Effective binder to cathode active material ratio

Abstract

The present disclosure relates to a method of forming electrodes for lithium- or sodium- ion cells. The method includes the dry deposition of an electroactive composition comprising cathode active material, a binder and optional conductive additives to a conductive foil. Importantly, the binder particles are smaller than the cathode active material particles. The methodology provides good packing densities compared with traditional dry deposition methodologies.

Description

[0001] EFFECTIVE BINDER TO CATHODE ACTIVE MATERIAL RATIO

[0002] FIELD

[0003] The present disclosure relates to a method of forming a cathode with high packing densities by dry deposition.

[0004] BACKGROUND

[0005] Rechargeable or 'secondary' batteries find widespread use as electrical power supplies and energy storage systems. For example, in automobiles, battery packs formed of a plurality of battery modules, wherein each battery module includes a plurality of electrochemical cells, are provided as a means of effective storage and utilization of electric power.

[0006] Conventionally, electrodes for lithium ion and sodium ion batteries have been produced using wet processing methods using a slurry, for example using N-methyl-2-pyrrolidone (NMP) and / or water as solvents. As a result, the amount of binder and the size of the binder particles has been optimised for slurry-based processing methods.

[0007] However, from an environmental point of view, the slurry-based processing methods are undesirable due to the increased amount of waste as well as the energy and time required in subsequent solvent removal steps.

[0008] A multimodal (preferably bimodal) distribution of cathode active material (CAM) particle size is typically used to achieve high packing density. However, having a CAM with a bimodal distribution of particle sizes increases the operating cost (OPEX), and capital cost (CAPEX). This is because bimodal materials require an extra processing line, an extra step for mixing the CAM materials together to achieve the bimodal distribution and additional quality management.

[0009] Therefore, there is an interest in forming cathodes by dry depositing particles using a method that allows for the use of cathode active material particles having a unimodal size distribution, while still obtaining high density cathodes.

[0010] SUMMARY

[0011] According to an aspect of the present disclosure, there is provided a dry process for forming a cathode, comprising I) providing a. a binder with a D50 particle diameter of 500 nm or less; b. a cathode active material with a D50 particle diameter of about 5 to about 15 microns; and c. optionally a conductive additive;

[0012] II) mixing the components provided in step (I) to form an electroactive composition;

[0013] III)depositing the electroactive composition by dry deposition onto a conductive foil to leave an electroactive layer on the conductive foil; and

[0014] IV) optionally calendering the product of step (III) to form a cathode.

[0015] According to a further aspect of the disclosure, there is provided an electroactive composition comprising 90 to 99.8 wt% CAM, from 0.02 to 6 wt% conductive additive, and from 0.05 to 3 wt% binder, wherein the binder covers from about 5% to about 50% of the surface of the cathode active material particle.

[0016] While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example and will be described in detail. It should be understood, however, that other embodiments, beyond the particular embodiments described, are possible as well. All modifications, equivalents, and alternative embodiments falling within the spirit and scope of the appended claims are covered as well.

[0017] The above discussion is not intended to represent every example embodiment or every implementation within the scope of the current or future Claim sets. Various example embodiments may be more completely understood in consideration of the following Detailed Description.

[0018] FIGURES SUMMARY

[0019] Figure 1A: SEM image of an electroactive composition wherein the CAM, the conductive additive and the binder is well mixed.

[0020] Figure IB: SEM image of an electroactive composition wherein the CAM, the conductive additive and the binder is poorly mixed.

[0021] Figure 2: Graph showing packing density of cells with varying compositions and different degrees of calendering. Figure 3: Graph showing the packing density and cathode active layer density as a function of CAM wt%.

[0022] Figure 4: Graph showing how cycling retention for cells comprising cathode active layers with different conductive additive wt%.

[0023] DETAILED DESCRIPTION

[0024] The disclosure relates to a method of forming a cathode using a dry deposition method. In particular, said method allows for preparation of cells having a high packing density, particularly while using cathode active material (CAM) having a unimodal or a bimodal size distribution.

[0025] It has surprisingly been found that a density of over 3.6 g / cm3can be achieved in electrodes formed by electrospray deposition (ESD) and a CAM with a D50 particle diameter of 8 to 12 pm, and a binder with a particle size (diameter) of less than 500 nm. This is surprising as densities of over 3.6 g / cm3are typically only achievable using wet processing conditions with bimodal CAM particles.

