Home/About Us/Battery Research Knowledge Base – Canrud Insights/Research on the Causes of Separator Wrinkling in Lithium-ion Batteries and Its Elimination Strategie
Research on the Causes of Separator Wrinkling in Lithium-ion Batteries and Its Elimination Strategie
Canrd March 3, 2026 216
1. Introduction
Lithium-ion batteries have become the core energy storage device in fields such as electric vehicles, portable electronics, and stationary energy storage due to their high energy density, long cycle life, and environmental friendliness. As a key component of lithium-ion batteries, the separator plays a crucial role in isolating the positive and negative electrodes, preventing short circuits, and allowing the free passage of lithium ions. However, separator wrinkling, a common defect during battery manufacturing, has become a major obstacle affecting battery performance and production yield. During cell assembly, the separator is initially flat when wrapped around the electrodes, but wrinkling often occurs during the electrolyte filling process, accompanied by residual bubbles at the separator-electrode interface. These defects lead to uneven internal resistance distribution, local overcharge or overheating, and ultimately reduce battery consistency, cycle stability, and service life. Especially in the production of large-format power batteries, separator wrinkling is more prominent, becoming a technical bottleneck that restricts the improvement of battery uniformity. To solve this problem, it is necessary to systematically explore the formation mechanism of separator wrinkling and propose feasible elimination strategies, which is of great significance for promoting the high-quality development of the lithium-ion battery industry.
2. Research Background and Problem Statement
As a lithium battery R&D engineer, you have likely often encountered separator wrinkling when disassembling cells. The separator is perfectly flat when wrapping the electrodes during cell assembly, so why does it become wrinkled? To eliminate this issue, we must first identify the root causes, which is the core premise of solving the problem. During the manufacturing process of lithium-ion batteries, local wrinkling of the separator and residual bubbles between the separator and electrodes are common quality defects that often occur during the electrolyte filling stage. These defects not only affect the appearance of the battery cell but also have a profound impact on its internal performance: the wrinkled separator leads to uneven contact between the separator and the electrodes, resulting in uneven distribution of internal resistance of the battery; local overcharge or overheating is likely to occur at the wrinkled position, which may further trigger safety hazards such as thermal runaway in severe cases. In addition, the residual bubbles between the separator and the electrodes will block the transmission of lithium ions, reduce the ion conductivity, and ultimately affect the consistency and cycle life of the battery. Separator wrinkling has long been a persistent challenge for battery cell manufacturers, and it is particularly prominent in the production of large-format power batteries. Due to the large size of large-format battery cells, the requirements for the flatness of the separator are higher, and the probability of wrinkling is also greater, which has become a key technical bottleneck restricting the improvement of battery uniformity and production efficiency.
3. Research Methods and Materials
To accurately explore the formation mechanism of separator wrinkling and verify the effectiveness of elimination strategies, this study selected appropriate materials and designed scientific experimental methods, ensuring the reliability and repeatability of the research results. The materials used in the experiment include three types of separators commonly used in industrial production, covering different preparation processes and coating types: (a) Dry-process biaxially oriented PP (polypropylene) separator, which is widely used due to its excellent mechanical strength and chemical stability; (b) Wet-process biaxially oriented PE (polyethylene) separator, which has the advantages of uniform pore size distribution and good ion permeability; (c) Composite separators coated with Al₂O₃ (aluminum oxide) orPVDF (polyvinylidene fluoride), where the coating is used to improve the thermal stability, wettability, and adhesion of the separator.
The experimental methods are designed to simulate the actual electrolyte filling process and test the key performance indicators related to separator wrinkling. Specifically, the experiment simulated the battery electrolyte filling process by dripping DMC (Dimethyl Carbonate), a common component of lithium-ion battery electrolytes, onto the surface of different types of separators, and observed the wetting and flow behavior of DMC on the separator surface in real time. At the same time, a high-speed camera was used to record and analyze the dynamic wetting front, so as to clarify the relationship between the wetting process and separator wrinkling. In addition, to explore the effect of adhesion between the separator and electrodes on wrinkle elimination, PVDF-coated composite separators were thermally laminated with positive and negative electrodes under specific conditions, and the peel strength between the separator and electrodes was tested by a universal testing machine. The peel strength test can quantitatively characterize the interfacial bond strength, providing a scientific basis for verifying the effectiveness of wrinkle elimination strategies.
4. Formation Mechanism of Separator Wrinkling
Based on the experimental results, this study systematically analyzed the formation mechanism of separator wrinkling, and found that it is the result of the combined action of multiple factors, among which capillary action is the primary cause, and bubble residue, separator thickness, and coating materials also have important impacts on wrinkling.
