High tap density lithium positive electrode active material, intermediate and process of preparation

Abstract

A lithium positive electrode active material intermediate including less than 80 wt % spinel phase and a net chemical composition of Li.sub.xNi.sub.yMn.sub.2-yO.sub.4-δ wherein 0.9≤x≤1.1; 0.4≤y≤0.5; and 0.1≤δ. Further, a process for the preparation of a lithium positive electrode active material with high tap density for a high voltage secondary battery where the cathode is fully or partially operated above 4.4 V vs. Li/Li+, comprising the steps of a)heating a precursor in a reducing atmosphere at a temperature of from 300° C. to 1200° C. to obtain a lithium positive electrode active material intermediate; b)heating the product of step a. in a non-reducing atmosphere at a temperature of from 300° C. to 1200° C.; wherein the mass of the product of step b. increases by at least 0.25% compared to the mass of the product of step a.

Claims

1. A process for the preparation of a lithium positive electrode active material for a high voltage secondary battery, where the cathode is fully or partially operated above 4.4 V vs. Li/Li+, said process comprising the steps of: a. heat treating a precursor comprising lithium in a reducing atmosphere at a temperature of between 300° C. and 750° C. to obtain the lithium positive electrode active material intermediate; b. heat treating the product of step a. in a non-reducing atmosphere at a temperature of between 700° C. and 1200° C.; wherein the mass of the product of step b. increases by at least 0.25% compared to the mass of the product of step a, wherein the reducing atmosphere is selected from the group consisting of a gas containing less than 15 vol % oxygen and a gas containing an added substance, which is capable of removing all or part of oxidizing species present in the gas.

2. A process according to claim 1, wherein the mass of the product of step b. increases by at least 0.75% compared to the mass of the product of step a.

3. A process according to claim 1, wherein the temperature of step a. is between 400° C. and 750° C.

4. A process according to claim 1, wherein the temperature of step b. is between 800° C. and 1200° C.

5. A process according to claim 1, wherein the reducing atmosphere is a gaseous composition containing less than 15 vol % oxygen.

6. A process according to claim 1, wherein the reducing atmosphere is created by adding a substance to the precursor composition, by decomposition of the precursor or by adding a gaseous composition to the atmosphere in order to remove all or part of any oxidising species present in the atmosphere.

7. A process according to claim 1, wherein the non-reducing atmosphere is a gaseous composition selected from the group consisting of air, and a composition comprising at least 5 vol % oxygen in an inert gas.

8. A process according to claim 1, wherein the lithium positive electrode active material has a capacity of greater than 110 mAhg.sup.−1.

9. A process according to claim 1, wherein the capacity of the lithium positive electrode active material decreases by no more than 7% over 100 cycles between from 3.5 to 5.0 V at 55° C.

10. A process according to claim 1, wherein the product of step b. has a tap density equal to or greater than 1.8 g cm.sup.−3.

11. A process according to claim 1, wherein the secondary particle size of the product of steps a. and b. has a D10 of greater than 100 μm, D50 of greater than 250 μm and D90 of greater than 800 μm.

12. A process according to claim 1, wherein the precursor is prepared by one or more processes selected from the group consisting of mechanically mixing starting materials to obtain a homogenous mixture, mixing a lithium starting material with a composition prepared by either mechanically mixing or co-precipitating starting materials to obtain a homogenous mixture.

13. A process according to claim 1, wherein the positive electrode active material comprises at least 95 wt % of spinel phase Li.sub.xNi.sub.yMn.sub.2-yO.sub.4; wherein 0.9≤x≤1.1, and 0.4≤y≤0.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the tap densities of the products of each step (a.-c.) for Examples 13 and 7;

(2) FIG. 2 shows the variation of the amount of spinel phase of the products of steps a. c. of Example 1-3 and 7;

(3) FIG. 3 shows the composition of phases of the lithium positive electrode active material intermediate for samples A-G of Example 8;

(4) FIG. 4 shows the variation of the tap density of the product of samples A-G of Example 8;

(5) FIG. 5 shows a thermogravimetric analysis (TGA) curve of an oxidation of a lithium positive electrode active material intermediate.

