Samples of the layered cathode materials, Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 (x= 1/12,1/4,5/12, and 1/2), were synthesized at 900 degrees C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF6 EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau Dear 4.5 V. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 samples with x= 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 degrees C in EC/DEC. Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 degrees C. However, their high reactivity is similar to that of Li0.5CoO2 (4.2 V) with 1 mu m particle size. Therefore, Li[NixLi(1/3-2x/3)Mn(2/3-x/3)]O-2 samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO2 (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased. (C) 2005 Elsevier Ltd. All rights reserved.