GB2614734A

GB2614734A - Binder free carbon coated Al-foil for battery applications

Info

Publication number: GB2614734A
Application number: GB2200478.2A
Authority: GB
Inventors: Fotedar Sameer
Original assignee: Morrow Batteries AS
Current assignee: Morrow Batteries AS
Priority date: 2022-01-14
Filing date: 2022-01-14
Publication date: 2023-07-19

Abstract

A method for the manufacture of an electrode suitable for use as the cathode or anode in a battery, comprises (i) obtaining a Cu or Al foil and subjecting the foil to a surface roughening step; (ii) applying a carbonaceous coating on the foil by dry powder coating; and (iii) applying an electron active material to the carbonaceous coating. Preferably, the foil is Al and the carbonaceous coating is 100% carbon and applied to both sides of the foil. The electron active material may be a layered oxide (such as lithium conbalt oxide), a polyanion (such as lithium iron phosphate) or a spinel (such as lithium manganese oxide). The carbon coating may be polished to adjust its thickness. An electrode suitable for use as the cathode in a battery is also disclosed. The electrode comprises a Cu or Al foil, a carbonaceous coating in direct contact with the foil, which is free of binder and solvent, and an electrode active material in direct contact with the carbonaceous coating. The electrode can be use in a battery, especially a Li-ion battery.

Description

Binder free carbon coated Al-foil for battery applications This invention relates to the coating of Al current collectors with a carbonaceous coating for use as the cathode in a battery, such as in a Li ion battery. In particular, the invention relates to a fast, efficient process for coating an Al current collector with a carbonaceous coating layer and the subsequent coating thereof with an electrode active material. The resulting electrode acts as a high specific energy cathode, e.g. for a Li-ion battery.

The carbonaceous coating process also has utility in the coating of anode current collector material such as copper.

Background of the Invention

Use of secondary batteries has increased significantly in recent years as a result of global demand for consumer electronics such as laptops and cellular phones has escalated. Efforts to develop technologies with green chemistries, such as electrical vehicles, have further fuelled the demand for rechargeable batteries with high energy densities.

The most popular rechargeable battery is a lithium ion battery. Compared to other types of rechargeable batteries, lithium ion batteries achieve high energy densities and have a minimal self-discharge. Due to these beneficial properties, lithium ion batteries have been used in transportation, back-up storage, defence and aerospace applications.

The three primary functional components of a battery such as a lithium-ion battery are the positive electrode, negative electrode and electrolyte. Generally, the negative electrode of a conventional lithium-ion cell is made from carbon. The positive electrode is typically a metal oxide or phosphate. The electrolyte is a lithium salt in an organic solvent. The electrochemical roles of the electrodes reverse between anode and cathode, depending on the direction of current flow through the cell.

The most common commercially used anode (negative electrode) is graphite, which forms LiC6 in its fully lithiated state. Highly porous graphite composites are commonly used. More recently, lithium titanates have been used as anodes in secondary Li ion batteries.

The positive electrode is generally one of three materials: a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spine! (such as lithium manganese oxide).

The electrolyte is typically a mixture of organic carbonates such as ethylene carbonate or diethyl carbonate containing complexes of lithium ions. These non-aqueous electrolytes generally use non-coordinating anion salts such as lithium hexafluorophosphate (LiPF6), lithium hexafluoroarsenate monohydrate (LiA5F6), lithium perchlorate (LiCI04), lithium tetrafluoroborate (UBE°, and lithium triflate (LiCF3S03).

The external faces of the electrodes are usually equipped with some means to collect the charge generated by the battery during discharge, and to permit connection to an external power source during the recharging of the lithium ion battery. These current collectors are usually made of stainless steel, iron-nickel alloys, copper foil, aluminium and similar relatively inexpensive metals. Typically, an Al current collector is used with the cathode and a copper current collector is used with the anode.

Secondary batteries typically use a cathode which comprises a current collector comprising an aluminium foil bonded to the electrode active material. An electrode active material layer is formed by applying a coating fluid on the Al foil It is known that the performance of a cell which uses an electrode comprising a current collector having such an electrode active material layer varies depending on the surface conditions of the metal foil and hence the adhesion of the electrode active material to the current collector.