[0026] The cathode comprises cathode active material, a binder, and optionally conductive additives.

[0027] CATHODE ACTIVE MATERIAL

[0028] The term "cathode active material" (CAM) is to be understood as an electrochemical species which can be oxidised and reduced in a system which enables a cell to produce electric energy during discharge. The role of the cathode active material is to reversibly intercalate (or otherwise bind) ions (such as lithium or sodium ions) during cell charge and discharge cycles.

[0029] The cathode active material may comprise a suitable material for use as a cathode active material in the cathode of a cell, for example a lithium ion cell or a sodium ion cell.

[0030] The cathode active material may comprise an intercalation material, such as a lithium intercalation material, for example a lithium metal oxide which may include lithium and a transition metal.

[0031] The cathode active material may comprise any one or a mixture of two or more of lithium manganese oxide, lithium nickel oxide, lithium cobalt oxide, lithium nickel manganese oxide (LNMO), lithium nickel cobalt oxide, lithium nickel manganese cobalt (NMC) oxide, lithium iron phosphate (LFP), lithium iron manganese phosphate (LFMP) and lithium nickel cobalt aluminium oxide (NCA).

[0032] Preferably, the cathode active material is a transition metal complex such as layered lithium metal oxide (LiMO2) cathode materials, wherein the metal is typically nickel. Even more preferably, the cathode active material is a lithium nickel manganese cobalt oxide (NMC), such as a high nickel lithium nickel manganese cobalt oxide (high Ni NMC) or lithium rich lithium nickel manganese cobalt oxide (Li rich NMC).

[0033] In some embodiments, the cathode active material comprises lithium nickel cobalt manganese oxides (NMC) LibNii-x-y-zCOxMnyAzO2wherein 0<x+y+z< l, A is an element other than Li, Ni, Co, Mn or O and wherein 0<z<0.05, preferably 0<z<0.03, more preferably 0.001<z<0.01, and wherein 0.9<b< 1.2. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0034] In preferred embodiments, the NMC cathode materials are lithium rich. As such, the cathode active material typically comprises lithium nickel cobalt manganese oxides (NMC) represented by the formula LibNii-x-y-zCOxMnyAzO2wherein 0<x+y+z< l, A is an element other than Li, Ni, Co, Mn or O and wherein 0<z<0.05, preferably 0<z<0.03, more preferably 0.001<z<0.01, and wherein 1.05<b< 1.2. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0035] In preferred embodiments, the NMC cathode materials are high in nickel. As such, the cathode active material typically comprises lithium nickel cobalt manganese oxides (NMC) represented by the formula LibNii-x-y-zCOxMnyAzO2, wherein 0<x+y+z<0.4, preferably 0<x+y+z<0.25, and wherein 0<z<0.05, preferably 0<z<0.03, more preferably 0.001<z<0.01, and wherein 0.9<b< 1.2. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0036] The ratio of the nickel of the high-nickel NMC material may range from 33 mol% to 98 mol%, in terms of the non-lithium metals. Preferably, the ratio may range from 60 mol% to 95 mol%, even more preferably, the ratio may range from 80 mol% to 95 mol%, in terms of the non-lithium metals. In preferred embodiments, the NMC cathode materials is defined as LibNii-x-yCOxMnyAzO2, wherein 0<x+y<0.4, preferably 0<x+y<0.25, and wherein 0<z<0.05, preferably 0.002 <z< 0.03, more preferably 0.001<z<0.01, and wherein 0.9<b< 1.1. A is one or more chosen from the group Al, B, Zr, Ba, Ca, Ti, Mg, Ta, Nb, V, Fe, Ru, Re, Pt and Mo. Preferably, A is chosen from the group Al and Zr.

[0037] Alternatively, the cathode active material may comprise a suitable material for use as a cathode active material in the cathode of a sodium ion cell.

[0038] The cathode active material of a sodium ion cell may comprise any one or a mixture of two or more sodium containing oxides, sodium containing cyanides, or any combination thereof.

[0039] For example, the cathode active material may comprise sodium transition metal cyanides having six cyanide groups per formula unit. Each of these cyanide groups connect transition metals within the cathode active material to form a framework with large voids that allow intercalation and de-intercalation of sodium-ions.

[0040] Exemplary sodium transition metal cyanides include Prussian Blue (PB) and its derivatives, i.e. Prussian Blue Analogues (PBA). Even more preferably, the cathode active material comprises Prussian Blue Analogues (PBA).

