Layered oxide materials for batteries

Abstract

Materials are presented of the formula:
A.sub.xM.sub.yM.sup.i.sub.ziO.sub.2d,
where A is sodium or a mixed alkali metal including sodium as a major constituent; x>0.5; M is a transition metal; y>0; M.sup.i, for i=1, 2, 3 . . . n, is a metal or germanium; z.sub.1>0 z.sub.i0 for each i=2, 3 . . . n; 0<d0.5;
the values of x, y, z.sub.i and d are such as to maintain charge neutrality; and the values of y, z.sub.i and d are such that y+z.sub.i>(2d). The formula includes compounds that are oxygen deficient. Further the oxidation states may or may not be integers i.e. they may be whole numbers or fractions or a combination of whole numbers and fractions and may be averaged over different crystallographic sites in the material. Such materials are useful, for example, as electrode materials in rechargeable battery applications.

Claims

1. A composition having the formula
A.sub.xM.sub.yM.sup.i.sub.ziO.sub.2d, where A is sodium or a mixed alkali metal including sodium as a major constituent; 0.5x0.76; M is nickel; y>0; M.sup.i.sub.zi, for i=1, 2, 3 . . . n is M.sup.1.sub.z1M.sup.2.sub.z2M.sup.3.sub.z3 . . . M.sup.n.sub.zn, and wherein each of M.sup.1.sub.z1M.sup.2.sub.z2M.sup.3.sub.z3 . . . M.sup.n.sub.zn is a metal or germanium; z1>0 zi0 for each i=2, 3 . . . n; 0<d0.5; the values of x, y, zi for i=1, 2, 3 . . . n, and d are such as to maintain charge neutrality; and the values of y, zi for i=1, 2, 3 . . . n, and d are such that y+zi>(2d).

2. A composition as claimed in claim 1 in which M.sup.i, for i=1, 2, 3 . . . n, is a transition metal, an alkali metal or an alkaline earth metal.

3. A composition as claimed in claim 1 wherein M.sup.i, for i=1, 2, 3 . . . n, is selected from the group consisting of: nickel, iron, cobalt, manganese, titanium, aluminium, magnesium and zirconium.

4. A composition as claimed in claim 1 in which 0.5x0.67.

5. A composition as claimed in claim 1 in which 0.67x0.76.

6. A composition having the formula
A.sub.xM.sub.yM.sup.1.sub.ziM.sup.i.sub.ziO.sub.2d, where A is sodium or a mixed alkali metal including sodium as a major constituent; 0.76x1.3; M is a nickel; y>0; M.sup.1 is an alkali metal or a mixed alkali metal; z1>0; M.sup.i.sub.zi, for i=2, 3, 4 . . . n is M.sup.2.sub.z2M.sup.3.sub.z3M.sup.4.sub.z4 . . . M.sup.n.sub.zn, wherein M.sup.2.sub.z2 is a transition metal and each of M.sup.2.sub.z2M.sup.3.sub.z3M.sup.4.sub.z3 . . . M.sup.n.sub.zn, is a metal or germanium; z2>0; zi0 for each i=3, 4 . . . n; 0<d0.5; the values of x, y, z1, zi for i=2, 3, 4 . . . n, and d are such as to maintain charge neutrality; the values of y, z1, zi, for i=2, 3, 4 . . . n, and d are such that y+1+zi>/(2d); and the composition adopts a layered oxide structure in which the alkali metal atoms A are coordinated by oxygen in a prismatic or octahedral environment, and in which the alkali metal atoms M.sup.1 adopts a coordination complementary to that adopted by the other M.sup.i transition metal element(s).

7. A composition as claimed in claim 6 in which M.sup.i, for i=3, 4 . . . n, is a transition metal, an alkali metal or an alkaline earth metal.

8. A composition as claimed in claim 6 wherein M.sup.1 is selected from the group consisting of: sodium, lithium, and a mixture of sodium and lithium.

9. A composition as claimed in claim 6 wherein M.sup.i, for i=2, 3 . . . n, is selected from the group consisting of: nickel, iron, cobalt, manganese, titanium, aluminium, magnesium and zirconium.

10. A composition as claimed in claim 6 in which 0.9x.

11. A composition as claimed in claim 6 in which x1.3.

12. A composition as claimed in claim 6 in which z20.3.

13. A composition having the formula
A.sub.xM.sub.yM.sup.i.sub.ziO.sub.2d, where A is sodium or a mixed alkali metal including sodium as a major constituent; 0.5x1; M is a transition metal not including nickel; y>0; M.sup.i.sub.zi, for i=1, 2, 3 . . . n is M.sup.1.sub.z1M.sup.2.sub.z2M.sup.3.sub.z3 . . . M.sup.n.sub.zn, and wherein each of M.sup.1.sub.z1M.sup.2.sub.z2M.sup.3.sub.z3 . . . M.sup.n.sub.zn is germanium or a metal not including nickel; zi0 for each i=1, 2, 3 . . . n; 0<d0.5; the values of x, y, zi for i=1, 2, 3 . . . n, and d are such as to maintain charge neutrality; and the values of y, zi for i=1, 2, 3 . . . n, and d are such that y+zi>(2d).

