Doped nickelate compounds

Abstract

The invention relates to novel electrodes containing one or more active materials comprising: A.sub.aM.sup.1.sub.vM.sup.2.sub.wM.sup.3.sub.xM.sup.4.sub.YM.sup.5.sub.zO.sub.2C(Formula 1) wherein A comprises either sodium or a mixed alkali metal in which sodium is the constituent; M.sup.1 is nickel in oxidation state less than or equal to 4+, M.sup.2 comprises a metal in oxidation state less than or equal to 4+, M.sup.3 comprises a metal in oxidation state 2+, M.sup.4 comprises a metal in oxidation state less than or equal to 4+, and M.sup.5 comprises a metal in oxidation state 3+ wherein 0a1 v>0 at least one of w and y is >0 x0 z0 c>0.1 where (a, v, w, x, y, z and c) are chosen to maintain electroneutrality. Such materials are useful, for example, as electrode materials in sodium-ion battery applications.
A.sub.aM.sup.1.sub.VM.sup.2.sub.WM.sup.3.sub.XM.sup.4.sub.YM.sup.5.sub.ZO.sub.2c(Formula 1)

Claims

1. A compound comprising:
A.sub.aM.sup.1.sub.VM.sup.2.sub.WM.sup.3.sub.XM.sup.4.sub.yM.sup.5.sub.ZO.sub.2c(Formula 1) wherein A comprises sodium; M.sup.1 is nickel in oxidation state greater than 0 to less than or equal to 4+, M.sup.2 comprises a metal in oxidation state greater than 0 to less than or equal to 4+, M.sup.3 comprises a metal in oxidation state 2+, M.sup.4 comprises a metal in oxidation state greater than 0 to less than or equal to 4+, and M.sup.5 comprises a metal in oxidation state 3+ wherein 0a1 v>0 at least one of w and y is >0 wherein one of x and z is 0, and the other of x and z is >0; c>0.1 where (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.

2. The compound according to claim 1, comprising:
A.sub.aM.sup.1.sub.VM.sup.2.sub.WM.sup.3.sub.XM.sup.4.sub.yM.sup.5.sub.ZO.sub.2c(Formula 1) wherein A comprises sodium; M.sup.1 is nickel in oxidation state 4+, M.sup.2 comprises a metal in oxidation state 4+, M.sup.3 comprises a metal in oxidation state 2+, M.sup.4 comprises a metal in oxidation state 4+, and M.sup.5 comprises a metal in oxidation state 3+.

3. The compound according to claim 1 comprising:
A.sub.aM.sup.1.sub.VM.sup.2.sub.WM.sup.3.sub.XM.sup.4.sub.yM.sup.5.sub.ZO.sub.2c(Formula 1) wherein 0<v<0.5; 0<w0.5; 0x<0.5; 0<y<0.5; c0.11.

4. The compound according to claim 1 wherein M.sup.2 comprises a metal selected from one or more of manganese, titanium and zirconium; M.sup.3 comprises a metal selected from one or more of magnesium, calcium, copper, zinc and cobalt; M.sup.4 comprises a metal selected from one or more of manganese, titanium and zirconium; and M.sup.5 comprises a metal selected from one or more of aluminium, iron, cobalt, molybdenum, chromium, vanadium, scandium and yttrium.

5. An electrode comprising the compound according to claim 1.

6. An energy storage device comprising the compound according to claim 1.

7. A rechargeable battery comprising the electrode according to claim 5.

8. An electrochemical device comprising the electrode according to claim 5.

9. An electrochromic device comprising the electrode according to claim 5.

10. The electrode according to claim 5 used in conjunction with a counter electrode and one or more electrolyte materials.

