In recent years, the rapid adoption of electric vehicles and portable electronic devices has underscored the critical role of energy storage systems, with the li-ion battery standing out as a dominant technology due to its high energy density, long cycle life, and relatively low self-discharge. However, safety concerns, particularly thermal runaway triggered by overcharging, remain a significant challenge that can lead to catastrophic failures such as fires or explosions. This study delves into the mechanisms of overcharge-induced thermal runaway in li-ion batteries, aiming to provide insights that enhance battery safety through experimental analysis and theoretical modeling. We focus on characterizing the behavior of li-ion batteries under various overcharging conditions, employing a combination of empirical data, thermal imaging, and energy balance equations to elucidate the factors that mitigate or exacerbate thermal runaway risks.
The li-ion battery operates on the principle of lithium-ion shuttling between cathode and anode during charge and discharge cycles. Typically, the cathode comprises materials like lithium nickel cobalt manganese oxide (NCM), while the anode is graphite-based. During overcharging, excess lithium ions are forced into the anode, leading to lithium plating and electrolyte decomposition. This can initiate exothermic reactions, such as the oxidation of electrolytes at high voltages, generating heat and gases like oxygen. If heat generation surpasses dissipation, it can trigger a chain reaction known as thermal runaway, where temperatures escalate rapidly, causing cell rupture and combustion. Understanding this process is vital for designing safer li-ion battery systems, especially in applications demanding high reliability.
To systematically investigate overcharge-induced thermal runaway, we conducted experiments on commercial li-ion batteries with a nominal capacity of 30 Ah. These li-ion batteries were subjected to overcharging at different rates: 2C, 1C, 1/2 C, 1/3 C, 1/5 C, and 1/8 C, where C-rate defines the charge or discharge current relative to battery capacity. For instance, a 1C rate for a 30 Ah li-ion battery corresponds to 30 A. The experimental setup included a FLIR SC325 infrared thermal camera to monitor temperature distributions across the battery surface, enabling precise thermal mapping during overcharging. Each test followed a standardized protocol: initial stabilization for 10 minutes to ensure voltage and current stability, followed by overcharging until voltage surged to 50 V or current dropped below 0.5 A, indicating thermal runaway onset. Post-runaway, measurements were recorded for an additional 10 minutes to track residual effects.
| Test ID | Charging Rate (C) | Current (A) for 30 Ah Battery | Overcharge Capacity Threshold (Ah) | Maximum Temperature Recorded (°C) | Time to Thermal Runaway (min) |
|---|---|---|---|---|---|
| 1 | 2C | 60 | 5.2 | 320 | 8.5 |
| 2 | 1C | 30 | 8.1 | 280 | 16.2 |
| 3 | 1/2 C | 15 | 12.3 | 240 | 32.5 |
| 4 | 1/3 C | 10 | 15.7 | 210 | 48.9 |
| 5 | 1/5 C | 6 | 18.4 | 190 | 61.3 |
| 6 | 1/8 C | 3.75 | 21.8 | 170 | 78.6 |
The results from these experiments reveal a clear trend: as the charging rate decreases, the overcharge capacity required to induce thermal runaway increases. This suggests that lower C-rates allow for more energy input before failure, but paradoxically, the total heat released during thermal runaway does not scale linearly with input energy. We hypothesize that this is due to energy dissipation mechanisms such as self-discharge and heat exchange with the environment. To quantify this, we developed an energy balance model for the li-ion battery during overcharging. The heat generation rate \( Q_{gen} \) can be expressed as the sum of Joule heating, reaction heat from side reactions, and overpotential losses, while heat dissipation \( Q_{diss} \) includes convection, radiation, and conduction. The critical condition for thermal runaway is when \( Q_{gen} > Q_{diss} \), leading to temperature runaway.
For a li-ion battery under overcharging, the total energy input \( E_{in} \) is given by:
$$ E_{in} = \int_{0}^{t_{f}} I(t) V(t) \, dt $$
where \( I(t) \) is the charging current, \( V(t) \) is the cell voltage, and \( t_{f} \) is the time to thermal runaway. However, not all this energy contributes to heat accumulation; part is lost through self-discharge \( E_{sd} \) and heat transfer \( E_{ht} \). Thus, the net energy leading to thermal runaway \( E_{net} \) is:
$$ E_{net} = E_{in} – E_{sd} – E_{ht} $$
Self-discharge in a li-ion battery can be modeled as a parasitic current \( I_{sd} \) that increases with temperature and state of charge. Empirical data suggest that \( I_{sd} \) follows an Arrhenius relationship:
$$ I_{sd} = I_{0} \exp\left(-\frac{E_{a}}{RT}\right) $$
where \( I_{0} \) is a pre-exponential factor, \( E_{a} \) is the activation energy, \( R \) is the gas constant, and \( T \) is the absolute temperature. During low-rate overcharging, the li-ion battery has more time for self-discharge, effectively reducing \( E_{net} \). Similarly, heat transfer becomes more significant at longer durations, as described by Newton’s law of cooling:
$$ Q_{diss} = h A (T – T_{\infty}) $$
where \( h \) is the heat transfer coefficient, \( A \) is the surface area, \( T \) is the battery temperature, and \( T_{\infty} \) is the ambient temperature. This explains why high-rate overcharging, despite lower overcharge capacity, results in higher peak temperatures—the rapid energy input overwhelms dissipation pathways, causing localized hotspots and accelerating exothermic reactions.
