NO20210855A1

NO20210855A1 - Composite anode material from silicon kerf and method for production thereof

Info

Publication number: NO20210855A1
Application number: NO20210855A
Authority: NO
Inventors: Anne-Karin Søiland; Bridget Catherine Deveney; Stian Madshus
Original assignee: Vianode AS
Priority date: 2021-07-02
Filing date: 2021-07-02
Publication date: 2023-01-03

Description

Field of the invention

The present invention relates to a composite anode material from silicon kerf, a method for production thereof, and use thereof in lithium-ion batteries.

Background of the invention

Silicon has been intensively studied in the past decades in the purpose of increasing the capacity of the conventional anode material graphite for Li-ion batteries. In the later years, the focus has turned to composite materials as a solution to integrate silicon into the anode, i.e. the negative electrode, since it can provide a matrix that can alleviate the volume changes of silicon upon charging and discharging. This will significantly increase the capacity retention upon cycling. Here reference is made to e.g.: Chae, S-H. Choi, N. Kim, J. Sung and J. Cho, "Integration of Graphite and Silicon Anodes for the Commercialization of High-Energy Lithium-Ion Batteries", Angew. Chem. Int. Ed., 59, 2020, pages 110-135; and I. Wu, Y. Cao, H. Zhao, J. Mao, Z. Guo, "The critical role of carbon in marrying silicon and graphite anodes for high-energy lithium-ion batteries", Carbon Energy, 2019;1, pages 57-76. Most of the concepts involves a graphite matrix and carbonaceous coating of the composite where pitch and sugars are frequently used.

A large industrial by-product, for instance of from the photovoltaic (PV) industry, is "kerf" which is the small silicon particles (chips) formed during the wire cutting of blocks (both multi crystalline and mono crystalline) into wafers. More than 40% of the silicon from the blocks is transformed into kerf during the wire cutting process, which constituted about 200000 tons in 2019. The silicon itself has a high purity with a size around 1 µm and can therefore be used directly into a composite material or be milled down to smaller size before utilization. In terms of circularity and CO2-footprint, the use of kerf into Li-ion batteries would be very beneficial.

In the wire cutting process of the blocks, a cutting fluid (cooling lubricant) additive is used. This additive is normally composed of several different components, which are mainly long chained hydrocarbons such as mineral oil and paraffins. The kerf is often collected as a filter cake with high moisture contents (up to 40%). After drying of the filter cake, the kerf still contains remnants of the cutting fluid around the particles.

CN111326723A discloses a silicon-carbon composite negative electrode material for a lithium ion battery using silica mud as the source of silicon. In CN111326723A, the remnants of the cutting fluid have been removed by acid washing and a non-ionic surfactant is subsequently added to create an amorphous carbon coating layer upon heating at 650-1000 <o>C.

WO2020140602A1 discloses preparation of porous silicon material from purified silicon waste from diamond cutting for use in a battery negative electrode material. In WO2020140602A1, the silicon is cleaned and made porous before further processing by use of a salt solution and hydrofluoric acid. Carbon coating of the silicon particles is obtained by adding a carbon material to form a slurry and heat treatment at 700-1200 <o>C in protective atmosphere.

Similar for CN111326723A and WO2020140602A1 are the need of cleaning steps of the kerf and addition of a carbonaceous substance to the kerf, and then heat treatment to form an amorphous carbon layer.

It is desirable to avoid cleaning steps, as they are time consuming and costly, as well as involving use of chemicals that may be hazardous and not environmentally friendly.

Poor cycling performance remains a major challenge when using silicon, either alone or in a composite with graphite, as anode materials for Li-ion batteries.

Therefore, there is a desire to find a material that has good reversible capacity as well as good cycling performance produced in an appropriate and efficient way.

Now, the inventors have surprisingly found that by creating a carbon coating on kerf particles with a specific heat treatment step and without cleaning and addition of external carbonaceous substance to the kerf, a silicon-graphite composite material where capacity and cycling properties are significantly improved can be obtained.

