Related applications

This application claims priority from GB patent application 2112262.7 filed on 27 August 2021 , the contents of which are incorporated by reference herein in their entirety.

Field of the Invention

The present invention generally relates to processes for making cathode active materials for secondary lithium-ion batteries, in particular lithium transition metal oxide materials, and to apparatus suitable for use in processes for making such cathode active materials.

Background of the Invention

Cathodes in secondary lithium-ion batteries contain cathode active materials into which lithium ions are deintercalated and intercalated during the charging and discharging process. The production of cathode active materials typically involves a high temperature heat treatment or calcination step. This heat treatment step may be used to form the desired crystal structure. For example, lithium transition metal oxide materials are produced by mixing transition metal precursors, such as hydroxides or oxyhydroxides, with a source of lithium, and then heat treating the mixture. During the heat treatment process, the transition metal precursor is lithiated and typically undergoes a crystal structure transformation to form the desired crystalline structure. Heat treatment may also be used during processes to modify the surface of cathode active material particles and / or to introduce dopants into the material structure.

Heat treatment is typically carried out on a manufacturing scale using industrial furnaces. Roller hearth kilns have been widely used in industrial settings. The use of such kilns typically involves loading a precursor of a cathode active material into a ceramic saggar (or crucible). The saggar then moves through the kiln leading to heat treatment of the precursor and conversion into the cathode active material. This type of process involves significant amounts of industrial waste due to the limited lifespan of the ceramic saggars. Energy requirements are also high due to the heat capacity of the saggars.

An alternative is to use a rotary tube furnace, also known as a rotary kiln or a rotary calciner. Tubes for rotary tube furnaces are typically made from metal alloys, although they can be manufactured from other materials such as ceramic. In use, such tubes are typically set at an incline to the horizontal and then rotated around their longitudinal axis. The metal precursor of the cathode active material is then fed into the upper end of the tube. As the tube rotates the material gradually moves towards the lower end, typically undergoing stirring and mixing. Heat is typically introduced through the tube wall via external heaters which may be electrical or for example gas burners. The rotary kiln may be set up such that there are distinct temperature zones which the precursor materials move through during the heat treatment process. This type of process offers advantages with respect to industrial waste however, can have significant drawbacks. Firstly, high temperature contact between the metal tube and precursor materials (typically in the presence of one or more lithium compounds) can lead to rapid corrosion of the metal surface. Secondly, the use of metal tubes, where the surface is actively corroding during operation, can lead to metal contamination of the formed cathode active material, leading to detrimental effects on cathode active material performance once it is incorporated onto a battery. These drawbacks have led to low levels of adoption of rotary tube furnaces in the cathode materials industry.

EP3362756B1 (BASF SE) describes rotary tubes comprising a double wall construction where the interior wall is a ceramic composite and the external wall is metal. The benefit of such tubes is the reduction of contamination in cathode active materials and protection from corrosion. Such rotary tubes however may suffer from a limited lifespan due to the difference in the thermal expansion co-efficient between the coated base metal and ceramic coating material upon heating to, cooling from, and at the typical kiln operating temperatures. This can lead to degradation of the ceramic surface layer and therefore to peeling from the base metal upon heating, exposing the original metallic material.

Fully ceramic tubes are very delicate and require careful control of temperature changes to prevent fracture, they cannot be manufactured to a suitable size for commercial kilns and cannot be used with build-up removal devices (knockers). In addition, the use of either a monolithic or double wall tube design containing a ceramic material may result in poor heat transfer that may pose operational problems and may result in a shorter lifespan.

There remains a need for improved processes for making cathode active materials for secondary lithium-ion batteries, and apparatus suitable for use in such processes, which address the problems set out hereinbefore.

Summary of the Invention

The present inventors have surprisingly found that by combining the use of chromium- containing alloys and the application of an aluminium diffusion coating, materials are formed which show good corrosion resistance to metal precursors of cathode active materials during high temperature heat treatment and also provide reduced metal contamination of the formed cathode active materials. The coated alloys offer suitable metallurgical and mechanical stability, and suitable external oxidation resistance.

Therefore, in a first aspect of the invention, there is provided a process for preparing a cathode active material for a secondary lithium-ion battery, the process comprising the step of the heat treatment of a metal precursor of the cathode active material in a vessel, wherein the inner surface of the vessel is formed from a chromium-containing alloy with an aluminium diffusion coating.

The use of a vessel formed from a chromium-containing alloy with an aluminium diffusion coating in processes of cathode active material preparation offers process efficiencies whilst achieving low levels of metal contamination, in particular chromium contamination. The use of an aluminium diffusion coating as a means to reduce metal contamination, for example in comparison to ceramic lined or multi-layer vessels, also provides the advantage that the vessel offers a high level of heat transfer and high structural stability during repeated heat treatment heating and cooling cycles.

