Anode material, battery, and method for producing the anode material

Abstract

An anode material (2) is provided having several particles (10), each particle (10) having a core (12) and a shell (14) surrounding the core (12), the shell (14) containing silicon, carbon and nitrogen. Additionally, a battery (6) and a method for making the anode material (2) are provided.

Description

[Technical Field]

[0001] The present invention relates to anode materials, batteries having anodes made from such anode materials, and methods for making the anode materials. [Background technology]

[0002] Anode material is used for manufacturing the anode that is part of the battery.Battery is used for example in electric vehicles, in this case to supply electric energy to the electric drive train.An example of the battery is a lithium ion battery, in which silicon or carbon is used as the anode material.

[0003] A battery has one or more cells, each of which has two electrodes: an anode and a cathode. The performance of the battery and each individual cell depends, among other things, on the electrodes and the materials used. For example, a combination of silicon particles and carbon can be used as the anode material. The use of silicon results in particularly high energy density. However, the use of pure silicon as an anode material also has various drawbacks. For example, silicon has poor electrical conductivity and is therefore electrically insulating. Furthermore, silicon undergoes a dramatic volume change upon lithiation, making it susceptible to the permanent capture of lithium (so-called "lithium trapping") and the unstable and continuous formation of a solid electrolyte interphase (SEI, or "solid electrolyte interphase") on the silicon surface. Pure silicon is also at risk from mechanical decomposition (e.g., breaking or crushing).

[0004] WO2020 / 260332 A1 describes an anode having particles with a silicon-based core surrounded by two carbon coatings with different densities.

[0005] US Patent Application Publication No. 2010 / 0310941 (US2010 / 0310941 A1) describes a method for producing an anode material, in which carbon nanotubes (abbreviated CNTs, "carbon nanotubes") are first produced, which are subsequently coated with silicon. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2020 / 260332 [Patent Document 2] US Patent Application Publication No. 2010 / 0310941 Summary of the Invention [Problem to be solved by the invention]

[0007] Against this background, the object of the present invention is to provide an improved anode material, in which one or more of the disadvantages mentioned at the beginning are to be avoided or at least reduced as much as possible.Furthermore, a battery having the corresponding anode material, as well as a method for producing the anode material, are to be provided. [Means for solving the problem]

[0008] According to the invention, this problem is solved by an anode material having the features of claim 1, by a battery having the features of claim 9, and by a method having the features of claim 10. Advantageous embodiments, further configurations and variants are the subject of the dependent claims. Statements made regarding the anode material apply correspondingly to the battery and to the method, and vice versa.

[0009] The anode material according to the invention is adapted for use in the anode of a battery. The battery is preferably a lithium-ion battery, and hereinafter, without limiting generality, it is assumed that the battery is a lithium-ion battery. The anode material comprises several particles. "Several" is understood to mean "one or more," and typically the anode material comprises a large number of particles. The number of particles per gram of anode material depends, inter alia, on the size of the individual particles. The individual particles preferably have an average diameter of 50 nm to 50 μm, particularly preferably 1 μm to 20 μm.

[0010] Each particle has a core and a shell that directly surrounds the core, i.e., without any intervening space or layers. The core itself is preferably solid, i.e., not hollow. The shell contains silicon, carbon, and nitrogen.

[0011] This preferably means that the shell consists only of silicon, carbon, nitrogen, and / or their compounds, and therefore does not contain any other substances, materials, and / or elements. Apart from the above elements silicon, nitrogen, and carbon, the shell preferably contains no further elements. Therefore, the shell is also referred to as a SiCxNy layer, i.e., a layer of a combination of silicon, carbon, and nitrogen, where x and y indicate the number of carbon or nitrogen particles relative to silicon. In a preferred embodiment, the particle number x = 0.515 and the particle number y = 0.046. This corresponds to a composition of Si:C:N = 64 atomic %: 33 atomic %: 3 atomic %. Thus, the carbon content is usually significantly higher (i.e., at least 5 times) than the nitrogen content, with silicon typically accounting for the largest proportion overall. The above composition does not necessarily have to be adhered to, and a more suitable particle number can be achieved, particularly by varying the above particle number by 0.5 to 2 times.

