Investigation of sodium insertion–extraction in olivine Na_xFePO₄ (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 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 Fe(2+)/Fe(3+) alignment. HRXRD data for NaxFePO4 (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.

Rate dependence of the reaction mechanism in olivine NaFePO4 Na-ion cathode material

“…From DFT calculations, it has been shown that the charge mechanism observed for all rates (a single‐phase domain until Na 2/3 FePO 4 followed by a 2‐phase reaction between Na 2/3‐β FePO 4 and Na 0+δ FePO 4 ) corresponds to the thermodynamic equilibrium path. Therefore, in the 3‐phase region observed upon discharge, the system is driven away from equilibrium . This is further confirmed by the evolution of the XRD pattern as a function of the relaxation time of a partially discharged electrode ( x ≈ 0.25, C/20), presented in Figure .…”

“…In both cases, varying miscibility limits were observed during the 2‐phase regions, allowing the system to greatly reduce the volume mismatch between phases with different Na content . Theoretical calculations confirmed that the phase transformation sequence followed upon charge corresponds to the thermodynamic equilibrium path . This suggests that the path followed upon discharge at low rate departs from the thermodynamic equilibrium, while in the case of LiFePO 4 the non‐equilibrium transformation path is only observed at high rates.…”

Phosphate Framework Electrode Materials for Sodium Ion Batteries

“…The Na insertion and deinsertion occur through different mechanisms due to the huge volumetric mismatch between FePO 4 and NaFePO 4 (17.58% difference in unit volume) 39. A Na 5/6 FePO 4 intermediate phase was also revealed by DFT calculations and high resolution synchrotron X‐ray diffraction 40. Yamada's group determined the composition‐temperature phase diagram of the FePO 4 /NaFePO 4 system and found that Na x FePO 4 (0 < x < 2/3) appeared in a two‐phase region, while Na x FePO 4 (2/3 < x < 1) is in solid‐solution region 41.…”

Effects of Mn‐Doping on the Structural and Electrochemical Properties of Na3Ni2SbO6 for Sodium‐Ion Battery.

“…Our preliminary searches for possible inequivalence among independent atomic positions automatically generated from P1 symmetry resulted in negligible differences in the range of standard convergence criteria of the electronic relaxation cycle (10 −5 eV/atom); therefore, the net formation energies and changes in unit cell parameters calculated in this study are assumed to be comparable to each other due to the crystallographically equivalent positions of Ni and Na in the C2/m symmetry of Na 3 Ni 2 SbO 6 . The formation energy ( E f ) of an intermediate phase Na y Ni 2 SbO 6 with respect to the phase separation to the portion of the two end members, O′3‐Na a Ni 2 SbO 6 and P′3‐Na b Ni 2 SbO 6 , could be described as follows: …”