The High Temperature Phase Diagram of Li1 + x Mn2 − x O 4 and Its Implications

Using thermal gravimetric analysis, we recently showed that there is a temperature dependent phase transition in which involves oxygen loss. The temperature of the phase transition, T cl , depends linearly on x over the range from x = 0 to x = 0.20. Here we discuss the origin of the phase transition and show that it is caused by the disproportionation of into , oxygen, and where x ' < x . X‐ray diffraction measurements of samples quenched from above T cl show that this produces . Furthermore, the weight loss of the samples above T cl can be accounted for by a simple model which describes the disproportionation reaction. Calculations of the rate of change of sample mass with temperature, dM/dT , agree well with the measured data. A phase diagram for the maximum attainable x in is presented as a function of temperature for samples heated in air. These results show that the optimum method for preparing materials with low specific surface area and large values of x is a two‐step method. The first step involves heating materials with x about equal to zero to about 850 to 900°C to lower the surface area, and the second involves reacting this intermediate with additional lithium salt near 600°C, which avoids formation. Such materials with x near 0.15 show excellent capacity retention vs. cycle number in secondary lithium coin cells even at 55°C.