Method to synthesize Na-based electroactive materials

Abstract

The invention relates to a process for the preparation of sodium-based solid compounds, such as sodium-based solid alloys and sodium-based crystalline phases by ball-milling using metallic sodium as starting material. The invention also relates to some sodium-based crystalline P′2-phases and to Na-based vanadium phosphates phases (Na.sub.(3+y)V.sub.2(PO.sub.4).sub.3) with 0<y≤3 and Na-based vanadium fluorophosphates phases (Na.sub.(3+z)V.sub.2(PO.sub.4).sub.2F.sub.3) with 0<z≤3, in particular Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3, obtained by such a process and to their use, as active material for positive electrode, in a Na-ion battery.

Claims

1. Process for the preparation of Na-based solid alloys or of Na-based crystalline phases selected in the group consisting of Na-based crystalline P′2-phases, Na-based solid crystalline phases of formula Na.sub.(3+y)V.sub.2(PO.sub.4).sub.3 with 0<y≤3 and Na-based solid crystalline phases of formula Na.sub.(3+z)V.sub.2(PO.sub.4).sub.2F.sub.3 with 0<z≤3, said process comprising at least one step of ball-milling metallic sodium with a stoichiometric amount of a powder of at least one alloying solid element (X), or of at least one solid Na-based crystalline phase selected in the group consisting of solid Na-based crystalline P2-phases, Na.sub.3V.sub.2(PO.sub.4).sub.3 and Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 respectively, said step of ball-milling being carried out in a dry atmosphere and without heating.

2. The process according to claim 1, wherein the at least one alloying element is selected from the group consisting of P, Sn, Sb, Pb, and C.

3. The process according to claim 1, for the preparation of Na.sub.3P, Na.sub.3Sb, Na.sub.15Sn.sub.4, Na.sub.15Pb.sub.4, and Na.sub.xC with 0.01<x<0.2.

4. The process according to claim 1, wherein Na-based solid crystalline P′2-phases are selected from the group consisting of P2 type layered crystalline Na-phases comprising Na and at least one oxide of at least one element selected from the group consisting of Fe, Mn, Co, Ni, P, S, Mn, V, Ti, and in which the amount of sodium per formula after the ball-milling process has been increased with regard to the amount of sodium initially present in the P2-phase.

5. The process according to claim 1 or, for the preparation of NaFe.sub.0.5Mn.sub.0.5O.sub.2, NaMnO.sub.2, NaCoO.sub.2, NaCo.sub.0.67Mn.sub.0.33O.sub.2, NaNi.sub.1/3Mn.sub.2/3O.sub.2.

6. The process according to claim 1, for the preparation of: i) NaFe.sub.0.5Mn.sub.0.5O.sub.2, and the ball milling step is carried out with metallic sodium and powder of Na.sub.0.67Fe.sub.0.5Mn.sub.0.5O.sub.2; or ii) Na.sub.(3+y)V.sub.2(PO.sub.4).sub.3 with 0<y≤3 and the ball milling step is carried out with metallic sodium and powder of Na.sub.3V.sub.2(PO.sub.4).sub.3; or iii) Na.sub.(3+z)V.sub.2(PO.sub.4).sub.2F.sub.3 with 0<z≤3 and the ball milling step is carried out with metallic sodium and powder of Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3.

7. The process according to claim 1, wherein the step of ball-milling is carried out in a glove box filled with an inert gas.

8. The process according to claim 1, wherein the step of ball-milling is performed at a temperature ranging from 25 to 80° C.

9. The process according to claim 1, wherein the ball-milling step is carried out in a hard steel ball-miller jar containing a weight of milling-balls (W.sub.mb) such as the weight ratio W.sub.mb/W.sub.S, with W.sub.S being the total weight of solid materials contained in the jar, ranges from about 10 to 60.

10. The process according to claim 1, wherein the ball milling step is carried out in a ball-miller operating by centrifuging movements of the balls at a rotation speed set at a value ranging from 200 and 1000 rotations per minute.

11. The process according to claim 1, wherein the effective duration of the ball-milling step varies from 0.1 to 50 hours.

