Comprehensive guide to lithium battery diaphragms

With the wide application of lithium batteries in many fields, from electric vehicles to portable electronic devices to large-scale energy storage systems, the requirements for lithium battery performance and safety are increasingly stringent. Battery separator as a key component of lithium battery, its performance is directly related to the overall performance of the battery.

1、the working principle of lithium battery and the key position of battery diaphragm

(1) The indispensability of the battery diaphragm in the lithium battery structure

The lithium battery consists of a positive electrode, a negative electrode, an electrolyte and a battery separator. The positive electrode material is usually a transition metal oxide containing lithium, which has a high potential and can provide lithium ions; The negative electrode materials are mostly carbon materials such as graphite, which can accommodate the insertion and removal of lithium ions. The electrolyte acts as a medium for the transport of lithium ions. The battery separator is located between the positive and negative electrodes, like a precision "firewall", separating the positive and negative electrodes with different potentials, avoiding the direct conduction of electrons to form a short circuit, ensuring that the internal electrochemical reaction of the battery can only be carried out through the migration of lithium ions in the electrolyte, thus ensuring the safety and effectiveness of the battery charging and discharging process.

2、the main role of the battery diaphragm

(1) Isolate the positive and negative electrodes to prevent short circuit

Physical isolation function

The battery separator has good insulation and mechanical strength, which can effectively block the direct contact of positive and negative electrodes at the microscopic level. The diaphragm maintains its integrity even when the battery is subjected to external shock, vibration, or in a complex operating environment, preventing short circuits between the positive and negative electrodes due to contact.

Microporous structure and ion conduction

The cell diaphragm is not completely closed, but has a large number of uniformly distributed microporous structures. The size and distribution of these microholes are carefully designed to ensure the smooth passage of lithium ions to achieve the charge and discharge function of the battery, but also to prevent the passage of larger particles or impurities to avoid short circuit of the positive and negative electrodes.

(2) Ensure ion transport and improve battery performancea

Promote lithium ion migration

In the process of charging and discharging lithium batteries, lithium ions need to shuttle quickly between positive and negative electrodes. The microporous structure of the battery diaphragm provides a low-resistance migration channel for lithium ions, so that lithium ions can efficiently diffuse in the electrolyte and reach the surface of the positive and negative electrodes for embedding and removal reactions. The high ion conductivity characteristics of the diaphragm can reduce the energy loss during the lithium ion migration process, thereby improving the charge and discharge efficiency of the battery.

The internal resistance of the battery is affected

The internal resistance of the battery is one of the important indicators to measure the performance of the battery, which directly affects the charging and discharging efficiency and heating condition of the battery. The resistance characteristics of the battery diaphragm have a significant effect on the internal resistance of the battery. If the resistance of the diaphragm is too high, it will increase the resistance of lithium ion migration, resulting in an increase in the internal resistance of the battery, generate more heat during the charge and discharge process, and reduce the energy conversion efficiency and cycle life of the battery.

(3) Strengthening the safety performance of the battery

Thermal stability and high temperature protection

In some special cases, such as battery overcharge, overdischarge or internal short circuit, the battery will generate a lot of heat, resulting in a sharp rise in temperature. The thermal stability of the battery diaphragm plays a key role at this point. The high-quality battery diaphragm can maintain its structural integrity and microporous characteristics in a high temperature environment, and will not shrink, melt or break due to heat, so as to continue to maintain the isolation of the positive and negative electrodes, prevent further deterioration of short circuit and avoid thermal runaway of the battery.

Prevent dendrites from penetrating

During the long-term use of lithium batteries, especially under adverse conditions such as low temperature charging or overcharging, lithium dendrites may grow on the surface of the negative electrode. Lithium dendrites are dendritic deposits of metallic lithium that, if left unchecked, can penetrate the battery diaphragm and cause a short circuit in the positive and negative electrodes, triggering battery failure. The appropriate thickness and mechanical strength of the battery diaphragm can effectively resist the penetration of lithium dendrites and protect the safety of the battery.

3、battery diaphragm material and performance requirements

(1) Common battery diaphragm materials

Polyolefin materials

Polyolefin materials, such as polyethylene (PE) and polypropylene (PP), are currently the most widely used battery separator materials. They have good chemical stability, mechanical properties and processing properties, and the cost is relatively low. Polyethylene diaphragm has high flexibility and ionic conductivity, while polypropylene diaphragm has better heat resistance and mechanical strength.

(2) The performance requirements of the battery diaphragm

Porosity and pore size distribution

Porosity refers to the proportion of the volume of micropores in the membrane to the total volume, which directly affects the migration rate of lithium ions. In general, higher porosity is conducive to the passage of lithium ions, but too high porosity may reduce the mechanical strength of the diaphragm.

Mechanical property

Battery diaphragms need to have good mechanical properties, including tensile strength, puncture strength and tear strength. Tensile strength can ensure that the diaphragm will not break due to tension during battery assembly and use; The puncture strength can prevent the diaphragm from being pierced by sharp particles in the positive and negative electrode materials; The tearing strength ensures that the diaphragm will not tear easily when subjected to local external forces.

Thermal performance

The thermal properties mainly include thermal shrinkage rate and thermal decomposition temperature. Thermal shrinkage refers to the degree to which the size of the diaphragm shrinks when heated, and a lower thermal shrinkage can ensure that the diaphragm maintains its shape and isolation performance in a high temperature environment.

4、the development trend of battery separator

(1) Direction of technological innovation

Ultra-thin balance with high strength

In order to improve the energy density of lithium batteries, the battery separator has a trend of ultra-thin development. However, thinning of the diaphragm reduces its mechanical strength and increases the risk of short circuits. Therefore, how to achieve ultra-thin while ensuring sufficient mechanical strength is one of the focuses of current research.

Multifunctional integration

The future battery separator will not only be limited to isolation and ion transport functions, but also have more additional functions.

5、Conclusion

As a key component of lithium battery, battery separator plays an irreplaceable role in isolating positive and negative electrodes, ensuring ion transport and improving battery safety performance. Its material selection, performance indicators and technological innovation trends have a profound impact on the development of lithium batteries.