Positive electrode active material comprising particles with a sulfur surface layer

Abstract

The present invention relates to a positive electrode active material comprising Li, Ni, one or both of Mn and Co, and O, wherein the surface layer comprises S and a transition metal. It was found during the temperature and pressure treatment that a sulfur layer is formed between the positive electrode material and sulfide solid electrolyte which strongly enhances the ionic conductivity and the electrochemical performance.

Description

[0001] POSITIVE ELECTRODE ACTIVE MATERIAL COMPRISING PARTICLES WITH A SULFUR SURFACE LAYER

[0002] TECHNICAL FIELD

[0003] The present invention relates to a positive electrode active material comprising Li, Ni, one or both of Mn and Co, and 0, wherein the surface layer comprises S and a transition metal.

[0004] BACKGROUND

[0005] The three primary functional components of a lithium-ion battery are the anode, the cathode, and the electrolyte. While many variations exist, the anode of a conventional lithium-ion cell is typically made from carbon or metallic lithium, the cathode is typically made from transition metal oxides (in particular, oxides of cobalt, nickel and / or manganese), and the electrolyte is typically a non-aqueous solvent containing a lithium salt. For example, mixtures of organic carbonates with lithium hexafluorophosphate are well known liquid electrolytes for lithium-ion batteries.

[0006] A significant disadvantage of liquid electrolytes is that the compositions containing said electrolytes, in particular the solvents are inflammable, which poses a large safety risk during normal operation of the battery containing said liquid electrolytes in for example an electric vehicle, and in particular in case of an incident. Another disadvantage inherent to the liquid nature of the electrolyte are associated with risks of leakage and with increased risk of environmental pollution in case of a spill or leakage.

[0007] Recently, efforts have been made to develop solid electrolytes which allow the provision of a solid-state lithium-ion battery. Such solid-state batteries have significantly reduced EHS (environmental, health and safety) hazards. The solid-state electrolyte can act as an electrolyte as well as a separator, physically separating the anode and cathode material in order to prevent short-circuiting.

[0008] While in a liquid electrolyte based Li-ion battery, thanks to its fluidity, the liquid electrolyte impregnates the positive electrode and gets into very intimate contact with the positive electrode active material, allowing low electrode polarization and high utilization rate of active material, in a solid state electrolyte based battery, without liquid fluidity, it is challenging to obtain similar intimate contact between the solid electrolyte material and the positive electrode active material. On top, the repeated electrode expansion and contraction during cycling further deteriorates the mechanical particle-to-particle contact between the solid electrolyte and positive electrode active material particles.

[0009] As a consequence, high polarization and low utilization rate of active materials are conventional drawbacks in solid-state lithium batteries.

[0010] An important key to realize solid state lithium batteries with competitive performances thus relies on the construction of a stable and intimate interface between the positive electrode active material and the solid electrolyte material. A way to achieve this is to prepare a positive electrode layer that includes an intimate mix of positive electrode active material together with the solid electrolyte material. As a result, the surface area of interface between solid electrolyte and positive electrode active material is increased resulting in a better and more stable contact between said solid electrolyte and the positive electrode active material particles.

[0011] However, it is known that the positive electrode active material reacts with the solid electrolyte creating a resistance against the movement of Li ions across the interface. It has been suggested in the prior art that a way to mitigate this problem is by treating the positive electrode active material particles surface with a buffer layer such as LiNbCh.

[0012] US2011 / 027661 Al discloses a positive electrode composite active material comprising a positive electrode active material, such as LiCoOz with a niobium coating and a sulfide solid electrolyte, such as U7P3S11.

[0013] US2014 / 287324 Al discloses a positive electrode composite active material comprising a positive electrode active material, such as NMC111, and a sulfide solid electrolyte, such as LPS.

[0014] It is an object of the present invention to provide an improved positive electrode composite active material, in particular a positive electrode composite active material having an improved electronic conductivity and electrochemical performance.

[0015] SUMMARY OF THE INVENTION

[0016] The present inventors have found that one or more objects of the invention can be achieved by the provision of a positive electrode active material comprising a lithium nickel manganese / cobalt oxide (e.g. NMC) having a surface layer containing metals (e.g. Zr or Nb). As will be shown in the appended examples, it was surprisingly found that when the positive electrode active material and the electrolyte material are subjected to a heat and / or a pressure treatment a sulfur interlayer is formed between them. It was found that the formation of this sulfur layer strongly enhances the electronic conductivity and the electrochemical performance. Without wishing to be bound by any theory, the present inventors believe that by applying a heat treatment and / or applying a high pressure to the mixture containing the positive electrode active material and the sulfide solid electrolyte, an elemental diffusion of sulfur in the surface layers takes place. The present inventors believe that at least part of the metal oxides, such as niobium oxides, present in the surface layer in the form of inter alia NbzOs and / or LiNbCh transform into niobium(oxy)sulfides LiNbOaiSpi, wherein pi > 0 and al+pi = 3, and / or NbzOoiSyi, wherein yl > 0 and col+yl = 5.

