Electrochemical Performance of Lithium-Ion Batteries As a Function of Linked Influences Originating from the Dispersing and Calendering Step
Authors: Dreger Henning , Froböse Linus , Haselrieder Wolfgang , Kwade Arno 
Published: February 2020

Lithium ion batteries for PHEV`s, HEV`s and EV`s have to meet different requirements like high energy density, high power density and always low production costs at high durability. As wide-spread standard, electrodes for lithium ion batteries are produced by mixing and dispersing active materials with conductive additives, binder and solvent. In a subsequent step, the coated electrodes are calendered to homogenize the electrode thickness. These two process steps strongly influence the electrode’s electrochemical performance in the battery cell. Thus, high quality mixing, deagglomeration and dispersing of all solid components in the solvent and a high-precision calendering has to be ensured to produce high performance electrodes.

One important aspect in the dispersing process is the deagglomeration of carbon black. Modifying the agglomerate size and shape of carbon black has a strong influence on the electrochemical performance due to structural changes of electrode properties like electrode porosity and conductivity. On the other hand, these structural changes also occur in the calendering step, where electrodes are compressed and electrode thickness and density are adjusted. For this reason, our work focuses on experiments regarding the changes in electrochemical performance due to different deagglomeration degrees of carbon black and subsequent calendering process step.

The electrodes were processed with a continuously operated twin-screw extruder. Suspensions with differently deagglomerated carbon black were produced by varying the tensile stress acting inside the suspension and the residence time through changes in the tip speed of extruder screws and volumetric flow rate of solids and solvent at constant solids content in the suspension.

In order to examine the influence of carbon black on the electrochemical performance, the agglomerate size of different suspensions was identified via laser diffraction analysis using a Horiba LA-960 Laser Particle Size Analyzer with two light sources, a laser diode with 650 nm and a light emitting diode (LED) with 405 nm wavelength. The analysis displayed a strong bimodal distribution with a shifting peak in the single-digit µm-range representing carbon black agglomerates and a stationary peak at a few hundred nanometers representing smaller carbon black aggregates.

A pilot-scale coater with a combined reverse-roll comma bar system and a dryer length of 6 meters was used to produce electrodes continuously out of selected suspensions. Afterwards, the electrodes were compressed with a pilot-scale calender. In this step, a density of 2.8 g/cm³ on cathode side and 1.5 g/cm³ on anode side was set as standard. Structural properties of differently produced electrodes were analyzed using mercury porosimetry and electrode conductivity analysis. Mercury porosimetry was used to visualize changes in the pore structure of electrodes, especially in a range of 0.1 to 4 µm pore diameter. In this pore size range the varying agglomerate sizes and the calendaring step have a great impact on the electrode´s pore structure.

Changing the size of carbon black agglomerates also significantly influences the properties of the carbon black conductive network which results in a modified electrode conductivity: A finer distribution and better deagglomeration of carbon black lead to higher electrode conductivity. This is beneficial to some degree for the electrochemical performance at higher c-rates. On the other hand, the calendering process also changes the electrodes structure by modifying the electrodes thickness, porosity and, in conclusion, the electrodes density. Of greater significance is the fact, that the calendering step leads to a larger contact surface between active material particles, carbon black and current collector. Within certain limits, this results in a better electrochemical performance. Linking these influences of different process steps reveals possible interactions which lead to an optimized process control.

The effect of structural changes on the electrochemical performance of electrodes was examined performing c-rate-tests up to 5 C and long-term cycling at 1C in full cells. The influence of changes in electrode properties became apparent especially at higher c-rates, where limiting effects in ionic and electrode conductivity, influenced by electrode porosity and conductivity, become more important. Good electrochemical performance only is attainable with suitable electrode porosity and well-developed conductivity network of carbon black.

Summing up all experimental results we are able to relate process parameters and structure properties of the electrodes and link these structural changes with the resulting electrochemical performance. Significant differences in electrochemical performance were traceable to structural changes in the electrodes, which necessitate distinct process know-how in the different process steps. This process-structure-property-relationship enables a deeper understanding for the continuously operating industrial processes of extrusion and calendering leading to an optimized production of electrodes for lithium ion batteries.

Figure 1