EP4670226A1

EP4670226A1 - Aluminum oxide dispersions for use as a separator coating in secondary batteries

Info

Publication number: EP4670226A1
Application number: EP24704437.3A
Authority: EP
Inventors: Daniel Ness; Daniel ESKEN; Maximilian CORNELIUS; Lukas MOHR; Manja DÖRING; Gabriele BERGMANN
Original assignee: Evonik Operations GmbH
Current assignee: Evonik Operations GmbH
Priority date: 2023-02-21
Filing date: 2024-02-12
Publication date: 2025-12-31
Also published as: KR20250155011A; TW202440465A; WO2024175389A1; JP2026505995A

Abstract

An aqueous dispersion containing aluminum oxide powder, characterized in that the aluminum oxide powder is present in the form of aggregated primary particles with an aggregate particle size distribution with a median aggregate particle size (D50) of 220 nm to 500 nm, as determined by dynamic light scattering measurement, and wherein the aluminum oxide powder has a BET of 10 m²/g to 100 m²/g, in an amount of at least 20 wt.%, preferably 40 to 60 wt.%, and more preferably 50 to 60 wt.% based on the total weight of the dispersion, and that the dispersion further comprises: - at least one carboxylic acid from the group consisting of dicarboxylic acids and/or hydroxy tricarboxylic acids having from 2 to 7 carbon atoms, and - at least one amino alcohol having from one to six carbon atoms, preferably DMEA (dimethylethanolamine), 2-Amino-2-Methyl-1 -Propanol.

Description

Aluminum oxide dispersions for use as separator coating in secondary batteries

Field of the Invention

The invention relates generally to an aluminum oxide dispersion, a coating for a secondary battery separator using the aluminum oxide dispersion, and methods for making the dispersion and the coating. The invention further relates to a secondary battery and an electrical apparatus employing the separator.

Background of the Invention

Secondary batteries, such as lithium-ion batteries use separators made of polyolefins in the form of membranes because of the good mechanical properties, chemical stability, and low cost of the polyolefins. The polyolefin separators are coated with inorganic materials for enhancing their thermal stability and improving the overall safety of the battery cells.

Moisture inside a lithium-ion battery damages the lithium-ion battery performance. Thus, materials used for the separators of lithium-ion batteries should meet strict moisture requirements. Typically, an upper limit of 2000 ppm residual moisture is acceptable, however, lowering the moisture even further is desirable.

Dispersions of aluminum oxide in aqueous media are well-known. For example, EP1987095B1 and US2008264299 describe aqueous aluminum oxide dispersions and their use as coatings in various applications. However, these dispersions employ various substances that are hygroscopic. For example, the dispersions described in EP19870951 B1 contain organophosphonic components and sodium hydroxide which are hygroscopic and, therefore, the remaining water content after drying is too high for a lithium-ion battery separator coating. Furthermore, the sodium ions may also change the electrical performance of the lithium-ion battery. Also, the aluminum oxide dispersions described in US2008264299 use sodium hydrogen phosphate as additive which is hygroscopic and can increase the moisture in a ceramic separator coating.

EP1771518 describes an aqueous dispersion obtainable by placing in water an aggregated titanium dioxide powder having a specific surface area of from 20 to 150 m2/g in such an amount that the desired dispersion comprises at least 20 wt.% titanium dioxide, and at least one amino alcohol having from 1 to 6 carbon atoms, and at least one carboxylic acid from the group comprising dibasic carboxylic acids and/or hydroxycarboxylic acids having from 2 to 6 carbon atoms. However, titanium oxide can not be used as coating for a separator since it irreversibly binds lithium ions thus reducing the capacity of the battery.

Continuous higher safety and performance requirements for the secondary batteries require further improvements in the materials used for the coating of the separators.

Summary of the Invention

It has been found that parameters such as the specific surface area and the size of the aggregate particles of pyrogenically obtained aluminum oxides can have a significant effect in the amount of binder used in a coating formulation of the aluminum oxide and also on the properties of the coating and, especially, on the residual moisture and the air permeability of the coating.

An object of the invention is to use a nanostructured, pyrogenic aluminum oxide (AI2O3) which has a low BET specific surface area, and at the same time a large aggregate particle size distribution for forming a highly filled and stable dispersion that is particularly suitable for forming a ceramic coating of a lithium-ion battery separator. “Low” BET specific surface area means a BET specific surface area of 10 to 100 m²/g, preferably 10 to 90 m²/g, preferably 30 to 90 m²/g, and more preferably 30 to 55 m²/g. “Large” aggregate particle size distribution means an aggregate particle size distribution having a median particle size (D50) of 220 nm to 500 nm, preferably of 230 to 400 nm, and, more preferably of 250 to 300 nm as determined by dynamic light scattering measurement.

Separators for which this coating is particularly suitable are made of a polyolefin material such as, for example, polyethylene and polypropylene. However, the coating can also be used with other plastic material separators such as separators made of nylon, PET (polyethylene terephthalate), PVC (polyvinyl chloride), PI (polyimide), PVDF (polyvinylidene fluoride) and PEEK (polyether ether ketone).

The dispersion of the aluminum oxide according to the invention shows high compatibility with a variety of different lithium-ion battery binders. Thin and homogeneous coatings having a thickness of less than 5 pm, preferably less than 3 pm, and more preferably less than 2 pm with a lowest limit of 500 nm can be obtained on top of a separator with a low moisture content of less than 1000 ppm and high air permeability as indicated by a low Gurley number of less than 100 s.

According to a first aspect of the present invention, an aqueous dispersion containing aluminum oxide powder is provided, the aqueous dispersion characterized in that the aluminum oxide powder is present in the form of aggregated primary particles with an aggregate particle size distribution with a median aggregate particle size (D50) of 220 nm to 500 nm, preferably of 230 to 400 nm, and more preferably of 250 nm to 300 nm as determined by dynamic light scattering measurement, and wherein the aluminum oxide powder has a BET of 10 m²/g to 100 m²/g, in an amount of at least 20 wt.%, preferably 40 to 60 wt.%, and more preferably 50 to 60 wt.% based on the total weight of the dispersion, and that the dispersion further comprises: at least one carboxylic acid from the group consisting of dicarboxylic acids and/or hydroxy tricarboxylic acids having from 2 to 7 carbon atoms, and at least one amino alcohol having from one to six carbon atoms, preferably at least one of DMEA (dimethylethanolamine), and 2-Amino-2-Methyl-1 -Propanol.

