As the density requirements of lithium batteries increase and the quality requirements of lithium battery separators further improve, dry film-making will gradually be replaced by wet film-making.
The separator is an important component of lithium-ion batteries, often referred to as battery separator, separator paper, or ion separation membrane, and is located in the upstream part of the new energy vehicle industry chain. Depending on the manufacturing process, they are generally divided into dry-process separators and wet-process separators. Dry-process separators can be further divided into dry-process single-stretch separators and dry-process double-stretch separators, or dry-process single-layer separators and dry-process multilayer separators.
The main raw material for separators is polyolefin resin. Depending on the process, dry-process separators generally use PP as the raw material, and sometimes dry-process multilayer separators also use PP and PE co-extrusion. Wet-process separators generally use ultra-high molecular weight polyethylene (UHMWPE) as the separator body, paraffin oil as the pore-forming agent, and dichloromethane as the extraction liquid.
In recent years, with the successful domestic production of separators, their prices have dropped rapidly, and their proportion in the total cost of lithium battery materials has also decreased, generally to around 7-15%. Generally speaking, due to the high unit cost of positive and negative electrode materials in ternary lithium batteries, the separator cost accounts for less than 10%, while in lithium iron phosphate batteries, the unit cost of positive and negative electrode materials is relatively low, with the separator cost accounting for around 15%. Among lithium battery materials, separator technology has relatively high barriers to entry and gross profit margins, and it was also the last material to achieve domestic production.
Paraffin oil serves as the diaphragm body, pore-forming agent, and dichloromethane as the extraction liquid.
The lithium-ion battery separator is a crucial inner component that affects key performance characteristics such as capacity, cycle performance, and charge/discharge current density. A high-performance separator needs to isolate the positive and negative electrodes to prevent short circuits while allowing lithium-ion conduction. It must also possess high-temperature self-closing properties to block current and prevent explosions during overcharging or excessively high temperatures. Furthermore, it must be characterized by high strength, fire resistance, good durability, and non-toxicity.
Generally speaking, based on the main function of the diaphragm, the diaphragm has high performance requirements in terms of safety and permeability:
1. Provide safety for the battery. It must have good insulation to prevent short circuits caused by contact between the positive and negative electrodes or by punctures from burrs, particles, or dendrites. Therefore, the separator needs to have certain tensile and puncture strength, be resistant to tearing, and maintain dimensional stability under sudden high-temperature conditions to prevent melting and shrinkage that could lead to large-area short circuits and thermal runaway.
2. It provides microporous channels for lithium batteries to achieve charge/discharge functions and rate performance. Therefore, the separator must have high porosity, and the porosity characteristics restrict the migration of lithium ions in the battery, which is reflected in the parameter of conductivity.
For power batteries used in new energy vehicles, due to the high requirements for battery safety and energy density of the entire vehicle, the separators used in power batteries generally require the following:
1. Enhanced safety, including thermal stability, electrochemical stability, and resistance to punctures and short circuits;
2. Improved consistency, including thickness, pore size, and pore size distribution;
3. Ideal porosity and pore structure;
4. Stronger liquid absorption capacity and lower resistance;
5. Higher energy density will require a larger electrochemical stability window, meaning that high voltage resistance (0-5V) is also a future trend.
