IEEE Electrification - December 2021 - 23

first filling an enclosed space. This is then followed up by a
thermal runaway event, generating yet more gas and providing
an ignition source for the built-up gases.
Recent scenarios of fuel-air ignition driving a catastrophic
failure are evident in battery systems fires that
occurred in the last few years. When first responders
entered the space, the accumulated gases ignited, resulting
in an explosion that destroyed the container and
injured several of the responding firefighters. Accident
analysis reports suggest that the container was filled with
flammable gases. While it is likely difficult to fully account
for every scenario, this demonstrates the need for education
on some specific hazards inherent to large battery
systems, particularly for system
installers and first responders.
Significantly, the primary source
for these gases is the breakdown of
the electrolyte necessary for the
normal function of Li-ion batteries.
Research is ongoing to try and find
a replacement for this electrolyte,
but the potential solutions identified
to date severely reduce the
power capability and lifetime of the
battery or are cost-prohibitive to
manufacture. A potential solution
includes the use of all solid-state
electrolytes, which are particularly
attractive as they have the potential
to enable a lithium metal anode,
dramatically increasing the available
energy in batteries. However,
even the best solid-state electrolytes
100
40
60
80
20
DEC
EC
EMC
DMC
EMC
DMC
DEC
EC
currently exhibit significantly lower current capability
than traditional electrolytes, limiting their use for high
power applications.
There are several leading system design strategies for
mitigating potential hazards from gas venting from Liion
cells. The simplest strategy is to prevent thermal runaway
propagation as the gas vented from single cell
failure is unlikely to be sufficient for hazardous combustion.
As many battery systems need to be located in a
temperature controlled, and hence enclosed space, the
next best strategy is to limit the maximum quantity of
batteries to below the threshold where their vented
gas would reach the low explosivity limit (LEL) in the
Figure 3. The molar percentage of gas decomposition components detected from the thermal
breakdown of diethyl carbonate (DEC), ethyl carbonate (EC), ethyl methyl carbonate (EMC), and
dimethyl carbonate (DMC) all with 1 M LiPF6 salt. (Courtesy of F. Meier and M. Hargather, New
Mexico Tech.)
1
20 mm
10 mm
10 mm
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
(a)
(b)
(c)
Figure 4. Image shows time of arrival of vented material from an 18650 cell after failure: (a) gas phase plume, b) low viscosity (2.57 cP) electrolyte
ejection, and c) high viscosity (329 cP) electrolyte ejection.
IEEE Electrification Magazine / DECEMBER 2021
23
Time of Arrival (ms)
CO2
H2
CO
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
Ethylether
Fluoroethane

IEEE Electrification - December 2021

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