Abstract
The invention relates to doped nickelate-containing compounds 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.2-c wherein A comprises either sodium or a mixed alkali metal in which sodium is the major constituent; 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 0≤a<1, v>0, at least one of w and y is >0 x≥0, z≥0 wherein c is determined by a range selected from 0<c≤0.1 and wherein (a, v, w, x, y, z and c) are chosen to maintain electroneutrality.
Claims
1. A doped nickelate-containing 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.2-c wherein A is either sodium or a mixed alkali metal in which sodium is the major constituent; M.sup.1 is nickel in oxidation state from 2+ 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+, selected from one or more of tin, manganese, titanium, and zirconium; M.sup.3 comprises a metal in oxidation state 2+, selected from one or more of magnesium, calcium, copper, zinc, and cobalt; M.sup.4 comprises a metal in oxidation state greater than 0 to less than or equal to 4+, selected from one or more of tin, manganese, titanium, and zirconium; M.sup.5 comprises a metal in oxidation state 3+, selected from one or more of aluminum, iron, cobalt, molybdenum, chromium, vanadium, scandium, and yttrium; 0≤a<1; v>0; at least one of w and y is >0; x>0; z≥0; 0<c≤0.1; wherein v+w+x+y+z=1; and wherein a, v, w, x, y, z and c are chosen to maintain electroneutrality.
2. The doped nickelate-containing compound according to claim 1 selected from: Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2-c; Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2-c; Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2-c; Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2-c; Na.sub.0.2Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2-c; Na.sub.0.2Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2-c; Na.sub.0.2Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2-c; Ni.sub.0.325Mn.sub.0.325Mg.sub.0.1625Ti.sub.0.1875O.sub.2-c; Na.sub.0.2Ni.sub.0.325 Mn.sub.0.325Mg.sub.0.1625 Ti.sub.0.1875O.sub.2-c; Na.sub.0.2Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2-c; Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2-c; and Na.sub.0.2Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2-c.
3. An application device selected from an energy storage device, a battery, a rechargeable battery, an electrochemical device, and an electrochromic device, comprising the doped nickelate-containing compound of claim 1.
4. A process for making the doped nickelate-containing compound of claim 1, comprising the step of heating a reaction mixture comprising suitable starting materials that are present in the correct stoichiometric amounts so as to be substantially fully consumed during the process, and so as to produce the doped nickelate-containing compound substantially in the absence of side-reaction products and/or unreacted starting materials.
5. An application device selected from an energy storage device, a battery, a rechargeable battery, an electrochemical device, and an electrochromic device comprising one or more doped nickelate-containing compounds made by the process of claim 4.
6. An electrochemical cell comprising one or more doped nickelate-containing compounds made by the process of claim 4.
7. An electrochemical cell comprising one or more doped nickelate-containing compounds according to claim 1.
8. A method of using the electrochemical cell of claim 7 comprising cycling the electrochemical cell at a voltage within the normal voltage limits based on the Ni.sup.2+ to Ni.sup.4+ redox couple.
