In the rapidly evolving landscape of energy storage technologies, the sodium-ion battery has emerged as a pivotal alternative to lithium-ion systems, particularly for large-scale applications. The driving force behind this shift is the abundant and geographically widespread availability of sodium resources, which mitigates supply chain constraints and cost volatility associated with lithium. However, as the commercialization of sodium-ion battery technology accelerates, a thorough understanding of its operational behavior—especially under high-stress conditions like elevated discharge rates—becomes paramount. The electrochemical reactions within a sodium-ion battery, both reversible and irreversible, generate heat during charge and discharge cycles. This heat accumulation elevates cell temperature, potentially leading to accelerated material degradation, performance decay, and, in extreme scenarios, safety hazards such as thermal runaway. Therefore, this study aims to delve deeply into the electrochemical performance and thermal behavior of two distinct types of commercial sodium-ion batteries—namely, energy-type and power-type variants—when subjected to various discharge rates. By employing experimental characterization and simulation modeling, we seek to elucidate the intrinsic differences between these sodium-ion battery designs and provide foundational data for developing effective thermal management strategies, thereby ensuring the safe and efficient deployment of sodium-ion battery systems in real-world applications.
The experimental framework of this investigation centers on two commercially available prismatic sodium-ion battery cells, identical in physical dimensions and weight but engineered for different performance priorities. One sodium-ion battery is optimized for high energy density (energy-type), while the other is tailored for high power output (power-type). Key specifications are summarized in Table 1.
| Parameter | Power-Type Sodium-Ion Battery | Energy-Type Sodium-Ion Battery |
|---|---|---|
| Nominal Capacity (Ah) | 170 | 185 |
| Energy Density (Wh/kg) | ≥ 105 | ≥ 110 |
| Nominal Voltage (V) | 3.0 | 3.0 |
| Charge Cut-off Voltage (V) | 3.75 | 3.85 |
| Discharge Cut-off Voltage (V) | 2.0 | 2.0 |
| Mass (kg) | 4.9 | 4.9 |
| Dimensions (Thickness × Width × Height, mm) | 72 × 173.7 × 207 | 72 × 173.7 × 207 |
To accurately monitor the thermal response of each sodium-ion battery, K-type thermocouples were affixed at three strategic locations on the cell surface: near the positive terminal (T1), at the geometric center (T2), and near the bottom (T3). A battery testing system was employed to control charge-discharge protocols and record electrical parameters (voltage, current, capacity) in real-time, while a temperature acquisition unit logged the thermal data.
The electrochemical characterization commenced with determining the actual capacity of each sodium-ion battery through three consecutive formation cycles at a 0.2 C rate. Subsequently, the internal resistance as a function of state-of-charge (SOC) was quantified using the Hybrid Pulse Power Characteristic (HPPC) test method. This technique involves applying a sequence of discharge and charge pulses at different SOC levels. The resulting voltage response allows for the separation of the total internal resistance ($R_{tot}$) into its ohmic ($R_o$) and polarization ($R_p$) components, calculated as follows:
$$ R_o = \frac{U_1 – U_2}{I_d} $$
$$ R_p = \frac{U_2 – U_3}{I_d} $$
$$ R_{tot} = \frac{U_1 – U_3}{I_d} = R_o + R_p $$
where $I_d$ is the discharge pulse current, $U_1$ is the voltage immediately before the pulse, $U_2$ is the voltage at the instant the pulse is applied (capturing the ohmic drop), and $U_3$ is the voltage at the end of the pulse.