11. The electrode according to claim 10 wherein the electrolyte material comprises an aqueous electrolyte material.

12. The electrode according to claim 10 wherein the electrolyte material comprises a non-aqueous electrolyte material.

13. A method of optimizing the specific charge capacity of an oxide-containing cathode compound according to claim 1 in a sodium-ion cell comprising: a) forming a sodium-ion cell comprising an oxide-containing cathode compound having the formula:
A.sub.aM.sup.1.sub.vM.sup.2.sub.wM.sup.3.sub.xM.sup.4.sub.yM.sup.5.sub.zO.sub.2(Formula 2) wherein Acomprises sodium; M.sup.1 is nickel in oxidation state 2+, M.sup.2 comprises a metal in oxidation state 4+, M.sup.3 comprises a metal in oxidation state 2+, M.sup.4 comprises a metal in oxidation state 4+, and M.sup.5 comprises a metal in oxidation state 3+ 1a<2; 0<v<0.5; 0<w0.5; 0x<0.5; 0y<0.5; z0; wherein one of x and z is 0, and the other of x and z is >0; and wherein v, w, x, y and z are all chosen to maintain electroneutrality b) optionally charging said cell to a cell voltage within the conventional theoretical capacity based on the Ni.sup.2+/Ni.sup.4+ redox couple; c) charging said cell to a cell voltage that results in the active cathode material being charged beyond its conventional theoretical capacity based on the Ni.sup.2+/Ni.sup.4+ redox couple; and d) degassing the resulting Na-ion cell to remove gasses formed during the charging process.

14. A method of using an oxide-containing cathode compound of claim 1 in a Na-ion cell comprising: a) forming a Na-ion cell comprising an active oxide-containing cathode material having the formula:
A.sub.aM.sup.1.sub.vM.sup.2.sub.wM.sup.3.sub.xM.sup.4.sub.yM.sup.5.sub.zO.sub.2(Formula 2) wherein Acomprises sodium; M.sup.1 is nickel in oxidation state 2+, M.sup.2 comprises a metal in oxidation state 4+, M.sup.3 comprises a metal in oxidation state 2+, M.sup.4 comprises a metal in oxidation state 4+, and M.sup.5 comprises a metal in oxidation state 3+ 1a<2; 0<v<0.5; 0<w0.5; 0x<0.5; 0y<0.5; z0; wherein one of x and z is 0, and the other of x and z is 0; and wherein v, w, x, y and z are all choses to maintain electroneutrality b) optionally charging said cell to a cell voltage within the conventional theoretical capacity based on the Ni.sup.2+/Ni.sup.4+ redox couple; c) charging said Na-ion cell to a cell voltage that results in the active oxide-containing cathode material of Formula 2 being charged beyond its conventional theoretical capacity based on the Ni.sup.2+/Ni.sup.4+ redox couple; d) degassing the resulting Na-ion cell to remove gasses formed during the charging process; and e) cycling the Na-ion cell from step b) at a voltage within the normal voltage limits based on the Ni.sup.2+/Ni.sup.4+ redox couple.

15. A method of using an oxide-containing cathode compound of claim 1 in a Na-ion cell comprising the steps of: a) forming a Na-ion cell comprising the oxide-containing cathode compound of Formula 1; and b) cycling the Na-ion cell from step a) at a voltage within the normal voltage limits based on the Ni.sup.2+/Ni.sup.4+ redox couple.

16. The compound according to claim 1 comprising: Na.sub.0.2Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.1.85.

17. A compound comprising:
A.sub.aM.sup.1.sub.VM.sup.2.sub.WM.sup.3.sub.XM.sup.4.sub.yM.sup.5.sub.ZO.sub.2c(Formula 1) wherein A comprises sodium; M.sup.1 is nickel is oxidation state equal to 4+, M.sup.2 comprises a metal in oxidation state equal to 4+, M.sup.3 comprises a metal in oxidation state 2+, M.sup.4 comprises a metal in oxidation state equal to 4+, and M.sup.5 comprises a metal in oxidation state 3+ wherein a=0 v>0 at least one of w and y is >0 wherein one of x and z is 0, and the other x and z is >0; 0.1<c0.5 where (a, v, w, x, y, z, and c) are choses to maintain electroneutrality.

