Particle Contamination in Commercial Lithium-Ion Cells—Risk Assessment with Focus on Internal Short Circuits and Replication by Currently Discussed Trigger Methods

Abstract

A possible contamination with impurities or material weak points generated in cell production of lithium-ion batteries increases the risk of spontaneous internal short circuits (ISC). An ISC can lead to a sudden thermal runaway (TR) of the cell, thereby making these faults especially dangerous. Evaluation regarding the criticality of an ISC, the development of detection methods for timely fault warning and possible protection concepts require a realistic failure replication for general validation. Various trigger methods are currently discussed to reproduce these ISC failure cases, but without considering a valid basis for the practice-relevant particle properties. In order to provide such a basis for the evaluation and further development of trigger methods, in this paper, the possibilities of detecting impurity particles in production were reviewed and real particles from pouch cells of an established cell manufacturer were analysed. The results indicate that several metallic particles with a significant size up to 1 mm × 1.7 mm could be found between the cell layers. This evidence shows that contamination with impurity particles cannot be completely prevented in cell production, as a result of which particle-induced ISC must be expected and the need for an application-oriented triggering method currently exists. The cause of TR events in the field often cannot be identified. However, it is noticeable that such faults often occur during the charging process. A new interesting hypothesis for this so-far unexplained phenomenon is presented here. Based on all findings, the current trigger methods for replicating an external particle-induced ISC were evaluated in significant detail and specific improvements are identified. Here, it is shown that all current trigger methods for ISC replication exhibit weaknesses regarding reproducibility, which results mainly from the scattering random ISC contact resistance.

Full text

Citation: Grabow, J.; Klink, J.;
Benger, R.; Hauer, I.; Beck, H.-P.
Particle Contamination in
Commercial Lithium-Ion Cells—Risk
Assessment with Focus on Internal
Short Circuits and Replication by
Currently Discussed Trigger
Methods. Batteries 2023,9, 9.
https://doi.org/10.3390/
batteries9010009
Academic Editor: Ivana Hasa
Received: 8 November 2022
Revised: 9 December 2022
Accepted: 20 December 2022
Published: 23 December 2022
Copyright: © 2022 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
batteries
Article
Particle Contamination in Commercial Lithium-Ion Cells—Risk
Assessment with Focus on Internal Short Circuits and Replication
by Currently Discussed Trigger Methods
Jens Grabow 1,* , Jacob Klink 1, Ralf Benger 1, Ines Hauer 2and Hans-Peter Beck 2
1Research Center Energy Storage Technologies, Clausthal University of Technology, Am Stollen 19A,
D-38640 Goslar, Germany
2Institute of Electrical Power Engineering and Electrical Energy Engineering, Clausthal University of
Technology, Leibnizstraße 28, D-38678 Clausthal-Zellerfeld, Germany
*Correspondence: jens.grabow@tu-clausthal.de
Abstract:
A possible contamination with impurities or material weak points generated in cell produc-
tion of lithium-ion batteries increases the risk of spontaneous internal short circuits (ISC). An ISC can
lead to a sudden thermal runaway (TR) of the cell, thereby making these faults especially dangerous.
Evaluation regarding the criticality of an ISC, the development of detection methods for timely fault
warning and possible protection concepts require a realistic failure replication for general validation.
Various trigger methods are currently discussed to reproduce these ISC failure cases, but without
considering a valid basis for the practice-relevant particle properties. In order to provide such a basis
for the evaluation and further development of trigger methods, in this paper, the possibilities of
detecting impurity particles in production were reviewed and real particles from pouch cells of an
established cell manufacturer were analysed. The results indicate that several metallic particles with
a significant size up to 1 mm
×
1.7 mm could be found between the cell layers. This evidence shows
that contamination with impurity particles cannot be completely prevented in cell production, as
a result of which particle-induced ISC must be expected and the need for an application-oriented
triggering method currently exists. The cause of TR events in the field often cannot be identified.
However, it is noticeable that such faults often occur during the charging process. A new interesting
hypothesis for this so-far unexplained phenomenon is presented here. Based on all findings, the
current trigger methods for replicating an external particle-induced ISC were evaluated in significant
detail and specific improvements are identified. Here, it is shown that all current trigger methods for
ISC replication exhibit weaknesses regarding reproducibility, which results mainly from the scattering
random ISC contact resistance.
Keywords:
lithium-ion battery; battery safety; internal short circuit; cell impurities; particle contamination;
trigger methods
1. Introduction
The lithium-ion battery is a widely used energy storage device for a range of differ-
ent applications [
1
,
2
]. The future production volume of lithium-ion batteries will further
increase, especially due to the expected expansion of usage in electric vehicles and sta-
tionary storage systems [
1
,
3
]. However, lithium-ion batteries have a frequently discussed
safety problem due to the limited thermal stability of some of the used cell components
(electrolytes, active materials and separator) [
4
,
5
] and, as a result, safety-critical conditions
are reported repeatably in field operation (field failures) [
6
,
7
]. In the worst-case scenario,
the spontaneous exothermic combustion of cell internals, so-called thermal runaway (TR),
occurs, resulting in the release of partly toxic gases and large amounts of energy [
8
,
9
].
The risk is further increased by the possibility of thermal propagation (TP) to adjacent cells
in larger energy storage systems [10].
Batteries 2023,9, 9. https://doi.org/10.3390/batteries9010009 https://www.mdpi.com/journal/batteries

