Method for producing a silicon-carbon anode

Abstract

Method for producing a silicon-carbon anode (14) wherein the method comprises the following steps: a) Producing a pyrolyzable coating on a substrate by applying a mixture consisting of silicon particles, conductive additives and a carbon-containing and pyrolyzable component, wherein the component (i) is selected from the group comprising waxes, paraffins and long-chain fatty acids and the mixture is applied as a paste; or (ii) is a pyrolyzable organic polymer and the mixture is applied as a melt; and b) Pyrolysis of the coating.

Description

[0001] The invention relates to a method for producing a silicon-carbon anode for rechargeable lithium-ion batteries.

[0002] Rechargeable lithium batteries have become a ubiquitous energy source for mobile electronic devices. They are used in hybrid and electric vehicles and are a key component of energy storage solutions for renewable energy. To meet the ever-increasing energy demands of these applications, new electrode materials are needed that increase energy density beyond that of currently available lithium batteries.

[0003] Lithium-ion secondary batteries are particularly attractive energy storage devices with high gravimetric and volumetric capacity and the ability to deliver high power. They have become ubiquitous energy sources for electric and hybrid electric vehicles. This has led to intense interest in the development of battery electrodes with high gravimetric and volumetric capacity to improve the energy density of the current generation of lithium batteries. The present application deals with specific anode materials that promise increased capacity.

[0004] From an energy density perspective, lithium metal is the best anode material. However, the electroplating of dendritic lithium during charging can lead to a short circuit, raising significant safety concerns for lithium metal anodes. The most commonly used material for commercial secondary lithium-ion battery anodes is graphite, which can hold a maximum of one lithium atom per six carbon atoms. The volumetric expansion during lithium incorporation between the planar graphite layers is slightly more than 10%, resulting in high reversibility and stable capacity over repeated cycles. Nevertheless, the theoretical capacity of graphite is low compared to other potential anode materials, such as lithium alloys of silicon or tin, limiting its power density.

[0005] Silicon is a promising alternative to graphite anodes with high capacity. It has a low discharge potential (~370 mV compared to Li / Li). + This makes it suitable for high-power applications when combined with common cathode materials such as LiCoO2 or LiMn2O4. It is abundant, non-toxic, and can be alloyed with up to 4.4 lithium atoms per silicon atom. The theoretical capacity of the fully lithiated alloy Li4.4Si is 4212 mAhg. -1, which is an order of magnitude higher than that of graphite. However, the commercial use of silicon in lithium cells is limited by silicon's low cycle stability. The large volume change during lithium insertion leads to high internal stresses, pulverization of the electrode, and subsequent loss of electrical contact between the active material and the current collector. This challenge can be overcome by silicon nanostructures, which allow for slight strain relaxation to counteract electrode fragmentation and potentially offer the additional benefits of short lithium diffusion distances and improved mass transport.

[0006] Lithium batteries generally consist of electrochemical cells connected in parallel or series to achieve the desired current and voltage characteristics. Each cell contains a positive electrode (cathode) and a negative electrode (anode), separated by an electrically insulating separator that is permeable to lithium ions. Ion conduction occurs via an electrolyte. The anode and cathode are connected by an external circuit. During charging, electrons flow from the cathode to the anode through the external circuit, while lithium ions deintercalate from the cathode and migrate through the electrolyte to the anode to maintain charge neutrality. In a silicon anode, the lithium ions are alloyed with the silicon, and the anode expands until it reaches the desired state of charge. Discharging is simply the reverse of this process.The anode undergoes volume contraction when lithium ions are released. The ions migrate back through the electrolyte and are incorporated at the cathode, while the electrons move through the external circuit to the cathode, performing useful work in the process.

[0007] Particle-based anodes, in which electrochemically active silicon nanoparticles are mixed with conductive additives and binders, have the advantage that the often simple particle syntheses are easily scalable. The capacity of particulate anodes containing conductive additives and silicon-based composite nanoparticles increases with increasing silicon content. Reducing the particle size improves cycle stability. Most research in the field of silicon composite anodes focuses on carbon matrices because carbon is abundant, its chemistry is well understood, and it offers advantages over other potential matrix materials. For example, carbon is highly conductive, enabling efficient electron transport, and is also lightweight and ductile, allowing it to accommodate the volume expansion of the active material.

