WO2024189227A1 - Method using a directional solidification furnace to produce 3n or higher purity silicon suitable for the manufacture of li-ion battery anodes - Google Patents

Abstract

The present invention relates to a method for manufacturing 3N or higher purity silicon usable for the manufacture of anodes of lithium-ion batteries, the method comprising a directed solidification step (104) in a directional solidification furnace in order to purify a metallurgical silicon feedstock of 2N purity.

Description

METHOD USING DIRECTIONAL SOLIDIFICATION FURNACE FOR PRODUCING SILICON OF 3N OR HIGHER PURITY SUITABLE FOR MANUFACTURING LI-ION BATTERY ANODE

FIELD OF THE INVENTION

The present invention relates to the general technical field of the production of crystalline material by directional solidification.

In particular, the present invention relates to the technical field of silicon manufacturing processes including a phase of forming a silicon block carried out in a crystallization furnace including a crucible (also known as a “directional solidification furnace”), the phase of forming a block comprising a directional solidification step.

The process described below makes it possible to create a silicon block suitable for the formation of a powder of micrometric silicon particles of purity 3N or higher (4N, 5N, 6N, etc.). Such a powder can be used for the manufacture of lithium-ion battery anodes.

For the record:

- metallurgical grade silicon (or “MG-Si” acronym for the Anglo-Saxon expression “Metallurgical Grade Silicon”) has a purity of 99% (or “2N” for the total number of “9”),

- solar grade silicon (or “SoG-Si” acronym for the Anglo-Saxon expression “Solar Grade Silicon”) has a purity of between 99.999 9% (6N) and 99.999 999 999 (11 N),

- electronic grade silicon (or "EG-Si" acronym for the Anglo-Saxon expression “Electronic Grade Silicon") has a purity of between 99.999 999 999 9% (12N) and 99.999 999 999 99 (13N). BACKGROUND OF THE INVENTION

1. Use of silicon in lithium-ion batteries

Lithium-ion batteries have enjoyed enormous commercial success, particularly in the fields of electric vehicles and portable electronics.

The operation of such a battery is based on the reversible exchange of a lithium ion between:

- a positive electrode, the cathode, and

- a negative electrode, the anode.

Today, lithium-ion battery technology relies heavily on the use of graphite-based anodes. However, it turns out that an anode using graphite-based material has a theoretical limit of 372 mAh/g of specific energy capacity, which limits the potential for future increases in specific capacity.

To overcome this drawback, several research studies have turned to the use of silicon (Si) powder as a potential anode material for lithium-ion batteries. Indeed, silicon reversibly intercalates and de-intercalates lithium ions through a reaction between silicon and lithium, 4Si + 15Li — > LiisSi4, corresponding to a theoretical capacity of 3576 mAh/g (capacity 10 times higher than that of anodes using a graphite-based material).

However, silicon cannot be used directly for the production of lithium-ion battery anodes. Indeed, anodes composed exclusively of silicon are not stable during cycling due to the high volume expansion of silicon: during the lithiation and delithiation phases, the silicon particles undergo a volume variation of the order of 300%, which induces very high mechanical stresses causing pulverization of the anodes composed exclusively of silicon. To improve the performance of silicon in cycling, it has already been proposed to produce lithium-ion battery anodes from silicon (Si) coated in non-stoichiometric silicon oxide (SiOx), the SiOx making it possible to constrain the Si to limit its deformation.

Such a compromise makes it possible to limit the risks of deterioration of the anodes, at the expense of the loss of part of the capacity of the silicon.

2. Process for obtaining silicon particles

To be suitable for the manufacture of lithium-ion battery anodes, silicon:

- must have a purity greater than or equal to 3N (hereinafter referred to as “3N+”), and

- must be reduced to the state of powder of micrometric scale particles (i.e. the largest dimension of each particle is less than 50 microns, in particular between 0.5 and 20 pm, preferably substantially equal to 5 pm).

The conventional technique for producing a powder of micrometric particles of silicon of purity 3N+ consists of implementing acid treatments on metallurgical grade silicon of purity 2N.

In particular, an example of a process for the production of 3N+ purity silicon powder includes the following steps:

- production of liquid silicon by carbo-reduction from quartz and carbon-based reducers (charcoal, coal, petroleum coke) in an arc furnace,

- addition of calcium and/or magnesium (between 1 and 5% by weight) to liquid silicon,

- very rapid and controlled solidification of the SICa or SiMg alloy to obtain an ingot, - crushing and grinding the ingot to obtain a powder of silicon particles of 2N purity and dimensions between 30 pm and 500 pm,

- acid attack of the powder of silicon particles of purity 2N from one (or more) acid(s) - such as hydrofluoric acid (HF), hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3) - to dissolve the impurities present on the surface of the particles of the powder and obtain a powder of silicon particles of purity 3N+.

