Are Lithium Ion Cells Intrinsically Safe?

Thomas H. Dubaniewicz, Jr. and Joseph P. DuCarme

The authors are with the Office of Mine Safety and Health Research,

National Institute for Occupational Safety and Health, Pittsburgh, PA 15236

USA (e-mail: tcd5@cdc.gov; jld6@cdc.gov; Joseph.DuCarme@cdc.hhs.gov).

Abstract—National Institute for Occupational Safety and

Health researchers are studying the potential for Li-ion-battery

thermal runaway from an internal short circuit in equipment

approved as permissible for use in underground coal mines. Researchers used a plastic wedge to induce internal short circuits

for thermal runaway susceptibility evaluation purposes, which

proved to be a more severe test than the flat plate method for

selected Li-ion cells. Researchers conducted cell crush tests within

a 20-L chamber filled with 6.5% CH4–air to simulate the mining

hazard. Results indicate that LG Chem ICR18650S2 LiCoO2 cells

pose a CH4 explosion hazard from a cell internal short circuit.

Under specified test conditions, A123 Systems 26650 LiFePO4 cells

were safer than the LG Chem ICR18650S2 LiCoO2 cells at a

conservative statistical significance level.

I. INTRODUCTION

(

GAS OR DUST explosions are some of the greatest hazards faced by underground coal mine workers. Methane

CH4) gas is released during the mining process and accumulates in areas that are not well ventilated. Coal dust accumulations can form explosive dust clouds when entrained into the

air. CH4 ignitions or explosives can disperse coal dust layers

into the atmosphere that subsequently ignite and propagate as

powerful explosions. Although fatal coal mine explosions in the

U.S. may be less frequent in modern times, they can account for

large numbers of deaths, as sadly evidenced by the recent Upper

Big Branch disaster, with 29 killed [1].

The mining equipment used in certain locations of underground coal mines, where there is a normal risk of exposure

to CH4, must be approved as permissible by the Mine Safety

and Health Administration (MSHA). The term “permissible”

refers to equipment that meets specifications for construction

and maintenance, ensuring that such equipment will not cause

a mine explosion or fire (30 CFR 75.2) [2]. Permissible equipment may be evaluated as intrinsically safe (IS). The 30 CFR

18.2 interprets IS as incapable of releasing enough electrical

or thermal energy under normal or abnormal conditions to

cause ignition of CH4 or natural gas and air of the most easily

ignitable composition.

An MSHA-permissible cap lamp powered by a lithium ion

(Li-ion) battery pack ignited a fire at the National Institute

for Occupational Safety and Health (NIOSH) Pittsburgh Safety

Research Coal Mine (SRCM) office building [3]. The newly

purchased cap lamp was placed on a wooden pallet and caught

fire overnight while on charge. The melted red plastic from

the cap lamp battery enclosure was found spattered across the

office, indicative of an explosive thermal runaway event. There

was no indication of user abuse that might have contributed

to the event. A definitive failure mechanism causing the fire

was not determined. Concerns over fire and explosion hazards prompted MSHA to issue a program information bulletin

regarding special precautions that should be observed when

charging Li-ion batteries or equipment containing Li-ion batteries [4].

In recent years, NIOSH has been engaged in new technology

development of mine communications and tracking equipment

(CTE) to satisfy mandates of the 2006 Mine Improvement

and New Emergency Response (MINER) Act. A consequence

of the MINER Act is the proliferation of backup batteries

in underground coal mines to power stationary CTE during

mine emergencies that may involve ventilation disruption and

CH4 buildup. An additional fire or explosion hazard involves

underground stationary CTE that is on charge and unattended

for long periods of time and may be exposed to CH4 and/or coal

dust under normal operation. Stationary battery-powered CTE

in coal mines may also be susceptible to mechanical damage,

such as from roof falls. The 2001 explosion disaster at the Jim

Walter Resources #5 mine involved a battery damaged by a roof

fall [5].

Most recently, the MSHA Approval and Certification Center

is investigating a Li-ion-powered cap lamp that caught fire in

an underground coal mine. A written report of the incident was

not publically available as of this writing.

NIOSH researchers are studying Li-ion-battery thermal runaway potential of MSHA-permissible equipment in order to

develop safety recommendations for underground coal mine

applications. The study includes an experimental evaluation

of potential thermal runaway initiating events of cells placed

within CH4–air atmospheres to simulate the mining 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. Researchers conceived

a new method of inducing an internal short circuit for thermal

runaway susceptibility evaluation purposes that was thought to

overcome limitations of two other commonly used methods.

This work summarizes the literature findings and experimental
observations from the first phase of the study, along with

interim recommendations.1

1The findings and conclusions in this paper are those of the authors and do

not necessarily represent the views of the NIOSH.

