Quantifying the Influence of Carbon Black De-Agglomeration on the Electrochemical Performance of Lithium-Ion Battery

Abstract

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. This process step strongly influences the electrode’s electrochemical performance in the subsequently produced battery cell. Thus, high quality mixing, de-agglomeration and dispersing of all solid components in the solvent has to be ensured to produce high performance electrodes. One important aspect in the dispersing process is the de-agglomeration of carbon black. Modifying the agglomerate size and shape of carbon black has a strong influence on electrochemical performance due to structural changes of electrode properties like electrode porosity and conductivity. For this reason, our work focuses on experiments regarding the changes in electrochemical performance due to different de-agglomeration degrees of carbon black processed with a continuously operated twin-screw extruder, suitable for large scale production of suspensions with NCM and artificial graphite as active material. Suspensions with differently de-agglomerated 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 (N-methyl-pyrrolidone) at constant solids content in the suspension. In order to examine the influence of the carbon black agglomerate sizes on electrochemical performance, a specific preparation of the suspension samples became necessary: The carbon black is separated from coarser active material particles via centrifugation and stabilized with a suitable stabilizer, which was used to preserve the sizes of the carbon black agglomerates and to prevent reagglomeration in the diluted suspension required for particle size analysis. In the next step, 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, structural properties of electrodes produced out of differently stressed suspensions 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 have a great impact on the electrode´s pore structure due to carbon black agglomerates acting as spacers between active material particles. Changing the size of carbon black agglomerates also significantly influences the properties of the carbon black conductive network resulting in modified electrode conductivity: A finer distribution and better de-agglomeration of carbon black lead to higher electrode conductivity which is beneficial to some degree for the electrochemical performance at higher c-rates, especially regarding the cathode. The effect of structural changes on the electrochemical performance of electrodes was examined performing c-rate-tests up to 5 C in half and full cells. The influence of these 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 dispersing step. This process-structure-property-relationship leads to a deeper understanding for the continuously operating industrial process of extrusion and for the optimized production of electrodes for lithium ion batteries. Figure 1