Synthesis and characterization of layered Li1−x(Ni0.5Co0.5)1−yFeyO2(0 ≤ (1 − x) ≤ 1 and 0 ≤ y ≤ 0.2) cathodes
Abstract Layered Li(Ni 0.5 Co 0.5 ) 1− y Fe y O 2 cathodes with 0 ≤  y  ≤ 0.2 have been synthesized by firing the coprecipitated hydroxides of the transition metals and lithium hydroxide at 700 °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 Li 1− x (Ni 0.5 Co 0.5 ) 1− y Fe y O 2 samples indicates that the initial O3 structure is maintained down to a lithium content (1 −  x ) ≈ 0.3. For (1 −  x )  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 ) ≈ 0.3 compared to the Li 1− x CoO 2 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 LiCoO 2 system.