This work presents the diffraction features of lithium-manganese oxide in extreme pressure and temperature conditions used as positive electrode materials in lithium-ion batteries. Energy-dispersive X-ray diffraction yield reliable description of material lattice, its distortion and chemical stability under high pressure and at high temperature (HP/HT). The phase evolution as a function of pressure and temperature is reported and analyzed in the LiMn2O4 sample. A comparison with another tetragonal spinel shows the influence of the Jahn–Teller effect on the HP/HT structure of this class of materials.