The discharge performance of each sodium-ion battery was evaluated at four constant current rates: 0.2 C, 0.5 C, 1.0 C, and 1.5 C. For each test, the cell was first fully charged using a constant current-constant voltage (CC-CV) procedure and then discharged at the specified C-rate to the cut-off voltage. Voltage profiles, delivered capacity, and energy were recorded. Simultaneously, surface temperatures at points T1, T2, and T3 were monitored to analyze the thermal behavior. The average surface temperature ($T_{ave}$) and maximum temperature difference ($\Delta T$) were computed:
$$ T_{ave} = \frac{T_1 + T_2 + T_3}{3} $$
$$ \Delta T = \max(T_1, T_2, T_3) – \min(T_1, T_2, T_3) $$
To model the heat generation and dissipation within the sodium-ion battery, a thermal model based on the energy conservation equation was established. The general heat equation for the battery is:
$$ \rho C_b \frac{\partial T}{\partial t} = \lambda \nabla^2 T + Q_{tot} $$
where $\rho$ is density, $C_b$ is specific heat capacity, $\lambda$ is thermal conductivity, and $Q_{tot}$ is the total volumetric heat generation rate. The total heat generation ($Q_{tot}$) comprises heat from the battery core ($Q_{batt}$) and heat from the tabs ($Q_{tab}$):
$$ Q_{tot} = Q_{tab} + Q_{batt} $$
The heat generation in the tabs (positive and negative) is primarily Joule heating:
$$ q_{pos} = \frac{I^2 R_{pos}}{V_{pos}} = \frac{I^2 \rho_{pos} l_{pos}}{V_{pos} S_{pos}}, \quad q_{neg} = \frac{I^2 R_{neg}}{V_{neg}} = \frac{I^2 \rho_{neg} l_{neg}}{V_{neg} S_{neg}} $$
The heat generation within the battery core is given by Bernardi’s formulation:
$$ Q_{batt} = I^2 R + I T \frac{dU_0}{dT} $$
where $I$ is the current, $R$ is the internal resistance, $T$ is absolute temperature, and $dU_0/dT$ is the entropic heat coefficient. For the sodium-ion battery under study, the entropic coefficient was determined experimentally and treated as a constant value of -0.5 mV/K.
The internal resistance profiles derived from HPPC tests for both sodium-ion battery types are presented in Table 2 and graphically analyzed. A clear trend is observed: the total internal resistance increases as the SOC decreases for both cells. However, the power-type sodium-ion battery generally exhibits lower total resistance across most of the SOC range compared to the energy-type sodium-ion battery. This aligns with the design philosophy where power-oriented cells minimize impedance to facilitate high-rate capability. The ohmic and polarization resistances show complex, non-linear variations with SOC, indicating the dynamic interplay between ionic conductivity, charge transfer kinetics, and mass transport limitations within the sodium-ion battery electrodes and electrolyte.
| SOC | Power-Type Sodium-Ion Battery | Energy-Type Sodium-Ion Battery | ||
|---|---|---|---|---|
| $R_o$ | $R_p$ | $R_o$ | $R_p$ | |
| 1.0 | 0.255 | 0.440 | 0.260 | 0.430 |
| 0.8 | 0.245 | 0.510 | 0.250 | 0.520 |
| 0.6 | 0.265 | 0.495 | 0.270 | 0.540 |
| 0.4 | 0.280 | 0.580 | 0.290 | 0.620 |
| 0.2 | 0.310 | 0.720 | 0.330 | 0.780 |
| 0.1 | 0.350 | 0.680 | 0.380 | 0.730 |
The discharge voltage profiles for both sodium-ion battery types at different C-rates reveal characteristic shapes. At a given discharge rate, the voltage decreases in three stages: an initial rapid drop due to activation polarization, a quasi-linear decline dominated by ohmic polarization, and a final steep drop influenced by concentration polarization as sodium-ion diffusion becomes limiting at low SOC. Higher discharge rates exacerbate the voltage polarization, leading to lower cutoff voltages being reached earlier and thus reducing the utilizable capacity. This is quantitatively summarized in Table 3, which lists the delivered capacity and energy at each discharge rate relative to the baseline 0.2 C values.
