Identifying Migration Channels and Bottlenecks in Monoclinic NASICON‐Type Solid Electrolytes with Hierarchical Ion‐Transport Algorithms

Abstract

Monoclinic natrium superionic conductors (NASICON; Na 3 Zr 2 Si 2 PO 12 ) are well-known Na-ion solid electrolytes which have been studied for 40 years. However, due to the low symmetry of the crystal structure, identifying the migration channels of monoclinic NASICON accurately still remains unsolved. Here, a cross-verified study of Na + diffusion pathways in monoclinic NASICON by integrating geometric analysis of channels and bottlenecks, bond-valence energy landscapes analysis, and ab initio molecular dynamics simulations is presented. The diffusion limiting bottlenecks, the anisotropy of conductivity, and the time and temperature dependence of Na + distribution over the channels are characterized and strategies for improving both bulk and total conductivity of monoclinic NASICON-type solid electrolytes are proposed. This set of hierarchical ion-transport algorithms not only shows the efficiency and practicality in revealing the ion transport behavior in monoclinic NASICON-type materials but also provides guidelines for optimizing their conductive properties that can be readily extended to other solid electrolytes.

The origin of high Na+ ion conductivity in Na1+xZr2SixP3−xO12 NASICON materials

“…Bottlenecks formed by ZrO 6 octahedral and PO 4 tetrahedral edges determine the migration energy. 28,45,48,51,[53][54][55] The high conductivity of compositions with x = 2-2.5 is not only attributed to the high concentration of mobile charge carriers, but also to the widening of the bottleneck that promotes the ion mobility. 28,32,40,45,47,54 Furthermore, increasing sodium ion concentration enhances the correlated migration by increasing sodium-sodium coulombic repulsions.…”

“…28,45,48,51,[53][54][55] The high conductivity of compositions with x = 2-2.5 is not only attributed to the high concentration of mobile charge carriers, but also to the widening of the bottleneck that promotes the ion mobility. 28,32,40,45,47,54 Furthermore, increasing sodium ion concentration enhances the correlated migration by increasing sodium-sodium coulombic repulsions. 49,52 This type of collective migration appears also in fast lithium ion conductors, such as in NASICON-, 59,60 LISI-CON- 61,62 and garnet [63][64][65] type structures.…”

“…The local minimum along the 1801 migration path in the structure with x = 3 corresponds to the mid-Na site suggesting that this site occurs due to high sodium content and resulting coulombic repulsion. 43,49,[51][52][53][54] The migration energy decreases with increasing angle of pathway due to less steric hindrance of the migrating sodium ions. In addition, the alignment of the sodium ions migrating in 1801 direction, which repel one another, may promote the correlated migration.…”

“…These results agree with previous studies which suggest that sodium ion diffusion increases with opening of the bottleneck. 40,42,47,54,88 For structures with composition x = 1 and x = 2 and different cation configurations, multiple migration pathways were investigated. The migration energies E mig,0 were extracted from the energy profiles of various configurations according to eqn (2).…”

Atomistic insight into the dopant impacts at the garnet Li7La3Zr2O12 solid electrolyte grain boundaries

“…S6† to understand the diffusion correlation of Li-ions in the investigated GB models, which has been widely discussed in previous studies on SE bulks. 17,58 G d ( t , r ) accounts for the average probability of finding one Li-ion ( i ) at a distance r from a site that was formerly occupied by another Li-ion ( j ) at time t . The build-up of G d ( t , r ) around r = 0 with the increase of t indicates that more and more Li-ions occupy the other Li-ion sites, showing highly concerted diffusion behaviour in the investigated GB models.…”