A li-rich, mn-rich cathode active material for rechargeable solid-state batteries

Abstract

This invention relates to a cathode active material for solid-state batteries comprising Li, M and O, wherein M comprises Ni, Mn, B, wherein the molar ratio of Li and M, Li / M, is between 1.20 and 1.40; and wherein B is present in a surface coating and the B content, calculated versus the total molar fraction of Ni, Mn, and B in the coating, measured by X-ray photoelectron spectroscopy XPS of at least 25 mol%.

Description

DESCRIPTIONTITLEA Li-RICH, Mn-RICH CATHODE ACTIVE MATERIAL FOR RECHARGEABLE SOLID- STATE BATTERIESTECHNICAL FIELD

[0001] This invention relates to a Li-rich and Mn-rich cathode active material for a rechargeable solid-state battery, which is B coated, preferably wherein the solid-state battery is a sulfide based solid-state battery. This invention further relates to a process for manufacturing such a cathode active material; and to a battery comprising said cathode active material.BACKGROUND ART

[0002] Energy storage systems such as rechargeable batteries are necessary for quickly storing and releasing high amounts of energy to adjust power output to demand. The same battery technology goes into electric vehicles, which run on stored electrical energy and reduce pollution compared to conventional vehicles with internal combustion engines. To meet proper demands, these batteries need to store high levels of energy with minimal weight, charge and discharge at fast rates, and go through many cycles without diminishing in performance. These requirements are respectively referred to as high energy density, rate capability, and cyclability, and the demands must be reached while the batteries remain affordable and safe. Particularly because of their high energy density and rate capability, many kinds of lithium-ion batteries (LIBs) are widely studied to meet these needs. The capacity and energy density of LIBs however remains limited by the available cathode active materials.

[0003] Lithium and manganese-rich (LMR) layered oxide is one of promising cathode active material for lithium-ion batteries due to its low cost originated from relatively lower Co content. However, the cost of the LMR layered oxide has to be further decreased and its energy density should be further improved.

[0004] All-solid-state lithium-ion batteries (ASSB) represent a promising battery technology thanks to the replacement of the volatile and flammable state-of-the-art liquid electrolyte by a solid electrolyte.

[0005] Therefore, there remains a need to provide a Li-rich and Mn-rich cathode active material, which is Co-free to make more affordable solid-state batteries with improved capacity, energy density, cycle life, and lower irreversible capacity.

[0006] It is therefore an object of the present invention to provide a cathode active material having good electrochemical properties, indicated by high discharge capacity (DQ) better cycle life, and high energy density for use in a solid-state rechargeable battery, preferably a sulfide based solid-state battery.

[0007] It is a further object of the present invention to provide a process for manufacturing said cathode active material.

[0008] It is a further object of the present invention to provide a solid-state battery comprising said cathode active material.SUMMARY

[0009] In a first aspect, the object of the invention is achieved by providing a cathode active material for rechargeable solid-state batteries consisting of Li, M and O, wherein M comprises: a. Ni in a molar ratio xi , wherein 0.03 < xi < 0.50 relative to M, b. Mn in a molar ratio yi , wherein 0.40 < yi < 0.95 relative to M, and c. B in a molar ratio zi , wherein 0.001 < zi < 0.10 relative to M, wherein the content of xi , yi , and zi is measured by Inductively Coupled Plasma - Optical Emission Spectroscopy ICP-OES; and xi+yi+zi is 1.00, the molar ratio of Li to M is between 1 .00 and 1 .60; and wherein B is present in a surface coating and the B content, calculated versus the total molar fraction of Ni, Mn, and B in the coating, measured by X-ray photoelectron spectroscopy XPS of at least 25 mol%..

[0010] Preferably, the rechargeable solid-state battery is a sulfide based solid-state battery.

[0011] It was found that the boron surface coating enables high reversible capacity, improved initial charge capacity CQ1 , initial discharge capacity DQ1 and reduced irreversible capacity loss Qin- of Li-, Mn-rich cathode material in sulfide solid-state batteriesthanks to better interfacial stability between the cathode and the solid electrolyte. This improvement is not seen in a lithium ion secondary battery based using a liquid electrolyte.

[0012] A high-manganese-based positive electrode active material according to the present invention may improve stability and initial capacity of an all-solid-state lithium secondary battery, and may also improve cycle characteristics of the battery by mitigating side reactions when incorporated into a cathode-solid electrolyte composite. An extra benefit of the invention is that the surface coating enables stable high voltage operation of the positive electrode material and enables additional reversible capacity to be achieved from anionic redox reactions.

[0013] The coating enhances the interfacial stability between the cathode active material and solid electrolyte, enabling high capacity and cycle life to be achieved compared to the bare material.

[0014] In a further aspect, the present invention provides a process for manufacturing said cathode active material, in particular through a solid-state synthesis route.

[0015] In a further aspect the present invention provides a battery comprising said cathode active material.Brief description of the figures

[0016] Figure 1 : SEM images of CEX1

[0017] Figure 2: SEM images of EX1 .1

[0018] Figure 3: Differential capacity profiles recorded during the first charge and discharge cycle of CEX1 , EX1 .1 and Ex1 .2.DETAILED DESCRIPTION

[0019] 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 and accompanying drawings.