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 the XRD profile for the compound NaNi.sub.0.5Mn.sub.0.5O.sub.2 used in comparative Example 1;
(3) FIG. 1(B) 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.0.5Mn.sub.0.5O.sub.2 cell, cycled between 1.5 to 4.0V at 30° C.;
(4) FIG. 1(C) shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//NaNi.sub.0.5Mn.sub.0.5O.sub.2 cell, cycled between 1.5 to 4.0V at 30° C.;
(5) FIG. 1(D) shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//NaNi.sub.0.5Mn.sub.0.5O.sub.2 cell, cycled between 1.5 to 4.0V at 30° C.;
(6) FIG. 1(E) shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//NaNi.sub.0.5Mn.sub.0.5O.sub.2 cell, cycled between 1.5 to 4.0V at 30° C.;
(7) FIG. 2(A) is the XRD profile for the compound Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 used in Example 2;
(8) FIG. 2(B) 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//Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(9) FIG. 2(C) shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na.sub.0.95Ni.sub.0.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(10) FIG. 2(D) shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(11) FIG. 2(E) shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(12) FIG. 3(A) is the XRD profile for the compound Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 used in Example 3;
(13) FIG. 3(B) 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//Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(14) FIG. 3(C) shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(15) FIG. 3(D) shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(16) FIG. 3(E) shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(17) FIG. 4(A) is the XRD profile for the compound Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 used in Example 4;
(18) FIG. 4(B) 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//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(19) FIG. 4(C) shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(20) FIG. 4(D) shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(21) FIG. 4(E) shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(22) FIG. 5(A) is the XRD profile for the compound Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 used in Example 5;
(23) FIG. 5(B) 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//Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(24) FIG. 5(C) shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(25) FIG. 5(D) shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(26) FIG. 5(E) shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(27) FIG. 6(A) is the XRD profile for the material Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 used in Example 6;
(28) FIG. 6(B) 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//Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(29) FIG. 6(C) shows the Cycle Life Performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for a Hard Carbon//Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(30) FIG. 6(D) shows the Third Cycle Discharge Voltage Profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for a Hard Carbon//Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.;
(31) FIG. 6(E) shows the Third Cycle Differential Capacity Profiles (Differential Capacity [mAh/g/V]) versus Na-ion Cell voltage [V]) for a Hard Carbon//Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 cell, cycled between 1.0 to 4.2V at 30° C.; and
(32) FIG. 7 is a comparison of XRD profiles for a) the precursor starting material, Na.sub.0.95Ni.sub.0.3167Ti.sub.0.3167Mg.sub.0.1583Mn.sub.0.2083O.sub.2, b) an electrode, originally containing Na.sub.0.95Ni.sub.0.3167Ti.sub.0.3167Mg.sub.0.1583Mn.sub.0.2083O.sub.2, recovered from a cell after charging to a specific capacity of 139 mAh/g and c) an electrode, originally containing Na.sub.0.95Ni.sub.0.3167Ti.sub.0.3167Mg.sub.0.1583Mn.sub.0.2083O.sub.2 recovered from a cell after charging to a specific capacity of 192 mAh/g.
DETAILED DESCRIPTION
(33) Any convenient process may be used to make the compounds of Formula 1 described above. For example, the following general method may be used:
(34) General Method:
(35) 1) Intimately mix together the starting materials in the correct stoichiometric ratio, to ensure that the required product is formed and ideally with the minimum amount of side reactions and unreacted starting materials, 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.
(36) Table 1 below lists the starting materials and heating conditions used to prepare a comparative compound NaNi.sub.0.5Mn.sub.0.5O.sub.2 (Example 1) and compounds of Formula 1 (Examples 2 to 6).
(37) TABLE-US-00001 TABLE 1 STARTING COMPOUND MATERIALS FURNACE CONDITIONS 1 COMPARATIVE NaNi.sub.0.5Mn.sub.0.5O.sub.2 0.5 Na.sub.2CO.sub.3 1) Air/900° C., dwell time of 8 0.5 NiCO.sub.3 hours. 0.5 MnO.sub.2 2) Air/900° C., dwell time of 8 hours. COMPOUNDS OF FORMULA 1 2 Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 0.475 Na.sub.2CO.sub.3 1) Air/900° C., dwell time of 8 0.3167 NiCO.sub.3 hours. 0.3167 MnO.sub.2 0.1583 Mg(OH).sub.2 0.2083 TiO.sub.2 3 Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 0.463 Na.sub.2CO.sub.3 1) Air/900° C., dwell time of 10 0.308 NiCO.sub.3 hours. 0.308 MnO.sub.2 0.154 Mg(OH).sub.2 0.229 TiO.sub.2 4 Na.sub.0.9Ni.sub.0.30Mn.sub.0.30Mg.sub.0.150Ti.sub.0.25O.sub.2 0.45 Na.sub.2CO.sub.3 1) Air/900° C., dwell time of 10 0.3 NiCO.sub.3 hours 0.3 MnO.sub.2 0.15 Mg(OH).sub.2 0.25 TiO.sub.2 5 Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 0.425 Na.sub.2CO.sub.3 1) Air/900° C., dwell time of 10 0.283 NiCO.sub.3 hours 0.283 MnO.sub.2 0.142 Mg(OH).sub.2 0.292 TiO.sub.2 6 Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 0.833 Na.sub.2CO.sub.3 1) Air/900° C., dwell time of 10 0.317 NiCO.sub.3 hours 0.467 TiO.sub.2, 0.1 Mg(OH).sub.2 0.117 MnO.sub.2
(38) 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 they yield materials which are highly thermodynamically stable and which are extremely effective and safer when used as cathode materials in electrochemical cells (e.g. Na-ion electrochemical cells).
