Characterization of the carbon-coated LiNi1-y Co y PO4 solid solution synthesized by a non-aqueous sol-gel route

Nanosize carbon-coated samples of the solid solution LiNi1-y Co (y) PO4 were prepared and investigated with respect to high-voltage cathode materials (4-5 V) for Li-ion batteries. For the syntheses, a modified non-aqueous sol-gel approach under argon atmosphere was applied. Crystal structure and morphology of the products were analyzed by X-ray powder diffraction, scanning electron microscopy, and Raman spectroscopy. Phase pure samples crystallize in the olivine-type structure with a linear relation between lattice parameters and chemical composition. The mean crystal size of the particles is smaller than 300 nm with increasing agglomeration towards samples with higher nickel content. Effects of the Ni/Co ratio on P-O and M-O bonds were explored by FTIR and UV/VIS measurements, too. Substitution of Ni by Co causes larger cell volumes, longer atomic distances, and weaker P-O and M-O bonds. Additional studies focus on the mechanism of formation of LiNiPO4 and the conditions for the formation of the phosphide by-products Ni3P and Ni12P5 by carbothermal reduction. The electrochemical properties of all compounds were investigated by galvanostatic charge/discharge measurements and differential capacity analysis. The small particles of the sol-gel process lead to a superior capacity in comparison to a common solid-state synthesis. Cyclability of the whole solid-solution is evaluated for the first time. For a Ni content of 33 %, the voltage is shifted to a higher level and a positive effect on capacity retention is observed.