14. A composition as claimed in claim 13 wherein M.sup.i, for i=1, 2, 3 . . . n, is selected from the group consisting of: nickel, iron, cobalt, manganese, titanium, aluminium, magnesium, calcium and zirconium.

15. A composition as claimed in claim 1 in which A is a mixed alkali metal including sodium and lithium.

16. A composition as claimed in claim 6 in which A is a mixed alkali metal including sodium and lithium.

17. A composition as claimed in claim 13 in which A is a mixed alkali metal including sodium and lithium.

18. A composition as claimed in claim 1 in which A is sodium.

19. A composition as claimed in claim 6 in which A is sodium.

20. A composition as claimed in claim 13 in which A is sodium.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1(A) shows Powder X-ray diffraction pattern of NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 prepared according to Example 1;

(2) FIG. 1(B) shows the TGA-STA data obtained for NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 (Example 1) heated to a temperature of 800 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(3) FIG. 1(C) Powder X-ray diffraction patterns of NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 after being heated to 800 C. and cooled in either air or nitrogen.

(4) FIG. 1(D) shows the first three charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 1 after heating to 800 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(5) FIG. 1(E) shows the Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 1 heated to a temperature of 800 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(6) FIG. 2(A) shows Powder X-ray diffraction pattern of NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2 prepared according to Example 2.

(7) FIG. 2(B) shows the TGA-STA data obtained for NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2 (Example 2) heated to a temperature of 800 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(8) FIG. 2(C) Powder X-ray diffraction patterns of NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2 after being heated to 800 C. and cooled in either air or nitrogen.

(9) FIG. 2(D) shows the first three charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 2 after heating to 800 C. under a constant flow of air (Solid line) or under a constant flow of nitrogen (dashed line).

(10) FIG. 2(E) shows the cycle life (Cathode specific capacity [mAh/g] vs cycle number) for the cathode material prepared according to Example 2 after heating to 800 C. under a constant flow of air (triangles) or under a constant flow of nitrogen (circles). Dotted line represents discharge and solid line represents charge.

(11) FIG. 2(F) shows the Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 2 heated to a temperature of 800 C. under a constant flow of air (solid line) or under a constant flow of nitrogen (dashed line).

(12) FIG. 3(A) shows Powder X-ray diffraction pattern of NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2 prepared according to Example 3.

(13) FIG. 3(B) shows the TGA-STA data obtained for NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2 (Example 3) heated to a temperature of 800 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(14) FIG. 3(C) Powder X-ray diffraction patterns of NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2 after being heated to 800 C. and cooled in either air or nitrogen.

(15) FIG. 3(D) shows the first four charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 3 after heating to 800 C. under a constant flow of air (solid line) or under a constant flow of nitrogen (dashed line).

(16) FIG. 3(E) shows the cycle life (Cathode specific capacity [mAh/g] vs cycle number) for the cathode material prepared according to Example 3 after heating to 800 C. under a constant flow of air (triangles) or under a constant flow of nitrogen (circles). Dotted line represents discharge and solid line represents charge.

(17) FIG. 3(F) shows the Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 3 heated to a temperature of 800 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(18) FIG. 4(A) shows Powder X-ray diffraction pattern of Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2 prepared according to Example 4;

(19) FIG. 4(B) shows the TGA-STA data obtained for Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2 (Example 4) heated to a temperature of 900 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(20) FIG. 4(C) Powder X-ray diffraction patterns of Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2 after being heated to 900 C. and cooled in either air or nitrogen.

(21) FIG. 4(D) shows the first four charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 4 after heating to 900 C. under a constant flow of air (solid line) or under a constant flow of nitrogen (dashed line).

(22) FIG. 4(E) shows the cycle life (Cathode specific capacity [mAh/g] vs cycle number) for the cathode material prepared according to Example 4 after heating to 900 C. under a constant flow of air (circles) or under a constant flow of nitrogen (triangles). Dotted line represents discharge and solid line represents charge.

(23) FIG. 4(F) shows the Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 4 heated to a temperature of 900 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(24) FIG. 5(A) shows Powder X-ray diffraction pattern of Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2 prepared according to Example 5;

(25) FIG. 5(B) shows the TGA-STA data obtained for Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2 (Example 5) heated to a temperature of 900 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(26) FIG. 5(C) Powder X-ray diffraction patterns of Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2 after being heated to 900 C. and cooled in either air or nitrogen.