[0012] The silicon of the shell serves here in particular as an active material that absorbs and releases lithium ions during charging and discharging (this is called lithiation and delithiation), so that the anode material is a silicon-based anode material.

[0013] SiC x N y The above-mentioned anode materials based on have a particularly high energy density, in particular above 1000 mAh / g, which allows particularly rapid charging and discharging of batteries containing such anode materials.

[0014] Preferably, the silicon in the shell (more precisely at least a first portion of the silicon in the shell) forms a multiplicity of nanoparticles, in particular nanoparticles each consisting solely of silicon and therefore also referred to as silicon nanoparticles.

[0015] "Nanoparticles" are understood in particular to mean particles having a size of about 1 nm to several hundred nm, in any case with spatial dimensions (i.e., length, width or diameter) of less than 1 μm. Here, nanoparticles preferably have an average diameter in the range of 0.5 nm to 5 nm, particularly preferably 1 nm to 2 nm, and are therefore relatively small nanoparticles. Here, the dimensions of the particles in each of the three spatial directions are not necessarily considered to be identical, but may differ from one another, for example, by up to an order of magnitude.

[0016] It is emphasized that the above nanoparticles are different from the aforementioned particles having a core and a shell. That is, the nanoparticles are part of the shell, and as a result, the particles of the anode material are typically one or more orders of magnitude larger than the nanoparticles, and the shell contains a large number of nanoparticles. In a suitable embodiment, the shell has a thickness in the range of 100 nm to 200 nm, with an average thickness of 150 nm being particularly preferred.

[0017] Preferably, the silicon in the shell (or more precisely, the second portion of the silicon in the shell) forms an inert matrix together with carbon and nitrogen, in which the nanoparticles are embedded. The nanoparticles are active materials, i.e., they contain silicon that undergoes lithiation and delithiation during cycling. In contrast, the inert matrix is ​​an inactive material that does not undergo corresponding lithiation and delithiation during cycling.

[0018] The present invention is based on the observation that, fundamentally, increasing the energy density of batteries while reducing their installation space is essential. This is particularly relevant for all types of electric vehicles. For lithium-ion batteries, carbon or silicon can be used as the anode material. However, graphite (one form of carbon), for example, has a low energy density, with a theoretical maximum of only 372 mAh / g. In contrast, silicon has an energy density that is an order of magnitude higher, theoretically reaching a maximum of 3579 mAh / g. Therefore, the use of silicon is fundamentally advantageous. However, silicon also has several drawbacks, particularly large volume changes during lithiation and delithiation (i.e., during charging and discharging, and also during cycling), as well as low electrical conductivity. This leads to further problems, particularly the irreversible capture of lithium (so-called "lithium trapping"), mechanical decomposition of the anode material in the form of fracture or shattering, electrical insulation, and the unstable and continuous formation of a solid electrolyte interphase (so-called SEI, "solid electrolyte interphase"). Overall, this results in poor electrochemical properties, particularly large and irreversible capacity loss during the first cycle (i.e., charge-discharge cycle) and during long cycles, increased internal electrical resistance, poor cycling efficiency, and a dramatic decrease in capacity with increasing cycle number.

[0019] In order to reduce the drawbacks of using silicon, various solutions are conceivable and advantageous. In suitable embodiments, the extent or size of silicon is adapted, particularly reduced. For example, as already described above as advantageous, silicon is used in the form of nanoparticles to reduce mechanical loads and their effects (breakage and crushing). Furthermore, in this way, faster diffusion of lithium ions and faster electron transfer are achieved. Generally, it is also advantageous to coat the silicon of the active material with a material having as high a conductivity as possible, thereby correspondingly improving the overall conductivity, thereby reducing side reactions on the silicon surface (of the active material in general) and further reducing the risk of mechanical damage. Furthermore, it is particularly advantageous to dilute silicon, which is an active material for lithium, with an inactive material for lithium. In this way, the volume change during lithiation and delithiation is reduced, because the inactive material does not contribute accordingly. Diluting active silicon with an inactive material may also overall improve the conductivity of the anode material. Special void design (or "void engineering"), i.e., the intentional creation of cavities in the anode material, is further advantageous, which accommodates volume changes and also limits the buildup of the solid electrolyte interfacial phase. Similar to the combinations of active and inactive materials already described, it is also advantageous to use special compounds containing carbon, for example in the form of graphite, as the active material. Such anode materials are usually particularly well calendered, so that high electrode densities can be achieved.