18. The compound according to claim 17 comprising: Ni.sub.0.33Mn.sub.0.33M.sup.3.sub.0.167Ti.sub.0.167O.sub.1.83.

19. The compound according to claim 17 comprising one or more of: Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.1.75; Ni.sub.0.33Mn.sub.0.33Mg.sub.0.167Ti.sub.0.167O.sub.1.83; Ni.sub.0.33Mn.sub.0.33Cu.sub.0.167Ti.sub.0.167O.sub.1.83; Ni.sub.0.33Mn.sub.0.33Zn.sub.0.167Ti.sub.0.167O.sub.1.83; Ni.sub.0.33Mn.sub.0.33Ca.sub.0.167Ti.sub.0.167O.sub.1.83; Ni.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.1.85; Ni.sub.0.3Mn.sub.0.3Mg.sub.0.2Ti.sub.0.2O.sub.1.8; Ni.sub.0.35Mn.sub.0.35Mg.sub.0.05Ti.sub.0.05Al.sub.0.2O.sub.1.85; Ni.sub.0.33Mn.sub.0.33Mg.sub.0.11Ti.sub.0.11Al.sub.0.11O.sub.1.83; Ni.sub.0.3Mn.sub.0.3Mg.sub.0.05Ti.sub.0.05Al.sub.0.3O.sub.1.8; Ni.sub.0.35Mn.sub.0.35Mg.sub.0.1Ti.sub.0.1Al.sub.0.1O.sub.1.85; Ni.sub.0.3Mn.sub.0.3Mg.sub.0.1Ti.sub.0.1Al.sub.0.2O.sub.1.8; Ni.sub.0.33Mn.sub.0.33Al.sub.0.33O.sub.1.83; Ni.sub.0.35Mg.sub.0.15Mn.sub.0.5O.sub.1.85; Ni.sub.0.33Mg.sub.0.167Mn.sub.0.5O.sub.1.83; Ni.sub.0.3Mg.sub.0.2Mn.sub.0.5O.sub.1.8; Ni.sub.0.35Mg.sub.0.15Mn.sub.0.5O.sub.1.85; Ni.sub.0.3Mg.sub.0.2Mn.sub.0.5O.sub.1.8.

20. A method of producing a compound of:
A.sub.aM.sup.1.sub.VM.sup.2.sub.WM.sup.3.sub.XM.sup.4.sub.yM.sup.5.sub.ZO.sub.2c(Formula 1) comprising the step of charging an electrochemical cell containing one or more active cathode materials of Formula 2 beyond the conventional theoretical specific capacity as determined by the Ni.sup.2+/Ni.sup.4+ redox couple; wherein Formula 2 is defined as:
A.sub.aM.sup.1.sub.vM.sup.2.sub.wM.sup.3.sub.xM.sup.4.sub.yM.sup.5.sub.zO.sub.2(Formula 2) wherein A comprises sodium; M.sup.1 is nickel in oxidation state 2+, M.sup.2 comprises a metal in oxidation state 4+, M.sup.3 comprises a metal in oxidation state 2+, M.sup.4 comprises a metal in oxidation state 4+, and M.sup.5 comprises a metal in oxidation state 3+ 1a<2; 0<v<0.5; 0<w0.5; 0x<0.5; 0y<0.5; z0; wherein on of x and z is 0, and the other of x and z is >0; and wherein v, w, x, y and z are all chosen to maintain electroneutrality.

21. A method of producing a compound of:
A.sub.aM.sup.1.sub.VM.sup.2.sub.WM.sup.3.sub.XM.sup.4.sub.yM.sup.5.sub.ZO.sub.2c(Formula 1) comprising the step of causing a net loss of Na.sub.2O from one or more active cathode materials of Formula 2; wherein Formula 2 is defined as:
A.sub.aM.sup.1.sub.vM.sup.2.sub.wM.sup.3.sub.xM.sup.4.sub.yM.sup.5.sub.zO.sub.2(Formula 2) wherein A comprises sodium; M.sup.1 is nickel in oxidation state 2+, M.sup.2 comprises a metal in oxidation state 4+, M.sup.3 comprises a metal in oxidation state 2+, M.sup.4 comprises a metal in oxidation state 4+, and M.sup.5 comprises a metal in oxidation state 3+ 1a<2; 0<v<0.5; 0<w0.5; 0x<0.5; 0y<0.5; z0; wherein one of x and z is 0, and the other of x and z is >0; and wherein v, w, x, y and z are all chosen to maintain electroneutrality.

22. A method of making an oxygen deficient oxide-containing cathode compound for use in a Na-ion cell comprising: a) forming an electrochemical cell comprising an oxide-containing cathode compound; b) charging the oxide-containing cathode compound in said electrochemical cell to cause the loss of oxygen from the oxide-containing cathode compound and thereby form the oxygen deficient oxide-containing cathode compound.

