Condensed polyanion electrode

Abstract

The invention relates to electrodes that contain active materials of the formula: Na.sub.aX.sub.bM.sub.cM.sub.d(condensed polyanion).sub.e(anion).sub.f; where X is one or more of Na+, Li+ and K+; M is one or more transition metals; M is one or more non-transition metals; and where a>b; c>0; d0; e1 and f0. Such electrodes are useful in, for example, sodium ion battery applications.

Claims

1. An electrode, wherein the electrode comprises: an active material capable of reversibly storing sodium ions, wherein the active material comprises: Na.sub.aX.sub.bM.sub.cM.sub.d(condensed polyanion).sub.e(anion).sub.f; where X is one or more of Na.sup.+, Li.sup.+ and K.sup.+; M is one or more transition metals; M is one or more non-transition metals; where a>b; c>0; d>0; e>1 and f>0 and where the condensed polyanion comprises one or more phosphorus moieties selected from P.sub.2O.sub.7.sup.4, P.sub.3O.sub.9.sup.5 and P.sub.4O.sub.11.sup.6; and a binder configured to bind at least a portion of the active material to form the electrode.

2. The electrode according to claim 1, wherein: the transition metal is selected from one or more of titanium, vanadium, niobium, tantalum, hafnium, chromium, molybdenum, tungsten, manganese, iron, osmium, cobalt, nickel, palladium, platinum, copper, silver, gold, zinc and cadmium; the optional non-transition metal is selected from one or more of magnesium, calcium, beryllium, strontium, barium, aluminium and boron; and the anion is selected from one or more of borate, nitrate, silicate, arsenate, sulfate, vanadate, niobate, molybdate, tungstate, phosphate, carbonate, fluorophosphate, fluorosulfate, halide and hydroxide.

3. The electrode according to claim 1, wherein the active material is Na.sub.aX.sub.bM.sub.cM.sub.dP.sub.2O.sub.7(PO.sub.4).sub.2, where a>b; a+b=4; c+d=3, and each of the metals represented by M and M has an oxidation state of +2.

4. The electrode according to claim 1, wherein the active material comprises Na.sub.aX.sub.bM.sub.cM.sub.dP.sub.2O.sub.7(PO.sub.4).sub.2, where a>b; a+b=4; c+d=3; and wherein M.sub.ccomprises iron as Fe.sup.2+.

5. The electrode according to claim 1, wherein the active compound comprises Na.sub.aX.sub.bM.sub.cM.sub.d(P.sub.2O.sub.7).sub.2(PO.sub.4).sub.2, where a>b; a+b=4; c+d=5; and wherein M.sub.c comprises iron as Fe.sup.2+.

6. The electrode according to claim 1 wherein one or more of the condensed polyanions is a hetero-ligand condensed polyanion.

7. The electrode according to claim 6, wherein one or more ligands of the hetero-ligand condensed polyanion are halide-containing moieties.

8. An energy storage device comprising the electrode according to claim 1.

9. An electrochemical cell, comprising the energy storage device according to claim 8, wherein the electrochemical cell is a sodium ion cell, a sodium metal cell, a non-aqueous ion cell, or an aqueous ion cell.

10. A rechargeable battery comprising the electrode according to claim 1.

11. An electrochemical device comprising the electrode according to claim 1.

12. An electrochromic device comprising the electrode according to claim 1.

13. The electrode according to claim 1, wherein the active material is Na.sub.aX.sub.b M.sub.cM.sub.d(P.sub.2O.sub.7).sub.4(PO.sub.4), where a>b; a+b=7; c+d=4, and each of the metals represented by M and M has an oxidation state of +3.

14. The electrode according to claim 1, wherein the active material is Na.sub.aX.sub.bM.sub.cM.sub.d(P.sub.2O.sub.7).sub.2(PO.sub.4).sub.2, where a>b; a+b=4; c+d=5 and each of the metals represented by M and M has an oxidation state of +2.

15. The electrode according to claim 1, wherein the active material is Na.sub.aX.sub.bM.sub.cM.sub.dP.sub.2O.sub.7(PO.sub.4).sub.2, where a>b; a+b=4; c+d=3; and wherein M.sub.ccomprises one or more transition metals.