Batteries 2023,9, 9 2 of 23
The onset of a safety-critical condition can be triggered by mechanical, electrical or
thermal failures [
11
,
12
]. With the addition of passive protection devices [
13
], a battery
management system (BMS) with intelligent fault detection [
14
,
15
], a robust mechanical
design [
16
] or with appropriate cooling solutions [
17
] on system level, the risk of many
faults can be reduced to a lower and acceptable level. One of these faults, which is
suspected to be responsible for up to 90% of field failures [
18
], is the internal short circuit
(ISC). Here, the characteristic short circuit is formed inside the battery casing, e.g., by
dendrites, poor production and assembly quality or by particles from the production
process itself [
19
]. Early warning of internal short circuits is challenging in practice [
15
,
20
].
Suitable countermeasures to minimise damage are currently not established, which further
increases the criticality of ISC [
5
]. Alongside this, the failure is spontaneous within the
permitted operating limits [
21
]. Impurities, in the case of external particles, can either
cause local mechanical damage to the separator or, in the case of an electrically conductive
particle such as iron (Fe) or nickel (Ni) located on the cathode, can form dendrites over a
longer time period [
22
–
25
]. This leads first to dissolution (oxidation) with the formation of
ions at the cathode and then to deposition (reduction), possibly in the form of dendrites,
on the anode [23,26].
In addition to the ISC risk, impurity particles in the cell can lead to accelerated ageing
behaviour and a decrease in performance [
27
–
29
]. The recall of 26,700 plug-in hybrid
electric vehicles (PHEV) initiated in October 2020 due to possible impurities underlines
this issue’s relevance in application even more [30].
Due to the layered structure of a cell with positive and negative electrodes—consisting
both of current collector and active material—there are four different possible contact types
for an ISC [
31
]. A large variance in resulting contact resistances is possible as a result of
the strongly different electrical conductivities of the current collector material aluminium
(Al) and copper (Cu) compared to the active materials of the cathode (Ca) and anode
(An) [
32
]. For an ISC case with surface-related high power density, a rapid melting of the
contact point occurs, which is known as the fusing phenomenon [
19
,
32
]. For the criticality of
different short circuit types, in addition to the heat output, the possible heat dissipation
and initiation of exothermic side reactions must also be considered [
31
,
33
]. In consequence,
the ISC between Al and An (Al
↔
An) is the most critical ISC case, especially when the cell
has a high state of charge (SOC) [
19
,
31
,
33
,
34
]. If the local heat generation is high enough,
exothermic self-accelerating reactions occur, which can lead to a thermal runaway within a
short time [11,32,35].
As a general source for possible impurities, the hardware, i.e., the complete set of
machines used in production [
29
], or the cutting process of the electrode layers [
23
,
36
] are
considered. In cell production, many moving parts of machines are present, which creates
a risk of contamination due to possible material abrasion or collisions [
3
,
37
]. Thus, in cell
production, focus must always be given to a high-quality standard in the various production
steps, also in order to increase production efficiency and minimise scrap [
28
,
38
,
39
]. There
are various approaches published in the literature for the detection of impurities, but to our
best knowledge, no publicly available information exists regarding how far these methods
are used in commercial cell production [3,39].
For example, J. Kurfer et al. developed a photo-optical system which scans the coating
surface (electrode layer coated with active material) and detects particles down to a size of
100
µ
m [
36
], which is a bit larger than the usual thickness of the electrode layer [
40
]. Using
Infrared thermography (IRT), D. Mohanty et al. were able to detect particles with a size
of
≤
2 mm [
29
]. A. Etiemble et al. used X-ray technology with a resolution of 20
µ
m to
determine the electrode thickness as well as for the localisation of thickness changes caused
by surface impurities [
41
]. Furthermore, the computed tomography (CT) scan has been
found suitable for detecting contaminants in manufactured cells. The detectable particle
size depends on the resolution of the scanner, such that Y. Wu et al. were able to detect
particles with a size of 100
µ
m [
42
] and G. Qian et al. even significantly smaller particles
(about 20
µ
m to 50
µ
m) [
38
]. The CT scan is also suitable as a basis for detection with

Batteries 2023,9, 9 3 of 23
the assistance of neural networks, which was investigated in more detail for this specific
application by O. Badmos et al. [
43
]. In addition, J. Robinson et al. successfully utilised
ultrasonic waves to detect local cell defects with a size of 20 µm to 200 µm [44].
For the presented methods, specifically prepared cells with intentional impurities or
cells that had already failed quality control due to increased self-discharge were analysed.
To our best knowledge, there is no specific documentation of particles sizes and types that
might be introduced within the production process without immediate short-circuit cre-
ation.
Many safety-critical failure cases in practice are reported to occur during the charging
process [
6
,
45
–
48
]. However, the exact cause of these field failures remains unknown due to
the limited analysis possibilities for a burned battery after the TR. In particular, the so-called
sudden-death ISC [
35
] or ghost failures [
49
], in which no prior signals are present, cannot
be plausibly explained at present, and, therefore, there is a need for new hypotheses [
19
],
which will be presented in the following:
Due to ageing processes, such as side reactions with gaseous reaction products or
solid electrolyte interface (SEI) layer expansion, there is an irreversible increase in the
internal cell pressure correlated to the cell age, which is known as the
swelling force [50–53]
.
For example, Y. Li et al. observed that after 1000 full cycles with 80% state of health
(SOH) remaining, the swelling force increased four times compared to the new state (100%
SOH) [
50
]. J. Cannarella et al. found that the swelling force of small (0.5 Ah) pouch cells
increased from 0 MPa to 0.6 MPa when the cell has reached an SOH of 90% [
51
]; therefore,
the aging-related increase in force reaches significant values. In addition, during the
charging process, a reversible expansion in cell volume occurs—known as breathing—which
ranges between 2% and 4% [
52
,
54
,
55
]. This force is particularly strong in the case of high
current rates and/or a large pre-stress [
56
,
57
]. In summary, it can be concluded that the
effective impact force on a hypothetical impurity particle increases due to parallel ageing
processes and with each charging cycle, as visualised in Figure 1. Moreover, the mechanical
stability of the separator also decreases with cell ageing [
58
–
60
], as indicated by the red
curve in Figure 1. For example, X. Zhang et al. observed a reduction in strength by up to
30% after 1200 full cycles compared to the new condition in a punch test [
59
]. M. Sprenger
et al. also noticed a decrease by 25% in tensile strength, 50% in failure strain and 30% in
Youngs modulus when investigating aged cells at an SOH of 90% [60].
Thus, if there is an external particle between the cell layers, the risk of separator
puncture and subsequent ISC increases with battery lifetime, because the resistivity of the
separator and the load on the particle converge to each other.
The increase in force due to the charging process of one cycle is significantly greater
than the constant aging-related growth of the swelling force [
56
]; thus, there is a higher
probability of ISC occurring in the high SOC condition. Due to the large amount of electric
energy and the reduced thermal stability of the cell materials, an ISC at a high SOC poses a
significantly larger safety risk [61–64].
Understanding the development, fault behaviour and consequences of an ISC is a
significant aspect of battery safety [
19
,
35
,
65
]. Due to its rare and stochastic [
23
] behaviour,
ISC research requires methods for the intentional creation of such field failures [
12
,
66
]. Thus,
a wide variety of trigger methods are discussed for investigating ISC behaviour [
22
,
35
,
67
].
Methods that are suitable for the imitation of particle-induced ISC are summarised in the
following Table 1, with special focus on the corresponding geometric and material-specific
parameters in anticipation of the later evaluation of the short-circuit conditions.
The melting wax layer of the PCM (phase change material) from NREL /NASA or
deforming bimetal SMA (shape memory alloy) is considered as particularly suitable in the
literature [
19
,
22
]. However, a ceramic nail with a nickel tip is also supposed to be much
more suitable for simulating practical ISC compared to the conventional penetration test
with a steel nail [68,69].

