Sodium-ion diffusion in alluaudite Na5In(MoO4)4

Abstract Alluaudite phases are very attractive as both cathode and electrolyte materials for rechargeable sodium-ion batteries. In this work, the combined experimental and DFT studies have been performed to establish the diffusion mechanism in alluaudite-like compound Na5In(MoO4)4. The ionic conductivity was found to reach 3.3 × 10−4 S/сm at 687 K, with an activation energy of 0.66 eV. The sodium diffusion mechanisms have been revealed from the analysis of the 23Na NMR spectra along with the DFT estimations of Na-ion migration barriers. Our results predict that one-dimensional diffusion of sodium in the separate channels along the c-axis is accompanied by the cross-linking jumps providing two-dimensional diffusion in the bc-plane. It is clearly demonstrated that the indium deficiency favors 2D diffusion, but sharply increases the energy barrier for 1D diffusion. Comparison of our results for Na5In(MoO4)4 and related Na5Sc(MoO4)4 shows that the type and deficiency of M-metal in the NaxMy(MoO4)z alluaudites can control the sodium diffusion. The present work highlights the key aspects of cation influence on the diffusion properties in alluaudite materials.