16. The electrode according to claim 1, wherein the active material is Na.sub.4Mn.sub.2F.sub.6(P.sub.2O.sub.7).

17. The electrode according to claim 2, wherein the non-transition metal is selected from beryllium and boron, and wherein d>0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described with reference to the following drawings in which:

(2) FIG. 1 is an XRD pattern for Na.sub.4Mn.sub.3(P.sub.2O.sub.7)(PO.sub.4).sub.2 prepared according to Example 4c;

(3) FIG. 2 is an XRD pattern for Na.sub.4Co.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) prepared according to Example 5c;

(4) FIG. 3 is an XRD pattern for Na.sub.4Ni.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) prepared according to Example 6c;

(5) FIG. 4 shows the first cycle constant current data for an electrode according to the present invention comprising Na.sub.4Mn.sub.3(PO.sub.4).sub.2P.sub.2O.sub.7 prepared according to Example 4c;

(6) FIG. 5 is an XRD pattern for Na.sub.4Fe.sub.3P.sub.2O.sub.7(PO.sub.4).sub.2 prepared according to Example 7;

(7) FIG. 6 is an XRD pattern for Na.sub.7V.sub.4(P.sub.2O.sub.7).sub.4PO.sub.4 prepared according to Example 8;

(8) FIG. 7 shows the first cycle constant current data for the Na.sub.4Fe.sub.3P.sub.2O.sub.7(PO.sub.4).sub.2 active material;

(9) FIG. 8 shows the first cycle constant current data for the Na.sub.7V.sub.4(P.sub.2O.sub.7).sub.4PO.sub.4 active material;

(10) FIG. 8 shows the first cycle constant current data for the Na.sub.7V.sub.4(P.sub.2O.sub.7).sub.4PO.sub.4 active material;

(11) FIG. 9 is an XRD pattern for Na.sub.7V.sub.3(P.sub.2O.sub.7).sub.4 prepared according to Example 9; and

(12) FIG. 10 shows the first cycle constant current data for the Na.sub.7V.sub.3(P.sub.2O.sub.7).sub.4 active material.

DETAILED DESCRIPTION

(13) Active materials used in the present invention are prepared on a laboratory scale using the following generic method:

(14) Generic Synthesis Method:

(15) The required amounts of the precursor materials are intimately mixed together and then the resulting precursor mixture is pelletized using a hydraulic press. The pelletized material is then heated in a tube furnace or a chamber furnace using either a flowing inert atmosphere (e.g. argon or nitrogen) or an ambient air atmosphere, at a furnace temperature of between about 500 C. to about 1000 C. until reaction product forms, as determined by X-ray diffraction spectroscopy. When cool, the reaction product is removed from the furnace and ground into a powder.

(16) Using the above method, active materials used in the present invention were prepared as summarised below in Examples 1 to 6:

EXAMPLE 1

(17) TABLE-US-00001 TARGET MATERIAL: Na.sub.2MnMo.sub.2O.sub.8 Starting materials: Na.sub.2CO.sub.3 (0.57 g) MnCO.sub.3 (0.89 g) MoO.sub.3 (2.22 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Ramp rate: 5 C./min; temperature: 650 C.; dwell time: 6 hours

EXAMPLE 2

(18) TABLE-US-00002 TARGET MATERIAL: Na.sub.2NiMo.sub.2O.sub.8 Starting materials: Na.sub.2CO.sub.3 (0.75 g) NiO (0.53 g) MoO.sub.3 (2.03 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Ramp rate: 3 C./min; temperature: 650 C.; dwell time: 6 hours

EXAMPLE 3

(19) TABLE-US-00003 TARGET MATERIAL: Na.sub.2CoMo.sub.2O.sub.8 Starting materials: Na.sub.2CO.sub.3 (0.75 g) CoCO.sub.3 (0.84 g) MoO.sub.3 (2.03 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Ramp rate: 5 C./min; temperature: 650 C.; dwell time: 6 hours