Batteries 2023,9, 9 4 of 23
Figure 1.
Exemplary presentation of the relationship between the increasing effective particle force
(ageing and cycling) and the reduced separator stability (ageing). As soon as the mechanical stability
(failure force) of the separator reaches a level lower than the force impact on an impurity particle,
a critical area is reached, which is marked as a critical zone.
Table 1.
Overview of current approaches to replicate a close-to-the-field ISC with special focus on the
replicated geometric and material-specific short-circuit conditions.
No. Name Description Refs.
1
Melting wax layer (phase
change material—PCM)
• Wax-coated Cu-puck is inserted into cell
• ISC is triggered by melting insulation wax layer (at ∼57 ◦C)
• Puck properties: contact area 0.32 mm to 4 mm; thickness 25µm
•
Cu puck is located between two pads corresponding to the current collectors
with a diameter of 11mm. The thickness of these pads is between 25
µ
m and
76
µ
m, depending on the ISC type, so that the active materials become connected
[70–74]
2 Low-melting-point alloy • Tin/Bismuth/Indium (Sn/Bi/In) alloy with a melting point of around 60 °C
•
Electrically conductive liquid metal bridged the separator isolation. Short circuit
area is 1 mm2. Metal coating 0.2 mm thick
[75]
3
Bi-metal (shape memory
alloy—SMA)
• Sharp arrow of SMA bends up and pieces the separator at around 70 ◦C)
•
Nickel–titanium (Ni-Ti) alloy with a ground area of 7.5 mm
×
7.5 mm and a
thickness of 0.2 mm
[22,76]
4
External force on internal
particle (Battery Associa-
tion of Japan—BAJ)
• Ni particle with L-shape (0.2 mm height, 0.1 mm width, 1 mm long each side)
• Fe particle 0.25 mm to 1 mm
• Part of IEC 62133-2:2017, IEC 62660-3:2016, JIS C8714
[69,77–79]
5
Inserting steel ball ex-
ternally/blunt rod/IIISC
Test
• Pressing a steel ball (∅2 mm) into the cell
•
Similar mechanical stress on the cell as in the blunt rod test or indentation-
induced internal short circuit (IIISC) test
• Voltage drop is caused by pressure on internal cell layers
• Critical ISC occurs due to deformation of the electrode current collector
[80–83]
6
Slots in separator and
electrode materials
• Small holes of 2mm in diameter provide an ISC when pressure is applied
•
Temporary isolation of the defect by Kapton
®
tape, which is removed immedi-
ately before local force is introduced
[64,80]
7
Penetration: ceramic Nail
with Ni tip
• Nail diameter: 1 mm to 3 mm
• Tip properties: angle 28◦to 45◦; length 0.3 mm to 0.35 mm
• Penetration stop: voltage drop 2 mV to 5 mV
•
Ceramic nail leads to less heat dissipation from ISC location than full metal nail
[68,69,84]
8
Slow penetration with
small needle
• Penetration with steel needle with a diameter of ≈1.25 mm
• Slow penetration speed (0.02 mm s−1)
[85,86]

Batteries 2023,9, 9 5 of 23
The evaluation of different trigger methods to replicate these impurity-triggered
internal fault conditions has not been made publicly available, yet. The direct link between
failure case and failure replication represents a novel basis for the selection and further
development of suitable trigger methods for field-realistic ISC. Thanks to the cooperation
with a battery systems manufacturer, we gained unique and rare access to pouch cells that
passed the cell manufactures quality control but had particle inclusions visually detected
by input control, thereby gaining the rare opportunity to analyse indisputable realistic field
impurities. Both the corresponding risk under increasing tension force (see above) and
in-depth ex-situ analyses were conducted. The results of this paper show that impurity
particles in cell production cannot be completely prevented and must be considered as a
possible cause for the feared internal short circuit.
The remainder of this paper is structured as follows: In Section 2, both the cells under
investigation and utilised analysis methods are described. Based on the particles presented
in Section 3, first, potential sources and corresponding risks are identified in Section 3.3,
followed by the comparison of current trigger methods with the findings (Section 3.4).
The conclusions derived are presented in Section 4.
2. Materials and Methods
For the investigation, six 53Ah high-energy pouch cells with the cell chemistry nickel
manganese cobalt (NMC)/Graphite were used. For an overview, the cells with the particle
inclusions are listed in Table 2.
Table 2. List of the investigated cells with corresponding particle impurities.
Cell No. 1 2 3 4 5 6
Particle No. 1.1 2.1 3.1 4.1 4.2 4.3 4.4 5.1 5.2 5.3 6.1
First, the inclusions were classified, according to their visibility characteristics on
the outer pouch foil, into the categories poor,medium and good. Then, the increase in
tension force due to changes in battery state (SOH and SOC) was investigated using three
tension configurations. The cells were then disassembled and the identified particles were
analysed using optical microscope and SEM (scanning electron microscopy) with EDX
(energy dispersive X-ray spectroscopy). The process steps are shown in Figure 2.
Figure 2.
Sequence of continuing process steps for analysis of particle impurities (from left to right).
2.1. Tensioning
As a safety precaution, the cells were located in a special storage chamber for lithium-
ion batteries (manufacturer: Stöbich; type: Stainlock L). To monitor the cell state, the tem-
perature and the cell voltage were recorded by a datalogger (manufacturer: Graphtec; type:
midiLogger GL 240) with a sample rate of 100 ms and a resolution of 1mV, in order to detect
possible ISCs. For temperature measurement, 0.5 mm thick type-K mantle thermometers
(manufacturer: TC Direct; type: 405-008) were used. One sensor per cell was positioned
between the cell tabs with thermal insulation from the outside. The temperature of the
chamber was not actively controlled, so that possible temperature fluctuations, e.g., day–