(27) FIG. 5(D) shows the first charge-discharge cycle voltage profile (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 5 after heating to 900 C. under a constant flow of air (solid line) or under a constant flow of nitrogen (dashed line).

(28) FIG. 5(E) shows the Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 5 heated to a temperature of 900 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(29) FIG. 6(A) shows Powder X-ray diffraction pattern of Na Fe.sub.0.5Ti.sub.0.125Mn.sub.0.125Mg.sub.0.25O.sub.2 prepared according to Example 5;

(30) FIG. 6(B) shows the TGA-STA data obtained for Na Fe.sub.0.5Ti.sub.0.125Mn.sub.0.125Mg.sub.0.25O.sub.2 (Example 6) heated to a temperature of 850 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

(31) FIG. 6(C) Powder X-ray diffraction patterns of Na Fe.sub.0.5Ti.sub.0.125Mn.sub.0.125Mg.sub.0.25 O.sub.2 after being heated to 850 C. and cooled in either air or nitrogen.

(32) FIG. 6(D) shows the first three charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) for the cathode material prepared according to Example 6 after heating to 850 C. under a constant flow of air (solid line) or under a constant flow of nitrogen (dashed line).

(33) FIG. 6(E) shows the cycle life (Cathode specific capacity [mAh/g] vs cycle number) for the cathode material prepared according to Example 6 after heating to 850 C. under a constant flow of air (circles) or under a constant flow of nitrogen (triangles). Dotted line represents discharge and solid line represents charge.

(34) FIG. 6(F) shows the Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) for the cathode material prepared according to Example 6 heated to a temperature of 800 C. under a constant flow of air (dashed line) or under a constant flow of nitrogen (solid line).

DETAILED DESCRIPTION

(35) The materials according to the present invention are prepared using the following generic method:

(36) Synthesis Method:

(37) The required amounts of the precursor materials are intimately mixed together and either pressed into a pellet or retained as a free flowing powder. The resulting mixture is then heated in a tube furnace or a chamber furnace using either an ambient air atmosphere, or a flowing inert atmosphere (e.g. argon or nitrogen), at a furnace temperature of between 400 C. and 1500 C. until reaction product forms; for some materials a single heating step is used and for others (as indicated below in Table 1) more than one heating step is used. Different cooling protocols can be used to induce oxygen non-stoichiometry in the materials. Sample may be heated and cooled under different atmospheres as indicated in table 1. When cool, the reaction product is removed from the furnace and ground into a powder prior to characterisation.

(38) Using the above generic method, active materials were prepared, Examples 1 to 11, as summarised below in Table 1:

(39) TABLE-US-00001 TARGET STARTING Ex. COMPOUND MATERIALS FURNACE CONDITIONS 1 NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2d TiO.sub.2 (d = 0) = 900 C., 8 h, Air Na.sub.2CO.sub.3 (d > 0) = 800 C., 1 h, N.sub.2 NiCO.sub.3 MnCO.sub.3 2 NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2d TiO.sub.2 1.sup.st firing (d = 0) = 900 C., 8 h, Air Na.sub.2CO.sub.3 2.sup.nd firing (d > 0) = 800 C., 1 h, N.sub.2 NiCO.sub.3 MnCO.sub.3 3 NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2d TiO.sub.2 1.sup.st firing (d = 0) = 900 C., 8 h, Air Na.sub.2CO.sub.3 2.sup.nd firing (d > 0) = 800 C., 1 h, N.sub.2 NiCO.sub.3 MnCO.sub.3 4 Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2d Na.sub.2CO.sub.3 1.sup.st firing (d = 0) = 900 C., 8 h, Air NiCO.sub.3 2.sup.nd firing (d > 0) = 800 C., 1 h, N.sub.2 MnCO.sub.3 5 Na.sub.0.76 Mn.sub.0.65Co.sub.0.18Ni.sub.0.17 O.sub.2d Na.sub.2CO.sub.3 1.sup.st firing (d = 0) = 900 C., 8 h, Air TiO.sub.2 2.sup.nd firing (d > 0) = 800 C., 1 h, N.sub.2 NiCO.sub.3 MnCO.sub.3 CoCO.sub.3 6 Na Fe.sub.0.5 Ti.sub.0.125 Mn.sub.0.125Mg.sub.0.25 Na.sub.2CO.sub.3 1.sup.st firing (d = 0) = 900 C., 12 h, Air O.sub.2d TiO.sub.2 2.sup.nd firing (d > 0) = 800 C., 1 h, N.sub.2 MgCO.sub.3 Na Fe.sub.0.5 Ti.sub.0.125 Mn.sub.0.125Mg.sub.0.25 O.sub.1.98 MnCO.sub.3 Fe.sub.2O.sub.3 7 Na Fe.sub.0.5 Ti.sub.0.125 Mn.sub.0.125Mg.sub.0.25 Directly synthesised as d > 0 O.sub.1.98 1.sup.st firing 900 C. for 12 h in Air followed by cooling to room temperature under N.sub.2 or Argon 8 NaNi.sub.0.5Ti.sub.0.5O.sub.2d TiO.sub.2 Directly synthesised as d > 0 Na.sub.2CO.sub.3 1.sup.st firing 900 C. for 12 h in Air NiCO.sub.3 followed by cooling to room temperature under N.sub.2 or Argon 9 NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2d TiO.sub.2 Directly synthesised as d > 0 Na.sub.2CO.sub.3 1.sup.st firing 900 C. for 12 h in Air NiCO.sub.3 followed by cooling to room MnCO.sub.3 temperature under N.sub.2 or Argon 10 Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2d Na.sub.2CO.sub.3 Directly synthesised as d > 0 NiCO.sub.3 1.sup.st firing 900 C. for 12 h in Air MnCO.sub.3 followed by cooling to room temperature under N.sub.2 or Argon 11 Na Mg.sub.x Mn.sub.x Co.sub.1(x+y)O.sub.2d MgCO.sub.3 Directly synthesised as d > 0 MnCO.sub.3 1.sup.st firing 900 C. for 12 h in Air CoCO.sub.3 followed by cooling to room Na.sub.2CO.sub.3 temperature under N.sub.2 or Argon

