Relationships between Structure, Composition, and Electrochemical Properties in LiNiₓMn₂–ₓO₄ [x = 0.37, 0.43, 0.49, 0.52, and 0.56] Spinel Cathodes for Lithium Ion Batteries

A series of off-stoichiometric LiNiₓMn₂–ₓO₄ (x = 0.37, 0.43, 0.49, 0.52, and 0.56) spinels are prepared by adjusting Mn/Ni molar ratio and are used to investigate the correlations between Mn³⁺ content, structural ordering degree, oxygen vacancies, impurities, and electrochemical properties in these spinels through inductive coupled plasma atomic emission spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, Rietveld refinement of the X-ray diffraction data, galvanostatic charge/discharge test, and first-principles computation. Results show that the relationships between these factors in the off-stoichiometric LiNiₓMn₂–ₓO₄ spinels are obviously different from those in common oxygen-deficient LiNi₀.₅Mn₁.₅O₄₋δ spinel due to their different Mn³⁺ formation mechanisms. Specifically, structural ordering degree and oxygen vacancy concentration almost remain constant when Mn³⁺ content varies in an obvious manner, which is attributed to the fact that the prolonged annealing (600 °C, 12 h) combined with slow cooling (1 °C/min) steps during LiNiₓMn₂–ₓO₄ preparation can order the distribution of Ni²⁺ and Mn⁴⁺ ions in spinel structure and compensate the oxygen loss due to calcining at 800 °C. Electrochemical properties (capacity, first Coulombic efficiency, and rate capability) are significantly improved with an increase in Mn³⁺ content because the increase of Mn³⁺ can reduce rock-salt impurity and improve electronic conductivity and Li⁺ diffusion in the LiNiₓMn₂–ₓO₄ structure.