(39) Product Analysis Using XRD
(40) 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.
(41) The general XRD operating conditions used to analyse the materials are as follows:
(42) Slits sizes: 1 mm, 1 mm, 0.1 mm
(43) Range: 2θ=5°-60°
(44) X-ray Wavelength=1.5418 Å (Angstroms) (Cu Kα)
(45) Speed: 1.0 seconds/step
(46) Increment: 0.025°
(47) Electrochemical Results
(48) The target materials were tested using a Na-ion test cell using a hard carbon anode. Cells may be made using the following procedures:
(49) A Na-ion electrochemical test cell containing the active material is constructed as follows:
(50) Generic Procedure to Make a Hard Carbon Na-Ion Cell
(51) 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 is used as the binder, and N-Methyl-2-pyrrolidone (NMP) is employed as the solvent. The slurry is then cast onto aluminium foil and heated until most of the solvent evaporates and an electrode film is formed. The electrode is then dried under dynamic vacuum at about 120° C. The electrode film contains the following components, expressed in percent by weight: 88% active material, 6% Super P carbon, and 6% PVdF binder.
(52) 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 is used as the binder, and N-Methyl-2-pyrrolidone (NMP) is employed as the solvent. The slurry is then cast onto aluminium foil and heated until most of the solvent evaporates and an electrode film is formed. The electrode is then dried further under dynamic vacuum at about 120° C. The electrode film contains the following components, expressed in percent by weight: 89% active material, 2% Super P carbon, and 9% PVdF binder.
(53) Cell Testing
(54) The cells are tested as follows, using Constant Current Cycling techniques.
(55) 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, alkali ions are extracted from the cathode active material. During discharge, alkali ions are re-inserted into the cathode active material.
(56) Convenient Method for Overcharging the Electrochemical Cells
(57) The cell is charged under constant current conditions to a pre-set cell voltage limit, typically 4.2V. At the end of this constant current stage, the cell is further charged at a constant voltage (CV—potentiostatic) (typically 4.2V) until the cell current drops to less than 1% of the constant current value. This CV step ensures that the cell is fully charged. Following this charging process the cell is discharged to a lower pre-set voltage limit (typically 1.0V).
Example 1
Comparative Compound NaNi.SUB.0.5.Mn.SUB.0.5.O.SUB.2
(58) In this comparative Example, it is shown that no anomalous capacity behaviour is observed when the process of the present invention is performed on NaNi.sub.0.5Mn.sub.0.5O.sub.2. This is because the [Ni] activity is sufficient to allow (theoretically) all of the Na to be extracted using the Ni.sup.2+/Ni.sup.4+ oxidation process. As FIGS. 1(B) to 1(E) illustrate and as explained below, no anomalous capacity is observed for this compound when an electrochemical cell containing it is charged beyond the conventional theoretical specific capacity as determined by the Ni.sup.2+/Ni.sup.4+ redox couple.
(59) A Hard Carbon//NaNi.sub.0.5Mn.sub.0.5O.sub.2 cell was prepared as follows, charged to 4V and held potentiostatically (constant voltage step) at 4V. The data shown in FIGS. 1(B) to 1(E) are derived from the constant current cycling data for a NaNi.sub.0.5Mn.sub.0.5O.sub.2 compound in a Na-ion cell (Cell #204056) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO.sub.4 in propylene carbonate. The constant current data were collected at an approximate current density of 0.1 mA/cm.sup.2 between voltage limits of 1.50 and 4.00 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.0 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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.
