For lithium battery series, due to the organic solvent system as the electrolyte, it is necessary to have organic solvent resistant membrane material. Generally, high strength thin film polyolefin porous membrane is used. Lithium ion battery separator has a large number of zigzag through micropores, which can ensure the free passage of electrolyte ions to form a charging and discharging circuit; when the battery is overcharged or the temperature rises, the diaphragm separates the positive and negative electrodes of the battery through the closed cell function to prevent direct contact and short circuit, so as to block the current conduction and prevent the battery from overheating or even explosion.
(1) It has electrical insulation to ensure effective mechanical isolation of positive and negative electrodes;
(2) It has certain pore size and porosity to ensure low resistance and high ionic conductivity, uniform pore size and distribution, good current density uniformity, permeability and lithium ion permeability;
(3) Because the solvent of the electrolyte is organic compound with strong polarity, the diaphragm must be resistant to electrolyte corrosion, have sufficient chemical and electrochemical stability, do not react with the electrolyte, and cannot affect the chemical properties of the electrolyte;
(4) It has good wettability to the electrolyte and has enough moisture absorption capacity;
(5) It has enough mechanical properties, including puncture strength, tensile strength, etc., but the thickness is as small as possible;
(6) Space stability and smoothness are good;
(7) Thermal stability and automatic shut-off protection performance are good, and the power battery has higher requirements for diaphragm, and composite membrane is usually used.
(8) The heat shrinks of the diaphragm should be small, otherwise it will cause short circuit, and then lead to thermal runaway of the battery.
(9) The diaphragm should have melting integrity after closing to avoid short circuit caused by electrode contact.
The diaphragm production process includes raw material formula and rapid formula adjustment, microporous preparation technology, complete set of equipment independent design, etc.
Micropore preparation technology is the core of lithium-ion battery separator preparation process, which includes dry unidirectional stretching, dry biaxial stretching and wet process.
|Comparison of different preparation methods of diaphragm|
|Item||Dry process||Wet process||Notes|
|Mode of production||Uniaxial tension||Biaxial tension||Asynchronous stretching||Synchronous stretching||-|
|Process principle||Wafer separation||Crystal transformation||Thermally induced phase separation||-|
|Thickness||14-40μm||9-20μm||Small thickness can reduce resistance and increase battery density|
|Pore size distribution||0.01-0.3μm||0.01-0.1μm||Narrow pore size distribution and good permeability|
|porosity||30-40%||35-45%||The porosity should be as large as possible|
|Closed cell temperature||145℃||130℃||Prevent the battery from overheating|
|Fusing temperature||170℃||150℃||Prevent short circuit caused by melting|
|Puncture intensity||200-550gf||300-650gf||Prevent short circuit caused by puncture|
|Transverse tensile strength||<200MPa||130-250MPa||Adequate tensile strength|
|Longitudinal tensile strength||130-260MPa||140-260MPa|
|Transverse thermal contraction||<1.5%||<4%||Smaller shrinkage@120℃|
|Longitudinal thermal contraction||<3%||<3%|