High-temperature storage of lithium manganese spinels was examined by SEM observations for Li1.1Mn1.9O4 and lambda-MnO2 single crystals using 1 M LiPF6 in EC: DEC (1:1) electrolyte solution. Starting single crystals of Li1.1Mn1.9O4 and lambda-MnO2 were synthesized by a flux method and subsequent chemical oxidation reaction. After being stored at 80 degrees C for 10 days, the Li1.1Mn1.9O4 crystals became fragile, and many surface grooves and holes forming a honeycomb structure were observed. Single-crystal X-ray structure analysis (R = 6.1%) d using 95 observed reflections for lambda-MnO2 after the storage revealed a manganese-deficient structure having the structural formula of Mn0.95O2. (c) 2005 The Electrochemical Society.