In recent papers, our laboratory has reported new positive-electrode materials based on the Li[NixCo1-2xMnx]O-2 series. These materials show good reversible capacity (170 mAh/g between 3.0 and 4.4 V) and very little capacity loss over 50 cycles. Most interesting, they appear to be much less reactive with electrolyte at high temperatures than LiyCoO2 at the same potential. To investigate the reason for the large increase in thermal stability of these materials the morphology and the reactivity with electrolyte was measured for a number of samples in the Li[NixCo1-2xMnx]O-2 series synthesized at 900degreesC. Materials with x > 0 showed a large decrease in reactivity and, surprisingly, much smaller primary particles and tap density as compared to the x 5 0 sample. For samples with x greater than or equal to 0.075 the morphology and reactivity remained roughly the same. The reactivity for x greater than or equal to 0.075 was significantly improved compared to LiCoO2. We conclude that viable alternatives to LiCoO2 that appear to be safer are available in the range 0.075 < x < 0.4. (C) 2003 The Electrochemical Society.