Patent Description:
There is a need for a Ni-rich NMC cathode material with further improved electrochemical properties to meet the requirements for batteries for use in the automotive and portable electronic device applications. In the framework of the present invention, a Ni-rich NMC compound or material is a LiM'O<NUM> cathode material wherein the molar content of Ni is of at least <NUM> mol%.

The first cycle efficiency (EF) is one of the key indices for performance evaluation of a secondary battery. The EF is a value obtained by dividing the initial discharge capacity (DQ1) by the initial charge capacity (CQ1) multiplied by <NUM> (%). A secondary battery having a high EF suffers a smaller loss of lithium ions accompanying the initial charging/discharging and is more likely to have a large capacity per volume and weight. Therefore, it is desirable for a secondary battery having as high an EF as possible
There have already been many efforts to improve the electrochemical properties of positive electrode active material, such as a core shell structure of the positive electrode active material. In this respect, <CIT> to Umicore discloses positive electrode active materials having a higher Co and lower Ni content in the shell of the positive electrode active materials with improved electrochemical properties. However, the Ni content of the positive electrode active material of Example <NUM> (EX1-P1) of <CIT> is only <NUM> mol% as compared to the total metal content and the Ni content of the positive electrode material of Example <NUM> (EX2-P1) of <CIT> is <NUM> mol% as compared to the total metal content. Comparative Example <NUM> of <CIT> discloses a positive electrode active material (CEX2-P1) having a Ni content of <NUM> mol% as compared to the total metal content of the positive electrode active material. However, CEX2-P1 is prepared by a metal hydroxide precursor having a Co content in a shell less than <NUM> mol% as compared to the total metal content in the shell. Therefore, it is expected that the positive electrode active material CEX2-P1 does not have the core-shell structure due to the Co diffusion during a heating step. That is the reason why the CEX2-P1 has inferior electrochemical properties.

Whilst achieving good electrochemical properties, the manufacturing costs can still be improved. An important cost factor is the total concentration of Co in a positive electrode active material.

Consequently, the present invention aims at providing a Ni-rich positive electrode active material (i.e. comprising at least <NUM> mol% of Ni) having excellent electrochemical properties, such as an initial discharge capacity (DQ1) higher than <NUM> mAh/g and first cycle efficiency (EF) higher than <NUM>%.

This objective is achieved by providing a positive electrode active material suitable for lithium-ion rechargeable batteries, wherein positive electrode active material comprising Li, M', and oxygen, wherein M' comprises:.

A positive electrode active material is defined herein as a material which is electrochemically active in a positive electrode. By active material, it must be understood a material capable to capture and release Li ions when subjected to a voltage change over a predetermined period of time.

The present invention concerns the following embodiments:.

In a first aspect, the present invention provides a positive electrode active material suitable for lithium-ion rechargeable batteries, said positive electrode active material comprising Li, M', and oxygen, wherein M' comprises:.

Preferably, the Ni content x ≥ <NUM> mol% and more preferably x ≥ <NUM> mol%, relative to M'.

Preferably, the Ni content x ≤ <NUM> mol% and more preferably x ≤ <NUM> mol%, relative to M'.

Preferably, the Co content y > <NUM> mol %, more preferably y ≥ <NUM> mol% and even more preferably y ≥ <NUM> mol%, relative to M'.

Preferably, the Mn content z ><NUM> mol%, more preferably ≥ <NUM> mol% and even more preferably z ≥ <NUM> mol%, relative to M'.

In another embodiment, said Ni in a content x is between <NUM> mol% and <NUM> mol% relative to M' and said Co in a content y is between <NUM> mol% and <NUM> mol% relative to M'.

In a preferred embodiment, the positive electrode active material of the present invention comprises a lithium transition metal oxide powder.

In a second embodiment, preferably according to the Embodiment <NUM>, the positive electrode active material of the present invention comprises Al in a content b between <NUM> mol% and <NUM> mol%, relative to M'.

Preferably, the Al content b is ≥ <NUM> mol%, more preferably b ≥ <NUM> mol%, and most preferably b ≥ <NUM> mol%, relative to M'.

Preferably, the Al content b is ≤ <NUM> mol%, more preferably b ≤ <NUM> mol%, and most preferably b is ≤ <NUM> mol%, relative to M'.