23. The method according to claim 22 comprising charging the Na-ion cell until the oxide-containing cathode compound loses sodium ions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1(A) is a comparison of XRD profiles for a) the precursor starting material, NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2, b) an electrode, originally containing NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2, recovered from a cell after charging to 120 mAh/g and c) an electrode, originally containing NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 recovered from a cell after charging to 230 mAh/g;

(3) FIG. 1(B) shows the First Charge Process (Na-ion cell Voltage [V] versus Cathode Specific Charge Capacity [mAh/g]) for a Hard Carbon//NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 cell using an electrolyte of 0.5M sodium perchlorate in propylene carbonate (PC), charging to 230 mAh/g;

(4) FIG. 2(A) shows the Charge-Discharge Voltage Profile for the first 4 cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for a Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell, cycled between 1.5 to 4.2V;

(5) FIG. 2(B) shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell, cycled between 1.5 to 4.2V;

(6) FIG. 2(C) shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell, cycled between 1.5 to 4.2V;

(7) FIG. 2(D) shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//NaNi.sub.0.35Mn.sub.o35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell, cycled between 1.5 to 4.2V;

(8) FIG. 3(A) shows the Charge-Discharge Voltage Profiles for the first 4 cycles (Na-ion cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for a Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell, cycled between 1.5 to 4.4V;

(9) FIG. 3(B) shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell, cycled between 1.5 to 4.4V;

(10) FIG. 3(C) shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell, cycled between 1.5 to 4.4V;

(11) FIG. 3(D) shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell, cycled between 1.5 to 4.4V;

(12) FIG. 4(A) shows the changes observed in the a and c cell parameters for NaNi.sub.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 as pristine material, material charged to 120 mAh/g and material charged to 230 mAh/g; and

(13) FIG. 4(B) shows the changes in unit cell volume for NaNi.sub.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 as pristine material, material charged to 120 mAh/g and material charged to 230 mAh/g.

DETAILED DESCRIPTION

(14) Any convenient process may be used to make the precursor materials (Formula 2) described above. For example, the following general method may be used:

(15) General Method:

(16) 1) Intimately mix together the starting materials in the correct stoichiometric ratio and press into a pellet. 2) Heat the resulting mixture in a 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. 3) Allow the product to cool before grinding it to a powder.

(17) Table 1 below lists the starting materials and heating conditions used to prepare example precursor materials 1 to 12 of Formula 2.

