In the core components of lithium-ion batteries, the lithium battery separator may seem like a “simple barrier” that isolates the positive and negative electrodes, but it is actually a key factor determining battery capacity, cycle performance, charge-discharge efficiency, and storage stability. Even subtle changes in its porosity and thickness will transmit to the overall electrical performance of the battery like a “butterfly effect.”
Currently, the mainstream lithium battery separators in the industry are mainly made of PP, PE, and PP/PE/PP composite materials, with preparation processes mainly divided into two categories: wet biaxial stretching and dry uniaxial stretching. To clarify the specific influence rules of separator porosity and thickness on battery performance, and to compare the performance differences between dry and wet separators, researchers conducted systematic experiments by preparing PE/PP separators with different parameters and matching them with LiFePO₄/graphite soft-pack batteries. Comprehensive performance tests and analyses were carried out, covering separator physical properties, battery internal resistance, self-discharge characteristics, and low-temperature discharge capacity, providing accurate experimental basis for separator selection and parameter optimization in scientific research and industrial production.
Preparation of Dry and Wet Separators: Process Differences Shape Structural Distinctions
The preparation process of the lithium battery separator directly determines its microstructure, which in turn determines its performance. As the two mainstream preparation processes, wet and dry methods have distinct differences in operational procedures and finished product structures. When preparing PE separators using the wet process, polyethylene powder and paraffin oil are mixed in proportion, heated, and extruded through a die. After cooling-induced phase separation to form micropores, the film is pressed and heated to a temperature close to its melting point. Through biaxial stretching and paraffin oil extraction, a separator with a uniform pore structure is obtained. For dry PP separator preparation, PP powder is melt-extruded and cast to form a highly oriented film, micro-defects are induced by low-temperature stretching, and then micropores are expanded by high-temperature stretching to finally obtain the dry separator.
These process differences directly lead to variations in micro-morphology and pore size: wet biaxially stretched separators have rounder pore shapes with a pore size of approximately 40nm; dry uniaxially stretched separators, which are only fully stretched transversely, have elongated pore shapes with a smaller pore size of about 25nm. The pore size, in turn, becomes a core factor affecting the lithium battery separator’s air permeability and ion permeability. For more in-depth research on separator preparation processes, you can refer to the research published by the Journal of Power Sources, a leading authority in energy storage technology.
Porosity and Thickness: The “Dual Core Parameters” of Separator Physical Properties
The key physical properties of the lithium battery separator, such as air permeability and wettability, show a clear linear correlation with porosity and thickness—a rule that has been accurately verified in tests on wet separators. Among wet PE separators with the same thickness, porosity is significantly negatively correlated with air permeability: when porosity increases from 37% to 52%, the air permeability value decreases from 195s/100mL to 89s/100mL, with a linear fitting degree as high as 0.9998. For wet PE separators with similar porosity and tortuosity, thickness is positively correlated with air permeability: when thickness increases from 12μm to 25μm, the air permeability value rises from 155s/100mL to 286s/100mL, with a linear fitting degree of 0.9545.
Meanwhile, there are obvious differences in wettability between dry and wet lithium battery separators: the contact angle of the separator has no significant correlation with porosity, but the contact angle of wet separators is significantly smaller than that of dry separators, indicating that wet separators have more sufficient contact with electrolyte and better wettability. Under the conditions of similar porosity and tortuosity, dry separators have much higher air permeability values than wet separators due to their smaller pore size, which is a major feature of their physical properties. For detailed research on separator wettability, the Journal of the Electrochemical Society provides valuable insights.
Internal Resistance Changes: “Reverse Regulation” by Porosity and Thickness
The alternating current resistance (ACR) and direct current resistance (DCR) of a battery directly affect its charge-discharge efficiency and electrical performance, and the porosity and thickness of the lithium battery separator exert a “reverse regulation” effect on battery internal resistance—a rule clearly confirmed in experiments. For wet separators with the same thickness, the higher the porosity, the smaller the battery’s ACR and DCR. This is because a higher porosity provides a smoother channel for lithium ion migration, reducing the resistance of ion transmission. For wet separators with similar porosity and tortuosity, the greater the thickness, the larger the battery’s ACR and DCR, as the increased thickness prolongs the lithium ion transmission path and thereby increases transmission resistance.
