Positive electrode active material powder and method for manufacturing a positive electrode active material powder

Abstract

The present invention relates to a positive electrode active material powder comprising secondary particles comprising a plurality of primary particles, wherein the positive electrode active material powder comprises a lithium metal oxide comprising lithium (Li), M',and oxygen (O), wherein the first lithium metal oxide has a layered α-NaFeO2 structure; wherein M' comprises titanium (Ti) and at least one element selected from the group consisting of nickel (Ni) and manganese (Mn); wherein a grain boundary is present between adjacent primary particles of the secondary particles; wherein a concentration of Ti in the grain boundary is greater than a concentration of Ti in the adjacent primary particles; and wherein the positive electrode active material powder has a surface area between 0.3 m2 / g and 1.2 m2 / g as determined by BET measurement.

Description

[0001] POSITIVE ELECTRODE ACTIVE MATERIAL POWDER AND METHOD FOR MANUFACTURING A POSITIVE ELECTRODE ACTIVE MATERIAL POWDER

[0002] TECHNICAL FIELD AND BACKGROUND

[0003] The present invention relates to a positive electrode active material powder comprising secondary particles comprising a plurality of primary particles, wherein the positive electrode active material powder comprises a first lithium metal oxide having a layered o-NaFeO? structure.

[0004] A lithium transition metal oxide having a layered o-NaFeO? structure has cation mixing between lithium (Li) and transition metals, and the increase of the cation mixing results in a problem of low initial discharge capacity caused by the hampering of the diffusion of Li ions.

[0005] Accordingly, it is demanded that a positive electrode active material powder having a layered o-NaFeO? structure should exhibit superior electrochemical properties such as high initial discharge capacity.

[0006] It is a first object of the present invention to provide a positive electrode active material powder which can solve the above-mentioned problem. Specifically, the first object is achieved by providing a positive electrode active material powder comprising a first lithium metal oxide having a layered o-NaFeO? structure, wherein a concentration of titanium (Ti) in a grain boundary present between two adjacent primary particles of secondary particles is greater than a concentration of Ti in the adjacent primary particles, and wherein the positive electrode active material powder has a surface area between 0.5 m2 / g and 1.2 m2 / g as determined by BET measurement.

[0007] It is a second object of the present invention to provide a method for manufacturing the positive electrode active material powder according to the present invention.

[0008] It is a third object of the present invention to provide a battery comprising the positive electrode active material powder according to the present invention.

[0009] It is a fourth object of the present invention to provide a use of the battery according to the present invention.

[0010] SUMMARY OF THE INVENTION

[0011] The first object is achieved by providing a positive electrode active material powder suitable for lithium-ion rechargeable batteries, comprising secondary particles comprising a plurality of primary particles, wherein the positive electrode active material powder comprises a first lithium metal oxide comprising Li, M', and oxygen (O), wherein the first lithium metal oxide has a layered o- NaFeO? structure; wherein M' comprises Ti and at least one element selected from the group consisting of nickel (Ni) and manganese (Mn); wherein a grain boundary is present between adjacent primary particles of the secondary particles; wherein a concentration of Ti in the grain boundary is greater than a concentration of Ti in the adjacent primary particles; and wherein the positive electrode active material powder has a surface area between 0.5 m2 / g and 1.2 m2 / g as determined by BET measurement.

[0012] The second object is achieved by providing a method for manufacturing of the positive electrode active material powder according to the present invention, wherein said method comprises consecutive steps of:

[0013] Step 1) mixing a second lithium metal oxide comprising Li, M', and O with an aqueous solution to obtain a slurry, filtering, and then drying said slurry to obtain a dried powder, Step 2) mixing the dried powder with a compound comprising Ti, preferably TiO?, wherein said compound comprises Ti in an amount between 300 ppm to 4000 ppm with respect to the weight of the dried powder, to obtain a mixture, and

[0014] Step 3) heating the mixture in an oxidizing atmosphere at a temperature between 450°C and 650°C so as to obtain the positive electrode active material powder.

[0015] The third object is achieved by a battery comprising the positive electrode active material powder according to the present invention.

[0016] The fourth object is achieved by a use of the battery according to the present invention in an electric vehicle or in a hybrid electric vehicle.

[0017] The positive electrode active material powder according to the present invention has superior electrochemical properties such as high initial discharge capacity (DQ1), low irreversible capacity (Qirr) and low capacity fading (QF).

