Further study of the intrinsic safety of internally shorted lithium and

lithium-ion cells within methane-air

Thomas H. Dubaniewicz Jr. *,

* Corresponding author.

E-mail address: tcd5@cdc.gov (T.H. Dubaniewicz).

Joseph P. DuCarme

National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research, P.O. Box 18070, Pittsburgh, PA 15236, USA

abstract

National Institute for Occupational Safety and Health (NIOSH) researchers continue to study the potential

for lithium and lithium-ion battery thermal runaway from an internal short circuit in equipment for use

in underground coal mines. Researchers conducted cell crush tests using a plastic wedge within a 20-L

explosion-containment chamber filled with 6.5% CH4-air to simulate the mining hazard. The present

work extends earlier findings to include a study of LiFePO4 cells crushed while under charge, prismatic

form factor LiCoO2 cells, primary spiral-wound constructed LiMnO2 cells, and crush speed influence on

thermal runaway susceptibility. The plastic wedge crush was a more severe test than the flat plate crush

with a prismatic format cell. Test results indicate that prismatic Saft MP 174565 LiCoO2 and primary

spiral-wound Saft FRIWO M52EX LiMnO2 cells pose a CH4-air ignition hazard from internal short circuit.

Under specified test conditions, A123 systems ANR26650M1A LiFePO4 cylindrical cells produced no

chamber ignitions while under a charge of up to 5 A. Common spiral-wound cell separators are too thin

to meet intrinsic safety standards provisions for distance through solid insulation, suggesting that a hard

internal short circuit within these cells should be considered for intrinsic safety evaluation purposes,

even as a non-countable fault. Observed flames from a LiMnO2 spiral-wound cell after a chamber ignition

within an inert atmosphere indicate a sustained exothermic reaction within the cell. The influence of

crush speed on ignitions under specified test conditions was not statistically significant.

1. Introduction

Lithium-ion (Li-ion) batteries have become one of the most

popular types of rechargeable battery for portable electronics. Liion technology provides enhanced energy storage capabilities

that lengthen device runtime, shorten the recharge time, and

extend the life of the battery. Beyond consumer electronics, Li-ion

batteries are now growing in popularity for underground mine

safety equipment such as cap lamps and communications and

tracking equipment. Chemistry, performance, cost, and safety

characteristics vary across lithium battery types. Portable electronics often use Li-ion batteries with lithium cobalt oxide (LiCoO2)

cathodes, which offer high energy density, but have well-known

thermal runaway safety concerns. Safety concerns specific to underground coal mine fire and explosion hazards from LiCoO2 cells

were described previously (Dubaniewicz and DuCarme, 2013). A Liion powered Mine Safety and Health Administration (MSHA)

permissible device was involved in a thermal event in an

underground coal mine. An investigative report of the incident is

not publically available.

Researchers with the National Institute for Occupational Safety

and Health, Office of Mine Safety and Health Research (NIOSH,

OMSHR) continue to study Li-ion battery thermal runaway potential in order to develop safety recommendations for underground

coal mine applications.1

1 The findings and conclusions in this report are those of the authors and do not

necessarily represent the views of the National Institute for Occupational Safety and

Health.

The study includes an experimental evaluation of potential thermal runaway initiating events of cells placed

within CH4-air atmospheres to simulate a mining explosion hazard.

The study focused on internal short circuits induced by external

mechanical damage, with this failure mechanism known to produce thermal runaway in Li-ion cells. NIOSH researchers previously

reported on a new method of inducing internal short circuit for

thermal runaway susceptibility evaluation purposes that overcomes some limitations of the flat plate and nail penetration

methods (Dubaniewicz and DuCarme, 2013). The present work

extends earlier findings to include a study of:
 LiFePO4 cells crushed while under charge.

 prismatic form factor LiCoO2 cells.

 crush speed influence on thermal runaway susceptibility.

 primary, spiral-wound constructed LiMnO2 cells.

2. Background/literature review

Li-ion thermal runaway hazards continue to be a concern. Wang

et al., (2012) reviewed Li-ion battery fire and explosion accidents,

and reported tens of thousands of mobile phone fires or explosions

from various causes since 2006. They also listed several accidents

involving large format LiFePO4 batteries used in electric vehicles.

Underwriters Laboratories (UL) (2013a) found that since March

2012, the Consumer Product Safety Commission (CPSC) documented 467 reported incidents that identified Li-ion cells as the

battery type involved, with 353 of those being incidents involving

fire/burn hazards. The UL report date suggests the CPSC incidents

occurred within a period of slightly more than 1 year. The UL report

emphasized the need to update existing standards and create new

ones as our information and knowledge of potential Li-ion battery

hazards increase. Li-ion battery failures grounded the Boeing 787

airliner fleet for several months over thermal runaway concerns

with the batteries (NTSB, 2013). Li-ion or lithium cells were

possibly linked to several cargo plane incidents, including two fatal

crashes (GCAA, 2013) (Brett, 2011).

Several studies suggest the need for more emphasis on the

safety aspects of Li-ion technology. Barnett et al., (2013) assert that

Li-ion battery safety issues are not met with the same scientific and

technical rigor that apply to other aspects of Li-ion technology.

