The quest for electrochemical energy storage systems that bridge the gap between high-energy batteries and high-power supercapacitors has led to the development of hybrid devices. Among these, sodium-ion battery capacitors (SIBatCs) represent a promising class, synergistically coupling the high energy density inherent to sodium-ion battery (SIB) electrode materials with the rapid kinetics and long cycle life characteristic of sodium-ion capacitor (SIC) components. This integration positions SIBatCs as formidable candidates for applications demanding both substantial energy storage and high-power delivery, such as energy recovery systems, grid frequency regulation, and fast-charging transportation. The operational principle hinges on combining a battery-type electrode (typically for sodium-ion storage via intercalation/conversion) with a capacitive-type electrode (for charge storage via electrical double-layer formation or surface redox reactions) within a single cell.
A critical yet often underexplored component influencing the performance, cost, and thermal management of these devices is the current collector. Current collectors serve as the essential backbone, providing mechanical support for the active material coatings and establishing efficient electronic pathways for current flow. In lithium-based systems, copper foil is exclusively used for the negative electrode due to its excellent conductivity and stability at low potentials, while aluminum foil is reserved for the positive electrode to avoid corrosive alloying with lithium. The landscape shifts for sodium-based systems. Since sodium does not readily form alloys with aluminum, aluminum foil emerges as a viable, lightweight, and lower-cost alternative to copper for the negative electrode in sodium-ion battery and hybrid capacitor configurations. This presents a significant opportunity for reducing material cost and increasing the gravimetric energy density of the device.
However, the transition from copper to aluminum as the negative current collector is not without potential challenges. Aluminum has a higher electrical resistivity (approximately 62% higher than copper) and a lower thermal conductivity. For high-power devices like sodium-ion battery capacitors, which are designed for rapid charge/discharge cycles, efficient current collection and heat dissipation are paramount. The question arises: can aluminum foil, potentially with increased thickness to compensate for its higher resistivity, meet the stringent requirements for power performance, cycle life, and thermal stability in commercial SIBatCs? Furthermore, a comprehensive evaluation must consider not just electrochemical metrics but also the impact on overall system cost.
This investigation systematically addresses these questions. We designed and fabricated two classes of practical, commercially-relevant pouch-type sodium-ion battery capacitors: a Power-type optimized for high-rate capability and an Energy-type optimized for higher capacity. For each class, we directly compared the performance of cells employing standard 10 μm copper foil against those using 20 μm aluminum foil as the negative electrode current collector. A rigorous testing protocol was employed to evaluate key parameters including initial efficiency, rate capability, low-temperature discharge, high-temperature storage, DC and pulse power, and long-term cycling stability. Electrochemical impedance spectroscopy was used to probe interfacial changes. Finally, a detailed cost analysis was performed to quantify the economic benefits. This comprehensive study provides critical insights and guidelines for the engineering development of cost-effective, high-performance sodium-ion battery capacitors.
Experimental Design and Cell Configuration
The sodium-ion battery capacitors in this study were constructed using a sophisticated electrode design that combines two energy storage mechanisms in the positive electrode. The core cell chemistry and construction parameters are summarized below.
Electrode Composition and Fabrication:
- Positive Electrode: A composite blend of O3-type layered oxide NaNi1/3Fe1/3Mn1/3O2 (NFM) and high-surface-area activated carbon (AC) served as the dual-functioning positive active material. This design leverages the high capacity of the NFM (battery-type material) and the fast surface-driven processes of the AC (capacitor-type material). The slurry consisted of the NFM/AC mixture, a conductive additive composite (Super P carbon black and multi-walled carbon nanotubes), and polyvinylidene fluoride (PVDF) binder in N-methyl-2-pyrrolidone (NMP), coated onto a 20 μm aluminum foil current collector.
- Negative Electrode: Hard carbon (HC) was used as the sodium-ion intercalation host for the negative electrode. The slurry contained HC active material and PVDF binder in NMP, coated onto either 10 μm copper foil or 20 μm aluminum foil for the comparative study.
- Cell Assembly: The electrodes were processed through standard industry steps: coating, drying, calendaring, slitting, tab welding, stacking, vacuum drying, electrolyte filling, sealing, formation, aging, and final shaping. A 22 μm thick cellulose/PET composite separator was used. The electrolyte was 1.0 M sodium hexafluorophosphate (NaPF6) dissolved in a mixture of organic carbonates with functional additives.
Cell Design Philosophy:
Two distinct SIBatC designs were realized by controlling the electrode areal loading (mass of active material per unit area):
- Power-type SIBatC: Featuring lower areal loadings on both electrodes to minimize ionic diffusion distances, thereby maximizing power capability. The nominal design capacity was 6 Ah (or 10000 F).