[0020] 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”.

[0021] The term “cathode active material” (CAM, also known as a positive electrode active material) as used herein and in the claims 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. The term “anode active material” (also known as negative electrode active material) as used herein and in the claims is defined as a material which is electrochemically active in a negative electrode or anode.

[0022] In the framework of the present invention, at% signifies atomic percentage. The at% or “atomic percent” of a given element expression of a concentration means how many percent of all atoms in the concerned compound are atoms of said element. The designation at% is equivalent to mol% or “molar percent”.

[0023] An "all-solid-state secondary battery" can mean a secondary battery in which a cathode, an anode, and an electrolyte are formed together, and the electrolyte can be divided into an organic (polymer)-based all-solid-state secondary battery based on a polymer (e.g., polyethylene oxide, etc.), and an inorganic all-solid-state secondary battery based on Li-P-S, etc.

[0024] Here within a range “from X to Y” or “between X and Y” includes the endpoints X and Y.

[0025] 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.Li- and Mn-rich (LMR) cathode active material

[0026] In a first aspect, the object of the invention is achieved by providing a cathode active material for solid-state batteries comprising Li, M and 0, wherein M comprises:Ni in a molar ratio xi , wherein 0.03 < xi < 0.50 relative to M,Mn in a molar ratio yi, wherein 0.40 < yi < 0.95 relative to M, andB in a molar ratio zi , wherein 0.001 < zi < 0.10 relative to M, wherein the content of xi , yi , and zi is measured by ICP-OES; and xi+yi+zi is 1.00, the molar ratio of Li and M is between 1.20 and 1.40; and wherein B is present in a surface coating and wherein and the B content, calculated versus the total molar fraction of Ni, Mn, and B in the coating, measured by X-ray photoelectron spectroscopy XPS of at least 25 mol%, preferably at least 40 mol%.

[0027] Cathode active material according the present invention, which is a high- manganese-based cathode active material, may improve stability and initial capacity of an all-solid-state lithium secondary battery, and may also improve cycle characteristics of the battery by mitigating side reactions when incorporated into a cathode-solid electrolyte composite. An extra benefit of the invention is that the surface coating enables stable high voltage operation of the positive electrode material and enables additional reversible capacity to be achieved from anionic redox reactions. The coating enhances the interfacial stability between the cathode active material and solid electrolyte, enabling high capacity and cycle life to be achieved compared to the bare material. After coating there is no change in the morphology of the particles as can be seen from Figure 1 and Figure 2.

[0028] The cathode active material of the present invention may in particular comprise secondary particles comprising a plurality of primary particles. In particular the each of the secondary particles in the positive electrode active material powder according to the present disclosure may consist of at least twenty, even at least thirty primary particles.

[0029] As can be seen from Figure 2, the positive electrode active material of the present invention comprises essentially polycrystalline particles.

[0030] In the framework of the present disclosure, the particles, wherein each of the secondary particles consisting of such a plurality of primary particle, are referred to as polycrystalline particles. At least about 30% of the particles, more preferably at least aboutin a Scanning Electron Microscope (SEM) image may be polycrystalline particles. The number of primary particles constituting the monolithic particle is determined in a field of view of at least about 45 pm x at least about 60 pm (i.e. of at least about 2700 pm2), preferably of: at least about 100 pm x about 100 pm (i.e. of at least about 10,000 pm2). The particles in the image may 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 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.

[0031] A preferred embodiment is the cathode active material according to the invention, wherein the molar ratio of Li to M (Li / M) is between 1 .20 and 1 .40. In particular, Li / M may be at least 1.25, at least 1.26, at least 1.27, at least 1.28, or at least 1.29. In particular, Li / M may be at most 1 .39, at most 1 .38, at most 1 .37, at most 1 .36, or at most 1 .35.

[0032] In one embodiment of the invention, this cathode active material has a composition according to a general formula (I): LiwNix2Mny2Bz2O2, a. wherein 0.03 < x2 < 0.50, 0.40 < y2 < 0.95, 0.001 < z2 < 0.10, and 1 .20 < w < 1.40, b. wherein x2+y2+z2 is 1.00, and the content of Li, Ni, Mn and B is measured by ICP-OES; c. and wherein B is present in a surface coating.

[0033] In another embodiment, this cathode active material has a composition according to a general formula (I), wherein 0.03 < x20.50, 0.50 < y2 0.70, 0.005 < Z2 0.05, 1 .20 < w < 1.40.

[0034] In another preferred embodiment of the cathode active material according to the invention, the molar ratio of B zi or Z2 is in the range of 0.001 < zi or Z2 < 0.10, preferably in the range of 0.005 < zi or Z2 < 0.05, more preferably in the range of 0.0025 < zi or Z2 < 0.02. In particular, the molar ratio of B zi or Z2 is about 0.004, about 0.005, about 0.008, about 0.01 , about 0.02, or about 0.03.

[0035] In another preferred embodiment of the cathode active material according to the invention, the molar ratio of Ni xi or x2 is at most 0.50.