[0020] Among the above solutions, coating of active materials, combination of active and inactive materials, and the use of special compounds are considered to be particularly promising and advantageous. This is especially true because the use of silicon nanoparticles alone and the void design disadvantageously leads to a reduced energy density, and the corresponding battery requires more installation space for the same energy density. Furthermore, the void design leads to a large irreversible capacity loss during the first cycle. This is due to the fact that silicon oxide (i.e., SiO xAlthough this problem can be avoided to some extent by using silicon oxide (where x represents the number of oxygen particles relative to silicon), silicon oxide has further drawbacks, as will be described further below. For example, it is advantageous to embed silicon nanoparticles in an inert matrix made of silicon oxide to at least partially suppress the volume change during charging and discharging. Alternatively or additionally, the inert matrix contains carbon, i.e., graphite. The use of a compound made of silicon and carbon is also advantageous accordingly.

[0021] Although the inert matrix (i.e., the inert material in which the active material is embedded) is fundamentally advantageous as part of the anode material, it also has drawbacks in terms of electrochemical properties. For example, when using silicon oxide, Li2O and Li4SiO4 can be formed in side reactions, which leads to a decrease in Coulombic efficiency. This process is also irreversible. In the case of silicon carbide, silicon nanoparticles are produced, for example, by mechanical grinding, which usually results in the formation of silicon oxide on the surface of the silicon nanoparticles, resulting in the above-mentioned drawbacks. Furthermore, the inert matrix made of carbon becomes brittle by grinding and has low mechanical strength, meaning it is easily destroyed by repeated charge / discharge cycles.

[0022] A further solution is to prelithiate the anode material before fabricating the battery to compensate for the loss of lithium on the first cycle, but the use of lithium is generally problematic and dangerous, which only becomes more adversely severe with increasing amounts.

[0023] Here, silicon nanoparticles are replaced with silicon carbide (SiC x ) or silicon nitride (SiN x It has been observed that embedding the silane in ethylene (for silicon carbide) or ammonia (for SiN) is particularly advantageous and results in a particularly suitable anode material. Such anode materials can be prepared in a suitable embodiment by chemical vapor deposition, thus advantageously by dissolving monosilane in ethylene (for silicon carbide) or ammonia (for SiN) xThe anode materials are produced by the pyrolysis of a gas mixture containing silicon carbide in combination with silicon dioxide (for example, SiO 2 ). Such anode materials advantageously have a reduced amount of irreversible reaction products and a higher reversible capacity. Anode materials having silicon carbide as an inert matrix are further characterized by particularly high mechanical strength, especially compared to simple carbon, and also have high electrical conductivity. In contrast, anode materials having silicon nitride as an inert matrix are characterized by particularly high ionic conductivity, especially compared to silicon oxide or pure carbon, and also have improved electrochemical behavior.

[0024] Therefore, carbon and nitrogen are combined with silicon to form an anode material, thereby combining the above-mentioned advantages of using both of these elements together with silicon.Here, carbon and nitrogen are particularly involved in forming an inert matrix, into which at least a portion of silicon is embedded as an active material, advantageously as nanoparticles.In this regard, in a preferred embodiment, carbon, nitrogen and silicon particularly form various compounds, which then together form an inert matrix.Preferably, the inert matrix is ​​formed from a number of regions made of nitrogen-doped carbon (i.e., carbon forms a lattice with nitrogen interspersed therein), silicon nitride (SiN), silicon carbide (SiC), and / or silicon carbonitride (SiCN), particularly preferably made from all of these compounds.SiC x N y Anode materials with such inert matrices based on, for example, SiN x Only, SiC x Experiments have confirmed that the half-cell and full-cell configurations have improved cycle stability compared to the use of SiC alone. x N y Anode materials based on these have improved electrochemical properties relative to the other compositions mentioned above.

