Every manufacturer in the computer, communication and consumer electronics market segment would like to avoid image problems caused by prematurely failing or even burning batteries.

Also, nobody in the 3C industry wants to have to replace lithium-ion batteries while they are still under warranty. That is why many manufacturers aim for a life expectancy of three to five years for their batteries. To ensure this, no liquid electrolyte solution may escape from a battery cell, nor may humidity penetrate – the electrolyte could react with water to form hydrofluoric acid.

However, checking filled lithium-ion cells for leaks during production is a challenge. Until now, there have hardly been any suitable methods for this because they are indirect. Inficon , a specialist in leak testing with a development and production site in Cologne, has now developed an innovative method that directly detects the smallest leaks in battery cells through escaping electrolyte solvent.

Battery cell failure modes

Lithium-ion cells can be differentiated according to the shape of their housing. On the one hand there are cells with rigid, stable housings. These include round and button cells. The other category is made up of cells with a soft, bag-like exterior: pouch cells. All these cell types have two damage mechanisms in common: If electrolyte escapes from the cell, its capacity is reduced – the battery life is shortened. And when moisture from the air enters the cell, the electrolyte can react with water to form hydrofluoric acid – leading to further leaks in the cell’s case and further reducing its lifespan.

There is another damage mechanism in the soft pouch cells. All cell types are usually filled with electrolyte up to a maximum of atmospheric pressure. In round and button cells with a solid housing, there is a pressure close to atmospheric pressure, and in pouch cells with a soft housing, the aim is even a significant negative pressure. Leaks in pouch cells therefore also cause them to fill with penetrating air – and thus lose mechanical stability and capacity.

Inaccurate pressure drop test

Up to now, leak tests on ready-to-use filled cells have been hampered either by the insensitivity of the method – as in pressure decay testing – or by the unreliability in this application – as in helium bombing. In the supposedly inexpensive pressure drop test, a test chamber is filled with air up to a defined overpressure of a few bars and measured over a defined time interval to see whether there are any pressure changes because air penetrates the cell through a leak. In practice, limit leak rates of up to 10 Determine mbar∙l/s. A major problem with this process, however, is its susceptibility to temperature fluctuations. If the temperature rises by only a fraction of a degree during the test, leaks remain undetected, but if the temperature falls, the pressure drop test detects phantom leaks.

Unreliable helium bombing

Helium bombing is a method that, in principle, brings with it a high sensitivity, but proves to be unreliable in this special application scenario of cell testing. During bombing, the battery cell is placed in a vacuum chamber and exposed to a helium atmosphere with a pressure of around 5 bar. This allows the helium tracer gas to enter the cell through any leaks. The test gas is detected in a subsequent step when the helium that has penetrated escapes again into the vacuum chamber, which has now been evacuated. However, the exact leak location and the position of the battery cell are crucial for the success of the bombing method. The light helium may rise in the liquid electrolyte and is then no longer directly in front of the leak.

The sniffer leak search also fails

If only because the electrolyte solution is never filled into the cells at overpressure, sniffer leak detection also fails in the application scenario of cell production. The principle of sniffer leak detection consists of sucking in gas escaping at a leak point with a sniffer tip so that it can be detected. In addition, in this case – under atmospheric external pressure and at a room temperature of 20 degrees Celsius – the vapor pressure of the electrolyte solvent escaping from a leak in the cell wall is simply too low. For solvents such as ethyl methyl carbonate (EMC) or dimethyl carbonate (DMC), the vapor pressure under the conditions described is only 43 or 53 mbar. With diethyl carbonate (DEC) it is only 13 mbar. A direct detection of leaking electrolyte solvent is therefore not possible with the conventional sniffer leak detection. The situation is different only when the filled cell is tested in a vacuum chamber.

Detect escaping solvent directly

Inficon uses this effect in its new test method for completely filled battery cells. Because if the cells are in a vacuum, in the event of a leak, enough solvent can escape into the vacuum chamber, where it evaporates quickly and is easily detectable. In this way, the new ELT3000 from Inficon directly detects all common electrolyte solvents when they escape from the cell: whether DMC, DEC or EMC – although mixtures of these solvents are very often used for battery cells. The innovative method also detects leaks in lithium-ion cells with rigid housings, i.e. round and button cells, as well as in soft pouch cells.