Doughty and Roth (2012) propose that safety is often a property

determined after the development phase of Li-ion technology, and

that safety and thermal stability should become a prime consideration in the initial development and material selection phase. Roth

and Orendorff (2012) reviewed research of nonflammable electrolytes, and contend that “electrolyte additives proposed to reduce

gas generation and mitigate flammability have not gained much

traction, in general, because of the tradeoff in performance.” UL

notes that current safety standards do not address the potential

impact of battery aging, and initial results have led the company to

expand its safety research program. For example, UL (2013b)

observed that aging adversely impacted the safety performance

of a selected 2.8 Ah 18650 type LiCoO2 cell.

Spiral-wound Li-ion cells use a thin separator material to insulate the anode from the cathode (Fig. 1). Physical damage to this

separator causes an internal short circuit that may lead to catastrophic heating events. Some separator materials have shutdown

properties that can provide a margin of safety against certain failure

modes, including internal short circuits, that result in an elevated

cell temperature. However, some potential safety issues remain.

Fig. 1. A drawing of a common spiral-wound Li-ion cell construction with a thin

separator material.

Baldwin (2009) reviewed separator materials and functions for

lithium-based batteries used in aerospace missions, and suggests

the shutdown mechanism would provide very little protection

from an internal short circuit accompanied by a rapid internal

heating rate. The thickness of commercial separator materials

reviewed were <30 mm. Orendorff (2012) discussed challenges with

designing safe Li-ion cell separators, including some tradeoffs between mechanical robustness and porosity/transport (performance) properties, primarily for large format cells. The thicknesses

of four varieties of commercial separators reviewed were approximately 25 mm. Orendorff concluded that ceramic/polymer composites and high melting point polymer materials offer some

improvement in thermal stability and abuse tolerance for Li-ion cell

separators but, in general, there needs to be more evaluation work

dedicated to quantifying the safety impact of new separators,

particularly for large format cells.

Barnett et al., (2013) emphasize internal short circuit hazards

from metallic dendritic growth through separators, where metal

particles on (and possibly in) the cathode dissolve and plate out on

the anode, growing back through the separator, leading to an internal short circuit. They note that ceramic layers are sometimes

implemented as porous coatings on electrodes or as separators as a

means of enhancing Li-ion battery safety. In limited testing of cells

containing ceramic layers, they observed formation of internal

shorts from dendritic growth, including internal short circuits that

matured to thermal runaway.

Recent studies (Ong et al., 2010) (Zaghib et al., 2012) demonstrate some safety aspects of LiFePO4 positive electrode materials

compared to some other materials. The relative safety of LiFePO4 is

generally attributed to limited O2 release upon high temperature

decomposition. Previous research (Dubaniewicz and DuCarme,

2013) found that selected LiFePO4 cells did not cause ignition

when crushed within CH4-air mixtures under specified test conditions. In the current study, additional tests were conducted to

crush selected LiFePO4 cells while charging within normal charging

conditions. These tests simulated a CH4 ignition hazard involving

underground stationary Li-ion battery-powered equipment that is

on charge and unattended, while maintaining a single fault condition of a crush-induced internal short circuit.

NIOSH previously contracted with QinetiQ North America (QNA)

to perform a safety assessment of emergency backup batteries and

battery charging systems for underground mining applications

(QNA, 2009). QNA reviewed safety aspects of several primary

lithium cell chemistries, including LiMnO2. The company reported

that LiMnO2 cells have shown resilience against many types of

abuse testing, including short circuit, over-discharge, puncture, and

crush. Overall, LiMnO2 has been proven as one of the most robust

primary cell types on the market today, still in use despite being

one of the first-pioneered lithium technologies. Further, LiMnO2 is

one of the safest of Li technologies as long as the supplier is

reputable and battery pack design has been appropriately tested.

QNA concluded that LiMnO2 is a good candidate power source for

an underground primary-cell application. In the current study, researchers included a commercial LiMnO2 cell marketed for intrinsically safe (IS) equipment, for evaluation as a potentially safe cell

for powering IS mining equipment. An intrinsic safety evaluation

test report for the cell was obtained for comparison purposes

(DEKRA, 2011).

Li-ion cell vent gases may be flammable or inert, and the volume

of vent gases from thermal runaway is substantial. Roth and

Orendorff (2012) and Roth (2008) found that cell venting before

thermal runaway is achieved may release flammable solvent vapor

into the surrounding environment, which may then be ignited by

an adequate ignition source. In contrast, they also found that the

decomposition vent gases from Li-ion cells undergoing thermal
runaway are not inherently combustible, and may act to inert the

surrounding atmosphere. Further, thermal runaway typically produces high-rate gas generation from the decomposition of the

organic solvents. Measurements of gas released from cells during

thermal runaway using several different cathode materials showed

“that the volumes of gas released at the end of the thermal runaway

peak (typically 350 C) were all nearly equal”, and approximately

1200 mL/Ah.

