before cell builds.

What happens if cells are not fully wet before formation?

Dry areas are not used, and current concentrates where the electrode is wet. Hagemeister and colleagues found strong lithium plating in pouch cells charged before complete wetting, and set about 98% wetting as the threshold to begin formation.

Can contact angle be measured on porous electrodes?

Only briefly. Porous electrodes in lithium-ion batteries soak up the drop, so a static angle is not meaningful for long. Read the angle at a fixed short time and record absorption time; the rate is the more useful number for electrodes.

Do we need a glovebox?

LiPF6 carbonate electrolytes react with moisture and form HF, so most teams working on lithium batteries handle them in a glovebox or dry room. Plan where the instrument will sit before you buy. Ionic liquid and aqueous electrolytes, as in our case study, are easier to handle.

Which standards apply?

We know of no battery specific wetting standard. Contact angle follows the general sessile drop method and surface tension the pendant drop method. Our guide to the best contact angle goniometer for industrial QA compares instruments for lot release.

Business impact

What changes when you screen electrolyte wetting before cell builds

Typical changes in how teams work. Benchmarks are given only where a published source exists.

Before and with wetting screening

Metric	Before Dropometer	With Dropometer	Indicative Benchmark
Additive screening	Cell builds for each candidate	Surface tension and contact angle first	No published benchmark; track on your own candidates
Wetting and formation time	Fixed long rest	Materials chosen for faster wetting	10 to 14 day wetting and formation baseline (Wood et al., 2015)
Process window	Trial and error	Lab wetting data plus cell checks	1.5 h full wetting vs under 40% with poor settings (Günter et al., 2022)
Separator lot release	Supplier certificate only	Contact angle and absorption gate	No published benchmark; track on your own lots

See the check on your own materials

Send a separator or electrode that wets well and one that does not. We will show the difference on a call.

Book a 15 minute demo

Estimate saved development trials

Enter your own numbers. The result is an estimate, not a Droplet Lab claim.

Instrument cost ($) Each Dropometer unit is $5,000; the default models 1 unit.

Iterations / batches / trials per month

Material cost per iteration ($) Your own electrolyte, electrode and separator cost per trial.

Technician hours per iteration Your own hours to prepare, measure and analyse one trial.

Loaded lab rate ($/hr)

Trials you expect to save with wetting data (%) No published benchmark. Use your own estimate and start low if unsure.

Result

~0

Iterations saved / month

~0

Monthly savings

~0

Payback period

~0

First year net benefit

Monthly savings = materials saved + technician time saved from reduced iterations.

Limits

What wetting readings cannot tell you about a battery

Honest limits, so you know when to reach for another test.

Moisture changes LiPF6 electrolytes; uncontrolled air spoils readings.

Porous samples absorb drops; read at a fixed time and record absorption.

A drop on a sample is not the same as wetting inside a wound or stacked cell, or the full battery system.

Metal needles can react with some electrolytes; use compatible needles.

It does not measure conductivity, ion transport in the electrolyte, the viscosity of the electrolyte or SEI quality.

Gates do not transfer between electrolytes, separators or electrode designs.

Surface tension and contact angle rank electrolytes and materials by wetting. Ion transport, SEI, gas, electrolyte distribution in the cell, the performance of lithium-ion batteries and overall battery performance still need electrochemical tests and cell builds.

Book a 15 minute demo Request a quote

How this page was created

Editorial and technical transparency notes for this page.

Transparency Details 4 checklist items

01

Drafting assistance

Drafted with Claude Opus 5.5 (Anthropic) using web search for sources, then edited by the Droplet Lab team.

02

Technical review

Reviewed and edited for technical accuracy by the Droplet Lab Team.

03

Verification steps

Standard identifiers, units, thresholds and key procedural claims are checked against cited sources before publication.

04

Updates

Reviewed every 12 months or when the underlying method changes.

Report a correction

Spotted an issue in this summary? Send a correction request and our team will review it.

Correction Request

We work hard to keep this page accurate and up to date. If you spot an error (wrong revision/year, missing requirement, incorrect interpretation, or broken link), tell us and we'll review it.

Contact us to report a correction

Sources

References

1.

Wood, D. L. III, Li, J., Daniel, C. Prospects for reducing the processing cost of lithium ion batteries. Journal of Power Sources 275:234 to 242, 2015. https://doi.org/10.1016/j.jpowsour.2014.11.019

2.

