DE102020202504A1 - Anode material, battery with such an anode material and method for producing an anode material - Google Patents

Abstract

An anode material (8) is specified which has a number of silicon particles (10), each silicon particle (10) being surrounded by a first shell (12), the first shell (12) made of porous carbon (14) is, wherein the first shell (12) additionally has an electrically conductive material (18) which is embedded in the carbon (14). A battery (2) and a method for producing an anode material (8) are also specified.

Description

An anode material is used to manufacture an anode, which in turn is part of a battery. Batteries are used, for example, in an electric vehicle, where they provide electrical energy for an electric drive train. An example of a battery is a lithium-ion battery in which silicon or carbon, for example, is used as the anode material.

A battery has one or more cells, each cell has two electrodes, namely an anode and a cathode. The performance of the battery and each individual cell also depends in particular on the electrodes and the materials used. For example, it is possible to use a combination of silicon particles and carbon as the anode material. The use of silicon results in a particularly high energy density. The problem, however, is that when the battery is repeatedly charged and discharged, i.e. when the silicon is lithiated, the silicon particles repeatedly expand and shrink, i.e. experience a change in volume. This results in mechanical stress. The silicon and the carbon form an electrical network that is stressed and possibly damaged by the expansion and shrinkage, with a negative effect on the performance of the battery. The battery therefore has poor so-called cycle stability.

In Baasner et al., “Sulfur: an intermediate template for advanced silicon anode architectures”, Journal of Materials Chemistry A, 2018 it is described that nanostructured silicon-carbon compositions with free volume between a silicon core and a carbon shell can absorb the change in volume. It is also described that a free volume can be obtained by first applying sulfur and then sucrose to the silicon cores in order to then remove the sulfur at the same time during the polymerization of the sucrose.

In Xie et al., "Core-shell yolk-shell Si @ C @ Void @ C nanohybrids as advanced lithium ion battery anodes with good electronic conductivity and corrosion resistance", Journal of Power Sources, 2016 , it is described that a so-called yolk shell structure, in which silicon is arranged within a free volume in a carbon shell, has some disadvantages, namely a low electrical contact between silicon and carbon on the one hand and the risk of corrosion of the silicon by electrolytes from outside on the other. In contrast, a so-called core-shell structure, in which the carbon is applied directly to the silicon, has the disadvantages already described when there is a change in volume. A structure is therefore described in which the silicon of the yolk shell structure is passivated by an additional carbon shell.

In Yu-Guo et al., "Superior Electrode Performance of Nanostructured Mesoporous TiO2 (Anatase) through Efficient Hierarchical Mixed Conducting Networks", Advanced Materials, 2007, an anode material is described in which mesoporous TiO2 is additionally added RuO2. The mesoporous TiO2 has channels that are metallized with crystalline RuO2.

In the US 2017/0162868 A1 describes a method in which a thin metal layer is applied to silicon particles, by means of which the electrical conductivity on the surface of the silicon particles can be increased. Carbon is then applied again. By means of an etching process with hydrogen fluoride (HF), nanopores are then produced, which can absorb a change in volume.

In the US 2013/0122401 A1 a layer structure is described which has a substrate made of titanium, for example, as well as a RuO2 coating on at least part of the substrate and a plurality of platinum nanoparticles on the RuO2 coating. A fuel cell is also described which has an anode with the above-mentioned layer structure.

Against this background, it is an object to provide an improved anode material. The anode material should enable an anode with the highest possible electrical conductivity and a battery with the highest possible capacity. A corresponding battery and a method for producing the anode material are also to be specified.

The object is achieved according to the invention by an anode material with the features according to claim 1, by a battery with the features according to claim 8 and by a method with the features according to claim 9. Advantageous configurations, developments and variants are the subject of the subclaims. The statements in connection with the anode material also apply mutatis mutandis to the battery and to the method and vice versa.

