EP4315454A1

EP4315454A1 - Cathode material and process

Info

Publication number: EP4315454A1
Application number: EP22713721.3A
Authority: EP
Inventors: Rainald Forbert; Gerhard Nuspl
Original assignee: Johnson Matthey PLC
Current assignee: Epsilon Carbon Private Ltd
Priority date: 2021-03-30
Filing date: 2022-03-21
Publication date: 2024-02-07
Also published as: GB202104495D0; WO2022208049A1

Abstract

The present invention provides a composition comprising carbon-coated particles of micro- agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate, the lithium metal phosphates having general formula: LiFe1-xMxPO4 in which 0≤x≤1 and M is one or more selected from Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg, wherein the particle size distribution of the composition has: a) when measured using a Malvern MasterSizer 2000 in ethanol, 3 or more peaks having at least peak modes at 0.1 to 1 μm, 1.2 to 5 μm, and 5 to 20 μm, and 25 volume % of the particles having a particle size of 1 μm or less and 75 volume% of the particles having a particle size of 4 p.m or less; and b) when measured using a Malvern MasterSizer 2000 in air at a gas pressure of 0.2 bar, a particle size distribution comprising 2 or more peaks having at least peak modes at 0.1 to 2 μm, and 10 to 20 μm, and 25 volume % of the particles having a particle size of 1 p.m or less and 75 volume% of the particles having a particle size of 12 μm or less.

