In recent years, lithium-ion batteries (LIBs) have emerged as pivotal components in energy storage systems, particularly for applications requiring high energy density and long cycle life. Among various LIB chemistries, lithium iron phosphate (LiFePO4) based energy storage cells have gained significant attention due to their inherent safety, cost-effectiveness, and environmental friendliness. However, the performance degradation of these energy storage cells under elevated temperatures remains a critical challenge, limiting their operational lifespan and reliability in real-world scenarios. Understanding the underlying aging mechanisms is essential for optimizing battery design and enhancing the durability of energy storage systems. This study focuses on investigating the degradation processes in commercial 280 Ah LiFePO4/graphite energy storage cells subjected to high-temperature cycling at 45°C, with an emphasis on identifying the primary factors contributing to capacity fade and material deterioration.
We conducted a comprehensive analysis of the electrochemical behavior and material properties of energy storage cells throughout their lifecycle, from initial state to 60% state-of-health (SOH). The SOH is defined as the ratio of the actual capacity to the initial capacity, expressed as: $$ \text{SOH} = \frac{C_{\text{actual}}}{C_{\text{initial}}} \times 100\% $$ where \( C_{\text{actual}} \) is the measured capacity and \( C_{\text{initial}} \) is the nominal capacity. Our experimental approach involved cycling the energy storage cells under controlled conditions, followed by post-mortem analysis of the electrodes and electrolytes. The key parameters monitored during cycling included capacity retention, voltage profiles, and differential voltage (dQ/dV) curves, which provide insights into the electrochemical processes within the energy storage cells.
The cycling tests were performed using a constant current-constant voltage (CC-CV) protocol. Specifically, the energy storage cells were charged at 1 C rate to 3.65 V, held at constant voltage until the current dropped below 0.05 C, rested for 30 minutes, and then discharged at 1 C to a cutoff voltage of 2.5 V. This cycle was repeated continuously at 45°C to accelerate aging. The capacity fade over cycles was recorded, and cells at 100%, 90%, and 60% SOH were disassembled in an inert atmosphere for detailed material characterization. The disassembled electrodes were rinsed with dimethyl carbonate (DMC), dried, and punched into discs for half-cell assembly. These half-cells, configured as CR2032 coin cells with lithium metal as the counter electrode, were evaluated at 0.1 C rate to assess the specific capacities of the positive and negative electrodes separately.
To quantify the degradation mechanisms, we employed various analytical techniques. Scanning electron microscopy (SEM) was used to examine the morphological changes in the electrodes. X-ray diffraction (XRD) and Raman spectroscopy provided insights into the structural evolution of the active materials. X-ray photoelectron spectroscopy (XPS) with depth profiling was utilized to analyze the composition and thickness of the solid electrolyte interphase (SEI) layer. Additionally, gas chromatography-mass spectrometry (GC-MS) and ion chromatography (IC) were applied to monitor changes in the electrolyte composition. The integration of these methods allowed us to correlate electrochemical performance with material properties in the energy storage cells.
The cycling performance of the energy storage cells at 45°C revealed a progressive capacity loss, as summarized in Table 1. The initial capacity of the fresh cells was approximately 280 Ah, which decreased to around 162 Ah after 4750 cycles, corresponding to a SOH of 58%. The capacity fade was not linear; instead, it accelerated beyond 67% SOH, indicating a threshold where degradation mechanisms become more pronounced. This nonlinear behavior underscores the complexity of aging in energy storage cells under thermal stress.
| Cycle Number | Capacity (Ah) | SOH (%) |
|---|---|---|
| 0 | 280.0 | 100.0 |
| 1000 | 252.0 | 90.0 |
| 3000 | 196.0 | 70.0 |
| 4750 | 162.4 | 58.0 |
The voltage profiles during charging and discharging exhibited increasing polarization with cycling, manifested as a shortening of the voltage plateaus and a rise in overpotentials. This can be mathematically described by the equation for cell polarization: $$ \eta = V_{\text{charge}} – V_{\text{discharge}} $$ where \( \eta \) represents the overpotential, which increases as internal resistance rises due to material degradation. The dQ/dV curves, derived from the charge cycles, displayed distinct peaks corresponding to the staging transitions in the graphite anode. Specifically, three characteristic peaks were observed at voltages around 3.30 V, 3.35 V, and 3.40 V, denoted as Peak 1, Peak 2, and Peak 3, respectively. These peaks are associated with the lithiation stages of graphite, and their evolution provided critical insights into the degradation of energy storage cells.
As cycling progressed, the intensities of Peak 2 and Peak 3 diminished significantly, with Peak 3 disappearing entirely by 80% SOH. The area under each peak, which corresponds to the charge capacity associated with each lithiation stage, was calculated to quantify the loss. The total charge capacity \( Q_{\text{total}} \) can be expressed as: $$ Q_{\text{total}} = \int \frac{dQ}{dV} dV $$ where the integral is taken over the voltage range of interest. The reduction in peak areas indicated a decline in accessible lithium and structural changes in the graphite. For instance, the area under Peak 2 decreased by 53% from 100% to 60% SOH, highlighting the role of anode degradation in energy storage cells.
