A study into the extracted ion number for NASICON structured Na₃V₂(PO₄)₃ in sodium-ion batteries

Abstract

Excellent C-rate and cycling performance with a high specific capacity of 117.6 mA h g(-1) have been achieved on NASICON-structure Na3V2(PO4)3 sodium-ion batteries. Two different Na sites, namely Na(1) and Na(2), are reported in the open three-dimensional framework, of which the ions at the Na(2) sites should be mainly responsible for the electrochemical properties. It is vitally important and interesting to find that there are two kinds of possible ion occupation of Na ions in Na3V2(PO4)3 and the investigation of ion-extraction number is firstly explored by discussing ion occupations with the help of first-principles calculations. The ion occupation of 0.75 for all Na sites is suitable for the configuration of [Na3V2(PO4)3]2, and the two-step extraction process accompanied by structure reorganization can account for the theoretical capacity of Na3V2(PO4)3.

Discovery of a Sodium-Ordered Form of Na3V2(PO4)3 below Ambient Temperature

“…Contrary to recent reports based on DFT calculation 4,17 , where the optimized occupancy for Na(1) and Na(2) are respectively 1 and 0.67, we found here the experimental occupancy factors for Na (1) and Na (2), refined from Rietveld refinement to be 0.75(3) and 0.70(4), respectively, which gives rise to ; α-Na 3 Fe 2 (PO 4 ) 3 18 ) . It is reminiscent of the well-known M(2) M(1) M(2) path discussed in ref.…”

“…Wyckoff (2) 0.0121(10) 0.0150 (9) 0.1326 (17) 0.0075 (9) 0.0174 (5) 0.0348 (5) In the refined crystal structure, two independent Na + ions (Na(1) and Na(2)) are located on the 6b (0, 0, 0) (6-fold coordination) and 18e (0.6349, 0, 1/4) (8-fold coordination) Wyckoff sites respectively. Contrary to recent reports based on DFT calculation 4,17 , where the optimized occupancy for Na(1) and Na(2) are respectively 1 and 0.67, we found here the experimental occupancy factors for Na (1) and Na (2), refined from Rietveld refinement to be 0.75(3) and 0.70(4), respectively, which gives rise to ; α-Na 3 Fe 2 (PO 4 ) 3 18 ) .…”

An Efficient Evaluation of F-doped Polyanion Cathode Materials with Long Cycle Life for Na-Ion Batteries Applications

“…15 samples were 5.68 × 10 −10 cm 2 s −1 and 6.37 × 10 −10 cm 2 s −1 for anodic and 6.03 × 10 −10 cm 2 s −1 and 6.46 × 10 −10 cm 2 s −1 for cathodic reaction, respectively. These results are in consistent with NVP reports 36,37 .

Figure 4Electrochemical performance of the bare and F-doped NVP as cathode materials for half-cell configuration: ( a and b ) CV curves at different scan rates of NVP and NVP-F 0 . 15 in the voltage range of 2.8–3.8 V vs. Na + /Na: different cycle charge/discharge profiles of ( c ) NVP and ( d ) NVP-F 0 .

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“…CV curves illustrate noticeably anodic peaks move to a lower potential, while the cathodic peaks slightly shift to higher potential, as a result indicates the inductive effect characteristic of NVP 34,35 . The diffusion coefficient of sodium-ion in NVP could be calculated from the linear relationship between their peak currents ( i p ) and the square root of scan rates ( v 1/2 ) using the following Randles-Sevcik equation via CV analyses: 26,36,37 .where, i p is the peak current (A), m is the mass of the active cathode material, F is Faraday constant, R is gas constant, T is absolute temperature, n is number of electrons involved in the reaction ( n  = 2), A is the effective contact area between the electrode and the electrolyte, as obtained from the cathode material multiplied with the active mass ratio, and C is the concentration of Na ions in the cathode, as calculated from the crystallographic cell parameters of NVP. CV curves of bare NVP and NVP-F 0 .…”

Phosphate Framework Electrode Materials for Sodium Ion Batteries

“…Song et al used first principle calculations to explore the Na ion migration pathways and occupations. According to their study, two pathways along the x and y directions and one possible curved route for migration were favored with a 3D transport characteristics, and the ion occupation of 0.75 for all Na sites was suitable for the configuration of [Na 3 V 2 (PO 4 ) 3 ] 2 68, 76. The crystal and electronic structures, electrochemical properties and diffusion mechanism of NASICON‐type Na 3 V 2 (PO 4 ) 3 have been investigated based on the hybrid density functional Heyd–Scuseria–Ernzerhof (HSE06) by Ohno's group.…”