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May 27 2025
How do the porosity and permeability of the separator affect the performance of lithium-ion batteries? In-depth analysis of the key parameters of battery cell design
As the core technology of modern energy storage, the performance of lithium-ion batteries is highly dependent on material selection and process design. As one of the four main materials of lithium-ion batteries, the porosity and permeability of the separator directly affect the internal resistance, rate performance and safety of the battery cell. This article will deeply explore the correlation mechanism of these two parameters and their practical impact on lithium-ion battery cells.
1. Definition and correlation of membrane porosity and air permeability
(1) Permeability (Gurley value)
Permeability measures the rate at which air passes through the diaphragm under a certain pressure. The lower the value, the denser the pore structure of the diaphragm. Its core influencing factors include porosity, pore size distribution and pore tortuosity. For example, although a high-porosity diaphragm improves the efficiency of lithium ion transmission, it may increase air flow resistance due to the interlacing of pores, resulting in a decrease in permeability.
(2) Porosity
Porosity refers to the volume ratio of pores in the diaphragm, which directly affects the electrolyte infiltration and ion migration path of lithium-ion batteries. Experiments show that when the porosity increases from 35% to 55%, the lithium ion migration resistance decreases by 18%, but excessive pores may cause excessive absorption of electrolyte and increase the risk of battery cell expansion.
(3)Relationship between the two
Porosity and permeability are negatively correlated. Although a high-porosity diaphragm provides more lithium ion channels, it is necessary to balance the pore connectivity to avoid deterioration of permeability.
Permeability measures the rate at which air passes through the diaphragm under a certain pressure. The lower the value, the denser the pore structure of the diaphragm. Its core influencing factors include porosity, pore size distribution and pore tortuosity. For example, although a high-porosity diaphragm improves the efficiency of lithium ion transmission, it may increase air flow resistance due to the interlacing of pores, resulting in a decrease in permeability.
(2) Porosity
Porosity refers to the volume ratio of pores in the diaphragm, which directly affects the electrolyte infiltration and ion migration path of lithium-ion batteries. Experiments show that when the porosity increases from 35% to 55%, the lithium ion migration resistance decreases by 18%, but excessive pores may cause excessive absorption of electrolyte and increase the risk of battery cell expansion.
(3)Relationship between the two
Porosity and permeability are negatively correlated. Although a high-porosity diaphragm provides more lithium ion channels, it is necessary to balance the pore connectivity to avoid deterioration of permeability.
2. Effect of porosity on the performance of lithium-ion battery cells
(1) Internal resistance and charge-discharge efficiency
Advantages of high porosity: Increase lithium ion transmission channels, reduce internal resistance, and improve rate discharge capability (such as fast charging scenarios).
Risk points: Porosity that is too high (>60%) can easily lead to electrolyte retention, aggravate high-temperature expansion, and even cause lithium dendrites to pierce the diaphragm.
(2) Rate performance and cycle life
High-voltage systems such as lithium iron phosphate batteries are sensitive to the porosity of the diaphragm. Porosity optimization can shorten the lithium ion migration path by 20%, reduce polarization effects, and thus extend the cycle life.
(3) Self-discharge and thermal stability
Self-discharge at room temperature: For every 5% increase in porosity, the self-discharge rate at room temperature increases by 3%-5%, mainly due to the increase in ion "leakage current".
High-temperature safety: Diaphragms with a porosity of 35%-55% can absorb more electrolyte and suppress local thermal runaway at high temperatures, but closed-cell structure failure must be avoided.
Advantages of high porosity: Increase lithium ion transmission channels, reduce internal resistance, and improve rate discharge capability (such as fast charging scenarios).
Risk points: Porosity that is too high (>60%) can easily lead to electrolyte retention, aggravate high-temperature expansion, and even cause lithium dendrites to pierce the diaphragm.
(2) Rate performance and cycle life
High-voltage systems such as lithium iron phosphate batteries are sensitive to the porosity of the diaphragm. Porosity optimization can shorten the lithium ion migration path by 20%, reduce polarization effects, and thus extend the cycle life.
(3) Self-discharge and thermal stability
Self-discharge at room temperature: For every 5% increase in porosity, the self-discharge rate at room temperature increases by 3%-5%, mainly due to the increase in ion "leakage current".
High-temperature safety: Diaphragms with a porosity of 35%-55% can absorb more electrolyte and suppress local thermal runaway at high temperatures, but closed-cell structure failure must be avoided.
3. How to balance porosity and air permeability?
(1) Process adaptability
Dry/wet diaphragms need to match different porosity requirements. For example, wet diaphragms achieve higher porosity uniformity through biaxial stretching, which is suitable for high energy density lithium-ion batteries.
(2) Testing standards
Gurley value test (such as 100mL air/1.2kPa pressure) needs to be combined with liquid absorption porosity measurement, and dual indicators are linked for optimization.
Dry/wet diaphragms need to match different porosity requirements. For example, wet diaphragms achieve higher porosity uniformity through biaxial stretching, which is suitable for high energy density lithium-ion batteries.
(2) Testing standards
Gurley value test (such as 100mL air/1.2kPa pressure) needs to be combined with liquid absorption porosity measurement, and dual indicators are linked for optimization.
