Liquid process for preparing a vanadium phosphate-carbon composite material

Abstract

The invention relates to a process for the preparation of a vanadium-carbon phosphate composite material, a vanadium-carbon phosphate composite material obtained according to the process, and to the uses of the composite material, especially as a precursor for the synthesis of electrochemically-active materials, electrode or active anode material.

Claims

1. Process for the preparation of a vanadium phosphate-carbon composite material corresponding to the formula VPO.sub.4/C, comprising the following steps: i) mixing a vanadium precursor, H.sub.3PO.sub.4, a compound A selected from a compound comprising at least one carboxylic acid function and a polysaccharide compound in an aqueous solvent, it being understood that when the compound comprising at least one carboxylic acid function is different from a carbon precursor, the mixture further comprises a carbon precursor compound, ii) heating the mixture of step i) to a temperature of 35 C. to 100 C., to form a solid residue, and iii) heating the solid residue to a temperature above 850 C.

2. Process according to claim 1, wherein the vanadium precursor is V.sub.2O.sub.5.

3. Process according to claim 1, wherein the compound comprising at least one carboxylic acid function comprises from 2 to 10 carbon atoms.

4. Process according to claim 1, wherein the compound comprising at least one carboxylic acid function is a saturated carboxylic acid or polycarboxylic acid chosen from oxalic acid, citric acid, glycolic acid, lactic acid, tartaric acid, malic acid, succinic acid, glycolic acid, malonic acid, glutaric acid, adipic acid, isocitric acid, oxalosuccinic acid and tricarballylic acid.

5. Process according to claim 1, wherein the molar ratio (compound comprising at least one carboxylic acid function/vanadium element in the vanadium precursor) varies from 1 to 2.

6. Process according to claim 1, wherein the carbon precursor compound is chosen from ethylene glycol and glycerol.

7. Process according to claim 1, wherein the molar ratio (carbon precursor compound/vanadium element in the vanadium precursor) varies from 0.05 to 2.

8. Process according to claim 1, wherein the polysaccharide compound is agar-agar.

9. Process according to claim 1, wherein the mixture of stage i) comprises: either citric acid, or oxalic acid and ethylene glycol or glycerol, or agar-agar.

10. Process according to claim 1, wherein the mixture of step i) further comprises a binder.

11. Process according to claim 9, wherein the binder is agar-agar when the compound comprising at least one carboxylic acid function is used.

12. Process according to claim 1, wherein step iii) lasts at most 8 h.

13. Process according to claim 1, wherein step iii) is carried out at a temperature of between 880 C. and 900 C.

14. Vanadium phosphate-carbon composite material, obtained according to a process as defined in claim 1, and that it comprises particles of VPO.sub.4 coated with an amorphous carbon layer.

15. Use of a vanadium phosphate-carbon composite material obtained according to a process as defined in claim 1, as a precursor for the preparation of electrochemically-active electrode materials.

16. Use of a vanadium phosphate-carbon composite material obtained according to a process as defined in claim 1, as anode active material.

17. Composite material of formula Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3/C, obtained from a composite material of vanadium phosphate and carbon of formula VPO.sub.4/C as defined in claim 14.

18. Composite material of formula Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3/C obtained according to a process as defined in claim 1 and having the following lattice parameters: a=9.0294(2) , b=9.0445(2) , c=10.7528(2) in the Amam crystalline system.

Description

EXAMPLES

[0079] The raw materials used in the examples are listed below: [0080] H.sub.3PO.sub.4, Alfa Aesar, 85% in water, [0081] V.sub.2O.sub.5, Alfa Aesar, 99.2%, [0082] citric acid, Alfa Aesar, 99+%, [0083] oxalic acid, Sigma Aldrich, 98%, [0084] ethylene glycol, Fluka, >99.5%, [0085] agar-agar, Fisher BioReagents, BP2641-1 [0086] Na.sub.3PO.sub.4, Acros Organic, pure anhydrous, [0087] NaF, Sigma Aldrich,>99%, [0088] distilled water, and [0089] argon 5.0, Messer.

[0090] Unless otherwise specified, all materials were used as received from the manufacturers.

Example 1

Preparation of a Composite Material 1 of the VPO.SUB.4./C Formula According to the Process According to the Invention

[0091] 4.04 g of vanadium oxide (V.sub.2O.sub.5), 5.12 g of phosphoric acid (H.sub.3PO.sub.4), 4.2 g of oxalic acid and 0.9 g of ethylene glycol were mixed in a beaker with 20 ml of distilled water.

