Sintering aid

Abstract

A sintering aid for lowering the sintering temperature of ceramic materials comprises two phases where the first phase has the formula Li3-xB1-xCxO3 with 0.4<x<1, the second phase has the formula Li3B

Description

Field of the invention The present invention relates to a sintering aid for use in lowering the sintering temperature of ceramic materials, and the use thereof in the manufacture of a component for use in an electrochemical cell. Background to the invention A lithium-ion electrochemical cell is a type of rechargeable electrochemical cell having two electrodes that are each capable of reversibly storing lithium ions (Li+). During discharge of the cell, lithium ions move from the negative electrode (anode) to the positive electrode (cathode), and the direction is reversed when the cell is charged. The electrodes are separated by an electrolyte separator, which allows for ionic transport. Each of these components is typically formed as a film, deposited in sequence on a supporting substrate. Typically, Li-ion electrochemical cells include additional components such as current collectors, interface modifiers and / or encapsulations or other protective elements. In manufacture, the components may be deposited, for example, in the order of cathode current collector, cathode, electrolyte, anode, anode current collector and encapsulation. However, the order of deposition may be varied, providing that the resulting device contains a positive electrode (cathode), an electrolyte separator and a negative electrode (anode). In certain cases, the negative electrode is not present in the electrochemical cell immediately after assembly of the cell, but is instead provided as a lithium metal anode formed during initial charging of the electrochemical cell. Rechargeable electrochemical cells are also known that use other charge-carrying metal ions in place of Li+ ions, for example, rechargeable electrochemical cells are known that comprise Na+ or Mg2+ as the charge-carrying metal ions. In the case of large format lithium ion batteries (e.g. for use in electric vehicles), components such as cathodes may be manufactured using a method comprising the steps of: providing a substrate; forming a slurry containing particles of a ceramic material (for example, particles of an electrode active material and / or particles of an electrolyte material); depositing the slurry onto the substrate; optionally drying the slurry; and sintering the slurry. In the case that the lithium ion battery is a solid state battery, a similar process may be used during the manufacture of the electrolyte separator (as used herein, the term “solid state battery” refers to a battery in which the electrolyte separator comprises a ceramic electrolyte phase and optionally also a gel polymer electrolyte). In certain cases, the ceramic material may have a high melting point, such that high temperatures (for example, over 850°C, or in certain cases over 1000°C) would typically be required to sinter the material. However, it is desirable to be able to sinter the components at temperatures that are lower than this, for example, so as to reduce costs and / or environmental impact, and / or limit material degradation (for example, by limiting lithium loss during sintering), and / or to expand the range of materials that may be used alongside the component, for example, to expand the range of materials that may be used as a substrate for the component or to allow for the use of carbon-based conductive additives. It is known to add sintering aids to the slurry. Typically, the sintering aid is provided by an inorganic, non-metallic material having a melting point of 900°C or less. During the sintering process, the sintering aid typically acts to bond the particles of the ceramic material. It is known to use Li3-xBi.xCxO3 compounds as sintering aids (for example, Li2.3C0.7B0.3O3). It is desirable to develop sintering aids that help to improve the performance of the electrochemical cell. Summary of the invention Surprisingly, it has been found that use of a sintering aid comprising Li3-xBi.xCxO3 in combination with U3BO3 as a minor phase may result in improved performance of an electrochemical cell. For example, it is thought that the use of such a sintering aid may result in electrochemical cells having higher capacities and / or higher charging rates and / or increased power. Therefore, in a first aspect, the present invention may provide a sintering aid for use in lowering the sintering temperature of ceramic materials, the sintering aid comprising a first phase and a second phase, the first phase having the formula Li3-xBi.xCxO3, wherein 0.4<x<1, and the second