Description

Cathode Material and Process Field of the Invention The present invention relates to a lithium metal phosphate material and a method for producing a lithium metal phosphate material. More specifically, the present invention relates to lithium metal phosphate materials for use in secondary lithium ion batteries, and a process for producing lithium metal phosphate. Background of the Invention Lithium metal phosphates with olivine structures have emerged as promising cathode materials in secondary lithium ion batteries. Advantages of lithium metal phosphates compared with other lithium compounds include the fact that they are relatively benign environmentally and have excellent safety properties during battery handling and operation. It is desirable that lithium metal phosphate materials can be formed into electrodes having high densities. Denser cathodes may provide secondary lithium ion batteries with improved gravimetric capacities, which in turn provide batteries which can hold more energy. It is known to provide lithium metal phosphate materials as a mixture of large particles and small particles to improve particle packing when the lithium metal phosphate is formed into an electrode. Typically, large particles are present as secondary agglomerates of primary particles, whilst small particles are present as primary particles or fragments of primary particles, in the form of powders. Agglomerated lithium metal phosphates may be prepared by a hydrothermal process and a milling step such as that described in WO2014/14140323A1 which is incorporated herein in its entirety by reference. A hydrothermal process is also described in WO2005/051840A1 which is incorporated herein in its entirety by reference. Powder lithium metal phosphate may be prepared according to the process described in WO2005/051840A1 the contents of which are incorporated herein in their entirety by reference. US2015072230A1 discloses a cathode material with a multimodal particle size distribution formed by combining materials with two or more different particle size distributions. US2012156560A1 discloses a mixture of small particles and large particles of a layered lithium metal oxide material. There remains a need for lithium metal phosphate materials which may be formed into electrodes having superior electrode densities. Summary of the Invention The present inventors have surprisingly found that by providing specific particle compositions the electrochemical properties of lithium metal phosphates can be tuned to provide a particulate electrode material composition which may be formed into an electrode having superior electrode density. In a first aspect of the present invention there is provided a composition comprising carbon- coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate, the lithium metal phosphates having general formula: LiFe1-xMxPO4 in which 0≤x≤1 and M is one or more selected from Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg, wherein the particle size distribution of the composition has: a) when measured using a Malvern MasterSizer 2000 in ethanol, 3 or more peaks having at least peak modes at 0.1 to 1 μm, 1.2 to 5 μm, and 5 to 20 μm, and 25 volume % of the particles having a particle size of 1 μm or less and 75 volume% of the particles having a particle size of 4 μm or less; and b) when measured using a Malvern MasterSizer 2000 in air at a gas pressure of 0.2 bar, a particle size distribution comprising 2 or more peaks having at least peak modes at 0.1 to 2 μm, and 10 to 20 μm, and 25 volume % of the particles having a particle size of 1 μm or less and 75 volume% of the particles having a particle size of 12 μm or less. It has surprisingly been found that compositions of the invention have a particle size distribution which allows the lithium metal phosphate to be formed into electrodes having superior electrode densities. Furthermore, compositions of the invention have been found to produce electrodes with superior volumetric and gravimetric capacities compared to those of the prior art. Laser diffraction techniques are commonly used to determine the particle size distribution of particulate materials, for example using a Malvern MasterSizer 2000. The particle size distribution of the composition of the invention may be analysed by suspending it in a carrier fluid such as a liquid (e.g. ethanol), or a gas (e.g. air). The choice of carrier fluid has been found to influence the physical behaviour of the composition during analysis. Different apparent particle size distributions are realised depending on the choice of carrier fluid. It is thought that smaller particles (e.g. powder particles, or primary particles) of carbon-coated lithium metal phosphate agglomerate when air is the carrier fluid. Conversely, agglomerated particles (e.g. secondary agglomerates of primary particles, or micro-agglomerates) may be broken up when ethanol is the carrier fluid. The present inventors further believe that a “perfect” carrier fluid may not exist and so taking particle size measurements in more than one carrier fluid is sometimes necessary. Material of the prior art (e.g. compositions formed by milling and/or sifting agglomerated materials) sometimes appear to be comprised of a mixture of large (e.g. agglomerated) particles and smaller particles (e.g. powder particles, fragments of agglomerated particles, or primary particles) when analysed by laser diffraction techniques (e.g. using a Malvern MasterSizer). Typically, these materials may exhibit a bimodal or multimodal particle size distribution when analysed using laser diffraction techniques (e.g. using a Malvern MasterSizer 2000). Particle size analysis typically show the smaller particles as peaks, or modes, between about 0.1 to 2 μm, whilst the large (e.g. agglomerated) particles may appear as peaks between about 5 to 30 μm. However, these materials may not in fact exist as a mixture of large (e.g. agglomerated) particles and smaller particles (e.g. powder, or primary particles), despite the bimodal/multimodal appearance of their particle size distributions. Without being bound to any specific theory it is believed that the smaller particles are weakly bound to, and remain associated with, the large (e.g. agglomerated) particle under most conditions. In other words, the smaller powder particles seen in the particle size distribution analysis may not actually be present as “free” particles. Consequently, these smaller particles (e.g. powder particles, or primary particles) may not be involved in packing of the carbon-coated lithium metal phosphate during formation/compaction into an electrode. The composition of the present invention comprises at least two types of particle: i) micro- agglomerates, and ii) powder particles. The micro-agglomerate particles and the powder particles of the composition are believed to be present independently of one another, and may both be regarded as “free” particles. In other words, the micro-agglomerate particles and the powder particles of the composition are not substantially associated with one another by chemical and/or physical bonds. This combination of micro-agglomerates and powder particles, as defined by their particle size distribution, gives rise to electrodes with very high densities, for example a density of 2.27 g/cm³ or more, such as 2.3 g/cm³ or more when the composition of the invention is formed into an electrode using a hydraulic press using a pressure of about 220 MPa for about 60 seconds, or 2.5 g/cm³ or more, such as 2.52 g/cm³ or more, when the composition of the invention is formed into an electrode using a hydraulic press using a pressure of about 750 MPa for about 120 seconds. Micro-agglomerates are typically formed of a plurality of primary, or powder, particles. Micro- agglomerates typically have a smaller D₅₀ particle size than secondary agglomerates found in compositions which have been prepared by traditional sifting processes, such as those described in WO2014/14140323A1. Compositions comprising micro-agglomerates and powder particles may be prepared using a process