[0018] BRIEF DESCRIPTION OF THE FIGURES

[0019] Figure la is the cross-sectional STEM image of EX2 at a first position.

[0020] Figure lb is the EDS map scan image of Figure la.

[0021] Figure 1c is the line scan profile of Ti from the position as indicated by the arrow in the Figure lb.

[0022] Figure 2a is the cross-sectional STEM image of EX2 at a second position Figure 2b is the EDS map scan image of Figure 2a.

[0023] Figure 2c is the line scan profile of Ti from the position as indicated by the arrow in the Figure 2b.

[0024] DETAILED DESCRIPTION

[0025] 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. In contrast, the present invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description.

[0026] As used herein, a range of values "from X to Y" includes the endpoints X and Y.

[0027] Positive Electrode Active Material Powder

[0028] In a first aspect, the present invention relates to a positive electrode active material powder suitable for lithium-ion rechargeable batteries, comprising secondary particles comprising a plurality of primary particles, wherein the positive electrode active material powder comprises a first lithium metal oxide comprising Li, M', and O, wherein the first lithium metal oxide has a layered o-NaFeO? structure; wherein M' comprises Ti and at least one element selected from the group consisting of Ni and Mn; wherein a grain boundary is present between adjacent primary particles of the secondary particles; wherein a concentration of Ti in the grain boundary is greater than a concentration of Ti in the adjacent primary particles; and wherein the positive electrode active material powder has a surface area between 0.5 m2 / g and 1.2 m2 / g as determined by BET measurement.

[0029] The grain boundary is on a surface of the primary particles ( / .e., crystallites). Thus, the shape of the grain boundary is defined by the shape of the primary particles adjacent to the grain boundary and may approximate a rectilinear shape such as polygon when viewed in crosssection.

[0030] The dimensions of the grain boundary are not particularly limited. A length and a width of the grain boundary may each independently be about 50 to about 1000 nm, preferably about 60 to about 900 nm, and more preferably about 70 to about 800 nm, when viewed in cross-section. The length and width of the grain boundaries may be perpendicular to each other and may be parallel to the surface of the adjacent crystallite. A thickness of the grain boundary may be about 5 to about 100 nm, preferably about 10 to about 80 nm, and more preferably about 20 to about 50 nm. The thickness of the grain boundary may be perpendicular to the length and the width of the grain boundary and may be perpendicular to the surface of the adjacent crystallite. The thickness of the grain boundary is defined by the Ti concentration profile through a line scan crossing the grain boundary. The Ti concentration profiles may be obtained by STEM-EDS measurement as described in this specification. Specifically, Ti concentration profile obtained by STEM-EDS during a line scan crossing the grain boundary has a maximum value between a right point and a left point. The right point corresponds to the first occurrence, where Ti concentration reaches the average Ti concentration of a positive electrode active material powder obtained by Inductively Coupled Plasma - Optical Emission Analysis (ICP-OES) as described in this specification, from the point of the maximum value on the right side. The left point corresponds to the first occurrence, where Ti concentration reaches the average Ti concentration of a positive electrode active material powder obtained by ICP-OES, from the point of the maximum value on the left side. The thickness of the grain boundary is the distance from the right point to the left point.

[0031] The ratio of the maximum Ti concentration in the grain boundary, relative to M', to the concentration of Ti obtained by ICP-OES may be 5 or more, preferably 7 or more, more preferably 9 or more, still more preferably 11 or more. Said ratio may be 50 or less, preferably 40 or less, more preferably 30 or less, still more preferably 25 or less. Specifically, said ratio may range from 5 to 50, preferably from 7 to 40, more preferably from 9 to 30, still more preferably from 11 to 25. If said ratio is below 5, Ti enrichment may not be sufficient to improve the electrochemical performance of batteries. If said ratio is above 50, the mechanical stability at the grain boundary may deteriorate.

[0032] The first lithium metal oxide comprising Li, M', and O has a layered o-NaFeO? structure. In the layered o-NaFeO? structure, hexagonal metal oxide layers are separated by planes of alkali metals. The metal oxide layers form metal centered oxygen octahedra which are separated by alkali metal ions, and the metal oxide layers are laterally offset to provide a three-layer structure. In this structure, the alkali metal atoms occupy the so called "3a" sites in the structure (x=0, y=0, and z=0), the metal atoms occupy the "3b" sites (x=0, y=0, and z=0.5), and the oxygen atoms occupy the "6c" sites (x=0, y=0, and z=0.25). The coordinates of the atoms and the cell parameters can vary according to the metal composition.