The aqueous dispersion may have a viscosity of less than 100 mPas (millipascal), preferably 100 to 10 mPas and, more preferably 60 to 15 mPas. The aluminum oxide powder may have a BET surface area of 10 to 100 m²/g, preferably 10 to 90 m²/g, preferably of 30 to 90 m²/g, and more preferably 30 to 55 m²/g.

The aluminum oxide aggregated primary particles have a monomodal particle size distribution with a Z-avg (v) at less than 500 nm, preferably between 500 nm to 150 nm, and more preferably between 350 nm to 180 nm.

In an embodiment, for example, the at least one carboxylic acid may be at least one of citric acid, and oxalic acid, the at least one amino alcohol may be at least one of dimethylethanolamine (DMEA) and 2-Amino-2-Methyl-1 -Propanol, and the aluminum oxide powder may be of pyrogenic origin, meaning that it is a pyrogenic aluminum oxide powder.

The aqueous dispersion may be further characterized in that it is free of sodium dihydrogen phosphate, phosphonic acids, or hygroscopic substances.

The at least one amino alcohol may be present in the dispersion in an amount of 2 from 2.5 to 8.0 pmol/m of the aluminum oxide specific surface area.

The at least one carboxylic acid may be present in an amount of from 1.0 to 4.0 2 pmol/m of the aluminum oxide specific surface area.

The aqueous dispersion is stable for at least 1 month, preferably at least 6 months in a pH range of 5 to 10.

According to another aspect of the present invention, a coating composition for a secondary battery separator is provided, further characterized in that the coating composition is obtained by adding a binder and a wetting agent into the aqueous dispersion.

The coating composition may comprise: the aluminum oxide particles in an amount of between 5 wt.% and 70 wt.%, preferably between 10 wt.% and 60 wt.%, and more preferably between 15 wt.% and 50 wt.% based on the total weight of the coating composition, a binder in an amount between 1 and 20 wt.%, preferably between 2 and 15 wt.%, and more preferably between 3 and 10 wt.% based on the total weight of the coating composition, and a wetting agent in an amount of 0.01 to 1 .0 wt.% based on the total weight of the coating composition.

According to yet another aspect of the invention, a process for the preparation of the aqueous dispersion is characterized in that:

- the aggregated aluminum oxide powder, corresponding to the desired amount in the dispersion, the at least one amino alcohol, and the at least one carboxylic acid are placed in water to create a water slurry, producing a pre-dispersion by introducing energy in the water slurry in an amount of less than the energy required for the formation of the dispersion, and producing the dispersion by introducing the pre-dispersion in a high-energy mill and grinding the pre-dispersion by means of the high-energy mill at a pressure of at least 500 bar.

The energy introduced in the water slurry to produce the pre-dispersion may be less than 1000 kJ/m³. In an embodiment, the pre-dispersion may be divided into at least two partial streams, wherein these partial streams are introduced in the high-energy mill and are let off via a nozzle to come together in a gas- or liquid-filled reaction space.

The invention also is directed to the use of the aqueous dispersion for the forming of a coating layer on at least one surface of a separator of a secondary battery,

- wherein the separator is a thin membrane made of at least one of polyolefin, nylon, PET (polyethylene terephthalate), PVC (polyvinyl chloride), PI (polyimide), PVDF (polyvinylidene fluoride) and PEEK (polyether ether ketone) and

- wherein the coating formed on the separator has a moisture of less than 1000 ppm and an air permeability defined by a Gurley value up to 100 s, preferably up to 50 s and, more preferably up to 30 s.

The battery may be any secondary battery such as, for example, a lithium-ion battery. In an embodiment, the battery is a lithium-ion battery and the separator is made of a polyolefin including, for example, at least one of polypropylene, polyethylene or any combination thereof.

The separator may comprise: a membrane comprising at least one polyolefin, and a coating layer obtained by applying a coating composition employing the aluminum oxide dispersion on at least one surface of the membrane and applying heat to dry the coating layer, wherein the coating layer formed on the separator has a moisture of less than 1000 ppm and an air permeability defined by a Gurley value up to 100 s, preferably up to 50 s and, more preferably up to 30 s as measured with the coating formed on both sides of the separator.

In an embodiment, the membrane may be made of polyethylene and may have a thickness of from 3 to 20 pm, wherein the coating layer may be homogeneous having a thickness of less than 5 pm, preferably less than 3 pm, and more preferably less than 2 pm with a lowest limit of 500 nm.

The invention is further directed to a secondary battery comprising the separator and to an apparatus comprising the secondary battery. The apparatus may comprise, for example, an electric or electronic device such as a mobile phone, an electronic watch, a key fab, a laptop computer, a desktop computer, a computer pad, a power tool, a vacuum cleaner, an electric lawn mower, an electric appliance, an electric vehicle and the like.

These and other features and advantages of the present invention will become understood by those skilled in the art of the invention from the following detailed description in conjunction with the following figures.

Brief Description of the Figures

Fig. 1 is a simplified schematic diagram of a method of making an aluminum oxide dispersion according to an embodiment of the present invention.

Fig. 2 illustrates a process of making the aluminum oxide powder according to an embodiment of the present invention.

Fig. 3 is a simplified schematic diagram of a method of making a coating composition for a lithium-ion separator using an aluminum oxide dispersion according to an embodiment of the present invention.

Fig. 4 is a simplified schematic diagram of a lithium-ion battery with a separator coated on both sides thereof with an aluminum oxide coating according to an embodiment of the present invention.

Fig. 5A-5C illustrate aggregate particle size distributions for aluminum oxide powders in aqueous dispersion according to embodiments of the present invention.

Fig. 5D illustrates an aggregate particle size distribution for an aluminum oxide powder employed in an aqueous dispersion prepared according to a comparative example.

Detailed Description

Referring now to figure 1 , a method for making a dispersion containing an aluminum oxide is provided according to an embodiment. Accordingly, the method may comprise: placing in water an aggregated aluminum oxide, at least one amino alcohol having from 1 to 6 carbon atoms, and at least one carboxylic acid from the group consisting of dicarboxylic acids and/or hydroxy tricarboxylic acids having from 2 to 7 carbons according to step 102, producing a pre-dispersion by introducing energy in an amount of less than the energy required for the formation of the dispersion according to step 104, and

- then, producing a dispersion by introducing the pre-dispersion in a high energy mill and grinding the pre-dispersion by means of the high-energy mill at a pressure of at least 500 bar according to step 106.

