Performance Requirements for Diaphragms

The separator is one of the key inner layer components in lithium-ion batteries. After absorbing the electrolyte, it can isolate the positive and negative electrodes to prevent short circuits and also conduct Li+. The separator blocks current conduction through closed pores and prevents explosions during overcharge or temperature rise. The advantages of separator performance determine the interfacial structure and internal resistance of the battery, which in turn affect key characteristics such as battery capacity, cycle performance, and charge-discharge current density. The separator with excellent performance plays an important role in improving the comprehensive performance of the battery.

1. Uniform thickness and small size

2. Mechanical properties

The mechanical properties of the diaphragm are an important factor affecting its application. If the diaphragm is ruptured, a short circuit will occur, reducing the yield. The mechanical strength of the diaphragm can be measured by puncture resistance and tensile strength.

3. Permeation performance

The permeation performance can be characterized by the amount of gas in the diaphragm under a certain time and pressure, which mainly reflects the patency of lithium ions permeating through the diaphragm. The permeability of the membrane is the result of the combined effects of the internal substructure of the membrane, such as the membrane porosity, pore size, pore shape and pore tortuosity. The pore distribution should be uniform. If the pore size is too small, the resistance will increase, and if the pore size is too large, the positive and negative electrodes will be easily contacted or short-circuited by dendrites.

4. Wettability and wetting Speed

Better wettability is conducive to improving the affinity of the separator and the electrolyte, expanding the contact surface between the separator and the electrolyte, thereby increasing the ionic conductivity and improving the charge-discharge performance and capacity of the battery. The wettability of the separator to the electrolyte can be measured by measuring its liquid absorption rate and liquid holding rate.

5. Chemical stability

The separator should maintain long-term stability in the electrolyte and not react with the electrolyte and electrode materials. Its chemical stability is evaluated by measuring the resistance to electrolyte corrosion and the expansion and shrinkage ratio.

6. Thermal stability

The battery will release heat during the charging and discharging process, especially when it is short-circuited or overcharged, a large amount of heat will be released. Therefore, when the temperature increases, the diaphragm should maintain the original integrity and certain mechanical properties, and play the role of isolating the positive and negative electrodes and preventing short circuits.

7. Safety protection performance

The thermal shutdown feature can provide an additional safety protection for Li-ion batteries. The main parameters of this function are closed cell temperature and film breaking temperature. Closing temperature is the temperature at which the micropores are closed, which is the closing temperature. Refers to the self-heating of the exothermic reaction inside the battery, overcharge or external short circuit of the battery, which will generate a lot of heat. Due to the thermoplasticity of the material, when the temperature is close to the melting point of the polymer, the micropores close to form a thermal shutdown, thereby blocking the continued transmission of ions to form an open circuit, which plays a role in protecting the battery. The film breaking temperature refers to the self-heating inside the battery, and the external short circuit increases the internal temperature of the battery. After the closing temperature is exceeded, the micropores block and block the current flow, and the temperature of the hot melt performance further rises, resulting in rupture of the diaphragm and short circuit of the battery. The temperature at which it breaks is the film breaking temperature. It is desirable for the separator to have a lower closed cell temperature and a higher rupture temperature.

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