phase having the formula U3BO3; wherein the second phase is present in an amount of 0.1 to 30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 0.1 to 10 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 0.1 to 8 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 0.5 to 30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 1 to 30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 2 to 30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 0.5-30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 1-10 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 2-8 wt% relative to the total weight of the first and second phases. In certain cases, the first phase has the formula Li3-xBi.xCxO3, wherein 0.5<x<1. In certain cases, 0.6<x<1. In certain cases, 0.6<x<0.9. In certain cases, 0.6<x<0.8. Typically, the sintering aid is provided in particulate form. In certain cases, the particles may have a d50 particle size in the range 0.1-20 pm. For example, the particles may have a d50 particle size of about 0.5 pm. The d50 particle size may be measured using laser diffraction of a liquid dispersion of the particles, following ISO 13320:2020. In a second aspect, the present invention may provide a component for use in an electrochemical cell, the component comprising ceramic particles that remain solid at temperatures up to 850°C or more, the ceramic particles being bonded by means of a sintering aid, wherein the sintering aid comprises a first phase and a second phase, the first phase having the formula Li3-xBi.xCxO3, wherein 0.4<x<1, and the second phase having the formula U3BO3; wherein the second phase is present in an amount of 0.1 to 30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 0.1 to 10 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 0.1 to 8 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 0.5 to 30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 1 to 30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 2 to 30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 0.5-30 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 1-10 wt% relative to the total weight of the first and second phases. In certain cases, the second phase is present in an amount of 2-8 wt% relative to the total weight of the first and second phases. In certain cases, the first phase has the formula Li3-xBi.xCxO3, wherein 0.5<x<1. In certain cases, 0.6<x<1. In certain cases, 0.6<x<0.9. In certain cases, 0.6<x<0.8. In certain cases, the component is an electrode, for example, a cathode. In certain cases, the component is an electrode for a solid state electrochemical cell. In certain cases, the component is a bulk electrolyte layer, that is, a layer that is configured to provide a separator between the two electrodes of the electrochemical cell. In certain cases, at least a portion of the ceramic particles that remain solid at temperatures up to 850°C or more comprise a solid electrolyte material. Such particles may assist in improving ion conduction, which may be useful in a bulk electrolyte layer and / or in an electrode. Typically, the solid electrolyte material has a greater ionic conductivity than the sintering aid. Typically, the electrolyte material is a lithium-containing electrolyte material. For example, the electrolyte material may be a lithium garnet electrolyte material. In certain cases, the electrolyte material may be a lithium-containing oxide material. In certain cases, the electrolyte material is a material selected from the group consisting of: lithium lanthanum zirconium oxide (LLZO) and cation-doped LLZO, wherein the cation dopant may be selected from the group consisting of tantalum, barium, yttrium, zinc, niobium, aluminium, germanium, strontium, gallium, titanium, and combinations thereof. In a preferred embodiment, the electrolyte material has the formula LiaLa3Zr2-yMyOi2, wherein 6<a<8; 0<y<1; and M is selected from the group consisting of Ta, Ba, Y, Zn, Nb, Al, Ge, Sr, Ga, Ti, and combinations thereof. In certain cases, M is Nb or Ta. In the case that the component is a cathode, at least a portion of the ceramic particles that remain solid at temperatures up to 850°C or more may comprise a cathode active material that is provided by a compound containing the cations of lithium and one or more transition metals, and an anion selected from the group consisting of: oxide anion, sulphide anion, and polyanions. Examples of suitable polyanions include phosphates, fluorophosphates (including PO4F), and fluorosulphonates (including SO4F). Examples of suitable cathode active materials include lithium nickel cobalt aluminium oxide (LiNi0.8Co0.15AI0.05O2); lithium cobalt oxide (LiCoO2); lithium iron phosphate (LiFePO4); lithium manganese nickel oxide (LiMn1.5Nio.5O4); lithium cobalt phosphate (UCOPO4); lithium nickel cobalt manganese oxide (LiNipCoqMnrO2 wherein 0<p<1; 0<q<1; 0<r<1 and p+q+r=1); vanadium oxide (V2O5); LiVOPO4; Li3V2(PO4)3; and combinations thereof. Metal chalcogenides such as TiSs, NbSes, LiTiS2 and combinations thereof may also provide suitable cathode active materials. Preferably, the cathode active material is selected from the group consisting of LiNipMnqCorO2, wherein 0.2<p<0.4; 0.2<q<0.4; and 0.2<r<0.4; and LiNipMnqCorO2, wherein 0.5<p<0.7; 0.1<q<0.3; and 0.1<r<0.3. Typically, the sintering aid is present in an amount of 1-40 wt% relative to the total weight of the ceramic particles and the sintering aid, preferably in an amount of 10-30 wt%. Typically, the ceramic particles that remain solid at temperatures up to 850°C or more have a d50 size in the range 10 nm to 50 pm, as measured using laser diffraction of a liquid dispersion of the particles, following ISO 13320:2020. For example, the ceramic particles may have a d50 size in the range 100 nm to 40 pm. In certain cases, the ceramic particles may have a d50 size in the range 1-40 pm. In certain cases, the ceramic particles may have a d50 size in the range 2-20 pm. In certain cases, the ceramic particles that remain solid at temperatures up to 850°C additionally remain solid at temperatures up to 1000°C. In the case that the component is an electrode, the thickness of the electrode is typically in the range 20-1000 pm; preferably 50-1000 pm; more preferably 70-1000 pm. In the case that the component is an electrode, the component may comprise an electronically-conductive constituent. When present, the electronically-conductive constituent typically has an electronic conductivity of at least 10-4 Scm-1, determined through DC decay measurement at 25°C. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 10-3 Scm-1. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 10-2 Scm-1. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 10-1 Scot1. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 1 Scm-1. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 10 Scm-1. When present, the electronically-conductive constituent may comprise a material selected from the group consisting of: carbon black, acetylene black, activated carbon, carbon nanotubes, carbon fibres, titanium nitride, indium tin oxide; antimony tin oxide; vanadium pentoxide; non-stoichiometric molybdenum nitride; aluminium-doped zinc oxide; tantalum carbide; and mixtures thereof. Alternatively, the electronically-conductive constituent may be provided by a metal powder. In a third aspect, the present invention may provide an electrochemical cell comprising a component according to the second aspect of the invention. In certain cases, the electrochemical cell may comprise two components according to the second aspect of the invention, for example, a cathode according to the second aspect of the invention, as well as a bulk electrolyte layer according to the second aspect of the invention. In a fourth aspect, the present invention may provide a method of making the sintering aid according to the first aspect of the invention, comprising the steps of mixing particles of a first phase with particles of a second phase, the first phase having the formula Li3-xBi.xCxO3, wherein 0.4<x<1, and the second phase having the formula U3BO3; wherein the second phase is present in an amount of 0.1 to 30 wt% relative to the total weight of the first and second phases. The sintering aid prepared according to the method of the fourth aspect of the invention may have one or more of the optional features of the sintering aid according to the first aspect of the invention. In a fifth aspect, the present invention may provide a method of making the component according to the second aspect of the invention, comprising the steps of: forming a slurry comprising ceramic particles that remain solid at temperatures up to 850°C or more and a sintering aid according to the first aspect of the invention; depositing the slurry on a substrate; and sintering the slurry at a temperature in the range of 500-750°C. During the sintering process, the sintering aid softens and / or partially melts, so as to bond the ceramic particles, which remain solid during the sintering process. In certain cases, the method may comprise a further step, after the step of depositing the slurry on the substrate and before the step of sintering the slurry, of drying the slurry. The drying step is generally carried out at a temperature in the range 70-400°C. In certain cases, the drying step is carried out at a temperature in the range 150-400°C. For the avoidance of doubt, the sintering temperature is the maximum temperature reached during the sintering step. In certain cases, the sintering temperature may be in the range of 550-750°C. In certain cases, the sintering temperature may be in the range of 600-700°C. The sintering time may be 1-4 hours. In the case that the component is an electrode, the slurry may contain an electronically-conductive constituent. When present, the electronically-conductive constituent typically has an electronic conductivity of at least 10-4 Scot1, determined through DC decay measurement at 25°C. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 10'3 Scm-1. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 10-2 Scot1. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 10-1 Scot1. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 1 Scm-1. In certain embodiments, the electronic conductivity of the electronically-conductive constituent may be at least 10 Scm-1. When present, the electronically-conductive constituent may comprise a material selected from the group consisting of: carbon black, acetylene black, activated carbon, carbon nanotubes, carbon fibres, titanium nitride, indium tin oxide; antimony tin oxide; vanadium pentoxide; non-stoichiometric molybdenum nitride; aluminium-doped zinc oxide; tantalum carbide; and mixtures thereof. Alternatively, the electronically-conductive constituent may be provided by a metal powder. Typically, the slurry further comprises an organic binder phase and a solvent for the organic binder phase. The choice of the organic binder is not particularly limited, as long as it performs the function of providing the slurry with a degree of mechanical strength after deposition and before sintering. For example, the organic binder may be selected from the group consisting of vinyl polymers (including polyvinyl alcohol, polyvinyl butyral, polyvinyl chloride, polyvinyl butanol, polyvinyl acetate and vinyl chloride-acetate); acrylic polymers (including polyacrylate esters, polymethyl methacrylate, and polyethyl methacrylate); cellulose binders (including ethyl cellulose, methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, nitrocellulose, and cellulose acetate-butyrate); polypropylene carbonate; polyethylene carbonate; polyethylene oxide; and combinations thereof. Preferably, the organic binder is selected from the group consisting of ethyl cellulose, polypropylene carbonate, polyethylene carbonate, polyvinyl alcohol, polyethylene oxide, carboxymethyl cellulose, and combinations thereof. In general, the amount of organic binder in the slurry is in the range 1-20 wt% relative to the total amount of solid material in the slurry, preferably in the range 5-15 wt%. The solvent is typically an organic solvent and may be selected from the group consisting of terpineol, benzyl alcohol, toluene, xylenes, ethanol, methanol, methyl ethyl ketone, ethylene glycol ethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monomethyl ether, diethylene glycol ethyl isobutyl ether, neopentyl glycol monoisobutyrate, diethylene glycol monobutyl ether, ethylene glycol ether, diethylene glycol monohexyl ether, propylene glycol monobutyl ether, diethylene glycol monoethyl ether acetic ester, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, butyl carbitol acetate, acetate 2-butoxy ethyl ester, acetate 2-ethoxy ethyl ester, acetate 2-methoxyl group ethyl ester; 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate (Texanol™), C10-C40 alcohols, ethyl lactate, dipropylene glycol monomethyl ether, 2 ethyl hexanoic acid, tri-methyl hexanoic acid, tetrahydrofurfuryl alcohol, furfuryl alcohol, 2-(benzyloxy) ethanol, 2-phenoxyethyl alcohol, 2-(methoxymethoxy) ethanol, triethylene glycol monomethyl ether, triethylene glycol, diethylene glycol monobutyl acetic ester, butyl carbitol acetate, phenylium ester, phenoxy group ethylhexoate, glycol monomethyl phenyl ether, diethylene glycol phenyl ether, glycol monomethyl benzylic ether, diethylene glycol single-benzyl ether, propylene glycol phenyl ether, benzyl glycol, phenylacetic acid methyl esters, phenylacetic acid ethyl ester, ethyl benzoate, methyl benzoate, gamma-butyrolactone, dimethyl sulfoxide (DMSO), N-methyl pyrrolidone, N-methylacetamide, ethanamide, N-dimethylformamide, N-methylformamide, methane amide, and combinations thereof. The slurry may further comprise a dispersant and / or a plasticiser. The component prepared using the method of the fifth aspect of the invention may have one or more of the optional features of the component according to the second aspect of the invention. In the case that the component is an electrode, the substrate is typically a metal or metal alloy sheet. For example, the