[0041] Exemplary Prussian blue analogues include Prussian White, Turnbull's blue, potassium ferricyanide, and potassium ferrocyanide.

[0042] In some embodiments, the cathode active material comprises Prussian Blue Analogoues (PBAs) having a formula of AxPy[Rz(CN)6]w, where A is a sodium ion, and P and R are transition metals. Prussian Blue Analogues (PBA) offer many opportunities for structural variation and hence the properties are highly tunable. For example, the stoichiometry may vary: l<x<2, 0<y<2, l<z<2, and l<w<2.

[0043] The transition metals P and R may each be selected from manganese (Mn), Iron (Fe), Aluminium (Al), Titanium (Ti), Nickel (Ni), Vanadium (V) and Cobalt (Co). Preferably, R is iron (Fe).

[0044] Preferred are Prussian Blue Analogues (PBA) selected from Fe-Fe-PBA, Mn-Fe-PBA, Fe-Ni- PBA, Ni-PBA, Co-PBA, or any combination thereof, especially Fe-Fe-PBA. Prussian Blue Analogous (PMA) may be complexed to water.

[0045] Typically, the D50 of the cathode active material particles is in the range of from about 5 pm to about 15 pm, such as from about 8 pm to about 12 pm, such as from about 9 pm to about 11 pm.

[0046] Additionally, the DIO of the cathode active material particles is typically in the range of from about 0.1 pm to 12 pm, such as from 5 pm to 10 pm, for instance from 6 pm to 9 pm.

[0047] The D90 of the cathode active material particles is typically in the range of from about 8 pm to 20 pm, such as from 10 pm to 15 pm, for instance from 11 pm to 14 pm.

[0048] The cathode active material may have a bimodal particle size distribution. That is, it has two maximum values in the particle size distribution. The size of the small particles is typically in the range of from about 0.1 pm to 10 pm, preferably from about 4 pm to about 6 pm. The size of the large particles is typically in the range of from about 5 pm to 20 pm, preferably from about 10 pm to about 15 pm.

[0049] Preferably, the cathode active material of the present invention has a unimodal particle size distribution.

[0050] Typically, the D50 of the unimodal cathode active material particles is in the range of from about 5 pm to 15 pm, for example from 8 pm to about 12 pm, such as from about 9 pm to about 11 pm.

[0051] Preferably, the unimodal particles have a D50 of about 8 pm, about 9 pm, about 10 pm, about 11 pm, or around 12 pm. Even more preferably, the D50 is about 10 pm.

[0052] Preferably, the CAM particles are substantially spherical or spherical. Without wishing to be bound by theory, it is believed that the use of spherical or substantially spherical cathode active material compared to cuboid cathode active material allows for better packing, resulting in increased packing density.

[0053] By "substantially spherical" is meant a shape that is approximately a sphere, with only small deviations from a perfect sphere by no more than about 20% in any given dimension. Typically, polycrystalline CAM particles are spherical, while single crystal CAM particles have a cuboid shape.

[0054] In an embodiment, the CAM comprises polycrystalline spherical particles.

[0055] To assess the shape of CAM particles, the aspect ratio is a key metric. This ratio compares the longest dimension to the shortest dimension of a particle, with a perfect sphere having an aspect ratio of 1.

[0056] The aspect ratio is determined by analyzing images captured via a microscope. For instance, using software like Image!, one can measure the longest and shortest dimensions of a particle.

[0057] The aspect ratio is then calculated as:

[0058] Shortest Dimension

[0059] Aspect Ratio = -

[0060] Longest Dimension

[0061] To obtain a representative value for a powder sample, the average aspect ratio is calculated from at least 10 particles.

[0062] Preferably, the CAM particles have an average aspect ratio below 1.5, preferably below 1.4, and more desirably below 1.3, 1.2, or even 1.1.

[0063] In an embodiment, the electroactive composition may comprise cathode active material in an amount of, by weight, from about 60-99.9 wt%, for example from about 70-99.9 wt%, from about 80-99.8 wt%, from about 90-99.6 wt%, or from about 92-99 wt%. Preferably the cathode comprises from about 94-99 wt% cathode active material, such as from 95- 98.8 wt%, even more preferably the cathode comprises from about 98 to about 98.5 wt% cathode active material.

[0064] CONDUCTIVE ADDITIVES

[0065] The role of the optional conductive additive is to improve the electronic properties of the electrode and to provide an electrical connection between the electrode active material of an electrode.