Batteries 2023,9, 9 6 of 23
night period, were able to influence the cell voltage due to entropy effects [
87
,
88
]. However,
due to the placement of the storage chamber inside a laboratory and its massive walls, this
effect was damped. At the beginning of the voltage measurement, the initial voltage
UInit
with the corresponding temperature value was set as a comparative value. Later, after 44 h
to 52 h, the voltage
UEnd
was determined when the cell reached the same temperature as in
the initial state to minimise thermal effects. For verification, an intact cell (
Cellref
) without
particle inclusions was taken as a reference. The cells were delivered with a voltage of
3.59 V to 3.67 V, which corresponds to a low SOC. In total, the three states listed in Table 3
were investigated.
Table 3. Tension configuration for evaluating the ISC risk of particle impurities.
No. Areal Pressure Intended Purpose
1 0 MPa Reference to prove constant voltage/no ISC
2 0.25 MPa Typical tension force applied to pouch cells
3 1 MPa
Reproduction of increased tension force due to higher SOC or
increased swelling force (ageing)
In each configuration, the cells were monitored for at least 48 h to ensure a constant
pressure state in the cell [
89
]. The first unstrained state (configuration No. 1) was used
to demonstrate an intact cell by constant cell voltage. For the second state (configuration
No. 2), the cells were tensioned with an area force of 0.25 MPa, which is typical for au-
tomotive pouch cells [
56
,
90
]. Therefore, the cells were clamped between two 8 mm thick
aluminium plates and screwed with eight M8 screws with a torque of 1.5 Nm using a torque
spanner (manufacturer: Wera; type: 7440), which is illustrated schematically in Figure 3.
The relationship between the torque and the area force was previously determined with a
load cell (manufacturer: Burster; type: 8526-6002). In the final configuration, No. 3, the area
force was increased by a factor of 4 to 1 MPa. A further increase in the tension force was
not considered due to the limited bending stiffness of the aluminium plates.
The cells were tested in two batches incorporating the cells
Cell1
,
Cell2
,
Cell3
and
Cell4
,
Cell5
,
Cell6
, respectively. The reference cell (
Cellref
) was measured both times in parallel.
However, charging of the cell was not performed due to the following reasons:
• Significant increase in TR risk if ISC is created at high SOC state [62];
• Losing the opportunity for any ex-situ analysis of realistic particles in case of TR;
•
Overall significant effort required for handling the potential TR of large-format (53 Ah)
cells.
In contrast to the low SOC, in which the cell was tensioned, there was a significantly
increased probability of a TR if an ISC occurred in the high SOC [
62
]. In the case of a TR
of a cell, the analysis of the impurity particle would be practically impossible and would
result in reduced knowledge benefit. Furthermore, the risk of a TR should be kept low due
to the possible severity of damage for these large-format cells.
Figure 3.
Schematic setup of tensioned cell for configuration No. 2 and No. 3 (see Table 3) for voltage
and temperature sensor placement.

Batteries 2023,9, 9 7 of 23
2.2. Disassembling
The cells were discharged to 2.7 V (0% SOC) before disassembly to reduce the safety
risk [
91
]. With a ceramic cutter, the cell layers at the particle inclusion were removed
with an approximate area-size of 5
×
6 cm layer per layer until the particle was reached.
Both depths in terms of number of layers and position within one layer was documented.
For further analysis, the samples were stored in glass cups.
2.3. Particle Analysis
For the optical analysis, a digital microscope (manufacturer: Keyence; type: VHX-5000)
was used and the particle size was determined. Furthermore, images were taken with an
SEM from Zeiss (type: Evo). An EDX from Bruker (type: Quantax 800—XFlash 6) was used
for material analysis.
3. Results
3.1. Tensioning
No change in cell tension was measured in any of the individual tensing states (No. 1–3)
after eliminating the influence of temperature changes. Even when the complete measure-
ment time of about 10 days is considered (
Uinit, config No. 1
to
Uend, config No. 3
), no significant
change in voltage can be measured, as shown in Table 4. With a resolution of 1 mV, the mea-
sured value may fluctuate by
±
1 mV. By applying a filter, the trend of the voltage curve
was determined as ∆U.
Table 4.
Individual cell voltages of investigated contaminated cells (
Cell1
–
Cell6
) and the reference
cell (
Cellref
). Values taken before (
Uinit, config No. 1
) and after (
Uend, config No. 3
) tension was applied to
maximise the observation time for possible voltage changes. Calculation of
∆U
by filter application
and trend analysis for the diagnosis of smallest voltage changes.
Cellref Cell1Cell2Cell3Cellref Cell4Cell5Cell6
Uinit, config No. 1
/V
3.594 3.665 3.663 3.672 3.594 3.664 3.591 3.591
Uend, config No. 3
/V
3.594 3.665 3.663 3.672 3.595 3.663 3.591 3.591
∆Ufilter/V −
0.145
0.032 −
0.012
0.167 0.29 −
0.744
0.414 0.253
Here, a maximum voltage change
∆U
of
−
0.744 mV was calculated for
Cell4
. For the
reference cell, a deviation of
−
0.14 mV resulted in the first batch and a difference of
0.29 mV in the second batch. Thus, the fluctuation in the cell voltages can be explained
by a combination of limited measurement accuracy, residual entropy effects [
88
,
92
] due to
temperature influence and self-discharge [
93
]. An existing significant ISC can be excluded
at this point. Due to the already visible bending of the aluminium plates in configuration
No. 3 (1 MPa), an inhomogeneous distribution of the surface force can already be assumed.
However, since the increase in force as a result of changes in SOH and SOC would be
distributed more homogeneously over the cell surface [
53
], the non-existing ISC here cannot
be used to draw representative conclusions about the high mechanical-load tolerance of
the separator and its principally lower criticality.
3.2. Disassembling and Particle Identification
When the cells were disassembled, electrically conductive and non-conductive parti-
cles were found inside. The impurity particles were located between different cell layers, as
shown schematically in Figure 4—also refer to Table 5for specification of the involved layer
components. For a possible faster ageing and performance decrease due to electrochemical
inactivity, non-conductive particles are less relevant than conductive particles [
29
]. Conduc-
tive particles are much more critical for a potential risk of an ISC, since they become part
of the current flow [
19
]. In the following, positions in which conductive metallic particles
were found are focused on. The complete set of particles is then summarised in Table 6.

Batteries 2023,9, 9 8 of 23
Figure 4.
Position of the discovered particles in the pouch cell structure. Numbering in ascending
order from position Ato position Ewith ascending cell-layer depth.
Table 5.
Nomenclature of identified particle positions with respect to cell layer involved and type of
involved layer material.
Pos. Particle between Pos. Particle between
APouchfoil ↔Isolation D2. Anode ↔2. Separator
BIsolation ↔1. Al-collector E6. Separator ↔6. Cathode
C1. Cathode ↔1. Separator
3.2.1. Particle 2.1
The localisation and removal of particle 2.1 is shown in Figure 5. Particle 2.1 was
located at position E and created a clear bump in the overlying layers (Figure 5left). In the
separator layers 4–6, increasingly stronger separator deformations and, in the centre at
the particle position, transparent areas were visible. The transparent separator areas result
from the pressure-induced pore plugging and lead to locally hindered ion transport in the
cell, which can affect Li plating [
27
]. Separator layer 6 had no visible holes, but the pressure
spots and warps showed signs of starting mechanical wear (Figure 5center), which would
be further stressed by cyclic loading [
58
]. Particle 2.1 was clearly pressed into the active
material, but had no direct contact with the Al collector. The following separator layers, 7
and 8, also showed local pressure spots.
Figure 5.
Sequential disassembly of cell layers for localisation of particle 2.1 of cell 2. Left: view of
aluminium collector of the 1st cell layer, centre: view of particle under separator of the 6th cell layer,
right: view of particle 2.1 (position E).
Figure 6a,b shows the images from the optical microscope. As indicated by the scale,
the particle had a width of 1 mm, a length of 1.7 mm and a thickness of about 0.8 mm.
The surface appears relatively rough. With the EDX-analysis realised in the yellow marked
region of Figure 6c, a clear detection of Al and magnesium (Mg) is visible in Figure 6d,
which means that this particle was an Al–Mg alloy. This result also corresponds to the