Leak rate and leak diameter

The new method detects leaks up to a helium equivalent leak rate of 1∙10^-6 mbar∙l/s. In the case of soft pouch cells with, for example, 400 mbar internal pressure and a film thickness of around 150 µm, this results in a minimum detectable leak diameter of 1.9 µm. In stable cells with a wall thickness of 1 mm and an internal pressure of 1000 mbar, for example, the new method identifies leaks down to a diameter of 1.7 µm. As a rule, whole drops of the electrolyte solution cannot escape from leaks of this size – and no humidity can penetrate into the cell either. A leak test against the limit leak rate of 1∙10^-6 mbar∙l/s to ensure that the service life of three to five years that the 3C industry is aiming for for its lithium-ion batteries is actually achieved.

Vacuum testing for all cell types

The new Inficon test system for the direct detection of leaking solvent consists of several components: a gas detection system for electrolyte solvents (the Gas Detection Unit, GDU) and a control unit for the gas flows (the Gas Control Unit, GCU). In addition, there is the vacuum chamber in which the cells are subjected to the testing process. Inficon supplies various test chambers for tests on round and button cells, but also a chamber for tests on the soft, more sensitive pouch cells. If the battery cells are in the respective chamber, the test starts at the push of a button. The control unit then creates a vacuum of 5 mbar absolute in the chamber. The pressure difference to the inside of the cell, which is filled with electrolyte under a pressure of a few hundred mbar, ensures that the electrolyte solution escapes from the cell through any leaks and the solvent content evaporates in the vacuum of the test chamber. The mass spectrometer of the gas detection system then detects this solvent – and thus the leak.

Vacuum testing on soft pouch cells

Until now, vacuum tests on the soft pouch cells have been impossible because the pressure difference between the inside of the cell and the vacuum in the test chamber would cause the cells to expand and be damaged. This problem is solved with the flexible FTC3000 test chamber, which Inficon has patented. A film membrane nestles closely against the cell surface during evacuation, thus stabilizing the sensitive cells. This prevents the pouch cell from bloating or even bursting and enables a quick and reliable vacuum check. The flexible test chamber is also a good tool for development departments, which sometimes have to test cell prototypes of the most varied shapes for leaks. Last but not least, the ELT3000 is designed for use in automated production lines.

Always traceable results

Whether with a rigid or flexible chamber: the new test device from Inficon minimizes human error sources and can be operated intuitively thanks to the simple test procedure and its touch display. It can be reliably calibrated with the help of a special E-Check test leak – for later verification of different solvents. The detection system compares the result of each test with a previously defined rejection limit and indicates leaks immediately. Assigning test results to the specific test item is also extremely easy. To do this, a barcode scanner is connected to the standardized interface of the device, with which each cell can be recorded individually. The system then links the exact test results with the respective part ID and a time stamp.

Batch testing and short test cycles

For the use of the new test method in large series production, it makes sense to design the test chamber individually. The duration of a test cycle ultimately depends on the size of the test chamber and whether a user wants to use various protective mechanisms such as a rinsing phase between two cycles. Typically, the test cycle time for the smaller chambers, as offered by Inficon itself, is in the range of 45 to 60 seconds. 10 to 30 seconds of this is pumping time, and the actual measuring process takes 10 seconds.

For tests in large chambers, it is recommended to use additional external pumps for rough evacuation to reduce cycle times. In industrial cell production in particular, it makes sense to automatically load a larger, individually designed chamber, for example using a robot arm or pneumatic grippers, in order to test several cells in a batch. Experienced system integrators are able to design such vacuum chambers and integrate them into test systems. Inficon also works with system integrators to automate test chambers with flexible film membranes.

Quality-assured cell production

Thanks to its mass spectrometer and vacuum method, the new device from Inficon can detect leaks a thousand times smaller than conventional pressure methods when testing filled lithiumion cells for leaks. At the same time, the new method delivers highly reliable results, which helium bombing cannot do in this application. The direct detection of leaking electrolyte solvent opens up completely new possibilities for quality assurance in cell production – indispensable for the desired battery life, whether in smartphones, wearables or medical devices.

(dr Yessica Brachthäuser, Application Engineer Batteries & E-Mobility, Inficon)