Benefits of Tuned Binder Distribution in Multi-Layered Thick NMC811 Cathodes
Authors: Neidhart Lukas , Fröhlich Katja , Winter Franz , Jahn Marcus 
Published: November 2022

Maximizing the electrode thickness is an inevitable target to reach when improving the energy density of Li-ion batteries [1,2]. Unfortunately, the fabrication of electrodes of several hundreds of micrometer thickness is often accompanied by detrimental side-effects. A critical process step in thick electrode production is the drying of the coated layer. During the evaporation of the solvent, light components of the slurry such as binder and carbon black can be dragged to the top of the coating [3,4]. The lack of liquid linkage throughout the layer leads to binder particles being evenly spread across the coating after the drying process is concluded. Inhomogeneous distribution of binder can not only cause delamination, but also negatively influences the Li-ion transport pathways through the electrode. To ensure ideal mechanical stability and satisfactory electrochemical performance, binder migration needs to be suppressed effectively.
Multi-layering has proven itself as a technique within aqueous processing to overcome the limitations arising for thick electrodes. Subsequent coating of layers offers opportunities to establish selective material compositions at specific regions within the electrode (Figure 1a). This novel coating method can be considered a steppingstone to further optimize high loading electrodes. We have previously demonstrated that by multi-layering an increase of 40% in adhesion can be achieved compared to single-layer electrodes. Moreover, the assembled cells show superior performances at rate capability tests. Especially for low current densities (0.2C) an increase of up to 20% in specific discharge capacity was reached through multi-layering (Figure 1b) [5].
Within aqueous electrode fabrication, the use of sodium carboxy-methyl cellulose (CMC) as binder material is common, due to its role as a thickening agent of the slurry and solubility in water. However, due to its brittleness, the usage of CMC requires additional binders to maintain flexibility of the coating. The ratio of active and inactive materials within the cell is strongly correlated to its capacity and the addition of these binders ultimately results in a decreased energy density of the cell. Besides keeping the particles together, the main purpose of the binder is to guarantee good adhesion to the substrate. Therefore, a high concentration is not necessarily needed at the top of the coating and the overall amount of binder material should be kept at a minimum.
Bi-layered cathodes consisting of Ni-rich lithium nickel manganese cobalt oxides (NMC811) as active material were fabricated. The concentration of poly(methyl)acrylate (PMA) binder was changed in the top layer and its effect on structural properties and electrochemical performance was investigated. Water-based processed cathodes with high loadings of more than 8 mAhcm-2 were tested in full cell configurations.
Reduction of the binder content leads to an increase in specific discharge capacity of up to 40% compared to single-layered and 25% to unmodified multi-layered electrodes at a C-rate of 1C (Figure 1b).

Acknowledgements

This work was financially supported by the Austrian Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology (bmk).
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Figure 1