1、The definition and essence of lithium battery separators

The lithium battery separator is a thin film material with a microporous structure, located between the positive and negative electrodes of the lithium battery. Its core function is to physically isolate the positive and negative electrodes to prevent direct contact between the two electrodes and cause a short circuit. At the same time, it allows lithium ions in the electrolyte to pass freely, achieving ion conduction during the battery charging and discharging process. In terms of material, it is usually made of high-molecular polymers (such as polypropylene, polyethylene, etc.) or composite materials, with a thickness generally ranging from 10 to 50 microns. Although as thin as a cicada's wing, it shoulders the important mission of ensuring the safe and stable operation of the battery.

2、The core function of lithium battery separators

(1) Safety isolation eliminates the risk of short circuits

Direct contact between the positive and negative electrodes can trigger intense electrochemical reactions, generating a large amount of heat and even causing the battery to explode. The lithium battery separator, with its insulating property and physical barrier function, separates the positive and negative electrodes, effectively avoiding the occurrence of such dangerous situations. For instance, when the battery is subjected to external force impact or has abnormal internal structure, the stable structure of the separator can minimize the possibility of contact between the positive and negative electrodes to the greatest extent, adding a crucial line of defense for battery safety.

(2) Ion conduction ensures charging and discharging efficiency

During the charging and discharging process of the battery, lithium ions need to migrate back and forth between the positive and negative electrodes. The microporous structure on the separator provides a dedicated transmission channel for lithium ions. Parameters such as pore size, uniformity of distribution and porosity directly affect the conduction speed and efficiency of ions. High-quality separators enable lithium ions to pass through quickly and smoothly, thereby enhancing the battery's charging and discharging rates as well as energy conversion efficiency. Studies show that if the ion conduction performance of the separator is improved by 10%, the charge and discharge efficiency of the battery can be correspondingly increased by 8% to 10%.

(3) Maintain the electrolyte and stabilize the battery environment

The separator has excellent liquid absorption and retention capabilities, capable of adsorbing a large amount of electrolyte and providing a stable electrochemical environment inside the battery. This not only helps maintain the normal chemical reactions of the battery, but also reduces the evaporation and loss of the electrolyte, thereby extending the service life of the battery. Meanwhile, a stable electrolyte environment is also conducive to improving the battery's cycle performance, enabling the battery to maintain good performance even after multiple charge and discharge cycles.

(4) Control battery expansion and enhance structural stability

During the charging and discharging process of the battery, the positive and negative electrode materials will undergo volume changes, which may cause the internal structure of the battery to expand and deform. The separator has certain mechanical strength and flexibility, which can to a certain extent suppress this expansion, maintain the stability of the internal structure of the battery, and thereby improve the overall reliability of the battery. For instance, in lithium batteries with high energy density, the structural stability of the separator is crucial for preventing problems such as short circuits caused by battery expansion.

3、The main types of lithium battery separators

(1) Polypropylene pp separators

PP partitions have excellent high-temperature resistance, with a melting point of over 160℃, enabling them to operate stably in high-temperature environments. It has strong chemical stability and is not prone to react with the electrolyte, making it suitable for scenarios with high requirements for safety and high-temperature resistance, such as electric vehicle batteries and energy storage batteries. However, the pore size of the PP separator is relatively large and its resistance is high, which to some extent affects the charging and discharging efficiency of the battery.

(2) Polyethylene PE Separator

The pore size of the PE separator is small, its resistance is low, and it has excellent ion conduction performance, which can increase the charging and discharging speed and energy density of the battery. It has good toughness and is suitable for use in the consumer electronics field with high requirements for battery performance, such as mobile phone and laptop batteries. However, the high-temperature resistance of PE partitions is relatively poor, with a melting point of approximately 130℃. They are prone to deformation in high-temperature environments, which limits their application in some high-temperature scenarios.

(3) Composite partition

In order to integrate the advantages of PP and PE partitions, composite partitions emerged. It is usually composed of two layers of materials, PP and PE. The upper PP layer provides high-temperature resistance, while the lower PE layer offers good ion conductivity and toughness. The composite separator combines the advantages of both and can perform well in different usage environments. It is suitable for lithium batteries that have high requirements for both safety and performance.

(4) Other new types of partitions

With the continuous development of lithium battery technology, new types of separator materials are constantly emerging, such as ceramic-coated separators, non-woven fabric separators, and polymer electrolyte separators. The ceramic-coated separator improves the high-temperature resistance and mechanical strength of the separator by coating a layer of ceramic particles on the surface of the traditional separator, and at the same time enhances the adsorption capacity for the electrolyte. Non-woven fabric separators have good air permeability and flexibility, and are suitable for use in some lithium batteries with special structures. Polymer electrolyte separators combine the functions of electrolytes and separators, and are expected to achieve higher energy density and safety.

