How carboxymethylcellulose adsorption and porous active material particles diminish the adhesion of graphite–silicon anodes in lithium-ion batteries

Due to its adsorption on graphite and its superior thickening properties, carboxymethylcellulose (CMC) is widely used as a dispersant and rheology modifier in water-based anode slurries for lithium-ion batteries. CMC also provides cohesion to the dry anode layer but exhibits poor adhesion to the copper foil necessitating the addition of styrene-butadiene rubber (SBR) as an adhesion promoter. High adhesion between the electrode layer and the current collector is crucial in electrode fabrication, especially for electrodes with higher mass loadings. In this work, we investigate how a polymeric binder, originally intended as a thickening and dispersing agent, can significantly affect the adhesive strength between the anode layer and the current collector. Our results reveal that CMC, by adsorbing onto active material particles (graphite, micro-silicon or nano-silicon), indirectly influences the anode adhesion. The adsorbed CMC layer hinders the direct binding of SBR to the active material particles, thereby creating the weakest link between the active layer and the current collector. This effect is more pronounced the higher the CMC molecular weight. Moreover, we could show that graphite–nano-silicon composite anodes exhibit a significantly reduced adhesion to the copper foil despite a low adsorption of CMC on nano-silicon, since a large fraction of SBR particles are trapped in the porous, micron-sized nano-silicon aggregates. Our findings highlight the importance of considering thickener adsorption on active material particles within electrode design, as a factor that exerts an indirect, albeit significant, influence on anode adhesion.