Cobalt Content Optimization of Layered 0.6Li[Li1/3Mn2/3]O2-0.4LiNi0.5-xMn0.5-xCo2xO2 (0 ≤ x ≤ 0.5) Cathode Materials Prepared by the Carbonate Coprecipitation

A series of layered 0.6Li[Li1/3Mn2/3]O-2-0.4LiNi(0.5-x)Mn(0.5-x)Co(2x)O(2) (0 <= x <= 0.5) materials are prepared by a carbonate coprecipitation method. The variation of the structural, morphological and electrochemical characteristics of these materials with Co content (x) is investigated. Each sample is single phase with layered structure in space group R (3) over barm. The lattice parameter a, c and V decrease, while the c/a ratio increases with an increase in Co content. This result suggests that higher cobalt content facilitates the formation of better hexagonal structure, which is beneficial to the electrochemical properties. However, the increase of Co content is always accompanied by the decrease of Ni and Mn content, which leads to a decrease in the Ni2+/Ni4+ redox couple and an increase in the structural instability, thus it is detrimental to the charge/discharge capacity and cycling stability. The former positive factor manifests itself when the cobalt content is relative low (x <= 0.12), while the latter negative factor becomes dominant when the cobalt content is higher (x >= 0.25). Therefore, only the materials with low cobalt content of 0.03 <= x <= 0.12 have good electrochemical properties including high capacity, good rate capacity and excellent cycling stability. The 0.6Li[Li1/3Mn2/3]O-2-0.4LiNi(0.38)Mn(0.38)Co(0.24)O(2) (x = 0.12) sample shows the highest initial capacity of 218 mAh . g(-1) and almost 100% capacity retention over fifty cycles, which is the best cobalt content in the series. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.012210jes] All rights reserved.