LITHIUM NICKEL MANGANESE COBALT COMPOSITE OXIDE AS A POSITIVE ELECTRODE ACTIVE MATERIAL FOR RECHARGEABLE LITHIUM ION BATTERIES

Abstract

A positive electrode active material powder suitable for lithium-ion batteries, comprising lithium transition metal-based oxide particles, said particles comprising a core and a surface layer, said surface layer being on top of said core, said particle comprising the elements: Li, M′ and oxygen, wherein M′ has a formula: M′=Ni.sub.zMn.sub.yCo.sub.xA.sub.k, wherein A is a dopant, 0.80≤z≤0.90, 0.05≤y≤0.20, 0.05≤x≤0.20, x+y+z+k=1, and 0≤k≤0.01, said positive electrode active material powder having a median particle size D50 ranging from 5 μm to 15 μm and a surface layer thickness ranging from 10 nm to 200 nm, said surface layer comprising sulfate ion (SO.sub.4.sup.2−) in a content superior or equal to 6.78.Math.z−4.83 wt % and inferior or equal to 6.78.Math.z−4.33 wt % with respect to the total weight of the positive electrode active material.

Claims

1-15. (canceled)

16. A positive electrode active material powder for lithium-ion batteries, comprising lithium transition metal-based oxide particles, said particles comprising a core and a surface layer, said surface layer being on top of said core, said particle comprising the elements: Li, M′ and oxygen, wherein M′ has a formula: M′=Ni.sub.zMn.sub.yCo.sub.xA.sub.k, wherein A is a dopant, 0.80≤z≤0.90, 0.05≤y≤0.20, 0.05≤x≤0.20, x+y+z+k=1, and 0≤k≤0.01, said positive electrode active material powder having a median particle size D50 ranging from 5 μm to 15 μm and a surface layer thickness ranging from 10 nm to 200 nm, said surface layer comprising sulfate ion (SO.sub.4.sup.2−) in a content superior or equal to 6.78.Math.−4.83 wt % and inferior or equal to 6.78.Math.−4.33 wt % with respect to the total weight of the positive electrode active material.

17. The positive electrode active material powder according to claim 16, wherein said lithium transition metal-based oxide particles further comprise aluminum and have an Al surface coverage A1/A2 that is superior or equal to 100, wherein Al is an atomic ratio Al/(Ni+Mn+Co+Al+S) of the elements Al, Ni, Mn, Co, and S contained in the surface layer, said atomic ratio A1 being obtained by XPS spectrum analysis and wherein A2 is an atomic ratio Al/(Ni+Mn+Co+Al+S) of the total Al, Ni, Mn, Co and S contained the particles and obtained by ICP.

18. The positive electrode active material powder according to claim 16, having a carbon content of less than 200 ppm.

19. The positive electrode active material powder according to claim 16, having a Li/(Ni+Mn+Co+A) atomic ratio or a Li/(Ni+Mn+Co+A+Al) atomic ratio superior or equal to 0.96 and inferior or equal to 1.05.

20. The positive electrode active material powder according to claim 17, wherein said surface layer exhibits an Al2p peak with a maximum peak intensity in the binding energies range from 73.0±0.2 eV to 74.5±0.2 eV, said intensity being obtained by XPS spectrum analysis.

21. The positive electrode active material powder according to claim 17, wherein said surface layer of lithium transition metal-based oxide particles comprises a LiAlO.sub.2 phase and a LiM″.sub.1-aAl.sub.aO.sub.2 phase wherein M″ comprising Ni, Mn, and Co, said LiAlO.sub.2 phase being present in the surface layer in a content superior or equal to 0.10 at % and inferior or equal to 0.30 at % with respect to the total atomic content of M′ in the positive electrode active material powder, said LiM″.sub.1-aAl.sub.aO.sub.2 phase being present in the surface layer in a content inferior to 0.14 at % with respect to the total atomic content of M′ in the positive electrode active material powder.

22. The positive electrode active material powder according to claim 16, wherein said lithium transition metal-based oxide particles have a sulfate ion surface coverage S 1/S2 that is superior to 0.85 and inferior or equal to 2.00, wherein S1 is an amount of sulfate ion contained in the surface layer, and wherein S2 is a total amount of sulfate ion contained in the particles, both obtained by ICP.

23. The positive electrode active material powder according to claim 17, having a general formula: Li.sub.1+a′(Ni.sub.z′Mn.sub.y′Co.sub.x′Al.sub.vS.sub.w).sub.1-kA.sub.k).sub.1-a′O.sub.2, wherein only A is a dopant, wherein 0.80≤z′≤0.90, 0.05≤y′≤0.20, 0.05≤x′≤0.20, x′+y′+z′+v+w+k=1, 0.0018≤v≤0.0053, 0.006≤w≤0.012, −0.05≤a′≤0.05, and 0≤k≤0.01.

24. The positive electrode active material powder according to claim 16, wherein A is either one or more of Al, B, S, Mg, Zr, Nb, W, Si, Ba, Sr, Ca, Zn, Cr, V, Y, and Ti, wherein the amount of each of the elements of A is superior to 100 ppm with respect to the total weight of the positive electrode active material powder.

25. The positive electrode active material powder according to claim 16, wherein the thickness of the surface layer corresponds to a minimal distance D defined either as:
D(in nm)=L.sub.S1−L.sub.S2, wherein L.sub.S1 is a first point location at the center of a particle, L.sub.S2 is a second point location in a line defined between said first point location and a geometric center of said particle, wherein a content of S is measured by TEM-EDS at the second point location L.sub.S2 is superior or equal to 0 at % and inferior or equal to 5.0 at % of a content of S measured at the first point location, said second content of S (S.sub.2) being defined as:
S.sub.2(in at %)=S.sub.3±0.1 at %, S.sub.3 being a content of S (in at %) at a third point location (LS.sub.3) in said line, said third point being located at any location between the geometric center of said particle and the second point location L.sub.S2.

26. The positive electrode active material powder according to claim 25, wherein:
S.sub.1−S.sub.2≥10.0 at % S.sub.1 being the first content of S (in at %) at the first point location (LS.sub.1).

