Carbon-coated Li4Ti5O12 nanowires showing high rate capability as an anode material for rechargeable sodium batteries

Abstract

This is the first report where crystalline carbon-coated Li4Ti5O12 nanowires are employed as an anode material for sodium-ion batteries. The Li4Ti5O12 nanowires are synthesized via a two-step ionic exchange process from Na2Ti3O7 nanowires to form hydrous lithium titanate nanowires, where excessive lithium oxide is adhered on the surface of the nanowires. The nanowire products are consequently heated to form Li4Ti5O12, and the resultant nanowires are subsequently coated by pitch as the carbon source. X-ray diffraction (XRD) and electron microscopic studies reveal that the carboncoated Li4Ti5O12 nanowires are highly crystalline products and that their nanowire features have been modified with carbon nanolayers (<10 nm in thickness). As a result, the electronic conductivity is approximately 3 x 10(-1) S c(-1). The delivered capacities are about 168 mAh g(-1) at a rate of 0.2 C (35 mA g(-1)), 117 mAh g(-1) at a rate of 10 C, 88 mAh g(-1) at a rate of 30 C, 67 mAh g(-1) at a rate of 50 C, and 38 mAh g(-1) at a rate of 100 C; these conductivity values are superior to those achieved with bare Li4Ti5O12. Continuous cycling testing reveals outstanding cycling stability, showing 96.3% capacity retention after cycles. Ex-situ XRD and X-ray photoelectron spectroscopic studies indicate that the electrode reaction is followed by Na+ insertion and extraction, accompanied by the Ti4+/3+ redox couple. We believe that the excellent high rate capacity and rechargeability upon cycling result from the unique morphology of the highly crystalline Li4Ti5O12 nanowires assisted by conducting thin carbon layers.