Disadvantages of such a process include its high cost, as well as its unfavourable environmental footprint due to the use of toxic acid(s). Indeed, its implementation requires specific adapted installations. Furthermore, the treatment of gases and effluents produced when the granules are subjected to an acid attack is costly due to environmental regulations.

Another disadvantage of such a process is that the obtained 3N+ purity silicon particle powder has a low grain boundary density. However, it has recently been shown that grain boundaries are favorable to the cycling resistance of silicon anodes (see Sarkar et al., “Micro-macroscopic modeling of a lithium ion battery by considering grain boundaries of active materials”, Electrochimica Acta 393 (2021)). This low grain boundary density is due in particular to the fact that grinding occurs mainly at the grain boundaries, so that the acid attack of the 2N purity silicon particle powder also leads to a removal of the grain boundaries.

3. Objective of the present invention

An aim of the present invention is to propose a method for manufacturing 3N+ quality silicon making it possible to overcome at least one of the aforementioned drawbacks.

In particular, an aim of the present invention is to propose a method for manufacturing 3N+ quality silicon which is:

- inexpensive, and whose environmental impact is limited.

Such a process comprises a phase of forming a silicon block by directional solidification from which it is possible to obtain a powder of micrometric silicon particles including a high density of grain boundaries.

This powder can be used in particular in the manufacture of lithium-ion battery anodes. Furthermore, for the manufacture of lithium-ion battery anodes, the powder obtained from the process according to the invention has better characteristics than the powder of silicon particles of purity 3N+ obtained by conventional techniques implementing an acid attack step.

BRIEF DESCRIPTION OF THE INVENTION

For this purpose, the invention proposes a method for manufacturing silicon of 3N purity or higher, the method comprising a phase of forming a silicon block by directional solidification in a directional solidification furnace including a crucible, remarkable in that said formation phase comprises:

- a step of depositing a germinating agent to line a bottom of the crucible, said germinating agent including a number of germination centers per square centimeter greater than or equal to 1,000, and

- a step of directed solidification of a silicon charge to form a silicon ingot, and

- a step of trimming the ingot to form the silicon block of 3N purity or higher.

Thus, the method according to the invention proposes the use of a directional solidification furnace for the manufacture of silicon of purity 3N+ usable for the manufacture of Li-ion battery anodes. Such a directional solidification furnace is typically used for the manufacture of silicon blocks of significantly higher quality. In particular, a directional solidification furnace is used for the manufacture of quasi-monocrystalline blocks (also called mono-like silicon) with a purity greater than or equal to 99.999% (or “6N” for the total number of “9s”), or high-performance multicrystalline blocks with optimized germination.

The use of a directional solidification furnace for the formation of a silicon block of quality lower than 6N therefore goes against the teachings provided to those skilled in the art in the literature.

Furthermore, the inventors propose using a germinating agent having at least 1000 germination centers per square centimeter in order to promote the formation of grain boundaries in the silicon block obtained by directed solidification during the formation phase thereof. The use of a germinating agent that generates such a high density of grain boundaries also goes against the teachings provided by the literature to those skilled in the art.

The inventors have in fact discovered that increasing the number of grain boundaries in the silicon block obtained by directed solidification allows:

- to facilitate its pulverization to obtain a powder of micrometric particles of silicon of purity 3N+, and

- to obtain a high density of grain boundaries in the powder of micrometric particles of silicon of purity 3N+ (i.e. higher than the grain boundary density of powders of silicon particles of purity 3N obtained by conventional techniques implementing an acid attack step).

It is thus possible to avoid the implementation of an acid attack step used in conventional processes to, on the one hand, dissolve impurities and, on the other hand, form micrometric-sized particles. Preferred but non-limiting aspects of the method according to the invention are the following:

- the germinating agent may comprise a nitride-based fluid medium including particles with a size of between 1 and 200 microns, and preferably between 1 and 100 microns, the deposition step comprising the following sub-steps: o application of the fluid medium to the bottom and the side walls of the crucible, the medium being applied in a quantity sufficient to provide, upon drying, a film on the surface of which particles with a size of between 1 and 200 microns extend in projection, o exposure of the crucible treated according to the preceding sub-step to a heat treatment in an oxidizing atmosphere and under conditions sufficient to cause the formation of a layer of silica and a layer of oxidized silicon nitride Si2N2O on the surface of the particles with a size of between 1 and 200 microns;