27. The positive electrode active material powder according to claim 26, wherein Al is present in the surface layer in a content l defined as: $l\left(\mathrm{mol}\%\right)={\left(\frac{\mathrm{Al}}{M^{*}}\right)}_{\mathrm{ICP}}\times \left({\mathrm{Al}}_{\mathrm{surface}}/{\mathrm{Al}}_{\mathrm{total}}\right)$ with: ${\left(\frac{\mathrm{Al}}{M^{*}}\right)}_{\mathrm{ICP}}$ is the atomic ratio of Al content on M* content in the powder measured by ICP, and $A{l}_{\mathrm{surface}}/A{l}_{\mathrm{total}}=\frac{\frac{4}{3}{\pi \left(\mathrm{Area}2\right)}^3-\frac{4}{3}{\pi \left(\mathrm{Area}2-\mathrm{Area}1\right)}^3}{\frac{4}{3}{\pi \left(\mathrm{Area}2\right)}^3}\times 100$ wherein: Al.sub.surface is the content of Al in at % in the surface layer measured by EDS; Al.sub.total the total content of Al in at % in the particles of said powder measured by EDS; Area1 is the integral of the Al/M* content measured by cross-section-TEM-EDS over D: $\mathrm{Area}1\left(\mathrm{at}\%/\mathrm{at}\%\right)={\int }_{x=0}^{x=D}\frac{\mathrm{Al}}{M^{*}}\left(x\right),$ where: Al(x) is the atomic content of Al at a point x of a cross sectional particle measured by cross-section-TEM EDS, M*(x) is the atomic content of Ni, Mn, Co, Al, and S, at a point x of a cross sectional particle measured by cross-section-TEM EDS, and x is the distance expressed in nm measured by TEM between said first and second point locations. Area2 is the integral of the Al/M* content measured by cross-SEM EDS over a distance C: $\mathrm{Area}2\left(\mathrm{at}\%/\mathrm{at}\%\right)={\int }_{x=0}^{x=C}\frac{\mathrm{Al}}{M^{*}}\left(x\right),$ where: Al(x) is the atomic content of Al at a point x of a cross sectional particle measured by cross-section-TEM EDS, M*(x) is the atomic content of Ni, Mn, Co, Al, and S, at a point x of a cross sectional particle measured by cross-section-TEM EDS, and x is the distance expressed in nm and measured by TEM between said first point location (at x=0 nm) and the geometric center of said particle (at x=C).

28. Process for the manufacturing of a positive electrode active material according to claim 16, comprising the consecutive steps of a) preparing a lithium transition metal-based oxide compound, b) mixing said lithium transition metal-based oxide compound with a source of sulfate ion, and with water, thereby obtaining a mixture, and c) heating the mixture in an oxidizing atmosphere in a furnace at a temperature between 350° C. and less than 500° C., for a time between 1 hour and 10 hours so as to obtain the positive electrode active material powder according to the present invention.

29. A battery comprising the positive electrode active material powder according to claim 16.

30. An electric vehicle or a hybrid electric vehicle comprising the battery according to claim 29.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0126] FIG. 1A. SEM image of particles with monolithic morphology

[0127] FIG. 1B. SEM image of particles with polycrystalline morphology

[0128] FIG. 2. Schematic image of the positive electrode active material particle cross section wherein L.sub.S1 is the first point location at the periphery of the particle, L.sub.S2 is the second point location at the imaginary line between the geometric center of the said particle and the L.sub.S1. D is the thickness of the surface layer defined as the distance between L.sub.S1 and L.sub.S2 (cfr. Embodiment 10).

[0129] FIG. 3A. XPS Al peak deconvolution of EX1 before fitting process (x-axis: binding energy, y-axis: count)

[0130] FIG. 3B. XPS Al peak deconvolution of EX1 after fitting process (x-axis: binding energy, y-axis: count)

[0131] FIG. 4A. Al2p and Ni3p XPS spectra of EX1 and CEX1.1 (x-axis: binding energy, y-axis: count)

[0132] FIG. 4B. Binding energy range for Al2p3 XPS peaks each for of LiM″.sub.1-aAl.sub.aO.sub.2, LiAlO.sub.2, and Al.sub.2O.sub.3 phases. The vertical solid line shows the Al2p3 XPS peak position for EX1.

[0133] FIG. 5A. TEM-EDS analysis result of S/M* of EX1 (x-axis: distance where 0 is the starting point of the surface layer, y-axis: element in atomic ratio)

[0134] FIG. 5B. TEM-EDS analysis result of Al/M* of EX1 (x-axis: distance where 0 is the starting point of the surface layer, y-axis: element in atomic ratio)

[0135] FIG. 6. Graph showing Ni/M′ content (z) of positive electrode active material in at % (x axis) versus S1 (SO.sub.4.sup.2− content in the surface layer in wt %; y axis). The patterned area shows the claim range in this invention.

DETAILED DESCRIPTION

[0136] In the drawings and the following detailed description, preferred embodiments are described so as to enable the practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. The invention includes numerous alternatives, modifications and equivalents that are apparent from consideration of the following detailed description and accompanying drawings.

[0137] A) ICP Analysis

[0138] The Li, Ni, Mn, Co, Al, and S contents of the positive electrode active material powder are measured with the Inductively Coupled Plasma (ICP) method by using an Agillent ICP 720-ES. 2 g of product powder sample is dissolved into 10 mL of high purity hydrochloric acid in an Erlenmeyer flask. The flask is covered by a glass and heated on a hot plate at 380° C. until complete dissolution of the precursor. After being cooled to room temperature, the solution of the Erlenmeyer flask is poured into a 250 mL volumetric flask. Afterwards, the volumetric flask is filled with deionized water up to the 250 mL mark, followed by complete homogenization. An appropriate amount of solution is taken out by pipette and transferred into a 250 mL volumetric flask for the 2.sup.nd dilution, where the volumetric flask is filled with internal standard and 10% hydrochloric acid up to the 250 mL mark and then homogenized. Finally, this 50 mL solution is used for ICP measurement. The atomic ratio of Al to the total amount of Ni, Mn, Co, Al, and S (Al/(Ni+Mn+Co+Al+S) (at %)) is named A2.

[0139] The amount of sulfate ion (SO.sub.4.sup.2−) in the entire powder, which is named S2, is obtained by the below equation:

[00012]$\mathrm{Sulfate}\mathrm{ion}\left(\mathrm{ppm}\right)=S\left(\mathrm{wt}\%\right)\times \frac{M{W}_{{\mathrm{SO}}_4}}{M{W}_S}\times \frac{10000\mathrm{ppm}}{1\mathrm{wt}\%}$

[0140] *MW.sub.SO4/MWs, molecular weight of SO.sub.4 divided by molecular weight of elemental S, is 2.996 value.

