LITHIUM POSITIVE ELECTRODE ACTIVE MATERIAL

Abstract

The present invention relates to a lithium positive electrode active material for a high voltage secondary battery, where the lithium positive electrode active material comprising a spinel, and the spinel has a chemical composition of Li.sub.xNi.sub.yMn.sub.2-yO.sub.4, wherein: 0.95≤x≤1.05; and 0.43≤y≤0.47. The lithium positive electrode active material is synthesized from precursors containing Li, Ni, and Mn in a ratio Li:Ni:Mn:X:Y:2−Y, wherein: 0.95≤X≤1.05; and 0.42≤Y<0.5. The present invention also relates to a process of preparing the lithium positive electrode active material as well as a secondary battery comprising the lithium positive electrode active material.

Claims

1. A lithium positive electrode active material for a high voltage secondary battery, said lithium positive electrode active material comprising a spinel, said spinel having a chemical composition of Li.sub.xNi.sub.yMn.sub.2−yO.sub.4, wherein: 0.95≤x≤1.05; and 0.43≤y≤0.47; and wherein said lithium positive electrode active material is synthesized from precursors containing Li, Ni, and Mn in a ratio Li:Ni:Mn: X:Y:2−Y, wherein: 0.95≤X≤1.05; and 0.42≤Y<0.5.

2. The lithium positive electrode active material according to claim 1, where y.Math.0.97<Y<y.Math.1.06.

3. The lithium positive electrode active material according to claim 1, where 0.42≤Y<0.49.

4. The lithium positive electrode active material according to claim 1, where at least 90 wt % of said spinel is crystallized in disordered space group Fd-3m.

5. The lithium positive electrode active material according to claim 1, wherein said lithium positive electrode active material in a half-cell has a difference of at least 50 mV between the potentials at 25% and 75% of the capacity above 4.3 V during discharge with a current of around 29 mA/g.

6. The lithium positive electrode active material according to claim 1, wherein said spinel constitutes at least 94 wt % of said lithium positive electrode active material.

7. The lithium positive electrode active material according to claim 1, wherein said lithium positive electrode active material is calcined so that the lattice parameter a is between 8.171 and 8.183 Å.

8. The lithium positive electrode active material according to claim 7, wherein the lattice parameter a is between (−0.1932y+8.2613) Å and 8.183 Å.

9. The lithium positive electrode active material according to claim 7, wherein the lattice parameter a is between (−0.1932y+8.2613) Å and (−0.1932y+8.2667) Å.

10. The lithium positive electrode active material according to claim 7, wherein the lattice parameter a is between (−0.1932y+8.2613) Å and (−0.1932y+8.2641) Å.

11. The lithium positive electrode active material according to claim 1, wherein said lithium positive electrode active material has a tap density equal to or greater than 2.2 g/cm.sup.3.

12. The lithium positive electrode active material according to claim 1, wherein the lithium positive electrode active material is made up of particles and wherein D50 of the particles of said lithium positive electrode active material satisfies: 3 μm<D50<12 μm.

13. The lithium positive electrode active material according to claim 1, wherein the BET area of said lithium positive electrode active material is below 1.5 m.sup.2/g.

14. The lithium positive electrode active material according to claim 1, wherein the lithium positive electrode active material is made up of particles, said particles being characterized by an average aspect ratio below 1.6.

15. The lithium positive electrode active material according to claim 1, wherein the lithium positive electrode active material is made up of particles, said particles being characterized by a roughness below 1.35.

16. The lithium positive electrode active material according to claim 1, wherein the lithium positive electrode active material is made up of particles, said particles being characterized by a circularity above 0.6.

17. The lithium positive electrode active material according to claim 1, wherein the lithium positive electrode active material is made up of particles, said particles being characterized by a solidity above 0.8.

18. The lithium positive electrode active material according to claim 1, wherein the lithium positive electrode active material is made up of particles, said particles being characterized by a porosity below 3%.

19. The lithium positive electrode active material according to claim 1, wherein 0.99x≤1.01.

20. The lithium positive electrode active material according to claim 1, wherein said lithium positive electrode active material has a capacity of at least 138 mAh/g.

21. The lithium positive electrode active material according to claim 1, wherein the capacity of said lithium positive electrode active material in a half cell decreases by no more than 4% over 100 cycles between 3.5 to 5.0 V at 55° C.

22. The lithium positive electrode active material according to claim 1, wherein y is determined by means of a method selected from the group consisting of electrochemical determination, X-ray diffraction and scanning transmission electron microscopy (STEM) in combination with energy dispersive X-ray spectroscopy (EDS).

23. The lithium positive electrode active material according to claim 1, wherein 0.43≤y<0.45.

24. A process for the preparation of a lithium positive electrode active material according to claim 1, said process comprising the steps of: a. providing a precursor for preparing said lithium positive electrode active material having a chemical composition of Li.sub.xNi.sub.yMn.sub.2−yO.sub.4 wherein 0.95≤x≤1.05; and 0.43≤y≤0.47, wherein said precursor contains Li, Ni and Mn in a ratio Li:Ni:Mn: X:Y:2−Y, wherein: 0.95≤X≤1.05; and 0.42≤Y<0.5; b. sintering the precursors of step a by heating the precursors to a temperature of between 500° C. and 1200° C. to provide a sintered product, c. cooling the sintered product of step b to room temperature.

25. The process according to claim 24, wherein part of step b is carried out in a reducing atmosphere.

26. The process according to claim 24, wherein said temperature of step b is between 850° C. and 1100° C.

27. The process according to claim 24, wherein during the cooling of step c, the temperature is maintained in an interval between 750° C. and 650° C. for a sufficient amount of time to obtain at least 94% phase purity of said lithium positive electrode active material.

28. The process according to claim 24, where y.Math.0.97<Y<y.Math.1.06.

29. The process according to claim 24, where 0.42≤Y<0.49.

30. The process according to claim 24, wherein at least one of the precursors is a precipitated compound.

31. The process according to claim 24, wherein the precipitated compound is a co-precipitated compound of Ni and Mn formed in a Ni—Mn co-precipitation step.

