In recent years, the application of lithium-ion battery technology has expanded significantly from traditional consumer electronics to electric vehicles, energy storage systems, and electric bicycles. Among various cathode materials, lithium iron phosphate (LiFePO4) has gained prominence due to its long cycle life, inherent safety, and cost-effectiveness. However, the performance degradation of lithium-ion battery systems over time remains a critical concern, particularly under abuse conditions such as overcharging. This study investigates the inhomogeneous reactions within a commercial LiFePO4 lithium-ion battery after cycling and subsequent overcharge testing, focusing on internal reaction uniformity and interface changes. The safety of lithium-ion battery packs is paramount, and understanding degradation mechanisms is essential for improving design and management strategies.
The degradation of lithium-ion battery performance is often attributed to inhomogeneous electrochemical reactions and interfacial evolution, which can lead to safety hazards under abusive scenarios. Overcharging is a common abuse condition that can induce thermal runaway, fires, or explosions in lithium-ion battery systems. Standard safety tests, such as those outlined in regulations like GB 38031—2020, typically assess new lithium-ion battery units, but aged lithium-ion battery cells may exhibit different behaviors due to cumulative damage. In this work, I subjected a cycled commercial LiFePO4 lithium-ion battery to an overcharge test per standard protocols, followed by detailed analysis using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The goal is to elucidate the spatial inhomogeneity in reaction dynamics and material changes, which are crucial for predicting failure in real-world lithium-ion battery applications.
The experimental approach involved a commercial soft-pack LiFePO4 lithium-ion battery with a nominal capacity of 20 Ah. The lithium-ion battery was first cycled under controlled conditions to simulate aging. Cycling was performed using a constant current-constant voltage (CC-CV) charge protocol and constant current discharge. The charge process involved a 0.5 C (10 A) constant current charge to 3.65 V, followed by a constant voltage charge until the current dropped to 0.05 C (1 A). Discharge was conducted at 1.0 C to a cut-off voltage of 2.50 V. This cycle was repeated to induce degradation, mimicking long-term use of the lithium-ion battery in electric vehicle applications.
The capacity fade of the lithium-ion battery over cycles can be modeled using an exponential decay function, commonly applied to lithium-ion battery systems:
$$C_n = C_0 \cdot e^{-k \cdot n}$$
where \(C_n\) is the capacity after \(n\) cycles, \(C_0\) is the initial capacity, and \(k\) is the degradation rate constant. For the studied lithium-ion battery, after 1,015 cycles, the capacity retention was 82.72%, indicating significant aging. This decay in lithium-ion battery performance is linked to loss of active lithium, material detachment, and increased internal resistance, all exacerbated by inhomogeneous reactions.
Following cycling, the aged lithium-ion battery was subjected to an overcharge test. According to safety standards, the lithium-ion battery was charged at 1.0 C to 110% of its termination voltage (4.00 V). During the test, surface temperatures were monitored at multiple points using thermocouples, as uneven heating can reflect internal inhomogeneities in the lithium-ion battery. The voltage and temperature profiles during overcharging are critical for understanding thermal behavior in abused lithium-ion battery systems.
The power generation during charging or discharging in a lithium-ion battery can be described by Joule heating:
$$P = I^2 R$$
where \(P\) is the thermal power, \(I\) is the current, and \(R\) is the internal resistance. Inhomogeneous current distribution due to electrode imperfections or aging can lead to localized hot spots, increasing the risk of failure in the lithium-ion battery.
After the overcharge test, the lithium-ion battery was disassembled in an argon-filled glovebox to prevent contamination. The cathode, anode, and separator were separated, and electrode samples from different regions (upper, middle, lower) were collected for analysis. These samples were washed with dimethyl carbonate (DMC) to remove residual electrolytes and lithium salts. The inhomogeneity in reaction was assessed through half-cell electrochemical tests, SEM imaging, and XPS analysis.
The electrochemical performance of selected regions was evaluated by assembling coin-type half-cells. For cathode samples, the specific charge capacity (lithium deintercalation capacity) was measured, while for anode samples, the specific charge and discharge capacities (lithium intercalation/deintercalation) were determined. The data are summarized in Table 1, highlighting the spatial variations in the lithium-ion battery electrodes.
| Region | Position | Cathode Specific Charge Capacity (mAh/g) | Anode Specific Charge Capacity (mAh/g) | Anode Specific Discharge Capacity (mAh/g) |
|---|---|---|---|---|
| Upper | C1 (Cathode), A1 (Anode) | 29.0 | 195.8 | 210.5 |
| Middle | C2 (Cathode), A2 (Anode) | 9.4 | 206.6 | 225.3 |
| Lower | C3 (Cathode), A3 (Anode) | 30.7 | 183.0 | 235.8 |
The table reveals significant inhomogeneity in the lithium-ion battery. The cathode’s middle region (C2) shows the lowest specific charge capacity (9.4 mAh/g), indicating reduced lithium deintercalation capability. Conversely, the anode’s middle region (A2) has a higher specific charge capacity (206.6 mAh/g), but the lower region (A3) exhibits the lowest anode specific charge capacity (183.0 mAh/g) and the highest discharge capacity, suggesting lithium metal deposition. This asymmetry underscores the complex degradation pathways in aged lithium-ion battery systems.
