Changes in the structure and physical properties of the solid solution LiNi1−xMnxO2 with variation in its composition

Abstract

The layered oxides LiNi_1−xMn_xO₂ (x = 0.1–0.5) were synthesized and characterized using synchrotron X-ray diffraction, TOF neutron diffraction, SQUID magnetometry, ICP spectroscopy, XAFS, and electrochemical measurements. All the samples were single-phase and adopted the α-NaFeO₂ structure; LiNi_1−xMn_xO₂ can be represented as Li(Ni²⁺_xNi³⁺_1−2xMn⁴⁺_x)O₂. Structural analysis using synchrotron and neutron diffraction data demonstrated that the lattice parameters of LiNi_0.5Mn_0.5O₂ are a = 2.892 Å and c = 14.302 Å and that the chemical composition can be expressed by referring to the Wyckoff positions 3a and 3b as [Li_0.91Ni_0.09]_3a[Li_0.09Mn_0.5Ni_0.41]_3bO₂. The lattice parameters a and c and the fraction of Ni at the 3a site of LiNi_1−xMn_xO₂ increased with Mn content up to the x = 0.4 composition and then showed little change between x = 0.4 and 0.5. An increase in the Ni–O distance was observed with increasing x. The appearance of ferromagnetism was clearly observed at x = 0.4–0.5 as the Ni²⁺ and Mn⁴⁺ content increased. The discharge capacity of the Li/LiNi_1−xMn_xO₂ cell decreased from 190 mAh g⁻¹ (x = 0.1) to 140 mAh g⁻¹ (x = 0.5).