32. The process according to claim 31, wherein, said precursor in the form of a co-precipitated Ni—Mn has been prepared in a precipitation step, wherein a first solution of a Ni containing starting material, a second solution of a Mn containing starting material and a third solution of a precipitating anion are added simultaneously to a liquid reaction medium in a reactor in such amounts that in relation to the added Ni, each of Mn and the precipitating anion are added in a ratio of from 1:10 to 10:1, relative to the stoichiometric amounts of the precipitate.

33. The process according to claim 32, wherein the first, second and third solutions are added to the reaction medium amounts calibrated so as to maintain the pH of the reaction mixture at alkaline pH of between 8.0 and 10.0.

34. The process of claim 32, wherein said first, second and third solutions are added to the reaction mixture over a prolonged period of between 2.0 and 11 hours.

35. The process of claim 32, wherein said first, second and third solutions are added to the reaction mixture under vigorous stirring providing a power input of from 2 W/L to 25 W/L.

36. A secondary battery comprising the lithium positive electrode active material according to claim 1.

Description

SHORT DESCRIPTION OF THE FIGURES

[0067] FIG. 1a shows experimental data on the relation between the nickel content in the spinel and the degradation for a range of lithium positive electrode active materials;

[0068] FIG. 1b shows experimental data on the relation between the 4V plateau of the lithium positive electrode active material in a half cell and the degradation for a range of lithium positive electrode active materials;

[0069] FIG. 1c shows experimental data on the relation between the lattice parameter a in the spinel of the lithium positive electrode active material and the degradation for a range of lithium positive electrode active materials;

[0070] FIG. 2a shows experimental data on the relation between the nickel content in the spinel and the lattice parameter a of the spinel for a range of lithium positive electrode active materials;

[0071] FIG. 2b shows experimental data on the relation between the 4V plateau of the lithium positive electrode active material in a half cell and the lattice parameter a of the spinel for a range of lithium positive electrode active materials;

[0072] FIG. 3 shows experimental data on the relation between cation ordering parameters determined using Raman spectroscopy and electrochemistry, respectively;

[0073] FIG. 4 shows experimental data on the relation between degradation and the discharge difference in a half-cell between the potentials at 25% and 75% of the capacity above 4.3 V during discharge with a current of around 29 mA/g for a range of lithium positive electrode active materials;

[0074] FIG. 5a shows the relationship between circularity and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;

[0075] FIG. 5b shows the relationship between roughness and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;

[0076] FIG. 5c shows the relationship between average diameter and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;

[0077] FIG. 5d shows the relationship between aspect ratio and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;

[0078] FIG. 5e shows the relationship between solidity and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;

[0079] FIG. 5f shows the relationship between porosity and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry;

[0080] FIGS. 6a and 6b show the relationship between capacity and the voltage for a half cell with the lithium positive electrode active material during discharging and charging for determination of 4V plateau and dV, respectively;

[0081] FIGS. 7a and 7b are SEM images at different magnifications levels of one of the materials depicted in FIGS. 5a-5f;

[0082] FIGS. 8a and 8b are SEM images at different magnifications levels of a second of the materials depicted in FIGS. 5a-5f;

[0083] FIGS. 9a and 9b are SEM images at different magnifications levels of a third of the materials depicted in FIGS. 5a-5f;

[0084] FIGS. 10a and 10b are SEM images at different magnifications levels of a fourth of the materials depicted in FIGS. 5a-5f;

[0085] FIG. 11 shows the Ni content of the spinel, Niy, measured by scanning transmission electron microscopy energy dispersive x-ray spectroscopy (STEM-EDS) compared to values from electro chemistry (EC) for three samples with different Niy;

[0086] FIG. 12 shows the heating profile used to obtain the cathode electrode active material described in Example 2;

[0087] FIG. 13 shows a Raman spectrum of an ordered sample. The four grey areas are used to calculate the degree of ordering.

[0088] FIG. 14a and FIG. 14b show SEM images of a material of the invention in perspective and in cross-section, respectively.

[0089] FIG. 15a and FIG. 15b show SEM images of a commercial material in perspective and in cross-section, respectively.

DETAILED DESCRIPTION OF THE FIGURES

[0090] FIG. 1a shows experimental data on the relation between degradation and the nickel content (the value y in Li.sub.xNi.sub.yMn.sub.2-yO.sub.4, indicated in FIG. 1a as “Niy”) in the spinel for a range of lithium positive electrode active materials. All samples show a capacity of at least 138 mAh/g when discharged at 74 mA/g (0.5 C) in half cells at 55° C. between 3.5 V and 5 V as described in Example A. The degradation is measured in half cells at 55° C. and stated as degradation per 100 full charge and discharge cycles between 3.5 V and 5 V as described in Example A. Degradation is affected by several factors, which causes variation, but a line or curve to guide has been drawn to emphasize that at a given Ni content of the spinel, a minimum degradation rate exists and the minimum degradation rate decreases with decreasing Ni content. Thus, it is not possible to provide a lithium positive electrode active material with a lower degradation rate than the minimum degradation rate; however, inhomogeneities, morphologies and/or too much ordering in a lithium positive electrode active material may make it difficult to reach the minimum degradation rate. To explain some of these other parameters, four samples (black squares) have been produced to investigate how morphology affects degradation as discussed in Example 4.

[0091] FIG. 1b shows experimental data on the relation between the 4V plateau of the lithium positive electrode active material in a half cell and the degradation for a range of lithium positive electrode active materials. All samples show a capacity of at least 138 mAh/g when discharged at 74 mA/g (0.5 C) in half cells at 55° C. between 3.5 V and 5 V as described in Example A. The degradation is measured in half cells at 55° C. and stated as degradation per 100 full charge and discharge cycles between 3.5 V and 5 V as described in Example A. Also in FIG. 1b, a line or curve to guide has been drawn to emphasize that at a given 4V plateau, a minimum degradation rate exists and the minimum degradation rate decreases with increasing 4V plateau. The four samples indicated with black squares in FIG. 1a, are also shown as black squares in FIG. 1b.