[0008] Silicon-carbon composites are typically produced by mechanical milling of the active and matrix materials or by pyrolysis of carbon and silicon precursors to obtain silicon in a carbon-containing matrix. The uniform carbon deposition during pyrolysis or extended ball milling in many composites results in close contact between carbon and silicon. Silicon-carbon composites are known to be produced by pyrolysis of organic starting materials mixed with nanoparticulate silicon or by direct pyrolysis of organosilicon starting materials. For example, nanocomposites using various polymers as the carbon source have been reported.Silicon-carbon composites produced by pyrolysis and used as anode material in lithium-ion batteries are disclosed by way of example in US 2016 / 0365567 A1, WO 2021 / 009031 A1, US 11114660 B1, US 2022 / 0013782 A1 and US 2021 / 0384495 A1.

[0009] DE 10 2019 133 014 A1 describes a process for manufacturing an electrode for an electrochemical cell. The process involves crosslinking a first mixture comprising a polymeric binder and an electroactive material containing silicon, lithium, graphite, and a combination thereof, to form a crosslinked intermediate electrode containing the electroactive material dispersed in the polymeric binder, wherein at least a portion of the polymeric binder is crosslinked. The process further includes carbonizing the crosslinked intermediate electrode to form the electrode.

[0010] DE 10 2020 003 354 A1 discloses a process for producing a silicon-carbon composite material. The process provides, in particular, for coating silicon particles with a suitable layer of carbon before their use in a battery electrode. For this purpose, a mixture of silicon particles and carbon compound is first thermally processed at 120-350°C (step A) and finally pyrolyzed at over 750°C (step B).

[0011] In US 2022 / 0013782 A1, an aqueous paste containing a water-based phenol binder and silicon powder is applied to a collector and pyrolyzed.

[0012] CN 1 08 565 451 A discloses a process for producing silicon-carbon cathode material. The manufacturing process involves melting a carbon source and adding graphite and silicon powder to the melt. The resulting mixture is carbonized and then crushed.

[0013] The invention is based on the objective of designing the production of silicon-carbon anodes by pyrolysis in such a way that the use of solvents or binders can be dispensed with in the industrial manufacturing process.

[0014] This problem is solved by the inventive method for producing a silicon-carbon anode according to claim 1. The method comprises the following steps: a) Producing a pyrolyzable coating on a substrate by applying a mixture consisting of silicon particles, conductive additives and a carbon-containing and pyrolyzable component, wherein the component (i) is selected from the group comprising waxes, paraffins and long-chain fatty acids and the mixture is applied as a paste; or (ii) is a pyrolyzable organic polymer and the mixture is applied as a melt; and b) Pyrolysis of the coating.

[0015] In other words, the inventive method for producing the silicon-carbon anode completely eliminates the need for solvents or binders. This makes the industrial manufacturing process of rechargeable lithium-ion batteries safer. Furthermore, it avoids the costs associated with disposing of solvent and binder waste.

[0016] The process involves the direct deposition of a mixture prepared for anode production onto a substrate. This mixture consists of silicon particles, conductive additives, and a carbon-containing, pyrolyzable component. The silicon-carbon composite used as the anode in the rechargeable lithium-ion battery is thus obtained by pyrolysis of organic starting materials (namely, the carbon-containing, pyrolyzable component) into which nanoparticulate or microparticulate silicon is mixed and which further contains conductive additives to increase or ensure electrical conductivity. The substrate can be electrically conductive and serve as a current collector. Metal foils, for example, made of copper, can be used as substrates.

[0017] In one variant of the process, the carbon-containing and pyrolyzable component is selected from the group comprising waxes, paraffins, and long-chain fatty acids, including mixtures thereof. The mixture produced for the coating is applied as a paste. The pyrolytic decomposition of the organic component yields, in addition to the conductive carbon black, the carbon component of the silicon-carbon anode material.

[0018] A paste is a solid-liquid mixture (suspension) with a high solids content. Pastes are no longer flowable but spreadable, meaning they have a correspondingly low viscosity. The silicon particles and the conductive additive are present as solids. The wax, paraffin, or long-chain fatty acid serves as the low-viscosity, but still liquid, component of the paste.

