TECHNICAL FIELD
The invention relates to a method and a system for leak testing one or more finished
5 battery cells. This method enables to check the leak tightness of the entire cell, or even
only a portion thereof, when parts (such as anode, cathode and separator) and chemical
substances (such as electrolytes) necessary to its operation have already been inserted in
the cell and the cell has been sealed.
The leak testing is generally important for all the different types of batteries in order
10 to guarantee the absence of leakages of electrolytes contained in the battery that may be
corrosive and whose leakage may in any case affect overtime the correct functionalities of
the battery.
This type of testing is even more fundamental to batteries, such as lithium-ion
batteries, for which it is absolutely necessary to ensure that the environmental humidity or
15 other chemical compounds coming from the outside cannot enter the battery, more
particularly one or more cells forming the battery. This is to prevent the chemical
substances, which are inside the battery or may form during its use, from coming into
contact with water or other chemical compounds and reacting violently, or in any case
affecting the functionalities of the battery.
20
BACKGROUND ART
Different types of techniques are currently used for leak testing batteries. Some of
them require that each cell inside the battery has an inlet hole, that is an opening in the
cell casing which makes the interior of the cell accessible and places it in communication
25 with the external environment.
The leak test is thus carried out on an unfinished cells, i.e. a cell that has not been
sealed yet.
Other techniques enable to carry out the leak test on one or more finished battery
cells, i.e. cells in which the electrolytes have already been inserted and which have
30 already been sealed.
Many of these techniques require that an additional gas, for example helium, is
inserted in the cell together with the electrolytes for the sole purpose of leak testing. Such
additional gas serves as tracer gas or indicator gas, that is it is the gas that will be
detected by the mass spectrometer in the event of a leak.
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After the cell has been placed inside the vacuum chamber, it is possible to detect by
means of a mass spectrometer the tracer gas that possibly leaks from the cell into the
vacuum chamber due to a leak in the cell and thus detect the presence of leaks in the cell.
Some examples of prior art are described hereinafter.
a. Testing the battery cell using air
It is possible to test only the casing of the cell by supplying compressed air inside
the cell, sealing the cell inlet hole and measuring the pressure drop or the flow rate
possibly caused by a leak in the casing.
b. Testing of the battery cell before the electrolyte is inserted into the cell by using
10 helium as tracer gas
It is possible to test the empty cell casing or the casing that already contains anodes,
cathodes and separators at a stage in which the electrolyte has not yet been inserted and
the inlet hole through which the electrolyte is inserted in the cell is thus still open.
The cell is inserted in a vacuum chamber or in a storage chamber (depending on the
15 leakage level that must be detected). Helium under pressure is then inserted inside the
cell and the quantity of helium flowing from the inside of the cell to the chamber through
the possible leak is measured by means of a mass spectrometer.
The main drawback of the two above-mentioned techniques is that the leak test is
carried out on an unfinished cell, that is a cell which has not yet been sealed. It is
20 therefore not possible to make a test on a cell in which the electrolyte has already been
inserted and whose inlet hole has been sealed permanently.
c. Testing the finished cell, after the electrolyte has been inserted and the cell has
been sealed, by inserting helium as tracer gas in the cell during the step of the electrolyte
insertion
25 As helium is an inert gas, it is theoretically possible - for the purposes of the leak test
that will be carried out subsequently - adding a certain quantity of helium inside the cell
while the electrolyte is inserted and before the inlet hole of the cell is sealed. After the cell
has been positioned inside a vacuum chamber, it is possible to detect by means of a
mass spectrometer the helium that possibly flows from the cell to the vacuum chamber
30 through a possible leak.
The main drawback of this technique is that it is necessary to insert - for the sole
purpose of leak testing - a further gas, that is the tracer gas, into the cell during a different
stage of the battery production that occurs before the leak test stage and is typically
performed by a third party. Moreover, the described technique might be applied to certain
35 types of batteries having a rigid casing with an inner free space which can contain the
tracer gas, but it might not be applied to pouch cells.