- the particle concentration in the fluid medium may be between 20 and 60% by weight of the fluid medium;

- the germinating agent may comprise a powder composed of particles with a size of between 1 and 200 microns, and preferably between 1 and 100 microns, the deposition step comprising a sub-step consisting of placing the powder at the bottom of the crucible to obtain a layer of particles with a thickness of between 1 and 10 millimeters;

- particles with a size between 1 and 200 microns can be chosen from: o silicon-based particles such as SiC particles, SiO particles, SiO2 particles, Si3N4 particles, o alumina AI2O3 particles, o ceramic particles;

- the germinating agent may comprise at least one silicon plate obtained by sintering a micrometric powder with a compound having a melting temperature greater than or equal to the melting temperature of silicon, the deposition step comprising a sub-step consisting of arranging said and at least one plate at the bottom of the crucible; - the directed solidification step may comprise a sub-step of injecting, onto the surface of the silicon charge in the liquid state, a gas including a carbon-based compound, such as carbon dioxide CO2;

- advantageously, during the injection sub-step: o the gas can be injected at a flow rate of between 0.01 and 1.5 L/min, preferably of the order of 1 L/min, and o the concentration of the gas in carbon-based compound can be 100%;

- the directed solidification step may also include another sub-step of injecting a gas including an argon-based compound;

- advantageously, during the other injection sub-step: o the gas can be injected at a flow rate of between 0.5 and 150 L/min, and o the concentration of the gas in carbon-based compound can be between 1% and 20%;

- the directed solidification step may comprise a sub-step consisting of establishing a thermal gradient between the bottom of the crucible and an upper opening of the crucible opposite the bottom, the thermal gradient being between 10°C/cm and 50°C/cm, preferably between 10°C/cm and 20°C/cm, and even more preferably substantially equal to 15°C/cm;

- the block formation phase may also include a step of cooling the ingot at a rate of between 100°C/h and 400°C/h, preferably between 200°C/h and 300°C/h;

- the method may also comprise a phase of pulverizing the silicon block to obtain a silicon powder;

- the pulverization phase may comprise a step of crushing the silicon block into pieces of larger dimensions less than 3 mm, and a mechanical grinding step, such as fluidized bed grinding to reduce the size of the silicon particles to micrometric and submicron sizes.

BRIEF DESCRIPTION OF THE DRAWINGS Other advantages and characteristics of the method according to the invention will emerge more clearly from the following description of several variant embodiments, given as non-limiting examples, from the attached drawings in which:

- Figure 1 is a schematic phase representation of a silicon manufacturing process according to the invention,

- Figure 2 is a schematic representation of an example of a directional solidification furnace used during the implementation of the silicon manufacturing process,

- Figure 3 is a detailed schematic representation of the silicon manufacturing process according to the invention,

- Figure 4 is a schematic representation illustrating a separation front between a liquid phase and a solid phase.

DETAILED DESCRIPTION OF THE INVENTION

We will now describe various examples of embodiments of the invention with reference to the figures. In these various figures, equivalent elements are designated by the same numerical reference.

1. General

With reference to FIG. 1, the silicon manufacturing process comprises a phase 10 of forming a block of silicon of purity 3N+ from a charge of silicon of purity 2N, and optionally a phase 20 of pulverizing said block of silicon of purity 3N+.

In the following text, the following terms shall apply:

- “silicon ingot” means a piece of silicon of 2N purity in the solid state obtained by directional solidification of a charge of silicon of 2N purity in the liquid state, and by

- “silicon block”, a piece of silicon of purity 3N+ in the solid state obtained after trimming an upper part of a “silicon ingot” of purity 2N in the solid state. This process allows:

- on the one hand to purify the silicon charge (obtaining a silicon block of 3N+ purity at the end of the process), and

- on the other hand, to improve the characteristics of the 3N+ purity silicon block obtained to make it compatible with use in the production of Li-ion battery anodes.

Advantageously, the formation phase 10 of the silicon block of purity 3N+ is implemented in a directional solidification furnace which will now be presented in more detail.

2. Directional solidification furnace

The directional solidification furnace may be of any type known to those skilled in the art.

As shown in Figure 2, such a directional solidification furnace comprises:

- a crucible 1 placed in

- graphite plates surrounding the crucible to limit its deformation

- a thermally insulating structure 2 containing the crucible 1,

- a heating system 3, and

- a cooling system 4.

