Method for preparing a NA3V2(PO4)2F3 particulate material

Abstract

A method for preparing a Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material, including at least the steps: a) reducing the vanadium oxide, V.sub.2O.sub.5, under a reducing atmosphere in the absence of elementary carbon and in the presence of at least one phosphate anion precursor in order to form vanadium phosphate, VPO.sub.4; and b) exposing, under an inert atmosphere, a mixture of the VPO.sub.4 material obtained in step a) with an effective amount of sodium fluoride, NaF, and at least one hydrocarbon- and oxygen-containing compound which is a source of elementary carbon, to temperature conditions that are favourable for calcining said mixture so as to form said Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 compound. Also, a related electrode material, an electrode and a secondary sodium battery using the presented material.

Claims

1. A method for preparing a Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material, comprising at least the steps of: a) reducing the vanadium oxide, V.sub.2O.sub.5, under a reducing atmosphere in the absence of elemental carbon and in the presence of at least one phosphate-anion precursor so as to form vanadium phosphate, VPO.sub.4, and b) exposing, under an inert atmosphere, a mixture of the VPO.sub.4 material obtained in step a) with an effective amount of sodium fluoride, NaF, and of at least one compound consisting of carbon, hydrogen and oxygen atoms, which is a source of elemental carbon, under temperature conditions suitable for the calcination of said mixture so as to form said Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 compound, characterized in that it is devoided of a mechanical compression step between step a) and step b).

2. The method as claimed in claim 1, wherein the reducing atmosphere uses dihydrogen as reducing agent.

3. The method as claimed in claim 1, wherein the phosphate-anion precursor is chosen from H.sub.3PO.sub.4, H(NH.sub.4).sub.2PO.sub.4 and H.sub.2NH.sub.4PO.sub.4.

4. The method as claimed in claim 1, wherein step a) is carried out under an argon atmosphere enriched with 2% dihydrogen, and at a temperature of approximately 800° C.

5. The method as claimed in claim 1, wherein the compound consisting of carbon, hydrogen and oxygen atoms of step b) is chosen from sugars and carbohydrates.

6. The method as claimed in claim 1, wherein the calcination of step b) is carried out at 800° C. under an inert atmosphere.

7. The method as claimed in claim 1, wherein the Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material is present in the form of primary particles with an average dimension of less than 2 μm and which are constituent particles of aggregates.

8. A Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material made up of primary particles with an average dimension of less than 2 μm, and surface-coated with conductive carbon, said material having an orthorhombic lattice of Amam space group with the following lattice parameters: a is between 9.028 and 9.030, b is between 9.044 and 9.046, c is greater than or equal to 10.749.

9. The material as claimed in claim 8, containing from 0.5% to 5% by weight of conductive carbon relative to its total weight.

10. The material as claimed in claim 8, wherein the particles are in the form of aggregates.

11. The material as claimed in claim 8, having a BET specific surface area at least equal to 1 m.sup.2/g.

12. The material as claimed in claim 8, having a V.sup.4+ cation content at most equal to 1% by weight.

13. A Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material made up of primary particles with an average dimension of less than 2 μm, and surface-coated with conductive carbon, said material having an orthorhombic lattice of Amam space group with the following lattice parameters: a is between 9.028 and 9.030, b is between 9.044 and 9.046, c is greater than or equal to 10.749, obtained according to the method as defined in claim 1.

14. An active material of an electrode comprising at least one material as defined in claim 8.

15. An electrode totally or partially made up of a material as defined in claim 8.

16. The electrode as claimed in claim 15, also comprising a polymer or binder.

17. A secondary sodium or sodium-ion battery comprising an electrode as claimed in claim 16.

18. The method as claimed in claim 1, wherein the compound consisting in of carbon, hydrogen and oxygen atoms of step b) is a cellulose-based derivative.

19. The material as claimed in claim 9, wherein said primary particles have an average dimension between 200 and 2000 nm.

20. The material as claimed in claim 9, said material having an orthorhombic lattice of Amam space group with the following lattice parameters: a is substantially equal to 9.029, b is substantially equal to 9.045, c is substantially equal to 10.751.

21. The method as claimed in claim 1, wherein the phosphate-anion precursor is H.sub.2NH.sub.4PO.sub.4.

Description

FIGURES

(1) FIG. 1: X-ray diffraction characterization of Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 synthesized in example 1 from VPO.sub.4 (reduction with H.sub.2).

(2) FIG. 2: SEM photo of the Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material (NVPF-HC) in accordance with the invention obtained according to example 1 via the Ar/H.sub.2/cellulose route.

(3) FIG. 3: SEM photo of the Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material (NVPF-CB) not in accordance with the invention prepared by carbothermic reduction according to example 2.

(4) FIG. 4: Electrochemical performance level of the NVPF-CB material of example 2 (dark gray line) and NVPF-HC material of example 1 (light gray line).

LITERATURE REFERENCES

(5) Ref 1: Kuniko Chihara et al., Journal of Power Sources 227 (2013) 80-85 Ref. 2: Paula Serras et al., J. Mater. Chem., 2012, 22, 22301

(6) Materials and Methods

(7) The EPR spectra are produced using a Broker EMX spectrometer equipped with an ER-4192-ST and ER-4131 VT cavity at 100 k.