12. The process according to claim 1, for the preparation of Na-based solid alloys in which the alloying element (X) is different from carbon, wherein the ball-milling step is performed in the presence said alloying element and further in the presence of carbon powder, said process leading to a Na.sub.3X/C composite in which particles of an alloy Na.sub.3X are embedded in a carbon matrix.

13. The process according to claim 12, wherein, the amount of carbon powder varies from 5 to 50 weight % with regard to the total amount of solid materials contained in the ball-miller.

14. The process according to claim 1, wherein molar ratios of metallic sodium/Na-based crystalline P2-phases, metallic sodium/Na.sub.3V.sub.2(PO.sub.4).sub.3 or metallic sodium/Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 vary from about 0.1 to about 2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an XRD pattern from Example 1;

(2) FIG. 2 is an XRD patterns of the Na.sub.3P/C composite from Example 4;

(3) FIG. 3 is an XRD pattern of the composite from Example 5;

(4) FIG. 4 is an XRD pattern of the composite from Example 6;

(5) FIG. 5A is an XRD patterns of the P′2-phase from Example 7;

(6) FIG. 5B is a detailed view of FIG. 5a between 15 and 17.5 degrees;

(7) FIG. 6 is an XRD patterns of Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 from Example 8;

(8) FIG. 7 is a synchrotron XRD pattern of pure Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 from Example 8;

(9) FIG. 8 is a corresponding crystal structure from FIG. 7;

(10) FIG. 9 is a table of a structural parameters of Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 deduced from the combined Rietveld refinement of the X-Ray diffraction patterns in Example 8;

(11) FIG. 10 is an XRD patterns of the composite material from Example 9;

(12) FIG. 11 is a charge/discharge curve in the 1st and 2nd cycle from Example 9;

(13) FIG. 12 is XRD patterns of the composite material from Example 10;

(14) FIG. 13 is a charge/discharge curve in the 1st and 2nd cycles from Example 10; and

(15) FIG. 14 is a table for the voltage (in V) as a function of capacity (in mAh/g) from Example 11.

DETAILED DESCRIPTION

(16) The present invention is illustrated in more detail in the examples below, but it is not limited to said examples.

(17) Powder purity and crystallinity were examined by X-ray diffraction (XRD), with a Bruker D8 Advance diffractometer operating in Bragg-Brentano geometry with a Cu Kα radiation and equipped with a power spectral density detector.

Example 1

Preparation of Na.SUB.3.P by Ball-Milling

(18) In this example, Na.sub.3P has been prepared.

(19) Stoichiometric amounts of metallic sodium as bulk (1.38 g, Sigma) and red phosphorus (0.62 g, Alfa, 325 mesh) were filled into a hard steel ball-milled jar of a Spex® 8000M ball-miller (30 cm.sup.3) in an Ar-filled glove box and equipped with seven hard steel balls each having a weight of 7 g and a diameter of 12 mm. These solid materials were ball milled for 2-10 h to obtain Na.sub.3P particles.

(20) The XRD pattern of the composite thus obtained is shown in FIG. 1 annexed (intensity (in arbitrary units) is a function of angle 2θ (in degrees)). It indicates that a pure phase of Na.sub.3P is successfully produced without any other intermediate phase.

Example 2

Preparation of Na.SUB.3.Sb by Ball-Milling

(21) In this example, Na.sub.3Sb has been prepared.

(22) The preparation of Na.sub.3Sb was basically the same as that of Na.sub.3P, only replacing P with Sb (Alfa, 325 mesh). 0.69 g of metallic sodium as bulk and 1.22 g of Sb was filled into the same ball-milled jar and ball milled for 2-10 h to produce Na.sub.3Sb.

Example 3

Preparation of Na.SUB.15.Sn.SUB.4 .by Ball-Milling

(23) In this example, Na.sub.15Sn.sub.4 has been prepared.

(24) The preparation of Na.sub.15Sn.sub.4 was basically the same as that of Na.sub.3P only replacing P with Sn. 0.83 g of metallic sodium as bulk and 1.17 g of Sn (Alfa, 325 mesh) was filled into the same ball-milled jar and ball milled for 2-10 h to produce Na.sub.15Sn.sub.4.