- Energy-type SIBatC: Featuring higher areal loadings to increase the total amount of active material per unit area, thereby maximizing energy density. The nominal design capacity was 8 Ah (or 12000 F).
For each type, two variants were built: one with copper (Cu) and one with aluminum (Al) as the negative current collector. This resulted in four cell groups: Power-Cu, Power-Al, Energy-Cu, and Energy-Al. Crucially, the negative-to-positive capacity ratio (N/P ratio) was kept identical within each pair (Power or Energy) to ensure a fair comparison focused solely on the current collector effect.
Electrochemical Performance Evaluation Protocol
A comprehensive suite of tests was conducted on a Neware battery test system, with impedance measurements performed on a Princeton Applied Research potentiostat. All tests, unless specified otherwise, used a voltage window of 2.0 V to 4.0 V.
1. Formation, Capacity, and Internal Resistance:
Cells underwent a low-current formation process to stabilize interfaces. The static capacity ($C_F$) and DC internal resistance ($R_{DC}$) were determined from a standardized test involving a 5C constant-current charge to 4.0 V followed by a 5C constant-current discharge to 2.0 V. The formulas used were:
$$C_F = \frac{I \Delta t}{\Delta V}$$
$$R_{DC} = \frac{U_R – U_i}{\Delta I}$$
where $I$ is the discharge current, $\Delta t$ is the discharge time between two voltage points, $\Delta V$ is the corresponding voltage difference, $U_R$ is the cell voltage at the end of the charge, and $U_i$ is the cell voltage 100 ms after the discharge begins with a current step $\Delta I$. The maximum theoretical power density ($P_{max}$) was calculated as:
$$P_{max} = \frac{U_R^2}{4m \times R_{DC}}$$
where $m$ is the cell mass.
2. Rate Capability Test:
Cells were charged and discharged at progressively increasing C-rates (where 1C = nominal capacity in Amperes). The Power-type cells were tested from 1C to 30C, while Energy-type cells were tested from 1C to 10C. The capacity retention at each rate was recorded.
3. Low-Temperature Discharge:
Fully charged cells were placed in a temperature chamber and allowed to equilibrate for 6 hours at various temperatures (25°C, 0°C, -20°C, -30°C). They were then discharged at a 3C rate to 2.0 V.
4. High-Temperature Storage:
Fully charged cells were stored at 60°C for 14 days. The open-circuit voltage (OCV) was monitored to assess self-discharge. The AC internal resistance ($R_{AC}$) at 1 kHz was measured before and after storage (after cooling to room temperature).
5. Pulse Power Performance (10s at 50% SOC):
Cells were adjusted to a 50% state-of-charge (SOC, ~3.25V). The maximum current that could be applied for 10 seconds without exceeding the voltage limits (4.0 V on charge, 2.0 V on discharge) was determined. The corresponding 10-second pulse charge/discharge power was calculated.
6. Cycle Life Testing:
- Standard Cycle Life: Power-type cells were cycled at a 6C rate and Energy-type at a 4.5C rate (both at 36A) between 2.3 V and 3.8 V for 4000 cycles.
- Pulse Cycle Life: Power-type cells were subjected to an extremely demanding 16C (96A) pulse cycling profile for 4000 cycles to stress-test the high-power durability and thermal management.
Cell surface temperatures were monitored at multiple points during cycling tests using thermocouples.
7. Electrochemical Impedance Spectroscopy (EIS):
EIS measurements were conducted on fresh cells and after cycle life testing over a frequency range of 100 kHz to 50 mHz with a 5 mV amplitude. The resulting Nyquist plots were fitted to an equivalent circuit model to extract parameters like ohmic resistance ($R_{\Omega}$), interfacial film resistance ($R_{F}$), and charge transfer resistance ($R_{ct}$).
Results and Discussion: Performance Comparison
The following sections present a detailed comparison of the electrochemical behavior of sodium-ion battery capacitors employing copper versus aluminum negative current collectors.
Basic Characteristics and Energy/Power Metrics
The fundamental parameters of the four cell groups are consolidated in Table 1. The use of aluminum foil, with its lower density (2.7 g cm-3 vs. 8.9 g cm-3 for copper), directly led to a reduction in total cell mass. Consequently, despite having nearly identical capacity and energy content in Watt-hours, the gravimetric energy density ($E_m$) of cells with aluminum foil was significantly higher.