[0141] To investigate the S content in the surface of lithium transition metal-based oxide particles according to the invention, washing and filtering processes are performed. 5 g of the positive electrode active material powder and 100 g of ultrapure water are measured out in a beaker. The electrode active material powder is dispersed in the water for 5 minutes at 25° C. using a magnetic stirrer. The dispersion is vacuum filtered, and the filtered solution is analyzed by the above ICP measurement to determine the amount of S present in the surface layer. The amount of the remaining S in the washed powder after the washing and filtering processes is defined as a dopant (as it corresponds to the content of S present in the core) and the amount of the removed S from the washed powder after the washing and filtering processes is defined as the amount of S present in the surface layer. The amount of sulfate ion (SO.sub.4.sup.2−) in the particle surface layer is obtained by multiplying the amount of S from this analysis by 2.996 and 10000 ppm, which is named S1. A sulfate ion surface coverage, S1/S2, is calculated by dividing S1 by S2.

[0142] B) X-Ray Photoelectron Spectroscopy Analysis

[0143] In the present invention, X-ray photoelectron spectroscopy (XPS) is used to identify and to determine the content (in at %) of each of the Al-based compounds or phases present in the surface layer of the cathode material particles according to the invention.

[0144] Such an identification includes to perform: i) a fitting of Al2p peaks identified by XPS (cfr. section B2—XPS peak treatment) followed by ii) a quantitative phase analysis by calculating the content in the surface layer of each of the compounds identified by the fitting of the Al2p peaks (cfr. section B3—content of Al-based compounds).

[0145] Also, XPS is used in the framework of the present invention to measure an Al surface coverage value which indicates the degree of homogeneity of said Al distribution in the surface layer of the particles according to the present invention.

[0146] The below-provided procedure is applicable to any cathode material according to the present invention and to the comparative examples described herein.

[0147] B1) XPS Measurement Conditions

[0148] For the surface analysis of lithium transition metal-based oxide particles, XPS measurement is carried out using a Thermo K-α+ (Thermo Scientific) spectrometer. Monochromatic Al Kα radiation (hu=1486.6 eV) is used with a spot size of 400 μm and measurement angle of 45°. Wide survey scan to identify elements present at the surface is conducted at 200 eV pass energy. C1s peak having a maximum intensity (or centered) at 284.8 eV is used as a calibrate peak position after data collection. Accurate narrow-scans are performed afterwards at 50 eV for at least 10 scans for each identified element to determine the precise surface composition.

[0149] B2) XPS Peak Treatment

[0150] In XPS measurements, the signal is acquired from the first few nanometers (e.g. 1 nm to 10 nm) of the sample surface layer. Therefore, all elements measured by XPS are contained in the surface layer. It is therefore assumed that the surface layer has a homogeneous distribution of the identified phases.

[0151] The quantitative phase analysis of XPS raw data is based on the treatment of XPS signals leading to a deconvolution of XPS peaks and to the determination of the contributions of the existing Al-based compounds to the deconvoluted peaks.

[0152] The XPS peak deconvolution is conducted to obtain the different contributions of atomic Al-based compounds including LiM″.sub.1-aAl.sub.aO.sub.2, LiAlO.sub.2, and Al.sub.2O.sub.3 in the surface layer of the investigated positive electrode active material particles. Al.sub.2O.sub.3 compound results from the heating of Al.sub.2(SO.sub.4).sub.3 that has not reacted with the lithium of the first sintered lithium transition metal-based oxide compound.

[0153] The XPS peaks measured for the positive electrode active material powder according to the invention are essentially a combination of multiple sub-peaks located within a narrow range of binding energies. An Al2p peak having a maximum intensity appearing (or centered) at a range of binding energies going from 70 eV to 79 eV consists of contributions from different sub-peaks of different Al containing compounds. The location (position of the maximum intensity) of each sub-peak is different from each other.

[0154] The XPS signal treatment including XPS peak deconvolution process in this invention follows the steps provided hereunder:

[0155] Step 1) removal of background by a linear function,

[0156] Step 2) deciding an equation of a fitting model,

[0157] Step 3) deciding the constraints of variables in the equation of a fitting model,

[0158] Step 4) deciding the initial values of variables before a calculation,

[0159] Step 5) executing the calculation

[0160] Step 1) Removal of Background by a Linear Function

[0161] In this invention, the XPS signal treatment is performed using a spectrum of an Al2p narrow scan in the range from 65±0.5 eV to 85±0.5 eV, wherein the spectrum comprises an Al2p peak having a maximum intensity (or being centered) in a range from 70 eV to 85 eV and overlaps with Ni3p peaks, each of these peaks having a maximum intensity (or being centered) in a range from 65 eV to 71 eV. The background of the measured data point is linearly baselined at the range from 65.0±0.5 eV to 81.5±0.5 eV.

[0162] Step 2) Deciding an Equation of a Fitting Model

[0163] There are four sub-peaks of a Ni3p peak and three sub-peaks of an Al2p peak having a maximum intensity in the range from 65.0±0.5 eV to 81.5±0.5 eV. The peaks are labelled as Ni3p1, Ni3p1 satellite, Ni3p2, Ni3p2 satellite, Al peak1, Al peak2, and Al peak3. The satellite peak is a minor additional peak appearing at a few eV higher binding energy than its primary peak. It is associated with the unfiltered X-Ray source from anode material in the XPS instrument. Al peaks 1 to 3 correspond to the compounds present in the particle surface layer, each are related to the: i) LiM″.sub.1-aAl.sub.aO.sub.2, ii) LiAlO.sub.2, and iii) Al.sub.2O.sub.3 phases, respectively.

[0164] Table 1.1 shows the reference of the maximum peak intensity position range for the LiM″.sub.1-aAl.sub.aO.sub.2, LiAlO.sub.2, and Al.sub.2O.sub.3 phases. The range of binding energy of Al peak1 varies with the amount of Al doped in the structure.

TABLE-US-00001 TABLE 1.1 XPS peak reference Binding Energy range Compound Peak (eV) attributed Literature reference Al peak1 72.6-73.1 LiM″.sub.1-aAl.sub.aO.sub.2 Chem. Mater. Vol. 19, No. 23, 5748- Al peak2 73.5-73.9 LiAlO.sub.2 5757, 2007; J. Electrochem. Soc., 154 (12) A1088-1099, 2007; and Chem. Mater. Vol. 21, No. 23, 5607-5616, 2009. Al peak3 73.9-74.3 Al.sub.2O.sub.3 Moulder, J. F., Handbook of XPS, Perkin-Elmer, 1992

[0165] The equation of a fitting model is according to the pseudo-Voigt equation which is a combination of Gaussian and Lorentzian functions commonly used for XPS peak fitting. The equation is:

[00013]$y=y_o+A^{\prime }\left[m_u\frac{2}{\pi }\frac{w}{4{\left(x-x_c\right)}^2+w^2}+\left(1-m_u\right)\frac{\sqrt{4\ln 2}}{\sqrt{\pi }w}{e}^{\frac{4ln2}{w^2}{\left(x-x_c\right)}^2}\right]$

[0166] with y.sub.o=an offset, x.sub.c=a center position of the sub-peak, A′=an area of the sub-peak, w=a width of sub-peaks (full width at half maximum or FWHM), and m.sub.u=a profile shape factor.

