Solef® PVDF for Li-Ion Batteries
Solef® PVDF Binders Improve Battery Performance
Lithium batteries are a challenging application for most polymeric materials, as they demand long-term reliability as well as chemical and electrochemical resistance in the specific chemical environment of Li-ion cells. In the case of automotive applications, higher temperature performance are also required.
Solef® PVDF is a partially fluorinated semi-crystalline polymer with excellent thermo-mechanical and chemical properties. It brings many advantages to the lithium battery industry when used as a binder in the formulation of electrodes as well as in the design of the separator.
Solef® PVDF is already well assessed in many specialty applications such as oil and gas, semiconductors, membranes for water filtration, plumbing, architectural coatings and photovoltaics.
- Solef® PVDF is electrochemically stable in the full range of voltage between 0 and 5 V vs Li+/Li, which guarantees its safe use in the electrochemical environment of the lithium cell.
- Thermogravimetric analysis shows that Solef® PVDF resins are stable at high temperature: no thermal degradation occurs before 420 °C for short term treatments.
- The shelf life of Solef® PVDF is infinite. According to ISO 9080 extrapolation standard, Solef® PVDF pipes are stable for more than 50 years under 25 MPa at room temperature.
Solef® PVDF Grades
It is important to choose the right Solef® PVDF grade in order to achieve the targeted chemical resistance. Thanks to its high crystallinity levels, homopolymer PVDF offers high resistance in typical electrolytes used in lithium batteries. PVDF copolymers, characterized by lower crystallinity, are soluble in a wider range of solvents and show different levels of swelling in organic carbonates. This property make them suitable for manufacturing the separator in gel polymer type batteries.
The high purity of Solef® PVDF is a guarantee for more safety. The Solef® PVDF has been used for more than 15 years in the high purity industry, including many semiconductor applications. Therefore Solvay Specialty Polymers has consolidated its experience on how to guarantee a very low level of contaminations in Solef® PVDF resins. Strict production conditions and quality control rules enable Solvay Specialty Polymers to reach a strong position among the leaders in the semiconductor industry.
Innovation from Solvay
The R&D expertise of Solvay Specialty Polymers in fluorinated chemistry and polymerization technology is continuously focused on the development of new tailored solutions in order to fulfill increasing requirements for safety and performance in the growing lithium batteries market.
|Property||Unit||PVDF Homopolymer||PVDF Homopolymer||Modified PVDF||PVDF Copolymer||Test Method|
|Solef® 6010||Solef® 6020||Solef® 5130||Solef® 21216|
|1st generation binder||2nd generation binder and porous separator||3rd generation binder for automotive application (or high adhesion)||Flexible binder and gel polymer separator|
|Molecular weight||Da||300,000 – 330,000||670,000 – 700,000||1,000,000 – 1,200,000||570,000 – 600,000||GPC*|
|Melting point||°C||170 – 175||170 – 175||158 – 166||130 – 136||ASTM D3418|
|Heat of fusion (ΔHf)||J/g||58 – 66||55 – 65||40 – 48||20 – 28||ASTM D3418|
|Glass transition (Tg)||°C||– 40||– 40||– 40||– 40||DMTA|
|Tensile modulus at 23 °C||MPa||1,700 – 2,500||1,300 – 2,000||1,000 – 1,500||400 – 600||ASTM D638
|Volume resistivity||Ohm · cm||≥ 1·1014||≥ 1·1014||≥ 1·1014||≥ 1·1014||ASTM D257
*Molecular weight data were obtained by gel permeation chromatography in dimethylacetamide (DMAC), calibrated using a polystyrene standard. The results are useful for a relative comparison.
Typical property values are reported in this document. They should not be interpreted as material specifications.
Electrochemical stability of Solef® PVDF homopolymer
Thermogravimetric analysis of Solef® PVDF homopolymer
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.
Binders for Stable Electrodes
Among other polymers, Solef® PVDF is one of the preferred choices as binder material for electrodes thanks to its stable and reliable performance.
In particular Solef® PVDF guarantees:
- Electrochemical stability from 0 to 5 V vs Li+/Li
- Solubility in NMP for easy processing
- Chemical resistance in the electrolyte
- Suitable cohesion between active materials
- Durable adhesion to the current collector
Adhesion is a key property which determines final performance of batteries especially at long term. A good binder guarantees the homogeneous dispersion of active materials and conductive carbon together with stable bonding to the metallic collector.
In order to evaluate the binder effect, some tests have been performed. Cathodes have been prepared from NMP slurry in standard conditions (LiCoO2 as active component, 5 % of carbon black and a fixed amount of Solef® PVDF), then coated onto an aluminum foil and dried in oven at 130 °C. Adhesion has been measured by peeling test following ASTM D903.
It is possible to notice the effect of molecular weight and binder content on mechanical consistency of electrodes. The temperature of drying, the chemistry and quantity of active material as well as post-treatments play also a role in the determination of adhesion performance and may be optimized for improving electrodes quality.
Adhesion to cathode with LiCoO2
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.
Processing is a key factor for the lithium battery industry. Some evaluations have been performed for a better understanding of the parameters to be controlled in order to optimize processing and performance of PVDF.
The first step of electrodes manufacturing is the dissolution of PVDF in an organic solvent such as NMP. Some guidelines may be taken into consideration for improving the efficiency of this process.
- The method for adding the powder to the solution plays a major role in dissolution ease and duration: in particular it is advised to slowly add the powder to the solution while stirring.
- Mixing speed, geometry of the stirrer and temperature of the solution play key roles in the kinetics of dissolution. It is advised to give enough time to the polymer to completely dissolve in the solvent; a slight heating of the solution can improve the dissolution time.
- It is important to use dry materials and solvents and to operate in a dry environment in order to improve the dissolution process.
Solef® PVDF grades in NMP
Flow curves measured by rheometer RFS III; T = 25 °C, concentration 8 % w/w
Slurry viscosity – LiCoO2
PVDF grade, concentration of the solution and temperature are key factors in determining the solution viscosity. Slurry viscosity is furthermore affected by the chemical nature and the concentration of active materials, as well as slurry viscosity. It is therefore recommended to optimize PVDF concentration and slurry formulation in order to reach suitable processing conditions. For example, some results of slurry viscosity are reported, where slurries have been prepared with the same active material (LiCoO2) but with different polymer content. It may be also necessary to optimize the total solid content of the slurry in order to obtain the targeted slurry viscosity for the coating process. Besides, the nature of active materials and its particle size also play an important role in the determination of slurry viscosity.
Slurry for electrodes coating