[0025] In addition to the shell already described in detail, the particles of the anode material also have a core. Its nature is not particularly important here, but an embodiment in which the core consists solely of carbon is reasonable. An embodiment in which the core is solid is also advantageous. Carbon generally forms, in particular, a framework for the shell, supporting and / or stabilizing it. Therefore, a carbon core, whether solid or not, is also referred to as a carbon-containing framework. The core preferably has an average diameter one to two orders of magnitude greater than the thickness of the shell. In a suitable embodiment, the core has an average diameter of 500 nm to 50 μm, with average diameters of 1 μm to 10 μm, especially 5 μm, being particularly suitable.

[0026] In a preferred embodiment, each particle has a carbon cover, in which the core and the shell are enclosed.Therefore, the core is, so to speak, double-coated, where the shell forms the first inner layer and the carbon cover forms the second additional layer.The cover is preferably directly connected to the shell.The carbon cover particularly serves as a filler material between various shells (and the cores surrounded by them), separating them from each other.Optimally, the particle covers grow together and / or bond to form a network in which the cores with shells are embedded, similar to individual particles in which nanoparticles are embedded in an inert matrix.

[0027] Various embodiments of the core and shell of each form are conceivable and suitable. Advantageously, the shell follows the outer contour of the core and has essentially the same layer thickness at each position (i.e., within a tolerance of less than 5%). In this way, the shell forms a homogeneous shell for the core. Preferred are embodiments in which the core and shell of each particle are formed together in the shape of a plate or rod. However, it is also basically suitable for the core and shell to be spherical.

[0028] The battery according to the invention has an anode made from an anode material as described above. The battery is preferably a lithium-ion battery. The battery preferably serves to supply an electric drive for a vehicle.

[0029] The present invention relates to a method for producing an anode material, particularly an anode material as described above. The anode material comprises several particles, each of which has a core and a shell surrounding the core. The shell is produced by vapor deposition, particularly chemical vapor deposition (CVD), from a silicon-containing gas, a carbon-containing gas, and a nitrogen-containing gas. The silicon-containing gas is preferably monosilane (SiH4). The carbon-containing gas is preferably ethene (C2H4). The nitrogen-containing gas is preferably ammonia (NH3). Suitably, all three gases are used simultaneously, i.e., in a common process step, and in particular not in separate, distinct process steps. The relative volumetric flow rates of the three gases and the respective amounts used can precisely adjust the exact composition and morphology of the anode material, particularly the particles. Further parameters affecting the production and the resulting anode material are the temperature and time during production, as well as the size and morphology of the anode material core. Optimal parameters may vary depending on the application of the anode material. Reasonably, the respective optimum parameters for manufacturing are determined by experiment.

[0030] The present invention will be described in more detail below with reference to the accompanying drawings. [Brief explanation of the drawings]

[0031] [Figure 1] Figure 1 shows a cutout of the anode material. [Figure 2] Figure 2 shows the battery. [Figure 3] FIG. 3 shows a portion of the shell of a particle of the anode material from FIG. [Figure 4] Figure 4 shows the voltage profiles of the three half-cells. [Figure 5] FIG. 5 shows the specific capacity and coulombic efficiency of the voltage profiles of the three half-cells from FIG. [Figure 6] FIG. 6 shows the storage capacity ratios of the three half-cells from FIG. [Figure 7] Figure 7 shows the storage capacity ratios for the three full cells. [Figure 8] FIG. 8 illustrates chemical vapor deposition for producing the anode material from FIG. [Example]

[0032] FIG. 1 shows a cutout of an anode material 2 configured for use in an anode 4 of a battery 6, such as that shown in FIG. 2. The battery 6 further comprises a cathode 8, here illustratively a lithium-ion battery. The anode material 2 comprises several particles 10. "Several" is understood to mean "one or more," and typically the anode material 2 comprises a large number of particles 10. The individual particles 10, in the example shown, have an average diameter of 50 nm to 50 μm, which is obtained as an average from different diameters D1.