[0092] The resulting mixture was heated to 85 C. with magnetic stirring for 12 h to evaporate the water. The resulting residue was heated to 890 C. for 1 h in a quartz tube under an argon atmosphere.

[0093] The tube was then cooled to room temperature using water.

[0094] The composite material 1 obtained in the form of a powder was analyzed by X-ray diffraction (XRD) using a diffractometer sold under the trade name D8 by Bruker (CuK radiation). The samples were scanned between 16 and 50 2.

[0095] FIG. 1 represents an X-ray diffraction diagram of the composite material 1 of formula VPO.sub.4/C.

[0096] All the diffraction peaks of FIG. 1 were indexed in the Cmcm crystal system with the following lattice parameters: a=5.2399(4) , b=7.7886(6) , and c=6.2956(4) , which is consistent with the description given by Glaum et al. [Zeitschrift fuer Kristallographie (1979-2010), 1992, 198, 41-47].

[0097] The amount of carbon in the composite material 1 of the formula VPO.sub.4/C was analyzed by thermogravimetric analysis (TGA). A heating rate of about 10 C. per minute was used from about 25 C. to about 680 C. and a step at 680 C. for 1 hour was performed. The composition of the gas phase was monitored in parallel with mass spectroscopic (MS) heating. It was approximately 4.8% by weight, based on the total weight of composite material.

[0098] The composite material 1 was also analyzed by transmission electron microscopy (TEM) using a microscope sold under the trade name FEI TECNAI G2 by the company FEI.

[0099] FIG. 2 represents a TEM image of the composite material 1. It confirms the presence of a carbon shell with a thickness of about 5 nm, enveloping the vanadium phosphate.

Example 2

Preparation of a Composite Material 2 of the VPO.SUB.4./C Formula According to the Process According to the Invention

[0100] 4.04 g of vanadium oxide (V.sub.2O.sub.5), 5.12 g of phosphoric acid (H.sub.3PO.sub.4) and 5.6 g of citric acid were mixed in a beaker with 20 ml of distilled water.

[0101] The resulting mixture was heated to 85 C. with magnetic stirring for 12 h to evaporate the water. The resulting residue was heated to 890 C. for 1 h in a quartz tube under an argon atmosphere.

[0102] The tube was then cooled to room temperature using water.

[0103] The composite material 2 obtained in the form of a powder was analyzed by X-ray diffraction (XRD) using an apparatus as described in Example 1. The samples were scanned between 16 and 50 2.

[0104] The X-ray diffraction pattern of the composite material 2 of the formula VPO.sub.4/C was similar to that obtained for the composite material of Example 1 (see FIG. 1).

[0105] The TEM image of the composite material 2 of the formula VPO.sub.4/C was similar to that obtained for the composite material of Example 1 (see FIG. 2.

[0106] The amount of carbon in the composite material 2 of the formula VPO.sub.4/C was analyzed by ATG as in Example 1. It was 4.5% by weight approximately, relative to the total mass of composite material.

Comparative Example 3

Preparation of a Material A According to a Method not in Accordance with the Invention

[0107] 4.04 g of vanadium oxide (V.sub.2O.sub.5), 5.12 g of phosphoric acid (H.sub.3PO.sub.4), 4.2 g of oxalic acid and 0.9 g of ethylene glycol were mixed in a beaker with 20 ml of distilled water.

[0108] The resulting mixture was heated to 85 C. with magnetic stirring for 12 h to evaporate the water. The resulting residue was heated to 850 C. for 10 h in a quartz tube under an argon atmosphere.

[0109] The tube was then cooled to room temperature using water.

[0110] The material A obtained in the form of a powder was analyzed by X-ray diffraction (XRD) using an apparatus as described in Example 1. The samples were scanned between 20 and 40 2.

[0111] FIG. 3 represents an X-ray diffraction pattern of material A, showing an amorphous material very different from composite materials 1 and 2 respectively obtained in Examples 1 and 2.

Example 4

Use of a Composite Material of Formula VPO.SUB.4./C Obtained According to a Process According to the Invention as a Precursor for the Preparation of Eelectrochemically-Active Electrode Materials

[0112] 4.1 Preparation of Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3/C

[0113] 4 g of a composite material of formula VPO.sub.4/C as obtained in Example 1 were mixed with 1.22 g of NaF for 12 h using a Turbula-type space mixer comprising a ball. Then, the resulting mixture was heated to 700 C. for 1 h in a quartz tube under an argon atmosphere. The tube was then cooled to room temperature using water.

