Electrochemical studies of substituted spinel LiAlyMn2-yO4-zFz for lithium secondary batteries

To improve the cycle performance of LiMn2O4 (Fd (3) over barm) as the cathode of 4 V class lithium secondary batteries, the cathode properties of the cubic spinel phases LiAlyMn2-yO4-zFz and LiAly.Mn2-yO4 synthesized at 750 degreesC were examined. Although the cycle performance of the LiAlyMn2-yO4 was improved by the substitution of Al3- for Mn3+ in the octahedral sites, the first discharge capacity was reduced considerably compared with that of the parent LiMn2O4. F-substituted spinel LiAlyMn2-yO4 F-z(z) can compensate for the theoretical capacity reduction in LiAlyMn2-yO4. The X-ray diffraction (XRD) data shows that the single-phase region of y and z in LiAly Mn2-yO4-zFz was 0 less than or equal to y less than or equal to 1/12 and 0 less than or equal to z less than or equal to 0.04. The shape of the voltammetric peaks in the 2-5 V region and the XRD data suggested that F-Al-substituted spinel can suppress the Jahn-Teller instability to improve the cycling performance better than LiMn2O4. The above results are further supported by an ac impedance experiment. We compared impedance spectra of the fresh and cycled electrode, measured at potentials close to the SSCV peak potential. The capacity decrease of these electrodes upon cycling is accompanied by a gradual increase in their charge-transfer resistance. (C) 2001 Elsevier Science B.V. All rights reserved.