For example, the pre-dispersion may be formed by introducing an energy into the aqueous slurry of less than 1000 kJ/m³ (kilojoule per cubic meter). In some embodiments, the pre-dispersion may be formed by introducing an energy into the aqueous slurry of 200 kJ/m³ or less.

In some embodiments, the pre-dispersion may be divided into at least two-part streams, and these part streams may be placed in a high energy mill under pressure of at least 500 bar, released via a nozzle and allowed to impinge on one another in a gas- or liquid-filled reaction chamber.

The aggregated aluminum oxide has a BET specific surface area from 10 to 100

2 2 2 m Zg, preferably 10- to 90 m /g, more preferably 30 to 90 m /g, and more preferably 30 to 55 m²/g.

The amino alcohol may be present in the dispersion in an amount of from 2.5 to 8.0 pmol/m (pmol per square meter) of the aluminum oxide specific surface area. The carboxylic acid may be present in an amount of from 1 .0 to 4.0 pmol/m of the aluminum oxide specific surface area.

Introducing energy in the water slurry for producing the pre-dispersion can be effected by using mechanical means. Suitable mechanical means for producing a dispersion, such as e.g. a pre-dispersion as described herein, are generally known in the art and may include, as illustrative but non-limiting examples, stirring, agitating, shaking and/or milling. In particular, shear conditions may be applied for introducing energy in the water slurry. Suitable devices for the preparation of the pre-dispersion may be, for example, rotor/stator machines or toothed discs. In a preferred embodiment, the pressure during the high-energy grinding process may be at least 2000 bar. Also, it is noted that it may be advantageous to expose the dispersion to the high-energy grinding process several times.

The invention provides an aqueous dispersion obtainable by the above process. The dispersion may comprise AI2O3 particles in an amount of at least 20 wt.% solid content in the total dispersion weight, preferably 40 to 60 wt.% and, and more preferably 50 to 60 wt.% solid content in the total dispersion with a low viscosity of less than 100 mPas, preferably 100 to 10 mPas, and more preferably 60 to 15 mPas. The solid content as this term is used here is the weight percentage of aluminum oxide particles in the dispersion.

The dispersion of the aluminum oxide may preferably have a monomodal particle size distribution with a Z-avg (v) at less than 500 nm, preferably between 500 nm to 150 nm, and, more preferably between 350 nm to 180 nm.

Furthermore, the dispersion is free of any components which are hygroscopic such as sodium dihydrogen phosphate or phosphonic acids which are used in some conventional metal oxide dispersions.

We have found that the large aggregate particle size of the aluminum oxide has an unexpectedly positive influence on the air permeability of the coating layer formed as described here from the aluminum oxide dispersion and results in a coating with lower overall moisture content that satisfies and exceeds the moisture requirement for a ceramic coating of a lithium-ion battery separator.

For the application of the separator ceramic coating, dispersions with a basic condition as indicated by a basic pH are preferred, more preferably with a pH of 8 to 10. It has been found that aluminum oxide dispersions within a basic pH range and, in particular, within the preferred range of 8 to 10 provide improved compatibility with the binder systems used in the coating slurry formulation.

The AI2O3 powder

The aggregate aluminum oxide powder may be a pyrogenically prepared aluminum oxide, also referred to as a fumed aluminum oxide powder or fumed alumina powder. Pyrogenically is here to be understood as meaning a powder obtainable by flame hydrolysis or flame oxidation. The powders so prepared consist of aggregates of sintered primary particles, which are formed first during the reaction. The aggregates of the sintered primary particles may also be referred to as secondary particles. A plurality of aggregates may subsequently form agglomerates. Owing to the reaction conditions, pyrogenically prepared powders exhibit only very low surface porosity and hydroxyl groups at the surface, up to 10 OH/nm2.

Although dispersions with mixed metal oxide powders containing the aluminum oxide powder may also be made, dispersions employing powders that contain aluminum oxide as the single metal oxide component are preferred.

According to the present invention, the aluminum oxide powder employed in the dispersion is characterized by: a BET surface area of 10 to 100 m²/g, preferably 10- to 90 m²/g, more preferably 30 tO to 90 m²/g, and more preferably 30 to 55 m²/g and a D50 of 220 to 500 nm, preferably of 230 to 400 nm, and more preferably of 250 nm to 300 nm as determined by dynamic light scattering measurement, wherein the aluminum oxide powder preferably has a monomodal distribution of the aggregate particle diameters.

The dispersions provided by the present invention are stable in a pH range from 5 to 10, meaning they are stable against sedimentation and re-agglomeration for at least a period of 1 month, preferably a period of 6 months.

The volume-related aggregate diameter (Z-avg (v)) is less than 500 nm and no particles greater than 1000 nm are detectable in the dispersion according to the invention by the conventional methods of light scattering for determining particle size distributions in dispersions, such as, for example, dynamic (e.g., Malvern Zetasizer or Horiba l-A-950)

Synthesis of the aluminum oxide

The aluminum oxide powder may preferably be produced by a pyrogenic process. Referring now to figure 2 an example of a pyrogenic process for the synthesis of the aluminum oxide powder is provided. The process comprises vaporizing aluminum chloride (AlCh) in evaporator 7 and feeding the vapors ‘a’ of the aluminum chloride into a mixing chamber 1 . The vapors may be transferred into the mixing chamber via an inert gas. Separately from the vapors of the aluminum chloride, combustion gas comprising hydrogen ‘b’ and primary air ‘c’ are introduced into the mixing chamber 1 . The air ‘c’ may optionally be enriched with oxygen. The air ‘c’ may optionally be pre-heated before supplied to the mixing chamber 1.

Aluminum oxide particles produced are in the form of aggregated primary particles, wherein the primary particles are free from pores and bear hydroxyl groups on their surface. Hydrochloric acid is formed as a byproduct in the conversion of the aluminum chloride most part of which is removed from the aluminum oxide particles via a steam treatment. Thus, only a very small amount of chlorine stays with the aluminum oxide particles.