[0114] The composite material 3 of formula Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3/C obtained in the form of a powder was analyzed by X-ray diffraction (XRD) using an apparatus as described in Example 1. The samples were scanned between 16 and 50 2. The Rietveld model was used to refine the lattice parameters of the materials.

[0115] FIG. 4 represents an X-ray diffraction pattern of the composite material 3 of formula Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3/C, as well as a TEM image of said composite material 3.

[0116] All the diffraction peaks of FIG. 4 were indexed in the Amam crystaline system with the following lattice parameters: a=9.0294(2) , b=9.0445(2) , c=10.7528(2) , which is in agreement with the description given by Bianchini et al. [Chem. Mater., 2015, 27, 8, 3009-3020].

[0117] The tamped density of the composite material Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3/C was about 1.3 g/cm.sup.3, measured using a volumeter sold under the trade name STAV II by the company J. Engelsmann A G with the following parameters: volume of 250 ml (ISO 787) and 1250 jolts.

[0118] For comparison, a composite material B of formula Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3/C was prepared from a VPO.sub.4/C obtained according to the method of Barker et al. [US2002/0192553, carbothermic reduction, Example 1(a)].

[0119] To this end, 5.40 g of V.sub.2O.sub.5, 6.83 g of NH.sub.4H.sub.2PO.sub.4 and 0.76 g of SP carbon were mixed, comminuted and converted into granules. Then the granules were heated in an oven under air up to 300 C. (temperature rise of 2 C. per minute) then the heating was maintained at 300 C. for 3 h and then at 800 C. for 8 h. The resulting mixture was cooled to room temperature. A VPO.sub.4/C black powder was thus obtained. The composite material B of formula Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3/C was prepared from this VPO.sub.4/C according to the same procedure as that described to produce the composite material 3.

[0120] The composite material 3 was analyzed from the point of view of its electrochemical performance and compared to the composite material B.

[0121] To do this, electrochemical tests were performed using cells of the button cell type. The electrodes in the form of a film were made in air from formulated inks comprising 87.1% by weight of active material (i.e. composite material 3 or B), 7.7% by weight of carbon and 5.2% by weight of PVF. The button cells were assembled in a glove box. The electrochemical cell included: [0122] an electrode film comprising the active material (i.e. composite material 3 or B), as a positive electrode, [0123] a sodium sheet, as a negative electrode, [0124] Whatman GF/D category 1823070 glass fibers, as a separator interposed between the positive and negative electrodes, and [0125] a solution comprising a sodium salt NaPF.sub.6 (approximately 1 mol/l) dissolved in a mixture of ethylene carbonate/dimethyl carbonate (ratio 1/1 by weight), and 3% by weight of fluoroethylene carbonate, of liquid electrolyte.

[0126] FIG. 5 shows the curve of the potential vs Na (in volts) as a function of the capacity (in mAh/g) with a current regime of 1 Na exchanged per hour of the composite material B (FIG. 5a) and the composite material 3 (FIG. 5b) and the capacitance curve (in mAh/g) as a function of the number of cycles of the composite material B (FIG. 5c) and the composite material 3 (FIG. 5d).

[0127] FIG. 5 clearly shows a good stability of the cycle when the active material is prepared from the composite material obtained according to the method of the invention.

[0128] 4.2 Preparation of Na.sub.3V.sub.z(PO.sub.4).sub.3/C

[0129] 4 g of VPO.sub.4 as obtained in Example 1 were mixed with 1.59 g of Na.sub.3PO.sub.4 for 12 h using a Turbula-type space mixer comprising a ball. Then, the resulting mixture was heated to 810 C. for 1 h in a quartz tube under an argon atmosphere.

[0130] The tube was then cooled to room temperature using water.

[0131] The composite material 4 of formula Na.sub.3V.sub.2(PO.sub.4).sub.3/C obtained in the form of a powder was analyzed by X-ray diffraction (XRD) using an apparatus as described in Example 1. The samples were scanned between 16 and 50 2.

[0132] FIG. 6 represents an X-ray diffraction diagram of the composite material 4 of formula Na.sub.3V.sub.2(PO.sub.4).sub.3/C, as well as a TEM image of said composite material 4.