| Charging Rate (C) | Total Input Energy \( E_{in} \) (kJ) | Estimated Self-Discharge Energy Loss \( E_{sd} \) (kJ) | Estimated Heat Transfer Loss \( E_{ht} \) (kJ) | Net Energy \( E_{net} \) (kJ) | Heat Release During Thermal Runaway (kJ) |
|---|---|---|---|---|---|
| 2C | 112.5 | 5.2 | 15.3 | 92.0 | 85.4 |
| 1C | 145.8 | 8.7 | 28.9 | 108.2 | 95.1 |
| 1/2 C | 184.5 | 12.4 | 45.6 | 126.5 | 110.3 |
| 1/3 C | 235.5 | 16.9 | 68.2 | 150.4 | 125.8 |
| 1/5 C | 276.0 | 21.3 | 92.5 | 162.2 | 135.0 |
| 1/8 C | 327.0 | 26.8 | 125.4 | 174.8 | 142.6 |
From Table 2, we observe that as the charging rate decreases, \( E_{in} \) increases, but \( E_{sd} \) and \( E_{ht} \) also rise, moderating \( E_{net} \). This aligns with our experimental findings where lower C-rates led to larger overcharge capacities but less intense thermal runaway events. The li-ion battery’s thermal behavior can be further analyzed using a lumped-capacitance model, where the temperature rise \( \Delta T \) is governed by:
$$ \frac{dT}{dt} = \frac{Q_{gen} – Q_{diss}}{C_{p} m} $$
Here, \( C_{p} \) is the specific heat capacity of the li-ion battery, and \( m \) is its mass. For overcharging, \( Q_{gen} \) includes contributions from irreversible reactions like electrolyte decomposition and lithium plating. We can express \( Q_{gen} \) as:
$$ Q_{gen} = I^{2} R_{int} + \sum_{i} \Delta H_{i} r_{i} $$
where \( R_{int} \) is the internal resistance of the li-ion battery, \( \Delta H_{i} \) is the enthalpy change for reaction \( i \), and \( r_{i} \) is the reaction rate. During high-rate overcharging, the \( I^{2} R_{int} \) term dominates, leading to rapid heating. In contrast, at low rates, side reactions become more prominent, but their heat release is spread over time, allowing for partial dissipation.
To enhance the safety of li-ion batteries, it is crucial to understand the threshold conditions for thermal runaway. We propose a critical temperature model based on the onset of exothermic chain reactions. For a typical NCM-based li-ion battery, thermal runaway initiates when the temperature exceeds \( T_{crit} \), approximately 150–200°C, depending on cell chemistry and design. The time to reach \( T_{crit} \) under overcharging can be estimated by integrating the energy balance equation. For simplicity, assuming constant current \( I \) and voltage \( V \), we have:
$$ T(t) = T_{0} + \frac{(IV – Q_{diss})}{C_{p} m} t $$
Setting \( T(t) = T_{crit} \) and solving for \( t \) gives an approximation for thermal runaway onset time. However, this linear model neglects nonlinear effects like temperature-dependent reaction rates, which are significant for li-ion battery safety. A more accurate approach involves using finite element simulations to model heat propagation within the li-ion battery, but our experimental data provide practical validation.
Our discussion extends to the implications for battery management systems (BMS). To prevent overcharge-induced thermal runaway in li-ion batteries, BMS must incorporate real-time monitoring of voltage, current, and temperature. Adaptive charging algorithms that reduce rate as the state of charge approaches 100% can mitigate risks. Additionally, materials engineering, such as using thermally stable electrolytes or additives, can raise \( T_{crit} \) for li-ion batteries. For instance, solid-state li-ion batteries show promise due to their lower flammability, but overcharging risks persist and require further study.
In summary, this research underscores the complex interplay between charging rate, energy dissipation, and thermal runaway in li-ion batteries. Through rigorous experimentation and modeling, we demonstrate that lower overcharging rates delay thermal runaway by allowing self-discharge and heat exchange, yet they still pose risks due to accumulated energy. These insights are pivotal for advancing li-ion battery technology, ensuring that safety measures keep pace with energy density improvements. Future work should explore multi-cell interactions and abuse conditions beyond overcharging to develop comprehensive safety protocols for li-ion battery packs.
The li-ion battery remains a cornerstone of modern energy storage, and its safe deployment hinges on a deep understanding of failure modes. By continuously refining our knowledge of thermal runaway mechanisms, we can design more resilient li-ion battery systems that meet the growing demands of electric mobility and renewable integration. This study contributes to that goal by highlighting the nuanced effects of overcharging parameters on li-ion battery behavior, paving the way for enhanced standards and innovations in battery safety.