Hydrogen b is preferably used in a small excess compared to the theoretical needed for the complete hydrolysis of the aluminum chloride following the equations:

2H₂ + O₂ 2H₂O

2AICI₃ + 3H₂O -> AI₂O₃ + 6HCI

The ratio of the hydrogen supplied over the stoichiometrically required hydrogen is referred to as a “gamma” ratio.

The ratio of the oxygen supplied over the stoichiometrically required oxygen is referred to as a “lambda” ratio.

The “gamma” and “lambda” ratio itself is commonly known to a person skilled in the art, e.g. as described in US8197791 B2.

The oxygen excess which is required for this reaction for full hydrolysis of the aluminum chloride, i.e. , the lambda ratio is from greater than 1 to 5.

The reaction mixture ‘d’ is fed in a central tube to a burner (inside reaction chamber 2) and ignited. The exit speed of the reaction mixture from the burner may range from 10 m/s to 100 m/s. The flame burns in a water-cooled reaction chamber 2. The reaction gases and solids from the reaction chamber are cooled down in the cooling coils 3 before entering a gas-solid separation unit 4, for example, a cyclone and/or filter. The aluminum oxide powder that is formed is then separated from the gas in the downstream gas-solid separation unit 4 and deposited at the bottom of the gas-solid separation unit 4 (e.g., a cyclone) and collected via a bottom outlet. The powder from 4 is transferred to the deacidification equipment 5 for removing the acid from the powder and the aluminum oxide powder is treated with air and steam which are fed in a counter-current direction inside the equipment 5 at an elevated temperature, for example, of about 700 °C. The temperature may not be limited to the 700 °C but may generally range from 400 °C to 900 °C. The purpose of the treating of the aluminum oxide powder with air and steam at the elevated temperature is to remove chlorine (e.g., HCL, chlorine gas) and adjust the pH value of the powder between a value of 2 to 7. Depending on the particular application intended for the aluminum powder, the powder pH can be adjusted to a particular value within the 2-7 range. The powder is collected in a silo 6 and can be further processed as may be needed, e.g., may be packed. The discharge rate of the reaction mixture from the mixing chamber into the reaction chamber may be at least 10 m/s.

The gamma ratio may be from 0.9 to 1 .6, preferably from 1 .0 to 1 .4, and more preferably from 1 .0 to 1 .2.

A range from 0.2 to 0.6 kg of AlC /m³ of gas may be used.

In a particular embodiment of the process according to the invention, a secondary gas consisting of air and/or nitrogen can be introduced into the reaction chamber. The ratio primary air/secondary gas preferably has values of from 10 to 0.5. The introduction of a secondary gas can help to avoid caking in the reaction chamber.

Dispersion

The amino alcohols in the dispersion according to the invention are preferably selected from the group comprising monoethanolamine, diethanolamine, triethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, N,N- dimethylisopropanolamine, 3-amino-1-propanol, 1-amino-2-propanol, 2-amino-2- methyl-1 -propanol and/or N,N-Dimethylethanolamine. Particular preference is given to N,N-Dimethylethanolamine, and 2-amino-2-methyl-1 -propanol.

The carboxylic acids in the dispersion according to the invention are preferably selected from the group comprising the dicarboxylic acids or hydroxy tricarboxylic acids oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, maleic acid, fumaric acid, lactic acid, malic acid, tartaric acid and/or citric acid. Citric acid and oxalic acid are particularly preferred.

In the dispersion according to the invention, the content of amino alcohol and of at least one dicarboxylic acid or hydroxy tricarboxylic acid is from 2.5 to 8.0 pmol/m² specific surface area of the aluminum oxide for amino alcohol and from 1 .0 to 4.0 pmol/m² specific surface area of the aluminum oxide for the carboxylic acid. In a preferred embodiment, the content of amino alcohol and of at least one dicarboxylic acid or hydroxy tricarboxylic acid may be from 3.0 to 6.0 pmol/m² specific surface area of the aluminum oxide for amino alcohol and from 1 .5 to 3.0 pmol/m specific surface area of the aluminum oxide for the carboxylic acid.

The dispersion according to the invention may optionally comprise at least one preservative for long-term stability and prevention of bacteria and algae growth. Suitable preservatives may include: aqueous formulations of 2-methylisothiazolin-3-one (MIT), benzisothiazolinone (BIT), 2-bromo-2- nitro-propane-1 ,3-diol, and 3(2H)-5-chloro-2-methyl- isothiazolone, and mixtures thereof, formaldehyde donors based on dimethylol- or trimethylolurea, formamide methylol, paraformaldehyde, bronopol, nitrilodibromopropionamide 1 ,3-di(hydroxymethyl)-5,5-dimethylhydantoin or hexahydrotriazines.

A particularly preferred preservative is ACTICIDE MV a commercially available mixture of CIT and MIT supplied by Thor, a UK company.

In the dispersion according to the invention, the preservative may be present from 0.02 to 0.4 wt.%, preferably 0.05 to 0.2 wt.% based on the total amount of the dispersion.

Coating

The invention also provides the use of the aluminum oxide dispersion according to the invention for the preparation of a coating for a separator of a secondary battery such as, for example, a lithium-ion battery.

The coating composition may comprise the dispersion according to the invention and at least one binder. The binder may preferably be an acrylate-based compound. Ideally, the binder should be hydrophilic. The coating composition can have a content of aluminum oxide particles of between 5 wt.% and 70 wt.%, preferably between 10 wt.% and 60 wt.%, and more preferably between 15 wt.% and 50 wt.% based on the total weight of the coating composition. The coating composition can furthermore have a binder content, based on the aluminum oxide powder, which is between 1 and 20 wt.%, preferably between 2 and 15 wt.%, and, more preferably, between 3 and 10 wt.%.

The coating composition may further include a wetting agent and optionally other auxiliary substances, such as pH buffers, and viscosity auxiliaries.

Referring now to figure 3, the coating composition can be prepared by adding, while stirring, the dispersion according to the invention to a solution of the binder according to step 302. A wetting agent is further added, and optionally other auxiliary substances such as additives according to step 304 while continuing stirring. The preparation of the coating composition further includes diluting the mixture until the desired ratio of aluminum oxide powder and binder and the desired total solids content are established according to step 306, and obtaining the coating composition comprising the aluminum oxide in an amount from 15 to 50 wt.%.