comprising a specific compaction step, and a specific milling and sifting step. Accordingly, in a second aspect of the invention there is provided a process for preparing a composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate, the lithium metal phosphates having general formula: LiFe1-xMxPO4 in which 0≤x≤1 and M is one or more selected from Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg, the process comprising the steps of: i) providing a particulate lithium metal phosphate and a carbon source; ii) compacting the particulate lithium metal phosphate and carbon source under a line force of from 400 kN/m to 1000 kN/m to produce a compacted lithium metal phosphate; iii) sieving the compacted lithium metal phosphate using a mesh with a nominal mesh size of 2 to 8 mm to remove fine particles and collecting a sieved lithium metal phosphate; iv) pyrolyzing the sieved lithium metal phosphate to produce a carbon-coated lithium metal phosphate; and v) milling and sifting the carbon-coated lithium metal phosphate under mild milling conditions to produce the composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate. It has surprisingly been found that the process of the second aspect of the invention produces a composition comprising both micro-agglomerate particles and powder particles which are present as substantially “free” particles. Furthermore, the process of the invention provides said particle composition in a simple process which does not require a wet-milling step. As such, the process is easy and economical to implement, and does not require the purchase of expensive wet mills. Without being bound by any sort of theory, it is believed that the steps of compacting the particulate carbon-coated lithium metal phosphate (step ii) and milling and sifting the carbon- coated lithium metal phosphate under mild milling and sifting conditions (step v) provides a material with an optimised particle size distribution which pack together to produce electrodes with superior densities. In a third aspect of the invention there is provided a cathode comprising a composition of the first aspect of the invention. In a fourth aspect of the invention there is provided a secondary lithium ion battery comprising a cathode of the invention. In a fifth aspect of the invention there is provided a lithium metal phosphate composition obtained or obtainable by a process of the invention. In a sixth aspect of the invention there is provided a process according to the second aspect of the invention, comprising the further step of forming the composition produced according to the second aspect into a cathode. In a seventh aspect of the invention there is provided a process according to the sixth aspect of the invention, comprising the further step of forming a battery from the cathode produced according to the sixth aspect. Brief Description of the Drawings Figure 1 shows the volume based particle size distribution of a composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate according to the invention, determined using a Malvern MasterSizer 2000 in ethanol. Figure 2 shows the volume based particle size distribution of a composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate according to the invention, determined using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. Figure 3 shows the volume based particle size distribution of carbon-coated particles of an agglomerated lithium iron phosphate (P2S2) available from Johnson Matthey, determined using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. Figure 4 shows the volume based particle size distribution of carbon-coated particles of powder lithium iron phosphate (P2), available from Johnson Matthey, as determined using a Malvern MasterSizer 2000 in ethanol. Figure 5 shows the volume based particle size distribution of carbon-coated particles of an agglomerated lithium iron phosphate (P2S2) available from Johnson Matthey, determined using a Malvern MasterSizer 2000 in ethanol. Figure 6 shows the volume based particle size distribution of carbon-coated particles of a powder lithium iron phosphate (P2), available from Johnson Matthey, as determined using a Malvern MasterSizer 2000 measured in air at a pressure of 0.2 bar. Detailed Description Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise. The present invention provides a composition comprising carbon-coated particles of micro- agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate. The lithium metal phosphate of the composition has general formula LiFe_1-xM_xPO₄. M may be one or more elements selected from the group comprising Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg. Suitably, M may be one or more elements selected from the group comprising Mn, Ni, Al, and Co. Preferably, M may be Al and/or Mn. In the general formula LiFe_1-xM_xPO₄, x may be greater than or equal to 0, greater than or equal to 0.05, greater than or equal to 0.1, greater than or equal to 0.2, or greater than or equal to 0.3. x may be less than or equal to 0.9, less than or equal to 0.7, less than or equal to 0.6, or less than or equal to 0.5. Typically, x may be greater than or equal to 0 and less than or equal to 0.9, greater than or equal to 0.05 and less than or equal to 0.7, greater than or equal to 0.1 and less than or equal to 0.6, greater than or equal to 0.2 and less than or equal to 0.5, or greater than or equal to 0.3 and less than or equal to 0.5. For example, x may be about 0.4. In preferred compositions of the invention, the lithium metal phosphate has the general formula LiFePO₄. In other words, x is 0, or essentially 0, and any inclusion of M is due to incidental impurities, for example impurities in the reagents used or from the manufacturing equipment used. The carbon-coated particles of the micro-agglomerated lithium metal phosphate may comprise agglomerates of primary particles and/or may be formed of a plurality of smaller particles, which may be, for example, primary particles. The carbon-coated particles of the powder lithium metal phosphate may be present in the form of essentially primary particle. In the context of the present invention, primary particles will be understood to refer to discrete particles which are themselves not substantially formed of agglomerates or aggregates of other particles. Primary particles typically have a particle size, for example a D₅₀ particle size, of from 0.1 to 0.5 μm. In the context of the present invention, micro-agglomerated particles will be understood to refer to particles formed of a plurality of smaller primary particles. The smaller primary particles are bound together by chemical and/or physical bonds to form a micro- agglomerate. Typically, micro-agglomerates have a particle size, for example a D50 particle size, of from 0.5 to 3 µm if measured in ethanol and of from 0.8 to 7 µm if measured in air at a gas pressure of 0.2 bar. As explained hereinabove, micro-agglomerates typically have a smaller particle size than secondary agglomerates such as those described in WO2014/14140323A1. The composition has a particle size distribution which is defined with respect to its particle size distribution as measured using a Malvern MasterSizer 2000 in two separate carrier fluids; a) in ethanol, and b) in air at a gas pressure of 0.2 bar. For the avoidance of doubt, particle size distributions referenced as having been measured in air at a gas pressure of 0.2 bar means 0.2 bar gauge. When the particle size distribution of the composition is described as being measured using air as the carrier fluid the composition may be dispersed in air with a Scirocco 2000 dry powder feeder/sample dispersion unit at a pressure of 0.2 bar and its particle size distribution measured using the Fraunhofer approximation. Where the particle size distributions of the composition is described as being measured in ethanol the composition may be dispersed in ethanol with a Hydro 2000S sample dispersion unit and its particle size distribution measured using the Mie theory with a particle refractive index of 1.52. It will be understood that peak