[0092] FIG. 1c shows experimental data on the relation between the lattice parameter a, “a axis”, in the spinel of the lithium positive electrode active material and the degradation for a range of lithium positive electrode active materials. All samples show a capacity of at least 138 mAh/g when discharged at 74 mA/g (0.5 C) in half cells at 55° C. between 3.5 V and 5 V as described in Example A. The degradation is measured in half cells at 55° C. and stated as degradation per 100 full charge and discharge cycles between 3.5 V and 5 V as described in Example A. Also in FIG. 1c, a line or curve to guide has been drawn to emphasize that for a given lattice parameter a, a minimum degradation rate exists and the minimum degradation rate decreases with increasing lattice parameter a. The four samples indicated with black squares in FIGS. 1a and 1b, are also shown as black squares in FIG. 1c. FIGS. 1a, 1b and 1c show relations between different parameters for the same samples.

[0093] FIG. 2a shows experimental data on the relation between the nickel content (via. the value y in Li.sub.xNi.sub.yMn.sub.2-yO.sub.4, indicated in FIG. 2a as “Niy”) in the spinel and the lattice parameter a of the spinel for a range of lithium positive electrode active materials. All samples show a capacity of at least 138 mAh/g when discharged at 74 mA/g (0.5 C) in half cells at 55° C. between 3.5 V and 5 V as described in Example A. From FIG. 2a it is seen that for the experimental data, a linear dependence exists between the content of nickel and the lattice parameter a. Small variations could occur due to variations in lithium content.

[0094] FIG. 2b shows experimental data on the relation between the 4V plateau of the lithium positive electrode active material in a half cell and the lattice parameter a of the spinel for a range of lithium positive electrode active materials. All samples show a capacity of at least 138 mAh/g when discharged at 74 mA/g (0.5 C) in half cells at 55° C. between 3.5 V and 5 V as described in Example A. FIGS. 2a and 2b show relations between different parameters for the same samples.

[0095] As discussed in Example C, a correlation exists between the Ni content in the spinel and the lattice parameter a of the spinel, because a lower amount of Ni will result in a higher content of Mn.sup.3+.

[0096] Thus, the inventors have realized that a close correlation exists between low degradation, the a parameter, the Ni content and the 4V plateau of the lithium positive electrode active material. This correlation may be used for selecting appropriate values of a parameter, Ni content to optimize the lithium positive electrode active material for specific applications.

[0097] FIG. 3 shows experimental data on the relation between cation ordering parameters determined using Raman spectroscopy and electrochemistry, respectively. The two methods are described in Example D, and it is seen a correlation exists. It has been observed that a disordered lithium positive electrode active material provides for a lower degradation compared to a similar material prepared as an ordered material. Even though the samples shown in FIG. 3 have some variation, a tendency exists indicating higher dV values correspond to lower Raman ordering values. The voltage difference, dV, is measured as described in relation to FIG. 6b. As used herein, the term “Raman ordering” is meant to denote a measurement of cation ordering within the lithium positive electrode active material based on Raman spectroscopy as described in Example D.

[0098] FIG. 4 shows experimental data on the relation between degradation and the discharge difference in a half-cell between the potentials at 25% and 75% of the capacity above 4.3 V during discharge with a current of around 29 mA/g for a range of lithium positive electrode active materials. The difference, dV, is measured as described Example D. In FIG. 4, it is shown that a relation exists between the difference dV and the degradation of the lithium positive electrode active materials. The difference dV is also denoted “plateau separation” and is a measure of the free energies related to insertion and removal of lithium at a given state of charge and this is influenced by whether the spinel phase is disordered or ordered. Even though the samples shown in FIG. 4 have some variation, a tendency exists indicating higher dV values correspond to lower degradation. Without being bound by theory, a plateau separation of at least 50 mV seems advantageous since this is related to whether the lithium positive electrode active material is in an ordered or a disordered phase and to the fade rate of a half cell with the lithium positive electrode active material.

[0099] FIGS. 5a-5f show the relationship between degradation and a range of parameters for the four samples indicated with black squares in FIGS. 1a-1c, 2a-2b and 4. These four samples of lithium positive electrode active materials have differing degradations values as it is clear from FIGS. 1a-1c and 2a-2b, but very similar spinel stoichiometries. Of the four samples shown in FIG. 5a-5f, the spinel of three of the samples has the spinel stoichiometry LiNi.sub.0.454Mn.sub.1.546O.sub.4, whilst the spinel of the fourth sample has the spinel stoichiometry LiNi.sub.0.449Mn.sub.1.551O.sub.4. The four samples are all prepared based on co-precipitated precursors and the particles are secondary particles.

[0100] FIG. 5a shows the relationship between circularity of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The circularity of a secondary particle is measured from the area and the perimeter of the particle shape as 4π*[Area]/[Perimeter].sup.2. Circularity describes both overall shape and surface roughness, where a higher value means more circular shape and smoother surface. A circle with a smooth surface has circularity 1. Average circularity is the arithmetic mean of the circularities of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagej.nih.gov). In FIG. 5a it is seen that higher value of circularity corresponds to lower degradation.

[0101] FIG. 5b shows the relationship between roughness of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The roughness of a secondary particle is measured as the ratio between the perimeter and the perimeter of an ellipse fitted to the particle shape. Roughness describes how rough the surface is, where a higher value means rougher surface. Average roughness is the arithmetic mean of the roughnesses of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagej.nih.gov). In FIG. 5b it is seen that lower value of roughness corresponds to lower degradation.

[0102] FIG. 5c shows the relationship between average diameter of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The diameter of a secondary particle is measured as the equivalent circle diameter, i.e. the diameter of a circle with the same area as the particle. Average diameter is the arithmetic mean of the diameters of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagej.nih.gov). In FIG. 5c it is seen that a lower average diameter to lower degradation. The average diameter of secondary particles is given in μm.

[0103] FIG. 5d shows the relationship between aspect ratio of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The aspect ratio of a secondary particle is measured from an ellipse fitted to the particle shape. The aspect ratio is defined as [Major axis]/[Minor Axis] where Major axis and Minor Axis are the major and minor axes of the fitted ellipse. Average aspect ratio is the arithmetic mean of the aspect ratios of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagej.nih.gov). In FIG. 5d it is seen that a lower aspect ratio in general corresponds to lower degradation.

