Structural, magnetic and lithium insertion properties of spinel-type Li2Mn3MO8 oxides (M = Mg, Co, Ni, Cu)

Single-phase compounds Li2Mn3MO8 (M = Mg, Co, Ni, Cu) have been synthesized and investigated as replacements of LiMn2O4 for lithium intercalation below 3 V. They all retain the spinel structure, with cation ordering on the octahedral M (16d) site for M = Mg only. Cell parameters vary as Co < Ni < Mg ≈ Cu < Mn and average M–O bond lengths as Co ≈ Ni < Cu < Mg < Mn. Lithium was intercalated both chemically and electrochemically. Electrochemical potential step spectroscopy shows features typical of a two-phase intercalation reaction, in spite of a manganese valence range mostly above the accepted Jahn–Teller distortion limit (50% Mn3+). The tetragonal distortion is only noticeable at high intercalation levels. It yields c/a distortion values much lower for M = Co or Ni than for unsubstituted LiMn2O4. However, no improvement in electrochemical cyclability was obtained. Magnetic susceptibility measurements show features typical of frustrated systems, as expected for the 16d sublattice, and confirm that chemical intercalation reaches lithium contents close to the theoretical limit (one additional Li per AB2O4 formula unit). For cobalt substitution, bond length and Curie constant analysis both lead to a charge distribution Li2[(Mn4+)2Mn3+Co3+]O8 rather than Li2[(Mn4+)3Co2+]O8.