In a third embodiment, preferably according to the Embodiment <NUM> or Embodiment <NUM>, the positive electrode active material of the present invention comprises Ni content Niedge and Co content Coedge as measured by cross-sectional EDS (CS-EDS) at the edge of the secondary particle of the positive electrode active material, wherein Ni and Co contents are expressed as molar fractions compared to the sum of Ni, Mn, and Co content as measured by CS-EDS at the edge of the secondary particle of the positive electrode active material, wherein the positive electrode active material has a Ni content Nicenter and Co content COcenter measured by CS-EDS at the center of the secondary particle of the positive electrode active material, wherein Ni and Co contents are expressed as molar fractions compared to the sum of Ni, Mn, and Co content as measured by CS- EDS at the center of the secondary particle of the positive electrode active material,.

In the framework of this invention, the external edge of the secondary particle of the positive electrode active material is the boundary or external limit distinguishing the secondary particle from its external environment. The molar fraction of an element in the center of a secondary particle is determined by EDS measurement of the cross-sectional sample at the center part of the secondary particle. The center part of the secondary particle is the center point of the longest axis in a secondary particle in the cross-section.

A secondary particle taken for the CS-EDS measurement typically has a diameter of D50±<NUM>, as determined by particle size distribution analysis.

Preferably, the Niedge / Nicenter ≤ <NUM>.

Preferably, the Niedge / Nicenter > <NUM>, and more preferably Niedge / Nicenter > <NUM>.

Preferably, the Coedge / COcenter > <NUM>, and more preferably Coedge / COcenter > <NUM>.

Preferably, the Coedge / COcenter < <NUM>, and more preferably COedge / COcenter < <NUM>.

Preferably, the difference between Niedge and Nicenter is at least <NUM> mol% and a difference between Coedge and COcenter is at least <NUM> mol%, thereby showing Ni and Co concentration gradients from the edge to the center of the secondary particle of the positive electrode active material.

Preferably, the ratio COedge/C<NUM>/<NUM> is smaller than the ratio Coedge/Cocenter, wherein C<NUM>/<NUM> is a Co content expressed as mol% relative to the sum of Ni, Mn, and Co content as measured by CS-EDS at ¾ distance from the edge of the secondary particle to the center of the secondary particle.

Preferably, the ratio Niedge/Ni<NUM>/<NUM> is larger than the ratio Niedge/Nicenter, wherein Ni<NUM>/<NUM> is a Ni content expressed as mol% relative to the sum of Ni, Mn, and Co content as measured by CS-EDS at ¾ distance from the edge of the secondary particle to the center of the secondary particle.

Preferably, the positive electrode active material has a cobalt gradient slope (mol%/µm) wherein
<NUM> ≤ cobalt gradient slope ≤ <NUM>, preferably <NUM> ≤ cobalt gradient slope ≤ <NUM> and the cobalt gradient slope is represented by the following formula : <MAT>.

In the framework of this invention, material having a concentration gradient indicating a material having a difference in Co and Ni concentration between their center and their edge, wherein, said Ni and Co contents are expressed as mol% relative to the sum of Ni, Mn, and Co content as measured by CS- EDS at the center or edge of the secondary particle of the positive electrode active material.

Preferably, a Mn content Mnedge as measured by cross-sectional EDS (CS-EDS) at the edge of the secondary particle of the positive electrode active material, wherein a Mn content is expressed as mol% relative to the sum of Ni, Mn, and Co content as measured by CS-EDS at the edge of the secondary particle of the positive electrode active material, wherein Mnedge is higher than <NUM> mol%.

In a fourth aspect, preferably according to the Embodiments <NUM> to <NUM>, the positive electrode active material of the present invention comprises secondary particles that typically have an average crystallite size of at least <NUM>, as determined by XRD.

Preferably, the secondary particles of the positive electrode active material have an average crystallite size of at least <NUM>, more preferably at least <NUM> as determined by XRD.

Preferably, the secondary particles of the positive electrode active material have an average crystallite size of at most <NUM>, more preferably at most <NUM> and most preferably at most <NUM> as determined by XRD.

In a fifth aspect, preferably according to the Embodiments <NUM> to <NUM>, the positive electrode active material of the present invention comprises the element other than Li, O, Ni, Co, Mn, and Al in a content a is between <NUM> mol% and <NUM> mol%, and preferably a is between <NUM> mol% and <NUM> mol%, relative to M'.

In another aspect, the element other than Li, O, Ni, Co, Mn, and Al is preferably selected from the group consisting of: B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W, Zn and Zr, and most preferably is S.

