Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 18, Issue 18, Pages 13045-13051Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c6cp00762g
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Funding
- Ministerio de Economia y Competitividad (MINECO) of the Spanish Government through Proyecto I+D Retos program [ENE2013-44330-R]
- MINECO through a Ramon y Cajal fellowship
- Marie Curie Career Integration Grant FP7-PEOPLE-CIG: Project NanoWGS
- Royal Society through the Newton Alumnus scheme
- MINECO [FPDI-2013-17329]
- U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-AC02-06CH11357]
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Olivine NaFePO4 has recently attracted the attention of the scientific community as a promising cathode material for Na-ion batteries. In this work we combine density functional theory (DFT) calculations and high resolution synchrotron X-ray diffraction (HRXRD) experiments to study the phase stability of NaxFePO4 along the whole range of sodium compositions (0 <= x <= 1). DFT calculations reveal the existence of two intermediate structures governing the phase stability at x = 2/3 and x = 5/6. This is in contrast to isostructural LiFePO4, which is a broadly used cathode in Li-ion batteries. Na2/3FePO4 and Na5/6FePO4 ground states both align vacancies diagonally within the ab plane, coupled to a Fe2+/Fe3+ alignment. HRXRD data for NaxFePO(4) (2/3 < x < 1) materials show common superstructure reflections up to x = 5/6 within the studied compositions. The computed intercalation voltage profile shows a voltage difference of 0.16 V between NaFePO4 and Na2/3FePO4 in agreement with the voltage discontinuity observed experimentally during electrochemical insertion.
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