Date: 2026-11-10 hits: 104
I. Introduction
In the last two years, there has been a near-industry consensus regarding large cylindrical cells, large pouch cells, and energy storage cells with capacities exceeding 500Ah: increasing cell size is an effective way to achieve superior energy density and cost performance. However, as cell dimensions grow, a seemingly minor step in the production process—electrolyte wetting—has increasingly become a bottleneck that constrains production capacity. When the same electrolyte formulation and electrode sheets are used in larger cells, the time required for complete wetting can extend from several hours to tens of hours. If not managed properly, "dry zones"—areas where wetting is insufficient—can emerge within the cell.
II. The Nature of Wetting
The essence of wetting lies in capillary action. The electrolyte penetrates the electrode sheets and separators driven by capillary pressure within the pores, rather than by external mechanical force. This process can be described by the classic Lucas-Washburn equation, which states that the square of the wetting depth is directly proportional to time.
In other words, doubling the wetting depth does not merely double the required time; it quadruples it. As electrode sheets become thicker and the number of winding turns or stacked layers increases, the path the electrolyte must travel grows longer. Consequently, the wetting time does not increase linearly, but rather expands exponentially (following a square-law relationship). This is the primary reason why wetting is slow in large-format cells—it is a matter of fundamental physical laws, not a reflection of the quality of the manufacturing process.
III. The Impact of Compaction Density
A second challenge arises from compaction density. In recent years, compaction density has steadily increased as manufacturers seek to pack in more active material to achieve higher volumetric energy density. However, higher compaction density results in lower porosity, smaller pore sizes, and greater tortuosity—the internal pathways become narrower and more convoluted, making it significantly more difficult for the electrolyte to penetrate the structure.
IV. "Dry Zones" Within the Cell Structure
A third layer of difficulty lies within the cell structure itself. The core of a wound cell or the central layers of a stacked cell are the points furthest from the electrolyte entryways, making them inherently prone to becoming "dry zones" in terms of wetting. When the electrolyte first makes contact with the electrodes, mismatches in capillary forces and the tension between the separator and the electrodes can cause the separator to wrinkle. Alternatively, gases released during the process may become trapped in the gaps between the electrodes and the separator, forming bubbles. These wrinkles and bubbles, in turn, obstruct the path of the electrolyte, further slowing down the wetting process and creating a self-perpetuating, vicious cycle.
V. Consequences of Inadequate Wetting
1. Inside the battery cell: Areas that are not sufficiently wetted by the electrolyte fail to form a complete Solid Electrolyte Interphase (SEI) layer. This leads to poor local ionic conductivity and uneven current density during the formation process. In a fully charged state, these regions are highly susceptible to localized lithium plating; upon disassembly, such areas often appear as distinct black spots.
2. Performance and safety implications: On a broader scale, this results in poor cell consistency and reduced cycle life. In severe cases, it can lead to cell swelling or electrolyte leakage, posing a risk of thermal runaway.
3. Impact on production and costs: For the production line, inadequate wetting translates to higher defect rates during formation and longer required resting periods. This directly slows down the mass production throughput for large-format cells and drives up costs.
VI. Assessing Wetting Quality
Since it is difficult to gauge the quality of wetting with the naked eye, the industry has developed several characterization methods:
1. Gravimetric method: An electrode sheet is suspended from a balance and immersed in the electrolyte; the change in weight over time is used to track the wetting process.
2. Height-based method: High-speed cameras are used to record the height to which the electrolyte rises along the electrode sheet, providing a direct visual representation of the wetting speed and extent.
3. Impedance method: A reference electrode is embedded within the cell. By monitoring the impedance of the positive and negative electrodes relative to this reference, the degree of wetting can be indirectly assessed.
VII. Summary
While achieving proper wetting in large-capacity battery cells is a significant challenge, it is not insurmountable.
Process-side: Increasing the duration of resting periods—both at room temperature and at elevated temperatures—can help. Additionally, enhancing the thermal-compression bonding between the separator and the electrode sheets can effectively counteract wrinkling caused by capillary forces.
Electrode-side: Techniques such as laser-perforation or the design of a gradient porous structure with low tortuosity can be employed, effectively creating "shortcuts" for the electrolyte to penetrate the structure.
Electrolyte-side: Wetting additives can be incorporated into the electrolyte formulation to reduce surface tension and contact angle, thereby strengthening the capillary driving force.