II. LITERATURE REVIEW

QinetiQ North America (QNA) [6] and Roth et al. [7]

describe a three-stage thermal runaway process observed for

certain Li-ion-cell designs thought to achieve high levels of

performance and safety. These high energy density devices

contained electrodes separated by a polymer of perhaps a few

tens of micrometers thick, immersed in a flammable organic

electrolyte. Cathode and anode active materials begin to react

exothermically with the electrolyte above a certain threshold

temperature. If the heat is not dissipated, the temperature continues to rise above a certain onset temperature when the cell

enters stage two, characterized by rapid heat release. Venting

and release of smoke may occur during stage two. Additional

heating causes the cell to enter stage three, in which highrate cathode reactions cause the temperature to rise rapidly and

flame or explosion to occur. This final stage is thermal runaway,

which is characterized by a self-heating rate of 10 ◦C/min

or more.

Standard UL 1642 [8] provides requirements for the

construction, performance, testing, and marking of primary

(nonrechargeable) and secondary (rechargeable) batteries containing lithium in various forms (metallic, alloy, or ion). Test

evaluations include electrical, mechanical, and environmental

tests. Mechanical integrity evaluations include a crush test,

where samples of batteries are squeezed between two flat

surfaces until 13 kN (3000 lb) is reached, at which point the

force is released. Battery safety standards IEC 62133 and UL

2054 specify a similar crush test [9]. The criterion for passing

the UL 1642 crush test is that the samples shall not explode

or catch fire. The UL 1642 interpretation of explosion is when

the cell or battery contents are forcibly expelled and the cell or

battery is torn or split into two or more pieces.

There have been numerous reported field failures of Li-ionbattery-powered equipment. NIOSH previously contracted with

QNA to perform a safety assessment of emergency backup

batteries and battery charging systems for underground mining

applications [6]. With regard to the Li-ion-battery thermal

runaway hazard, QNA concluded from the Consumer Product Safety Commission recall data that more is required for

acceptable safety than UL testing provides. To help address

the mining safety issue, QNA discussed the merits of experimental research to crush or puncture Li-ion cells within

explosive CH4–air atmospheres. Mikolajczak et al. [9] 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.

Tobishima [10] reviewed Li-ion-cell thermal runaway events,

safety evaluation methods, and materials to improve thermal

stability. When describing the UL 1642 crush test, Tobishima

recommended an alternative test that was asserted to be harder

to pass. One key feature of the alternative test was crushing

the cell to less than half of its original thickness. The UL 1642

crush test does not specify a depth of crush.

Tobishima [10] described a nail penetration test, indicating

that it was a very important test for simulating internal short

circuits linked to many actual accidents. However, some studies [9], [11]–[14] suggest that the nail test’s ability to create

thermal runaway from internal short may be defeated by the

nail itself. The described weaknesses of the nail penetration

test include thermal or electrical energy dissipation through the

metallic nail alone or through the metallic nail in contact with

an exterior metallic cell can. Loud et al. [14] observed that a nail

penetration test allowed gas and electrolyte to exit the cell at the

point of penetration. Furthermore, UL 1642 does not prescribe

a nail penetration test.

Megerle et al. [15] demonstrated thermal runaway using an

aggressive iterative crush protocol. They observed that crush

tests inducing case cracking generally produced less severe

results than tests without case cracking.

Mikolajczak et al. [9] 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 the findings in [13]. Mikolajczak et al. [9] provided two

explanations for the ease of producing thermal runaway from

an edge crush, one relating to the inability to penetrate the

separator in the case of the conventional crush test and the other

to electrical or thermal energy dissipation through a nail in the

case of perpendicular metallic nail penetration. For cylindrical

cells, the UL 1642 crush test is applied with the cell’s longitudinal axis parallel to the flat plates only. The direction of

force is perpendicular to the electrode surface within the cylindrical cell.

An apparently common safety misconception involves the

crush test and internal shorting. Arora et al. [16] indicate that

the standard cell crush test involves applying a force to the cell’s

enclosure until an internal short circuit is achieved. However,

the UL 1642 crush test is stopped as soon as a predetermined

force is applied without regard to achieving internal shorting.

Mikolajczak et al. [9] found that, when crush damage is perpendicular to the cell, it may not cause penetration of the separator,

and thus, minimal or no internal shorting occurs.

UL 1642 [17] and IEC 62133 standard committees are considering a forced internal short-circuit (FISC) test for inclusion

in newer editions of UL 1642 and IEC 62133. The FISC test

being considered for IEC 62133 [18] is based on FISC methods

described in [19]. A nickel particle is inserted inside the cell,

and a press machine is used to compress the cell in an attempt

to force the nickel particle to create an internal short circuit. The

FISC test is stopped after a period of time once a voltage drop of

at least 50 mV occurs, which is defined as attaining an internal

short circuit. The FISC test is also stopped if the press force

reaches 800 N for cylindrical cells or 400 N for prismatic cells,

without creating an internal short. The acceptance criterion is

no fire. Thus, the proposed FISC test procedures suggest that

a cell may pass the test without exhibiting any indication of

an internal short (press force limited) or by exhibiting limited

shorting indicated by a voltage drop as small as 50 mV.
Balakrishnan et al. [20] reviewed safety mechanisms in Liion batteries. One such mechanism is a pressure relief vent often

built into the cell. Under severe mechanical and electrical abuse

conditions, the vent may provide a safe means of releasing

internal pressure before the cell reaches excessively high temperatures. However, Roth et al. [7] described a thermal runaway

process where cell venting may be followed by explosive

decomposition. Because of the uncertainty surrounding the cell

vent effectiveness, researchers used high-speed video recordings in the NIOSH research study to observe the effectiveness

of the cell venting process as the cell was crushed.