The use of a vessel formed from a chromium-containing alloy with an aluminium diffusion coating in processes of active cathode material preparation also offers protection of the underlaying alloy where this would be detrimentally damaged by the presence of sulphur or sulphur containing species that are part of the precursor feed materials.

In a second aspect of the invention there is provided a rotary furnace tube with an inner surface formed from a chromium-containing alloy with an aluminium diffusion coating.

In a third aspect of the invention there is provided the use of a rotary furnace tube according to the second aspect for the manufacture of cathode active materials for secondary lithium- ion batteries.

In a fourth aspect of the invention there is provided the use of a container, such as a saggar or crucible, with an inner surface formed from a chromium-containing alloy with an aluminium diffusion coating, for the manufacture of cathode active materials for secondary lithium-ion batteries.

In a fifth aspect of the invention there is provided a cathode active material, such as a lithium transition metal oxide material, prepared by a process according to the first aspect.

Another aspect of the invention is the use of a vessel during the heat treatment of a metal precursor of a cathode active material to inhibit corrosion of the vessel during the heat treatment, wherein the inner surface of the vessel is formed from a chromium-containing alloy with an aluminium diffusion coating. The vessel may be a container, such as a saggar or crucible, or a rotary furnace tube.

Another aspect of the invention is the use of a vessel during the heat treatment of a metal precursor of a cathode active material to reduce metal (e.g. chromium) contamination of the formed cathode active material, wherein the inner surface of the vessel is formed from a chromium-containing alloy with an aluminium diffusion coating. The vessel may be a container, such as a saggar or crucible, or a rotary furnace tube.

Detailed Description

Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

The present invention provides a process for preparing a cathode active material for a secondary lithium-ion battery which comprising the step of heat treatment of a metal precursor of the cathode active material in a vessel.

Typically, the cathode active material is a lithium composite metal oxide, such as a lithium transition metal oxide. The term “lithium transition metal oxide” as used herein means a mixed metal oxide comprising lithium and at least one transition metal. Advantageously, the process may be used to prepare lithium transition metal oxides comprising nickel, cobalt, manganese, or combinations thereof. Preferred lithium transition metal oxides are lithiated spinels, cation-disordered rocksalt transition metal oxides, and lithiated transition metal oxides having a layered structure (typically having an a-NaFeC>2-type structure).

In some embodiments, the cathode active material is a lithium transition metal oxide with a composition according to the Formula 1 :

Li _c Ni _x C0yMn _w M _z O2±d

Formula 1 wherein:

0.30 < x < 1.0; 0 < y < 0.5; 0 < z < 0.2; 0 < w < 0.7; 0.8 < c < 1.4; -0.2 < d < 0.2; M is one or more selected from Mg, Al, B, V, Ti, Zr, Sr, Ca, Ce, Cu, Sn, Cr, Fe, Ga, Si, W, Mo, Ta, Y, Sc, Nb, Pb, Ru, Rh and Zn and combinations thereof; or 0.5 < x < 1.0, 0 < y < 0.3, 0 < z < 0.1 , 0 < w < 0.5, 0.9 < c < 1.1, -0.2 < d < 0.2; M is one or more selected from Mg, Al, B, V, Ti, Zr, Sr, Ca, Ce, Cu, Sn, Cr, Fe, Ga, Si, W, Mo, Ta, Y, Sc, Nb, Pb, Ru, Rh and Zn and combinations thereof; or

0.7 < x < 0.95; 0 < y < 0.2; 0 < z < 0.1; 0 < w < 0.3; 0.9 < c < 1.1 ; -0.2 < d < 0.2; M is one or more selected from Mg, Al, B, V, Ti, Zr, Sr, Ca, Ce, Cu, Sn, Cr, Fe, Ga, Si, W, Mo, Ta, Y, Sc, Nb, Pb, Ru, Rh and Zn and combinations thereof; or

0.8 < x < 0.95; 0 < y < 0.2; 0 < z < 0.05; 0 < w < 0.2; 0.9 < c < 1.1; -0.2 < d < 0.2; M is one or more selected from Mg, Al, B, V, Ti, Zr, Sr, Ca, Ce, Cu, Sn, Cr, Fe, Ga, Si, W, Mo, Ta, Y, Sc, Nb, Pb, Ru, Rh and Zn and combinations thereof; or