[0017] In a first aspect an object of the present invention is achieved by providing a positive electrode active material comprising Li, Ni, one or both of Mn and Co, and 0, wherein the surface layer comprises S and M', wherein M' is B, Zr, Nb or a combination thereof. In another aspect of the invention, there is provided a battery comprising a positive electrode comprising the positive electrode active material as described herein, a solid electrolyte and an anode.

[0018] In another aspect of the invention, there is provided the use of a battery comprising the positive electrode active material of the invention in motor vehicles, bicycles operated by electric motor, robots, aircraft (for example unmanned aerial vehicles including drones), ships, satellites or stationary energy stores.

[0019] BRIEF DESCRIPTION OF THE FIGURES

[0020] Figure 1 : a) TEM-EDX of EXI, b) EDX Spectrum showing elemental diffusion of Nb and S in the surface layer.

[0021] Figure 2: Two TEM-EDX images of EX2 (a and b).

[0022] Figure 3: Electrochemical performance of the positive electrode active material of EXI.

[0023] Figure 4: Electrochemical performance of the positive electrode active material of EX2 (2) as compared to the comparative example CEX2 (1). DETAILED DESCRIPTION

[0024] 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. But to the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description.

[0025] The term "comprising", as used herein and in the claims, 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".

[0026] The term "solid-state battery" as used herein refers to a cell or a battery that includes only solid or substantially solid-state components such as positive electrode composite solid active material (e.g. anode, cathode and solid electrolyte).

[0027] The term "positive electrode active material" and "cathode active material" are synonyms and can be used interchangeably throughout this document.

[0028] The term "argyrodite-type crystal structure" as used herein refers to a crystal structure having a crystal structure or system similar to naturally existing AgsGeSe and U7S6 (Argyrodite). The argyrodite-type crystal structure may be of orthorhombic symmetry and described in the F-43m space group. In some embodiments the argyrodite-type crystal structure may also be empirically determined for example, by X-ray diffraction by observing diffraction peaks around at 20=15.5±1°, 18±1°, 26±1°, 30.5±l° and 32 ±1° using CuKa-ray wavelength. X-Ray diffraction (XRD) as referred to herein, refers to XRD experiments performed using Bruker D8 diffractometers equipped with Cu (Kal-Ka2) radiation in a 0-0 configuration. Preferably, an air-tight sample holder with a dome type is used. Preferably, the patterns were collected between 20 = 10 ° - 55 ° with a step size of 0.02 °.

[0029] Electronic conductivity as referred to herein, refers to the electronic conductivity determined at 25 °C, unless described otherwise. It is preferably determined on cold- pressed samples in a 13 mm die at 375 MPa. The spectra were recorded using Biologic VMP3 potentiosat. Preferably, the relative density of the pellet was 85 to 95% and the thickness was 2.5 mm approximately.

[0030] Transmission electron microscope - Energy Dispersive X-Ray Analysis (TEM-EDX) as referred to herein, refers to FEI TitanThemis aberration-corrected TEM equipped with ThermoFischer Super-X EDX detectors, used at 200 kV. The sample area is reduced to 256x256 pixels using 235k x magnification, a spot size of 11 and a 30mm aperture lens. The EDX analyses are carried out at low doses using specific acquisition conditions, such as long acquisition times (lh-lh30), with a large number of short passes and a short dwell time at each position (dwell time 3-10 ps / pixel).

[0031] Positive Electrode Active Material

[0032] In a first aspect of the present invention a positive electrode active material is provided comprising Li, Ni, one or both of Mn and Co, and O, wherein the positive electrode active material comprises particles having a surface layer covering at least a part of a surface of the particles, wherein the surface layer comprises S and M', wherein M' is B, Zr, Nb or a combination thereof.

[0033] A preferred embodiment is the positive electrode active material of the invention having a layered structure, preferably a layered structure of the a-NaFeO2 type, preferably a layered structure of the a-NaFeO2 type having a R-3m space group.

[0034] In preferred embodiments the positive electrode active material is according to the invention comprising Li, S, Ml, and O, wherein Ml comprises

[0035] - Ni in a content xl, wherein 55.0 at% < xl < 95.0 at%, relative to Ml;

[0036] - Mn in a content yl, wherein 0.0 at% < yl < 40.0 at%, relative to Ml; - Co in a content zl, wherein 0.0 at% < zl < 40.0 at%, relative to Ml;

[0037] - M' in a content al, wherein 0.0 at% < al < 5.0 at%, relative to Ml;

[0038] - DI in a content bl, wherein 0.0 at% < bl < 2.0 at%, relative to Ml, wherein

[0039] DI is at least one element other than Li, Ni, Mn, Co, S and O;

[0040] - wherein xl + yl + zl + al + bl is 100.0 at%; preferably wherein Ml comprises:

[0041] - Ni in a content xl, wherein 60.0 at% < xl < 90.0 at%, relative to Ml;

[0042] - Mn in a content yl, wherein 0.0 at% < yl < 30.0 at%, relative to Ml;

[0043] - Co in a content zl, wherein 0.0 at% < zl < 30.0 at%, relative to Ml;

[0044] - M' in a content al, wherein 0.01 at% < al < 1.5 at%, relative to Ml;

[0045] - DI in a content bl, wherein 0.0 at% < bl < 2.0 at%, relative to Ml; more preferably wherein Ml comprises:

[0046] - Ni in a content xl, wherein 75.0 at% < xl < 88.0 at%, relative to Ml;

[0047] - Mn in a content yl, wherein 5.0 at% < yl < 15.0 at%, relative to Ml;

[0048] - Co in a content zl, wherein 5.0 at% < zl < 15.0 at%, relative to Ml;

[0049] - M' in a content al, wherein 0.02 at% < al < 1.0 at%, relative to Ml;

[0050] - DI in a content bl, wherein 0.0 at% < bl < 1.0 at%, relative to Ml; even more preferably wherein Ml comprises:

[0051] - Ni in a content xl is about 85.0 at%, relative to Ml;

[0052] - Mn in a content yl is about 7.0 at%, relative to Ml;

[0053] - Co in a content zl is about 7.0 at%, relative to Ml;

[0054] - Nb in a content al is about 1.0 at%, relative to Ml; or Zr in a content al is about 0.6 at%, relative to Ml; and

[0055] - DI in a content bl is about 0.0 at%, relative to Ml.

[0056] As appreciated by the skilled person the amount of Ni, Mn, Co, M' and DI in the positive electrode active material is measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Worded differently xl, yl, zl, al and bl are measured by ICP-OES. For example, but not limiting to the invention, an Agilent ICP 720-ES is used in the ICP-OES analysis.

[0057] A preferred embodiment is the positive electrode active material of the invention having a Li / Ml molar ratio, preferably a Li / (Ni + Mn+Co) molar ratio (mol / mol), > 0.90, preferably > 0.92, more preferably > 0.95. A preferred embodiment is the positive electrode active material of the invention having a Li / Ml molar ratio, preferably a Li / (Ni + Mn+Co) molar ratio (mol / mol), < 1.10, preferably < 1.08, more preferably < 1.05. A preferred embodiment is the positive electrode composite active material of the invention having a Li / Ml molar ratio, preferably a Li / (Ni + Mn+Co) molar ratio (mol / mol), in the range of 0.90 - 1.10, preferably in the range of 0.92 - 1.08, more preferably in the range of 0.95 - 1.05.

[0058] In certain preferred embodiments Ml consists of Ni, Mn, Co, M' and DI in the ratios as defined herein.

[0059] As is known to the skilled person, the positive electrode active material of the invention can comprise impurities or be doped or contain metals on the surface resulting in an overall positive electrode active material comprising one or more elements other than Li, Ni, Mn, Co, M' and O, which is reflected in the parameter "DI" used herein. A preferred embodiment is the positive electrode active material according to the invention comprising DI, wherein DI is at least one element selected from the group consisting of Al, Ba, Ca, Ce, Cr, Fe, La, Mg, Mo, Sr, Ti, V, W, Y, and Zn; preferably Al, Ti, Cr, Y, and W; more preferably Al, Ti, and W.

[0060] In certain preferred embodiments the positive electrode active material is according to the invention, wherein bl = 0.0 mol%, relative to Ml.

[0061] In preferred embodiments the positive electrode active material is according to the invention, wherein the positive electrode active material is at least partially covered with a surface layer, wherein the surface layer comprises S and M', wherein M' is B, Zr, Nb or a combination thereof. In more preferred embodiments M' is B, Zr or Nb. In certain preferred embodiments M' is Zr or Nb.

[0062] In a highly preferred embodiment the positive electrode active material comprises S and a lithium transition metal oxide according to formula (I):

[0063] Uw2Nix2Mny2COz2M'a2D2b2O2 (I) wherein 0.90 < w2 < 1.10, preferably 0.92 < w2 < 1.08, more preferably 0.95

[0064] < w" < 1.05; wherein 0.55 < x2 < 0.95, preferably 0.60 < x2 < 0.90, more preferably 0.75

[0065] < x2 < 0.88; wherein 0.0 < y2 < 0.40, preferably 0.0 < y2 < 0.30, more preferably 0.05 < y2 < 0.10; wherein 0.0 < z2 < 0.40, preferably 0.0 < z2 < 0.30, more preferably 0.05 < z2 < 0.10; wherein 0.0 < a2 < 0.02, preferably 0.001 < a2 < 0.015, more preferably 0.002 < a2 < 0.02; wherein 0.0 < b2 < 0.02, preferably 0.0 < b2 < 0.001, more preferably b2 is about 0.0; and wherein x2+y2+z2+a2+b2 = 1.0.

