X-ray amorphous manganese oxides were prepared by reduction of sodium
permanganate by lithium iodide in aqueous medium (MnOx-I) and by decomposition
of manganese carbonate at moderate temperature (MnOx-C). TEM showed that these
materials are not amorphous, but nanostructured, with a prominent spinel
substructure in MnOx-C. These materials intercalate lithium with capacities up
to 200 mAh/g at first cycle (potential window 1.8-4.3 V) and 175 mAh/g at 100th
cycle. Best performances for MnOx-C are obtained with cobalt doping. Potential
electrochemical spectroscopy shows that the initial discharge induces a 2-phase
transformation in MnOx-C phases, but not in MnOx-I ones. EXAFS and XANES
confirm the participation of manganese in the redox process, with variations in
local structure much smaller than in known long-range crystallized manganese
oxides. X-ray absorption spectroscopy also shows that cobalt in MnOx-C is
divalent and does not participate in the electrochemical reaction