Batteries 2023,9, 9 9 of 23
colouring of images in Figure 6a,b. The fluorine (F) and phosphorus (P) components have
their origin in the conducting salt LiPF6and appear on all samples.
(a) (b)
(c) (d)
Figure 6.
Characterisation of discovered particle 2.1 by (
a
) optical image, (
b
) optical image with size
measurement, (c) SEM-image, (d) EDX-analysis.
In principle, an Al-particle could result from laser cutting of the Al-cathode collec-
tor [
43
]. However, the particle was significantly larger than the usual thickness of an
electrode collector, at 10
µ
m to 25
µ
m [
40
,
94
]. Therefore, the origin of this particle is
assumed to be machine abrasion [29], which is supported by its size and shape.
The size and location of the particle pose a significant safety risk to the cell due to
the high electric conductivity of aluminium (
∼
37.7
×
10
6
S m
−1
) [
32
]. The lower melting
temperature, of 660
◦
C, compared to other metals favours a fusing process; however, based
on the particle size, this process would only locally interrupt the internal short circuit.
The danger of a continuous, critically developing internal short-circuit situation would still
exist [80,95]. Consequently, the risk caused by particle 2.1 is rated as high.
3.2.2. Particle 5.1, 5.2 and 5.3
In
Cell5
, three particles (5.1, 5.2 and 5.3) were located in position
C
, as shown in
Figure 7. The close locations of the particles suggest that the origin of the particles is linked.
With a size of 3.5 mm
×
1.6 mm, particle 5.1 was the largest particle found in this
analysis. In the detailed images in Figure 8a, the bulky shape, but with a mostly smooth
surface, of particle 5.1 is visible. Inside the separator, the particle contour is clearly identified
as a pressure zone in Figure 8b.

Batteries 2023,9, 9 10 of 23
Figure 7.
Sequential disassembly of cell layers for localisation of particles 5.1, 5.2 and 5.3 of cell 5.
Left: view of cathode active material of the 1st cell layer, right: view of separator of 1st cell layer.
(position C).
(a) (b)
(c) (d)
Figure 8.
Characterisation of discovered particle 5.1 by (
a
) optical image with size measurement, (
b
)
optical measurement of generated pressure area in the separator, (c) SEM image, (d) EDX analysis.
Particle 5.1 consisted of a non-conductive material and, consequently, the SEM image
(Figure 8c) was charged—as indicated by bright spots and the lower contrast. The over-
average Flur (F) component of the EDX analysis (Figure 8d) is not purely related to the
conducting salt (
LiPF6
). Consequently, it is reasonable to assume that the particle originates
from a fluorinated polymer such as PTFE (Polytetrafluoroethylene), which is also consistent
with the colouring. Due to the low electrical conductivity of particle 5.1, the risk of a
mechanically induced separator rupture can be classified as reduced. Nevertheless, the size
of particle 5.1 leads to structural stresses in many other electrode layers, resulting in an
increased risk of a separator defect and a potentially critical contact between the anode and
cathode [
19
,
33
]. However, the additional mechanical stress due to increased tension force
in condition No. 3 (1 MPa) did not result in any separator damage, so the risk is classified
in the medium range.

Batteries 2023,9, 9 11 of 23
Particle 5.2 also caused clear structural changes in the above separator layer. This
particle 5.2 showed a triangular shape with peaks which are slightly curved upwards and
the dimension was 1.4
×
1.5 mm. The surface shows a thin black coating in Figure 9a.
The particle edges, on the other hand, were rough and look like a broken edge (Figure 9b).
From the EDX-analysis, Figure 9c, increased Fe amounts and traces of Ti were detected. It
is, therefore, highly probable that particle 5.2 was an Fe particle in which titanium had been
used as an additive [
96
]. Particle 5.2 was potentially dangerous for a separator puncture
due to its partially peaked corners. The good electrical conductivity (
∼
9.9
×
10
6
S m
−1
)
and the high melting point of Fe (
∼
1500
◦
C) created a high risk of a safety-critical failure
should an ISC occur [32].
(a) (b) (c)
Figure 9.
Characterisation of discovered particle 5.2 by (
a
) optical image with size measurement,
(b) SEM-image, (c) EDX analysis.
Particle 5.3—measuring 0.3 mm
×
0.7 mm—was significantly smaller than the adjacent
particles 5.1 and 5.2 (Figure 10a)). The crystalline structures on the particle consisted of
conducting salt (
LiPF6
). The colouring and EDX analysis (Figure 10c) indicate that the
particle was carbon (C), presumably corresponding to the anode active material (graphite).
The safety risk of this particle, 5.3, is considered low due to the smaller size of the com-
paratively low electric conductivity (
∼
100 S m
−1
) and mechanical stability of graphite
compared to the metal particles (2.1 and 5.2) [22,97].
(a) (b) (c)
Figure 10.
Characterisation of discovered particle 5.3 by (
a
) optical image with size measurement,
(b) SEM image, (c) EDX analysis.
Polymer particle 5.1 and Fe-particle 5.2 could have been created by a mechanical
abrasion with a machine used in production [
23
,
29
]. The graphite particle, 5.3, could have
been already adhered to a machine and fallen on the cell layer due to collision impact.

