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Paul R. Shearing et al.
Characterising thermal runaway within lithium-ion cells by inducing and
monitoring internal short circuits
Volume 10 Number 6 June 2017 Pages 1287–1542
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©
The Royal Society of Chemistry 2017 Energy Environ. Sci., 2017, 10, 1377--1388 | 1377
Cite this: Energy Environ. Sci.,
2017, 10,1377
Characterising thermal runaway within lithium-ion
cells by inducing and monitoring internal short
circuits†
Donal P. Finegan,
a
Eric Darcy,
b
Matthew Keyser,
c
Bernhard Tjaden,
a
Thomas M. M. Heenan,
a
Rhodri Jervis,
a
Josh J. Bailey,
a
Romeo Malik,
d
Nghia T. Vo,
e
Oxana V. Magdysyuk,
e
Robert Atwood,
e
Michael Drakopoulos,
e
Marco DiMichiel,
f
Alexander Rack,
f
Gareth Hinds,
g
Dan J. L. Brett
a
and
Paul R. Shearing
*
a
Lithium-ion batteries are being used in increasingly demanding applications where safety and reliability are of
utmost importance. Thermal runaway presents the greatest safety hazard, and needs to be fully understood
in order to progress towards safer cell and battery designs. Here, we demonstrate the application of an
internal short circuiting device for controlled, on-demand, initiation of thermal runaway. Through its use,
the location and timing of thermal runaway initiation is pre-determined, allowing analysis of the
nucleation and propagation of failure within 18 650 cells through the use of high-speed X-ray imaging at
2000 frames per second. The c ause of unfavoura ble occurrences such a s sidewall rupture, cell bursting, and
cell-to-cell propagation within modules is elucidated, and steps towards improved safety of 18 650 cells and
batteries are discussed.
Broader context
From portable electronics to grid-scale storage, high energy density Li-ion batteries are ubiquitous in today’s society. Such cells can and do fail, sometimes catastrophically,
releasing large amounts of energy. To facilitate safer and more reliable cell designs, the importance of understanding failure mechanisms of Li-ion cells is widely recognised.
Here, we demonstrate the application of a novel device that is capable of generating an internal short circuit within commercial cell designs, on-demand, and at a pre-
determined location. This enables us to test more effectively the ability of safety devices of cells and modules to withstand ‘worst-case’ failure scenarios. By combining the
use of this device with high-speed X-ray imaging at 2000 frames per second, we cha racterise for the first time the initiation and propagation of thermal runaway from a
knownlocationwithinaLi-ioncell.TheinsightsachievedinthisstudyareexpectedtoguidethedesignanddevelopmentofsaferandmorereliableLi-ion cells.
1. Introduction
The demand for Li-ion batteries is expected to rapidly increase over
the coming years,
1
but concerns over their safety and reliability
hinders their uptake, in particular for applications that require
exceptionally low risk, such as communications
2
and space
exploration.
3,4
Although catastrophic failure of lithium-ion cells
is extremely rare, recent high-profile events have demonstrated
the high socioeconomic risks associated with battery failure; for
example, the recall of an entire product line of smartphones
5
and the grounding of an aircraft fleet
6
following the catastrophic
failure of Li-ion batteries. Improving the safety and reliability of
Li-ion batteries is essential for ‘mission critical’ applications.
Internal short circuits (ISCs) within Li-ion cells can lead to
thermal runaway by providing enough heat to trigger a series of
exothermic reactions,
7–12
and are consequently of great concern
to battery manufacturers, particularly since they can stem from
latent defects from manufacturing, which are difficult to
detect.
13
The maximum temperature reached during a short
circuit, and therefore the likelihood of a short circuit leading to
a
Electrochemical Innovation Lab, Department of Chemical Engineering,
University College London, Torrington Place, London, WC1E 7JE, UK.