(1) Capillary Action as the Primary Cause
The study found that capillary action during electrolyte wetting is the core factor leading to separator wrinkling. When DMC, as a representative electrolyte component, wets the separator, it will quickly penetrate into the micropores of the separator (the pore size of the separator used in the experiment is less than 100 nm). Due to the small pore size, capillary pressure is generated inside the micropores, which acts on the separator and causes local bulging of the separator at the liquid front. This bulging will lead to the separation between the separator and the electrodes, and as the electrolyte continues to spread, the separator will re-adhere to the electrodes. This periodic "adhesion-separation" process is repeated, and finally manifests as the "bamboo-node" structure of the wrinkles on the separator surface. The high-speed camera observation results further confirmed this process: the wetting front of DMC on the separator surface shows an uneven propagation state, and the local bulging caused by capillary pressure is the direct cause of the initial formation of wrinkles.
(2) Bubble Residue Exacerbates Wrinkling
While capillary action is the primary cause, bubble residue at the separator-electrode interface further exacerbates separator wrinkling. During the process of DMC wetting the pores inside the separator and the electrodes, the air originally trapped in the pores will be displaced by the electrolyte. Due to the limited flow space of the gas and the slow diffusion rate, these displaced gases will accumulate at the interface between the separator and the electrodes, forming small bubbles of different sizes. These bubbles will exert a certain pressure on the separator, further pushing the separator to deform locally, expanding the existing wrinkles or forming new wrinkles. The experimental results show that when the electrolyte filling speed is too fast, the amount of trapped air increases, the number of bubbles at the interface increases significantly, and the degree of separator wrinkling is also more serious, which fully indicates that bubble residue is an important factor exacerbating wrinkling.
(3) Relationship Between Separator Thickness and Wrinkle Spacing
Separator thickness is another key factor affecting the characteristics of separator wrinkling. Through comparative experiments on separators of different thicknesses (5 μm, 10 μm, 15 μm, 20 μm), it was found that there is a clear correlation between separator thickness and wrinkle spacing: as the separator thickness increases, the wrinkle spacing increases, and the number of wrinkles per unit area decreases. This is because thicker separators have higher mechanical rigidity, which can resist the deformation caused by capillary pressure to a certain extent, so the wrinkles formed are wider and fewer. However, it should be noted that with the increase of separator thickness, the conformity between the separator and the electrodes becomes worse, resulting in an increase in interfacial contact resistance, which will affect the ion transmission efficiency and battery performance. This indicates that simply increasing the separator thickness is not a fundamental solution to eliminate wrinkling, and it may even bring negative impacts on battery performance, so it is not feasible in practical industrial applications.
(4) Influence of Coating Materials
The type of separator coating material also has a significant impact on the formation of wrinkles, which is mainly reflected in the differences in rigidity and wettability of the separator after coating. For Al₂O₃-coated separators, due to the high rigidity of Al₂O₃ particles, the overall rigidity of the separator is significantly increased after coating, which can reduce the deformation degree caused by capillary pressure, thus showing a larger wrinkle spacing. For PVDF-coated separators, PVDF has good wettability with electrolytes, which can accelerate the wetting speed of the electrolyte on the separator surface, reduce the uneven propagation of the wetting front, and thus show a smaller wrinkle spacing. However, experimental results show that neither Al₂O₃-coated nor PVDF-coated separators can completely eliminate wrinkles, indicating that coating modification alone is not sufficient to solve the wrinkling problem, and it is necessary to combine other strategies to achieve complete wrinkle elimination.
5. Strategies for Wrinkle Elimination and Experimental Verification
Based on the formation mechanism of separator wrinkling, this study proposed a wrinkle elimination strategy centered on enhancing the interfacial bond strength between the separator and electrodes, and verified its effectiveness through systematic experiments. The core idea of this strategy is to counteract the capillary pressure that causes wrinkling by improving the adhesion between the separator and electrodes, thereby suppressing the local bulging and deformation of the separator.
(1) Thermal Lamination of Composite Separator with Electrodes
To enhance the adhesive force between the separator and electrodes, this study adopted the method of thermal lamination of composite separators with positive and negative electrodes. Specifically, PVDF/PP composite separators (which have good adhesion and wettability) were selected, and thermal lamination was carried out with positive and negative electrodes at 90°C under different pressures (5 MPa, 10 MPa, 15 MPa, 20 MPa). The experimental process controlled the lamination time to 30 minutes to ensure sufficient contact and bonding between the separator and electrodes. The principle of thermal lamination is that under a certain temperature and pressure, the PVDF coating on the separator surface melts slightly, forming a strong bond with the active material of the electrodes after cooling, thereby improving the interfacial bond strength. The experimental results show that thermal lamination can effectively enhance the adhesion between the separator and electrodes, and the choice of lamination pressure has a significant impact on the bonding effect: with the increase of lamination pressure, the adhesion between the separator and electrodes first increases and then tends to be stable.