Günter, F. J. et al. Influence of pressure and temperature on the electrolyte filling of lithium-ion cells: experiment, model and method. Journal of Power Sources 517:230668, 2022. https://doi.org/10.1016/j.jpowsour.2021.230668

3.

Hofmann, A., Kaufmann, C., Müller, M., Hanemann, T. Interaction of high flash point electrolytes and PE based separators for Li-ion batteries. International Journal of Molecular Sciences 16:20258 to 20276, 2015. https://doi.org/10.3390/ijms160920258

4.

Hagemeister, J. et al. Lean cell finalization in lithium-ion battery production: determining the required electrolyte wetting degree to begin the formation. Energy Technology 11(5):2200686, 2023. https://doi.org/10.1002/ente.202200686

5.

Sheng, Y. et al. Effect of calendering on electrode wettability in lithium-ion batteries. Frontiers in Energy Research 2:56, 2014. https://doi.org/10.3389/fenrg.2014.00056

6.

Davoodabadi, A. et al. Journal of Power Sources 424:193 to 203, 2019 (electrolyte wetting of graphite and NMC532 electrodes). https://doi.org/10.1016/j.jpowsour.2019.03.115

7.

Lux, S. F. et al. The mechanism of HF formation in LiPF6 based organic carbonate electrolytes. Electrochemistry Communications 14:47 to 50, 2012.

https://doi.org/10.1016/j.elecom.2011.10.026

8.

Chen, X. et al. Contact angle measurement with a smartphone. Review of Scientific Instruments 89(3):035117, 2018.

https://pubs.aip.org/aip/rsi/article-abstract/89/3/035117/368179/Contact-angle-measurement-with-a-smartphone

9.

Surface tension measurement with a smartphone using a pendant drop. Colloids and Surfaces A: Physicochemical and Engineering Aspects.

https://www.sciencedirect.com/science/article/abs/pii/S0927775717307744

10.

Droplet Lab validation papers

11.

Customer publications that cite the Dropometer

12.

Battery materials case study

13.

Transportation guide

Electrolyte Wetting and Electrolyte Additive Selection for Lithium-Ion Batteries: Optimization of Electrolyte Filling and Wetting on the Separator and Positive Electrode in Battery Production

Featured Studies & Publications

Why electrolyte wetting matters in battery manufacturing

10 to 14 days

1.5 h vs under 40%

46.2° to 15.2°

What this electrolyte wetting check does and what it does not

Evidence Box

What electrolyte wetting question are you trying to answer?

Electrolyte R&D

Separator or electrode supplier

Cell production engineer

Is this the right test for your electrolyte wetting problem?

Good fit if

Less relevant if

How to optimise electrolyte wetting and additive selection

Expert perspective on additive screening

Why filling liquid electrolytes in lithium-ion batteries is slow

Why electrolyte wetting is slow and what to check

Not sure what is slowing your wetting?

What an electrolyte wetting record contains

Materials

Conditions

Method settings

Readings

Decision

Drop images

What to measure for electrolyte wetting

Electrolyte surface tension by pendant drop

Contact angle at a fixed read time

Absorption time into separator or electrode

Surface tension against additive concentration

Validated measurement approach for electrolyte wetting

How to screen electrolyte additives and separators by wetting

Plan safe handling

Lock the method

Baseline the base electrolyte

Screen additives and materials

Confirm in cells

Release lots and trend

Download the electrolyte wetting and additive screening SOP

Example electrolyte additive and separator screening log

Example electrolyte additive and separator screening log

The order to check signals when wetting slows

Electrolyte surface tension

Separator contact angle

Electrode absorption time

Atmosphere and storage

Filling process settings

Common questions about electrolyte wetting and additives

What changes when you screen electrolyte wetting before cell builds

Before and with wetting screening

See the check on your own materials

Estimate saved development trials

Estimate saved development trials and lab cost.

What wetting readings cannot tell you about a battery

Evidence and related battery wetting workflows

Battery materials R&D team

Dropometer

Contact angle measurement

Surface tension measurement

Battery wetting and electrolyte spreading

Corrosive liquids

LIG electrode QC

How this page was created

Drafting assistance

Technical review

Verification steps

Updates

Report a correction

References