| Cell ID | Mass (g) | Thickness (mm) | 1C Capacity (Ah) | 1C Energy (Wh) | $C_F$ (F) | $E_m$ (Wh kg-1) | $P_{max}$ (W kg-1) |
|---|---|---|---|---|---|---|---|
| Power-Cu | 230 | 9.0 | 5.80 | 18.19 | 9710 | 78.9 | 10002.1 |
| Power-Al | 213 | 9.3 | 5.96 | 18.68 | 9792 | 87.8 | 11317.4 |
| Energy-Cu | 275 | 11.1 | 8.24 | 25.80 | 12712 | 93.8 | 8708.2 |
| Energy-Al | 256 | 11.4 | 8.23 | 25.74 | 12732 | 100.5 | 9574.3 |
The initial coulombic efficiency (ICE) and AC internal resistance ($R_{AC}$) were very similar between Cu and Al counterparts, with Al cells showing a marginal advantage (ICE ~0.1-0.3% higher, $R_{AC}$ ~1.5% lower). This indicates that the choice of current collector does not adversely affect the initial electrode formation processes in this sodium-ion battery capacitor system. The discharge curves were quasi-linear, characteristic of hybrid capacitor systems. The calculated maximum power density ($P_{max}$) revealed a clear benefit for the aluminum-based cells due to their lower mass, with improvements of 13.1% for the Power-type and 9.9% for the Energy-type. This highlights a primary advantage: for a given power output, the sodium-ion battery capacitor with an aluminum negative current collector will be lighter.
Rate Capability and Low-Temperature Behavior
The rate performance tests, shown in Figure 1 for Power-type cells, indicated that up to 20C, the capacity retention of Power-Cu and Power-Al cells was nearly identical. However, at the extreme rate of 30C, the Power-Al cell showed a more pronounced capacity fade (71.6% retention) compared to the Power-Cu cell (76.9% retention). A similar trend was observed for Energy-type cells at the highest tested rates (8C-10C). This suggests that under ultra-high current loads, the higher resistivity and potentially inferior thermal conductivity of the aluminum foil-current tab system may lead to greater polarization and capacity loss compared to the copper system.
At low temperatures, the kinetics of any sodium-ion battery or capacitor slow down. All cells exhibited expected capacity reduction as temperature decreased to -30°C. Interestingly, the capacity retention at -30°C was comparable between Cu and Al variants (~80% for Power-type, ~75-79% for Energy-type). However, temperature monitoring during the -30°C discharge revealed that cells with aluminum foil/negative tab experienced a slightly higher internal temperature rise (by ~1°C) by the end of the discharge. This can be attributed to the higher specific heat capacity of aluminum, meaning it requires more energy to change its temperature, and its lower thermal conductivity, which hinders heat dissipation from the cell core. The slightly warmer internal condition of the Al cells may have contributed to their marginally better capacity retention at this extreme temperature.
High-Temperature Storage and Stability
Accelerated aging tests at 60°C for 14 days showed nearly indistinguishable behavior between current collector types. The voltage decay rate (self-discharge, or K-value) was virtually identical for paired cells. After storage, the capacity retention and percentage increase in $R_{AC}$ were also remarkably similar. For instance, Power-Cu and Power-Al showed capacity retentions of 94.0% and 94.3%, and $R_{AC}$ growth of 11.3% and 11.4%, respectively. This is a critical finding, indicating that the long-term chemical stability of the cell, including electrolyte decomposition and passive film evolution, is not negatively impacted by replacing copper with aluminum as the negative current collector in this sodium-ion battery capacitor chemistry.
Pulse Power and Cycle Life Analysis
The 10-second pulse power test at 50% SOC confirmed the high-power capability of these sodium-ion battery capacitors. While the absolute currents achieved were slightly higher for Cu-based cells, the gravimetric power densities were 6-10% superior for Al-based cells due to their lower mass. The Power-Al cell, for example, delivered pulse charge and discharge power densities of 3318 W kg-1 and 2301 W kg-1, respectively.
Long-term cycling stability is a hallmark of capacitor-like devices. Under standard cycling conditions (6C for Power-type, 4.5C for Energy-type), all cells demonstrated excellent capacity retention after 4000 cycles, with final values between 87% and 92%. There was no significant difference attributable to the current collector material. However, temperature monitoring during cycling revealed a consistent and important trend: cells with aluminum foil consistently operated at higher surface temperatures than their copper counterparts. The temperature difference was most pronounced at the negative tab location (Point D). This observation became even more stark during the ultra-high rate (16C) pulse cycling test on Power-type cells. While both Power-Cu and Power-Al maintained similar capacity retention (~77.5%) after 4000 harsh cycles, the temperature swing during each pulse was significantly larger for the Power-Al cell (6-8°C vs. 4-5°C for Power-Cu).
