As the global energy landscape shifts toward renewable sources, large-scale energy storage technologies have become pivotal. Among these, sodium-ion batteries have emerged as a promising alternative to lithium-ion batteries due to their resource abundance, cost-effectiveness, and robust electrochemical performance, particularly for grid-scale applications. However, as sodium-ion battery capacities increase and commercialization accelerates, safety concerns, especially thermal runaway under abuse conditions, pose significant barriers to widespread adoption. Thermal runaway is a critical safety issue where internal heat generation exceeds dissipation, leading to uncontrolled temperature rise and irreversible reactions. While lithium-ion battery thermal runaway has been extensively studied, research on sodium-ion batteries remains in its infancy, with limited systematic comparisons of large-format commercial sodium-ion batteries under varied abuse scenarios. This work aims to address this gap by investigating the thermal runaway characteristics of sodium-ion batteries under thermal and electrical abuse, leveraging experimental data to elucidate mechanisms and inform safety strategies.
The safety of sodium-ion batteries is paramount, given their potential for large-scale deployment. Thermal runaway in sodium-ion batteries can be triggered by various factors, including external heating, overcharging, mechanical damage, or internal defects. Under thermal abuse, such as external heating, localized overheating can initiate exothermic reactions, while electrical abuse, like overcharging, leads to electrochemical instability and gas accumulation. Understanding these processes is crucial for designing safer sodium-ion battery systems. This study focuses on comparing these two common abuse conditions, providing insights into temperature distribution, voltage evolution, and expansion force dynamics during thermal runaway events in sodium-ion batteries.
To explore the thermal runaway behavior of sodium-ion batteries, experimental tests were conducted on commercial prismatic sodium-ion batteries with a nominal capacity of 185 Ah. The sodium-ion battery samples utilized layered oxide cathodes and hard carbon anodes, typical in modern sodium-ion battery designs. Two abuse conditions were employed: thermal abuse via a heating plate and electrical abuse via continuous overcharging. Key parameters, including temperature at multiple points, voltage, and expansion force, were monitored using high-frequency data acquisition systems. For thermal abuse, an 800 W heating plate was applied to induce localized heating, while for electrical abuse, a constant-current charge at 0.5C rate was maintained beyond the cutoff voltage to simulate overcharging. The sodium-ion battery samples were preconditioned to full state-of-charge (SOC) to replicate real-world operating conditions, and all tests were performed in a controlled environment to ensure consistency. The data collected provide a comprehensive view of the sodium-ion battery response under stress, enabling a detailed comparison of thermal runaway triggers and progression.
The thermal runaway process in sodium-ion batteries is governed by complex interplay between heat generation and dissipation. The heat generation rate in a sodium-ion battery can be expressed as the sum of irreversible and reversible contributions. A simplified model for heat generation during abuse conditions is given by:
$$ \dot{Q}_{gen} = I^2 R + \dot{Q}_{chem} $$
where \( \dot{Q}_{gen} \) is the total heat generation rate, \( I \) is the current, \( R \) is the internal resistance, and \( \dot{Q}_{chem} \) represents the chemical heat release from exothermic reactions such as solid electrolyte interphase (SEI) decomposition, electrode-electrolyte reactions, and gas evolution. In sodium-ion batteries, these reactions may differ from lithium-ion systems due to material properties, but the fundamental principles remain similar. The temperature change in the sodium-ion battery can be described by the energy balance equation:
$$ m C_p \frac{dT}{dt} = \dot{Q}_{gen} – \dot{Q}_{loss} $$
where \( m \) is the mass of the sodium-ion battery, \( C_p \) is the specific heat capacity, \( T \) is temperature, \( t \) is time, and \( \dot{Q}_{loss} \) is the heat loss to the surroundings. Under abuse conditions, \( \dot{Q}_{gen} \) often overwhelms \( \dot{Q}_{loss} \), leading to thermal runaway. For sodium-ion batteries, the kinetics of these reactions may vary with abuse type, influencing the onset and severity of thermal runaway.