(6) FIG. 6 shows the secondary particle size distribution of the lithium positive electrode active material intermediate after the first heating step (step a.) of Example 11;

(7) FIGS. 7a and 7b show SEM images of the lithium positive electrode active material intermediate (product of step a.) of Examples 1 and 7, respectively;

(8) FIGS. 8 and 9 show power tests and electrochemical cycling tests at 25° C. and 55° C. of the lithium positive electrode active materials described in Examples 1, 7 and 11; and

(9) FIGS. 10 and 11 show voltage curves of specific electrochemical cycles at 25° C. and 55° C. of the lithium positive electrode active materials described in Examples 1, 7 and 11.

EXAMPLE A

Method of Electrochemical Testing of Lithium Positive Electrode Active Materials Prepared According to Examples 1, 7 and 11

(10) Electrochemical tests have been realized in 2032 type coin cells, using thin composite positive electrodes and metallic lithium negative electrodes (half-cells). The thin composite positive electrodes were prepared by thoroughly mixing 84 wt % of lithium positive electrode active material (prepared according to Examples 1, 7 and 11) with 8 wt % Super C65 carbon black (Timcal) and 8 wt % PVdF binder (polyvinylidene difluoride, Sigma Aldrich) in NMP (N-methyl-pyrrolidone) to form a slurry. The slurries were spread onto carbon coated aluminum foils using a doctor blade with a 100 μm gap and dried for 12 hours at 80° C. to form films. Electrodes with a diameter of 14 mm and a loading of approximately 4 mg of lithium positive electrode active material were cut from the dried films, pressed in a hydraulic pellet press (diameter 20 mm; 5 tonnes) and subjected to 10 hours drying at 120° C. under vacuum in an argon filled glove box.

(11) Coin cells were assembled in argon filled glove box (<1 ppm O.sub.2 and H.sub.2O) using two polymer separators (Toray V25EKD and Freudenberg FS2192-11SG) and electrolyte containing 1 molar LiPF.sub.6 in EC:DMC (1:1 in weight). Two 135 μm thick lithium disks were used as counter electrodes and the pressure in the cells were regulated with a stainless steel disk spacer and disk spring on the negative electrode side. Electrochemical lithium insertion and extraction was monitored with an automatic cycling data recording system (Maccor) operating in galvanostatic mode.

(12) A power test was programmed to run the following cycles: 3 cycles 0.2 C/0.2 C (charge/discharge), 3 cycles 0.5 C/0.2 C, 5 cycles 0.5 C/0.5 C, 5 cycles 0.5 C/1 C, 5 cycles 0.5 C/2 C, 5 cycles 0.5 C/5 C, 5 cycles 0.5 C/10 C, and then 0.5 C/1 C cycles with a 0.2 C/0.2 C cycle every 20.sup.th cycle. C-rates were calculated based on the theoretical specific capacity of the material of 148 mAhg.sup.−1 so that e.g. 0.2 C corresponds to 29.6 mAg.sup.−1 and 10 C corresponds to 1.48 Ag.sup.−1.

Example 1

Method of Preparing Lithium Positive Electrode Active Material

(13) Mn.sub.3O.sub.4 (240 g corresponding to 3.16 mol Mn), basic Ni(OH).sub.x(CO.sub.3).sub.y (131 g corresponding to 0.92 mol Ni), Li.sub.2CO.sub.3 (76.8 g corresponding to 2.08 mol Li) were weighed and ball-milled (600 rpm for 30 min with reverse rotation) in a planetary ball mill. The mixture was then dried at 120° C. for 12 hours. Graphite [0.86 g; C-NERGY Graphite (Low Fe content) from TimCal] was added to the mixture of starting materials (40 g) and mixed in a mortar for 15 min to obtain a precursor. The precursor was heated (step a.) in a 50 mL crucible with lid for 3 hours at 900° C., followed by cooling of 1° C./min to room temperature. Upon cooling, the product had contracted into a hard lump that was broken down in a mortar. After further grinding for 15 min achieving a D50 of approximately 250 μm. The tap density of the material was 2.7 g cm.sup.−3.

(14) The lithium positive electrode active material intermediate was heated for a second time (step b.) for 3 hours in air at 900° C., followed by cooling to room temperature. The tap density of the material was 2.4 g cm.sup.−3.