modes, as used herein, may also be referred to as peak maxima. Generally, in the present invention peak modes at higher particle size may be considered to be associated with micro-agglomerate particles, whereas peak modes at lower particle size may be considered to be associated with powder particles. However, as will be understood by the person skilled in the art, this assignment may not be definitive for all compositions of the invention, which is defined according to particle size distribution of the claims. For the avoidance of doubt values of particle size given as 25 volume percent or less, 50 volume percent or less, and 75 volume percent or less correspond to a particle size value where 25 volume percent, 50 volume percent, and 75 volume percent, respectively, have a value of or below the given particle size. Said values may be calculated by integrating the area under the particle size distribution plot. The start and end points from peak modes defined below, for particle size distribution measured in ethanol, may be combined in any order to form a new range. The composition has a multimodal particle size distribution when measured in ethanol using a Malvern MasterSizer 2000. The particle size distribution of the composition as measured using a Malvern MasterSizer 2000 in ethanol comprises 3 or more peaks. The peak modes of the 3 or more peaks are present at from 0.1 to 1 μm, from 1.2 to 5 μm, and from 5 to 20 μm. The peak modes of the 3 or more peaks may be present at from 0.15 to 0.9 μm, from 1.3 to 3 μm, and from 7 to 18 μm. The peak modes of the 3 or more peaks may be present at from 0.2 to 0.7 μm, from 1.4 to 2.5 μm, and from 8 to 16 μm. The peak modes of the 3 or more peaks may be present at from 0.25 to 0.5 μm, from 1.5 to 2 μm, and from 10 to 14 μm. 25 volume percent of the composition has a particle size of 1 μm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol. For example, 25 volume percent of the composition may have a particle size of 0.7 μm or less, 0.5 μm or less, 0.4 μm or less, or 0.3 µm or less. 50 volume percent of the composition may have a particle size of 1.5 μm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol. For example, 50 volume percent of the composition may have a particle size of 1.3 μm or less, 1.2 μm or less, 1.1 μm or less, or 1 µm or less. 75 volume percent of the composition has a particle size of 4 μm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol. For example, 75 volume percent of the composition may have a particle size of 3.5 μm or less, 3 μm or less, or 2.5 µm or less. The start and end points from peak modes defined below, for particle size distributions measured in air at a pressure of 0.2 bar, may be combined in any order to form a new range. The composition has a multimodal particle size distribution when measured in air at a gas pressure of 0.2 bar using a Malvern MasterSizer 2000. For example, the composition may have a bimodal particle size distribution when measured in air at a gas pressure of 0.2 bar using a Malvern MasterSizer 2000. The particle size distribution of the composition as measured using a Malvern MasterSizer 2000 in air at a gas pressure of 0.2 bar typically comprises 2 or more peaks. The peak modes of the 2 or more peaks are present at from 0.1 to 2 μm, and 10 to 20 μm. The peak modes of the 2 or more peaks may be present at from 0.3 to 1.8 μm, and 11 to 18 μm. The peak modes of the 2 or more peaks may be present at from 0.5 to 1.5 μm, and 12 to 17 μm. The peak modes of the 2 or more peaks may be present at from 0.8 to 1.2 μm, and 13 to 16 μm. 25 volume percent of the composition has a particle size of 1 μm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 25 volume percent of the composition have a particle size of 0.9 μm or less, 0.8 μm or less, or 0.75 µm or less. 50 volume percent of the composition typically have a particle size of 7 μm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 50 volume percent of the composition may have a particle size of 4 μm or less, 3 μm or less, 2 µm or less, 1.8 µm or less, 1.6 µm or less, or 1.4 µm or less. 75 volume percent of the composition has a particle size of 12 μm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar. For example, 75 volume percent of the composition have a particle size of 10 μm or less, 8 μm or less, 7 µm or less, 5 µm or less, or 3 µm or less. The BET surface area of the composition may be 4 m²/g or more, 5 m²/g or more, 6 m²/g or more, or 7 m²/g or more. The carbon-coated particles of the powder lithium metal phosphate may have a BET surface area of 14 m²/g or less, 12 m²/g or less, 10 m²/g or less, or 9 m²/g or less. For example, the BET surface area of the composition may be from 4 to 14 m²/g, from 5 to 12 m²/g, from 6 to 10 m²/g, or from 7 to 9 m²/g. The particles of lithium metal phosphate of the composition comprise an electrically conductive carbon coating on at least a part of the surface of the particles. The weight percentage of carbon present based on the total weight of the composition is typically between 1 and 3 wt.%, for example, between 1.5 and 2.5 wt.%, such as 1.7 to 2.3 wt.%. The second aspect of the invention provides a process for preparing a composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate, the lithium metal phosphates having general formula: LiFe_1-xM_xPO₄ in which 0≤x≤1 and M is one or more selected from Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg Particulate lithium metal phosphate may be prepared using a hydrothermal process. Such a method involves the combination of an iron (II) source with at least one lithium source, at least one phosphate source, and optionally at least one source of M, and obtaining particulate lithium metal phosphate under hydrothermal conditions. Suitable iron (II) sources include iron sulphate (FeSO₄), typically in the form of a hydrate, and iron oxalate. Suitable lithium sources include lithium carbonate (Li₂CO₃), lithium hydrogen phosphate (Li₂HPO₄), lithium hydroxide (LiOH), lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI), lithium phosphate (Li₃PO₄) or mixtures thereof. Lithium hydroxide may be preferred. Suitable phosphate sources include phosphoric acid, metaphosphoric acid, pyro-phosphoric acid, triphosphoric acid, tetraphosphoric acid, hydrogen phosphates or dihydrogen phosphates, such as ammonium phosphate or ammonium dihydrogen phosphate, lithium phosphate or iron phosphate or any desired mixtures thereof. Phosphoric acid is particularly preferred. Suitable sources of M, if applicable, include sulphates and / or oxides of M or mixtures thereof. It will be understood by the skilled person that M may also be present in the iron (II) source, the lithium source, or the phosphate source, and therefore an additional source of M may not need to be added to achieve the desired level of M in the lithium metal phosphate. Where M comprises aluminium, suitable aluminium sources include aluminium hydroxide (Al(OH)3), aluminium chloride (AlCl3), aluminium sulphate (Al2(SO4)3*xH2O (typically 0 ≤ x ≤ 18)) , and aluminium oxide (Al2O3). Aluminium hydroxide or aluminium sulphate may be particularly preferred. Where M comprises manganese, suitable manganese sources include manganese nitrate (Mn(NO3)2), manganese carbonate (Mn(CO3)2), and manganese sulphate (MnSO4). In the context of the present invention, the term hydrothermal conditions is to be understood to refer to treatment of the precursor mixture at a temperature above room temperature and a steam pressure of above 1 bar. The hydrothermal treatment can be carried out in a manner known to the person skilled in the art, for example as described in WO2005/051840 the content of which is hereby incorporated by reference. It is preferable for the hydrothermal treatment to be carried out at temperatures of between 100 to 250° C, in particular from 100 to 180° C and at a steam pressure of from 1 bar to 40 bar, in particular at a steam pressure from 1 bar to 10 bar. The precursor mixture is typically reacted in a tightly closed or pressure-resistant vessel. The reaction preferably takes place in an inert