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Transcript
INFORMATIONAL PAPER – DRAFT presented at the First Workshop, Brussels, Belgium, March
17-18, 2015.
An Overview of Internal Short-Circuit Screening Tests for Lithium Batteries and a Proposal for Test T.6
Modification
Transmitted by the experts in the United States
1. There are eight test procedures listed in the UN Manual of Criteria and Tests (the UN Manual)
for lithium cells and batteries to pass the compliance requirements for transportation in
countries concerned with harmonizing their regulations with the UN Model Regulations.
2. Cell and battery failures could be caused by external short circuit, internal short circuit,
overcharge, over-discharge, elevated temperatures.
3. Despite extensive testings that take place following the procedures in the UN Manual, lithium
battery incidents continue to occur.
4. It is difficult to determine in some cases whether the incidents resulting in smoke or fire are due
to external short circuiting or internal short circuiting that would be difficult to predict or detect
at the time of manufacture.
5. The Test T.6: Impact/Crush test in the UN Manual states its purpose being “simulating
mechanical abuse from an impact or crush that may result in an internal short circuit.”
6. The Test T.6: Impact test was stated to “simulate an impact” in the revised fifth edition but has
been changed to the Test T.6: Impact/Crush test with a purpose as indicated above related to an
“internal short circuit” by inserting a Test Procedure – Crush (applicable to prismatic, pouch,
coin/button cells and cylindrical cells not more than 20 mm in diameter) which prescribes
crushing a cell or component cell by applying force up to 13 kN at a designated speed.
7. In this Test T.6: Impact/Crush test, the application of the pressure is stopped when the voltage
drops by 100 mV or more, or the cell is deformed by at least 50 % of its original thickness.
8. The criterion for passing is that the external temperature does to exceed 170 oC and there is no
disassembly and no fire within six hours after the test.
9. The cell and battery failures attributable to internal short-circuiting can come from various
causes including shock or dropping; the formation of dendrite; existence of impurities due to
poor quality control during manufacturing; separator failure; and experiencing of
impact/puncture.
10. Other cases of internal short-circuiting caused by voltage anomalies such as the imbalance of
voltage among cells or the high charging voltage applied to cells can be managed and controlled
by battery management systems rather than their prevention that should be handled by cell or
battery testing.
11. Not all of internal short-circuiting could result in thermal runaway. Available data show that
“nail penetration” tests produced variable results and did not reflect the failure mode by which
internal shorts result in thermal runaway the occurrence of which depended on cell chemistries;
internal short-circuiting induced by “indentation” caused thermal runaway in some cases; and
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forced internal short-circuiting techniques with nickel particles inserted as recommended in IEC
(IEC 62133, 2nd Edition) did not cause thermal runaway though internal short-circuiting occurred.
Another simulation technique pursued by the US Department of Energy’s National Renewable
Energy Laboratory (NREL) is the use of the patented “internal short-circuit device” which is
inserted within the cell producing an internal short. The device is developed as an analysis tool
for battery manufacturers for simulating internal shorts. The utility of this technique is to
trigger an internal short to the extent that it will cause thermal runway while the cells
manufactured with shut-off separators may not cause thermal runaway.
Another system proposed is the "propagation test" to test that a thermal runaway cell does
propagate to a battery system by insulating the cells not to affect the other cells and by venting
out generated gases properly when thermal runaway occurs.
The propagation test is not considered a test procure to prevent occurrence of internal shortcircuiting and thermal runaway but rather a system design to prevent propagation of a single
cell thermal runaway to adjacent cells.
Available data indicate that, though internal short-circuiting was initiated, the nail penetration
did not cause thermal runaway in all cases; and the "forced techniques" (such as IEC 62133 2nd
Edition) did not cause thermal runaway while the use of the "indentation technique" resulted in
a fire or explosion by more than 50 % of the time from the chosen sample size.
Recent research data indicate that the integrity of the separator plays an important role in the
way the dendrite creates an internal short-circuit.
The data show that an internal short-circuiting can be prevented or delayed by preventing or
delaying dendrite penetration or growth; and that ductile separators retard the penetration of
dendrite through the separator which forms a barrier for the dendrite that migrates from the
anode where it is grown to the cathode.
In order to test for the integrity of the separator membranes, a test procedure that will test for
the separator to endure stress and strain during and under various operating cycles (such as
variations in temperatures, aging, deformation, bending, etc.), and the separator bending test is
proposed.
In this procedure, the ductile separator will elongate without breaking while the non-ductile
separator will crack or break which result in significant penetration of dendrite through the
separator cracks and an eventual thermal runaway.
The occurrence of thermal runaway in such tests can be attributable to the mechanical failures
of membrane integrity.
Membranes are susceptible to temperature and stress, and tend to become brittle at low
temperatures, and can elongate or soften at elevated temperatures, but should retain their
integrity at these conditions including these temperatures and applied stresses.
It is desirable for the membranes to sustain limited shrinkage at operating conditions, to provide
good resistance to puncture, to be easily wettable, and to be compatible with various
electrolytes and electrode materials, and should be tested for these properties.
One of the simplest tests to test for these properties is bend the membrane in a manner that
will test for its ductility, brittleness, resistance to rupture, and stress-strain.
24. From these observations, the following is proposed as an addition to the exiting T.6 test to
strengthen its "purpose" of testing for an "internal short circuit."
PROPOSAL
25. Insert 38.3.4.6.4 in 38.3.4.6 as follows:
38.3.4.6.4 - Test Procedure – Separator Integrity
A cell or component cell is placed on a flat smooth surface. A blunt stainless steel
rod (Type 316) of a suitable length with a tip diameter of 1 mm is placed vertically on top of the
sample. The rod is pressed slowly at the highest point of sample at a controlled rate (0.05 mm per
second) normal to the horizontal supporting surface. The pressing is to be continued until the cell
casing is deformed to effect the bending of underlying separator membranes. When the tip of the
rod has penetrated and pieced the cell casing, the pressing is stopped. The test is performed at
temperatures of -10 oC and 25 oC each using a total of three cells at a 100% state of charge. The
three cells must be stored at each test temperature for at least 48 hours before the test.
(Note: "Requirement" does not change.)