[0167] Step 3) Deciding the Constraints of Variables in the Equation of a Fitting Model

[0168] The constraints of five variables (y.sub.0, x.sub.c, A′, w, m.sub.u) are described below:

[0169] y.sub.0 (Offset): [0170] y.sub.0 of all 7 sub-peaks are 0.

[0171] x.sub.c (a center position of the sub-peak): [0172] X.sub.c of Ni3p1≥66.0 eV; [0173] X.sub.c of Ni3p1≤X.sub.c of Ni3p1 satellite−0.7 eV; [0174] X.sub.c of Ni3p1 satellite≤X.sub.c of Ni3p2−0.7 eV; [0175] X.sub.c of Ni3p2≤72 eV [0176] X.sub.c of Ni3p2≤X.sub.c of Ni3p2 satellite−0.7 eV; [0177] 72.3 eV≤X.sub.c of Al peak1≤73.3 eV; [0178] 73.5 eV≤X.sub.c of Al peak2≤73.9 eV; and [0179] 73.9 eV≤X.sub.c of Al peak3≤74.3 eV.

[0180] The ranges of X.sub.c for Al peaks 1 to 3 are determined from Chem. Mater. Vol. 19, No. 23, 5748-5757, 2007; J. Electrochem. Soc., 154 (12) A1088-1099, 2007; and Chem. Mater. Vol. 21, No. 23, 5607-5616, 2009.

[0181] A′ (Area of Sub-Peaks): [0182] A′ of Ni3p1*0.1≤A′ of Ni3p1 satellite*1.2≤A′ of Ni3p1; [0183] A′ of Ni3p2*0.1≤A′ of Ni3p2 satellite; and [0184] A′ of all 7 sub-peak are superior to 1.0.

[0185] w (Width of Sub-Peaks): [0186] 1.2≤w≤1.8

[0187] M.sub.u (Profile Shape Factor): [0188] 0.1≤m.sub.u≤0.9

[0189] Step 4) Deciding the Initial Values of Variables Before a Calculation

[0190] A calculation for fitting sub-peaks is reproducible when the initial values of variables are obtained by the following procedure: [0191] 1) The initial values of y.sub.0, w, m.sub.u are respectively set to 0.0, 1.5, and 0.7. [0192] 2) The initial values of x.sub.c of the sub-peaks Ni3p1, Ni3p1 satellite, Ni3p2, Ni3p2 satellite, Al peak1, Al peak2, and Al peak3 are 67.0 eV, 68.0 eV, 69.0 eV, 70.0 eV, 73.0 eV, 73.7 eV, and 74.3 eV, respectively. [0193] 3) The initial values of A′ of the sub-peaks Ni3p1, Ni3p1 satellite, Ni3p2, and Ni3p2 satellite are obtained by the following additional procedure: [0194] 3.a) The A′ of the sub-peak of Ni3p1 is a maximum peak intensity of a Ni3p peak multiplied by a 1.5 factor, wherein the shape of the peak is estimated as a triangle having a base of 3 eV. [0195] 3.b) The A′ of the sub-peak of Ni3p2 is 60% of that of Ni3p1. [0196] 3.c) The A′ of the sub-peak of Ni3p1 satellite is 80% of that of Ni3p1. [0197] 3.d) The A′ of the sub-peak of Ni3p2 satellite is 80% of that of Ni3p2. [0198] 4) The initial values of A′ of the sub-peaks Al peak1, Al peak2, and Al peak3 are obtained by the following procedure:

[0199] 4.a) A′ values of the three sub-peaks of Al2p are calculated according to the following equation:

A′=Fraction Factor (FF)×Estimated Area×Normalization Factor (NF)

[0200] A fraction Factor (FF) is a function of x.sub.c of three sub-peaks of Al2p in the range from x.sub.o to x.sub.n where x.sub.o=72.8 eV and x.sub.n=74.6 eV. The intensity of Al peak1 linearly decreases from x.sub.n to x.sub.o.

[0201] 4.b) The intensity of Al peak3 linearly increases from x.sub.n to x.sub.o. The intensity of Al peak2, which is located between Al peak1 and Al peak3, has therefore the highest intensity at its center 73.7 eV. The Fraction Factor (FF) for each sub-peak is calculated according to the below equations:

[00014]$\mathrm{FF}\mathrm{of}\mathrm{Al}\mathrm{peak}1=\frac{x_n-x_c}{x_n-x_o};$$\mathrm{FF}\mathrm{of}\mathrm{Al}\mathrm{peak}2=-4\times {\left(\left(\frac{x_n-x_c}{x_n-x_o}\right)-0.5\right)}^2;\mathrm{and}$$\mathrm{FF}\mathrm{of}\mathrm{Al}\mathrm{peak}3=x_n-\left(\frac{x_n-x_c}{x_n-x_o}\right).$

[0202] The estimated area is a maximum peak intensity of an Al2p peak*2.5, wherein the shape of the peak is estimated as a triangle having a base of 5 eV.

[0203] 4.c) A Normalization Factor (NF) is added to subtract the overlapping area from the total calculated peak when the sub-peaks are summed. It is important because the first two components in the peak area (A′) equation (Fraction Factor and Estimated Area) include some overlapped regions which render the calculated intensity excessively high. In the calculation method, sub-peaks are simplified so as to be considered like a triangle shape with a height t and a base b.

