Polythiophene-Wrapped Olivine NaFePO4 as a Cathode for Na-Ion Batteries

The surface of olivine NaFePO4 was modified with polythiophene (PTh) to develop a high-performance cathode material for use in Na-ion batteries. The Rietveld refinement results of the prepared material reveal that PTh-coated NaFePO4 belongs to a space group of Pnma with lattice parameters of a = 10.40656 Å, b = 6.22821 Å, and c = 4.94971 Å. Uncoated NaFePO4 delivers a discharge capacity of 108 mAh g–1 at a current density of 10 mA g–1 within a voltage range of 2.2–4.0 V. Conversely, the PTh-coated NaFePO4 electrode exhibits significantly improved electrochemical performance, where it exhibits a discharge capacity of 142 mAh g–1 and a stable cycle life over 100 cycles, with a capacity retention of 94%. The NaFePO4/PTh electrode also exhibits satisfactory performance at high current densities, and reversible capacities of 70 mAh g–1 at 150 mA g–1 and 42 mAh g–1 at 300 mA g–1 are obtained compared with negligible capacities without coating. The related electrochemical reaction mechanism has been investigated using in situ X-ray absorption spectroscopy (XAS), which revealed a systematic change of Fe valence and reversible contraction/expansion of Fe–O octahedra upon desodiation/sodiation. The ex situ X-ray diffraction (XRD) results suggest that the deintercalation in NaFePO4/PTh electrodes proceeds through a stable intermediate phase and the lattice parameters show a reversible contraction/expansion of unit cell during cycling.