Lithium-nickel-manganese oxides, LiNi_1/2Mn_1/2O₂, with layered crystal structure have been considered as next generation of cathode materials for lithium ion batteries. The improvement of their electrochemical performance requires a detailed study on the local cationic distribution of Li⁺, Ni²⁺ and Mn⁴⁺ ions.

The aim of this contribution is to study the cationic distribution in LiNi_1/2Mn_1/2O₂ at long- and short scale range. While the Rietveld refinement of the X-ray powder diffraction patterns allows determining the extent of Li⁺ and Ni²⁺ disorder between lithium and transition metal layers, the distribution of Ni²⁺ and Mn⁴⁺ in the transition metal layers was accessed by X-band EPR spectroscopy.

LiNi_1/2Mn_1/2O₂ with layered structure were synthesized by solid state reaction between lithium hydroxide and mixed Ni,Mn oxides. Two types of mixed Ni,Mn oxides were used: an ilmenite-type oxide obtained from co-precipitated Ni,Mn carbonates and a spinel-type oxide obtained from freeze-dried Ni,Mn citrates. The temperature of the solid state reaction between LiOH and Ni,Mn oxides was varied between 800 and 950 ^oC.

It was found that the extent of Li/Ni mixing between layers decreases with increasing the preparation temperature and is insensitive towards the precursor used. For oxides annealed at 950 ^oC, the EPR spectroscopy reveals the formation of large 180^o Ni²⁺-O-Ni²⁺ magnetic clusters, which include Ni²⁺ ions from both lithium and transition metal layers. This means that at lower synthesis temperatures, where the extent of the Li and Ni mixing is higher, Ni²⁺ ions from both layers are distributed in a way to avoid the formation of large 180^o Ni²⁺-O-Ni²⁺ magnetic clusters.

For LiNi_1/2Mn_1/2O₂, an EPR response from Mn⁴⁺ ions has only been detected, while Ni²⁺ ions remain EPR silent. The EPR line width increases proportionally with the Ni-to-Mn ratio in the first coordination sphere of Mn⁴⁺. Analysis of the EPR line width allows determining the Ni-to-Mn ratio in the first coordination sphere of Mn⁴⁺ ions. It appears that the local Ni-to-Mn ratio depends mainly on the precursor used, but not on the synthesis temperature. LiNi_1/2Mn_1/2O₂ obtained from NiMnO₃-ilmenite displays a lower Ni-to-Mn ratio as compared to LiNi_1/2Mn_1/2O₂ obtained from Ni,Mn spinels. In addition, the different type of cationic distribution in the transition metal layers can be related with the thermal stability of LiNi_1/2Mn_1/2O₂. Between 800 and 950 ^oC, ex-spinel LiNi_1/2Mn_1/2O₂ is stable, while ex-ilmenite LiNi_1/2Mn_1/2O₂ is decomposed above 900 ^oC into Li₂MnO₃ and a Ni-rich layered oxide.

Authors are grateful to EC for a grant within the FAME project (FAME FP6-500159-1), to the Centre of Competence MISSION (SSA, EC-INCO-CT-2005-016414) and to the National Science Fund of Bulgaria (Contract no. Ch1701/2007).

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