PEG-coated Fe3O4 for li-Ion battery anodes: Effects of crystallite size and surface chemistry

Abstract

Battery electrodes are complex mesoscale systems comprised of an active material, conductive agent, current collector, and polymeric binder. Previous work focused on enhancing electron and ion transport in high capacity anode systems by introducing poly[3-(potassium-4- butanoate) thiophene] (PPBT) as a binder component and a polyethylene glycol (PEG) surface coating on magnetite (Fe3O4) nanoparticles. The PPBT/PEG system will be utilized in this work, which takes a closer look at the active material, Fe3O4, and examines the effects of surface chemistry and crystallite size (10 nm vs. 20 nm) on battery performance. Variations in surface chemistry are due to the synthesis methods used for Fe3O4, which use ammonium hydroxide or triethylamine as a base. XRD and TEM initially characterized the active materials to confirm the magnetite phase and crystallite size. DLS and zeta potential measurements demonstrated aggregate size and colloidal stability. SEM images of the electrodes, which are composed of Fe3O4 particles, carbon additives, and the PPBT binder, indicate that the bases produce different morphologies. The Fe3O4 particles synthesized with ammonium hydroxide appear more dispersed relative to those made with triethylamine, which could have a significant impact on the battery performance. Furthermore, XPS and FTIR data indicate that these bases produce difference chemical interactions within the electrode. Electrochemical testing demonstrates that the triethylamine-based electrode has a higher capacity and better capacity retention over 100 cycles at 0.3C as compared to the ammonium hydroxide-based electrode. With regards to differences in active material size, the electrodes with 20 nm crystallite size Fe3O4 initially have a higher capacity, but the electrodes with 10 nm crystallite size Fe3O4 have better capacity retention over 100 cycles at 0.3C. Rate capability testing and electrical impedance confirm the superior performance of triethylamine derived electrodes and the 10 nm crystallite size.