The separator is an important component of lithium-ion batteries and is also commonly referred to as battery separator, separator paper, ion separation membrane, etc. The main function of the separator in lithium battery materials is to isolate the positive and negative electrodes to prevent problems such as battery self-discharge and short circuit between the two electrodes. Since the safety, permeability, porosity and membrane thickness of the separator will affect the ion conductivity and mechanical strength, different design methods must be applied to manufacture different products, such as single-layer, double-layer and triple-layer separators.
What is a lithium battery separator?
In the structure of a lithium battery, the separator is one of the key internal components. The performance of the separator determines the battery's interface structure, internal resistance, and other characteristics, directly affecting the battery's capacity, cycle life, and safety performance. A high-performance separator plays an important role in improving the overall performance of lithium batteries.
The main function of the separator is to separate the positive and negative electrodes of the battery, preventing short circuits caused by contact between the electrodes. It also allows electrolyte ions to pass through. The separator material is non-conductive, and its physicochemical properties have a significant impact on battery performance. Different types of batteries use different separators. For lithium-ion batteries, since the electrolyte is an organic solvent system, separator materials resistant to organic solvents are required; generally, high-strength, thin-film polyolefin porous membranes are used.
Requirements for lithium battery separators
1. It has electronic insulation properties, ensuring mechanical isolation between the positive and negative electrodes.
2. It has a certain pore size and porosity to ensure low resistance and high ionic conductivity, and has good permeability to lithium ions.
3. Since the solvent of the electrolyte is a highly polar organic compound, the diaphragm must be resistant to electrolyte corrosion and have sufficient chemical and electrochemical stability.
4. It has good wettability to electrolytes and sufficient liquid absorption and moisturizing ability.
5. It should have sufficient mechanical properties, including puncture strength and tensile strength, but the thickness should be as small as possible.
6. It has good spatial stability and flatness.
7. Excellent thermal stability and automatic shutdown protection performance. Power lithium batteries have higher requirements for separators, and composite membranes are usually used.
Preparation method of lithium battery separator
The preparation methods of lithium battery separators are divided into two categories: dry method and wet method.
The dry process involves melting, extruding, and blowing polyolefin resin into a crystalline polymer film. After crystallization and annealing, a highly oriented multilayer structure is obtained. Further stretching at high temperature peels off the crystalline interfaces, forming a porous structure that increases the pore size of the film. The dry process includes uniaxial stretching and biaxial stretching.
The wet process, also known as the phase separation process or thermally induced phase separation process, involves mixing liquid hydrocarbons or some small molecules with polyolefin resin, heating and melting them to form a homogeneous mixture, then cooling to separate the phases and pressing them into membranes. The membranes are then heated to near their melting point and subjected to biaxial stretching to orient the molecular chains. Finally, the mixture is kept at this temperature for a certain period of time, and the residual solvent is washed away with volatile substances. This process can produce interconnected microporous membrane materials.
Development Trends of Lithium Iron Phosphate Battery Separator Industry
● The lithium iron phosphate battery separator industry has broad market prospects and huge development potential. Currently, the industry's low-end products have entered a competitive stage, and corporate profits are becoming increasingly even. Furthermore, industry consolidation and market segmentation are nearing completion. In the future, the overall price of lithium iron phosphate battery separators will further decline, while more product innovations will meet the diverse needs of downstream industries.
● With technological advancements and societal development, safety issues will inevitably receive increasing attention. High-temperature resistant composite separators that maintain membrane integrity even after large-area short circuits between the positive and negative electrodes during charging and discharging will undoubtedly gain market favor. To improve the safety of lithium iron phosphate batteries, coating polyolefin separators with ceramic or other nanomaterials, or employing new matrix materials, is becoming a future technological development trend.
● Developing ultra-thin separators to improve energy density per unit area. As the market develops, lighter and smaller 3C electronic products will continue to enter our lives. These products require thinner lithium-ion battery separators to make batteries. At the same time, the pursuit of capacity in lithium batteries also requires thinner and lighter films. However, the production and preparation of ultra-thin separators have very high requirements for equipment and processes.
Among lithium battery materials, separator technology has relatively high barriers to entry and gross profit margins, and it is also the last material to achieve domestic production. As the technological level of domestically produced lithium battery separator equipment improves, the gap in product quality with imported equipment will be further narrowed, and the cost-effectiveness advantage and substitution effect of domestically produced equipment will become increasingly apparent.
