Experimental Study on High-Temperature Cycling Aging of Large-Capacity Lithium Iron Phosphate Batteries

Large-capacity lithium iron phosphate (LFP) batteries are widely used in energy storage systems and electric vehicles due to their low cost, long lifespan, and high safety. However, the lifespan of batteries gradually decreases during their usage, especially due to internal heat generation and exposure to high temperatures, which leads to rapid capacity degradation. In-depth research is needed on the degradation characteristics of large-capacity LFP batteries under high temperatures. To study the degradation characteristics of large-capacity LFP batteries at high temperatures, a commercial 135Ah LFP battery is selected for 45°C high-temperature dynamic cycling aging experiments and 25°C reference performance experiments. A detailed analysis of the degradation process is conducted by examining the patterns of changes in charge-discharge voltage curves, capacity, internal resistance, open circuit voltage (OCV), and incremental capacity curve. The study uncovers that the OCV displays diverse degradation patterns at different states of charge (SOC). Furthermore, it identifies the loss of lithium inventory and active material as the fundamental factors contributing to the degradation observed during high-temperature cycling. This study provides references for developing battery life prediction algorithms and designing long-cycle-life battery cells.


Introduction
Large-capacity lithium iron phosphate (LFP) batteries are increasingly used in mobile energy storage systems, such as electric vehicles, and stationary energy storage systems, such as storage power stations [1].However, batteries experience aging and degradation during operation [2,3].The degradation modes of batteries primarily include the loss of lithium inventory (LLI) [4], the loss of active material (LAM) [5], electrolyte loss [6], and ohmic resistance increment (ORI) [7].The LLI and LAM result in a decrease in battery capacity.In the final stage of the battery's lifespan, the loss of excess electrolytes leads to a sharp decline in capacity, causing issues such as voltage sag [8].An increase in internal resistance reduces the battery's power output and, under the same charge and discharge cut-off voltage, decreases available capacity [9].Due to internal heat generation and exposure to high-temperature environments, batteries often operate at elevated temperatures.High temperatures accelerate the degradation of the battery, leading to a rapid decrease in capacity and an increase in resistance.There is more research on the degradation mechanisms of ternary and small-capacity lithium-ion batteries.In comparison, there is relatively less research on the high-temperature degradation of large-capacity LFP batteries.The degradation patterns of large-capacity lithium iron phosphate batteries are not yet clear.
To explore the degradation mechanisms of large-capacity LFP batteries, we select a commercial 135 Ah LFP battery and establish a battery aging experimental platform.Based on this experimental platform, we design the 45°C high-temperature New European Driving Cycle (NEDC) and Federal Urban Driving Schedule (FUDS) aging experiments and 25°C reference performance experiments.Through a comprehensive analysis of five aspects, including charge-discharge voltage, capacity degradation, increased internal resistance, open circuit voltage (OCV) changes, and Incremental Capacity (IC) curve variations, we thoroughly examined the characteristics and parameter variations during the hightemperature aging process.

Battery experiment platform
The research focused on a commercial 135 Ah LFP battery.The cathode material of the battery is lithium iron phosphate and the cathode material is graphite.The charging cut-off voltage is 3.8 V and the discharge cut-off voltage is 2 V.The battery experimental platform is shown in Figure 1 (a).The battery is placed inside a programmable temperature chamber, and the current sampling lines of the charge-discharge system are connected to the battery to collect the current and voltage of the battery during the experiment.The sampling data are sent to the main control computer through controller area network (CAN) communication for data logging.The maximum sampling current of the charge-discharge system is 300 A, and the maximum sampling voltage is 5 V.The temperature chamber can be adjusted from -40°C to 150°C.

Battery experiment contents
To achieve cyclic aging of the battery at high temperatures, the ambient temperature is adjusted to 45°C.The battery is subjected to 1C constant current constant voltage charging (CCCV) with a cut-off current of 1/20C, and NEDC and FUDS dynamic discharge, as illustrated in Figure 1 (b).Each minor cycle of high-temperature aging refers to 1C constant current constant voltage charging followed by NEDC discharge or 1C constant current constant voltage charging followed by FUDS discharge.These two steps constitute one main cycle.At the 50th, 150th, and 250th minor cycles, reference performance experiments are conducted at 25°C ambient temperature.The reference performance experiments include capacity testing, Hybrid Pulse Power Characteristic (HPPC) testing, and OCV testing, as illustrated in Figure 1 (b), Figure 1 (c), and Figure 1 (d), respectively.The capacity test involves 1/3C CCCV and 1/3C CC discharge, with three cycles performed to obtain the average discharge capacity.HPPC testing involves 10-second bi-directional pulses of 1/3C and 1C at intervals of 0.1 state of charge (SOC) to obtain the voltage pulse response for calculating the internal resistance parameters.To ensure that the battery is not overcharged or over-discharged, discharge pulses are only performed at 1 SOC, and charge pulses are only performed at 0 SOC.LFP batteries exhibit hysteresis in OCV, with the OCV during charging higher than discharge.To obtain the OCV during the aging process, the battery is subjected to a current of 1/3C, and the SOC is adjusted by 0.05 at each step, allowing it to rest for two hours before recording the OCV value.The SOC adjustment path is as follows: 1→0.95…→0→…→0.95→1.

