In recent years, the rapid advancement of energy storage technologies has positioned sodium-ion batteries as a promising alternative to lithium-ion batteries due to their cost-effectiveness, abundant raw materials, and robust performance across a wide temperature range. As someone deeply involved in electrochemical safety research, I have observed a growing interest in commercializing sodium-ion battery systems for applications ranging from electric vehicles to grid-scale storage. However, the pace of safety assessments, particularly regarding thermal runaway hazards, has not kept up with this commercialization drive. Thermal runaway—a condition where excessive heat generation leads to catastrophic failure—poses significant risks, including fires and explosions, due to the release of flammable gases. This study aims to bridge that gap by providing a detailed analysis of thermal runaway behavior and gas emission characteristics under various abuse conditions for a high-capacity 180 Ah sodium-ion battery. By employing adiabatic calorimetry and closed pressure vessel tests, we seek to elucidate the temperature dynamics, pressure changes, and gas composition during failure events, thereby offering insights crucial for the safe design and risk mitigation of sodium-ion battery systems.
The fundamental chemistry of sodium-ion batteries mirrors that of lithium-ion batteries, comprising a cathode, anode, separator, and electrolyte. For the sodium-ion battery under investigation, the cathode is a layered oxide, while the anode consists of soft carbon, both immersed in an organic carbonate-based electrolyte with sodium salts. This configuration, while efficient for energy storage, introduces vulnerabilities when subjected to mechanical, thermal, or electrical abuses. In our experiments, we focused on thermal and electrical abuses, as these are common triggers in real-world scenarios. The sodium-ion battery sample had a nominal capacity of 180 Ah, an energy density of approximately 110 Wh/kg, and a weight of around 4900 g, all at 100% state of charge (SOC). Understanding the thermal stability of such large-format sodium-ion battery units is essential, as their failure can release substantial energy and hazardous gases.
To systematically evaluate the safety of this sodium-ion battery, we conducted three sets of experiments: adiabatic self-heating tests, external heating tests, and overcharge tests. Each test was designed to simulate specific abuse conditions, with data collected on temperature, voltage, pressure, and gas composition. The adiabatic tests utilized an accelerating rate calorimeter (ARC) to measure self-heating onset and thermal runaway progression under near-zero heat loss conditions. The external heating and overcharge tests were performed in a 320 L sealed pressure vessel equipped with heating plates, temperature sensors, pressure transducers, and gas sampling ports. This setup allowed us to capture the entire failure process, from initial heating to gas emission, in a controlled environment. Throughout this study, we emphasize the behavior of the sodium-ion battery under these stresses, comparing it to known characteristics of lithium-ion batteries to contextualize the findings.
In the adiabatic experiment, the sodium-ion battery was placed in the ARC with thermocouples attached to its surface to monitor temperature changes. The device was set to detect a self-heating rate threshold of 0.02 °C/min, initiating tracking when this rate was exceeded. The temperature evolution revealed three distinct phases: self-heating onset, safety vent opening, and thermal runaway. The self-heating onset temperature ($T_{\text{onset}}$) was identified at 115.92 °C, indicating the point where internal exothermic reactions began to outpace heat dissipation. As the temperature rose, internal pressure built up due to gas generation from electrolyte decomposition and electrode interactions, leading to venting at approximately 200 °C. Notably, for this sodium-ion battery, the venting event coincided closely with the thermal runaway trigger temperature ($T_{\text{tr}}$) of 201.30 °C, a behavior reminiscent of some lithium-ion chemistries. The thermal runaway peak temperature ($T_{\text{max}}$) reached 444.82 °C, with a maximum heating rate of 2353.08 °C/min. The mass loss post-test was 22.80%, attributed to the ejection of materials and gases. The temperature data from various locations on the sodium-ion battery are summarized in Table 1, highlighting the uniformity and intensity of heating during failure.
| Measurement Point | Location on Sodium-Ion Battery | Maximum Temperature (°C) |
|---|---|---|
| T1 | Large Surface | 444.82 |
| T2 | Positive Terminal Side | 454.22 |
| T3 | Negative Terminal Side | 453.22 |
| T4 | Negative Tab | 360.63 |
| T5 | Near Safety Vent | 367.79 |
The adiabatic data can be further analyzed using the Arrhenius equation to estimate the activation energy of the dominant exothermic reactions. The self-heating rate ($\frac{dT}{dt}$) is related to temperature ($T$) by:
$$\frac{dT}{dt} = A \exp\left(-\frac{E_a}{RT}\right)$$
where $A$ is the pre-exponential factor, $E_a$ is the activation energy, and $R$ is the gas constant. By plotting $\ln(\frac{dT}{dt})$ against $1/T$, we can derive $E_a$ for the sodium-ion battery’s decomposition processes, which typically ranges from 50 to 150 kJ/mol for such systems. This kinetic analysis helps in modeling thermal runaway propagation in larger sodium-ion battery packs.
