Positive electrode for lithium-metal rechargeable battery

Abstract

The present disclosure concerns a positive electrode for a lithium-metal rechargeable battery, comprising a current collector plate bearing an active material coating, the active material comprising a first positive electrode active material, having a particle size distribution value D501 lower than or equal to 2 μm, in particular measured by PSD, a conductive agent, and a binder, wherein the positive electrode has a surface roughness Sa value Sa ≤ 140nm; wherein Sa is measured according to ISO25178:2016 on the active material coating, wherein the first positive electrode active material comprises Li, M', and O, wherein M' comprises Ni, Mn, Co, and optionally at least one element selected from Al, B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W, Zn, and Zr.

Description

DESCRIPTIONTitlePositive Electrode for lithium-metal rechargeable batteryTechnical field

[0001] The present disclosure a low surface roughness positive electrode for a lithium-metal rechargeable battery, comprising a positive electrode active material, a conductive agent, and a binder, wherein said positive electrode active material has low particle diameter.Description of related art

[0002] Lithium-metal rechargeable batteries are batteries that use lithium metal or lithium-alloys as anodes or negative electrodes and are being developed as an alternative to lithium-ion batteries with the prospect of providing significantly higher energy density.

[0003] While increasing the capacity of lithium metal secondary batteries is a major objective in the industry, a major drawback of currently used systems is the sudden loss of capacity when undergoing charge-discharge cycles. Such a drop of capacity is usually noticeable after only a few cycles. It has been foiund that the lithium metal or lithium alloy anodes may form lithium dendrites that lead to this issue which is specific to lithium metal rechargeable batteries and does not occur in lithium-ion batteries that do not have lithum metal or lithium-alloy anodes.

[0004] Coated lithium anodes have been disclosed that may reduce the occurrence of lithium dendrite upon cycling and reduce the such observed capacity drops. US2002 / 0192148 discloses a LiF protective layer, KR..20030042288 discloses a cross-linked polymer protective film and US2016 / 0372743 Al discloses a single layer polymer protective film.

[0005] However, lithium dendrite penetration of these coatings may still occur on charge as lithium is plated at the anode. One of the results thereof being an abrupt loss of capacity.

[0006] There is thus still a need in the art to improve the capacity retention, that is to reduce the drop of discharge capacity, particularly in the first 5 and / or 10 charge cycles.Summary

[0007] The inventors have found that the retention capacity of a lithium metal rechargeable battery may be improved by a. a positive electrode for a lithium-metal rechargeable battery, comprising a current collector plate bearing an active material coating, the active material comprising : i. a first positive electrode active material, having a particle size distribution value D50i lower than or equal to 2 pm, in particular measured by particle size distribution (PSD), ii. a conductive agent, and iii. a binder, wherein the positive electrode has a surface roughness Sa value Sa < 140nm; wherein Sa is measured according to ISO25178:2016 on the active material coating.

[0008] The Sa roughness value is measured according to ISO 25178:2016. Sa expresses the arithmetical mean of the difference of height of each point of a surface compared to the mean height of said surface.

[0009] The inventors have indeed found that a positive electrode comprising positive electrode material of low D50 and having limited surface roughness leads to improvement of the capacity retention in lithium metal rechargeable batteries.

[0010] As mentioned herein above, a lithium-metal rechrgeable battery is a battery comprising an anode (or negative electrode) of lithium metal or a lithium alloy.Short description of drawings

[0011] Figures 1 shows three dimensional (3D) images of a positive electrode not according to the present disclosure.

[0012] Figures 2 and 3 show three dimensional 3D images of different positive electrodes according to different embodiments of the present disclosure.Detailed Description

[0013] In the following detailed description, preferred embodiments are described in detail to enable practice of the present disclosure. Although the present disclosure is described with reference to these specific preferred embodiments, it willbe understood that the present disclosure is not limited to these preferred embodiments. To the contrary, the present disclosure includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description and accompanying drawings. Unless otherwise indicated, it is not meant that the alternatives, modifications, and equivalents described herein are understood as separate, non-combinable, embodiments. That is, provided it is technically feasible, the different parts of the present disclosure may be combined with one another.

[0014] "ICP-OES" as used herein refers to Inductively Coupled Plasma - Optical Emission Spectrometry. The method of determining metal compositions using ICP- OES and its meaning are described herein below.