Batteries 2023,9, 9 12 of 23
3.3. Evaluation of All Particles
Further particles were found in the cells, which are—together with the particles
presented—summarised in Table 6regarding their characteristic properties.
Particle 1.1 had a fibre-like structure, which is glued together in the particle centre.
The safety risk of particle 1.1 is classified as low as a result of its non-electrical conduc-
tivity. Due to the colourless and transparent structure, the fibres could originate from
separator production. However, this assumption is not supported by the fact that the
fibres have a thickness of 25 µm to 35 µm, which is significantly larger than the structures
of the commercially used separators, which have a thickness of approximately 20
µ
m to
25 µm [13,94].
Particle 3.1 also belonged to the category of conductive particles, but due to its more
paste-like structure, the risk of separator penetration was significantly lower and the
general risk is, therefore, rated as medium. Particles 4.1, 4.2, 4.3 and 4.4 were taken from
outside the active electrochemical cell area. Due to the small size, low conductivity and
minor mechanical stability of the active materials, the outgoing risk is rated as low [
22
,
97
].
Particle 6.1 was also removed from position
A
of the
Cell6
and had a brownish-yellow
round structure. Increased silicon (Si) contents were detected in the EDX analysis. It can be
assumed that the Si content originates from a silicate used as a flux in soldering, e.g., of the
cell tabs [98]. The generated risk level of Particle 6.1 is evaluated as low.
One of the properties is the visual appearance on the cell surface. Figure 11a clearly
shows that particle 5.1 caused a more prominent bump on the pouch foil than did particle
5.2 and one significantly heavier than particle 5.3 did. In connection with the particle sizes,
it is clear that smaller particles are more difficult to detect by visual appearance from the
outside. However, larger impurities such as 2.1 are also more difficult to detect with a
deeper layer positioning (Figure 11b), since the deeper position smooths out the uplift and
makes it less prominent.
(a) Particle position C(b) Particle position E
Figure 11.
External in-situ visibility of particles found in Figures 7a and 5b dependent of depth in
cell layers and particle size (Particle 5.1 > 5.2 > 5.3).
Considering the position of all found particles (Table 6), it is noticeable that the particles
appeared more frequently in the upper cell layers. On the other hand, only larger and more
mechanically stable particles were found in the deeper cell layers (
D
and
E
). Position
E
is located at approximately 10% of the total cell thickness, which means no particles
could be found—when we look at the top and bottom side—at approximately 80% of the
cell volume. Agglomerations of anode- or cathode-active material have a similar structural
stability, as a result of which impurity particles adapt to the active material applied to the
collector [
97
]. Therefore, it is reasonable to assume that deeper lying and smaller particles
cannot be detected by visual inspection and find their way into commercial use. This
fact is particularly risky in the case of metal particles. For cells with a prismatic or round
housing, a visual inspection of this kind is not possible anyway. The size of the particles
found suggests that only limited methods for detecting impurities are used in the cell
production by the manufacturer of these cells or that the particles found their way between
the electrode layers in a subsequent production step.

Batteries 2023,9, 9 13 of 23
Table 6. Overview of characteristic properties of all analysed impurity particles.
Cell Particle Image
In-Situ
Visibility
Particle
Location Size (w ×l)
Material
(Assumption) Risk Level
1 1.1 Medium A0.5 ×0.85 mm Polymer (Separator) Low
2 2.1 Medium E1.0 ×1.7 mm Aluminium-Magnesium High
3 3.1 Medium D1.0 ×1.5 mm Aluminium Medium
4 4.1 Medium A0.3 ×0.35 mm Graphite Low
4.2 Low A0.2 ×0.45mm NMC Low
4.3 Medium A0.5 ×1.3mm Graphite Low
4.4 Low B0.5 ×0.95mm NMC Low
5 5.1 Good C1.6 ×3.5mm Teflon Medium
5.2 Medium C1.4 ×1.5mm Iron High
5.3 Low C0.3 ×0.75mm Graphite Low
6 6.1 Good A0.25 ×0.3mm Silicon (Flux) Low

Batteries 2023,9, 9 14 of 23
3.4. Evaluation of Current Trigger Methods for Particle-Induced ISC
The metal particles 2.1 and 5.2 found in this study, with an edge length of <1 mm,
were significantly larger than the particles presented by G. Qian et al. with a size in the
micrometre range (approx. 20
µ
m to 50
µ
m edge length) [
38
]. The smallest particle inclusion
found in this study (Particle 6.1) had a size of 0.25
×
0.3 mm. It can be assumed that the
occurring particle inclusions have a wide particle size distribution, in reality. Particles in the
size of the separator thickness can only create an ISC between the active materials (Ca
↔
An),
which is considered to have lower safety criticality [
19
,
33
]. However, larger particles above
∼
60
µ
m can create an ISC between one collector and the active material of the counterpart
electrode [
40
]. In particular, a short circuit between the aluminium collector of the cathode
and the anode active material (Al
↔
An) is considered particularly safety-critical [
19
,
33
,
72
]
and has to be reproduced as a worst-case scenario during battery safety tests. It can be
expected that the separator will be penetrated by larger particles (such as particle 2.1 or
5.2) only locally, similar to a pinch test [
99
], and, consequently, the resulting contact area
will most likely be smaller than the particle diameter. The resulting contact resistance is
responsible for the criticality of the short circuit and will be affected by some variation,
mainly due to the size of the contact area [
32
,
100
], the material [
32
], the contact force [
80
,
101
]
and the amount of available electrolyte [
85
], as a result of which each occurring ISC has a
certain uniqueness. M. Chen et al. investigated different contact resistances by a
∼
20 times
repeated puncture with a steel needle and obtained a contact resistance of 2.5
±
1.5
Ω
between the needle and An/Cu and a contact resistance of
20.3 ± −12.4 Ω
between the
needle and Al/Lithium Cobalt Oxide (
LiCoO2
), which represent significant scatter [
85
].
Depending on the technical properties, all trigger methods presented in Table 7are expected
to show scattering of at least this order of magnitude.
Table 7.
Evaluation of trigger methods for the replication of particle-induced internal short circuits
with special respect to the characteristics of the created ISC and scientific quality of the method.
No.
Name
Variation in ISC Types
Adaptabilty of
ISC resistance
Locality of ISC
Adaptability of
General Cell Parameter
(T, SOC, SOH, etc.)
Reproducibility
Transferability to
Other Cell Designs
Preparation Effort
1 PCM ++ + ++ o + o -
2 Low-melting-point alloy + o + o o o -
3 SMA + - + o o/+ o -
4 Particle-BAJ o o/+ o + o/+ o -
5
Inserting steel ball externally/blunt
rod/IIISC test
- - o - o - +
6
Slots in separator and electrode mate-
rial
++ + o + + - -
7 Penetration: Ceramic nail with Ni tip -/o - - + +/o +/o +
8 Slow penetration with small needle -/o -/o o + + o o/+
The PCM device
1
presented by NREL can be used to create contact surfaces of
various sizes with diameters in the range of 0.32 mm to 4 mm. The smaller contact area
is considered to be well-suited for reproducing practical fault cases. Conceptually, it is
possible to adapt the thickness or material of the puck through which the current flows in

