New rock salt-related oxides Li3M2RuO6 (M=Co, Ni): Synthesis, structure, magnetism and electrochemistry

Abstract

We describe the synthesis, crystal structure, magnetic and electrochemical characterization of new rock salt-related oxides of formula, Li 3 M 2 RuO 6 ( M =Co, Ni). The M =Co oxide adopts the LiCoO 2 ( R- 3 m ) structure, where sheets of LiO 6 and (Co 2 /Ru)O 6 octahedra are alternately stacked along the c -direction. The M =Ni oxide also adopts a similar layered structure related to Li 2 TiO 3 , where partial mixing of Li and Ni/Ru atoms lowers the symmetry to monoclinic ( C 2 /c ). Magnetic susceptibility measurements reveal that in Li 3 Co 2 RuO 6 , the oxidation states of transition metal ions are Co 3+ (S=0), Co 2+ (S=1/2) and Ru 4+ (S=1), all of them in low-spin configuration and at 10 K, the material orders antiferromagnetically. Analogous Li 3 Ni 2 RuO 6 presents a ferrimagnetic behavior with a Curie temperature of 100 K. The differences in the magnetic behavior have been explained in terms of differences in the crystal structure. Electrochemical studies correlate well with both magnetic properties and crystal structure. Li-transition metal intermixing may be at the origin of the more impeded oxidation of Li 3 Ni 2 RuO 6 when compared to Li 3 Co 2 RuO 6 . Interestingly high first charge capacities (between ca. 160 and 180 mAh g −1 ) corresponding to ca. 2/3 of theoretical capacity are reached albeit, in both cases, capacity retention and cyclability are not satisfactory enough to consider these materials as alternatives to LiCoO 2 . Two new rock salt related oxides of formula, Li 3 M 2 RuO 6 , ( M =Co, Ni) have been prepared. The M =Co oxide adopts the LiCoO 2 ( R- 3 m ) structure and the M =Ni oxide adopts a similar layered structure related to Li 2 TiO 3, monoclinic ( C 2 /c ), with partial mixing of Li and Ni/Ru atoms. For Li 3 Co 2 RuO 6 , oxidation state for Ru is 4+ and antiferromagnetic (AFM) order is found below 10 K while for the analogous Li 3 Ni 2 RuO 6 , Ru oxidation state is 5+ and a ferrimagnetic (FM) behavior with a Curie temperature of 100 K is found. Electrochemical studies correlate well with both magnetic properties and crystal structure. • New Ruthenium rock salt-related oxides of formula, Li 3 M 2 RuO 6 ( M =Co, Ni) were studied. • Structurally different: rhombohedral α-NaFeO 2 -type (Co) and monoclinic Li 2 TiO 3 -type (Ni) • Magnetic behavior is different: the Co sample is AFM while the Ni one is ferrimagnetic. • Ruthenium oxidation states are different: 4+ in the Co sample and 5+ in the Ni one. • Electrochemical studies correlate well with magnetic properties and crystal structures.