[0033] As can be seen from Figure 1, each particle 10 has a core 12 and a shell 14 immediately surrounding the core 12. The shell 14 contains silicon, carbon, and nitrogen, and here consists exclusively of silicon, carbon, nitrogen, and / or compounds thereof. Besides the above elements silicon, nitrogen, and carbon, the shell 14 advantageously has no further elements. Thus, the shell 14 is preferably SiC x N y It is also referred to as a layer, i.e. a layer made of a combination of silicon with carbon and nitrogen.

[0034] The silicon of the shell 14 serves here as the active material, which absorbs and releases lithium ions accordingly during charging and discharging (this is called lithiation and delithiation), and the anode material 2 is therefore a silicon-based anode material 2.

[0035] In the illustrated embodiment, the silicon within the shell 14 forms a number of nanoparticles 16. This can be seen in Figure 3, which shows an enlarged view of a cutout from Figure 1, so that the shell 14 of the particle 10 can be seen in detail. The nanoparticles 16 each consist solely of silicon and are therefore also referred to as silicon nanoparticles. Here, the nanoparticles 16 have an average diameter in the range of 1 nm to 2 nm, which is taken as an average from the diameter D2, which may vary.

[0036] The nanoparticles 16 are part of the shell 14, so that the particles 10 of the anode material 2 are typically one or more orders of magnitude larger than the nanoparticles 16. The shell 14 has a thickness D3 in the range of, for example, 100 nm to 200 nm.

[0037] The silicon in shell 14, together with carbon and nitrogen, forms an inert matrix 18 in which nanoparticles 16 are embedded. Nanoparticles 16 are active material, i.e., they contain silicon that is lithiated and delithiated during cycling. In contrast, inert matrix 18 is an inactive material that is not correspondingly lithiated and delithiated during cycling.

[0038] As previously indicated, carbon and nitrogen are combined with silicon to form anode material 2, where they participate in the formation of inert matrix 18, into which at least a portion of the silicon is embedded as active material, here as nanoparticles 16. In this regard, in the illustrated example, carbon, nitrogen, and silicon form various compounds that then together form inert matrix 18, where inert matrix 18 is formed from multiple regions made of nitrogen-doped carbon 20, silicon nitride 22, silicon carbide 24, and silicon carbonitride 26. In FIG. 3, for clarity, no explicit distinction is made between nitrogen-doped carbon 20 and silicon carbide 24, and regions containing carbon and / or carbon-containing compounds other than SiCN are combined for brevity.

[0039] Figures 4 to 7 show the SiC x N yThe anode material 2 has an inert matrix 18 based on SiN x Only, SiC x The experimental results show that both the half-cell configuration (see Figures 4-6) and the full-cell configuration (see Figure 7) have improved cycle stability compared to the use of only the SiN x , SiC x and SiC x N y Figure 5 shows the voltage profile of the anode manufactured by Electrolyte ... For example, the charge current is 0.2 C (C = A / Ah) for the first cycle, 0.5 C for the 2nd to 4th cycles, 1 C for the 5th to 7th cycles, 2 C for the 8th to 10th cycles, and 3 C for the 11th to 13th cycles. Finally, Figure 7 also shows the storage capacity ratio 36 as a function of cycle number 34, but for a full-cell configuration, i.e., a SiN x , SiC x and SiC x N y Three full cells with anodes made of ZnO are shown.

[0040] In addition to the shell 14 already described in detail, the particles 10 of the anode material 2 also have a core 12. In the example shown, the core 12 consists solely of carbon, which generally forms a framework for the shell 14 and holds and / or stabilizes it. The core 12 has, for example, an average diameter that is one to two orders of magnitude larger than the thickness D3 of the shell 14, which is given as the average of the different diameters D4.

