What is a separator?
separator
The separator is a crucial component of lithium-ion batteries. It's a microporous membrane used to separate the positive and negative electrodes, and is a functional polymer material with a nanoscale microporous structure, as shown in Figure 7-9. The performance of the separator determines the battery's interface structure, internal resistance, and other characteristics, directly affecting its capacity, cycle life, and safety. A high-performance separator plays a vital role in improving the overall performance of the battery. Its main function is to prevent short circuits caused by electrode contact while allowing electrolyte ions to pass through. 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.
The performance of the lithium-ion battery separator determines the battery's interface structure and internal resistance, directly affecting its capacity, cycle life, and safety. The technical requirements for lithium-ion battery separators are:
1) Insulation performance: The separator is an electrically conductive insulator, ensuring mechanical isolation between the positive and negative electrodes.
2) High ionic conductivity: It must minimize resistance to the movement of electrolyte ions, possessing a certain pore size and porosity to ensure low resistance and high ionic conductivity, allowing for good lithium-ion permeability.
3) Chemical stability and electrochemical inertness: It must be sufficiently stable against the electrolyte, potential impurities, electrode reactants, and electrode reaction products, preventing dissolution or degradation. Since the electrolyte solvent is a strongly polar organic compound, the separator must be resistant to electrolyte corrosion and possess sufficient chemical and electrochemical stability.
4) Minimal repulsion from the electrolyte: It must have good wettability to the electrolyte and sufficient liquid absorption and moisture retention capacity.
5) High mechanical strength is required to prevent tearing and deformation during processing. Sufficient mechanical properties, including puncture strength and tensile strength, are essential, while minimizing thickness.
6) Dimensional stability is crucial. Minimal dimensional change below the melting point is necessary to prevent short circuits between the positive and negative electrodes. Good spatial stability and flatness, along with high uniformity in thickness and pore size, are also important.
7) Good thermal stability and automatic shutdown protection are essential. Power batteries place even higher demands on the separator, typically employing composite membranes.
8) Minimal thermal shrinkage is required to effectively prevent the migration of particles, colloids, or other soluble substances between the positive and negative electrodes. Otherwise, short circuits can occur, leading to battery thermal runaway.
Different lithium-ion battery systems and applications have different requirements for separators.
