Lithium iron phosphate (LiFePO4) batteries, often paired with graphite, are widely recognized for their exceptional characteristics as power lithium-ion batteries. They offer high safety, extended cycle life, and environmental friendliness, making them a popular choice for applications in electric transportation, solar-powered streetlights, and energy grid storage. Like all batteries, LiFePO4/graphite-based power lithium-ion batteries face the challenge of mid to long-term storage during production, sales, and usage. Understanding how to set storage conditions to minimize capacity loss, power loss, and impedance increase before and after storage is of significant importance when assessing storage performance factors.
A study conducted by Li Jia and others explored the impact of storage conditions on LiCoO2/graphite and Li(Ni1/3Co1/3Mn1/3)O2/graphite-based batteries under different state-of-charge (SOC) levels and temperatures. They compared the batteries' capacity, charge-discharge characteristics, power, cycling performance, overcharge resistance, thermal stability, and analyzed changes in electrode materials, separators, and electrochemical impedance before and after storage. Their findings indicated that higher storage temperatures and SOC levels led to more severe battery performance degradation.
In another study involving commercially available 200mAh pouch cells, researchers investigated LiFePO4/graphite-based batteries stored at 55°C under various SOC levels. They compared the battery's electrical properties and safety performance before and after storage and analyzed structural changes in the positive electrode material. This research aimed to reveal the variation patterns in the storage performance of LiFePO4/graphite-based power lithium-ion batteries at high temperatures (55°C).
In our study, we focused on cylindrical 9Ah aluminum-cased LiFePO4/graphite-based power lithium-ion batteries. We conducted experiments to evaluate changes in battery capacity, voltage, AC internal resistance, and other characteristics during and after storage at temperatures of 55°C, 45°C, 23°C, and -10°C. Additionally, we placed particular emphasis on testing parameters such as DC internal resistance, power capability, constant current charge-to-discharge ratio, Coulombic/energy efficiency at 45°C and 23°C before and after storage. We also analyzed how changes in these parameters affected the overall performance of LiFePO4/graphite-based power lithium-ion battery packs. Our study aimed to determine the optimal storage conditions for these batteries.
Experimental Setup:
- We used cylindrical 32131-type aluminum-cased LiFePO4/graphite batteries with a rated capacity of 9Ah, featuring LiFePO4 as the positive electrode material and artificial graphite as the negative electrode material.
- For storage experiments at 45/55°C, we utilized a DHP200-type electrically heated constant-temperature incubator.
- Low-temperature storage was conducted in a refrigerator.
- Electrical performance testing equipment included the CT-3008W-5V100A-TF testing cabinet for capacity testing and the HIOKI3554 battery internal resistance tester (AC 1kHz) for AC internal resistance testing.
Experiment 1 (Storage Experiment):
- We selected over 60 individual batteries and subjected them to three weeks of charge-discharge cycles at room temperature with a 4500mA (0.5C) current within the voltage range of 3.65V to 2V. This step aimed to obtain the initial capacity values of the batteries.
- The batteries were brought to 100%, 50%, and 0% SOC states (20 batteries for each SOC level) before the end of the cycles.
- After allowing the batteries to stabilize for 15 hours, we measured their voltage, AC internal resistance, and other basic data.
- Subsequently, we placed 12 batteries from each SOC state into separate environments: 55°C oven, 45°C oven, 23°C air-conditioned room, and -10°C refrigerator.
- During the storage period, we conducted internal resistance and voltage tests on these batteries every 7 days.
- After the storage period ended, we subjected the batteries to three weeks of charge-discharge cycles at room temperature under the same conditions as the initial cycles. This allowed us to assess the batteries' post-storage capacity.
Experiment 2 (Storage Experiment):
- We selected over 45 individual batteries and subjected them to three weeks of charge-discharge cycles at room temperature with a 9000mA (1C) current within the voltage range of 3.65V to 2V. This step aimed to obtain the initial capacity values of the batteries.
- We also conducted HPPC (Hybrid Pulse Power Characterization) testing on the batteries according to the FreedomCAR standard, which involves high-current pulses and discharges.
- The batteries were brought to 100%, 80%, 50%, 30%, and 0% SOC states (9 batteries for each SOC level) before the end of the cycles.
- After allowing the batteries to stabilize for 15 hours, we measured their voltage, AC internal resistance, and other basic data.
- Subsequently, we placed 15 batteries from each SOC state into separate environments: 45°C oven and 23°C air-conditioned room.
- During the storage period, we conducted AC internal resistance, 1C capacity, and HPPC tests on these batteries every 28 days.
These experiments aimed to gain insights into the effects of different storage conditions on the performance of LiFePO4/graphite-based power lithium-ion batteries. The results will contribute to optimizing the storage practices for these batteries and enhancing their overall performance.
2. Data and Discussion
2.1 Changes in Battery Voltage, Internal Resistance, and Capacity During 28 Days of Storage in Experiment 1
Figure 1 illustrates the changes in open-circuit voltage (OCV) of the experimental batteries during 28 days of storage. It is apparent from Figure 1 that the OCV variations during storage at different temperatures and state-of-charge (SOC) levels are not significant. The best consistency is observed at 50% SOC, while the greatest variation is seen at 0% SOC. This variation is largely attributed to the polarization of LiFePO4/graphite-based batteries at different SOC levels. Generally, these batteries exhibit the most significant polarization at 0% SOC (fully discharged) and the least at 50% SOC. The voltage changes at different temperatures under 0% SOC conditions also show that higher temperatures facilitate rapid stabilization of the polarization state. Leveraging this principle, during the assembly of battery packs, batteries with similar polarization states can be selected quickly by warming them.
