Structure and Electrochemical Properties of LiFe_xMn_2-xO₄ (0 ≤ x ≤ 0.5) Spinel as 5 V Electrode Material for Lithium Batteries

An electroactive spinel solid solution, LiFe_xMn_2-xO₄ (0 ≤ x ≤ 0.5), was prepared and investigated. X-ray Rietveld refinements showed a small amount of transition metal ions in tetrahedral 8a sites in Fe-substituted samples. The ⁵⁷Fe Mössbauer spectrum of LiFe_0.5Mn_1.5O₄ at 300 K is composed of two doublets and attributed to trivalent Fe. In the voltage range 3.0 and 5.3 V vs. Li/Li⁺, two reversible plateaus appeared at 4.9 and 3.9 V during discharge. Combined data of X-ray diffraction, Mn K-edge X-ray absorption near-edge structure (XANES), and ⁵⁷Fe Mössbauer spectroscopy obtained in situ during charge and discharge demonstrate that the plateau around 4 V on charging is associated with the oxidation of Mn³⁺ to Mn⁴⁺ and the plateau around 5 V with the partial oxidation of Fe³⁺ to Fe⁴⁺.