[0033] M' in the first lithium metal oxide comprises Ti and at least one element selected from the group consisting of Ni and Mn. The concentration of Ti in the grain boundary is greater than the concentration of Ti in the primary particles. Ti may be concentrated on the grain boundary, and thus, Ti may barely exist in the primary particles. Furthermore, the concentration of Ti may exhibit a gradient in which the concentration of Ti decreases gradually from the grain boundary between the primary particles toward the center portion of the primary particles. The concentration of Ti in the grain boundary may be measured by Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive X-ray Spectroscopy (EDS) as described in this specification.

[0034] The positive electrode active material powder according to the present invention has a surface area of 0.3 m2 / g or more, preferably 0.4 m2 / g or more, more preferably 0.5 m2 / g or more, still more preferably 0.6 m2 / g or more as determined by BET measurement described in this specification. The positive electrode active material powder according to the present invention has a surface area of 2.0 m2 / g or less, preferably 1.5 m2 / g or less, more preferably 1.2 m2 / g or less, still more preferably 1.1 m2 / g or less as determined by BET measurement described in this specification. Specifically, the positive electrode active material powder according to the present invention has a surface area between 0.3 m2 / g and 1.2 m2 / g, preferably between 0.4 m2 / g and 1.2 m2 / g, more preferably 0.5 m2 / g and 1.2 m2 / g, still more preferably 0.6 m2 / g and 1.1 m2 / g as determined by BET measurement described in this specification. As the surface area of the positive electrode active material powder increases, the area of a region in which a side reaction may occur also increases. This side reaction may cause a phase transformation in which the crystal structure of the lithium composite oxide constituting the positive electrode active material powder is changed. Such a phase transformation of the crystal structure in the surface of the positive electrode active material powder is one of the causes of reducing the electrochemical characteristics, such as the lifespan characteristics, of the lithium secondary battery.

[0035] The inventors of the present invention have found out that a combination of the Ti enrichment in the grain boundary and the surface area of the positive electrode active material powder ranging from 0.5 m2 / g and 1.2 m2 / g causes superior electrochemical properties of cell. In detail, the positive electrode active material powder according to the present invention exhibits high initial discharge capacity (DQ1), low irreversible capacity (Qirr) and low capacity fading (QF). Specifically, residual Li such as U2CO3 and LiOH on the surface of a positive electrode active material may be converted to form a cathode electrolyte interphase (CEI), and thus, the interphase resistance may be increased. Furthermore, residual Li may decompose to produce gases that may cause surface degradation of a positive electrode active material. In order to remove residual Li, the surface of a positive electrode active material may be washed with an aqueous solution, but the surface area of the positive electrode active material may be increased due to washing. Without wishing to be bound by any theory, Ti enrichment in the grain boundary may offset the deteriorated electrochemical performances of the positive electrode active material caused by the increased surface area after washing to remove residual Li. In a preferred embodiment, M' comprises:

[0036] Ni in a content x, wherein 70.0 < x < 100.0 at%, relative to M', Mn in a content y, wherein 0.0 < y < 11.0 at%, relative to M', Co in a content z, wherein 0.0 < z < 11.0 at%, relative to M',

[0037] - Ti in a content a, wherein 0.0 < a < 2.0 at%, relative to M',

[0038] - Al in a content b, wherein 0.0 < b < 2.0 at%, relative to M',

[0039] Zr in a content c, wherein 0.0 < c < 2.0 at%, relative to M',

[0040] D in a content d, wherein 0.0 < d < 2.0 at%, relative to M', wherein D is at least one element selected from the group consisting of B, Ba, Ca, Ce, Cr, Fe, La, Mg, Mo, Nb, S, Sr, V, W, Y, and Zn; wherein x, y, z, a, b, c, and d are measured by ICP-OES; and wherein x+y+z+a+b+c+d is 100.0 at%.

[0041] As the content of Ni in the first lithium metal oxide increases, the amount of Li by-products present on the surface of the positive electrode active material powder increases, and thus, gelation in the production of a positive electrode paste and gas generation during charging and discharging of a lithium secondary battery may be reduced.

[0042] In the framework of the present invention, at% signifies atomic percentage. The at% or "atomic percent" of a given element means a percentage of atoms of said element among all atoms in a claimed composition.