(18) TABLE-US-00001 TABLE 1 PRECURSOR MATERIAL OF STARTING FURNACE FORMULA 2 MATERIALS CONDITIONS i NaNi.sub.0.45Mn.sub.0.45Mg.sub.0.05Ti.sub.0.05O.sub.2 Na.sub.2CO.sub.3 1) Air/800 C., dwell NiCO.sub.3 time of 8 hours. Mg(OH).sub.2 2) Air/900 C., dwell MnO.sub.2 time of 8 hours. TiO.sub.2 ii NaNi.sub.0.40Mn.sub.0.40Mg.sub.0.10Ti.sub.0.10O.sub.2 Na.sub.2CO.sub.3 1) Air/800 C., dwell NiCO.sub.3 time of 8 hours. Mg(OH).sub.2 2) Air/900 C., dwell MnO.sub.2 time of 8 hours. TiO.sub.2 iii NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 Na.sub.2CO.sub.3 1) Air/900 C., dwell NiCO.sub.3 time of 8 hours. MnO.sub.2 2) Air/900 C., dwell Mg(OH).sub.2 time of 8 hours. TiO.sub.2 iv NaNi.sub.0.30Mn.sub.0.30Mg.sub.0.20Ti.sub.0.20O.sub.2 Na.sub.2CO.sub.3 1) Air/900 C., dwell NiCO.sub.3 time of 8 hours MnO.sub.2 2) Air/900 C., dwell Mg(OH).sub.2 time of 8 hours. TiO.sub.2 v NaNi.sub.0.40Ti.sub.0.50Mg.sub.0.10O.sub.2 Na.sub.2CO.sub.2 1)Air/900 C., dwell NiCO.sub.3 time of 8 hours TiO.sub.2 2)/900 C., dwell time Mg(OH).sub.2 of 8 hours. vi NaNi.sub.0.40Ti.sub.0.40Mg.sub.0.10Mn.sub.0.10O.sub.2 Na.sub.2CO.sub.3 1)Air/900 C., dwell NiCO.sub.3 time of 8 hours TiO.sub.2, 2) Air/900 C., dwell Mg(OH).sub.2 time of 8 hours. MnO.sub.2 vii NaNi.sub.0.40Mn.sub.0.40Mg.sub.0.05Ti.sub.0.05Al.sub.0.1O.sub.2 Na.sub.2CO.sub.3 1) Air/800 C., dwell NiCO.sub.3 time of 8 hours Mg(OH).sub.2 2) Air/900 C., dwell MnO.sub.2 time of 8 hours. TiO.sub.2 Al(OH).sub.3 viii NaNi.sub.0.45Mn.sub.0.45Cu.sub.0.05Ti.sub.0.05O.sub.2 Na.sub.2CO.sub.3 1) Air/900 C., dwell NiCO.sub.3 time of 8 hours MnO.sub.2 2) Air/900 C., dwell CuO time of 8 hours. TiO.sub.2 ix NaNi.sub.0.40Mn.sub.0.40Ca.sub.0.10Ti.sub.0.10O.sub.2 Na.sub.2CO.sub.3 1) Air/900 C., dwell NiCO.sub.3 time of 8 hours MnO.sub.2 2) Air/900 C., dwell CaCO.sub.3 time of 8 hours. TiO.sub.2 3) Air/950 C., dwell time of 8 hours. x NaNi.sub.0.40Mn.sub.0.40Zn.sub.0.10Ti.sub.0.10O.sub.2 Na.sub.2CO.sub.3 1) Air/900 C., dwell NiCO.sub.3 time of 8 hours MnO.sub.2 2) Air/900 C., dwell CuO time of 8 hours. TiO.sub.2 xi NaNi.sub.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 Na.sub.2CO.sub.3 1) Air/900 C., dwell NiCO.sub.3 time of 8 hours TiO.sub.2 2) Air/950 C., dwell Mg(OH).sub.2 time of 8 hours MnO.sub.2 xii NaNi.sub.0.33Mn.sub.0.33Mg.sub.0.167Ti.sub.0.167O.sub.2 Na.sub.2CO.sub.3 1) Air/900 C., dwell NiCO.sub.3 time of 8 hours TiO.sub.2, Mg(OH).sub.2 MnO.sub.2

Example 1

The Preparation of Ni0.25Ti0.25Mg0.25Mn0.25O1.75

(19) Active precursor doped nickelate material, NaNi.sub.0.25Ti.sub.0.2525Mg.sub.0.25Mn.sub.0.25O.sub.2 (compound xi in Table 1), prepared using the general method described above, was made into a hard carbon anode/MaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 cathode cell using an electrolyte comprising 0.5M NaClO.sub.4 in propylene carbonate, see FIG. 1(A)a). Following a charge process to 230 mAh/g, the cathode material was removed from the cell, washed several times in clean dimethyl carbonate and then dried at 70 C. Looking at FIG. 1(A)c), it is clear that the material obtained following the charge process to 230 mAh/g is not the same as that shown in either FIG. 1(A)b) (charged to 120 mAh/g) or that shown by the original active precursor doped nickelate material, NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 (as shown in FIG. 1(A)a). The proposed composition for the product obtained following the charge process to 230 mAh/g is Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.1.75, as determined by the following mass loss experiment.

(20) Mass Loss Experiment to determine the composition of Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.1.75.

(21) Cathode material=NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 Anode=Hard Carbon Electrolyte=0.5 M NaClO.sub.4 in Propylene Carbonate. Active Mass of cathode in as-prepared cell=21.5 mg

(22) Following the charge process to 230 mAh/g, the cathode electrode disk was removed from the cell, washed several times in clean dimethyl carbonate to remove the electrolyte and then dried at 70 C.

(23) The Active Mass of the washed cathode after first charge process shown in FIG. 1(B)=16.1 mg

(24) Thus the Active Mass loss=(21.5 mg16.1 mg)=5.4 mg which equates to: % mass loss=25.1%

(25) If charge process was just by Na-ion extraction then mass loss should be: Starting composition=NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 Molecular weight=101.4 g/mol

(26) Based on only the Ni.sup.2+ to Ni.sup.4+ redox process, on cell charge it is only possible to extract 0.5 Na-ion per formula unit. i.e.
NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2.fwdarw.Na.sub.0.5Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2+0.5 Na.sup.++0.5 e.sup.(1)

(27) Therefore, the theoretical capacity based on this reaction (1) may be given by the following:
Theoretical Specific Capacity=(0.596485)/(101.43.6)=132 mAh/g

(28) Thus the Expected % Mass loss for reaction (1)=(11.5/101.4)100=11.3%

(29) The charge process as shown in FIG. 1(B), corresponds to an actual cathode specific capacity of 230 mAh/gi.e. far in excess of the expected theoretical specific capacity of 132 mAh/g.