| Discharge Rate (C) | Power-Type Sodium-Ion Battery | Energy-Type Sodium-Ion Battery | ||
|---|---|---|---|---|
| Capacity (Ah) | Energy Retention vs. 0.2C | Capacity (Ah) | Energy Retention vs. 0.2C | |
| 0.2 | 186.0 | 100.0% | 195.2 | 100.0% |
| 0.5 | 180.5 | 97.1% | 188.9 | 96.8% |
| 1.0 | 167.9 | 90.2% | 180.5 | 92.5% |
| 1.5 | 152.1 | 81.7% | 167.9 | 86.0% |
While the energy-type sodium-ion battery demonstrates superior capacity and energy retention at lower rates, its performance degrades more sharply at the highest rate of 1.5 C compared to the power-type sodium-ion battery. The power-type cell shows a more linear decline in output with increasing rate, indicative of its robust high-power design. This fundamental difference in rate capability has direct implications for the thermal behavior of the sodium-ion battery, as higher currents inevitably lead to greater Joule heating.
The thermal response of the sodium-ion battery is a critical safety and performance metric. Figure 1 (represented conceptually by the temperature data) illustrates the evolution of the average surface temperature ($T_{ave}$) during discharge at different rates. A strong correlation between discharge rate and temperature rise is evident. For the power-type sodium-ion battery, $T_{ave}$ increases from 26.0°C at 0.2 C to 44.6°C at 1.5 C. For the energy-type sodium-ion battery, the rise is more pronounced, from 28.1°C to 49.2°C under the same conditions. Notably, at the 1.5 C rate, the energy-type sodium-ion battery’s temperature exceeds 45°C, which is generally considered the upper safe operating limit for many cell chemistries, highlighting a potential thermal risk for this sodium-ion battery design under high-load conditions.
The rate of temperature increase, expressed as $dT_{ave}/dDOD$ (change in average temperature per unit change in depth-of-discharge), provides further insight. This parameter was calculated from the linear portions of the temperature curves and is plotted against discharge rate. The data fits well to an exponential decay model:
For Power-Type Sodium-Ion Battery: $$ \frac{dT_{ave}}{dDOD} = 28.78 + (-31.18) \exp\left(-\frac{\text{Rate}}{0.980}\right) $$
For Energy-Type Sodium-Ion Battery: $$ \frac{dT_{ave}}{dDOD} = 40.89 + (-42.21) \exp\left(-\frac{\text{Rate}}{1.354}\right) $$
These equations confirm that the energy-type sodium-ion battery consistently exhibits a higher intrinsic heating rate across all discharge rates. The spatial temperature distribution on the sodium-ion battery surface is non-uniform, as captured by the three thermocouples. In all tests, the region near the positive terminal (T1) recorded the highest temperature, followed by the center (T2) and the bottom (T3). This gradient is attributed to the concentrated Joule heating at the current collection tabs and the thermal conduction path within the cell. The maximum temperature difference ($\Delta T$) within a single sodium-ion battery increases with discharge rate, as shown in Table 4. At 1.5 C, the $\Delta T$ for the energy-type sodium-ion battery reaches 5.8°C, which is significant for cell longevity and uniformity, whereas the power-type cell maintains a lower $\Delta T$ of 4.0°C.
| Discharge Rate (C) | Power-Type Sodium-Ion Battery $\Delta T_{max}$ (°C) | Energy-Type Sodium-Ion Battery $\Delta T_{max}$ (°C) |
|---|---|---|
| 0.5 | 1.0 | 1.0 |
| 1.0 | 3.1 | 2.6 |
| 1.5 | 4.0 | 5.8 |
To extend the analysis from single-cell behavior to practical battery packs, a liquid cooling simulation model was developed. The model focuses on a module composed of multiple sodium-ion battery cells, incorporating a variable-cross-section symmetric flow channel cold plate design attached to the base of the cells. This design aims to enhance heat removal from the central, typically hotter regions of the module. The governing equations for fluid flow and heat transfer were solved numerically, with boundary conditions set to an ambient temperature of 25°C and a convective heat transfer coefficient of 5 W/(m²·K) for exposed surfaces. Water was used as the coolant, with material properties listed in Table 5.
