The performance of the separator is deeply related to the safety of lithium batteries, such as the tensile strength, thermal shrinkage rate, puncture strength, etc. of the separator, which are directly related to the performance of the battery cell. Ceramic coatings, as the most widely used coatings on the market, have the advantages of good stability and low thermal shrinkage. This article will briefly explain the influence of the thickness of the diaphragm ceramic layer on the safety performance of the battery cell.

As the thickness of the diaphragm becomes thinner, ceramic separator coating has become a necessary additional process. Single sided or double-sided coating, symmetrical or asymmetrical coating, have an impact on the tensile strength, thermal shrinkage rate, and puncture strength of the diaphragm.

I. Definition

1. Tensile strength: The maximum stress (in MPa) that a diaphragm can withstand during the stretching process, reflecting the material's ability to resist tearing.

2. Thermal shrinkage rate: The size shrinkage ratio (%) of the diaphragm after being maintained at high temperature for a certain period of time, reflecting the thermal stability of the material.

3. Puncture strength: The minimum force (unit: N) required for the diaphragm to be pierced by a steel needle (such as φ 1mm), reflecting the material's ability to resist penetration by sharp objects.

II. The influence of different diaphragm parameters on the safety of battery cells:

1. Tensile strength: Tensile strength determines the mechanical stability of the diaphragm, and the direct reaction is the tension of the diaphragm. If the tensile strength is not sufficient, it may cause breakage or microcracks due to insufficient or fluctuating tension in the battery cell winding process, which can seriously lead to misalignment of the pole pieces and directly cause short circuits; When square battery cells are packaged in a housing, the diaphragm needs to withstand the compression stress of the electrode group. Low strength separators may rupture due to compression deformation, especially in large-sized battery cells (such as energy storage cells) where the risk is higher.

2. Thermal shrinkage rate: The thermal shrinkage rate reflects the thermal stability of the diaphragm. It is particularly evident at high temperatures. In tests such as overcharging and thermal shock, we will find that membranes with low thermal shrinkage are more likely to pass the test. This is because the contraction of the diaphragm due to heat may cause direct contact between the positive and negative electrode plates, generating a large amount of heat. Causing the risk of thermal runaway.

3. Puncture strength: The ability of the reaction material to resist penetration by sharp objects. The most important ones are burrs, metal dust, etc. If the puncture strength is too low, it will cause foreign objects such as burrs and dust to pierce the diaphragm during the production process, forming a short circuit and affecting the safety performance of the battery cell. And the electrode material may crack due to volume changes during cycling, and the crack tip may pierce the diaphragm. In addition, there are higher requirements for the puncture strength of the diaphragm in high-energy density and high-pressure solid systems.

III The advantages of increasing the thickness of the ceramic layer on the performance of the battery cell:

①Increasing the thickness of the ceramic layer undoubtedly brings great improvements to the safety performance of the battery cell. Increasing the thickness of the ceramic layer will improve the overall tensile strength and wear resistance of the diaphragm, reducing mechanical damage during winding or laminating processes.

② Increasing the thickness of the ceramic layer can significantly enhance the high temperature resistance of the diaphragm and greatly improve the thermal safety performance of the battery cell. Ceramic particles delay heat transfer through physical barriers. The ceramic layer can maintain structural stability at high temperatures, preventing the diaphragm from short circuiting the positive and negative electrodes due to thermal shrinkage.

③ Increasing the thickness of the ceramic layer can enhance the puncture strength of the diaphragm. A thick ceramic coating layer can provide stronger mechanical support and inhibit lithium dendrites from piercing the membrane.

④ Increasing the thickness of the ceramic layer can increase the liquid absorption capacity of the battery, which is beneficial for storing electrolyte and has a positive effect on cell cycling.

Disadvantage:

① Increasing the thickness of the ceramic layer will lead to an increase in cost, and the thicker the ceramic layer, the higher the cost. Double sided ceramics are also more expensive than single-sided ceramics.

② To a certain extent, it affects the energy density of the battery cell. The ceramic coating on the diaphragm does not play a role in storage capacity, but it increases the weight of the battery cell.

③ Thickening the ceramic layer will also affect the migration of lithium ions to a certain extent, and a thick coating means that the migration path of lithium batteries is longer.