[0066] As is known to the skilled person, the positive electrode active material of the invention can comprise impurities or be doped or contain metals on the surface resulting in an overall positive electrode active material comprising one or more elements other than Li, Ni, Mn, Co, M', S and O, which is reflected in the parameter "D2" used herein. A preferred embodiment is the positive electrode active material according to the invention comprising D2, wherein D2 is at least one element selected from the group consisting of Al, Ba, Ca, Ce, Cr, Fe, La, Mg, Mo, Sr, Ti, V, W, Y, and Zn; preferably Al, Ti, Cr, Y, and W; more preferably Al, Ti, and W.

[0067] In certain preferred embodiments the positive electrode active material is according to the invention, wherein b2 = 0.0, relative to M2.

[0068] As appreciated by the skilled person w2, x2, y2, z2, a2 and b2 in the positive electrode active material are measured by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). For example, but not limiting to the invention, an Agilent ICP 720-ES is used in the ICP-OES analysis.

[0069] In preferred embodiments the positive electrode active material is according to the invention comprising single particles and / or secondary particles, wherein each of the single particles consist of only one primary particle and each of the secondary particles consist of at least two primary particles and at most twenty primary particles as observed in a SEM image. Preferably, at least 30% of the particles, more preferably at least 50% of the particles, constituting the powder observed in a SEM image are the single particles and / or the secondary particles. The number of primary particles constituting the single particles and / or the secondary particles are determined in a field of view of at least 45 pm x at least 60 pm ( / .e. of at least 2700 pm2), preferably of: at least 100 pm x 100 pm (i.e. of at least 10,000 pm2. 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. In the context of the present invention primary particles are 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.

[0070] In preferred embodiments the positive electrode active material is according to the invention, wherein the surface layer has a thickness of at least 5 nm, preferably at least 10 nm, more preferably at least 20 nm, even more preferably at least 25 nm. In preferred embodiments the positive electrode active material is according to the invention, wherein the surface layer has a thickness of less than 60 nm, preferably less than 50 nm, even more preferably less than 40 nm, most preferably less than 35 nm. In preferred embodiments the positive electrode active material is according to the invention, wherein the surface layer has a thickness between 5 and 60 nm, preferably 10 and 50 nm, more preferably between 20 and 40 nm, even more preferably between 25 and 35 nm, as determined by TEM-EDX.

[0071] In preferred embodiments the positive electrode active material is according to the invention, wherein the surface layer covers at least 70 % of the surface of the positive electrode active material, preferably at least 80 %, more preferably at least 90%, even more preferably 95%, most preferably the whole surface of the positive electrode active material.

[0072] In certain preferred embodiments the surface layer consist of S and M', wherein M' is B, Zr, Nb or a combination thereof.

[0073] In certain preferred embodiments M' = Nb. As understood by the present inventors the heat treatment step and / or high pressure step results in an elemental diffusion of sulfur into the surface layer thereby transforming the niobium oxide compounds present in the surface layer, such as LiNbCh or NbzOs, into niobium (oxy)sulfide compounds such as LiNbOzS or NbzOzSs. Hence, in certain preferred embodiments the surface layer comprises niobium (oxy)sulfide compounds, preferably LiNbOaiSpi, wherein pi >0 and al+pi = 3, or derivatives thereof, and / or Nb2OffliSyior derivatives thereof, wherein yl >0 and col+yl = 5. Examples of the niobium (oxy)sulfide compounds are LiNbOzS, LiNbOSz, LiNbSs, NbzC S, NbzChSz, NbOzSs, NbOS4 or NbSs. In certain preferred embodiments M' = Zr. As understood by the present inventors the heat treatment step and / or high pressure step results in an elemental diffusion of sulfur into the surface layer thereby transforming the zirconium oxide compounds present in the surface layer, such as LizZrCh or ZrO2, into zirconium (oxy)sulfide compounds such as LizZrOzS or ZrOS. Hence, in certain preferred embodiments the surface layer comprises zirconium (oxy)sulfides compounds, preferably Li2ZrOa2Sp2or derivatives thereof, wherein p2 > 0 and a2+p2 = 3; and / or ZrOffl2Sy2or derivatives thereof, wherein y2 > 0 and o2+y2 = 2. Examples of the zirconium (oxy)sulfide compounds are LizZrOzS, LizZrOSz, LizZrSs, ZrOS or ZrSz.

[0074] Batery

[0075] A second aspect of the invention concerns a battery comprising a positive electrode, a solid electrolyte and a negative electrode.

[0076] The positive electrode comprises the positive electrode active material according to the invention, as described herein.

[0077] The solid electrolyte according to the invention is disposed between the positive electrode and the negative electrode. The solid electrolyte may be a known solid electrolyte used in solid-state lithium ion batteries, and examples thereof include a sulfide solid electrolyte, an oxide solid electrolyte, and a halide solid electrolyte, and among them, a sulfide solid electrolyte is preferable.