[0043] ICP-OES provides weight percent (wt%) of each element included in a material whose composition is determined by this technique. Conversion from wt% to at% is as follows: at% of a first element Ei Eati) in a material can be converted from a given wt% of said first element Ei Ewti) in said material by applying the following formula, wherein Eawiis a standard atomic weight (molecular weight) of the first element Ei, Ewti is wt% of an ithelement Ei, Eawiis a standard atomic weight (molecular weight) of said ithelement Ei, and n is an integer which represents the number of types of all elements included in the material.

[0044] In a preferred embodiment, M' comprises:

[0045] Ni in a content x, wherein 0 < x < 30. 0 at%, relative to M',

[0046] Mn in a content y, wherein 50.0 < y < 100.0 at%, relative to M',

[0047] Co in a content z, wherein 0.0 < z < 12.0 at%, relative to M',

[0048] - Ti in a content a, wherein 0.0 < a < 2.0 at%, relative to M', - Al in a content b, wherein 0.0 < b < 2.0 at%, relative to M',

[0049] Zr in a content c, wherein 0.0 < c < 2.0 at%, relative to M',

[0050] D in a content d, wherein 0.0 < d < 2.0 at%, relative to M', wherein D is at least one element selected from the group consisting of B, Ba, Ca, Ce, Cr, Fe, La, Mg,

[0051] Mo, Nb, S, Sr, V, W, Y, and Zn; wherein x, y, z, a, b, c, and d are measured by ICP-OES; and wherein x+y+z+a+b+c+d is 100.0 at%.

[0052] In a preferred embodiment, the first lithium metal oxide comprises LiOH in a content of at most 0.7 wt.%, preferably at most 0.6 wt.%, more preferably at most 0.5 wt.%, relative to a total weight of the first lithium metal oxide, and wherein a content of LiOH is measured by acid-base titration as described in this specification.

[0053] In a preferred embodiment, the first lithium metal oxide comprises U2CO3 in a content of at most 0.3 wt.%, preferably at most 0.25 wt.%, more preferably at most 0.2 wt.%, relative to a total weight of the first lithium metal oxide, and wherein a content of U2CO3 is measured by acid-base titration as described in this specification.

[0054] In a preferred embodiment, 0.0 < a < 1.0 at%.

[0055] In a preferred embodiment, 0.01 < b < 5.0 at% and 0.01 < c < 5.0 at%.

[0056] Method for Manufacturing Positive Electrode Active Material Powder

[0057] In a second aspect, the present invention relates to a method for manufacturing of the positive electrode active material powder according to the first aspect, wherein said method comprises consecutive steps of:

[0058] Step 1) mixing a second lithium metal oxide comprising Li, M", and O with an aqueous solution to obtain a slurry, filtering, and then drying said slurry to obtain a dried powder, wherein M" is identical to M' except for Ti,

[0059] Step 2) mixing the dried powder with a compound comprising Ti, preferably TiC>2, wherein said compound comprises Ti in an amount between 300 ppm and 4000 ppm with respect to the weight of the dried powder, to obtain a mixture, and

[0060] Step 3) heating the mixture in an oxidizing atmosphere at a temperature between 450°C and 650°C so as to obtain the positive electrode active material powder.

[0061] The second lithium metal oxide may be prepared by lithiating a (oxy) hydroxide comprising M". The surface area of the second lithium metal oxide may be reduced in Step 1), and the surface area of the positive electrode active material powder may be further reduced in Step 2) and Step 3). Ti enrichment in the grain boundary is achieved by Step 2) and Step 3). The temperature in Step 3) may be 450°C or more, preferably 500°C or more, more preferably 550°C or more, and may be 650°C or less, preferably 630°C or less, more preferably 600°C or less. If the temperature in Step 3) is below 450°C or above 650°C, and / or if the amount of Ti added in Step 2) is below 300 ppm or above 4000 ppm, Ti may not be sufficiently enriched in the grain boundary. The surface area of a positive electrode active material powder may decrease as the temperature in Step 3) increases. Without wishing to be bound by any theory, if the temperature in Step 3) is above 650°C, the reduced surface area of the positive electrode active material may not lead to improved electrochemical performances due to insufficient Ti enrichment in the grain boundary.

[0062] In a preferred embodiment, said Ti is in an amount between 500 ppm and 3000 ppm.