Thermal runaway susceptibility is dependent upon the cell

state of charge (SOC) and capacity. Cai et al. [12] reported

on simulations and experiments of internal short circuits in Liion and Li-ion–polymer cells induced by “pinching” the cell

between two metallic spheres. They found that the internal

short-circuit risk of thermal runaway increased with cell capacity and SOC for three types of prismatic form factor cells

studied. White et al. [21] conducted accelerating-rate calorimetry (ARC) tests of selected 18650-type cells and observed that

the onset temperature of a 2.1-Ah LiCoO2 cell at 82% SOC

was comparable to that of a 1.1-Ah LiFePO4 cell at full SOC.

They concluded that self-heating onset temperature and selfheating rate were functions of cell energy. In order to factor

out these SOC and capacity dependences, in the current study,

researchers tested two cell designs of similar capacity, fully

charged.

Several studies [9], [22] indicate that preheated cells are more

susceptible to thermal runaway. Santhanagopalan et al. [22]

developed an electrochemical thermal model and conducted

experiments of Li-ion-cell internal short circuits. The thermal

model indicates that a cell passing a short-circuit test at room

temperature has a greater tendency toward runaway for higher

values of the initial cell temperature. They reported that the

margin for safety is reduced in a preheated cell. A short between

a lithiated (charged) anode material and an aluminum current

collector produced the most heat of the short-circuit scenarios

studied. Mikolajczak et al. [9] explain that high ambient temperatures or adiabatic insulation will increase the likelihood that

any given internal fault can drive a cell to thermal runaway and

thus increase the energy available to heat the cell. IEC 60079-

11 [23] and ACRI2001 [24] specify a maximum ambient temperature of 40 ◦C for IS evaluation purposes, unless otherwise

specified. Thus, 40 ◦C may be considered within the specified

ambient temperature range for “normal” operation of most IS

equipment. 40 ◦C is below the Li-ion-cell self-heating onset

temperatures [6], [7]. In contrast, the UL 1642 crush test is

conducted at room temperature.

Some newer Li-ion chemistries are proving to be safer than

others. QNA [6] reviewed ARC data for several different Liion chemistries, including the data published by Roth [25].

LiMn2O4 and LiFePO4 chemistries displayed higher onset temperatures than several others studied, showing that they were

more resistant to thermal abuse. The peak self-heating rate was

the lowest for the LiFePO4 cell by a wide margin, leading QNA

to conclude that LiFePO4 cells were the safest on the market.

A common LiCoO2 cell chemistry was the least stable of those

studied. QNA went on to recommend that either LiMn2O4 or

LiFePO4 may be sufficiently safe for underground coal mining

applications. Chen and Richardson [26] describe the thermal

stability of LiFePO4 and its charged counterpart, FePO4, as

“remarkable” and instrumental in its commercialization as a Liion-battery cathode material. Hund and Ingersoll [27] reported

no sparks or fire from an overvoltage charge abuse test of a

LiFePO4 cell. Scrosati and Garche [28] suggested that LiFePO4

may be the cathode chemistry of choice for some hybrid

electric vehicle projects, for safety reasons. Based on these

findings about potential safety benefits of LiFePO4 chemistries,

researchers included a commercial LiFePO4 cell in the NIOSH

research study for evaluation as a potentially safer cell for

powering IS mining equipment.

Magison [29] describes IS equipment and wiring as not capable of releasing sufficient electrical or thermal energy under

normal or abnormal conditions to cause ignition of a specific

hazardous atmospheric mixture in its most easily ignited concentration. Hazardous atmospheres in underground coal mines

include CH4–air mixtures and coal dust. MSHA requires [30]

cap lamp components to comply with ACRI2001 [24] criteria

for the evaluation and test of IS equipment. ACRI2001 contains

provisions for lithium batteries, including a requirement that

lithium batteries shall not explode or cause a fire when tested

as per UL 1642. MSHA relies on UL 1642 cell level evaluations to help ensure intrinsic safety in potentially explosive

atmospheres.

Somewhat similar criteria for IS equipment used in other

U.S. industries are found in standard ANSI/ISA 60079-11.

This is the U.S. adopted version of the IEC 60079-11 [23].

IEC 60079-11 recognizes that some lithium types of cells or

batteries may explode. The standard notes that “Cells that

comply with the requirements of UL 1642 or IEC 62133 or

other relevant safety standards are considered to meet this

requirement.” IEC 60079-11 permits battery packs containing

serial and parallel configurations of Li-ion cells. All of these IS

standards, national or international, for all applicable industries,

rely on other cell level safety standards for ensuring intrinsic

safety of lithium-battery-powered equipment used in potentially explosive environments. The IEC Technical Committee

31 has established a working group to address the safety of

electrochemical cells and batteries in equipment for explosive

atmospheres.