The anode material is designed for use in an anode of a battery. The anode material has a number of silicon particles. “A number of” is understood to mean “one or more”; the anode material typically has a large number of silicon particles. The number of Silicon particles per gram of anode material depends in particular on the size of the individual silicon particles. In the case of silicon particles with an average diameter of 20 nm, for example, 1 g of anode material contains 10 ^ 17 silicon particles. A single silicon particle preferably has an average diameter in the range from 10 nm to 100 nm. The silicon particles consist in particular of pure silicon, ie Si, or of silicon oxide, especially silicon dioxide, ie SiO2, or a mixture of silicon and silicon oxide.

A respective silicon particle is surrounded, i.e. encased, by a first shell. This first shell is made of porous carbon. The first shell preferably has a thickness in the range from 50 nm to 200 nm, for example 100 nm. The carbon is preferably in the form of graphite, alternatively, for example, in the form of amorphous carbon, for example as so-called acetylene black, or graphene oxide. The carbon is porous insofar as the carbon forms a layer around the silicon particle which has several pores. A single pore generally forms a hole or recess in the carbon layer. For example, a pore is continuous and thus designed as a channel and then connects an outside of the shell to an inside of the shell, the silicon particle lying on the inside. Alternatively or additionally, non-continuous pores are also possible. Through the porous carbon, especially through its pores, an exchange of charge carriers is possible overall, in particular of Li ions. A respective pore has, in particular, a diameter in the range from 1 nm to 5 nm.

The first shell additionally has an electrically conductive material which is embedded in the carbon. The electrically conductive material is not carbon, but a different material. Because the carbon is porous, further functional materials can be embedded in it, in this case the electrically conductive material, which is arranged in particular within the pores, that is to say occupies free pores in the layer of carbon, and thus metallizes them. All pores do not necessarily have to be provided with the electrically conductive material. The pores are also expediently not completely closed by the electrically conductive material. For example, “cryogenic decomposition” or a sol-gel process are suitable for applying the electrically conductive material.

The carbon and the electrically conductive material of the first shell in particular form a mechanical and, at the same time, electrical network. The additional electrically conductive material results in a surface modification which leads to a significant improvement in the electrical and ionic conductivity in the anode material. The electrically conductive material is in this respect a surface-active substance and supports the charge transport in the anode material, especially the transport of Li ions to and from the respective silicon particle. In addition, the electrically conductive material also mechanically stabilizes the first shell in that the electrically conductive material is connected to the carbon. The anode material thus withstands a change in volume of the silicon particularly well. Overall, an anode material with a particularly large silicon content can also be implemented as a result, which results in a correspondingly increased capacitance for an anode with a corresponding anode material.

In principle, it is also possible with a combination of silicon and carbon to select the relative proportion of silicon to be particularly low in order to keep the volume change and the associated mechanical load on the network correspondingly low. The proportion of silicon can be limited especially with certain specifications regarding the cycle stability of the cell and the battery as a whole. However, this has the disadvantage that it again limits the capacity of the individual cells and thus the overall performance of the battery.

Alternatively, it is also possible to structure the anode in such a way that it has a geometric structure that absorbs the change in volume. However, the production of such an anode is complex and typically includes an etching process with hydrogen fluoride (HF), which is extremely problematic in handling. In addition, such production is slow and expensive.

In contrast, by embedding the electrically conductive material in the carbon, an improved anode material is implemented in a particularly simple manner, from which an anode with particularly good capacity, cycle stability and conductivity can be manufactured, with corresponding advantages for a battery which has one or more such anodes contains.

The electrically conductive material is preferably ruthenium oxide, particularly preferably ruthenium dioxide, ie RuO2. Ruthenium oxide is a metallic conductive oxide. Compared to other electrically conductive materials, ruthenium oxide has the particular advantage that it interacts particularly well with the silicon or silicon oxide of the silicon particles and therefore preferably occupies the pores in the carbon when it is applied to the carbon. In the case of silicon particles, ruthenium oxide can therefore be embedded particularly well in a carbon layer. Furthermore, ruthenium oxide is distinguished particularly good electrical properties and particularly good mechanical stability, especially in combination with the carbon. In addition, ruthenium oxide has particularly good conductivity, especially for Li ions.

