JAHN-TELLER STRUCTURAL PHASE-TRANSITION AROUND 280K IN LIMN2O4

The structural phase transition of spinel LiMn2O4 (Fd3m at room temperature) was investigated using thermal analysis and powder x-ray diffraction. The phase transition was observed around T-t=280K with a temperature hysteresis of about 10K. It was revealed that the low temperature phase (T<T-t) is a mixture of Fd3m (c/a=1) and I4(1)/amd (c/a=1.011). The phase transformation from Fd3m to I4(1)/amd proceeds with the decrease in temperature until the volume fraction of the I4(1)/amd phase saturates at 65% at around 260K, just below T-t. The single phase of I4(1)/amd never appeared even at 220K, far below T-t. This phase transition is of a first-order character and is owing to the cooperative Jahn-Teller effect of Mn3+. Cell performance utilizing the LiMn2O4 cathode is also discussed.