SIMULATION OF INTERNAL SHORT
CIRCUITS IN LITHIUM-ION CELLS
Simulation of Internal Short Circuits in Lithium-Ion Cells
Simulation of Internal Short Circuits
in Lithium-Ion Cells
Abstract
Understanding the root causes and mitigating the safety hazards associated with
internal short circuits in lithium-ion cells is an active area of research and a needed
White Paper by
Alvin Wu
update for battery safety standards. For battery safety standards, there is a need for a
Mahmood Tabaddor, PhD
safety performance of lithium-ion cells under the conditions of an internal short circuit.
Judith Jeevarajan,1 PhD
practical, safe and reliable test method for battery safety standards that can assess the
In this paper, we provide details of a proposed test method which relies on external,
localized indentation of cell casing to induce an internal fault that might simulate some
field failures.
Carl Wang, PhD
Corporate Research, UL LLC
1
NASA Johnson Space Center
Introduction
Lithium-ion cells have been powering a wide range of electrical and electronic devices
from consumer products and medical equipment to automotive systems and space
applications. However, some highly publicized failures of products powered by
lithium-ion cells, such as laptops and electronic toys, have been reported.2 In some
instances, a large number of products have been recalled. A portion of these failures
has been linked to overheating of the battery resulting in fire and explosion. Fires
on some cargo planes carrying bulk shipments of lithium-ion cells have regulators
(United Nations and Federal Aviation Authority in the US) concerned about safe
handling procedures.3 Insurance companies have funded research to support fire
protection strategies for bulk storage of lithium-ion batteries.1 Finally, the most recent
and publicized fire incidents involving a GM Volt and two Boeing 787 airplanes, have
brought this technology under intense governmental scrutiny. Overall, these incidents
have catalyzed a great deal of research and design activities aimed at trying to
understand the causes of such failures and help guide safer cell designs. 4
Though the lithium-ion cell is designed with integrated passive safeguards (and active
safeguards in the case of pack designs), the sheer number of lithium-ion cells, the
complexity of the cell electrochemistry, and the numerous usage conditions present
challenges not only to the design of safe cells, but to the design of tests for battery
safety standards. Moreover, lithium-ion cells are not only being used in small numbers
to power an individual hand-held device, but many thousands of these commercial,
off-the-shelf (COTS) cells are packaged together (with monitoring systems) to
power electric vehicles and even spacecraft. In some cases, such as transportation,
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Simulation of Internal Short Circuits in Lithium-Ion Cells
customized lithium-ion cells are being
designed and employed where field
usage history, which exists for COTS, is
not available yet. Considering the effort
in understanding and mitigating the
failure of a single cell, these challenges
are likely to increase manyfold for
modules/packs. The ability to access and
translate the results of battery safety
research and field failure information
into a consensus-based safety standard
is a challenge that can only be addressed
through open cooperation between
governmental research organizations,
cell manufacturers, safety stakeholders,
and standards organizations. In this
paper, we describe one promising new
internal short circuit simulation method
that is being developed for battery
safety standards by UL through such a
cooperative approach with other battery
research organizations.
Lithium-Ion Cells
Batteries basically convert chemical
Basics of Cell Operation
energy into electrical energy during
(secondary) lithium-ion batteries
(redox) reactions. For lithium-ion cells,
lithium batteries are not. The main
is generated by the migration of
lithium-ion cell are the cathode, anode,
battery is cycled, these lithium cations
The electrochemically active materials
active electrode materials. Specifically,
oxide (such as LiCoO2) for the cathode
anode during a charging condition
materials are each bound to a metal foil
discharging condition. Electrons travel via
electrodes are electrically isolated from
equilibrate charge difference between
film within a non-aqueous electrolyte.
current.6
In the terminology of batteries,
discharge through oxidation/reduction
are rechargeable while (primary)
the current between the electrodes
electrochemical components of a
positively charged lithium-ions. As a
separator and electrolyte (Figure 1).
are inserted into or extracted from the
for the electrodes are a lithium metal
lithium-ions, travel from cathode to
and graphitic carbon for the anode. These
and in the reverse direction during a
current collector through a binder. The
external circuitry driven by a load to help
one another by a micro-porous separator
the electrodes thereby generating a
The electrolyte allows only for transfer
of ions and can be liquid, gel, polymer, or
solid (such as ceramic).
