Exploring Factors Limiting Three-Na⁺ Extraction from Na₃V₂(PO₄)₃

Abstract

NASICON-type Na 3 V 2 (PO 4 ) 3 is a promising cathode material for Na-ion batteries. Although it is well known that two Na + can be extracted from Na 3 V 2 (PO 4 ) 3 by charging the cathode material, an electrochemical three-Na + extraction has not been reported yet, to the best of our knowledge. In this work, we studied factors that limit the three-Na + extraction from Na 3 V 2 (PO 4 ) 3 . In DFT calculations, the voltage of the third-Na + extraction is predicted to be more than 4.5 V vs. Na + /Na 0 , which is above the potential windows of the conventional organic electrolytes. Our study of Na 3 V 1.5 Al 0.5 (PO 4 ) 3 reveals that such a high voltage is required when Na ions are extracted from Na1 sites in the NASICON structure. From NEB calculations, the activation energy of the Na + extraction from the Na1 site is predicted to be 753 meV for NaV 2 (PO 4 ) 3 . Ab-initio molecular dynamics simulations also suggest that the Na ions which remain in NaV 2 (PO 4 ) 3 are kinetically locked up in Na1 sites. Our results indicate that the three-Na + extraction is limited due to the high voltage and the large activation energy. We also compare Na 3 V 2 (PO 4 ) 3 with Li 3 V 2 (PO 4 ) 3 , in which the three-Li + extraction has been reported.

Phase stability and sodium-vacancy orderings in a NaSICON electrode

“…Generally, the computed voltage proles at 0 K are in agreement with previous reports. 12,34,52,53 Apart from a small ($50 mV) previously unreported voltage step at x ¼ 2 (N 2 VP), the voltage proles for 1 # x # 3 at different temperatures display a similar shape. Although there is good qualitative agreement between theory and experiment for the entire Na composition range, all the predicted voltage curves at the selected temperatures underestimate the average voltage across 1 # x # 3, and overestimate the average voltage in the 3 # x # 4 range compared to experiment.…”

“…[11][12][13]39 Our data indicates that, at x ¼ 1, all Na ions only occupy Na(1) sites, and their extraction may affect the structural integrity of N 1 VP, thus suppressing the chemical or electrochemical extraction of the last Na + . Indeed, Na migration between Na(1) sites at N 1 VP is hindered by high migration barriers ($755 meV, computed previously), 52 which usually indicate highly stable arrangement of atoms. Furthermore, the thermodynamic instability of the V 2 (PO 4 ) 3 framework with respect decomposition into VPO 5 + VP 2 O 7 can also account for the lack of reproduceable extraction of the last Na + from N 1 VP.…”

“…Huang et al and Ishado et al independently used nudged elastic band calculations to estimate Na migration barriers between Na(1) and Na(2) sites ($420 meV, and $304 meV, respectively) in N 3 VP, which suggest facile Na mobility in N 3 VP as opposed to Na migration in N 1 VP ($755 meV). 52,55 During the construction of the N x VP phase diagram, we isolated two novel low-temperature stable Na/vacancy orderings at the compositions N 2 VP and N 3.5 VP. Other low energy structures are visible from an analysis of the convex hull of Fig.…”

“…It is impossible to extract the 4 th Na ion from N 1 VP although the V(IV)/V(V) redox couple is theoretically accessible. 21,33,34 In the R 3c N x VP, two distinct Na sites exist: Na(1) and Na (2). The Na(1) site is 6-coordinated and sandwiched between 2 VO 6 octahedra, whereas Na(2) (yellow or black-outlined circles in Fig.…”

Eldfellite-type cathode material, NaV(SO4)2, for Na-ion batteries

“…13 The potential difference of 1 V between NaV(SO 4 ) 2 and NaFe(SO 4 ) 2 was reasonably attributed to the typical potential gap between V 2+ / V 3+ and Fe 2+ /Fe 3+ , similar to the difference observed between the NASICON-type materials Na 3 V 2 (PO 4 ) 3 (1.5 or 1.6 V) and Na 3 Fe 2 (PO 4 ) 3 (2.5 V). [18][19][20][44][45][46] Different electrolyte types did not affect the discharge-charge properties in the sodiation/ desodiation process from the initial state of NaV(SO 4 ) 2 , as shown in Fig. 3a and c.…”