As a key material for lithium-ion batteries, the battery separator serves as an electronic isolation function, preventing direct contact between the positive and negative electrodes and allowing lithium ions in the electrolyte to pass freely. At the same time, the separator is also very important to ensure the safe operation of the battery. use. Under special circumstances, such as accidents, punctures, battery abuse, etc., partial breakage of the diaphragm will cause direct contact between the positive and negative electrodes, which will trigger a violent battery reaction and cause the battery to catch fire and explode.

01 Layer-by-layer testing experienced by lithium battery separators

1. Thickness

With the increase of battery energy density, the battery diaphragm is getting thinner and thinner, and the accuracy of measurement is also higher and higher. Due to the soft material of the diaphragm, the measurement is difficult to be accurate, and there is no standard method that can be referred to.

Test method: Choose different test methods according to the type of diaphragm, and measure several points to ensure the consistency of diaphragm thickness.

2. Curvature

Refers to the arc shape that appears after the diaphragm is slit. When the arc shape is obvious, it will cause the laminations to be uneven, and the vortex shape will appear during winding, which causes the pole pieces to be exposed and short-circuit.

Test method: Spread the diaphragm flat on the table and compare the parallelism with the edge of the steel ruler.

3. Air permeability

The time required for a certain volume of air to pass through the diaphragm under certain conditions is also called the Gurley value.

Test method: generally use ASTM test method.

4. Porosity

The proportion of the void volume to the entire volume;

Test method: liquid absorption calculation method or mercury intrusion test method;

5. Pore size distribution

Test method: Use a capillary flow analyzer to measure, use inert gas to break through the wetted diaphragm, measure the pressure value of the gas flowing out, and obtain the aperture parameter through calculation.

6. Infiltration

Test method: contact angle measurement method.

7. Surface condition

Test method: SEM.

8. Mechanical properties

1) Tensile strength and elongation: reflect the mechanical properties of the diaphragm in the transverse direction (TD) and the longitudinal direction (MD). The force that stretches to the diaphragm until it breaks is generally measured with a tensile tester. It can be seen that there are Significant difference

2) Puncture strength: Evaluate the force of external sharp objects penetrating the diaphragm, which is strongly related to the safety performance of the battery and can be measured with special testing equipment.

9. Thermal performance

1) Thermal shrinkage rate: The change rate of the diaphragm size before and after heating is also divided into transverse (TD) and longitudinal (MD) shrinkage rates. Now the test temperature and time of each manufacturer are different, with 85°C 2h, 90°C 24h, 105℃2h, 130℃0.5h, etc., different tests can be carried out according to different requirements; with the application of ceramic diaphragm, the thermal shrinkage rate of the diaphragm is getting lower and lower;

2) DSC test: It is important to test the temperature of closed pores and membrane rupture of the diaphragm, and measure it with a differential scanning calorimeter.

10. Electrical performance

Assemble the separator, positive and negative electrodes, and electrolyte to compare the performance of different separators by testing the rate, high and low temperature, storage, circulation, internal resistance, and safety.

02The advantages and disadvantages of polyethylene (PE) and polypropylene (PP) films

my country's lithium-ion battery separator industry is in a stage of rapid development, and wet-process separators have gradually become the mainstream technical route, but at the same time, the overall technical level of domestic separators and the technical level of international first-line companies still have a big gap.

Currently, the commercialized lithium ion battery separators on the market are mainly microporous polyolefin separators based on polyethylene (PE) and polypropylene (PP).

Advantages: PE and PP films are widely used at present due to their low cost, good mechanical properties, excellent chemical stability and electrochemical stability.

Disadvantages:

Its lyophobic surface and low surface energy result in poor wettability of this type of separator to the electrolyte, which affects the cycle life of the battery.

