Cross-Linked Polyphosphazene Binders for Silicon Anodes

Silicon is an attractive anode material for next-generation lithium-ion batteries because of its high theoretical capacity, but its large volume change during repeated lithiation and delithiation causes particle pulverization, unstable solid-electrolyte interphase formation, and rapid electrode failure. Herein, an ion-conducting cross-linked polyphosphazene binder was designed to simultaneously improve the mechanical integrity and interfacial stability of silicon anodes. Carboxyl-functionalized polyphosphazene was converted into a water-processable ammonium salt and subsequently cross-linked with a PEG-based diamine to form PCG-X binders with different cross-linking densities. The polyphosphazene backbone and carboxyl groups provide strong adhesion and hydrogen-bonding interactions with silicon particles, while the flexible PEG segments construct ion-transport pathways and accommodate dynamic volume changes. Among the prepared binders, PCG-7.5 exhibited the most balanced properties, including a high elastic modulus, strong adhesion, and an ionic conductivity of approximately 10⁻³ S cm⁻¹, which was higher than that of conventional poly(acrylic acid) (PAA). Silicon electrodes containing PCG-7.5 showed improved cycling stability and rate capability compared with PAA-based electrodes. Cross-sectional SEM images further revealed that the electrode thickness increased only from 17.2 to 22.5 μm after cycling, whereas the PAA electrode expanded from 21.2 to 36.5 μm, demonstrating effective suppression of electrode swelling and structural degradation. XPS analysis also indicated a distinct and more controlled interfacial chemistry for the PCG-7.5 electrode. These results demonstrate that the synergistic combination of mechanical reinforcement, strong interfacial bonding, and lithium-ion conduction in the cross-linked polyphosphazene network offers an effective binder strategy for durable, high-performance silicon anodes.