(30) Thus the following overall charge mechanism is:
NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2.fwdarw.Na.sub.0.5Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2+0.5 Na.sup.++0.5 e.sup.(1)
followed by:
Na.sub.0.5Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2.fwdarw.Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.1.75+0.5 Na.sup.++0.125 O.sub.2+0.5 e.sup.(2)

(31) Looking at an overall process that relies on the complete extraction of Na:
NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2.fwdarw.Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.1.75+1.0 Na.sup.++0.125 O.sub.2+1.0 e.sup.(3)

(32) The theoretical capacity based on this reaction (3) may be given by:
Theoretical Specific Capacity=(1.096485)/(101.43.6)=264 mAh/g.

(33) This compares well with the actual capacity achieved of 230 mAh/g.

(34) Looking also at the Expected mass loss for reaction (3)=((101.4-74.4)/101.4)100%=26.5%. Again this percentage mass loss is very close to the 25.1% which is observed.

(35) Thus on the basis that there is close correspondence between theoretical and actual results for both Specific Capacity and Expected Mass Loss, the Applicant has been able to determine with a high degree of certainty that Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.1.75 is obtained when NaNi.sub.0.25Ti.sub.0.2525Mg.sub.0.25Mn.sub.0.25O.sub.2 is charged to the end of the second unconventional voltage plateau.

(36) From reaction (3) above, it is proposed that the anomalous capacity arises as a result of the loss of Na.sub.2O, i.e. the production of active Nations plus the liberation of O.sub.2, and this produces a new layered oxide active material, Ni.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.1.75, as confirmed by comparing XRD FIG. 1Aa) with that of FIG. 1Ac). This is surprising because it is not the usual charging mechanism i.e. a simple Na.sup.+ extraction from the cathode, but is a structural change that releases Na.sup.+ and oxygen from the material to produce a new composition.

Example 2

The Preparation of NaaNi0.2535Mn0.2535Mg0.15Ti0.15O2c

(37) The data shown in FIGS. 2A, 2B, 2C and 2D are derived from the constant current cycling data for a NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 (compound iii in Table 1) active precursor cathode material in a Na-ion cell where this cathode material is coupled with a Hard Carbon (Carbotron P/J) anode material. The electrolyte used is a 0.5 M solution of NaClO.sub.4 in propylene carbonate. The constant current data are collected at an approximate current density of 0.10 mA/cm.sup.2 between voltage limits of 1.50 and 4.20 V. To fully charge the Na-ion cell it, is potentiostatically held at 4.2 V at the end of the constant current charging process. The testing is carried out at room temperature. During the cell charging process, sodium-ions are extracted from the cathode active material, and inserted into the Hard Carbon anode. During the subsequent discharge process, sodium-ions are extracted from the Hard Carbon and re-inserted into the cathode active material.

(38) FIG. 2A shows the first four charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell. During the first charge process an anomalously high charge capacity is realizeda cathode specific capacity of 215 mAh/g is achieved this figure is significantly larger than the theoretical specific capacity (which amounts to 178 mAh/g, based on the Ni.sup.2+ to Ni.sup.4+ redox couple) for the NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 active material. In particular, a two section voltage profile is clearly evident during this initial cell charge step. At cell voltages lower than about 4.0 V a sloping profile is evident, presumably reflecting the conventional Na extraction process from the NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 precursor material. At cell voltages greater than about 4.0 V, a more flat voltage region is evident which presumably reflects a new Na extraction process (i.e. not based on the Ni.sup.2+ to Ni.sup.4+ redox couple) occurring for the NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 precursor material. The active material that produces the anomalously high specific capacity is understood to be Na.sub.aNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2-c. Importantly, as demonstrated below, this two section charge behaviour is not evident on subsequent cell charge profiles.

(39) FIG. 2(B) shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell. The cell shows good reversibility with the delivered cathode specific capacity reaching around 135 mAh/g after 4 cycles.

