Li-Ion Electrode Microstructure Evolution during Drying and Calendering

Abstract

The complicated microstructure of a Li-ion electrode is the result of intrinsic material properties and fabrication steps. This work presents experiments and simulations that illuminate microstructure evolution during the drying and calendering fabrication steps and highlights important factors during manufacture of a high-power electrode.Li-ion material interactions include self-interactions and interactions with other materials. For example, smaller active particles can have higher attraction, which makes them more susceptible to form agglomerates. Therefore, manufacturing processes are modified to account for such material interactions to make coatings that are uniform and minimize surface defects and potential delaminations. During mixing and coating operations, deficiencies are more visually apparent, whereas defects generated during the processes of drying and calendering may not be as obvious.Different and sometimes opposing theories attempt to explain microstructure changes during the drying step. A well-known theory is binder migration, proposing that movement of the solvent during evaporation redistributes the carbon and binder. In this work, binder migration and other theories are investigated quantitatively and compared to new experimental and simulation results.The spatially-resolved ionic and electronic conductivity properties of an electrode containing nickel manganese cobalt (NMC) active material were experimentally determined. In simulation, a multi-phase smoothed particle model developed within LAMMPS was used to simulate all of the mixing, coating, drying, and calendering processing steps.Results show that increased drying temperature can have negative effects such as decreased ionic (Figure 1) and electronic conductivities for two NMC cathodes with identical compositions that are mixed and coated through the same processes but dried at two different temperatures.The calendering process can also change the microstructure in unexpected ways. Our work shows that the pressure of calendering causes redistribution of the soft and small carbon/binder domains. This non-uniformity, in turn, changes the overall transport properties of the Li-ion electrode.Figure 1. Ionic conductivity maps of two uncalendered NMC cathodes reflecting the difference between the samples dried at (a) 20 °C and (b) 150 °C. Figure 1