[0036] In another preferred embodiment of the cathode active material according to the invention, the molar ratio of Mn yi or y2 is in the range of 0.40 < yi or y2 < 0.95, preferably in the range of 0.50 < yi or y2 < 0.70.

[0037] In another preferred embodiment of the cathode active material according to the invention, the molar ratio of Li w is in the range of 1 .20 to 1 .40 . Advantageously, w may be at least 1 .25, at least 1 .26, at least 1 .27, at least 1 .28, or at least 1 .29. In particular, w may be at most 1 .39, at most 1 .38, at most 1 .37, at most 1 .36, or at most 1 .35.

[0038] As appreciated by the skilled person the amount of xi , yi , zi , X2, y2, Z2 and w are determined by Inductively coupled plasma atomic emission spectrometry (ICP-AES), in particular the amounts of Li, Ni, Mn and B. For example, but not limited to this invention, a PerkinElmer NexION 2000 ICP mass spectrometer can be used for ICP-AES measurements. Inductively coupled plasma atomic emission spectrometry (ICP-AES) is also known as Inductively coupled plasma optical emission spectrometry (ICP-OES).

[0039] In this aspect, the present invention provides a cathode active material for a solid- state rechargeable battery comprising a positive electrode active material according to that described in the first aspect of the invention. Preferably, the solid-state rechargeable battery is a sulfide based solid-state battery and the solid electrolyte is a sulfide based electrolyte.Method for manufacturing a cathode active material

[0040] In a further object, the present invention provides a process for manufacturing said cathode active material through a solid-state synthesis route.

[0041] In another aspect, present invention relates to a method for manufacturing a cathode active material, wherein the method comprises the following steps:1 ) mixing a transition metal source, wherein the transition metal consist of Ni and Mn, mixed with lithium source;2) heating the mixture from Step 1 ) to a temperature in the range of 300 to 600 °C; and then to a temperature in the range of 700 to 1100 °C under air atmosphere for a duration of 1 to 24 h;3) coating the product from Step 2) with a boron compound;4) heating the dried powder from Step 3) at a temperature in the range of 200 to 900 °C for a duration of 3 to 10 h under air to obtain the cathode active material; after sintering in step 2, the mixture is cooled.

[0042] The transition metal powder as nickel and manganese source could be their respective nitrates, halides, oxides, carbonates apart from hydroxides.

[0043] The lithium source is at least one of lithium carbonate, lithium hydroxide monohydrate, lithium oxalate, lithium nitrate.

[0044] In one embodiment the coating in step 3 is a dry coating, and the product from step 2 is mixed with a boron compound using a mixer; in particular acoustic mixer.

[0045] In another embodiment the coating in step 3 is a wet coating, and the product from step 2 is dispersed in a solution containing boron and lithium sources, dissolved in a suitable solvent. A suitable solvent may be ethanol or propanol, the solid content of the mixture may be 60 - 70% by weight. Then the solution is stirred and heated under vacuum conditions. Further an additional drying step is carried out to remove any residual solvent.

[0046] The method with its wet coating step is unique over the prior art in the additional Li source added during the coating process and in that the coating reagents are dissolved in ethanol or propanol.

[0047] The coating enhances the interfacial stability between the CAM and solid electrolyte, enabling high capacity and cycle life to be achieved compared to the bare material.

[0048] In a preferred embodiment the method comprises a wet coating process and comprises the following steps:1 ) First mixing: A transition metal hydroxide powder, wherein the transition metal consist of Ni and Mn having molar ratio Ni / Mn in the range of 0.05 / 0.95 to 0.5 / 0.5 is mixed with lithium hydroxide such that the lithium to transition metal molar ratio is in the range of 1 .2 - 1 .6.2) First heating: The mixture from Step 1 ) is pre-heated to a temperature in the range of 300 to 600 °C, in particular at about 5 °C / min ramping rate, under air atmosphere. The temperature is further increased to a temperature in the range of 700 to 1100 °C, in particular at 1.5 to 3 °C / min ramping rate, under airatmosphere and maintained at a temperature in the range of 700 to 1100 °C for a duration of 1 h to 24 h. After this sintering, the mixture is cooled.3) Coating: The product from Step 2) is dispersed in a solution of a boron source with an amount of 1 to 5 mol% of B, relative to the total molar contents of Ni and Mn , and Li source, source with an amount of 1 to 5 mol% of Li, relative to the total molar contents of Ni and Mn which are dissolved in an solvent to obtain a second mixture. The flask is transferred into a water bath. Then, the solution is stirred and heated to 40 °C under vacuum conditions of for example about -0.4 bar for about 60 min.3b) Drying: The resulting slurry from Step 3) is dried at a temperature of about 110 °C under vacuum conditions for about 12 h.4) Second heating: The dried powder from Step 3b) is heated at a temperature in the range of 200 to 900 °C (preferably in the range of 350 to 700 °C) for a duration of 3 to 10 h under air to obtain a positive electrode active material.

[0049] The boron source may in particular be selected from boric acid, boron oxide, and ammonium tetraborate.

[0050] The lithium source may in particular be LiOH, Li2CO3, and lithium ethoxide.