(60) FIG. 1(B) shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//NaNi.sub.0.5Mn.sub.0.5O.sub.2 cell (Cell #204056). During the first charge process a cathode specific capacity of 162 mAh/g is achieved for the NaNi.sub.0.5Mn.sub.0.5O.sub.2 active material. At cell voltages lower than about 3.7 V, a sloping profile is evident reflecting the Na extraction process proceeding based on the Ni.sup.2+ to Ni.sup.4+ redox couple from the NaNi.sub.0.5Mn.sub.0.5O.sub.2 compound. The first discharge process is equivalent to a cathode specific capacity of 86 mAh/g. This discharge process is based on the Ni.sup.4+ to Ni.sup.2+ reduction process within the cathode material.
(61) FIG. 1(C) shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//NaNi.sub.0.5Mn.sub.0.5O.sub.2 cell (Cell #204056). The cell shows good reversibility with the delivered cathode specific capacity being around 85 mAh/g after 4 cycles.
(62) FIG. 1(D) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//NaNi.sub.0.5Mn.sub.0.5O.sub.2 cell (Cell #204056). The cathode specific capacity in this cycle corresponds to 86 mAh/g.
(63) FIG. 1(E) shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//NaNi.sub.0.5Mn.sub.0.5O.sub.2 cell (Cell #204056). These symmetrical data demonstrate the reversibility of the ion-insertion reactions in this Na-ion cell.
(64) In conclusion, this comparative Example shows that NaNi.sub.0.5Mn.sub.0.5O.sub.2, although containing more Ni than any of the experimental examples, delivers far lower reversible specific capacity. The Na:Ni ratio (2:1) is set at a level where all of the redox processes that are occurring during cell charge and discharge are solely based on the Ni.sup.2+ to Ni.sup.4+ redox couple in the cathode active material. There is no anomalous capacity behaviour, and this limits the cathode reversible capacity.
Example 2
The Effect of Charging Na.SUB.0.95.Ni.SUB.0.3167.Mn.SUB.0.3167.Mg.SUB.0.1583.Ti.SUB.0.2083.O.SUB.2 .Beyond the Conventional Theoretical Capacity as Determined by the Ni.SUP.2+ to Ni.SUP.4+ Redox Couple, in Accordance with the Process of the Present Invention
(65) A Hard Carbon//Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention.
(66) The data shown in FIGS. 2(B) to 2(E) are derived from the constant current cycling data for a Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 compound in a Na-ion cell (Cell #311033) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO.sub.4 in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm.sup.2 between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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.
(67) FIG. 2(B) shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell (Cell #311033). During the first charge process an anomalously high charge capacity is realized—a cathode specific capacity of 214 mAh/g is achieved—a figure which is significantly larger than the theoretical specific capacity (based on the Ni.sup.2+ to Ni.sup.4+ redox couple) for the Na.sub.0.95Ni.sub.0.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 compound. 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 reflecting the conventional Na extraction process based on the Ni.sup.2+ to Ni.sup.4+ redox couple from the Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 compound. 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 Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 compound. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles. The first discharge process is equivalent to a cathode specific capacity of 158 mAh/g.
(68) FIG. 2(C) shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na.sub.0.95Ni.sub.0.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell (Cell #311033). The cell shows good reversibility with the delivered cathode specific capacity being around 150 mAh/g after 4 cycles.
(69) FIG. 2(D) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell (Cell #311033). The cathode specific capacity in this cycle corresponds to 152 mAh/g.
(70) FIG. 2(E) shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cell (Cell #311033). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell.
(71) In conclusion, the electrochemical data obtained in Example 2 demonstrate high active material specific capacity and excellent reversibility. The data are far superior to those shown in the comparative Example 1, in which both the Ni- and Na-contents are higher.
Example 3
The Effect of Charging Na.SUB.0.925.Ni.SUB.0.308.Mn.SUB.0.308.Mg.SUB.0.154.Ti.SUB.0.229.O.SUB.2 .Beyond the Conventional Theoretical Capacity as Determined by the Ni.SUP.2+ to Ni.SUP.4+ Redox Couple, in Accordance with the Process of the Present Invention
(72) A Hard Carbon//Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention.