[0204] The locations of a maximum intensity are x.sub.c for Al peak1, Al peak2, and Al peak3 which are 73.0V, 73.7 eV, and 74.3 eV for, respectively. All sub-peaks are assumed to have the same size and shape with base is set to be 3 eV. Normalization factor for each sub-peak is calculated as:

[00015]$\begin{array}{c}\mathrm{NF}\mathrm{of}\mathrm{Al}\mathrm{peak}1=\frac{\mathrm{Al}\mathrm{peak}1}{\mathrm{overlapped}\mathrm{Al}\mathrm{peak}1}\\ =\frac{\left(\frac{1}{2}.Math.3.Math.t\right)}{\left(\frac{1}{2}.Math.3.Math.t\right)++\left(\frac{1}{2}.Math.2.3.Math.\frac{23}{30}t\right)}\\ =0.57;\end{array}$$\begin{array}{c}\mathrm{NF}\mathrm{of}\mathrm{Al}\mathrm{peak}2=\frac{\mathrm{Al}\mathrm{peak}2}{\mathrm{overlapped}\mathrm{Al}\mathrm{peak}2}\\ =\frac{\left(\frac{1}{2}.Math.3.Math.t\right)}{\begin{array}{c}\left(\frac{1}{2}.Math.3.Math.t\right)+\left(\frac{1}{2}.Math.2.3.Math.\frac{23}{30}t\right)+\\ \left(\frac{1}{2}.Math.2.4.Math.\frac{24}{30}t\right)+\left(\frac{1}{2}.Math.1.7.Math.\frac{17}{30}t\right)\end{array}}\\ =0.39;\mathrm{and}\end{array}$$\begin{array}{c}\mathrm{NF}\mathrm{of}\mathrm{Al}\mathrm{peak}3=\frac{\mathrm{Al}\mathrm{peak}3}{\mathrm{overlapped}\mathrm{Al}\mathrm{peak}3}\\ =\frac{\left(\frac{1}{2}.Math.3.Math.t\right)}{\begin{array}{c}\left(\frac{1}{2}.Math.3.Math.t\right)+\left(\frac{1}{2}.Math.2.4.Math.\frac{24}{30}t\right)+\\ \left(\frac{1}{2}.Math.1.7.Math.\frac{17}{30}t\right)\end{array}}\\ =0.51.\end{array}$

[0205] Table 1.2 shows the example of initial values of variables for EX1.

TABLE-US-00002 TABLE 1.2 Example of initial values of variables for EX1 Ni3p1 Ni3p2 Al Al Al Parameters Ni3p1 satellite Ni3p2 satellite peak1 peak2 peak3 y.sub.0 0 0 0 0 0 0 0 x.sub.c 67.0 68.0 69.0 70.0 73.0 73.7 74.3 A′ 5966.6 4773.3 3579.9 2864.0 5721.7 11443.4 8582.6 w 1.5 1.5 1.5 1.5 1.5 1.5 1.5 m.sub.u 0.7 0.7 0.7 0.7 0.7 0.7 0.7

[0206] Step 5) Executing the Calculation

[0207] The peak deconvolution process is assisted by a Solver tool, embedded in the Microsoft Excel software Version 1808. The minimum value of a target cell is set as the objective of the Solver calculation. The target cell returns the sum of squares of differences between a measured curve and a calculated curve. The calculation is terminated when the correlation coefficient between a measured curve and a calculated curve reaches 99.5% or more. When the number is closer to 100% it shows the shape of a calculated curve is closely matched with the shape of a measured curve. Otherwise, iterations will continue to reach the minimum value of the objective.

[0208] An Al2p peak of EX1 before and after fitting process is shown in FIG. 3A (x-axis: binding energy, y-axis: count) and FIG. 3B (x-axis: binding energy, y-axis: count), respectively.

[0209] The result of calculated variables is shown in Table 1.3.

TABLE-US-00003 TABLE 1.3 Calculated parameter after fitting for EX1 Ni3p1 Ni3p2 Al Al Al Parameter Ni3p1 satellite Ni3p2 satellite peak1 peak2 peak3 y.sub.0 0 0 0 0 0 0 0 x.sub.c 67.1 68.4 69.9 70.6 73.2 73.8 74.3 A′ 6863.7 5414.2 2545.1 2202.9 4248.1 7240.5 6401.8 w 1.8 1.7 1.8 1.8 1.8 1.4 1.4 m.sub.u 0.6 0.6 0.8 0.8 0.8 0.8 0.7

[0210] B3) Content of Al-Based Compounds Linked to Identified Al Sub-Peaks 1 to 3

[0211] The ratio of A′ (area) of each Al sub-peak is directly converted to the relative atomic ratio among corresponding Al compounds in a surface layer by dividing the area of each Al sub-peak by the sum of areas of all three Al sub-peaks. The amount of LiM″.sub.1-aAl.sub.aO.sub.2, LiAlO.sub.2, and Al.sub.2O.sub.3 is then provided with respect to the total atomic content of M′ in the positive electrode active material powder.

[0212] For example, the relative atomic ratio of Al peak1 (LiM″.sub.1-aAl.sub.aO.sub.2): Al peak2 (LiAlO.sub.2): Al peak3 (Al.sub.2O.sub.3) is 23.7 at %:40.5 at %:35.8 at % in the surface layer of EX1 based on Table 1.2. Since the total content of aluminum is contained in the surface layer of EX1 and is obtained by ICP analysis, the amount of LiM″.sub.1-aAl.sub.aO.sub.2, LiAlO.sub.2, and Al.sub.2O.sub.3 with respect to the total atomic content of M′ of the positive electrode active material powder is obtained by multiplying the atomic percentage of Al/M′ in the positive electrode active material powder (measured by ICP) and the relative atomic ratio of each Al sub-peaks (measured by XPS). For example, the amount of LiAlO.sub.2 in EX1 is 0.377 (at %) (Al/M′)*40.5% (LiAlO.sub.2/(LiM″.sub.1-aAl.sub.aO.sub.2+LiAlO.sub.2+Al.sub.2O.sub.3)=0.15 at %.

[0213] B4) XPS Peak Integration and Coverage

[0214] All primary peaks for other elements except Al2p are fitted using the Thermo Scientific Avantage software with a Smart background function. The Smart background is a Shirley-type baseline with the constraint that background intensity must be lower than the data point intensity. An Al2p peak integrated area is calculated as the sum of areas of Al peak1, Al peak2, and Al peak3 in B2) XPS deconvolution process. Scofield relative sensitivity library is used for the calculation of atomic fractions from the integrated peak area. The atomic ratio of Al to the total amount of Ni, Mn, Co, Al, and S (Al/(Ni+Mn+Co+Al+S) (at %/at %) is named A1.

[0215] The Al surface coverage value is calculated as the fraction of Al on the surface of particle (A1), measured by XPS, divided by the Al fraction in the particle (A2), measured by ICP. The surface coverage of the positive electrode active material by Al is calculated as follow:

[00016]$\mathrm{Al}\mathrm{surface}\mathrm{coverage}=\frac{\mathrm{Al}/M^{*}\left(\mathrm{XPS}\right)}{\mathrm{Al}/M^{*}\left(\mathrm{ICP}\right)}=\frac{A1}{A2}$

[0216] Where M* is the total atomic fraction of Ni, Mn, Co, Al, and S of the positive electrode active material particles.