There is still a need for high power cathodes to match fast anode materials, such as lithium-titanate ([TO) and highly porous graphite composites. Achieving high power cathodes requires eliminating the cathode impedances which are generated especially at high current drains. This will also help improve the thermal safety of the batteries by suppressing the ohmic heating.

Carbon coating of Al foils is a relatively new procedure in the field of electrochemical energy storage/conversion systems. This procedure is known to improve the performance of a cathode in a Li ion battery. To increase the mechanical strength and electronic conductivity, the Al foils are typically carbon coated therefore. The most common coating process involves a mixture of carbonaceous material and a binder dissolved in a solvent, followed by its application onto the metal surface as described in US 2013/0323589. The coating 3 -slurry may be applied to the Al substrate using any suitable method. The coating may be applied in multiple layers.

A problem with a slurry coating prepared by mixing the carbonaceous material and a binder is its inherently poor electrical conductivity due to the binders being non-conductive. The binder also increases the electrode mass. The binder doesn't take part in charging process and thus deteriorates the mass specific energy and power density of the battery. Moreover, having the carbonaceous layer dried out prior to electrode active material application creates an interface that may diminish the electrode performance. In addition, solvents used to dissolve the binder need to be recovered (and dried out) which is energy and capital intensive process.

Another known method to prepare carbon coated metal foils is the ionization of the carbon atoms (positive) that are subsequently attached to the negatively polarized foil surfaces. US 2012/0237782 describes such a coating procedure in which a gas mixed with carbon atoms is fed into a vacuum chamber and electric field is applied to yield positively charged carbon atoms. The carbon atoms with positive charge are drawn to the negatively charged aluminium foil. The carbon atoms are hence firmly attached to the aluminium surface. This method avoids the use of binder; however, it is complicated and has a very high cost.

US 7,327,556 also discloses a method for the production of aluminium substrate coated with a carbon layer. To increase the adhesion, an aluminium carbide layer is developed between the aluminium substrate and the carbon layer. The disadvantage of this method is that a long thermal treatment under methane and acetylene is required, thus limiting the production efficiency and increasing the cost.

Similarly, US 8,644,006 discloses a method for carbon coating aluminium surfaces. This involves the building of a metal layer on the aluminium surface via a carbide of the metal on the metal layer and finally the carbon layer on the metal carbide. For practical purposes, this method may not be employed in Li-ion battery electrodes.

Surface roughening/treatment of metal foils used as current collectors is generally conducted prior to coating with active electrode material to enhance the adhesion of the coating and increase the foil surface area. W000/007253 discloses a method to increase the surface area. It comprises the treatment of an Al foil in an acidic or basic solution at a range of temperatures, washing with water and drying, 4 -resulting in a highly porous surface oxide layer. Similarly, US 2013/0323589 also discusses a surface treatment method in acidic or basic solutions. On the contrary to W000/007253, this method results in a more compact and continuous oxide layer which facilitates the adhesion with the cast containing the active material.

On the basis of the available literature and prior art described above, a brief overview of the shortcomings of state of the art coatings is listed below; High resistance coatings due to the use of non-conductive binders Use of organic solvents that need recovery systems and health and safety issues Time, energy and cost intensive processes that limit the industrial applicability Non-transparent coating that induces challenges during industrial electrode manufacturing The present invention overcomes these shortcomings by using a binder-free and solvent-free, low cost, low energy process for coating the Al current collector.

The coating process is simple and rapid. The resulting coated Al foil can then be coated with an electrode active material to form a cathode for a battery such as a Li ion battery. No intermediate layer is required between the Al foil and the carbonaceous layer.

Moreover, the process described herein for coating an Al current conductor can also be used to coat the current conductor used in connection with an anode, for example, a copper foil. In order to improve adherence of the electrode active material to the current collector, it is also possible to use a carbonaceous coating in the anode as well.

Summary of Invention

The present invention therefore relates to an aluminium or copper current collector coated with carbonaceous material, and its use in an electrode structure with an active electrode material such as a metal oxide. Such an electrode structure may be present in a battery exploiting said coated copper or aluminium material, such as a Li ion battery. The invention primarily relates to a manufacturing method of said coated copper or aluminium foil and the electrode structure. The method enhances the adhesion between the Al or Cu foil and the electrode active material and also reduces corrosion and enhances electronic conductivity of the electrode structure.