(73) The data shown in FIGS. 3(B) to 3(E) are derived from the constant current cycling data for a Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 compound in a Na-ion cell (Cell #312001) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO.sub.4 in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm.sup.2 between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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.
(74) FIG. 3(B) shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 cell (Cell #312001). During the first charge process above approximately 4 V an anomalously high charge capacity is realized—a cathode specific capacity of 199 mAh/g is achieved a figure which is significantly larger than the theoretical specific capacity (based on the Ni.sup.2+ to Ni.sup.4+ redox couple) for the Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.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 reflecting the conventional Na extraction process based on the Ni.sup.2+ to Ni.sup.4+ redox couple from the Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.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 Na.sub.0.925Ni.sub.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 active material. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles. The first discharge process is equivalent to a cathode specific capacity of 150 mAh/g.
(75) FIG. 3(C) shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na.sub.0.925Ni.sub.0.308 Mn.sub.0.308 Mg.sub.0.154Ti.sub.0.229O.sub.2 cell (Cell #312001). The cell shows good reversibility with the delivered cathode specific capacity being around 145 mAh/g after 4 cycles.
(76) FIG. 3(D) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.925Ni.sub.0.308 Mn.sub.0.308 Mg.sub.0.154Ti.sub.0.229O.sub.2 cell (Cell #312001). The cathode specific capacity in this cycle corresponds to 147 mAh/g.
(77) FIG. 3(E) shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na.sub.0.925Ni.sub.0.308 Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.sub.2 cell (Cell #312001). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell.
(78) In conclusion, the electrochemical data obtained in Example 3 demonstrate high active material specific capacity and excellent reversibility. The data are far superior to those shown in the comparative Example 1, in which both the Ni- and Na-contents are higher.
Example 4
The Effect of Charging Na.SUB.0.9.Ni.SUB.0.3.Mn.SUB.0.3.Mg.SUB.0.15.Ti.SUB.0.25.O.SUB.2 .Beyond the Conventional Theoretical Capacity as Determined by the Ni.SUP.2+ to Ni.SUP.4+ Redox Couple, in Accordance with the Process of the Present Invention
(79) A Hard Carbon//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention.
(80) The data shown in FIGS. 4(B) to 4(E) are derived from the constant current cycling data for a Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 active material in a Na-ion cell (Cell #311069) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO.sub.4 in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm.sup.2 between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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.
(81) FIG. 4(B) shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell (Cell #311069). During the first charge process an anomalously high charge capacity is realized—a cathode specific capacity of 197 mAh/g is achieved a figure which is significantly larger than the theoretical specific capacity (based on the Ni.sup.2+ to Ni.sup.4+ redox couple) for the Na.sub.0.925Ni.sub.0.0.308Mn.sub.0.308Mg.sub.0.154Ti.sub.0.229O.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 reflecting the conventional Na extraction process based on the Ni.sup.2+ to Ni.sup.4+ redox couple from the Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.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 Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 active material. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles. The first discharge process is equivalent to a cathode specific capacity of 140 mAh/g.
(82) FIG. 4(C) shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell (Cell #311069). The cell shows good reversibility with the delivered cathode specific capacity being around 133 mAh/g after 4 cycles.
(83) FIG. 4(D) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell (Cell #311069). The cathode specific capacity in this cycle corresponds to 134 mAh/g.
(84) FIG. 4(E) shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na.sub.0.9Ni.sub.0.3Mn.sub.0.3Mg.sub.0.15Ti.sub.0.25O.sub.2 cell (Cell #311069). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell.
(85) In conclusion, the electrochemical data obtained in Example 4 demonstrate high active material specific capacity and excellent reversibility. The data are far superior to those shown in the comparative Example 1, in which both the Ni- and Na-contents are higher.
Example 5
The Effect of Charging Na.SUB.0.85.Ni.SUB.0.283.Mn.SUB.0.283.Mg.SUB.0.142.Ti.SUB.0.292.O.SUB.2 .Beyond the Conventional Theoretical Capacity as Determined by the Ni.SUP.2+ to Ni.SUP.4+ Redox Couple, in Accordance with the Process of the Present Invention
(86) A Hard Carbon//Na.sub.0.85Ni.sub.0.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention.