E-mail: p.shear ing@ucl.ac.uk
b
NASA-Johnson Space Center, Houston, TX, 77058, USA
c
National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden,
CO 80401, USA
d
Warwick Manufacturing Group, University of Warwick, Coventry CV4 7AL, UK
e
Diamond Light Source, The Harwell Science and Innovation Campus, Didcot,
Oxfordshire OX110DE, UK
f
ESRF The European Synchrotron, 71 Rue des Martyrs, 38000 Grenoble, France
g
National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW,
UK
† Electronic supplemen tary information (ESI) available. See DOI: 10.1039/c7 ee00385 d
Received 9th February 2017,
Accepted 17th March 2017
DOI: 10.1039/c7ee00385d
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thermal runaway, depends on the electrical and thermal con-
duction properties of the materials that make unintentional
contact.
14,15
For example, contact between the positive Al
current collector and the negative Cu current collector would
provide the most rapid discharge and heat generation, but the
heat would dissipate relatively quickly due to the high thermal
conductivity of the materials. A short circuit between the Al
current collector and the carbon negative electrode would also
result in rapid discharge and heat generation due to the two
materials having high electrical conductivities; however, the
relatively low thermal conductivity of the porous carbon electrode
would lead to locally higher temperatures, and therefore an
increased risk of thermal runaway initiating.
The risks associated with the occurrence of thermal runaway
depend on the design of the cell or module; pouch cells that are
not contained within a rigid shell tend to swell and catch fire,
whereas 18 650 cylindrical cells, which consist of a spiral-wound
electrode assembly contained within a rigid steel container, can fail
via violent rupture or explosion due to the container acting as a
pressure vessel.
16
Numerous safety devices are included in today’s
18 650 cells to help avoid or mitigate catastrophic failure, such as
pressure relief vents, current interrupt devices (CIDs) and positive
temperature coefficient (PTC) current-limiting switches.
17
Similarly,
external safety devices such as battery management systems (BMSs)
and temperature control in modules and systems help avoid
unfavourable conditions. Whilst these devices can help maintain
safe operating conditions, they cannot prevent the occurrence of
ISCs in defective cells, which remain a risk in battery-powered
systems where cell-to-cell propagation of thermal runaway can
occur. An improved understandingofthethermalandstructural
dynamics associated with ISCs would help manufacturers develop
safer battery designs and modules to minimise the risk associated
with such failures. However, internal short circuiting and the
process of thermal runaway, as well as the influence of mechanical
designs and safety features on the failure mechanism, are not yet
well-understood.
Over the past decade, X-ray radiography and computed tomo-
graphy (CT) have emerged as effective, non-destructive tools for
characterising battery degradation and failure.
18,19
In particul ar,
the rapidly advancing capabilities of X-ray CT systems
20,21
present
scope for capturing failure events and degradation mechanisms
across multiple length scales
9
and over very short time periods.
16,22
The high-speed imaging capability of synchrotrons allows tem-
poral studies of evolving material microstructures or device
architectures in 3D, which has recently been used to reveal
degradation mechanisms from the electrode particle
23
to the
full cell level.
16,24
However, characterising the process of thermal
runaway is extremely challenging, primarily due to the rapid
change in material architecture and the unpredictability of the
location at which thermal runaway initiates.
16
This needs to be
known in order to set the field-of-view of the X-ray and orienta-
tion of the cell such that the initiation and propagation of
thermal runaway can be successfully imaged.
Numerous attempts to replicate an ISC have previously been
made. Maleki et al.
25
induced an ISC through nail penetration,
indentation and pinching of cells. Orendorff et al.
26
described
an ISC device that involves placing a low melting point metal
between the positive electrode and the separator, which when
melted would penetrate through the separator and cause a
short circuit; a technique that has not yet been proved to be
reliable. The 2015 International Electrotechnical Commission
(IEC) testing standard
27
describes a technique for forced internal
short circuiting (FISC) by incorporating a metallic particle into the
electrode assembly and subjecting it to compression; however,
this method involves dismantling the cell and does not provide
information about actual cell behaviour during an internal short,
which is determined by the cell design and integrated safety
devices. Hence, there is a need for a reliable test method that
replicates the behaviour of an unmodified commercial cell design
undergoing an ISC to better understand thermal runaway propaga-
tion and evaluate the efficacy of safety devices.