Standard IEC 62133 recognizes cell crushing as reasonably

foreseeable misuse (IEC, 2012). IEC 62133 edition 2.0 contains

revised crush test procedures. A new test provision indicates that

once a 10% deformation with flat plates has occurred, the force is to

be released. This new provision suggests that a cell may deform by

10% at forces significantly less than the 13 kN maximum force, at

which point the force is released. Previous research (Dubaniewicz

and DuCarme, 2013) found that a sample of cylindrical Li-ion

cells crushed by flat plates deformed by approximately 18%

without any adverse outcomes; similar cells readily ignited when

crushed by a plastic wedge. IEC 62133 edition 2 has also eliminated

a narrow side crush test of prismatic cells. Mikolajczak et al., (2011)

demonstrated that crushing the edge of cells is more likely to cause

cell thermal runaway than crush or penetration perpendicular to

electrode surfaces, in agreement with findings by Maleki and

Howard (2009). In the current study, researchers included a sample of prismatic cells to extend previous research to compare crush

characteristics between a flat plate and a plastic wedge method.

Barnett et al., (2013) mention impact speed in a discussion of

potential “gaming” of cell level safety standard tests. Crush or nail

penetration speeds identified in the literature vary significantly. An

amended 5th edition of the UN manual of tests specifies a 1.5-cm/s

flat plate crush speed (UN, 2011). Neither IEC 62133 ed. 2.0 nor UL

1642 ed. 5 (UL, 2013c) specify a flat plate crush speed. The IEC 62133

ed. 2.0 forced internal short circuit test specifies a much slower 0.1-

mm/s crush speed. A proposed UL indentation-induced internal

short circuit test indicates a constant speed in the range of

0.01e0.1 mm/s (UL, 2013d). Spek and Hosseinifar (2012) discussed

efforts to improve nail penetration test methods, and concluded

that a 100-cm/s nail speed showed a strong tendency to produce

adverse outcomes (100%, 6 samples) in large format Li-ion cells

tested. Although not included in the conclusions, much slower nail

speeds (1.1 cm/s) produced a similar rate of adverse outcomes

(100%, 4 samples). Ichimura (2007) suggests that a slower nail

penetration speed may produce more adverse results due to internal resistance considerations. Spotnitz and Franklin (2003) reported on nail tests performed by Dahn using 18650 LiCoO2 cells.

Using a nail with an embedded thermocouple, the nail temperature

exceeded 600 C when forced slowly into the cell to a depth of

<4.5 mm. However, with a fast, deep nail penetration (7.5 mm), the

nail temperature did not exceed 140 C. Penetration speed was not

specified. Based on Dahn's results, perhaps a sufficiently slow

penetration may produce thermal runaway before maximum

penetration is achieved, minimizing the influence of penetration

depth. In testing reported here, interim results with a prismatic cell

produced a limited number of ignitions, providing an opportunity

to run another set of tests with similar cells using a different crush

speed, to assess the influence of selected crush speeds using the

Fisher's Exact Test (Rosner, 1990).

ACRI2001 (MSHA, 2008) and ANSI/ISA-60079-11 (ANSI/ISA,

2012) intrinsic safety standards provide requirements for separation distances through solid insulation. ACRI2001 stipulates a 0.5-

mm minimum distance through solid insulation for voltages up

to 60 V. If the separation between two conductors is less than onethird of this 0.5-mm value, ACRI2001 stipulates that the conductors

are to be considered normally connected, if connection impairs

intrinsic safety. Such shorting of conductors is not to be considered

a fault in the analysis. That is, such a short is considered as a noncountable fault, allowing MSHA to apply up to two countable faults

for intrinsic safety evaluation purposes. ANSI/ISA-60079-11 ANNEX

F lists a 0.2-mm separation distance through solid insulation, for all

over-voltages listed, for both “ia” and “ib” Levels of Protection. If

separation distances are less than one-third of the values specified,

they shall be considered as subject to non-countable short-circuit

faults, if this impairs intrinsic safety. Neither ACRI2001 nor ANSI/

ISA 60079-11 requirements for distance through solid insulation

are currently applied to internal Li-ion cell constructions. These

standards rely on other cell level standards to help ensure intrinsic

safety in potentially explosive environments (Dubaniewicz and

DuCarme, 2013).

Standard ANSI/ISA 60079-0 (ANSI/ISA, 2013) provides general

requirements for explosion protection techniques covered by the

ANSI/ISA 60079 series of standards. These are the US adopted

versions of the IEC 60079 series of standards. ANSI/ISA 60079-

0 indicates that additional consideration and additional testing may

be required for equipment operating outside of normal air oxygen

content, i.e., oxygen enriched atmospheres. Further, potential

ignition sources from exothermic chemical reactions are not

addressed by the standard, and equipment with such ignition

sources should be subjected to a hazard analysis that identifies and

lists all of the potential sources of ignition by the electrical equipment and the measures to be applied to prevent them from

becoming effective. Ong et al., (2010) found that LiCoO2 may release

significant amounts of O2 during thermal runaway decomposition.

Ignition pressures on the order of 10.8 MPa (1560 psi) have been

reported for certain spiral-wound LiCoO2 cells (Jhu et al., 2011) (Yen

et al., 2011). The excessive O2 generation and the high thermal

runaway pressures reported suggest that certain spiral-wound

LiCoO2 cells may be considered as strong oxidizers and potential

exothermic chemical reaction ignition sources for ANSI/ISA 60079-

0 purposes.

