O3-type Na[Fe1/3Ni1/3Ti1/3]O2 cathode material for rechargeable sodium ion batteries

Abstract

A Na[Fe_1/3Ni_1/3Ti_1/3]O₂ cathode material for sodium-ion batteries has been synthesized by a solid-state reaction method. The obtained Na[Fe_1/3Ni_1/3Ti_1/3]O₂ shows an O3-type structure, and delivers a discharge capacity of 117 mA h g⁻¹ at a current density of 10 mA g⁻¹ in a range of 1.5–4.0 V at 20 °C. Furthermore, the Na[Fe_1/3Ni_1/3Ti_1/3]O₂ cathode material shows good rate capability and cycling stability. The working and structural transition mechanisms of the Na[Fe_1/3Ni_1/3Ti_1/3]O₂ material are examined by ex situ X-ray absorption spectroscopy (XAS) and in situ X-ray diffraction (XRD) methods. The valence state of Fe ions in the Na[Fe_1/3Ni_1/3Ti_1/3]O₂ material is estimated to be 2.67+. The main redox couple is Ni²⁺/Ni⁴⁺, but the Fe²⁺/Fe³⁺ contributes a little as well at voltages below 2.0 V. The original O3 phase transforms to a P3 phase during sodium extraction with good reversibility, but a slightly irreversible change of lattice parameters may lead to capacity decay during long-term cycling. Moreover, the gas evolution during the first charge/discharge process is analyzed by using an operando mass spectrometry technique. The obvious release of CO₂ gas at the end of the charge process may be the other origin of the capacity decay. Nevertheless, the absence of O₂ evolution indicates an improved safety of the Na/Na[Fe_1/3Ni_1/3Ti_1/3]O₂ cell.