[0078] In preferred embodiments the positive electrode composite active material is according to the invention, wherein the sulfide solid electrolyte comprises Li, P and S.

[0079] In certain preferred embodiments the positive electrode active material is according to the invention, wherein the sulfide solid electrolyte is according to formula (II)

[0080] Li7-yPS6-yXy (II) wherein 0.0 < y < 1 and X is F, Cl, Br or I.

[0081] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein 0 < y < 1.0, preferably 0.05 < y < 1.0, more preferably 0.5 < y < 1. In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein 0.25 < y < 1, preferably 0.5 < y < 1, more preferably 0.85 < y < 1, even more preferably y=l.

[0082] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein X is Cl, Br or I, preferably X is Cl or Br, more preferably X is Cl.

[0083] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein at least 50 mol% of X represents Cl, preferably at least 80 mol% of X represents Cl, most preferably X represents Cl.

[0084] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein X represents Cl, Br or I and wherein at least 50 mol% of X represents Cl, preferably at least 80 mol% of X represents Cl.

[0085] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein at least 50 mol% of X represents Br, preferably at least 80 mol% of X represents Br, most preferably X represents Br.

[0086] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein X represents Cl, Br or I and wherein at least 50 mol% of X represents Br, preferably at least 80 mol% of X represents Br.

[0087] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein at least 50 mol% of X represents I, preferably at least 80 mol% of X represents I, most preferably X represents I.

[0088] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein X represents Cl, Br or I and wherein at least 50 mol% of X represents I, preferably at least 80 mol% of X represents I.

[0089] In preferred embodiments the sulfide solid electrolyte is according to formula (II) , wherein y= 1, and

[0090] X is Cl. In certain preferred embodiments the sulfide solid electrolyte has an argyrodite-type crystal structure.

[0091] In certain preferred embodiments the sulfide solid electrolyte has a purity of at least 90%, preferably at least 95%, more preferably at least 99%, as determined by XR.D.

[0092] The negative electrode according to the invention contains at least a negative electrode active material. The negative electrode active material can contain a further solid electrolyte, preferably the sulfide solid electrolyte as described herein. The negative electrode active material is not particularly limited, and examples thereof include a metal active material, a carbon active material, and an oxide active material. In a preferred embodiment the battery is a solid-state battery, preferably a lithium solid-state battery.

[0093] Use

[0094] A third aspect of the invention concerns a use of the positive electrode active material according to the invention in a battery, preferably a solid-state-battery, most preferably a lithium solid-state-battery.

[0095] A fourth aspect of the present invention concerns a use of the battery according to the invention in either one of a portable computer, a tablet, a mobile phone, an energy storage system, an electric vehicle or in a hybrid electric vehicle, preferably in an electric vehicle or in a hybrid electric vehicle

[0096] EXAMPLES

[0097] Description of methods

[0098] Synthesis protocol for the positive electrode active material

[0099] Positive electrode active material with Nb in surface layer (NMC811-Nb)

[0100] • Step 1 : Co-precipitation

[0101] A transition metal-based precursor with a metal composition Ni0.85Mn0.07Co0.08 (NMC811) was 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. • Step 2: First mixing

[0102] The precursor prepared from Step 1 was mixed with LiOH in an industrial blender to obtain a first mixture having a lithium to metal (Ni, Mn, and Co) ratio of 0.96.

[0103] • Step 3: First heating

[0104] The first mixture from Step 2 was heated at 890 °C for 10 hours in an oxidizing atmosphere to obtain a first heated product.

[0105] • Step 4: Wet bead milling

[0106] The first heated product from Step 3 was bead milled in water followed by drying and sieving process to obtain a milled product. The bead milling solid to solution weight ratio was 6:4 and was conducted for 20 minutes.

[0107] • Step 5: Second mixing

[0108] The milled product obtained from Step 4 was mixed in an industrial blender with 5.6 mol% Li from LiOH, each with respect to the total molar contents of Ni, Mn, and Co in the milled product to obtain a second mixture.

[0109] • Step 6: Second heating

[0110] The second mixture from Step 5 was heated at 760 °C for 10 hours in an oxidizing atmosphere followed by crushing and sieving to obtain a second heated product (NMC811).

[0111] • Step 7: Surface treatment

[0112] About 700 grams of the second heated product obtained from Step 6 is placed in a rotating container and then sprayed with a solution made from 35 grams ethanol, 21.23 grams Nb ethoxide, and 3.47 grams of Li ethoxide using 60°C air carrier with dosing speed of 35mL / minute and 40 bar spray pressure to obtain a coated powder.

[0113] • Step 8: Third heating

[0114] The coated powder obtained from Step 7 is heated at 350°C for 5 hours in an oxygen atmosphere to obtain MNC811-Nb. Positive electrode active material with Zr in surface layer (NMC811-Zr)

[0115] • Step 1 : Co-precipitation

[0116] 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.

[0117] • Step 2: Mixing

[0118] 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.