As described above, the coating composition may be obtained by creating a mixture of a binder, a wetting agent, the dispersion of the aluminum oxide, and optionally of other additives, preferably while stirring. Figure 3 illustrates one example of a sequence of adding the various parts of the coating composition. It is noted though that the sequence of the addition of the various parts of the mixture may be changed. For example, the mixture may be formed by adding, while stirring, a binder and a wetting agent into the dispersion of the aluminum oxide. As another example, the binder and the wetting agent may be mixed first, and the aluminum oxide may be added to the mixture of the binder and the wetting agent while stirring. The mixture is optionally stirred for a certain period of time (from a few minutes to an hour) to ensure homogeneity and, if necessary, subsequently deaerated. In some embodiments deaeration may be performed in vacuum. Other additives are to be understood as including, for example, pH buffers, and viscosity auxiliaries.

The process may comprise, prior to adding the binder and the wetting agent into the dispersion, diluting the dispersion to a desired solid content based on the total weight of the dispersion prior to the addition of the binder and the wetting agent. The desired solid content is highly dependent on the used coating process. For example, the dilution of the dispersion may range from 15 to 50 wt. % solid content, i.e. , aluminum oxide as shown in step 308. The coating composition is particularly suitable for forming a thin coating film over a lithium-ion battery separator.

In some embodiments, the coating composition obtained from the above process may comprise the aluminum oxide in an amount of 5 to 50 wt.% based on the total weight of the coating composition, the binder in an amount of 1 to 15 wt.% based on the weight of the aluminum oxide, and the wetting agent in an amount of 0.01 to 1 .0 wt.% based on the total weight of the coating composition.

The binder may be selected from any suitable binders. For example, some preferred binders may include acrylate-based compounds. Some examples of suitable binders may include polyvinyl alcohol, carboxymethylcellulose (CMC), styrenebutadiene rubber (SBR) and modified polyvinylidene fluoride (PVDF).

The binder is added in the coating composition to adhere the oxide particles to the polymer surface and to adhere the oxide particles among each other.

Any suitable wetting agents may be used, including, for example, acetylenic glycols and their alkoxylates, ethoxylates, or ethylene oxide-propylene oxide copolymers octyl- or nonylphenols, and fluorosurfactants (also known as fluorinated surfactants) including, for example, perfluorosulfonic acids, such as perfluorooctanesulfonic acid (PFOS) and the perfluorocarboxylic acids like perfluorooctanoic acid (PFOA). Other suitable wetting agents include eugenol polyethers and eugenol polyether siloxanes described in US patent no. 9993786B2. Yet other suitable wetting agents may include lithium dodecylbenzene sulfonate, a non-ionic fluorosurfactant, and lithium stearate.

The purpose of adding the wetting agent in the coating composition is to lower the surface tension of the aqueous coating slurry and to make it compatible with the substrate surface on which the coating is applied. For example, in some embodiments the substrate is the surface of the lithium-ion battery separator which may be made of a polyolefin material.

Applying the Coating on the Separator

The coating composition containing the aluminum oxide is used to apply a thin and homogeneous ceramic coating on top of a polyolefin separator of a secondary battery such as a lithium-ion battery. The polyolefin separator may typically have a thickness of from 3 to 20 pm. For applying the coating, according to some embodiments, the separator membrane may be fixed on a vacuum table and a slot-die may be positioned over the separator with a suitable gap between the slot-die and the separator. Upon start of the coating process the speed of the slot-die and the fluid rate of the coating formulation are set to desired values for controlling the thickness of the applied coating on the separator surface. Once the surface of the separator is covered with a thin film of the coating composition, the wet coated lithium- ion battery separator is dried at an elevated temperature for a specified amount of time to remove any residual moisture. The process can be repeated for application of an additional ceramic layer on the other side of the separator.

The formed coating film may have a thickness of 0.2 pm to 10 pm, preferably 0.5 pm to 5 pm, and more preferably 1 pm to 3 pm. The formed coating exhibits an overall increase in air permeability as indicated by a Gurley number of up to 100 s, preferably up to 50 s and, more preferably up to 30 s.

The coating film is homogeneous meaning that it is readily applied on the surface of the separator and forms a uniform thickness film having a uniform appearance and air permeability on its entire area.

Figure 4 illustrates a lithium-ion battery generally designated with numeral 300 including a separator made from a membrane 316 and coated with the coating layer 318. The coating layer 318 is formed on both sides of the membrane 316 using the process described above. The lithium-ion battery 300 can be used in electronic and electrical apparatuses 400 including, for example, mobile phones, computers (lap top computers, desk top computers, computer pads), electronic watches, key fabs, electric appliances, power tools, vacuum cleaners, electric lawn mowers and electric vehicles. The lithium- ion battery 300 further includes an active cathode material 312 on a cathode plate 310, and an anode active material 322 on an anode plate 320. An electrolyte 324 is placed around the separator and between the anode and cathode active materials 322 and 312.

Examples

Analytical Procedures

Size distribution measurement (Malvern)

Dynamic light scattering (DLS) is a technique in physics that can be used to determine the size distribution profile of small particles in suspension or polymers in solution. This equipment can be used to measure the particle size of dispersed material (inorganic nanoparticles or polymeric spheres, e.g.) in the range from 3 nm to around 6 pm. The measurement is based on the Brownian motion of the particles within the medium and the scattering of incident laser light due to a difference in refraction index of liquid and solid material.

For measurement of the particle size distribution a Malvern Zetasizer nano S is used and the Z-avg (v) value taken as average value for the distribution. In a plastic cuvette, a single droplet of the as-produced dispersion is diluted with distilled water so that a slightly turbid solution results. In an automated mode this has been measured using the aforementioned equipment.

BET

The BET specific surface area of the particles is determined in accordance with DIN 66131.

Viscosity measurement

The dynamic viscosity was measured with the Physica MCR 300 from Anton Paar using the rotational viscosity method and a single gap cylinder CC 27.

The motor of the viscometer drives a bob inside a fixed cup. The rotational speed of the bob is preset and produces a certain motor torque that is needed to rotate the measuring bob. This torque must overcome the viscous forces of the tested substance and is therefore a measure for its viscosity.

Data are measured at a shear rate of 100 s-1 and 23 °C.

Stability determination

The stability of dispersions is determined by visual inspection and control of the viscosity after certain time (1 week and 1 month). A good stability is noted, when there is no visual effect like sedimentation, flocculation, or separation within the dispersion and when the viscosity remains below 100 mPas as after production. All samples are stored under room temperature.