The heat distortion temperature is relatively low (the heat distortion temperature of PE is 80～85℃, and that of PP is 100℃). When the temperature is too high, the diaphragm will undergo severe heat shrinkage. Therefore, this type of diaphragm is not suitable for use in high temperature environments and cannot meet the current 3C. Requirements for the use of products and power lithium batteries.

03New lithium battery diaphragm material

In response to the development needs of lithium-ion battery technology, researchers have developed a variety of new lithium battery diaphragm materials. The non-woven membrane is used to oriented or randomly arrange the fibers through a non-woven method to form a web structure, which is then reinforced into a film by chemical or physical methods, so that it has good air permeability and liquid absorption.

Natural materials and synthetic materials have been widely used in the preparation of non-woven membranes. Natural materials mainly include cellulose and its derivatives. Synthetic materials include polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), and polyvinylidene fluoride (PVDF). Vinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyamide (PA), polyimide (PI), aramid (meta-aramid, PMIA; para-aramid PPTA), etc.

1. Polyethylene terephthalate

Polyethylene terephthalate (PET) is a material with excellent mechanical properties, thermodynamic properties, and electrical insulation properties. The most representative product of PET separators is a composite membrane coated with ceramic particles and developed by Degussa in Germany. It exhibits excellent heat resistance and the closed cell temperature is as high as 220°C.

Xiangtan University uses electrostatic spinning to prepare PET membrane. Its melting point is much higher than that of PE membrane, at 255℃, maximum tensile strength is 12Mpa, porosity reaches 89%, and liquid absorption rate reaches 500%, which is much higher than the Celgard membrane on the market. The ionic conductivity reaches 2.27×10-3Scm-1, and the cycle performance is better than Celgard diaphragm. The porous fiber structure of PET diaphragm remains stable after 50 cycles of battery cycle.

2. Polyimide

Polyimide (PI) is also one of the polymers with good comprehensive properties. It has excellent thermal stability, high porosity, and good high temperature resistance. It can be used for a long time at -200~300℃. Miao et al. (2013) used the electrospinning method to produce PI nanofiber membranes at a degradation temperature of 500°C, and will not undergo aging and thermal shrinkage at a high temperature of 150°C. Due to the strong polarity of PI and good wettability to the electrolyte, the manufactured diaphragm exhibits the best liquid absorption rate. Compared with the Celgard diaphragm, the PI diaphragm made by electrospinning has lower impedance and higher rate performance, and the capacity retention rate is still 100% after 100 cycles of charging and discharging at 0.2C.

3. Poly(p-phenylene benzodiazole)

The new polymer material PBO (poly-p-phenylene benzodiazole) is an organic fiber with excellent mechanical properties, thermal stability, and flame retardancy. Its matrix is a linear chain structure polymer, which does not decompose below 650°C, has ultra-high strength and modulus, and is an ideal heat-resistant and impact-resistant fiber material.

Hao Xiaoming et al. prepared PBO nanoporous membranes by phase inversion method. The ultimate strength of the diaphragm can reach 525Mpa, the Young's modulus is 20GPa, the thermal stability can reach 600℃, the contact angle of the diaphragm is 20, which is less than the 45 contact angle of the Celgard2400 diaphragm, and the ion conductivity is 2.3×10-4Scm- 1. It performs better than commercial Celgard2400 diaphragm under cycling conditions.

However, due to the difficult manufacturing process of PBO fibrils, there are only a handful of companies worldwide that produce excellent PBO fibers, and they all use monomer polymerization. The PBO fibers produced are difficult to be used in lithium ion battery separators due to strong acid treatment. field.

4. Composite film

The sandwich structure of the non-woven composite membrane prepared by the hot pressing method significantly improves the shedding of the ceramic layer and the self-discharge of the battery. (Some pictures are quoted from the president of the Lithium Battery Alliance)

The PI nanofiber membrane prepared by the electrospinning method not only improves the mechanical strength of the diaphragm itself, but also significantly improves the absorption of electrolyte and ion conductivity.