[0066] Suitable conductive additives include acetylene black, carbon black, graphene, graphite, mesocarbon microbead (MCMB), pitch-based carbon, coke powders, carbon nanotubes or metallic powders. These conductive additives may be used alone or in combination. Preferred conductive additives are selected from carbon black, graphite, carbon nanotubes, or mixtures thereof. Preferably, the conductive additive is carbon black.

[0067] In an embodiment, the electroactive composition may comprise 0-10 wt% conductive additive based on the combined weight of the CAM, the conductive additives and the binder. For example, the electrode may comprise from about 0.02-8 wt%, from about 0.05-6 wt%, or from about 0.1-5 wt% conductive additive. Preferably the electrode comprises from about 0.2 to 4 wt% conductive additive, even more preferably 0.5 wt% to 3 wt% conductive additive, such as from 0.6 wt% to 2.5 wt% conductive additive, or from 0.75 wt% 2 wt% conductive additive. Most preferably the electrode comprises about 0.75 wt%, about 1 wt%, about 1.5 wt% or about 2 wt% conductive additive.

[0068] BINDER

[0069] The role of the binder is to adhesively connect all the electrode materials for long-term charge / discharge cycling.

[0070] Exemplary binders include carboxymethyl cellulose binder (CMC), styrene butadiene rubber (SBR), hydrogenated nitrile butadiene rubber (HNBR), polyvinylidene fluoride (PVDF), and / or copolymers thereof.

[0071] Preferably, the binder is polyvinylidene fluoride (PVDF).

[0072] Typically, the binder of the disclosure is provided as small particles. By small particles is meant particles having a diameter of 0.5 micron or less.

[0073] For example, the binder may have a D50 of from about 10 nm to about 500 nm, such as from about 50 nm to about 300 nm, for example from about 75 nm to about 250 nm, such as from 90 nm to about 200 nm.

[0074] Even more preferably, the small particles are provided as small particles having a D50 of about 100 nm or about 200 nm.

[0075] Additionally, the binder particles may have a D90 of from about 60 nm to about 600 nm, such as from about 80 nm to about 325 nm, for example from about 90 nm to about 275 nm, such as from 100 nm to about 250 nm.

[0076] The binder particles are smaller than the CAM particles. For example, the ratio between the D50 of the binder particles and the D50 of the CAM particles may be from about 1 :5 to about 1:200, from about 1 : 10 to about 1: 150, from about 1:20 to about 1: 100, from about 1:40 to about 1:75, or from about 1:45 to about 1:55. Preferably, the ratio between the D50 of the binder particles and the D50 of the CAM particles is about 1:50.

[0077] Without wishing to be bound by theory, it is believed that the use of smaller binder particles compared to the size of CAM particles, allows the smaller binder particles to coat the surface of the comparably large CAM particles. This in turn means that a smaller amount of binder is needed compared to traditional dry deposition methodologies. A low binder amount is desirable, as the binder does not contribute to energy storage. Hence, lowering the amount of binder, will increase the energy density of a cell.

[0078] Typically, the binder is prepared using emulsion polymerisation. The use of emulsion polymerisation ensures segregation of the propagating polymer chains which in turn suppresses termination of the chains. It is therefore possible to achieve relatively high molecular weight polymers.

[0079] Further, the fact that the polymerisation reaction proceeds in micelles and droplets ensures that there is an upper boundary for the size of the produced polymer particles. That means that emulsion polymerisation may be used to obtain polymer particles having diameters ranging from 10 to 500 nm, with the particles typically containing relatively high molecular weight polymers.

[0080] Overall, the use of emulsion polymerisation allows for adequate control of the size distribution of the formed polymer particles.

[0081] The binder particles may loosely bind to one another to form agglomerates. The agglomerates typically have a D50 of about 1 pm to about 10 pm. The binding within the agglomerates is relatively weak, so mixing easily breaks up the agglomerates to release individual particles.

[0082] In some embodiments, the binder may initially be provided as larger particles. For example, the binder particles may have a size which is similar to or larger than the CAM particles. In such a case, pre-milling of the binder may be carried out ensure the binder particles are sufficiently small. By sufficiently small is meant particles which fulfil the above mentioned size requirements. Pre-milling is however not essential, and typically any binder agglomerates will be broken apart when the binder is mixed with the CAM and optional conductive additive. Preferably, the binder is present in a small amount such as below 5 wt%, for example below 3 wt%, such as below 1 wt%, for example below 0.5 wt% based on the combined weight of the CAM, binder and optional conductive additives.

