In a battery electrode slurry the mixture of heavy solid particles in suspension along with the need for extensive processing and thin coating layers requires a specific rheological profile. Viscosity and shear thinning properties help with mixing, yield stress and zero-shear viscosity aid with stability. A whole host of rheological properties such as surface tension and extensional viscosity as well as viscosity and yield stress come into play when using slurries as a thin film electrode coating and can have a significant impact on battery performance.
Contact us to discuss approaches for characterising your battery electrode slurries.
The Rheology of an Electrode Slurry is Intrinsically Tied to It’s Handling Properties
Electrode slurries are a mix of solid conductive particles along with active materials, polymer binders and a solvent medium. This combination of heavy solid particles in a liquid medium makes handling, processing and storage of electrode slurries a particularly difficult task. The ideal slurry has a low viscosity to be mixed well, but a high enough viscosity to slow the settling of solids and maintain homogeneity. It has to yield well during mixing and create a homogenous mixture, but once mixed it needs to resist yielding in order to hold those particles in place. Once coated onto an electrode, the slurry needs to level well to create a thin, even film layer and maintain that levelling while drying.
Fortunately, with the use of plasticisers, formulators have the tools to improve the rheological characteristics of their electrode slurry in order to achieve specific handling properties. By profiling the rheology of benchmark formulations, chemists can target specific rheological characteristics and ensure their electrode slurry mixes, disperses and stores as well as expected from a modern high quality product.
Mixing and Storage – Viscosity, Shear Thinning and Yield Stress
Having a highly viscous medium for an electrode slurry can impart many benefits to stability, storage and application. This viscosity can be a complication during mixing however, as it can be difficult to achieve a uniform dispersion with higher viscosity solutions, which can affect battery performance. Many materials, and indeed many electrode slurries, possess a shear thinning property. A material which reduces its viscosity under shear can be described as shear thinning. Shear thinning can be an incredibly useful property for an electrode slurry. As force is applied to the mixture, its viscosity lowers and the particles are free to disperse evenly throughout, resulting in a homogenous mixture. Once mixing has concluded, the viscosity of the mixture can increase again to aid in holding the particles in space, helping to maintain homogeneity.
Once the mixing is completed and you have achieved an even dispersion, maintaining that dispersion during storage is a separate challenge. Once again, there are rheological properties that can be targeted within your formulation to aid with stability, and maintain an even particle dispersion.
Zero shear viscosity describes the viscosity of the slurry at rest, such as when it is in storage. While a slurry displays very obvious fluid properties, at very low stresses it has the potential to react as a solid. This zero shear viscosity can aid with stability, preventing the solid particles from settling to the bottom of the solution.
The yield stress of the slurry is the amount of force needed to elicit significant flow. On a large scale, yield stress can be a valuable measurement to aid in application and defect reduction. On a micro-scale, however, yield stress can be incredibly valuable property to aid with stability. The weight of the solid particles needs to overcome the yield stress of the liquid part of the slurry in order to fall to the bottom. If the yield stress is high enough, the particles are unable to fall, and won’t be able to sediment out.
Rheology is Key to Creating Even Layer Thin Films In Any Printing Process
The rheology of coating and coating defects is an expansive area – we have written about it before (read about it here). In a battery slurry, these defects can be hugely detrimental to the final performance of the electrode. The slurry must level well and dry into a thin film without defects to ensure an even coating layer and contact area with the electrode. While the coating method selected to create this film will play a part in the minimisation of defects, rheology plays a vital part when selecting and tuning coating systems. Viscosity, zero-shear viscosity and yield stress play a part here, as do other factors such as cure time, extensional viscosity and interfacial rheology (surface tension and surface energy).
At all stages of an electrode slurry’s life there are a wide range of rheological properties which can be measured and controlled in order to elicit specific characteristics in the finished product. Mixing, storage, coating and drying are all processes that are affected by the slurry’s rheology. Viscosity, yield stress and shear thinning properties can aid with creating and maintaining homogenisation within the slurry. All those properties as well as interfacial rheology measurements such as surface tension play a part in the handling, coating and drying properties of the slurry, affecting how it spreads, levels and forms a film. A combination of all these rheological factors can be vital for understanding and improving key handling properties of your electrode slurry as well as improving battery performance.
Should you wish to discuss testing of your own battery electrode slurries please feel welcome to contact us.