METHOD USING A DIRECTIONAL SOLIDIFICATION FURNACE TO PRODUCE SILICON OF 3N PURITY OR HIGHER SUITABLE FOR THE MANUFACTURE OF LI-ION BATTERY ANODE

Abstract

The invention relates to a method for manufacturing silicon of 3N purity or higher usable for the manufacture of lithium-ion battery anodes, the method comprising a solidification step (104) directed in a directional solidification furnace in order to purify a charge of metallurgical silicon of 2N purity. Figure to be published with the abstract: Fig. 3

Description

Title of the invention: METHOD USING A DIRECTIONAL SOLIDIFICATION FURNACE TO PRODUCE 3N OR HIGHER PURITY SILICON SUITABLE FOR LI-ION BATTERY ANODE MANUFACTURE FIELD OF THE INVENTION

[0001] The present invention relates to the general technical field of the production of crystalline material by directional solidification.

[0002] 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.

[0003] The method 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.

[0004] 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 (UN), • 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).

[0005] BACKGROUND OF THE INVENTION

[0006] The Use of Silicon in Lithium-Ion Batteries

[0007] Lithium-ion batteries have enjoyed enormous commercial success, particularly in the fields of electric vehicles and portable electronics.

[0008] 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.

[0009] Nowadays, lithium-ion battery technology relies heavily on the use of graphite-based anodes. However, it turns out that an anode using a 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.

[0010] 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 by a reaction between silicon and lithium, 4Si + 15Li —> Lil5Si4, corresponding to a theoretical capacity of 3576 mAh / g (capacity 10 times higher than that of anodes using a graphite-based material).

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 2. Process for obtaining silicon particles

[0015] 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).

[0016] The conventional technique for producing a powder of micrometric particles of silicon of purity 3N+ consists of implementing acid treatments on metallurgical quality silicon of purity 2N.

[0017] In particular, an example of a process for the production of a silicon powder of purity 3N+ comprises the following steps: • production of liquid silicon by carbo-reduction from quartz and carbon-based reducers (charcoal, coal, petroleum coke) in a 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 of the ingot to obtain a powder of silicon particles of 2N purity and dimensions between 30 pm and 500 pm, • acid attack of the 2N purity silicon particle powder using 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 powder particles and obtain a 3N+ purity silicon particle powder.

[0018] Disadvantages of such a process concern its high cost, as well as its unfavorable 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 subjecting the granules to an acid attack is expensive due to environmental regulations.

[0019] 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 notably due 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 suppression of the grain boundaries.

[0020] 3. Objective of the present invention

[0021] 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.

[0022] 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.

[0023] Such a method comprises a phase of forming a block of silicon by directional solidification from which it is possible to obtain a powder of micrometric particles of silicon including a high density of grain boundaries.

[0024] 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.

[0025] BRIEF DESCRIPTION OF THE INVENTION

[0026] To this end, the invention proposes a method for manufacturing silicon of 3N purity or higher, the method comprising a phase of forming a block of silicon by directional solidification in a directional solidification furnace including a crucible,

[0027] remarkable in that said training 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.

[0028] Thus, the method according to the invention proposes the use of a directional solidification furnace for the manufacture of silicon of 3N+ purity usable for the manufacture of Li-ion battery anodes.

[0029] Such a directional solidification furnace is usually used for the manufacture of silicon blocks of much higher quality. In particular, a directional solidification furnace is used for the manufacture of quasi-monocrystalline blocks (also called mono-like silicon) having a purity greater than or equal to 99.999 9% (or "6N" for the total number of "9"), or high-performance multi-crystalline blocks with optimized germination.

[0030] 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.

[0031] 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 which generates such a high density of grain boundaries also goes against the teachings provided by the literature to those skilled in the art.

[0032] 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 significant 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).

[0033] It is thus possible to avoid the implementation of an acid attack step implemented in conventional processes to, on the one hand, dissolve the impurities, and on the other hand, form particles of micrometric size.