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Viewed from one aspect the invention provides a method for the manufacture of an electrode suitable for use as the anode or cathode in a battery, said process comprising (i) obtaining a Cu or Al foil and subjecting said foil to a surface roughening step; applying a carbonaceous coating on said foil by dry powder coating; (iii) applying to said carbonaceous coating an electron active material.

In one aspect, the invention provides a method for the manufacture of an electrode suitable for use as the cathode, e.g. in a Li ion battery, said process comprising (i) obtaining an Al foil and subjecting said foil to a surface roughening step; (ii) applying a carbonaceous coating on said foil by dry powder coating; (iii) applying to said carbonaceous coating an electron active material. Viewed from another aspect the invention provides an electrode obtained by the process hereinbefore defined.

Viewed from another aspect the invention provides an electrode suitable for use as the anode or cathode in a battery, comprising, in this order, a Cu or Al foil; (ii) a carbonaceous coating in direct contact with said Cu or Al foil, said coating being free of binder and solvent; (iii) an electron active material in direct contact with said carbonaceous coating.

In particular, the invention provides an electrode suitable for use as the cathode in a Li ion battery, comprising, in this order, an Al foil; a carbonaceous coating in direct contact with said Al foil, said coating being free of binder and solvent; (iii) an electron active material in direct contact with said carbonaceous coating.

Viewed from another aspect the invention provides a battery, such as a Li ion battery comprising an electrode as hereinbefore defined.

In particular the process of the invention reduces the electrode impedance by minimizing the charge transfer resistance from the Cu or Al foil, which in turn can reduce the overall impedance in a full cell battery.

Detailed Description of Invention 6 -

This invention relates to a new process for coating a Cu or Al foil current collector with a carbonaceous coating and the subsequent use of that coated current collector to prepare an anode or cathode electrode for a battery, such as Li ion battery. Whilst the invention is generally described with reference to Li ion batteries it will be appreciated that the anodes and catodes described herein might also have utility in other battery types.

Moreover, whilst the invention is primarily described with reference to Al foils and hence cathodes, the same principles also apply to the manufacture of copper foils with carbonaceous coatings and their use in anodes.

The invention requires the use of an Al or Cu foil. By an Al or Cu foil is meant a foil in which at least 60 wt% of the foil is Al or Cu respectively. It is possible therefore for other metals to be present in the foil but the major element present is Al or Cu. Ideally, the foil is 90 wt% or more Al or Cu respectively.

An Al foil is conventionally used as current collector for cathode electrodes in Li-ion batteries. The Al foil of use in the invention may comprise more than 95% Al by mass, preferably at least 99 wt%, the remainder being other metals including, but not limited to Cu, Mg, Si, Fe, Zn, V, Ga and Ti.

A Cu foil is conventionally used as current collector for anode electrodes in Li-ion batteries. The Cu foil of use in the invention may comprise more than 95% Cu by mass, preferably at least Cu wt%, the remainder being other metals including, but not limited to Al, Mg, Si, Fe, Zn, V, Ga and Ti.

Most preferably, the foil consists essentially of Al or Cu (i.e. is pure Al or Cu other than the presence of minor impurities).

There is no limitation, other than the practical battery constraints, on the thickness of the foil used in this invention. Preferably 1-20 pm thick Cu or Al foils are used. The current collector may be an etched Cu or Al foil. The foil can also have holes, e.g. in the form of a grid or a net, in order to minimize the mass per unit area of the foil.

To prepare an electrode, typically, the starting Cu or Al foil is attached to a smooth surface prior to coating. The Cu or Al foil is subjected to a surface roughening process, typically a mechanical surface roughening process e.g. with a grinding paper or similar, before application of the carbonaceous coating. Preferably, the grain size of the roughening tool is between 600 and 4800, most preferably between 800 and 1500. This roughening treatment facilitates the adhesion of carbonaceous material to the foil and improves contact between the foil 7 -and carbonaceous material so that electrical resistance between the two layers is eliminated or significantly reduced.