[0119] • Step 3: Heating

[0120] The mixture is heated at 890 °C for 10 hours in an oxidizing atmosphere to obtain a heated product.

[0121] • Step 4: Wet bead milling

[0122] 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.

[0123] • Step 5: Second mixing

[0124] The milled product obtained from Step 5 was mixed in an industrial blender with 1.5 mol% Co from CO3O4 and 8.5 mol% Li from LiOH, each with respect to the total molar contents of Ni, Mn, and Co in the milled product to obtain a second mixture.

[0125] • Step 6: Second heating

[0126] The second mixture from Step 5 was heated at 760 °C for 10 hours under an oxidizing atmosphere followed by crushing and sieving with 250 ppm of alumina powder to obtain a second heated product.

[0127] • Step 7: Wet mixing

[0128] Step 7a) to Step 7c) below was applied to introduce Zr into the positive electrode active material: o Step 7a) Zr solution preparation: 0.8 mol% of Zr from Zr-propoxide (70 wt.% Zr-propoxide in n-propanol solution), 1.6 mol% of Li from Li- ethoxide powder, each with respect to the total molar contents of Ni, Mn, and Co in the second heated product, and ethanol solvent were mixed to form a solution. The amount of ethanol solvent was 40 wt.% of the total weight of the designated second heated product to mix in the Step 7b). o Step 7b) Mixing: the second heated product obtained from Step 6) was mixed with Zr solution prepared in Step 7a) for 20 minutes in a heatable reactor. o Step 7c) Heating: 70°C heat was applied to reactor in Step 7b) while at the same time reactor was connected to a vacuum pump to evaporate volatile phases. A dried powder was obtained from this step.

[0129] • Step 8: Fourth heating

[0130] The dried powder from Step 7c) was heated at 350 °C for 6 hours under an oxygen atmosphere to obtain NMC811-Zr.

[0131] ICP-OES protocol :

[0132] The cathode 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 2nd dilution, where the volumetric flask is filled with internal standard and 10% hydrochloric acid up to the 250 mL mark and then homogenized. Finally, this solution is used for ICP- OES measurement. The contents of elements are expressed as at%.

[0133] Table 1 : ICP-OES values for NMC811-Nb and NMC811-Zr.

[0134] Synthesis protocol for the sulfur containing positive electrode active material

[0135] • Step 1 : Mixing protocol of the positive electrode active material and the sulfide solid electrolyte

[0136] All the synthesis work and sample treatment were carried out in Ar filled glovebox with O2 and H2O levels <0.1 ppm. Different formulations of composite active material were prepared, in which the weight ratio (w / w%) of the NMC811-Nb and NMC811-Zr with the sulfide solid electrolyte were respectively 72:28 and 96:4. The positive active material and the sulfide solid electrolyte, LiePSsCI, were weighted to obtain a 5 g batch mixture. The mixture was transferred into a Restch PM 100 using 50 mL zirconia ball-milling jar along with 30g of zirconia balls of 0.8 mm diameter (the ball : powder ratio was 6: 1 in mass). The mixture was initially milled at 100 rpm for 3 hours to obtain a homogeneous mixture. Each cycle constituted in 5-minute milling and 5-minute rest and reversing the direction of milling for every cycle. After the 3 hours of milling time, the ball-milling jars were opened in the glovebox to recover the final mixture of NMC811-Nb or NMC811-Zr and SSE. At the end of the ball milling step, approximately 96 wt% of the material was recovered.

[0137] • Step 2: Firing / Heat treatment o Heat treatment using Hot pressing (EXI)

[0138] The mixture powder of NMC811-Nb and LiePSsCI (after BM) was uniaxially pressed into a 13 mm pellet in WC mold. And then the mold is placed in hot pressing equipment (Fontijne) which were placed under vacuum (0,1 bar). The temperature increased to 250 °C at a ramp rate of 20 °C / min with a pressure of 375 MPa, held for 1 hours, and cooled to room temperature at a ramp rate of 20 °C / min. The reacted pellets were then pulverized using a pestle and mortar and stored in the glovebox for further analysis. o Heat treatment using Hot pressing (CEX1.1)

[0139] The mixture powder of NMC811 and LiePSsCI (after BM) was uniaxially pressed into a 13 mm pellet in WC mold. And then the mold is placed in hot pressing equipment (Fontijne) which were placed under vacuum (0,1 bar). The temperature increased to 250 °C at a ramp rate of 20 °C / min with a pressure of 375 MPa, held for 1 hours, and cooled to room temperature at a ramp rate of 20 °C / min. The reacted pellets were then pulverized using a pestle and mortar and stored in the glovebox for further analysis. o Cold treatment using Cold pressing (CEX1.2)

