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 degrees C only spinel with the composition Li1+xMn2-xO4+delta (delta approximate to 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, Delta x, at a given T and a given p(O-2) is much smaller than generally accepted. We find that 0.05 < Delta x < 0.13. The spinel Li1+xMn2-xO4+delta has a negligible oxygen nonstoichiometric range, delta < 0.02. Between 880 and 980 degrees C a variety of different tetragonal Li-Mn spinel phases Li1-xMn2+xO4 exist. Though commonly attributed as oxygen and lithium deficient spinel, our results indicate the absence of any significant oxygen deficiency. Below 400 degrees C spinel appears to be stable only as Li4Mn5O12 and LiMn1.75O4. A phase diagram of the second kind is presented for the Li-Mn-O system (Figure 1).