Influence of isovalent ions (Ca and Mg) on the properties of LiCo0.9M0.1PO4 powders

LiCo0.9M0.1PO4 (M = Co2+, Mg2+, Ca2+) compounds are synthesized by Pechini-assisted sol-gel process and after annealing at 300 °C for 5 min in air, then at 730 °C for 12 h in nitrogen (2-steps annealing process). The XRD-patterns reveal the presence of LiCoPO4 as major crystalline phase and of CoP2O7 (M = Co), of Co2P (M = Mg), of Co2P, Li3PO4, (Ca,Co)3(PO4)2 (M = Ca) as impurities. The morphological investigation shows a homogenous spongy-like structure for M = Co, and a more compact structure with spherical particles separated on the surface in Mg- and Ca-containing systems. In the voltammetric curves, a very good reversibility with mean peak maxima in the cathodic region of 4.4 V for M = Co, 4.5 V for M = Ca and 4.6 V for M = Mg respectively are observed. The discharge specific capacities, at a discharge rate of C/10 and room temperature, were 60 mAh g−1 for M = Co, 36 mAh g−1 for M = Mg and 68 mAh g−1 for M = Ca respectively. The electrochemical impedance spectroscopy data reveal a decrease of the electrical resistance and the improvement of the Li-ion conductivity after the addition of Ca and Mg-ions into the LiCoPO4 system.