Date: 2026-08-11 hits: 107
During the R&D process, it is often observed that system designs that perform excellently in small pouch cells exhibit significant performance degradation when applied to large pouch cells. Below, we will briefly discuss this phenomenon.
I. Differences in Current Collector Resistance
Many people believe that doubling the electrode size doubles the current collector resistance, but the situation is far more complex. Small pouch cells have a small size, a relatively short path from the electrode to the tab, and a relatively uniform current distribution, resulting in minimal contribution from the current collector's surface resistance. In large pouch cells, the current converges from the far end of the electrode to the tab, resulting in a long transmission path and continuous accumulation of current density along the path.
According to the resistance formula, the resistance of a single current path is proportional to its length. However, the current distribution in large electrodes is convergent, with the current density near the tab reaching more than twice that at the far end. Local Joule heating and resistance losses increase quadratically.
II. Planar Concentration Polarization
In small pouch cells, the focus is on mass transfer along the thickness direction, where the small planar size and rapid ion diffusion make it negligible. This is not the case for large pouch cells. During high-current discharge, lithium ions in the center of the electrode are consumed rapidly. Ions in the electrolyte must diffuse laterally from the edges to replenish the electrolyte, a path that can be tens of centimeters long. The mass transfer resistance can even exceed that in the thickness direction. The result is severe SOC differentiation in different areas of the electrode: the edges haven't fully delithilated before the center is already deeply discharged, leading to a significant decrease in the utilization rate of the active material.
III. Uneven Interlayer Pressure Pouch cells lack rigid shell constraints, and the pressure in the thickness direction relies entirely on external clamping.
1. Small Pressure Gap: Pouch cells have fewer layers, sometimes even using a direct winding structure. Pressure is transmitted uniformly from the surface to the inside, resulting in consistent interfacial impedance. Large pouch cells have dozens of layers, and the external pressure decreases layer by layer inwards, leading to insufficient contact pressure at the center and the formation of tiny gaps. Once a gap appears, the local interfacial impedance spikes, preventing the active material from being utilized effectively.
2. Electrode Slippage and Wrinkling: During cycling, the electrode repeatedly expands and contracts, resulting in greater displacement space in the center layer. This also makes it prone to relative slippage, wrinkling, or misalignment. Therefore, in the industry, almost all large flexible packaging materials require hot pressing and module clamping structures to compress the layers with external force and minimize impedance delamination.
IV. Accumulation of Expansion Stress
Both the positive and negative electrodes expand and contract during charging and discharging. Small pouch cells have fewer layers, resulting in less total expansion and easier stress release; large pouch cells have a higher layer count, leading to millimeter-level total expansion and considerable internal shear stress.
During high-rate charging and discharging, uneven expansion along the thickness direction can occur, creating an expansion stress gradient. Accumulation to a certain extent can lead to interlayer delamination, electrode cracking, and conductive network breakage.
Pouch aluminum-plastic films lack rigid constraints, allowing for greater expansion freedom and more pronounced interlayer shear displacement.
V. Thermal-Electro-Chemical Coupling Effect
Small pouch cells have a large surface area, resulting in good heat dissipation and relatively uniform temperature. Large pouch cells have poor heat dissipation at the center, where the temperature can be 5 to 8°C higher than the edges. Higher temperatures lead to higher conductivity and faster reactions, causing current to concentrate further towards the center; this current concentration generates more Joule heating, further increasing the temperature and creating positive feedback. High-temperature regions experience accelerated SE growth and intensified side reactions, while low-temperature regions exhibit greater polarization and are more prone to lithium plating.
Over the long term, the aging rates vary significantly across different regions, and the overall battery life is determined by the worst-performing hotspot area. This is one of the key reasons why the cycle life of large pouch cells is generally inferior to that of small pouch cells.
VI. Uneven SEI Film Formation
Small pouch cells have a uniform current distribution, and SE grows synchronously during formation, resulting in consistent thickness and density. Large pouch cells have uneven current density, with rapid and thick film formation near the tabs, while the film is thin and incomplete at the far ends.
This inherent difference is continuously amplified during cycling.
Thin SE areas continuously consume active lithium, becoming thicker and thicker; excessively thick areas have high impedance and large polarization, making them prone to lithium plating. Ultimately, the interface impedance uniformity deteriorates, leading to premature aging in certain areas and dragging down the overall lifespan. The rapid increase in internal resistance in the early stages of cycling in many large pouch cells is largely due to compensating for this uneven SEI.
VII. Summary
In conclusion, the performance differences between large and small pouch cells are mainly the result of multiple effects, including nonlinear growth in current collector resistance, planar concentration polarization, uneven interlayer pressure, accumulated expansion stress, amplified thermoelectric coupling, and uneven SEI film formation. Each factor can be ignored in small cells, but becomes a dominant factor in large cells.