[0133] All the diffraction peaks of FIG. 6 were indexed in the R-3c crystaline system with the following lattice parameters: a=8.7217(2) , b=8.7217(2) and c=21.8485(7) , which is consistent with the description given by Zatovsky et al. [Acta Crystallographica, Section E., Structure Reports Online, 2010, 66, 2, pi12-pi12].

[0134] For comparison, a composite material C of formula Na.sub.3V.sub.2(PO.sub.4).sub.3/C was prepared from a VPO.sub.4/C obtained according to the method of Barker et al. [US2002/0192553, carbothermic reduction, Example 1 (a)]. The VPO.sub.4/C was therefore prepared according to a process identical to that described in Example 4.1 above, and then the composite material C of formula Na.sub.3V.sub.2(PO.sub.4).sub.3 was prepared from this VPO.sub.4/C in the same manner as that described for producing the composite material 4.

[0135] The composite material 4 was analyzed from the point of view of its electrochemical performance and compared to the composite material C.

[0136] To do this, electrochemical tests were performed using cells of the button cell type. The electrodes in the form of a film were made in air from formulated inks comprising 85.5% (respectively 80%) by weight of composite material 4 (respectively by weight of composite material C), 9.8% by weight of carbon (respectively 14.2%) by weight of carbon and 4.7% (respectively 5.8%) by weight of PVF. The button cells were assembled in a glove box. The electrochemical cell comprised: [0137] an electrode film comprising the active material (i.e. composite material 4 or C), as a positive electrode, [0138] a sodium sheet, as a negative electrode, [0139] Whatman GF/D category 1823070 glass fibers, as a separator interposed between the positive and negative electrodes, and [0140] a solution comprising a sodium salt NaPF.sub.6 (approximately 1 mol/l) dissolved in a mixture of ethylene carbonate/dimethyl carbonate (ratio 1/1 by weight), and 3% by weight of fluoroethylene carbonate, of liquid electrolyte.

[0141] FIG. 7 shows the curve of the potential vs Na (in volts) as a function of the capacity (in mAh/g) with a current regime of C/10 of the composite material C (FIG. 5a) and of the composite material 4 (FIG. b) and the capacitance curve (in mAh/g) as a function of the number of cycles of the composite material C (FIG. 5c) and of the composite material 4 (FIG. 5d).

[0142] FIG. 7 clearly shows a good stability of the cycling when the active material is prepared from the composite material obtained according to the method of the invention.

[0143] 4.3 Preparation of LiV(PO.sub.4)F/C

[0144] 4 g of VPO.sub.4 as obtained in Example 1 were mixed with 0.68 g of LiF for 12 h using a Turbula-type space mixer comprising a ball. Then, the resulting mixture was heated to 700 C. for 1 h in a quartz tube under an argon atmosphere.

[0145] The tube was then cooled to room temperature using water.

[0146] The composite material of formula LiV(PO.sub.4)F/C obtained in the form of a powder was analyzed by X-ray diffraction (XRD) using an apparatus as described in Example 1. The samples were scanned between 16 and 50 2.

[0147] FIG. 8 represents an X-ray diffraction pattern of the composite material of formula LiV(PO.sub.4)F/C, as well as a TEM image of said composite material 5.

[0148] All the diffraction peaks of FIG. 8 were indexed in the P-1 crystalline system with the following lattice parameters: a=5,1751(5) , b=5,3041(4) , c=7, 2481(6) , =107.507(4), =107.847(5) and =98.450(4), which is consistent with the description given by Ateba Mba et al. [Chemistry of Materials, 2012, 24, 6, 1223-1234].

[0149] For comparison, a composite material D of formula LiV(PO.sub.4)F/C_was prepared from a VPO.sub.4/C obtained according to the method of Barker et al. [US2002/0192553, carbothermic reduction, Example 1(a)]. VPO.sub.4/C was therefore prepared according to a process identical to that described in Example 4.1 above, and then the composite material D of formula LiV(PO.sub.4)F/C_was prepared from this VPO.sub.4/C according to the same procedure as that described to produce the composite material 5.

[0150] The composite material 5 was analyzed from the point of view of its electrochemical performance and compared to the composite material D.

