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

Abstract

A positive electrode active material powder for lithium-ion rechargeable batteries, comprising: a first fraction of polycrystalline particles, each consisting of more than twenty primary particles; a second fraction of first particles and a third fraction of second particles, the first and second particles each consisting of one to twenty primary particles, the polycrystalline particles comprising: Ni in a content of 60.0 to 100.0 at%, Co in a content of 0.0 to 20.0 at%, and Mn in a content of 0.0 to 20.0 at%; the first particles comprising: Ni in a content of 60.0 to 100.0 at%, Co in a content of 0.0 to 20.0 at%, and Mn in a content of 0.0 to 20.0 at%; the second particles comprising: Ni in a content of 0.0 to 10.0 at%, Co in a content of 80.0 to 100.0 at%, and Mn in a content of 0.0 to 10.0 at%.

Description

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

[0002] The present invention relates to a positive electrode active material for batteries comprising lithium, oxygen, nickel, cobalt and manganese. This invention also relates to a method for manufacturing a positive electrode active material, a battery comprising said positive electrode active material and the use of said battery.

[0003] BACKGROUND

[0004] As the development of small and lightweight electronic products, electronic devices, communication devices and the like has advanced rapidly and a need for electric vehicles has widely emerged with respect to environmental issues, there is a demand for improvement of performance of secondary batteries used as power sources for these products. Among these, a lithium secondary battery has come into the spotlight as a high-performance battery due to the high energy density and a high reference electrode potential.

[0005] During the charging process of a secondary battery, lithium ions are removed from the positive electrode, transported through the electrolyte, and are inserted into the anode while electrons are removed from the positive electrode and injected into the anode through an external circuit (charger). During the use or discharge of a secondary battery, lithium ions are removed from the anode, transported through the electrolyte, and are inserted into the positive electrode, while electrons flow through an external circuit to provide electric work.

[0006] Commonly used positive electrode active materials are lithium transition metal oxides. During the charging and / or discharging of the lithium battery, the delithiated positive electrode active material can slowly react with electrolyte leading to a gradual degradation of the electrochemical performance of lithium batteries using such positive electrode active materials.

[0007] There is therefore a need to limit this gradual degradation and thereby improve the cycle life of batteries, in other words the number of charge / discharge cycles, while still having sufficient capacity.

[0008] It is well known that not only the chemical composition of the positive electrode active material, but also the physical characteristics of the particles of the positive electrode active material play a role in determining the performance of a battery.

[0009] For instance, in WO2019166350 a mixture is made of two kinds of positive electrode active materials both containing Li, Ni, Co and Mn: a relatively coarse, polycrystalline powder and a relatively fine monolithic powder with similar compositions. In addition, when a positive electrode is manufactured by applying a slurry containing a positive electrode active material onto both sides of a positive electrode current collector followed by passing the slurry-applied collector through two pressing rolls, it is preferable to lower a nip pressure, which is a repulsive force against applied pressure when the electrode passes through the two pressing rolls during calendaring, since high nip pressure may cause damage to the positive electrode.

[0010] It is an object of the present invention to provide a positive electrode active material having an improved cycle life and causing low nip pressure of a positive electrode.

[0011] It is another object of the present invention to provide a method for manufacturing a positive electrode active material having an improved cycle life and causing low nip pressure of a positive electrode.

[0012] It is another object of the present invention to provide a battery comprising said positive electrode active material.

[0013] It is another object of the present invention to provide a use of said battery.

[0014] SUMMARY OF THE INVENTION

[0015] The object of the invention is achieved by providing a positive electrode active material powder for a lithium-ion rechargeable battery comprising: a first fraction of polycrystalline particles, wherein each of the polycrystalline particles consists of more than twenty primary particles as observed in a SEM image; a second fraction of first particles, wherein each of the first particles consists of at least one primary particle and at most twenty primary particles as observed in a SEM image; and a third fraction of second particles, wherein each of the second particles consists of at least one primary particle and at most twenty primary particles as observed in a SEM image, wherein the polycrystalline particles comprise: Ni in a content xl, wherein 60.0 at% < xl < 100.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles, Co in a content yl, wherein 0.0 at% < yl < 20.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles, and Mn in a content zl, wherein 0.0 at% < zl < 20.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles; wherein the first particles comprise: Ni in a content x2, wherein 60.0 at% < x2 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the first particles, Co in a content y2, wherein 0.0 at% < y2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the first particles, and Mn in a content z2, wherein 0.0 at% < z2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the first particles; and wherein the second particles comprise: Ni in a content x3, wherein 0.0 at% < x3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the second particles, Co in a content y3, wherein 80.0 at% < y3 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the second particles, and Mn in a content z3, wherein 0.0 at% < z3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the second particles.