[0063] In a preferred embodiment, the temperature in Step 3) is between 500 and 600°C.

[0064] In a preferred embodiment, the second lithium metal oxide further comprises Al and Zr.

[0065] In a preferred embodiment, the aqueous solution is deionized water.

[0066] Batery

[0067] In a third aspect, the present invention relates to a battery comprising the positive electrode active material powder according to the first aspect.

[0068] Use of Batery

[0069] In a fourth aspect, the present invention relates to a use of the battery according to the third aspect.

[0070] As appreciated by a person skilled in the art, all embodiments directed to the positive electrode active material according to the first aspect may apply mutatis mutandis to the second, third and fourth aspects.

[0071] EXPERIMENTAL TESTS USED IN THE EXAMPLES

[0072] The following analysis methods are used in the Examples:

[0073] A) Particle size distribution (PSD) analysis

[0074] The PSD is measured using a Malvern Mastersizer 3000 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 50% of the cumulative volume % distribution.

[0075] B) Inductively coupled plasma - optical emission analysis (ICP-OES) analysis

[0076] The positive electrode active material examples as described herein below are measured by the Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES) method using an Agillent ICP 720-OES. 1 gram of a powder sample of each example is dissolved into 50 mL high purity hydrochloric acid in an Erlenmeyer flask. The flask is covered by a watch glass and heated on a hot plate at 380°C 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 250 mL volumetric flask. Afterwards, the volumetric flask is filled with DI 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 2nddilution, where the volumetric flask is filled with internal standard and 10% hydrochloric acid up to the 250 mL mark and then homogenized. Finally, this solution is used for ICP-OES measurement. The contents of Ni, Mn, Co, and Ti are expressed as at% of the total of these contents.

[0077] C) Coin cell testing

[0078] Cl. Coin cell preparation

[0079] 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 96.5: 1.5:2.0 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 170 pm 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. IM LiPF6in 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.

[0080] C2. Testing method

[0081] 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 1. Each cell is cycled at 25°C using a Toscat-3100 computer-controlled galvanostatic cycling station (from Toyo).

[0082] The schedule uses a 1C current definition of 220 mA / g in the 4.3 V to 3.0 V / Li metal window range. The capacity fading rate (QF) is obtained according to below equation. 100 wherein DQ1 is the discharge capacity at the first cycle, DQ7 is the discharge capacity at the 7th cycle, DQ34 is the discharge capacity at the 34th cycle.

[0083] The irreversible capacity (Qirr)(%) is obtained according to an equation below: 100(%).

[0084] Wherein CQ1 is the charge capacity at the first cycle.

[0085] Table 1. Cycling schedule for Coin cell testing method

[0086] D) Surface base analysis

[0087] In the measurement of soluble base content by pH titration, two steps are performed: (a) the reparation of solution, and (b) pH titration. The detailed explanation of each step is as follows: Step (a): The preparation of solution: powder is immersed in deionized water and stirred for 10 min in a sealed glass flask containing 100 ml of deionized water. The amount of positive electrode active material powder is 4 grams. After stirring, to dissolve the base, the suspension of powder in water is filtered to get a clear solution.

[0088] Step (b): pH titration: 90 ml of the clear solution prepared in step (a) is used for pH titration by using 0.1M HCI. The flow rate is 0.5 ml / min and the pH value is recorded each 3 seconds. The pH titration profile (pH value as a function of added HCI) shows two clear equivalence (or inflection) points. The first equivalence point (corresponding to a HCI quantity of EPl) at around pH 7.4 results from the reaction of OH’ and COs2’ with H+. The second equivalence point (corresponding to a HCI quantity of EP2) at around pH 4.7 results from the reaction of HCO3’ with H+. It is assumed that the dissolved base in deionized water is either LiOH (with a quantity 2*EP1-EP2) or IJ2CO3 (with a quantity 2*(EP2-EP1)). The obtained values for LiOH and U2CO3 are the result of the reaction of the surface with deionized water.

[0089] E) Specific surface area analysis

[0090] The specific surface area of the positive electrode active material is measured with the Bruanauer-Emmett-Teller (BET) method by using a Micromeritics Tristar II 3020. A powder sample is heated at 300 °C under a nitrogen (N2) gas for 1 hour prior to the measurement in order to remove adsorbed species. The dried powder is put into the sample tube. The sample is then de-gassed at 30 °C for 10 minutes. The instrument performs the nitrogen adsorption test at 77 K. By obtaining the nitrogen isothermal absorption / desorption curve, the total specific surface area of the sample in m2 / g is derived.