EXAMPLE 4a

(20) TABLE-US-00004 TARGET MATERIAL: Na.sub.4Mn.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.29 g) MnCO.sub.3 (1.67 g) NH.sub.4H.sub.2PO.sub.4 (1.11 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Ramp rate: 3 C./min; temperature: 300 C.; dwell time: 6 hours

EXAMPLE 4b

(21) TABLE-US-00005 TARGET MATERIAL: Na.sub.4Mn.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.29 g) MnCO.sub.3 (1.67 g) NH.sub.4H.sub.2PO.sub.4 (1.11 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Conditions of Example 4a, followed by a ramp rate: 3 C./min; temperature: 500 C.; dwell time: 6 hours

EXAMPLE 4c

(22) TABLE-US-00006 TARGET MATERIAL: Na.sub.4Mn.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.29 g) MnCO.sub.3 (1.67 g) NH.sub.4H.sub.2PO.sub.4 (1.11 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Conditions of Example 4b, followed by a ramp rate: 3 C./min; temperature: 700 C.; dwell time: 6 hours

EXAMPLE 5a

(23) TABLE-US-00007 TARGET MATERIAL: Na.sub.4Co.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.26 g) CoCO.sub.3 (1.69 g) NH.sub.4H.sub.2PO.sub.4 (1.09 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Ramp rate: 3 C./min; temperature: 300 C.; dwell time: 6 hours

EXAMPLE 5b

(24) TABLE-US-00008 TARGET MATERIAL: Na.sub.4Co.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.26 g) CoCO.sub.3 (1.69 g) NH.sub.4H.sub.2PO.sub.4 (1.09 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Conditions of Example 5a followed by a ramp rate: 3 C./min; temperature: 500 C.; dwell time: 6 hours

EXAMPLE 5c

(25) TABLE-US-00009 TARGET MATERIAL: Na.sub.4CO.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.26 g) CoCO.sub.3 (1.69 g) NH.sub.4H.sub.2PO.sub.4 (1.09 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Conditions of Example 5b followed by a ramp rate: 3 C./min; temperature: 700 C.; dwell time: 6 hours

EXAMPLE 6a

(26) TABLE-US-00010 TARGET MATERIAL: Na.sub.4Ni.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.26 g) NiCO.sub.3 (1.69 g) NH.sub.4H.sub.2PO.sub.4 (1.09 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Ramp rate: 3 C./min; temperature: 300 C.; dwell time: 6 hours

EXAMPLE 6b

(27) TABLE-US-00011 TARGET MATERIAL: Na.sub.4Ni.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.26 g) NiCO.sub.3 (1.69 g) NH.sub.4H.sub.2PO.sub.4 (1.09 g) Furnace Parameters: Muffle furnace Gas type (Ambient air) Conditions of Example 6a followed by a ramp rate: 3 C./min; temperature: 500 C.; dwell time: 6 hours

EXAMPLE 6c

(28) TABLE-US-00012 TARGET MATERIAL: Na.sub.4Ni.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7) Starting materials: Na.sub.4P.sub.2O.sub.7 (1.26 g) NiCO.sub.3 (1.69 g) NH.sub.4H.sub.2PO.sub.4 (1.09 g) Furnace Parameters: Muffle furnace, Gas type (Ambient air) Conditions of Example 6b followed by a ramp rate: 3 C./min; temperature: 700 C.; dwell time: 6 hours

EXAMPLE 7

(29) TABLE-US-00013 TARGET MATERIAL: Na.sub.4Fe.sub.3P.sub.2O.sub.7(PO.sub.4).sub.2 Starting materials: Na.sub.4P.sub.2O.sub.7 (0.85 g) FeC.sub.2O.sub.42H.sub.2O (1.73 g) NH.sub.4H.sub.2PO.sub.4 (0.74 g) Furnace Parameters: Tube furnace, Gas type (argon) Ramp rate 5 C./min; temperature 300 C.; dwell time 6 hours, followed by 500 C. for 6 hours

