IEEE Electrification - September 2020 - 65

Temperature Effects on Fast
Charging

Start

Voltage (V)

Controlling temperature during fast
charging of Li-ion cells has a signifi2-C Charge
cant role in improving the performance and cycle life of batteries.
No
No
Temp >
Voltage ≥
Charge Capacity
Researchers have reported that a
60 °C?
4.35 V?
≥ 2.5 Ah?
higher operating temperature
Yes
Yes
Yes
improves the sluggish kinetics of
the battery. Yang et al. demonstratTwo-h Rest
ed that increasing the cell temperature from 25 to 45 °C increases Li
C/3 Discharge to 2.75 V
intercalation kinetics by a factor of
6.5 and Li diffusivity in graphite
One-h Rest
and electrolyte by factors of 2.4 and
1.4, respectively. This enabled the
Stop
Li-plating-free charging current to
be increased from 1.5 to 3 C in highFigure 1. A flowchart of the cycling profile.
energy NMC Li-ion cells.
To better understand the temperature effect on the performance of Li-ion batteries, an
experimental study was conducted using commercially
4.35
available 5Ah 21700 NMC Li-ion cells. The cells were cycled
at an ambient temperature of 35 and 45 °C inside thermal
3.95
chambers. Each cell was equipped with a thermocouple to
monitor the surface temperature of the cell. The cycling
3.55
profile (the flowchart in Figure 1) was as follows:
1)	The sequence started with a charge at a 2-C charging
3.15
35 °C
rate. The charge capacity was limited to 2.5 Ah, corre45 °C
sponding to 50% SoC in 15 min. The charge was initi2.75
0
0.5
1
1.5
2
2.5
ated from 2.75 V and limited to 4.35 V for safety
Capacity
(Ah)
concerns. The charging step was also stopped if the
surface temperature of the cell reached 60 °C. In this
Figure 2. The voltage versus charged capacity during charge at 2-C rate.
case, the tests skipped to the next step.
2)	The cells were rested for 2 h after the charge to let the
battery cool down to ambient temperature.
After 145 cycles, the charge step was stopped by the
3)	The cells were then discharged at a C/3 rate to 2.75 V.
temperature limitation rather than voltage limitation
4)	After another 1-h post-discharge rest period, the
[(Figure 3(b)]. This was an interesting finding that supsequence was repeated across 150 cycles.
ported Yang's work.
The voltage profile of both cells during the charge at
Challenge: Robust Thermal Management System
different ambient temperatures are plotted in Figure 2.
for Fast Charge at Elevated Temperatures
The voltage curve at 45 °C was always lower than the one
Thermal management systems (TMSs) play a critical role
at 35 °C, which implies the overall DCIR of the cell was
in the performance and safety aspects of Li-ion energy
reduced at 45 °C ambient temperature. The higher thermal
storage devices. This was evident from the previous secagitation at elevated temperature increased the dissociation where cells overheated during the cycling test at 45 °C
tion of ion pairs in the electrolyte, improving the conducambient temperature. The surface temperature of the cell
tivity of the electrolyte, and thus reduced the DCIR of the
reached the safety temperature limitation (60 °C) at 2-C
electrolyte. The increased Li-ion intercalation kinetics and
charging rates because of the lack of a TMS.
diffusivity in graphite also could have contributed to the
A local rise in temperature and uneven distribution of
improved overall DCIR.
temperature in the battery pack can also affect the perforAs for degradation, the cycling test results showed that
mance, reduce the cycle life, and may lead to thermal runa higher operation temperature can improve the cycle
away of the cells; therefore, large temperature differences
life at a 2-C charging rate. At 35 °C ambient temperature,
across cells are not preferred. Saw et al. has shown that a
after 49 cycles, the cell was unable to finish the charging
5 °C gradient across the pack can lead to a 10% degradastep, and voltage reached the safety limit [Figure 3(a)]. In
tion of power capability and an increase of 25% in thermal
comparison, the cell at 45 °C completed 145 full cycles.
	

IEEE Elec trific ation Magazine / S EP T EM BE R 2 0 2 0

65



IEEE Electrification - September 2020

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