Cation ordering and electrochemical properties of the cathode materials LiZn_xMn_2-xO₄, 0 < x ≤ 0.5: A ⁶Li magic-angle spinning NMR spectroscopy and diffraction study

The local and long-range structures of LiZn_xMn_2-xO₄ (x = 0.1, 0.2, 0.3, 0.5) have been studied by using ⁶Li 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(OMn^3.5+)_12-y(OMn⁴⁺)_y, Mn^3.5+ 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 °C 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 Li_0.5Zn_0.5[Mn_1.5Li_0.5] 0₄ was refined by using neutron and X-ray powder diffraction data in the space group P2₁3 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 Li_0.5Zn_0.5[Mn_1.5Li_0.5] O₄), as the concentration of Zn increases. The ⁶Li NMR spectra of LiZn_0.1Mn_1.9O₄ 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.