28
Here, the characterisation of an ISC device developed at NASA
andNRELbyKeyserandDarcy
29
is presented. The ISC device,
which is implanted inside 18 650 test cells during assembly, can
achieve an on-demand failure at a pre-determined location within
the cell, which is representative of a latent defect, ‘in-field’ failure.
By being able to control the location at which thermal runaway
initiates, ‘worst-case’ failure scenarios, such as cell bursting or side-
wall rupture can be induced, allowing insight into cell design
vulnerabilities. The provision of ISCs at a pre-determined location
allows replication of initiation and analysis of thermal runaway
propagation through the use of X-ray imaging.
25
Until now, due
to the small field-of-view associated with high-speed imaging,
capturing the rapid events surrounding initiation and propaga-
tion of thermal runaway was left to chance. The initiation and
propagation of thermal runaway resulting from an ISC is captured,
using high-speed (ca. 2000 frames per second) X-ray imaging at The
European Synchrotron (ESRF)
22,30
and Diamond Light Source (DLS)
synchrotron. This has provided unprecedented insights into the
failure mechanisms and the in fluence of cell design
16
on the safety
of 18 650 cells. Furthermore, 18 650 cells with an ISC device, located
in such a way as to induce sidewall rupture, are placed in 2 2
modules of 18 650 cells in order to study cell-to-cell propagation
and the safety concerns that such a mode of thermal runaway may
cause for multi-cell modules.
2. Experimental
2.1. Internal short circuiting device
The ISC device
29
was designed to generate an electrical short
between two electrically conducting layers. In this work, the ISC
device connects the negative electrode active material to the
positive electrode current collector, bypassing the less electrically
conductive positive electrode active material (Fig. 1). This has
been suggested to be the type of short that is most likely to result
in thermal runaway, as shown by Santhanagopalan et al.
15
and
confirmed by Shoesmith,
31
since the negative carbon electrode
maintains high electrical conductivity (to cater for a rapid short
circuit and heat generation), but a lower thermal conductivity
than the Cu current collector (resulting in a lower rate of heat
dissipation).
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The ISC device used in this work consisted of four layers
(Fig. 1): a 76 mm thick Al pad with a diameter of 11.11 mm, a
thin (10–20 mm) layer of electrically conductive wax (consisting
of a mixture of microcrystalline and paraffin wax) that melts at
57 1C, a 25 mm thick Cu puck with a diameter of 3.18 mm, and a
25 mm thick Cu pad with a diameter of 11.11 mm. The crucial
component of the ISC device was the thin (10–20 mm) and
uniform wax layer on the side of the Al pad adjacent to the
Cu puck whose deposition was achieved through a spin coating
process. The device also included a separator layer that matched
the separator of the implanted cell, making up the gap between
the Cu and Al pads outside the area of the Cu puck. Activation of
the ISC device generally occurs when the wax melts (at 57 1C) and
is expected to be wicked away by the surrounding separator,
providing an electronically conducting path between the negative
carbon electrode and the positive Al current collector.
Once the ISC device had been assembled, implantation began
by unwinding dry spiral-wound electrode assemblies to uncover
the desired short location. In this study, the positive electrode
coating was removed to expose the Al current collector (Fig. 1a)
by unravelling the electrode, placing a nylon template with a
14.29 mm diameter hole on the surface of the electrode layer,
and using an acetone-soaked cloth to remove the electrode active
material. In a dry electrode assembly, the Al side of the
assembled ISC device was then seated into the hole in contact
with the Al current collector (Fig. 1a). A corresponding, oversized
and aligned hole was made in the cell separator to allow the Cu pad
side of the ISC device to contact the negative electrode active
material. The separator of the device and of the cell, were adhered
together with a small amount of glue to secure the device in place
during rewinding of the electrode assembly. Once the electrode
assembly had been rewound, an isolation resistance test was
performed before returning to the cell assembly process.