2. 1. Crucible

The crucible 1 comprises a bottom 11 and one (or more) side wall(s) 12 defining an upper opening of the crucible 1.

For the solar industry, the crucibles usually used for the manufacture of silicon ingots are made of vitreous silica (or "fused silica" in English) in order to limit the contamination brought to the silicon by the crucible during the solidification of the silicon. Such crucibles have the disadvantage of cracking when the silicon cools, which does not allow them to be reused.

On the contrary, in the context of the present invention, the crucible 1 may optionally be composed of removable plates made of graphite to allow reuse of the crucible once the silicon block has been formed. Indeed, the carbon contamination of the silicon by the crucible is not critical for the intended applications, namely the manufacture of silicon (of purity 3N+) usable in the production of Li-ion battery anodes.

Optionally, the crucible 1 may include a cover (not shown) - for example made of graphite - intended to cover the upper opening. This allows:

- to promote the formation of carbon monoxide (CO) in a volume of crucible 1 located above the initial silicon charge of purity 2N, and

- to confine this carbon monoxide to promote the dissolution of carbon in the silicon charge in the liquid state and thus increase the quantity of carbon contained in the silicon charge in the liquid state.

2.2. Thermally insulating structure

The thermally insulating structure 2 makes it possible to reduce thermal losses during the formation phase 10 of the silicon block.

The thermally insulating structure 2 is of a type known to those skilled in the art and will not be described in more detail below. It may, for example, be composed of thick graphite felt plates arranged around the crucible 1.

2.3. Heating system

The heating system 3 can be of the resistive type or of the inductive type. In the embodiment illustrated in FIG. 2, the heating system 3 comprises one (or more) graphite resistor(s) arranged in an upper part of the thermally insulating structure 2.

In certain embodiments, the heating system 3 may also comprise one (or more) graphite resistor(s) arranged at the side walls of the crucible 1. This allows better control of the temperature inside the crucible 1 over its entire height.

The heating system may also comprise one (or more) graphite resistor(s) arranged in a lower part of the thermally insulating structure 2, in particular under the bottom 11 of the crucible 1. In this case, the heating resistor(s) located above the crucible 1 is (are) configured to generate heat greater than the heating resistor(s) located below the crucible 1 in order to create a vertical temperature gradient in the crucible 1, as will be described in more detail below.

2.4. Cooling system

The directional solidification furnace further comprises a cooling system 4 arranged in a lower part of the thermally insulating housing 2.

This cooling system 4 is for example composed of one (or more) heat exchanger(s) in which a heat transfer fluid circulates. This (or these) heat exchanger(s) can be mounted under the bottom 11 of the crucible 1.

The integration of a cooling system 4 makes it possible to extract heat and precisely control the temperature in the crucible 1 in order to control the vertical temperature gradient between the bottom 11 of the crucible 1 and its upper opening.

3. Presentation of the method according to the invention As previously stated, the 3N+ purity silicon manufacturing process includes:

- a phase of formation 10 of a silicon block, and possibly

- a phase of crushing and pulverizing 20 of the silicon block.

3. 1. Formation phase of a silicon block

Phase 10 of forming a silicon block makes it possible to:

- generate crystalline defects (grain boundaries, dislocations) in the silicon once it has solidified, and

- segregate the impurities contained in the metallurgical grade silicon used at the input of the process in order to purify it.

To enable the generation of crystal defects, the formation phase 10 comprises a deposition step 101 in the crucible 1, of a germinating agent including a number of germination centers per square centimeter greater than or equal to 1000.

In the context of the present invention, the term "nucleation center" means a particle (called "nuclei") on the surface of which a charge of molten silicon crystallizes during its transition from the liquid state to the solid state to form a grain having a given crystalline orientation. During solidification, a nucleation center is a point (or a region) within a material where the crystallization process begins (start of liquid-solid phase change). Each nucleation center can be generated by impurities, surface defects, local temperature disturbances or by intentionally added nuclei. The nucleation is said to be "homogeneous" when the nucleation centers are made of silicon, and "heterogeneous" when the nucleation centers are of different chemical nature. To enable the segregation of impurities, the formation phase 10 comprises a directional solidification step 104 of a silicon charge to form a silicon ingot of 2N purity.

In the context of the present invention, the term “directional solidification” means the control of the germination and growth of solid crystals in molten silicon during its transition from the liquid state to the solid state.

3.1.1. Step of depositing an terminating agent

The deposition step 101 of a germinating agent makes it possible to generate a maximum density of crystalline defects - in particular grain boundaries - in the silicon ingot once it has solidified.

