Impact of the F^– for O^2– Substitution in Na₃V₂(PO₄)₂F_3–yO_y on Their Transport Properties and Electrochemical Performance

Particle nanosizing and coating with an ionic liquid: two routes to improve the transport properties of Na3V2(PO4)2FO2

“…A common feature between the electrical behaviors of these samples is the distorted semi-circles probably caused by a joint contribution of grains and grain boundaries in the high- and middle-frequency ranges, respectively. 39 In contrast, NVPFO 2 -Iono (Fig. 8c) presents an impedance response completely different from that of the other samples and even less resistive than that of the sample NVPFO 2 -Nano.…”

“…3, all materials possess the typical signature of Na 3 V 2 (PO 4 )FO 2 with peaks at 539 cm −1 , 943 cm −1 , 1010 cm −1 and 1052 cm −1 attributed to the vibrations of V–O, VO and phosphate groups respectively. 39,44,45 As reported in our previous work on the topochemical reaction mechanism happening during the ionothermal synthesis of NVPFO 2 , the ionic liquid functionalizes the particle surface to create an IL coating, which can be seen as an “artificial solid electrolyte interface”. 40 This latter, which in some cases stabilizes the electrode material upon cycling and favors ionic adsorption on the surface and/or alkaline diffusion through the material, 46–48 is observed on the NVPFO 2 -Iono spectrum (Fig.…”

“…Overall, the dependence of the electrical conductivity on temperature for NVPFO 2 -Nano seems to present a low ( T < 180 °C) and a high ( T > 180 °C) temperature regime, due to the dominating electronic transport at low temperatures and the mixed ionic–electronic conducting behavior at higher temperatures. 39 The activation energy of the high-temperature regime is considerably higher than that of the low-temperature one (Fig. 9).…”

“…34,35 However, this synthesis process leads most often to partial oxidation of vanadium and to a V 3+ /V 4+ ratio difficult to be controlled in the final composition, which drastically affects the energy storage performance and thus makes it difficult to evaluate the sole influence of morphology. 34,[36][37][38][39] Recently, Olchowka et al have reported an original ionothermal synthesis method leading to NVPFO 2 particles coated with an EMI TFSI ionic liquid (IL). 40 Their energy storage performance appears promising, even if the effect of IL coating is not fully understood yet.…”

Thermal stability of Na3-xV2(PO4)2F3-yOy: influence of F- for O2- substitution and degradation mechanisms

“…5 Additionally, NVPF can easily accommodate various chemical substitutions such as O 2-for Fthat allows the preparation of the solid-solution Na3V 3+ 2-yV 4+ y(PO4)2F3-yOy (0 ≤ y ≤ 2). [28][29][30][31][32][33][34] Depending on the synthesis conditions, this substitution can be either finely tuned or not, leading to discrepancies in lattice parameters and electrochemical signatures reported in the literature. 6,16,35 It was previously demonstrated that O 2for Fsubstitution in Na3V 3+ 2-yV 4+ y(PO4)2F3-yOy does not significantly impact the specific capacity since two Na + ions can be reversibly extracted whatever the oxygen content.…”