[0051] The solvent may be an alcohol. The solvent may in particular be selected from ethanol and propanol.

[0052] The second mixture may have solid content of 60 to 70% by weight.

[0053] In another embodiment the method comprises a dry coating process and comprises the following steps:1 ) First mixing: A transition metal hydroxide powder, wherein the transition metal consist of Ni and Mn having molar ratio Ni / Mn in the range of 0.05 / 0.95 to 0.5 / 0.5, is mixed with lithium hydroxide such that the lithium to transition metal molar ratio is in the range of 1 .2 - 1 .6.2) First heating: The mixture from Step 1 ) is pre-heated to a temperature in the range of 300 to 600 °C, in particular at about 5 °C / min ramping rate, under air atmosphere. The temperature is further increased to a temperature in the range of 700 to 1100 °C, in particular at 1.5 to 3 °C / min ramping rate, under airatmosphere and maintained at a temperature in the range of 700 to 1100 °C for a duration of 1 h to 24 h. After this sintering, the mixture is cooled.3) Coating: The product from Step 2) is mixed with H3BO3 in an amount of 1 to 5 mol% of B, relative to the total molar contents of Ni and Mn to obtain a second mixture in an acoustic ram mixer for 10 minutes.4) Second heating: The dried powder from Step 3) is heated at a temperature of 200 to 900 °C (preferably of 350 to 700 °C) for a duration of 3 to 10 h under air to obtain a positive electrode active material.

[0054] Preferred boron source in this embodiment is H3BO3.

[0055] In the methods of the present invention, the transition metal hydroxide powder, may be mixed with lithium hydroxide such that the lithium to transition metal molar ratio is in the range of 1 .2 to 1 .4Battery

[0056] Specifically, an all-solid-state battery according to the present invention includes a positive electrode comprising a mixture of the cathode active material and sulfide-based solid electrolyte; a negative electrode, and sulfide-based solid electrolyte interposed between the positive electrode and the negative electrode.

[0057] In a further object, the present invention provides a battery comprising said cathode active material, in particular the cathode active material according to the first aspect of the invention. Preferably, the solid-state rechargeable battery is a sulfide based solid-state battery.

[0058] In a preferred embodiment the battery is a lithium-ion battery, preferably a lithium- ion rechargeable battery. Preferably the battery comprises a positive electrode comprising the active material according to the first aspect of the invention, a negative electrode, an solid electrolyte, and a separator.

[0059] Preferably, the solid-state battery further comprises an anode comprising anode active material. Suitable electrochemically active anode materials are those known in the art. For example, the anode may comprise graphitic carbon, metallic lithium or a metal alloy comprising lithium such as Li-ln alloy, as the anode active material.

[0060] In additional aspect the present invention concerns a use of the battery according to the last aspect of the invention in a portable electronic device, such as portablecomputer, 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.EXAMPLES and EXPERIMENTAL TESTS

[0061] The invention is described below in greater details with reference to examples, but the invention is not limited in any way by these examples, as long as it does not exceed the scope and spirit of the present invention.Experimental tests used in the examples

[0062] Scanning Electron Microscopy (SEM) - The morphology of positive electrode active materials is analyzed by a Scanning Electron Microscopy (SEM) technique. The measurement is performed using a Jeol JSM-F100 under a high vacuum environment of 8x1 O’6Pa at 25 °C. 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 dry air to remove the excess powder.

[0063] Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES) - The composition of a positive electrode active material powder is measured by the inductively coupled plasma (ICP) method using an Agilent 720 ICP-OES. 1 gram of powder sample is dissolved into 50 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 watch glass and heated on a hot plate at 380 °C until the powder is completely dissolved. After being cooled to room temperature, the solution from the Erlenmeyer flask is poured into a first 250 mL volumetric flask. Afterwards, the first volumetric flask is filled with deionized water up to the 250 mL mark, followed by a complete homogenization process (1stdilution). An appropriate amount of the solution from the first volumetric flask is taken out by a pipette and transferred into a second 250 mL volumetric flask for the 2nddilution, where the second volumetric flask is filled with an internal standard element and 10 % hydrochloric acid up to the 250 mL mark and then homogenized. Finally, this solution is used for ICP measurement.

[0064] X-ray Photoelectron Spectroscopy (XPS) - In the present invention, X-ray photoelectron spectroscopy (XPS) is used to analyze the surface of positive electrodeactive material powder particles. In XPS measurement, the signal is acquired from the first few nanometers (e.g., 1 nm to 10 nm) of the uppermost part of a sample, i.e., surface layer. Therefore, all elements measured by XPS are contained in the surface layer.

[0065] For the surface analysis of positive electrode active material powder particles, XPS measurement is carried out using a Thermo K-a+ spectrometer. Monochromatic Al Ka radiation (hu = 1486.6 eV) is used with a spot size of 400 pm and measurement angle of 45°. A wide survey scan to identify elements present at the surface is conducted at 200 eV pass energy. C1 s peak having a maximum intensity (or centered) at a binding energy of 284.8 eV is used as a calibrate peak position after data collection. Accurate narrow scans are performed afterwards at 50 eV for at least 10 scans for each identified element to determine the precise surface composition.