(87) The data shown in FIGS. 5(B) to 5(E) are derived from the constant current cycling data for a Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 active material in a Na-ion cell (Cell #311068) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO.sub.4 in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm.sup.2 between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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.
(88) FIG. 5(B) shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell (Cell #311068). During the first charge process an anomalously high charge capacity is realized—a cathode specific capacity of 187 mAh/g is achieved a figure which is significantly larger than the theoretical specific capacity (based on the Ni.sup.2+ to Ni.sup.4+ redox couple) for the Na.sub.0.85Ni.sub.0.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.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 reflecting the conventional Na extraction process based on the Ni.sup.2+ to Ni.sup.4+ redox couple from the Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.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 Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 active material. Importantly, this two section charge behaviour is not evident on subsequent cell charge profiles. The first discharge process is equivalent to a cathode specific capacity of 134 mAh/g.
(89) FIG. 5(C) shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell (Cell #311068). The cell shows good reversibility with the delivered cathode specific capacity being around 131 mAh/g after 4 cycles.
(90) FIG. 5D) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell (Cell #311068). The cathode specific capacity in this cycle corresponds to 131 mAh/g.
(91) FIG. 5(E) shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na.sub.0.85Ni.sub.0.283Mn.sub.0.283Mg.sub.0.142Ti.sub.0.292O.sub.2 cell (Cell #311068). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell.
(92) In conclusion, the electrochemical data obtained in Example 5 demonstrate high active material specific capacity and excellent reversibility. The data are far superior to those shown in the comparative Example 1, in which both the Ni- and Na-contents are higher.
Example 6
Effecting Oxygen Loss from Na.SUB.0.833.Ni.SUB.0.317.Mn.SUB.0.467.Mg.SUB.0.1.Ti.SUB.0.117.O.SUB.2 .in Accordance with the Process of the Present Invention
(93) A Hard Carbon/Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 cell was prepared as described below and then overcharged and held at 4.2V in accordance with the process of the present invention.
(94) The data shown in FIGS. 6(B) to 6(E) are derived from the constant current cycling data for a Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 active material in a Na-ion cell (Cell #311063) where this cathode material was coupled with a capacity balanced Hard Carbon (Carbotron P/J) anode material. The electrolyte used was a 0.5 M solution of NaClO.sub.4 in propylene carbonate. The constant current data were collected at an approximate current density of 0.125 mA/cm.sup.2 between voltage limits of 1.00 and 4.20 V. To fully charge the cell the Na-ion cell was potentiostatically held at 4.2 V at the end of the constant current charging process until the current dropped to less than 1% of the constant current value. The cell testing was carried out at 30° C. 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.
(95) FIG. 6(B) shows the first 4 charge-discharge cycles (Na-ion Cell Voltage [V] versus Cumulative Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 cell (Cell #311063). During the first charge process a cathode specific capacity of 160 mAh/g is achieved for the Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 active material. The first discharge process is equivalent to a cathode specific capacity of 122 mAh/g.
(96) FIG. 6(C) shows the cycle life performance (Cathode Specific Capacity [mAh/g] versus Cycle Number) for the Hard Carbon//Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 cell (Cell #311063). The cell shows good reversibility with the delivered cathode specific capacity being around 122 mAh/g after 4 cycles.
(97) FIG. 6D) shows the third cycle discharge voltage profile (Na-ion Cell Voltage [V] versus Cathode Specific Capacity [mAh/g]) for the Hard Carbon//Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1Ti.sub.0.117O.sub.2 cell (Cell #311063). The cathode specific capacity in this cycle corresponds to 122 mAh/g.
(98) FIG. 6(E) shows the third cycle differential capacity profiles (Differential Capacity [mAh/g/V] versus Na-ion Cell Voltage [V]) for the Hard Carbon//Na.sub.0.833Ni.sub.0.317Mn.sub.0.467Mg.sub.0.1 Ti.sub.0.117O.sub.2 cell (Cell #311063). These symmetrical data demonstrate the excellent reversibility of the ion-insertion reactions in this Na-ion cell.