[0217] The surface coverage by Al indicates the degree of coverage of the positive active electrode active material particles by aluminum. If the Al surface coverage value is high, the Al compound covers the surface with a homogenous distribution.

[0218] C) Carbon Measurement

[0219] The content of carbon of the positive electrode active material powder is measured by Horiba EMIA-320V carbon/sulfur analyzer. 1 g of hNMC powder is placed in a ceramic crucible in a high frequency induction furnace. 1.5 g of tungsten and 0.3 g of tin as accelerators are added into the crucible. The powder is heated at a programmable temperature. Gases produced during the combustion are then analyzed by four Infrared detectors. The analysis of low and high CO.sub.2 and CO determines carbon concentration.

[0220] D) PSD Measurement

[0221] A particle-size distribution (PSD) is measured using a Malvern Mastersizer 3000 with Hydro MV wet dispersion accessory after dispersing the powder in an aqueous medium. In order to improve the dispersion of the powder, sufficient ultrasonic irradiation and stirring is applied, and an appropriate surfactant is introduced. D10, D50, and D90 are defined as the secondary particle size at 10%, 50%, and 90% of the cumulative volume% distribution. A span is defined as: span=(D90−D10)/D50.

[0222] E) TEM-EDS Measurement

[0223] To examine the Al and S distributions within a lithium transition metal-based oxide particle, cross-sectional TEM lamellas of particles are prepared by a Helios Nanolab 450hp Dual Beam Scanning Electron Microscope-Focused Ion Beam (SEM-FIB) (FEI, USA, https://www.nanolabtechnologies.com/helios-nanolab-450-fei/). A Ga ion beam is used with 30 kV voltage and 30 pA-7 nA current. The obtained etched sample has a dimension of 5×8 μm with 100 nm surface layer thickness. Using the prepared (etched) sample, the surface property from the top to the center of the lithium transition metal-based oxide particle is analyzed by TEM and energy-dispersive X-ray spectroscopy (EDS). The TEM-EDS line scan is performed on a JEM-2100F (JEOL, https://www.jeol.co.jp/en/products/detail/JEM-2100F.html) with X-Max.sup.N 80T (Oxford instruments, https://nano.oxinst.com/products/x-max/x-max). An EDS analysis of the lithium transition metal-based oxide particle provides the quantitative element analysis of the cross-section. The Al and S are normalized by M* where M* is the total atomic fraction of Ni, Mn, Co, Al, and S.

[0224] The measured line scan of Al/M* and S/M* as a function of a linear distance in a cross section of particle is smoothed by Savitzhky-Golay method with the points of 20 using Origin 9.1 software so as to mitigate intrinsic analytical error of EDS.

[0225] F) Coin Cell Testing

[0226] F1) Coin Cell Preparation

[0227] For the preparation of a positive electrode, a slurry that contains a positive electrode active material powder, conductor (Super P, Timcal), binder (KF #9305, Kureha)—with a formulation of 90:5:5 by weight—in a solvent (NMP, Mitsubishi) is prepared by a high-speed homogenizer. The homogenized slurry is spread on one side of an aluminum foil using a doctor blade coater with a 230 μm gap. The slurry coated foil is dried in an oven at 120° C. and then pressed using a calendaring tool. Then it is dried again in a vacuum oven to completely remove the remaining solvent in the electrode film. A coin cell is assembled in an argon-filled glovebox. A separator (Celgard 2320) is located between a positive electrode and a piece of lithium foil used as a negative electrode. 1M LiPF.sub.6 in EC/DMC (1:2) is used as electrolyte and is dropped between separator and electrodes. Then, the coin cell is completely sealed to prevent leakage of the electrolyte.

[0228] F2) Testing Method

[0229] The testing method is a conventional “constant cut-off voltage” test. The conventional coin cell test in the present invention follows the schedule shown in Table 2. Each cell is cycled at 25° C. using a Toscat-3100 computer-controlled galvanostatic cycling station (from Toyo).

[0230] The schedule uses a 1 C current definition of 220 mA/g. The initial charge capacity (CQ1) and discharge capacity (DQ1) are measured in constant current mode (CC) at C rate of 0.1 C in the 4.3V to 3.0V/Li metal window range.

[0231] The irreversible capacity IRRQ is expressed in % as follows:

[00017]$\mathrm{IRRQ}\left(\%\right)=\frac{\left(\mathrm{CQ}1-\mathrm{DQ}1\right)}{\mathrm{CQ}1}\times 100$

TABLE-US-00004 TABLE 2 Cycling schedule for Coin cell testing method Charge Discharge V/Li V/Li C End Rest metal C End Rest metal Rate current (min) (V) Rate current (min) (V) 0.1 — 30 4.3 0.1 — 30 3.0

[0232] G) Results

[0233] The invention is further illustrated by the following (non-limitative) examples:

EXAMPLES

Comparative Example

Without Surface Treatment

[0234] Polycrystalline hNMC powders (CEX1.1), having the formula Li.sub.1+a(Ni.sub.0.80Mn.sub.0.10Co.sub.0.10).sub.1-aO.sub.2 are obtained through a double sintering process which is a solid-state reaction between a lithium source and a transition metal-based source running as follows:

[0235] 1) Co-precipitation: a transition metal-based hydroxide precursor M′O.sub.0.24(OH).sub.1.76 with metal composition M′=Ni.sub.0.80Mn.sub.0.10Co.sub.0.10 is prepared by a co-precipitation process in a large-scale continuous stirred tank reactor (CSTR) with mixed nickel-manganese-cobalt sulfates, sodium hydroxide, and ammonia.

[0236] 2) Blending: the transition metal-based hydroxide and LiOH as a lithium source are homogenously blended at a lithium to metal M′ (Li/M′) ratio of 1.01 in an industrial blending equipment.

[0237] 3) 1.sup.st sintering: the blend is sintered at 730° C. for 12 hours under an oxygen atmosphere. The sintered powder is crushed, classified, and sieved so as to obtain a sintered intermediate product.

[0238] 4) 2.sup.nd sintering: the intermediate product is sintered at 830° C. for 12 hours under an oxygen atmosphere so as to obtain a sintered powder of agglomerated primary particles. The sintered powder is crushed, classified, and sieved so as to obtain CEX1.1 having a formula Li.sub.1.005M′.sub.0.995O.sub.2 (a=0.005) with M′=Ni.sub.0.80Mn.sub.0.10Co.sub.0.10. CEX1.1 has a D50 of 12.0 μm and a span of 1.24. CEX1.1 comprises a trace of sulfur obtained from the metal sulfate sources in the Step 1) co-precipitation process.