In the following, it is assumed, without loss of generality, that the electrically conductive material is ruthenium oxide.

The electrically conductive material is preferably in the form of nanoparticles. Nanoparticles are understood to mean, in particular, particles with an average diameter in the range from 10 nm to 100 nm, for example 50 nm. Nanoparticles fit particularly well into the porous carbon and are therefore particularly suitable for embedding. The nanoparticles are expediently approximately as large as the pores.

The first shell is expediently applied directly to a respective silicon particle. In other words: a respective silicon particle has a surface on which the first shell is applied so that it is connected to the surface. The shell then forms a particularly effective protective layer around the silicon particle. Due to the electrically conductive material embedded in the carbon, particularly good conductivity, especially of Li ions, into the silicon particle is guaranteed. The high mechanical stability of the first shell also enables a repeated change in volume to a large extent, especially when the silicon is lithiated.

Alternatively, the first shell encloses a volume in which the silicon particle is arranged. The volume is larger than the silicon particle in the non-expanded state and expediently corresponds approximately to that volume which the silicon particle occupies in the maximally expanded state. In this way, the negative effects of a change in volume, in particular the mechanical load on the first shell, can be effectively avoided.

In a particularly advantageous embodiment, a respective silicon particle is additionally surrounded by a second shell made of porous carbon in such a way that a free space is formed between the first and the second shell, which space serves as an expansion volume for the silicon particle. Advantages result from the above explanations. Overall, the two covers now form a double shell, so to speak. In this case, the additional inner shell ensures, in particular, good electrical contact between the silicon particle and the outer shell. One, inner shell surrounds the silicon particle, the free space is formed around this arrangement and, finally, the other, outer shell is formed around it. The inner shell is expediently applied directly to the silicon particle. The free space is preferably dimensioned in such a way that it is completely filled by the silicon particle in the maximally expanded state and together with the inner shell. In the non-expanded state, the free space leads accordingly to a spacing of the first shell from the second shell.

Typically, a silicon particle shows a change in volume of about 300% on the transition from the non-expanded state to the maximally expanded state. The free space and the outer shell are preferably dimensioned accordingly.

The second shell preferably has a thickness in the range from 1 nm to 20 nm, particularly preferably from 5 nm to 10 nm. In principle, an embodiment is also suitable in which the first and the second shell have similar or identical thicknesses.

In the embodiment with first and second sheaths, it is initially irrelevant which of the two sheaths is on the inside and which is on the outside, i.e. in which case the electrically conductive material is additionally embedded. Both possible configurations are fundamentally advantageous. An embodiment is particularly useful in which an electrically conductive material is also embedded in the carbon in the second shell. The electrically conductive material in the second shell is preferably the same material as in the first shell, but this is not essential per se.

In a possible and suitable embodiment, a respective silicon particle is then preferably directly surrounded by a shell made of carbon with electrically conductive material embedded therein, so that a coated silicon particle is formed which is then surrounded by a further shell made of carbon with electrically conductive material embedded therein, wherein a free space is formed as an expansion volume between the two shells in the non-expanded state of the silicon particle. The silicon particle and the inner shell then form a core, so that a core-shell structure is implemented in combination with the second shell.

The shells do not necessarily have to be designed individually as discrete shells for each of the silicon particles. Rather, an embodiment is suitable in which, in the case of only one shell in each case, the first shells and, in the case of two shells, the outer shells are connected to one another and form a superordinate network, with chambers in each of which a silicon particle is then placed arranged. Expediently, only one silicon particle is arranged in each chamber and the chamber also forms a free space as an expansion volume. The higher-level network then ensures a particularly high mechanical stability.

A battery expediently has an anode which is produced from an anode material as described above. The battery is preferably designed for use in an electric or hybrid vehicle, especially a non-rail passenger vehicle with an electric drive train. The battery is preferably a Li-ion battery.