On this basis, the present invention has been provided.

Summary of the invention

It is a main object of the present invention to provide a new composite anode material from silicon kerf for use in lithium-ion batteries showing high cycle efficiency and reversible capacity.

Another object of the present invention is to provide a method for producing such a composite anode material from silicon kerf utilising the carbon source which is already there due to the cutting fluid to give the protective coating layer around the particles, a process which gives circularity benefits and cost efficiency.

Still another object of the present invention is to provide a method for producing such a composite anode material from silicon kerf without cleaning steps to remove the cutting fluid avoiding use of hazardous and environmentally unfriendly chemicals.

These and other objects are obtained by subject-matter as defined in the accompanying claims.

Definitions

It is to be understood that the herein disclosed invention is not limited to the particular component parts of the material described or steps of the methods described since such material and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only and is not intended to be limiting.

It should be noted that, as used in the specification and the appended claim, the articles "a", "an", "the", and "said" are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several means, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

The terms “lithium-ion batteries”, “Li-ion batteries” and “LIBs” as used herein have the same meaning and are used interchangeably in the present disclosure.

The terms “silicon kerf”, “kerf”, “kerf particles” and “particles” are used interchangeably in the present disclosure for the small silicon particles (chips) formed during the wire cutting of blocks into wafers.

The terms “cutting fluid additive” and “cutting fluid” are used interchangeably herein and mean cooling lubricant or coolant that can include one or more of the following components:

Mineral oil, severely hydrotreated, naphthenic

Chlorinated paraffins

Emulsifier based on fatty acids with alkanoamines

1-Phenoxypropan-2-ol

Alkoxylated ester of polymerized fatty acids

For example, Blasocut® from Blaser Swisslube is a mineral oil based cutting fluid in use for the photovoltaic industry.

The term “d50” as used herein means median diameter or medium value of the particle size distribution.

Brief description of the figures

Figure 1: Shows results from Raman spectroscopy on samples of kerf not subjected to heat treatment (“kerf as_is”) and kerf subjected to heat treatment (“kerf heat_treated”), respectively.

Figure 2: Shows results from Raman spectroscopy on the sample of kerf subjected to heat treatment (“kerf heat_treated”), where the region with amorphous carbon signatures is enlarged.

Figure 3: Shows a block diagram of the processing route for preparing composite material comprising silicon kerf not subjected to heat treatment (“kerf as_is”), i.e. for comparison.

Figure 4: Shows a block diagram of one processing route for preparation of composite material according to the invention comprising heat treatment of silicon kerf (“kerf heat_treated”); Alternative a.

Figure 5: Shows a block diagram of another processing route for preparation of composite material according to the invention comprising heat treatment of silicon kerf (“kerf heat_treated”); Alternative b.

Figure 6: Shows initial capacities in the silicon graphite composite materials according to the invention as a function of heat treatment temperature of the kerf. A silicon graphite composite material with kerf not subjected to heat treatment (“kerf as_is”) is also shown for comparison.

Figure 7: Shows first cycle efficiencies of the silicon graphite composite materials according to the invention as a function of heat treatment temperature of the kerf particles. A silicon graphite composite material with kerf not subjected to heat treatment (“kerf as_is”) is also shown for comparison.

Figure 8: Shows cycling data for half cells with composites made with “kerf as_is” (comparison) and “kerf heat_treated” as anode material, cycled with cut-off voltages at 5mV and 50 mV.

Detailed description of the invention

The concept of the present invention is to utilize the hydrocarbons surrounding the kerf particles to create a carbon coating around them. This is made by heating the kerf particles at high temperature under inert conditions, which decomposes the hydrocarbons on the surface of the particles, leaving a carbon coating (carbon residue) on the surface of the kerf particles. In this way, both the mechanical and chemical integrity of the silicon are preserved, and the electronic conductivity is enhanced. This helps to improve the cycling performance and reversible capacity in silicon-graphite composite, used as anode materials for Li-ion batteries.