This Cu or Al foil is then coated with carbonaceous material. The coating is ideally uniform, i.e. the thickness of the carbonaceous coating is constant. The coating ideally covers the whole of at least one surface of the Cu or Al foil. Ideally the foil is coated on both sides.

The coating process requires the application of a powder coating on the foil.

Application of the powder coating might simply involve sprinkling of powdery carbonaceous materials over the surface of the foil. Any known powder coating application process may be used. Key here is that the carbonaceous powder coating consists of carbonaceous material only. There is no binder or solvent present. Surprisingly, the inventors have found that the use of binders and solvents can be completely avoided if the coating is applied as a powder coating. The carbonaceous layer and carbonaceous coating preferably consists of carbon.

Conveniently, the carbonaceous powder is applied and a rotary polishing apparatus, such as a car polisher, can be used to ensure even application of the carbonaceous powder on the foil. Suitable rotation speeds for the polisher are 5000 to 15000 rpm, e.g. 10000 to 15000 rpm. The polishing step can also be used to control the thickness of the carbonaceous coating. The coating may be applied quite thickly and polished down to a target thickness. The polishing action causes exfoliation of graphite layers.

It may also be possible to apply the carbonaceous powder by spraying or dipping. The basic principle of spray application is that a powder/air mixture is fed from a hopper through a spray gun, which blasts it onto the substrate to be coated.

The coated piece is then sent to the oven for curing. The powder is attracted to the substrate through electric charge. The most convenient spray technologies are corona charging gun (which apply a charge to the powder as it leaves the gun) or tribostatic spray application with tribo charging gun.

Dipping is an alternative powder coating process, e.g. using a fluidised bed application. These methods may however lead to unacceptably thick coatings and hence the use of a rotary polisher to control the thickness of the carbonaceous layer is preferred.

The particle size of the carbonaceous powder may vary and different particle sizes might be used depending on the target thickness of the carbonaceous layer. As the carbonaceous layer is polished the initial coating thickness can be 8 -reduced to a desired level. The particle size of the carbonaceous powder may be 1 to 30 pm in diameter, such as 5 to 20 pm.

The amount of carbonaceous powder required varies depending on the required thickness of the carbonaceous layer however, a suitable amount of carbonaceous powder is 0.5 to 2.0 g per 100 cm2of foil. Remarkably, under the application of pressure from the rotary polisher the carbonaceous powder exfoliates to allow the preparation of a very thin layer on the foil. The carbonaceous powder adheres to the foil without binder or solvent being required. Without wishing to be limited by theory, mechanical interlocking adhesion forces ensure adhesion between the powder and foil.

The present invention therefore exploits carbonaceous materials having a high electronic conductivity to reduce the charge transfer resistance at the interface between the foil and carbonaceous material.

The term carbonaceous material defines a material containing elemental carbon as the predominant component. Carbonaceous materials preferably contain wt% or more elemental carbon. Ideally the only component is elemental carbon. The carbonaceous material may be any form of carbon such as graphite, including, but not limited to, graphite (natural and synthetic), graphene, coke and other materials that are rich in carbon, e.g. carbon black such as acetylene black, furnace black, or Ketjen black, carbon nanofibers, carbon nanotubes and mixtures thereof.

The thickness of the carbonaceous layer can range from nano to micrometers, e.g. 1 nm to 20 microns, such as 1 to 200 nm, especially 10 to 100 nm. The ratio of the thickness of the said coating to that of the said foil is preferably within the range of 0.000001 to 0.001:1, most preferably from 0.000001 to 0.00001:1. The foil is therefore considerably thicker.

It is key that the subsequent electrode active material is applied to a surface on which the carbonaceous coating has been applied.

An electrode active material can be applied, such as cast, onto the said foil coated with carbonaceous material. The carbonaceous material therefore enhances the adhesion between the foil and the electrode active material, thus reducing the electrode impedance by minimizing the charge transfer resistance from/to the foil.

