Dependence of Electrochemical Behavior on Concentration and Annealing Temperature of Li[sub x]CoPO[sub 4] Phase-Grown LiNi[sub 0.8]Co[sub 0.16]Al[sub 0.04]O[sub 2] Cathode Materials

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Abstract
LixCoPO4LixCoPO4 phase-grown LiNi0.86Co0.1Al0.04O2LiNi0.86Co0.1Al0.04O2 cathode materials were prepared by varying the coating concentration of precipitated Co3(PO4)2Co3(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, 3wt%3wt% coating showed no decrease in the first discharge capacity (193mAh∕g)(193mAh∕g) and a much improved capacity retention (90%) at a 1C1C cycling rate after 40 cycles, compared to the bare cathode ( 193mAh∕g193mAh∕g and 44% for first discharge capacity and capacity retention, respectively). Using this concentration, the annealing time was varied from 2 to 7h7h at 700, 800, and 900°C. The coated cathode annealed at 700°C for 7h7h showed comparable electrochemical properties and moisture uptake to that annealed at 700°C for 5h5h . Even after annealing at 700°C for 7h7h , P ions remained on the particle surface with a similar coating thickness to that at 700°C for 5h5h . 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 Li2CO3Li2CO3 and LiOHLiOH 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 Li2CO3Li2CO3 and LiOHLiOH . Thermal stability was also greatly improved by the coating, and the sample annealed at 700°C for 7h7h exhibited a decreased total heat capacity (240J∕g)(240J∕g) compared to the 5h5h annealed samples (380J∕g)(380J∕g) even though those values were much smaller than the bare sample (980J∕g)(980J∕g) .

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