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 be tween Li4Mn5O12 and Co2MnO4 including Li2CoMn3O8, (or LiCo1/2Mn3/2O4) and w ith the general formula, Li1-XCo1/2+3X/2Mn3/2-X/2O4: -0.17 less than or equ al to X less than or equal to 0.84, forms in air at 800 degreesC with a fin al heating at 600 degreesC. From the combined data of powder X-ray diffract ion (XRD) and X-ray absorption near edge spectroscopy (XANES), the solid so lutions have the structural formulae [Li+](8a)[Li- X+Co1/2+3X/23+Mn1/2+3X/2 3+Mn1-2X4+](16d)O-4 for X less than or equal to 0 and [Li1 - X+CoX2+](8a)[C o1/2+X/23+Mn1/2+X/23+Mn1-X4+](16d)O-4 for X greater than or equal to 0, in space group Fd (3) over barm; 1:3 ordering of cations on octahedral 16d sit es may occur for compositions around X = 0, but was not detected by XRD. Ob served effective magnetic moment values from magnetic susceptibility data a re consistent with these formulae. Weiss temperatures become increasingly n egative with increasing X, with evidence for spontaneous magnetisation belo w ca. 150 K, suggesting the presence of ferromagnetic interaction between C o2+ and Mn3+ or Mn4+ for samples with X > 0. Impedance data on pellets with blocking electrodes demonstrate a modest level of semiconductivity which m ay involve mainly Mn3+/Mn4+ situated in adjacent octahedral 16d sites. Pote ntial profiles for electrochemical cells, Li/Li1-XCo1/2+3X/2Mn3/2-X/2O4: -0 .17 less than or equal to X less than or equal to 0.18, reveal a charge/dis charge 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 electrochemica l activity as a cathode possibly because the Co that resides in tetrahedral 8a sites blocks Li+ conduction through the pathway: tetrahedral 8a site-em pty octahedral 16c site. For X < 0, the first charging cycle is not reprodu ced on subsequent discharge/charge cycles; reasons for this are not underst ood. In order to explain the large capacities of charge/discharge for X < 0 , some additional redox process, possibly O2-/O-, appears to be necessary.