3. Methods and materials

NIOSH researchers conceived a new method of inducing internal

short circuit for thermal runaway susceptibility evaluation purposes that was thought to overcome limitations of the flat plate and

nail penetration methods (Dubaniewicz and DuCarme, 2013). The

new method employs a 90 wedge-shaped plastic fixture in place of

the UL flat plate to compress the cell. A plastic material was selected

to minimize electrical and thermal energy dissipation by the test

fixture itself. The wedge shape and point angle are designed to be

robust enough to crush the cell without significant damage to the

plastic wedge itself. A plastic fixture simulates external forces

applied to plastic-wrapped or encased battery packs which are

common (Mikolajczak et al., 2011). Numerous field failures have

been linked to latent mechanical damage (Mikolajczak et al., 2011).

Researchers also conducted a series of tests with flat plates to

simulate the UL 1642 crush tests as an experimental control for

tests with the plastic wedge. A hydraulic ram extension with a 13-

mm-thick flat plate was fabricated from steel and used to perform

these tests.

3.1. Methods

A custom hydraulic press (Fig. 2), designed and fabricated to fit

inside a 20-L explosion-containment chamber, was used for these

experiments. The press uses a small single-acting hydraulic cylinder with a 43-mm bore and 54-mm stroke. The cylinder ram is

retracted by an internal spring when hydraulic fluid is released. A

cutaway view of the press is shown below. The top and bottom

plates are connected with a steel tube (not shown for clarity) that
Fig. 2. Drawings of the plastic wedge and flat plate crush test fixtures.

has access holes for installing the battery and plastic wedge or flat

plate. A commercially available 0.75-kW hydraulic power unit

supplies the hydraulic pressure, and the system relief valve is set to

allow 13 kN of force at the cylinder ram. The press is controlled

from a solenoid-operated four-way hydraulic valve. A bleed-in flow

control is used to regulate the speed of the ram. The complete

hydraulic system schematic is shown in Fig. 3.

Tests using the plastic wedge were completed at two different

crush speeds. The nominal rates were 5 mm/s for the fast ram speed

and 0.5 mm/s for the slow rate. Due to temperature and viscosity

variations, the actual ram speed varies by a slight amount from test

to test as documented in the Results section. Ram speed adjustment

precision was limited with this setup because it is an open loop

control system. Researchers investigated using a hydraulic cylinder

with an internal linear variable differential transformer to enable

closed loop control of the ram speed. Unfortunately, no such cylinder could be found that would fit inside the chamber. Viatran

model 248 pressure transducer measurements were used to

calculate cylinder force.

Cell crush tests were conducted within the 20-L chamber filled

with 6.5% CH4-air. The concentration of the gas-air mixture was

determined by partial pressures and set to 100 kPa (14.5 psi) at

room temperature. The CH4 source purity was specified as 99.97%.

Subsequent tests at 40 C raised the chamber pressure slightly

above atmospheric, and researchers bled off a small amount of gas

to maintain 100 kPa (14.5 psi) after the temperature stabilized. The

term “overpressure” refers to the peak chamber pressure above

100 kPa. A fan within the chamber provided mixing. Chamber

headspace volume was approximately 18 L. Crush tests with the

plastic wedge were conducted at 40 C. Researchers waited at least

1 h after the cell surface temperature reached 38 C before conducting ignition tests at 40 C. Crush tests with flat plates were

conducted at room temperatures per UL 1642. The rationale for

chamber temperature selections was explained previously

(Dubaniewicz and DuCarme, 2013). To determine if a flammable

atmosphere was still present, a hot-surface furnace igniter placed

inside the chamber was activated after tests that did not result in an

ignition.

A personal computer-based Labview data acquisition program

recorded cell voltages, pressure transducer signals, and thermocouple measurements. A chamber pressure transducer detected

ignitions in conjunction with a high-speed video camera. The criterion for ignition was an overpressure of at least 50 kPa (7.25 psi), a

value derived from previous explosibility research (Cashdollar and

Hertzberg, 1985) (Cashdollar, 2000). Sapphire windows allowed

viewing inside the chamber. A high-speed video camera (NAC

model 512SC) recorded tests at 250 frames per second. A white LED

fixture was placed within the chamber to provide illumination for

video recording. An independent laboratory calibrated in-house

Fig. 3. Hydraulic schematic.

Table 1

Cell specifications and conditioning summary data.

M1A MP M52EX

Charge, discharge voltage 3.8, 2.0 4.2, 2.5 e, 2.0

Charge or discharge constant current (A) 3 2.4 0.2

End charge current (mA) 50 50 e

Rated capacity (Ah) 3.2 4.8 5.1

Measured discharge capacity (% rated) >100% >96% 101.7%

Measured resistance range (mU) 23 to 29 41 to 51 120

Measured separator thickness (mm) 20 20 40

multimeters and a thermocouple calibration cell traceable to the

National Institute of Standards and Technology (NIST). A battery

analyzer was calibrated by the manufacturer. Kapton tape was used

to attach the thermocouple to the cell can, and to cover the bottom

metal platen for insulation purposes.