[0015] "XPS" as used herein refers to X-ray Photoelectron Spectroscopy. The method of determining metal compositions using XPS and its meaning are described herein below.

[0016] "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. 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%, as is well known to the person skilled in the art, 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 Eawi is a standard atomic weight (molecular weight) of the first element Ei, Ewti is wt% of an ithelement Ei, Eawi is 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.

[0017] The term "about" as used herein referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of + / -20% or less, preferably + / -10% or less, more preferably + / -5% or less, even more preferably + / -1% or less, and still more preferably + / -0.1% or less of and from the specified value, in so far such variations are appropriate to perform in the present disclosure. However, it is to be understood that the value to which the modifier "about" refers is itself also specifically disclosed.

[0018] The term "D50" as used herein refers to a particle size at 50% of cumulative volume% distribution when measured by laser scattering method. The method of measuring D50 by laser scattering method is described herein below. The same applies of course to D50i and D502, which apply to the first and second positive electrode active material powders respectively.

[0019] As used herein, a range of values "from X to Y" includes the endpoints X and Y.

[0020] "Milling" as used herein is the action of reducing the size of particles by a mechanical action submitting the particles to a stress. Some cracks will appear under the stress, and subsequently the particle will be broken in different parts.

[0021] "Homogeneous" as used herein may refer to a state of a blend with different powdered substances in which the components are uniformly distributed throughout the mixture. In other words, every sample of the homogeneous mixture may exhibit almost the same proportions of each substance.

[0022] The active material may be present on at least part of a major face of the current collector plate of the positive electrode.

[0023] In an embodiment, D50i is lower than or equal to 1.7 pm, in particular D50i is lower than or equal to 1.5 pm.

[0024] In an embodiment, D50i is at least 0.5 pm, in particular at least 0.8 pm, or even at least 1.0 pm.

[0025] In an embodiment, the first positive electrode active material comprises Li, M', and 0, wherein M' comprises: a. Ni in a content x, wherein 45 at% < x < 98 at%, relative to M', b. Mn in a content y, wherein 0 at% < y < 25 at%, relative to M', c. Co in a content z, wherein 0 at% < z < 25 at%, relative to M', and d. D in a content a, wherein 0 at% < a < 5 at%, relative to M', wherein D comprises at least one element selected from Al, B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W, Zn, and Zr, wherein x, y, z, and a are measured by ICP-OES, wherein x+y+z+a is 100 at%.

[0026] In an embodiment the first positive electrode active material comprises first particles, wherein each of the first particles consists of at least one first primary particle and at most twenty first primary particles. The term "monolithic particles" are hereby defined as particles consisting of at least one primary particle and at most twenty primary particles.

[0027] The number of primary particles constituting the monolithic particle is determined by scanning electron microscopy (SEM) in a field of view of at least about 45 pm x at least about 60 pm ( / .e. of at least about 2700 pm2), preferably of: at least about 100 pm x about 100 pm ( / .e. of at least about 10,000 pm2). The particles in the image may be well distributed therefore avoiding overlapping 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. In the context of the present disclosure, primary particles may be distinguished from each other in a SEM image by observing grain boundaries between the primary particles. A grain boundary is defined as the interface between two primary particles, preferably wherein the atomic planes of the two primary particles are aligned to different orientations and meet as a crystalline discontinuity.

[0028] Monolithic particles can maintain their morphological integrity in the absence of anisotropic forces even if operated under extreme conditions, and thus, microcracks responsible for the degradation of a cathode active material powder, e.g., the decreased cycle life and the increased impedance, can be reduced.

[0029] In an embodiment the first positive electrode active material comprises Ni in a content x, wherein 55 at% < x < 95 at%, preferably 65 at% < x < 92 at%, and more preferably 70 at% < x <90 at%, relative to M'.

[0030] In an embodiment the first positive electrode active material comprises Mn in a content y, wherein 2 at% < y < 20 at%, preferably 4 at% < y < 15 at%, and more preferably 5 at% < y < 10 at%, relative to M'.

[0031] In an embodiment the first positive electrode active material comprises Co in a content z, wherein 2 at% < z < 20 at%, preferably 4 at% < z < 15 at%, and more preferably 5 at% < z < 10 at%, relative to M'.