A comparison of the internal resistance of dry and wet lithium battery separators also yields clear results: under conditions of similar thickness and porosity, batteries assembled with dry separators have lower ACR and DCR than those with wet separators, which is closely related to the higher air permeability of dry separators. Our previous article onlithium battery internal resistance optimization further elaborates on the interaction between separator parameters and battery resistance.
Self-Discharge Characteristics: “Trade-Off” Between Porosity and Thickness
The room-temperature and high-temperature self-discharge of a battery is a core indicator of its storage stability. The impact of the lithium battery separator’s porosity and thickness on this indicator presents a “trade-off” rule, which is consistent in both room-temperature and high-temperature environments. Among wet separators with the same thickness and similar pore structure, the higher the porosity, the more significant the battery’s room-temperature and high-temperature physical self-discharge. Experimental data show that when porosity increases from 37% to 52%, the voltage drop of the battery at 25℃ for 30 days increases from 8.33mV to 9.40mV, and the physical self-discharge rate at 55℃ for 7 days rises from 0.88% to 1.11%, indicating a clear linear positive correlation between porosity and self-discharge.
For wet separators with similar porosity and tortuosity, the greater the thickness, the milder the battery’s self-discharge. When thickness increases from 12μm to 25μm, the voltage drop of the battery at 25℃ for 30 days decreases from 8.67mV to 7.80mV, and the physical self-discharge rate at 55℃ for 7 days drops from 0.99% to 0.90%, showing a linear negative correlation between thickness and self-discharge.
A comparison of the self-discharge of dry and wet lithium battery separators shows that under the same thickness, batteries assembled with dry separators have slightly higher room-temperature voltage drop and high-temperature physical self-discharge rate than those with wet separators. This phenomenon is related to the large number of straight pores caused by the pore-forming method of dry separators, which accelerate the battery’s self-discharge process to a certain extent.
Low-Temperature Discharge: “Reverse Impact” of Porosity and Thickness
In a low-temperature environment of -20℃, the reaction activity of the active materials and electrolyte of lithium-ion batteries is greatly reduced, and the discharge capacity is significantly affected. The impact of the lithium battery separator’s porosity and thickness on the battery’s low-temperature discharge capacity is consistent with the internal resistance rule, showing a “reverse impact” characteristic. For wet separators with the same thickness, the higher the porosity, the lower the battery’s capacity retention rate at -20℃; as porosity increases from 37% to 52%, the battery’s low-temperature capacity retention rate decreases significantly. For wet separators with similar porosity and tortuosity, the greater the thickness, the higher the battery’s low-temperature capacity retention rate.
The core reason for this rule is that batteries assembled with lithium battery separators of small porosity and large thickness have higher internal resistance, generating more heat during the discharge process, which to a certain extent improves the reaction activity of active materials and electrolyte, thereby increasing the low-temperature discharge capacity. The comparison of low-temperature performance between dry and wet separators shows that under the same thickness, batteries assembled with wet separators have a slightly higher low-temperature capacity retention rate than those with dry separators, giving wet separators a certain advantage in low-temperature application scenarios. For more information on low-temperature battery performance, refer to the guidelines provided by the Institute of Electrical and Electronics Engineers (IEEE).
Core Conclusions: Optimization Logic and Selection Reference for Separator Parameters
This comprehensive experimental study clarifies the core relationship between the porosity, thickness of the lithium battery separator and battery performance, and also identifies the performance differences between dry and wet separators. It provides three core references for the optimization of separator parameters and process selection in scientific research and industrial production:
For wet separators, porosity is negatively correlated with air permeability and battery internal resistance, and positively correlated with self-discharge; thickness is positively correlated with air permeability and battery internal resistance, and negatively correlated with self-discharge. The optimization of porosity and thickness needs to achieve a “trade-off balance” according to the battery’s application scenario.
The core advantage of dry lithium battery separators is lower battery internal resistance, while wet separators have more advantages in wettability, self-discharge control, and low-temperature discharge performance. The selection of the two types of separators needs to match the battery’s usage requirements.
The preparation process of the separator determines its microstructure, the structure determines its physical properties, and the physical properties ultimately transmit to the battery performance. This “process-structure-performance” relationship is the core logic for the research and production of lithium battery separators.
As lithium batteries develop towards higher energy density, higher cycle stability, and wider temperature adaptability, the parameter optimization and process innovation of the lithium battery separator will become important research directions. The rules revealed in this study provide key experimental support for the precise design and customized development of subsequent separators, helping to further improve the performance of lithium-ion batteries.