Electrochemical, electrical and magnetic properties and valence state distributions in the high voltage spinel cathode solid solutions Li1 − XCo1/2 + 3X/2Mn3/2 − X/2O4: −0.33 ≤ X ≤ 1

An extensive series of spinel solid solutions covering most of the range between Li4Mn5O12 and Co2MnO4 including Li2CoMn3O8, (or LiCo1/2Mn3/2O4) and with the general formula, Li1 − XCo1/2 + 3X/2Mn3/2 − X/2O4: −0.17 ≤ X ≤ 0.84, forms in air at 800 °C with a final heating at 600 °C. From the combined data of powder X-ray diffraction (XRD) and X-ray absorption near edge spectroscopy (XANES), the solid solutions have the structural formulae [Li+]8a[Li+− XCo3+1/2 + 3X/2Mn3+1/2 + 3X/2Mn4+1 − 2X]16dO4 for X ≤ 0 and [Li+1 − XCo2+X]8a[Co3+1/2 + X/2Mn3+1/2 + X/2Mn4+1 − X]16dO4 for X ≥ 0, in space group Fdm; 1∶3 ordering of cations on octahedral 16d sites may occur for compositions around X = 0, but was not detected by XRD. Observed effective magnetic moment values from magnetic susceptibility data are consistent with these formulae. Weiss temperatures become increasingly negative with increasing X, with evidence for spontaneous magnetisation below ca. 150 K, suggesting the presence of ferromagnetic interaction between Co2+ and Mn3+ or Mn4+ for samples with X > 0. Impedance data on pellets with blocking electrodes demonstrate a modest level of semiconductivity which may involve mainly Mn3+/Mn4+ situated in adjacent octahedral 16d sites. Potential profiles for electrochemical cells, Li/Li1 − XCo1/2 + 3X/2Mn3/2 − X/2O4: −0.17 ≤ X ≤ 0.18, reveal a charge/discharge plateau centred on ca. 4.0 V, and a second plateau centred on ca. 5.1 V which shows a maximum discharge capacity of ca. 62 mA h g−1 at X = 0, i.e., Li2CoMn3O8. For X ≫ 0, the solid solutions lose their electrochemical activity as a cathode possibly because the Co that resides in tetrahedral 8a sites blocks Li+ conduction through the pathway: tetrahedral 8a site–empty octahedral 16c site. For X < 0, the first charging cycle is not reproduced on subsequent discharge/charge cycles; reasons for this are not understood. In order to explain the large capacities of charge/discharge for X < 0, some additional redox process, possibly O2−/O−, appears to be necessary.

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