(263f) The Connection between Slurry Rheology and Electrochemical Performance of Graphite Anodes in Lithium-Ion Batteries

Most current lithium-ion battery anodes are prepared from a slurry containing graphite, small amounts of conductive carbon black (CB), polyvinylidene fluoride as binder, and n-methyl-2-pyrrolidone as solvent. The rheology of the four-component slurry is an important indicator of the underlying microstructure, that affects the thickness of the coated layer as well as the structure of the dried electrode. Electrode structure, in turn, has consequences on the electrochemical performance of the anode. Early indicators of battery success or failure are highly valuable for reducing R&D costs and testing time. Here, we correlate slurry rheology to anode electrochemical performance through a rheological procedure congruent to the coating process to the properties of electrodes coated under the same conditions before and after cycling.

We modulate the rheology of the slurry by choosing three different commercially available CBs that are used in lithium-ion batteries. Small amplitude oscillatory shear was applied to the slurries before and after an applied coating shear to measure changes in storage modulus (G’). Graphite electrodes were coated at the same coating shear rate in a laboratory-scale battery manufacturing process, built into CR2032 coin cells with a lithium counter electrode, and cycled ten times. Thickness and surface resistance were measured for the graphite anodes before and after cycling.

We show that the storage modulus at coating is a predictor of electrochemical performance. In graphite electrodes, viscoelasticity is both imparted by and dependent upon CB. An increase in G’ during coating shear correlates to an increase in CB percolation, lower electrode resistance, and higher discharge capacity. A considerable decrease in slurry G’ during coating indicates a breakdown of the CB network, producing electrodes of higher resistance, with poorer electrochemical performance. Rheological modeling of graphite slurries under expected coating conditions is therefore of considerable importance for battery research and development. Novel carbon black conductive additives must also have a favorable slurry storage modulus under a range of coating shear rates to provide maximum enhancement to electrochemical performance.