[0104] FIG. 5e shows the relationship between solidity of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The solidity of a secondary particle is defined as the ratio between the particle area and the area of the convex area, i.e. [Area]/[Convex Area]. The convex area can be thought of as the shape resulting from wrapping a rubber band around the particle. The more concave features in a particle's surface, the higher is the convex area and the lower is the solidity. Average solidity is the arithmetic mean of the solidities of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagej.nih.gov). In FIG. 5e it is seen that higher values of solidity correspond to lower degradation.

[0105] FIG. 5f shows the relationship between porosity of secondary particles and degradation for four samples of a lithium positive electrode active material according to the invention and with substantially the same spinel stoichiometry. The porosity of a secondary particle is the percentage of the internal area that appears with dark contrast in the SEM image, where dark contrast is interpreted as a porosity, i.e. a hole inside the particle. Average porosity is the arithmetic mean of the porosities of all secondary particles measured in a sample. Calculated using the software ImageJ (https://imagej.nih.gov).

[0106] In FIG. 5f it is seen that a lower value of porosity in general corresponds to lower degradation.

[0107] FIGS. 6a and 6b show the relationship between capacity and the voltage for a half cell with the lithium positive electrode active material during discharging and charging for determination of 4V plateau and dV, respectively. The measurement used as example to calculate the two parameters is based on the lithium positive electrode active material described in Example 2. The 4V plateau is used to describe the capacity around 4V compared to the total capacity. This ratio may vary slightly between charge and discharge, and thus the value is determined as an average of the two. Using variable names from the figure, the 4V plateau is calculated as (Q.sub.cha.sup.4V+(Q.sup.tot.sub.dis−Q.sup.4V.sub.dis)/(2*Q.sup.tot.sub.dis). Based on the example, the value is calculated as: (11.0+(138.8−123.1))/(2*138.8)=9.6%. The plateau separation, dV, between the two plateaus at around 4.7 V is calculated as the difference in voltage between the potentials at 25% and 75% of the discharge capacity between 4.3 V and 5 V during discharge at 29.6 mA/g. Using the example shown in FIG. 6b, this is calculated as 4.718V−4.662 V=56 mV.

[0108] FIGS. 7a-10b are SEM images at two different magnification levels for the four materials indicated with black squares in FIGS. 1a-1c and 2a-2b. These four materials have differing degradations values as it is clear from FIGS. 1a-1c and 2a-2b. In the samples of FIGS. 7a, 7b, 9a, 9b, 10a and 10b, the spinel has the stoichiometry LiNi.sub.0.454Mn.sub.1.546O.sub.4, whilst the spinel of the sample of FIGS. 8a and 8b has the stoichiometry LiNi.sub.0.449Mn.sub.1.551O.sub.4.

[0109] FIGS. 7a and 7b are SEM images at two different magnification levels of one of the samples depicted in FIGS. 1a-1c, 2a-2b and 5a-5f. The sample shown in FIGS. 7a and 7b is the lithium positive electrode active material having a degradation of 7.2%. The sample material was embedded in epoxy and polished to a flat surface in order to image cross sections of the secondary particles of the lithium positive electrode active material. Images were acquired using an acceleration voltage of 8 kV and the backscatter electron detector. Pixel size: a) 0.216 μm/pixel and b) 0.054 μm/pixel.

[0110] FIGS. 8a and 8b are SEM images at two different magnifications levels second of the samples depicted in FIGS. 1a-1c, 2a-2b and FIGS. 5a-5f. The sample shown in FIGS. 8a and 8b is the lithium positive electrode active material having a degradation of 6.2%. The sample material was embedded in epoxy and polished to a flat surface in order to image cross sections of the secondary particles of the lithium positive electrode active material. Images were acquired using an acceleration voltage of 8 kV and the backscatter electron detector. Pixel size: a) 0.216 μm/pixel and b) 0.054 μm/pixel.

[0111] FIGS. 9a and 9b are SEM images at two different magnifications levels of third of the samples depicted in FIGS. 1a-1c, 2a-2b and 5a-5f. The sample shown in FIGS. 9a and 9b is the lithium positive electrode active material having a degradation of 4.6%. The sample material was embedded in epoxy and polished to a flat surface in order to image cross sections of the secondary particles of the lithium positive electrode active material. Images were acquired using an acceleration voltage of 8 kV and the backscatter electron detector. Pixel size: a) 0.216 μm/pixel and b) 0.054 μm/pixel.

[0112] FIGS. 10a and 10b are SEM images at different magnifications levels of a fourth of the samples depicted in FIGS. 1a-1c, 2a-2b and 5a-5f. The sample shown in FIGS. 10a and 10b is the lithium positive electrode active material having a degradation of 3.2%. The sample material was embedded in epoxy and polished to a flat surface in order to image cross sections of the secondary particles of the lithium positive electrode active material. Images were acquired using an acceleration voltage of 8 kV and the backscatter electron detector. Pixel size: a) 0.216 μm/pixel and b) 0.054 μm/pixel.

[0113] FIG. 11 shows the Ni content of the spinel, Niy, measured by scanning transmission electron microscopy energy dispersive x-ray spectroscopy (STEM-EDS) compared to values from electro chemistry (EC) for three samples with different values of Niy. STEM-EDS directly measures the elemental composition of a material and EC indirectly measures the composition from the size of the 4V charge plateau. The comparison shows that the two methods agree and that the 4V charge plateau is indeed directly related to the composition of the spinel phase. Therefore, the determination of the 4V charge plateau is a valid method for determining the composition of the spinel.

[0114] FIG. 12 shows the heating profile used to obtain the cathode electrode active material described in Example 2. The temperature is measured with a thermocouple in close proximity of the powder bed. The heating is divided in two stages as described in Example 2.