[0151] To do this, electrochemical tests were performed using cells of the button cell type. The electrodes in the form of a film were made in air from formulated inks comprising 86.5% (respectively 87.1%) by mass of composite material 5 (respectively by weight of composite material D), 8.7% by weight of carbon (respectively 7.7%) by weight of carbon and 4.8% (respectively 5.2%) by weight of PVF. The button cells were assembled in a glove box. The electrochemical cell comprised: [0152] an electrode film comprising the active material (i.e. composite material 5 or C), as a positive electrode, [0153] a lithium sheet, as a negative electrode, [0154] Whatman GF/D category 1823070 glass fibers, as a separator interposed between the positive and negative electrodes, and [0155] a solution comprising an LiPF.sub.6 sodium salt (approximately 1 mol/l) dissolved in a mixture of ethylene carbonate/dimethyl carbonate (ratio 1/1 by weight), and 3% by weight of fluoroethylene carbonate, of liquid electrolyte.

[0156] FIG. 9 shows the curve of the potential vs Li (in volts) as a function of the capacity (in mAh/g) with a current regime of C of the composite material D (FIG. 9a) and of the composite material 5 (FIG. 9b) and the capacitance curve (in mAh/g) as a function of the number of cycles of the composite material D (FIG. 9c) and of the composite material 5 (FIG. 9d).

[0157] FIG. 9 clearly shows a good stability of the cycling when the active material is prepared from the composite material obtained according to the method of the invention.

Example 5

Preparation of a Composite Material 6 of the VPO.SUB.4./C Formula According to the Invention

[0158] According to the Invention 4.04 g of vanadium oxide (V.sub.2O.sub.5), 5.12 g of phosphoric acid (Na.sub.3PO.sub.4) and 2 g of agar in a beaker with 50 ml of distilled water.

[0159] The resulting mixture was heated to 80 C. with magnetic stirring for 12 h to evaporate the water. The resulting residue was heated to 890 C. for 1 h in a quartz tube under an argon atmosphere.

[0160] The tube was then cooled to room temperature using water.

[0161] The use of the agar-agar makes it possible at the same time to overcome the evolution of gas generated by the decomposition of the compound comprising at least one carboxylic acid function (compound A.sub.1) and the precursor of carbon (compound B) if it exists. used in Examples 1 and 2 when in contact with phosphoric acid; and to limit the volume expansion of the mixture observed during the rise in temperature to 890 C. as shown in FIG. 10 (10a: residue obtained during the temperature rise in Examples 1 and 2; 10b: residue obtained during the rise in temperature in Example 5).

[0162] The composite material 6 obtained in the form of a powder was analyzed by X-ray diffraction (XRD) using an apparatus as described in Example 1. The samples were scanned between 16 and 50 2.

[0163] The X-ray diffraction pattern of the composite material 6 of the formula VPO.sub.4/C was similar to that obtained for the composite material of Example 1 (see FIG. 1).

[0164] The TEM image of the composite material 6 of the formula VPO.sub.4/C was similar to that obtained for the composite material of Example 1 (see FIG. 2).

[0165] The amount of carbon in the composite material 6 of formula VPO.sub.4/C was analyzed by ATG as in Example 1. It was about 5% by weight, based on the total mass of material.

Example 7

Preparation of a Composite Material 7 of Formula VPO.SUB.4./C According to the Process According to the Invention

[0166] 4.04 g of vanadium oxide (V.sub.2O.sub.5), 5.12 g of phosphoric acid (H.sub.3PO.sub.4), 5.4 g of citric acid and 0.8 g of agar-agar were mixed in a beaker with 30 ml of distilled water.

[0167] The resulting mixture was heated to 85 C. with magnetic stirring for 12 h to evaporate the water. The resulting residue was heated to 890 C. for 1 h in a quartz tube under an argon atmosphere.

[0168] The tube was then cooled to room temperature using water.

[0169] The composite material 7 obtained in the form of a powder was analyzed by X-ray diffraction (XRD) using an apparatus as described in Example 1. The samples were scanned between 16and 50 2.

[0170] The X-ray diffraction pattern of the composite material 7 of the formula VPO.sub.4/C was similar to that obtained for the composite material of Example 1 (see FIG. 1).

[0171] The TEM image of the composite material 7 of the formula VPO.sub.4/C was similar to that obtained for the composite material of Example 1 (see FIG. 2).

[0172] The amount of carbon in the composite material 7 of formula VPO.sub.4/C was analyzed by ATG as in Example 1. It was about 5% by weight, based on the total mass of composite material.