According to the present invention, no efforts are made to remove the cutting fluid. Instead, the cutting fluid is utilized as a carbon source to create an amorphous carbon coating surrounding the particles. Since the cutting fluid is well attached to the particles, removal of it would necessitate e.g. careful cleaning. The inventors have found that the cutting fluid can serve as a very efficient source for carbon coating as it is already well spread out on the particles surfaces. The process to achieve this coating layer is by decomposing the hydrocarbons with a heat treatment of the kerf under an inert gas atmosphere such as N2 or Ar. Thus, this concept utilizes the carbon source which is already there to give the protective coating layer around the particles.

The kerf particles used as starting material in the present invention are generated during diamond wire cutting/wafering of silicon ingots (Cz- or multicrystalline). When the silicon ingots are cut into blocks and wafers with a diamond wire cutting process a cutting fluid is used in small amounts, often in a content around a few percentages. The kerf particles are after the cutting process extracted from the slurry by filtering techniques utilized by e.g. the photovoltaic (PV) industry, which gives the kerf in the form of a wet filter cake. After drying of the filter cake to remove water, it is broken apart in a high frequency mixer or another suitable equipment to separate the individual kerf particles, and optionally in a milling process to mill them further down in size. There is no cleaning step of the kerf particles, meaning that the remnants of the cutting fluid are still there as a film surrounding the particles.

The content of the cutting fluid in the kerf without any cleaning step is usually, in terms of measured carbon contents in weight% (with LECO analyses), in the range of 1 to 10 wt%. This invention utilizes the cutting fluid as a carbon source to create an amorphous carbon coating surrounding the particles, which will protect the silicon against degradation reactions with the electrolyte in LIBs, thereby creating a more controlled solid electrolyte interface (SEI). The process to achieve this coating layer is by decomposing the hydrocarbons with a heat treatment of the kerf in the range of 200-1100 <o>C, preferably 400-800 <o>C, under an inert gas atmosphere, e.g. N2 or Ar.

Resulting analyses have shown that the remaining amorphous carbon content is typically between 0.8-1 wt% (see Example 1). Clear effects from the heat treatment on initial capacity and first cycle efficiency are shown (see Figures 6 and 7) and also on cycling (see Figure 8).

In a first aspect, the present invention provides a composite anode material, comprising silicon kerf in a d50 size range of 10 nm – 2�m having a content of amorphous carbon of 0.4 to 2.5 wt% based on combustion infrared analyses (LECO), a carbonaceous binder, a conductive agent, and natural or synthetic graphite particles or agglomerates with d50 sizes ranging from 0.5 μm – 15 μm.

In a preferred embodiment, the composite material of the invention, the size range with d50 of the silicon kerf is 50 nm to 1 μm.

In another preferred embodiment, the composite material comprises silicon kerf with a content of amorphous carbon of 0.5-1.5 wt% based on combustion infrared analyses (LECO).

The carbonaceous binder included in the composite material of the invention, may be selected from the group consisting of petroleum pitch, coal pitch, bitumen, kraft lignin, hydrolyzed lignin, ammonium-, calcium- or sodium-lignosulfonates, glucose, fructose, sucrose, aromatic hydrocarbons, aromatic lipids, cellulose or any mixture thereof.

The conductive agent included in the composite material of the invention, may be selected from carbon blacks, carbon nano tubes, acetylene black, vapour grown carbon fibre, graphene, or a mixture thereof.

In a preferred embodiment, the composite material of the invention comprises natural or synthetic graphite particles or agglomerates with d50 sizes ranging from 2-8 μm.

In an embodiment of the invention, the composite material comprises natural or synthetic graphite particles or agglomerates with in quantities of 10 – 90 wt%, preferably 30 – 70 wt%, and more preferably 40 - 60 wt%.