[0115] FIG. 13 shows a Raman spectrum of an ordered spinel. The four grey areas between 151 cm.sup.−1-172 cm.sup.−1, 385 cm.sup.−1-420 cm.sup.−1, 482 cm.sup.−1-505 cm.sup.−1 and 627 cm.sup.−1-639 cm.sup.−1, respectively, are used to calculate the degree of ordering.

EXAMPLES

[0116] In the following, exemplary and non-limiting embodiments of the invention are described in the form of experimental data. Examples 1-5 relate to methods of preparation of the lithium positive electrode active material. Example A describes a method of electrochemical testing, Example B describes SEM based measurement of morphological parameters, Example C describes three methods to determine the content of Mn and Ni in the spinel, and Example D describes two methods used to determine the degree of cation ordering in the spinel.

Example 1: Synthesis of Lithium Positive Electrode Active Material

[0117] A metal ion solution of NiSO.sub.4 and MnSO.sub.4 with a Ni:Mn atomic ratio of 1:3.18 is prepared by dissolving 7.1 kg of NiSO.sub.4.7H.sub.2O and 15.1 kg of MnSO.sub.4.H.sub.2O in 48.5 kg water. In a separate container, a carbonate solution is prepared by dissolving 11.2 kg of Na.sub.2CO.sub.3 in 51.0 kg water. No ammonia or other chelating agents are used. The metal ion solution and the carbonate solution are added separately with around 3 L/h each into a reactor provided with vigorous stirring (400 rpm), pH between 8.8 and 9.5 and a temperature of 35° C. The volume of the reactor is 40 liters. The product is removed from the reactor after 4 hours and divided into six. Precipitation is continued on one of the six batches for around 4 hours, after which it is divided into two. Precipitation is continued on each of the two batches until the desired Ni,Mn-carbonate precursor is obtained. This procedure is followed for the remaining five samples. The precursor is filtrated and washed to remove Na.sub.2SO.sub.4.

[0118] Precursors in the form of 4667 g co-precipitated Ni,Mn-carbonate (Ni:0.478, Mn:1.522) produced as described above and 716 g Li.sub.2CO.sub.3 (corresponding to Li:Ni:Mn=1.00:0.478:1.522) 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.

[0119] The powder mix is heated in a furnace with nitrogen flow with a ramp of 2.5° C./min to 550° C. The powder is heated 4 hours at 550° C. Hereafter the powder is treated for 9 hours in air at 550° C. The temperature is increased to 950° C. with a ramp of 2.5° C./min. A temperature of 950° C. is maintained for 10 hours and decreased to 700° C. with a ramp of 2.5° C./min. A temperature of 700° C. is maintained for 4 hours and decreased to room temperature with a ramp of 2.5° C./min.

[0120] Subsequently, 20 g powder is heated to 900° C. in oxygen enriched air (90% O.sub.2) with a ramp of 2.5° C./min. A temperature of 900° C. is maintained for 1 hour and decreased to 750° C. with a ramp of 2.5° C./min to 750° C. A temperature of 750° C. is maintained for 4 hours and decreased to room temperature with a ramp of 2.5° C./min.

[0121] The powder is again de-agglomerated by shaking for 6 min in a paint shaker and passed through a 45-micron sieve resulting in lithium positive electrode active material consisting of 97.7% LNMO, 1.5% O3 and 0.8% rock salt. Using methods described in Example A and C, the stoichiometry of the spinel is determined to be LiNi.sub.0.47Mn.sub.1.53O.sub.4, the 4V plateau constitute 6% of the total discharge capacity and the degradation at 55° C. is measured to be 4% per 100 cycles in half cells. Relevant parameters are listed in Table 1 below.

Example 2: Synthesis of Lithium Positive Electrode Active Material

[0122] Precursors in the form of 529 g co-precipitated Ni,Mn-carbonate (Ni:0.46, Mn:1.54) produced as described in Example 1 and 83.1 g Li.sub.2CO.sub.3 (corresponding to Li:Ni:Mn=1.00:0.46:1.54) 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 or-der to obtain a free flowing homogeneous powder mix.

[0123] The powder mix is heated in a muffle furnace with nitrogen flow with a ramp of around 1° C./min to 550° C. A temperature of 550° C. is maintained for 3 hours and cooled to room temperature with a ramp of around 1° C./min.

[0124] This product is de-agglomerated by shaking for 6 min. in a paint shaker, passed through a 45-micron sieve and distributed in a 10-25 mm layer in alumina crucibles. The powder is heated in a muffle furnace in air with a ramp of 2.5° C./min to 670° C. A temperature of 670° C. is maintained for 6 hours and increased further to 900° C. with a ramp of 2.5° C./min. A temperature of 900° C. is maintained for 10 hours and decreased to 700° C. with a ramp of 2.5° C./min. A temperature of 700° C. is maintained for 4 hours and decreased to room temperature with a ramp of 2.5° C./min.

[0125] The powder is again de-agglomerated by shaking for 6 min. in a paint shaker and passed through a 45-micron sieve resulting in lithium positive electrode active material consisting of 98.9% LNMO, 0.5% 03 and 0.6% rock salt. Using methods described in Example A and C, the stoichiometry of the spinel is determined to be LiNi.sub.0.45Mn.sub.1.55O.sub.4, the 4V plateau constitute 10% of the total discharge capacity and the degradation at 55° C. is measured to be 3% per 100 cycles in half cells. Relevant parameters are listed in Table 1 below.

Example 3: Synthesis of Lithium Positive Electrode Active Material

[0126] Precursors in the form of 1400 g co-precipitated Ni,Mn-carbonate (Ni:0.47, Mn:1.53) as produced in Example 1 and 211 g Li.sub.2CO.sub.3 (corresponding to Li:Ni:Mn=0.98:0.47:1.53) 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.

[0127] The powder mix is heated in a furnace with nitrogen flow with a ramp of 2° C./min to 600° C. A temperature of 600° C. is maintained for 6 hours. Hereafter the powder is heated for 12 hours in air at 600° C. The temperature is increased to 900° C. with a ramp of 2° C./min. A temperature of 900° C. is maintained for 5 hours and decreased to 750° C. with a ramp of 2° C./min. A temperature of 750° C. is maintained for 8 hours and decreased to room temperature with a ramp of 2° C./min.