Example 4

Preparation of a Na.SUB.3.P/C Composite by Ball-Milling

(25) In this example, a composite comprising Na.sub.3P alloy particles embedded in a carbon matrix has been prepared.

(26) Na.sub.3P was synthesized through ball milling of metallic sodium (Sigma) and red phosphorus (Alfa, 325 mesh) using a Spex 8000M ball-miller Stoichiometric amounts of precursors (0.69 g of metallic sodium as bulk and 0.31 g of red phosphorus) were filled into a hard steel ball-milled jar (30 cm.sup.3) in an Ar-filled glove box and equipped with five hard steel balls each having a weight of 7 g and a diameter of 12 mm.

(27) The ball-to-powder weight ratio was maintained between 30-50 during the whole ball-milling step.

(28) After 2 hours of ball-milling, 0.43 g of carbon black (30 wt. %) were added into the jar to produce Na.sub.3P/C composites with Na.sub.3P particles embedded in the carbon matrix.

(29) For achieving uniform distribution of Na.sub.3P in carbon, the ball milling was pursued for 10 h. The XRD patterns of the Na.sub.3P/C composite thus obtained is reported in FIG. 2 annexed (intensity (in arbitrary units) is a function of angle 2θ (in degrees)). The pattern given by FIG. 2 indicates that a pure phase of Na.sub.3P is successfully produced without any other intermediate phase.

Example 5

Preparation of a Na.SUB.3.Sb/C Composite by Ball-Milling

(30) In this example, a composite comprising Na.sub.3Sb alloy particles embedded in a carbon matrix has been prepared.

(31) The preparation of Na.sub.3Sb/C composite was basically the same as that of Na.sub.3P/C in example 4, only replacing P with 0.64 g of Sb (Alfa, 325 mesh). The amount of Na was changed to 0.36 g accordingly.

(32) The XRD pattern of the composite thus obtained is shown in FIG. 3 annexed (intensity (in arbitrary units) is a function of angle 2θ (in degrees)). The pattern given by FIG. 3 proves the successful production of Na.sub.3Sb with minor impurity as indicated by a star, which may be because of the intermediate phase of Na.sub.xSb (x<3).

Example 6

Preparation of Na.SUB.15.Sn.SUB.4./C by Ball-Milling

(33) In this example, a composite comprising Na.sub.15Sn.sub.4 alloy particles embedded in a carbon matrix has been prepared.

(34) Na.sub.15Sn.sub.4/C was synthesized through ball milling of metallic sodium (Sigma), Sn (Alfa, 325 mesh) and carbon black using Spex 8000M ball-miller Stoichiometric amounts of Na.sub.15Sn.sub.4 precursors (0.42 g of metallic sodium and 0.58 g of Sn) and 0.43 g of carbon black (30 wt %) were filled into a hard steel ball-milled jar (30 cm.sup.3) in an Ar-filled glove box and equipped with six hard steel balls each having a weight of 7 g and a diameter of 12 mm.

(35) The XRD pattern of the composite thus obtained is shown in FIG. 4 annexed (intensity (in arbitrary units) is a function of angle 2θ (in degrees)). The pattern given by FIG. 4 proves the successful production of Na.sub.15Sn.sub.4 with minor impurity as indicated by a star, which may be because of the intermediate phase of Na.sub.xSn.sub.4 (x<15).

Example 7

Preparation of Na[Fe.SUB.1/2.Mn.SUB.1/2.]O.SUB.2 .by Ball Milling

(36) The P2 type phase Na.sub.0.67[FeMn].sub.0.5O.sub.2 (NFMO) was first prepared by traditional solid state reactions according to the method disclosed by Yabuuchi N. et al. (Nature Materials, 2012, 11, 512-517).

(37) Stoichiometric amounts of metallic Na and Na.sub.0.67[FeMn].sub.0.5O.sub.2 were ball milled to make the P′2-phase Na[Fe.sub.1/2Mn.sub.1/2]O.sub.2. 1.03 g of Na.sub.0.67[FeMn].sub.0.5O.sub.2 and 0.076 g of metallic Na were filled into a hard steel ball-milled jar (30 cm.sup.3) (Spex® 8000M) in an Ar-filled glove box and equipped with five hard steel balls each having a weight of 7 g and a diameter of 12 mm. The corresponding P′2-phase Na[Fe.sub.1/2Mn.sub.1/2]O.sub.2 started to appear after 20 min of ball milling. To fully transfer P2 into P′2, it is possible to longer time ball milling to 2 h or add excess amount of metallic Na.