Solid content

The solid content of dispersions according to this invention are determined using a muffle furnace. A defined amount of dispersion (3 g) is weighed into a crucible. The water is first evaporated by treatment of the dispersion in the furnace at 105 °C for 120 min and subsequently all other components are burned off by treatment at 1000 °C for 120 min. The remaining weight is measured and by division through the original value the solid content is achieved.

Air permeability

Air permeability was measured according to the Gurley method. The Gurley number is the time in seconds it takes for 100 cc (cubic centimeters) of air to pass through one-square inch membrane when a constant pressure of 4.88 inches of water is applied.

The air permeability of uncoated and coated separators was measured using the model 4110N GURLEY Densometer from Gurley Precision Instruments. 100 cc of air was pressed through the separator membranes and the time was recorded. The shorter the time, the higher the air permeability (= lower Gurley number) and the better the Li-ion conductivity. Gurley measurements were performed on ceramic coated separators and on the related uncoated separator to directly compare the influence of the ceramic coating on the air permeability performance.

Water Content

Determination of water content was by Karl Fischer analysis. The water content was measured using the device C30S from Mettler-Toledo. For the KF measurements, a temperature of 300 °C for a duration of 10 minutes was pre-set. 0.1 g of each sample (oxide powder, dried dispersion, dried coating slurry and ceramic coated separator) was used for the test series and multiple determinations were made. The samples were directly weighed into the KF vials.

Examples

Preparation of Aluminum Oxide Powder

The aggregate particle size distribution of the powder was determined by DLS (dynamic light scattering).

The BET specific surface area of the particles was determined in accordance with DIN 66131. gamma ratio = H2 supplied/stoichiometrically required H2 lambda ratio = O2 supplied/stoichiometrically required O2

Example 1 inventive:

Aluminum chloride (AICI3) is vaporized and the vapors are transferred into a mixing chamber. Separately from the raw material, combustion gas including hydrogen and primary air are introduced into the mixing chamber. The combustion gas has an excess of 1.05 of hydrogen compared to the theoretical needed for the complete hydrolysis of the AICI3 following the equations:

2H₂ + O₂ 2H₂O

2AICI₃ + 3H₂O -> AI₂O₃ + 6HCI

The oxygen excess for this reaction for obtaining full hydrolysis of AICI3 is 1.04. Hence, the gamma and lambda ratio values were 1.05 and 1.04 respectively.

The reaction mixture was fed in a central tube to a burner and ignited. The exit speed of the reaction mixture from the burner was 33.7 m/s. The flame burned in a water-cooled reaction chamber. The powder formed was deposited in a downstream cyclone and filter and then treated with air and steam in counter-current at approximately 700°C.

The obtained powder had a BET specific surface area of 44 m²/g. The aggregate particle size distribution obtained by dynamic light scattering is shown in figure 1 and had a D50 value of 260 nm.

Example 2 comparative:

Comparative example 1 was carried out according to the procedure described in example 1 except that the combustion gas had an excess of 2.3 hydrogen compared to the theoretical needed for the complete hydrolysis of the aluminum chloride following the equations:

The oxygen excess for this reaction for obtaining full hydrolysis of AICI3 was 0.95. The gamma and lambda ratio values for example 2 were 2.3 and 1.04 respectively.

The reaction mixture was fed in a central tube to a burner and ignited. The exit speed of the reaction mixture from the burner was 37.6 m/s. The flame burned in a water-cooled reaction chamber. The powder formed was deposited in a downstream cyclone and filter and then treated with air and steam in counter-current at 700°C.

The obtained powder had a BET specific surface area of 48 m²/g.

The aggregate particle size distribution obtained by dynamic light scattering is shown in figure 1 and had a D50 value of 206 nm. Measurement of Aggregate size distribution:

Aggregate size distribution is determined via DLS measurement using a SYMPATEC

NANOPHOX equipment. For analysis, 20 gr dispersion of 1 wt.% aluminum oxide in water is prepared via ultrasound treatment using an ultrasound equipment (Hielscher UP400St, 50% amplitude) for 5 minutes. For accurate measurement using the Nanophox, a single scattered light ratio in the range of 20 to 80% is set by diluting between 100 to 500 mg of this as- produced dispersion with distilled water to 2.5 g total amount.

More examples were carried out analogously to example 1 , with the key parameters of the reaction condition and the physicochemical values of the produced aluminum oxide powders included in Table 1. Examples 1 , and 3 are working examples according to the invention. Examples 2, and 4 are comparative examples.

Table 1: Starting substances and amounts; physico-chemical values of the aluminum oxide powders

In table 1 , *VB = exit speed from the burner. Also, the gamma and lambda values are based on the core gases of primary air, hydrogen, and inert gas. The concentration (“c”) of AI2O3 in the total gas volume based on the core gases which are all gases which go through the core tube (i.e. , the AICI3 (as gas phase) and the PH2; primary air). All other gases (MH2 and secondary air) are not considered for the concentration of the concentration of the AhOsCore. The concentration of the AI2O3 (all) refers to the concentration of the AI2O3 based on the overall gas flows including the MH2 and secondary air. It is noted that the MH2 and secondary air are do not contribute to product changes; only the core gases contribute to product changes.

Preparation of aluminum oxide dispersions

Example D1:

An amount of 0.85 kg of citric acid, (corresponding to 2.0 pmol/m² of citric acid per specific surface area of the AI2O3) and 48.3 kg demineralized water are placed in a reaction vessel. Then, 0.85 kg of the 2-Amino-2-Methyl-1 -Propanol (corresponding to 4.2 pmol/m² of the amino alcohol per specific surface area of the AI2O3) is added proportionally to the added amount of powder in order to obtain a flowable predispersion. To that end, 50.0 kg of the aluminum oxide powder of example 1 having a BET specific surface area of 45 m²/g is drawn in via the suction pipe of an Ystral Conti- TDS 3 under shear conditions and, when the drawing in is complete, the pre-dispersion is subjected to shear for 30 minutes at 3000 rpm.