[0128] The powder is again de-agglomerated by shaking for 6 min. in a paint shaker and passed through a 45-micron sieve resulting in lithium positive electrode active material consisting of 98.1% LNMO, 1.4% 03 and 0.5% rock salt. Using methods described in Example A and C, the stoichiometry of the spinel is determined to be LiNi.sub.0.43Mn.sub.1.57O.sub.4, the 4V plateau constitutes 13% of the total discharge capacity and the degradation at 55° C. is measured to be 2% per 100 cycles in half cells. Relevant parameters are listed in Table 1 below.

Example 4: Synthesis of Lithium Positive Electrode Active Material

[0129] Four samples have been synthesized in order to obtain different morphologies of the particles, while maintaining the same Ni content in the spinel. The four samples are included in FIGS. 1a-1c, 2a-2b and 4 as black squares, FIGS. 7a-10b show SEM images of particle cross sections, and FIGS. 5a-5f show the relationship between degradation and a range of parameters related to morphology for the four samples. Relevant parameters are listed in Table 1 below. Precursors for all samples has been co-precipitated as described in Example 1, using slightly different variations. As an example, the precursor of sample 2 in Table 2 as shown in FIGS. 8a and 8b is produced with a stirring of 200 rpm corresponding to around 2.6 W/L in a filled reactor and the precursor of sample 4 in Table 2 as shown in FIGS. 10a and 10b is produced with a stirring of 400 rpm corresponding to around 10 W/L in a filled reactor.

Example 5: Synthesis of Lithium Positive Electrode Active Material

[0130] Additional samples have been prepared as Examples 1-3 using different precursors and different calcination programs. FIG. 1a shows the correlation between degradation per 100 cycles at 55° C. measured in half cells as described in Example A and the Ni content in the spinel. The Ni content in the spinel is determined electrochemically as described in Example C. FIG. 1b shows the correlation between degradation per 100 cycles at 55° C. measured in half cells as described in Example A and the 4V plateau. FIG. 1c shows the correlation between degradation at 55° C. measured in half cells as described in Example A and the lattice parameter a in the spinel. Table 1 below contains the Ni content, Niy, the lattice parameter, a, the 4V plateau, the capacity, degradation and the difference, dV, between the two Ni-plateaus as described in Example D for the samples described in Examples 1-5.

TABLE-US-00001 TABLE 1 a-axis 4 V Capacity Degradation dV Niy (Å) plateau (mAh/g) per 100 cycles (mV) Examples 1-3 0.47 8.173 6% 140 4% 59 0.45 8.176 10%  140 3% 56 0.43 8.180 13%  140 2% 68 Example 4 0.454 8.175 9% 136 7% 58 0.449 8.175 10%  135 6% 58 0.454 8.174 9% 138 5% 62 0.454 8.174 9% 138 3% 57 Example 5 0.43 8.180 13%  138 2% 68 0.44 8.178 13%  138 2% 71 0.44 8.178 12%  138 2% 64 0.46 8.175 9% 140 3% 56 0.46 8.174 8% 141 4% 43 0.47 8.171 5% 142 6% 37 0.48 8.171 5% 138 6% 34 0.48 8.170 4% 139 8% 35 0.48 8.170 3% 139 10%  32 0.49 8.168 2% 138 17%  31

Example 6: Determination of Shape Using Scanning Electron Microscopy: Comparison of Sample According to the Invention (Sample 4) and Commercial Sample

[0131] Sample 4 as discussed in Example 4 and a sample of a commercial product of lithium positive electrode active material were compared using Scanning Electron Microscopy (SEM).

[0132] FIG. 14a and FIG. 14b show SEM images of the Sample 4 in perspective and in cross-section, respectively, and FIG. 15a and FIG. 15b show SEM images of the commercial sample in perspective and in cross-section, respectively. As will appear from FIG. 14a and FIG. 14b, the particles of Sample 4 are highly spherical and highly uniform in their internal structure. In comparison, the particles of the commercial sample (FIG. 15a and FIG. 15b) are not spherical and appear to have a high degree of agglomeration.

Example A: Method of Electrochemical Testing of Lithium Positive Electrode Active Materials Prepared According to Examples 1 to 5

[0133] 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 1-4) 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 100-200 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 8 mg of lithium positive electrode active material were cut from the dried films, pressed in a hydraulic pellet press (diameter 20 mm; 3 tonnes) and subjected to hours drying at 120° C. under vacuum in an argon filled glove box.

[0134] 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 250 μm thick lithium disks were used as counter electrodes and the pressure in the cells were regulated with two stainless steel disk spacers and a disk spring on the negative electrode side. Electrochemical lithium insertion and extraction were monitored with an automatic cycling data recording system (Maccor) operating in galvanostatic mode.

[0135] The electrochemical test contains 6 formation cycles (3 cycles 0.2 C/0.2 C (charge/discharge) and 3 cycles 0.5 C/0.2 C), 25 power test cycles (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 120 0.5 C/1 C cycles to measure degradation. C-rates were calculated based on the theoretical specific capacity of the lithium positive electrode active material of 147 mAhg.sup.−1; thus, for example 0.2 C corresponds to 29.6 mAg.sup.−1 and 10 C corresponds to 1.47 Ag.sup.−1. The voltage separation of the two plateaus at 4.7 V, dV, and the 4V plateau are calculated based on cycle 3, the capacity is calculated based on cycle 7, and the degradation is calculated between cycle 33 and cycle 133.

Example B: Method of Measuring Particle Size and Shape Using Scanning Electron Microscopy

[0136] To prepare samples for scanning electron microscopy (SEM), the lithium positive electrode active material was embedded in epoxy and polished to a flat surface in order to image cross sections of the particles. SEM images acquired of the embedded cross sections were used to measure particle size and shape of different samples in order to evaluate the correlation between particle shape and degradation for samples with substantially the same stoichiometry of the spinel phase. In the samples of FIGS. 7a, 7b, 9a, 9b, 10a and 10b, the spinel has the stoichiometry LiNi.sub.0.454Mn.sub.1.546O.sub.4, whilst the spinel of the sample of FIGS. 8a and 8b has the stoichiometry LiNi.sub.0.449Mn.sub.1.551O.sub.4.