(38) The XRD patterns of the P′2-phase Na[Fe.sub.1/2Mn.sub.1/2]O.sub.2 thus obtained and of initial P2 phase Na.sub.0.67[FeMn].sub.0.5O.sub.2 are given in FIG. 5a annexed (intensity (in arbitrary units) is a function of angle 2θ (in degrees)). FIG. 5b is a detailed view of FIG. 5a between 15 and 17.5 degrees. FIG. 5a and FIG. 5b confirm the successful production of Na[Fe.sub.1/2Mn.sub.1/2]O.sub.2, in which a shift of (002) peak from 15.8 to 16.3° was observed in addition to the appearance of two peaks at 33 and 37°.

Example 8

Preparation of Na.SUB.4.V.SUB.2.(PO.SUB.4.).SUB.2.F.SUB.3 .by Ball Milling

(39) Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 was first prepared by traditional solid state reactions according to the method disclosed by L. Croguennec et al. (Chemistry of Materials, 2014, 26, 4238-4247).

(40) Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 were ball milled with an excess molar amount of metallic Na (2Na per Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3) to make Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3. 1.0 g of Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 and 0.11 g of metallic Na were filled into a hard steel ball-milled jar (30 cm.sup.3) (Spex® 8000M) in an Ar-filled glove box and equipped with four hard steel balls, each having a weight of 7 g and a diameter of 12 mm Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 was obtained after 3 h of ball milling.

(41) The XRD patterns of Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 thus obtained and of initial Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 are given in FIG. 6 annexed [intensity (in arbitrary units) as a function of angle 2θ (in degrees)]. The synchrotron XRD pattern of pure Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 is shown in FIG. 7 with corresponding crystal structure shown in FIG. 8. VO.sub.4F.sub.2 octahedral and PO.sub.4 tetrahedral are colored in dark grey and light-grey, respectively. Na atoms are shown as light-grey balls. Structural parameters of Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 deduced from the combined Rietveld refinement of the X-Ray diffraction pattern are given in FIG. 9.

(42) More particularly, the material of formula Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 is composed of particles of a fluorophosphate that crystallizes in the Amam space group.

(43) The Synchrotron X-ray diffraction patterns of the pure Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 phase measured at ESRF on ID22 with λ=0.3543 Å shows that the diffraction peaks can be indexed in the same orthorhombic cell as for the pristine Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3, but with different lattice parameters, i.e. a=9.2208(2) Å, b=9.2641(2) Å, c=10.6036(2) Å.

Example 9

Preparation of a Composite Material 1 Comprising 0.5 Mole of Na.SUB.4.V.SUB.2.(PO.SUB.4.).SUB.2.F.SUB.3 .and 0.5 Mole of Na.SUB.3.V.SUB.2.(PO.SUB.4.).SUB.2.F.SUB.3 .by Ball-Milling

(44) Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 was prepared according to Example 8.

(45) Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 was ball milled with a stoichiometric amount of Na (0.5Na per Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3) to make a composite material 1 comprising Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 and Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3. 1.0 g of Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 and 0.0275 g of metallic Na were filled into a hard steel ball-milled jar (30 cm.sup.3) (Spex® 8000M) in an Ar-filled glove box and equipped with four hard steel balls, each having a weight of 7 g and a diameter of 12 mm. A composite material 1 comprising 0.5 mole of Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 and 0.5 mole of Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 was obtained after 0.5 h of ball milling.

(46) The XRD patterns of the composite material 1 thus obtained and of initial Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 are given in FIG. 10 annexed [intensity (in arbitrary units) as a function of angle 2θ (in degrees)].