(2) Relationship Between Peel Strength and Wrinkle Elimination
To quantitatively characterize the relationship between interfacial bond strength and wrinkle elimination effect, the peel strength between the separator and electrodes after thermal lamination was tested, and the wrinkle status of the separator was observed. The experimental results show that there is a clear threshold relationship between peel strength and wrinkle elimination: when the peel strength is less than 10 mN/cm, the number of wrinkles on the separator surface decreases compared with the unlaminated group, but wrinkles still exist, and the degree of wrinkling is relatively mild; when the peel strength is greater than 15 mN/cm, the wrinkles on the separator surface are completely eliminated, and the separator remains flat. In addition, there is a difference in the required peel strength between positive and negative electrodes: for graphite anodes, due to the strong van der Waals force between the graphite surface and the PVDF coating, wrinkles can be completely eliminated even when the peel strength is as low as about 2 mN/cm, which indicates that the interfacial interaction between the anode and the separator is stronger than that between the cathode and the separator. This finding provides a basis for optimizing the thermal lamination process for different electrodes.
(3) Mechanism Summary
Combined with the experimental results, the mechanism of wrinkle elimination can be summarized as follows: the generation of separator wrinkles is essentially the result of capillary forces exceeding the adhesive force between the separator and electrodes. During the electrolyte filling process, capillary pressure generated by electrolyte penetration causes local bulging of the separator; if the adhesive force between the separator and electrodes is insufficient to resist this capillary pressure, the separator will be separated from the electrodes, forming wrinkles. By adopting the thermal lamination method, the interfacial bond strength between the separator and electrodes is significantly increased, and the enhanced adhesive force can effectively counteract the capillary pressure, preventing the local bulging and deformation of the separator, thereby achieving the purpose of eliminating wrinkles. This mechanism clarifies the core logic of wrinkle elimination, providing a theoretical basis for the industrial application of this strategy.
6. Conclusions and Significance
Through systematic experiments and analysis, this study clarified the formation mechanism of separator wrinkling in lithium-ion batteries and proposed an effective elimination strategy, which has important theoretical and practical significance for improving battery performance and production yield.
(1) Clear Cause of Wrinkling
The study clearly points out that separator wrinkling is primarily caused by the combined effects of capillary action during electrolyte wetting and bubble residue at the separator-electrode interface. At the same time, it is related to the type of separator, thickness, and coating material: different types of separators have different wettability and rigidity, leading to differences in wrinkling characteristics; separator thickness affects wrinkle spacing and number; coating materials affect the rigidity and wettability of the separator, thereby influencing the formation of wrinkles. It is worth noting that the study found that stress relaxation caused by the swelling of the amorphous region of the separator is not the main cause of wrinkling, which corrects the previous misunderstanding of the wrinkling mechanism and provides a clear direction for solving the wrinkling problem.
(2) Effective Solution
The study proposed an effective solution to eliminate separator wrinkling: using PVDF-coated composite separators and performing thermal lamination with positive and negative electrodes. By controlling the thermal lamination conditions (90°C, appropriate pressure), the interfacial bond strength between the separator and electrodes is increased, and when the peel strength is greater than 15 mN/cm (or about 2 mN/cm for graphite anodes), wrinkles can be completely eliminated. This solution is targeted, as it directly addresses the core cause of wrinkling (capillary force exceeding adhesive force), and the elimination effect is stable and reliable, which has been verified by a large number of experiments.
(3) High Process Feasibility
One of the prominent advantages of the proposed wrinkle elimination strategy is its high process feasibility. This method does not require significant changes to the existing separator production process, and only needs to optimize the cell assembly process by adding a thermal lamination step. The thermal lamination equipment used is mature in the industrial field, and the process parameters (temperature, pressure, time) are easy to control and adjust, which can be quickly integrated into the existing battery production line. Compared with other strategies that require modifying the separator structure or changing the electrolyte formula, this method has lower cost and higher industrial application value.
(4) Improvement to Battery Performance
Eliminating separator wrinkling has a positive impact on improving battery performance. After wrinkles are eliminated, the uniformity of internal interfacial contact between the separator and electrodes is significantly improved, which effectively reduces the uneven distribution of internal resistance of the battery; at the same time, the elimination of residual bubbles promotes the smooth transmission of lithium ions, reduces the risk of local overcharge/over-discharge, and thus enhances the consistency and cycle stability of the battery. The experimental results show that after adopting this wrinkle elimination strategy, the cycle life of the battery is increased by about 15% compared with the unprocessed group, and the consistency of battery capacity is significantly improved, which has important practical significance for promoting the application of lithium-ion batteries in high-demand fields such as electric vehicles.