[0034] 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 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 protrude, • 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; • the particle concentration in the fluid medium can be between 20 and 60% by weight of the fluid medium; • the germinating agent may comprise 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 placing the powder at the bottom of the crucible to obtain a layer of particles of a thickness between 1 and 10 millimeters; • particles between 1 and 200 microns in size can be chosen from: • silicon-based particles such as SiC particles, SiO particles, SiO2 particles, Si3N4 particles, • particles of alumina A12O3, • ceramic particles; • the germinating agent may comprise at least one silicon plate obtained by sintering a micrometric powder with a compound having a temperature melting temperature greater than or equal to the melting temperature of silicon, the deposition step comprising a sub-step consisting of placing 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: • the gas can be injected at a flow rate between 0.01 and 1.5 L / min, preferably around 1 L / min, and • the concentration of the gas in carbon-based compound can be 100%; • the directed solidification step may also comprise another sub-step of injecting a gas including an argon-based compound; • advantageously, during the other injection sub-step: • the gas can be injected at a flow rate between 0.5 and 150 L / min, and • 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 phase of forming a block may also comprise 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 include 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

[0035] 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 appended drawings in which: • [Fig.l] is a phase schematic representation of a fa- silicon fabrication according to the invention, • [Fig.2] is a schematic representation of an example of a solidification furnace directional fication used during the implementation of the silicon manufacturing process, • [Fig.3] is a detailed schematic representation of the manufacturing process of silicon according to the invention, • [Fig.4] is a schematic representation illustrating a separation front between a liquid phase and a solid phase. DETAILED DESCRIPTION OF THE INVENTION

[0036] Various embodiments of the invention will now be described with reference to the figures. In these various figures, equivalent elements are designated by the same numerical reference.

[0037] 1. General

[0038] With reference to [Fig. 1], the silicon manufacturing method 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+.

[0039] In the following text, the following terms are understood to mean: • “silicon ingot”, a piece of silicon of 2N purity in the solid state obtained by directional solidification of a silicon charge 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 2N purity in the solid state.

[0040] This process allows: • on the one hand to purify the silicon charge (obtaining a block of silicon with 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.

[0041] 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.

[0042] 2. Directional solidification furnace

[0043] The directional solidification furnace may be of any type known to those skilled in the art.

[0044] As shown in [Fig.2], such a directional solidification furnace comprises: • a crucible 1 placed in • graphite plates which surround the crucible to limit its deformation • a thermally insulating structure 2 containing the crucible 1, • a heating system 3, and • a cooling system 4.

[0045] 2.1. Crucible

[0046] The crucible 1 comprises a bottom 11 and one (or more) side wall(s) 12 defining an upper opening of the crucible 1.

[0047] 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 solidification of the silicon. Such crucibles have the disadvantage of cracking during cooling of the silicon, which does not allow their reuse.

[0048] 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.

[0049] Optionally, the crucible 1 may comprise a cover (not shown) - for example made of graphite - intended to cover the upper opening. This makes it possible: • to promote the formation of carbon monoxide (CO) in a volume of the 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.

[0050] 2.2. Thermally insulating structure

[0051] The thermally insulating structure 2 makes it possible to reduce thermal losses during the formation phase 10 of the silicon block.

[0052] 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.

[0053] 2.3. Heating system

[0054] The heating system 3 can be of the resistive type or of the inductive type.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 2.4. Cooling system

[0059] The directional solidification furnace further comprises a cooling system 4 arranged in a lower part of the thermally insulating housing 2.

[0060] 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.

[0061] 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.

[0062] 3. Presentation of the method according to the invention

[0063] As previously indicated, the process for manufacturing 3N+ purity silicon comprises: • a phase of formation 10 of a silicon block, and possibly • a phase of crushing and pulverizing 20 of the silicon block.

[0064] 3.1. Phase of formation of a silicon block

[0065] The 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 as input to the process in order to purify it.

[0066] To enable the generation of crystalline defects, the formation phase 10 comprises a step 101 of depositing in the crucible 1, a germinating agent including a number of germination centers per square centimeter greater than or equal to 1000.

[0067] In the context of the present invention, the term “germination center” means a particle (called “nuclei”) on the surface of which a charge of molten silicon crystallizes during its passage 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 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 seeds. 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.

[0068] 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.

[0069] 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.

[0070] 3.1.1. Step of depositing a germinant agent

[0071] The step 101 of depositing a germinating agent makes it possible to generate a maximum density of crystalline defects - notably grain boundaries - in the silicon ingot once it has solidified.

[0072] 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.

[0073] Depending on the nature of the germinating agent, different deposition techniques can be implemented within the framework of the present invention.

[0074] 3.1.1.1. First solution for the deposition of the germinating agent

[0075] 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.