or protective gas atmosphere. Examples of suitable inert gases include nitrogen, argon, carbon dioxide, carbon monoxide or mixtures thereof. The hydrothermal treatment may, for example, be carried out for 0.5 to 15 hours, in particular for 6 to 11 hours. Purely as a non-limiting example, the following specific conditions may be selected: 1.5 hour heat-up time from 50° C (temperature of the precursor mixture) to 160° C, 10 hour hydrothermal treatment at 160° C, 3 hours cooling from 160° C to 30° C In the context of the present invention the particles of the lithium metal phosphate are carbon-coated. In order to form the carbon coating, the lithium metal phosphate formed by the hydrothermal process is typically mixed with a carbon source and then dried before being heated in a pyrolysis step (e.g. step iv of the process of the invention). The nature of the carbon source is not particularly limited in the present invention. The carbon source is typically a carbon-containing compound which decomposes to a carbonaceous residue when exposed to the pyrolysis step (e.g. step iv of the process of the invention). For example, the carbon source may be one or more of starch, maltodextrin, gelatine, polyol, sugar (such as mannose, fructose, sucrose, lactose, glucose, galactose), and carbon-based polymers such as polyacrylate, polyvinyl acetate (PVA), glucono delta- lactone (GDL), and polyvinyl butyrate (PVB). The carbon source may be an elemental carbon, such as one or more of graphite, carbon black, acetylene black, carbon nanotubes and carbon fibres (such as vapour grown carbon fibres, VGCF). It may be preferred that more than one carbon source is used. It may be particularly preferred that graphite and maltodextrin are the carbon source. The amount of carbon source added is not particularly limited in the present invention. For example, the amount of carbon source added may be selected to yield particles of the agglomerated or the powder lithium metal phosphate with a carbon content of between 1 and 3 wt.%, for example, between 1.5 and 2.5 wt.%, such as 1.7 to 2.3 wt. The amount of carbon source added may be in the range from 3 to 10 wt.% based on the weight of the particulate lithium metal phosphate, for example from 4 to 7 wt.%, depending on the nature of the carbon precursor, and its carbonisation yield. The skilled person will understand that the carbon source may be combined with the lithium metal phosphate by any one of a number of means. For example, the particles of lithium metal phosphate may be mixed with the carbon source, following hydrothermal synthesis, in the presence of a liquid medium, such as water, and the mixture then dried, for example spray dried. It will also be understood by the skilled person that in some cases it may be preferable that the carbon source is added to the precursor mixture prior to hydrothermal treatment. In such a case, it will be understood that the addition of a carbon source after hydrothermal treatment may be no longer required. Optionally, a surfactant may be combined with the lithium metal phosphate prior to the drying step. The surfactant may be the carbon source or may be added in addition to any one or more of the carbon sources described above. It may be preferred that the surfactant is a non-ionic surfactant. It may be preferred that the surfactant only comprises carbon, hydrogen, and oxygen atoms. It may be preferred that the surfactant is a solid at room temperature. The process of the invention may comprise the step of drying, preferably spray drying, the slurry of the lithium metal phosphate and carbon source to produce a particulate lithium metal phosphate and carbon source in a dry form. The skilled person is able to select suitable conditions for drying, for example spray drying, the slurry comprising the particles of the lithium metal phosphate and carbon source. Typically, a spray drier with a pneumatic nozzle is used having a gas pressure of from 2-8 bar gauge, an inlet temperature of from 200 to 500 °C, and an outlet temperature of from 100 to 150 °C. Typically, a spray drier with a rotary atomizer may be employed. Preferably, following drying, a carbon source is present as a layer on the surface of the particulate lithium metal phosphate. Following pyrolysis, the carbon source, e.g. the layer of the carbon source, forms an electrically conductive carbon-coating on the lithium metal phosphate. The process of the invention comprises the step of providing a particulate lithium metal phosphate and a carbon source. The step of providing the particulate lithium metal phosphate and the carbon source typically involves supplying the particulate lithium metal phosphate and the carbon source as a dry powder to suitable compaction equipment. Any suitable compaction equipment may be used to apply the compaction force to the particulate lithium metal phosphate and a carbon source. For example, the compaction equipment may be a compactor, granulator, or a roller press. Preferably, the compaction equipment is a roller press, such as a Hosokawa Bepex roller press Pharmapaktor L200/50P. The term “line force” will be recognised by the person skilled in the art. Line force describes the case where two cylinders of length L with infinite Young’s modulus and parallel axes are pressed against each other over their complete length, L. The line force may then be calculated as the force between the two cylinders divided by the length, L. The line force may be a fictitious dimension. According to continuum mechanics a so called “Hertzian pressure” evolves when two cylinders of length L with finite Young’s modulus and parallel axes are pressing against each other. However, in the case of a roller press compacting a solid powder the physical states are very complex and not constant with regard to time even if the force of the two rolls pressing against the solid powder is kept constant. Therefore, the concept of the line force is used in describing the process of the invention. The process of the invention comprises the step of compacting the particulate lithium metal phosphate and carbon source to produce a compacted lithium metal phosphate. The step of compacting the particulate lithium metal phosphate and carbon source is carried out under a line force of greater than or equal to 400 kN/m, greater than or equal to 500 kN/m, or greater than or equal to 600 kN/m. The step of compacting the particulate lithium metal phosphate and carbon source is carried out under a line force of less than or equal to 1000 kN/m. The step of compacting the particulate lithium metal phosphate and carbon source may be carried out under a line force of less than or equal to 900 kN/m, or less than or equal to 800 kN/m. For example, the step of compacting the particulate lithium metal phosphate and carbon source may be carried out under a line force of from 400 to 1000 kN/m, from 500 to 900 kN/m, or from 600 to 800 kN/m. In preferred processes of the invention the step of compacting the particulate lithium metal phosphate and carbon source is carried out under a line force of from 680 to 720 kN/m, such as about 700 kN/m. The step of compacting the particulate lithium metal phosphate and carbon source has the effect of causing the particles of lithium metal phosphate to adhere together. It has been found than when a compaction line force of less than 400 kN/m is used, for example 300 kN/m or less, that the particles of the particulate lithium metal phosphate and carbon source are not sufficiently adhered and comprise small particles which resemble uncompacted material. Conversely, when a compaction line force of 1000 kN/m or more is used, the material is over compacted resulting in large secondary agglomerate particles which may be difficult to break apart into micro-agglomerates which may result in inferior electrochemical properties. Surprisingly, a compaction line force of from 400 kN/m to 1000 kN/m, for example about 700 kN/m, provides a composition which can be formed into a denser electrode than when a higher compaction line force of greater than 1000 kN/m is used. In other