Several studies [31], [32] found that Li-ion cells can produce

high explosion pressures. Over concerns regarding laboratory

equipment damage and injury to staff, Yen et al. [31] studied explosive failures of Li-ion cells that may occur when subjected to

typical safety standards abuse tests. The concern was prompted

by a chemical hood that was destroyed by the blast pressure

of failed cells. They conducted an overtemperature abuse test

of a 2.6-Ah Li-ion cell and measured 10.89 MPa (1580 psi)

gauge pressure in an adiabatic calorimeter as the cell temperature reached approximately 200 ◦C. Although not specified in

the paper, a coauthor indicated that the chamber volume was

17 mL. Jhu et al. [32] also studied the explosive failure of Liion cells placed within an adiabatic calorimeter. A fully charged

2.6-Ah LiCoO2 18650 cell initiated self-heating at 129.1 ◦C,

began thermal runaway at 160 ◦C, and peaked at 903 ◦C.

Chamber gauge pressure peaked at 10.797 MPa (1565.9 psi),
Fig. 1. Drawing of plastic wedge crush fixture with LG Chem cell.

which destroyed their chamber. Subsequent tests used a

150-mL expansion chamber to reduce peak pressures to protect

the equipment and laboratory personnel. The 150-mL expansion chamber reduced the peak gauge pressure to 2446 kPa

(354.7 psi) during a test similar to the test that produced the

10.797-MPa gauge pressure. Jhu et al. [32] concluded that an

internal short circuit could lead to violent fire and explosion.

With these safety concerns in mind, in the current study, researchers conducted cell crush tests within a 20-L chamber with

no CH4 and measured chamber pressures to assess the potential

for personal injury.

III. METHODS

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. 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 over

metal to minimize electrical and thermal energy dissipation by

the test fixture itself. The wedge shape and point angle were

selected because they were thought to be robust enough to crush

the cell without significant damage to the plastic wedge itself.

A plastic fixture simulates external forces applied to plasticwrapped or plastic-encased battery packs which are common

[9]. 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.

Researchers designed and fabricated a custom crush fixture

to fit inside a 20-L chamber for these experiments. The press

uses a small single-acting hydraulic cylinder with a 43-mm

bore and a 54-mm stroke. The cylinder incorporates an internal

return spring; this retracts the cylinder ram when hydraulic fluid

is released. A cutaway view of the press is shown in Fig. 1.

The top and bottom plates are connected with a steel tube (not

shown for clarity). This tube has access holes for installing

the battery and Delrin [33] plastic wedge. The wedge is made

from a 12.7-mm-thick sheet of Delrin by machining a 45◦ bevel

on each side to create a 90◦ pointed edge. A commercially

available 0.75-kW hydraulic power unit supplies the required

flow. The system relief valve is set to produce 13 kN of force

at the cylinder ram. The press is controlled from a solenoidoperated hydraulic four-way valve. A bleed-in flow control is

used to regulate the speed of the ram and was set to about

TABLE I

DELRIN PROPERTIES AND CHARACTERISTICS

Property Value

Yield Stress, Mpa 71

Tensile Strength, Mpa 96

Shear Strenght, Mpa 62

Tensile Modulus, Mpa 3000

Surface Resistivity, Ω >1E15

Thermal conductivity, W/m-K 0.33

Hardness, Rockwell M scale 92

Melt temperature, oC >200

Fig. 2. Drawing of flat plate crush fixture with LG Chem cell.

5 mm/s. Viatran model 248 pressure transducer measurements

are used to calculate cylinder force.

The wedge and wedge holder were fabricated from Delrin

[33]. This material was selected for its mechanical, thermal

insulating, and electrical resistance properties as well as its

ability to be readily machined. This plastic has high mechanical

strength and rigidity, excellent resistance to solvents and chemicals, and good electrical and thermal insulating characteristics.

The material’s properties are listed in Table I. The wedge is

considered a sacrificial element, and a new one is installed for

each test.

For all tests, the bottom platen was fabricated from steel and

coated with an insulating paint. Its thickness was calculated to

position the battery so that, at full ram extension, the Delrin

wedge would penetrate about 66% into the battery’s diameter.

Penetration was greater than 50%, as recommended by Tobishima [10].

A ram extension with a 13-mm-thick flat plate was fabricated

from steel and used to perform tests that simulated the UL 1642

crush test Fig. 2. The plate was also coated with an insulating

paint. The extension length and bottom platen height were

selected to allow only a slight clearance between them at full

ram extension.

An IEC Ex certificate [34] for the manufacturer and model

of cap lamp that caught fire at the NIOSH SRCM office [3]

listed an LG Chem ICR18650S2 or a Sanyo UR 18650F cell

as approved for use with the cap lamp. Samples of LG Chem

model ICR18650S2, cylindrical 18650 format LiCoO2 cells

(LG Chem cells) were purchased through a battery distributor.

Codes on the LG Chem cells indicated that they were manufactured seven years prior to these ignition tests. A specification

sheet for the LG Chem cells [35] describes safety tests that the

cell should pass, including a flat plate crush as per UL 1642
and a nail penetration test. These tests were specified at room 2

temperature. A123 Systems2

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

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 26650A cylindrical 26650

format LiFePO4 cells (A123 cells) [36] with similar rated

capacity were selected for comparative testing and purchased

directly from the manufacturer. A123 Systems indicated that

the A123 cells were compliant with all UL 1642 single-cell tests

and were manufactured four years prior to these ignition tests.

