Structural and electrochemical properties of new nanospherical manganese oxides for lithium batteries

The products of decomposition of manganese carbonate with and without doping by copper or cobalt, at temperatures <500 degrees C in air were studied. Doped systems with 20 atom% Cu or Co give new nanometric manganese oxides agglomerated in submicronic spheres at 370 degrees C. Transmission electron microscopy (TEM) shows that these X-ray amorphous compounds are nanocrystalline with grain size in the 10 nm range and a spinet substructure. The electrochemical behaviour of these materials in lithium cells was studied. Whereas non-doped manganese oxide exhibits poor intercalation capabilities, the freshly co-precipitated Cu or Co doped materials can be cycled successfully around 3 V vs. Li/Li+. Step-potential electrochemical spectroscopy shows that the initial discharge gives rise to a two-phase transition and is followed by stable, reversible, single-phase cycling. Best results are obtained on a cobalt-doped manganese oxide (Co : Mn = 1 : 5), which can sustain more than 100 charge-discharge cycles with a 175 mA h g(-1) capacity in the 1.8-4.2 V range. XAS spectra were measured on pristine and electrochemically discharged materials, showing that (1) in the cobalt-doped material, cobalt is divalent and manganese is the only redox-active species, (2) the variations in local structure around Mn on discharge are much smaller than in long-range ordered compounds such as Li-Mn-O spinels.