A current limiting power supply (Acopian EP015MX500) was

connected to selected LiFePO4 cells for crush tests while under

charge. These tests simulated a CH4 ignition hazard involving underground stationary Li-ion battery powered equipment that is on

charge and unattended. The power supply voltage was set to the

maximum cell charge voltage recommended by the cell manufacturer. The test protocol called for limiting the power supply current

to the maximum charge current recommended by the manufacturer, or 5 A, whichever amperage was less. The 5-A limit would

maintain the charging circuit parameters within the safe region of

the resistive circuit CH4 ignition curves of ACRI2001 and ANSI/ISA

60079-11. The maximum recommended charge current for the
selected LiFePO4 cells exceeded 5 A, so the 5-A limit was selected

for these tests. The charging parameters maintained the single fault

condition of an internal short circuit induced by crush.

A Vencon UBA5 Battery Analyzer preconditioned and analyzed

cells for testing (Table 1). A software package provided user configurable routines for constant current, constant voltage cycling,

and for measuring resistance and capacity. The resistance routine

uses a two-step DC measurement. Cell resistance was measured on

fully charged cells.

Researchers disassembled a sample of each cell and measured

the thickness of the cell separator using a scanning electron microscope (SEM) (Table 1).

3.2. Lithium-ion cells

A123 systems2

2 Mention of any company or product does not constitute endorsement by the

National Institute for Occupational Safety and Health (NIOSH). In addition, citations

to Web sites external to NIOSH do not constitute NIOSH endorsement of the

sponsoring organizations or their programs or products. Furthermore, NIOSH is not

responsible for the content of these Web sites. All Web addresses referenced in this

document were accessible as of the publication date.

model ANR26650M1A cylindrical 26650 format

LiFePO4 cells (M1A cells) were selected for crush tests while under

charge. The M1A cells were conditioned prior to the test (Table 1)

and fully charged before placing the cells in the 20-L chamber. The

power supply was connected to the cell just prior to the crush test.

The power supply was set to 3.8 V with a 5-A current limit. The M1A

cells were purchased through a battery distributor.

A Saft MP 174565 IntegrationTM medium prismatic cell is marketed for use in potentially explosible atmospheres as compliant

with the IEC 60079-11 intrinsic safety standard (class T4 assignment between 20 C and þ40 C) (Saft, 2009). The medium

prismatic spiral-wound cell features a LiCoO2-based cathode and

organic solvent electrolyte. Built-in redundant safety protections

include a shutdown separator, circuit breaker, and safety vent. Both

cell terminals are located at the top of the cell. Samples of the cells

were purchased through a battery distributor. Received cells were

specified by the battery distributor as MP 174565 (MP cells).

Saft FRIWO model M52EX cylindrical “C” format LiMnO2 primary cells (M52EX cells) are designed to be in compliance with the

IEC 60079-11 intrinsic safety standard (FRIWO, 2012) (Saft, 2012)

(DEKRA, 2011). The spiral-wound constructed M52EX cell features

several safety devices, including a shutdown separator and safety

vent. A sample of M52EX cells were purchased through a battery

distributor. Each cell had a label warning of fire, explosion, and

severe burn hazard as well as do not recharge, crush, disassemble,

heat above 100 C, or incinerate. A cell was discharged for a capacity

measurement (Table 1.). A separate cell was used for the resistance

measurement. Neither of these two cells was crush tested.

Researchers purchased more cells of each type described above

than tested. A random sequence generator provided a random set

of serial numbers assigned to all purchased cells. Tested cells were a

random sample of those purchased.

4. Results

4.1. M1A LiFePO4 cells

Researchers conducted a series of ten plastic wedge crush tests

of the fully charged M1A cells while connected to a charging circuit.

The crush apparatus was placed in the 20-L chamber filled with

6.5% CH4-air and the cell was maintained at 40 C. Charge current

was limited to 5 A to maintain charging parameters within the safe

region of the IS resistive circuit ignition curves. The charging circuit

was connected a few seconds before the crush. None of the M1A

cells crushed under charge ignited the chamber atmosphere. Highspeed video showed all M1A cells rupturing and producing smoke

near the point where the wedge penetrated the can (Fig. 4). Two

cells ruptured such that the cells opened electrically within a few

seconds and stopped the charge current; these two tests were

repeated with other cells. The charge circuit was disconnected after

2 min accompanied by a significant cell temperature drop. These

ruptured cells exposed the cell's internals and cooled relatively

quickly compared to the MP and M52EX cells. One test produced a

few luminous sparks without igniting the chamber atmosphere.

Voltage and current time traces indicated that the cells hardshorted before the cell ruptured. These tests produced chamber

overpressures of <20 kPa (2.9 psi) when the cells ruptured

(Table 2). The furnace igniter subsequently ignited the chamber

atmosphere for all of these tests, confirming a flammable atmosphere was present.

4.2. MP prismatic LiCoO2 cells

Researchers conducted three series of crush tests with the

prismatic cells. The first series used flat plates to simulate the UL

1642 method as a comparison point for tests using the plastic

wedge. The second series used the plastic wedge at a crush rate

similar to that used for the flat plates. Interim results with this

second series of tests produced a limited number of ignitions,

providing an opportunity to conduct a third series of tests with a

slower crush rate, to assess the potential influence of selected crush

speeds using the Fisher's exact test.

