New Solef® 5130 for High Energy and Large Format Cells
Solvay has taken advantage of its expertise in fluorine chemistry and polymerization technologies for designing a new generation of fluoropolymers, Solef® 5130. This high-performance fluoropolymer is designed for use as a binder and is especially tailored for high-demanding applications, where it is necessary to guarantee best performance during battery operation.
As result, the new Solef® 5130 combines the effect of ultra-high molecular weight with the benefit of the polar functional groups distributed in the polymer chain. The reinforced intermolecular interactions between polymer, active materials and metal collector result in increased performance in terms of adhesion and chemical resistance in electrolyte. These effects are translated into higher energy density, better power performance and longer cycle life of batteries for the automotive industry.
The polymer has been tested in combination with different active materials. Especially new oxides, which find an increasing interest in large battery applications, and graphite for anode have been considered. Electrodes have been prepared with standard conditions from NMP solutions, then coated onto a metal foil and dried in an oven at 130 °C. Adhesion has been measured by peeling test following ASTM D903. In all cases Solef® 5130 shows superior adhesion properties compared with PVDF standard homopolymer with high MW.
Therefore it is possible to significantly reduce binder content, giving access to higher energy density as well as lower internal resistance. An example obtained by reducing binder content with LiFePO4 as active material is reported.
Reinforced intermolecular interactions
Adhesion to electrodes
Adhesion to cathode with LiFePO4
Battery Performance with Solef® 5130
PVDF standard grades as Solef® 6020 are recognized on the market and already offer stable performance when utilized as binder in cathode and anode for consumer application.
For the automotive industry it is necessary to assure better performance especially at high depth of discharge in the case of electric vehicles (EV), while the stability at higher current rates for short cycles and lower depth of discharge is requested for hybrid electrical vehicles (HEV).
All advantages of Solef® 5130 may be appreciated when reduced binder content is utilized for high energy applications: cells are characterized by higher capacity and at the same time long life is guaranteed.
It can be noted that the initial capacity is increased by more than 5 % with a lower binder content. In our testing this percentage increases significantly after 50 cycles.
When high power performance are needed, still Solef® 5130 provides more stable performance after time. An example of the improvement of cycle life obtained with the new polymer is shown on the following page.
Electrodes and coin cells
Cycle life for energy applications
Galvanostatic cycles on coin cell test samples 1C, 2.5 – 4.0 V; 80 % DoD at RT; electrode composition: PVDF binder, 10 % super P, LiFePO4; discharge capacity is normalized for electrodes weight
Cycle life for power applications
Galvanostatic cycles on coin cell test samples 2C, 2.5 – 4.0 V; 40 % DoD at RT; electrode composition: 8 % PVDF binder, 10 % super P, 82 % LiFePO4; cycle life: cycle number when cell capacity reaches 80 % of the initial capacity
Impedance after cycling
Longer Battery Life with Solef® 5130
How can Solef® 5130 guarantee the constancy of its improved performance?
An important aspect which is linked to the long-term stability is the chemical resistance in the aggressive environment of a lithium-ion cell, which contains organic carbonates and lithium salt. Especially at high temperature, the weight uptake of Solef® 5130 is very low, as reported in the following paragraph. Molecular weight plays an important role in determining this property.
The breakthrough in term of adhesion properties in harsh conditions is linked to the chemical nature of the polymer. In order to demonstrate this aspect, a test of stability after immersion in the electrolyte was performed. Cathodes prepared with different binder content of homopolymer PVDF and Solef® 5130 were dipped in the electrolyte mixture (EC/DMC 1:1) at 90 °C for 5 days. The resistance of electrodes after immersion was determined by comparing the weight of the active material retained on the metal collector before and after the test.
Although the test conditions (free electrode film dipped in an excess of electrolyte) are much more severe than those in a real battery, it can be taken as an indicator of the stability given by the binder. Solef® 5130 demonstrates excellent performance, ensuring long-term battery stability, where other binder grades do not.
Cathode – LiCoO2 – active material
Binder stability in electrolyte immersion
after treatment of coin cells at 85 °C for 2 days
For example, coin cell test samples have been stored at 85 °C for 2 days and the visual appearance of electrodes is reported in the picture. This result can be easily scaled-up on pouch or prismatic cells.