In the method for producing an anode material, a number of silicon particles are each surrounded by a first shell, and the first shell is initially produced from porous carbon. An electrically conductive material is then embedded in the carbon of the first shell. For example, what is known as “cryogenic decomposition” or a sol-gel process are suitable for this purpose.

As already mentioned above, a configuration with a double shell is particularly advantageous. For this purpose, in a suitable embodiment of the method, a respective silicon particle is additionally surrounded by a second shell made of porous carbon in such a way that a free space is formed between the first and second shell, which space serves as an expansion volume for the silicon particle. The free space is created by applying a sacrificial layer to that of the two shells that is further inside, i.e. the inner shell that is closer to the silicon particle. The other of the two casings is then applied to this sacrificial layer. The sacrificial layer is then removed, leaving the two shells with the space in between.

The sacrificial layer is preferably removed by means of a pyrolysis process. A pyrolysis process is significantly simpler, safer and more cost-effective than, for example, an etching process with hydrogen fluoride. In a suitable embodiment, the sacrificial layer is produced from sulfur, which is particularly suitable for removal by means of a pyrolysis process.

In a suitable embodiment, the outer shell is produced by first applying a sacrificial layer to the inner shell, onto which the second shell is then applied. The sacrificial layer is then removed. In a particularly preferred embodiment, the sacrificial layer is made from sulfur and removed by means of a pyrolysis process, e.g. at a temperature of 800 ° C.

• 1 eine Batterie,
• 2 ein Siliziumpartikel,
• 3 das Siliziumpartikel aus 2 mit einer ersten Hülle,
• 4 die Anordnung aus 3 mit einer zusätzlichen Opferschicht,
• 5 die Anordnung aus 4 mit einer zweiten Hülle,
• 6 ein Siliziumpartikel mit zwei Hüllen und in nicht-ausgedehntem Zustand,
• 7 die Anordnung aus 6 mit dem Siliziumpartikel in maximal ausgedehntem Zustand.

Exemplary embodiments of the invention are explained in more detail below with reference to a drawing. They each show schematically:

• 1 a battery,
• 2 a silicon particle,
• 3 the silicon particle from 2 with a first shell,
• 4th the arrangement 3 with an additional sacrificial layer,
• 5 the arrangement 4th with a second cover,
• 6th a silicon particle with two shells and in a non-expanded state,
• 7th the arrangement 6th with the silicon particle in a maximally expanded state.

1 shows an example of a battery 2 with two electrodes 4th , 6th . One of the two electrodes 4th , 6th is an anode 4th , which are made of an anode material 8th is made. An embodiment of such an anode material 8th is partially in the 6th , 7th shown, which show a single silicon particle in different states of expansion. An exemplary embodiment of a method for producing the anode material 8th is explained below using the 2 until 5 is explained. The figures each show sectional views.

The battery 2 in 1 is designed as an example for use in an electric or hybrid vehicle that is not explicitly shown, especially a non-rail passenger vehicle with an electric drive train, and is used there to supply the drive train with electrical energy. Here is the battery 2 a Li-ion battery.

The anode material 8th has a number of silicon particles 10 on, where “a number of” is basically understood to mean “one or more” and where the anode material typically has a large number of silicon particles 10 having. An example of a single silicon particle 10 is in 2 shown. A single silicon particle 10 has a mean diameter in the exemplary embodiment shown D1 in the range from 10 nm to 100 nm. The silicon particles 10 consist of pure silicon or silicon oxide, especially silicon dioxide, or a mixture of both.

A respective silicon particle 10 is from a first shell 12th surrounded, ie enveloped. This first shell 12th is made of porous carbon 14th produced and has a thickness here D2 in the range of 50 nm up to 200 nm. The carbon 14th is porous in that it has a layer around the silicon particle 10 around which forms several pores 16 has, whereby an exchange of charge carriers, in particular Li ions, is possible overall. A respective pore 16 has a diameter, for example D3 in the range from 5 nm to 10 nm.