0.8 < x < 0.95; 0.05 < y < 0.2; 0 < z < 0.05; w = 0; 0.9 < c < 1.1; -0.2 < d < 0.2; M is one or more selected from Mg, Al, B, V, Ti, Zr, Sr, Ca, Ce, Cu, Sn, Cr, Fe, Ga, Si, W, Mo, Ta, Y, Sc, Nb, Pb, Ru, Rh and Zn and combinations thereof; or

0.8 < x < 0.95; 0.05 < y < 0.2; 0 < z < 0.05; w = 0; 0.9 < c < 1.1; -0.2 < d < 0.2; and M is Mg and optionally one or more selected from Al, B, V, Ti, Zr, Sr, Ca, Ce, Cu, Sn, Cr, Fe, Ga, Si, W, Mo, Ta, Y, Sc, Nb, Pb, Ru, Rh and Zn and combinations thereof.

It may be preferred that in Formula 1 , x + y + w = 1 or about 1 (e.g. 0.98 < x + y + w < 1.02). It may be preferred that x + y + z + w = 1 or about 1 (e.g. 0.98 < x + y + z + w < 1.05 or 1.03).

In some embodiments, the lithium transition metal oxide is a compound of the general formula Lii _+s (M2) ₂ O4-r, where r is advantageously in the range from 0 to 0.4, and s is advantageously in the range from 0 to 0.4; M2 is selected from among one or more metals of groups 3 to 132 of the Periodic Table, for example B, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Mo, with Mn, Co and Ni and combinations thereof being preferred. Particular preference is given to combinations of Ni and Mn, such as doped or un-doped LiMn2- _x Ni _x O4, where x is advantageously in the range from 0 to 1 .

Typically, the metal precursor of the cathode active material is a metal oxide, metal hydroxide, metal oxyhydroxide, or metal phosphate, such as a transition metal oxide, hydroxide, oxyhydroxide or phosphate. Typically, the metal precursor of the cathode active material is an oxide, hydroxide, oxyhydroxide, or phosphate comprising iron, nickel, cobalt, manganese, magnesium, aluminium, zirconium, titanium or a mixture of two or more thereof. The metal precursor of the cathode active material may be uncoated or coated, for example with one or more metal-containing compounds. The metal precursor may be in particulate form. The metal precursor may be prepared by a process known in the art, for example by (co-)precipitation of transition metal hydroxides by the reaction of transition metal salts with sodium hydroxide under basic conditions.

The process comprises the step of heat treatment of the metal precursor. Such heat treatment involves heating the precursor, for example to dry the metal precursor, to precalcine and / or calcine the precursor (in the presence or absence of lithium), or to apply a heat treatment after a coating or doping step.

The heat treatment is typically carried out in a carbon dioxide (CO2) free atmosphere. For example, the atmosphere may be carbon dioxide free air, which may be a mixture of oxygen and nitrogen. Preferably, the atmosphere is an oxidising atmosphere. As used herein, the term “CC free” is intended to include atmospheres including less than 100 ppm CO2, e.g. less than 50 ppm CO2, less than 20 ppm CO2 or less than 10 ppm CO2. These CO2 levels may be achieved by using a CO2 scrubber to remove CO2.

It may be preferred that the heat treatment is carried out in a mixture of O2 and an inert gas, such as N2. In some embodiments, the mixture comprises N2 and O2 in a volume ratio of from 0:100 to 100:0. It may be preferred that the mixture comprises a high concentration of oxygen, for example a volume ratio of inert gas (such as nitrogen) to oxygen of from 30:70 to 5:95.

It may be further preferred that the heat treatment is carried out in an atmosphere of at least 20 vol% oxygen (at standard pressure and temperature). In such cases the balance of gas is typically an inert gas, such as nitrogen.

Suitably, the heat treatment step involves lithiation of a precursor to form a cathode active material, such as a lithium transition metal oxide material. Therefore, it may be preferred that the process comprises the steps of:

(i) mixing a metal precursor of a cathode active material with a lithium source;

(ii) heat treatment of the mixture in a vessel, wherein the inner surface of the vessel is formed from a chromium-containing alloy with an aluminium diffusion coating.

It may be preferred that the metal precursor is transition metal hydroxide or oxyhydroxide. It may be further preferred that the process comprises the steps of: (i) mixing a transition metal hydroxide or oxyhydroxide precursor of a lithium transition metal oxide with a lithium source;

(ii) heat treatment of the mixture in a vessel, wherein the inner surface of the vessel is formed from a chromium-containing alloy with an aluminium diffusion coating.

The lithium source comprises lithium ions and a suitable inorganic or organic counter-anion. Suitably the lithium source comprises one or more lithium compounds selected from lithium carbonate, lithium oxide, lithium hydroxide, lithium chloride, lithium nitrate, lithium sulfate, lithium hydrogen carbonate, lithium acetate, lithium fluoride, lithium bromide, lithium iodide and lithium peroxide. In some embodiments, the lithium source is selected from one or more of lithium carbonate and lithium hydroxide. In some embodiments, the lithium source is lithium hydroxide.

