Battery Technology - May 2021 - 10

Li-Ion Batteries

4.0

20
0

3.5

-40

3.0
Voltage

-60

Current
2.5

-80

Current (A)

Voltage (V)

-20

-100
2.0
-120
1.5

-140
0

200

400

600

800

1000

Time (s)
Figure 3. Current and voltage characteristics of LFP 2.3 Ah under test during charge and discharge. (US Army CCDC Ground Vehicle Systems Center)

3s 120A 8s 0A

2s 120A 8s 0A

Temperature (C)

10
0

500

1000

Time (s)
Figure 4. Temperature characteristics of test during charge and discharge. Heating occurs during discharge at 120-A pulses. Cooling occurs during charge at 10 A. Ambient temperature is
10 °C. (US Army CCDC Ground Vehicle Systems Center)

capacity loss is based on initial cycle
capacity during the pulse profile.
The three-second 120-A pulse shows
higher degradation (32%) than the twosecond 120-A pulse (22%) after 250 cycles. The degradation is substantially
higher than the expected degradation
(1,000 cycles), which may occur due to
higher temperature.

Degradation Mitigation
It appears that lithium loss due to SEI
growth is the dominant loss mechanism. The SEI growth is accelerated due
to the higher temperature produced by
joule heating, increasing with pulse
duration. The SEI growth increases with
temperature, as the reaction rate of
electrolyte decomposition is assumed
to follow an Arrhenius-type dependence. Considering that SEI growth
increases with temperature, improved
heat removal should mitigate pulseinduced degradation.
To better evaluate heat removal, a
thermal model of the 26650 LFP cell
was developed. Utilizing the high-pulse
discharge data and Hybrid Pulse Power
Characterization Test data, a 2-RC equivalent circuit was fit. Utilizing the energy
equation, it is possible to estimate the
temperature profile in the cell. Utilizing
an ANSYS computational fluid dynamics (CFD) tool and cell data, it is possible to estimate the temperature profile
on the cell.
Due to the cell construction, the cell's
thermal conductivity is lower in the radial direction than the axial direction.
However, although cell surface temperature is usually monitored, it is critical to
estimate the internal cell temperature.
Although certain parts are cooler due to
the tabs, the hottest part of the cell is
towards the center.
It is important to design the cell to
ensure enough heat is removed to avoid
thermal hot spots and subsequent degradation. This can be done by alternative
cell design and cooling.

Conclusion
A snapshot of the discharge with temperature is shown in Figure 4. The heat
generation due to the three-second 120A pulse increases the cell skin temperature to 58 °C. This is significantly higher

than the 41 °C maximum observed during the two-second pulse profile. Both
cells cool during the charge profile due
to the 10 °C ambient air cooling. The
degradation data is shown in Figure 5;

Based on a high-pulse discharge profile, accelerated degradation was observed on LFP cells. The degradation
increased with the duration of the pulse.
Based on analysis, it appears that lithium
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