In a further aspect, the positive electrode active material of the present invention preferably comprises S in a content a between <NUM> mol% and <NUM> mol%, most preferably S in a content a between <NUM> mol% and <NUM> mol%, and even more preferably S in a content a between <NUM> mol% and <NUM> mol%, relative to M'.

In a sixth aspect, the present invention provides a battery comprising the positive electrode active material of the present invention.

In a seventh aspect, the present invention provides the use of a battery according to the present invention in a portable computer, a tablet, a mobile phone, an electrically powered vehicle, or an energy storage system.

The Ni-rich NMC cathode materials according to the present invention typically have one or more of the following advantages of an improved first cycle efficiency (EF), cycle stability and thermal stability which promote a higher level of safety. This is believed to be achieved by the positive electrode material having a difference in cobalt and nickel concentration between their center and their edge, wherein the Ni content in the edge is less than that of the center and the Co content in the edge is more than that of the center of particle, and also that the secondary particles of the positive electrode material have a specific average crystallite size.

Typically, the positive electrode material of the present invention comprises secondary particle having a median size D50 of at least <NUM>, and preferably of at least <NUM> as determined by laser diffraction particle size analysis.

Preferably, said material has a secondary particle median size D50 of at most <NUM>, and preferably of at most <NUM> as determined by laser diffraction particle size analysis.

It is clear that further product embodiments according to the invention may be provided by combining features that are covered by the different product embodiments described before.

In a further aspect of the present invention, the positive electrode material of the present invention may be prepared by a method comprising the steps of:.

The advantage of using the specific heating temperature in the final step of the method of the present invention is that prevents or limit is crystallite growth of the secondary particles and ensures that the difference in cobalt and nickel concentration between the center and the edge of the precursor is retained in the positive electrode material.

Typically, the first metal sources are transition metal salts, and preferably sulfates of the M' elements Ni, Mn and/or Co.

The base typically used is an alkali compound, such as an alkali hydroxide e.g. sodium hydroxide, and/or ammonia.

The lithium source which may be used comprises LiOH, Li<NUM>O and/or LiOH.

Second metal source used to prepare the third M' based precursor is typically a transition metal salt, and preferably a sulfate of the M' elements Mn and/or Co.

Typically, the heating step is carried out for a time between <NUM> and <NUM> hours.

Optionally, an element containing compound can be added to the positive
electrode material. Preferably, said element containing compound is added in the mixing step together with the lithium source to M'-based precursor having a difference in cobalt and nickel concentration between the center and the edge. Alternatively, said element containing compound may be mixed together with the M'-based precursor having a difference in cobalt and nickel concentration between the center and the edge prior to the mixing step.

Preferably, the element of the element compound is an element other than Li, O, Ni, Co, Mn, and Al, and more preferably is selected from the group consisting of: B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W, Zn and Zr.

In addition, the method described herein above may comprise the following steps of:.

Preferably, the positive electrode active material comprises S in an amount of <NUM> mol% to <NUM> mol%, relative to M'.

In the following detailed description, preferred embodiments are described in detail to enable practice of the present invention. Although the present invention is described with reference to these specific preferred embodiments, it will be understood that the present invention is not limited to these preferred embodiments. To the contrary, the present invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description.

The following analysis methods are used in the Examples:.

The PSD is measured using a Malvern Mastersizer <NUM> with Hydro MV wet dispersion accessory after dispersing examples as described herein below of positive electrode active material powders in an aqueous medium. To improve the dispersion of the positive electrode active material powder examples, sufficient ultrasonic irradiation and stirring is applied, and an appropriate surfactant is introduced. D50 is defined as the particle size at <NUM>% of the cumulative volume % distribution.

The positive electrode active material examples as described herein below are measured by the inductively coupled plasma (ICP) method using an Agillent ICP <NUM>-ES. <NUM> gram of a powder sample of each example is dissolved into <NUM> high purity hydrochloric acid in an Erlenmeyer flask. The flask is covered by a watch glass and heated on a hot plate at <NUM> until complete dissolution of the sample. After being cooled to room temperature, the solution and the rinsing water of Erlenmeyer flask are transferred to a <NUM> volumetric flask. Afterwards, the volumetric flask is filled with DI water up to the <NUM> mark, followed by complete homogenization. An appropriate amount of solution is taken out by pipette and transferred into a <NUM> volumetric flask for the <NUM>nd dilution, where the volumetric flask is filled with internal standard and <NUM>% hydrochloric acid up to the <NUM> mark and then homogenized. Finally, this solution is used for ICP measurement. The Ni, Co, Mn, Al, and Element other than Li, Ni, Mn, Co, O and Al contents (x, y, z, b and a contents, respectively) measured is expressed as mol% of the total of these contents.