[0137] SEM images were acquired using an acceleration voltage of 8 kV and the backscatter electron detector. Images were acquired at low and high magnification with pixel sizes 0.216 μm/pixel (FIGS. 7a, 8a, 9a, 10a) and 0.054 μm/pixel (FIG. 7b, 8b, 9b, 10b), respectively. The low magnification images were used for measuring particle size and shape.

[0138] SEM images were analyzed using the software ImageJ (https://imagej.nih.gov). The procedure was the following: [0139] Median filter, with 1 pixel radius; [0140] Sharpen; [0141] Threshold using the Otsu algorithm; and [0142] Analyze particles: Only particles with area larger than 3 μm.sup.2 considered.

[0143] The step of analyzing particles includes measuring area and perimeter for each particle and calculating a best fit ellipse having the same area as the particle. Area, perimeter and fitted ellipse are then used to calculate a number of descriptors for size and shape for each particle in the SEM image: [0144] Diameter: Equivalent circle diameter, i.e. the diameter of a circle with the same area as the particle. [0145] Aspect ratio: The aspect ratio of the particle's fitted ellipse, i.e. [Major axis]/[Minor Axis]. [0146] Roughness: Ratio between measured perimeter and the perimeter of the fitted ellipse. Describes the surface roughness of the particle. [0147] Circularity: 4π*[Area]/[Perimeter].sup.2. Circularity describes overall shape and surface roughness. A circle with a smooth surface has a circularity of 1. [0148] Solidity: [Area]/[Convex Area]. Convex area can be thought of as the shape resulting from wrapping a rubber band around the particle. The more concave features in a particle's surface, the higher is the convex area and the lower is the solidity. [0149] Porosity: The percentage of the internal area of a particle that appears with dark contrast in the SEM image, where dark contrast is interpreted as a porosity, i.e. a hole inside the particle.

[0150] The sample average value of these descriptors is shown in the table below for the four samples with substantially the same spinel stoichiometry and different degradation. Degradation is measured in a half cell as the decrease in capacity after 100 cycles between 3.5 to 5.0 V at 55° C.

TABLE-US-00002 TABLE 2 Number of Aspect Sample particles Diameter ratio Roughness Circularity Solidity Porosity Degradation 1 633 10.1 μm  1.46 1.32 0.58 0.85 1.9% 7% 2 764 9.8 μm 1.56 1.29 0.59 0.86 1.6% 6% 3 896 9.1 μm 1.41 1.26 0.63 0.87 2.0% 5% 4 1250 7.7 μm 1.39 1.19 0.71 0.89 1.5% 3%

[0151] As described in relation to FIGS. 5a-5f, degradation as a function of the six descriptors shows a correlation in such a way that a lithium positive electrode active material with a low degradation is characterized by one or more of the following parameters: Low diameter, low roughness, low aspect ratio, high circularity, high solidity and low porosity. Optimally, a lithium positive electrode active material would fulfill most of or all of the six descriptors: Low diameter, low roughness, low aspect ratio, high circularity, high solidity and low porosity. Preferably, diameter is below 10 μm, roughness is below 1.35, circularity is above 0.6 and solidity is above 0.8.

Example C: Determination of the Ni and Mn Content in the Spinel

[0152] As described above, depending on the preparation of the lithium positive electrode active material, the content of Ni and Mn in the spinel of the lithium positive electrode active material may be different from the bulk values that can be determined using ICP among others. Example C demonstrates that the Ni and Mn content in the spinel of the lithium positive electrode active material may be determined using three different methods based on electrochemistry, diffraction and electron microscopy, respectively.

[0153] The methods based on electrochemistry and diffraction exploit that variations in the Mn/Ni ratio change the ratio between Mn.sup.3+ and Mn.sup.4+. This is apparent by calculating the average oxidation state of Mn in Li.sub.xNi.sub.yMn.sub.2−yO.sub.4 as (4*2−1*x−2*y)/(2−y) based on the assumption that the oxidation state of Li is 1+, Ni is 2+ and O is −2. Using this, the formula can be written as Li.sup.+1Ni.sup.+2.sub.yMn.sup.+3.sub.1−2yMn.sup.+4.sub.1+yO.sub.4 in the case of x=1, and a similar expression for x different from 1.

[0154] Electrochemically, Mn.sup.3+ can be oxidized reversibly to Mn.sup.4+ and back by extraction and insertion of Li.sup.+ during cycling, and Ni.sup.2+ can be oxidized reversibly to Ni.sup.4+ and back by extraction and insertion of Li.sup.+ during cycling. It is thus possible to extract (and subsequently insert) two Li.sup.+ per Ni.sup.2+ and one Li.sup.+ per Mn.sup.3+. Based on the formula Li.sup.+1Ni.sup.+2.sub.yMn.sup.+3.sub.1−2yMn.sup.+4.sub.1+yO.sub.4 in the case of x=1, the share of capacity related to Mn activity compared to the total capacity is thus given by (1−2y)/(1−2y+2y)=(1−2y). As an example y=0 corresponds to 0% capacity related to Mn activity and y=0.45 and 0.4 corresponds to 10% and 20% of the total capacity coming from Mn activity, respectively.

