Dependence of electrochemical Behavior on concentration and annealing temperature of LixCoPO4 phase-grown LiNi0.8Co0.16Al0.04O2 cathode materials

LixCoPO4 phase-grown LiNi0.86Co0.1Al0.04O2 cathode materials were prepared by varying the coating concentration of precipitated Co-3(PO4)(2) nanoparticles and the annealing temperature. These materials were characterized by analyzing their electrochemical properties, moisture uptake, and thermal stability. Among the various coating concentrations, 3 wt % coating showed no decrease in the first discharge capacity (193 mAh/g) and a much improved capacity retention (90%) at a 1 C cycling rate after 40 cycles, compared to the bare cathode (193 mAh/g and 44% for first discharge capacity and capacity retention, respectively). Using this concentration, the annealing time was varied from 2 to 7 h at 700, 800, and 900 degrees C. The coated cathode annealed at 700 degrees C for 7 It showed comparable electrochemical properties and moisture uptake to that annealed at 700 degrees C for 5 h. Even after annealing at 700 degrees C for 7 h, P ions remained on the particle surface with a similar coating thickness to that at 700 degrees C for 5 h. However, increasing the annealing temperature led to a further decrease in electrochemical properties and was related to deeper diffusion of P ions into the bulk particle, which likely formed a solid solution consisting of bulk elements and P elements. The amounts of Li2CO3 and LiOH formation of the coated samples upon exposure to air with a relative humidity of 40% showed a similar result when annealing at 700 degrees C, but increasing the annealing temperature to 800 and 900 degrees C resulted in a significant increase of the amount of Li2CO3 and LiOH. Thermal stability was also greatly improved by the coating, and the sample annealed at 700 degrees C for 7 h exhibited a decreased total heat capacity (240 J/g) compared to the 5 It annealed samples (380 J/g) even though those values were much smaller than the bare sample (980 J/g). (C) 2008 The Electrochemical Society.