(15) In Examples 1-3, 7 and 11 (corresponding to Tables 1-3 and 8) an additional step c. has been added. Step c. comprises heating the product of step b for a third time for 10 hours at 900° C. with slow cooling to 700° C. (1° C. per minute), followed by cooling to room temperature. The tap density of the material was 2.1 g cm.sup.−3 (d50 approx. 70 um). The phase composition of the lithium positive electrode active material obtained comprised more than 95 wt % spinel phase.

(16) TABLE-US-00001 TABLE 1 Heating Tap Mass Heating time Density Yield change Spinel Step atmosphere (hours) (g cm.sup.−3) (%) (%) (wt %) a. Reducing  3 2.7 79.6% N/A <80 b. Non-reducing  3 2.4 81.4% 1.8 >80 c. Non-reducing 10 2.1 82.9% 1.5 >95

(17) “Spinel” means spinel phase.

(18) “Yield” means the mass of the product of step a, b or c compared to the total mass of the starting materials excluding any added substances to create the reducing atmosphere; i.e. graphite for Examples 1-3 and 7.

(19) “Mass change” means the difference in mass between the products of step a. and b., or b. and c relative to the mass of the starting materials.

(20) The mass change between the mass of the intermediate obtained after the first heating (step a.) under a reducing atmosphere and the material obtained after the second heating under a non-reducing atmosphere (step b.) is +1.8%. The total mass change between the products of steps a. and c. is +3.3%. A +3.3% mass increase corresponds to a change in δ of 0.4 in the formula Li.sub.xNi.sub.yMn.sub.2-yO.sub.4-δ.

Example 2

Method of Preparing Lithium Positive Electrode Active Material

(21) The procedure as described in Example 1 was followed; the heating times were varied as shown in Table 2.

(22) TABLE-US-00002 TABLE 2 Heating Tap Mass Heating time Density Yield change Spinel Step atmosphere (hours) (g cm.sup.−3) (%) (%) (wt %) a. Reducing  3 2.6 79.8 N/A <80 b. Non-reducing 10 2.2 82.2 2.4 >80 c. Non-reducing 10 2.0 83.1 0.9 >95

(23) The mass change between the mass of the intermediate obtained after the first heating under a reducing atmosphere and the material obtained after the second heating under a non-reducing atmosphere is 2.4%. The total mass change between the products of steps a. and c. is +3.3%. A+3.3% mass increase corresponds to a change in δ of 0.4 in the formula Li.sub.xNi.sub.yMn.sub.2-yO.sub.4-δ.

Example 3

Method of Preparing Lithium Positive Electrode Active Material

(24) The procedure as described in Example 1 was followed; the heating times were varied as shown in Table 3.

(25) TABLE-US-00003 TABLE 3 Heating Tap Mass Heating time Density Yield change Spinel Step atmosphere (hours) (g cm.sup.−3) (%) (%) (wt %) a. Reducing 10 2.0 82.5 N/A <90 b. Non-reducing 10 1.9 82.9 0.4 >90 c. Non-reducing  3 1.9 83.0 0.1 >95

(26) The mass change between the mass of the intermediate obtained after the first heating under a reducing atmosphere and the material obtained after the second heating under a non-reducing atmosphere is +0.4%. The total mass change between the products of steps a. and c. is +0.5%. A +0.5% mass increase corresponds to a change in δ of 0.06 in the formula Li.sub.xNi.sub.yMn.sub.2-yO.sub.4-δ.

(27) It should be noted that the first heating step (step a.) of Example 3 has a duration of 10 hours, in comparison to 3 hours for Examples 1 and 2. Example 3 has a higher amount of the intermediate material in the spinel phase than Examples 1 and 2, this amount of spinel material is caused by a change in the concentration of the reducing atmosphere over time, therefore the material that is heated for a longer duration experiences a less reducing (more oxidizing) atmosphere as the heating proceeds. Therefore the spinel phase of the intermediate corresponds to the concentration of oxidizing species in the reducing atmosphere, rather than directly corresponding to the duration of heating.

Example 4

Method of Preparing Precursor for Lithium Positive Electrode Active Material By Co-Precipitation

(28) A metal solution of NiSO.sub.4 and MnSO.sub.4 with a Ni:Mn atomic ratio of 1:3 was prepared by dissolving 258 g of NiSO.sub.4.7H.sub.2O and 521 g of MnSO.sub.4H.sub.2O in 1775 g water. In a separate flask, an alkaline solution was prepared by dissolving 2862 g of Na.sub.2CO.sub.3.10H.sub.2O and 68 g NH.sub.3.H.sub.2O in 2995 g water. The acid and the base are added separately into a continuously stirred tank reactor provided with vigorous stirring (650 rpm) and a temperature of 50° C. The volume of the reactor was 1 litre.