In a second aspect of the invention, a method for preparation of a composite anode material as described above is provided. The method according to the invention comprises the following steps:

- providing silicon kerf particles from industrial by-products;

- subjecting the kerf particles to heat treatment at a temperature in the range of 200 to 1100 <o>C for a minimum of 30 minutes to a maximum of 10 hours under inert gas atmosphere;

- mixing the heat treated kerf particles with a carbonaceous binder, a conductive agents, and natural or synthetic graphite particles or agglomerates; and

- subjecting the mixed material to carbonization at a temperature in the range of 500 – 1200 <o>C for 30 min to 10 hours under inert gas atmosphere.

In one embodiment of the method, the kerf particles are provided by drying of silicon kerf in form of a wet filter cake from industrial by-products, and subsequent crushing of the filter cake into kerf particles.

In an embodiment of the method, the kerf particles obtain in said crushing step may be further milled down to smaller size before subjecting the kerf particles to heat treatment. This alternative, Alternative a, is shown in Figure 4.

In another embodiment of the method, the kerf particles obtain in the crushing step may be further milled down to smaller size after the kerf particles have been subjecting to heat treatment. This alternative, Alternative b, is shown in Figure 5.

In the crushing step, optionally supplemented with a milling step as described above, the filter cake is separated into particles with size range d50 from 10 nm to 2 μm, preferably 50 nm to 1 μm. A high frequency mixer or another suitable equipment is used to separate the individual kerf particles.

In case of Alternative b as shown in Figure 5, the kerf particles obtained by crushing in process step #2’’ is typically of size d50 of ~1 μm. When further milling the heat treated particles by using an appropriate milling equipment in process step # 4’’, the d50 size range of the particles is of 10-500 nm, preferably 50 nm to 200 nm.

In a preferred embodiment of the method, the kerf particles are subjected to heat treatment at a temperature in the range of 400 to 800 <o>C.

In another preferred embodiment of the method, the kerf particles are subjected to heat treatment for a period of 1 hour to 3 hours.

The mixing step is carried out by using a high frequency mixer, a conical powder mixer or another suitable type of mixer or mixing equipment.

The carbonaceous binder introduced in the mixing step is selected from the group consisting of petroleum pitch, coal pitch, bitumen, kraft lignin, hydrolyzed lignin, ammonium-, calcium- or sodium- lignosulfonates, glucose, fructose, sucrose, aromatic hydrocarbons, aromatic lipids, cellulose and mixtures thereof.

The conductive agent introduced in the mixing step is selected from the group consisting of carbon blacks, carbon nano tubes, acetylene black, vapour grown carbon fibre, graphene, and mixtures thereof.

The natural or synthetic graphite particles or agglomerates introduced in the mixing step have d50 sizes ranging from 0.5 μm – 15 μm, preferably 2-8 μm.

In a preferred embodiment of the method, the mixed material obtained in the mixing step is subjected to carbonization at a temperature in the range of 800 -1100 <o>C.

In another preferred embodiment of the method, the mixed material obtained in the mixing step is subjected to carbonization for a period of 1 hour to 3 hours.

According to one embodiment of the invention, the method is carried out by the processing route as shown in Figure 4, comprising process steps #1’ to #5’:

Process step # 1’: drying of silicon kerf in form of a wet filter cake from industrial by-products;

Process step # 2’: crushing, and optionally milling, the filter cake into kerf particles with size range d50 from 10 nm to 2 μm, preferably 50 nm to 1 μm;

Process step # 3’: subjecting the kerf particles to heat treatment at a temperature in the range of 200 to 1100 <o>C, preferably 400 to 800 <o>C for a minimum of 30 min. to a maximum of 10 hours, preferably 1 to 3 hours under inert gas atmosphere;

Process step # 4’: mixing the heat treated kerf particles with a carbonaceous, conductive agents, and natural or synthetic graphite particles or agglomerates with d50 sizes ranging from 0.5 μm – 15 μm, preferably 2-8 μm; and

Process step # 5’: subjecting the mixed material to carbonization at a temperature in the range of 500 – 1200 <o>C, preferably 800 -1100 <o>C for 30 min. to 10 hours, preferably 1 to 3 hours under inert gas atmosphere.