[0140] The mixture powder of NMC811-Nb and LiePSsCI (after BM) was uniaxially pressed into a 13 mm pellet in WC mold. And then the mold is placed in hot pressing equipment (Fontijne) which were placed under vacuum (0,1 bar). The temperature increased to 25 °C at a ramp rate of 20 °C / min with a pressure of 375 MPa, held for 1 hours, and cooled to room temperature at a ramp rate of 20 °C / min. The reacted pellets were then pulverized using a pestle and mortar and stored in the glovebox for further analysis. o Cold treatment using Cold pressing (CEX1.3)

[0141] The mixture powder of NMC811 and LiePSsCI (after BM) was uniaxially pressed into a 13 mm pellet in WC mold. And then the mold is placed in hot pressing equipment (Fontijne) which were placed under vacuum (0,1 bar). The temperature increased to 25 °C at a ramp rate of 20 °C / min with a pressure of 375 MPa, held for 1 hours, and cooled to room temperature at a ramp rate of 20 °C / min. The reacted pellets were then pulverized using a pestle and mortar and stored in the glovebox for further analysis. o Heat treatment using Heat treatment (EX2)

[0142] The mixture powder of NMC811-Zr and LiePSsCI (after BM) was placed in dried quartz tubes which were then closed under Ar and placed in a furnace (Nabertherm) for heat treatment. The temperature of the furnace was slowly increased to 250 °C at a ramp rate of 2 °C / min, held for 5 hours, and naturally cooled to room temperature. The reacted powders were then pulverized using a pestle and mortar and stored in the glovebox for further analysis.

[0143] X-ray diffraction

[0144] The powder X-ray diffraction patterns were collected using Bruker D8 diffractometers equipped with either Cu (Koi-Koz) radiation in a 0-0 configuration. An air-tight sample holder with dome type was used for the measurements. The patterns were collected between 20 = 10° - 55° with a step size of 0.02°. XR.D measurement for EXI demonstrated that before and after hot pressing the XRD graph are almost identical.

[0145] TEM-EDX

[0146] A cross-sectional particle sample was prepared by using, for example, Focused Ion Beam (FIB) technique. A cross-sectional particle sample should contain ideally only one cathode active material particle so that other components such as a solid electrolyte in a cathode do not affect the EDX result. The cross-sectional particle sample was analyzed by FEI TitanThemis aberration-corrected transmission electron microscope equipped with ThermoFischer Super-X EDX detectors, used at 200 kV. The sample area is reduced to 256x256 pixels using 235k x magnification, a spot size of 11 and a 30mm aperture lens. The EDX analyses presented were carried out at low doses using specific acquisition conditions. These involved long acquisition times (lh-lh30), with a large number of short passes and a short dwell time at each position (dwell time 3-10 ps / pixel).

[0147] The cross-sectional EDX elemental mapping results of Zr, Nb, and S were used to identify the existence of the surface layer. The surface layer is defined as a layer having relatively higher concentration of Zr and S or Nb and S compared to overall concentration as measured by ICP-OES. The thickness of the surface layer was directly measured by observing the EDX elemental mapping images for Zr, Nb, and S. Figure 1 (b) shows another way to measure a thickness of the surface layer by using an EDS line profiling technique.

[0148] The coverage of the surface layer was obtained by below equation;

[0149] Length of a covered surface / the perimeter of a cross-sectional particle wherein the covered surface is covered by both Zr and S; or both Nb and S. The covered surface is identified by manual observation of EDS elemental mapping images.

[0150] Electrochemical conductivity

[0151] About 500 mg of the mixture NMC811-Nb + LieRSsCI (CEX2), NMC811-Zr + LiePSsCI (CEX3), EXI and EX2 was uniaxially cold-pressed in a 13 mm die at 375 MPa. The thickness was 2.5 mm approximately. The pellet is assembled with Ni discs (not pressed, just in contact) connected to the aluminum tabs of the pouch cell. Cells are tested at 23 °C using Wagner DC polarization technique: 1 V is applied using a BioLogic VMP3 potentiostat. The residual current obtained after 3400 s is used to calculate the electronic conductivity.

[0152] Battery testing

[0153] First step, to make the separator, about 150 mg of pure LiePSsCI was uniaxially cold- pressed in a 13 mm die at 75 MPa. Then, the positive electrode active material NMC811-Nb + LiePSsCI (CEX2) or NMC811-Zr + LiePSsCI (CEX3) or the positive electrode composite active material CEX1.1-1.3, EXI or EX2 is weighted and placed on top of the separator to achieve a NMC loading level between 15 to 30 mg / cm2. Then, the sandwich was again uniaxially cold-pressed in a 13 mm die at 375 MPa. Finally, Li metal anode is placed under the separator and the newly assembled positive electrode / solid electrolyte layer / negative electrode sandwich is again pressed at 75 MPa.

[0154] This assembly is sealed in a pouch foil under 0.1 bar of dry air. The cells are cycled under a stacking pressure of 9 MPa applied with jigs in a temperature-controlled chamber.