For the preparation of a positive electrode for each example described below, a slurry that contains an example of the positive electrode active material as described herein, a conductor (Super P, Timcal) and a binder (KF#<NUM>, Kureha) - with a formulation of <NUM>:<NUM>:<NUM> by weight - in a solvent (NMP, Mitsubishi) is prepared using a high-speed homogenizer. The homogenized slurry is spread on one side of an aluminum foil using a doctor blade coater with a <NUM> gap. The slurry-coated foil is dried in an oven at <NUM> 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 <NUM>) is located between the positive electrode and a piece of lithium foil used as a negative electrode. <NUM> LiPF<NUM> in EC/DMC (<NUM>:<NUM>) is used as electrolyte and is dropped between separator and electrodes. Then, the coin cell is completely sealed to prevent leakage of the electrolyte.

Each cell is cycled at <NUM> using Toscat-<NUM> computer-controlled galvanostatic cycling stations (from Toyo). The coin cell testing schedule used to evaluate samples is detailed in Table <NUM>. The schedules use a <NUM> C current definition of <NUM> mA/g and comprise the evaluation of rate performance at <NUM> C in the <NUM>~<NUM> V/Li metal window range. The initial charge capacity (CQ1) and discharge capacity (DQ1) are measured in constant current mode (CC). The first cycle efficiency (EF) is expressed in % as: <MAT>.

Cross-sections of the positive electrode active material examples as described herein below are prepared by an ion beam cross-section polisher (CP) instrument JEOL (IB-0920CP). The instrument uses argon gas as beam source.

To prepare the specimen, a small amount of a positive electrode active material powder is mixed with a resin and hardener, then the mixture is heated for <NUM> minutes on a hot plate. After heating, it is placed into the ion beam instrument for cutting and the settings are adjusted in a standard procedure, with a voltage of <NUM> kV for a <NUM> hours duration.

Using the examples of the positive electrode active materials prepared according to method D1) above, the concentration of Ni, Mn, and Co from the edge to the center of the positive electrode material secondary particles is analyzed by energy-dispersive X-ray spectroscopy (EDS). A secondary particle with a diameter around D50 value as measured by PSD according to Section A) is selected for analysis for each of the examples. The EDS is performed by JEOL JSM 7100F SEM equipment with a <NUM><NUM> X-MaxN EDS sensor from Oxford instruments. An EDS analysis of the positive electrode active material secondary particles provides the quantitative element analysis of the cross-section wherein it is assumed that particles are spherical. A straight line is set from the edge to the center point of the secondary particle and multiples points are set along the line with about <NUM> distance between each point. Ni, Mn, and Co concentrations are measured at every point and expressed as a mol% relative to the sum of Ni, Mn, and Co content at each point.

The X-ray diffraction pattern of the positive electrode active material powder examples as described herein below is collected with a Rigaku X-Ray Diffractometer Ultima <NUM> using a Cu Ka radiation source (<NUM> kV, <NUM> mA) emitting at a wavelength of <NUM>Å. The instrument configuration is set at: a <NUM>° Soller slit (SS), a <NUM> divergent height limiting slit (DHLS), a <NUM>° divergence slit (DS) and a <NUM> reception slit (RS). The diameter of the goniometer is <NUM>. For the XRD, diffraction patterns are obtained in the range of <NUM> - <NUM>° (2θ) with a scan speed of <NUM>° per min and a step-size of <NUM>° per scan.

The average crystallite size is determined by the XRD measurement of the positive electrode active material secondary particles. It has a good correlation with an average primary particle size of the positive electrode active material secondary particles. Therefore, the average crystallite size obtained by XRD is often used as a relative parameter to estimate the primary particle size of the secondary particles.

The average crystallite size of the secondary particles of the positive electrode active material examples as described herein below is determined according to the following steps:.

Fitting function is according to the pseudo-Voigt equation, a mix of Gaussian and Lorentzian line shape. The equation is: <MAT> with yo=offset, xc=center position of the peak, A=peak area, w=peak width (full width half maximum), and mu=profile shape factor. These five parameters are the variable cells set in the Solver tools.