substrate may be provided by an aluminium sheet or a stainless steel sheet. In the case that the component is a bulk electrolyte layer, the substrate is typically an electrode. In certain cases, the electrode may be a component according to the second aspect of the invention and / or may have been prepared using the method of the fifth aspect of the invention. The sintering aid used in the method of the fifth aspect of the invention may have one or more of the optional features of the sintering aid according to the first aspect of the invention. The component prepared according to the method of the fifth aspect of the invention may have one or more of the optional features of the component according to the second aspect of the invention. Detailed description The invention will now be described by way of example with reference to the following Figure in which: Figure 1 shows a graph of voltage against discharge capacity for an electrochemical cell according to a first embodiment of the third aspect of the invention and an electrochemical cell according to a Comparative Example of the invention. Example 1 Manufacture of sintering aid Single phase LBCO particles having the formula Li2.3B0.3C0.7O3 were obtained from William Blythe Ltd. U3BO3 particles were physically mixed with the LBCO particles in an amount of 5 wt% relative to the total weight of the Li2.3B0.3C0.7O3 and U3BO3. Preparation of electrochemical cell Preparation of the cathode The slurry for the cathode was prepared by mixing the constituents listed in Table 1 with an organic solvent for the binder phase. Table 1: constituents of slurry for the cathode Slurry constituent Function Amount Particles of NMC Cathode active material 56 wt% Particles of LLZTO Electrolyte material 26 wt% Particles of LBCO and U3BO3 Sintering aid 16 wt% Ethyl cellulose Binder phase 2 wt% The NMC cathode active material had the chemical formula LiNi0.33Mn0.33Co0.33O2. The LLZTO electrolyte had the chemical formula Li6.4La3Zr1.4Tao.6O12. 5 The slurry for the cathode was cast onto a metal foil using a screen printing process and subsequently dried. 4-6 layers of slurry were cast in total before drying to form a first green body. 10 Preparation of the bulk electrolyte layer The slurry for the bulk electrolyte layer was prepared by mixing the constituents listed in Table 2 with an organic solvent for the binder phase. Table 2: constituents of slurry for the bulk electrolyte layer Slurry constituent Function Amount Particles of LLZTO Ceramic ion-conducting material 75-85 wt% Particles of LBCO and U3BO3 Sintering aid 13-23 wt% Ethyl cellulose Binder phase 2 wt% 15 The slurry for the bulk electrolyte layer was cast onto the first green body using a screen printing process and subsequently dried. 4-6 layers of slurry were cast in total before drying to form the second green body. The stacked metal foil, first green body and second green body were sintered in a Carbolite GSM 1100 furnace at a temperature of 600-700°C. The cathode and bulk electrolyte layer each have a thickness after sintering of about 20-30 pm. The exposed surface of the bulk electrolyte has an area of about 25 cm2. Addition of the anode layer An anode-side stack was provided comprising a graphite anode layer and a metal current collector layer in a stacked arrangement. The exposed surface of the graphite sheet was glued to the exposed surface of the bulk electrolyte layer using a polymer gel electrolyte. Comparative Example In a Comparative Example, an electrochemical cell was prepared as described above in relation to Example 1, with the difference that the sintering aid consisted only of particles of LBCO (that is, no U3BO3 particles were added). The cells of Example 1 and the Comparative Example contained the same loading of cathode active material per unit area of the cathode. Table 3: Comparison of capacities of Example 1 and Comparative Example cells Measured discharge capacity after first cycle (mAh) Example 1 8.9 Comparative Example 7.8 The cells of Example 1 and the Comparative Example were cycled according to the following protocol: • 2 cycles at a C-rate of C / 20 • 1 cycle at a C-rate of C / 10 • 1 cycle at a C-rate of C / 5 • 10 cycles at a C-rate of 1C • 1 cycle at a C-rate of C / 10. (A C-rate of 1C means that the battery is charged or discharged using a current that would theoretically achieve full charge from zero in one hour). Figure 1 shows the discharge capacities that were achieved for the cells of Example 1 and the Comparative Example at the first cycle at a C-rate of C / 20 and the first cycle at a C-rate of 1C. This shows that the cell of Example 1 exhibits higher discharge capacities and a better c-rate capability.