(8) The SEM characterization is carried out by means of a Zeiss LEO 1530 scanning microscope.

(9) The XR characterizations are carried out using an Empyrean PANalytical diffractometer which has a copper cathode.

Example 1

(10) VPO.sub.4 is obtained beforehand by carrying out premixing of the V.sub.2O.sub.5 (110 g) and NH.sub.4H.sub.2PO.sub.4 (140 g) precursors in a mill. The resulting mixture is then heated in an oven at a heating rate of 10° C./minute up to 800° C. and maintained at this temperature for 3 hours under an argon atmosphere enriched with 2% H.sub.2. The gray powder thus obtained was characterized by X-ray diffraction.

(11) The Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material (>100 g) was then prepared from a mixture of the VPO.sub.4 (160 g) as prepared above, with NaF (70 g), under the stoichiometric conditions (2:3), and of cellulose (23 g). This mixture was calcined under an argon atmosphere at 800° C. for 1 hour. At the end of this calcination step, the material obtained is removed from the oven at 800° C. to be rapidly cooled. The Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material (NVPF-HC) is then washed with water and dried at 80° C. for 24 hours.

(12) FIG. 1 hereinafter reports the X-ray diffraction characterization of this product, indexed in an orthorhombic lattice (Amam space group) having the parameters a=9.02940(2) Å, b=9.04483(2) Å and c=10.75145(2) Å.

(13) FIG. 2 reports the SEM characterization of this material in accordance with the invention.

Comparative Example 2

(14) A Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 material was also prepared according to the protocol described in example 1, but by favoring a carbothermic reduction. It is also referred to hereinafter as NVPF-CB.

(15) The essential difference compared with the protocol of example 1 consists of the use of carbon black (Timcal super C65, 18 g) which results in the formation of a particulate surface carbon of the product thus formed. Contrary to the invention, the surface carbon of the primary particles is in the form of a heterogeneous and not very dense deposit.

(16) FIG. 3 reports the SEM characterization of this material.

Example 3: Characterization of Na.SUB.3.V.SUB.2.(Po.SUB.4.).SUB.2.F.SUB.3 .According to the Invention (NVPF-H) Versus Na.SUB.3.V.SUB.2.(PO.SUB.4.).SUB.2.F.SUB.3 .According to Example 2 (NVPF-CB)

(17) The characterization of these two materials reveals a certain number of structural and morphological differences, the most significant of which are detectable by EPR spectroscopy, XRD and SEM.

(18) The comparison of FIGS. 2 and 3 makes it possible to highlight these differences.

(19) Thus, the SEM analysis reveals a clear difference in terms of primary-particle size and of the carbon-based surface coating of these particles.

(20) The primary particles of the material synthesized using carbon black (NVPF-CB) have an average size of greater than 2 μm, whereas they are less than 2 micrometers, preferably less than 1 micrometer, more preferably between 200 and 600 nm for the particles of the material according to the invention (NVPF-H).

(21) The presence of a carbon-based coating is also noted.

(22) It is also observed, by laser particle size analysis (measuring apparatus: Malvern Mastersizer S model MSS), that the agglomerates of the NVPF-CB material have a volume average diameter d(v0.5) which is much greater than 25 μm. On the other hand, the volume average diameter d(v0.5) of the agglomerates of the material according to the invention is less than 10 μm.

(23) A significant difference between the two materials is also observed by comparison of their respective EPR spectra.

(24) The NVPF-H material advantageously reveals a much higher V.sup.3+ content, in particular greater than or equal to 99%.

(25) As explained in detail in the description, a V.sup.4+ content is attributed to the oxidized species of Na.sub.3V.sub.7(PO.sub.4).sub.2F.sub.3, which is Na.sub.3V.sub.2O.sub.x(PO.sub.4).sub.2F.sub.3-x.

(26) This oxidized species was characterized by Seras et al. (ref. 2). The incorporation of a carbon-source material during the second synthesis step, and which results in the formation of a carbon-based coating of the primary particles, clearly efficiently protects the Na.sub.3V.sub.7(PO.sub.4).sub.2F.sub.3 against the phenomenon of oxidation to Na.sub.3V.sub.2O.sub.x(PO.sub.4).sub.2F.sub.3-x.

(27) It is noted that the NVPF-CB material has, on the other hand, a V.sup.4+ content of about 1% to 5%.

(28) These results therefore clearly reveal the advantage of the method according to the invention which makes it possible to dispense with the compression steps that may not be envisioned on the scale of an industrial production, while at the same time guaranteeing the production of an NVPF-H material with a V.sup.4+ content that is not very high since it is at most equal to 10%.

(29) The electrochemical performance levels of the two materials were also tested in galvanostatic mode at a constant current density of 12.8 mA/g between the voltage limits 2 V and 4.3 V. FIG. 4 reports these measurements.

(30) The specific capacity and also the irreversibility in the first cycle of each material were determined in a button cell using a hard carbon electrode as anode.

(31) It emerges that the NVPF-CB and NVPF-H materials have, respectively, an initial specific capacity of 122 mAh/g and 128 mAh/g and an irreversibility of 30% and 23%.