Electrochemical studies of substituted spinel LiAlyMn2-yO4-zFz for lithiumsecondary batteries

To improve the cycle performance of LiMn2O4 (Fd (3) over barm) as the catho de 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 d egreesC were examined. Although the cycle performance of the LiAlyMn2-yO4 w as improved by the substitution of Al3- for Mn3+ in the octahedral sites, t he 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 diffract ion (XRD) data shows that the single-phase region of y and z in LiAly Mn2-y O4-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 voltammetri c peaks in the 2-5 V region and the XRD data suggested that F-Al-substitute d spinel can suppress the Jahn-Teller instability to improve the cycling pe rformance better than LiMn2O4. The above results are further supported by a n ac impedance experiment. We compared impedance spectra of the fresh and c ycled electrode, measured at potentials close to the SSCV peak potential. T he capacity decrease of these electrodes upon cycling is accompanied by a g radual increase in their charge-transfer resistance. (C) 2001 Elsevier Scie nce B.V. All rights reserved.