Figure 2 presents the changes in AC internal resistance during the 28-day storage period. It is evident that AC internal resistance decreases with increasing temperature because higher temperatures enhance the conductivity of various components inside the battery. However, after storage and testing at room temperature, there is not much difference in internal resistance among all the batteries. Nevertheless, noticeable changes in AC internal resistance are observed after storage at different SOC levels and temperatures. Batteries stored at high temperatures (45/55°C) and 100% SOC conditions show a significant increase in internal resistance. This increase is attributed to the thickening of the solid electrolyte interface (SEI) on the graphite negative electrode surface and trace decomposition of LiPF6 electrolyte, resulting in the formation of high-impedance inorganic salts such as LiF.
Table 1 provides data on capacity changes in the batteries after 28 days of storage. The data reveals that lower SOC levels are more favorable for capacity retention compared to higher SOC levels. Except for the case of 0% SOC at low temperatures (-10°C), where some capacity loss is observed, batteries at 0% SOC generally show an increase in capacity. This phenomenon might be due to the formation of new interfaces as a result of secondary particle cracking in the positive electrode material after storage, which regains the ability to intercalate and deintercalate lithium ions. In fact, this phenomenon also occurs in batteries that are cycled directly without undergoing storage. Such batteries often exhibit gradual capacity increases during the initial cycles.
2.2 Changes in AC Internal Resistance During 3 Months of Storage in Experiment 2
Figures 3 and 4 show the rate of change in AC internal resistance for batteries stored for three months at 25°C and 45°C, respectively, at different SOC levels (100%, 80%, 50%, 30%, 0%). It is consistent with the results from Experiment 1 that batteries stored at high temperatures (45°C) exhibit a more significant increase in AC internal resistance compared to those stored at 25°C. Furthermore, the SOC level that has the least impact on battery internal resistance during storage differs between the two temperatures. At 25°C, the SOC level with the least impact is 80% SOC, while at 45°C, all SOC levels experience substantial increases in internal resistance, with 30% SOC showing a relatively smaller increase. Literature [5] reports that changes in battery internal resistance during storage are primarily associated with the charge transfer resistance of the positive electrode LiFePO4, which varies with temperature and SOC. Our test results indicate that the charge transfer resistance of the positive electrode LiFePO4 increases more rapidly with increasing temperature and as SOC approaches the extremes (0% and 100%). When these factors act together on the battery, the results depicted in Figures 3 and 4 are observed, where the SOC levels with the least impact on battery internal resistance differ depending on the temperature.
2.3 Changes in 1C Capacity During 3 Months of Storage in Experiment 2
Figures 5 and 6 illustrate the rate of change in 1C capacity for batteries stored for three months at 25°C and 45°C, respectively, at different SOC levels (100%, 80%, 50%, 30%, 0%). Similar to the results in Experiment 1, batteries stored at high temperatures (45°C) exhibit a more pronounced decline in 1C capacity compared to those stored at 25°C. Storage at 0% SOC has the least impact on capacity loss.
The changes in capacity of LiFePO4/graphite-based power lithium-ion batteries during storage result from the interplay of three factors: (1) the formation of irreversible capacity on the negative electrode, mainly due to the loss of active lithium within the graphite electrode during extended storage, leading to the formation of inactive lithium (dead lithium). This negatively affects capacity and tends to be more pronounced at higher SOC levels. (2) According to Ohm's law: U = UR + Ur = I(R + r), where U is the battery electromotive force, UR is the terminal voltage of the circuit, Ur is the internal voltage loss of the battery, R is the external circuit resistance, r is the battery's internal resistance, and I is the discharge current. The increase in internal resistance during storage, which reduces the terminal voltage UR during discharge, results in a shorter discharge duration and, consequently, a decrease in capacity. This effect is also negative. (3) As storage time increases, the secondary particles within the positive electrode LiFePO4 begin to crack, forming fresh interfaces and regaining the activity to intercalate and deintercalate lithium ions. This effect is positive and may lead to capacity increases, particularly during the early stages of storage. Based on the analysis of these three factors, it becomes evident that the test data reflects the combined effects of these influences.
2.4 Changes in DC Internal Resistance During 3 Months of Storage in Experiment 2
Figures 7 and 8 display the rate of change in DC internal resistance for batteries stored for three months at 25°C and 45°C, respectively, at different SOC levels (100%, 80%, 50%, 30%, 0%). Interestingly, the batteries stored at the higher temperature (45°C) exhibit slightly lower DC internal resistance compared to those stored at 25°C. The trend in DC internal resistance change with respect to SOC is similar under different temperature storage conditions, with the highest increase occurring at 0% SOC, followed by 100%, 30%, 80%, and 50% SOC.
2.5 Changes in Power Capability During 3 Months of Storage in Experiment 2
Figures 9, 10, 11, and 12 present the rate of change in discharge power capability and feedback power capability for batteries stored for three months at 25°C and 45°C, respectively, at different SOC levels (100%, 80%, 50%, 30%, 0%). Consistent with previous observations, batteries stored at the higher temperature (45°C) exhibit a more pronounced reduction in power capability compared to those stored at 25°C. The power capability change with SOC follows a similar pattern under different temperature storage conditions, with the highest reduction occurring at 0% SOC, followed by 100%, 30%, 80%, and 50% SOC.
From the results obtained, it is evident that changes in battery power capability closely mirror the variations in battery
