Conjugated Imine Polymer Synthesized via Step-Growth Metathesis for Highly Stable Silicon Nanoparticle Anodes in Lithium-Ion Batteries

We have developed a new method to synthesize polyphenylmethanimine (polyPMI) as a linear or a hyperbranched, conjugated polymer using an aldehyde-imine metathesis reaction. We detail the reaction mechanisms of this polymerization by characterizing a red-shift in its absorption spectrum as polymer conjugation length increases and verify that this optical shift results from extended p-condensation using density functional theory. This new synthetic approach provides a polymer that can potentially be depolymerized for facile recyclability and is compatible with air- and water-sensitive chemistries. As an example of the utility of this new approach, we demonstrate that this polymer can be directly grown on silicon nanoparticles to create silicon anodes for lithium-ion batteries with a high degree of electrochemical interfacial passivation. These silicon anodes exhibit Coulombic efficiencies above 99.9% and can accommodate silicon nanoparticle expansion and contraction during lithiation and delithiation as demonstrated by stable reversible capacities for 500 cycles. Finally, we demonstrate that polyPMI facilitates the formation of a lithium fluoride rich solid electrolyte interphase that remains chemically and mechanically stable after long term cycling.