With the increasing global demand for renewable energy, electrochemical energy storage technology, particularly lithium-ion batteries, has become a critical supporting technology due to its high energy density, excellent cycle performance, and good adaptability. Electrochemical energy storage systems play a key role in balancing supply and demand, improving energy efficiency, and promoting the integration of renewable energy sources. However, as the installed capacity of these systems continues to grow, the cost has become a significant challenge. Lithium-ion batteries and their connectors account for approximately 70% of the total cost of an energy storage system. Reducing the cost of batteries and management systems, along with improving integration efficiency, are effective ways to lower the overall system cost. Increasing the capacity of a single cell is a key method for achieving this cost reduction. The market demand for large-capacity energy storage cells has driven the capacity of single cells from 280 Ah to 430 Ah, necessitating intensified research and development efforts for larger cells.
The safety of large-capacity lithium-ion batteries is a major focus of the industry and a primary factor limiting their application in the energy storage sector. Issues such as thermal runaway, overcharging, and short circuits are more prominent in large-capacity cells, posing risks to personnel safety and equipment integrity. Therefore, developing large-capacity lithium-ion batteries that meet safety standards with high energy density and reliability is a critical task for both academia and industry.
This study compares the performance of a self-developed 430 Ah large-capacity lithium iron phosphate (LFP) energy storage cell with the commonly used 280 Ah cell, according to the standard GB/T 36276—2023. The objective is to provide a reliable, safe, and cost-effective solution for large-capacity electrochemical energy storage cells, offering theoretical guidance and practical insights for future developments in this field.
Experimental Setup
Materials and Cell Manufacturing
The positive electrode material used was high-purity LiFePO4 (99.95% purity). The material was first processed in a mixer at 450 rpm for 60 minutes, and then mixed with conductive carbon black (99.99% purity) at a mass ratio of 96.3:1 at 200 rpm for 180 minutes. The final mixture was stored in an argon atmosphere to ensure stability.
For the electrode preparation, polyvinylidene fluoride (PVDF) binder was dissolved in N-methylpyrrolidone (NMP), and then the processed LiFePO4 mixture, conductive carbon black, and carbon nanotubes (CNTs) were added and uniformly mixed. The slurry was coated onto 13 μm thick aluminum foil and dried at 120°C under a vacuum of 133 Pa for 24 hours. The positive electrode was pressed to a thickness of (150±3) μm, with an active material mass fraction of 96.3%.
For the negative electrode, high-purity graphite was mixed with conductive agent SP-C65 at a mass ratio of 96:0.8. Carboxymethyl cellulose (CMC, 99.50%) and styrene-butadiene rubber (SBR) were added as binders. The slurry was coated onto 6 μm thick copper foil, dried, and pressed to a thickness of (128±3) μm, resulting in an active material mass fraction of 96%.
The 430 Ah large-capacity energy storage cell, designed as a large flat plate with a lamination structure, used a 14 μm thick coated separator. This separator was based on a 9 μm polyethylene (PE) base membrane with a porosity of 45%, combined with a 3 μm ceramic coating and a 2 μm PVDF adhesive coating. The electrolyte used was 1 mol/L LiPF6 in a solvent mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) in a volume ratio of 2:5:3. The cell dimensions were 530 mm × 218 mm × 32 mm, with a rated capacity of 430 Ah.
For comparison, a 280 Ah prismatic lithium-ion cell was manufactured with dimensions of 173 mm × 207 mm × 71 mm. This cell used a winding structure with a 12 μm ceramic separator (9 μm PE base membrane with a 3 μm ceramic coating). The materials and electrolyte were the same as those used in the 430 Ah cell. This control group allows for the study of the effects of different structural designs and separators on cell performance.
Testing Methods
Cell performance and cycling tests were conducted using a BTS-600 charge-discharge system. A programmable temperature chamber was used to simulate actual operating conditions. Voltage and temperature changes were monitored with a data recorder. Safety tests, including nail penetration and crush tests, were performed using a needle puncture and extrusion machine. Overcharge and over-discharge tests were conducted with the charge-discharge system. All safety tests followed the GB/T 36276—2023 standard, with the nail penetration test referenced from GB/T 31484—2015.
For safety testing, eight 430 Ah cells and eight 280 Ah cells were placed in an environment at (25±2) °C for 5 hours. After reaching thermal equilibrium, the 430 Ah cells were discharged at a constant power of 688 W (rated power 0.5P) to the cut-off voltage (2.50 V), rested for 10 minutes, and then charged at 688 W to the cut-off voltage (3.65 V). The 280 Ah cells were charged at 488 W using the same procedure.
Results and Discussion
Comparison of Basic Cell Performance
The performance of the 280 Ah and 430 Ah energy storage cells is compared in the following table. The testing conditions were in accordance with GB/T 36276—2023, with a rated charge/discharge power of 0.5P.
| Parameter | 280 Ah Cell | 430 Ah Cell |
|---|---|---|
| 0.5P Discharge Energy (25 °C) / Wh | 976 | 1,377 |
| 0.5P Energy Efficiency (25 °C) / % | 94.3 | 94.8 |
| 0.5P Low-Temp Discharge Energy (5 °C) / Wh | 782 | 1,143 |
| 0.5P Low-Temp Energy Efficiency (5 °C) / % | 80.5 | 81.5 |
| 0.5P High-Temp Discharge Energy (45 °C) / Wh | 987 | 1,474 |
The data shows that the 430 Ah energy storage cell has a room temperature discharge energy of 1,377 Wh and an energy efficiency of 94.8%, compared to 976 Wh and 94.3% for the 280 Ah cell. The high and low-temperature discharge energies for both cells exceed 80% of the rated energy, meeting the design requirements. The 430 Ah cell exhibits higher energy efficiency at both room and low temperatures. This indicates that the increase in capacity did not lead to a performance degradation. The primary reason is that the 430 Ah cell employs a lamination process, which, unlike the winding process, applies near-zero tension to the electrode assembly. This preserves the porosity of the separator, keeping it consistent with the raw material. Furthermore, the specially coated separator, with its PVDF and ceramic layers, ensures a tight contact between the separator and the electrode surfaces, reducing the ion diffusion distance within the cell and lowering internal resistance. This structural and material design enhances the electrochemical performance of the large-capacity energy storage cell.