[0239] Optionally, a source of dopant can be added in the co-precipitation process in Step 1) or in the blending step in the Step 2) together with lithium source. Dopant can be added, for instance, to improve the electrochemical properties of the positive electrode active material powder product.

[0240] CEX1.1 is not containing a surface treatment and thus is not according to the present invention.

Example 1

Surface Treatment—EX1

[0241] EX1, which is according to the present invention, is prepared by the following procedure. 1 kg of CEX1.1 is blended with an aluminum and sulfate ion solution, which is prepared by dissolving 11.68 g Al.sub.2(SO.sub.4).sub.3.16H.sub.2O into 29.66 mL deionized water. The prepared blend is heated at 375° C. for 8 hours under an oxygen atmosphere. After heating, the powder is crushed, classified, and sieved so as to obtain EX1. Accordingly, the hNMC compound, EX1, contains about 1000 ppm Al with respect to total weight of EX1. The preparation of EX1 from CEX1.1 is also called, in the framework of the present invention, a surface treatment.

Comparative Example CEX 1.2

[0242] CEX1.2 is prepared according to the same method as EX1, except that 23 g Al.sub.2(SO.sub.4).sub.3.16H.sub.2O is added. CEX1.2 contains about 2000 ppm Al with respect to total weight of CEX1.2.

[0243] CEX1.2 is not according to the present invention.

Polycrystalline and Monolithic Embodiment of Example EX1

[0244] EX1 has a polycrystalline morphology. This morphology can be modified into a monolithic morphology by applying the following steps 4a and 4b after the 2.sup.nd sintering [i.e. step 4 of the manufacturing process of CEX1.1 according to the Example 1] and before the subsequent crushing, classifying and sieving steps:

[0245] 4a) subjecting the sintered intermediate product to a wet ball milling step whereby agglomerated primary particles are deagglomerated and a slurry comprising deagglomerated primary particles is obtained, and

[0246] 4b) separating the deagglomerated primary particles from the slurry, and heat treating the deagglomerated primary particles at a temperature between 300° C. and at least 200° C. below the temperature in the 2.sup.nd sintering step (4) [i.e. of at most 630° C.], whereby single crystal monolithic particles are obtained.

[0247] Such a treatment to convert polycrystalline particles into monolithic particles is known from EX1 of WO2019/185349, in page 20, line 11 to page 20, line 33.

Comparative Example CEX2

[0248] A polycrystalline hNMC powder having a Ni to M′ atomic ratio of 86 at % with a formula Li.sub.1+a(Ni.sub.0.86Mn.sub.0.04Co.sub.0.10).sub.1-aO.sub.2 is prepared in to identify the surface treatment effect as follows:

[0249] 1) Co-precipitation: a transition metal-based hydroxide precursor M′O.sub.0.16(OH).sub.1.84 with metal composition M′=Ni.sub.0.86Mn.sub.0.04Co.sub.0.10 is prepared by a co-precipitation process in a CSTR with mixed nickel-manganese-cobalt sulfates, sodium hydroxide, and ammonia.

[0250] 2) Blending: to obtain an intermediate product, the mixed transition metal-based precursor prepared from Step 1) and LiOH.H.sub.2O as a lithium source are homogenously blended at a Li/M′ ratio of 1.02 in an industrial blending equipment.

[0251] 3) Sintering: the blend is sintered at 765° C. for 12 hours under an oxygen atmosphere.

[0252] After the sintering, the sintered powder is classified and sieved so as to obtain a non-agglomerated hNMC powder.

[0253] The final hNMC powder, which named CEX2, has the formula Li.sub.1.002M′.sub.0.998O.sub.2 and its D50 and span are 11.2 μm and 0.53, respectively.

[0254] CEX2 is not according to the present invention.

Example EX2.1, EX2.2, EX2.3

[0255] EX2.1 is prepared according to the same method as EX1 except that CEX2 is used instead of CEX1.1.

[0256] EX2.1 is according to the present invention.

[0257] EX2.2 is prepared according to the same method as EX1 except that CEX2 is used instead of CEX1.1 and 17 g Al.sub.2(SO.sub.4).sub.3.16H.sub.2O is added. EX2.2 contains about 1500 ppm Al with respect to total weight of EX2.2.

[0258] EX2.2 is according to the present invention.

[0259] EX2.3 is prepared according to the same method as EX1 except that 717 ppm S is additionally added during blending using 98 wt % H.sub.2SO.sub.4 together with Al.sub.2(SO.sub.4).sub.3.

[0260] EX2.3 is according to the present invention.

TABLE-US-00005 TABLE 3 Summary of composition and electrochemical properties of the examples and comparative examples ICP Sulfate ion According Entire hNMC powder Surface layer surface Coin cell Link to Sample to Ni/M′ Al S S2 S* S1 coverage Carbon DQ1 IRRQ features ID invention (at. %) (wt %) (wt %) (wt %) (wt %) (wt %) S1/S2 (ppm) (mAh/g) (%) of claim CEX1.1 No 0.80 0.0015 0.167 0.500 0.124 0.371 0.74 270 195.3 13.9 Low S CEX1.2 No 0.80 0.1854 0.473 1.418 0.384 1.140 0.80 58 192.6 14.9 High S EX1 Yes 0.80 0.1058 0.324 0.969 0.281 0.841 0.87 108 205.0 9.7 In this invention CEX2 No 0.86 0.0025 0.248 0.744 0.181 0.543 0.73 745 207.6 11.2 Low S EX2.1 Yes 0.86 0.1086 0.410 1.228 0.370 1.109 0.90 499 212.3 8.1 In this invention EX2.2 Yes 0.86 0.1710 0.537 1.610 0.497 1.490 0.93 475 213.3 8.1 In this invention EX2.3 Yes 0.86 0.0930 0.456 1.367 0.416 1.248 1.71 512 217.4 7.4 In this invention Ni/M′: Ni content in the positive electrode active material particles S*: amount of S in the surface layer S1 and S2: amount of sulfate ion obtained according to the following equations: S.sub.1 = S*(wt. %) × 2.996 × 10000 ppm, S.sub.2 = S(wt. %) × 2.996 × 10000 ppm

[0261] Table 3 summarizes the composition and electrochemical properties of the examples and comparative examples. Furthermore, the relationship between Ni content and SO.sub.4.sup.2− in the surface layer (S1) is depicted in FIG. 6. The table 3 and FIG. 6 show that a certain amount of S1 is required to improve electrochemical properties of the positive electrode active material wherein the improvement is a function of the Ni content in the positive electrode active material. A positive electrode active material with a higher Ni content requires more SO.sub.4.sup.2−.