(29) The product was continuously removed from the reactor, so that the residence time of the reactants in the reactor was 30 minutes.

Example 5

Method of Preparing Lithium Positive Electrode Active Material

(30) Precursors in the form of 5626 g co-precipitated Ni,Mn-carbonate with a Ni:Mn atomic ratio of 1:3 and 884 g Li.sub.2CO.sub.3 (corresponding to a Li:Ni/Mn ratio of 1:2) are mixed with ethanol to form a viscous slurry. The slurry is shaken in a paint shaker for 3 min. in order to obtain full de-agglomeration and mixing of the particulate materials. The slurry is poured into trays and left to dry at 80° C. The dried material is further de-agglomerated by shaking in a paint shaker for 1 min. in order to obtain a free flowing homogeneous powder mix.

(31) The powder mix is sintered in a muffle furnace with Nitrogen flow. The heating profile is given in Table 4.

(32) TABLE-US-00004 TABLE 4 Temperature Ramp Dwell Gas flow (° C.) (° C./min) (hours) Gas (L/min)  RT-260 2 Nitrogen 0.5 260-450 0.5 Nitrogen 0.5 450-700 2 Nitrogen 0.5 700 2.5 Nitrogen 0.5 700 4 Air 0.5 700-RT  natural cooling Air 0.5

(33) This intermediate product is de-agglomerated by shaking for 6 min. in a paint shaker and passed through a 45 micron sieve. The powder is sintered in a standard furnace in air according to the heating profile given in Table 5. The powder is distributed in alumina crucibles.

(34) TABLE-US-00005 TABLE 5 Temperature Ramp Dwell Gas flow (° C.) (° C./min) (hours) Gas (L/min)  RT-900 5 Air 2 900 14 Air 2 900-700 2.5 Air 2 700 4 Air 2 700-RT  Natural cooling Air 2

(35) The powder is again de-agglomerated by shaking for 6 min in a paint shaker and passed through a 45 micron sieve resulting in a lithium positive electrode active material consisting of 96.6 wt % spinel phase, 2.9 wt % 03 phase and 0.5 wt % rock salt phase. The tap density was determined to be 2.4 g cm.sup.−3.

Example 6

Method of Preparing Lithium Positive Electrode Active Material

(36) Precursors in the form of 5626 g co-precipitated Ni,Mn-carbonate with a Ni:Mn atomic ratio of 1:3 and 884 g Li.sub.2CO.sub.3 (corresponding to a Li:Ni/Mn ratio of 1:2) are mixed with ethanol to form a viscous slurry. The slurry is shaken in a paint shaker for 3 min. in order to obtain full de-agglomeration and mixing of the particulate materials. The slurry is poured into trays and left to dry at 80° C. The dried material is further de-agglomerated by shaking in a paint shaker for 1 min in order to obtain a free flowing homogeneous powder mix.

(37) The powder mix is sintered in a muffle furnace with nitrogen flow. The heating profile is given in Table 6.

(38) TABLE-US-00006 TABLE 6 Temperature Ramp Dwell Gas flow (° C.) (° C./min) (hours) Gas (L/min)  RT-260 2 Nitrogen 0.5 260-450 0.5 Nitrogen 0.5 450-700 2 Nitrogen 0.5 700 2.5 Nitrogen 0.5 700-RT  natural cooling Nitrogen 0.5

(39) This intermediate product is sintered in a standard furnace in air according to the heating profile given in Table 7. The material is distributed in alumina crucibles.

(40) TABLE-US-00007 TABLE 7 Temperature Ramp Dwell Gas flow (° C.) (° C./min) (hours) Gas (L/min)  RT-700 5 Air 8 700 6 Air 8 700-900 5 Air 8 900 14 Air 8 900-700 2.5 Air 8 700 4 Air 8 700-RT  Natural cooling Air 8

(41) The powder is de-agglomerated by shaking for 6 min in a paint shaker and passed through a 45 micron sieve resulting in a lithium positive electrode active material consisting of 96.2 wt % spinel phase, 2.8 wt % 03 phase and 1.0 wt % rock salt phase. The tap density was determined to be 2.4 g cm.sup.−3.

