Microstructure of Li1+xMn2–xO4 spinels obtained from metal-organic precursors

Abstract

A metal-organic precursor method was adopted for the preparation of lithium-manganese spinel oxides. EPR spectra of Mn ²⁺ in solutions and in freeze-dried compositions, as well as IR and DSC of freeze-dried compositions, show that α-hydroxyorganic acids (such as lactic, malic and citric acids) chelate Mn ²⁺ via hydroxy and carboxylate groups. At 450 °C, thermal decomposition of Li-Mn-organic acid precursors with Li/Mn=1.05/1.95 leads to the formation of non-stoichiometric Li[Li _y Mn _{2–y– Δ} □ _Δ ]O ₄ spinels (0.03<y<0.05; 0.02<δ<0.05). EPR of Mn ⁴⁺ was used to throw light on the microstructure of the spinels obtained. For the annealed spinels at 750 °C there is a transition from uniform to non-uniform distribution of the excess Mn ⁴⁺ ions and the corresponding metal vacancies. As a result, the bulk of the spinel becomes close to the stoichiometric composition Li[Li _0.05 Mn _1.95 ]O ₄ , whereas the spinel surface accommodates the excess Mn ⁴⁺ ions. The formation of Mn ⁴⁺ -rich surface regions depends both on the cooling rate and on the organic acid used in the precursors. Lactate precursors are most appropriate for the preparation of lithium-manganese spinels with a homogeneous cation distribution.