7. Common Questions (FAQs)
Q1: Will separator wrinkling affect the safety of lithium-ion batteries?
Yes, separator wrinkling will affect the safety of lithium-ion batteries. Wrinkling leads to uneven contact between the separator and electrodes, resulting in local overcharge or overheating at the wrinkled position. In severe cases, this local overheating may cause the separator to melt, leading to a short circuit between the positive and negative electrodes, and even triggering safety hazards such as thermal runaway. Especially for large-format power batteries, the impact of wrinkling on safety is more prominent, so it is necessary to strictly control the occurrence of separator wrinkling.
Q2: Can the problem of separator wrinkling be solved by adjusting the electrolyte filling speed?
Adjusting the electrolyte filling speed can alleviate separator wrinkling to a certain extent but cannot completely eliminate it. When the filling speed is too fast, a large amount of air is easily trapped at the separator-electrode interface, forming bubbles and exacerbating wrinkling; reducing the filling speed can reduce the amount of trapped air, thereby reducing the degree of wrinkling. However, capillary action, the primary cause of wrinkling, cannot be eliminated by adjusting the filling speed alone. Therefore, adjusting the filling speed can only be used as an auxiliary measure, and it is necessary to combine other strategies such as thermal lamination to completely eliminate wrinkling.
Q3: Is there a significant difference in the wrinkling tendency between dry-process and wet-process separators?
Yes, there is a significant difference in the wrinkling tendency between dry-process and wet-process separators. Dry-process biaxially oriented PP separators have higher mechanical rigidity but relatively poor wettability with electrolytes, so the wrinkles formed are usually wider and fewer; wet-process biaxially oriented PE separators have better wettability and uniform pore size distribution, but lower mechanical rigidity, so the wrinkles formed are narrower and more numerous. In practical applications, the choice of separator type should be combined with the battery design and production process to reduce the risk of wrinkling.
Q4: Will the thermal lamination process affect the performance of the separator or electrodes?
Under appropriate process parameters (90°C, appropriate pressure, 30 minutes), the thermal lamination process will not affect the performance of the separator or electrodes. The PVDF coating on the surface of the composite separator melts slightly at 90°C, and forms a strong bond with the electrode active material after cooling, without damaging the microporous structure of the separator (which ensures the ion permeability of the separator) and without affecting the electrochemical performance of the electrodes. The experimental results show that the thermal lamination process has no negative impact on the mechanical strength, ion conductivity of the separator, or the capacity and cycle performance of the electrodes.
Q5: Can other coating materials (such as SiO₂) be used instead of PVDF to eliminate separator wrinkling?
At present, PVDF is more suitable for eliminating separator wrinkling than other coating materials such as SiO₂. PVDF has good wettability with electrolytes and strong adhesion to electrode active materials, which can effectively enhance the interfacial bond strength between the separator and electrodes after thermal lamination. Although SiO₂-coated separators also have good thermal stability, their wettability and adhesion are worse than PVDF-coated separators, so even after thermal lamination, it is difficult to achieve the peel strength required for complete wrinkle elimination. Therefore, PVDF is the preferred coating material for the wrinkle elimination strategy proposed in this study.
8.Canrd Brief Introduction
Canrd applies advanced battery R&D technologies (core team members from CATL) and leverages the strong supply chain in China to support overseas companies in achieving fast and efficient R&D.
We provide a full range of R&D services including laboratory materials, electrodes, customized dry cells, material evaluation, performance testing, and coin cell / pouch cell / cylindrical cell pilot lines.
We provide a full range of R&D services including laboratory materials, electrodes, customized dry cells, material evaluation, performance testing, and coin cell / pouch cell / cylindrical cell pilot lines.
Previous
Effect of Conductive...Related Products & Services
Use Canrud products and R&D services to turn battery knowledge into practical experiments.
Coin Cell Case
ProductCoin cell cases for validating materials and electrochemical concepts from the article.
Use coin cell cases in your next test →Sodium Electrolyte
CategoryElectrolytes for sodium-ion battery research, compatibility tests, and cell validation.
Shop sodium electrolyte for validation →Cell Fabrication
ServiceGet support building dry cells, assembled cells, or custom formats for R&D validation.
Request cell fabrication support →Experimental Materials
CategoryBrowse materials for follow-up experiments, benchmarking, and product development.
Browse experimental battery materials →