Some relevant constraints are specified in the calculation following:
Kα1 and Ka2 peak width, wherein w Kα1 ≤ <NUM>°, w Kα2 ≤ <NUM>°, and w Kα1 = w Kα2; Integrated area ratio between Kα1 and Ka2, wherein A Kα2 ≤ A Kα1 * <NUM>; Kα1 and Ka2 peak position, wherein XcKα1 = XcKα2 - d, wherein d can be calculated according to Rachinger equation (<NPL>): <MAT> Wherein, λ is wavelengths of Cu Ka = <NUM>Å, λ1 is wavelengths of Cu Kα1 = <NUM>Å, λ2 is wavelengths of Cu Ka2 =<NUM>Å (<NPL>), and θ is the half of the center point of the selected 2θ range in Step <NUM>) (θ for LaB<NUM> is <NUM>°/<NUM>=<NUM>° and θ for the active material is <NUM>°/<NUM>=<NUM>°). Therefore, the value of d is <NUM>° for LaB<NUM>, and <NUM>° for the positive electrode active material.

Input value table is a set of initial data used as a starter to improve the fitting and obtain repeatable result. It involves prediction of parameter value based on estimation. Table <NUM> shows the example of input value table for EX1. <NUM>, an example of a positive electrode material according to the present invention.

In the calculation, y<NUM> offset is always zero since input data is linearly baselined to <NUM>. The peak positions are organized to place Kα1 on the lower 2θ than Ka2. mu and w are set as <NUM> and <NUM>, respectively. The XRD peak area in the range of <NUM>°-<NUM>° is assumed to be a triangle shaped with <NUM>° base and maximum intensity of the baselined peak as the triangle height. Kα1 area is <NUM>/<NUM> of the calculated total XRD peak area and Ka2 area is <NUM>/<NUM> of the calculated total XRD peak area.

The minimum value of SUMXMY2 is set as the objective in the Solver calculation. This function returns the sum of squares of differences between two array values. In this case, the difference is between real and calculated values. Calculation is terminated when the goodness of fitting R<NUM> reached <NUM>% or more. Otherwise, iteration will continue to reach the minimum value of the objective.

The diffractogram of LaB<NUM> is shown in <FIG>. The example of XRD peak of EX1. <NUM> after fitting process is shown in <FIG> (x-axis: 2θ, y-axis: intensity). The result of calculated parameter is shown in Table <NUM>.

From this step, maximum intensity of Kα1 peak each for LaB<NUM> and the positive electrode active material are obtained and labelled as ILaB6 and Iactive material, respectively.

Step <NUM>) Calculating integral breadth according to equation: <MAT> <MAT>.

From this step, integral breadths of LaB<NUM> and the positive electrode active material are obtained and labelled as IBLaB6 and IBactive material, respectively.

Step <NUM>) Correcting IB of positive electrode active material from the instrument broadening according to equation: <MAT> Wherein β is the corrected IBactive material.

Step <NUM>) Calculating the average crystallite size of the secondary particles of the positive electrode active material by using a Scherrer equation: <MAT>, wherein τ is the average crystallite size in nm as calculated from XRD, λ is the X-Ray wavelength in nm, K is the Scherrer constant which set as <NUM>, θ is xc of positive electrode active material Kα1 in radians as obtained from Step <NUM>, and β is the corrected IBactive material obtained from Step <NUM>).

The present invention is further illustrated in the following examples:.

<NUM> is an example of a positive electrode material according to the present invention which was prepared through a solid-state reaction between a lithium source and a transition metal-based source precursor A to prepare a positive electrode material according to the present invention by the following method steps:.

<NUM> is an example of a positive electrode material according to the present invention which was prepared according to the same method as EX1. <NUM> except that the heating temperature at the heating step <NUM>) was <NUM>. <NUM> had a composition of Ni:Mn:Co = <NUM>:<NUM>:<NUM> (in mol%), as determined by ICP analysis, and D50 of around <NUM>, as determined by PSD analysis.

CEX1 is a comparative example of a positive electrode material which was prepared according to the same method as EX1. <NUM> except that the heating temperature used in the heating step <NUM>) was <NUM>. CEX1 had a composition of Ni:Mn:Co = <NUM>:<NUM>:<NUM> (in mol%), as determined by ICP analysis, and D50 of around <NUM>, as determined by PSD analysis.

CEX2 is a comparative example of a positive electrode material which was obtained through a solid-state reaction between a lithium source and a transition metal-based source precursor B in the following method steps:.