The first shell 12th additionally has an electrically conductive material 18th on which in the carbon 14th is embedded and free pores 16 in the layer of carbon 14th occupied. All pores do not necessarily have to be 16 with the electrically conductive material 18th be provided. The carbon 14th and the electrically conductive material 18th the first shell 12th form a mechanical and electrical network at the same time. An example of a silicon particle 10 with a first shell 12th made of carbon 14th , in which electrically conductive material 18th is embedded is in 2 shown.

In the present case, the electrically conductive material is 18th Ruthenium oxide, especially ruthenium dioxide. This occurs with the silicon or silicon oxide of the silicon particles 10 interacting and occupied when applied to the carbon 14th prefers the pores 16 .

The electrically conductive material 18th is also present in the exemplary embodiments shown in the form of nanoparticles, here specifically with a mean diameter D4 in the range from 5 nm to 10 nm.

In the exemplary embodiments shown, the first shell is 12th directly to a respective silicon particle 10 applied. In an alternative, not shown, the first envelope encloses 12th on the other hand, a volume in which the silicon particle 10 is arranged, the volume being greater than the silicon particle 10 in a non-expanded state and wherein the volume further corresponds approximately to that volume which the silicon particle 10 occupies in a maximally expanded state.

In the present case, there are the two aforementioned configurations with one directly on the silicon particle 10 Applied shell on the one hand and a spaced-apart shell on the other hand combined by how specifically in the 6th and 7th a respective silicon particle can be seen 10 in addition, from a second shell 20th made of porous carbon 14th that is surrounded between the first shell 12th and the second shell 20th a free space 22nd is formed, which as an expansion volume for the silicon particle 10 serves. In total, the two shells form 12th , 20th a double shell. It does not matter which of the two covers 12th , 20th inside and which is outside, in which shell 12th so additionally the electrically conductive material 18th is embedded. In the embodiments shown is also in the carbon 14th in the second shell 20th an electrically conductive material 18th embedded, which is also the same material here 18th like in the first shell 12th is. However, this is not mandatory per se. Next has the second shell 20th a fat one here D5 in the range from 10 nm to 20 nm. In principle, an embodiment is also suitable in which the first and second sheaths have different thicknesses.

In each case it forms one of the two shells 12th , 20th an inner shell 24 and the other of the two covers 12th , 20th is then an outer shell 26th . The inner shell 24 surrounds the silicon particle 10 Around this arrangement is the free space 22nd formed and therefore finally the outer shell 26th . The free space 22nd is in the present case dimensioned in such a way that it is separated from the silicon particle 10 in a maximally expanded state as in 7th shown and together with the inner shell 24 is filled out in full. In a non-expanded state as in 6th shown is the lead 22nd corresponding to a spacing of the first shell 12th from the second shell 20th .

the 2 until 6th also show, in the order shown, an exemplary method for producing the anode material 6th . It is made of a number of silicon particles 10 as in 2 shown went out. The silicon particles 10 will be like in 3 each shown with a first cover 12th surrounded, which initially made of porous carbon 14th is produced, in which then an electrically conductive material 18th is embedded, here especially ruthenium oxide in the form of nanoparticles. For example, a so-called “cryogenic decomposition” or a sol-gel process are suitable for embedding.

For the formation of a double shell, a respective silicon particle is additionally covered by a second shell 20th made of porous carbon 14th surrounded, namely in such a way that between the first shell 12th and the second shell 20th a free space 22nd is formed, which as an expansion volume for the silicon particle 10 serves. Thereby the free space 22nd formed by placing on the inner shell 24 , which is also the first shell here 12th is, a sacrificial layer 28 for example from sulfur is applied. This is in 4th shown. On this sacrificial layer 28 is then as in 5 shown the outer shell 26th applied, which here is the second shell 20th is. The sacrificial layer 28 is then removed, for example by means of a pyrolysis process, so that the two shells 12th , 20th with the free space 22nd remain in between, as in the 6th and 7th shown.