(99) The Applicant does not wish to be rigidly bound to the following explanation but it is their current belief that the process of overcharging 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 some of the remaining sodium-ions. The loss of both sodium and oxygen in this way appears to explain an observed reduction in both the a- and c-parameters of the unit cell, and the resulting overall reduction in unit cell volume.
Example 7
The Preparation of Na.SUB.0.2075.Ni.SUB.0.3167.Mn.SUB.0.3167.Mg.SUB.0.1583.Ti.SUB.0.2083.O.SUB.1.9454 .Using the Process of the Present Invention i.e. from a Cell Showing Anomalous Capacity Behaviour Fully Charged
(100) Active precursor doped nickelate-containing compound Na.sub.0.95Ni.sub.0.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 (compound 2 in Table 1), was prepared using the conditions outlined in Table 1 and produced the XRD shown in FIG. 7. The asterisks in FIG. 7 indicate peak positions for aluminium, as the aluminium current collector may contribute some intensity to XRD patterns (b) and (c).
(101) This compound was made into a cell (#412004) comprising hard carbon (Carbotron P(J)) anode//Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cathode and using an electrolyte comprising 0.5M NaPF.sub.6 in EC/DEC/PC (1:1:1). The Active Mass of the cathode in the as-prepared cell=53.6 mg. Following a charge process to a specific capacity of 192 mAh/g (i.e. incorporating the anomalous capacity process of the present invention) (XRD shown in FIG. 7c) produced), the cathode electrode was removed from the cell, washed several times in clean dimethyl carbonate to remove the electrolyte and then dried at 70° C. The Active Mass of the washed cathode after this first charge process=44.2 mg.
(102) Looking at FIG. 7c), it is clear that the material obtained following the charge process to a specific capacity of 192 mAh/g is not the same as that shown in either FIG. 7b) (charged to a specific capacity of 139 mAh/g) which does not include an anomalous capacity process, only a conventional Ni.sup.2+ to Ni.sup.4+ charging process, or that shown by the original active precursor doped nickelate-containing material Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 (shown in FIG. 7a)). The proposed composition for the product obtained following the overcharge process to a specific capacity of 192 mAh/g is Na.sub.0.2075Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.1.9454, as determined by the following mass loss experiment.
(103) Mass Loss Experiment to Evidence the Formation of Na.sub.0.2075Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.1.9454.
(104) Using the information above, the Active Mass Loss following overcharging the above cell (#412004) containing Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cathode material=(53.6 mg 44.2 mg)=9.4 mg.
(105) Thus, the Measured % Mass Loss=17.5%
(106) If the charge process was just by Na-ion extraction then mass loss should be:
(107) Starting compound Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2=
(108) Molecular Weight=103.6
(109) Based on only Ni.sup.2+ to Ni.sup.4+ redox process, on cell charge it is only possible to extract 0.6333Na ions per formula unit, i.e.
Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2.fwdarw.Na.sub.0.3167Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2+0.6333 Na.sup.++0.6333e.sup.− (1)
(110) Therefore, the theoretical capacity based on this reaction (1) may be given by the following:
(111) Theoretical Specific Capacity=(0.6333×96485)/(103.6×3.6)=164 mAh/g
(112) Thus the Expected % Mass loss for reaction (1)=(14.6/103.6)×100=14.0%
(113) The charge process for Cell #412004 corresponds to an actual cathode specific capacity of 192 mAh/g i.e. far in excess of the expected theoretical specific capacity of 164 mAh/g.