[0091] F) Carbon analysis

[0092] The content of carbon of the positive electrode active material powder is measured by Horiba Emia-Expert carbon / sulfur analyzer. 1 gram of the positive electrode active material powder is placed in a ceramic crucible in a high frequency induction furnace. 1.5 grams of tungsten and 0.2 grams of tin are added into the crucible as accelerators. The powder is heated at a programmable temperature wherein gases produced during the combustion are then analyzed by Infrared detectors. The analysis of CO2 and CO determines the carbon concentration.

[0093] G) Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive X- ray Spectroscopy (EDS) measurement

[0094] To prepare a lamella for cross-sectional Scanning Transmission Electron Microscopy (STEM) and Energy Dispersive X-ray Spectroscopy (EDS), the positive electrode active material particle sample is coated with 25 nm layer of carbon (Leica EM ACE600 coater) before Focused Ion Beam (FIB) preparation. FIB lamella is prepared on a Cu Omniprobe TEM grid, using a Thermo Fisher Helios FIB-SEM with Ga ion beam first at 30 kV, then at 8 kV and at the final thinning step at 2 kV and 39 pA. The lamella size is about 4.2 x 6.6 um, the thickness is about 50-100 nm.

[0095] The lamella is transferred to the Ar filled glove box in the vacuum transfer box. The TEM vacuum transfer holder (Gatan) is assembled in the glove box. The High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) and Energy Dispersive X-ray spectroscopy (EDS) are performed on an aberration corrected FEI Titan transmission electron microscope at 300 kV, using a Super X detector. The screen current is 150 pA with acquisition time of 20 min in the map size of 570x570 nm.

[0096] For EDX map scans acquisition and data processing, Esprit Quantax software version 1.9, Bruker, is used. For line scan profiles, the following elements were considered: Ni (Ni-K line at 7.47 keV), Mn (Mn-K line at 5.90 keV), Co (Co-K line at 6.93 keV) and Ti (Ti-K line at 4.51 keV).

[0097] The thickness of the grain boundary is determined through a Ti concentration profile from a line scan crossing the grain boundary. The amount of Ti is normalized by the total atomic fraction of Ni, Mn, Co, and Ti. The thickness of the grain boundary is measured as a width of the maximum Ti peak along the baseline of the line scan profile crossing the grain boundary.

[0098] EXAMPLES

[0099] The present invention is further illustrated in the following examples:

[0100] Comparative Examples 1 to 4

[0101] Comparative Example 1 (CEX1) was prepared according to the following steps:

[0102] 1) Co-precipitation : a transition metal-based precursor with metal composition of Ni0.91Mn0.04Co0.04AI0.01 was prepared by a co-precipitation process in a large-scale continuous stirred tank reactor (CSTR) with mixed nickel-manganese-cobalt-aluminum sulfates, sodium hydroxide, and ammonia.

[0103] 2) First mixing: 5000 grams of the precursor prepared from Step 1), 24.9 grams of ZrO?, and

[0104] 1354.6 grams of LiOH as a lithium source were homogenously mixed to obtain a first mixture with a lithium to metal (Ni, Mn, Co, and Al) ratio of 1.06. The amount of ZrO? was determined such that the content of Zr with respect to the weight of the precursor would be 3500 ppm.

[0105] 3) First heating : the first mixture from Step 2) was heated at 710°C for 12 hours under an oxygen atmosphere followed by crushing and sieving to obtain a first heated product.

[0106] 4) Washing: the first heated product from Step 3) was mixed with 5°C water for 10 min to form a slurry. The ratio of the first heated product to water is 1 : 1. The slurry was filtered, dried at 140°C in vacuum for 10 hours, and sieved to obtain a washed powder.

[0107] 5) Second mixing : 500 grams of the washed powder from Step 4) was mixed with 0.88 grams of TiO? powder to obtain a second mixture comprising Ti of 1000 ppm with respect to the weight of the washed powder.

[0108] 6) Second heating : the second mixture from Step 5) was heated at 300°C for 5 hours under an oxygen atmosphere followed by crushing and sieving to obtain CEX1 having D50 of around

[0109] 11.6 pm.

[0110] CEX2 was prepared according to the same method as CEX1 except that the second heating temperature was 400°C.

