An investigation of Li0.6Na2.4V2(PO4)2F3 cathode with NASICON structure in lithium-ion battery

Binary NaCl–NaF and NaCl–LiF Flux-Mediated Growth of Mixed-Valence (V3+/4+) NASICON-Type Na3V2(PO4)2F2.5O0.5 and Na2.4Li0.6V2(PO4)2F2.5O0.5 for Highly Reversible Na- and Li-Ion Storage

“…4−10 More importantly, sodium vanadium fluorophosphates offer high energy density and operating voltage owing to the combined inductive effect of both phosphate groups/ fluorine atoms coupled with larger ionicity of the M−F bond compared to the M−O bond. 11,12 The crystal structure of Na 3 V 2 (PO 4 ) 2 F 3 has P4 2/ mnm 3 and Amam 13 space groups with a three-dimensional (3D) framework of V 2 O 8 F 3 bi-octahedra connected through PO 4 tetrahedral units via the O atom, where the Na ions occupy the large tunnel sites along the [110] and [1][2][3][4][5][6][7][8][9][10] directions. In addition, for a typical Na 3 V 2 O 2y (PO 4 ) 2 F 3−2y NASICON structural unit, y can vary from 0 to 1; for a value of y = 0, the vanadium exist as V +3 in Na 3 V 2 (PO 4 ) 2 F 3 end-member, while y = 1 corresponds to Na 3 V 2 (PO 4 ) 2 O 2 F end-member with V +4 oxidation state, and as mixed-valence Na 3 V 2 O 2y (PO 4 ) 2 F 3−2y (0 ≤ y ≤ 1) solid solution with V +3/+4 with varying O/F ratios.…”

“…32,33 Meanwhile, partial ion-exchanged (Na/Li) 3 V 2 (PO 4 ) 2 F 3 -type compounds of end-members, e.g., Na 3 V 2 (PO 4 ) 2 F 3 and Na 3 (VO) 2 (PO 4 ) 2 F, have garnered much attention as cathode for both LIB and SIB. 5,34 In this context, the Tang and Kosova groups have synthesized Li 0.6 Na 2.4 V 2 (PO 4 ) 2 F 3 from Na 3 V 2 (PO 4 ) 2 F 3 and Na 3−x Li x V 2 (PO 4 ) 2 F 3 by the ion-exchange method and investigated their structural, electronic, and electrochemical properties for both LIB and SIB. 35,36 More interestingly, these samples are characterized by a 4-fold increase in electrical conductivity over the pristine Na 3 V 2 (PO 4 ) 2 F 3 , with improved rate performance.…”

“…Nevertheless, Na cells always suffer from inadequate cycle life and power capability that solely depends on the intrinsic structural properties of electrode materials, particularly cathode materials. In the last decade, much effort has been devoted toward polyanionic phosphate materials such as Li 3 V 2 (PO 4 ) 3 , LiVPO 4 F, NaVPO 4 F, and Na 3 V 2 (PO 4 ) 2 F 3 as cathode for both LIBs and SIBs due to their robust structural framework and stability. − Especially, the Na 3 V 2 (PO 4 ) 2 F 3 with NASICON framework received significant attention in both LIBs and SIBs due to their large reversible capacity, good ionic conductivity, thermal stability, and a high operating voltage of ∼4.1 V vs Li + /Li (3.8 V vs Na/Na + ). − More importantly, sodium vanadium fluorophosphates offer high energy density and operating voltage owing to the combined inductive effect of both phosphate groups/fluorine atoms coupled with larger ionicity of the M–F bond compared to the M–O bond. , The crystal structure of Na 3 V 2 (PO 4 ) 2 F 3 has P 4 2/ mnm 3 and Amam space groups with a three-dimensional (3D) framework of V 2 O 8 F 3 bi-octahedra connected through PO 4 tetrahedral units via the O atom, where the Na ions occupy the large tunnel sites along the [110] and [1-10] directions. In addition, for a typical Na 3 V 2 O 2 y (PO 4 ) 2 F 3–2 y NASICON structural unit, y can vary from 0 to 1; for a value of y = 0, the vanadium exist as V +3 in Na 3 V 2 (PO 4 ) 2 F 3 end-member, while y = 1 corresponds to Na 3 V 2 (PO 4 ) 2 O 2 F end-member with V +4 oxidation state, and as mixed-valence Na 3 V 2 O 2 y (PO 4 ) 2 F 3–2 y (0 ≤ y ≤ 1) solid solution with V +3/+4 with varying O/F ratios. , The Na 3 V 2 (PO 4 ) 2 F 3 and Na 3 V 2 (PO 4 ) 2 O 2 F end-members were extensively studied for both Li-ion and Na-ion cells because of their stable and tunable crystal structures. − Even though sodium fluorophosphate-based materials show good ionic conductivity with little lattice expansion with their 3D open-framework structures, they suffer from poor electronic conductivity (∼10 –12 S cm –1 ), which affects the high rate capability and cycling stability.…”

Achieving the Stable Structure and Superior Performance of Na3V2(PO4)2O2F Cathodes via Na-Site Regulation

“…As a fascinating method, ion doping has been demonstrated to successfully improve the electrochemical properties of many electrode materials. − For polyanionic compounds, they are mainly composed of alkali metals, transition metals, and anionic groups. Generally, the electrochemical performance of polyanionic cathode materials can be effectively enhanced by the substitution of alien ions at the transition site. ,,− Similarly, alkali-site substitution using a dopant with similar chemical characteristics has been investigated in recent years.…”

Ag3V2(PO4)2F3, a new compound obtained by Ag+/Na+ ion exchange into the Na3V2(PO4)2F3 framework

“…These tunnels are responsible for the facile sodium extraction from Na3V2(PO4)2F3 and for its promising electrochemical activity as a positive electrode for Na-ion batteries, but also for the relative ease of ion exchange with silver without consequent disruption of the 3D framework of the material. Interestingly, there are reports in the literature about ion exchange of Na with Li, where partial exchanges have been obtained 33,34 and this process has even been recently studied in situ for the parent compound Na3V IV 2(PO4)2FO2: Park et al showed that the ion exchange process is biphasic 35 . In the following, we detail the structural modifications that are induced by the Ag + /Na + ionic exchange to achieve the Ag3V2(PO4)2F3 composition; moreover, the crystal structure is reported for the first time, for both the room-temperature and high-temperature polymorphs.…”