Nanostructured Cathode Materials by Wet Chemical Processing for Efficient Lithium Ion Batteries

This research has been focused on the development of solution-based methods to fabricate cathode materials with well controlled nanostructures and significantly enhanced Li+ intercalation properties for Li-ion batteries. The synthesis methods include sol-gel processing, electrodeposition and hydrothermal growth, and the materials studied are either vanadium pentoxide or lithium iron phosphate. Various nanostructured cathode films of different processing methods and morphologies have been studied: (1) The nanostructured V 2O5 thin film cathodes were prepared via electrodeposition from vanadium precursor on fluorine-doped tin oxide coated glasses followed by annealing at 500 °C in the air. The unique highly folded nanostructure of the V2O5 film cathodes showed high discharge capacity of 240 mAh/g at discharge rate of 200 mA/g for 200 cycles, with high energy density of 900 Wh/kg and power density of over 28 kW/kg. (2) To increase the thickness of the electrodeposited V2O5 film cathodes without losing high electrochemical performance, the porous nanostructured V2O5 film cathodes were successfully fabricated after introducing surfactant to the deposition precursor. They maintained a high specific energy density of 755 Wh/kg and a power density of 25 kW/kg at increased film thickness, which demonstrated much enhanced charge transfer kinetics and Li+ diffusion process in the porous nanostructure. Moreover the free-standing V2O5 films were successfully prepared and they can be directly assembled into coin cell batteries as cathodes without adding any conductive additives or binders. (3) Moreover the vanadium dioxide VO2 (B) nanorods and vanadium pentoxide V2O5 nanodiscs were fabricated by hydrothermal process and thermal treatment in air. The V2O5 nanodiscs exhibited high discharge capacity of 281 mAh/g at 50 mA/g, and the energy density and power density were calculated to be 790 Wh/kg when discharged at 50 mA/g and 8.9 kW/kg at 2.4 A/g. (4) The uniform and crack-free LiFePO4/carbon nanocomposite films were readily obtained by spreading sol on Pt-coated Si wafer followed by thermal treatment in nitrogen, and they showed high Li+ intercalation capacity of over 200 mAh/g at a current density of 200 mA/ for over 20 cycles. The enhanced Li+ intercalation capacity and energy density, improved rate capability, high power density and better cyclic stability observed in the nanostructured V2O5 film cathodes and LiFePO 4/carbon nanocomposite film could be ascribed to the well-constructed and tuned micro- or nano-structure, controlled materials crystallinity and introduced desired defects on the surface and/or in the bulk. The fabrication of binder-less and additive-free nanostructured film electrodes via sol-gel processing was also highlighted. The nanostructured LiFePO4/carbon nanocomposite film and V2O5 film cathodes from this work have been either tested with three-electrode set-up, or assembled in coin cells that are typically used in the labs; however such cathode films are readily scale-up and applicable for the fabrication of commercial Li-ion batteries with a little device fabrication efforts.