The U.S. Army has been pursuing vehicle electrification to achieve enhanced combat effectiveness. The benefits include new capabilities that require high-power pulse duty cycles. However, as the vehicle platform size decreases, the Energy Storage System (ESS) pulse power discharge rates (>40 C-rate) to support system requirements can be significantly greater than commercial ESS.

Results are reported of high-power pulse duty cycles on lithium-iron phosphate cells that show a dramatic loss in lifetime performance. For two-second and three-second pulse duration tests, the observed degradation is 22% and 32%, respectively. Although these cells were thermally managed in a convective chamber at 10 °C, the 2-second pulse showed a 31 °C temperature rise and the 3-second pulse a 48 °C temperature rise.

The decreased lifetime is attributed to increased lithium loss due to the increased temperature during pulse discharging. To mitigate the thermally induced degradation, the use of different thermal management systems and alternative cell design is discussed and recommended.

Introduction

The U.S. Army’s pursuit of vehicle electrification is to realize benefits of significant fuel savings/range extension, increased silent watch/mobility, and new capabilities in Electronic Warfare (EW), high-power sensors, and Directed Energy (DE) systems.

Figure 1. Battery capacity normalized with platform size to provide silent mobility and directed energy capabilities. (US Army CCDC Ground Vehicle Systems Center)

The discharge rates for silent mobility — a 30-kW DE and 100-kW DE capability using a Hybrid Electric Vehicle (HEV) configuration — are shown in Figure 1 along with examples of commercial systems. (Note: Standard industry practice is to define charging/discharging by C rates. By definition, a 1 C-rate discharge is equivalent to a discharge current that will discharge the entire battery in one hour.)

The silent mobility power requirement has been normalized for a hybrid-electric combat vehicle platform weight (3.9 kW/t) and the battery pack is proportionally sized (0.6 kWh/t). Thus, the discharge rate for the silent mobility capability is constant across different platform sizes. This discharge rate can be met using existing HEV ESS solutions. However, as the platform size decreases, the ESS discharge rates for DE capabilities increase significantly beyond standard HEV ESS solutions. In these pulse power applications, the high-power pulse duty cycles can have discharge rates that are significantly higher (>10 C) than commercial HEV ESS systems, resulting in increased thermal and electrical stress.

Previous Work

There has been limited published experimental work on high-rate discharge. However, Wong et al. tested LiNixCoyAl1-x-yO2 (NCA) and LiFePO4 (LFP) for pulse at high rate. For the LFP cells tested at 15 C discharge rate, the rapid cell capacity decay was attributed to the increase in cell resistance.

Cell degradation theory and prediction is critically important to multiple commercial applications and is an active area of research. Models have been proposed based on empirical and physics-based aging mechanisms.

Based on previous work, the cell degrades due to the consumption of active Li material via solid electrolyte inter-phase (SEI) growth. As shown in Figure 2, the electrolyte reacts and consumes lithium to form an insoluble interface that decreases cell capacity.

Figure 2. The desired electrochemical reaction is the lithium intercalation in graphite but lithium can also react with components of the electrolyte to form a solid-electrolyte interphase. (US Army CCDC Ground Vehicle Systems Center)

Experimental

Based on the use of LFP cells in commercial pulse power applications, such as power tools with a long lifetime, a 26650 LFP cell was selected.

The cells were attached to an A&D/ BITRODE electronic load with thermocouples affixed to the cell negative tab and cell skin surface. The cells were then placed into a thermal chamber for environmental control at 10 °C for automated lifetime testing.

Results

Figure 3 shows the cell’s voltage and current response to a load profile, as shown in Table 2, with a 120-A pulse for two seconds. It can be seen that the cell can sustain the pulse for six minutes before it reaches the 2V discharge limit. The initial capacity with this profile was 1.84 Ah. The cell under a 120-A pulse for 3s shows a similar profile, with an initial capacity of 1.95 Ah.

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

A snapshot of the discharge with temperature is shown in Figure 4. The heat generation due to the three-second 120-A 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; capacity loss is based on initial cycle capacity during the pulse profile.

Figure 5. LFP 2.3 Ah cell capacity loss with 120-A pulsing for 2 seconds and 3 seconds. The degradation is significantly lower than the 1,000-cycle design target. (US Army CCDC Ground Vehicle Systems Center)

The three-second 120-A pulse shows higher degradation (32%) than the two-second 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 pulse-induced 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

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 loss due to SEI growth is the dominant loss mechanism.

The SEI growth is accelerated due to the higher temperature observed with joule heating. This SEI growth can be mitigated by appropriate cell design and alternate cooling including immersion/evaporative cooling. Future work involves model development to predict failure at different conditions and additional cell characterization techniques to further elucidate degradation mechanisms and identify mitigation measures including improved thermal management systems.

This is a condensed version of SAE Technical Paper 2020-01-0452, “Accelerated Degradation of Li-Ion Batteries for High Rate Discharge Applications,” authored by Tony Thampan, Yi Ding, and Laurence Toomey of the Army Combat Capabilities Development Command (CCDC) Ground Vehicle Systems Center.

The paper can be ordered or downloaded from SAE International here .


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This article first appeared in the May, 2021 issue of Battery Technology Magazine.

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