Claims

06 08 251. A sintering aid for use in lowering the sintering temperature of ceramic materials, the sintering aid comprising a first phase and a second phase, the first phase having the formula Li3-xBi-xCxO3, wherein 0.4<x<1, and the second phase having the formula U3BO3; wherein5 the second phase is present in an amount of 1 to 10 wt% relative to the total weight of the first and second phases.

2. A sintering aid according to claim 1, wherein the second phase is present in an amount of 2 to 8 wt% relative to the total weight of the first and second phases.

103. A sintering aid according to any one of the preceding claims, wherein 0.5<x<1.

4. A sintering aid according to claim 3, wherein 0.6<x<1.15 5. A sintering aid according to any one of the preceding claims, wherein the sinteringaid is provided in particulate form, the particles having a d50 particle size in the range 0.1-20 pm, measured using laser diffraction of a liquid dispersion of the particles, following ISO 13320:2020.20 6. A component for use in an electrochemical cell, the component comprising ceramicparticles that remain solid at temperatures up to 850°C or more, the ceramic particles being bonded by means of a sintering aid, wherein the sintering aid comprises a first phase and a second phase, the first phase having the formula Li3-xBi.xCxO3, wherein 0.4<x<1, and the second phase having the formula U3BO3; wherein the second phase is present in an amount25 of 1 to 10 wt% relative to the total weight of the first and second phases;wherein the component is a cathode or a bulk electrolyte layer.06 08 257. A component according to claim 6, wherein the component is a bulk electrolyte layer and the ceramic particles that remain solid at temperatures up to 850°C or more comprise a solid electrolyte material having the formula LiaLa3Zr2-yMyOi2, wherein 6<a<8; 0<y<1; and M is selected from the group consisting of Ta, Ba, Y, Zn, Nb, Al, Ge, Sr, Ga, Ti, and5 combinations thereof.

8. A component according to claim 6, wherein the component is a cathode and the ceramic particles that remain solid at temperatures up to 850°C or more comprise a cathode active material having the formula LiNipMnqCorO2, wherein 0.2<p<0.4; 0.2<q<0.4; and10 0.2<r<0.4.

9. A component according to claim 6, wherein the component is a cathode and the ceramic particles that remain solid at temperatures up to 850°C or more comprise a cathode active material having the formula LiNipMnqCorO2, wherein 0.5<p<0.7; 0.1<q<0.3; and15 0.1<r<0.3.

10. A component according to any one of claims 6-9, wherein the sintering aid is present in an amount of 1-40 wt% relative to the total weight of the ceramic particles and the sintering aid.2011. A component according to claim 10, wherein the sintering aid is present in an amount of 10-30 wt% relative to the total weight of the ceramic particles and the sintering aid.

12. An electrochemical cell comprising a component according to any one of claims 6-11. 2513. A method of making the sintering aid according to any one of claims 1-5, comprising the steps of mixing particles of a first phase with particles of a second phase, the first phasehaving the formula Li3-xBi.xCxO3, wherein 0.4<x<1, and the second phase having the formulaU3BO3.

14. A method of making the component according to any one of claims 6-11, comprising5 the steps of:forming a slurry comprising ceramic particles that remain solid at temperatures up to850°C or more and a sintering aid according to any one of claims 1-5;depositing the slurry on a substrate; andsintering the slurry at a temperature in the range of 500-750°C.1006 08 25

Images