(40) One example of a method of manufacturing a composition of the invention is an indirect route in which we first mix the precursors and then fire the mixture to produce a stoichiometric layered oxide. To form the oxygen non-stoichiometric form a secondary processing step is used. The secondary processing step can take one of two forms, the material may be re heated under air to a temperature close to the formation temperature of the material and cooled under a flow of nitrogen. This method relies on preventing the re-uptake of oxygen in the material. A broadly similar method can also be used in which the secondary processing step can be undertake under an inert atmosphere such as an atmosphere consisting, or consisting substantially of, an inert gas or a mixture of inert gases. Examples of suitable inert gases include nitrogen, and argon and the other noble gases. That is, in one example, both the heating and cooling steps can be conducted under a nitrogen atmosphere to yield the non-stoichiometric form of the oxide.

(41) Another example of a method of manufacturing a composition of the invention is a single step process in which we mix the precursor materials together, heat the mixture to an appropriate temperature and holding for a specified time to allow the layered oxide to form. At this stage the layered oxide will be in a metastable state. Simply changing the atmosphere to an inert atmosphere (for example by putting the reaction chamber under a nitrogen atmosphere) at the end of the formation of the material and cooling to room temperature yield the oxygen deficient form of the material without the need for secondary processing.

(42) Generic Procedure to Make a Sodium Metal Electrochemical Test Cell:

(43) Electrochemical cells were prepared using conventional electrochemical testing techniques. Materials were either tested as powder, pressed pellets or as cast electrodes, each testing methodology used is highlighted alongside the example materials. To prepare an electrode of the test material the sample was prepared using a solvent-casting technique, from a slurry containing the active material, conductive carbon, binder and solvent. The conductive carbon used is Super P C65 (Timcal). PVdF (e.g. Kynar) is used as the binder, and NMP (N-Methyl-2-pyrrolidone, Anhydrous, Sigma, UK) is used as the solvent. The slurry is then cast onto an aluminium current collector using the Doctor-blade technique. The electrode is then dried under Vacuum at about 80-120 C. The electrode film contains the following components, expressed in percent by weight: 75% active material, 18% Super P carbon, and 7% Kynar binder. Optionally, this ratio can be varied to optimise the electrode properties such as, adhesion, resistivity and porosity. The electrolyte comprises a 0.5 or 1.0 M solution of NaClO.sub.4 in propylene carbonate (PC), and can also be any suitable or known electrolyte or mixture thereof. A glass fibre separator (e.g. Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard 2400) wetted by the electrolyte is interposed between the positive and negative electrodes forming the electrochemical test cell. Typically, cells were symmetrically charged and discharged galvanostatically at a rate of 5-10 mA/g.

