LiNi(M)O₂ Layered Oxides: Positive Electrode Materials for Lithium Batteries

Abstract

High oxidation states of transition metals in layered oxides have attracted the attention of many research groups for application in high voltage electrochemical cells. The layered structure is preferred for its flexibility to adopt the geometry of the guest species by expansion of the interlayer spacing or by sliding of the slabs with respect to each other which may result in changes in stacking order. The structure of the AMO₂ (A⁺ = alkaline ion, M³⁺ = transition ion) layered compounds is based on a close-packed network of oxygen atoms with the A⁺ and M³⁺ ions ordering on alternating (111) planes of the cubic rock-salt structure (Fig. 1). The (111) ordering introduces a slight distortion of the lattice to hexagonal symmetry. While all NaMO₂ ternary oxides exhibit such a layered structure, as a result of the large difference in size between Na⁺ and M³⁺, in the case of lithium derivatives the 2D order is only observed for M = V, Cr, Co, Ni (Fig. 2). The use of lithium layered oxides as intercalation electrode materials was mentioned by Goodenough in the early 80’s [1]. Since then, a tremendous amount of work has been devoted around the world to the study of LiNiO₂ and LiCoO₂ [2–3].The cobalt phase, easy to prepare and stable in a 2D structure, already exists in commercial devices while the LiNiO₂ phase, cheaper, represents a promising alternative. However, one of the main disadvantage of the nickelate is that it is difficult to prepare due to the tendency of non-stoichiometry as a result of the presence of an excess nickel leading to the Li_1-zNi_1+zO₂ formula (Fig. 3). The extra nickel ions which are located in the lithium sites in the interslab space, break the 2D ordering, thus hindering the lithium diffusion during the cell cycling [4]. In addition to the capacity fading, the instability of the oxidized nickel phase lowers the safety of the battery. Therefore, to improve the performance of the battery which relates to a stabilization of the 2D structure, over the last few years, a large number of studies have been devoted to the cationic substitution for nickel. In this paper, attention will be focused on the effect that substitution of Co, Fe and Al for Ni has on the structure and electrochemical performances.