[0032] In an embodiment the first positive electrode active material comprises D wherein D comprises Nb, wherein 0 at% < a < 4 at%, preferably 0.2 at% < a < 3.5 at%, and more preferably 0.3 at% < a < 3 at%, relative to M'.

[0033] In an embodiment, the positive electrode has a Sa value of said positive electrode is lower than or equal to 130 nm, preferably lower than or equal to 120 nm, more preferably lower than or equal to 110 nm.

[0034] In the present disclosure, the conductive agent is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, graphite; Carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used. In an embodiment of the positive electrode, the conductive agent comprises carbon black powder, carbon nanotube, or carbon nanofiber.

[0035] The binder is a component that assists in the bonding of the active material and the conductive material and in the bonding to the current collector.

[0036] The binder may include one or more of polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene (PE), and polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene- propylene-diene monomer, styrene-butadiene rubber, fluorine rubber, various copolymers, etc. In an embodiment of the positive electrode, the binder comprises polyvinylidene fluoride (PVDF).

[0037] In an embodiment, the active material of the positive electrode further comprises a second positive electrode active material having a particle size distribution value D502 higher than 2 pm, in particular measured by PSD.

[0038] In an embodiment, D502 is not more than 15pm, advantageously not more than 10pm, more advantageously not more than 7pm.

[0039] In an embodiment, D502 is at least 3pm, advantageously at least 3.3pm.

[0040] In an embodiment, the ratio D502 / D50i ranges from 1.5: 1 to 5: 1, in particular from 1.5: 1 to 3.5: 1.

[0041] In an embodiment, the mass ratio of the second positive electrode active material to the first positive electrode active material is in the range from 1 :99 to 35:65.

[0042] The amounts of binder and conductive material may be adjusted so as to achieve sufficient cohesion and electronic conductivity of the positive electrode. Generally, the amount of positive electrode material in th electrode should be kept as high as possible. Too little binder can lead to poor adhesion, causing active material detachment and increased internal resistance. Too much binder can reduce the electrode's conductivity due to the binder's electrochemical inertness. Using too much conductive agent can reduce electrode density and capacity, while too little may lead to poor electron transport and lower rate capability.

[0043] In an embodiment the second positive electrode active material comprises Li, M", and 0, wherein M" comprises: a. Ni in a content x", wherein 45 at% < x" < 98 at%, relative to M", b. Mn in a content y", wherein 0 at% < y" < 25 at%, relative to M", c. Co in a content z", wherein 0 at% < z" < 25 at%, relative to M", and d. D" in a content a", wherein 0 at% < a" < 5 at%, relative to M", wherein D comprises at least one element selected from Al, B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W, Zn, and Zr, wherein x", y", z", and a" are measured by ICP-OES, wherein x"+y"+z"+a" is 100 at%.

[0044] In an embodiment the second positive electrode active material comprises second particles, wherein each of the second particles consists of at least one second primary particle and at most twenty second primary particles.

[0045] In an embodiment the second positive electrode active material comprises Ni in a content x", wherein 55 at% < x" < 95 at%, preferably 65 at% < x" < 92 at%, and more preferably 70 at% < x" <90 at%, relative to M".

[0046] In an embodiment the second positive electrode active material comprises Mn in a content y", wherein 2 at% < y" < 20 at%, preferably 4 at% < y"< 15 at%, and more preferably 5 at% < y" < 10 at%, relative to M".

[0047] In an embodiment the second positive electrode active material comprises Co in a content z", wherein 2 at% < z" < 20 at%, preferably 4 at% < z"< 15 at%, and more preferably 5 at% < z" < 10 at%, relative to M'.

[0048] In an embodiment the second positive electrode active material comprises D" wherein D" comprises Nb, wherein 0 at% < a" < 4 at%, preferably 0.2 at% < a" < 3.5 at%, and more preferably 0.3 at% < a" < 3 at%, relative to M'.

[0049] The first and second positive electrode materials may have the composition or different compositions.