(47) The composite material 1 was used as a positive electrode in a coin cell to evaluate the electrochemical performance. A Na foil was used as the counter electrode, with 1M NaClO.sub.4 in EC/DMC (1:1 by volume) as the liquid electrolyte and glass fiber (Whatman®, GF/D) as the separator. The cells were tested in a VMP system (Biologic S.A., Claix, France). The charge/discharge curve in the 1st and 2nd cycle is shown in FIG. 11 annexed [Voltage (V) as a function of x in Na.sub.xV.sub.2(PO.sub.4).sub.2F.sub.3]. The Na that extracted from first plateau at about 1.5V corresponds to the transformation of the composite material 1 to Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3.

Example 10

Preparation of a Composite Material 2 Comprising 0.25 Mole of Na.SUB.4.V.SUB.2.(PO.SUB.4.).SUB.2.F.SUB.3 .and 0.75 Mole of Na.SUB.3.V.SUB.2.(PO.SUB.4.).SUB.2.F.SUB.3 .by Ball-Milling

(48) Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 was prepared according to Example 8.

(49) Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 was ball milled with a stoichiometric amount of Na (0.25Na per Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3) to make a composite material 2 comprising Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 and Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3. 1.0 g of Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 and 0.0138 g of metallic Na were filled into a hard steel ball-milled jar (30 cm.sup.3) (Spex® 8000M) in an Ar-filled glove box and equipped with four hard steel balls, each having a weight of 7 g and a diameter of 12 mm A composite material 2 comprising 0.25 mole of Na.sub.4V.sub.2(PO.sub.4).sub.2F.sub.3 and 0.75 mole of Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 was obtained after 0.5 h of ball milling.

(50) The XRD patterns of the composite material 2 thus obtained and of initial Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 are given in FIG. 12 annexed [intensity (in arbitrary units) as a function of angle 2θ (in degrees)].

(51) The electrochemical performance of the composite material 2 thus obtained was tested in a Na cell identical to the one used in example 9 with the composite material and Na metal as working and counter electrodes, respectively. The charge/discharge curve in the 1st and 2nd cycles is shown in FIG. 13 annexed [Voltage (V) as a function of x in Na.sub.xV.sub.2(PO.sub.4).sub.2F.sub.3]. The Na that extracted from first plateau at about 1.5V corresponds to the transformation of the composite material 2 to Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3.

Example 11

Preparation of a Full Cell with the Composite Material 1 Prepared According to Example 9 as Positive Electrode Material

(52) In this example, the composite material 1 prepared according to example 9, has been used as a positive electrode active material to assemble a Na-ion battery comprising hard carbon as a negative electrode and 1M NaClO.sub.4 in EC/DMC (1:1 by volume) as a liquid electrolyte.

(53) 1) Preparation of Hard Carbon Anode

(54) Hard carbon was produced by pyrolysis of glucose according to the method disclosed by J. R. Dahn et al. (Journal of the Electrochemical Society, 2000, 147, 1271)

(55) 2) Assembly of the Na-Ion Battery

(56) Hard carbon and the composite material 1 prepared according to the method given in example 9, were used as anode and cathode, respectively, in a coin cell to evaluate the electrochemical performances of Na-ion batteries. A Na foil was used as the counter electrode, with 1M NaClO.sub.4 in EC/DMC (1:1 by volume) as the electrolyte and glass fiber (Whatman®, GF/D) as the separator. The cell was tested in a VMP system (Biologic S.A., Claix, France).

(57) For comparison purpose, a comparative Na-ion battery not forming part of the present invention has also been prepared using the Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 solid crystalline phase as positive electrode active material instead of the composite material of example 9.

(58) 3) Cycling Performances

(59) Each battery was sealed and then a reducing current corresponding to a rate of 0.2 C was applied. The capacity was calculated based on the weight of the positive electrode.

(60) For each Na-ion battery, the cycling performances were studied. They are reported on FIG. 14 annexed on which the voltage (in V) is a function of capacity (in mAh/g). These results show that there is large irreversible capacity in the first cycle for the battery comprising only Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3 as positive electrode active material (battery not forming part of the present invention) and a coulombic efficiency of about 70% in the 1st cycle. The majority capacity loss is attributed to the SEI formation on the carbon electrode which consumed Na. For the battery according to the invention, i.e. comprising the composite material 1, the results show a capacity increased from 89 to 110 mAh/g, thus demonstrating that the use of such a composite material 1 makes it possible to fight against irreversible is capacity in Na-ion batteries.