Magnetic properties of the cathode material Na₄Ni₇(PO₄)₆ for sodium-ion batteries

Full text: The magnetic structures and properties of sodium nickel phosphate Na₄Ni₇(PO₄)₆ have been investigated using magnetometry and low-temperature neutron diffraction. According to the magnetic susceptibility analysis and variable temperature neutron diffraction measurements, Na₄Ni₇(PO₄)₆ presents three successive antiferromagnetic ordering (Phase I, Phase II, and Phase III) at 9.1 - 17 K, 4.6 - 9.1 K and < 4.6 K with the magnetic ordering vector [0, 1, 1/2], [0, 2/3, 1/2], and [0.076, 2/3, 1/2] refer to the nuclear structure, respectively. The magnetic ordering shows distinct ferromagnetic (FM) Ni2+ strips and anti-ferromagnetic (AFM) arrangements between FM strips. The moment amplitude of all strips is same in Phase I but varies in Phase II. Phase III is an incommensurate one and should have a similar spin arrangement with Phase II. Na₄Ni₇(PO₄)₆ has large indirect band gap (2.39eV from DFT calculations and 2.07eV from reflection spectrum analysis). An obvious Na+ conductivity with diffusion activation energy of 1.04 eV was observed from impedance analysis of sintered Na₄Ni₇(PO₄)₆ pellet. (author)