Energy storage lithium batteries, particularly lithium iron phosphate (LiFePO4)/graphite systems, are critical for applications in electric vehicles and grid storage due to their safety and long cycle life. However, performance degradation at elevated temperatures remains a significant challenge, limiting their operational lifespan. In this study, we investigate the capacity fading mechanisms of a 280 Ah energy storage lithium battery under high-temperature cycling at 45°C. By combining electrochemical analysis with material characterization, we aim to elucidate the primary factors contributing to decay, with a focus on the role of graphite anode degradation and active lithium loss. The findings provide insights into designing more durable energy storage lithium battery systems for high-temperature environments.
The electrochemical performance of the energy storage lithium battery was evaluated through cyclic charge-discharge tests at 45°C. The capacity retention as a function of cycle number is summarized in Table 1, showing a rapid decline beyond 67% state-of-health (SOH), where SOH is defined as the ratio of actual capacity to initial capacity. This accelerated fading in later cycles indicates intensified internal side reactions, such as electrolyte decomposition and solid electrolyte interphase (SEI) growth, which are common in energy storage lithium battery systems under thermal stress.
| Cycle Number | Capacity (Ah) | SOH (%) |
|---|---|---|
| 0 | 280 | 100 |
| 1000 | 252 | 90 |
| 3000 | 196 | 70 |
| 4750 | 162.4 | 58 |
To quantify the capacity loss, we employed differential voltage (dQ/dV) analysis during charging. The dQ/dV curves exhibit three characteristic peaks corresponding to the staging transitions of lithium intercalation into graphite. The evolution of these peaks with cycling is described by the following empirical model for peak area decay: $$ A(n) = A_0 \cdot \exp(-\beta n) $$ where \( A(n) \) is the peak area at cycle \( n \), \( A_0 \) is the initial area, and \( \beta \) is the decay constant. As summarized in Table 2, the areas of peaks 2 and 3 decrease significantly by 53% and 100%, respectively, at 60% SOH, indicating substantial graphite structure damage and active lithium loss in the energy storage lithium battery.
| Peak | Voltage Range (V) | Area at 100% SOH (Ah/V) | Area at 60% SOH (Ah/V) | Reduction (%) |
|---|---|---|---|---|
| Peak 1 | 3.30-3.35 | 0.15 | 0.12 | 20 |
| Peak 2 | 3.36-3.42 | 0.25 | 0.12 | 52 |
| Peak 3 | 3.43-3.50 | 0.20 | 0.00 | 100 |
Post-cycling analysis of electrodes from the energy storage lithium battery revealed critical material changes. For the LiFePO4 cathode, X-ray diffraction (XRD) patterns showed increased intensity of FePO4 phases, suggesting lithium vacancy formation. The degree of lithium loss can be modeled using the phase fraction equation: $$ f_{\text{FePO}_4} = \frac{I_{(020)}}{I_{(020)} + I_{\text{LiFePO}_4}} $$ where \( I \) represents peak intensity. In half-cell tests, the specific capacity of cycled cathodes decreased from 158 mAh/g (100% SOH) to 147.6 mAh/g (60% SOH), implying a 4% active material loss. This highlights that while cathode degradation is minor, it contributes to the overall capacity fade in energy storage lithium batteries.
In contrast, the graphite anode exhibited severe degradation. Scanning electron microscopy (SEM) images showed surface roughening and cracking, while XRD and Raman spectroscopy confirmed structural disorder. The Raman D/G intensity ratio increased from 0.302 to 0.859, indicating graphitization loss. The specific capacity in half-cells dropped by 45.5%, from 336 mAh/g to 196.2 mAh/g, which we attribute to graphite fracture and SEI growth. The SEI thickness, estimated from X-ray photoelectron spectroscopy (XPS) depth profiling, increased from 43.7 nm to 82.5 nm, consuming active lithium. The SEI growth kinetics can be described by: $$ \delta_{\text{SEI}} = \delta_0 + k \sqrt{t} $$ where \( \delta_{\text{SEI}} \) is the thickness, \( \delta_0 \) is the initial thickness, \( k \) is the growth rate constant, and \( t \) is time. This equation underscores the continuous lithium depletion in energy storage lithium battery systems under high temperatures.
Further analysis of the electrolyte in the energy storage lithium battery using gas chromatography-mass spectrometry (GC-MS) and ion chromatography (IC) revealed decomposition of solvents and additives. As shown in Table 3, the relative content of ethylene carbonate (EC) decreased from 41.6% to 35%, while lithium salt (LiPF6) content dropped from 14% to 10.9%. Additives like vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were fully consumed, exacerbating SEI instability. These changes correlate with increased cell polarization and resistance, further accelerating the decay of the energy storage lithium battery.
| Component | Mass Fraction at 100% SOH (%) | Mass Fraction at 60% SOH (%) | Change (%) |
|---|---|---|---|
| EC | 41.6 | 35.0 | -15.9 |
| EMC | 12.3 | 9.4 | -23.6 |
| LiPF6 | 14.0 | 10.9 | -22.1 |
| VC | 1.5 | 0.0 | -100 |
| FEC | 1.2 | 0.0 | -100 |
The separator in the energy storage lithium battery also showed degradation, with clogged pores and reduced porosity. Permeability tests indicated a rise in gas passage time from 190 s/100 mL (pristine) to 526 s/100 mL in inner regions, impeding ion transport. This micro-scale blockage contributes to increased internal resistance and capacity loss in energy storage lithium batteries. The overall capacity fade mechanism can be summarized by a combined model: $$ \Delta C_{\text{total}} = \Delta C_{\text{Li}} + \Delta C_{\text{anode}} + \Delta C_{\text{cathode}} + \Delta C_{\text{resistance}} $$ where \( \Delta C_{\text{Li}} \) is active lithium loss, \( \Delta C_{\text{anode}} \) and \( \Delta C_{\text{cathode}} \) are electrode material losses, and \( \Delta C_{\text{resistance}} \) is due to increased impedance. At 60% SOH, graphite anode degradation accounts for approximately 45.5% of the total capacity loss, underscoring its dominance in high-temperature aging of energy storage lithium batteries.
In conclusion, the high-temperature degradation of energy storage lithium batteries is primarily driven by graphite anode failure, which triggers cascading effects like SEI thickening, electrolyte decomposition, and separator clogging. Our results emphasize the need for improved anode materials and electrolyte formulations to enhance the thermal resilience of energy storage lithium battery technologies. Future work should focus on real-time monitoring and advanced materials to mitigate these issues, ensuring longer service life for energy storage lithium battery systems in demanding applications.