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d. Testing the finished cell after the electrolyte has been inserted and the inlet hole
of the cell has been sealed by using a technique called "bombing"
When it is not possible to add helium or another tracer gas inside the cell during the
electrolyte insertion step, it is possible to use the technique called "bombing" to carry out
5 the leak test employing helium as tracer gas.
The battery cell is first inserted into a bombing chamber in which helium under
pressure is injected. If a leak is present in the cell, a part of the helium flows from the
chamber inside the cell. The quantity of helium entering the cell depends not only from the
free space inside the cell but also by the pressure level inside the chamber and the time of
10 bombing.
15
After the bombing step, the battery cell is inserted into a vacuum chamber where the
leakage level is measured by tracing by means of a mass spectrometer the quantity of
helium flowing from the inside of the cell to the vacuum chamber through the leak present
in the cell.
The main drawbacks of this technique are the overall cycle time, which could be very
long and not compatible with industrial processes, and the acceptable overpressure level
in the bombing chamber which must not exceed levels which could permanently
deforming the casing of the cell.
Other techniques enable to identify possible leaks in one or more cells by detecting
20 substances that are already present inside the cell, without the need to insert additional
gas in the cell for the sole purpose of leak testing.
Such techniques share the need to prevent residues of gases and/vapors from
forming or remaining inside the leak testing system. Such residues could affect the leak
detection performed by the detecting and measuring instrument and pollute the detecting
25 and measuring instrument and even the entire leak testing system over time.
30
Furthermore, such techniques share the need to ensure a reliable and repeatable
measurement of the leak rate of a cell in the shortest time possible even in the case of
very small leak.

CLAIMS
PCT/EP2021/069442
1. A method for leak testing a battery cell, said cell being sealed and comprising
parts and substances which are necessary to the operation of the battery cell, by
5 means of a leak test system (1 00) comprising a vacuum chamber (4) and a
detecting and measuring instrument (1 ), the method including the following steps,
not necessarily in that order:
- positioning said cell in such a way that at least one portion of the cell (2) is in the
vacuum chamber (4);
10 -sealing said vacuum chamber (4);
- starting a vacuum creation phase in which the pressure inside the vacuum
chamber (4) is decreased to a lower level than the level of pressure inside the cell
to be checked, said lower level being such that the gases and/or vapors deriving
from the parts and/or the substances inside the cell leak from said cell in cases of
15 leak;
- starting a stabilization phase in which said gases and/or vapors leak from the cell
in cases of leak;
- starting an acceleration phase in which an auxiliary gas is fed into the vacuum
chamber (4) pushing the gases and/or vapors leaking from the cell towards the
20 detecting and measuring instrument (1 ); and
25
-detecting the leak rate by means of the detecting and measuring instrument (1 );
characterized in that the acceleration phase comprises raising the pressure inside
the leak test system (1 00), thereby increasing the flow of the gases and/or vapors
leaking from the cell towards the detecting and measuring instrument (1 ).
2. The method according to claim 1, wherein the pressure inside the leak test
system (1 00) is raised by feeding the auxiliary gas into the vacuum chamber (4)
impulsively and in a predetermined quantity.
30 3. The method according to claim 2, wherein the fed quantity of auxiliary gas is
proportional to the volume of the vacuum chamber (4).
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4. The method according to any one of the preceding claims, wherein the pressure
inside the leak test system (1 00) is raised by actuating mechanical means which
reduce the internal volume of the leak test system (1 00) causing a compression of
said gases and/or vapors leaking from the cell.
5. The method according to claim 4, wherein the mechanical means comprise a
piston (11) placed inside the vacuum chamber (4).
6. The method according to claim 5, wherein the mechanical pieces comprise a
10 piston (13) placed between the vacuum chamber (4) and the detecting and
measuring instrument (1 ).
7. The method according to any one of the preceding claims, wherein the gases
and/or vapors detected by the detecting and measuring instrument (1) consist of
15 dimethyl carbonate.
8. The method according to any one of the preceding claims, wherein the auxiliary
gas consist of argon.