[0066] Curve fitting is done with CasaXPS Version 2.3.19PR1.0 using a Shirley-type background treatment and Scofield sensitivity factors. The fitting parameters are according to Table 2a. Line shape GL(30) is the Gaussian / Lorentzian product formula with 70% Gaussian line and 30% Lorentzian line. LA(a, |3, m) is an asymmetric line-shape where a and [3 define tail spreading of the peak and m define the width.

[0067] Table 1 XPS fitting parameter for Ni2p3, Mn2p3, and B1s.EXAMPLES

[0068] The present invention is further illustrated by the following examples.Comparative example 1 (Li1.28Nio.32Mno.68O2)

[0069] A positive electrode active material labelled as CEX1 is prepared according to the following steps:Step 1 ) First mixing: A transition metal hydroxide powder, wherein the transition metal consist of Ni and Mn having molar ratio of Ni:Mn = 38:62, is mixed withlithium hydroxide such that the lithium to transition metal molar ratio of 1 .3 for 30 minutes at 1000 RPM.Step 2) First heating: The mixture from Step 1 is pre-heated to 500 °C at 5 °C / min ramping under air atmosphere. The temperature is further increased to 925 °C at 2 °C / min ramping rate under air atmosphere and maintained at 925 °C for 10 h. After sintering, the mixture is cooled naturally to obtain CEX1 .

[0070] The prepared CEX1 is analyzed using Inductively Coupled Plasma (ICP) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), and solid-state cell testing to evaluate composition, surface characteristics, morphology, and electrochemical performance.Example 1

[0071] A positive electrode active material labelled as EX1 is prepared according to the following steps:Step 1 ) First mixing: A transition metal hydroxide powder, wherein the transition metal consist of Ni and Mn having molar ratio of Ni:Mn = 38:62, is mixed with lithium hydroxide such that the lithium to transition metal molar ratio of 1 .3 for 30 minutes at 1000 RPM.Step 2) First heating: The mixture from Step 1 was pre-heated to 500 °C at 5 °C / min ramping under air atmosphere. The temperature is further increased to 925 °C at 2 °C / min ramping rate under air atmosphere and maintained at 925 °C for 10 h. After sintering, the mixture is cooled naturally.Step 3) Coating: The first heated product from Step 2) is dispersed in a solution of 2 mol% (relative to sum of Ni and Mn) H3BO3, and 6 mol% (relative to sum of Ni and Mn) of LiOH, which was dissolved in ethanol (70 % solid content), each with respect to the total molar contents of Ni and Mn to obtain a second mixture. The flask is transferred into a water bath. Then, the solution is stirred and heated to 40 °C under vacuum conditions of -0.4 bar for 60min.Step 4) Drying: The resulting slurry from Step 3) is dried under at 110 °C under vacuum conditions for 12 h.Step 5) Second heating: The dried powder from Step 4) is heated at 350 °C for 7 h under air to obtain EX1 .1 .

[0072] EX1 .2 is prepared according to the same method as EX1 .1 except that the second heating temperature in Step 5) is 700 °C.

[0073] The prepared examples, EX1.1 and EX1.2, are analyzed using Inductively Coupled Plasma (ICP) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), and solid-state cell testing to evaluate their composition, surface characteristics, morphology, and electrochemical performance.Example 2

[0074] A positive electrode active material labelled as EX2 is prepared according to the following steps:Step 1 ) First mixing: A transition metal hydroxide powder, wherein the transition metal consist of Ni and Mn having molar ratio of Ni:Mn = 38:62, is mixed with lithium hydroxide such that the lithium to transition metal molar ratio of 1 .3 for 30 minutes at 1000 RPM.Step 2) First heating: The mixture from Step 1 ) is pre-heated to 500 °C at 5 °C / min ramping under air atmosphere. The temperature is further increased to 925 °C at 2 °C / min ramping rate under air atmosphere and maintained at 925 °C for 10 h. After sintering, the mixture is cooled naturally.Step 3) Coating: The first heated product from Step 2) is dispersed in a solution of 1 mol% H3BO3, and 3 mol % of LiOH, which is dissolved in ethanol (70 % solid content), each with respect to the total molar contents of Ni and Mn to obtain a second mixture. The flask is transferred into a water bath. Then, the solution is stirred and heated to 40 °C under vacuum conditions of -0.4 bar for 60 min.Step 4) Drying: The resulting slurry from Step 3) is dried under at 110 °C under vacuum conditions for 12 h.Step 5) Second heating: The dried powder from Step 4) is heated at 350 °C for 7 h under air to obtain EX2.1 .

[0075] EX2.2 is prepared according to the same method as EX2.1 except that the second heating temperature in Step 5) is 500 °C.

[0076] EX2.3 is prepared according to the same method as EX2.1 except that the second heating temperature in Step 5) is 700 °C.

