X-Ray Inspection in Lithium-Ion Pouch Cell Manufacturing

X-ray inspection lets a quality engineer look inside a sealed pouch cell without cutting it open. Dense materials such as metal foils, tabs and metallic particles absorb more X-rays than the surrounding polymer and electrolyte, so the cell internals appear as a grey-scale image that can be measured and judged.

Original EngineerX process map: the 11 main steps of pouch cell manufacturing in the order used on this site; the highlighted step is the subject of this article.

In a typical line this check comes after the OCV test and before the final quality inspection (FQI). By this point the cell is sealed, filled, formed and degassed, so any hidden structural fault that was built in during winding, stacking or packing is still present and can still be caught before the cell leaves the plant.

Why this stage matters

Many serious faults cannot be seen from outside. A cell can look perfect and still carry a shifted electrode, a bent tab or a metal particle between the layers. An OCV reading may even look normal because an early micro-short does not always pull the voltage down immediately.

X-ray inspection gives direct evidence of the internal build. It supports three goals:

Safety: finding conditions that could lead to an internal short circuit or local heating in the field.
Performance: confirming that the electrode overlap is correct, because poor overlap wastes capacity and spreads current unevenly.
Process feedback: a repeated pattern, such as every cell shifted the same way, points back to a machine setting upstream.

How the process works

The operator or loading robot places the cell on a carrier so that the cell body sits in the beam path with a fixed orientation.
The carrier moves into a shielded cabinet and the door interlock closes. The X-ray tube cannot switch on while the door is open.
The tube and detector are set to the recipe for that cell model, including tube voltage, current and exposure time.
The system captures one or more images, often a top view and sometimes an angled view, so that layer ends and tab roots are visible.
Software measures the required features, for example the offset between anode and cathode edges at each corner, and compares them with the limits in the recipe.
Cells that pass move on towards FQI. Cells that fail are marked electronically, diverted to a hold area and reviewed by a trained person.
The image is stored against the cell barcode so that it can be retrieved later during a customer complaint or a failure investigation.

Key process parameters

Parameter	What it controls	How it is checked
Tube voltage and current	Penetration and contrast of the image	Recipe lock and a periodic test piece with known features
Exposure time	Image brightness and noise	Image quality indicator or reference sample at shift start
Cell position and orientation	Repeatability of measurements	Fixture check and software alignment marks
Image calibration (pixel size)	Accuracy of measured distances	Calibration gauge or master cell, as per supplier procedure
Acceptance limits	Pass or fail decision	Taken from the cell maker's specification and loaded in the recipe
Inspection speed (cycle time)	Throughput versus image quality	Trial runs and review of false-call rate

Typical settings depend on cell design, materials and equipment supplier, so the actual values must come from the machine recipe and the cell specification.

Typical defects, causes and corrective actions

Electrode misalignment or offset

The image shows an anode edge that does not extend beyond the cathode edge by the designed margin. Causes include electrode strip wander during stacking, worn grippers, or movement of the jelly roll during insertion. Corrective action: check and re-teach the alignment camera, replace worn pick-up parts, and review the handling steps after stacking.

Tab position or bending

The tab root may sit away from its nominal position or be folded. Causes are tab welding offset, rough handling, or contact with fixtures. Corrective action: verify the welder position, adjust transfer fixtures and add guards where the tab touches the tooling.

Layer deformation or wrinkles

Waves in the electrode stack or crushed corners can show as irregular lines. Causes are excessive pressing force, foreign matter under the press plate, or tension problems during winding. Corrective action: inspect the press surfaces and reset the force.

Foreign dense particles

A bright spot that is not part of the design may be a metal fragment. Causes include burrs from slitting or tab cutting, or dust from tooling. Corrective action: isolate the cell, trace the lot, and improve cleaning and burr control at the source.

Missing or double layers

Layer counting on the edge can reveal a missing or extra electrode. The cause is a feeding or pick-up error in the stacker. Corrective action: stop the line, check feeders and sensors, and sort the affected batch.

Quality checks and measurements

Daily check of the system with a reference cell or test piece that has known features.
Verification that the pass and fail decision works, by running a deliberately marked reject sample.
Measurement of electrode overlap at defined points, normally on all corners and along the tab side.
Review of false calls: if too many good cells fail, the limits or image settings need adjustment; if bad cells escape, limits are too loose.
Traceability: each image is linked to the cell identity and saved for the period set by the plant quality procedure.

Safety and handling notes

X-ray equipment must be operated only inside its shielded cabinet with working door interlocks. Never bypass an interlock.
Radiation safety rules from the national regulator apply, for example the atomic energy regulator in India, including trained operators and periodic survey of the cabinet.
Cells at this stage are charged. Use insulated handling, avoid dropping or pressing the cell and never lay a cell on metal trays that can short the tabs.
Any cell showing swelling, leakage or heat must be kept in a designated fire-safe container away from other cells.
Electrolyte is flammable and irritating. If a pouch leaks, follow the plant spill procedure and wear the specified gloves and eye protection.

Interview questions

Q1. Why is X-ray used when OCV is already measured?
OCV shows only voltage. It cannot show electrode offset, tab position or particles, so X-ray adds structural information.

Q2. What makes a metal particle visible in an image?
Metals absorb X-rays more strongly than polymers, carbon or electrolyte, so they appear brighter or darker depending on the display convention.

Q3. What is anode overhang and why is it checked?
It is the margin by which the anode extends beyond the cathode. If it is too small, lithium can deposit at the edge during charging, which is a safety and life risk.

Q4. Where does X-ray sit in the process flow?
After OCV testing and before FQI, when the cell is already sealed and formed.

Q5. What do you do if a cell fails X-ray?
Mark and hold it, review the image, record the defect type, and investigate the upstream station if a pattern appears.

Q6. Is X-ray inspection destructive?
No. It is a non-destructive test, and cells continue to the next stage if they pass.

FAQs

Does X-ray damage the cell?

Inspection doses used in production machines are controlled by the equipment design and the cell maker's approved procedure. Normal use is non-destructive, and cells are released after passing.

Can X-ray find every defect?

No. It is good at geometry and dense foreign matter. It cannot reliably show electrolyte wetting quality, small chemical impurities or all micro-shorts, which is why it is combined with OCV, leakage and other tests.

Is every cell scanned?

This depends on the customer requirement and plant plan. Some lines inspect every cell, others inspect a defined sample, as decided in the control plan.