A surface treatment process based on a mild acidic solution was utilized to stabilize the surface of the LiMn1.5Ni0.5O4-delta (LMNO) spinel cathode material and to improve its elevated temperature performance. To characterize the failure mechanism of the LMNO spinel at an elevated temperature (60 degrees C), the effect of the Mn3+ content and the charge/discharge state storage conditions were studied. It was shown that the existence of Mn3+ is necessary for an improved elevated temperature performance. It was also identified that one of the main degradation mechanisms at an elevated temperature was the systematic impedance rise rather than the intrinsic capacity loss. The results of the charged state storage at 60 degrees C demonstrated the worst condition for the spinel materials; however, the surface-treated materials presented an improved elevated temperature cycling and a much less impedance increase than the untreated spinel after 4 weeks of storage at 60 degrees C. X-ray diffraction, X-ray photoelectron spectroscopy, high resolution transmission electron microscopy, and electron energy loss spectroscopy were utilized to characterize the effect of surface treatment on the crystal structure and morphology of the acid-treated material.