Investigation of sodium insertion–extraction in olivine NaxFePO4 (0 ≤ x ≤ 1) using first-principles calculations

Abstract

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 Na_xFePO₄ 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. Na_2/3FePO₄ and Na_5/6FePO₄ ground states both align vacancies diagonally within the ab plane, coupled to a Fe²⁺/Fe³⁺ alignment. HRXRD data for Na_xFePO₄ (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 Na_2/3FePO₄ in agreement with the voltage discontinuity observed experimentally during electrochemical insertion.