The application of the electrode active material can be achieved using any convenient method and can be adapted to take account of the nature of electrode active material. The use of tape casting is preferred. 9 -

The electrode active material can cover the whole of the relevant side of the foil or only partially cover the carbonaceous coating on the foil. Ideally, the electrode active material forms a continuous second layer on the carbonaceous layer. The thickness of the electrode active material is independent of carbonaceous layer. A thickness of 100 nm to 20 microns is possible.

Suitable electrode active materials are known in the art. For a cathode, suitable electrode active materials include metal salts such as Li mixed metal oxides, e.g. lithium manganese oxide.

For an anode, suitable electrode active materials include graphite and lithium titanates.

The resulting electrode can be used in a battery. The general design of a half cell is known in the art. In a Li ion battery, the anode and the cathode electrodes are placed opposite to each other such that the Li ions are inserted/removed to/from the appropriate electrode during charging/discharging. A porous ionically conductive film may be sandwiched between the electrodes to prevent electrical shorting. A non-aqueous electrolyte is also used. The electrolyte comprises a solvent selected from the group consisting of organic ethers, mixtures of organic ethers, propylene carbonate, ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate. Improved charge/discharge voltage profiles, and thus rate performance, are achieved by utilizing the said cathode.

The Al foil current collector used to construct an electrode structure for Li-ion batteries is preferably at least partially coated with an ultrathin layer of carbonaceous material. Coating the Al foil with a carbonaceous material yields higher surface area, and therefore enhances the adhesion between the electrode active material and the Al foil. Increasing the surface area and thus the adhesion reduces the interfacial impedance and hence leads to a rapid electron transfer rate to/from current collector in the said battery. The use of the Al foil coated with said carbonaceous material, reduces the sheet resistance by a factor of at least two. The presence of carbon at the interphase enables rapid electron transfer during both charge and discharge process and hence improves the rate performance of the electrode.

It is also preferred if the Cu foil used as current collector in the anode has a carbonaceous coating as described herein.

The invention will now be described with reference to the following non

limiting examples.

-10 -

Example

A 12 pm thick Al foil is roughened prior to the coating. Roughening is achieved by sandpaper 1000 grit. Sandpaper is gently rubbed on the aluminium foil surface and subsequently the sanded Aluminium foil is cleaned with ethanol. The foil has a surface area of 100 cm2.

1g graphite powder is spread evenly on the aluminium foil. A rotary polisher is used to coat and compact the surface with graphite. A suitable rotation speed is 12000 rpm. The polisher is operated for 10 minutes to evenly coat the aluminium foil with graphite. The sheer stress generated by circular action of polisher causes exfoliation of graphite layers.

The graphite coating layer that is formed has a thickness of up to 100 nm. The electrode active material is then applied by tape casting.

Claims

Claims 1. A method for the manufacture of an electrode suitable for use as the cathode or anode in battery, said process comprising obtaining a Cu or Al foil and subjecting said foil to a surface roughening step; applying a carbonaceous coating on said foil by dry powder coating; (iii) applying to said carbonaceous coating an electron active material.
2. A method as claimed in any preceding claim wherein the foil is an Al foil.
3. A method as claimed in any preceding claim wherein the carbonaceous coating consists of carbon. (i.e. is 100% carbon)
4. A method as claimed in any preceding claim wherein the electron active material is a layered oxide (such as lithium cobalt oxide), a polyanion (such as lithium iron phosphate) or a spine! (such as lithium manganese oxide).
5. A method as claimed in any preceding claim wherein the carbonaceous coating is 10 to 100 nm in thickness.
6. A method as claimed in any preceding claim wherein the carbonaceous coating is applied to both sides of the foil.
7. A method as claimed in any preceding claim wherein the carbonaceous coating is polished, e.g. with a rotary polishing device, to adjust the thickness of the carbonaceous coating.
8. A method as claimed in any preceding claim wherein the foil is 1 to 20 microns in thickness.
9. An electrode suitable for use as the cathode in a battery, comprising, in this order, (i) a Cu or Al foil; -12 -(ii) a carbonaceous coating in direct contact with said Al foil, said coating being free of binder and solvent; (iii) an electron active material in direct contact with said carbonaceous coating.
10. A battery, such as a Li ion battery, comprising an electrode as hereinbefore defined.