[0016] Alternatively, the object of the invention is achieved by a positive electrode active material powder for a lithium-ion rechargeable battery comprising: a first fraction of particles, wherein an average equivalent diameter of the particles in the first fraction is between 8.0 pm and 18.0 pm, as determined by SEM image analysis, a second fraction of particles, wherein an average equivalent diameter of the particles in the second fraction is between 2.0 pm and 8.0 pm, as determined by SEM image analysis.; and a third fraction of particles, wherein an average equivalent diameter of the particles in the third fraction is between 1.0 pm and 5.0 pm, as determined by SEM image analysis, wherein the particles in the first fraction comprise: Ni in a content xl, wherein 60.0 at% < xl < 100.0 at%, relative to a total amount of Ni, Co and Mn in the particles in the first fraction, Co in a content yl, wherein 0.0 at% < yl < 20.0 at%, relative to a total amount of Ni, Co and Mn the particles in the first fraction, and Mn in a content zl, wherein 0.0 at% < zl < 20.0 at%, relative to a total amount of Ni, Co and Mn the particles in the first fraction; wherein the particles in the second fraction comprise: Ni in a content x2, wherein 60.0 at% < x2 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the particles in the second fraction, Co in a content y2, wherein 0.0 at% < y2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the particles in the second fraction, and Mn in a content z2, wherein 0.0 at% < z2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the particles in the second fraction; and wherein the particles in the third fraction comprise: Ni in a content x3, wherein 0.0 at% < x3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the particles in the third fraction, Co in a content y3, wherein 80.0 at% < y3 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the particles in the third fraction, and Mn in a content z3, wherein 0.0 at% < z3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the particles in the third fraction.

[0017] The positive electrode active material of the invention has a smaller deterioration of its capacity per charge / discharge cycle, resulting in a longer battery life while causing low nip pressure of a positive electrode.

[0018] The term 'at%' means 'atomic percent'. This is equivalent to the better-known concept of 'molar percent'. The term 'at%' is used because formally the SEM-EDS method, which is the method used to determine compositions in this document, reports results as relative abundance of atoms.

[0019] In a further aspect the invention provides a method for preparing a positive electrode active material powder for a lithium-ion rechargeable battery, preferably the positive electrode active material powder according to the invention, comprising mixing a first fraction, a second fraction and a third fraction, wherein: the first fraction comprises polycrystalline particles, wherein each of the polycrystalline particles consists of more than 20 primary particles as observed in a SEM image; the second fraction comprises first particles, wherein each of the first particles consists of at least one primary particle and at most twenty primary particles as observed in a SEM image; and the third fraction comprises second particles, wherein each of the second particles consists of at least one primary particle and at most twenty primary particles as observed in a SEM image, wherein the polycrystalline particles comprise: Ni in a content xl, wherein 60.0 at% < xl < 100.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles, Co in a content yl, wherein 0.0 at% < yl < 20.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles, and Mn in a content zl, wherein 0.0 at% < zl < 20.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles; wherein the first particles comprise: Ni in a content x2, wherein 60.0 at% < x2 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the first particles, Co in a content y2, wherein 0.0 at% < y2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the first particles, and Mn in a content z2, wherein 0.0 at% < z2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the first particles; and wherein the second particles comprise: Ni in a content x3, wherein 0.0 at% < x3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the second particles, Co in a content y3, wherein 80.0 at% < y3 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the second particles, and Mn in a content z3, wherein 0.0 at% < z3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the second particles.

[0020] In a further aspect the invention provides a battery comprising said positive electrode active material.

[0021] In a further aspect the invention provides a use of said battery in an electric vehicle or in a hybrid electric vehicle.

[0022] DETAILED DESCRIPTION OF THE INVENTION

[0023] In the following detailed description, preferred embodiments are described in detail to enable 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. To the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description.

[0024] The term "comprising", as used in this document, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It needs to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a composition comprising components A and B" should not be limited to compositions consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the composition are A and B. Accordingly, the terms "comprising" and "including" encompass the more restrictive terms "consisting essentially of" and "consisting of".

[0025] The term "a positive electrode active material", also known as cathode active material, as in this document is defined as a material which is electrochemically active in a positive electrode or cathode. By active material, it must be understood to be a material capable to capture and release Li ions when subjected to a voltage change over a predetermined period of time.