To further increase the likelihood of a thermal runaway
response and maximise its effects, the cells were made with a
single-layer polypropylene separator (with no shutdown feature).
Previous cells made with a tri-layer shutdown separator were
found to effectively inhibit ionic flow and reduce the chance of
catastrophic thermal runaway response to collector–collector
shorts.
32
In these implantations, the wax-coated aluminium
pad was oriented towards the centre of the electrode assembly.
Two different batches of 18 650 test cells were i mplanted and are
showninFig.2;onewiththeISCdevice placed at mid-longitudinal
height (Fig. 2a) and six winds (from a total of 18) into the spiral-
wound electrode assembly (henceforth referred to as ‘ISC 1’), and the
second with the ISC device at mid-longitudinal height and three
winds into the electrode asse mbly (hencefo rth r eferred to as ‘ISC 2’).
Two radial depths were tested to examine the influence of the
location of failure initiation on the failure mechanism of the cells.
The ISC 1 cells had a capacity of 2.4 A h with an int ernal cylindrical
mandrel (Fig. 2b), and the ISC 2 cells had a capacity of 3.5 A h with
no internal mandrel (Fig. 2c). To date, this batch of over 36 cells has
yielded 100% success at inducing thermal runaway response with
16 cells activated at 100% state of charge (SOC). In order to safely
ship these test cells, it was verified that activation at 0% SOC results
in a hard short but only a small temperature rise.
High-resolution synchrotron X-ray CT (imaging details are
provided in Section 2.3) was used for quality inspection (Fig. 2b–e).
Fig. 2d and e show that there is a significant distance between the
circumference of the Al pad and the positive electrode material; in
the immediate vicinity of the circumfere nce of the pad, the separator
layer is seen to make contact directly with the Al current collector.
Furthermore, a pinch point is observed between the Cu and Al pad
where the separator may be under additional compressive strain;
this could w eaken and close the pores of the separator around t hese
regions. This pinch point is expected to affect the propagation of
thermal runaway, and is discussed in the Results section.
2.2. Setup, heating and temperature measurement
A modified nail penetration tester (MSK-800-TE9002, MTI Corpora-
tion, USA) with infra-red and X-ray transparent windows was used
to secure the cells for imaging and to act as a containment system
for the failing cells. Further details and images of the system are
provided in the ESI.† The 18 650 cells were held firmly in place by
hydraulic clamps to keep the region of interest in the field-of-view
for the entire duration of the battery failure.
The test cells were heated using a custom, 12 mm tall,
circumferential NiCr wire heater with a length equivalent to a
5 O resistance located at the bottom side wall of the cell (Fig. 2a).
The high resistance coil was connected to a DC power supply that
wasoperatedat10Vand1.83A,providingaheatingpowerof
18.3 W (of which ca. 16.7 W was applied to the cell and the rest was
Fig. 1 (a) Exploded view of the individual components of the ISC device, where the positive electrode material is etched away for seating the Al pad.
(b) 3D illustration showing where the components of the ISC device are inserted into the spiral-wound electrode assembly of an 18 650 cell.
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dissipated in the cables and connections). The power was applied
immediately for the ISC 1 cells and gradually ramped up to the
required power for the ISC 2 cells. After prolonged heating at a
power of 18.3 W, the cells in which the ISC device had not activated
at the expected temperature of ca. 60 1C, were exposed to gradually
increasing power until activation occurred. This delayed response is
suspected to occur as a result of insufficient winding tension in the
electrode assembly for the Cu puck to press through the wax.