EX2 is an example of a positive electrode material according to the present invention which was prepared through following method steps:.

The results of the experimental tests used on the examples described herein above are as follows:.

Table <NUM> summarizes the ICP values of S and Al, average crystallite sizes and electrochemical properties of EX1. <NUM>, EX1. <NUM>, CEX1, CEX2, and EX2. It was demonstrated that positive electrode active material EX1. <NUM> and EX1. <NUM> prepared from precursor A and prepared at a firing temperature between <NUM> to <NUM> showed the highest DQ1 and EF. The benefit in the lower EF values was also linked with the average crystallite size of the secondary particles being lower than <NUM>, as calculated by XRD method in the Section E). A firing temperature higher than <NUM> was found to be disadvantageous since it promoted the growth of the average crystallite size of the secondary particles to values exceeding <NUM>, as shown in Table <NUM> for CEX1.

EX2 was obtained by mixing EX1. <NUM> with Al<NUM>(SO<NUM>)<NUM> followed by <NUM> heating. The treatment further improved DQ1 and EF over those of EX1. <NUM> showing the presence of both Al and S in the positive electrode active material of EX2 is beneficial for the electrochemical properties.

<NUM> prepared from precursor A lithiated at <NUM> showed higher DQ1 and EF value than the CEX2 prepared from precursor B at the same firing temperature. From Table <NUM> it can be concluded that a precursor with concentration gradient characteristic is favorable to prepare positive electrode active material with the improved electrochemical properties.

<FIG> show the CS-EDS analysis as described in section D) for EX1. <NUM>, EX1. <NUM>, CEX1, and CEX2, respectively. The analysis is conducted to investigate difference in cobalt and nickel concentration between the center and the edge in the positive electrode active material. The mol% of Ni, Mn, and Co each at the edge and at the center of a secondary particle for each of these examples is shown in Table <NUM>. <NUM> manufactured at <NUM> showed Ni and Co gradients from the edge to the center of the positive electrode active material secondary particle. Ni and Co concentration gradients were observed from the edge (at <NUM>) to the center of EX1. <NUM> wherein Ni concentration at the edge was lower than the center. Conversely, Co concentration at the edge of EX1. <NUM> was higher than the concentration in the center following the composition of the precursor A. On the other hand, both Ni and Co concentration gradients were not observed in CEX1 which was prepared at a higher lithiation temperature of <NUM> indicating that the difference in cobalt and nickel concentration between the center and the edge in precursor disappeared when the firing temperature was higher than <NUM>. CEX2 manufactured from a precursor B having no concentration gradient which consequently also showed no Ni and Co concentration gradients.

Claim 1:
A positive electrode active material suitable for lithium-ion rechargeable batteries, said positive electrode active material comprising Li, M', and oxygen, wherein M' comprises:
- Ni in a content x between <NUM> mol% and <NUM> mol%, relative to M',
- Co in a content y between <NUM> mol% and <NUM> mol%, relative to M',
- Mn in a content z between <NUM> mol% and <NUM> mol%, relative to M',
- Al in a content b between <NUM> mol% and <NUM> mol%, relative to M',
- Element other than Li, Ni, Mn, Co, O and Al in a content a between <NUM> mol% and <NUM> mol%, relative to M',
- wherein x, y, z, a and b contents are measured by ICP,
- wherein x+y+z+a+b is <NUM> mol%,
wherein said positive electrode active material comprises secondary particles comprising a plurality of primary particles,
wherein the positive electrode active material has a Ni content Niedge and Co content Coedge as measured by cross-sectional EDS (CS-EDS) at the edge of the secondary particle of the positive electrode active material, wherein Ni and Co contents are expressed as mol% relative to the sum of Ni, Mn, and Co content as measured by CS-EDS at the edge of the secondary particle of the positive electrode active material,
wherein the positive electrode active material has a Ni content Nicenter and Co content COcenter measured by CS-EDS at the center of the secondary particle of the positive electrode active material, wherein Ni and Co contents are expressed as mol% relative to the sum of Ni, Mn, and Co content as measured by CS- EDS at the center of the secondary particle of the positive electrode active material,
wherein the ratio Niedge / Nicenter < <NUM>,
wherein the ratio Coedge / COcenter > <NUM>,
wherein said secondary particles have an average crystallite size of at least <NUM> and at most <NUM>, as determined by XRD.