2
as the use the BEVs and wearable equipment continue to
be more commonplace. The challenges are compounded in
applications that implement large banks of LIB. The main
cause of the catastrophic failure of LIB is a thermal runway,
which happens when the heat generated by the exothermic
reaction outpaces the heat dissipated by the battery which
leads to increased reaction rates, temperatures, and pres-
sures. If no mitigation efforts are made, the accelerating
self-reinforcing process will lead to the venting of toxic and
flammable gases and elevated temperature and in turn fires
and explosions [9]. The initial TR can be caused by many
internal or external factors such as internal shorts, mechani-
cal damage, exposure to heat, surging currents, and faulty
wiring. The severity of these failures is also dependent on
the discharge rate, state of charge, and internal chemistries
of the LIB (cathode anode, electrolyte, separator) [10].
Large equipment powered by LIB has the additional risk of
a cascade of thermal runaways caused by a single battery cell
in TR and generating enough heat to drive the neighboring
batteries to catastrophic failure. Explosion-proof enclosures
are required by the Mine Safety and Health Administration
(MSHA) to contain an internal methane-air explosion and
stop the propagation of the explosion into the explosible
mine atmosphere [11]. The enclosure must be capable of
withstanding a minimum pressure of 150 psig (10.3 barg),
and the outside surfaces of the XP container are not allowed
to reach 150 °C. If any pressure peak exceeds 125 psig, the
manufacturer must either conduct static pressure tests on
the enclosure to ensure that it can withstand a static pres-
sure twice as high as the highest value recorded in any previ-
ous tests or make constructional changes that will lower the
pressure to 125 psig or less [12]. Prior testing of other LIB,
namely iron phosphate (LFP) and nickel manganese cobalt
(NMC), has been conducted in containers of varying vol-
umes. The results of this past NIOSH research showed an
inverse power relationship between the peak pressures and
the amount of free space within the containers, but due to
the inherent variations in the different battery chemistries,
it was found that NMC batteries required more than eight
times the amount of free space per cell volume than LFP to
meet the 125 psig XP enclosure specifications [13, 14]. The
current study is a continuation of this previous research in
which the composition of the LTO type LIB was character-
ized and the battery cells were tested in various-sized con-
tainers to determine how the specific chemistry influenced
the relationship between peak pressure and the amount of
free space and how it compares to the other LIB chemistries.
METHODS
The LIB selected for this study were cylindrical spiral-
wound type 18650 manufactured by Hua Hui Energy. The
cells are the same cells that were studied by Yuan et al. [15]
with a nominal voltage and rated capacity of 2.4 V and
1.3 Ah, respectively. Table 1 shows the comparative energy
density of LIB cells
The battery cell composition was characterized by scan-
ning electron microscopy (SEM Model S-4800, Hitachi,
Tokyo, Japan) and energy dispersive x-ray spectroscopy
(EDS Bruker Quantax, Madison, Wisconsin). The samples
for SEM/EDS analysis were extracted from the cell after
fully discharging it for personnel safety and were mounted
on 25-mm aluminum posts with conductive carbon tape.
The separator was coated with gold and palladium to avoid
image distortion from a lack of sample conductivity, while
the anode and cathode remained uncoated due to their
high conductivity. Particle size was measured with ImageJ
software (Version 1.48v, National Institutes of Health).
An Arbin Multi-channel potentiostat/galvanostat
(MSTAT, Arbin Instruments, College Station, Texas) was
used to cycle the cells through three charge-discharge cycles
before bringing the cells to a full state of charge. The mea-
sured discharge capacity of the cells was at least 95% of the
rated capacity.
The LIB TR tests were conducted using an accelerat-
ing rate calorimeter (ARC) (model EV+, Thermal Hazard
Technology, Milton Keynes, United Kingdom) (Figure 1)
and a stand-alone data acquisition system (model DI-720,
DATAQ Instruments, Akron, Ohio) with a faster measure-
ment rate to capture peak thermal runaway pressures.
The testing consisted of placing a fully charged LIB
cell and in some cases three LIB cells, which passed the
95% capacity screening, into sealed a container and driving
Table 1. Summary of LIB cell characteristic
Type LTO LFP NMC
Size 18650 18650 26650 18650
Voltage, V 2.4 3.2 3.2 3.67
Capacity, Ah 1.3 1.5 3.8 3.2
Energy, Wh 3.1 4.8 12.2 11.7
Approx. cell weight, g 38 41 88 45
Energy density, Wh/kg 82 117 138 260
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