Striking impact of Na insertion on structural and electronic properties of the electrode material Na_2+x V₆O₁₆

The Na_2.55V₆O₁₆ structure is a promising material for sodium ion batteries due to a significant capacity and stability at high current rates, but its cycle stability for Na application is significantly lower than that obtained with Li (de-)insertion. In this work, we present the results of density functional theory (DFT) calculations on the structural and electronic properties of Na_2+x V₆O₁₆ compounds up to x  =  2. As x increases, we evidence a strong deformation of the tetrahedral sites occupied by sodium ions, leading to new highly stable sites for these inserted ions at x  =  2. Comparing with Li₄V₆O₁₆, we demonstrate that the stability of these new Na sites can be attributed to the electrostatic interactions between sodium ions and atoms of the host structure, the evolution of the oxidation degree of Vanadium atoms being another indicator of such effects.