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.
5
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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 /EP2021/069441
1. A leak test system (1 00) for leak testing at least one battery cell, said cell being
sealed and comprising parts and substances which are necessary to the operation
5 of the battery cell, the system comprising the following components:
-a vacuum chamber (4),
-a vacuum pump (6) connected to said vacuum chamber (4),
-a detecting and measuring system (1) connected to said vacuum chamber (4) to
detect gases and/or vapors which can leak from said cell, said gases and/or
10 vapors deriving from the parts and/or the substances inside in the cell, and
-a capillary element (2) which connects the detecting and measuring system (1) to
the vacuum chamber (4) and through which the gases and/or vapors leaking from
said cell flow,
characterized in that said leak test system ( 1 00) further comprises heating devices
15 adapted to heat at least one of said components of the leak test system (1 00) to
prevent residues of the gases and/or vapors leaking from the cell from being
deposited on the internal walls of said components, thereby making the leak test
system (1 00) substantially memory-free.
20 2. The leak test system (1 00) according to claim 1, wherein said capillary element
(2) is heated by said heating devices.
3. The leak test system (1 00) according to claim 2, further comprising a connection
valve (3) at the entrance of the capillary element (2) which enables or interrupts
25 the connection between the capillary element (2) and the vacuum chamber (4) so
as to prevent the capillary element (2) and the detecting and measuring system (1)
from being contaminated.
4. The leak test system (1 00) according to claim 3, wherein said heating devices
30 are adapted to heat said connection valve (3) and/or said vacuum chamber (4).
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5. The leak test system (1 00) according to any one of claims 2 to 4, wherein the
heating devices comprise the detecting and measuring instrument (1 ), and the
capillary element (2) is heated by said detecting and measuring instrument (1 ).
5 6. The leak test system (1 00) according to any one of the preceding claims,
wherein said heating devices further comprise a heat generator (7).
7. The leak test system (1 00) according to any one of the preceding claims, further
including mechanical means whose actuation reduces the internal volume of at
10 least one of the components of the leak test system ( 1 00) and causes the
compression of said gases and/or vapors, thereby pushing them towards the
detecting and measuring instrument (1 ).
8. The leak test system (1 00) according to claim 7, wherein said mechanical
15 means comprise a piston (11, 12) positioned inside the vacuum chamber (4) or
between the vacuum chamber (4) and the detecting and measuring instrument (1 ).
9. The leak test system (1 00) according to any one of the preceding claims,
wherein the gases and/or vapors detected by the detecting and measuring
20 instrument (1) consist of dimethyl carbonate.
25
10 The leak test system ( 1 00) according to any one of the preceding claims,
further comprising a container of auxiliary gas adapted to push said gases and/or
vapors towards the detecting and measuring instrument (1 ).
11. 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
means of a leak test system (1 00) comprising a plurality of components including a
vacuum chamber (4) and a detecting and measuring instrument (1 ), the method
30 comprising 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);
-sealing said vacuum chamber (4);
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-decreasing the pressure inside the vacuum chamber (4) 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 in the
cell leak from said cell in cases of leak;
5 - detecting said gases and/or vapors by means of the detecting and measuring
instrument ( 1 ); and
- identifying the presence of leaks by comparing values corresponding to said
detected gases and/or vapors to predetermined threshold values;
characterized in that the method further comprises the step of making the leak test
10 system (1 00) substantially memory-free by heating at least one of said
components of the leak test system (1 00) by means of heating devices to prevent
residues of gases and/or vapors escaping from the cell from depositing on the
internal walls of said components.