Typically, the heat treatment of the mixture of the metal precursor and the lithium source is typically performed by heating to a temperature of at least 600 °C, at least 650 °C, at least 670 °C or at least 680 °C. The heat treatment is typically performed at a temperature of 1000 °C or less, 900 °C or less, 850 °C or less, or 800 °C or less. For example, the heat treatment may be performed at a temperature in the range of 600 to 1000 °C, 650 to 1000 °C, 650 to 800 °C or 650 to 750 °C.

Heat treatment of the mixture is typically performed for a period of 1 hour or more, 2 hours or more, 3 hours or more, 4 hours or more, 5 hours or more, or 5.5 hours or more. The heat treatment is typically performed for a period of 20 hours or less, 15 hours or less, 10 hours or less, 8 hours or less, 7 hours or less or 6.5 hours or less. For example, the heat treatment may be performed for between 1 and 20 hours.

For example, the heat treatment of the mixture may be carried out at a temperature in the range from 600 to 1000 ^e C for a period of 1 to 10 hours.

Suitably, the heat treatment step involves pre-calcination of a metal precursor of a cathode active material, in the presence or absence of lithium. Therefore, it may be preferred that the process comprises the steps of:

(i) pre-calcination of a metal precursor of a cathode active material, such as a transition metal hydroxide or transition metal oxide, in a vessel, optionally in the presence of a lithium source, to form a pre-calcined intermediate, wherein the inner surface of the vessel is formed from a chromium-containing alloy with an aluminium diffusion coating; (ii) high-temperature calcination of the pre-calcined intermediate.

It may be preferred that the high-temperature calcination is also carried out in a vessel in which the inner surface of the vessel is formed from a chromium-containing alloy with an aluminium diffusion coating.

The pre-calcination step (i) typically includes a heating stage during which the temperature is increased, and a hold phase during which the temperature is maintained at an elevated level. The hold phase of the pre-calcination is typically performed at a temperature of at least 275 °C, at least 290 °C, at least 300 °C, at least 320 °C, at least 330 °C or at least 350 °C. The hold phase of the pre-calcination is typically performed at a temperature of 600 °C or less, 550 °C or less, 525 °C or less, 500 °C or less, or 475 °C or less. For example, the hold phase of the pre-calcination may be performed at a temperature in the range of 275 to 600 °C, 290 to 550 °C, 300 to 500 °C, 320 to 450 °C, or 330 to 450 °C.

The hold phase of the pre-calcination step is typically performed for a period of 50 minutes or more, 60 minutes or more, 70 minutes or more, or 80 minutes or more. The hold phase of the pre-calcination is typically performed for a period of 5 hours or less, 4 hours or less, or 3 hours or less. For example, the hold phase of the pre-calcination step may be carried out for between 1 and 4 hours, such as between 1 .5 and 3 hours.

For example, the pre-calcination step may be carried out at a temperature in the range from 300 ^e C to 500 ^e C for a period of 1 to 3 hours.

The high-temperature calcination step (ii) typically includes a heating stage during which the temperature is increased, and a hold phase during which the temperature is maintained at an elevated level. The hold phase of the high-temperature calcination is typically performed at a temperature of at least 600 °C, at least 650 °C, at least 670 °C or at least 680 °C. The hold phase of the high-temperature calcination is typically performed at a temperature of 1000 °C or less, 900 °C or less, 850 °C or less, 800 °C or less, or 750 °C or less. For example, the hold phase of the high-temperature calcination may be performed at a temperature in the range of 600 to 1000 °C, 600 to 800 °C, 650 to 800 °C, 650 to 750 °C, or 670 to 750 °C.

The hold phase of the high-temperature calcination step is typically performed for a period of 2 hours or more, 3 hours or more, 4 hours or more, 5 hours or more, or 5.5 hours or more. The hold phase of the high-temperature calcination is typically performed for a period of 20 hours or less, 10 hours or less, 8 hours or less, 7 hours or less or 6.5 hours or less. For example, the hold phase of the high-temperature calcination may be performed for between 4 and 10 hours, such as between 5 and 7 hours. For example, the high-temperature calcination may be carried out at a temperature in the range from 600 to 800 ^e C for a period of 5 to 7 hours.