Example 7

Comparative Example

(42) The procedure as described in Example 1 was followed; however, the atmosphere of steps a., b. and c. were all non-reducing. The heating times were varied as shown in Table 8.

(43) TABLE-US-00008 TABLE 8 Heating Tap Mass Heating time Density Yield change Spinel Step atmosphere (hours) (g cm.sup.−3) (%) (%) (wt %) a. Non-reducing  3 1.5 82.8 N/A >95 b. Non-reducing  3 1.5 82.9 +0.1% >95 c. Non-reducing 10 1.6 82.8 −0.1% >95

(44) The mass change between the mass of the intermediate obtained after the first heating under a non-reducing atmosphere (step a.) and the material obtained after the second heating under a non-reducing atmosphere (step b.) is +0.1%. The total mass change between the products of steps a. and c. is +0.0%. A 0% mass increase corresponds to a constant value of δ in the formula Li.sub.xNi.sub.yMn.sub.2-yO.sub.4-δ.

(45) FIG. 1 shows the tap densities of the products of each step (a.-c.) for Examples 1-3 and 7. FIG. 1 illustrates that Examples 1-3 show exceptional tap densities for the product of each step. Example 7 illustrates the slight increase (0.1 g cm.sup.−3) in tap density with time when heated.

(46) FIG. 2 shows the variation of the amount of spinel phase of the products of steps a. c. of Example 1-3 and 7. Examples 1 and 2 demonstrate a significantly smaller amount of spinel phase, 60 wt % and 65 wt % respectively, in the lithium positive electrode active material intermediate (product of step a.).

(47) FIG. 7a shows a SEM image of the lithium positive electrode active material intermediate (product of step a.) of Example 1; i.e. where step a. is carried out under a reducing atmosphere.

(48) FIG. 7b shows a SEM image of the lithium positive electrode active material intermediate (product of step a.) of comparative Example 7; i.e. where step a. is carried out under a non-reducing atmosphere.

Example 8

(49) Example 3 was repeated with varying amounts of graphite present in the precursor, see Table 9.

(50) TABLE-US-00009 TABLE 9 Amount of graphite present in the precursor of Example 3. Sample Graphite (g) A - Comparative Example 0.0 B 0.2 C 0.4 D 0.6 E 0.8 F 1.0 G 3.0

(51) FIG. 3 shows the composition of phases of the lithium positive electrode active material intermediate for samples A-G of Example 8. FIG. 3 shows the correlation between the composition of phases of the lithium positive electrode active material intermediate (product of step a.) and the amount of oxygen present in the reducing atmosphere. The carbon present in the composition reacts with oxygen in the atmosphere of the sealed crucible and forms a carbon-oxygen compound such as carbon monoxide or carbon dioxide. Increasing the amount of graphite present in the composition to be calcined under a reducing atmosphere correlates to reducing the amount of oxygen present in the reducing atmosphere. FIG. 3 illustrates that the increase in graphite (i.e. decrease in oxygen present in the atmosphere of the process) results in a product comprising a lithium positive electrode active material intermediate (product of step a.) comprising metal oxide phases such as LiMnO.sub.2 and Mn.sub.3O.sub.4, and impurities such as NiO/MnO rock-salt and other rock-salt solid-solutions of the form Ni.sub.xMn.sub.1-xO for x larger than or equal to 0 and smaller than or equal to 1 [e.g.: MnO, Mn.sub.2Ni.sub.3O.sub.5, Mn.sub.4Ni.sub.6O.sub.10].

(52) FIG. 4 shows the variation of the tap density of the product of samples A-G of Example 8.

(53) Similarly to FIG. 3, FIG. 4 illustrates the correlation between decreasing the percentage of oxygen in the atmosphere of the reducing atmosphere and the tap density of the material of the product of Example 8.

Example 9

In-Situ XRD Measurements During Synthesis

(54) Phase distribution during the heat treatment process determined by in-situ XRD. A 0.2 g sample of mixed carbonate precursors is heated 1° C./min in Nitrogen atmosphere and in Air respectively in an in-situ X-ray diffraction set-up. XRD-scans were performed at 50° C. intervals from 25° C. to 900° C. The phase assembly has been identified and quantified using Rietveld refinement at each temperature step. The results are given in Table 10 and 11.

