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

Abstract

The products of decomposition of manganese carbonate with and without doping by copper or cobalt, at temperatures <500 °C in air were studied. Doped systems with 20 atom% Cu or Co give new nanometric manganese oxides agglomerated in submicronic spheres at 370 °C. Transmission electron microscopy (TEM) shows that these X-ray amorphous compounds are nanocrystalline with grain size in the 10 nm range and a spinel 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.