Suitably, a vessel as described herein may be also be used during heat treatment steps used to form surface-modified cathode active materials, such as surface-modified lithium transition metal oxide materials. Typically, the surface modification results from contacting a core material, such as a transition metal hydroxide or a lithium transition metal oxide, with one or more metal-containing compounds and then heat treatment of the surface-treated metal precursor material. The metal-containing compounds may be in solution, and in such context herein the term “compound” refers to the corresponding dissolved species, or may, for example by in powder form. The inclusion of a modified surface layer in the cathode active material offers protection against material degradation through electrolyte interaction, reduction in surface impurity levels and improvements in capacity retention.

Therefore, it may be preferred that the process comprises the steps of:

(i) contacting a metal precursor of a cathode active material with one or more metalcontaining compounds to form a surface-treated metal precursor; and

(ii) heat treatment of the surface-treated metal precursor.

It may be preferred that the surface-treated metal precursor is mixed with a lithium source (as described hereinbefore) prior to heat treatment step (ii).

The heat treatment of surface-treated metal precursors has particular utility for forming surface-modified lithium transition metal oxide materials, such as those according to Formula 1 . In such cases the metal precursor is typically a lithium transition metal oxide material or may be, for example, a transition metal hydroxide or oxyhydroxide.

Herein, the terms “surface-modified” refers to a particulate material which comprises a core material which has undergone a surface modification or surface enrichment process to increase the concentration of an element at or near to the surface of the particles. The term “enriched surface layer” refers to a layer of material at or near to the surface of the particles which contains a greater concentration of at least one element (such as at least one of aluminium and cobalt) than the remaining material of the particle, i.e. the core of the particle.

The metal-containing compounds are typically metal salts, such as inorganic metal salts, for example oxides, nitrates, sulfates or acetates. Nitrates may be particularly preferred. It may be preferred that the one or more metal-containing compound are selected from compounds comprising one or more of Mg, Al, B, V, Ti, Zr, Sr, Ca, Ce, Cu, Sn, Cr, Fe, Ga, Si, W, Mo, Ta, Y, Sc, Nb, Pb, Ru, Rh and Zn, optionally in combination with a lithium-containing compound. It may be further preferred that the surface-modified cathode active material is a compound according to Formula 1 as set out hereinbefore and that the one or more metal-containing compounds are selected from compounds comprising one or more of Mg, Al, B, V, Ti, Zr, Sr, Ca, Ce, Cu, Sn, Cr, Fe, Ga, Si, W, Mo, Ta, Y, Sc, Nb, Pb, Ru, Rh and Zn, optionally in combination with a lithium-containing compound.

It may be preferred that the metal-containing compound is a cobalt-containing compound, for example an inorganic cobalt salt, such as cobalt nitrate (or a hydrate thereof). It may also be preferred that the metal-containing compound is an aluminium-containing compound for example an inorganic aluminium salt, such as aluminium oxide or aluminium nitrate (or a hydrate thereof). It may be further preferred that, in addition to the aluminium-containing compound and / or cobalt-containing compound, the metal precursor of the cathode active material is contacted with a lithium-containing compound, such as an inorganic lithium salt, for example lithium nitrate.

The metal precursor of the active material is contacted with one or more metal-containing compounds is contacted to form a surface-treated metal precursor. The precursor and one or more metal containing compounds may be contacted, for example, by dry mixing or by mixing in the presence of one or more liquids. The precursor and / or the one or more metalcontaining compounds, such as an aluminium-containing compound, and / or a cobalt- containing compound (and optionally lithium containing compound), may be provided in solution, for example in aqueous solution.

The mixture of the metal precursor of the cathode active material with a metal-containing compound may be heated, for example to a temperature of at least 40 °C, e.g. at least 50 °C. The temperature may be less than 100 °C or less than 80 °C. Where the metalcontaining compound is provided in solution, the mixture of the solution with the intermediate may be dried, e.g. by evaporation of the solvent or by spray drying, to yield the surface- treated metal precursor.

The surface-treated metal precursor of the cathode active material is then heat treated in a vessel which is configured such that the vessel material in contact with the metal precursor is a chromium-containing alloy with an aluminium diffusion coating. Typically, the heat treatment of the surface-treated metal precursor of the cathode active material is carried out at a temperature of at least 400 °C, at least 500 °C, at least 600 °C or at least 650 °C. The temperature may be less 1000 °C or less, 900 °C or less, 800 °C or less or 750 °C or less. The material to be calcined may be at a temperature of 400 °C, at least 500 °C, at least 600 °C or at least 650 °C for a period of at least 30 minutes, at least 1 hour or at least 2 hours. The period may be less than 24 hour, or less than 12 hours. It may be preferred that the heat treatment is carried out at a temperature in the range of and including 400 to 1000 °C for a period of 30 mins to 12 hours.

