Cation ordering and electrochemical properties of the cathode materials LiZnxMn2-xO4, 0 < x ≤ 0.5:: A 6Li magic-angle spinning NMR spectroscopy and diffraction study

The local and long-range structures of LiZnxMn2-xO4 (x = 0.1, 0.2, 0.3, 0.5) have been studied by using Li-6 magic-angle spinning (MAS) NMR spectroscopy and X-ray and neutron powder diffraction. For all the samples, a 6Li resonance at approximately 2000 ppm is observed due to the presence of Li in the octahedral sites of the spinel structure, indicating that Zn substitutes for lithium in the tetrahedral sites. For low doping levels (x = 0. 1 and 0.2), several resonances at 500-700 ppm are seen, which are assigned to lithium cations in tetrahedral sites with local environments such as Li(OMn3.5+)(12-y)(OMn4+)(y), Mn3.5+ indicating a Mn ion with an average oxidation state of 3.5. The x = 0.3 sample is not obtained as a single phase; instead, two phases with Zn-doping levels corresponding to x 0.2 and 0.5 are observed. Different cation-ordering schemes are observed for the x 0.5 sample, depending on the condition of synthesis: slow cooling from 700 degreesC to room temperature results in cation ordering in both tetrahedral and octahedral sites, whereas the sample quenched from high temperature shows no long-range cation ordering on either site. The structure of ordered Li0.5Zn0.5[Mn1.5Li0.5]O-4 was refined by using neutron and X-ray powder diffraction data in the space group P2(1)3 and shows complete ordering on the octahedral site but some partial disorder or cation vacancies on the tetrahedral site. The variable temperature 6Li NMR spectra are consistent with a gradual change in the nature of the magnetic interactions between the manganese spins, from antiferromagnetic (for the sample with the lowest Zn-doping level) to ferromagnetic (for ordered Li0.5Zn0.5[Mn1.5Li0.5]O-4), as the concentration of Zn increases. The Li-6 NMR spectra of LiZn0.1Mn1.9O4 during the first charging cycle shows a gradual shift in the peak position to higher frequency, rather than a series of discrete resonances. This is consistent with the potential vs charging profiles, where the two potential plateaus that are typically seen in the charging/discharging potential profiles of LiMn2O4 are no longer observed. Li ions remain in the octahedral site throughout.