The temperature distribution during overcharging further illustrates inhomogeneity. Table 2 lists the maximum surface temperatures recorded at different points on the lithium-ion battery during the test.
| Thermocouple Point | Location Relative to Terminals | Maximum Temperature (°C) |
|---|---|---|
| 1 | Near positive terminal | 39.5 |
| 2 | Center, upper side | 38.2 |
| 3 | Near negative terminal | 40.1 |
| 4 | Lower side | 35.8 |
Points near the terminals (1 and 3) showed higher temperatures, likely due to greater current density and resistive heating in those regions. This thermal gradient can accelerate localized degradation in the lithium-ion battery, such as SEI growth or lithium plating.
The SEM analysis of the graphite anode after overcharging revealed distinct morphological differences across regions. In the lower region (A3), the surface was covered with deposits, indicative of lithium metal plating, while the upper and middle regions (A1 and A2) displayed relatively clean graphite particles. This visual evidence confirms the inhomogeneous reaction distribution in the lithium-ion battery anode, which aligns with the electrochemical data.
XPS analysis provided insights into the chemical composition of the solid electrolyte interphase (SEI) on the anode. The C 1s, O 1s, and F 1s spectra were deconvoluted to identify functional groups. For instance, the C 1s spectrum includes peaks for C-C/C-H (284.8 eV, from graphite), C-O (286.3 eV), C=O (288.3 eV), and CO32- (289.7 eV). The relative atomic percentages of key components are summarized in Table 3 for different anode regions.
| Region | C-C/C-H (%) | C-O (%) | C=O (%) | CO32- (%) | LiF (%) |
|---|---|---|---|---|---|
| A1 (Upper) | 45.2 | 22.1 | 15.3 | 10.4 | 7.0 |
| A2 (Middle) | 42.8 | 23.5 | 16.0 | 11.0 | 6.7 |
| A3 (Lower) | 38.5 | 28.7 | 18.2 | 8.9 | 5.7 |
The lower region (A3) shows higher percentages of C-O and C=O bonds, suggesting a thicker organic-rich SEI layer, while LiF content is lower. This composition can increase impedance and promote lithium deposition, as observed in the electrochemical tests. The inhomogeneity in SEI composition across the lithium-ion battery anode underscores the role of localized conditions in degradation.
To quantify the inhomogeneity in reaction kinetics, I applied a diffusion model for lithium ions in the graphite anode. The diffusion equation in spherical coordinates for a particle can be expressed as:
$$\frac{\partial c}{\partial t} = D \left( \frac{\partial^2 c}{\partial r^2} + \frac{2}{r} \frac{\partial c}{\partial r} \right)$$
where \(c\) is the lithium concentration, \(D\) is the diffusion coefficient, \(r\) is the radial coordinate, and \(t\) is time. Inhomogeneous aging can reduce \(D\) in certain regions, leading to concentration gradients and lithium plating. For the overcharged lithium-ion battery, regions with slower diffusion may exhibit higher overpotentials, triggering side reactions.
The capacity loss in the lithium-ion battery over cycles can also be linked to active material loss. Assuming first-order kinetics for material degradation, the remaining active material mass \(m\) after \(n\) cycles is:
$$m = m_0 \cdot e^{-\alpha n}$$
where \(m_0\) is the initial mass and \(\alpha\) is the degradation constant. Combined with lithium loss, this contributes to the overall capacity fade in the lithium-ion battery.
During overcharging, the voltage rise beyond the normal cut-off can be described by the Nernst equation for the cathode:
$$E = E^0 – \frac{RT}{F} \ln \left( \frac{1-x}{x} \right)$$
where \(E\) is the electrode potential, \(E^0\) is the standard potential, \(R\) is the gas constant, \(T\) is temperature, \(F\) is Faraday’s constant, and \(x\) is the fraction of lithium in LiFePO4. Overcharging pushes \(x\) to near zero, increasing \(E\) and potentially causing electrolyte oxidation. However, for LiFePO4, the flat voltage plateau mitigates this, but inhomogeneities can lead to localized overpotentials in the lithium-ion battery.
The thermal behavior during overcharging is critical for lithium-ion battery safety. The heat generation rate \(\dot{Q}\) can be modeled as:
$$\dot{Q} = I(E – U) + I^2 R$$
where \(U\) is the open-circuit voltage. The first term represents reversible heat, and the second is irreversible Joule heat. Inhomogeneous reactions can cause uneven heat distribution, leading to hot spots that compromise the lithium-ion battery integrity.
My analysis confirms that aged lithium-ion battery cells exhibit significant internal inhomogeneity, which exacerbates under overcharge conditions. The cathode and anode reaction uniformity is compromised, with some regions showing limited lithium deintercalation or intercalation capacity. This inhomogeneity is reflected in temperature profiles, morphological changes, and SEI composition variations. Such insights are vital for developing better management systems for lithium-ion battery packs, especially in electric vehicles where safety is paramount.
Future work should focus on real-time monitoring of inhomogeneity in lithium-ion battery systems using sensors or electrochemical impedance spectroscopy. Additionally, materials engineering to enhance uniformity, such as graded electrodes or improved electrolytes, could mitigate these issues. The lithium-ion battery industry must prioritize inhomogeneity studies to advance safety and longevity.
In conclusion, this study highlights the critical role of inhomogeneous reactions in the degradation and safety of commercial LiFePO4 lithium-ion battery cells. Through combined electrochemical, morphological, and spectroscopic analysis, I demonstrated that overcharging an aged lithium-ion battery reveals spatial variations in performance and material changes. These findings underscore the need for standardized tests that account for aging effects in lithium-ion battery safety assessments. As lithium-ion battery technology continues to evolve, addressing inhomogeneity will be key to unlocking higher performance and reliability in diverse applications.