Flexible Battery Separators
Solef® PVDF grades for battery separators offer all the advantages of a high purity resin with the suitable chemical and electrochemical stability critical in the aggressive environment of a lithium battery.
Different technologies may be employed for manufacturing separator membranes, either dense gelled or porous ones.
Advantages of Solef® PVDF Grades
Due to its physical-chemical properties, PVDF can be processed by different processing methods including solutions-based and melt extrusion technologies.
It offers various advantages when it is utilized in the design of separator, as main component or even just as a thin coated layer:
- Electrochemical stability from 0 to 5 V vs Li+/Li
- Solubility for easy processing
- Fast wettability of the membrane
- Controlled leakage of electrolytes
- Durable adhesion with electrodes
- Flexibility for lamination process
According to the specific design of the separator and to the manufacturing process, it is possible to select the most appropriate Solef® PVDF grade.
|Grade||Type of Polymer||Application||Molecular Weight [kDa]||Melting Temp. [°C]|
|Solef® 1015||PVDF standard homopolymer||Porous separator for high ionic conductivity||570 – 600||170 – 175|
|Solef® 6020||PVDF standard homopolymer||Porous separator for high ionic conductivity||670 – 700||170 – 175|
|Solef® 21216||PVDF-HFP copolymer acetone soluble||GEL polymer technology for safety and flexibility or coating of polyolefin separator||570 – 600||130 – 136|
|Solef® 21216||PVDF-HFP copolymer acetone soluble||GEL polymer technology for safety and flexibility or coating of polyolefin separator||290 – 310||130 – 136|
|Solef® 31508||PVDF-CTFE copolymers acetone soluble||Coating of polyolefin separator||270 – 290||167 – 171|
Some differences may be highlighted between PVDF homo and copolymers. Homopolymers are soluble in a limited number of solvents such as NMP, DMAc, DMF, TEP and DMSO and they show low swelling ability in electrolytes. Their melting temperature and crystallinity are high, determining superior mechanical properties and high thermal resistance.
Copolymers are soluble in a wider range of solvents including, for instance, acetone, MEK and THF for easier processing. They have high swelling ability which allows the design of gel polymer membranes or coating adhesive layers.
The high compatibility of PVDF with electrolytes guarantees fast wettability and suitable ionic conductivity. Due to the different crystallinity, PVDF homopolymers can uptake lower amounts of electrolytes and maintain the original shape, while copolymers can reach high swelling levels especially at high temperatures.
For a comparison among various Solef® homopolymer and copolymer grades, swelling experiments have been performed on molded test samples, which have been immersed in a standard mixture of electrolyte (EC/DMC/DEC 2:2:1) and LiPF6 1 M for 30 days. Maximum swelling values give an indication of the compatibility of the polymer with a specific electrolyte.