words, it has surprisingly been found that the selection of a lower compaction line force may give rise to a composition which can be formed into a denser electrode. The process of the invention comprises the step of sieving the compacted lithium metal phosphate using a mesh with a nominal mesh size of 2 to 8 mm, for example 3 mm to 5 mm, or 3.5 mm to 4.5 mm, to remove fine particles and collecting a sieved lithium metal phosphate. The sieving may be carried out in, for example, a tumbling sieve with a mesh filter size of 2 mm to 8 mm, for example 3 mm to 5 mm, or 3.5 mm to 4.5 mm. Material which is separated in the sieving process (i.e. fine particle material which bypasses the mesh) may be recirculated to the compaction step and be combined with material to be compacted. For the avoidance of doubt, the sieved lithium metal phosphate comprises the desired product in the sieving process, and the fine particles which bypass the sieve may be discarded or recirculated to the compaction step as described hereinabove. The process of the invention comprises the step of pyrolyzing the sieved lithium metal phosphate to produce a carbon-coated lithium metal phosphate. The step of pyrolyzing the sieved lithium metal phosphate performs two functions. Firstly, it results in pyrolysis of the carbon source to form a conductive carbon coating on the lithium metal phosphate particles. Secondly, it improves the crystallinity and/or heals potential defects of the lithium metal phosphate crystals. Typically, the step of pyrolyzing the sieved lithium metal phosphate is carried out in an inert atmosphere, for example in an inert gas such as argon. It may alternatively be carried out in a reducing atmosphere. It is typically carried out at a temperature in the range from 550°C to 800 °C, e.g. from 630 °C to 780 °C, or from 650 °C or 700 °C to 780 °C. Typically, the step of pyrolyzing the sieved lithium metal phosphate is carried out for a period of 0.4 to 10 hours. The heating time depends on the scale of manufacture (i.e. where larger quantities are prepared, longer heating times may be preferred). At a commercial scale, 0.5 to 3 hours may be suitable, for example. Suitably, the step of pyrolyzing the sieved lithium metal phosphate is carried out in a rotary kiln. Following the step of pyrolyzing the sieved lithium metal phosphate, the carbon-coated lithium metal phosphate is subjected to a milling and sifting step. The step of milling and sifting the carbon-coated lithium metal phosphate produces a composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate. The milling and sifting step may comprise simultaneously milling and sifting the carbon-coated lithium metal phosphate in a combined milling and sifting step. In preferred processes of the invention the composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate is a composition of the first aspect of the invention. Suitably, the milling and sifting step may be carried out in a turbo mill combined with an air classifier or a sifter, an impact mill combined with an air classifier or a sifter, or a jet mill combined with an air classifier or a sifter. Alternatively, the milling and sifting step may be carried out in an impact mill with an integrated dynamic air classifier, or a jet-mill with an integrated rotary sifter. Preferably, the milling and sifting step is performed using a jet-mill with an integrated rotary sifter. As will be understood by the person skilled in the art, the exact conditions under which the milling and sifting step are carried out will vary depending upon the equipment selected to carry out the milling and sifting step. For instance, the conditions under which the milling and sifting step are carried out may vary depending on the size, make, or specific configuration of the equipment selected. The step of milling and sifting the carbon-coated lithium metal phosphate is carried out under mild milling conditions. Mild milling conditions in the context of the present disclosure means conditions which are sufficiently harsh to produce micro-agglomerates from agglomerated particles, but not so harsh as to convert micro-agglomerates into exclusively powder particles. The step of milling and sifting the carbon-coated lithium metal phosphate under mild milling conditions produces a composition comprising carbon-coated particles of micro- agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate. In certain preferred processes of the invention the composition produced by the step of milling and sifting the carbon-coated lithium metal phosphate under mild milling conditions is a composition according to the first aspect of the invention. By way of an example, mild milling conditions may be achieved using an AFG 100 jet mill, supplied by Hosokawa-Alpine. Mild milling conditions may be achieved using an AFG 100 jet mill operated at a pressure of from 0.25 to 2.5 bar, more preferably from 0.5 to 2 bar, most preferably from 0.7 to 1.5 bar, even more preferably from 0.8 to 1.2 bar. For example, the sifting jet-milling may be carried out using an AFG 100 jet mill operated at a pressure of about 1 bar. Mild milling conditions may be achieved using an AFG 100 jet mill having nozzles with a nozzle size of 2-5 mm, such as 3-4 mm, for example 3 mm. Suitable sifting conditions may be achieved using an AFG 100 jet mill and a sifter operated in the speed range from 3000 to 5000 rpm, preferably from 3250 to 4750 rpm, more preferably from 3500 to 4500 rpm, most preferably from 3750 to 4250 rpm. As described above, the conditions under which the milling and sifting step are carried out will vary depending upon the equipment selected to carry out the milling and sifting step. On the basis of the disclosure hereinabove the skilled person would be able to select mild milling and sifting conditions if alternative milling and sifting equipment were to be selected. The mild milling and sifting conditions may be selected to produce a composition according to the first aspect of the invention. It has surprisingly been found that by providing a process comprising the compaction step (i.e. step ii of the process) and the mild milling and sifting step (i.e. step v of the process), in combination, that a composition comprising micro-agglomerates and powder particles may be obtained. As described hereinabove such compositions, comprising micro-agglomerates and powder particles, allow electrodes with enhanced densities to be obtained. The process of the invention has the advantage that it can be carried out without the need to carry out a wet milling step on the lithium metal phosphate. Accordingly, the process of the invention may not comprise a wet-milling step. The composition of the first aspect of the invention may be prepared by a process according to the second aspect of the invention. The process or use of the present invention may further comprise the step of forming an electrode (typically a cathode) comprising the composition of the invention. Typically, this is carried out by forming a slurry of the composition, applying the slurry to the surface of a current collector (e.g. an aluminium current collector), and optionally processing (e.g. calendaring) to increase the density of the electrode. The slurry may comprise one or more of a solvent, a binder, carbon material and further additives. The process or use of the present invention may further comprise constructing a battery or electrochemical cell including the electrode comprising the composition. The battery or cell typically further comprises an anode and an electrolyte. The battery or cell may typically be a secondary (rechargeable) lithium (e.g. lithium ion) battery. Electrodes may be formed by any means known in the art. Typically, a slurry of the lithium metal phosphate composition of the invention, a conductive additive and a binder are mixed in a solvent to produce a slurry. The slurry may be coated onto a current collector (e.g. an aluminium sheet) and dried to obtain an electrode. As described above, suitable solvents for producing a slurry include N-methyl-2-pyrrolidone. Suitable binders