4.2.1. MP cells, flat plates

Researchers conducted five flat plate crush tests using the Saft

MP cells placed within 6.5% CH4-air. These tests were conducted at

room temperature per UL 1642. The crush was applied to the wide

side of the cells using 13 kN cylinder force and approximately

6 mm/s crush speed. These flat plate tests produced no ignitions in

five attempts. No cell venting, temperature increase, chamber

overpressure, or voltage drop was apparent from the high-speed

videos or sensor time traces. After a 5-min wait period, the

chamber atmosphere was ignited by the furnace igniter for these

flat plate tests, verifying a flammable atmosphere was present. The

flat plate compressed the cells by approximately 4%. These flat plate

tests provided some assurance that the MP cell samples were

compliant with UL 1642 crush test requirements.

Fig. 4. An M1A cell crushed under charge ruptured but did not ignite the chamber

atmosphere.
Table 2

Summary data, M1A cells crushed under charge, plastic wedge, 40 C, 6.5% CH4-air

non-ignitions.

Cylinder force at short circuit kN (lbf) 1.83 to 3.18 (411e716)

Crush speed avg. (range) mm/s 3.9 (3.0e5.3)

Chamber overpressure kPa (psi) <20 (2.9)

Peak cell can temperatures C 67 to 174

4.2.2. MP cells, plastic wedge

Researchers conducted a series of ten plastic wedge crush tests

using the Saft MP cells placed within 6.5% CH4-air at 40 C ambient.

The crush was applied to the wide side of the cells, with the wedge

tip parallel to the top of the cells. The crush speed was approximately 6 mm/s and cell penetration was limited to approximately

66%. The cells hard-shorted to a voltage of <50 mV in all ten tests

and ignited the chamber atmosphere in three of these tests. Table 3

lists summary data for the three chamber ignitions. The wedge tip

penetrated the cell can for all tests. After a 5-min wait period

accompanied by a significant temperature drop, the furnace igniter

ignited the chamber atmosphere for cells that did not ignite the

chamber atmosphere.

4.2.3. MP cells, plastic wedge, slower crush

The next series of ten tests with the Saft MP cells and plastic

wedge used a slower crush speed of approximately 0.43 mm/s (avg.

of crush speeds in Table 4.). The cells were placed within 6.5% CH4-

air at 40 C. The crush was again applied to the wide side of the cells

with the wedge tip parallel to the top of the cells (Fig. 5). Cell

penetration was limited to approximately 66%. The cells hardshorted to a negative voltage level in all ten tests. The wedge tip

penetrated the cell can for all tests. Evidence of thermal runaway

was observed in six of the ten tests, with cell can temperatures

exceeding 372 C and chamber overpressures exceeding the

selected ignition criteria of 50 kPa (7.25 psi). Three of these thermal

runaway events produced chamber overpressures ranging from

655 to 676 kPa. Three other thermal runaway events produced

lower overpressures ranging from 65 to 72 kPa. The high-speed

video recordings for the low overpressure thermal runaway

events were obscured by smoke, but a bright flash was discernible

through the smoke. The furnace igniter was unable to ignite the

chamber atmosphere following the three lower overpressure

thermal runaway events, indicating that there was no longer a

flammable atmosphere within the chamber. The lower and higher

overpressure thermal runaway events produced similar peak cell

can temperatures.

Four of the slower crush tests produced no evidence of thermal

runaway, with peak temperatures of <104 C and chamber overpressures of <2 kPa. The high-speed video recordings were

obscured by smoke without the bright flash observed for the six

thermal runaway events. After a 5-min wait period accompanied by

a significant temperature drop, the furnace igniter ignited the

chamber atmosphere for these four tests, producing overpressures

of 579e655 kPa (Table 4).

4.3. M52EX primary LiMnO2 cells

Researchers next conducted a series of plastic wedge crush tests

using the M52EX cells placed within 6.5% CH4-air at 40 C. The

crush speed was approximately 0.6 mm/s and cell penetration was

limited to approximately 66%. A slower crush speed was selected

for this series because slower crush speeds produced twice as many

ignitions as the faster crush speeds with the MP cells. The slower

crush induced deeper voltage drops as well. The series was stopped

after observing a fourth ignition in six tests per the testing protocol

Table 3

Summary data, MP cells, plastic wedge, 6 mm/s crush speed, 40 C, 6.5% CH4-air

ignitions.

Cylinder force at short circuit kN (lbf) 6.72 to 7.65 (1510e1720)

Crush speed avg. (range) mm/s 6.0 (4.8e7.3)

Chamber overpressure kPa (psi) 648 to 696 (94e101)

Peak cell can temperatures C 327 to 510

Table 4

Summary data for MP cells crushed by the plastic wedge, slower crush speeds.

Higher pressure Lower pressure No thermal

thermal runaway thermal runaway runaway

Cylinder force at short 6.54 to 7.30 (1470

e1640)

7.03 to 7.87 (1580

e1770)

6.72 to 7.78

circuit kN (lbf) (1510e1750)

Crush speed avg. 0.33 (0.2e0.46) 0.49 (0.44e0.58) 0.46 (0.3

(range) mm/s e0.66)

Chamber 655 to 676 (95e98) 65 to 72 (9.4e10.4) <2 (0.3)

overpressure kPa

(psi)

Furnace igniter e e 579 to 655 (84

overpressure kPa e95)

(psi)

Peak cell can 390 to 427 372 to 423 102 to 104

temperatures C

Fig. 5. An MP cell crushed by the plastic wedge at a slower speed that produced a

lower overpressure thermal event. The cell swelled significantly.