Safety Performance Comparison
To assess the safety of the larger-capacity energy storage cell, both the 280 Ah and 430 Ah cells were subjected to a series of safety tests per GB/T 36276—2023. The results are summarized in the following tables.
| Test Item | 430 Ah Cell | 280 Ah Cell |
|---|---|---|
| Overcharge Test | Pass | Pass |
| Overdischarge Test | Pass | Pass |
| Short Circuit Test | Pass | Pass |
| Crush Test | Pass | Pass |
| Drop Test | Pass | Pass |
| Adiabatic Temperature Rise Test | Pass | Pass |
| Thermal Runaway Test | Pass | Pass |
| Nail Penetration Test | Pass | Pass |
| Test Item | 280 Ah Cell | 430 Ah Cell |
|---|---|---|
| Overcharge | 25.4 | 17.3 |
| Overdischarge | 136.2 | 93.3 |
| Short Circuit | 259.6 | 236.5 |
| Nail Penetration | 281.6 | 2.2 |
| Thermal Runaway | 327.0 | 233.7 |
Both cells passed all safety tests. However, the 430 Ah energy storage cell consistently showed lower temperature rises than the 280 Ah cell, indicating superior safety performance. The most significant difference was observed in the nail penetration test. The 430 Ah cell exhibited a temperature rise of only 2.2 °C, reaching a maximum surface temperature of 20.5 °C, with no smoke or sparks observed. In contrast, the 280 Ah cell temperature rose rapidly to 281 °C, causing deformation of the cell’s blue insulating film. This superior thermal stability of the 430 Ah energy storage cell is attributed to its design. The lamination structure results in a thinner cell (32 mm) with fewer electrode layers, reducing the contact area with the nail and the number of internal short circuits. The larger surface area facilitates faster heat dissipation, minimizing heat accumulation. Additionally, the ceramic and PVDF coating on the separator enhances its stability at high temperatures, preventing shrinkage and mitigating the risk of large-scale short circuits that lead to thermal runaway. The 280 Ah cell, with its thicker winding structure (71 mm), has a larger contact area with the nail, more short-circuit points, and less efficient radial heat transfer, leading to higher temperatures and a greater risk of thermal runaway.
The nail penetration test data demonstrates that through optimized design of the structure and separator, a large-capacity energy storage cell can achieve a high level of safety.
Cycle Life Performance Comparison
To evaluate the effect of increased capacity on cycle life, the energy retention rates of both cells were compared at 25 °C under a 0.5P charge/discharge power. The results are presented in the table below.
| Cycle Number | 430 Ah Cell Energy Retention (%) | 280 Ah Cell Energy Retention (%) |
|---|---|---|
| 450 | 98.5 | 95.5 |
After 450 cycles, the 430 Ah energy storage cell retained 98.5% of its initial energy, which is 3% higher than the 95.5% retention of the 280 Ah cell. The larger-capacity cell did not show a decline in cycle performance. The lamination structure and optimized separator design contribute to an extended cycle life. In the 280 Ah cell, the winding structure causes uneven stress distribution during charge/discharge cycles, leading to electrode deformation and wrinkling, particularly at the edges. This can result in poor contact between the electrode and separator, causing localized lithium plating and capacity fade. The variable tension during winding also leads to non-uniform ion diffusion and current distribution, accelerating degradation. In contrast, the lamination structure of the 430 Ah energy storage cell ensures uniform stress and prevents electrode wrinkling. The coated separator adheres closely to the electrodes, eliminating gaps and reducing the risk of lithium plating. The flat interfaces within the cell allow for synchronous expansion and contraction of the electrodes, ensuring uniform current distribution and ion diffusion, thereby improving performance and extending cycle life.
Conclusion
Through the use of an internal lamination structure and an optimized coated separator, we successfully developed a 430 Ah large-capacity prismatic energy storage cell using LiFePO4 as the cathode active material and artificial graphite as the anode active material. Performance tests demonstrated that, despite the increase in capacity, the optimized 430 Ah energy storage cell exhibits superior performance compared to the 280 Ah cell.
Safety tests confirmed that the 430 Ah energy storage cell with a lamination structure fully complies with national safety standards. It successfully passed the stringent nail penetration test with no smoke or sparks, and the temperature rise during the test was less than 3 °C.
Cycle life tests revealed that the 430 Ah energy storage cell maintained a high cycle life, showing no performance degradation due to the capacity increase. The cell retained 98.5% of its energy after 450 cycles.
This research demonstrates that using an internal lamination structure and separator optimization for a large flat-plate style battery can simultaneously enhance capacity, safety, and performance. The key to this improved performance lies in the design’s ability to manage heat, ensure uniform stress, and optimize ionic transport. The findings provide a valuable reference for the development of high-capacity energy storage cells, highlighting that structural innovations are crucial for mitigating the safety and performance challenges associated with scaling up cell capacity.
$$ \eta = \frac{E_{\text{discharge}}}{E_{\text{charge}}} \times 100\% $$
$$ Q = C_p \cdot m \cdot \Delta T $$
$$ \text{Capacity Retention} = \frac{E_{n}}{E_{0}} \times 100\% $$