Layered Li(Ni0.5Co0.5)(1-y)FeyO2 cathodes with 0 <= y <= 0.2 have been synthesized by firing the coprecipitated hydroxides of the transition metals and lithium hydroxide at 700 degrees C and characterized as cathode materials for lithium ion batteries to various cutoff charge voltages (up to 4.5 V). While the y=0.05 sample shows an improvement in capacity, cyclability, and rate capability, those with y=0.1 and 0.2 exhibit a decline in electrochemical performance compared to the y=0 sample. Structural characterization of the chemically delithiated Li1-x(Ni0.5Co0.5)(1-y)FeyO2 samples indicates that the initial O3 structure is maintained down to a lithium content (1-x)approximate to 0.3. For (1-x)< 0.3, while a P3 type phase is formed for the y=0 sample, an O1 type phase is formed for the y=0.05, 0.1 and 0.2 samples. Monitoring the average oxidation state of the transition metal ions with lithium contents (1-x) reveals that the system is chemically more stable down to a lower lithium content (1-x)approximate to 0.3 compared to the Li1-xCoO2 system. The improved structural and chemical stabilities appear to lead to better cyclability to higher cutoff charge voltages compared to that found before with the LiCoO2 system. (c) 2006 Elsevier B.V All rights reserved.