Charge-discharge voltage variation
The charge and discharge voltage curves of the battery at 25°C after 50, 150, and 250 high-temperature cycles are shown in Figure 2. As the number of cycles increases, the charging voltage plateau increases, the discharging voltage plateau decreases, and the time taken to reach the upper cut-off voltage during charging decreases.This indicates that during high-temperature aging, the internal resistance of the battery increases.Figure 3. Capacity-cycle curve.

Capacity degradation
To investigate the capacity degradation during high-temperature cycling, the capacity test results are used to plot the capacity versus cycle number curve, as shown in Figure 3.Under 250 cycles of hightemperature aging, the battery capacity exhibits linear degradation, with the charging capacity slightly larger than the discharging capacity.This is because side reactions, such as the decomposition and formation of the solid electrolyte interface (SEI), occur during the battery discharge process, resulting in a reduction.The ratio of the discharging capacity to the charging capacity is known as the coulombic efficiency, .In the three reference performance experiments, the coulombic efficiency for charging and discharging is 0.9986, indicating that the side reaction rate did not increase.where R char/dis is the charge/discharge resistance, V e is the end value of the pulse voltage of 1s/10s, V i is the initial value of the pulse voltage of 1s/10s, I is the pulse current, and the charge is positive.The 10 s internal resistance is significantly higher than the 1 s internal resistance due to more pronounced polarization phenomena occurring within the battery at the 10 s time scale.Analyzing the trends in 1 s and 10 s internal resistances, it can be observed that the internal resistance increases with the number of cycles, and the internal resistance values at the 1C rate are generally lower than those at the 1/3C rate.Higher current rates lead to increased internal temperature rise and enhanced electrochemical reaction activity, resulting in lower internal resistance.

Open circuit voltage variation
The OCV also undergoes corresponding changes during the high-temperature aging process, as shown in Figure 5.The OCV of LFP batteries is related to their charging and discharging paths, and different charge-discharge paths result in different OCV values [10].The aging behavior of OCV varies in different SOC ranges.In the 0.05 SOC to 0.3 SOC range, the OCV increases with the number of cycles, as shown in Figure 5 (b).In the 0.55 SOC to 0.75 SOC range, the OCV decreases with the number of cycles, as significant changes occur between the two plateau regions.However, in the 0.95 SOC to 1 SOC range, the OCV increases with the number of cycles, contrary to the trends observed in the first two SOC ranges.

Analysis of incremental capacity
The incremental capacity (IC) is the derivative curve of capacity to voltage, as shown in (2).The IC curve is widely used in the analysis of battery aging mechanisms.
where Q is the capacity, and V is the voltage.This research calculates the IC curves based on the voltage curves obtained from capacity testing.The results are illustrated in Figure 6, where Figure 6 (a) represents the changes in the IC curve during charging, and Figure 6 (b) represents the changes during discharging.The charging IC curve has three peaks: the first peak appears in the voltage range of 3.25 V to 3.3V, the second peak appears in the range of 3.3V to 3.4V, and the third peak appears between 3.4 V and 3.5 V.With increasing cycles, the charging IC curve shifts to the right, indicating an increase in battery internal resistance leads to higher charging voltage.Under 150 cycles, the first two peaks do not show significant changes.In contrast, the third peak decreases significantly, suggesting that the primary degradation mechanism in the 50-150 cycle range is the LLI, which may be related to the consumption of lithium ions in the formation of SEI film.In the 250-cycle voltage IC curve, all three peaks show a significant decrease, indicating a loss of active material (LAM) occurring between 150 and 250 cycles.
Similarly, the discharging IC curve exhibits three peaks: the first peak appears in the voltage range of 3.1 V to 3.15 V, the second peak appears in the range of 3.2 V to 3.25 V, and the third peak occurs around 3.28 V.With an increasing number of cycles, the discharging IC curve shifts to the left, indicating a continuous increase in internal resistance.

Conclusions
This research conducts high-temperature dynamic aging and reference performance experiments on a commercial 135 Ah LFP battery.Detailed analysis is performed on the changes in voltage, capacity, internal resistance, OCV, and IC curves to investigate the aging characteristics.The capacity exhibited a linear decay pattern under 250 cycles of NEDC and FUDS high-temperature cycling.The OCV decreased continuously in the low SOC region and gradually increased in the high SOC region as the aging progressed.The increase in internal resistance resulted in higher charging voltages and a rightward shift of the charging IC curve, while the discharging IC curve shifted leftward.The IC curve variations analysis revealed that the LLI and the LAM are the core factors contributing to the aging mechanism in high-temperature cycling.

Figure 1 .
Figure 1.Battery experiments (a) Experiment platform (b) Cycling experiment (c) Capacity experiment (d) Hybrid pulse power characteristic experiment (e) Open circuit voltage experiment.

Figure 4 . 4
Figure 4. Ohmic resistance (a) 1s charge resistance (b) 1s discharge resistance (c) 10s charge resistance (d) 10s discharge resistance.The internal resistance values are calculated based on the HPPC test results for 1/3C and 1C with 1s and 10s pulse durations to in vestigate the degradation of internal resistance during high-temperature cycling.The equations for internal resistance are shown in (1), and the calculation results are shown in Figure 4.