(55) TABLE-US-00010 TABLE 10 Phase distribution of mixed carbonate precursors during a heat treatment process in nitrogen. The phase fractions are listed as relative fractions by weight, determined by Rietveld refinement. In all listed structures, M represents Mn, Ni or a mix of these and 0 ≤ x ≤ 1. Temp M.sub.3O.sub.4 M.sub.3O.sub.4 [° C.] Li.sub.2CO.sub.3 MCO.sub.3 Li.sub.xM.sub.2O.sub.4 (s) (h) Li.sub.xMO Ni 25 22 78 50 22 78 100 22 78 150 23 77 200 25 75 250 27 73 300 32 68 350 25 25 37 14 400 18 24 51 8 450 7 50 25 17 500 4 54 24 18 550 5 47 14 34 600 6 15 16 64 650 100 700 99 1 750 98 2 800 97 3 850 97 3 900 98 2 25 97 3

(56) TABLE-US-00011 TABLE 11 Phase distribution of mixed carbonate precursors during a heat treatment process in air. The phase fractions are listed as relative fractions by weight, determined by Rietveld refinement. In all listed structures, M represents Mn, Ni or a mix of these and 0 ≤ x ≤ 1. Temp M.sub.3O.sub.4 [° C.] Li.sub.2CO.sub.3 MCO.sub.3 Li.sub.xM.sub.2O.sub.4 (s) Li.sub.xM.sub.2O.sub.3 Li.sub.xMO Ni 25 17 83 50 17 83 100 17 83 150 17 83 200 20 80 250 20 80 300 21 79 350 24 76 400 11 34 14 41 450 5 60 2 33 500 2 71 1 26 550 75 5 20 600 76 9 15 650 80 13 7 700 82 16 2 750 80 16 4 800 80 12 8 850 72 11 17 900 54 16 30 25 94 3 3

(57) “Li.sub.2CO.sub.3” means a crystal lattice described by space group C2/c with the lattice parameters a, b, c and β around 8.4 Å, 5.0 Å, 6.2 Å and 115°, respectively.

(58) “MCO.sub.3” means a crystal lattice described by space group R-3c with the lattice parameters a and b around 4.8 Å and 15.5 Å, respectively.

(59) “Li.sub.xM.sub.2O.sub.4” means a crystal lattice described by the space groups P4.sub.332 and Fd-3m for the cation ordered and disordered phase, respectively, with the lattice parameter a around 8.2 Å.

(60) “M.sub.2O.sub.3” means a crystal lattice described by space group R-3R with the lattice parameters a and b around 5.4 Å and 54.1 Å, respectively.

(61) “M.sub.3O.sub.4 (s)” means a crystal lattice described by space group Fd-3 mS with the lattice parameter a around 8.4 Å.

(62) “M.sub.3O.sub.4 (h)” means a crystal lattice described by space group I41/amdZ with the lattice parameters a and c around 5.8 Å and 9.5 Å, respectively.

(63) “Li.sub.xMO” means a crystal lattice described by space group Fm-3m with the lattice parameter a around 4.2 Å. In some cases two Li.sub.xMO phases are identified with slightly different values of the lattice parameter a.

(64) “Ni” means a crystal lattice described by space group Fm-3m with the lattice parameter a around 3.6 Å.

(65) “Li.sub.2CO.sub.3” and “MCO.sub.3” describe the precursor, “Li.sub.xM.sub.2O.sub.4” and “M.sub.3O.sub.4 (s)” describe two different spinel phases, “Li.sub.xMO” describes the rocksalt phases and “Ni” describes metallic nickel.

Example 10

Weight Increase of Intermediate Product During Heat Treatment in Air

(66) FIG. 5 shows a TGA measurement of the oxidation (heat treatment in air) of the product from a heat treatment process in reducing atmosphere from room temperature to 900° C. and down to room temperature. Such a product is an example of a lithium positive electrode active material intermediate. A 1 g sample was tested with an air flow of 150 ml/min and a heating rate of 5° C./min.

