Characterization of the calendering process for compaction of electrodes for lithium-ion batteries

Calendering is the common compaction process for lithium-ion battery electrodes and has a substantial impact on the pore structure and therefore the electrochemical performance of Lithium-ion battery cells. For a targeted determination of the performance-optimized pore structure, it is of decisive importance to be able to comprehensively control the compaction process. Thus, during continuous calendering of graphite anodes and lithium nickel cobalt manganese oxide (NCM) cathodes the applied line load is tracked and varied at different speeds to compact the electrode to several coating densities. The generated pore structures are measured via mercury intrusion, resulting in quite similar porosities, while the densities of graphite and NCM diverge greatly. The porosities, measured via mercury intrusion, are compared to geometrical determined data, identifying compaction resistant closed pores in the NCM. On the basis of the measured porosity reduction the compaction behavior of the line load was found to be describable by an exponential model equation. The model parameters quantify the different compaction resistances of the cathodes and anodes in converging to a maximal density, respectively minimal porosity. This substantial difference could be traced to the closer packing of the spherical and harder NCM particles. The calendering speed showed only an insignificant impact up to speeds of 5 m/min.