[0050] In an embodiment, the positive electrode further comprises a current collector. The positive electrode current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery. The current collector may comprise stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon coated aluminum or stainless steel coated with carbon, nickel, titanium or silver. Advantageously, the positive electrode current collector comprises aluminum, in particular an aluminum foil, a carbon coated aluminum foil or a stainless steel foil. Examples of JIS steel types include SUS304, SUS316L, and SUS310S for austenite type steel, and SUS436, SUS444, and SUS447J1 for ferrite type steel.

[0051] The present disclosure further concerns a method for preparing a positive electrode for a lithium-metal rechargeable battery, in particular a positive electrode according to an embodiment or combination of embodiments disclosed herein, comprising: a. Providing electrode precursors comprising a first positive electrode active material, a conductive agent, a binder mixed homogeneously in a solvent, wherein the first positive electrode active material has a particle size distribution value D50i lower than or equal to 2 pm, in particular measured by PSD; b. mixing the precursors so as to form a mixture; c. spreading the mixture on a current collector; andd. drying and pressing the current collector.

[0052] In the method of the present disclosure the electrode precursors further comprise a second positive electrode active material having a particle size distribution value D502 higher than 2 pm, in particular measured by PSD.

[0053] In the method of the present disclosure, the first and second positive electrode active materials may have the composition of any embodiment or combination of embodiments of the first and second positive electrode active materials disclosed herein in relation to the positive electrode.

[0054] In the method of the present disclosure, the first and second positive electrode active materials may have the particle size distribution value ranges D50i and D502 respectively composition of any embodiment or combination of embodiments of the first and second positive electrode active materials disclosed herein in relation to the positive electrode.

[0055] In the preset disclosure the first and second positive electrode active materials combined may be included in a total amount of 60% to 99% by weight, preferably 70% to 99% by weight, and more preferably 80% to 98% by weight, based on the total weight of solids excluding the solvent in the slurry for the positive electrode.

[0056] In the present disclosure, the binder may be contained in an amount of 1% to 20% by weight, preferably 1% to 15% by weight, more preferably 1% to 10% by weight, based on the total weight of solids excluding the solvent in the slurry for the positive electrode.

[0057] In the present disclosure, the conductive material may be contained in an amount of 1% to 20% by weight, preferably 1% to 15% by weight, more preferably 1% to 10% by weight, based on the total weight of solids excluding the solvent in the slurry for the positive electrode.

[0058] In the present disclosure, the solvent may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that achieves a desirable viscosity when including the positive electrode active material(s), binder and conductive material. For example, the solid concentration including the positive electrode active material and optionally the binder and the conductive material is 50% to 95% by weight, preferably 70% to 95% by weight, more preferably 70% to 90% by weight.

[0059] The porosity of the positive electrode of the present disclosure may be less than about 30%.

[0060] The present disclosure further concerns a lithium metal secondary battery comprising a positive electrode according to according to an embodiment or combination of embodiments disclosed herein.

[0061] The lithium metal secondary battery of the present disclosure may comprise a Li or Li alloy negative electrode. Example Li alloys are Li-Si, Li-Sn, Li- Ge, Li-AI, Li-In, Li-Mg.

[0062] The lithium metal secondary battery of the present disclosure may comprise a liquid electrolyte. The Li or Li alloy negative electrode may in particular be in direct contact with the liquid electrolyte.

[0063] The present invention further concerns the use of a battery according to any embodiment of the present disclosure in a portable electronic device, such as portable computer, tablet, or mobile phone, in a power tool, in an energy storage system, in an uninterruptible power supply, an electric vehicle, or hybrid electric vehicle. Electric vehicles and hybrid electric vehicles include, Plug-in Hybrid Electric Vehicles, Extended Range Electric Vehicles. Electric vehicles include vehicles for passenger and freight, marine, air, aerospace and ground transportation and mobile machinery.