[0026] The term "a positive electrode" as used in this document is defined as a material comprising a positive electrode active material also in addition to other components added to the positive electrode active material, which are not electrochemically active, in particular conductivity agents such as carbon black or binders such as PVDF.

[0027] In a preferred embodiment the average equivalent diameter of the polycrystalline particles in the first fraction is at least 8.0 pm, preferably at least 10.0 pm and more preferably at least 12.0 pm, as determined by SEM image analysis.

[0028] In a preferred embodiment the average equivalent diameter of the polycrystalline particles in the first fraction is at most 18.0 pm, preferably at most 16.0 pm and more preferably at most 15.0 pm, as determined by SEM image analysis.

[0029] In a preferred embodiment the average equivalent diameter of the first particles in the second fraction is at least 2.0 pm, preferably at least 3.0 pm and more preferably at least 3.5 pm, as determined by SEM image analysis.

[0030] In a preferred embodiment the average equivalent diameter of the first particles in the second fraction is at most 8.0 pm, preferably at most 7.0 pm and more preferably at most 6.5 pm, as determined by SEM image analysis.

[0031] In a preferred embodiment the average equivalent diameter of the second particles in the third fraction is at least 1.0 pm, preferably at least 2.0 pm and more preferably at least 2.5 pm, as determined by SEM image analysis. In a preferred embodiment the average equivalent diameter of the second particles in the third fraction is at most 5.0 pm, preferably at most 4.0 pm and more preferably at most 3.5 pm, as determined by SEM image analysis.

[0032] In a preferred embodiment a weight ratio of the first fraction relative to the positive electrode active material powder is at least 50.0 wt%, preferably at least 55.0 wt% and more preferably at least 60.0 wt%.

[0033] In a preferred embodiment a weight ratio of the first fraction relative to the positive electrode active material powder is at most 95.0 wt%, preferably at most 85.0 wt% and more preferably at most 80.0 wt%.

[0034] In a preferred embodiment a weight ratio of the second fraction relative to the positive electrode active material powder is at least 2.5 wt%, preferably at least 7.5 wt% and more preferably at least 10.0 wt%.

[0035] In a preferred embodiment a weight ratio of the second fraction relative to the positive electrode active material powder is at most 25.0 wt%, preferably at most 22.5 wt% and more preferably at most 20.0 wt%.

[0036] In a preferred embodiment a weight ratio of the third fraction relative to the positive electrode active material powder is at least 2.5 wt%, preferably at least 7.5 wt% and more preferably at least 10.0 wt%.

[0037] In a preferred embodiment a weight ratio of the third fraction relative to the positive electrode active material powder is at most 25.0 wt%, preferably at most 22.5 wt% and more preferably at most 20.0 wt%.

[0038] In a preferred embodiment a weight ratio of the total of the first fraction and the second fraction and the third fraction relative to the positive electrode active material powder is at least 95.0%, preferably at least 99.0%, and most preferably 100%.

[0039] In a preferred embodiment, 65.0 at% < xl < 99.0 at%, 0.5 at% < yl < 17.5 at%, and 0.5 at% < zl < 17.5 at%.

[0040] In a preferred embodiment, 65.0 at% < x2 < 99.0 at%, 0.5 at% < y2 < 17.5 at%, and 0.5 at% < z2 < 17.5 at%. In a preferred embodiment, 0.0 at% < x3 < 2.5 at%, 95.0 at% < y3 < 100.0 at%, and 0.0 at% < z3 < 2.5 at%.

[0041] In a preferred embodiment the particles in the first fraction are represented by formula (I):

[0042] LiaiNibiCociMndiO2 (I) wherein 0.90 < al < 1.10, preferably 0.92 < al < 1.08, more preferably 0.95 < al < 1.05, wherein 0.60 < bl < 1.00, preferably 0.65 < bl < 0.99, more preferably 0.70 < bl < 0.98, wherein 0.00 < cl < 0.20, preferably 0.05 < cl < 0.17, more preferably 0.08 < cl < 0.15, wherein 0.00 < dl < 0.20, preferably 0.05 < dl < 0.17, more preferably 0.08 < dl < 0.15, and wherein bl+cl+dl = 1.00.