According to another embodiment of the invention, the method is carried out by the processing route as shown in Figure 5, comprising process steps #1’’ to #6’’:

Process step # 1’’: drying of silicon kerf in form of a wet filter cake from industrial by-products;

Process step # 2’’: crushing the filter cake into kerf particles with size typically d50 of ~1 μm;

Process step # 3’’: subjecting the kerf particles to heat treatment at a temperature in the range of 200 to 1100 <o>C, more preferably 400 to 800 <o>C for a minimum of 30 min to a maximum of 10 hours, preferably 1 to 3 hours under inert gas atmosphere;

Process step # 4’’: milling the heat treated kerf particles to d50 size range of 10-500 nm, preferably 50 nm to 200 nm;

Process step # 5’’: mixing the heat treated and milled kerf particles with a carbonaceous binder, conductive agents, and natural or synthetic graphite particles or agglomerates with d50 sizes ranging from 0.5 μm – 15 μm, more preferably 2-8 μm; and

Process step # 6’’: subjecting the mixed material to carbonization at a temperature in the range of 500 – 1200 <o>C, preferably 800 -1100 <o>C for 30 min. to 10 hours, preferably 1 to 3 hours under inert gas atmosphere.

In a preferred embodiment of the method, the silicon kerf particles are provided from by-products of the photovoltaic (PV) industry.

In a third aspect, the present invention provides a composite anode material from silicon kerf, prepared by the method described above.

In a fourth aspect, the invention provides use of the composite anode material as defined above in Li-ion batteries.

The invention is explained in more detail in the examples below. The examples are only meant to be illustrative and shall not be considered as limiting.

Example 1

The kerf was in the form of a dried filter cake, and the kerf particles were separated first in a high frequency mixer. A part of the kerf was then heat treated to between 400-1100 °C under an inert gas atmosphere for 2 hours at maximum temperature (“kerf heat_treated”). The other part of the kerf was not subjected to any heat treatment (“kerf as_is”).

Table 1: Contents of amorphous carbon in kerf as a function of heat treatment temperature.

Raman spectroscopy analyses (Figures 1 and 2) show clear signs of amorphous carbon, with broad bands at 1350, 1500 and 1600 cm<-1 >for the heat treated kerf, while there is no signs of amorphous carbon for the sample without heat treatment, clearly demonstrating that amorphous carbon is formed during the heat treatment.

Broader bands at 1350, 1500 and 1600 cm<-1 >are significative of amorphous carbon, and as shown in Figure 1, only visible on some scans for heat treated kerf. Figure 2 shows the region with amorphous carbon signature enlarged for the heat treated kerf sample, a) all scans for heat treated kerf and b) scans with carbon signature included baseline for heat treated kerf.

Example 2

Composites of graphite particles (63 wt%), kerf (silicon) (15 wt%), pitch (20 wt%) and carbon black (2 wt%) have been made with kerf from diamond wire cutting of Cz-ingots from PV industry. The kerf was in the form of a dried filter cake, and the kerf particles were separated first in a high frequency mixer. A part of the kerf was then heat treated to between 400-1100 °C under an inert gas atmosphere for 2 hours at maximum temperature (“kerf heat_treated”), ref. Figure 4. The other part of the kerf was not subjected to any heat treatment (“kerf as_is”) and used directly in the making of the composites, ref. Figure 3.

Both “kerf heat_treated” and “kerf as_is” were made into composites with identical compositions and identical production method as described in the next section, only the heat pre-treatment of the kerf itself was different.

For making the composites, the kerf were all mixed dry in a high frequency mixer and then subjected to a carbonization step; comprising a lower temperature hold for 1-2 hours followed by a controlled heating, at a rate in the range 1-10 °C/min, to a maximum temperature of 1000 °C, where a second hold at 1-2 hours occurs, all under inert gas atmosphere.

After this process, the materials were crushed and sieved at 45 µm, and half cells were made of the materials and electrochemically tested.

