Micro Vs. Nano: Impact of Particle Size on the Processing of Si Anode Slurries

Abstract

In the quest for ever greater storage capacity to satisfy the increasing demand for high-energy-density storage in electric vehicles and large-scale grid storage, anodes comprising Si have the promise of significantly increasing the energy density of Li-ion batteries [1]. Either as a stand-alone active material or as a minor component in a more traditional graphite matrix, Si can through alloying incorporate far more Li than can be intercalated in graphite [1]. This, however, comes at significant volume expansion of the Si particles, resulting in cracking and disintegrating during repeated cycling [2]. This can be counteracted by using nm-sized Si particles which minimizes the impact of severe volume changes during cycling [2]. We have been studying the processing of environmentally benign water-based slurries for Si-containing anodes and note that the Si particle size not only affects the cycling performance, but also significantly affect the processing characteristics. Notably, the rheological properties, i.e., the shear stress and apparent viscosity at the shear rate of electrode coating and the viscoelastic properties, are all determined by the Si particles – even when these are only present as a minor component in a graphite matrix. Whereas µm-sized Si particles result in low-viscosity slurries that are characterized by liquid-like behavior, the slurries containing nm-sized Si particles show both higher viscosity and notable viscoelasticity. These differences are most pronounced at low shear rates, which in the context of electrode processing will affect how the slurry will either flow under its own weight or retain its overall geometry between application and drying. These vastly differing properties appear to be related to the surface area of the Si particles within the slurry, where the polar surface groups of the native oxide on the particles interact strongly with the surrounding water, leading to the observed differences in rheological behavior. Thackeray MM, Wolverton C, Isaacs ED. Energy Environ. Sci. 2012; 5: 7854Casimir A, Zhang H, Ogoke O, et al. Nano Energy 2016; 27: 359–376