Identification of high ionic conducting members (M(iii) = Co, Ni) in the new lithium rich Li4.50M0.50TeO6 (M(iii) = Co, Ni, In) series

Abstract

Many new lithium-excess compounds with rock-salt related structures have been extensively studied in recent years to discover high-capacity electrode materials for lithium-ion batteries. In the present work, lithium rich layered tellurates, Li_4.50M_0.50TeO₆ (M(III) = Co, Ni, In), are added to the existing series of Li_4.50M_0.50TeO₆ (M(III) = Cr, Mn, Fe, Al, and Ga) oxides. Structural investigations revealed their stabilization in the space group C2/m with a new cationic ordering. The structure consists of (Li_1.50M_0.50TeO₆)³⁻ honeycomb arrays along the ab plane by the edge sharing of TeO₆ with (Li/M)O₆ octahedra. The honeycomb arrays are separated by the intermediate layer of Li alone in Li_4.50Co_0.50TeO₆. On the other hand, in the Ni and In analogues, the interlayer region consists of Li with Te, and Li with In ions, respectively. XPS studies confirmed the +3 oxidation state of Co and Ni ions. The appearance of a strong band at 680 nm resulting from LMCT (O → Co) in the UV-vis DRS data of the Li_4.50Co_0.50TeO₆ sample further indicated the presence of Co³⁺ (d⁶, low spin) ions. The absence of characteristic Ni²⁺ bands at around ∼650 and 740 nm supported Ni³⁺ ions. Li_4.50Co_0.50TeO₆ showed diamagnetic behaviour, while Li_4.50Ni_0.50TeO₆ displayed paramagnetic nature. A negative θ (−14(2)) K has been obtained in the temperature region of 300–100 K for Li_4.50Ni_0.50TeO₆ representing dominant antiferromagnetic interactions. At 2 K, Li_4.50Ni_0.50TeO₆ unveiled a non-linear trend with no significant hysteresis and nearly saturation at 5 T field indicating the existence of additional interactions. Li_4.50Co_0.50TeO₆ and Li_4.50Ni_0.50TeO₆ exhibited significant conductivity values of 0.016 and 0.003 S cm⁻¹, respectively, at 300 °C, thereby opening up further studies in this direction.