PVdF Molecular Weight as Key to Reduced Solvent Consumption

The fluoropolymer polyvinylidene difluoride (PVdF) is considered the state-of-the-art binder for positive electrodes in lithium ion batteries. However, the expensive, harmful organic solvent N-methylpyrrolidone (NMP) is required for its processing during cell production. A promising approach to reduce the NMP content – and thereby lowering energy consumption in the existing manufacturing process – is to modify the properties of the binder. A team from MEET Battery Research Center at the University of Münster has therefore investigated how the molecular weight of PVdF influences the properties of the electrode paste and the composite electrode, as well as the electrochemical performance of the cathodes (based on lithium-nickel-manganese-cobalt oxide: NMC622).

Advantages of Low Molecular Weight

The researchers discovered that low-molecular-weight PVdF allows a substantially higher solid content during processing. “We reduced the mass fraction of NMP within the electrode paste by 33 percent,” explains MEET researcher Johanna Kauling. As a result, the absolute NMP consumption in electrode production was decreased by 41 percent.  Dr Markus Börner, head of the MEET research division “Cell System”, adds: “Reducing NMP is a mid-term strategy to lower energy costs and promote safer, more environmentally friendly battery production processes without requiring extensive infrastructure adjustments.”

In addition, the research team analyzed two different  particle morphologies of the active material NMC622: single-crystalline and polycrystalline. Low-molecular-weight PVdF yielded particularly promising results in polycrystalline systems. In these systems, among other things, binder migration processes were suppressed, leading to a homogeneous distribution of the carbon-binder domain. The resulting electrode structure enabled an extended cycle life by 37 percent. The study thus demonstrates that the purpose-driven selection of a binder can both reduce material consumption in production and improve the electrode's performance.

Detailed Results Available Online

The full study was published by the authors Johanna Kauling, Dr Candeniz Gercek, Jonas Fechner and Dr Markus Börner, MEET Battery Research Center, as well as Prof. Dr Martin Winter, MEET Battery Research Center and Helmholtz Institute Münster of Forschungszentrum Jülich, in the journal “Advanced Energy and Sustainability Research”.