(44) The materials described herein can also be tested as powders, where the active material is mixed with a conductive additive, either by hand mixing or in a ball mill. The conductive carbon used is Super P C65 (Timcal). The electro active mixture contains the following components, expressed in percent by weight: 80% active material, 20% Super P carbon, this ratio can be varied to optimise the mixtures properties such as, resistivity and porosity. The electrolyte comprises a 0.5 or 1.0 M solution of NaClO.sub.4 in propylene carbonate (PC). A glass fibre separator (e.g. Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard 2400) wetted by the electrolyte is interposed between the positive and negative electrodes forming the electrochemical test cell. Typically, cells were symmetrically charged and discharged galvanostatically at a rate of 5-10 mA/g.

(45) Alternatively, materials described herein may also be tested as pressed pellets, where the active material is mixed with a conductive additive and a polymer binder, either by hand mixing or in a ball mill. The conductive carbon used is Super P C65 (Timcal). The electro active mixture contains the following components, expressed in percent by weight: 80% active material, 10% Super P Carbon, and 10% binder (PVdF or similar), this ratio can be varied to optimise the mixtures properties such as, resistivity, porosity and wetting behaviour of the pellet. The electrolyte comprises a 0.5 or 1.0 M solution of NaClO.sub.4 in propylene carbonate (PC). A glass fibre separator (e.g. Whatman, GF/A) or a porous polypropylene separator (e.g. Celgard 2400) wetted by the electrolyte is interposed between the positive and negative electrodes forming the electrochemical test cell. Typically, cells were symmetrically charged and discharged galvanostatically at a rate of 5-10 mA/g.

(46) Cell Testing:

(47) Electrochemical cells of materials prepared according to the procedures outlined in Table 1 were tested using Constant Current Cycling Techniques.

(48) The cell was cycled at a given current density (ca. 5-10 mA/g) between pre-set voltage limits as deemed appropriate for the material under test. A commercial battery cycler from Maccor Inc. (Tulsa, Okla., USA) was used. Cells were charged symmetrically between the upper and lower voltage limits at a constant current density. On charge sodium ions are extracted from the cathode and migrate to the anode. On discharge the reverse process occurs and Sodium ions are re-inserted into the cathode material.

(49) Structural Characterisation:

(50) All of the product materials were analysed by X-ray diffraction techniques using a Bruker D2 phaser powder diffractometer (fitted with a Lynxeye detector) to confirm that the desired target materials had been prepared, and also to establish the phase purity of the products and to determine the types of impurities present. From this information it is possible to determine the unit cell lattice parameters.

(51) The operating conditions used to obtain the powder diffraction patterns illustrated, are as follows:

(52) Range: 2=10-90

(53) X-ray Wavelength=1.5418 (Angstroms) (Cu K)

(54) Step size: 2=0.02

(55) Speed: 1.5 seconds/step

(56) Diffraction patterns were collected using sample holders which could allow measurement of diffraction under an inert atmosphere. The sample holder contributes to the observed diffraction patterns with large peaks centered at ca. 32=2 and ca. 50=2 and other smooth peak features can also be observed.

(57) Quantification of Oxygen Loss and Uptake:

(58) The loss or uptake of oxygen could be readily quantified using TGA-STA (Thermogravimetric analysis-simultaneous thermal analysis) using a Perkin Elmer STA 6000 equipped with a passivated Al.sub.2O.sub.3 crucible. To quantify the oxygen loss or uptake in a layered oxide sample the sample was reheated to a temperature less than or equal to the formation temperature. In a typical experiment the sample was heated at a rate of 20 C. min.sup.1 to a temperature less than or equal to the formation temperature, the sample was held at temperature for a period in the range 60 s-1 h to allow equilibrium of any metastable state, the sample was cooled at a rate of 20 C. min.sup.1 to room temperature. The heating and cooling protocol varied by sample as did the combination of flowing gasses and the point at which gas flows were changed. For example, to demonstrate oxygen loss in a stoichiometric sample the sample was heated and cooled under a constant nitrogen flow throughout the entire cycle. Similarly, to demonstrate oxygen uptake in a oxygen deficient sample (Examples 8-12) the sample cycle was completed under a constant flow of air. Control experiments were performed to confirm complete oxygen re-uptake or retention of oxygen deficiency by heating under an inert gas followed by cooling under oxygen or heating under air followed by heating under an inert gas, respectively.

DETAILED DESCRIPTION OF THE INVENTION

Results

(59) The present Applicant has found that not only are the oxidation states of the metal constituents in the compounds of the present invention a critical feature to the production of highly electrochemically active compounds but they have also confirmed that having particular transition metal constituents allows variable oxidation states (i.e. oxidation states which are not integers) in the same crystalline structure of the compound. It is known that that there are several possible layered structural forms which alkali metal/metal/oxides may adopt, including O3, P3 and P2. The Applicant has shown that the oxidation states for the metal constituents can allow oxidation states which are not integers to be stabilised in many structural forms including O2, P3 and P2 via the loss of oxygen from the material. This is achieved through incorporating a reducible transition metal within the material composition. The magnitude of the loss of oxygen may also be controlled by tailoring the synthesis of these materials. The applicant has also noted several benefits in the application of these materials in electrochemical devices. Materials which show oxygen deficiency generally show lower irreversibility on alkali metal intercalation and de intercalation, they also show similar capacity retention and similar intercalation potentials. The slightly lower oxygen content also results in dilation of the unit cell and realises a smaller volume change on electrochemical cycling, leading to improvements in cell capacity retention.