The processes as described hereinbefore comprise heat treatment of a metal precursor in a vessel. The type of vessels used for heat treatment during the production of cathode active materials are well known to the skilled person. Such vessels include crucibles, saggars, or other containers into which precursor materials are loaded before the vessel is placed into or travels through a kiln, and tubes, such as for a rotary tube furnace, through which precursor materials travel during the heat treatment process.

In some embodiments the vessel is a container, such as a crucible, tray, or saggar, in particular a saggar suitable for use in an industrial furnace. Typically, such vessels are open (i.e. that that do not have a lid), but it may be preferred that they are closed. In some embodiments such vessels have handles, but it may be preferred that they do not. Typically, such vessels have a base that is rectangular or square and it may be preferred that such vessels have a rectangular or square base with rounded angles. The container is configured such that the vessel material in contact with the metal precursor is a chromium-containing alloy with an aluminium diffusion coating, for example the container may be formed essentially of a chromium-containing alloy with an inner (and optionally outer) surface with an aluminium diffusion coating. The use of such vessels offers excellent heat transfer properties in combination with an enhanced lifetime. In some embodiments the container may be used as a liner for a ceramic saggar which, in use, is placed inside a ceramic saggar during heat treatment.

In some embodiments the vessel is a tube for a rotary tube furnace. The shape and dimensions of rotary furnace tubes are well known to the skilled person and such parameters are selected according to the required throughput of material and the required residence time of the material in the tube in order to achieve the desired product properties. Typically, such tubes have a length in the range of from 1 to 40 m. Typically, such tubes have an average diameter of from 100mm to 1600mm. The cross section may be circular or non-circular, with circular being preferred. The tube may include internal elements to improve mixing or to retain material in the tube. Such internal elements may be removable or attached to the tube. The tube material in contact with the metal precursor as it passes through the tube is formed from a chromium-containing alloy with an aluminium diffusion coating. It may be preferred that the tube is formed essentially of a chromium-containing alloy with an inner surface formed of an aluminium diffusion coating. In the context of the present invention ‘formed essentially of’ means that at least 90% of the weight of the vessel is either the chromium-containing alloy or the aluminium diffusion coating. In some embodiments, the tube for a rotary tube furnace is a multi-layered structure with the inner layer formed from a chromium-containing alloy with an aluminium diffusion coating.

The inner surface of the vessel is formed from a chromium-containing alloy with an aluminium diffusion coating. By ‘inner surface’ is meant herein the surface facing the interior of the vessel which is in contact with the metal precursor during the heat treatment. In the case of a tube for a rotary kiln this is the surface of the tube facing the interior of the tube.

The presence of chromium in the alloy provides oxidation resistance at high temperatures and can increase mechanical strength at high temperature. The use of a chromium- containing alloy additionally provides corrosion resistance which offers benefits if the aluminium diffusion coating degrades towards the end of vessel life.

It may be preferred that the chromium-containing alloy comprises an amount of chromium in the range of and including 0.5 wt% to 40 wt%. A chromium content less than 0.5 wt% does not provide appreciable benefits to strength and oxidation resistance at high temperatures. Chromium contents greater than 40 wt% can lead to poor mechanical properties and higher cost. It may be further preferred that the chromium-containing alloy comprises an amount of chromium in the range of and including 0.5 wt% to 30 wt%, 1 wt% to 30 wt%, 5 wt% to 30 wt%, 10 wt% to 30 wt%, or 15 wt% to 30 wt%.

In some embodiments, the chromium-containing alloy is a stainless steel. It may be further preferred that the chromium-containing alloy is a stainless steel with a chromium content in the range of and including 0.5 wt% to 40 wt%, such as in the range of and including 0.5 wt% to 30 wt%, 1 wt% to 30 wt%, 5 wt% to 30 wt%, 10 wt% to 30 wt%, or 15 wt % to 30 wt%.

It may be preferred that the chromium-containing alloy comprises nickel. The inclusion of nickel offers benefits associated with high strength and creep resistance at high temperatures and metallurgical stability when combined with alloying elements such as chromium. It may be further preferred that the chromium-containing alloy comprises an amount of nickel in the range of and including 1 to 80 wt%. It may be further preferred that the chromium-containing alloy comprises an amount of nickel in the range of and including 5 to 75 wt%, such as 5 to 60 wt%. It may be further preferred that the chromium-containing alloy is a stainless steel with a nickel content in the range of and including 5 to 75 wt%, such as between 5 and 60wt%.

It may be preferred that the chromium-containing alloy comprises chromium in an amount in the range of and including 14 to 30 wt%, such as between 14 and 26 wt% and nickel in the range of and including 5 to 72 wt%, with the mass balance formed from iron and optionally one or more dopant elements, such as one or more of B, Ce, Y, Nb, Zr, Ta, C, Al, Ti, Cu, Mn, Si and N.