The resulting dispersion has 50 wt.% of AI2O3 showing a monomodal particle size distribution with a Z-avg (v) value of 217 nm and a viscosity of 58 mPas (at a shear rate of 100 s-1) with good stability. The aggregate particle size distribution by volume of the aluminum oxide dispersion is provided in figure 5A. As it can be also seen from figure 5A the aluminum oxide powder has a generally bell shape curve and is free from particles with a diameter of more than 1000 nm. The aluminum powder is also free from particles with a diameter of less than 50 nm. Example D2:

The pre-dispersion produced as described in example D1 is fed in one pass through an UHDE high-pressure device HPD 4075-nano through diamond nozzles with a diameter of 0.20 mm at a pressure of 2500 bar.

The resulting dispersion has 50 wt.% of AI2O3 showing a monomodal particle size distribution with a Z-avg (v) value of 187 nm and a viscosity of 16 mPas (at a shear rate of 100 s-1) with good stability. The aggregate particle size distribution by volume of the aluminum oxide dispersion is provided in figure 5B.

Example D3:

The pre-dispersion produced as described in example D1 is fed in two passes through an UHDE high-pressure device HPD 4075-nano through diamond nozzles with a diameter of 0.20 mm at a pressure of 2500 bar.

The resulting dispersion has 50 wt.% of AI2O3 showing a monomodal particle size distribution with a Z-avg (v) value of 187 nm and a viscosity of 13 mPas (at a shear rate of 100 s-1) with good stability. The aggregate particle size distribution by volume of the aluminum oxide dispersion is provided in figure 5C.

Example D4:

In a 1 -liter batch containment, 8.5 g of carboxylic acid (citric acid, 2.0 pmol/m² AI2O3) and 483 g demineralized water are placed. 8.5 g of the amino alcohol (N,N- Dimethylethanolamine, 4.2 pmol/m² AI2O3) is added proportionally to the added amount of powder in order to obtain a flowable pre-dispersion. To that end, 500 g of the aluminum oxide powder from example 1 (45 m²/g BET) are added into the containment while an ultra-turrax equipment (IKA T 25) is shearing the dispersion with 6000 rpm and, when addition is complete, is then subjected to shear for 30 additional minutes. This dispersion is afterwards treated with ultrasound (Hielscher UP400St, 24 kHz with Ti-sonotrode) at an amplitude of 100% for 60 minutes.

The resulting dispersion has 50 wt.% of AI2O3 showing a monomodal particle size distribution with a Z-avg (v) value of 502 nm and a viscosity of 18 mPas (at a shear rate of 100 s-1) with good stability.

Comparative Example CD1: Preparation of Dispersion showing too high-water content in coating

A dispersion prepared using the AI2O3 powder from Example 1 (45 m²/g BET) is produced according to example 1 from US2008264299, the procedure of which is copied here. Instead of using AEROXI DE® Alu 65 (BET 65 m²/g) used in the example 1 of US2008264299, the AI2O3 powder from Example 1 (45 m²/g BET) of the present application was used. Accordingly, 34.7 kg of deionized water are placed in a 60 1 stainless steel batch vessel. Next, 7.0 kg of the AI2O3 powder from Example 1 (45 m²/g BET) are sucked in by means of the suction pipe of the Ystral Conti- TDS 3 under shear conditions. Further, 13.3 kg of a solution of 1 .80 kg of anhydrous citric acid, 1.49 kg of disodium hydrogen phosphate (Na2HPC>4) dihydrate and 10 kg of water are added and a further 65.0 kg of the AI2O3 powder from Example 1 (45 m²/g BET) are sucked in. After completion of the suction, the suction connector is closed and shearing at 3000 RPM is continued for a further 10 mins. After the grinding, 108 g of Acticide® MV, THOR Co., are added as a preservative. This pre-dispersion produced as described is fed in two passes through an UHDE high-pressure device HPD 4075- nano through diamond nozzles with a diameter of 0.20 mm at a pressure of 2500 bar. It yields a dispersion with 50 wt.% of AI2O3 showing a monomodal particle size distribution with a Z-avg (v) value of 106 nm (See Figure 5D) and a viscosity of 16 mPas (at a shear rate of 100 s-1) with good stability.

Comparative Example CD2: Preparation of Dispersion with higher BET Aluminum oxide

A dispersion according to example D4 but with use of AEROXI DE Alu 130 (having a BET specific surface area of 130 m²/g) and 24.6 g of citric acid as carboxylic acid and 24.6 g of 2-Amino-2-Methyl-1 -Propanol as amino alcohol could not be prepared. After the addition of 40 wt.% AI2O3 material the dispersion is too viscous for further processing.

Comparative Example CD3: Preparation of Dispersion with aluminum oxide only using a carboxylic acid

A dispersion according to example D4 but with use of only citric acid as carboxylic acid could not be prepared. After the addition of 20 wt.% AI2O3 material the dispersion is too viscous for further processing.

Preparation of the coating formulations:

The dispersions from the inventive and comparative Examples were used to prepare respective ceramic coating formulations as follows: First, the dispersion is diluted by adding water to a predetermined solid content of 20 wt.%, prior to addition of the respective binder. The amount of binder is adjusted to 3 wt.% based on the weight of the aluminum oxide powder. Finally, the wettability additive (fluorosurfactant) is added in an amount of 0.2 wt.% based on the total weight of the formulation. This final coating slurry is homogenized via gentle stirring for 15 minutes and then transferred to the slot-die coating machine.

Coating of the lithium-ion batery separator:

The coating formulations from the above Examples are used to apply a 2 pm thin and homogenous ceramic coating on top of a PE based separator available by ASAHI KASEI corporation of Japan under the trade name HIPORE AC 1681. The separator is a microporous membrane with highly uniform pores and a thickness of 16 pm. For applying the coating, the separator membrane is fixed on a vacuum table and the slot-die is adjusted with a gap to the separator of 15 pm. Upon start of the coating process, the speed of the slot-die is set to 0.4 m/min with a fluid rate of the coating formulation of 0.4 ml/min. The wet coated lithium-ion battery separator is dried at 50 °C for 2 hours. The process is repeated for application of an additional 2 pm thin layer on the other side of the separator. The same process was repeated for both the inventive and the comparative examples and the parameters and data are presented in the table 2.

Table 2

Coating compositions of dispersions D1-D3 and comparative dispersion CD1

As seen in Table 2, the water content of the coating compositions of dispersions D1-D3 is significantly reduced compared to the coating composition of the comparative dispersion CD1.