Because of the space 22nd is now a change in volume of the silicon particle 10 with the surrounding inner shell 24 possible without the outer shell 26th excessively mechanical stress. This can be seen when comparing 6th , which is the silicon particle 10 shows in a non-expanded state, and 7th , which is the silicon particle 10 shows in a maximally expanded state.

The shells 12th , 20th do not necessarily have to be discrete covers 12th , 20th individually for each of the silicon particles 10 be trained. Rather, it is based on 7th in an embodiment not explicitly shown, the outer shell 26th with further outer covers 26th around other silicon particles 10 connected around. An embodiment not explicitly shown is also possible, in which starting from only one respective shell 12th with or without space 22nd the shells 12th several silicon particles 10 are connected to each other. In any case, if there are several silicon particles 10 then through the covers 12th , 26th a superordinate network is formed, with chambers, in each of which a silicon particle is then formed 10 arranged.

List of reference symbols

2

battery

4th

Anode, electrode

6th

electrode

8th

Anode material

10

Silicon particles

12th

first shell

14th

carbon

16

pore

18th

electrically conductive material

20th

second shell

22nd

free space

24

inner shell

26th

outer shell

28

Sacrificial layer

D1

Diameter (of the silicon particle)

D2

Thickness (of the first shell)

D3

Diameter (of a pore)

D4

Diameter (of a nanoparticle)

D5

Thickness (of the second shell)

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• US 2017/0162868 A1 [0006]
• US 2013/0122401 A1 [0007]

Non-patent literature cited

• Baasner et al., "Sulfur: an intermediate template for advanced silicon anode architectures", Journal of Materials Chemistry A, 2018 [0003]
• Xie et al., "Core-shell yolk-shell Si @ C @ Void @ C nanohybrids as advanced lithium ion battery anodes with good electronic conductivity and corrosion resistance", Journal of Power Sources, 2016 [0004]

Claims (10)

Anode material (8) which has a number of silicon particles (10), - wherein a respective silicon particle (10) is surrounded by a first shell (12), - wherein the first shell (12) is made of porous carbon (14), - The first shell (12) additionally having an electrically conductive material (18) which is embedded in the carbon (14). Anode material (8) Claim 1 wherein the electrically conductive material (18) is ruthenium oxide. Anode material (8) Claim 1 or 2 wherein the electrically conductive material (18) is in the form of nanoparticles. Anode material (8) according to one of the Claims 1 until 3 , wherein the first shell (12) is applied directly to a respective silicon particle (10). Anode material (8) according to one of the Claims 1 until 4th , wherein a respective silicon particle (10) is additionally surrounded by a second shell (20) made of porous carbon (14) in such a way that a free space (22) is formed between the first shell (12) and the second shell (20), which serves as an expansion volume for the silicon particle (10). Anode material (8) Claim 5 wherein an electrically conductive material (18) is also embedded in the carbon (14) in the second shell (20), which is the same material (18) as in the first shell (12). Anode material (8) according to one of the Claims 1 until 6th , wherein a respective silicon particle (10) has an average diameter (D1) in the range from 10 nm to 100 nm. Battery (2) which has an anode (4) made of an anode material (8) according to one of the Claims 1 until 7th is made. Method for producing an anode material (8), - wherein a number of silicon particles (10) are each surrounded by a first shell (12), - First, the first shell (12) is made of porous carbon (14), - An electrically conductive material (18) is then embedded in the carbon (14) of the first shell (12). Procedure according to Claim 9 - wherein a respective silicon particle (10) is additionally surrounded by a second shell (20) made of porous carbon (14) in such a way that a free space (22) is formed between the first shell (12) and the second shell (20), which serves as the expansion volume for the silicon particle (10), - the free space (22) being formed by applying a sacrificial layer (28) to that of the two shells (12, 20) which is further inside, onto which the the other of the two shells (12, 20) is applied, and by then removing the sacrificial layer (28) by means of a pyrolysis process.

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