[0083] To secure sufficient binding, at least 0.005 wt%, for example at least 0.05 wt%, such as at least 0.1 wt% needs to be present in the electroactive composition.

[0084] Preferably, the binder is present in an amount of about 0.05 wt% to about 3 wt% based on the combined weight of the CAM, the conductive additives and the binder. Even more preferably, the binder is present in an amount of about 2 wt%, 1 wt%, or 0.5 wt%. Most preferably, the binder is present in an amount of about 0.5 wt%.

[0085] PARTICLE SIZE ANAL YSIS

[0086] The D10, D50 and D90 of the particles may be measured by any method known to the person skilled in the art. Suitably however laser diffraction is used. Suitable methodologies for measuring particle size and particle size distributions by laser diffraction are detailed in ISO 13320:2020.

[0087] The size distribution may be determined by laser diffraction for instance using a Malvern Mastersizer 3000 + .

[0088] Laser diffraction measures particle size distributions by measuring the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample. Large particles scatter light at small angles relative to the laser beam and small particles scatter light at large angles. The angular scattering intensity data is then analysed to calculate the size of the particles responsible for creating the scattering pattern, using the Mie theory of light scattering. The particle size is reported as a volume equivalent sphere diameter (i.e. D[4,3] value).

[0089] When using a Mastersizer, typically, the particle properties (D10, D50 and D90 particle sizes) are measured in a solid dispersion.

[0090] Preferably, the particle properties (D10, D50 and D90 particle sizes) are measured when dispersed in water.

[0091] Alternatively, a dry dispersion can be used. Suitably, the Mastersizer 3000+ Ultra allows particle size measurement using a dry powder, for instance dispersed in air. When measured using laser diffraction, the particle size is reported as a volume equivalent sphere diameter (i.e. the D[4,3] value).

[0092] In an example, the particles are suspended in water and analysed using a Mastersizer 3000, wherein the Mastersizer 3000 is set a follows:

[0093] Mode: Single mode

[0094] Laser light: Red and blue laser light

[0095] Measurement time: 10 seconds per measurement

[0096] Obscuration range: 4 to 7%

[0097] Stabilization time: 30 seconds

[0098] The particle size analysis is then be carried out using Mie theory (volume equivalent sphere). To use Mie theory, it is usually necessary to know the refractive index and adsorption index of the sample. These may be determined by any suitable method.

[0099] ELECTROACTIVE COMPOSITION

[0100] The electrode active material, the binder and the optional conductive additive combine to form the electroactive composition.

[0101] Upon deposition of the electroactive composition unto a conductive foil, an electroactive layer is formed on said conductive foil.

[0102] The electroactive composition is typically provided as a dry composition. By "dry composition" is meant a composition which is substantially free of solvent.

[0103] In one embodiment, the electrode active composition consists of or comprises from 60 to 99.9 wt% CAM, from 0 to 10 wt% conductive additive, and from 0.005 to 5 wt% binder, for example from 90 to 99.8 wt% CAM, from 0.02 to 6 wt% conductive additive, and from 0.05 to 3 wt% binder, such as from 94 to 99 wt% CAM, from 0.5 to 3 wt% conductive additive, and from 0.1 to 1 wt% binder.

[0104] In an embodiment, the mass ratio of cathode active material: bindenconductive additive is about 96:2:2, more preferably about 98: 1: 1, even more preferably about 98.5:0.75:0.75.

[0105] The optimal mass ratio of cathode active material:binder:conductive additive may differ depending on the types of and the size of the materials used. Therefore, the optimal mass ratio for one set of CAM, binder and conductive additive may differ from the optimal mass ratio for another set of CAM, binder and conductive additive.

[0106] Prior to deposition of the electroactive composition, the components of the electroactive composition are mixed. For example, mechanochemical mixing may be used.

[0107] Typically, the mixing results in the coating of binder onto the cathode active material. This means that all the components of the electroactive composition are equally distributed.

[0108] An electroactive composition wherein the binder and the conductive additive is coated onto the CAM is shown on Figure 1A. For comparison, Figure IB shows a picture of an electroactive composition, wherein the components of the electroactive composition are not equally distributed. Both pictures are recorded using scanning electron microscopy (SEM).

