Unraveling the Origin of the Anomalous Na+ Mobility in NASICON Solids: Insights from Molecular Dynamics Simulations

L Pradhan and PP Kumar, JOURNAL OF PHYSICAL CHEMISTRY C, 129, 13756-13767 (2025).

DOI: 10.1021/acs.jpcc.5c02759

Extensive molecular dynamics investigations of the NASICON solid solutions, Na1+x Zr2Si x P3-x O12, where 0 <= x <= 3, are carried out employing classical molecular dynamics (MD) simulations in the isothermal-isobaric (NPT) ensemble at 600 K. These calculations are complimented with additional sets of constrained MD simulations, which restrict the dynamics of the framework ions and/or the unit cell volume with composition. A comparison of Na+ mobility across these series of simulations establishes the significance of the dynamical correlation between Na+ ions with framework-oxygens, which gets compounded by the anomalous volume expansion of the unit cell with composition. Further, it is demonstrated that there are two competing factors sensitive to composition (x), namely, the Na+ interaction with the Si4+/P5+ ions of the framework and the mutual repulsion between the Na+ ions. However, they impact differently at the two dominant Na+ sites in the migration channel, resulting in a near barrier-free transport for Na+ around x = 2.5, which leads to the observed anomalous ionic conductivity of NASICON with composition. The results present insights into enhancing the ionic conductivity in similar covalent framework solids through targeted aliovalent substitutions.

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