For this purpose, the germinating agent chosen to line the bottom 11 and the side wall(s) of the crucible 1 comprises a number of germination centers per square centimeter greater than or equal to 1000.

Depending on the nature of the germinating agent, different deposition techniques can be implemented within the framework of the present invention.

3. 1.1.1. First solution for the deposition of the germinating agent

In a first variant of the present invention, the germinating agent may consist of a nitride-based fluid medium including particles of size between 1 and 200 microns.

In this case, a first solution for the deposition 101 of the germinating agent can include the following sub-steps:

- apply the fluid medium to the bottom and the side wall(s) of the crucible 1, then to dry the fluid medium including particles of size between 1 and 200 microns.

Advantageously, the fluid medium is applied in a quantity sufficient to provide, upon drying, a layer formed at least of particles of size between 1 and 200 microns, preferably between 1 and 100 pm.

The drying sub-step may consist of a heat treatment in an oxidizing atmosphere (and under sufficient conditions) to cause the formation of a layer of silica and a layer of oxidized silicon nitride Si2N2O on the surface of the particles with a size of between 1 and 200 microns. This makes it easier to demould the 2N purity silicon ingot obtained after solidification of the silicon in the liquid state.

The particle concentration in the fluid medium may be between 20 and 60% by weight of the fluid medium in order to have a sufficient number of germination centers while ensuring their adhesion to the bottom and the side wall(s) of the crucible 1.

In some embodiments, the particles contained in the fluid medium may be silicon or silicon-based particles such as SiC particles, SiO particles, SiC particles, SisN4 particles. These silicon-based particles may be derived from either:

- sawing waste from the solar industry (“kerfs”).

- grinding of metallurgical silicon to obtain particles of 50pm for example.

In other embodiments, the particles may be alumina particles AI2O3, or ceramic particles. In all cases, the particles chosen are particles allowing germination, on their surface, of silicon in the liquid state.

3. 1.1.2. Second solution for the deposit of the terminating agent In a second variant of the present invention, the germinating agent may consist of a powder of particles of size between 1 and 200 microns, and preferably between 1 and 100 microns. These particles may be of the same type as the particles described above (particles based on silicon or alumina or ceramic).

In this case, a second solution for the deposition 101 of the germinating agent may consist in arranging (by spraying, dusting, etc.) the powder on the bottom 11 of the crucible 1 to obtain a layer of particles with a thickness of between 1 and 10 millimeters. Such a thickness makes it possible to guarantee the presence of germination centers between the crucible and the liquid silicon.

3.1 .1 .3. Third solution for the deposition of the germinating agent

Finally, the germinating agent may consist of one (or more) plate(s) of agglomerated silicon obtained for example by sintering (or any other technique known to those skilled in the art) of a micrometric powder (from 1 to 20 pm) with a compound having a melting temperature greater than or equal to the melting temperature of silicon. The silicon plate will have a high open porosity, hence the presence of a significant quantity of germination center.

In this case, a third solution for the deposition 101 of the germinating agent may consist of placing the plate(s) on the bottom 11 (and optionally on the side wall(s) 12) of the crucible 1.

When the germinating agent consists of a single plate, the latter is placed at the bottom of the crucible. When the germinating agent comprises several plates, they are arranged side by side to line the bottom 11 (and optionally on the side wall(s) 12) of the crucible 1, limiting the spaces between two adjacent plates. Once the germinating agent has been deposited, and prior to the implementation of the directional solidification step, the process comprises various conventional steps known to those skilled in the art.

3.1.2. Crucible loading step

In particular, the method comprises a step 102 of loading the crucible 1 (for example manually) with silicon of purity 2N (metallurgical quality).

The 2N purity metallurgical silicon charge may be placed in solid form in the crucible 1. In particular, the loading step 102 may consist of depositing pieces or granules of 2N purity silicon in the solid state.

The use of a metallurgical grade silicon charge (2N purity) at the process input makes it possible to reduce the manufacturing cost of 3N+ purity silicon.

The composition of this metallurgical grade silicon charge can be such that:

- the iron concentration of the silicon charge is less than 0.2% ([Fe]<0.2%),

- the aluminum concentration of the silicon charge is less than 0.2% ([AI]<0.2%), and

- the calcium concentration of the silicon charge is less than 0.2% ([Ca]<0.02%).

This silicon charge may possibly have an iron content of less than 2000 parts per million by weight (or “ppmw”, the acronym for the Anglo-Saxon expression “part per million weight”).

