[PO-O2] Poster Session 2

[P2-80] Potential cathode material Na_xVOPO₄ for rechargeable Sodium - ion batteries: DFT investigation

^○Duc Huu Luong¹, An Van Dinh^1,2, Yoshitada Morikawa³, Yoji Shibutani^2,1 (1.Nano Technology Program, Vietnam Japan University, Viet Nam, 2.Center for Atomic and Molecular Technology, Graduate School of Engineering, Osaka University, Japan, 3.Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, Japan)

Using the density functional calculations, we investigated the crystal and electronic structures, electrochemical properties and ion diffusion mechanism of Sodium - polaron complex in Sodium Vanadyl phosphate NaVPO₅. A calculated voltage of 3.77 V (GGA+U) and 3.58 V (HSE06) corresponding to a redox reaction of potential couple V^4+/V⁵⁺ were in good agreement with experimental result [1]. The diffusion mechanism of charge carriers was explored using GGA+U. In the charge process, a Sodium ion is removed from the crystal structure so that the Sodium vacancy appears and a positive small polaron forms at one the two first nearest VO₆ octahedron. The diffusion of Na+ ion which is accompanied by a positive small polaron is described by three elementary diffusion processes, including single, crossing and parallel diffusions [2]. With the smallest activation energy of 395 meV, the pathway of Sodium diffusion along the [010] direction is the most favorable diffusion pathway and it is significantly higher than previous calculation which did not mention the small polaron formation. In the discharge process, Sodium ion is intercalated to structure of β-VOPO₄, then the negative small polaron forms at one of the nearest neighbour VO₆ octahedron. In addition, the elementary diffusion process of sodium ion is more favorable in the [010] direction. However, because of about 10% smaller volume, the diffusion activation energy (627 meV) is significantly higher than those required in the charge process. Compared with the other materials, it is obviously that this material would perform as well as some common materials for cathode such as Olivine.
[1] G. He, A. Huq, W. H. Kan, and A. Manthiramm, Chem. Mater. 2016, 28, 1503−1512
[2] V. A. Dinh et al. App. Phys. Express, 2012, 5, 045801