Structural and Magnetic Nature for Fully Delithiated Li_<i>x</i>NiO₂: Comparative Study between Chemically and Electrochemically Prepared Samples

Abstract

Structural and magnetic properties of two Li x NiO 2 samples with x ≤ 0.1 have been studied by a powder X-ray diffraction, magnetic susceptibility (χ), and muon-spin rotation/relaxation (μSR) measurements. One of the two samples was prepared by an electrochemical (EC) reaction in a nonaqueous lithium cell, whereas the other sample by a chemical (C) reaction in a HNO 3 aqueous solution. Although the crystal structure of both C-Li 0.01 NiO 2 and EC-Li 0.10 NiO 2 samples were assigned as a mixture of a cubic close-packed phase and a hexagonal close-packed phase, the effective magnetic moment (μ eff ) of Ni ions for C-Li 0.01 NiO 2 was estimated as μ eff = 1.43 μ B and was very close to that for EC-Li 0.5 NiO 2 (μ eff = 1.39 μ B ). This implies that the vacant tetrahedral or octahedral sites in C-Li 0.01 NiO 2 are partially occupied by H + ions. Actually, the zero-field μSR time spectrum in the paramagnetic state for C-Li 0.01 NiO 2 exhibited a large relaxation compared to those for EC-Li 0.5 NiO 2 and EC-Li 0.10 NiO 2 . Furthermore, a pryrolysis gas chromatography/mass spectroscopy analysis confirmed the existence of H + ions in the C-Li 0.01 NiO 2 crystalline lattice. The actual composition of C-Li 0.01 NiO 2 is, thus, determined to be H ∼0.5 Li 0.01 NiO 2 .