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.