Explaining Performance-Limiting Mechanisms in Fluorophosphate Na-Ion Battery Cathodes through Inactive Transition-Metal Mixing and First-Principles Mobility Calculations

Abstract

Fluorophosphate cathodes are currently one of the most promising polyanionic sodium-ion battery cathodes and exhibit specific energies not far below oxide cathodes.To further improve fluorophosphate cathodes their capacity must be increased, which might be possible since some sodium (Na) remains unextracted in these cathodes during cycling. In this study we attempt to answer the question of what specific mechanism limits fluorophosphate cathode capacity, which could stem from either redox-limiting or site-limiting behavior. This paper reports the synthesis, electrochemical characterization, and computational examination of Na 3 GaV(PO 4 ) 2 F 3 . This test system, which was designed explicitly for uncovering the limiting factors in these structures, exhibits reversible insertion of Na + and redox activity for V 2+ through V 5+ during electrochemical cycling, indicating that fluorophosphate cathodes are not fundamentally redox-limited and must be site-limited. First-principles calculations indicate that large diffusion barriers at high sodiations impose a kinetic limit on Na + insertion in fluorophosphate cathodes, but further investigation is needed to determine capacity limits on Na + extraction. From our combined results we also propose possible routes to improve future fluorophosphate cathodes.

Advanced Cathode Materials for Sodium‐Ion Batteries: What Determines Our Choices?

“…Matts et al reported the synthesis, electrochemical performance, and computational investigation of Na 3 VGaPO 4 ) 2 F 3 to uncover the limiting factors in these NASICON structures. [163] Figure 9 shows the electrochemical cycling features of Na x V 2 (PO 4 ) 2 F 3 and Na x VGa(PO 4 ) 2 F 3 , respectively, illustrating reversible Na + insertion and vanadium redox states V 2+ through V 5+ , which reveals that Na x V 2 (PO 4 ) 2 F 3 is not primarily redox-limited and should be site-restricted. Na 3 VGaPO 4 ) 2 F 3 can achieve secondcycle charge and discharge capacities of 144 and 141 mA h g −1 , respectively with three plateaus at 4.2, 3.7, and 1.4 V, confirming the redox state of V 2+ through V 5+ .…”

“…Na 3 VGaPO 4 ) 2 F 3 can achieve secondcycle charge and discharge capacities of 144 and 141 mA h g −1 , respectively with three plateaus at 4.2, 3.7, and 1.4 V, confirming the redox state of V 2+ through V 5+ . [163] Furthermore, based on Figure 9, a sodium-rich-phase Na 4 V 2 (PO 4 ) 2 F 3 can be designed to make fluorophosphate cathodes competitive with a theoretical capacity up to 256 mA h g −1 assuming all the sodium can be extracted.…”

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

“…This in turn induces a homogeneous electrostatic repulsion along the a and b axes, and thus favors the tetragonal symmetry of the structure. DFT calculations by Matts et al 43 shed light on the migration barriers of sodium into the structure of Na3V2(PO4)2F3. In particular, they calculated a very low migration barrier for sodium along the rings (Figure 5c in 43 , but that can also be seen in Figure 4), of the order of 20 meV.…”

“…DFT calculations by Matts et al 43 shed light on the migration barriers of sodium into the structure of Na3V2(PO4)2F3. In particular, they calculated a very low migration barrier for sodium along the rings (Figure 5c in 43 , but that can also be seen in Figure 4), of the order of 20 meV. This is of the same order of the thermal energy at room T, usually estimated as 25 meV, which can well explain why the orthorhombic structure features sodium ions in partial occupancy positions.…”

Syntheses and nanocrystallization of NaF–M2O3–P2O5 NASICON‐like phosphate glasses (M = V, Ti, Fe)

“…Na 3 V 2 (PO 4 ) 3 and Na 3 V 2 (PO 4 ) 2 F 3 with the NASICON structure are among the most prospective materials for the Na-ion battery cathodes. 5 A substitution of one phosphate group with three ions of fluorine results in a compound with tetragonal structure 6 and may be additionally beneficial due to several reasons. First, it decreases the molar mass of the compound, which results in an increase in theoretical gravimetric capacity.…”