[0043] In a preferred embodiment the particles in the second fraction are represented by formula (II):

[0044] Lia2Nib2COc2Mnd2O2 (II) wherein 0.90 < a2 < 1.10, preferably 0.92 < a2 < 1.08, more preferably 0.95 < a2 < 1.05, wherein 0.60 < b2 < 1.00, preferably 0.65 < b2 < 0.99, more preferably 0.70 < b2 < 0.98, wherein 0.00 < c2 < 0.20, preferably 0.05 < c2 < 0.17, more preferably 0.08 < c2 < 0.15, wherein 0.00 < d2 < 0.20, preferably 0.05 < d2 < 0.17, more preferably 0.06 < d2 < 0.15, and wherein b2+c2+d2= 1.00.

[0045] In a preferred embodiment the particles in the third fraction are represented by formula (III): Lia3Nib3COc3Mnd3O2 (III) wherein 0.90 < a3 < 1.20, preferably 0.92 < a3 < 1.15, more preferably 0.95 < a3 < 1.12, wherein 0.00 < b3 < 0.10, preferably 0.00 < b3 < 0.03, more preferably 0.00 < b3 < 0.02, wherein 0.80 < c3 < 1.00, preferably 0.95 < c3 < 1.00, more preferably 0.98 < c3 < 1.00, wherein 0.00 < d3 < 0.10, preferably 0.00 < d3 < 0.03, more preferably 0.00 < d3 < 0.02, and wherein b3+c3+d3= 1.00.

[0046] In a preferred embodiment xl, x2, x3, yl, y2, y3, zl, z2 and z3 are as measured by scanning electron microscopy energy-dispersive X-ray spectroscopy.

[0047] In a preferred embodiment of the method the polycrystalline particles in the first fraction have a D50 which is between 8.0 pm and 18.0 pm as measured by laser diffraction, the first particles in the second fraction have a D50 which between 2.0 pm and 8.0 pm, and the second particles in the third fraction have a D50 which is between 1.0 pm and 5.0 pm, as measured by laser diffraction.

[0048] BRIEF DESCRIPTION OF THE FIGURES

[0049] Figure 1 is a SEM image of EXI. EXPERIMENTAL TESTS USED IN THE EXAMPLES

[0050] The following analysis methods are used in the Examples:

[0051] A) Scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM- EDS) analysis

[0052] The average equivalent diameter and the content of Ni, Mn, and Co based on a total amount of the transition metals of the positive electrode active material are analyzed by a scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS) technique. In particular, the average equivalent diameter is measured by the method according to Section C). A particle with a diameter around D50 value as measured by PSD according to Section B) is selected for analysis for each of the components. The SEM-EDS is performed on a JEOL JSM 7100F SEM equipment with a 50mm2X-MaxNEDS sensor from Oxford instruments. The particles in the image should be well distributed therefore avoiding overlap between particles. This can be achieved by pouring a small amount of powder sample to the adhesive attached on the SEM sample holder and blowing air to remove the excess powder.

[0053] First, second, and third fractions of the positive electrode active material are determined according to the criteria indicated in Table 1 below.

[0054] The number of primary particles is determined from observation of the SEM image where a primary particle count can be performed by the operator. An EDS mapping image can be used to assist the classifications of the fractions wherein a contrast color code can be assigned to distinguish Ni rich particles (which belong to the first fraction or the second fraction) and Co rich particle (which belong to the third fraction). The metal composition is then determined through EDS point scan analysis at the center of the isolated particle.

[0055] Finally, each metal composition is averaged from at least three different particles.

[0056] Table 1. Assignment of positive electrode active material fraction B) Particle size distribution (PSD) analysis

[0057] The particle size distribution (PSD) of the positive electrode active material powder is measured by laser diffraction particle size analysis using a Malvern Mastersizer 3000 with a Hydro MV wet dispersion accessory after having dispersed each of the powder samples in an aqueous medium. To improve the dispersion of the powder, 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 obtained from the Malvern Mastersizer 3000 with Hydro MV measurements.

[0058] C) Average equivalent diameter analysis

[0059] The average equivalent diameter of particle is calculated by using ImageJ software (ImageJ 1.52a, National Institutes of Health, USA) according to the following steps:

[0060] Step 1) Open the file containing SEM image of positive electrode active material as obtained in section A) with 2,000 times magnification.

[0061] Step 2) Set scale according to the SEM magnification.

[0062] Step 3) Draw lines following particle edges using 'polygon selections' tool for at least 3 particles that are not overlapped with another particles. The particles at the edges of image are to be excluded if truncated.

[0063] Step 4) Measure the area of the drawn particles selected from Set Measurements and Area box.

[0064] Step 5) Repeat step 1 to 4 to obtain area of particles from 3 different SEM images.