EXAMPLE 8

(30) TABLE-US-00014 TARGET MATERIAL: Na.sub.7V.sub.4(P.sub.2O.sub.7).sub.4PO.sub.4 Starting materials: Na.sub.2CO.sub.3 (1.61 g) V.sub.2O.sub.5 (1.57 g) NH.sub.4H.sub.2PO.sub.4 (0.26 g) C (0.26 g) Furnace Parameters: Tube furnace, Gas type (nitrogen) Ramp rate 5 C./min; temperature 300 C.; dwell time 2 hours, followed by 800 C. for 36 hours, then 800 C. for 8 hours and 800 C. for 30 hours with intermittent grinding

EXAMPLE 9

(31) TABLE-US-00015 TARGET MATERIAL: Na.sub.7V.sub.3(P.sub.2O.sub.7).sub.4 Starting materials: Na.sub.2HPO.sub.4 (2.89 g) Na.sub.4P.sub.2O.sub.7 (1.81 g) V.sub.2O.sub.5 (1.24 g) NH.sub.4H.sub.2PO.sub.4 (2.34 g) C (0.20 g) Furnace Parameters: Tube furnace, Gas type (nitrogen) Ramp rate 5 C./min; temperature 300 C.; dwell time 4 hours, followed by 650 C. for 8 hours, then 750 C. for 8 hours with intermittent grinding

(32) The resulting product materials were analysed by X-ray diffraction techniques using a Siemens D5000 XRD machine to confirm that the desired target materials had been prepared and to establish the phase purity of the product material and to determine the types of impurities present. From this information it is possible to determine the unit cell lattice parameters.

(33) The typical operating conditions used to obtain the XRD spectra illustrated in the figures are as follows: Slits sizes: 1 mm, 1 mm, 0.1 mm Range: 2=5-60 X-ray Wavelength=1.5418 (Angstroms) (Cu K) Speed: 2 seconds/step Increment: 0.015

(34) Results

(35) The target materials were tested in a lithium metal anode test electrochemical cell to determine their specific capacity and also to establish whether they have the potential to undergo charge and discharge cycles. A lithium metal anode test electrochemical cell containing the active material is constructed as follows:

(36) Generic Procedure For Making A Lithium Metal Test Electrochemical Cell

(37) The positive electrode is prepared by solvent-casting a slurry of the active material, conductive carbon, binder and solvent. The conductive carbon used is Super P (Timcal). PVdF co-polymer (e.g. Kynar Flex 2801, Elf Atochem Inc.) is used as the binder, and acetone is employed as the solvent. The slurry is then cast onto glass and a free-standing electrode film is formed as the solvent evaporates. The electrode film contains the following components, expressed in percent by weight: 80% active material, 8% Super P carbon, and 12% Kynar 2801 binder. Optionally, an aluminium current collector may be used to contact the positive electrode, or alternatively, metallic lithium on a copper current collector may be employed as the negative electrode. The electrolyte comprises one of the following: (i) a 1 M solution of LiPF.sub.6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) in a weight ratio of 2:1; (ii) a 1 M solution of LiPF.sub.6 in ethylene carbonate (EC) and diethyl carbonate (DEC) in a weight ratio of 1:1; or (iii) a 1 M solution of LiPF.sub.6 in propylene carbonate (PC). A glass fibre separator (Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard 2400) wetted by the electrolyte is interposed between the positive and negative electrodes.

(38) First Cycle Constant Current Data

(39) FIG. 4 shows the first cycle constant current data for the Na.sub.4Mn.sub.3(PO.sub.4).sub.2P.sub.2O.sub.7 active material (prepared in Example 4c). The Open Circuit Voltage (OCV) of the as-made cell was 3.22 V vs. Li. The constant current data were collected using a lithium metal counter electrode at a current density of 0.1 mA/cm.sup.2, between voltage limits of 1.00 and 4.60 V. The testing was carried out at room temperature. It is assumed that sodium is extracted from the active material during the initial charging of the cell. A charge equivalent to a material specific capacity of 57 mAh/g is extracted from the cell.

(40) It is expected from thermodynamic considerations that the sodium extracted from the Na.sub.4Mn.sub.3(PO.sub.4).sub.2P.sub.2O.sub.7 material during the initial charging process, enters the electrolyte, and is displaced by being plated onto the lithium metal anode (i.e. releasing more lithium into the electrolyte). Therefore, during the subsequent discharging of the cell, it is assumed that a mix of lithium and sodium is re-inserted into the material. The re-insertion process corresponds to 102 mAh/g; this indicates the reversibility of the extraction-insertion processes. The generally symmetrical nature of the charge-discharge curves shown in FIG. 4 further indicates the excellent reversibility of the system.

