Overview and future development trends of lithium battery separators

I.Definition of lithium-ion battery separator

Lithium-ion batteries are composed of four parts: positive electrode material, negative electrode material, electrolyte and separator . The diaphragm is one of the key inner components. The performance of the separator determines the interface structure, internal resistance, etc. of the battery, which directly affects the battery's capacity, cycle, safety performance and other characteristics. Excellent separators play an important role in improving the overall performance of the battery.

II. Main function of lithium-ion battery separator

The main function of the separator is to separate the positive and negative electrodes of the battery to prevent the two electrodes from contacting and causing a short circuit. In addition, it also has the function of allowing electrolyte ions to pass through. The separator material is non-conductive, and its physical and chemical properties have a great impact on the performance of the battery. Different types of batteries use different separators. For the lithium battery series, since the electrolyte is an organic solvent system, a separator material that is resistant to organic solvents is required. High-strength thin-film polyolefin porous membranes are generally used.

To sum it up simply in one sentence, the quality of the separator material can directly affect the performance of the battery. It can isolate the positive and negative electrodes and prevent conduction through current and short circuit burning of the battery. Therefore, whether it is protection or safety, the diaphragm plays a very important role.

III. Requirements for lithium battery separators

First. It has electronic insulation and ensures mechanical isolation of positive and negative electrodes.

Second. It has a certain pore size and porosity to ensure low resistance and high ion conductivity , and has good permeability to lithium ions.

Third. Since the solvent of the electrolyte is a highly polar organic compound , the separator must be resistant to electrolyte corrosion and have sufficient chemical and electrochemical stability.

Fourth. It has good wettability to electrolyte and has sufficient ability to absorb liquid and moisturize.

Fifth. It has sufficient mechanical properties, including puncture strength, tensile strength, etc., but the thickness is as small as possible.

Sixth. Good spatial stability and flatness.

Seventh. Good thermal stability and automatic shutdown protection performance. Power batteries have higher requirements for separators, and composite membranes are usually used.

IV. Diaphragm technology and future development

At present, the lithium battery separators produced in large scale mainly use PP, PE separators and coated separators based on these two separators. The corresponding production processes are dry method, wet method and coating. According to the different pore-forming mechanisms, the dry method is divided into two different processes: dry uniaxial stretching and dry biaxial stretching.

With the continuous improvement of lithium battery technology, the original PP and PE separators made by dry and wet processes have gradually been unable to meet the technical requirements of lithium batteries due to their material properties such as low melting point and poor heat resistance. In order to improve the safety and electrochemical performance of the base film, different coating processes are increasingly used in the field of separators. Currently, the types of coating materials on the market are mainly divided into three types: inorganic coating, organic coating, and organic + inorganic coating. Among them, the inorganic coating materials are mainly ceramics and boehmite, and the organic coating materials are polyvinylidene fluoride. , mainly aramid.

As one of the key materials for lithium-ion batteries, the market demand for separators is also growing rapidly. The future development of lithium-ion battery separators will mainly focus on:

First.Diversification of film materials. Biomass composite materials and special polymer materials are gradually used in battery separator products; the output power and safety performance of battery separators can be improved by compounding various separators or adding inorganic particles, PE micropowder, etc.

Second.Diversification of separator microporous structures and preparation methods. For example, the extraction method is used to extract the soluble substances in the substrate after film formation to prepare a microporous film; the electrospinning method is used to obtain a separator with smaller pores and higher porosity; the micropores are etched by heavy ion irradiation. The method prepares a diaphragm with uniform distribution and up-and-down pore channels.

Third. Pay attention to high-performance diaphragms with low cost and simple production process.

Currently, 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, using grafting, Surface modification, coating and other methods are used to seek separator materials with simple manufacturing processes that can greatly improve performance.