New investigation of fluorine-substituted spinel LiMn 2O 4− xF x by using sol–gel process

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To investigate the electrochemical properties of LiMn 2O 4− x F x (0 < x < 1) as a cathode material for 4-V class lithium secondary batteries, spinel phases synthesized at 800 °C are examined. Although the initial discharge capacity of LiMn 2O 4 is improved by substitution of F − for O 2− in the single-phase region, the cycle performance is reduced considerably compared with that of parent LiMn 2O 4. X-ray diffraction (XRD) data show that the single-phase region of x in LiMn 2O 4− x F x is 0 < x < 0.2. A secondary phase (Mn 2O 3) is observed in the region 0.4 < x < 1.0. Voltammetric peaks between 3 and 4.5 V region and XRD data suggest that the oxidation state of Mn in the parent LiMn 2O 4 increases at the expense of creating second phase (Mn 2O 3). These results also imply that a second phase spinel can improve the cycling performance of LiMn 2O 4 by suppressing the Jahn–Teller instability. The oxidation state of Mn in the single-phase spinel and the second-phase coexisting spinel is compared by means of XPS measurements. The electronic conductivity and activation energy of LiMn 2O 4 is also discussed.