Electrochemical Characterization of a Lithiated Mixed Nickel-Cobalt Oxide (LiNi0.5Co0.5O2) Prepared by Sol-Gel Process

Lithiated transition metal oxides having a layered structure and general formula LiMO(2), have been extensively studied as positive electrode active materials for lithium or lithium-ion batteries. In particular, lithium nickel dioxide (LiNiO(2)) and lithium cobalt dioxide (LiCoO(2)) present a layered structure with high diffusion coefficients for the lithium ion. This latter property is very important in order to realize practical devices having high discharge rates. LiNiO(2), compared with LiCoO(2), has the advantage to be a cheaper material with a higher specific capacity for lithium cycling, but its stability upon cycling can be greatly influenced by the displacement of Ni ions from the Ni layers to the Li planes as the content in lithium is reduced over a certain value. Recently, solid solutions such as LiNi(x)Co(1-x)O(2) have been proposed to offer a compromise between stability, cost and capacity. In this work we have studied LiNi(0.5)Co(0.5)O(2) prepared by the Complex Sol-Gel Process (CSGP). The advantage of this procedure toward the solid-state process is the high homogeneity in composition and in particle dimension of the synthesized compounds. The samples have been characterized electrochemically using chronopotentiometric, voltammetric and impedance measurements in liquid electrolyte. The results indicates that CSGP-synthesized LiNi(0.5)Co(0.5)O(2) shows good cyclability (after 1000 cycles about 2/3 of the initial capacity can still be cycled) only if the anodic potential is limited to about 4.2 V. The quite low values of the specific capacity (similar to 70 mAh/g at C/1 charge-discharge rate) can be justified by the non-complete calcination reaction, as suggested by X-ray measurements. Kinetic properties have been evaluated by Electrochemical Impedance Spectroscopy measurements, which have shown quite high values for the lithium chemical diffusion coefficient (10(-7) divided by 10(-8) cm(2)s(-1)) and its unexpected decrease as deintercalation proceeds from x=0.5 in LiNi(0.5)Co(0.5)O(2).