Samples of the layered cathode materials, Li[Ni x Li(1/3−2x/3)Mn(2/3−x/3)]O2 (x =1/12, 1/4, 5/12, and 1/2), were synthesized at 900°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 1M 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 4.45V and second, for samples charged to 4.8V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5V. Li[Ni x Li(1/3−2x/3)Mn(2/3−x/3)]O2 samples with x =1/4, 5/12 and 1/2 charged to 4.45V do not react appreciably till 190°C in EC/DEC. Li[Ni x Li(1/3−2x/3)Mn(2/3−x/3)]O2 samples charged to 4.8V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130°C. However, their high reactivity is similar to that of Li0.5CoO2 (4.2V) with 1μm particle size. Therefore, Li[Ni x Li(1/3−2x/3)Mn(2/3−x/3)]O2 samples showing specific capacity of up to 225mAh/g may be acceptable for replacing LiCoO2 (145mAh/g to 4.2V) from a safety point of view, if their particle size is increased.