"Wetting" in lithium-ion cells refers to the thorough saturation of the cell's porous electrode structures with electrolyte, a critical process that ensures efficient ionic conductivity & optimal contact between the electrodes & electrolyte. Uniform wetting is fundamental to the cell's functionality, as it directly influences ion transport and, consequently, the battery's overall performance. It also prevents the formation of dry spots, which can lead to localized heat generation and, in severe cases, thermal runaway.
Evaluating electrolyte wetting non-destructively during the formation stage of sealed li-ion cells is challenging. The X-ray computed tomography technique has historically stumbled, due to its insensitivity to electrolyte interactions & gaseous phenomena within the cell, & this is where "ultrasonic imaging" comes into play, offering potential for non-destructive evaluation. This method's merit is also in its speed, cost-effectiveness, and applicability to a range of cell formats. Its sensitivity is particularly noteworthy, as it detects early-stage electrolyte dry-out, or "unwetting," critical for understanding failure mechanisms in batteries.
By measuring local ultrasonic transmittance, one of the in-house developed ultrasonic scanning tools by Zhe D. et. al. translates these signals into a visual map of wetting processes. A color scale corresponding to the transmittance intensity distinguishes well-wetted regions, essentially quantifying the electrolyte distribution within the cell. Their results reveal the method’s precision in identifying the volume threshold for complete electrolyte saturation & the necessary time for electrolyte diffusion, a significant factor for cell assembly optimization. In examining aged cells, differences in ultrasonic transmittance provide insights into the state of the electrolyte & its stability under various cycling conditions. Unlike prior fixed-point ultrasonic measurements that offered average values over broader areas, this scanning technique portrays fine structural variations within the cell.
The image shows the gradient from insufficient wetting (in blue) to complete wetting (green to red) as a function of both electrolyte volume & time post-filling. For instance, a cell that requires at least 0.8 mL of electrolyte for saturation demonstrates distinct ultrasonic transmittance patterns before & after reaching this critical volume. Time-lapse imaging further illustrates the progressive improvement in wetting over a 24-hour period.
Such a tool for diagnosing the internal state of cells can offer manufacturers a powerful method to fine-tune their processes for optimized cell life, quality, & reliability.
Comments