The first studies of the impact of calendaring and lamination on full cell electrochemistry with Si@C-anodes

The electric vehicle industry requires batteries with high capacity and power to satisfy the current great market demand. For this reason, there is a great interest in the research and implementation of new materials inside the electrodes, which can improve the final performance of the battery. On the one hand, Ni-rich NMC material is being investigated in the cathode area to increase the capacity of the cell and at the same time reduce the critical cobalt content. On the other hand, silicon-based anodes are attracting increasing interest due to their high specific capacity (3579 mAh g^-1). However, due to the significant volume changes of silicon while charging and discharging, their application in industrial-scale electrode fabrication was limited, and the electrodes do not benefit from the high capacity of silicon. In addition to the manufacturing of the electrodes, the cell assembly process is also a crucial step that can greatly influence the performance and quality of the battery. After calendering, the electrode surface undergoes a morphological change and its relationship to battery performance is not yet fully investigated, especially for silicon-based anodes. In this work, the influence of calendering and lamination of anodic electrodes with high silicon content is studied.

For the manufacturing of the silicon-based anodes, a recipe was developed using silicon-carbon core-shell nanoparticles as the active material. The final silicon content of the electrode was 30% by mass. As conductive additive, the nano-graphene in platelet form were applied. In addition, a mixture of sodium carboxymethyl cellulose and a new fluoroplastic water-dispersion was used as aqueous binder. Our project partner provided the cathodes with high area loading (4.7 mAh cm^-2) and carried out the calendering process for the silicon-based anodes. There were two types of cell assembling in pouch format. The single cells were applied for the validation of calendering and bi-cells were used for the verifying of lamination. The lamination process was performed using our house standard parameters.

Scanning Emission Microscopy (SEM) carried out for the surface characterisation of the anodes. The properties of adhesion and cohesion were tested in 90° peel-tests. In addition, the porosity of the anodes was determined by the mercury porosimetry. Finally, electrochemical characterisation was performed on a battery measurement system (BaSyTec).

A double site coating of Si@C-anodes with an area loading above 5 mAh cm^-2 was achieved. The thickness of the electrodes without calendering was between 163 µm and 208 µm in consider of the structure of graphene nanoplatelets. After calendering, the thickness was reduced to 122 µm in respect of the assembling.

The calendering process on Si@C-anodes reduced the discharge capacity and the coulomb efficiency was 4% lower in the first formation cycle. However, the single cell after calendering showed 10% better cycle stability after 100 cycles at 0.5C.

The laminated cells indicated higher discharge capacity at low c-rate and the first coulomb efficiency is 10% higher comparing to the non-laminated ones. The cycle stability of laminated cells were 5% higher than non-laminated ones after 100 cycles at 0.5C.

The results of our research demonstrate the clear message, that lamination with the procedure now performed an electrochemical improvement on cells using silicon-based anodes and Ni-rich NMC cathodes in pouch format. The calendering process on the silicon-based anodes provided a smooth electrode surface, increased the adhesion ability, reduced the porosity and improved the cycle stability. However, the capacity of the anodes were lightly reduced, possibly due to the trapping of Si-particles during calendering process. The further work will focus on parameter optimization of calendering and lamination processes, and an optimized combination of both processes.