Lithium intercalation in Li-Mg-Mn-O and Li-Al-Mn-O spinels

LiAlMnO4 and LiMg0.5Mn1.5O4 have been investigated as replacements of LiMn2O4 for lithium intercalation below 3 V. The decomposition of acetate and carbonate precursors was studied by thermogravimetry. Solid state reactions yielded 'LiAlMnO4' with manganese valence <4 and always containing impurity phases such as LiAl5O8 or LiMn2O4. The intercalation behaviour was studied potentiostatically in lithium cells using both liquid and solid electrolytes, and by X-ray diffraction on intercalated cathodes. The structural and electrochemical behaviour of 'LiAlMnO4' is very similar to that of LiMn2O4, and gives lower capacities on cycling. Lithium intercalation in LiMg0.5Mn1.5O4 includes three reduction steps between 2.8 and 1.6 V, corresponding to the intercalation of ca. 0.58, 0.22 and 0.25 Li atoms, respectively. The total initial capacity is 180 mAh/g, but also drops on cycling, mainly due to the collapse of the second step corresponding to the critical threshold around Mn-+3.5, where the tetragonal distortion due to the Jahn-Teller effect takes place.