[0111] CEX3 was prepared according to the same method as CEX1 except that the first heated product in Step 3) was not washed as in Step 4) and directly used in Step 5) instead of the washed powder in Step 4), and that the second heating temperature was 600°C. CEX4 was prepared according to the same method as CEX1 except that the second heating temperature was 700°C.

[0112] Examples 1 to 3

[0113] EXI was prepared according to the same method as CEX1 except that the second heating temperature was 500°C.

[0114] EX2 was prepared according to the same method as CEX1 except that the second heating temperature was 600°C.

[0115] EX3 was prepared according to the same method as EX2 except that more TiO? powder was added in the second mixing to obtain Ti of 2500 ppm with respect to the weight of the washed powder.

[0116] Results

[0117] Table 2 summarizes the process and properties of the examples and the comparative examples.

[0118] Table 2. Summary of the process and properties of examples and comparative examples to the present invention.

[0119] Except for EX3, all of the examples and the comparative examples are positive electrode active material powders comprising Ti in an amount of about 0.2 at% with respect to the sum of the contents of Ni, Mn, Co, and Al in the positive electrode active material powder. The second heating temperature is varied from 300°C to 700°C. EXI to EX3, whose second heating temperatures range from 500°C to 600°C, showed superior electrochemical properties tested by coin cell. This is indicated by a higher DQ1, lower Qirr, and lower QF. EX3 is a positive electrode active material comprising Ti in an amount of about 0.44 at% and the second heating temperature was 600°C. Comparison of EX2 with EX3 shows that the second heating temperature have an effect on the electrochemical properties greater than the amount of Ti because EX2 and EX3 showed similar electrochemical properties even though the amounts of Ti in EX2 and EX3 were very different.

[0120] The content of residual Li on the surface of CEX3, which was prepared according to the same method as EX2 except that CEX3 was not washed with an aqueous solution, was significantly higher than those on the surfaces of EX2.

[0121] The electrochemical properties such as DQ1, Qirr and QF of CEX4, which was prepared according to the same method as EX2 except that the second heating temperature of CEX4 was 700°C, were not advanced over EX2. Without wishing to be bound by any theory, the second heating temperature of 700°C may not lead to improved electrochemical performances due to insufficient Ti enrichment in the grain boundary whereas the surface area of CEX4 was decreased to 0.31 m2 / g.

[0122] Ti enrichment in the grain boundaries is confirmed in Figures la, lb, 1c, 2a, 2b, and 2c. Figures la and 2a are the cross-sectional STEM images of EX2 in different locations. In each image, the boundaries between the adjacent primary particles are observed and marked by arrows. Figures lb and 2b are the EDS map scan image of corresponding Figures la and 2a, respectively. Figures lb and 2b show the Ti enrichment in the grain boundary. Specifically, the lighter area crossed by arrows are the Ti enriched areas which are located in the boundaries between the adjacent primary particles.

[0123] Figure 1c and 2c are the EDS line scan of the area crossed by arrows displaying concentration of Ti each for Figure lb and 2b, respectively. The arrows also indicate the scanning direction. Ti enrichment is clearly seen from each figure showing thickness of grain boundary in the range of 20 - 50 nm. Figure 1c shows that the ratio of the maximum Ti concentration in the grain boundary, relative to Ni, Mn, Co and Ti, to the concentration of Ti obtained by ICP-OES was about 20. Figure 2c shows that the ratio of the maximum Ti concentration in the grain boundary, relative to Ni, Mn, Co and Ti, to the concentration of Ti obtained by ICP-OES was about 14. This high ratio indicates that Ti was significantly enriched in the grain boundary since Ti was added after washing EX2 with an aqueous solution.

Claims

CLAIMS1. A positive electrode active material powder suitable for lithium-ion rechargeable batteries, comprising secondary particles comprising a plurality of primary particles, wherein the positive electrode active material powder comprises a first lithium metal oxide comprising lithium, M', and oxygen, wherein the first lithium metal oxide has a layered o- NaFeO? structure; wherein M' comprises Ti and at least one element selected from the group consisting of nickel and manganese; wherein a grain boundary is present between adjacent primary particles of the secondary particles; wherein a concentration of Ti in the grain boundary is greater than a concentration of Ti in the adjacent primary particles; wherein the positive electrode active material powder has a surface area between 0.5 m2 / g and 1.2 m2 / g as determined by BET measurement; and wherein a ratio of a maximum concentration of Ti in the grain boundary, relative to M', to a concentration of Ti obtained by ICP-OES is 5 or more.