Newer versions of the LG Chem 18650 and A123 26650 cells

have higher rated capacity.

Ignition experiments were conducted in a 20-L test chamber,

which can be used at initial pressures that are below, at, or

above atmospheric as long as the maximum explosion pressure

is less than 2100 kPa (305 psi). The CH4–air concentration

was set to 6.5% by volume as per IEC 60079-11 provisions

for testing potential thermal ignition sources in mines. 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%. The air source

was dried and hydrocarbon free with 20.9% O2 by volume.

Subsequent tests at 40 ◦C raised the chamber pressure slightly

above atmospheric, and researchers bled off a small amount of

the gas–air mixture to maintain 100 kPa after the temperature

stabilized. Heat tape was wrapped around the exterior of the

chamber. A fan within the chamber provided mixing. A furnace heating element placed inside the chamber was used to

ignite the atmosphere after tests that resulted in nonignitions,

confirming that a flammable atmosphere was present.

Researchers conducted cell crush tests with no CH4 in the

20-L chamber and measured chamber pressures to assess the

potential for personal injury [31], [32]. The open head space

volume inside the chamber was estimated to be 18 L, taking

into account the Li-ion cell, press, fan, and white LED fixture.

Chamber instrumentation included a gas pressure transducer

(Viatran model 218-28) and a 30-AWG 0.25-mm-diameter

(0.01-in-diameter) K-type thermocouple attached to the surface

of the Li-ion cell. IEC 60079-0 [37] considers surface temperature of equipment under test as stabilized when the rate

of temperature rise falls to 2 K/h. Researchers waited at least

1 h after the cell surface temperature reached 38 ◦C before

conducting ignition tests at 40 ◦C.

The thermocouple was inserted underneath plastic wrapping

around the LG Chem cell, against the metallic can that is

about 1.3 cm (0.5 in) from the cell end opposite the vent.

Cardboard insulation around the A123 cell was removed, and

the thermocouple was taped against the metallic can that is

about 1.3 cm from the cell end opposite the vent.

The chamber pressure transducer detected ignitions in conjunction with a high-speed video camera. The criterion for

ignition was a pressure rise of at least 50 kPa (7.25 psi), a

value derived from previous explosibility research [38], [39].

Sapphire windows allowed viewing inside the chamber. A highspeed video camera (NAC model 512SC) recorded tests at

50 frames/s. A white LED fixture was placed within the

chamber to provide illumination for video recording. The cell

venting observed prior to ignitions in this study indicated that

the cell vent was not an effective protection. Ignitions saturated (whited-out) the video recordings, whereas nonignitions

did not.

A personal-computer-based Labview data acquisition program recorded cell voltages, pressure transducer signals, and

thermocouple measurements. Data acquisition was initiated by

a signal from the hydraulic-solenoid-operated four-way valve.

An independent laboratory calibrated in-house multimeters

traceable to the National Institute of Standards and Technology.

Thermocouple measurements were checked in-house using a

thermocouple calibration cell.

A Vencon UBA5 Battery Analyzer preconditioned and analyzed cells for testing. A software package provided user

configurable routines for constant current, for constant-voltage

cycling, and for measuring resistance and capacity. The resistance routine uses a two-step dc measurement.

Researchers used Fisher’s exact test [40] to assess the relative safety between two cell designs. The null hypothesis

was that crushed samples of two different Li-ion-cell designs

were equally likely to ignite under similar test conditions. A

maximum of ten tests were planned for a particular cell design.

Only a cell design that produced zero ignition in ten attempts

was to be considered as a safer cell candidate. The alternative

hypothesis under this scenario was that the alternative cell

design was more likely to ignite than the safer cell candidate.

There was no possibility that the alternative cell design would

be observed to be less likely to ignite, indicating the use of a

one-tailed probability test. This implied that at least four ignitions in ten attempts with another cell design would be needed

to produce a conservative one-tailed significance value of p <

0.05, i.e., a less than 0.05 probability that the observed ignitions

and nonignitions came from the same underlying population,

thus rejecting the null hypothesis. The cell designs could then

be considered independent from an ignition standpoint, under

specified test conditions, at a conservative significance level. A

test series with a particular cell design was ended when either

four ignitions were observed or a total of ten cells were tested.

A series was stopped after four observed ignitions to save

wear and tear on the chamber instrumentation. An IBM SPSS

statistical package computed Fisher’s exact test p values using

ignition and nonignition data listed in a two-by-two table such

as that shown in Table V. The test assumes random sampling;

a random sequence generator provided a random set of serial

numbers assigned to cell samples.