In addition, the air permeability of the coating compositions made by the dispersions D1-D3 is higher compared to coating compositions produced by comparative dispersion CD1. Therefore, the overall performance of ceramic coated separators using the inventive, new aluminium oxide dispersions is improved significantly.

Although the invention has been described in reference to only specific examples, it should be understood that the invention is not limited only to the described specific examples. The skilled person after reading the present disclosure would be able to envisage a number of variations of the described examples and other examples that fall within the scope of the invention as defined in the following claims. For example, an element described as employed either alone or in combination with other features in an example may also be used with another combination of features in another example without departing from the scope of the disclosed and claimed invention.

Also, for example, although the invention has been described in reference primarily to lithium-ion batteries, it should be understood that the dispersion and coating composition employing the dispersion may also be used for separators of other secondary batteries such as, for example, sodium-ion type secondary batteries.

Claims

1 . An aqueous dispersion containing aluminum oxide powder, characterized in that the aluminum oxide powder is present in the form of aggregated primary particles with an aggregate particle size distribution with a median aggregate particle size (D50) of 220 nm to 500 nm, as determined by dynamic light scattering measurement, and wherein the aluminum oxide powder has a BET of 10 m²/g to 100 m²/g, in an amount of at least 20 wt.%, based on the total weight of the dispersion, and that the dispersion further comprises:

- at least one carboxylic acid from the group consisting of dicarboxylic acids and/or hydroxy tricarboxylic acids having from 2 to 7 carbon atoms, and

- at least one amino alcohol having from one to six carbon atoms.

2. The aqueous dispersion of claim 1 , further characterized in that: the dispersion has a viscosity of less than 100 mPas, preferably 100 to 10 mPas and, more preferably, 60 to 15 mPas, as determined with a rotational viscosity method, the aluminum oxide powder has BET surface area of 10 to 100 m²/g, preferably 10 to 90 m²/g, preferably 30 to 90 m²/g, and more preferably 30 to 55 m²/g, as determined in accordance with DIN 66131 , the aluminum oxide aggregated primary particles have a monomodal particle size distribution with a Z-avg (v) at less than 500 nm, preferably between 500 nm to 150 nm, and, more preferably, between 350 nm to 180 nm , as determined by dynamic light scattering measurement, and wherein the at least one carboxylic acid is at least one of citric acid, and oxalic acid, wherein the at least one amino alcohol is dimethylethanolamine (DMEA) and 2-Amino-2- Methyl-1 -Propanol, and wherein the aluminum oxide powder is a pyrogenic aluminum oxide powder.

3. The aqueous dispersion of any of the claims 1 or 2, further characterized in that: the dispersion is free of sodium dihydrogen phosphate, phosphonic acids, or hygroscopic substances, the at least one amino alcohol is present in the dispersion in an amount of from 2.5 to 8.0 pmol/m of the aluminum oxide specific surface area, and the at least one carboxylic acid is present in an amount of from 1.0 to 4.0 pmol/m of the aluminum oxide specific surface area.

4. The aqueous dispersion of any of the claims 1 to 3, which is stable for at least 1 month, preferably at least 6 months in a pH range of 5 to 10, as determined with a method as described in the specification.

5. A coating composition for a secondary battery separator, characterized in that the coating composition is obtained by adding a binder and a wetting agent into the aqueous dispersion of any of the claims 1 to 4.

6. The coating composition of claim 5, comprising: the aluminum oxide particles in an amount of between 5 wt.% and 70 wt.%, preferably between 10 wt.% and 60 wt.%, and, more preferably between 15 wt.% and 50 wt.% based on the total weight of the coating composition, the binder in an amount between 1 and 20 wt.%, preferably between 2 and 15 wt.%, and, more preferably between 3 and 10 wt.% based on the total weight of the coating composition, and a wetting agent.

7. Process for the preparation of the aqueous dispersion according to claims 1 to 4, characterized in that:

- the aggregated aluminum oxide powder, corresponding to the desired amount in the dispersion, the at least one amino alcohol, and the at least one carboxylic acid are placed in water to create a water slurry,

- a pre-dispersion is produced by introducing energy, by mechanical means, in the water slurry in an amount of less than the energy required for the formation of the dispersion, and

- then, producing the dispersion by introducing the pre-dispersion in a high-energy mill and grinding the pre-dispersion by means of the high-energy mill at a pressure of at least 500 bar.

8. The process according to claim 7 wherein the energy introduced in the water slurry to produce the pre-dispersion is less than 1000 kJ/m^.

9. Use of the coating composition according to claims 5 or 6 for the forming of a coating layer on at least one surface of a separator of a secondary battery, wherein the separator is a thin membrane made of at least one of polyolefin, nylon, PET (polyethylene terephthalate), PVC (polyvinyl chloride), PI (polyimide), PVDF (polyvinylidene fluoride) and PEEK (polyether ether ketone), wherein the coating formed on the separator has a moisture of less than 1000 ppm and an air permeability defined by a Gurley value up to 100 s, preferably up to 50 s and, more preferably up to 30 s, as determined with a method as described in the specification.

10. The use of the coating composition according to claim 9, wherein the separator is made of a polyolefin including at least one of polypropylene, polyethylene or any combination thereof.

11 . A separator for a lithium-ion battery cell, the separator comprising: a membrane comprising at least one polyolefin, and a coating layer obtained by applying the dispersion of any of the claims 1-4 or the coating composition of claims 5 to 6 on at least one surface of the membrane and drying the dispersion by applying heat to form the coating layer, wherein the coating layer formed on the separator has a moisture of less than 1000 ppm and an air permeability defined by a Gurley value up to 100 s, preferably up to 50 s and, more preferably up to 30 s as measured with the coating formed on both sides of the separator.

12. The separator of claim 11 , wherein the membrane is made of polyethylene and has a thickness of from 3 to 20 pm, wherein the coating layer is homogeneous having a thickness of less than 5 pm, preferably less than 3 pm, and more preferably less than 2 pm with a lowest limit of 500 nm.

13. A secondary battery comprising the separator of any of the preceding claims.

14. An apparatus comprising the secondary battery of claim 13, the apparatus comprising an electric or electronic device including a mobile phone, an electronic watch, a key fab, a laptop computer, a desktop computer, a computer pad, a power tool, a vacuum cleaner, an electric lawn mower, an electric appliance, and an electric vehicle.