(Invited) Overview of the Ordering Phenomena in Li and Na Layered Oxide Electrode Materials

Abstract

AX(M)O2 layered oxides (A = Li, Na) are considered as the best positive electrode materials either for Li ion and Na ion batteries. In most of the systems, the shape of the voltage curve shows change in slope of the curve which indicate the occurrence of structural transitions due to: (i) electronic configuration modification in the MO2 slab, (ii) change in the slab packing, (iii) alkali ion-vacancy ordering. Moreover, in AX(M,L)O2 materials a M/L ordering can occur within the (M,L)O2 slab for the pristine material.Lithium and sodium tends to be ordered in the deintercalated materials for peculiar compositions. Nevertheless, due to the larger ionic radii of Na+ vs Li+, the repulsive interaction between sodium ions are stronger for sodium, leading to many Na+/vacancy patterns in the interslab space.Moreover, transition element orderings can occur within the (L,M)O2 slab if the difference in size is significant and if the cation ratio corresponds to a specific composition (1/3, ½, 2/3...). Then an interesting behaviour is observed as the sodium/vacancy ordering in the interslab space can be st superimposed to the cation ordering.Another original situation is also observed for cations (d2 or d3) which like to form M-M bonds. In this case, the pattering of short bond and long bond in the MO2 slab is directly related to the Na+/vacancy distribution and to the M3+/M4+ ratio.