IV. RESULTS

A. Cell Conditioning

Cells were conditioned within the manufacturer’s recommendations [35], [36] at room temperature (Table II). Cells

were cycled through at least three charge–discharge cycles

using a constant-current constant-voltage routine. Measured

discharge capacity was slightly below the rated, and resistance

measurements were slightly above the rated. A123 Systems
TABLE II

CELL CONDITIONING SUMMARY DATA

LG Chem A123

Charge, discharge voltage 4.2, 3.0 3.8, 2.0

Charge or discharge constant current (A) 1.075 3.0

End charge current (mA) 50 50

Rated capacity (Ahr) 2.2 2.2

Measured discharge capacity (% rated) >95% >96%

Rated impedance @ 1 kHz (mΩ) 90 8

Measured resistance range (mΩ) 113 to 135 25 to 43

Cell age when tested (years) 7 4

TABLE III

FLAT PLATES VERSUS PLASTIC WEDGE, 6.5% CH4–AIR IGNITION

TESTS, LG CHEM CELLS

Ignitions Totals 

Plastic wedge, 40 oC 4 3 7
�

Flat plates, 25 oC 0 10 10
�

Totals 4 13 17
�

No ignitions

Fig. 3. Flat plate compressed the LG Chem cells about 18% without igniting

6.5% CH4–air.

indicates that Li-ion cells may show certain shelf life or

calendar-related aging effects where impedance grows with

time and capacity is lost over time. In addition, internal resistance and capacity measurement protocols may influence

results. The capacities of the two cell designs were similar

and greater than 95% of the rated (MSHA recommends battery

replacement when capacity fades to 80% of the rated [4]). The

cells were given a final top-off charge prior to ignition tests.

B. Plastic Wedge Versus Flat Plate Methods

The plastic wedge and UL simulated flat plate methods were

compared in CH4–air ignition tests using the LG Chem cells.

The cell was maintained at room temperature for the flat plate

tests as per UL 1642 and at 40 ◦C for the plastic wedge tests as

per IEC 60079-0 and ACRI2001. The chamber contained 6.5%

CH4–air at 100 kPa. The cylinder force for both the wedge and

flat plate was set to 13 kN as per UL 1642. The plastic wedge

tests were stopped after obtaining four ignitions. The flat plate

produced no ignitions in ten attempts (Table III). The chamber

atmosphere was ignited by the furnace element for these ten flat

plate tests, verifying that a flammable atmosphere was present.

The flat plate compressed the cells about 18% (Fig. 3).

Video recordings showed nonluminous material ejecting

from the cell vent prior to the four LG Chem cell CH4–air

ignitions. Three of the four cells ruptured and spewed luminous

sparks from the side of the can close to the plastic wedge, after

initial cell venting and prior to ignition (Fig. 4). The fourth can

did not rupture on the side but spewed sparks from the cell vent,

after initial cell venting and prior to ignition. Ignitions saturated

(whited-out) the video recordings.

Fig. 4. Ruptured LG Chem cell crushed by the plastic wedge that ignited 6.5%

CH4–air.

Fig. 5. Graph showing that an LG Chem cell hard shorted as the plastic wedge

applied force reached 4.75 kN (1065 lb).

Fig. 6. Graph showing that an LG Chem cell hard shorted prior to 6.5%

CH4–air ignition indicated by 586-kPA (85 psi) peak chamber pressure.

Figs. 5–7 show the time traces for an LG Chem cell CH4–air

ignition, crushed by the plastic wedge at 40 ◦C. Fig. 5 shows

the cell yielding and hard shorting at cylinder forces significantly less than 13 kN. Figs. 6 and 7 show the pressure

and temperature peaks from 6.5% CH4–air ignition occurring

shortly after the hard short. Summary data for the four LG

Chem cells that ignited CH4 are listed in Table IV. Measured

cell can temperatures were lower than the thermal runaway

or CH4–air flame temperatures due to the insulating nature
Fig. 7. Graph showing that an LG Chem cell hard shorted and the cell can

surface temperature reached 221 ◦C during a 6.5% CH4–air ignition.

TABLE IV

CHAMBER SENSOR SUMMARY DATA, 6.5% CH4–AIR IGNITIONS,

LG CHEM CELLS, PLASTIC WEDGE, 40 ◦C

Cylinder force at ignition kN (Lbs) 4.75 to 5.96 (1068 to 1340)

Peak chamber pressure, kPa (psi) 586 to 738 (85 to 107)

Peak cell can temperature, oC 221 to 360

Fig. 8. Photograph showing an LG Chem cell crushed by the plastic wedge

that did not rupture nor ignite 6.5% CH4–air at 40 ◦C ambient temperature.

The cell hard shorted.

of the can, plastic cover, and separation distance between the

thermocouple and short circuit.

Three LG Chem cell plastic wedge tests at 40 ◦C did not

produce ignitions (Fig. 8). These cells were hard shorted to a

few tenths of a volt within a few seconds after wedge contact.

The wedge tip penetrated the can in two of the three cells. There

were no measurable chamber pressure increases. Peak cell can

temperature measurements ranged from 84 ◦C to 103 ◦C. The

temperatures decayed over a 5-min waiting period, after which

the test was terminated and the chamber atmosphere was ignited

by the furnace element, verifying that a flammable atmosphere

was present.

The LG Chem cell flat plate tests produced no CH4–air

ignitions, no measurable cell can temperature increases, and

no apparent cell venting, as observed from the video. Seven of

the flat plate tests produced no cell voltage drops. Three tests

showed evidence of soft shorting, producing voltage drops of

22 mV or less. A cell voltage and cylinder force time trace for

one of the three soft shorts is shown in Fig. 9. Researchers

stopped the press extension temporarily when 13 kN was

reached. Fig. 9 shows that the cylinder force decayed over

time because of internal hydraulic system leakage. Researchers

then retracted the cylinder after a short period of time. These

three cells returned to their pretest voltage as the plate was

Fig. 9. Time traces of cell voltage and flat plate applied force, showing that a

soft short appeared and disappeared as the plate was extended and retracted.

