Journal
ACS APPLIED MATERIALS & INTERFACES
Volume 8, Issue 24, Pages 15422-15429Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsami.6b04014
Keywords
olivine NaFePO4; polythiophene; Na-ion batteries; X-ray absorption spectroscopy; Fe valence
Funding
- R&D Convergence Program of the National Research Council of Science & Technology (NST) of the Republic of Korea
- KIST Institutional Program [2E26330]
- Dongguk University
- Ministry of Science, ICT & Future Planning, Republic of Korea [2E26330] Funding Source: Korea Institute of Science & Technology Information (KISTI), National Science & Technology Information Service (NTIS)
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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 angstrom, b = 6.22821 angstrom, and c = 4.94971 angstrom. 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.
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