Magnesium-Ion Distribution and Dynamics in Magnesium Zirconium Orthophosphate

Thursday, 23 June 2016
Riverside Center (Hyatt Regency)
N. Kitamura, H. Kuwajima, N. Ishida, and Y. Idemoto (Tokyo University of Science)
The magnesium secondary battery has drawn much attention as a post lithium-ion secondary battery at recent years because of the higher theoretical volumetric energy density. Similarly to the lithium-ion battery, inorganic oxides such as MgCo2O4 [1] and MgFeSiO4 [2] can be regarded as one of the most promising candidates for a cathode material of the magnesium secondary battery. Due to a strong electrostatic interaction between Mg2+ and surrounding oxide anions, however, Mg2+ diffusion is deteriorated significantly in oxides and thus the battery performance is very low generally at room temperature. In order to overcome the problem, we have to carry out a systematic study on Mg2+ diffusion in solids and gain deeper understanding on Mg2+ dynamics.

      From such background, this work focuses on MgZr4(PO4)6-based Mg-ion conductors [3], and investigated Mg2+ behaviors in the crystal by means of a combination of the density functional theory (DFT) and the reverse Monte Carlo (RMC) modelling using synchrotron X-ray total scatterings.

      We synthesized Mg1-2x(Zr1-xNbx)4(PO4)6 with a conventional solid-state reaction method using MgHPO4·3H2O, ZrO(NO3)2·2H2O, Nb2O5 and NH4H2PO4 as starting materials. In the sintering process, we performed the spark plasma sintering (SPS: LABOX-315, Sinter Land Inc.) in order to prepare dense ceramics of the samples for electrical conductivity measurements. For refinements of crystal structures (average structures) of the samples, synchrotron X-ray Bragg profiles were recorded at BL19B2 installed at SPring-8, Japan, and then analyzed by the Rietveld and maximum-entropy method (MEM) techniques. From the refined unit cells, we constructed super cells comprised of 276 atoms or above, and Mg-ion distributions were determined experimentally by the RMC simulations for polycrystalline materials [4, 5] using the Bragg profiles and X-ray structure factors S(Q) simultaneously. The S(Q) were measured with BL04B2 at SPring-8, Japan, and then degraded by convolutions considering the simulation box sizes.

      As a theoretical approach, we performed the DFT calculations and the DFT-based molecular dynamic (MD) simulations with the CP2K program which combines the localized Gaussian basis set and plane waves for a dual GPW basis set. As the exchange-correlation energy functional, the generalized gradient approximation of PBE was utilized. In the DFT-based MD simulation, the NVT ensemble was adopted, and the temperature was controlled with a CSVR thermostat. Initial cells for these calculations were constructed on the basis of the average structures or the atomic-configuration snapshots obtained by the RMC modelling.

      From preliminary laboratorial X-ray diffraction measurements, it is confirmed that Mg1-2x(Zr1-xNbx)4(PO4)6 can be synthesized successfully at least within the Nb-content range of x=0~0.25. The Rietveld analysis using synchrotron X-ray data reveals that the Nb-substituted samples have a monoclinic average structure with a space group of P21/n which is the same as MgZr4(PO4)6. By using the refined unit cell, we constructed a super cell with 276 atoms and then performed DFT-base MD simulation to visualize Mg2+ diffusion. As a result, it is indicated that Mg2+ at an experimentally-determined crystallographic site with five-fold coordination migrates via another site. It is also found that a triangle formed by three oxide ions becomes a bottle neck for the migration. Such a diffusion pathway is supported by a bond-valence-sum (BVS) mapping using an atomic configuration relaxed energetically by the DFT.


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


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