Phase Diagram of Li−Mn−O Spinel in Air

Thermogravimetric analysis, solid electrolyte coulometry, and X-ray diffraction were used to examine the stable phases in the Li−Mn−O phase diagram. Our investigations concentrated on the equilibrium composition of the spinel phase in the system Li−Mn−O. Between 400 and 880 °C only spinel with the composition Li 1+ x Mn 2 - x O 4+ δ (δ ≈ 0) is stable. The lithium stoichiometry, x, is a function of temperature, T, and oxygen partial pressure, p (O 2 ). The value of x increases with decreasing temperature and increasing p (O 2 ). The possible lithium stoichiometry range, Δ x, at a given T and a given p (O 2 ) is much smaller than generally accepted. We find that 0.05 < Δ x < 0.13. The spinel Li 1+ x Mn 2 - x O 4+ δ has a negligible oxygen nonstoichiometric range, δ < 0.02. Between 880 and 980 °C a variety of different tetragonal Li−Mn spinel phases Li 1 - x Mn 2+ x O 4 exist. Though commonly attributed as oxygen and lithium deficient spinel, our results indicate the absence of any significant oxygen deficiency. Below 400 °C spinel appears to be stable only as Li 4 Mn 5 O 12 and LiMn 1.75 O 4 . A phase diagram of the second kind is presented for the Li−Mn−O system (Figure 1).