4、Selection methods for lithium battery separators

(1) Choose according to the battery type

The requirements for separators vary among different types of lithium batteries. For instance, lithium-ion batteries and lithium polymer batteries differ in structure and electrolyte composition, and it is necessary to select a separator that matches them. Lithium-ion batteries typically use liquid electrolytes, which have high requirements for the liquid absorption and retention capabilities of the separator. PP, PE or composite separators can be selected. Lithium polymer batteries use gel-like or solid electrolytes, and have stricter requirements for the mechanical strength and compatibility of the separator. New non-woven fabric separators or polymer electrolyte separators may be better choices.

(2) Consider the working environment

Temperature environment

If lithium batteries need to operate in high-temperature environments, such as near the engine compartment of a car or in high-temperature industrial Settings, PP separators or ceramic-coated separators with good high-temperature resistance should be selected to prevent the separators from deforming due to high temperatures and causing battery short circuits. In low-temperature environments, the viscosity of the electrolyte in batteries increases and the ion conduction speed slows down. At this time, PE separators or composite separators with larger pore diameters and lower resistance should be selected to improve the charging and discharging performance of batteries at low temperatures.

Mechanical stress environment

In some scenarios that are prone to mechanical vibration and shock, such as during the operation of electric vehicles or when portable electronic devices are frequently moved, the partition needs to have good mechanical strength and flexibility to withstand mechanical stress without damage. Composite partitions and non-woven fabric partitions perform well in this aspect and can adapt to complex mechanical stress environments.

(3) In combination with performance requirements

Energy density

For scenarios with high requirements for energy density, such as electric vehicles and unmanned aerial vehicles, it is necessary to select separators with low resistance and reasonable pore size distribution to reduce energy loss inside the battery, improve ion conduction efficiency, and thereby enhance the energy density of the battery. PE partitions and composite partitions have certain advantages in this aspect.

Cycle life

If lithium batteries need to be frequently charged and discharged, such as energy storage batteries and power tool batteries, the cycle stability of the separator is of vital importance. A separator with a stable structure and strong liquid absorption and retention capacity during multiple charge and discharge processes should be selected to reduce the loss of electrolyte and the aging of the separator, and to extend the cycle life of the battery. Ceramic-coated partitions and high-quality composite partitions perform well in terms of cycle life.

Safety

Safety is the primary consideration in the design and application of lithium batteries. For scenarios with extremely high safety requirements, such as electric vehicle batteries and aerospace batteries, separators with excellent high-temperature resistance and high puncture strength, such as ceramic-coated separators or multi-layer composite separators, should be selected to minimize the probability of safety accidents such as battery short circuits and fires to the greatest extent.

(4) Pay attention to the key performance parameters of the partition

Pore size and porosity

The pore size determines the ease with which lithium ions pass through, while the porosity affects the liquid absorption capacity of the separator and the number of ion conduction paths. Generally speaking, partitions with smaller pore diameters and moderate porosity have better ion conduction performance and safety. The appropriate combination of pore size and porosity should be selected based on the specific battery design and performance requirements.

Thickness and mechanical strength

The thickness of the separator affects the volume and energy density of the battery. Thinner separators can increase the content of active substances in the battery and enhance its energy density, but at the same time, they also impose higher requirements on its mechanical strength. Mechanical strength includes tensile strength, puncture strength, etc. Separators with high mechanical strength can better withstand the internal pressure and external mechanical stress of the battery, ensuring the structural stability of the battery.

Electrolyte compatibility

The separator must have good compatibility with the electrolyte and should not undergo chemical reactions with it, which could lead to a decline in separator performance or battery failure. When choosing a separator, it is necessary to understand the chemical properties of the separator material and the electrolyte used to ensure that no adverse reactions occur between the two.

As one of the core components of lithium batteries, the performance and quality of lithium battery separators directly affect the overall performance of the battery. When choosing lithium battery separators, it is necessary to comprehensively consider multiple factors such as battery type, working environment, performance requirements, and key performance parameters of the separator. At the same time, a reliable manufacturer should be selected to ensure that the chosen separator can meet the actual application needs. With the continuous advancement of lithium battery technology, the requirements for separators are also constantly increasing. It is believed that in the future, more high-performance and multi-functional new separators will be introduced, injecting new impetus into the development of the lithium battery industry.