The composite material made of kerf that had received heat treatment prior to making the composite (“kerf heat_treated”), showed surprisingly higher first cycle efficiency and reversible capacity than composite material made of kerf that had not received heat treatment (“kerf as_is”), see Figures 6 and 7. The capacity retention during cycling was much higher for composite material made of heat treated kerf compared to composite material made of kerf without heat treatment, see Figure 8.

The person skilled in the art realizes that the present invention is not limited to the preferred embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the disclosure, and the appended claims.

Claims

C l a i m s

1.

A composite anode material, comprising silicon kerf in a d50 size range of 10 nm – 2 μm having a content of amorphous carbon of 0.4 to 2.5 wt% based on combustion infrared analyses (LECO), a carbonaceous binder, a conductive agent, and natural or synthetic graphite particles or agglomerates with d50 sizes ranging from 0.5 μm – 15 μm.

2.

A composite anode material according to claim 1, wherein the silicon kerf is in the size range with d50 of 50 nm - 1 μm.

3.

A composite anode material according to claim 1 or 2, wherein the content of amorphous carbon of the silicon kerf is in the range of 0.5 to 1.5 wt% based on combustion infrared analyses (LECO).

4.

A composite anode material according to any one of claims 1 to 3, wherein the carbonaceous binder is selected from the group consisting of petroleum pitch, coal pitch, bitumen, kraft lignin, hydrolyzed lignin, ammonium-, calcium- or sodiumlignosulfonates, glucose, fructose, sucrose, aromatic hydrocarbons, aromatic lipids, cellulose or a mixture thereof.

5.

A composite anode material according to any one of claims 1 to 4, wherein the conductive agent is selected from carbon blacks, carbon nano tubes, acetylene black, vapour grown carbon fibre, graphene, or a mixture thereof.

6.

A composite anode material according to any one of claims 1 to 5, wherein the size range with d50 of the natural or synthetic graphite particles or agglomerates is ranging from 2 to8 μm.

7.

A composite anode material according to any one of claims 1 to 6, wherein the natural or synthetic graphite particles or agglomerates is present in quantities of 10 – 90 wt%, preferably 30 – 70 wt%, and more preferably 40 - 60 wt%.

8.

A method for producing of a composite anode material as defined in any one claims 1 to 7, comprising the following steps:

- providing silicon kerf particles from industrial by-products;

- subjecting the kerf particles to heat treatment at a temperature in the range of 200 to 1100 <o>C for a minimum of 30 min to a maximum of 10 hours under inert gas atmosphere;

- mixing the heat treated kerf particles with a carbonaceous binder, a conductive agents, and natural or synthetic graphite particles or agglomerates; and

- subjecting the mixed material to carbonization at a temperature in the range of 500 – 1200 <o>C for 30 min to 10 hours under inert gas atmosphere.

9.

The method according to claim 8, wherein a further step for milling of the kerf particles takes place before or after the heat treating of the particles.

10.

The method according to claim 8 or 9, wherein the kerf particles are subjected to heat treatment at a temperature in the range of 400 to 800 <o>C.

11.

The method according to any one of claims 8 to 10, wherein the kerf particles are subjected to heat treatment for a period of 1 hour to 3 hours.

12.

The method according to any one of claims 8 to 11, wherein the mixed material is subjected to carbonization at a temperature in the range of 800 to 1100 <o>C.

13.

The method according to any one of claims 8 to 12, wherein the mixed material is subjected to carbonization, for a period of 1 hour to 3 hours.

14.

The method according to any one of claims 8 to13, wherein the silicon kerf particles are provided by:

- drying of silicon kerf in form of a wet filter cake from industrial by-products; and

- crushing the filter cake into kerf particles.

The method according to any one of claims 8 to 14, wherein the silicon kerf particles are provided from by-products of the photovoltaic (PV) industry.

16.

A composite anode material from silicon kerf, obtainable by the method according to any one of claims 8 to 15.

17.

Use of a composite anode material as defined in any one of claims 1 to 7, in lithium-ion batteries.