5
may have been used in the current battery due to its excel-
lent safety features. For example, the LCMO cathode has
cycling stability that approaches the minimal strain proper-
ties of the LTO anode [27]. In addition, it has very high
thermal stability—the LCMO cathode withstood tempera-
tures up to 480 °C when paired with the LTO anode and a
solid-state (inflammable) electrolyte [28]. Because the LTO
and LCMO electrodes are thermally stable, the electrolyte
and separator become the main vulnerabilities.
The separator contained mostly carbon (Figure 5c)
and had the morphology of a porous polymer membrane
(Figure 6c) [29]. Porous polymer membranes have good
lithium-ion conductivity, but the organic membrane can
fail at high temperatures and can be vulnerable to punc-
tures by lithium dendrites [29, 30]. Overall, the character-
ization suggests that the separator had a composition with
relatively low thermal abuse tolerance but was balanced by
a robust anode and cathode.
Temperatures
Figure 7 shows the time plots of the recorded temperatures
and cell voltage of a single cell in a 1,175-ml container. The
cell temperature was gathered via a thermocouple secured
to the outer metal casing of the LTO battery with fiber tape
and copper wire. As expected, the container temperature
leads the gas temperature which leads the cell tempera-
ture during the heating process. It’s not until the venting
event after 90 minutes that the cell begins a self-heating
process where the internal exothermic chemical reactions
and internal shorts lead to increasing heat. Around the time
of the venting, a small voltage drop is observable. The rate
of the drop increases until an internal short causes a ther-
mal runway, possibly due to a thermally degraded polymer
separator.
The gas temperature was recorded via a thermocouple
hanging in the available free space within the container, and
the container thermocouple was secured to the outer body
of the sealed steel pipe with glass fiber tape. The maximum
recorded temperatures were found to be 161 °C for the gas
and 164 °C for the container. In the smaller containers,
109 ml and 71 ml, the gas and cell temperatures were not
able to be measured due to the lack of available free space.
Figure 8 shows a boxplot of the recorded gas, cell, and con-
tainer temperature at the peak of recorded pressure rates
during venting.
During much of the heating process, the cell tempera-
ture lags in both gas and container temperatures. It is not
until the cell begins self-heating that the cell temperature
climbs past the gas temperature, and in some cases, at the
time of venting, exceeds the temperature of the container.
Figure 6. SEM images of the LTO cell (a) anode, (b)
cathode, and (c) separator [16]
Figure 7. Temperature and voltage plot of LTO-18650 in a
1,175-ml container
Figure 8. Summary of gas, cell, and container temperatures
at venting
may have been used in the current battery due to its excel-
lent safety features. For example, the LCMO cathode has
cycling stability that approaches the minimal strain proper-
ties of the LTO anode [27]. In addition, it has very high
thermal stability—the LCMO cathode withstood tempera-
tures up to 480 °C when paired with the LTO anode and a
solid-state (inflammable) electrolyte [28]. Because the LTO
and LCMO electrodes are thermally stable, the electrolyte
and separator become the main vulnerabilities.
The separator contained mostly carbon (Figure 5c)
and had the morphology of a porous polymer membrane
(Figure 6c) [29]. Porous polymer membranes have good
lithium-ion conductivity, but the organic membrane can
fail at high temperatures and can be vulnerable to punc-
tures by lithium dendrites [29, 30]. Overall, the character-
ization suggests that the separator had a composition with
relatively low thermal abuse tolerance but was balanced by
a robust anode and cathode.
Temperatures
Figure 7 shows the time plots of the recorded temperatures
and cell voltage of a single cell in a 1,175-ml container. The
cell temperature was gathered via a thermocouple secured
to the outer metal casing of the LTO battery with fiber tape
and copper wire. As expected, the container temperature
leads the gas temperature which leads the cell tempera-
ture during the heating process. It’s not until the venting
event after 90 minutes that the cell begins a self-heating
process where the internal exothermic chemical reactions
and internal shorts lead to increasing heat. Around the time
of the venting, a small voltage drop is observable. The rate
of the drop increases until an internal short causes a ther-
mal runway, possibly due to a thermally degraded polymer
separator.
The gas temperature was recorded via a thermocouple
hanging in the available free space within the container, and
the container thermocouple was secured to the outer body
of the sealed steel pipe with glass fiber tape. The maximum
recorded temperatures were found to be 161 °C for the gas
and 164 °C for the container. In the smaller containers,
109 ml and 71 ml, the gas and cell temperatures were not
able to be measured due to the lack of available free space.
Figure 8 shows a boxplot of the recorded gas, cell, and con-
tainer temperature at the peak of recorded pressure rates
during venting.
During much of the heating process, the cell tempera-
ture lags in both gas and container temperatures. It is not
until the cell begins self-heating that the cell temperature
climbs past the gas temperature, and in some cases, at the
time of venting, exceeds the temperature of the container.
Figure 6. SEM images of the LTO cell (a) anode, (b)
cathode, and (c) separator [16]
Figure 7. Temperature and voltage plot of LTO-18650 in a
1,175-ml container
Figure 8. Summary of gas, cell, and container temperatures
at venting