Examination of Ion Conduction Mechanism and Electronic Structure in Mg Ion Conductor Mg_1-2x(Zr_1-xNb_x)₄(PO₄)₆

INTRODUCTION 

For the purpose of the realization of the rechargeable battery with superior safety, solid electrolytes have been studied actively due to their non-flammability and superior stability. This work paid attention to a cationic conductor having a strong zirconium phosphate frame in crystal structure as the electrolyte materials. In Mg_1-2x(Zr_1-xNb_x)₄(PO₄)₆ which is a Mg ion conductor, the substitution of the Nb⁵⁺ with higher valence for the Zr⁴⁺ decreases the charge-carrier concentration on the basis of the electroneutrality condition, but the Nb⁵⁺ substitution achieves improvement of the ionic conduction characteristic, i.e., the higher conductivity and the lower activation energy. It is necessary to examine a relation between the crystal structure and the ionic conduction of Mg_1-2x(Zr_1-xNb_x)₄(PO₄)₆ in order to develop of superior materials. However, it is difficult to analyze effects of the Nb⁵⁺ at the Zr⁴⁺ site on the diffusion mechanism in detail, since one cannot distinguish each atom occupying the same crystallography site by only experimental approaches like the conductivity measurement and the Rietveld analysis. Therefore, in this study, we examined ion diffusion mechanism in Mg_1-2x(Zr_1-xNb_x)₄(PO₄)₆ by the computational approach such as first-principles calculation and the first-principles molecular dynamics calculation in addition to experimental approach. Furthermore, we examined a relation between the bonding state and the ionic conduction from electronic structure analysis.

EXPERIMENTAL

Starting materials of MgHPO₄･3H₂O, ZrO(NO₃)₂･2H₂O, Nb₂O₅, and (NH₄)₂HPO₄ were mixed in a ratio of (1-2x):(4-4x):2x:(5+2x) and heated at 300°C for 5 h and then at 1200°C for 12 h in air. In addition, the obtained product was pulverized by a ball mill processing. The resulting powder was sintered in the form of disc by Spark Plasma Sintering Machine (LABOX-315). During the entire SPS process, a mechanical pressure of 50 MPa was applied, and the sintering was carried out at 1100°C for 5 min. The product was identified by powder XRD and the lattice constant was calculated. In addition, a morphology of the sintered pellet was investigated by SEM observation. The conductive characteristic of the sample was evaluated by measuring the resistance of the electrolyte by the AC impedance method. In order to determine an average structure of the sample, the synchrotron X-ray diffraction pattern was measured (BL02B2, SPring-8). The data was analyzed with the Rietveld technique using Rietan-FP. We made a structure model for the theoretical calculation based on a structure parameter of the average structure that we analyzed, and then examined a correlation of Mg²⁺ and Nb⁵⁺ by the first-principles calculation with the CP2K program. In addition, we visualized the conduction path of Mg²⁺by the first-principles molecular dynamics calculation. Furthermore, we performed electronic structure analysis (Density of states, electron density).

RESULTS AND DISCUSSION

From powder X-ray diffractions, it was found that a product could be attributed to a single phase of the Fe₂(SO₄)₃ type structure with space group of P2₁/n. In addition, as a result of a cross-section observation of the sample by SEM, the cavity was not seen. It was also confirmed that the sintering characteristics was enough for measurement of the electrochemistry properties, from the relative density by the Archimedes method. Furthermore, we performed crystal structure analysis by Rietveld analysis using a diffraction pattern obtained from the synchrotron X-ray diffraction measurement. Based on a refined crystal structure parameter, we built the multiple models with different distributions of Zr and Nb, and performed structure relaxation by the first-principles calculation. As a result, we demonstrated that the structure became stable when Nb⁵⁺ occupied the Zr1 site although the sample has the two crystallographic sites for Zr⁴⁺. It was also found that Mg²⁺ tended to exist in the vicinity of the ZrO₆ octahedron in comparison with the NbO₆ octahedron. Furthermore, by performing the first-principles molecular dynamics calculation, a diffusion path of the Mg²⁺ could be visualized (Fig.1). In order to gain deeper understanding on an effect of Nb⁵⁺ on the Mg²⁺ behavior, the electron density distribution and density of states indicated high ion bonding of Mg-O, high covalent bonding of P-O and higher covalent bonding of Nb-O than Zr-O. Furthermore, we evaluated the effective charge of each ion by space division of the electron density distribution on the basis of the Vonoroi method. As a result, it was found that oxygen of NbO₆ showed the lower effective charge than that of ZrO₆, and the Mg which was neighboring to NbO₆ had the higher effective charge than the Mg around ZrO₆.

This work was financially supported in part by the ALCA-SPRING project.

References

1) N. Imanaka, et al., Ionics, 7 440-446 (2001).

Figure 1