(60) The Invention Will Now be Described with Reference to the Example Materials.

(61) The invention relates to materials which have lost oxygen from their ideal stoichiometry. In the simplest embodiment of the invention oxygen loss may be induced in a stoichiometric layered oxide by post processing. This aspect of the present invention will now be described in reference to examples 1-11.

(62) With reference to Example 1. The data shown in FIG. 1A shows the Powder X-ray diffraction pattern of stoichiometric NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 showing the formation of an O3 layered oxide phase produced as described in Example 1 for the formation of a stoichiometric layered oxide. Post processing of this material leads to the loss of oxygen when post processing via heating under an inert atmosphere is undertaken.

(63) Oxygen loss in NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.2 Oxygen loss from the material was demonstrated using TGA-STA by reheating the sample to a temperature of 800 under a constant flow of N.sub.2. The mass loss associated with reheating the material under air and under N.sub.2 are compared in FIG. 1B. It can be seen that, while mass loss occurs owing to oxygen loss as the sample is heated in air, complete uptake of oxygen occurs in the sample heated in air upon subsequent cooling leading to no overall mass loss. In contrast the mass loss observed in the sample heated and subsequently cooled under nitrogen is 0.57% this equates to a stoichiometry if the mass loss is associated with oxygen of NaNi.sub.0.25Na.sub.0.17Mn.sub.4/12Ti.sub.3/12O.sub.1.96

(64) Electrochemically these materials show a benefit over samples which do not show nonstoichiometry in oxygen. The first three charge-discharge voltage profiles (Na-ion half cell Voltage against a sodium metal anode [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG. 1 D in which the sample produced by post processing in air and under N.sub.2 are compared. It can be seen FIG. 1 D that inducing oxygen deficiency within this material leads to an slight decrease in the cycling capacity. However, there is also a slight reduction in the irreversible capacity loss on the first cycle with similar capacity fade over the first few cycles. The average voltage of each cell is similar at 2.97 and 3.10 V vs Na/Na+ for the sample post processed in air and nitrogen, respectively. Calculation of specific energy density of the materials yields values of 353 and 421 Wh/kg for sample post processed in air and nitrogen, respectively. Showing that oxygen non-stoichiometry in this material leads to a specific energy density gain verses the stoichiometric oxide.

(65) The reduction of first cycle loss is also demonstrated in FIG. 1 E in which the differential capacity plot of the first electrochemical cycle is shown.

(66) With reference to Example 2 a material similar in structure to that given in Example 1. The data shown in FIG. 2A shows the Powder X-ray diffraction pattern of stoichiometric NaNi.sub.0.25Na.sub.0.17Mn.sub.2/12Ti.sub.5/12O.sub.2 showing the formation of an O3 layered oxide phase produced as described in Example 2 for the formation of a stoichiometric layered oxide.

(67) Oxygen loss from the material was demonstrated using TGA-STA by reheating the sample to a temperature of 800 under a constant flow of N.sub.2. The mass loss associated with reheating the material under air and under N.sub.2 are compared in FIG. 2B. In this material a mass loss of 0.4% was realised upon heating and subsequent cooling under N.sub.2, this equates to a stoichiometry if the mass loss is associated with oxygen of N Ni.sub.0.25Na.sub.0.17Mn.sub.0.166Ti.sub.0.416O.sub.1.97. It can be seen that complete uptake of oxygen occurs upon cooling of the sample heated in air leading to no overall mass loss. When compared to the mass loss shown in Example 1 the magnitude of oxygen deficiency within the material may be related to the content of a reducible transition metal within the structure. To confirm that no structural transitions occurred in the post processed sample a XRD pattern of the material is shown in FIG. 2C in which it can be seen that no structural transitions occur in the materials.