[0109] The binder covers from about 5% to about 50% of the surface of the cathode active material particle, for example from about 10% to about 40%, such as from 20% to about 30%.

[0110] FORMATION OF ELECTRODE

[0111] The present disclosure relates to a solvent-free method of forming electrodes for lithium- ion cells.

[0112] In particular, said method allows for preparation of cells having a high packing density, while using cathode active material (CAM) and a binder consisting of particles which are smaller than the CAM particles.

[0113] In an embodiment of the invention, a dry process for forming a cathode comprises the steps of

[0114] I) providing a. a binder with a D50 particle diameter of 500 nm or less, b. a cathode active material with a D50 particle diameter of about 5 to about 15 microns; c. a conductive additive;

[0115] II) mixing the components provided in step (I) to form an electroactive composition;

[0116] III)depositing the electroactive composition by dry deposition onto a conductive foil to leave an electroactive layer on the conductive foil; and IV) optionally calendering the product of step (III) to form a cathode.

[0117] The conductive foil acts as the cathode current collector. The cathode current collector may comprise a metal, such as aluminium, nickel or stainless steel.

[0118] Preferably, the conductive foil is a primer-coated conductive foil. The coating may function to improve conductivity at the interface with the electrode active material. The coating may also improve the peel strength (or adhesive properties) of the electrode active layer, reducing delamination of the electrode.

[0119] The primer coating typically comprises carbon and acrylic binder. Typically, the content of carbon is larger than the content of acrylic binder. For example, the primer coating comprises at least 50 wt% carbon, such as at least 75 wt% carbon, for example at least 80 wt% carbon, such as at least 90 wt% carbon, for example at least 95 wt% carbon.

[0120] The primer coating layer is typically from 0.5 pm to 3 pm thick, for example from 0.75 pm to 2 pm thick. Preferably, the primer coating layer is about 1 pm thick.

[0121] The deposition of the electroactive composition occurs via a dry-deposition method such as electrostatic deposition (ESD), cold plasma deposition or cold plasma spraying, to coat the current collector. Preferably, the coating process is electrostatic deposition (ESD).

[0122] The deposition of the electroactive composition typically occurs via electrostatic deposition (ESD). Typically, in ESD, particles are charged as they pass through a charging gun, and are deposited onto a substrate, wherein the substrate may be grounded. ESD provides further advantages, for example it may be used with a variety of different materials that have different particle shapes and / or sizes, it may be scaled up for large-scale manufacturing and / or it offers high deposition rates onto large surfaces.

[0123] ESD may involve depositing and / or spray coating the composition onto the conductive foil, for example using a spray gun system comprising a charging gun. Typical process parameters are used to control the deposition process in order to achieve an electrode with the desired properties, for example spray gun voltage, spray gun current, coating time, coating speed, deposition rate, nozzle size, air flow, coating passes (i.e. cycles), etc.

[0124] The composition may be mixed with fluidising air prior to and / or during deposition (e.g. using a powder feeding hopper). The composition may be charged prior to and / or during the deposition process (e.g. in the form of charged particles). Calendering is used to compress the deposited electroactive composition (the electroactive layer) to achieve higher densities.

[0125] After calendering, the thickness of the electroactive lay may be from about 20 pm to about 500 pm, such as from about 30 pm to about 350 pm, for example from about 50 pm to about 250 pm, such as from about 100 pm to about 200 pm.

[0126] The thickness of the bound electroactive composition may be measured by any method known to the skilled person. For example, profilometry or atomic force microscopy (AFM) may be used to determine the difference between the hight of the coated and the uncoated area. This difference should correspond to the thickness of the bound electroactive composition.

[0127] The structure may be incorporated into an electrochemical cell, for instance by rolling, folding or stacking.

[0128] The present disclosure further relates to a cathode obtained by the above methods.

[0129] PACKING DENSITY

[0130] To achieve high energy density within a battery, a high packing density is desirable.

[0131] While energy density describes the amount of energy stored per unit volume, packing density describes the mass of material per unit volume. Both quantities are measured after calendering.

[0132] Upon calendering, the conductive foil and the electroactive layer are tightly bound to one another. Therefore, the packing density is the density of the combined conductive foil and the electroactive layer. For that reason, the packing density may also be referred to as the total density.

[0133] By using the method of the disclosure, packing densities above 3.5 g / cm3, for example above 3.6 g / cm3, such as above 3.7 g / cm3are achievable.