(Dubaniewicz and DuCarme, 2013). The cells hard-shorted to a

voltage of <0.9 V for all tests. High-speed video showed fluid

flowing from the negative terminal vent after internal shorting for

all tests, and prior to ignition for tests that produced ignitions.

Flame emanated from the negative terminal vent for one chamber

ignition. Flames and sparks emanated from elsewhere through the

cell can for the other three chamber ignitions. In one test the cell

continued to emit flames and sparks through holes burned through

the cell can (Fig. 6) after the chamber ignition, suggesting the cell

was supplying its own oxygen source, as the chamber oxygen was

mostly consumed by the chamber ignition. Another chamber

ignition produced a relatively high 608 C peak temperature with a

745 kPa chamber overpressure (Table 5). The cell contents were

expelled during this test, likely allowing flame from the chamber

ignition to heat the inner metal can surface in contact with the

thermocouple.

5. Discussion

Mikolajczak et al, (2011) reported investigating hundreds of

thermal runaway events from the field. These included numerous

field failures caused by latent mechanical damage, particularly of

soft pouch cells where mild mechanical damage did not cause
immediate failure, but rather the cells failed during subsequent

cycling. With numerous new reports of Li-ion field failures, UL

emphasizes the need to update existing standards and create new

ones as our information and knowledge of potential Li-ion battery

hazards increase. Taken together, the new IEC 62133 edition 2 crush

test provisions suggest that edition 2 provides less stringent crush

hazard evaluation criteria than found in the previous edition. Flat

plate tests with the prismatic cell reported here and with cylindrical cells reported previously (Dubaniewicz and DuCarme, 2013)

produced no adverse results. Similar cells readily ignited the

chamber atmosphere with the plastic wedge crush, using a fraction

of the 13 kN force specified for the flat plate test. Field events and

laboratory research suggest a need for a more invasive (e.g. wedge)

crush test for cell level safety evaluations.

The fire triangle provides a simple model for describing thermal

runaway events (Fig. 7). Li-ion cell elements that may contribute to

thermal runaway are shown on the corresponding leg of the triangle. The three legs of the triangle are isolated by a thin separator

within spiral-wound constructed cells. Spiral-wound cells also

feature high current discharge, enhancing heat generation from

internal short circuits. A Li-ion cell thermal runaway event may be

prevented by removing any leg of the triangle.

The M1A cells crushed while under charge produced no chamber ignitions. A delithiated LiFePO4 cathode produces relatively

little O2 upon thermal decomposition (Ong et al., 2010) (Zaghib

et al., 2012), and thus inhibits the development of thermal

runaway inside of the cell by weakening the oxidizer leg of the

triangle. Other potential failure modes should be considered, as

appropriate. Thermal events involving large format LiFePO4 batteries have been reported (Wang et al., 2012). Larger capacity LiFePO4

cells may produce correspondingly more O2 and higher internal

short circuit currents than smaller capacity cells that might favor an

internal thermal reaction. As another example of a potential failure

mode, a cell breach may expose electrolyte fuel vapors to oxidizing

air, thus creating a flammable fuel-air mixture. Such a flammable

mixture may be ignited by a competent ignition source. The ANSI/

ISA 60079 series of explosion protection standards anticipate

equipment exposure to flammable atmospheres, and provide

design requirements that may serve to minimize the risk of ignition

under a number of foreseeable fault conditions external to a

LiFePO4 cell. A multi-phase safety approach for large capacity Li-ion

cells conceivably could provide a more robust cell-level safety solution. A weakly oxidizing cathode, flame retardant electrolyte and

anode, and shutdown separator combined would help to minimize

and isolate two legs of the triangle other than the heat source

represented by the large electrical capacity.

Tests with the MP cells produced two types of thermal events

characterized by higher or lower chamber overpressures. The lower

overpressure events produced an inert atmosphere as verified by

the furnace igniter. The lower overpressure events likely involved

cell thermal runaway with vent gases of sufficient volume to inert

the chamber atmosphere as described by Roth and Orendorff

(2012) and Roth (2008). All thermal events produced peak cell

can temperatures exceeding 327 C and met the ignition criteria of

a pressure rise of at least 50 kPa (7.25 psi).

Researchers used the Fisher's exact test to assess the potential

influence of selected crush speeds (6.0 vs. 0.43 mm/s) on ignitions

with the MP cells. The Fisher's exact test is used when data are

analyzed in the form of a 2  2 contingency table and the expected

value of at least one cell in the table is <5. The null hypothesis was

that crushed cell samples using two different speeds were equally

likely to ignite under otherwise similar test conditions. The selected

alternative hypothesis was that the slower crush speeds were more

likely to produce ignitions than the faster crush speeds, indicating

the use of a one-tailed probability test. An SPSS statistical package

computed Fisher's exact test p values using data listed in Table 6.

