Structural evolution of mixed valent (V³⁺/V⁴⁺) and V⁴⁺ sodium vanadium fluorophosphates as cathodes in sodium-ion batteries: comparisons, overcharging and mid-term cycling

Abstract

Sodium vanadium fluorophosphates belonging to the Na3V2O2x(PO4)2F3-2x family of compounds have recently shown very good electrochemical performance versus Na/Na + providing high working voltages (3.6 and 4.1 V) and good specific capacity values. In this work the electrochemical behaviour and structural evolution of two compositions, Na3V2O1.6(PO4)2F1.4 (V 3.8+ ) and Na3V2O2(PO4)2F (V 4+ ), are detailed using time-resolved in-situ synchrotron X-ray powder diffraction. For the first time in sodium-ion batteries the effects of overcharging and mid-term cycling are analyzed using this technique. Differences in the composition of both materials lead to different combinations of biphasic and singlephase reaction mechanisms while charging up to 4.3 V and overcharging up to 4.8 V. Moreover, the analysis of particle size broadening of both samples reveals the higher stress suffered by the V 4+ compared to the more disordered V 3.8+ sample. The more "flexible" structure of the V 3.8+ sample allows for maximum sodium extraction when overcharging up to 4.8 V while in the case of the V 4+ sample no evidence is shown of more sodium extraction between 4.3 V and 4.8 V. Furthermore, the analysis of both materials after 10 cycles shows the appearence of secondary phases due to the degradation of the material or the battery itself (e.g. electrolyte degradation). This study shows examples of the possible degradation mechanisms (and phases) while overcharging and mid-term cycling which is in turn crucial to making better electrodes, either based on these materials or generally in cathodes for sodium-ion batteries.

The rapid microwave-assisted hydrothermal synthesis of NASICON-structured Na3V2O2x(PO4)2F3−2x(0 <x≤ 1) cathode materials for Na-ion batteries

“…9b-e) of the selected patterns showed that the Na 3 V 2 O 2 (PO 4 ) 2 F material was characterized by a combination of two-phase and single-phase (de)intercalation mechanisms. In agreement with the previous operando data on Na 3 V 2 O 2 (PO 4 ) 2 F, 16,41 the rst charge proceeded through a very short single-phase region at the beginning of the rst charge. A new phase with similar crystallographic symmetry and unit cell parameters then appeared, and desodiation followed a two- phase mechanism until the extraction of 1Na.…”

Higher energy and safer sodium ion batteries via an electrochemically made disordered Na3V2(PO4)2F3 material

“…However, to make sodium ion cells based on NVPF technologically relevant, there is a need to increase their specific energy, which is less competitive than today’s Li-electrodes (~600 Wh kg −1 for LiCoO 2 -type materials), hence the various attempts to boost the capacity of NVPF electrodes. A first approach has consisted in the successful and reversible electrochemical insertion of 0.5 sodium ion in Na 3 V 2 (PO 4 ) 2 F 3 or 1 sodium ion in Na 3 V 2 (PO 4 ) 2 O 2 F at potentials of ~1.6 V, respectively 15–17 . Such a low potential limits the specific energy gain associated to the insertion of extra sodium ions but in contrast provides the feasibility to use Na 3+ x V 2 (PO 4 ) 2 F 3 composites as a sodium reservoir, as previously demonstrated, to compensate for the sodium loss at the carbon negative electrode during the first cycling 15 .…”

Large interlayer spacing vanadium oxide nanotubes as cathodes for high performance sodium ion batteries

“…35 Both of the samples have the board peak at 1623 and 3445 cm À1 which are assigned to H-O-H bending vibration and H-O stretching. 33,36 Remarkably, there are some differences between two samples. The intensity of the peaks at 2927 cm À1 association with organic templates in Fe-VO x -NTs has weakened signicantly, and the peak at 2853 cm À1 disappears (inset gure of Fig.…”