Batteries 2023,9, 9 15 of 23
the ISC case. The reproducibility is considered high compared to other trigger methods,
such as the nail test defined in current standards [
19
,
22
]. However, this trigger method
also has a certain variance in the results, which is based, in particular, on the already
discussed scattering of the short-circuit resistance. In [
71
], it is reported that 6 of 10 cells
were triggered to TR in a test series to produce an ISC of the Al
↔
An type. In two of these
cells, no activation of the device was registered. With the Al
↔
Cu ISC type, the device was
activated in 7 out of 10 cells, and one cell in total achieved a TR. In a further application with
9 round cells (type 18650), this device was activated in some cases only at a significantly
increased temperature of 100
◦
C [
74
]. L. Liu et al. tested the PCM in 8 cells, whereby one
cell was triggered at 29.5
◦
C and another cell was triggered only after extra pressure was
applied to the prepared location [22].
The melting Bi/Sn/In-alloy
2
can only be evaluated based on a single study. Here, only
one 18650 round cell with the rather uncritical short-circuit-type Ca
↔
An was tested [
75
].
Other experiments were conducted with coin cells and resulted in an ISC in 4 out of
5 cases. The contact resistances before short-circuit initiation varied between 10
Ω
and 35
Ω
.
The short circuit occurred at temperatures between 58
◦
C and 65
◦
C. From the voltage
curves, it can be deduced that different ISC resistances were generated.
In the first research performed with SMA
3
, four 1 Ah cells with ISC type Al
↔
An
were investigated [
76
]. The ISC was triggered between 58
◦
C to 72
◦
C accompanied with a
sudden voltage drop. All the investigated cells with this ISC type (Al
↔
An) went into a TR
with similar maximum temperatures. The other two investigated cells—shorted with type
Ca
↔
An—reached similar maximum temperatures of 70
◦
C. From the voltage curves, it can
be determined that the first cell already dropped from 4.2 V to 4 V after 2000 s, whereas
the second cell took double the time of
∼
4000 s for this voltage drop. In the investigations
by L. Liu et al., the SMA was triggered at a temperature between 60
◦
C and 80
◦
C [
22
].
However, one device only functioned when additional local pressure was applied.
Methods
1
,
2
and
3
trigger the ISC all by a temperature-related phase change.
In the case of positioning in deeper cell layers, the cell must be heated up significantly
before the trigger temperature is reached internally [
102
]. When applied to cells with a
hard case (prismatic design or round cells), heat distribution on the cell surface is enhanced,
which leads to a large heating zone for the cell. This heating of the cell results in a higher
criticality due to the significantly lowered internal cell resistance between 50
◦
C and
90 ◦C [103,104]
and a lower temperature difference before exothermic side reactions’ start
(80 ◦C to 105 ◦C) [105], which does not correspond to the practical application.
The technique BAJ
4
, which placed an electrically conductive external particle be-
tween the cell layers and then created an ISC by applying an external force, comes close
to the realistic failure case. However, the reconstruction of a single layer ISC requires
an iterative strategy to determine the correct force level. It should also be noted that for
every cell type and every particle type (size and shape), this value needs to be determined
individually [
77
]. A single-layer ISC was achieved with quite high accuracy in the studies
by K. Maeda [
69
]. In the study by H. Döring et al., two different particle sizes (
∅
0.25 mm
and 1 mm) were implemented in the cell. Then, a cylindrical stamp (
∅
150 mm) was used
to generate a large-area compressive load. The smaller 0.25 mm Fe particle did not lead to
an ISC even under a very high large-area pressure load of 154 kN (deformation to 50% of
the cell thickness). A larger Fe particle with 1 mm diameter caused a clear voltage drop at
55 kN. When the compressive load was further increased to 154 kN, the particle triggered
a TR. For testing cells with a hard case, the fully charged cell coil is removed out of the
case and the particle-prepared cell is then sealed in a plastic bag [
79
]. The original case
is not used further. A test of this type is required, for example, in the JIS C8714 and IEC
62,133 standards [
79
,
106
]. The approach of repeating the test five times, documented here,
is recommended. The reproducibility of this methodology is difficult to assess due to a lack
of data, but in comparison it is considered to be between moderate and good.
In method
5
, a steel ball is pressed into the cell. Due to the setup it will not participate
in the ISC, but will deform the cell internally to a point where the separator collapses and

Batteries 2023,9, 9 16 of 23
an ISC between Ca
↔
An is formed first [
80
]. With further applied load, the upper collector
bends into the active material below and creates more critical ISC conditions. For an SOC of
95%, ISC occurred quite reproducibly in four repetitions with a required force application
between 260 N to 280 N, whereas other voltage and temperature characteristics showed
different ISC resistances. In another study, J. Lamb et al. produced similar ISC conditions
with a blunt rod test, in which a blunt steel body with a diameter of 3 mm was pressed
into the cell until the voltage dropped [
81
]. Repeated runs also showed that the force
required to reach the ISC was very constant—ranging from 935 N to 939 N. However,
the ISC conditions were different, which is shown by the different maximum temperatures
of 99
◦
C (soft ISC) and 455
◦
C (hard ISC). Overall, under different experimental conditions,
soft ISCs were found to be present in about 25% (2 out of 8) of the tested cells, and, therefore,
different ISC characteristics must also be expected for this method. The performance of this
test type is well practicable on pouch cells. When the test type is transferred to cells with a
hard case, in particular such as prismatic cells, a large force is required to trigger the ISC
and the internal surface load will be less local, which does not correspond to the conditions
of an ISC triggered by a particle in the small mm range [66,81,83].
Method
6
, presented by P. Ramadass, creates a defect in the separator with a diameter
of 2 mm. A flat plunger is used to create the compression impact. Just before the plunger
touches the cell surface, the Kapton
®
tape is pulled out of the cell. This procedure is both
critical from a safety point of view, and can also only be transferred to a limited extent
to large-format cells. It was observed that a compressive load of about 333 N did not
lead to complete contacting of the previously prepared defective area. On the other hand,
an increased force led to a multi-layer ISC, which means that the optimal force design
requires experience or extensive pre-tests. The optional local removal of the active material
at the cathode also enables the more critical Al
↔
An ISC type, in addition to the Ca
↔
An
type. Due to a single test with identical parameters, the reproducibility can be evaluated
only to a minor extent. The presented results showed, at least for both short circuit types, an
increasing maximum temperature with increased SOC (
Tmax, 100% >Tmax, 80% >Tmax, 60%
),
which corresponds to a plausible behaviour. The experiments by B. Liu et al., with a
defect of 10 mm
2
in the separator, led to an untypical contact resistance of approx. 40%
lower for the ISC type Ca
↔
An than for the type Ca
↔
Cu [
80
]. T. Volck et al. tested the
contact resistances of all four short-circuit types (with and without electrolyte) with a larger
sample of ten single-layer pieces each [
101
]. For this purpose, a 1 mm
2
large and 0.2 mm
thick copper particle was implemented in a separator hole. The standard deviations of
the measured short-circuit resistances were typically about 50% of the mean value. For a
practical fault replication, the separator defect should be selected to be ≤2 mm.
External penetration by a nail (method
7
) with a nickel tip has the advantage that
the preparation effort—compared to the already-mentioned tests—is minimised. However,
for cells with a hard case, a reduction in the case thickness is foreseen [
68
]. In the studies by
K. Maeda et al. for the desired voltage drop of 2mV with a penetration speed of
0.05 mm s−1
,
typically two to five cell layers were short-circuited [
69
]. Nevertheless, the short-circuit
characteristic is clearly more local and, thus, more safety-critical than the nail test with
conventional steel nails [
84
]. The replication of different ISC types is not possible due to
the comparatively large Ni tip (∅∼0.3 mm).
Method
8
, with a thinner needle, also leads to a more local heat generation [
85
,
86
].
Depending on the cell configuration, either the anode or the cathode is penetrated first
and, thus, better contacted. Consequently, the cell structure predefines different critical ISC
scenarios. In the study by S. Huang et al., it was first possible to reproduce more gentle
voltage drops by a slow (0.02 mm s
−1
) needle penetration and—with further penetration—
more critical ISC conditions [
86
]. Similar to method
7
, it was possible to simulate different
contact resistances with varying penetration depths. The generation of relatively gentle
ISC can present the possibility of triggering ISC initiated by small particles in the size
range of 0.1 mm. Due to the limited mechanical stability of the needle, transferability to
cells with hard cases is only possible with prior case adaptation. All methods that insert

