Enabling Si-Dominant Anodes with Focus on Binder

Abstract

Since current chemistries in lithium ion batteries (LIBs) including graphite anode reached their energy density limits, a promising candidate on anode side is the abundantly available silicon with its enormous capacity, but showing several failure mechanisms based on its extensive swelling (>300%). This leads to excessive solid electrolyte interphase (SEI) formation and electrode delamination during cycling, which can be reduced by the modification of the active material itself, e.g. carbon-embedded silicon (Si/C), but still these materials show the same failure mechanisms in a less pronounced manner. Regarding the costs and availability, the next generation anode is desired to be based on metallurgical silicon, where one promising path to enable Si-dominant anodes is the design of advanced binder systems to maintain a stable network around the active material during cycling. However, in most studies new binders are mainly tested in electrodes with low active material content and low loadings while lacking of full cell data, which makes it impossible to estimate the capability and influence of the binder to mitigate the failure mechanisms in full cell operation.In this work, several modifications of the binder are combined with commercial Si/C material and bare micro-sized silicon with 100% silicon as active material. The electrodes are tested in half coin cells and small full pouch cells with ~60 mAh capacity. To examine the performance in detail and understand the underlying failure mechanisms, post-mortem analysis was conducted.The already commonly used polyacrylic acid (PAA) can be further adapted to the respective anode active material (AAM) via partial neutralization with e.g. LiOH to tune the interaction with the AAM and reduce the first cycle losses due to the reaction of the carboxylic groups with lithium. Nevertheless, the influence of PAA/LiPAA on the failure mechanisms and maintenance of the electrode, especially on full cell basis, are not yet fully understood. In this work, the effect of the partial neutralization on the full cell performance of a NMC811-Si/C cell is shown with the anode consisting of 91.5% Si/C material, 8% binder and 0.5% conductive additive (single-wall carbon nanotubes SWCNTs). The full cells show 3.5% higher FCE with 70% neutralized LiPAA in comparison to acidic PAA in the anode, while maintaining the same cycle life.