2. The positive electrode active material according to claim 1, wherein a thickness of the grain boundary is between 5 to 100 nm, preferably 10 to 80 nm, and more preferably 20 to 50 nm.

3. The positive electrode active material powder according to claim 1 or 2, wherein M' comprises:Ni in a content x, wherein 70.0 < x < 100.0 at%, relative to M', Mn in a content y, wherein 0.0 < y < 11.0 at%, relative to M', Co in a content z, wherein 0.0 < z < 11.0 at%, relative to M',- Ti in a content a, wherein 0.0 < a < 2.0 at%, relative to M',- Al in a content b, wherein 0.0 < b < 2.0 at%, relative to M',Zr in a content c, wherein 0.0 < c < 2.0 at%, relative to M',D in a content d, wherein 0.0 < d < 2.0 at%, relative to M', wherein D is at least one element selected from the group consisting of B, Ba, Ca, Ce, Cr, Fe, La, Mg, Mo, Nb, S, Sr, V, W, Y, and Zn; wherein x, y, z, a, b, c, and d are measured by ICP-OES; and wherein x+y+z+a+b+c+d is 100.0 at%.

4. The positive electrode active material powder according to claim 1 or 2, wherein M' comprises:Ni in a content x, wherein 0 < x < 30. 0 at%, relative to M',Mn in a content y, wherein 50.0 < y < 100.0 at%, relative to M', Co in a content z, wherein 0.0 < z < 12.0 at%, relative to M',- Ti in a content a, wherein 0.0 < a < 2.0 at%, relative to M',- Al in a content b, wherein 0.0 < b < 2.0 at%, relative to M',Zr in a content c, wherein 0.0 < c < 2.0 at%, relative to M',D in a content d, wherein 0.0 < d < 2.0 at%, relative to M', wherein D is at least one element selected from the group consisting of B, Ba, Ca, Ce, Cr, Fe, La, Mg, Mo, Nb, S, Sr, V, W, Y, and Zn; wherein x, y, z, a, b, c, and d are measured by ICP-OES; and wherein x+y+z+a+b+c+d is 100.0 at%.

5. The positive electrode active material powder according to any one of the previous claims, wherein the first lithium metal oxide comprises LiOH in a content of at most 0.7 wt.% relative to a total weight of the first lithium metal oxide, and wherein a content of LiOH is measured by acid-base titration.

6. The positive electrode active material powder according to any one of the previous claims, wherein the first lithium metal oxide comprises U2CO3 in a content of at most 0.3 wt.% relative to a total weight of the first lithium metal oxide, and wherein a content of U2CO3 is measured by acid-base titration.

7. The positive electrode active material powder according to any one of the previous claims, wherein 0.0 < a < 1.0 at%.

8. The positive electrode active material powder according to any one of the previous claims, wherein 0.01 < b < 5.0 at% and 0.01 < c < 5.0 at%.

9. The positive electrode active material powder according to any one of the previous claims, wherein the surface area is between 0.6 m2 / g and 1.1 m2 / g.

10. A method for manufacturing of the positive electrode active material powder according to any one of the previous claims, wherein said method comprises consecutive steps of:Step 1) mixing a second lithium metal oxide comprising lithium, M", and oxygen with an aqueous solution to obtain a slurry, filtering, and then drying said slurry to obtain a dried powder, wherein M" is identical to M' except for Ti,Step 2) mixing the dried powder with a compound comprising Ti, preferably TiC>2, wherein said compound comprises Ti in an amount between 300 ppm to 4000 ppm with respect to the weight of the dried powder, to obtain a mixture, andStep 3) heating the mixture in an oxidizing atmosphere at a temperature between 450°C and 650°C so as to obtain the positive electrode active material powder.

11. The method according to claim 10, wherein said Ti is in an amount between 500 ppm and 3000 ppm.

12. The method according to claim 11 or 12, wherein the temperature in Step 3) is between 500 and 600°C.

13. The method according to any one of claims 10 to 12, wherein the second lithium metal oxide further comprises Al and Zr.

14. A battery comprising the positive electrode active material powder according to any one of claims 1 to 9.

15. Use of the battery according to claim 14 in an electric vehicle or in a hybrid electric vehicle.

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