Research into the safety of lithium-ion
cells has been ongoing for over a decade
while battery safety standards have
been in existence for even longer.
Since lithium-ion cells bring together
highly energetic materials in contact
with a flammable electrolyte, when
exposed to many different abuse
conditions (vibration, high-temperature
environments, crushing, etc.), the
initiation of heat-generating reactions
which can lead to fire and explosion
are a concern. For that reason, the core
tests within consensus-based battery
safety standards, such as UL 16427 and
IEC 62133,8 subject cells to anticipated
abuse conditions. Cells are designed with
integrated passive safety features such
as a circuit interrupt device, a pressure
Figure 1: Components of a cylindrical lithium-ion cell (left), and, discharging mechanism for
lithium-ion cell (right) 5
vent and/or shutdown separator to help
mitigate an over-current condition or
excessive pressure buildup within the
cell.9 However, such features cannot
address all potential abuse conditions. In
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Simulation of Internal Short Circuits in Lithium-Ion Cells
addition, cell designers and researchers
Though only brief accounts of field
When portions of the cell affected by
options for improving cell safety, such as
the presence of manufacturing defects
(generally above 120°C), exothermic
have been investigating a range of
cell chemistries, including less reactive
materials for anode, cathode, and/
or electrolyte, without compromising
performance. Nevertheless, field
failures have been recorded involving
explosive release of energy along with
fire. There have been some ideas on the
underlying initiators for such failures,
known as thermal runaway, in cells.
One source describes some causes of
internal cell faults leading to field failures:
contamination, electrode damage,
mechanical damage, and thermal abuse.10
Internal Short Circuits (ISC)
A review of the publicly available
lithium-ion battery research shows a
strong focus on understanding and
mitigating cell failure modes, specifically
those due to internal short circuits.
18
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failures are available, in some cases,
has been noted to lead to internal
short circuits within the cell. A particle,
depending upon its size and morphology,
may pierce the separator allowing for
direct contact between electrodes
(internal short circuit). There is also
the possibility that the over-charge or
over-voltage subjection of lithium-ion
cells causes the formation of lithium
dendrites -- small, thin, crystal,needle-
like structures -- which may eventually
puncture the separator leading to an ISC.11
joule heating reach certain temperatures
reactions between the active electrode
materials and the electrolyte are initiated.
Depending upon the ability of the cell to
dissipate the heat, the heat generated
may continue to sustain these exothermic
reactions with a rapid increase in the
temperature of and pressure within
the cell (thermal runaway).12 In cases,
where the pressure buildup is relieved
via the safety vent, then expulsion of the
contents is still a chemical hazard.
However, once an ISC is established, a
The combined high pressure and lower
the cell. This is termed joule heating and
temperatures can compromise the integrity
depends upon the internal resistance of
volatile gases, which may further ignite. The
along with other factors as shown in the
lead to ignition of nearby flammable
localized heat source is generated within
modulus of the casing due to high
the extent of the heat produced locally
of the casing leading to explosive release of
the cell and state of charge (current flow)
high temperature of the cells could certainly
figure below.
objects. In the case of battery packs (or
Simulation of Internal Short Circuits in Lithium-Ion Cells
bundles of lithium-ion cells), the highly
manner. For example, the number of
battery safety standards such as UL 1642
a condition where one cell failure could
prior to testing may vary. However, a
fast-changing pace of lithium-ion cell
dense configuration of the cells creates
spread to adjacent cells without proper
external battery thermal management.