This thermal behavior is directly linked to the fundamental material properties of the current collectors, as summarized in Table 2.
| Property | Copper (10 μm) | Aluminum (20 μm) |
|---|---|---|
| Density (g cm-3) | 8.9 | 2.7 |
| Thermal Conductivity (W m-1 K-1) | 401 | 237 |
| Specific Heat Capacity (J g-1 °C-1) | 0.39 | 0.88 |
| Electrical Resistivity (Ω m) | 1.75 × 10-8 | 2.83 × 10-8 |
The copper foil and tab system acts as a more efficient “heat sink” due to its superior thermal conductivity (1.7 times that of Al). It can rapidly draw heat away from the electrode stack. Conversely, aluminum’s lower thermal conductivity and higher heat capacity cause it to retain more generated heat locally, leading to a higher operating cell temperature. This elevated temperature during high-rate cycling has direct electrochemical consequences.
Impedance Evolution and Degradation Mechanism
EIS analysis before and after cycling provided clear evidence of the thermal effect. The fitted impedance parameters showed that while the ohmic resistance ($R_{\Omega}$) and interfacial film resistance ($R_{F}$) changes were similar between Cu and Al cells, the charge transfer resistance ($R_{ct}$) increased more substantially in the cells with aluminum foil after cycling. For example, after standard cycling, $R_{ct}$ increased by 53.1% for Power-Cu but by 60.3% for Power-Al. The difference was even more pronounced for Energy-type cells (93.8% vs. 120.8% increase).
The growth in $R_{ct}$ is associated with kinetic limitations at the electrode-electrolyte interfaces. The consistently higher operating temperature in Al-based cells is hypothesized to accelerate minor side reactions at these interfaces, leading to a gradual buildup of less conductive species or a modification of the interfacial layer, thereby increasing the charge transfer barrier more rapidly than in the cooler-operating Cu-based cells. This finding does not necessarily preclude the use of aluminum but highlights a critical engineering consideration: thermal management is even more crucial for sodium-ion battery capacitors employing aluminum negative current collectors, especially in high-power applications. Effective module-level cooling strategies, such as direct tab cooling (“root absorption”) or dual-sided liquid cooling plates, are recommended to mitigate this effect and maximize cycle life.
Cost-Benefit Analysis
Beyond performance, the economic driver for replacing copper with aluminum is substantial. A detailed bill-of-materials analysis was performed based on commercial material prices and standard manufacturing yields. The results, focusing on the cost per unit of energy (Wh), are presented in Figure 2.
The analysis reveals that the current collector constitutes a major cost component when using copper foil, representing 28.8% and 23.6% of the total material cost for Power-Cu and Energy-Cu cells, respectively. This is second only to the cost of the positive active material. Substituting with aluminum foil dramatically reduces this contribution to 8.0% and 6.4%. Consequently, the overall unit energy cost drops from 1.21 RMB Wh-1 to 1.02 RMB Wh-1 for the Power-type (a 15.7% reduction) and from 1.00 RMB Wh-1 to 0.87 RMB Wh-1 for the Energy-type (a 13.0% reduction).
This cost advantage is coupled with the previously demonstrated 11.3% and 7.1% improvement in gravimetric energy density for Power-Al and Energy-Al cells, respectively. Furthermore, aluminum is more abundant and exhibits less price volatility than copper, offering greater supply chain security and cost predictability for large-scale production of sodium-ion battery capacitors.
Conclusion
This comprehensive study provides a definitive performance and cost comparison of copper versus aluminum foil as the negative electrode current collector in practical sodium-ion battery capacitors. The key conclusions are:
- Feasibility Confirmed: Aluminum foil (20 μm) is a viable direct replacement for copper foil (10 μm) as the negative current collector in both high-power and high-energy sodium-ion battery capacitor designs. It does not negatively impact initial efficiency, baseline capacity, low-temperature performance, or high-temperature storage stability.
- Performance Trade-off: Aluminum-based cells offer superior gravimetric energy and power density due to lower mass. However, under extreme high-rate or continuous high-power cycling, they exhibit greater internal temperature rise and a subsequently faster increase in charge transfer impedance ($R_{ct}$) compared to copper-based cells, attributable to aluminum’s inferior thermal conductivity and higher specific heat capacity.
- Cost Advantage: The transition from copper to aluminum delivers a significant economic benefit, reducing the unit energy cost by 13-16% while simultaneously increasing gravimetric energy density by 7-11%.
- Engineering Imperative: The successful implementation of aluminum foil in high-power sodium-ion battery capacitors necessitates prioritized thermal management at the module and pack level. Effective cooling strategies are essential to harness the cost and weight benefits of aluminum while maintaining the long-term cycle life expected from hybrid capacitor technologies.
In summary, aluminum foil stands as a compelling choice for the negative current collector in commercial sodium-ion battery capacitors, striking an excellent balance between performance, cost, and sustainability. This work provides a clear roadmap for developers, highlighting that with prudent thermal design, the advantages of aluminum can be fully realized, accelerating the adoption of cost-effective, high-performance sodium-ion battery capacitors for a wide range of energy storage applications.