(68) Electrochemically these materials can be differentiated in terms of performance. The first two charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG. 2 D in which the sample produced by post processing in air and under N.sub.2 are compared. It can be seen FIG. 2 D that inducing oxygen deficiency within this material leads to an increase in the cycling capacity, and a reduction in the irreversible capacity loss on the first cycle. Also suggesting a relationship between transition metal elements present and cycling capacity. The average voltage of each cell is similar at 3.09 and 3.12 V vs Na/Na+ for the sample post processed in air and nitrogen, respectively. Calculation of specific energy density yields 285 and 369 Wh/kg for the sample post processed in air or nitrogen, respectively. The increase in specific energy density shown in the oxygen deficient materials is a distinct advantage, in for example, an electrochemical cell. FIG. 2E compares the cycling capacities and capacity retention of the materials produced from Example 2. The Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown in FIG. 2 F in which the reduction of irreversibility in the material can be attributed to a peak centered at 4.15 V vs Na/Na+, this is usually attributed to oxygen loss in O3 layered oxide materials. From the differential capacity plot shown in FIG. 2 F we believe that oxygen deficient materials show lower oxygen loss on cycling to high voltages which may also be beneficial in manufacture of full cells based on these materials.

(69) With reference to Example 3, this material is a compositional variant of Examples 1 and 2. FIG. 3A shows the Powder X-ray diffraction pattern of stoichiometric NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.2 showing the formation of an O3 layered oxide phase produced as described in Example 3 for the formation of a stoichiometric layered. In this material a mass loss of 0.56% was realised when post processed by heating and cooling under N.sub.2 under the same conditions used in Examples 1 and 2, this equates to a stoichiometry if the mass loss is associated with oxygen of NaNi.sub.0.25Na.sub.0.17Mn.sub.5/12Ti.sub.2/12O.sub.1.96. Which is of similar magnitude to that observed in Examples 1 and 2.

(70) Electrochemically Example 3 shows similar material properties to those observed in Examples 1 and 2. However, this material may also be differentiated in terms of performance to the stoichiometric variant. The first four charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG. 3 D in which the sample produced by post processing in air and under N.sub.2 are compared. It can be seen FIG. 3 D that inducing oxygen deficiency within this material leads to an increase in the cycling capacity, and a reduction in the irreversible capacity loss on the first cycle. The average voltage of each cell is similar at 3.05 and 3.07 V vs Na/Na+ for the sample post processed in air and nitrogen, respectively. Calculation of specific energy density yields 367 and 451 Wh/kg for the sample post processed in air or nitrogen, respectively. The increase in specific energy density shown in the oxygen deficient materials is a distinct advantage, in for example, an electrochemical cell. FIG. 3E compares the cycling capacities and capacity retention of the materials produced from Example 2. The Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown in FIG. 2 F in which the reduction of irreversibility in the material can be attributed to a peak centered at 4.15 V vs Na/Na+, this is usually attributed to oxygen loss in O3 layered oxide materials. From the differential capacity plot shown in FIG. 3F we believe that oxygen deficient materials show lower oxygen loss on cycling to high voltages which may also be beneficial in manufacture of full cells based on these materials. We have demonstrated that this oxygen loss consistently results in a reduced first cycle loss and a slight increase in the average potential of a cell constructed from oxygen deficient layered oxides.

(71) With reference to Example 4, this material is an example of a layered oxide material which can be stabilized in an oxygen non-stoichiometric form which contains less than one Na atom per formula unit. FIG. 4A shows the Powder X-ray diffraction pattern of stoichiometric Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.2 showing the formation of an P2 layered oxide product. In this material a mass loss of 1.55% was realised when post processed by heating to 900 C. and cooling under N.sub.2 this equates to a stoichiometry if the mass loss is associated with oxygen of Na.sub.2/3Ni.sub.1/3Mn.sub.2/3O.sub.1.91 which is of greater magnitude to that observed in previous examples.

(72) Electrochemically Example 4 shows similar material properties to those observed in the O3 layered oxide materials described in examples 1-3. However, this material can also be clearly differentiated in terms of electrochemical performance to the stoichiometric variant. The first four charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG. 4 D in which the sample produced by post processing in air and under N.sub.2 are compared. It can be seen FIG. 4 D that inducing oxygen deficiency within this material leads to an increase in the cycling capacity 97 mAh/g and 87 mAh/g for the non-stoichiometric and stoichiometric samples. The average voltage is also increased by inducing oxygen non stoichiometry in the sample and this raises from 2.50V vs Na/Na+ to 2.71V vs Na/Na+ for the sample post processed in air, respectively. Calculation of specific energy density yields 262 and 215 Wh/kg for the sample post processed in nitrogen and air, respectively. FIG. 4E compares the cycling capacities and capacity retention of the materials produced from Example 4 in which it can be seen that the oxygen non-stoichiometric form of Example 4 shows higher capacity retention over the first few electrochemical cycles. The Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown in FIG. 4 F in which it can be seen that the irreversibility in this P2 material is similar between the stoichiometric and non-stoichiometric with a similar voltage profile.