Batteries 2023,9, 9 17 of 23
a third-party material inside the cell (method
4
,
7
and
8
) can vary the melting point
and, thus, the fusing behaviour through a selected material choice. However, mechanical
stability must also be considered in the realisation. The advantage of methods
7
and
8
is
the free parameter combination of SOC, temperature and SOH.
4. Conclusions
By in-situ and ex-situ analyses, six large-format pouch cells with visually detected
impurities were investigated. These impurities were identified not only as metallic con-
ductive particles with diameter
x≥
1 mm, but as smaller impurities (
x≤
1 mm) of active
materials and other non-conductive contaminants. Although methods for particle detection
in cell production have already been developed and published by the scientific community,
the findings indicate that these do not currently seem to be generally applied in cell pro-
duction. Thus, apparently, in cell production, inclusions of impurities cannot be completely
prevented. Due to the location of the impurities found within the outer 10% layers, it is
unlikely that all critical impurities can be identified in a subsequent visual inspection.
Using real compression, the safety risk of the defects was evaluated. Although mod-
erate compression did not caus an ISC of the investigated cells, structural changes in the
separator were identified in a subsequent analysis.
Based on the material properties, the individual origins of the particles—ranging most
likely from machine abrasion to material abrasion—were discussed. In addition, the specific
risk potential was classified, as summarised in Table 6. The risk of causing a safety-critical
ISC is considered high for the large (≥1 mm) metallic (Fe and Al) particles found.
With consideration of already-published behaviour of the internal cell tension during
operation and lifetime as well as the ageing process of the separator, the failure process
of sudden ISCs was deduced and described (see Figure 1). Thus, the probability of spon-
taneous ISC occurring in the critical high SOC increases with battery lifetime due to the
increasing force on the internal particle and the reduced separator stability, which describes
so far unexplained causes of failure, especially during or immediately after the charging
process. Nevertheless, further experimental tests are needed to replicate field ISCs, espe-
cially to investigate their causes and consequences, for developing detection methods and
new safety concepts, as well as for a general risk assessment.
Currently discussed test methods for this purpose were evaluated in terms of such
important parameters as reproducibility, possibilities of parameter adjustment and trans-
ferability, etc., based on published results. Due to varying contact resistances, significant
scattering must be expected with all considered methods. Since, even under laboratory
conditions, there is considerable variation in the essential contact resistance for the ISC,
independent of the method, a well-founded evaluation is only possible by increasing the
number of tests (n
≥
5) with the same test parameters. Overall, only a few experimental
studies are currently available for evaluation. In particular, the application to larger cells
has hardly been investigated and more related research investigations are required here.
At least for pouch cells, slow needle penetration (method
8
) is an interesting novel
trigger method with potential for further development. This includes, for example, stopping
needle penetration after the first ISC for a closer field-failure replication and monitoring the
ongoing progress. In the future, this approach will be investigated in detail by the authors
of this paper.
Since the position of an impurity particle must be assumed to be random, the most
safety-critical ISC case, Al
↔
An, must be replicated as a worst-case scenario. This is more
reliable than methods that prepare the cells internally. On the other hand, this ISC type
can also be replicated by precise needle penetration, assuming the cathode is the outer
cell layer.
Author Contributions:
Conceptualization, J.G. and J.K.; methodology, J.G.; investigation, J.G.; data
curation, J.G.; writing—original draft preparation, J.G.; writing—review and editing, J.G. and J.K.;
visualization, J.G.; supervision, H.-P.B. and I.H.; project administration, R.B.; funding acquisition, R.B.
All authors have read and agreed to the published version of the manuscript.

Batteries 2023,9, 9 18 of 23
Funding:
This research was founded by the Federal Ministry for Economic Affairs and Climate
Action of Germany in the project RiskBatt (project number: 03EI3010A).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
The data of this study as well as further information are available on
request by the corresponding author.
Acknowledgments:
We thank Dorian Hühne and Mahmoud Mhaidly for the experimental support.
The authors acknowledge the financial support of the Federal Ministry for Economic Affairs and
Energy of Germany in the project RiskBatt (project number 03EI3010A). We thank the support by
Open Access Publishing Fund of Clausthal University of Technology.
Conflicts of Interest:
The authors declare no conflict of interest. The founders had no role in the
design of the study; in the collection, analyses, or interpretation of data; in the writing of the
manuscript, or in the decision to publish the results.
Abbreviations
The following abbreviations are used in this manuscript:
Al Aluminium
An Anode
BAJ Battery Association of Japan
BMS Battery management system
C Carbon
Ca Cathode
Cu Copper
EDX Energy dispersive X-ray spectroscopy
F Flurine
Fe Iron
IIISC Indentation-induced internal short circuit
IRT Infrared thermography
ISC Internal short circuit
LiCoO2Lithium cobalt oxide
Mg Magnesium
Ni Nickel
NMC Nickel manganese cobalt
PCM Phase change material
P Phosphorus
PHEV Plug-in hybrid electric vehicle
PTFE Polytetrafluoroethylene
SEM Scanning electron microscopy
Si Silicon
SEI Solid electrolyte interphase
SOC State of charge
SOH State of health
TP Thermal propagation
TR Thermal runaway
Sn/Bi/In Tin/bismuth/indium
Ti Titanium
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