Na–Li–[V3O8] insertion electrodes: Structures and diffusion pathways

The potential insertion-electrode compounds Na1.2[V3O8] (NaV) and Na0.7Li0.7[V3O8] (NaLiV) were synthesized from mixtures of Na2CO3, Li2CO3 and V2O5, which were melted at 750° and subsequently cooled to room temperature. The structures of NaV and LiV contain sheets of polymerized (VOn) polyhedra, which are topologically identical to the sheet of polymerized polyhedra in Li1.2[V3O8] (LiV). Vanadium occurs in three different coordination environments: [2+3] V(1), [2+2+2] V(2) and [1+4+1] V(3). Calculated bond-valence sums indicate that V4+ occurs preferentially at the V(3) site, which agrees with the general observation that [6]-coordinated V4+ prefers [1+4+1]-rather than [2+2+2]-coordination. The M-cations Na and Li occur at three distinct sites, M(1), M(2) and M(3) between the vanadate sheets. The M(1)-site is fully occupied and has octahedral coordination. The M(2) sites are partly occupied in NaV and NaLiV, in which they occur in [4]- and [6]-coordination, respectively. Li partly occupies the M(3) site in NaLiV, in which it occurs in [3]-coordination. The M(2) and M(3) sites in NaLiV occur closer to the vanadate sheets than the M(2) sites in NaV and LiV. The shift in these cation positions is a result of the larger distance between the vanadate sheets in NaLiV than in LiV, which forces interstitial Li to move toward one of the vanadate sheets to satisfy its coordination requirements. Bond-valence maps for the interstitial cations Na and Li are presented for NaV, NaLiV and LiV. These maps are used to determine other potential cation positions in the interlayer and to map the regions of the structure where the Na and Li have their bond-valence requirements satisfied. These regions are potential pathways for Na and Li diffusion in these structures, and are used to explain chemical diffusion properties of Na and Li in the Na–Li–[V3O8] compounds.

Bond-valence map for Li in Na0.7Li0.7[V3O8]. Contour lines representing the bond-valence sum of 1.0 vu are marked with thick black lines and indicate potential diffusion pathways of Li.Figure optionsDownload as PowerPoint slide