include Solef Binder 5120. Suitable conductive additives are known to the person skilled in the art. Conductive additives include carbon blacks, such as graphite, or graphene. Typically, a slurry will be prepared comprising the lithium metal phosphate composition of the invention, a conductive additive and a binder in a respective weight ratio of 90:5:5. The slurry may be applied to a current collector, for example an aluminium sheet, using any method known in the art, to produce a coated current collector. For example, the slurry may be applied to the current collector using a doctor blade. The coated current collector may be dried in a first drying step to remove volatile components such as the solvent used in producing the slurry. The drying step typically involves heating the coated current collector to 50 °C to produce a partially dried current collector. The partially dried current collector may be subject to a second drying step to produce the electrode. The second drying step may involve heating the coated current collector to a temperature of from 50 to 250 °C, such as from 100 to 130 °C. A vacuum may optionally be used to assist drying. The invention further relates to an electrode, such as a cathode, comprising the composition of the invention. It has been found that when the composition of the invention is used in preparing an electrode, such as a cathode, a superior electrode density can be achieved relative to when compositions of the prior art are used. The electrode density achieved may depend upon the conditions under which the electrode is formed, for instance the compaction force used in preparation of the electrode. As will readily be understood, higher compaction forces or compaction times may result in denser electrodes. Accordingly, when a compaction pressure of 220 MPa for 60 seconds is used to prepare the electrode of the invention, it may be preferred that the electrode of the invention, such as a cathode, has a density of greater than or equal to 2.2 g/cm³, greater than or equal to 2.24 g/cm³, greater than or equal to 2.26 g/cm³, or greater than or equal to 2.28 g/cm³. It may be preferred that the electrode of the invention, such as a cathode, has a density of less than or equal to 2.4 g/cm³, less than or equal to 2.39 g/cm³, less than or equal to 2.38 g/cm³, or less than or equal to 2.37 g/cm³. For example, the electrode of the invention may have a density of from 2.2 to 2.4 g/cm³, from 2.24 g/cm³ to 2.39 g/cm³, from 2.26 g/cm³ to 2.38 g/cm³, or from 2.28 g/cm³ to 2.37 g/cm³. When a compaction pressure of 750 MPa for 120 seconds is used to prepare the electrode of the invention, it may be preferred that the electrode of the invention, such as a cathode, has a density of greater than or equal to 2.5 g/cm³, greater than or equal to 2.52 g/cm³, greater than or equal to 2.54 g/cm³, or greater than or equal to 2.56 g/cm³. It may be preferred that the electrode of the invention, such as a cathode, has a density of less than or equal to 2.75 g/cm³, less than or equal to 2.7 g/cm³, less than or equal to 2.68 g/cm³, or less than or equal to 2.65 g/cm³. For example, the electrode of the invention may have a density of from 2.5 to 2.75 g/cm³, from 2.52 g/cm³ to 2.7 g/cm³, from 2.54 g/cm³ to 2.68 g/cm³, or from 2.56 g/cm³ to 2.65 g/cm³. The invention further relates to a battery comprising an electrode comprising the composition of the invention. Examples Preparation of a composition according to the invention Lithium iron phosphate (30 kg), prepared via the hydrothermal route disclosed in WO2005/051840A1, was suspended in demineralized water (36.7 litres). Maltodextrin (1.4 kg) was added to the slurry and mixed until the maltodextrin dissolved. A source of elemental carbon and a surfactant were added to the slurry. The slurry was spray dried in a Nubilosa spray drier TT1 with a pneumatic nozzle type 940- 43 Form 0 S2 inner diameter 1.8 mm supplied by Schlick with air cap position 5 and an atomization pressure of 5 bar gauge, drying air flow rate of 300 m³/h, an inlet temperature of 300 °C, and an outlet temperature of 110 °C. The dried lithium metal phosphate and carbon source was compacted in a Hosokawa Bepex roller press Pharmapaktor L200/50P with a roller speed of 6 rpm and a line press force of between 680 and 720 kN/m. The compacted lithium metal phosphate was subjected to a sieving process using a tumbling sieve with a nominal mesh size of 4 mm. Fine material which bypassed the sieve was continuously recirculated to the roller press where it was added to the spray dried material entering the roller press. The compacted lithium metal phosphate was pyrolyzed in a rotary kiln under a nitrogen atmosphere at a temperature of between 700 to 800 °C for a period of 1.5 to 3 hours. The pyrolyzed carbon-coated lithium metal phosphate was mildly milled and sifted in a AFG 100 jet mill supplied by Hosokawa-Alpine with 3 mm nozzles, a pressure of 1 barg, and a sifter speed of 4000 rpm. Comparative examples were obtained from commercial sources. Comparative Example 2 consisted of carbon-coated particles of an agglomerated lithium iron phosphate material were obtained from Johnson Matthey under the trade name P2S2 (herein “agglomerated”, or “agglomerated material”). Comparative Example 3 consisted of carbon-coated particles of powder lithium iron phosphate were obtained from Johnson Matthey under the trade name P2 (herein “powder”, or “powder material”). Particle Size Distribution Particle size distributions of compositions according to the invention and comparative compositions were analysed using a Malvern MasterSizer 2000. Particle size distributions were measured in air at a pressure of 0.2 bar and in ethanol and presented hereinbelow. Where measurements were taken in air the material was dispersed with a Scirocco 2000 at a pressure of 0.2 bar and the particle size distribution measured using the Fraunhofer approximation. For measurements taken in ethanol the material was dispersed in ethanol with a Hydro 2000S and the particle size distribution measured using the Mie theory with a particle refractive index of 1.52 Table 1 shows the particle size which 25, 50, and 75 volume percent of particles possessed (i.e.25, 50, or 75 volume percent of particles have a particle size of this value or less) and the position of the peak modes. Table 1 Preparation of Electrodes Cathodes comprising a loading of between 11 to 12 mg/cm² of a composition of each of Example 1, Comparative Example 2, and Comparative Example 3 were manufactured according to the following general procedure. Each composition (5.4 g) was suspended in N- methyl-2-pyrrolidone (5.8 g) in a Thinky Mixer. As a binder, Solef 5130 (3 g of solution, 10 wt.% binder in NMP) and a conductive carbon, Super P Li, (0.3 g) were added to form a slurry. A doctor blade was used to coat the compositions on to an aluminium current collector. The target loading was 11-12 mg/cm². The electrodes were dried at 120 °C overnight in vacuum. Electrodes were pressed by means of a hydraulic press with a pressure of 220 MPa for 60 seconds. For comparison, electrodes were also pressed by means of a hydraulic press with a pressure of 750 MPa for 120 seconds. Electrode densities The electrode densities of the electrodes formed from Example 1 and Comparative Examples 2 and 3 are given in Table 2. Table 2 Electrochemical Testing Electrochemical cells, prepared using the electrodes pressed at 220 MPa for 60 seconds, were tested at a variety of charge/discharge rates (C-rates) from C/10 to 4C, in a voltage range between 2.5 and 4.2 volts. The direct current resistance (DCR) of electrodes comprising the electrode materials of Example 1 and Comparative Examples 2 and 3 were determined with respect to a current pulse using a Basytec test system. A 1C pulse with a 10 second duration was applied at a 50% state of charge (SOC). Resistances were calculated from the change in voltage and the current from the pulse. R0 was calculated from the values after 1 ms and DCR calculated after 10 seconds. The electrochemical test results are summarised in Table 3.

