Deep insights into the sodium transport mechanism and phase evolution in Na4VP2O9 cathode/anode material

LY Tang and YM Kang and M Shui, JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, 195, 112273 (2024).

DOI: 10.1016/j.jpcs.2024.112273

For the entire intercalation/deintercalation range of Na4VP2O9, quantum mechanics is applied to investigate sodium transport mechanism and a machine learning force field is used instead of ab initio method to offers an applicable method for the full understanding of the complicated Na (+) movements dynamically. It shows chain of '8' shaped units horizontally and repeated alternating round aggregate and '8' shaped long chain vertically. For primitive Na4VP2O9, facile Na+ transportation along a-axis is observed with an energy barrier of 0.16 V. As a cathode material, Na3VP2O9 also shows fast Na (+) diffusion with a minimal diffusion energy barrier 0.27 eV along a-axis. However, for the fully sodiated state of Na4VP2O9, Na+ diffusion in the lattice is significantly restricted with an energy barrier of 0.92 eV along b-axis. Accordingly, the sodium ion diffusion coefficients at room temperature for Na4VP2O9, Na4VP2O9 and Na4VP2O9 are 1.2 x 10 9 cm(2) s(-1), 5.9 x 10 9 cm(2) s(-1) and 8.2 x 10 11 cm(2) s(-1), respectively. The formation energy convex hull originated from the removal of Na1 sites and the occupation of E8cO8 cavities reveals an intermediate phase, Na3.5VP2O9, for the NaxVP2O9 (3 = x = 4) and three intermediate phases, Na4.5VP2O9, Na4.625VP2O9 and Na4.75VP2O9, for the NaxVP2O9 (4 = x = 5).

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