Battery Safety Standards
Battery safety standards contain a menu
of abuse tests. Table 1 shows the extent
of harmonization of some major safety
samples and the state of sample charge
review of international safety standards
related to lithium-ion cells reveals that,
despite harmonization across a large
number of abuse tests, only a few are
considering the inclusion of specially
designed internal short circuit tests.13
standards for batteries. Though it should
As product recalls undermine public
procedures in the various standards, the
the safe commercialization of lithium-ion
be noted that for similarly named test
confidence, it is imperative to promote
test details may not be strictly identical
cells by ensuring that consensus-based
Table 1: Survey of international battery safety standard abuse tests
page 5
and IEC 62133 effectively capture the
safety and design knowledge. Therefore,
the key activity for the safety standards
for lithium-ion cells is the development of
an internal short circuit test suitable for
such standards.
Quite often research-level testing cannot
meet the best-practices requirements
for acceptable safety tests for standards.
In addition, the challenges associated
with the myriad possible root causes for
Simulation of Internal Short Circuits in Lithium-Ion Cells
an ISC and the various configurations of
Currently, there are only two new
Generally, the disassembly of a cell is
and pouch) and multi-cell (module/
(ISC) tests, which are either under
in safety standards, especially, as this
lithium-ion cells (cylindrical, prismatic,
pack) configurations may preclude the
possibility of a single ISC-specific safety
test. A review of publicly available
ISC tests (Figure 2) shows that most
require either access to the cell during
manufacturing to insert a particle (FISC
test), compromise integrity of casing
14
(ITRI test) or rely upon disassembly of
a production cell to insert a particle
(SNL test).19
Figure 2: List of ISC tests for lithium-ion cells
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simulated internal short circuits
consideration or part of some
consensus-based standards. One is the
Forced Internal Short Circuit (FISC) test
and the other is based on an indentationtype approach. The FISC requires the
disassembly of the cell with testing
being conducted on the jelly-roll (cell
internals without the casing) making
it useful mainly for research studies.
not considered a best practice for tests
operation for a lithium-ion cell involves
a hazardous condition. Instead, we have
chosen an approach that will be called
an Indentation Induced ISC test. This test
is based on a history of indentation type
testing by UL in collaboration with NASA
15, 16
and more recently with Oak Ridge
National Laboratory (ORNL),20 with the
intention of bringing the best ideas into
a single ISC test method.
Simulation of Internal Short Circuits in Lithium-Ion Cells
Indentation Induced ISC Test
Indentation testing evolved from previous
methods that depended upon an object
either penetrating (nail penetration
test) or crushing (rod circumference
crush test) the cell.17 In the penetration
approach, there is little deformation of
the cell, however, the nail acts as a bridge
generating an ISC that is not localized.
The breach of the casing also dissipates
any hazardous pressure buildup. For
Figure 3: Pictures of Indentation Induced ISC test for cylindrical lithium-ion cells
the rod circumference crush test, the
deformations of the cell are large and
generally the safety mechanisms relieve
the pressure. Of course, both these
may be necessary if they match abuse
conditions that the cell can be subjected
to in the field. However, neither generates
the localized ISC within a closed cell that
is considered to simulate the field failures
noted previously.
The current Indentation Induced ISC test
setup for cylindrical lithium-ion cells
is shown in Figure 3. The cell is placed
in a holder that prevents rotation or
translation of the cell. An indenter with
Figure 4: CT scan images of cylindrical lithium-ion cell prior to testing (left) and single CT scan image
of cell after indentation (right)
a smooth profile presses from above
against the cell casing at a constant speed
(0.01 – 0.1 mm/s). Test measurements
include temperature of the casing surface
at a point near the indentation site,
distance traveled by indenter (amount
of cell casing deflection), applied force
through indenter, and open circuit
voltage. The cells can be at different
states of charge (SOC) or stages of
aging. This entire setup is placed within
a chamber that allows for control of
ambient temperature.
