Atomic Structure and Kinetics of NASICON Na_xV₂(PO₄)₃ Cathode for Sodium‐Ion Batteries

Abstract

Na 3 V 2 (PO 4 ) 3 is one of the most important cathode materials for sodium‐ion batteries, delivering about two Na extraction/insertion from/into the unit structure. To understand the mechanism of sodium storage, a detailed structure of rhombohedral Na 3 V 2 (PO 4 ) 3 and its sodium extracted phase of NaV 2 (PO 4 ) 3 are investigated at the atomic scale using a variety of advanced techniques. It is found that two different Na sites (6b, M1 and 18e, M2) with different coordination environments co‐exist in Na 3 V 2 (PO 4 ) 3 , whereas only one Na site (6b, M1) exists in NaV 2 (PO 4 ) 3 . When Na is extracted from Na 3 V 2 (PO 4 ) 3 to form NaV 2 (PO 4 ) 3 , Na + occupying the M2 site (CN = 8) is extracted and the rest of the Na remains at M1 site (CN = 6). In addition, the Na atoms are not randomly distributed, possibly with an ordered arrangement in M2 sites locally for Na 3 V 2 (PO 4 ) 3 . Na + ions at the M1 sites in Na 3 V 2 (PO 4 ) 3 tend to remain immobilized, suggesting a direct M2‐to‐M2 conduction pathway. Only Na occupying the M2 sites can be extracted, suggesting about two Na atoms able to be extracted from the Na 3 V 2 (PO 4 ) 3 structure.