Introduction to Lithium Battery Separator

1.Basic introduction of lithium battery separator

Lithium battery is composed of four main parts: positive electrode, negative electrode, separator and electrolyte. The separator is a thin film with a microporous structure, which is a key component in the lithium battery, and the key inner layer component with the most technical barriers in the lithium battery industry chain, the cost for accounts for about 10%-20%. During the electrolysis reaction, the lithium battery separator can be used to separate the positive and negative electrodes to prevent the occurrence of short circuits, while allowing the free passage of electrolyte ions. The lithium battery separator is immersed in the electrolyte, and there are a large number of micropores on the surface that allow lithium ions to pass through. The material, number and thickness of the micropores will affect the speed of lithium ions passing through the separator, which in turn affects the discharge rate, cycle life and other performance of the battery.

The production process of lithium battery separators is complex and the technical barriers are high. High-performance lithium batteries require separators with uniform thickness and excellent mechanical properties (including tensile strength and puncture resistance), air conductivity, and physical and chemical properties (including wettability, chemical stability, thermal stability, and safety). Whether the separator is excellent or not directly affects the capacity, cycle ability and safety performance of lithium batteries, and the separator with excellent performance plays an important role in improving the overall performance of the battery.

2.Market and application of lithium battery separator

According to the application field, lithium battery separators can be divided into power lithium battery separators, digital lithium battery separators and other functional separators. In the fields of new energy vehicles, energy storage power stations, aerospace, medical and electronic products, lithium battery separators are the most widely used.

Polyolefin is the most commonly used lithium battery separator material on the market, it can be classified as polyethylene (PE), polypropylene (PP), polyethylene (PE) and polypropylene ( PP) composite material. Among them, polyethylene products are mainly used in ternary lithium batteries, and polypropylene products are mainly used in lithium iron phosphate batteries. The technical routes for producing lithium battery separators include dry uniaxial stretching process, dry biaxial stretching process and wet process. Due to the poor performance of the dry biaxial stretching process, the separator can only be used in low-end batteries, so dry uniaxial stretching process and wet process are the main preparation processes now. According to physical and chemical properties, lithium battery separators can be divided into woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, rolled membranes, etc.

In 2021, the output of battery separators in China was 7.9 billion square meters, a year-on-year increase of 112.5%, of which the output of wet-process separators will be 6.1 billion square meters, accounting for 76.7%, a year-on-year increase of 132%; the output of dry-process separators will be 1.8 billion square meters, accounting for 23% , an increase of 67.7% year-on-year, and with the rapid growth of energy storage in the future, it will further bring about an incremental market for battery separators.

3.Principle of dry preparation process

Principle of dry method：

The principle of the dry method is to first melt the raw material of the polymer, and then crystallize the polymer melt under tensile stress during extrusion to form a lamellar structure perpendicular to the extrusion direction and parallel to the extrusion direction, and heat treatment to obtain a hard elastic material. After the polymer film with hard elasticity is stretched, the lamellae are separated to form slit-shaped micropores, and then the microporous film is obtained by heat setting. This process requires high precision control of the process, especially the stretching temperature is higher than the glass transition temperature of the polymer and lower than the crystallization temperature of the polymer. At present, it mainly includes dry uniaxial stretching and biaxial stretching processes. The main difficulty of the dry process is that the process control precision is strict, and the porosity control is difficult to grasp.

Principle of wet method

Compared with the dry method, the wet method requires an organic solvent. The basic process is to dissolve the polymer in a solvent with a high boiling point and low volatility at high temperature to form a homogeneous liquid, and then cool down, resulting in a liquid-solid phase of the solution, and then use volatile reagents to extract the high boiling point solvent, and obtain a polymer microporous membrane with a certain structure and shape after drying. In the manufacturing process of the microporous membrane for separator, unidirectional or biaxial stretching can be carried out before solvent extraction, after extraction, it can be shaped and wound to form a film, or it can be stretched after extraction. The ultra-high molecular weight polyethylene microporous membrane produced by this method has good mechanical properties.