As the indenter presses against the
casing, layers of separator, anode
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and cathode immediately below the
voltage (100-500 mV), there is a rapid
localized high curvature (Figure 4). The
high as 700°C) with an outcome involving
indentation region are deformed due to
resulting high stress/strain will lead to
a mechanical failure of the separator
allowing for direct contact between
increase in cell surface temperature (as
explosive release of gases and flames
(Figure 7).
electrodes at a distance only a few layers
Now the typical risk measure consists
Though this mechanical event cannot be
probability of failure. In forcing a failure,
the effect of the separator failure is a
measuring the severity of cell failure. This
(Figure 6). For some cells, seconds after
NASA in screening of COTS rechargeable
below the casing surface (Figure 5).
of severity of failure multiplied by
observed and documented in real time,
the Indentation Type ISC test is basically
sudden drop in the open circuit voltage
approach is one that has been adopted by
a measured drop in the open circuit
batteries for space applications. Cells that
Simulation of Internal Short Circuits in Lithium-Ion Cells
Figure 5: CT scan image of cell showing breakdown of layers directly below indentation region
Figure 6: Measurements taken during the indentation test for a cell undergoing
thermal runaway
Figure 7: Picture of cells experiencing thermal runaway (left) and one example of explosive failure of
a lithium-ion cell during indentation test (right)
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Simulation of Internal Short Circuits in Lithium-Ion Cells
do not perform well under this type of
apparent. This hole has been measured
cells subjected to this indentation type
stringent secondary testing schedule that
SOC, some localized heating is apparent.
performance (observed severity of failure)
test would then be subjected to a more
might help establish the probability of ISC
cell failure.
Though the method does not rely on a
particle-induced defect of the separator,
the key question is whether the damage
to have a radius of 1-2 mm. For the 50%
It was also determined that the puncture
penetrated several layers, again a feature
of the internal short circuit expected
for particle-related field failures. These
ISC test shows a correlation between test
to energy density, thermal stability of active
materials, material changes (such as STOBA
in Figure 9), and chemistry of the cell.
observations strongly suggest that the
Other Form Factors
conditions that might be representative
has been modified to test prismatic
test method is suitable for simulating ISC
The UL indentation test for ISC simulation
of field failures for lithium-ion cells.
and pouch cells. The main challenge for
inside, the test was run on a cell at 0%
Aside from simulating an internal short
of puncturing the soft casing, especially
the cell that would make measurements
the relevant design changes that might
to the separator is similar to that might
be expected in field failures involving
particles. To determine what is happening
and 50% SOC to help reduce damage to
impossible. Figure 8 shows the layers
within the cell where a puncture is
Figure 8: Cell windings subjected to indentation
under 0% SOC (top pictures) and 50% SOC
(bottom pictures)
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circuit, the test should be sensitive to
affect the resulting ISC performance. To
date, an analysis of results from many
pouch cells is the expected inevitability
when it inflates due to outgassing from
exothermic reactions set off by the ISC.
However, the trends in the data are very
similar to those for cylindrical cells.
Figure 9: Data from 18650 type NMC (1950 mAh) cell without STOBA (top) and with STOBA (bottom)
Simulation of Internal Short Circuits in Lithium-Ion Cells
Summary
Lithium-ion cells are expected to be a dominant portable energy/power source for
electrical and electronic devices for the near future. However, the energetic nature of
the active materials within a lithium-ion cell presents safety challenges. One of these
challenges is insight into thermal runaway within a cell believed to be responsible for
field failures over the last few years. As this research is ongoing, it presents a further
challenge to new test development for battery safety standards. Considering the effort
in understanding and mitigating the failure of a single cell, these challenges are likely to
be increased manyfold for battery modules and packs. For example, one commercially
available electric vehicle contains over 6,000 cylindrical (18650) lithium-ion cells.
As this is a very challenging area, an open and cooperative dialogue to share
information on the failure modes of lithium-ion cells and to help develop ISC test(s) for
consensus-based safety standards is ever-more important.
With the expected growth in using COTS for a variety of applications, it is important the
regardless of how the OEM may choose to subject the cells to further in-house testing
incoming cells be certified according to a suite of comprehensive abuse tests. This
approach is taken by NASA, whereby they select certified COTS cells and still subject the
cells to their space application-specific testing.
Even if the cell is customized for an application, subjecting the cell to tests prescribed by
safety standards (possibly with some modifications) could help filter and rank different
designs and provide greater confidence in safe field performance over the expected life
of the battery system.
Acknowledgments
The authors would like to acknowledge the tremendous support given by
Dr. Hsin Wang (ORNL).
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Simulation of Internal Short Circuits in Lithium-Ion Cells
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