It may be preferred that the chromium-containing alloy is selected from ferric stainless alloys (such as ferritic iron-chromium-aluminium (FeCrAI) alloys, for example Kanthal (RTM) APM , alloy 439 (UNS S43035), iron-based austenitic stainless alloys (such as alloys 321 (UNS S32100), 347 (UNS S34700), 304/304H/304L (UNS S30400/ S30409Z S30403), 309 (UNS S30900), 310 (UNS S31000), 314 (UNS S31400), 316/316L/316H/316Ti (UNS S31600/ S31603/ S31609/ S31635), nickel-based austenitic alloys (such as alloys 600 (UNS N06600), 602CA (UNS N 06025), 690 (UNS N0669), C276 (UNS N10276), 625 (UNS N06625)), or iron-nickel-chromium austenitic alloys (alloy 800 (UNS N08800), 800H (UNS N08810), 800HT (UNS N08811). Such alloys offer a combination of high oxidative resistance and strength at high temperature and have alloy chemistry compatible with the aluminium diffusion coating process. It may be further preferred that the chromium- containing alloy is selected from the group of alloys Kanthal APM, 439, 321 , 347, 304, 304H, 304L, 309, 310, 314, 316, 316L, 316H, 316Ti, 600, 602CA, 690, C276, 625, 800, 800H, and 800HT. It may be further preferred that the chromium-containing alloy is selected from the group of alloys 304, 800H, 602CA and C276.

The chromium-containing alloys have an aluminium diffusion coating. In the context of the present invention an aluminium diffusion coating is one produced by causing aluminium to react with and / or diffuse into the surface of a metallic substrate, thus, chemically altering the surface of the substrate.

Such coatings may be formed by subjecting the surface of the chromium-containing alloy to an aluminising process (also known as an alonising process or in situ chemical vapour deposition process) leading to the formation of a surface layer comprising a metal aluminide. Typically in such processes, the metal object to be treated is positioned in a container and immersed, filled with, or surrounded with a powder mixture containing aluminium (source), a halide salt such as ammonium chloride, ammonium fluoride, aluminium fluoride, sodium chloride or sodium iodide (the activator), and an inert diluent such as alumina (filler). The container is then sealed, and then heated under an inert atmosphere in a furnace. At elevated temperatures, the aluminium diffuses into the surface of the metal object resulting in a surface layer comprising a metal aluminide. After furnace cooling, the metal object is taken out of the vessel and excess powder is removed. An aluminisation process is described, for example, in W02005/106064A1 which is incorporated herein by reference. Aluminisation by pack cementation is described in ASTM B875 - 96(2018) which is also incorporated herein by reference.

Typically, the aluminium diffusion coating has a thickness in the range of and including 1 to 1000 pm. It may be preferred that the aluminium diffusion coating has a thickness of from around 5 to around 500 pm, such as around 10 to around 300 pm, 25 to 300 pm, or 50 to around 300 pm. The thickness of the aluminium diffusion coating may be assessed by cross-sectioning and assessment of the elemental composition by energy-dispersive X-ray spectroscopy (EDX).

Preferably, the aluminium content of the aluminium diffusion coating is in the range of and including 20 to 45 wt%, or more preferably in the range of and including 20 to 40 wt%. It has been found that an aluminium content within this range provides a significant reduction in the chromium contamination observed in cathode materials heat treated in contact with the coated alloy. The aluminium content may be determined by EDX analysis of a cross section of the aluminium diffusion coating. The analysis may be suitably carried out at a position about 1 micron from the coating surface (measured in a direction perpendicular to the surface). The aluminium wt% value is determined based on the total composition detected by the EDX analysis.

After the heat treatment step, the processes as set out hereinbefore may include one or more milling steps. The nature of the milling equipment is not particularly limited. For example, it may be a ball mill, a planetary ball mill or a rolling bed mill. The milling may be carried out until the particles reach the desired size. For example, the cathode active material may be milled until it has a D50 particle size of at least 3 pm, e.g. at least 5 pm. The cathode active material may be milled until it has a D50 particle size of 20 pm or less, e.g. 15 pm or less. The term D50 as used herein refers to the median particle diameter of the volume-weighted distribution. The D50 may be determined by using a laser diffraction method. For example, the D50 may be determined by suspending the particles in water and analysing using a Malvern Mastersizer 3000.