[0155] In LNMO, Mn.sup.3+/Mn.sup.4+ reactions are observed around 4 V vs. Li/Li.sup.+ and Ni.sup.2+/Ni.sup.4+ reactions are observed around 4.7 V vs. Li/Li.sup.+. It is therefore expected that the capacity measured between 3.5 V and 4.3 V vs. Li/Li.sup.+ compared to the total capacity between 3.5 V and 5 V vs. Li/Li.sup.+ corresponds to Mn activity. The capacity around 4V is determined using the third discharge at 29 mA/g (0.2 C) as described in Example A. During charge and discharge, the cell is not in equilibrium and the measured voltages may shift upwards during charge and downwards during discharge due to internal resistance in the cell. This effect is especially pronounced near sudden changes in cell voltage and the fraction of Mn-activity will therefore appear different depending on whether the analysis is based on a charge or a discharge. The true value will be between these two values and a reasonable estimate is the average between the two. FIG. 6a shows the discharge and charge voltage curves as a function of capacity for the third charge at 29 mA/g (0.2 C) as described in Example A. Using the capacities Q.sup.4V.sub.cha and Q.sup.4V.sub.dis corresponding to a voltage of 4.3 V during charge and discharge, respectively, and the total discharge capacity Q.sup.tot.sub.dis, the fraction of Mn-activity is given by (Q.sup.4V.sub.cha+(Q.sup.tot.sub.dis−Q.sup.4V.sub.dis))/(2*Q.sup.tot.sub.dis). This value is denoted “4V plateau”. The maximum and minimum values of the 4V plateau are given by (Q.sup.tot.sub.dis−Q.sup.4V.sub.dis) (Q.sup.tot.sub.dis)) and (Q.sup.4V.sub.cha)/(Q.sup.tot.sub.dis) respectively.

Diffraction

[0156] The size of Mn.sup.3+ and Mn.sup.4+ ions are different and this affect the lattice parameter of the spinel. Powder x-ray diffraction data were collected on a Phillips PW1800 instrument system in θ−2θ geometry working in Bragg-Brentano mode using Cu Kα radiation (λ=1.541 Å). The observed data needs to be corrected for experimental parameters contributing to shifts in the observed peak positions, which are used to calculate the lattice parameter. This is achieved using the full profile fundamental parameter approach as implemented in the TOPAS software from Bruker. As a result the spinel lattice parameter is determined with an uncertainty around 5/10000 Å, which is enough to determine the amount of Mn.sup.3+ and thus the amount of Mn and Ni.

Electron Microscopy

[0157] A direct measurement of the amount of Mn and Ni in the spinel is possible by elemental mapping using scanning transmission electron microscopy (STEM) in combination with energy dispersive x-ray spectroscopy (EDS). STEM-EDS has been used to measure the amount of Ni and Mn in three different samples, in order to compare the composition of the spinel phase with the values calculated from the 4V charge plateau in the electrochemical measurement.

[0158] STEM-EDS measurements were performed on a FEI Talos transmission electron microscope equipped with the ChemiSTEM EDS detector system. The microscope was operated in STEM mode with an acceleration voltage of 200 kV. Elemental maps were acquired and analyzed using the software Esprit 1.9 from Bruker. A standard-less quantification was performed using automatic background subtraction, series deconvolution and the Cliff-Lorimer method. Impurities or non-spinel phases in the sample were easily identified from a composition substantially different from the spinel, i.e. they are rich in either Mn or Ni, and the fact that they comprise a small fraction of the total sample. These non-spinel phases were not included in the quantification in order to strictly measure the composition of the spinel phase. The quantification provides atomic percentages of the elements present in the spinel phase. The amount of Ni in the spinel, Niy, was determined as Niy=2*Ni.sub.at%/(Ni.sub.at%+Mn.sub.at%) where Ni.sub.at% and Mn.sub.at% are the atomic percentages of Ni and Mn measured in the spinel.

[0159] Three samples prepared with different values of Niy were analyzed as shown in Table 3 below and in FIG. 11. Ni net chemical composition refers to the overall Ni content in the sample and Niy refers to the Ni content of the spinel phase as measured using STEM-EDS and the 4V charge plateau. The table shows a good agreement between the two measurements of Niy, confirming that the 4V charge plateau is indeed directly related to the composition of the spinel phase. Furthermore, the data shows that Niy is not necessarily identical to the net chemical composition, but rather determined by the conditions during calcination.

TABLE-US-00003 TABLE 3 Ni, net chemical Niy Niy 4 V Niy X-ray composition STEM-EDS charge plateau diffraction 0.46 0.461 0.458 0.458 0.5 0.450 0.444 0.446 0.46 0.474 0.473 0.477

[0160] As seen in FIG. 2a, a relation exists between the a-axis determined using XRD measurements and the ratio between Mn and Ni given by y as determined from the 4V plateau. The correspondence can be fitted with the line: a=−0.1932*y+8.2627. FIG. 2b shows the similar correspondence between the a-axis and the 4V plateau.

Example D: Quantification of Ordering

[0161] Cation ordering of Ni and Mn in the spinel of the lithium positive electrode active material can be determined by Raman spectroscopy as described in Ionics (2006) 12, pp 117-126. To quantify the degree of ordering, it is used that the two peaks around 162 cm.sup.−1 (151 cm.sup.−1-172 cm.sup.−1) and 395 cm.sup.−1 (385 cm.sup.−1-420 cm.sup.−1) are related to cation ordering and the two peaks around 496 cm.sup.−1 (482 cm.sup.−1-505 cm.sup.−1) and 636 cm.sup.−1 (627 cm.sup.−1-639 cm.sup.−1) are not depending on ordering. In a simple approach, the area of each peak is calculated as indicated in FIG. 13, and the ordering parameter can be calculated as the ratio (A.sub.1+A.sub.2)/(A.sub.3+A.sub.4). This method compensates for variations in background and signal strength. A fully ordered spinel has a value around 0.4 and a fully disordered spinel has a value around 0.1.

[0162] Another method to determine the degree of ordering is to measure the difference dV between the two voltage plateaus at around 4.7 V during 29.6 mA/g (0.2 C) discharge. This method requires sufficiently good materials and electrode fabrication in order to obtain flat and well separated plateaus as seen in FIGS. 6a and 6b. The difference is calculated as shown in FIG. 6b between the middle of each of the two plateaus around 4.7 V. The Q.sup.4V.sub.dis is determined as described in Example C and the middle of each of the two plateaus are determined at 25% of Q.sup.4V.sub.dis and 75% of Q.sup.4V.sub.dis. A fully ordered spinel has a value around 30 mV and a fully disordered spinel has a value around 60 mV.

[0163] FIG. 3 shows a comparison between the two ordering parameters that confirm a correlation. The correlation between dV and ordering is used in FIG. 4 to determine that cation ordering cause an increase in degradation.