[0064] The following analysis methods are used in the Examples and the Comparative Example.A) Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) measurement

[0065] The amount of Ni, Mn, and Co in the positive electrode active material powder is measured with the ICP-OES method by using an Agilent ICP 720-ES (Agilent Technologies). 2 grams of powder sample is dissolved into 10 mL of high purity hydrochloric acid (at least 37 wt% of HCI with respect to the total weight of solution) in an Erlenmeyer flask. The flask is covered by a glass and heated on a hot plate at 380 °C until complete dissolution of the precursor. After being cooled to room temperature, the solution of the Erlenmeyer flask is poured into a 250 mL volumetric flask. Afterwards, the volumetric flask is filled with deionized water up to the 250 mL mark, followed by complete homogenization.B) Particle size analysis

[0066] 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. In order to improve the dispersion of the powder, sufficient ultrasonic irradiation and stirring are applied, and an appropriate surfactant is introduced. D50 is defined as the particle size at 50% of the cumulative volume% distributions.C) Roughness of electrode

[0067] The roughness of the positive electrode is expressed as Sa (arithmetical mean height of a plane) and Sa is measured following ISO25178 :2016. Images obtained for different examples by Laser Microscope are shown below in figures 1 to 3.D) Coin cell testingD-l) Coin cell preparation

[0068] For the preparation of a positive electrode, a slurry that contains a positive electrode active material powder, conductive agent (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 or a piece of graphite used as a negative electrode. IM LiPFe in EC / DMC (1 :2) is used as electrolyte and is dropped between separator and electrodes. Then, the coin cell is completely sealed to prevent leakage of the electrolyte.D-2) Testing method

[0069] The testing method is a conventional "constant cut-off voltage" test. The conventional coin cell test in the present disclosure follows the schedule shown in Table 3. 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 to 3.0 V / Li metal window range. The retention capacity (%), R5 and R.10, are obtained according to the following equation below wherein DQ1 is the discharge capacity at the first cycle.R5 (%) = 100RIO (%) = 100

[0071] Table 1. Cycling schedule for coin cell testing methodEXAMPLES

[0072] The present disclosure is further illustrated in the following examples.Manufacturing method of a second positive electrode active material P

[0073] A positive electrode active material P is obtained through following steps:1) First mixing: a precursor having transition metal composition as Ni0.85Mn0.07Co0.08 in hydroxide or oxyhydroxide form is mixed homogeneously with LiOH, wherein the molar ratio of Li to total amount of Ni, Mn, and Co is 0.99, to prepare a first mixture.2) First heating: the first mixture from step 1) is heated at 750 °C for 12 hours under oxygen atmosphere followed by cooling, grinding, and sieving to prepare a first heated material.3) Wet ball milling: the first heated material is ball milled in an aqueous solution containing 0.5 mol% CoSO4 relative to the first heated material followed by filtering and drying to prepare a milled material.4) Second mixing: the milled material is mixed homogeneously with CO3O4 and LiOH, wherein 1.5 mol% Co relative to total amount of Ni, Mn, and Coin the milled material and the molar ratio of Li to total amount of Ni, Mn, and Co is 0.99 to prepare a second mixture.5) Second heating: the second mixture is heated at 730 °C for 12 hours followed by cooling and the cooled material is ground with AI2O3 which comprises 0.1 mol% Al relative to total amount of Ni, Mn, and Co in the second heated material and sieved so as to prepare a positive electrode active material P.Manufacturing method of a first positive electrode active material Q

[0074] A positive electrode EX1.1 comprising a positive electrode active material Q is obtained through following steps:1) First mixing: a precursor having transition metal composition as Ni0.85Mn0.07Co0.08 in hydroxide or oxyhydroxide form is mixed homogeneously with NbzOs and LiOH, wherein 0.5 mol% Nb is added relative to total amount of Ni, Mn, and Co in the precursor and the molar ratio of Li to total amount of Ni, Mn, and Co is 0.99, to prepare a first mixture.2) First heating: the first mixture is heated at 885 °C for 10 hours under oxygen atmosphere followed by cooling, grinding, and sieving to prepare a first heated material.3) Wet ball milling: the first heated material is ball milled in an aqueous solution followed by filtering and drying to prepare a milled material.4) Second heating: the milled material is heated at 700 °C for 10 hours under oxygen atmosphere, wherein increasing rate is 5 °C / min until reaching 350 °C and 2 °C / min until reaching 700 °C, followed by cooling, grinding, and sieving to prepare a positive electrode active material Q.Comparative Example 1.1 (CEX1.1)

[0075] A coin cell comprising the positive electrode active material P is prepared following the method described in D-l) coin cell preparation so as to obtain a coin cell CEX1.1, wherein a piece of Li is used as negative electrode.Example 1.1 (EX1.1)