[0077] The prepared examples, EX2.1 , EX2.2, and EX2.3, are analyzed using Inductively Coupled Plasma (ICP) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), and solid-state cell testing to evaluate their composition, surface characteristics, morphology, and electrochemical performance.Example 3

[0078] A positive electrode active material labelled as EX3 is prepared according to the following steps:Step 1 ) First mixing: A transition metal hydroxide powder, wherein the transition metal consist of Ni and Mn having molar ratio of Ni:Mn = 38:62, is mixed with lithium hydroxide such that the lithium to transition metal molar ratio of 1 .3 for 30 minutes at 1000 RPM.Step 2) First heating: The mixture from Step 1 ) is pre-heated to 500 °C at 5 °C / min ramping under air atmosphere. The temperature is further increased to 925 °C at 2 °C / min ramping rate under air atmosphere and maintained at 925 °C for 10 h. After sintering, the mixture is cooled naturally.Step 3) Coating: The first heated product from Step 2) is dispersed in a solution of 2 mol% (relative to sum of Ni and Mn) H3BO3 which is dissolved in ethanol (70% solid content), each with respect to the total molar contents of Ni and Mn to obtain a second mixture. The flask is transferred into a water bath. Then, the solution is stirred and heated to 40 °C under vacuum conditions of -0.4 bar for 60 min.Step 4) Drying: The resulting slurry from Step 3) is dried at 110 °C under vacuum conditions for 12 h.Step 5) Second heating: The dried powder from Step 4) is heated at 350 °C for 7 h under air to obtain EX3.1 .

[0079] EX3.2 is prepared according to the same method as EX3.1 except that the second heating temperature in Step 5) is 500 °C.

[0080] EX3.3 is prepared according to the same method as EX3.1 except that the second heating temperature in Step 5) is 700 °C.

[0081] The prepared examples, EX3.1 , EX3.2, and EX3.3, are analyzed using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), X-ray PhotoelectronSpectroscopy (XPS), Scanning Electron Microscopy (SEM), and solid-state cell testing to evaluate their composition, surface characteristics, morphology, and electrochemical performance.Example 4

[0082] A positive electrode active material labelled as EX4 is prepared according to the following steps:Step 1 ) First mixing: A transition metal hydroxide powder, wherein the transition metal consist of Ni and Mn having molar ratio of Ni:Mn = 38:62, is mixed with lithium hydroxide such that the lithium to transition metal molar ratio is 1 .3 for 30 minutes at 1000 RPM.Step 2) First heating: The mixture from Step 1 ) is pre-heated to 500 °C at 5 °C / min ramping under air atmosphere. The temperature is further increased to 925 °C at 2 °C / min ramping rate under air atmosphere and maintained at 925 °C for 10 h. After sintering, the mixture is cooled naturally.Step 3) Coating: The first heated product from Step 2) mixed with 2 mol % H3BO3, each with respect to the total molar contents of Ni and Mn to obtain a second mixture in an resonant acoustic RAM mixer.Step 4) Second heating: The mixture from Step 3) is heated at 350 °C for 7 h under air.

[0083] The prepared EX4 is analyzed using Inductively Coupled Plasma (ICP) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), and solid-state cell testing to evaluate composition, surface characteristics, morphology, and electrochemical performance.

[0084] The summary of the coating process for the examples and comparative example can be found in Table 2.

[0085] Table 2. Summary of the coating process for the preparation of cathode active material.Experimental testsSulfide solid-state battery

[0086] Positive electrode preparation - For the preparation of a positive electrode, a slurry contains positive electrode active material powder, Li-P-S based solid electrolyte [LiePSsCI (LPSCI)], carbon (Super-P, Timcal), and binder (RC-10, Arkema) - with a formulation of 64.0 : 30.0 : 3.0 : 3.0 by weight - in butyl acetate solvent is mixed in Ar- filled glove box. The slurry is casted on one side of an aluminum foil followed by drying the slurry coated foil in a vacuum oven to obtain a positive electrode. The obtained positive electrode is punched with a diameter of 10 nm wherein the active material loading amount is around 4 mg / cm2

[0087] Negative electrode preparation - For the preparation of a negative electrode, Li foil (diameter 3 mm, thickness 100 pm) is placed centered on the top of In foil (diameter 10 nm, thickness 100 pm) and pressed to form Li-ln alloy negative electrode.

[0088] Separator preparation - For the preparation of a separator which also has a function of the solid electrolyte in a battery, the Li-P-S based solid electrolyte is pelletized with a pressure of 250 MPa to obtain 100 pm pellet thickness.