[0065] Step 6) Calculated the particle diameter of each measured area by assuming the particle in the spherical shape following d = 2 x and obtain the average equivalent diameter for at least 3 particles.

[0066] D) Coin cell testing

[0067] DI) Coin cell preparation

[0068] 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. D2)Testing method

[0069] 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).

[0070] The schedule uses a 1C current definition of 220 mA / g in the 4.3 V / Li to 3.0 V / Li metal window range. The capacity fading rate (QF) is obtained according to below an equation below wherein DQ1 is the discharge capacity at the first cycle.

[0071] QF (% / 100 cycles') = 100 100

[0072] Table 2. Cycling schedule for coin cell testing method

[0073] E) Nip pressure measurement

[0074] A mixture composed of positive electrode active material, Super-P Li (IMERYS) as positive electrode conductive agents, H-NBR (ARANXEO) as additive, and polyvinylidene fluoride (PVDF S5130, Solvay) as a positive electrode binder are added to N-methyl-2-pyrrolidone (NMP) as a dispersion medium so that the mass ratio of the positive electrode active material powder: the positive electrode conductive agents: additive: binder is set at 94.9:3.0:0.15:2.0. Thereafter, the mixture is kneaded to prepare a positive electrode mixture slurry. The resulting positive electrode mixture slurry is then applied onto both sides of a positive electrode current collector, made of a 20 pm thick aluminum foil. The width of the applied area is 88.5 mm and the length is around 420 mm. Typical loading weight of a positive electrode active material is about 16.5±1 mg / cm2. The electrode is then dried and calendared to obtain the electrode density of 3.5 g / cm3.

[0075] A nip pressure is defined as a repulsive force against applied pressure when the electrode passes through the two pressing rolls during calendaring. The pressing rolls are installed vertically wherein the upper roll position is fixed and the bottom roll is movable. Two load cells (CDES-500, Bongshin Loadcell Co. Ltd.) are installed at each side of the bottom roll (right and left) to measure the force when an electrode is passed through the rolls. The force is recorded for around 100 mm length of electrode with measurement interval of 20 mm. Nip pressure is calculated by dividing the average force by the width of the applied area and is expressed in kgf / cm.

[0076] F) Single layer pouch cell testing

[0077] Fl) Single layer pouch cell preparation

[0078] 33 mAh pouch-type cells are prepared as follows: the positive electrode active material powder, Li-435 (Denka) as positive electrode conductive agents, and polyvinylidene fluoride (PVDF S5130, Solvay) as a positive electrode binder are added to N-methyl-2-pyrrolidone (NMP) as a dispersion medium so that the mass ratio of the positive electrode active material powder, the positive electrode conductive agents: PVDF: positive electrode binder is set at 96.4: 1.8: 1.8. Thereafter, the mixture is mixed to prepare a positive electrode mixture slurry. The resulting positive electrode mixture slurry is then applied onto one side of a positive electrode current collector, made of a 20 pm thick aluminum foil. The positive electrode is punched to obtain a sheet with total area of 11.7 cm2. Typical loading weight of a positive electrode active material is about 16.0±l mg / cm2. The electrode is then dried and calendared. In addition, an aluminum plate serving as a positive electrode current collector tab is arc- welded to an end portion of the positive electrode.

[0079] Commercially available negative electrodes are used. In short, a mixture of natural graphite: carbon (Super P, Imerys): carboxy-methyl-cellulose-sodium: and styrene-butadiene-rubber in a mass ratio of 95.5: 1.0: 1.5:2.0, is applied on one side of a copper foil. A nickel plate serving as a negative electrode current collector tab is arc-welded to an end portion of the negative electrode. Typical loading weight of a negative electrode active material is about 10 ± 1 mg / cm2.

[0080] Non-aqueous electrolyte is obtained by dissolving lithium hexafluorophosphate (LiPF6) salt at a concentration of 1.2 mol / L in a mixed solvent of ethylene carbonate (EC): ethyl methyl carbonate (EMC): and diethyl carbonated (DEC) in a volume ratio of 1: 1 : 1. It contains 1.0 wt% lithium difluorophosphate (LiPO2F2), and 1.0 wt% vinylene carbonate (VC) as additives. A sheet of the positive electrode, a sheet of the negative electrode, and a sheet of the microporous polymer separator (13 pm) interposed between them. The assembly and the electrolyte are then put in an aluminum laminated pouch in a dry room with dew point of - 50°C, so that a flat pouch-type lithium secondary battery is prepared. The design capacity of the secondary battery is 33 mAh when charged to 4.2 V. The cell testing procedure uses a 1 C current definition of 33 mA / g.

