Unravelling the Storage Degradation Mechanism of LiFePO₄–Graphite Batteries

Lithium iron phosphate-graphite (LFP-C) batteries are widely used in various fields due to their high energy density, low cost, high safety, good rate performance, long cycle life, and environmental friendliness. However, prolonged calendar aging leads to increased internal resistance and capacity decay, thereby shortening their service life. In this work, the impacts of storage temperature and SOC on the calendar aging performance of LFP-C batteries were investigated. The results show that the capacity decay is more severe under high temperature and high SOC conditions. The structural evolution of the anode and cathode materials, as well as the cathode–electrolyte interphase (CEI) and solid electrolyte interphase (SEI) layers were analyzed. It is indicated that the structure of LiFePO₄ cathode remains stable, while the degree of disorder in the graphite anode increases. The decomposition of the liquid electrolyte leads to an increase of LiF content in the CEI and SEI layers with decreasing Li₂CO₃ content in the SEI layer at 50 ℃.

In an inert atmosphere glove box, several LFP-C CR2025 coin cells were assembled. The cells were charged to the targeted SOC, and then divided into six groups and stored under different conditions (i.e., at 0℃-50%SOC, 0 ℃-100%SOC, 25 ℃ -50%SOC, 25 ℃-100%SOC, 50 ℃-50%SOC, and 50 ℃-100%SOC) for one month. After storage, each group of cells was cycled at 1C to assess the impact of different storage conditions on the electrochemical performance of the cells. Other cells were disassembled, and then cleaned to obtain the positive and negative electrodes. The capacity degradation mechanisms of batteries during calendar aging were characterized by X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy dispersive X-Ray spectroscopy (SEM-EDX), and X-ray photoelectron spectroscopy (XPS).

After 1-month storage, the LFP - graphite coin cells are charged and discharged at 1 C for 300 cycles. The results show that the cells stored at 50 ℃ cannot deliver any capacity. The cells stored at 0 ℃ exhibit a significantly higher discharge capacity, compared to the cells stored at 25 ℃. The cycling performance results of the cells after storage at different states of charge (SOC) indicate that the discharge capacity of coin cells stored at 50% SOC is higher than that of cells stored at 100% SOC.

It is revealed that there is a slight difference in the XRD patterns of LFP electrodes stored under various temperatures and SOC conditions, indicating that the structure of LFP maintains a high stability under various storage conditions. The XRD patterns of the graphite anodes also remain unchanged, and only the peaks attributed to the graphite and current collector Cu foil appear.

The results by Raman spectroscopy indicate that the LFP material remains stable after storage, while the degree of disorder in the graphite anode increases significantly and becomes more pronounced with the increase of storage temperatures and SOC. The results by FTIR reveal the side reaction products on the surfaces of both the cathode and anode materials (i.e., Li₂CO₃, (CH₂OCOOLi)₂, and ROCO₂Li).

The results by SEM-EDX indicate that there is a slight change in the elemental ratio on the surface of the cathode electrodes after storage under different conditions, further validating the stability of the LFP material. The surface of graphite particles becomes rough after storage, and the content of fluorine and phosphorus elements on the surface of graphite anodes increases under high-temperature and high-SOC conditions.

The results by XPS show that the content of LiF in the CEI layers increases with increasing the temperature and SOC. The storage temperature shows a more pronounced effect on the composition of the CEI rather than the SOC. The relative content of LiF and P—F in the SEI layers significantly increases, while the relative content of Li₂CO₃ decreases due to decomposition as the storage temperature increases.

LiFePO₄-graphite coin cells with different SOC (i.e., 50% and 100%) were stored at various temperatures (i.e., 0, 25 ℃ and 50 ℃) and for one month and then charged and discharged for 300 cycles. The results showed that cells stored at 50℃ were completely inoperative. The battery capacity retention followed an order of 0 ℃-50%SOC > 0 ℃-100%SOC > 25 ℃ -50%SOC > 2 ℃-100%SOC.

It was revealed that the LFP cathode exhibited a structural stability as the storage temperature and SOC increased. Conversely, the degree of disorder of graphite anode increased, having a detrimental effect on the lithium ion migration. It was evident that the temperature could have a dominant influence on the composition of the CEI rather than SOC. The relative content of Li₂CO₃ decreased, while the relative content of LiF decreased within the CEI layers as the storage temperatures and SOC increased. In addition, the relative content of LiF and P—F increased in the SEI layers with temperature, and the temperature had a more significant effect rather than SOC on the SEI layer.