(41) FIG. 7 shows the first cycle constant current data for the Na.sub.4Fe.sub.3(PO.sub.4).sub.2P.sub.2O.sub.7 active material (prepared as in Example 7). The Open Circuit Voltage (OCV) of the as-made cell was 2.93 V vs. Li. The constant current data were collected using a lithium metal counter electrode at a current density of 0.04 mA cm.sup.2, between voltage limits of 2.0 and 4.0 V vs. Li. The testing was carried out at room temperature. It is assumed that sodium is extracted from the active material during the initial charging of the cell. A charge equivalent to a material specific capacity of 130 mAh/g is extracted from the material.

(42) It is expected from thermodynamic considerations that the sodium extracted from the Na.sub.4Fe.sub.3(PO.sub.4).sub.2P.sub.2O.sub.7 material during the initial charging process, enters the electrolyte, and is displaced by being plated onto the lithium metal anode (i.e. releasing more lithium into the electrolyte). Therefore, during the subsequent discharging of the cell, it is assumed that a mix of lithium and sodium is re-inserted into the material. The re-insertion process corresponds to 111 mAh/g; this indicates the reversibility of the extraction-insertion processes. The generally symmetrical nature of the charge-discharge curves shown in FIG. 7 further indicates the excellent reversibility of the material.

(43) FIG. 8 shows the first cycle constant current data for the Na.sub.7V4(P.sub.2O.sub.7).sub.4PO.sub.4 active material (prepared as in Example 8). The Open Circuit Voltage (OCV) of the as-made cell was 3.20 V vs. Li. The constant current data were collected using a lithium metal counter electrode at a current density of 0.04 mA/cm.sup.2, between voltage limits of 3.0 and 4.4 V vs. Li. The testing was carried out at room temperature. It is assumed that sodium is extracted from the active material during the initial charging of the cell. A charge equivalent to a material specific capacity of 76 mAh/g is extracted from the material.

(44) It is expected from thermodynamic considerations that the sodium extracted from the Na.sub.7V4(P.sub.2O.sub.7).sub.4PO.sub.4 material during the initial charging process, enters the electrolyte, and is displaced by being plated onto the lithium metal anode (i.e. releasing more lithium into the electrolyte). Therefore, during the subsequent discharging of the cell, it is assumed that a mix of lithium and sodium is re-inserted into the material. The re-insertion process corresponds to 64 mAh/g; this indicates the reversibility of the extraction-insertion processes. The generally symmetrical nature of the charge-discharge curves shown in FIG. 8 further indicates the excellent reversibility of the material.

(45) FIG. 10 shows the first cycle constant current data for the Na.sub.7V.sub.3(P.sub.2O.sub.7).sub.4 active material (prepared as in Example 9). The Open Circuit Voltage (OCV) of the as-made cell was 3.15 V vs. Li. The constant current data were collected using a lithium metal counter electrode at a current density of 0.02 mA/cm.sup.2, between voltage limits of 3.0 and 4.7 V vs. Li. The testing was carried out at room temperature. It is assumed that sodium is extracted from the active material during the initial charging of the cell. A charge equivalent to a material specific capacity of 163 mAh/g is extracted from the material.

(46) It is expected from thermodynamic considerations that the sodium extracted from the Na.sub.7V.sub.3(P.sub.2O.sub.7).sub.4 material during the initial charging process, enters the electrolyte, and is displaced by being plated onto the lithium metal anode (i.e. releasing more lithium into the electrolyte). Therefore, during the subsequent discharging of the cell, it is assumed that a mix of lithium and sodium is re-inserted into the material. The re-insertion process corresponds to 71 mAh/g; this indicates the reversibility of the extraction-insertion processes. The generally symmetrical nature of the charge-discharge curves shown in FIG. 10 further indicates the reasonable reversibility of the material.