Date: 2025-10-17 hits: 124
The separator base film serves as the core channel for ion transport and a physical barrier in lithium batteries. Its thickness is not simply a dimensional parameter; it fundamentally influences the battery's overall performance by regulating ion migration efficiency, thermal response characteristics, interfacial stability, and the proportion of inactive materials.
A. Impact of Base Film Thickness on Internal Resistance and Rate Performance
The core function of the base film is to create porous channels for lithium ion migration. Its thickness, by varying the "migration path length" and "pore network resistance," directly determines ion transport efficiency, thereby affecting the battery's internal resistance and rate performance.
From a microscopic perspective, lithium ions in the base film must travel through the electrolyte within the pores, and the migration distance is linearly correlated with the base film thickness. Increasing base film thickness lengthens the lithium ion migration path from the positive electrode to the negative electrode, reducing the number of lithium ions reaching the negative electrode per unit time, directly leading to a decrease in ionic conductivity.
B. Safety Performance Impact
The base film is the battery's "first line of defense" against thermal runaway. Its thickness regulates the thermal closure response speed and physical strength, balancing the battery's thermal safety and mechanical impact resistance.
(1) Thermal Capacity
The thermal pore closure function of the base film relies on the melting properties of polymer materials (such as PE and PP): When the battery temperature rises to the base film's melting point (approximately 135°C for PE and 160°C for PP), the thermal motion of the polymer chains intensifies, causing the pore walls to melt and close, blocking ion transport and terminating the reaction. Thicker base films, due to their greater heat capacity, experience a slower temperature rise when absorbing the same amount of heat, leading to a delay in triggering thermal pore closure. Thicker base films require longer to accumulate sufficient heat to reach the melting temperature.
Thinner base films, however, have faster thermal conductivity and lower heat capacity, allowing them to quickly respond to temperature changes and close pores. This delay can prevent the battery from shutting off in the early stages of thermal runaway, allowing the electrolyte to continue decomposing and producing gas, increasing the risk of fire and explosion.
(2) Physical Capacity
As thickness increases, the base film's puncture resistance is positively correlated with its tensile strength. Thicker base films are better able to resist punctures from electrode burrs and active material particles. The thick base film can disperse the impact force and avoid pore rupture; the volume expansion during the battery cycle will produce extrusion stress on the base film. The thick base film has a larger tensile margin and can absorb stress through slight deformation, avoiding the separation of the base film and the electrode or pore collapse, reducing the risk of micro-short circuit.
C. Influence of Wetting Effects
The electrolyte must completely penetrate the base membrane pores and fill the electrode-diaphragm interface to establish a continuous ion transport channel. The base membrane thickness directly determines the electrolyte's "permeation resistance" and "permeation time": the pore network of a thick base membrane presents a "deep cavity structure," requiring the electrolyte to overcome capillary resistance along a longer path to fully penetrate. However, a thicker base membrane retains more electrolyte, which positively impacts cycling performance.
D. Cycling Performance
During long-term cycling, the repeated expansion and contraction of the positive and negative electrodes continuously exerts dynamic stress on the base membrane. A thicker base membrane, with its greater thickness margin, can absorb some of this stress through "elastic deformation," preventing delamination between the base membrane and the electrode interface. However, a thin base membrane has limited deformation margin and is easily squeezed and deformed by expansion stress, leading to pore structure destruction (such as pore size reduction and pore closure) or even base membrane rupture, resulting in direct contact between the positive and negative electrodes and a micro-short circuit. Furthermore, a thicker base membrane retains more electrolyte, which is beneficial for cycling performance.
E. Energy Density
The base film is a battery's "inactive material" (it does not participate in electrochemical reactions). Its thickness directly affects the mass and volume fraction of the inactive material in the battery. From the perspective of mass energy density, decreasing base film thickness reduces the mass fraction of the inactive material. Similarly, thicker base films reduce battery energy density.
F. Summary
The impact of base film thickness on battery performance is essentially a comprehensive regulation of multiple physical and chemical processes, including lithium ion migration, thermal response, interfacial interactions, and material fraction. Thin base films contribute to high energy density and high-rate performance by shortening ion pathways and reducing the inactive fraction. Thick base films ensure high safety and long cycle life by enhancing physical protection and improving expansion buffering capacity.
