Lithium ion battery separator – discussion of its functions and methods of preparation

Separator is one of the most critical components in the lithium ion battery structure, which directly affects the key characteristics of the battery such as capacity, cycle and safety performance.

The separator is the link with the highest technical barriers in lithium battery materials, generally accounting for about 10% of the total cost of the battery.

Next, this article will introduce the lithium ion battery separator, including its function, preparation method, test standard, etc.

The main role of the separator in lithium batteries

The separator is a functional membrane material with a microporous structure, and its thickness is generally 8-40 μm.

In the battery system, it plays the role of separating the positive and negative electrodes, blocking the passage of electrons in the circuit during charging and discharging, and allowing the free passage of lithium ions in the electrolyte.

The micropores can be selectively closed when the battery is charging and discharging or the temperature rises to limit excessive current and prevent short circuits. The performance of the micropore directly determines the overall performance of the battery.

Lithium ion battery separator performance requirements

Lithium ion battery separators have several key requirements to ensure battery safety and performance.Here is some requirements:
● Electronic insulation;
● Appropriate pore size and porosity;
● Good electrochemical stability and resistance to electrolyte corrosion;
● Good thermal stability, low closed cell temperature and high fusing temperature;
● Good affinity with the electrolyte, with a certain liquid absorption rate;
● Sufficient mechanical properties and small thickness;
● Good spatial stability and flatness.

Preparation method of lithium ion battery separator

Traditional lithium-ion battery separators are polyolefin separators, mostly single-layer or three-layer structures, such as single-layer PE, single-layer PP, PP/PE/PP composite films, etc. According to the conventional preparation process, it can be divided into dry process and wet process.

Dry process

The dry process is the most commonly used method. By means of extrusion and film blowing, the molten polyolefin resin is made into a sheet-like crystal film, and slit-shaped porous film is formed at high temperature by uniaxial stretching or biaxial stretching. structure.

Wet process

The wet process is also called phase separation method or thermal phase separation method in industry.

Its preparation principle is to heat and melt a mixture of low molecular weight substances (liquid hydrocarbons, paraffins, etc.) and high molecular weight substances (polyolefin resins) that are incompatible with each other at room temperature, so that the mixture forms a uniformly mixed liquid state, and is separated by cooling press to obtain microporous membrane material.

The three-dimensional structure of the wet film is more complex than that of the dry film, and the degree of microporous buckling is higher. However, due to the use of solvents in the production process, the wet method is relatively less environmentally friendly than the dry method, and has poor thermal stability, and the process is relatively complicated.

Lithium ion battery separator test standard

Referring to the regulations of the American Advanced Battery Alliance on the performance parameters of lithium-ion battery separators, the performance of battery separators can be divided into physical and chemical properties, mechanical properties, thermal properties, and electrochemical properties.

Physical and chemical properties

Physical and chemical properties include thickness, porosity, wettability, liquid absorption, etc.

● Thickness, as the most basic parameter of the battery separator, is inversely proportional to the permeability of lithium ions, so the thickness should be as small as possible when the mechanical properties meet the actual needs;

● Porosity refers to the percentage of the volume of micropores in the material to the total volume of the material, which is closely related to the gas permeability, liquid absorption rate, and electrochemical resistance of the battery separator;

● Wettability and liquid absorption rate are the ability of the separator to retain the electrolyte, so as to reduce the internal resistance of the battery and improve battery performance.

Mechanical properties

Mechanical properties mainly include puncture strength, mixed puncture strength and tensile strength.

The separator material must not only withstand the puncture force of the electrode mixture during the battery operation, but also meet the physical impact, puncture, wear, compression and tensile force during the production process due to curling, packaging, and fabrication.

Thermal properties

Thermal properties mainly include thermal closure temperature, fusing temperature and thermal shrinkage rate.

● The closed cell temperature is set by the special protection mechanism of the separator for the battery, that is, when the temperature exceeds the closed cell temperature, the micropores in the separator are closed, preventing the passage of lithium ions, and reducing the risk of short circuit to a certain extent;

● The melting temperature refers to the temperature at which the separator ruptures and short-circuits at high temperatures. The higher the temperature, the smaller the risk of short-circuiting.

Summary

With the ev battery cell market demand in the rapid growth, as one of the key materials of lithium-ion battery separator, is also undergoing rapid innovation.The future development of lithium-ion battery separators will mainly focus on:

(1) Diversification of types of materials for films. Biomass composite materials and special polymer materials are gradually used in battery separator products; output power and safety performance of battery separators can be improved by compounding various separators or adding inorganic particles and PE micropowder.

(2) Diversification of membrane microporous structure and preparation method. For example, the extraction method is used to extract the soluble substances in the substrate to prepare a microporous film; electrospinning method is used to obtain a separator with smaller pores and higher porosity; micropores are etched by heavy ion irradiation. Methods Prepare evenly distributed membranes with channels running through from top to bottom.

(3) Focus on high-performance separators with low cost and simple production process. At present, commercialized separators are mainly PE and PP films. Due to their own structure and cost constraints, their status as commercial battery separators is difficult to shake. Therefore, commercial battery separators still use PE and PP as base films. Surface modification, coating and other methods are used to seek separator materials with simple manufacturing process and greatly improved performance.

(4) Pay attention to the comprehensive performance and evaluation system of separator materials. With increasing attention to the physical and chemical properties, mechanical properties, thermal properties, and electrochemical properties of the separator, further improving the application evaluation of battery separator materials is also an important direction for current development.