Abstract

EPR of Mn⁴⁺ has been used to specify the electronic structure and cation distribution in LiCoMnO₄ compositions that belong to high-potential electrode materials. To prepare single phase LiCoMnO₄, two synthesis techniques have been adopted: solid state reactions and a lactate precursor method. EPR of Mn⁴⁺ shows that diamagnetic Co³⁺ (low-spin configuration, S = 0) and paramagnetic Mn⁴⁺ (S = 3/2) account for the electronic structure of LiCoMnO₄. Analysis of the EPR line width in terms of dipole–dipole and exchange interactions allows one to estimate the mean number of paramagnetic neighbours of Mn⁴⁺. It has been demonstrated that the Co³⁺/Mn⁴⁺ distribution in 16d spinel sites is sensitive towards the cooling rates. For the high-temperature LiCoMnO₄ phase obtained by air quenching from 750°C, three paramagnetic Mn⁴⁺ and three diamagnetic Co³⁺ give rise to the local coordination of Mn⁴⁺, indicating a random Co/Mn distribution. For the low-temperature LiCoMnO₄ phases obtained by air quenching from 600 °C and by slow-cooling from 750 to 25 °C, the mean paramagnetic number of Mn⁴⁺ decreases from 3 to 2, which is interpreted in the framework of short-range Co³⁺/Mn⁴⁺ ordering. The Co³⁺/Mn⁴⁺ octahedral ordering process is developed within a small-scale range (approximately up to a distance of the third metal shell) and does not affect the cubic spinel symmetry of LiCoMnO₄.