Structures and electronic states of nickel-rich oxides for lithium ion batteries

Abstract

A new superstructure of layered pristine LiNiO₂ (LNO) was obtained by first replicating 16 times the optimized structure of the 12-atom primitive cell of the Rm space group and then relaxing both cell parameters and internal positions. The structural optimization is accompanied by a very significant lowering of the internal energy. The Rm space group is the space group observed experimentally by XRD. In contrast to the structure commonly proposed in the literature, this new crystal structure, which still belongs to the Rm space group, shows size and charge disproportionation of the NiO₆ octahedra. The charge disproportionation results in a change in the Ni oxidation state from Ni³⁺ calculated in the 12-atom primitive unit cell, to Ni⁴⁺ and almost Ni²⁺. This is the first time that such charge disproportionation has been calculated for the Rm crystal structure of LNO. The Ni–O bond length distribution of the new structure agrees well with experimental values. Our results show that the choice of the simulation unit cell is important for determining the energetics of this class of oxide material, proposed for cathodes in lithium ion batteries (LIBs). We used this new structure as a template for the study of the structural and electronic changes induced by delithiation and by Mn for Ni cation substitution, originating the solid solutions LiNi_yMn_(1−y)O₂ (LNMO). Our results, surprisingly, agree well with existing experiments and explain observed trends better than previous studies.