23 Na and 31 P Solid-State NMR: A Key Tool to Study Local Environments in Na3V2(PO4)2F3-Y O y (0 ≤ y ≤ 2) Materials

Abstract

Sodium-ion batteries (SIBs) have recently been developed as alternate means of energy storage besides the conventional Lithium-ion batteries (LIBs). Several materials have greatly been studied as promising candidates for positive electrodes in SIBs, such as NaxCoO2, Na3V2(PO4)3, Na3V2(PO4)2F3 and Na3V2(PO4)2FO2.1-4 Among these materials, Na3V2(PO4)2F3 and Na3V2(PO4)2FO2 are in fact the two end-members of a family of materials with the general chemical formula Na3V2(PO4)2F3-y O y , where the y value can be varied between 0 and 2 with a linear evolution of the unit cell parameters.5 For y ≤ 1 the phase crystallizes in an orthorhombic system with a subtle difference between the a and b unit cell parameters, when y exceeds the value of 1, the difference between a and b becomes undistinguishable, which is evidenced by the merging of the (200) and (020) diffraction lines, and the structure can be described by a tetragonal unit cell (Space group P42/mnm). The structure of Na3V2(PO4)2F3 is characterized by the presence of V2O8F3 bioctahedral units linked together by PO4 tetragonal groups (Figure 2). A partial substitution of the two fluorine atoms on the terminal positions of the bioctahedral units by oxygen leads to the formation of other members of the Na3V2(PO4)2F3-y O y family (y < 2), while a complete substitution of these two fluorine by oxygen results in the formation of Na3V2(PO4)2FO2. In order to probe the structure at a local scale, 23Na and 31P solid-state NMR was used, as it is very sensitive to the local structures and the local electronic structure of paramagnetic materials for batteries, thanks to the Fermi contact interaction (interaction between the nuclear spin and the surrounding unpaired electron spins through chemical bonds). The end-member Na3V2(PO4)2F3 contains only V3+ giving rise to a highly shifted signal on 31P solid-state NMR at 6000 ppm, corresponding to P(OV3+)4 local environment where the four surrounding V3+ are able to transfer their electron spins on the Phosphorus nuclei. Upon oxygen substitution, more 31P NMR signals are observed at 4500, 3000, 1500, and ~0 ppm, which were assigned to P(OV3+)3(OV4+), P(OV3+)2(OV4+)2, P(OV3+)(OV4+)3, and P(OV4+)4 local environments, respectively (Figure 3). The presence of one V3+ in the second coordination sphere of a phosphorus atom gives a chemical shift of 1500 ppm and this effect is cumulative. In Na3V2(PO4)2FO2, only one 31P signal was recorded at around 0 ppm and was assigned to P(OV4+)4, which is the only phosphorus local environment in this composition. Even though an unpaired electron can be found on each V4+, no Fermi contact between V4+ and a nearby phosphorus nuclei was detected due to the lack of orbital overlapping between the orbital containing the spin of V4+ and the valence orbitals of the nearby phosphorus atom. In the same way 23Na solid-state NMR can also provide information on the local V3+/V4+ distribution. For a certain composition in the range of 0 < y < 2, five different phosphorus local environments coexist and their distributions are controlled by statistics, indicating that V3+ and V4+ do not tend to segregate in the bioctahedral units in these materials. DFT calculations were used to confirm the signal assignments and to understand the local electronic structure of V3+ and V4+ in this structural framework. We especially discuss the specific electronic structure of V4+ ion with a short vanadyl V=O bond.8 The oxygen substitution can indeed have a great impact on the electrochemical performance of the phases. Na3V2(PO4)2F3 and Na3V2(PO4)2FO2 undergo several bi-phasic steps upon desodiation with the formation of many intermediate phases whose crystal structures were reported in the literature.9,10 However, the desodiation of O-substituted phases (0 < y < 2) occurs through solid solution mechanism without the formation of intermediate phases. In this family, Na3V2(PO4)2F3 has the highest desodiation voltages (3.70 and 4.20 V vs. Na+/Na). Upon oxygen substitution, the voltages of the phase were shifted down and the lowest values were observed in Na3V2(PO4)2FO2.