Table 3 Results Table 2 shows that compositions of the invention, comprising carbon-coated particles of micro-agglomerated of lithium metal phosphate can be formed into denser electrodes as compared to carbon-coated particles of lithium metal phosphate consisting of agglomerated or powder materials. As a consequence, an electrode with greater gravimetric and volumetric capacity may be obtained. The superior electrochemical performance of the material of the invention can be seen in Table 3. Table 3 shows that compositions of the invention have superior electrochemical performance as compared to agglomerated or powder materials. In particular, compositions of the invention display significantly higher volumetric capacities.

Claims

Claims 1. A composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate, the lithium metal phosphates having general formula: LiFe_1-xM_xPO₄ in which 0≤x≤1 and M is one or more selected from Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg, wherein the particle size distribution of the composition has: a) when measured using a Malvern MasterSizer 2000 in ethanol, 3 or more peaks having at least peak modes at 0.1 to 1 μm, 1.2 to 5 μm, and 5 to 20 μm, and 25 volume % of the particles having a particle size of 1 μm or less and 75 volume% of the particles having a particle size of 4 μm or less; and b) when measured using a Malvern MasterSizer 2000 in air at a gas pressure of 0.2 bar, a particle size distribution comprising 2 or more peaks having at least peak modes at 0.1 to 2 μm, and 10 to 20 μm, and 25 volume % of the particles having a particle size of 1 μm or less and 75 volume% of the particles having a particle size of 12 μm or less. 2. A composition according to claim 1, wherein the particle size distribution of the composition has, when measured using a Malvern MasterSizer 2000 in ethanol, 3 or more peaks having at least peak modes: present at from 0.15 μm to 0.9 ^ μm, from 1.3 to 3 μm, and from 7 to 18 μm; present at from 0.2 to 0.7 μm, from 1.4 to 2.5 μm, and from 8 to 16 μm; or present at from 0.25 to 0.5 μm, from 1.5 to 2 μm, and from 10 to 14 μm. 3. A composition according to claim 1 or claim 2, wherein the particle size distribution of the composition has, when measured using a Malvern MasterSizer 2000 in air at a gas pressure of 0.2 bar,

2 or more peaks having at least peak modes: present at from 0.

3 to 1.8 μm, and 11 to 18 μm; at from 0.5 to 1.5 μm, and 12 to 17 μm; or present at from 0.8 to 1.2 μm, and 13 to 16 μm. 4. A composition according to any one of claims 1 to 3, wherein 25 volume percent of the particles of the composition have a particle size of 0.7 μm or less, 0.5 μm or less, 0.

4 ^ μm or less, or 0.3 µm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol.

5. A composition according to any one of the preceding claims, wherein 50 volume percent of the particles of the composition have a particle size of 1.5 μm or less, 1.3 μm or less, 1.2 μm or less, 1.1 μm or less, or 1 µm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol.

6. A composition according to any one of the preceding claims, wherein 75 volume percent of the particles of the composition have a particle size of 3.5 μm or less, 3 μm or less, or 2.5 µm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in ethanol.

7. A composition according any one of the preceding claims, wherein 25 volume percent of the particles of the composition have a particle size of 0.9 μm or less, 0.8 μm or less, or 0.75 µm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar.

8. A composition according to any one of the preceding claims, wherein 50 volume percent of the particles of the composition have a particle size of 7 μm or less, 4 μm or less, 3 μm or less, 2 μm or less, 1.8 μm or less, 1.6 μm or less, or 1.4 µm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar.

9. A composition according to any one of the preceding claims, wherein 75 volume percent of the particles of the composition have a particle size of 10 μm or less, 8 μm or less, 7 μm or less, 5 μm or less, or 3 μm or less when the particle size distribution of the composition is measured using a Malvern MasterSizer 2000 in air at a pressure of 0.2 bar.

10. A process for preparing a composition comprising carbon-coated particles of micro- agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate, the lithium metal phosphates having general formula: LiFe1-xMxPO4 in which 0≤x≤1 and M is one or more selected from Ni, Co, Mn, Ca, Zn, Al, B, Ti and Mg, the process comprising the steps of: i) providing a particulate lithium metal phosphate and a carbon source; ii) compacting the particulate lithium metal phosphate and carbon source under a line force of from 400 kN/m to 1000 kN/m to produce a compacted lithium metal phosphate; iii) sieving the compacted lithium metal phosphate using a mesh with a nominal mesh size of 2 to 8 mm to remove fine particles and collecting a sieved lithium metal phosphate; iv) pyrolyzing the sieved lithium metal phosphate to produce a carbon-coated lithium metal phosphate; and v) milling and sifting the carbon-coated lithium metal phosphate under mild milling conditions to produce the composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate.

11. A process according to claim 10 wherein the step of compacting the particulate lithium metal phosphate and carbon source is carried out under a line force of from 500 to 900 kN/m, or from 600 to 800 kN/m, from 680 to 720 kN/m, or about 700 kN/m.

12. A process according to any one of claims 10 to 11, wherein the composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate is a composition according to any one of claims 1 to 9.

13. A process according to any one of claims 10 to 12, wherein the process comprises the further step of forming a cathode comprising the composition comprising carbon-coated particles of micro-agglomerated lithium metal phosphate and carbon-coated particles of powder lithium metal phosphate of step v.

14. A cathode comprising the composition of any one of claims 1 to 9.

15. A secondary lithium ion battery comprising a cathode according to claim 14.