The one-tailed p value for Table 6 data is 0.1849. Although twice as

many thermal events were observed for the slower crush speeds,

the influence of crush speed is not considered to be statistically

significant for these tests. A wider variation in crush speeds

conceivably may produce more significant results. The selected

slower crush speeds were near the limit of the testing equipment.

M52EX cell intrinsic safety evaluations included a set of external

short circuit tests (DEKRA, 2011). Eleven cells were subjected to

external short circuit at various ambient temperatures ranging

from 25 C to þ70 C. The external short circuits produced

maximum short circuit currents ranging from 30 to 70 A, with

maximum cell temperatures ranging from 98 C to 113 C. No

visible changes to the cells were reported (no “bombage,” bursting,

damage, or leakage). By comparison, the plastic wedge produced

short circuits within the M52EX cells, which then frequently

ignited the chamber atmosphere.

Common Li-ion cell separators are less than 30 mm thick

(Baldwin, 2009) (Orendorff, 2012). These separators are too thin to

meet intrinsic safety standards provisions for distance through

solid insulation (MSHA, 2008) (ANSI/ISA, 2012). Results reported

here demonstrate that hard internal short circuits may impair

intrinsic safety, and suggest that a hard internal short circuit within

these spiral-wound Li-ion cells should be considered for intrinsic

safety evaluation purposes, even as a non-countable fault. Results

suggest that at least 4.8 Ah capacity for spiral wound LiCoO2 cells

and 5.1 Ah capacity for spiral wound LiMnO2 cells may pose a

thermal runaway hazard, even with a shutdown separator.

Comparing the M52EX results with those from the DEKRA report

indicate that external short circuits tests are not an adequate surrogate for evaluating internal short circuit hazards. Some steps are

being taken to help address Li-ion thermal runaway hazards in

explosion protection equipment design standards. The updated

ANSI/ISA 60079-0 edition 6 indicates that the use of spiral-wound,

lithiumecobalt-oxide cells is not recommended due to potential

thermal runaway hazards resulting from internal short circuits.

6. Conclusions

A review of the safety research literature, field events, and

laboratory testing suggest the need for a more invasive (e.g.,

wedge) crush test for cell level safety evaluations.

The plastic wedge crush was a more severe test than the flat

plate crush with a prismatic format LiCoO2 cell. The plastic wedge

produced deeper penetration and lower impedance (hard) shorting

while using a fraction of the applied force of the flat plates.

Testing reported here indicates that Saft MP 174565 and Saft

FRIWO M52EX cells pose a CH4-air ignition hazard from internal

Fig. 6. An M52EX cell crushed by the plastic wedge that ignited the chamber atmosphere with two holes burned through the side of the cell can.
Table 5

Summary data, M52EX cells, plastic wedge, 40 C, 6.5% CH4-air ignitions.

Cylinder force at short circuit kN (lbf) 3.49 to 4.27 (784e960)

Crush speed avg. (range) mm/s 0.58 (0.52e0.69)

Chamber overpressure kPa (psi) 421 to 745 (61e108)

Peak cell can temperatures C 340 to 608

Fig. 7. Fire triangle analogy for Li-ion cell thermal runaway.

Table 6

Fisher's exact test comparison of crush speed influence on ignition using the MP

cells.

MP cells Ignitions No ignitions Totals

Slower crush 6 4 10

Faster crush 3 7 10

Totals 9 11 20

short circuit caused by crushing damage, even with shutdown

separators.

Under specified test conditions, A123 systems ANR26650M1A

LiFePO4 cells produced no chamber ignitions while under a charge

of up to 5 A. These tests simulated a potential CH4 ignition hazard

involving underground stationary Li-ion battery powered equipment that may short circuit internally under normal charging

conditions. Other potential failure modes should be considered, as

appropriate.

The influence of crush speed on ignitions under specified test

conditions was not statistically significant. A wider variation in

crush speeds conceivably may produce more significant results. The

selected slower crush speeds were near the limit of the testing

equipment.

Common spiral-wound cell separators are too thin to meet

intrinsic safety standards provisions for distance through solid

insulation. Results with selected LiCoO2 and LiMnO2 spiral-wound

cells demonstrate that hard internal short circuits may impair

intrinsic safety as described within the ACRI2001 and ANSI/ISA

60079-11 standards, and suggest that a hard internal short circuit

within these spiral-wound Li-ion and lithium cells should be

considered for intrinsic safety evaluation purposes, even as a noncountable fault.

The literature indicates that spiral-wound LiCoO2 cells may

produce significant amounts of oxygen and high pressures during

thermal decomposition, suggesting that these cells may be

considered as potentially strong oxidizers and exothermic chemical

reaction ignition sources for ANSI/ISA 60079-0 purposes. Observed

flames from a LiMnO2 spiral-wound cell after a chamber ignition

within an inert atmosphere indicate a sustained exothermic

chemical reaction in this cell as well. LiFePO4 cells may offer a safer

alternative due to limited oxygen release upon thermal

decomposition.

Acknowledgments

The authors thank Diane Schwegler-Berry (NIOSH HELD) for the

SEM measurements, Bruce Whisner (NIOSH OMSHR) for data

acquisition software support, and Justin Srednicki (NIOSH OMSHR)

and Steven Metz for ignition testing support.

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