Fig. 10. Ruptured A123 cell crushed by a plastic wedge that did not ignite

6.5% CH4–air.

TABLE V

LG CHEM CELLS VERSUS A123 CELLS, 6.5% CH4–AIR IGNITION TEST

RESULTS, PLASTIC WEDGE, 40 ◦C

Ignitions Totals

LG Chem cells 4 3 7

A123 cells 0 10 10

Totals 4 13 17

retracted. Two of the three cells self-discharged completely,

and the third discharged by 1 V overnight without going into

thermal runaway.

C. A123 26650 Versus LG Chem ICR18650S2 Cells

Researchers conducted ten 6.5% CH4–air ignition tests using

the A123 cells and plastic wedge at 40 ◦C. These cells were

hard shorted to a few tenths of a volt within a few seconds after

wedge contact. None of the A123 cells ignited the chamber

atmosphere. Cell can temperatures peaked at 123 ◦C or less.

Video showed all A123 cells rupturing and producing smoke

near the point where the wedge penetrated the can (Fig. 10).

These tests produced chamber overpressures of 3.8 kPa

(0.58 psi) or less when the cells ruptured, indicating that the

chamber atmosphere did not ignite. The furnace igniter subsequently ignited the chamber atmosphere for all of these tests,

confirming that a flammable atmosphere was present. Table V

lists the A123 and LG Chem cell test data in Fisher’s exact test

No ignitions
Fig. 11. LG Chem cell crushed by the plastic wedge went into thermal

runaway and produced 482-kPa (70 psi) peak chamber pressure with no CH4

in the chamber.

two-by-two format. The one-tailed p value for Table V data is

p = 0.015. The two cell designs can be considered independent

from an ignition standpoint at a significance level of p < 0.05,

as discussed previously.

D. Cell Crush Tests With No CH4

An LG Chem cell and an A123 cell were each tested without

CH4 in the chamber using the plastic wedge at 40 ◦C. The LG

Chem cell went into thermal runaway, and the A123 cell did

not. The ruptured A123 cell produced a 2-kPa overpressure and

a peak can temperature of 86 ◦C. The LG Chem cell thermal

runaway produced 482-kPa (70 psi) chamber peak pressure

(Fig. 11) and 284 ◦C peak can temperature, and the high-speed

video recording was saturated.

V. DISCUSSION

ACRI2001 IS evaluations and ANSI/ISA 60079-11 level-ofprotection “ia” evaluations take into consideration up to two

countable faults. Two-countable-faults-evaluated IS equipment

is permitted for use in coal mines by MSHA and in other

industrial hazardous locations as per the National Electrical

Code, Articles 500 and 505 [41]. Mikolajczak et al. [9] indicate that some field failures involved mechanically induced

cell internal short circuits. Stationary battery-powered CTE in

coal mines may be susceptible to mechanical damage, such

as from roof falls. For the NIOSH study, researchers charged

cells within the manufacturer’s recommendations and kept the

ambient temperature within the normal range for IS evaluations.

The tests reported here simulated a single foreseeable internal

short-circuit fault from an external crush. The number of faults

considered was not excessive relative to IS evaluation criteria.

There was a clear distinction between ignitions and nonignitions in all tests. Cell ignitions produced pressures in excess of

480 kPa, and nonignitions produced pressures well below the

50-kPa threshold criterion. Furnace igniter ignitions produced

pressures in excess of 510 kPa and verified that a flammable

atmosphere was maintained in the chamber for all cell crush

tests that did not result in ignition of the chamber atmosphere.

TABLE VI

SELECTED CH4–AIR IGNITION TEMPERATURES (IN DEGREES CELSIUS)

AIT

(800 mL vol.) [44]

Heated air jet

(1 cm dia.) [42]

Heated nichrome wire

(0.1 cm dia.) [42]

600 1040 1220

The LG Chem cells readily ignited CH4–air, whereas the

A123 cells did not ignite in any test with the plastic wedge.

The two cell samples had similar capacities and were tested at

similar temperatures. The plastic wedge produced hard shorts

to a few tenths of a volt within a few seconds after wedge

contact for all tests. The difference in test outcomes was likely

due to the ability or inability of an internal short circuit to

initiate a thermal runaway chemical reaction within the cell.

The LG Chem cells were tested at 100% SOC, but different

results may be obtained if cell SOC were limited to significantly

lower values. Fisher’s exact test results indicate that the two cell

designs can be considered to be independent from an ignition

standpoint at a conservative significance level. The A123 cells

were the safer of the two cell designs, under specified test

conditions.

Ignition by a heated surface is a function of the heat source

dimensions [42]. Autoignition temperature (AIT) usually refers

to ignition temperature obtained within a heated vessel, where

the atmosphere is heated through the surrounding vessel walls.