[0155] Examples

[0156] TEM-EDX pictures demonstrate that the surface layer of the positive electrode active material has sulfur on the location as the Nb and Zr (see Figures 1 and 2 respectively). The thickness of the surface layer is around 30 nm as shown in Figure 1(b).

[0157] Table 2 displays the overall composition of the examples EX1-2 and CEX2-3 synthesized via the general synthesis protocol described above with their corresponding electronic conductivity. Cathode comprising CEX1.1, CEX1.2 or CEX1.3 did not demonstrate any cycling stability, whereas EXI performs over repeated charge and discharge cycles in a battery.

[0158] Table 2: electronic conductivity of EXI, EX2, CEX2 and CEX3.

[0159] Moreover, the cathode comprising CEX1.1, CEX1.2 or CEX1.3 did not demonstrate any cycling stability, whereas EXI performs over repeated charge and discharge cycles in a battery (see Figure 3).

[0160] A battery comprising EX2 shows a higher battery performance than a battery comprising CEX3 as positive electrode active material (see Figure 4).

Claims

CLAIMS1. A positive electrode active material comprising Li, Ml, S and 0, wherein Ml comprises- Ni in a content xl, wherein 55.0 at% < xl < 95.0 at%, relative to Ml;- Mn in a content yl, wherein 0.0 at% < yl < 40.0 at%, relative to Ml;- Co in a content zl, wherein 0.0 at% < zl < 40.0 at%, relative to Ml;- M' in a content al, wherein 0.0 at% < al < 5.0 at%, relative to Ml;- DI in a content bl, wherein 0.0 at% < bl < 2.0 at%, relative to Ml; wherein DI is at least one element other than Li, Ni, Mn, Co, M', S and O;- wherein xl, yl, zl, al and bl are measured by ICP-OES;- wherein xl + yl + zl + al + bl is 100.0 at%; and wherein the positive electrode active material comprises particles having a surface layer covering at least a part of a surface of the particles, wherein the surface layer comprises S and M', wherein M' is B, Zr, Nb or a combination thereof.

2. The positive electrode active material according to claim 1,- wherein 60.0 at% < xl < 90.0 at%, relative to Ml;- wherein 0.0 at% < yl < 30.0 at%, relative to Ml;- wherein 0.0 at% < zl < 30.0 at%, relative to Ml;- wherein 0.0 at% < al < 1.5 at%, relative Ml;- wherein 0.0 at% < bl < 2.0 at%, relative to Ml.

3. The positive electrode active material according to claim 1 or 2, wherein- wherein 75.0 at% < xl < 88.0 at%, relative to Ml;- wherein 5.0 at% < yl < 15.0 at%, relative to Ml;- wherein 5.0 at% < zl < 15.0 at%, relative to Ml;- wherein 0.02 at% < al < 2.0 at%, relative Ml;- wherein 0.0 at% < bl < 1.0 at%, relative to Ml.

4. The positive electrode active material according to any one of claims 1-3, wherein DI is at least one element selected from the group consisting of at least one element of the group consisting of: Al, Ba, Ca, Ce, Cr, Fe, La, Mg,Mo, S, Sr, Ti, V, W, Y, and Zn; preferably Al, Ti, Cr, S, Y, and W; more preferably Al, B, Ti, and W.

5. The positive electrode active material according to any one of claims 1-4, wherein bl is about 0.0 at%, relative to Ml.

6. The positive electrode active material according to any one of claims 1-5, wherein M' is Nb, Zr or B; more preferably Nb or Zr, most preferably Nb.

7. The positive electrode active material according to any one of the previous claims having a Li / Ml molar ratio in the range of 0.90 - 1.10, preferably in the range of 0.92 - 1.08, more preferably in the range of 0.95 - 1.05.

8. The positive electrode active material according to any one of the previous claims, wherein the positive electrode active material comprises single particles and / or secondary particles, wherein each of the single particles consist of only one primary particle and each of the secondary particles consist of at least two primary particles and at most twenty primary particles as observed in a SEM image.

9. The positive electrode active material according to any one of the previous claims, wherein the surface layer has a thickness between 10 and 50 nm, preferably between 20 and 40 nm, more preferably between 25 and 35 nm, most preferably about 30 nm.

10. The positive electrode active material according to any one of the previous claims, wherein the surface layer covers at least 70 % of the surface of the positive electrode active material, preferably at least 80 %, more preferably at least 90%, most preferably 95%, even most preferably the whole surface of the positive electrode active material.

11. The positive electrode active material according to any one of the previous claims, wherein M' = Zr and having an electronic conductivity of at least 0.11 mS / cm, preferably at least 0.13 mS / cm, most preferably at least 0.15 mS / cm.

12. The positive electrode active material according to any one of the previous claims, wherein M' = Nb and having an electronic conductivity of at least 1.0 mS / cm, preferably at least 2.0 mS / cm, most preferably at least 3.0 mS / cm.

13. A battery comprising a positive electrode comprising the positive electrode active material according to any of the claims 1-12, a solid electrolyte and a negative electrode.

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