Under thermal abuse conditions, the sodium-ion battery exhibited distinct thermal runaway characteristics. The process was divided into three phases based on temperature and voltage trends. Initially, external heating caused a gradual temperature rise, with the sodium-ion battery surface reaching approximately 268.61°C before safety vent opening. The voltage showed a slow decline from 3.599 V to 3.587 V, indicating minimal electrochemical activity early on. As heating continued, localized overheating triggered exothermic reactions, leading to a rapid temperature increase and voltage fluctuations. The expansion force peaked at 1213 kPa upon vent opening, after which it dropped due to gas release. The entire thermal runaway event lasted about 820 seconds, with peak temperatures exceeding 600°C in localized areas. This behavior highlights the role of heat diffusion in sodium-ion battery thermal runaway under thermal abuse.
In contrast, electrical abuse via overcharging resulted in a more prolonged and violent thermal runaway in the sodium-ion battery. The process also unfolded in three stages. During overcharging, the voltage steadily climbed from the nominal 3.85 V to a peak of 4.89 V, far beyond the safe cutoff, while temperature increased slowly initially. The sodium-ion battery experienced significant gas accumulation, with expansion force rising gradually to 2402 kPa before vent opening. This delay in venting allowed pressure to build, culminating in a sudden thermal runaway with flames and rapid temperature spikes above 500°C. The voltage collapsed to near zero after venting, indicating complete electrochemical failure. The entire event spanned approximately 6996 seconds, much longer than under thermal abuse, underscoring the cumulative nature of electrochemical instability in sodium-ion batteries during overcharging.
To quantify the differences, key parameters from both abuse conditions are summarized in Table 1. This comparison reveals critical insights into sodium-ion battery safety under varied stresses.
| Parameter | Thermal Abuse (Heating) | Electrical Abuse (Overcharging) |
|---|---|---|
| Peak Temperature (°C) | >600 (localized) | >500 (with flames) |
| Voltage at Onset (V) | ~3.59 | ~3.85 (rising to 4.89) |
| Maximum Expansion Force (kPa) | 1213 | 2402 |
| Safety Vent Opening Time (s) | ~597 | ~5040 |
| Total Duration (s) | 820 | 6996 |
| Temperature Distribution | Non-uniform, gradient from heated side | More uniform until sudden spike |
| Key Observations | Localized heating, gradual venting | Gas accumulation, delayed venting, flames |
The data in Table 1 illustrate that sodium-ion battery thermal runaway under electrical abuse involves higher pressures and longer durations, posing greater safety risks. The expansion force in overcharged sodium-ion batteries is nearly double that in heated ones, suggesting more intense gas generation from electrochemical reactions. This can be attributed to decomposition processes in sodium-ion battery components, such as cathode material breakdown and electrolyte oxidation. The time to vent opening is significantly longer in overcharging, allowing more heat and gas to accumulate, which exacerbates the severity. For sodium-ion batteries, this implies that electrical abuse may require more robust safety mechanisms, such as early pressure relief devices or advanced monitoring systems.
The thermal runaway kinetics in sodium-ion batteries can be further analyzed using reaction models. For instance, the heat release from exothermic reactions in a sodium-ion battery during abuse can be modeled with Arrhenius-type equations. The rate of chemical heat generation \( \dot{Q}_{chem} \) may be expressed as:
$$ \dot{Q}_{chem} = A \exp\left(-\frac{E_a}{RT}\right) \Delta H $$
where \( A \) is the pre-exponential factor, \( E_a \) is the activation energy, \( R \) is the gas constant, \( T \) is absolute temperature, and \( \Delta H \) is the enthalpy change. In sodium-ion batteries, the values of these parameters may differ between thermal and electrical abuse due to varying reaction pathways. Under thermal abuse, localized heating can accelerate reactions like SEI decomposition, whereas overcharging may promote cathode degradation and oxygen evolution. The cumulative heat release \( Q_{total} \) over time can be integrated to assess the severity:
$$ Q_{total} = \int_0^t \dot{Q}_{gen} \, dt $$
For the sodium-ion battery under overcharging, \( Q_{total} \) is likely higher due to the prolonged reaction time, aligning with the observed greater damage.
