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

Abstract

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.

Conductive Polymer Frameworks in Silicon Anodes for Advanced Lithium-Ion Batteries

“…Although graphite has high electrical conductivity (3 × 10 5 S m –1 ) and low synthesis cost, every six carbon (C) atoms in graphite can hold only one lithium (Li) atom, resulting in a low average theoretical specific capacity of 372 mAh g –1 . , Furthermore, a graphite anode is deficient in rate capability because it suffers from slow lithiation kinetics at high currents . As alternatives, anodes made of inorganic semiconductor materials [e.g., germanium and silicon (Si)] have been explored due to their high specific capacity. , Among those materials, Si is considered to be a promising option because it can generate a high theoretical capacity of up to 4200 mAh g –1 when lithiated to Li 4.4 Si, which is attributed to the ability of one Si atom to attract four Li ions. − Moreover, the discharge potential of Si (∼0.2 V vs Li/Li + ) is lower than that of most alloys and metal oxides, leading to high energy density . The favorability of Si is strengthened by its environmental friendliness, abundance on the earth’s crust, and nontoxicity …”