[0089] Cell assemby - A sulfide solid-state rechargeable battery is assembled in an Ar- fil led glovebox with such order from bottom to top: positive electrode comprising Al current collector with the coated part on the top - separator - negative electrode with Li side onthe top - Cu current collector. The stacked components are pressed together with a pressure of 250 MPa and placed in an external cage to prevent air exposure.[00901 Electrochemical testing - The testing method is a conventional “constant cut-off voltage” test. Each cell is cycled at 60 °C using a Toscat-3100 computer-controlled galvanostatic cycling station (from Toyo). The schedule uses a 1 C current definition of 160 mA / g. The initial charge capacity (CQ1 ) and discharge capacity (DQ1 ) are measured in constant current mode (CC) at C rate of 0.1 C in voltage range from 4.3 V to 2.5 V (Li / Li+) or from 3.7 V to 1 .9 V ( I n-Li / Li+). The percentage (%) of the irreversible capacity is obtained according to an equation below: 100(%).Coin cell

[0091] Electrochemical test using coin cells are performed on CEX1 , EX1.1 , and EX1.2 to compare with the performance of sulfide solid-state rechargeable cell. (Results in Table 4)

[0092] Coin cell preparation - A slurry comprising a positive electrode active material powder, a conductive material (Super P, Timcal), and a binder (KF#9700, Kureha) in a weight ratio of 83:8.0:8.0, respectively, in a solvent (NMP, Mitsubishi) is homogenized using a high-speed homogenizer. The homogenized slurry is then uniformly applied to one side of an aluminum foil substrate using a doctor blade coater with a gap of 230 pm. The coated foil is subsequently dried in an oven at a temperature of 120 °C. Following the drying step, the coated foil is pressed using a calendaring tool and then subjected to a second drying process in a vacuum oven to ensure complete removal of the solvent from the electrode film.

[0093] The coin cell is assembled within an argon-filled glovebox to prevent contamination. A separator (Celgard 2320) is placed between the positive electrode and a piece of lithium foil, which serves as the negative electrode. An electrolyte solution comprising 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 3:7 is added between the separator and the electrodes. The coin cell is then sealed to prevent electrolyte leakage, completing the assembly process.

[0094] Testing method - The tests for discharge capacity (DQ) and irreversible capacity (Qirr) are performed using coin cells with electrodes consisting of 83 wt% of the cathode active material described in this invention. The electrode loading is approximately 6mg / cm2The discharge capacity for the first cycle (DQ1 ) is measured within the voltage range of 4.8-2.0 V at a rate of 0.05C (expressed in mAh / g) at a temperature of 25°C. The discharge capacity of the sixth cycle (DQ6) is measured within the voltage range 4.6-2.0 V at 0.2C after a five cycle rate performance test (0.2 C, 0.33C, 0.5C, 1 C).

[0095] Table 3a. Summary of the composition and the corresponding electrochemical properties of examples and comparative examples* calculated versus the total molar fraction of N i, Mn and B, as analysed by ICP-OES ** calculated versus the total molar fraction of N i, Mn, and B as analysed by XPS

[0096] Table 3b. Summary of electrochemical properties of examples and comparative examples

[0097] Table 3a summarizes the composition of examples and comparative examples and Table 3b summarizes their corresponding electrochemical properties. EX1 to EX4 comprise of boron coated material, meanwhile CEX1 is an uncoated material. Comparison between CEX1 and EX1 - 4 shows that B addition is beneficial to improve solid-state rechargeable battery performance for positive electrode active material as the first cycle discharge capacity (DQ1 ) increases and the irreversible capacity (Qirr) decreases. Additionally, the annealing temperature used to form the coated material also impacts the observed performance (e.g. EX1.1 , 2.1 and 3.1 exhibit the highest DQ1 among their series). The discharge capacity at the sixth cycle (DQ6) also shows that all coated samples exhibit improved capacity retention and compared to the uncoated material CEX1 .

[0098] Results in Table 3b for EX3.1 ~3.3 show that DQ1 result is lower than CEX1 (uncoated CAM), however the efficiency is improved which prove that the inclusion of Li source during wet coating gives a large improvement in DQ1 result [see EX1 and EX2 comprising Li and B coating versus EX3 comprising only B coating],

[0099] Table 3a also summarizes the XPS analysis result of EX1 to EX4 showing B fraction with respect to the total atomic fraction of Ni and Mn. The table also compares the result with that of ICP. The atomic ratio from XPS is higher than 1 which indicates that B is enriched in the surface region of the positive electrode active material, as the XPS signal is acquired from the first few nanometers (e.g. 1 nm to 10 nm) of the uppermostpart of a sample, i.e. a surface layer. On the other hand, B atomic ratio from ICP measurement is obtained from the entire particles. Therefore, the ratio of XPS to ICP of higher than 1 indicates that the presence of element B is concentrated on the surface of the positive electrode active material.

[0100] Table 4. Electrochemical performance comparison on coin cell test and sulfide solid-state cell test.

[0101] Based on Table 4, B-coated LMR (EX1.1 and EX1.2) demonstrated enhanced discharge capacity and reduced irreversible capacity in solid-state cell applications when compared with the uncoated LMR (CEX1 ). This is highlighted in Figure 3 which highlights the increased intensity of the peak at 4.48 V in the differential capacity profile, corresponding to the anionic redox contribution expected from Li-rich cathode active materials. Conversely, in coin cell tests, B-coated LMR exhibited a slight increase in charge capacity and an increase in irreversible capacity when compared with uncoated LMR. This indicates that B-coated LMR is more suitable specifically for solid-state battery applications.