[0076] A coin cell comprising the positive electrode active material Q is prepared following the same method as CEX1.1 except that the positive electrode active material Q is used instead of the positive electrode active material P.Example 1.2 (EX1.2)

[0077] A coin cell comprising a mixture of the positive electrode active material P and Q is prepared the same method as CEX1.1 except that the positive electrode active material P and Q are mixed homogeneously to be used as a positive electrode active material, wherein the mass ratio of P and Q is 80:20.Comparative Example 1.2 (CEX1.2)

[0078] A coin cell comprising the positive electrode active material P is prepared following the same method as CEX1.1 except that a piece of graphite is used as a negative electrode.Comparative Example 1.3 (CEX1.3)

[0079] A coin cell comprising the positive electrode active material Q is prepared following the same method as EX1.1 except that a piece of graphite is used as a negative electrode.Comparative Example 1.4 (CEX1.4)

[0080] A coin cell comprising a mixture of the positive electrode active material P and Q is prepared following the same method as EX1.2 except that a piece of graphite is used as a negative electrode.

[0081] Table 2 below summarizes the particle size of the positive electrode active material, the roughness (Sa) of positive electrode surface, and QF analyzed by coin cell testing for all the examples and the comparative examples.

[0082] Table 2. Summary of PSD, Sa, and coin cell results

[0083] According to Table 2, EX1.1 and EX1.2 have a retention capacity for 5 cycles (R5) of 92.8 % and 90.7 % respectively while CEX1.1 has the lower R.5 of 76.2 %. The positive electrode used for CEX1.1 (Fig. 1) has the higher roughness (Sa) value of 147 nm and the positive electrode used for EX1.1 (Fig. 2) and EX1.2 (Fig. 3) have Sa values of only 78 nm and 108 nm respectively, which indicates the surface of the positive electrode for CEX1.1 is rougher than that of EX1.1 and EX1.2. Thus, it is confirmed that a battery comprising a positive electrode with a lower Sa exhibits the higher retention capacity.

[0084] When comparing the batteries CEX1.2, CEX1.3, and CEX1.4, they show the high retention capacity R.5 and RIO regardless of the roughness of the positive electrode surface. It is assumed that in EX1.1 and EX1.2 compared to CEX1.1 the lesser roughness of the positive electrode surface could lower the deterioration of the electrode resulting in the lower capacity fading for a Li metal battery.

[0085] Accordingly, it is confirmed that the positive electrode as described herein may provide an improved electrochemical stability during the Li metal battery cycling. This effect is not observed when the negative electrode is a graphite electrode, thus confirming that the issue is specific to batteries having a Li metal or Li alloy anode and thus in all likelihood related to Li dendrite formation.

Claims

CLAIMS

1. A positive electrode for a lithium-metal rechargeable battery, comprising a current collector plate bearing an active material coating, the active material comprising a. a first positive electrode active material, having a particle size distribution value D50i lower than or equal to 2 pm, b. a conductive agent, and c. a binder, wherein the positive electrode has a surface roughness Sa value Sa < 140nm; wherein Sa is measured according to ISO25178:2016 on the active material coating; and wherein the first positive electrode active material comprises Li, M', and 0, wherein M' comprises: a. Ni in a content x, wherein 45 at% < x < 98 at%, relative to M', b. Mn in a content y, wherein 0 at% < y < 25 at%, relative to M', c. Co in a content z, wherein 0 at% < z < 25 at%, relative to M', and d. D in a content a, wherein 0 at% < a < 5 at%, relative to M', wherein D comprises at least one element selected from Al, B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W, Zn, and Zr, wherein x, y, z, and a are measured by inductively coupled plasma - optical emission spectroscopy ICP-OES, wherein x+y+z+a is 100 at%.

2. The positive electrode according to claim 1, wherein the first positive electrode active material comprises first particles, wherein each of the first particles consists of at least one first primary particle and at most twenty first primary particles.

3. The positive electrode according to claim 1 or 2, wherein 55 at% < x < 95 at%, preferably 65 at% < x < 92 at%, and more preferably 70 at% < x <90 at%, relative to M'.

4. The positive electrode according to any one of claims 1 to 3, wherein 2 at% < y < 20 at%, preferably 4 at% < y < 15 at%, and more preferably 5 at% < y < 10 at%, relative to M'.