Another aspect is the gas generation dynamics in sodium-ion batteries during thermal runaway. The expansion force \( F \) relates to the internal pressure \( P \) and volume \( V \) of the sodium-ion battery, approximated by the ideal gas law for generated gases:
$$ P V = n R T $$
where \( n \) is the number of moles of gas produced. The rate of gas production \( \frac{dn}{dt} \) can be linked to electrochemical reactions, such as electrolyte decomposition or electrode material transformation. In sodium-ion batteries, overcharging often leads to more gas generation due to side reactions at high voltages, explaining the higher expansion force. The pressure buildup \( \frac{dP}{dt} \) before venting can be derived as:
$$ \frac{dP}{dt} = \frac{R T}{V} \frac{dn}{dt} + \frac{n R}{V} \frac{dT}{dt} $$
This equation shows that both gas production and temperature rise contribute to pressure increase. For the sodium-ion battery under heating, temperature dominates early on, while under overcharging, gas production plays a larger role due to extended reaction times.
The voltage behavior of sodium-ion batteries during thermal runaway also offers insights. Under thermal abuse, voltage decline is gradual initially, reflecting minor internal shorts or increased resistance. In contrast, overcharging causes voltage to rise until electrochemical instability triggers a sharp drop. The voltage \( V_{batt} \) of a sodium-ion battery can be modeled as:
$$ V_{batt} = E_{eq} – I R_{int} $$
where \( E_{eq} \) is the equilibrium potential and \( R_{int} \) is the internal resistance. During abuse, \( R_{int} \) may increase due to material degradation, and \( E_{eq} \) can shift from electrode reactions. For overcharged sodium-ion batteries, \( E_{eq} \) rises abnormally before collapse, indicating over-lithiation or phase changes in the cathode. The sudden voltage drop marks the point of internal short circuit or complete failure, a critical warning sign for sodium-ion battery safety systems.
To further compare the abuse conditions, Table 2 summarizes the key reaction mechanisms and their impact on sodium-ion battery thermal runaway. This highlights the multifaceted nature of failure in sodium-ion batteries.
| Abuse Condition | Primary Reactions | Effect on Sodium-Ion Battery | Typical Onset Temperature (°C) |
|---|---|---|---|
| Thermal Abuse | SEI decomposition, electrolyte oxidation, anode-cathode reactions | Localized heat, gas release, voltage fluctuation | ~150-250 |
| Electrical Abuse | Cathode over-oxidation, electrolyte reduction, gas evolution from side reactions | Gas accumulation, pressure build-up, voltage surge then collapse | ~40-80 (with delayed spike) |
The onset temperatures in Table 2 indicate that sodium-ion battery thermal runaway under thermal abuse starts at higher temperatures, but progresses quickly once triggered. For electrical abuse, the onset is lower in terms of temperature but involves a longer incubation period. This difference has implications for safety monitoring: sodium-ion batteries under heating may require temperature sensors near hot spots, while overcharged sodium-ion batteries need voltage and pressure monitoring to detect early warnings.
The expansion force data from the experiments can be used to estimate the internal gas volume in the sodium-ion battery. Assuming the sodium-ion battery casing behaves as a rigid container until venting, the force \( F \) measured by the sensor relates to pressure \( P \) via the area \( A \): \( P = F / A \). From the ideal gas law, the gas volume \( V_g \) at venting can be approximated as:
$$ V_g = \frac{n R T}{P} $$
For the sodium-ion battery under heating, with \( P \approx 1213 \, \text{kPa} \) and \( T \approx 541.61 \, \text{K} \) (268.61°C), and assuming \( n \) from typical electrolyte decomposition, we can calculate \( V_g \). Similarly, for overcharging, \( P \approx 2402 \, \text{kPa} \) and \( T \approx 773 \, \text{K} \) (500°C) yield a larger \( V_g \). This quantitative approach underscores the greater gas-related hazard in overcharged sodium-ion batteries.