(40) FIG. 2(C) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell. The cathode specific capacity in this cycle corresponds to 130 mAh/g.

(41) FIG. 2(D) shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell. These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell.

(42) Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 (Material=X0474A) Cell#204064

(43) The data shown in FIGS. 3(A), 3(B), 3(C) and 3(D) are derived from the constant current cycling data for a NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 (compound iii in Table 1)) active precursor material in a Na-ion cell where this cathode material is coupled with a Hard Carbon (Carbotron P/J) anode material. The electrolyte used is a 0.5 M solution of NaClO.sub.4 in propylene carbonate. The constant current data are collected at an approximate current density of 0.10 mA/cm.sup.2 between voltage limits of 1.50 V and 4.40 V. To fully charge the cell the Na-ion cell is potentiostatically held at 4.4 V at the end of the constant current charging process. The testing is carried out at room temperature. During the cell charging process, sodium ions are extracted from the cathode active material, and inserted into the Hard Carbon anode. During the subsequent discharge process, sodium-ions are extracted from the Hard Carbon and re-inserted into the cathode active material.

(44) FIG. 3(A) shows the first four charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell (Cell#204064). During the first charge process an anomalously high charge capacity is realizeda cathode specific capacity of 226 mAh/g is achieved a figure which is significantly larger than the theoretical specific capacity (which amounts to 178 mAh/g, based on the Ni.sup.2+ to Ni.sup.4+ redox couple) for the NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 active material. In particular, a two section voltage profile is clearly evident during this initial cell charge step. At cell voltages lower than about 4.0 V a sloping profile is evident, presumably reflecting the conventional Na extraction process from the NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 active material. At cell voltages greater than about 4.0 V, a more flat voltage region is evident which presumably reflects a new Na extraction process (i.e. not based on the Ni.sup.2+ to Ni.sup.4+ redox couple) occurring for the NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 active precursor material. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles.

(45) FIG. 3(B) shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell. The cell shows good reversibility with the delivered cathode specific capacity reaching around 130 mAh/g after 4 cycles.

(46) FIG. 3(C) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell. The cathode specific capacity in this cycle corresponds to 125 mAh/g.

(47) FIG. 3(D) shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//NaNi.sub.0.35Mn.sub.0.35Mg.sub.0.15Ti.sub.0.15O.sub.2 cell. These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell.

(48) Other sodium doped nickelate electrode materials were also found to exhibit anomalous charge capacities:

(49) TABLE-US-00002 TABLE 2 Theoretical Theoretical Charge Charge Capacity Capacity based on based on Actual Ni.sup.2+/Ni.sup.4+ removal of Charge only all Na Capacity Precursor [Formula 2] Product [Formula 1] [mAh/g] [mAh/g] [mAh/g] Example 3. Ni.sub.0.33Mn.sub.0.33Mg.sub.0.167Ti.sub.0.167O.sub.1.833 169 256 239 NaNi.sub.0.33Mn.sub.0.33Mg.sub.0.167Ti.sub.0.167O.sub.2 Example 4. Ni.sub.0.33Mn.sub.0.33Cu.sub.0.167Ti.sub.0.167O.sub.1.833 159 241 221 NaNi.sub.0.33Mn.sub.0.33Cu.sub.0.167Ti.sub.0.167O.sub.2

(50) It is desirable for electrode materials to be safe during charge and discharge in an energy storage device. Li-ion batteries in common use today undergo safety/abuse testing, the results of such tests revealing that lithium oxide-based cathode materials are liable to liberate oxygen, which is a major contributing factor to an undesirable process known as thermal runaway. Prior to the present invention it might have been expected that a similar thermal runaway process would also be observed for sodium oxide-based materials, and that such sodium oxide-based materials would be unsafe and/or rendered completely useless by overcharging in rechargeable battery applications. However, the present invention has surprisingly demonstrated that when these sodium-based materials are overcharged, the labile oxygen (which could contribute to thermal runaway) is caused to be removed from the structure; this yields the materials of the present invention which are highly thermodynamically stable and which are extremely effective and safe when used in reversible cathode materials.

(51) Product Analysis using XRD

(52) Analysis by X-ray diffraction techniques was conducted using a Siemens D5000 powder diffractometer to confirm that the desired target materials had been prepared, 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.

