Battery manufacture relies on the electrode materials being applied as a slurry, to form a film. The size and shape of particles within the slurry are critical to the production of the film and its final uniformity, via the viscosity of the slurry. In this application note, the Morphologi automated image analysis system is used to characterize efficiently the shape of particles in high and low viscosity electrode slurries. 

Introduction

Batteries are ubiquitous in modern life and our reliance on them has never been greater. Therefore, ensuring optimum battery performance through manufacturing control is of increasing significance. In previous application notes we have discussed the importance of controlling the size of particles used in the manufacture of battery materials [1] and the impact of carbon microstructure in graphite electrodes on battery performance [2].

Shape is also an important factor to consider and control, as irregular shaped particles not only reduce packing density, but they can lead to the formation of a high viscosity electrode slurry. In this third application note on batteries, we consider the role of size and shape on the viscosity of the electrode slurry.

Electrode composition

The typical structure of a battery electrode is given in Figure 1. The electrode is usually fabricated by applying a slurry of particles in suspension onto a metal foil. 

AN170103_Fig1

 Figure 1 – typical structure of the Li-ion battery.

The slurry in question is composed of electrode particles (anode or cathode), small carbon particles to aid conduction, and binder material (composed of solvent and polymer) to hold the structure together. The concentration of particles in the slurry is high, representing between 20-40 % of the total by weight. Consequently, the particle properties have a significant impact on the physical properties of the resultant slurry.

Particle properties and slurry viscosity

The viscosity, dispersibility, concentration and compactability of the slurry are important parameters in determining how effective the slurry will be during application. A high viscosity slurry causes difficulties in the coating process, and poor dispersibility results in low film uniformity; the concentration and compactability of the slurry controls the film density. Uniformity of the coating thickness and the layer density are important to ensure control over the ion transfer rate and life time (recharge cycle time) of the battery, while controlling the layer thickness enables a smaller battery to be produced.

As illustrated in Figure 2, the presence of a significant proportion of irregularly shaped particles will lead to a higher viscosity slurry due to the increased impact of particle friction and interlocking but also due to the extra flow energy required for the fluid to circumvent the particles.

AN170103_Fig 2 1

Figure 2 – irregularly shaped particles experience greater interlocking and friction, leading to a higher viscosity.

Particle shape also impacts on packing density since irregular particles pack less efficiently than spheres. Hence, fewer particles can be added to the liquid before the viscosity starts to increase, as illustrated in Figure 3. Further, a polydisperse sample will pack more efficiently than a monodisperse sample, at the same concentration, lowering viscosity. Smaller irregular particles, however, may increase viscosity due to their higher surface area, which will accentuate particle-particle and particle-liquid interactions. Therefore, it is important to be able to monitor and control the proportion of irregularly shaped particles and fine material within an electrode material sample to minimize the viscosity.  

AN170103_Fig3 1

Figure 3 - the influence of particle shape on viscosity.

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