High-Entropy NASICON Phosphates (Na<sub>3</sub>M<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> and NaMPO<sub>4</sub>O<sub><i>x</i></sub>, M = Ti, V, Mn, Cr, and Zr) for Sodium Electrochemistry
Bing Wu, Guorong Hou, Evgeniya Kovalska, Vlastimil Mazánek, Petr Marvan, Liping Liao, Lukáš Děkanovský, David Sedmidubský, Ivo Marek, Charles H. Hervoches, Zdeněk Sofer
Abstract
High-entropy materials, with complex compositions and unique cocktail characteristics, have recently drawn significant attention. Additionally, a family of sodium super ion conductors (NASICONs)-structured phosphates in energy storage areas shows a comprehensive application for traditional alkaline ion batteries and, in particular, solid-state electrolytes. However, there is no precedent in fabricating this kind of NASICON-type high-entropy phase. Here, we report the successful fabrication of two well-crystallized high-entropy phosphates, namely, Na3(Ti0.2V0.2Mn0.2Cr0.2Zr0.2)2(PO4)3 (HE-N3M2P3) and Na(Ti0.2V0.2Mn0.2Cr0.2Zr0.2)2PO4Ox (HE-NMP). The prepared materials in which the transition metals (TMs) of Ti, V, Mn, Cr, and Zr occupy the same 12c Wykoff position can form a structure analogous to R3̅c Na3V2(PO4)3 that is carefully determined by X-ray diffraction, neutron diffraction, and transmission electron microscopy. Further, their performance for sodium ion batteries and sodium-based solid-state electrolytes was evaluated. The HE-N3M2P3 might exhibit a promising electrochemical performance for sodium storage in terms of its structure resembling that of Na3V2(PO4)3. Meanwhile, the HE-NMP shows considerable electrochemical activity with numerous broad redox ranges during extraction and insertion of Na+, related to the coexistence of several TM elements. The evaluated temperature-dependent ionic conductivity for HE-NMP solid electrolyte varies from 10–6 to 10–5 S cm–1 from room temperature to 398.15 K, offering high potential for energy storage applications as a new high-entropy system.