[0134] In some embodiments, packing densities of about 3.5 g / cm3, about 3.6 g / cm3, about 3.7 g / cm3, and about 3.8 g / cm3are obtained. This is surprising as packing densities of over 3.6 g / cm3are typically only achievable using wet processing conditions with bimodal CAM particles.

[0135] Alternatively, the cathode active layer density may be determined. The cathode active layer is the density of the electroactive layer alone.

[0136] By using the method of the disclosure, cathode active layer densities above 3.5 g / cm3, for example above 3.6 g / cm3are achievable.

[0137] In some embodiments, cathode active layer densities of about 3.5 g / cm3, about 3.6 g / cm3, and about 3.7 g / cm3are obtained.

[0138] This is surprising as cathode active layer densities of over 3.6 g / cm3are typically only achievable using wet processing conditions with bimodal CAM particles.

[0139] The packing density (or the total density) may be measured using any method known to the skilled person. An exemplary method of determining the packing density includes measuring the thickness of the coated foil, the area of the foil and the weight of the foil. Then the packing density may be determined as the weight divided by the product of the thickness and the area.

[0140] The cathode active layer density may be determined by measuring the thickness and weight of the uncoated foil. These numbers are then subtracted from the thickness and weight of a coated foil having the same size (i.e. the area of the uncoated and the coated foil is the same). Then the cathode active layer density may be determined as the resulting weight divided by the product of the resulting thickness and the area.

[0141] CELL

[0142] The present disclosure also relates to cells comprising the cathode of the disclosure, for example a secondary lithium-ion cell. Such cells typically comprise a cathode, an anode, a separator disposed between the anode and the cathode, said cathode, anode and separator forming an electrode assembly, the cell further comprising a housing for the electrode assembly. The cell will typically comprise an electrolyte to facilitate the transport of lithium ions between the cathode and the anode. The housing is typically sealed to ensure the electrolyte is retained within the housing. Said housing usually includes terminals in electrical contact with the anode and cathode.

[0143] These cells may be combined to form a battery system (i.e. an array of cells). The disclosure also relates to an electrical device comprising a cell of the disclosure. For instance, the disclosure relates to a vehicle comprising a cell (or battery system) of the disclosure. The vehicle is preferably an electric vehicle, such as a car, truck, bus, scooter, motorbike, bicycle or the like, preferably a car, truck or bus.

[0144] EXAMPLE 1

[0145] Several cathodes were prepared using the methodology of the disclosure. Unimodal NMC with a D50 of 10 pm was used as CAM, conductive carbon was used as conductive additive, and PVDF with a particle size (D50) of 200 nm was used as binder.

[0146] CAM, conductive carbon and binder was measured according to Table 1, and then mixed in a IL mixer, equipped with 2 blades, rotating at about 36,000 rpm for about lmin.

[0147] The cathodes differed in the CAM :conductive carbon: binder mass ratio as well as in the gap setting during calendaring, see Table 1 below as well as Figure 2.

[0148] Table 1

[0149] As seen in Table 1 and in Figure 2, the gap setting is provided as a negative number. The negative gap means that the calendering rolls are forced together closer than their combined radii, so the rolls themselves are being slightly compressed. For reference, a gap setting of 0 pm would imply that the rolls are just touching, and anything negative is the rolls being moved closer than theoretically possible if they were non-compressible. This creates a very high pressure at the calendering nip which results in compression of the electrode active composition. The calendering rolls used for this example are made of steel.

[0150] Based on this experiment, it is evident that higher packing densities may be optained in electrodes formed by ESD using unimodal CAM with a D50 of 10 pm, and a binder with a particle size (D50) of 200 nm.

[0151] Furthermore, it is seen that even at low binder concentration, it is possible to obtain packing densities above 3.6 g / cm3. This is surprising as densities of over 3.6 g / cm3are typically only achievable using wet processing conditions with bimodal CAM particles.

[0152] EXAMPLE 2

[0153] Several cathodes were prepared using the methodology of the disclosure. Unimodal NMC with a D50 of 10 pm was used as CAM, conductive carbon was used as conductive additive, and PVDF with a D50 of 200 nm was used as binder.

[0154] The cathodes differed in the CAM wt%, while the same gap setting during calendaring was applied for each cathode. The wt% of CAM (based on the combined mass of CAM, conductive carbon and binder) and the packing density was determined for each of the cathodes, see Table 2 below:

[0155] Table 2

[0156] As is evident, high packing densities may be optained in electrodes formed by ESD using unimodal CAM with a D50 of 10 pm, and a binder with a particle size (D50) of 200 nm.