Once the crucible is filled with the metallurgical silicon charge, a melting step 103 of the metallurgical silicon charge is implemented.

3.1.3. Merging step The fusion step 103 makes it possible to change the silicon charge from the solid state to the liquid state.

To enable the melting of the metallurgical silicon charge, the temperature inside crucible 1 is increased above the melting temperature of silicon (1410°C).

More specifically, the heating system 3 is activated to increase the temperature of the metallurgical silicon charge from the upper opening (and possibly from the bottom 11) of the crucible 1.

The temperature rises following a temperature ramp to a temperature above 1450°C. The melting time is generally greater than 10 hours (typically 13 to 20 hours) depending on the height of the metallurgical silicon charge to be melted.

When the metallurgical silicon charge is completely melted (i.e., the silicon charge is in the liquid state), the directional solidification step 104 can be implemented.

3.1.4. Directional solidification stage

Directional solidification can exhibit a columnar structure or an equiaxed structure.

3. 1.4. 1. Advantage

Directional solidification step 104 serves to concentrate the impurities of the metallurgical silicon charge in the upper part of the silicon ingot, solidifying it as quickly as possible. This accumulation of impurities in the upper part of the silicon ingot is obtained by segregation.

Segregation is a physical phenomenon occurring during the solidification of a material. During the solidification of the molten silicon charge, impurities are released into the liquid phase 52.

In fact, the distribution of impurities follows Scheil's law:

With k = Cs/CI the impurity-specific segregation coefficient in Silicon. This specific segregation coefficient is known to those skilled in the art and will not be described in further detail below.

Thus, during the directed solidification of silicon, the impurities contained in the 2N purity silicon remain preferentially in the liquid phase 52. This makes it possible to obtain a block of solid silicon 53 of higher purity (3N+) after trimming the upper part of the silicon ingot.

The higher the solid fraction 53, the higher the concentration of impurities in the liquid phase 52.

Advantageously, the segregation phenomenon can be promoted by effective stirring of the silicon bath in the liquid state. This stirring can be obtained by convection. Alternatively, this stirring can be obtained by induction, for example by the application of an alternating, rotating, or sliding magnetic field. Alternatively, this stirring can be mechanical stirring or gas stirring.

3. 1.4.2. Principle To induce directional solidification of the 2N purity silicon charge in the liquid state, a low temperature difference is applied between the upper opening and the bottom 11 of the crucible 1.

The temperature at the upper opening of crucible 1 is higher than the temperature at the bottom 11 of crucible 1. This induces the solidification of silicon from the bottom 11 of the crucible towards the upper opening along a rising solidification front (solid/liquid interface).

In particular, the temperature ramps applied are such that:

- the Thaute temperature range (measured by a thermocouple positioned at the upper opening) above the silicon in the liquid state is between: o 1500°C, preferably 1450°C at the start of the directional solidification step, and o 1410°C at the end of the directional solidification step, the melting temperature of silicon being 1410°C;

- the temperature range Tlow (measured by a thermocouple positioned under the bottom 11 of the crucible 1) below the silicon in the solid state is between 1300°C (at the start of the directional solidification step) and 800°C (at the end of the directional solidification step).

Advantageously, the temperature at the surface of the silicon in the liquid state is maintained higher (at least) by 10°C compared to the temperature of the silicon at the rising solidification front. This makes it possible to limit the thermal gradient by avoiding the encapsulation of silicon in the liquid state in the silicon ingot in the solid state (i.e. imprisonment of liquid pockets of silicon in the solid silicon ingot).

Indeed, the encapsulation of silicon in the liquid state induces a risk of cracking of the ingot. To reduce the risk of encapsulation, a thermal gradient is applied between the bottom 11 and the upper opening of the crucible 1 throughout the directional solidification step.

Advantageously, the thermal gradient is between 10°C/cm and 50°C/cm, preferably between 10°C/cm and 20°C/cm, and even more preferably substantially equal to 15°C/cm. The application of a thermal gradient between 10°C/cm and 50°C/cm makes it possible to obtain an equiaxed structure and to increase the crystalline defects and the quantity of SiC precipitate in the silicon ingot obtained at the end of the directional solidification step, which improves the properties of the silicon block obtained after trimming, in particular in the context of the production of Li-ion battery anodes.

Furthermore, the application of a thermal gradient between 10°C/cm and 50°C/cm coupled with strong cooling (heat exchanger) makes it possible to obtain a solidification speed between 2 and 6 cm/h (unlike a directional solidification furnace for solar where the solidification speed is between 1 and 1.5 cm/h):

- which promotes the formation of crystalline defects (grain boundaries and dislocations) in the silicon ingot in the solid state,

- while ensuring “acceptable” purification of the silicon block in the solid state (purification by a factor of 10 or greater of the silicon block obtained by directional solidification compared to the metallurgical silicon charge initially introduced into the crucible 1).