Heated wire, rod, or tube ignition temperatures are normally

much higher than heated vessel AITs. Some CH4–air ignition temperature data are listed in Table VI. CH4–air AIT is

significantly higher than AITs for some typical Li-ion electrolyte components summarized in [9]. Mikolajczak et al. [9]

indicate that temperatures produced by cell thermal runaway

reactions are considered sufficient to cause hot surface ignition

of flammable mixtures but do not reach levels that will cause

the melting of pure copper (1080 ◦C). In some cases, very small

points of pure copper, nickel, or steel melting were found within

a cell, attributed to internal electrical arcing or shorting [9]. By

comparison, Kuchta [42] lists a 1220 ◦C ignition temperature

for CH4–air ignited by a heated Nichrome wire. CH4–air

ignition temperatures are significantly higher than those for

many industrially important explosive atmospheres [43]. The

results reported here should not be considered conservative for

explosive atmospheres with lower igniting temperatures than

CH4–air.

The flat plates produced no hard shorts within the LG Chem

cells at 13-kN applied force, and a few observed soft shorts returned to pretest voltage after the plate was retracted. These flat

plate tests provided some assurance that this particular sample

of cells was compliant with UL 1642 crush test requirements,

as specified [35].

The plastic wedge produced hard shorts in all tests reported

here with relative ease. The wedge crushed the cell to less than

half of its original thickness as recommended by Tobishima

[10], using a fraction of the applied force specified in UL 1642.

Researchers limited wedge penetration to approximately 66%

of the cell diameter. The pointed edge of the wedge overcame

the difficulty of penetrating the separator, as described by

Mikolajczak et al. [9]. The applied pressure did not damage
or melt the wedge tip significantly in tests that did not produce ignition. The plastic wedge tip melted during tests that

produced ignitions. A plastic fixture simulates external forces

applied to plastic-wrapped or plastic-encased battery packs

which are common [9]. The plastic wedge crush method does

not compensate for potential heat dissipation through a cell’s

metallic can. The plastic wedge minimizes energy dissipating

effects of the test fixture itself, compared to metallic nails.

Safety standards organizations may wish to consider the plastic

wedge crush test, in response to field failure reports involving

external mechanical damage [9].

The UL 1642 interpretation of explosion is when the cell or

battery contents are forcibly expelled and the cell or battery is

torn or split into two or more pieces. The A123 cells pressed by

the wedge ruptured and produced smoke but did not ignite the

chamber atmosphere. These results suggest that ignition of the

explosive atmosphere in question may be a more appropriate

failure criterion using the plastic wedge, for IS evaluation

purposes.

The MSHA-approved Li-ion cap lamp battery contains a

multicell battery pack within a sealed plastic enclosure. The LG

Chem cell thermal runaway test with no CH4 in the chamber

produced a significant chamber peak pressure of 482 kPa

(70 psi). Although the peak pressure was orders of magnitude

less than those reported in [31] and [32], the chamber provided

about 18 L of head space to allow expansion and reduce peak

pressure. Peak pressures will be significantly higher than that

found in this study within significantly smaller volume and

sealed enclosures and under otherwise similar test conditions.

Observed thermal runaway with the LG Chem ICR18650S2

cells fully charged and not charging suggests that the field failure of the MSHA-approved cap lamp in the NIOSH Pittsburgh

SRCM office conceivably could have involved an internal shortcircuit cell fault while under normal charge, and not necessarily

a charging fault.

Results suggest the plastic wedge test should be a more

severe test than the nail penetration test specified in [35], potentially due to elevated ambient temperatures and the electrical

and thermal insulating properties of the plastic.

VI. CONCLUSION

IS evaluated equipment powered by LG Chem ICR18650S2

LiCoO2 cells has posed a CH4 explosion hazard from cell

internal short circuit.

Under specified test conditions, the A123 26650A LiFePO4

cells were safer than the LG Chem ICR18650S2 LiCoO2 cells

at a conservative statistical significance level. Other potential

failure modes should be considered, as appropriate.

IS evaluated equipment powered by cells with similar

form factor, chemistry, and charge capacity to the LG Chem

ICR18650S2 should be reevaluated per an appropriate cell

internal short-circuit test within suitable atmospheric conditions

(gas mixture and ambient temperature).

There was a clear distinction between ignitions and nonignitions in all tests. Ignitions and nonignitions produced chamber

pressures well above and well below the threshold criterion,

respectively.

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

plate crush. The plastic wedge produced deeper penetration and

lower impedance (hard) shorting while using a fraction of the

applied force of the flat plates. The plastic wedge electrical

and thermal insulating properties minimize energy dissipating

effects of the test fixture itself. A plastic fixture simulates

external forces applied to plastic-wrapped or plastic-encased

battery packs which are common.

Vents in the LG Chem ICR18650S2 cells were not reliable

protection against thermal runaway in the plastic wedge tests.

Venting was observed prior to ignition in all LG Chem cell tests

that produced ignitions.

The LG Chem ICR18650S2 cell thermal runaway test with

no CH4 in the chamber produced significant chamber pressures

with relatively large head space volume.

ACKNOWLEDGMENT

The authors would like to thank S. Gallagher and

L. J. McWilliams for the help with the statistical analysis.

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