Redox reactions of cobalt, aluminum and titanium substituted lithium manganese spinel compounds in lithium cells

Single-phase Li[MxMn2−x]O4 (x=0.5 and M=Co, Al and Ti) spinel compounds were synthesized by solid-state reactions. The lattice parameter of Li[Co0.5Mn1.5]O4, Li[Al0.5Mn1.5]O4 and Li[Li0.04Ti0.49Mn1.47]O4 was 0.8134, 0.8150, and 0.8288 nm, respectively. The change in the lattice parameter relative to Li[Mn2]O4 was found consistent with the modified concentrations of Mn4+ ions upon Co3+, Al3+ and Ti4+ substitution. A reversible reaction at 5.1 V vs. lithium was observed for Li/Li[Co0.5Mn1.5]O4 cells, which was associated with the redox of Co3+/Co4+ ions. The presence of an oxidation reaction at 5.1 V (partially reversible upon reduction at 4.9 V) in Li/Li[Al0.5Mn1.5]O4 cells suggested that the near 5 V electrochemical reaction could not be attributed solely to the redox of substituted transition metal ions in the lithium manganese spinel compounds. In contrast, Li/Li[Li0.04Ti0.49Mn1.47]O4 cells, similar to Li/Li[Mn2]O4 cells, did not show any reactions at voltages above 4.6 V vs. lithium. It is believed that increase in the Mn4+ ion concentrations (or the Li/Mn3+ ratio) in the spinel framework upon substitution (like Co3+ and Al3+) is essential for the presence of electrochemical reactions near 5 V in substituted lithium manganese spinel compounds. This hypothesis is consistent with the mechanism proposed by Ohzuku et al. (J. Power Sources, 1999;81/82:90–94), which is related to compact crystal fields imposed by the cubic close-packed oxygen arrays linked with tetravalent manganese ions in the spinel framework.