[0081] F2) Cycle life test

[0082] F2-1) Pre-charging and formation

[0083] The non-aqueous electrolyte solution is impregnated into the prepared cell for 12 hours at room temperature. The cell is pre-charged with the current of 0.1 C until 30% of its theoretical capacity at room temperature. The cell is then degassed using a pressure of -760 mmHg for 30 seconds and the aluminum pouch is sealed. During measurement, the pouch is assembled in a press jig provided with silicon pad.

[0084] The battery is charged with a current of 0.33 C in CC mode (constant current) up to 4.2 V and CV mode (constant voltage) until a cut-off current of C / 20 is reached. The battery is discharged with a current of 0.33 C in CC mode down to 2.5 V. The charge discharge process is repeated for 3 times.

[0085] F2-2) Cycle life test

[0086] The cells are charged and discharged continuously under the following conditions at 45°C, to determine their charge-discharge cycle performance:

[0087] - Charge is performed in CC mode under 1 C rate up to 4.2 V, then CV mode until C / 20 is reached,

[0088] - The cell is then set to rest for 10 minutes,

[0089] - Discharge is done in CC mode at 1 C rate down to 2.5 V,

[0090] - The cell is then set to rest for 10 minutes,

[0091] - The charge-discharge cycles proceed until 400 cycles. Every 100 cycles, the discharge is done at 0.33 C rate in CC mode down to 2.5 V.

[0092] The internal resistance or direct current resistance (DCR) is measured at 2.5 C for 30 seconds at the beginning of every 100 cycles repetition and the end of 400thcycles at room temp. The cycle life is defined as the number of charge-discharge cycles when the capacity degrades to 80%.

[0093] EXAMPLES

[0094] The present invention is further illustrated in the following examples: Comparative Example 1

[0095] A positive electrode active material, further called CEX1, was prepared according to the following steps:

[0096] Step 1) Preparing powder A, which is a positive electrode active material comprising polycrystalline particles consist of more than twenty primary particles as observed in a SEM image, according to below steps: a) Co-precipitation: a transition metal-based precursor with metal composition of Ni0.80Mn0.10Co0.10 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. b) Mixing : the transition metal-based precursor is mixed with LiOH in an industrial blender to obtain a mixture having a lithium to metal (Ni, Mn, and Co) ratio of 1.01. c) Heating : the mixture is heated at 815°C for 8 hours in an oxidizing atmosphere. The heated powder is crushed, classified, and sieved to obtain a powder A having D50 of 12.4 pm.

[0097] Step 2) Preparing powder B, which is a positive electrode active material comprising particles consist of at least one primary particle and at most twenty primary particles as observed in a SEM image, according to below steps: a) Co-precipitation: a transition metal-based precursor with metal composition of Ni0.85Mn0.07Co0.08 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. b) Mixing : the transition metal-based precursor is mixed with LiOH in an industrial blender to obtain a mixture having a lithium to metal (Ni, Mn, and Co) ratio of 0.96. c) Heating : the mixture is heated at 890°C for 10 hours in an oxidizing atmosphere to obtain a heated product. d) Wet bead milling: The heated product is bead milled in a solution containing 0.5 mol% Co with respect to the total molar contents of Ni, Mn, and Co in the heated product followed by drying and sieving process to obtain a milled product. The bead milling solid to solution weight ratio was 6:4 and conducted for 20 minutes to obtain a powder B having a D50 of 3.1 pm.

[0098] Step 3) Preparing CEX1 by mixing 105 grams of powder A and 45 grams of powder B. The mixing ratio of a first powder and a second powder is 70: 30. Comparative Example 2

[0099] A positive electrode active material, further called CEX2, is prepared according to the following steps:

[0100] Step 1) Preparing powder C, which is a positive electrode active material comprising particles consist of at least one primary particle and at most twenty primary particles as observed in a SEM image, according to below steps: a) First heating: CO3O4 powder is heated at 550°C for 10 hours in a dry air to obtain a first heated powder. b) Mixing: the first heated powder is mixed with LiOH in an industrial blender to obtain a mixture having a lithium to cobalt ratio of 1.10. c) Second heating: the mixture is heated at 675°C for 10 hours in a dry air to obtain a second heated product. d) Wet milling: the second heated product is ball milled in water with solid to solution weight ratio of 1 : 1 for 40 hours using 1mm Zr balls followed by drying at 150°C for 15 hours to obtain a powder C having a D50 of 1.36 pm.

