Effect of carboxymethyl cellulose on the flow behavior of lithium-ion battery anode slurries and the electrical as well as mechanical properties of corresponding dry layers

Abstract

We present a holistic view on the role of polymeric binders in waterborne LiB anodes, including preparation and processing of wet slurries as well as microstructure, electrical conductivity and mechanical integrity of dry electrode layers. We focus on carboxymethyl cellulose (CMC), with respect to technical application the influence of soft, nano-particulate styrene–butadiene rubber (SBR) as secondary binder is also addressed. We discuss the influence of CMC concentration, molecular weight (Mw) and degree of substitution (DS) on flow behavior of anode slurries. Rheological data are not only relevant for processing, here we use them to characterize the adsorption of CMC on active material particles and dispersion of these particles in the slurry at technically relevant concentrations. The fraction of CMC adsorbed onto graphite particles increases with increasing Mw and decreasing DS. Electrical conductivity increases with Mw, i.e. with decreasing free polymer deteriorating conductive carbon black pathways. CMC does not contribute to the adhesion of electrode layers, irrespective of Mw or DS, technically feasible adhesion is inferred by SBR. Cohesive strength of anode layers, determined here for the first time under well-defined mechanical load, increases with increasing Mw and decreasing DS, i.e. with increasing fraction of adsorbed CMC and corresponding improved particle dispersion. Strong cohesion and high electrical conductivity are correlated to an alignment of graphite particles as revealed by electron microscopy, presumably enabled by higher particle mobility in well-dispersed slurries. Accordingly, targeted choice of CMC is a valuable means to control processing, electrical conductivity and mechanical strength of LiB electrodes.