[0019] Wax is a collective term for esters of long-chain fatty acids (C24-C36) (wax acids) with long-chain alcohols (fatty alcohols) [C15-C36; for example, cetyl alcohol (hexadecan-1-ol, C 16 H 33 OH) and stearyl alcohol (octadecan-1-ol, C 18 H 37 OH)], triterpene or steroid alcohols (for example, ambrein, betulin), which are widespread in plants and animals.

[0020] A long-chain fatty acid is, in particular, a saturated or mono- to triple-unsaturated, unbranched C10-C30 fatty acid with a melting point of 30°C or higher.

[0021] Paraffin refers to a mixture of acyclic alkanes with the general formula C n H 2n+2 , where n = 18 to 32.

[0022] Particularly preferred carbon-containing and pyrolizable components for the production of the paste include lauric acid, myristic acid, palmitic acid, and stearic acid, whose melting points are in the range of 43°C to 70°C. Paraffins with a melting point of 50°C to 80°C are also particularly preferred.

[0023] In another variation of the process, the applied mixture contains a pyrolyzable organic polymer in addition to silicon particles and conductive carbon black. This mixture is applied to the substrate as a melt. In other words, the organic polymer must be able to form a polymer melt when heated and can, in particular, be a thermoplastic polymer.

[0024] Preferred pyrolitable organic polymers for the melt preparation include acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA), polycarbonate (PC), polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyetheretherketone (PEEK). Acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA), polycarbonate (PC), and polyethylene (PE) are particularly preferred due to their melting points in the range of approximately 100°C to 170°C.

[0025] The mixture contains (nano- / microscale) silicon particles. The silicon particles preferably have a mean diameter D50 of 100 nm to 5 µm. The mean particle diameter (D50) can be determined by dynamic light scattering (DLS) according to ISO 22412. The equivalent diameter of a non-spherical particle corresponds to the diameter of a spherical particle exhibiting the same properties as the non-spherical particle under investigation.

[0026] Finally, the mixture contains a conductive additive. The conductive additive can be conductive carbon black and / or a carbon-based conductive material. Conductive carbon blacks, such as CNTs, graphene, or nanowires, are preferred. Conductive additives are well-known components of lithium-ion batteries. Conductive carbon black (also known as conductive industrial carbon black, conductivity carbon black, or carbon black) is a black specialty chemical available as a powder. It is produced in strictly controlled processes and contains more than 95% pure carbon. Conductive carbon black has highly branched aggregates that provide electrical conductivity in applications. The shape of the aggregates can vary, and a distinction is made between spherical, elliptical, linear, and branched aggregates. Conductive carbon blacks with linear and branched aggregates are particularly preferred because they exhibit higher electrical conductivity and are more easily dispersible.Conductive carbon blacks are produced using methods such as the furnace black process and thermal cracking, for example, the acetylene black process. Carbon-based conductive materials include carbon nanotubes (CNTs) and graphene.

[0027] The applied mixture preferably has the following composition: 1 to 95 wt.% silicon particles, preferably 40 to 90 wt.%; 0.1 to 15 wt.% conductive additive, preferably 0.5 to 5 wt.%; 0.1 to 30 wt.% of the carbon-containing and pyrolyzable component, preferably 1 to 20 wt.%; and Impurities with less than 1 wt.%.

[0028] The stated proportions refer to the total weight of the mixture. All proportions combined add up to 100% by weight.

[0029] In step b) of the process, the applied coating is converted into the desired silicon-carbon composite material by pyrolysis, forming the anode. The composite material is formed during the thermochemical transformation of the mixture. Pyrolysis preferably takes place in an oxygen-free atmosphere and at a temperature in the range of 200 °C to 1000 °C, preferably from 250 °C to 900 °C.

[0030] Further preferred embodiments of the invention will result from the other features mentioned in the dependent claims and the following description.

[0031] The various embodiments of the invention mentioned in this application can be combined with each other, unless otherwise stated in individual cases.

[0032] The invention is explained below using exemplary embodiments with reference to the accompanying drawings. These show: Fig. 1. A schematic diagram of a rechargeable lithium-ion battery. Fig. 2 a flowchart for the manufacturing process according to the invention of a silicon-carbon anode material.