[0101] Step 2) Preparing CEX2 by mixing 105 grams of powder A and 45 grams of powder C. The mixing ratio of a powder A and a powder C is 70:30.

[0102] Example 1

[0103] A positive electrode active material, further called EXI, is prepared by mixing 105 grams of powder A, 22.5 grams of powder B and 22.5 grams of powder C. The mixing ratio of a powder A: a powder B: a powder C is 70: 15: 15.

[0104] Results

[0105] Table 3 summarizes average equivalent diameter calculation and average metal composition for powder A, B, and C.

[0106] Table 3. Summary of the average equivalent diameter and average metal composition for powder A, B, and C Table 4 summarizes the mixing ratio and nip pressure of examples and comparative examples, and their corresponding electrochemical property. The nip pressure result shows the repulsive force of positive electrode against the applied pressure. The lowest nip pressure indicates less damage to the positive electrode.

[0107] Also, the electrochemical property of the positive electrode is improved as indicated by the low capacity fading rate (QF) as measured by coin cell test. Accordingly, the cycle life at 45°C as measured by a single layer pouch cell test is improved for EXI which is a mixture of powder A, powder B, and powder C.

[0108] Table 4. Summary of the mixing ratio, nip pressure, and the corresponding electrochemical properties of example and comparative examples.

Claims

CLAIMS1. A positive electrode active material powder for a lithium-ion rechargeable battery comprising: a first fraction of polycrystalline particles, wherein each of the polycrystalline particles consists of more than twenty primary particles as observed in a SEM image; a second fraction of first particles, wherein each of the first particles consists of at least one primary particle and at most twenty primary particles as observed in a SEM image; and a third fraction of second particles, wherein each of the second particles consists of at least one primary particle and at most twenty primary particles as observed in a SEM image, wherein the polycrystalline particles comprise:Ni in a content xl, wherein 60.0 at% < xl < 100.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles,Co in a content yl, wherein 0.0 at% < yl < 20.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles, andMn in a content zl, wherein 0.0 at% < zl < 20.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles; wherein the first particles comprise:Ni in a content x2, wherein 60.0 at% < x2 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the first particles,Co in a content y2, wherein 0.0 at% < y2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the first particles, andMn in a content z2, wherein 0.0 at% < z2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the first particles; and wherein the second particles comprise:Ni in a content x3, wherein 0.0 at% < x3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the second particles,Co in a content y3, wherein 80.0 at% < y3 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the second particles, andMn in a content z3, wherein 0.0 at% < z3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the second particles.

2. The positive electrode active material powder according to claim 1, wherein an average equivalent diameter of the polycrystalline particles in the first fraction is between 8.0 pm and 18.0 pm, preferably between 10.0 pm and 16.0 pm, more preferably between 12.0 pm and 15.0 pm, as determined by SEM image analysis.

3. The positive electrode active material powder according to claim 1 or 2, wherein an average equivalent diameter of the first particles in the second fraction is between 2.0 pm and 8.0 pm, preferably between 3.0 pm and 7.0 pm, more preferably between 3.5 pm and 6.5 pm, as determined by SEM image analysis.

4. The positive electrode active material powder according to any one of the previous claims, wherein an average equivalent diameter of the second particles in the third fraction is between 1.0 pm and 5.0 pm, preferably between 2.0 pm and 4.0 pm, more preferably between 2.5 pm and 3.5 pm, as determined by SEM image analysis.

5. The positive electrode active material powder according to any one of the previous claims, wherein a weight ratio of the first fraction relative to the positive electrode active material powder is between 50.0 wt% and 95.0 wt%, preferably between 55.0 wt% and 85.0 wt%, more preferably between 60.0 wt% and 80.0 wt%.

6. The positive electrode active material powder according to any one of the previous claims, wherein a weight ratio of the second fraction relative to the positive electrode active material powder is between 2.5 wt% and 25.0 wt%, preferably between 7.5 wt% and 22.5 wt%, and more preferably between 10.0 wt% and 20.0 wt%.

7. The positive electrode active material powder according to any one of the previous claims, wherein a weight ratio of the third fraction relative to the positive electrode active material powder is between 2.5 wt% and 25.0 wt%, preferably between 7.5 wt% and 22.5 wt%, and more preferably between 10.0 wt% and 20.0 wt%.

8. The positive electrode active material powder according to any one of the previous claims, wherein 65.0 at% < xl < 99.0 at%, 0.5 at% < yl < 17.5 at%, and 0.5 at% < zl < 17.5 at%.