| Material | Density (kg/m³) | Specific Heat Capacity (J/(kg·°C)) | Thermal Conductivity (W/(m·°C)) |
|---|---|---|---|
| Sodium-Ion Battery Cell | 2118 | 1070.5 | Anisotropic: X/Y=21.63, Z=2.11 |
| Cold Plate (Aluminum) | 2719 | 871 | 202.4 |
| Coolant (Water) | 998.2 | 4182 | 0.6 |
| Thermal Interface Pad | 2500 | 903 | 1.5 |
The simulation investigated the effect of coolant inlet velocity on the maximum temperature within the sodium-ion battery module during a 1.0 C discharge. The results, summarized in Table 6, demonstrate that increasing flow rate effectively reduces the peak temperature, but with diminishing returns. An inlet velocity of 0.5 m/s was identified as a practical optimum, balancing cooling performance against pumping power requirements. At this velocity, the maximum cell temperature was maintained below 40°C, ensuring a safe operating window for the sodium-ion battery pack.
| Coolant Inlet Velocity (m/s) | Maximum Cell Temperature in Module (°C) | Temperature Reduction vs. 0.1 m/s case (°C) |
|---|---|---|
| 0.1 | 43.9 | — |
| 0.3 | 40.1 | 3.8 |
| 0.5 | 39.0 | 4.9 |
| 0.7 | 38.4 | 5.5 |
The experimental and simulation findings collectively paint a comprehensive picture of sodium-ion battery behavior under stress. The power-type sodium-ion battery, with its lower internal resistance, demonstrates superior tolerance to high discharge rates from both an electrical and thermal perspective. Its more stable resistance profile and reduced heat generation rate make it inherently more suitable for applications requiring frequent high-power bursts. Conversely, the energy-type sodium-ion battery, while offering higher energy density at low rates, suffers from greater performance fade and more severe heating at elevated rates. The thermal model confirms that without active cooling, the energy-type sodium-ion battery can easily exceed safe temperature limits during high-rate operation, necessitating more aggressive thermal management for such designs.
The heat generation model, validated against experimental data, provides a valuable tool for predicting the thermal behavior of sodium-ion battery systems. The model accurately captures the contributions of both irreversible (Joule) and reversible (entropic) heating. For the sodium-ion battery studied, the entropic heat coefficient was found to be negative, indicating that the reversible reaction absorbs heat during discharge, which slightly mitigates overall temperature rise. However, the dominant factor remains the $I^2R$ Joule heating, which scales quadratically with current. This underscores the critical importance of minimizing internal resistance, especially for high-power sodium-ion battery applications.
From an application engineering standpoint, the choice between an energy-type and power-type sodium-ion battery must carefully weigh the specific duty cycle. For long-duration energy storage with modest power demands, the energy-type sodium-ion battery is advantageous. For applications involving frequent load following, peak shaving, or rapid charging/discharging, the power-type sodium-ion battery offers better overall efficiency and thermal stability. Furthermore, the thermal management system must be customized accordingly. For modules using energy-type sodium-ion batteries, liquid cooling with optimized flow channels, as simulated here, is highly recommended. For power-type sodium-ion batteries, depending on the operating profile, forced air convection or less intensive liquid cooling might suffice.
In conclusion, this detailed investigation into the electrochemical performance and thermal behavior of commercial sodium-ion batteries under varying discharge rates provides essential insights for the burgeoning field of sodium-based energy storage. The sodium-ion battery technology holds immense promise, but its successful integration relies on a deep understanding of its operational characteristics. We have demonstrated clear performance distinctions between energy-optimized and power-optimized sodium-ion battery designs, quantified their heat generation patterns, and developed a predictive thermal model coupled with an effective cooling strategy. These results form a crucial knowledge base for battery pack designers, system integrators, and safety engineers working to harness the full potential of the sodium-ion battery for a sustainable energy future. Future work should focus on long-term cycle life testing under combined electrical-thermal stress, abuse condition testing, and the development of advanced materials and cell designs that further reduce the intrinsic heat generation of the sodium-ion battery.