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

Abstract

Lix CoPO4 phase-grown LiNi0.86 Co0.1 Al0.04 O2 cathode materials were prepared by varying the coating concentration of precipitated Co3 (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 mAhg) and a much improved capacity retention (90%) at a 1 C cycling rate after 40 cycles, compared to the bare cathode (193 mAhg 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°C. The coated cathode annealed at 700°C for 7 h showed comparable electrochemical properties and moisture uptake to that annealed at 700°C for 5 h. Even after annealing at 700°C for 7 h, P ions remained on the particle surface with a similar coating thickness to that at 700°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 Li2 CO3 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°C, but increasing the annealing temperature to 800 and 900°C resulted in a significant increase of the amount of Li2 CO3 and LiOH. Thermal stability was also greatly improved by the coating, and the sample annealed at 700°C for 7 h exhibited a decreased total heat capacity (240 Jg) compared to the 5 h annealed samples (380 Jg) even though those values were much smaller than the bare sample (980 Jg).