3. 1.4.3. Optional sub-steps

Optionally, a sub-step of injecting a carbon-based gas can be implemented during the directional solidification step. This gas can be carbon dioxide CO2, and the injection can be carried out on the surface of the silicon charge in the liquid state. This injection can for example be carried out at a flow rate of between 0.01 and 1.5L/min, preferably of the order of 1 L/min for a carbon-based compound concentration of 100%.

The injection of a carbon-based gas allows the silicon to be contaminated with carbon, which induces the creation of small SiC precipitates generating “grit” type defects (equiaxed structure: small grains found in the form of clusters in the silicon ingot) promoting the formation of grain boundaries and dislocations in the silicon ingot obtained at the end of the directional solidification step.

A sub-step of injecting a gas including an argon-based compound (in addition to or replacing the sub-step of injecting carbon-based gas) may also be implemented during the directional solidification step. The argon-based gas may for example be injected at a flow rate of between 0.5 and 10 L/min. This argon-based gas may further comprise a carbon-based compound whose concentration is between 1% and 20%.

At the end of the directional solidification step 104, a solid state silicon ingot is obtained.

Advantageously, a rapid cooling step of the silicon ingot can be implemented to maximize the formation of dislocations in the solidified ingot due to the high thermal stresses undergone during cooling (the faster the ingot is cooled, the greater the number of dislocations generated). Of course, we still avoid cooling too quickly, which would lead to cracking of the ingot and/or the graphite crucible. We will therefore aim for cooling of the solid silicon between 100°C/h and 400°C/h, preferably between 200°C/h and 300°C/h. The annealing step will ideally be eliminated.

A step of demolding and trimming the silicon ingot is then implemented to obtain a silicon block of 3N+ purity. During trimming, the upper portion of the silicon ingot is cut off at a height of between 70 and 90% of the total height of the block. This portion of the silicon ingot contains the majority of impurities (this portion corresponds to the last volume of silicon that was solidified). The greater the thickness of the trimmed silicon layer, the purer the remaining silicon block (lower part of the ingot).

The weight of the remaining lower portion is between 400kg and 2,000kg. The concentrations of impurities in this remaining lower portion are:

- [Fe]< l OOppmw,

- [AI]<150ppmw,

- [Ca]<50ppmw,

- [Ni]<20ppmw,

- [Cu]<20ppmw,

- [O]< l OOOppmw,

- [N]<50 ppmw.

The remaining lower portion thus obtained can be sprayed during spraying phase 20.

3.2. Spraying phase

Spraying phase 20 produces a powder of micrometric silicon particles of 3N+ purity which can be used for the production of Li-ion battery anodes.

Spray phase 20 includes:

- a step of crushing the silicon block to obtain silicon granules of purity 3N+ and dimensions less than 3 mm, preferably between 50 pm and 500 pm, and even more preferably substantially equal to 50 pm,

- a mechanical grinding step, such as fluidized bed grinding to reduce the size of the silicon particles to micron and submicron sizes (including between 0.5 and 20 pm), for example at sizes less than 10 pm and preferably less than 5 pm.

Prior to the implementation of the crushing step, the pulverization phase may optionally comprise a step of cutting the silicon block into bricks to facilitate its handling, and/or a pre-crushing step to reduce the silicon block or bricks into pieces of sufficiently small sizes (i.e. between 3 mm and 500 mm) to allow their introduction into a crusher for the purpose of carrying out the crushing step.

The crushing step is conventionally known to those skilled in the art and will not be described in further detail below.

The grinding step is also known to those skilled in the art. Indeed, the fluidized bed grinding technique is described in particular in document WO 2012/014985.

This technique consists of arranging silicon granules of 3N+ purity in a chamber of a fluidized bed mill. A pressurized gas (nitrogen or air) supplied by a compressor (between 7 and 30 bars) is injected at high speed into the chamber by jet nozzles. The size reduction is accomplished through collisions between the particles by energy transmission.

The crushed particles thus produced are moved to a classification device in which an upward air flow circulates. Particles having a size greater than a threshold value (e.g. 15 pm, 10 pm or 5 pm in the preferred case) are rejected and particles having a size less than 10 pm (preferably less than 5 pm) are collected.