[0157] Furthermore, it is seen that by finetuning the weight ratios of the solid components within the cathode, improved densities may be obtained. This is visualised in Figure 3, where the packing density was plotted as a function of the CAM wt%. It appears that when using the CAM, conductive additive and binder of this example, a CAM wt% around 98.5 will result in the highest possible packing density and cathode active layer density for said combination of materials. For example, if the CAM wt% is higher, then too little binder is present to achieve efficient binding between the CAM particles. Hence a drop in density is seen.

[0158] EXAMPLE 3

[0159] Several cathodes were prepared using the methodology of the disclosure. Unimodal NMC with a D50 of 10 pm was used as CAM, conductive carbon was used as conductive additive, and PVDF with a D50 of 200 nm was used as binder.

[0160] The difference between the cathodes was the weight percentage of conductive carbon. The cathodes comprised either 0.5, 0.75, 1.0, 1.5 or 2.0 wt% carbon black. As is evident from Figure 4, too small quantities of conductive carbon results in poor cycle retention. That is 0.5 wt% conductive carbon results in inferior cycle retention compared to a cathode comprising at least 0.75 wt% conductive carbon. All the cathodes comprising from 0.75 to 2.0 wt% carbon black had a cycle retention of at least 95% after 50 cycles.

[0161] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0162] All embodiments of the invention and particular features mentioned herein may be taken in isolation or in combination with any other embodiments and / or particular features mentioned herein (hence describing more particular embodiments and particular features as disclosed herein) without departing from the disclosure of the invention.

[0163] As used herein, the term 'comprises' will take its usual meaning in the art, namely indicating that the component includes but is not limited to the relevant features (i.e. including, among other things). As such, the term 'comprises' will include references to the component consisting essentially of the relevant substance(s). Wherever the word 'about' is employed herein in the context of amounts, for example absolute amounts, weights, volumes, sizes, diameters etc., or relative amounts (e.g. percentages) of individual constituents in a composition or a component of a composition (including concentrations and ratios), timeframes, and parameters such as temperatures etc., it will be appreciated that such variables are approximate and as such may vary by ±10%, for example ±5% and preferably ±2% (e.g. ±1%) from the actual numbers specified herein. This is the case even if such numbers are presented as percentages in the first place (for example 'about 10%' may mean ±10% about the number 10, which is anything between 9% and 11%).'

Claims

CLAIMS1. A dry process for forming a cathode, comprisingI) providing a. a binder with a D50 particle diameter of 500 nm or less; b. a cathode active material with a D50 particle diameter of about 5 to about 15 microns; and c. optionally a conductive additive;II) mixing the components provided in step (I) to form an electroactive composition;III) depositing the electroactive composition by dry deposition onto a conductive foil to leave an electroactive layer on the conductive foil; andIV) optionally calendering the product of step (III) to form a cathode.

2. The process of claim 1, wherein the electrode active composition comprises, or consists of, from 90 to 99.8 wt% cathode active material (CAM), from 0.02 to 6 wt% conductive additive, and from 0.05 to 3 wt% binder.

3. The process of claim 1 or 2, wherein the cathode active material has a unimodal particle size distribution and / or wherein the CAM particles have an average aspect ratio below 1.3.

4. The process of any of the preceding claims, wherein the ratio between the D50 of the binder particles and the D50 of the CAM particles may be from about 1:5 to about 1:200.

5. The process of any of the preceding claims, wherein the electroactive composition is deposited using electrospray deposition (ESD).

6. The process of any of the preceding claims, wherein a packing density of above 3.6 g / cm3is obtained and / or wherein a cathode active layer density of above 3.6 g / cm3is obtained.

7. The process of any of the preceding claims, wherein the binder is polyvinylidene fluoride (PVDF), preferably, wherein the PVDF is made using an emulsion process.

8. An electroactive composition comprising 90 to 99.8 wt% CAM, from 0.02 to 6 wt% conductive additive, and from 0.05 to 3 wt% binder, wherein the binder covers from about 5% to about 50% of the surface of the cathode active material particle.

9. A cathode obtained by any of the methods of claims 1 to 7.

10. A cell, for example a secondary lithium-ion cell, comprising the cathode of claim 9.

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