[0102] Comparing the capacity vs. voltage profiles of the bare and B coated materials shows a distinct difference in behaviour particularly in the region > 4.3 V. The redox behaviour of Li-rich materials during (de-)lithiation can be divided into two regions, firstly cationic redox of the transition metals (e.g. Ni2+>Ni3+>Ni4+) analogous to conventional layered oxide cathode materials (e.g. NMC) as Li is extracted from the Li layer, and at higher potential vs. Li / Li+, anionic redox contribution from the oxygen sublattice as Li is extracted from the TM layer, which occurs beyond 4.3 V vs Li+ / Li. The effect of the coating is particularly pronounced in this high voltage region (4.3 - 4.8 V), which can be seen from the extended plateau observed in the potential vs. capacity plot (A), and the differential capacity plot (B) which demonstrate a clear increase in the contribution to capacity from anionic redox activity in the Li-rich cathode material.

[0103] Table 3b shows that discharge capacity after six cycles is significantly improved, which demonstrates that the B coated materials also deliver excellent cycle stability, in terms of both rate capability (ability to deliver reasonable capacity at high C-rates), and also capacity retention (consistent reversible on successive cycles). A common issue of sulfide solid-state battery systems is the narrow potential window in which the solid electrolyte is operable. However the coating here is able to ‘shield’ the solid electrolyte from the positive electrode material and clearly enables the operation of the sulfide solid- state battery at high potentials (up to 4.6 V) with stable capacity retention.

Claims

CLAIMS1. A cathode active material for solid-state batteries comprising Li, M and O, wherein M comprises:Ni in a molar ratio xi, wherein 0.03 < xi < 0.50 relative to M, Mn in a molar ratio yi, wherein 0.40 < yi < 0.95 relative to M, B in a molar ratio zi, wherein 0.001 < zi < 0.10 relative to M, wherein- the content of xi, yi, and zi is measured by Inductively Coupled Plasma - Optical Emission Spectroscopy ICP-OES;- xi+yi+zi is 1.00,- the molar ratio of Li and M, Li / M, is between 1.20 and 1.40; and wherein B is present in a surface coating and the B content, calculated versus the total molar fraction of Ni, Mn, and B in the coating, measured by X-ray photoelectron spectroscopy XPS of at least 25 mol%.

2. A cathode active material according to claim 1, having a composition according to a general formula (I): LiwNix2Mny2Bz2C>2, wherein 0.03 < X2 < 0.50, 0.40 < y2 < 0.95, 0.001 < Z2 < 0.10, 1.00 < w < 1.60, wherein X2+y2-i-Z2 is 1.00, and the content of Li, Ni, Mn and B is measured by ICP- OES; and wherein B is present in a surface coating.

3. A cathode active material for solid-state batteries according to claim 2, wherein material has a composition according to a general formula (I), wherein 0.03 < X2 < 0.50, 0.50 < y2 < 0.70, 0.005 < z2< 0.05, 1.20 < w < 1.40.

4. A cathode active material for solid-state batteries according to claim 1 or 2, wherein zi or Z2 is in the range of 0.005 < zi or Z2 < 0.05, or in the range of 0.0025 < zl or z2 < 0.02; or the molar ratio of B zi or Z2 is about 0.004, about 0.005, about 0.008, about 0.01, or 0.02, or about 0.03.

5. A cathode active material according to any one preceding claim, having a B content, calculated versus the total molar fraction of Ni, Mn, and B, in the coating measured by XPS of at least 40 mol%.

6. A cathode active material according to any one preceding claim, having a B content in the coating measured by XPS / ICP-OES of at least 30 mol% / mol%, preferably at least 50 mol% / mol%.

7. A method for manufacturing a cathode active material according to any one of the claims 1-6, wherein the method comprises the following steps:1) mixing a transition metal powder, wherein the transition metal consists of Ni and Mn, mixed with lithium source;2) heating the mixture from Step 1) up to a temperature in the range of 300 to 600 °C; and then up to a temperature in the range of 700 to 1100 °C under air atmosphere for a duration of 1 to 24 h;3) coating the product from Step 2) with a boron compound;4) heating the dried powder from Step 3) at a temperature of 200 to 900 °C for a duration of 3 to 10 h under air to obtain the cathode active material.

8. A method for manufacturing a cathode active material according to claim 7, wherein the coating in step 3 is a dry coating step and wherein the product from step 2) is mixed with a boron compound using a mixer.

9. A method for manufacturing a cathode active material according to claim 7, wherein the coating in step 3 is a wet coating step, wherein the product is dispersed in a solution containing boron and lithium sources, dissolved in a suitable solvent and wherein the solution is stirred and heated under vacuum conditions.

10. A method for manufacturing a cathode active material according to claim 9, wherein an additional drying step is further carried out to remove any residual solvent after the coating step.

11. A solid-state battery comprising the cathode active material according to any one of claims 1-6.

12. A solid-state battery comprising:- a positive electrode comprising a mixture of the cathode active material according to any one of claims 1-6 and a sulfide-based solid electrolyte,- a negative electrode and- a sulfide-based solid electrolyte interposed between the positive electrode and the negative electrode.

13. Use a battery according claim 11 or claim 12 in a portable electronic device, a portable computer, tablet, or mobile phone, in a power tool, in an energy storage system, in an uninterruptible power supply, in an electric vehicle, or in a hybrid electric vehicle.

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