[0262] XPS analysis is conducted to investigate the Al compounds and their distribution in the surface layer. EX1 and CEX1.1 are analyzed according to the procedure described in section B) XPS measurement.

[0263] As shown in FIG. 4A (x-axis: binding energy, y-axis: count), XPS spectra exhibits the peaks of Al2p and Ni3p having a maximum intensity at the range between 80 eV and 65 eV. The maximum peak intensity of Al2p of EX1 is at the binding energy of 73.78 eV. The Al2p peak position of EX1 having a maximum intensity are indicated in Table 5. As disclosed in Chem. Mater., 2017, 19, 5748-5757, if the Al2p peak position shifts to the lower binding energy while the Ni3p peak position does not change, it indicates the presence of LiAlO.sub.2 compound. A better battery performance, as shown in Table 3 is attributed to the appearance in Al2p peak in the range from 73.6 eV to 74.1 eV, corresponding to the presence of LiAlO.sub.2 compounds such as in EX1. FIG. 4B depicts the binding energy range for Al2p3 XPS peaks each for of LiM″.sub.1-aAl.sub.aO.sub.2, LiAlO.sub.2, and Al.sub.2O.sub.3 phases. The vertical solid line shows the Al2p3 XPS peak position for EX1.

[0264] The Al2p peak contains compounds such as LiM″.sub.1-aAl.sub.aO.sub.2, LiAlO.sub.2, and Al.sub.2O.sub.3, and the respective amounts of these compounds are quantified by the procedure described in section B2) XPS peak deconvolution. Table 4 shows the quantification of Al compounds. In the case of EX1, which has excellent electrochemical performances, it is confirmed that the content of Al peak2, which indicates the presence of a LiAlO.sub.2 phase, is superior or equal to 0.10 at % and inferior or equal to 0.30 at % with respect to the total atomic content of M′ of the positive electrode active material powder. In addition, the LiM″.sub.1-aAl.sub.aO.sub.2 content is inferior to 0.14 at % with respect to the total atomic content of M′ of the positive electrode active material powder.

[0265] In order to investigate the surface coverage by Al in the positive electrode active material powder, the atomic ratio of Al/M″ from XPS analysis (A1) is divided by the atomic ratio of Al/M″ from ICP analysis (A2), as indicated by B4) XPS peak integration and coverage. When the value of Al1/Al2 is higher than or equal to 100, it means the positive electrode active material compounds have Al uniformly distributed in the surface layer. As shown Table 5, EX1 have homogeneous Al distribution in the surface layer.

TABLE-US-00006 TABLE 4 XPS peak deconvolution of EX1 Atomic ratio/M′ (at %) Al peak1 According to (LiM″.sub.1-a Al peak2 Al peak3 Sample ID invention Al.sub.aO.sub.2 (LiAlO.sub.2) (Al.sub.2O.sub.3) EX1 No 0.09 0.15 0.13

TABLE-US-00007 TABLE 5 Al surface coverage of EX1 ICP XPS A2 Al2p peak Al Al surface According to Al/M* position Al/M* coverage Sample ID invention (at/at) (eV) (at/at) A1/A2 EX1 No 0.0038 73.78 0.528 139 M* = Ni + Mn + Co + Al + S

[0266] The surface layer of EX1 is confirmed as described in section E) TEM-EDS measurement, and the results is shown in FIG. 5A and FIG. 5B (x-axis: distance where 0 is the starting point of the surface layer, y-axis: element in atomic ratio). A cross-section sample prepared by SEM-FIB is investigated in a TEM/EDS equipment, and the Al and S content of the sample in an atomic ratio is obtained by dividing by the total amount of transition metals, Al, and S.

[0267] It is shown in FIG. 5A that the S/M* ratio reaches 0 at % at a linear distance (or distance) of 100 nm from the external edge of the surface layer of the particles. The thickness of the surface layer of EX1 particles is therefore of 100 nm.

[0268] FIG. 5B shows that the total content of aluminum is contained in the 100 nm thickness surface layer of EX1 particles.

[0269] This is indeed expected since CEX1.1 particles (before the surface treatment) do not contain aluminum.

[0270] In the case of a hNMC containing aluminum as a dopant present in its core, meaning before the surface treatment is applied, the amount of aluminum in a surface layer (Al.sub.surface) with respect to the total amount of aluminum in the positive electrode active material powder after the surface treatment is applied (Al.sub.total) of the hNMC according to the invention is obtained by the following procedure: [0271] 1) Firstly, the total amount of aluminum in the positive electrode active material powder (Al/M*.sub.ICP) is obtained by ICP analysis. [0272] 2) Secondly, an elemental line profile of a cross section of a particle is measured by techniques such as EDS and/or EELS (Electron Energy Loss Spectroscopy). [0273] 3) Thirdly, the thickness of the surface layer is determined based on the sulfur content evolution with respect to the distance from the external edge of the particles (a distance from the external edge of the surface layer to a point in the particle when S/M*=0 or S/M* is constant). [0274] 4) Fourthly, an Area1 parameter is obtained by integrating Al/M* on a distance in the surface layer in the one-dimensional EDS and/or EELS-based elemental smoothed line profile (cfr. FIG. 5B) and an Area2 parameter is obtained by integrating Al/M* by a distance from the external edge of the surface layer to the center of a particle in the one-dimensional EDS and/or EELS-based elemental line profile. Assuming that a particle is spherical and the surface layer is uniform, Area1 and Area2 are used to calculate the atomic ratio of Al.sub.surface to Al.sub.total by a following equation.

[00018]$A{l}_{\mathrm{surface}}/A{l}_{\mathrm{total}}=\frac{\frac{4}{3}{\pi \left(\mathrm{Area}2\right)}^3-\frac{4}{3}{\pi \left(\mathrm{Area}2-\mathrm{Area}1\right)}^3}{\frac{4}{3}{\pi \left(\mathrm{Area}2\right)}^3}\times 100\left(\mathrm{at}\%\right)$

[0275] The amount of aluminum in a surface layer with respect to total atomic content of M′ in the positive electrode active material powder is obtained by multiplying the Al/M*.sub.ICP ratio to the Al.sub.surface/Al.sub.total ratio, according to the following equation: Al/M*.sub.ICP*Al.sub.surface/Al.sub.total.