(73) With reference to Example 5, this material is a an example of a layered oxide material which can be stabilized in an oxygen non-stoichiometric form which contains less than one Na atom per formula unit and forms a P2 layered structure. FIG. 5A shows the Powder X-ray diffraction pattern of stoichiometric Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17O.sub.2 showing the formation of an P2 layered oxide product as described in Table 1. In this material a mass loss of 3.67% was realised when post processed by heating to 900 C. and cooling under N.sub.2 this equates to a stoichiometry if the mass loss is associated with oxygen of Na.sub.0.76Mn.sub.0.65Co.sub.0.18Ni.sub.0.17 O.sub.1.76 which is of greater magnitude to that observed in previous examples without a structure transition as shown in FIG. 5B.

(74) Electrochemically Example 5 shows similar material properties to those observed in the O3 layered oxide materials described in examples 1-4. Again, this material can be clearly differentiated electrochemically from the stoichiometric variant. The first cycle charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG. 5 D in which the sample produced by post processing in air and under N.sub.2 are compared. It can be seen FIG. 5 D that inducing oxygen deficiency within this material leads to an increase in the cycling capacity 120 mAh/g and 115 mAh/g for the non-stoichiometric and stoichiometric samples. However, the average voltage is reduced by inducing oxygen non stoichiometry in the sample and this reduces from 2.8V vs Na/Na+ to 2.3V vs Na/Na+. Calculation of specific energy density yields 268 and 321 Wh/kg for the sample post processed in nitrogen and air, respectively. This reduction in average voltage may be as a result of the significant proportion of reduced elements present in the material when in its oxygen non-stoichiometric form. The Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown in FIG. 5E in which it can be seen that there is a contribution to capacity originating from a low voltage reaction in the non-stoichiometric material.

(75) With reference to Example 6, this material is a an example of a layered oxide material which can be stabilized in an oxygen non-stoichiometric form which does not contain redox active +2 oxidation state metals and forms a O3 layered structure. FIG. 6A shows the Powder X-ray diffraction pattern of stoichiometric Na Fe.sub.0.5Ti.sub.0.125Mn.sub.0.125Mg.sub.0.25O.sub.2 showing the formation of an O3 layered oxide product as described in Table 1. In this material a mass loss of 0.18% was realised when post processed by heating to 900 C. and cooling under N.sub.2 this equates to a stoichiometry if the mass loss is associated with oxygen of Na Fe.sub.0.5Ti.sub.0.125 Mn.sub.0.125Mg.sub.0.25O.sub.1.98, which is of similar magnitude to that observed in previous

(76) Again, Example 6 can be clearly differentiated electrochemically from the stoichiometric variant. The first three cycle charge-discharge voltage profiles (Na-ion half cell Voltage [V vs Na/Na+] verses Cathode specific capacity [mAh/g]) are shown in FIG. 6 D in which the sample produced by post processing in air and under N.sub.2 are compared. It can be seen FIG. 6 D that inducing oxygen deficiency within this material leads to an increase in the cycling capacity 95 mAh/g and 90 mAh/g for the non-stoichiometric and stoichiometric samples. However, the average voltage is increased by inducing oxygen non stoichiometry in the sample and this increases from 3.12 to 3.16 V vs Na/Na+. Calculation of specific energy density yields 300 and 280 Wh/kg for the sample post processed in nitrogen and air, respectively. FIG. 6E compares the cycling capacities and capacity retention of the materials produced from Example 6 in which it can be seen that the oxygen non-stoichiometric form shows higher capacity retention over the first few electrochemical cycles. The Differential Capacity Profiles for the 1.sup.st charge cycle (Differential Capacity [mAh/g/V] verses Na-ion half Cell Voltage [V vs Na/Na+]) are shown in FIG. 6F in which it can be seen that both the stoichiometric and non-stoichiometric forms of this oxide show similar irreversibility on the first electrochemical cycle.

(77) Further to the embodiments of the invention described by Examples 1-6. Oxygen non-stoichiometry can be induced in materials by altering the atmosphere under which the reaction product quench to room temperature. Examples of this embodiment of the invention are described in Table 1 by the materials of Examples 7-11. These materials are produced by the single step process outlined above, in which the precursor materials are mixed and heated in air for the time period specified in Table 1. The atmosphere is then changed to an inert atmosphere as specified in Table 1 and the material is allowed to cool under the inert atmosphere, resulting in an oxygen deficient material.

INDUSTRIAL APPLICABILITY

(78) Electrodes according to the present invention are suitable for use in many different applications, energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices. Advantageously the electrodes according to the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise of either aqueous electrolyte or non-aqueous electrolytes or a mixture thereof.