Moving to the external heating test, the sodium-ion battery was subjected to dual-sided heating at a controlled rate of 7 °C/min until thermal runaway occurred. The temperature profiles, as shown in Figure 1 (though not referenced directly), indicated a rapid temperature rise starting at around 171.83 °C, leading to a peak of 484.51 °C at one of the large surfaces. The heating plates maintained a consistent input, but once the sodium-ion battery’s internal reactions accelerated, the temperature surged autonomously. The pressure inside the vessel increased from an initial 101.3 kPa to 142.6 kPa after cooling to 25 °C. Using the ideal gas law, we calculated the total gas emitted during this event. The ideal gas equation is:
$$n = \frac{pV}{RT}$$
where $n$ is the number of moles, $p$ is pressure, $V$ is volume (320 L), $R$ is 8.3145 L·kPa/(mol·K), and $T$ is 298.15 K. The initial moles were 12.9 mol, and the final moles were 18.4 mol, yielding a gas release of 5.5 mol. At standard conditions, this corresponds to 123.25 L of gas. The composition of this gas, analyzed via gas chromatography, is presented in Table 2. Hydrogen, carbon dioxide, and carbon monoxide dominated the mixture, underscoring the flammability and toxicity risks associated with sodium-ion battery failures.
| Gas Component | Volume Percentage (%) | Potential Hazard |
|---|---|---|
| Hydrogen (H₂) | 35.39 | Flammable, explosive |
| Carbon Dioxide (CO₂) | 30.95 | Asphyxiant |
| Carbon Monoxide (CO) | 19.16 | Toxic, flammable |
| Ethylene (C₂H₄) | 4.34 | Flammable |
| Other Gases | 10.16 | Varying risks |
The mass loss in this heating test was 24.98%, slightly higher than in the adiabatic case, likely due to more vigorous ejection of contents. Comparing these results to lithium-ion batteries, the sodium-ion battery’s thermal response under heating resembles that of lithium iron phosphate (LFP) batteries in terms of peak temperature but shows venting behavior similar to nickel-manganese-cobalt (NMC) batteries. This dual characteristic necessitates tailored safety strategies for sodium-ion battery deployments.
In the overcharge test, the sodium-ion battery was charged at a 0.5C rate (90 A) beyond its capacity limit until thermal runaway ensued. Overcharging induces oxidative reactions at the cathode and lithium plating at the anode, generating heat and gases. The voltage and temperature trajectories revealed a sharp voltage spike to 11.18 V at 6496 seconds, followed by a drop to zero as internal short circuits occurred. The temperature at this trigger point was approximately 115 °C, but it rapidly escalated to a maximum of 573.60 °C. The total charge input reached 163.51 Ah, corresponding to an SOC of 190.84%. The pressure in the vessel peaked at 518.40 kPa during the event and stabilized at 231.70 kPa after cooling. Applying the ideal gas law again, the gas release was computed as 200.26 L, significantly larger than in the heating test. The gas composition, detailed in Table 3, showed higher proportions of ethylene and carbon monoxide, indicating more severe electrolyte decomposition. The mass loss was 47.96%, reflecting extensive structural damage to the sodium-ion battery.
| Gas Component | Volume Percentage (%) | Notes on Formation |
|---|---|---|
| Carbon Dioxide (CO₂) | 29.08 | From carbonate electrolyte decomposition |
| Hydrogen (H₂) | 28.10 | From solvent reduction and reactions |
| Carbon Monoxide (CO) | 20.79 | Product of incomplete combustion |
| Ethylene (C₂H₄) | 14.43 | From ethylene carbonate reduction |
| Other Gases | 7.60 | Includes methane, ethane, etc. |
The overcharge test underscores the heightened hazard of electrical abuse for sodium-ion battery systems. The gas emission volume can be modeled using a simplified relation based on the charge input and battery capacity. For a sodium-ion battery, the total gas moles ($n_{\text{gas}}$) might correlate with the overcharge capacity ($Q_{\text{oc}}$) via:
$$n_{\text{gas}} = k \cdot Q_{\text{oc}}$$
where $k$ is an empirical constant specific to the sodium-ion battery chemistry. From our data, $k \approx 0.0336$ mol/Ah for this sodium-ion battery, indicating substantial gas generation per unit of overcharge. This emphasizes the need for robust battery management systems to prevent overcharge in sodium-ion battery packs.
Throughout these experiments, the sodium-ion battery demonstrated consistent thermal runaway characteristics, but with nuances depending on the abuse type. Under adiabatic conditions, the self-heating onset was relatively low, suggesting that thermal management must be effective even at moderate temperatures. In heating scenarios, the sodium-ion battery’s response was violent yet contained, with gas emissions dominated by hydrogen—a key concern for ventilation design. During overcharge, the sodium-ion battery exhibited extreme gas release and mass loss, highlighting the criticality of voltage control. To quantify the overall risk, we can define a hazard index ($HI$) for the sodium-ion battery as a function of temperature rise and gas volume:
$$HI = \alpha \cdot (T_{\text{max}} – T_{\text{onset}}) + \beta \cdot V_{\text{gas}}$$
where $\alpha$ and $\beta$ are weighting factors, and $V_{\text{gas}}$ is the gas volume in liters. For our sodium-ion battery, $HI$ values would be highest for overcharge, reinforcing the severity of electrical abuse.
In conclusion, this comprehensive study on a 180 Ah sodium-ion battery reveals critical insights into thermal runaway and gas emission under adiabatic, heating, and overcharge conditions. The sodium-ion battery’s self-heating begins near 116 °C, with thermal runaway triggering above 200 °C and peak temperatures reaching up to 574 °C. Gas emissions range from 123 L to 200 L, comprising flammable and toxic species like hydrogen, carbon monoxide, and ethylene. The sodium-ion battery’s behavior shares similarities with both LFP and NMC lithium-ion batteries, suggesting that safety protocols for sodium-ion battery systems should incorporate elements from both. Future work should focus on mitigating these risks through advanced materials, cooling systems, and failure detection algorithms. As sodium-ion battery technology continues to evolve, such safety assessments will be pivotal in ensuring its sustainable integration into the energy landscape. By understanding and addressing these hazards, we can unlock the full potential of sodium-ion battery technology while safeguarding against catastrophic failures.