(53) The XRD operating conditions used to analyse the precursor electrode materials are as follows: Slits sizes: 1 mm, 1 mm, 0.1 mm Range: 2=5 60 X-ray Wavelength=1.5418 (Angstoms) (Cu K) Speed: 1.0 seconds/step Increment: 0.025

(54) The XRD operating conditions used for ex-situ analysis of the electrodes are as follows: Slits sizes: 1 mm, 1 mm, 0.1 mm Range: 2=10 60 X-ray Wavelength=1.5418 (Angstoms) (Cu K) Speed: 8.0 seconds/step Increment: 0.015
Electrochemical Results

(55) The target materials were tested either i) using a lithium metal anode test cell, or ii) using a Na-ion test cell using a hard carbon anode. It is also possible to test using a Li-ion cell with a graphite anode. Cells may be made using the following procedures:

(56) Generic Procedure to Make a Lithium Metal Electrochemical Test Cell

(57) 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 is then dried further at about 80 C. 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. 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 1:1; (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.

(58) A Na-ion electrochemical test cell containing the active material is constructed as follows:

(59) Generic Procedure to Make a Hard Carbon Na-ion Cell

(60) 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 is then dried further at about 80 C. 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.

(61) The negative electrode is prepared by solvent-casting a slurry of the hard carbon active material (Carbotron P/J, supplied by Kureha), 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 is then dried further at about 80 C. The electrode film contains the following components, expressed in percent by weight: 84% active material, 4% Super P carbon, and 12% Kynar 2801 binder. Optionally, a copper current collector may be used to contact the negative electrode.

(62) Generic Procedure to Make a Graphite Li-Ion Cell

(63) 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 is then dried further at about 80 C. 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.

(64) The negative electrode is prepared by solvent-casting a slurry of the graphite active material (Crystalline Graphite, supplied by Conoco Inc.), 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 is then dried further at about 80 C. The electrode film contains the following components, expressed in percent by weight: 92% active material, 2% Super P carbon, and 6% Kynar 2801 binder. Optionally, a copper current collector may be used to contact the negative electrode.

(65) Cell Testing

(66) The cells are tested as follows, using Constant Current Cycling techniques.

(67) The cell is cycled at a given current density between pre-set voltage limits. A commercial battery cycler from Maccor Inc. (Tulsa, Okla., USA) is used. On charge, sodium (lithium)-ions are extracted from the cathode active material. During discharge, sodium (lithium)-ions are re-inserted into the cathode active material.

(68) Cell Parameters at Various States of Charge of an Electrode Originally Containing NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 (Precursor Material xiii)

(69) Although the materials may change structure upon charge, for the purposes of calculating the cell parameters given in Table 2, an R-3m space group was used for the calculation at all states of charge.

(70) TABLE-US-00003 TABLE 2 Calculated cell parameters for NaNi.sub.0.25Ti.sub.0.25Mg.sub.0.25Mn.sub.0.25O.sub.2 at different states of charge (standard deviation shown in brackets) a () c () Vol (.sup.3) Uncharged 2.9839(2) 16.076(2) 143.135 120 mAh/g 2.9149(5) 16.852(7) 143.185 230 mAh/g 2.873(1) 14.82(1) 122.326

(71) As the pristine material is charged to 120 mAh/g, nickel reduces in size as it is oxidised from Ni.sup.2+ (IR for CN 6=0.69 ) to Ni.sup.4+ (IR for CN 6=0.48 ), and this is reflected in the -parameter. The c-parameter, however, increases upon charging to 120 mAh/g, and this is due to an increase in repulsion between the electronegative oxygen-ions of adjacent layers, as the sodium-ions are removed.

(72) Upon further charging of this electrode, to 230 mAh/g, there is negligible further oxidation of the nickel, so the nickel oxidation state no longer contributes to the volume change of the unit cell. The Applicant does not wish to be rigidly bound to the following explanation but it is their current belief that the further application of an oxidative potential is capable of oxidising O.sup.2 in the lattice, thus releasing this from the structure as O.sub.2, and this is accompanied by a removal of the remaining sodium-ions. The loss of both sodium and oxygen in this way appears to explain the observed reduction in both the a- and c-parameters, and the resulting overall reduction in unit cell volume.

(73) These results are shown in FIGS. 4(A) and 4(B).