(67) The weight increases during the entire heating, except in the range from 500° C. to 600° C., where Li.sub.2CO.sub.3 decomposition dominates. The weight increase is a characteristic of the lithium positive electrode active material intermediate, but the amount may vary depending on the preparation and the amount of Li-precursor that will decompose during the heat treatment. Table 12 shows the mass at four characteristic points during the measurement. It is seen that the decomposition of Li.sub.2CO.sub.3 amounts to at least 4.05 wt %. The minimum oxidation weight gain thus amounts to 4.81 wt %. The net oxidation weigh gain of 0.76 wt % is masked by the loss contribution from decomposition of Li.sub.2CO.sub.3 in the temperature range 300-600° C.

(68) TABLE-US-00012 TABLE 12 Weight of a lithium positive electrode active material intermediate during heat treatment in air. Temp [° C.] Mass [mg] Gain [mg] Loss [mg] 1 30 1000.0 2 470 1031.8 31.8 3 580 991.3 40.5 4 30 1007.6 16.3 Total 48.1 (4.81%) 40.5 (4.05%)

Example 11

Method of Preparing Lithium Positive Electrode Active Material

(69) The method of Example 1 was followed; however, acetic acid [glacial, Sigma Aldrich] was added to the mixture of starting materials instead of graphite. The lithium positive electrode active material produced in this way had a tap density of 1.9 g cm.sup.−3 and 98 wt % of spinel phase.

(70) The material was subjected to charge-discharge cycling test; an initial specific discharge capacity of 130 mAhg.sup.−1 was obtained and a stability of less than 7% decrease in discharge capacity after 100 cycles between 3.5-5.0 V at 55° C.

(71) FIG. 6 shows the secondary particle size distribution of the lithium positive electrode material intermediate after the first heating step (step a.) of Example 11. After light grinding, the material has a particle size distribution corresponding to a D10 of greater than 100 μm, D50 of greater than 250 μm and D90 of greater than 800 μm (dashed line). After further grinding and classification (sieving) to less than 150 μm, a D10 of around 30 μm, D50 of around 120 μm and D90 of around 200 μm are obtained (solid line).

(72) TABLE-US-00013 TABLE 13 Power test (cycle 9-31) and electrochemical cycling test (from cycle 32) at 25° C. and 55° C. Temperature Atmosphere Capacity Fade of Power in heating per 100 Power Test step a. Example cycles capability 25 Non-reducing 7 2.3% 71% 25 Reducing 11 1.4% — 25 Reducing 1 1.1% 85% 55 Non-reducing 7 7.9% 76% 55 Reducing 11 4.2% 85% 55 Reducing 1 3.9% 83%

(73) “Power capability” means the specific discharge capacity available at 1.48 Ag.sup.−1 (10C) compared to the specific discharge capacity available at 148 mAg.sup.−1 (1C). Testing was measured between 3.5 to 5.0 V at discharge and charge rates according to Example A.

(74) FIG. 8 shows a power test (cycles 9-31) and electrochemical cycling test (from cycle 32) at 25° C. The lithium positive electrode active materials described in Examples 1 and 11 have exceptional stability wherein the capacity of the lithium positive electrode active material decreases by no more than 2% over 100 cycles between from 3.5 to 5.0 V at 25° C. at discharge and charge rates according to Example A. This is significantly better than the product described in the comparative Example 7.

(75) FIG. 9 shows power test (cycles 9-31) and electrochemical cycling test (from cycle 32) at 55° C. The lithium positive electrode active materials described in Examples 1 and 11 (with graphite and acetic acid) have good stability wherein the capacity of the lithium positive electrode active material decreases by around 4% over 100 cycles between from 3.5 to 5.0 V at 55° C. The lithium positive electrode active material described in the comparative Example 7 (without reducing atmosphere) has a much lower stability of 7.9% decrease in capacity over 100 cycles. Testing is performed according to Example A.

(76) FIG. 10 shows voltage curves of the specific electrochemical cycle 3 (0.2 C/0.2 C) and cycle 33 and 100 (both 0.5 C/1.0 C) at 25° C. of the lithium positive electrode active materials described in Examples 1, 7 and 11. Testing is performed according to Example A.

(77) FIG. 11 shows voltage curves of the specific electrochemical cycle 3 (0.2 C/0.2 C) and cycle 33 and 150 (both 0.5 C/1.0 C) at 55° C. of the lithium positive electrode active materials described in Examples 1, 7 and 11. Testing is performed according to Example A.