In recent papers, our laboratory has reported new positive-electrode materials based on the Li͓Ni x Co 1Ϫ2x Mn x ͔O 2 series. These materials show good reversible capacity ͑170 mAh/g between 3.0 and 4.4 V͒ and very little capacity loss over 50 cycles. Most interesting, they appear to be much less reactive with electrolyte at high temperatures than Li y CoO 2 at the same potential. To investigate the reason for the large increase in thermal stability of these materials the morphology and the

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... tivity with electrolyte was measured for a number of samples in the Li͓Ni x Co 1Ϫ2x Mn x ͔O 2 series synthesized at 900°C. Materials with x Ͼ 0 showed a large decrease in reactivity and, surprisingly, much smaller primary particles and tap density as compared to the x ϭ 0 sample. For samples with x у 0.075 the morphology and reactivity remained roughly the same. The reactivity for x у 0.075 was significantly improved compared to LiCoO 2 . We conclude that viable alternatives to LiCoO 2 that appear to be safer are available in the range 0.075 Ͻ x Ͻ 0.4.