Magnetic clusters in LiNi_1−yCo_yO₂ nanomaterials used as cathodes in lithium-ion batteries

We have prepared LiNi_1−yCo_yO₂ (0 ≤ y ≤ 1) compounds as polycrystalline nanomaterials (d ≈ 400 nm) using a low-temperature sol–gel method. XRD studies indicate that these materials are single phase for 0.2 ≤ y ≤ 1 with an ordered distribution of Li and Ni/Co in the layered structure. Nevertheless, as this technique provides only averaged structural information, it is still possible that locally there are some defects, among them disorder, that could affect the electrochemical behaviour of these materials. In fact, through FTIR spectroscopy we observe for the Li–O band a slight deviation from a linear behaviour for high nickel content (y ≤ 0.2), that is attributed to the presence of Ni cations in the predominantly lithium layers (octahedral interstices). In addition, by means of magnetic measurements, χ_m (T) and M(H), we detect in all the samples a ferrimagnetic signal, that gets smaller and smaller as the Co content increases, but that indeed reveals the presence of some Ni²⁺ ions occupying Li⁺ places, that would lead to the formation of small ferromagnetic islands. From those magnetic measurements we have estimated the size of those nanometric magnetic inhomogenities that decreases upon Co doping from R_{(y = 0)} = 3.5 nm to R_{(y = 0.4)} = 0.5 nm. This result confirms that the addition of Co³⁺ inhibits the presence of interlayer Ni²⁺ and therefore favours a better lamellar structure, only obtained for y > 0.4.