In terms of thermal management, the heat transfer within the sodium-ion battery during abuse is crucial. The temperature distribution can be modeled using Fourier’s law of heat conduction. For a simplified one-dimensional model across the sodium-ion battery thickness \( L \):
$$ \frac{\partial T}{\partial t} = \alpha \frac{\partial^2 T}{\partial x^2} + \frac{\dot{Q}_{gen}}{\rho C_p} $$
where \( \alpha \) is the thermal diffusivity, \( \rho \) is density, and \( x \) is the spatial coordinate. Under thermal abuse, the boundary condition at the heated side drives the gradient, leading to non-uniform temperatures as observed. For electrical abuse, the heat generation \( \dot{Q}_{gen} \) is more uniform initially, but localized hot spots may develop later due to reaction heterogeneity. This explains why sodium-ion battery temperatures under heating show a distinct gradient, while overcharging results in a more uniform rise until sudden spikes.
The safety implications for sodium-ion battery design are significant. Based on the findings, sodium-ion batteries under thermal abuse benefit from vents that open early to release pressure and prevent rupture. For electrical abuse, sodium-ion batteries require robust overcharge protection circuits and pressure relief mechanisms that activate before gas accumulation reaches critical levels. Additionally, thermal runaway propagation in sodium-ion battery packs may differ between abuse types; heating might cause cascading failures through thermal conduction, while overcharging could lead to synchronized failures due to voltage imbalances.
To encapsulate the severity, a risk index \( R \) for sodium-ion battery thermal runaway can be proposed, combining key parameters:
$$ R = \frac{T_{max} \cdot P_{max} \cdot t_{duration}}{V_{drop}} $$
where \( T_{max} \) is peak temperature, \( P_{max} \) is maximum expansion force, \( t_{duration} \) is event duration, and \( V_{drop} \) is voltage drop magnitude. Higher \( R \) values indicate greater hazard. For the tested sodium-ion battery, \( R \) would be higher for overcharging due to larger \( P_{max} \) and \( t_{duration} \), emphasizing the need for prioritized mitigation strategies for electrical abuse in sodium-ion battery systems.
In conclusion, this comparative analysis reveals that sodium-ion battery thermal runaway under thermal and electrical abuse exhibits distinct characteristics in terms of onset, progression, and severity. Thermal abuse in sodium-ion batteries leads to localized heating, earlier venting, and shorter duration, while electrical abuse involves cumulative electrochemical effects, delayed venting, higher pressures, and longer, more violent events. These insights underscore the importance of condition-specific safety measures for sodium-ion batteries. Future work should focus on modeling these processes with sodium-ion battery-specific parameters and developing advanced monitoring systems to prevent thermal runaway in large-scale sodium-ion battery energy storage. As sodium-ion battery technology advances, such safety studies will be crucial for ensuring reliable and secure deployment in renewable energy grids.
The study also highlights the broader context of sodium-ion battery safety research. Compared to lithium-ion batteries, sodium-ion batteries may exhibit different thermal runaway thresholds due to material properties, such as lower reactivity of sodium ions or varied electrolyte compositions. However, the fundamental principles of heat and mass transfer apply, making this analysis relevant for engineering safer sodium-ion battery packs. By integrating experimental data with mathematical models, we can better predict and mitigate thermal runaway risks in sodium-ion batteries, fostering their adoption in energy storage applications.
Ultimately, the goal is to enhance the safety profile of sodium-ion batteries through design improvements, such as optimized venting mechanisms, enhanced thermal management systems, and smart battery management systems that detect early signs of abuse. As the demand for sustainable energy solutions grows, sodium-ion batteries offer a viable path forward, provided their safety challenges are adequately addressed. This work contributes to that effort by providing a detailed comparison of thermal runaway under common abuse conditions, laying the groundwork for future innovations in sodium-ion battery technology.