The process of the present invention may further comprise the step of forming an electrode comprising the cathode active material. Typically, this is carried out by forming a slurry of the cathode active material, applying the slurry to the surface of a current collector (e.g. an aluminium current collector), and optionally processing (e.g. calendaring) to increase the density of the electrode. The slurry may comprise one or more of a solvent, a binder, carbon material and further additives.

Typically, the electrode of the present invention will have an electrode density of at least 2.5 g/cm ³ , at least 2.8 g/cm ³ , at least 3 g/cm ³ , or at least 3.3 g/cm ³ . It may have an electrode density of 4.5 g/cm ³ or less, or 4 g/cm ³ or less. The electrode density is the electrode density (mass/volume) of the electrode, not including the current collector the electrode is formed on. It therefore includes contributions from the active material, any additives, any additional carbon material, and any remaining binder.

The process of the present invention may further comprise constructing a battery or electrochemical cell including the electrode comprising the cathode active material. The battery or cell typically further comprises an anode and an electrolyte. The battery or cell may typically be a secondary (rechargeable) lithium (e.g. lithium ion) battery.

The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention and are not intended to limit its scope.

The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention and are not intended to limit its scope.

Metal

Four metal samples (stainless steel 304, 800H, 602CA and C276) were coated using a pack cementation aluminium diffusion coating process to provide a surface layer coating containing a metal aluminide with a target coating thickness of 100 pm.

Precursor

Samples of Li1.03Ni0.91Co0.08Mg0.01O2 were immersed in an aqueous solution of cobalt nitrate, aluminium nitrate and lithium nitrate. The mixture was then spray dried to yield a surface- treated lithium transition metal oxide precursor material.

Calcination trials - si heat treatment Each metal sample was placed in an alumina saggar and submerged in 10 grams of the surface-treated lithium transition metal oxide precursor powder. The same batch of spray dried powder was used throughout. The samples were then placed in a box furnace and calcined by heating to 450 °C at 10 °C /min then holding for 120 minutes followed by heating to 700 °C at 10 °C /min and holding for 120 minutes. The furnace was then allowed to cool until the sample reached 120 °C in CC>2-free air. After calcination, the metal sample was removed from the cathode active material powder and washed in 10 ml of deionised water to remove any attached powder from the metal sample. The cathode active material powder was then analysed for trace metal contamination by ICP-MS to determine the amount of chromium and iron contamination added during the calcination. These results were compared to a cathode active material powder sample calcined in an alumina saggar containing no metal sample and to cathode active material powder sample calcined in contact with metal samples with no aluminium diffusion coating.

ICP method

Samples of the calcined powder were digested in HF. Digestion was performed at 105 °C for a total of 2 hours using a sample digestion block. Analysis for iron and chromium was performed using an Agilent ICP-MS system using an internal standard for calibration.

Results

Results from the testing of powders are shown in Table 1 .

Table 1 - ICP-MS analysis of the lithium transition metal oxide powder after heat treatment

From the results, it can be seen that for the 800 H, 602Ca and 304 samples, the amount of chromium added during calcination was reduced by aluminisation of the metal alloy samples. This is particularly evident in the 602Ca results, where a reduction from 91 to <10 ppm chromium was recorded. The results for the C276 sample were within experimental error.

Calcination trials - multiple heat treatments To confirm that aluminising provided a benefit over multiple heat treatments, the metal samples were subjected to further calcinations in contact with batches of the surface-treated lithium transition metal oxide precursor material. The lithium transition metal oxide powder after each calcination was analysed for chromium content by ICP-MS and the measured chromium content values added together for each metal sample to provide a cumulative chromium amount. The results are shown in Table 2. This indicates a significant benefit of aluminising on the level of chromium contamination for each alloy. The levels of iron, copper and zinc contamination of the cathode active material are also consistently low following multiple heat treatments in the presence of the aluminised metal samples.

Table 2 - ICP-MS analysis of the lithium transition metal oxide powders in contact with C276 samples after repeated heat treatment

Surface and cross-section analysis

Inspections of the aluminised and untreated metal pieces were performed after the heat treatment to look for evidence of corrosion caused by heating the active material precursor at high temperatures. The aluminised surface of the metal samples were analysed by optical microscopy and scanning electron microscope analysis (SEM). Cross sections of the coupons were also prepared and analysed using energy dispersive x-ray (EDX) analysis. The composition was analysed for aluminium content at a position at a distance of 1 micron from the surface (three measurements taken and the mean value reported). The results for aluminium content are shown in Table 3.

Table 3 shows Al content in the surface region for each of the metal coupons.

The test results indicate that metal corrosion during calcination of a lithium transition metal oxide material is reduced by aluminisation of the metal surface and that chromium- containing alloys with an aluminium diffusion coating are suitable materials to use to form vessels used during the formation of cathode active materials.