We report on a rigorous comparative study of nano- and microparticles of LiMn1.5Ni0.5O4 spinel as cathode materials for Li-ion batteries. The stability of these materials in LiPF6/alkyl carbonate solutions in temperatures up to 70 degrees C was explored. Capacity, cycling, rate capabilities, and impedance behavior were also studied. The methods included X-ray diffraction, Raman, X-ray photelectron, Fourier transform infrared, and electron paramagnetic resonance spectroscopies, and electron microscopy, in conjunction with standard electrochemical techniques: voltammetry, chronopotentiometry, and impedance spectroscopy. These materials show an impressive stability in solutions at elevated temperature. The use of nanomaterials was advantageous for obtaining a better rate capability of LiMn1.5Ni0.5O4 electrodes. LiMn1.5Ni0.5O4 particles develop a unique surface chemistry in solutions that passivates and protects them from detrimental interactions with solution species at elevated temperatures. (C) 2007 The Electrochemical Society.