[0033] Fig. Figure 1 shows a highly schematic cross-sectional view of the basic structure of a rechargeable lithium-ion battery 10. The lithium-ion battery 10 comprises a positive electrode (cathode 12) and a negative electrode (anode 14), which are separated by an electrically insulating separator 16 that is permeable to lithium ions. Ion conduction occurs via an electrolyte. Anode 14 and cathode 12 are connected to each other via an external circuit. During charging, electrons flow from cathode 12 through the external circuit to anode 14, while lithium ions deintercalate from cathode 12 and migrate through the electrolyte to anode 14 to maintain charge neutrality. Discharging is simply the reverse of this process. The anode 14 undergoes volume contraction when lithium ions are released.The ions migrate back through the electrolyte and are stored at the cathode 12, while the electrons move through the external circuit to the cathode 12, performing useful work (load 20).

[0034] Anode 14 is a silicon-carbon anode, meaning it contains nanoparticulate silicon embedded in an electrically conductive carbon matrix. The silicon-carbon anode is produced via a pyrolytic process, the process steps of which are described using an exemplary embodiment in Fig. 2 are shown.

[0035] In step S100 of the process, a mixture of silicon particles, a conductive additive, and a carbon-containing, pyrolyzable component is produced. The components can be mixed using conventional mechanical methods. The aim is to obtain a mixture with the most homogeneous distribution possible of the silicon particles and the conductive additive.

[0036] If the carbon-containing and pyrolyzable component is a wax or paraffin, the viscosity of the mixture can be reduced by heating for the mixing process. If the component is an organic polymer, mixing with a melt of the polymer can take place in step S100.

[0037] In step S110, the generated, homogeneous mixture is applied to a substrate. The substrate can be, for example, a metal foil. Common mechanical coating techniques can also be used for this process step. Pastes or melts can be applied, for example, by roller coating, thermal spraying, slot nozzle coating, spray coating, or doctor blade coating.

[0038] In step S120, the coated substrate is heated to such an extent that the carbon-containing and pyrolyzable component is thermally decomposed into carbon. Accordingly, the process conditions necessary for the pyrolysis of the respective component prevail in this step. These are generally an oxygen-deficient or oxygen-free atmosphere and temperatures above 200 °C. At the end of the pyrolysis and after cooling, the silicon-carbon anode is available for the subsequent process steps in the production of lithium-ion batteries. Reference symbol list 10 Lithium-ion batteries 12 Cathode 14 Anode 16 Separator 20 Last S100 - S120 Procedure steps

Claims

Method for producing a silicon-carbon anode (14), the method comprising the following steps: a) producing a pyrolyzable coating on a substrate by applying a mixture consisting of silicon particles, conductive additives and a carbon-containing and pyrolyzable component, wherein the component (i) is selected from the group comprising waxes, paraffins and long-chain fatty acids and the mixture is applied as a paste; or (ii) is a pyrolyzable organic polymer and the mixture is applied as a melt; and b) pyrolysis of the coating. The method of claim 1, wherein the applied mixture has the following composition: 1 to 95 wt.% silicon particles, 0.1 to 15 wt.% conductive additives, 0.1 to 30 wt.% of the carbon-containing and pyrolyzable component, and impurities with less than 1 wt.%, wherein the proportions refer to the total weight of the mixture and all proportions add up to 100 wt.%. Method according to claim 1 or 2, wherein the silicon particles have a mean diameter D50 of 100 nm to 5 µm, determined by dynamic light scattering (DLS) according to ISO 22412. Method according to any of the preceding claims, wherein the pyrolitable organic polymer is selected from the group comprising: Acrylonitrile butadiene styrene (ABS), polymethyl methacrylate (PMMA), polycarbonate (PC), and polyethylene (PE). Method according to one of the preceding claims, wherein the carbon-containing and pyrolizable component for the production of the paste is lauric acid, myristic acid, palmitic acid, stearic acid or a paraffin with a melting point of 40°C to 80°C. Method according to any of the preceding claims, wherein the conductive additive is a conductive carbon black and / or a carbon-based conductive material. Method according to one of the preceding claims, wherein the pyrolysis takes place under an oxygen-free atmosphere and at a temperature in the range of 200 °C to 1000 °C.

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