The temperature of the surface of the cells was measured using a
thermal camera (FLIR SC5000MB, FLIR Systems France, Croissy-
Beaubourg, France), which was separated from the battery failure by
a 2 mm thick infra-red transparent sapphire window (set-up shown
in ESI†). The 18 650 cells were painted with a uniform layer of heat-
resistant black paint with a calibrated emissivity
33
of 0.96 over the
range of 40–180 1C. The calibrated temperature range of the thermal
camera that was used in this experiment was 15–250 1C. The InSb
detector of the thermal camera allowed detection of infra-red
wavelengths between 2.5 mmand5.1mm, the noise equivalent
temperature difference of the camera was o20 mK for the calibrated
range, and the measurement accuracy was 1 1Cor1%ofthe
measured temperature in degrees Celsius. Thermal images were
recorded at a rate of 25 Hz.
Due to prolonged exposure to high energy X-rays, the detector
readings from the thermal camera became less reliable, hence
for the ISC 2 cells temperature readings were gathered from a
fast response K-type thermocouple with a diameter of 0.5 mm
(product 406-534, TC Direct, Uxbridge, UK) that was applied to
the surface of the cells using aluminium tape.
2.3. Tomography and high-speed X-ray imaging
Lab-based X-ray CT. A tomogram of a full 18 650 cell with
an integrated ISC device was captured using a lab-based X-ray
CT system (Zeiss Xradia 520 Versa, Carl Zeiss Microscopy,
Pleasanton, CA, USA). A 0.4 optical magnification was used,
and the sample-to-source and source-to-detector distances
were 32.2 mm and 79.4 mm, respectively, giving a pixel size
of 19.82 mm. With an exposure time of 2 s and a source voltage
of 160 kV, 2001 images of 1024 1024 pixels were used to
reconstruct the individual tomograms. Five tomograms along
the length of the 18 650 cell were reconstructed using Zeiss
XMReconstructor software which uses a filtered back projection
(FBP) algorithm, and were stitched together (vertically) using
the software’s automatic stitching option.
Synchrotron X-ray CT. A higher resolution tomogram of the
section of the ISC 1 cell with the integrated ISC device was
achieved at beamline ID19 at The European Synchrotron
(ESRF), and a similar tomogram of the ISC 2 cell was captured
at beamline I12 at Diamond Light Source (DLS) synchrotron.
34
At the ESRF, the ISC 1 cell was imaged using a polychromatic
beam with a field-of-view (FOV) of 9.65 mm 15.20 mm
(horizontal vertical) which consisted of 1302 2048 pixels
with a pixel resolution of 7.42 mm. This FOV corresponded to
half the cell in the horizontal direction. The rotation axis of the
sample was located at the edge of the FOV such that by rotating
the sample 3601 it was possible to image and reconstruct the
entire sample.
35
A total of 3600 images with an exposure time of
0.2 s, taken at angular increments of 0.11, were used for the
reconstruction. A high-speed PCO.Dimax (PCO AG, Germany)
detector and a LuAG:Ce (Lu
3
Al
5
O
12
:Ce) scintillator were used
for capturing tomograms at the ESRF. The tomograms were
reconstructed using the standard FBP.
At DLS, the ISC 2 cell was imaged using a monochromatic
74 keV beam with a FOV of 20.22 mm 17.06 mm which
consisted of 2560 2160 pixels with a pixel resolution of
7.9 mm. A total of 2400 images, each with an exposure of 2.5 ms,
were taken for a 1801 rotation. A PCO.Edge detector (PCO AG,
Germany) and a LuAG:Ce scintillator were used for acquisition of
the tomograms. At DLS, the tomograms were reconstructed using
Fig. 2 (a) 3D reconstruction of a full 18 650 test cell showing the longitudinal location of the ISC device (yellow) and the NiCr heating wire (orange). (b)
3D reconstructions and associated YZ and XY slices with labelled regions of interest (A and B) showing placement of the ISC device six layers into the
electrode assembly (ISC 1), and (c) three layers into the electrode assembly (ISC 2). (d) Magnified view of the top end of the ISC pads from a YZ slice of the
ISC 1 cell labelled A, and (e) bottom end of the ISC pads labelled B. A pinch point between the Cu and Al pad is highlighted by arrows.
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