Understanding the Solution Dynamics and Binding of a PVDF Binder with Silicon, Graphite, and NMC Materials and the Influence on Cycling Performance.

The impact of the binding, solution structure, and solution dynamics of poly(vinylidene fluoride) (PVDF) with silicon on its performance as compared to traditional graphite and LiNiMnCoO (NMC) electrode materials was explored. Through refractive index (RI) measurements, the concentration of the binder adsorbed on the surface of electrode materials during electrode processing was determined to be less than half of the potentially available material resulting in excessive free binder in solution. Using ultrasmall-angle neutron scattering (USANS) and small-angle neutron scattering (SANS), it was found that PVDF forms a conformal coating over the entirety of the silicon particle. This is in direct contrast to graphite-PVDF and NMC-PVDF slurries, where PVDF only covers part of the graphite surface, and the PVDF chains make a network-like graphite-PVDF structure. Conversely, a thick layer of PVDF covers NMC particles, but the coating is porous, allowing for ion and electronic transport. The homogeneous coating of silicon breaks up percolation pathways, resulting in poor cycling performance of silicon materials as widely reported. These results indicate that the Si-PVDF interactions could be modified from a binder to a dispersant.