Lattice instability in Li(LixMn2-x)O-4

The lattice instability in the discharged state of lithium-rich spinel manganese compounds, Li(LixMn2-x)O-4, was investigated with respect to the Jahn-Teller structural phase transition (Fd3m to I4(1)/amd) around T-t = 280 K in LiMn2O4. A small amount of excess Li (0 < x < 0.035) substituted for Mn (16d sites) increased the average Mn valency, suppressing the Jahn-Teller effect and hence stabilizing the cubic phase to drastically decrease both the transition temperature T-t from 282 to 214 K and the latent heat Delta H from 226 to 29 cal/mol. No transition was observed in the region x > 0.035, which corresponds to merely <2% Li substitution for Mn. The lattice instability in the charged state was also investigated by comparing the theoretical and the experimental capacity of Li(LixMn2-x)O-4. The cell capacity showed a clear deviation from the theoretical value in the small x region (x < 0.15), indicating the mechanical lattice instability induced by scarcity of Li+ (8a). The value of Li substitution x which will maximize the cell performance is discussed. (C) 1996 Academic Press, Inc.