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

A Na[Fe1/3Ni1/3Ti1/3]O2 cathode material for sodium-ion batteries has been synthesized by a solid-state reaction method. The obtained Na[Fe1/3Ni1/3Ti1/3]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[Fe1/3Ni1/3Ti1/3]O2 cathode material shows good rate capability and cycling stability. The working and structural transition mechanisms of the Na[Fe1/3Ni1/3Ti1/3]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[Fe1/3Ni1/3Ti1/3]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[Fe1/3Ni1/3Ti1/3]O2 cell.