Thermodynamic stability of chemically delithiated Li(LixMn2-x)O4

Stoichiometric and excess-lithium spinels, Li(LixMn2-x)O-4 (x = 0.0, 0.1, 0.2), were treated with diluted H2SO4. The resulting products retained the spinel structure at reduced Lithium content; those with lowest lithium content underwent exothermic phase transitions, observed as peaks in differential scanning calorimetry (DSC) curves between 250 and 300 degrees C at heating rates of 10 degrees C/m. The phase transition of the delithiated stoichiometric spinel was followed by powder X-ray diffraction of heat-treated samples. The structural analysis shows that the oxygen atoms rearrange first (from cubic to hexagonal packing), leaving the manganese atoms in disordered positions. It is this rearrangement of oxygen atoms that releases most of the heat observed in DSC measurements. Eventually the manganese atoms took their expected positions in the rutile or beta-form of MnO2. The metastability of the delithiated compounds turned out not to be the origin of the capacity fade in electrochemical cells using stoichiometric spinel as the cathode.