[0041] Furthermore, in the embodiment shown, the particles 10 each have a carbon covering 38 within which the core 12 and shell 14 are encapsulated. The core 12 is thus doubly coated, with the shell 14 forming a first, inner layer and the carbon covering 38 forming a second, additional layer. The covering 38 is directly connected to the shell 14. The covering 38 also serves as a filler material between the various shells 14, spacing them apart, as best seen in FIG. 1 . The coverings 38 of the particles 10 now grow together and / or bond to form a network in which the cores 12 with their shells 14 are embedded, as well as individual particles 10 with nanoparticles 16 embedded in an inert matrix 18.

[0042] Various embodiments of the core and shell of each form are possible. Here, the shell 14 follows the outer contour of the core 12 and has essentially the same layer thickness D3 at each position. In this way, the shell 14 forms a homogeneous shell for the core 12. Furthermore, in the examples shown here, the core 12 and shell 14 of each particle 10 are formed together in the shape of a plate or rod. However, it is also possible that the core 12 and shell 14 are spherical, which is not shown here.

[0043] An example of a method for producing an anode material 2 is shown in Figure 8. Here, the shell 14 is produced in a chamber 40 using chemical vapor deposition (CVD) from a silicon-containing gas 42, a carbon-containing gas 44, and a nitrogen-containing gas 46. The silicon-containing gas 42 is here monosilane (SiH4). The carbon-containing gas 44 is here ethene (C2H4). The nitrogen-containing gas 46 is here ammonia (NH3). All three gases are used simultaneously, i.e., in a common method step, and not in separate, distinct method steps. [Explanation of symbols]

[0044] 2. Anode material 4 anodes 6 batteries 8 cathode 10 particles 12 cores 14 shells 16 Nanoparticles 18 Inert Matrix 20 Nitrogen-doped carbon 22 Silicon nitride 24 Silicon carbide 26 Silicon carbonitride 28 Voltage 30 Specific capacity 32 Coulombic efficiency 34 cycles 36 Storage capacity ratio 38 Cover 40 Chamber 42 Silicon-containing gases 44 Carbon-containing gases 46 Nitrogen-containing gases D1 diameter (particle) D2 diameter (nanoparticles) D3 Layer thickness (shell) D4 diameter (core)

Claims

1. An anode material (2) having several particles (10), a. each particle (10) has a core (12) and a shell (14) surrounding said core (12); b. the shell (14) comprises silicon, carbon, and nitrogen; The silicon in the shell (14) forms a multitude of nanoparticles (16); The silicon in the shell (14) forms, together with carbon and nitrogen, an inert matrix (18) in which the nanoparticles (16) are embedded. The anode material (2).

2. An anode material (2) as described in claim 1, wherein the nanoparticles (16) each consist solely of silicon.

3. The anode material (2) of claim 1, wherein the nanoparticles (16) have an average diameter in the range of 0.5 nm to 5 nm.

4. 2. The anode material (2) of claim 1, wherein the inert matrix (18) is formed from multiple regions of nitrogen-doped carbon (20), silicon nitride (22), silicon carbide (24), and silicon carbonitride (26).

5. 2. The anode material (2) according to claim 1, wherein the core (12) consists solely of carbon.

6. 2. The anode material (2) of claim 1, wherein each of said particles (10) has a carbon covering (38) encapsulating said core (12) and said shell (14).

7. 2. The anode material (2) according to claim 1, wherein the core (12) and shell (14) of each particle (10) are formed together in the shape of a plate or rod.

8. A battery (6) having an anode (4) made from the anode material (2) according to claim 1.

9. A method for producing an anode material (2) having several particles (10), comprising the steps of: a. each particle (10) has a core (12) and a shell (14) surrounding said core (12); b. the shell (14) is fabricated using vapor deposition from a silicon-containing gas (42), a carbon-containing gas (44), and a nitrogen-containing gas (46); The silicon in the shell (14) forms a multitude of nanoparticles (16); The silicon in the shell (14) forms, together with carbon and nitrogen, an inert matrix (18) in which the nanoparticles (16) are embedded. The method.

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