3.2.1. Conclusions

The main disadvantages of silicon Li-ion battery anodes include: - their manufacturing cost, and

- their resistance to cycling.

The method described above provides a solution to both of these problems.

In particular, the use of 2N purity metallurgical silicon as input to the process and its purification by directional solidification in a directional solidification furnace makes it possible to reduce the production cost of 3N+ quality silicon that can be used for the manufacture of lithium-ion battery anodes.

Furthermore, the application of a germinating agent integrating at least 1000 germination centers per square centimeter makes it possible to generate grain boundaries:

- facilitating the spraying of the silicon block obtained at the end of the directional solidification step, and

- increasing the cycling resistance of silicon Li-ion battery anodes.

The reader will understand that numerous modifications can be made to the invention described above without materially departing from the new teachings and advantages described here.

Claims

1. A method of manufacturing silicon of 3N purity or higher, the method comprising a phase of forming (10) a block of silicon by directional solidification in a directional solidification furnace including a crucible, characterized in that said forming phase comprises: - a step of depositing a germinating agent to line a bottom of the crucible, said germinating agent including a number of germination centers per square centimeter greater than or equal to 1,000, and - a step of directed solidification of a silicon charge to form a silicon ingot, and - a step of trimming the ingot to form the silicon block of 3N purity or higher. 2. Method according to claim 1, in which the germinating agent comprises a nitride-based fluid medium including particles of size between 1 and 200 microns, and preferably between 1 and 100 microns, the deposition step comprising the following sub-steps: - application of the fluid medium to the bottom and side walls of the crucible, the medium being applied in a quantity sufficient to provide, upon drying, a film on the surface of which particles of a size between 1 and 200 microns extend in projection, - exposure of the crucible treated according to the previous sub-step to a heat treatment in an oxidizing atmosphere and under conditions sufficient to cause the formation of a layer of silica and a layer of oxidized silicon nitride Si2N2O on the surface of particles of size between 1 and 200 microns. 3. Method according to the preceding claim, in which the particle concentration in the fluid medium is between 20 and 60% by weight of the fluid medium. 4. Method according to claim 1, in which the germinating agent comprises a powder composed of particles of a size between 1 and 200 microns, and preferably between 1 and 100 microns, the deposition step comprising a sub-step consisting of arranging the powder at the bottom of the crucible to obtain a layer of particles of a thickness between 1 and 10 millimeters. 5. Method according to any one of claims 2 to 4, in which the particles of size between 1 and 200 microns are chosen from: - silicon-based particles such as SiC particles, SiO particles, SiC particles, SisN4 particles, - alumina particles AI2O3, - ceramic particles. 6. Method according to claim 1, in which the germinating agent comprises at least one silicon plate obtained by sintering a micrometric powder with a compound having a melting temperature greater than or equal to the melting temperature of silicon, the deposition step comprising a sub-step consisting of arranging said and at least one plate at the bottom of the crucible. 7. Method according to any one of claims 1 to 6, in which the directed solidification step comprises a sub-step of injecting, onto the surface of the silicon charge in the liquid state, a gas including a carbon-based compound, such as carbon dioxide CO2. 8. Method according to claim 7, in which during the injection sub-step: - the gas is injected at a flow rate between 0.01 and 1.5 L/min, preferably of the order of 1 L/min, and - the concentration of the gas in carbon-based compound is 100%. 9. A method according to any one of claims 1 to 8, wherein the directed solidification step further comprises another sub-step of injecting a gas including an argon-based compound. 10. Method according to claim 9, in which during the other injection sub-step: - the gas is injected at a flow rate between 0.5 and 150 L/min, and - the concentration of carbon-based compounds in the gas is between 1% and 20%. 11. Method according to any one of claims 1 to 10, in which the directed solidification step comprises a sub-step consisting of establishing a thermal gradient between the bottom of the crucible and an upper opening of the crucible opposite the bottom, the thermal gradient being between 10°C/cm and 50°C/cm, preferably between 10°C/cm and 20°C/cm, and even more preferably substantially equal to 15°C/cm. 12. Method according to any one of claims 1 to 11, in which the phase of forming a block further comprises a step of cooling the ingot at a rate of between 100°C/h and 400°C/h, preferably between 200°C/h and 300°C/h. 13. Manufacturing method according to any one of claims 1 to 12, which further comprises a phase of spraying (20) the silicon block to obtain a silicon powder. 14. The manufacturing method of claim 13, wherein the pulverization phase comprises a mechanical grinding step, such as fluidized bed grinding to reduce the size of the silicon particles to micron and submicron sizes.