Post-mortem analysis of the positive electrodes revealed that LiFePO4 particles maintained their structural integrity up to 90% SOH, but at 60% SOH, microcracks and surface fractures were evident. XRD patterns showed an increase in the intensity of FePO4 phases, such as the (020) and (200) peaks, suggesting the formation of lithium vacancies due to irreversible lithium loss. The specific capacity of the positive electrodes, measured in half-cells, decreased marginally from 158 mAh/g at 100% SOH to 147.6 mAh/g at 60% SOH. This capacity loss can be attributed to active material degradation and is quantified by the formula: $$ \Delta C_{\text{positive}} = C_{\text{initial}} – C_{\text{aged}} $$ where \( \Delta C_{\text{positive}} \) is the capacity loss. Based on the initial charging capacity, the loss of active lithium accounted for approximately 49.2% of the total capacity fade, while active material loss contributed about 4%.
In contrast, the graphite anode exhibited more severe degradation. SEM images showed that the smooth surface of fresh graphite became rough and covered with deposits after cycling. At 60% SOH, pronounced grooves and cracks indicated mechanical failure of the graphite structure. XRD analysis revealed a shift in the (002) peak from 26.56° to 26.74°, indicating lattice distortion. Raman spectroscopy further confirmed the loss of structural order, with the D/G band intensity ratio increasing from 0.302 to 0.859, which is calculated as: $$ R = \frac{I_D}{I_G} $$ where \( I_D \) and \( I_G \) are the intensities of the D and G bands, respectively. This increase in \( R \) signifies enhanced disorder, which adversely affects the stability of the SEI layer and promotes further side reactions in energy storage cells.
The half-cell tests on the graphite electrodes demonstrated a substantial reduction in specific capacity, from 336 mAh/g at 100% SOH to 196.2 mAh/g at 60% SOH. This represents a capacity loss of about 45.5%, primarily due to graphite structural damage and SEI growth. The thickening of the SEI layer was confirmed by XPS depth profiling, which estimated the SEI thickness to increase from 43.7 nm in fresh cells to 82.5 nm in aged cells. The SEI growth consumes active lithium and electrolyte, leading to irreversible capacity loss. The rate of SEI growth can be modeled empirically as: $$ \delta_{\text{SEI}} = k \sqrt{t} $$ where \( \delta_{\text{SEI}} \) is the SEI thickness, \( k \) is a rate constant, and \( t \) is time or cycle number.
Electrolyte analysis indicated progressive decomposition of key components. The concentrations 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 entirely consumed by 60% SOH, exacerbating SEI instability. These changes are summarized in Table 2, which highlights the compositional shifts in the electrolyte of energy storage cells during aging.
| Component | 100% SOH (%) | 90% SOH (%) | 60% SOH (%) |
|---|---|---|---|
| EC | 41.6 | 39.2 | 35.0 |
| EMC | 12.3 | 11.5 | 9.4 |
| DMC | 31.1 | 30.8 | 29.7 |
| LiPF6 | 14.0 | 12.5 | 10.9 |
| VC | 0.5 | 0.2 | 0.0 |
| FEC | 0.5 | 0.2 | 0.0 |
Furthermore, examination of the separators revealed pore blockage and detachment of the Al2O3 coating on the cathode side. The gas permeability tests showed increased resistance to air flow, with the time for 100 mL of air to pass through rising from 190 s in fresh separators to 526 s in aged ones, particularly in the inner regions of the jellyroll. This blockage impedes ion transport, increasing internal resistance and contributing to power fade in energy storage cells. The relationship between permeability \( P \) and time \( t \) can be expressed as: $$ P = \frac{V}{t \cdot A} $$ where \( V \) is the volume of air, \( t \) is time, and \( A \) is the cross-sectional area.
Based on our findings, the degradation mechanism of LiFePO4 energy storage cells under high-temperature cycling involves a synergistic interplay of factors. Initially, capacity fade is dominated by active lithium loss due to SEI formation and reorganization. As cycling continues, graphite structural damage becomes the primary driver, leading to accelerated degradation. The structural failure of graphite not only reduces the anode’s capacity but also triggers continuous SEI growth, electrolyte decomposition, and separator clogging. This cascade effect is encapsulated in the following empirical model for capacity fade: $$ C_{\text{loss}} = A \cdot e^{-B \cdot \text{SOH}} + C \cdot \text{cycles} $$ where \( A \), \( B \), and \( C \) are constants derived from experimental data, representing the contributions of lithium loss and material degradation.
In conclusion, our study elucidates that the high-temperature aging of LiFePO4 energy storage cells is predominantly caused by graphite anode failure, which initiates a series of secondary reactions. The insights gained from this work can inform the development of more robust energy storage cells with enhanced thermal stability and longer service life. Future research should focus on optimizing graphite materials and electrolyte formulations to mitigate these degradation pathways in energy storage cells.