5. The positive electrode according to any one of claims 1 to 4, wherein 2 at% < z < 20 at%, preferably 4 at% < z < 15 at%, and more preferably 5 at% < z < 10 at%, relative to M'.

6. The positive electrode according to any one of claims 1 to 5, wherein D comprises Nb, wherein 0 at% < a < 4 at%, preferably 0.2 at% < a < 3.5 at%, and more preferably 0.3 at% < a < 3 at%, relative to M'.

7. The positive electrode according to any one of preceding claims, wherein Sa value of said positive electrode is lower than or equal to 130 nm, preferably lower than or equal to 120 nm, more preferably lower than or equal to 110 nm.

8. The positive electrode according to any one preceding claim, wherein the active material further comprises a second positive electrode active material having a particle size distribution value D502 higher than 2 pm and lower than or equal to 15 pm.

9. The positive electrode according to claim 8 wherein the mass ratio of the second positive electrode active material to the first positive electrode active material is in the range from 1 :99 to 35:65.

10. The positive electrode according to claim 8 or claim 9 wherein the second positive electrode active material comprises Li, M", and O, wherein M" comprises: a. Ni in a content x", wherein 45 at% < x" < 98 at%, relative to M", b. Mn in a content y", wherein 0 at% < y" < 25 at%, relative to M", c. Co in a content z", wherein 0 at% < z" < 25 at%, relative to M", and d. D" in a content a", wherein 0 at% < a" < 5 at%, relative to M", wherein D comprises at least one element selected from Al, B, Ba, Ca, Cr, Fe, Mg, Mo, Nb, S, Si, Sr, Ti, Y, V, W, Zn, and Zr, wherein x", y", z", and a" are measured by inductively coupled plasma - optical emission spectroscopy (ICP-OES),wherein x"+y"+z"+a" is 100 at%.

11. The positive electrode according to any one of claims 8 to 10, wherein the second positive electrode active material comprises second particles, wherein each of the second particles consists of at least one second primary particle and at most twenty second primary particles.

12. The positive electrode according to claim 10 or 11 wherein the second positive electrode active material comprises Ni in a content x", wherein 2 or 3, wherein 55 at% < x" < 95 at%, preferably 65 at% < x" < 92 at%, and more preferably 70 at% < x" <90 at%, relative to M".

13. The positive electrode according to any one of claims 10 to 12 wherein the second positive electrode active material comprises Mn in a content y", wherein 2 at% < y" < 20 at%, preferably 4 at% < y" < 15 at%, and more preferably 5 at% < y" < 10 at%, relative to M".

14. The positive electrode according to any one of claims 10 to 13 wherein the second positive electrode active material comprises Co in a content z", wherein 2 at% < z" < 20 at%, preferably 4 at% < z" < 15 at%, and more preferably 5 at% < z" < 10 at%, relative to M'.

15. The positive electrode according to any one of claims 10 to 14 wherein the second positive electrode active material comprises D" wherein D" comprises Nb, wherein 0 at% < a" < 4 at%, preferably 0.2 at% < a" < 3.5 at%, and more preferably 0.3 at% < a" < 3 at%, relative to M'.

16. A method for preparing a positive electrode for a lithium-metal rechargeable battery, comprising: a. Providing electrode precursors comprising a first positive electrode active material, a conductive agent, a binder mixed homogeneously in a solvent, wherein the first positive electrode active material has a particle size distribution value D50i lower than or equal to 2 pm and optionally comprising a second positive electrode active material having a particle size distribution value D502 higher than 2 pm; b. Mixing the electrode precursors so as to form a mixture; c. Spreading the mixture from a. on a current collector; and d. Drying and pressing the current collector.

17. A lithium metal secondary battery comprising the positive electrode according to any one of claims 1 to 15.

18. A lithium metal secondary battery according to claim 17 comprising a Li or a Li alloy negative electrode, wherein the Li alloy is preferably selected from the alloys Li-Si, Li-Sn, Li-Ge, Li-AI, Li-In, and Li-Mg.

19. Use of the battery according to claim 18 in a portable electronic device, a portable computer, a tablet, a mobile phone, a power tool, an energy storage system, an uninterruptible power supply, an electric vehicle, or hybrid electric vehicle.

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