9. The positive electrode active material powder according to any one of the previous claims, wherein 65.0 at% < x2 < 99.0 at%, 0.5 at% < y2 < 17.5 at%, and 0.5 at% < z2 < 17.5 at%.

10. The positive electrode active material powder according to any one of the previous claims, wherein 0.0 at% < x3 < 2.5 at%, 95.0 at% < y3 < 100.0 at%, and 0.0 at% < z3 < 2.5 at%.

11. The positive electrode active material powder according to any one of the previous claims, wherein the polycrystalline particles in the first fraction are represented by formula (I):LiaiNibiCociMndiO2 (I) wherein 0.90 < al < 1.10, preferably 0 .92 < al < 1.08, more preferably 0.95 < al< 1.05, wherein 0.60 < bl < 1.00, preferably 0 .65 < bl < 0.99, more preferably 0.70 < bl< 0.98, wherein 0.00 < cl < 0.20, preferably 0 .05 < cl < 0.17, more preferably 0.08 < cl< 0.15, wherein 0.00 < dl < 0.20, preferably 0 .05 < dl < 0.17, more preferably 0.08 < dl< 0.15, and wherein bl+cl+dl = 1.00.

12. The positive electrode active material powder according to any one of the previous claims, wherein the first particles in the second fraction are represented by formula (II): Lia2Nib2COc2Mnd2O2 (II) wherein 0.90 < a2 < 1.10, preferably 0.92 < a2 < 1.08, more preferably 0.95 < a2< 1.05, wherein 0.60 < b2 < 1.00, preferably 0.65 < b2 < 0.99, more preferably 0.70 < b2< 0.98, wherein 0.00 < c2 < 0.20, preferably 0.05 < c2 < 0.17, more preferably 0.08 < c2< 0.15, wherein 0.00 < d2 < 0.20, preferably 0.05 < d2 < 0.17, more preferably 0.06 < d2< 0.15, and wherein b2+c2+d2=1.00.

13. The positive electrode active material powder according to any one of the previous claims, wherein the second particles in the third fraction are represented by formula (III): Lia3Nib3COc3Mnd3O2 (III) wherein 0.90 < a3 < 1.20, preferably 0.92 < a3 < 1.15, more preferably 0.95 < a3< 1.12, wherein 0.00 < b3 < 0.10, preferably 0.00 < b3 < 0.03, more preferably 0.00 < b3< 0.02, wherein 0.80 < c3 < 1.00, preferably 0.95 < c3 < 1.00, more preferably 0.98 < c3< 1.00, wherein 0.00 < d3 < 0.10, preferably 0.00 < d3 < 0.03, more preferably 0.00 < d3< 0.02, and wherein b3+c3+d3=1.00.

14. A method for preparing a positive electrode active material powder for a lithium-ion rechargeable battery, preferably the positive electrode active material powder accordingto any one of the previous claims, comprising mixing a first fraction, a second fraction and a third fraction, wherein: the first fraction comprises polycrystalline particles, wherein each of the polycrystalline particles consists of more than twenty primary particles as observed in a SEM image; the second fraction comprises first particles, wherein each of the first particles consists of at least one primary particle and at most twenty primary particles as observed in a SEM image; and the third fraction comprises second particles, wherein each of the second particles consists of at least one primary particle and at most twenty primary particles as observed in a SEM image, wherein the polycrystalline particles comprise:Ni in a content xl, wherein 60.0 at% < xl < 100.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles,Co in a content yl, wherein 0.0 at% < yl < 20.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles, andMn in a content zl, wherein 0.0 at% < zl < 20.0 at%, relative to a total amount of Ni, Co and Mn in the polycrystalline particles; wherein the first particles comprise:Ni in a content x2, wherein 60.0 at% < x2 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the first particles,Co in a content y2, wherein 0.0 at% < y2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the first particles, andMn in a content z2, wherein 0.0 at% < z2 < 20.0 at%, relative to a total amount of Ni, Co and Mn in the first particles; and wherein the second particles comprise:Ni in a content x3, wherein 0.0 at% < x3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the second particles,Co in a content y3, wherein 80.0 at% < y3 < 100.0 at%, relative to a total amount of Ni, Co and Mn in the second particles, andMn in a content z3, wherein 0.0 at% < z3 < 10.0 at%, relative to a total amount of Ni, Co and Mn in the second particles.

15. A battery comprising the positive electrode active material powder according to any one of claims 1 to 13.

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