Ion-Conducting Cross-Linked Polyphosphazene Binders for High-Performance Silicon Anodes in Lithium-Ion Batteries

A cross-linked polyphosphazene-based polymeric material was prepared using NH3-neutralized poly[bis(4-carboxyphenoxy)phosphazene] (PCPP-NH3) as the base polymer and poly(ethylene glycol)bisamine (PEG-NH3) as a cross-linker for use as a silicon (Si) anode binder in lithium-ion batteries. PCPP was prepared by two-step polymer modification reactions from poly(dichlorophosphazene). When an optimum amount of PEG-NH3 (7.5 wt % of PCPP-NH3) was used as the cross-linker, the resulting cross-linked polyphosphazene exhibited the highest glass-transition temperature, elastic modulus, and peeling force values. The battery performance of the cells with the Si anode fabricated with this cross-linked polyphosphazene-based binder was found to be better than that of cells with Si anodes fabricated with the poly(acrylic acid) binder. The better performance of the cell with the polyphosphazene-based binder was ascribed to the composition and structure of the binder material: the flexible polyphosphazene backbone, the carboxylic acid group with hydrogen bonding ability, and the cross-linked network structure maintained the electrode structure during charge/discharge cycling of the cell. Furthermore, the ion-conducting PEG moieties increased the ion-transfer conductivity inside the Si anode.