Overview
Lithium battery separators serve as a protective barrier between the positive and negative electrodes, conducting ions but not electrons. Ideally, after injection and formation, the separator should maintain a perfect, flat fit with the electrode. However, often, upon disassembling the battery, the separator is found to be severely wrinkled. (This can also be clearly seen through wrinkles on the negative electrode.)
The following discusses separator wrinkles in three key points: the hazards of separator wrinkles, the causes of separator wrinkles, and solutions for separator wrinkles.
Hazards of Separator Wrinkles
Increased Internal Resistance: The separator's microporous structure in the wrinkled area is disrupted, obstructing the lithium ion transmission path. This increases internal resistance by 15%-30%, significantly reducing charge and discharge efficiency.
Capacity Fading: Uneven electrolyte infiltration in the wrinkled area reduces active material utilization. Experimental data from a certain ternary cathode battery showed a capacity fade rate of up to 8% per 100 cycles.
Lithium plating induces short circuits: Wrinkling increases local current density and the probability of lithium dendrite growth on the graphite anode surface, increasing the risk of short circuits.
Causes of diaphragm wrinkling
Material defects: Wrinkles are more likely to occur when the diaphragm surface roughness is greater than 0.3μm (Ra standard value 0.1-0.3μm) or the tensile strength is less than 300MPa.
Process defects: Excessive winding tension fluctuations exceeding ±3% can lead to uneven electrode roll tightness (standard requirement ±1%), oven temperature gradients greater than 5°C (allowable value ≤ 2°C), etc.
Improper hot pressing process: Improper hot pressing pressure, temperature, and time settings can lead to poor electrode roll formation.
Improper injection process: The injection process generally includes (primary vacuum injection - resting - pre-filling - secondary vacuum injection). If the vacuum pressure is too high or the speed is too fast, separation between the diaphragm and electrode can easily occur. At the same time, gas is generated after pre-charging, and vacuuming during secondary filling also draws out gas and some electrolyte. This gas extraction creates gas paths and noticeable wrinkles.
Incomplete electrolyte impregnation: Incomplete electrolyte impregnation results in dry areas, gaps, and bubbles between the electrode and diaphragm, making it prone to wrinkling during vacuuming. Electrode surface defects: Intrinsic defects in the electrode (such as bumps and pits) can affect the adhesion of the diaphragm to it, leading to electrode wrinkles.
Solutions for Diaphragm Wrinkling
The solutions often share similar causes, including improving the basic properties of the diaphragm and refining the winding, hot pressing, baking, and filling processes.
Optimal electrode impregnation can be achieved by extending the impregnation time and appropriately raising the impregnation temperature (e.g., 45°C).
In addition to process improvements, using coated diaphragms can effectively mitigate the problem of diaphragm wrinkling. For example, diaphragms coated on one side with PVDF significantly improve adhesion after hot pressing, completely eliminating or reducing wrinkles.
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