(114) Thus it is proposed that the following overall charge mechanism is:
Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2.fwdarw.Na.sub.0.3167Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2+0.6333 Na.sup.++0.6333e.sup.− (1)
followed by:
Na.sub.0.3167Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2.fwdarw.Na.sub.0.2075Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.1.9454+0.1091Na.sup.++0.0273O.sub.2+0.1e.sup.− (2)
(115) Looking at the overall process (by the Applicants mechanism) for the extraction of Na:
Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2.fwdarw.Na.sub.0.2075Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.1.9454+0.7425Na.sup.++0.0273O.sub.2+0.7425e.sup.− (3)
(116) Therefore, the theoretical capacity based on this reaction (3) may be given by:
(117) Theoretical Specific Capacity=(0.7425×96485)/(103.6×3.6)=192 mAh/g
(118) Also, the Expected mass loss for reaction (3)=((103.6−85.7)/103.6)×100%=17.3%. This percentage mass loss is extremely close to the 17.5% (as detailed above) which is observed.
(119) In conclusion, on the basis that there is very 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 Na.sub.0.2075Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.1.9454 is obtained when Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 is charged to the end of the second unconventional voltage plateau.
(120) From reaction (3) above, it is proposed that the anomalous capacity arises as a result of the net 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 Na.sub.0.2075Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.1.9454. This is surprising because it is not a 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 compound.
Example 8
Charging Na.SUB.0.95.Ni.SUB.0.3167.Mn.SUB.0.3167.Mg.SUB.0.1583.Ti.SUB.0.2083.O.SUB.2 .Up to the Conventional Theoretical Maximum for the Ni.SUP.2+ to Ni.SUP.4+ Redox Process, ie. From a Cell Showing No Anomalous Capacity Behaviour Partially Charged
(121) Active precursor doped nickelate-containing compound Na.sub.0.96Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 (compound 2 in Table 1), was prepared using the conditions outlined in Table 1 and produced the XRD shown in FIG. 7a)). This compound was made into a cell (#412003) comprising hard carbon (Carbotron P(J)) anode//Na.sub.0.96Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cathode and using an electrolyte comprising 0.5M NaPF.sub.6 in EC/DEC/PC (1:1:1). The Active Mass of the cathode in the as-prepared cell=51.0 mg.
(122) Following a charge process to a specific capacity of 139 mAh/g (i.e. not incorporating the anomalous capacity process of the present invention) instead just using a conventional Ni.sup.2+ to Ni.sup.4+ charging process (XRD shown in FIG. 7b) produced), the cathode electrode was removed from the cell, washed several times in clean dimethyl carbonate to remove the electrolyte and then dried at 70° C. The Active Mass of the washed cathode after this first charge process=45.0 mg.
(123) Looking at FIG. 7b), it is clear that the material obtained following the charge process to a specific capacity of 139 mAh/g is not the same as that shown in either FIG. 7c) (charged to a specific capacity of 192 mAh/g) which includes an anomalous capacity process, or that shown by the original active precursor doped nickelate-containing material Na.sub.0.96Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 (shown in FIG. 7a)). The proposed composition for the product obtained following the overcharge process to a specific capacity of 139 mAh/g is Na.sub.0.4125Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2, as determined by the following mass loss experiment.
(124) Mass Loss Experiment to Evidence the Formation of Na.sub.0.4125Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2.
(125) Using the information above, the Active Mass Loss following overcharging the above cell (#412003) containing Na.sub.0.96Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 cathode material=(51.0 mg 45.0 mg)=6 mg.
(126) Mesaured % Mass Loss=11.8%
(127) If charge process was just by Na ion extraction then mass loss should be:
(128) Starting compound=Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583 Ti.sub.0.2083O.sub.2
(129) Molecular weight=103.6
(130) Based on only the Ni.sup.2+ to Ni.sup.4+ redox process, the charge process may be given as:
Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2.fwdarw.Na.sub.0.4125Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2+0.5375Na.sup.++0.5375e.sup.− (1)
Expected % Mass Loss for Reaction (1)=(103.6−91.3)/103.6×100=11.9%
(131) In conclusion, on the basis that there is very close correspondence between theoretical and actual results for the Expected % Mass Loss 11.8% v 11.9%, the Applicant has been able to determine with a high degree of certainty that Na.sub.0.4125Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 is obtained when Na.sub.0.95Ni.sub.0.3167Mn.sub.0.3167Mg.sub.0.1583Ti.sub.0.2083O.sub.2 is charged to the convention theoretical charge process i.e. the Ni.sup.2+ to Ni.sup.4+ redox reaction takes place under these conditions.