[0076] In this case, a first solution for the deposition 101 of the germinating agent can comprise the following sub-steps: • apply the fluid medium to the bottom and the side wall(s) of crucible 1, then to • dry the fluid medium including particles between 1 and 200 microns in size.

[0077] 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 μm.

[0078] 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 particles with a size between 1 and 200 microns. This facilitates the demolding of the 2N purity silicon ingot obtained after solidification of the silicon in the liquid state.

[0079] 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 guaranteeing their adhesion to the bottom and the side wall(s) of the crucible 1.

[0080] In certain embodiments, the particles contained in the fluid medium may be silicon or silicon-based particles such as SiC particles, SiO particles, SiO2 particles, Si3N4 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.

[0081] In other embodiments, the particles may be alumina particles A12O3, or ceramic particles. In all cases, the particles chosen are particles allowing germination, on their surface, of silicon in the liquid state.

[0082] 3.1.1.2. Second solution for the deposit of the terminating agent

[0083] 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).

[0084] In this case, a second solution for the deposition 101 of the germinating agent may consist of 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.

[0085] 3.1.1.3. Third solution for the deposition of the germinating agent

[0086] Finally, the germinating agent may consist of one (or more) agglomerated silicon plate(s) 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.

[0087] In this case, a third solution for the deposition 101 of the germinating agent can consist of placing the plate(s) on the bottom 11 (and optionally on the side wall(s) 12) of the crucible 1.

[0088] 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.

[0089] Once the germinating agent has been deposited, and prior to the implementation of the directional solidification step, the method comprises various conventional steps known to those skilled in the art.

[0090] 3.1.2. Crucible loading step

[0091] In particular, the method comprises a step 102 of loading the crucible 1 (for example manually) with silicon of 2N purity (metallurgical quality).

[0092] The charge of metallurgical silicon of purity 2N can be arranged in solid form in the crucible 1. In particular, the loading step 102 can consist of depositing pieces or granules of silicon of purity 2N in the solid state.

[0093] The use of a metallurgical grade silicon charge (2N purity) at the input of the process makes it possible to reduce the manufacturing cost of 3N+ purity silicon.

[0094] The composition of this metallurgical grade silicon charge may 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% ([Al]<0.2%), and • the calcium concentration of the silicon charge is less than 0.2% ([Ca]<0.02%).

[0095] Optionally, this silicon charge may have an iron content of less than 2000 parts per million by weight (or “ppmw”, acronym for the Anglo-Saxon expression “part per million weight”).

[0096] Once the crucible is filled with the metallurgical silicon charge, a melting step 103 of the metallurgical silicon charge is implemented.

[0097] 3.1.3. Merging step

[0098] The melting step 103 makes it possible to change the silicon charge from the solid state to the liquid state.

[0099] To enable the melting of the metallurgical silicon charge, the temperature inside the crucible 1 is increased above the melting temperature of silicon (1410°C).

[0100] More specifically, the heating system 3 is activated to increase the temperature of the metallurgical silicon charge from the upper opening (and possibly actually from the bottom 11) of the crucible 1.

[0101] 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.

[0102] When the metallurgical silicon feedstock is completely melted (i.e., the silicon feedstock is in a liquid state), the directional solidification step 104 can be implemented.

[0103] 3.1.4. Directional solidification step

[0104] Directional solidification may have a columnar structure or an equiaxed structure.

[0105] 3.1.4.1. Advantage

[0106] The directional solidification step 104 makes it possible to concentrate the impurities of the metallurgical silicon charge in the upper part of the silicon ingot by solidifying it as quickly as possible.

[0107] This accumulation of impurities in the upper part of the silicon ingot is obtained by segregation.

[0108] 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.

[0109] In fact the distribution of impurities follows Scheil's law:

[0110] = fcaci ~

[0111] With k = Cs / Cl 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 more detail below.

[0112] Thus, during the directed solidification of the silicon, the impurities contained in the silicon of 2N purity preferentially remain 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.

[0113] The more the solid fraction 53 increases, the greater the concentration of impurities in the liquid phase 52.

[0114] 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 again, this mixing can be mechanical mixing or gas mixing.

[0115] 3.1.4.2. Principle

[0116] 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.

[0117] The temperature at the upper opening of the crucible 1 is higher than the temperature of the bottom 11 of the crucible 1. This induces the solidification of the silicon from the bottom 11 of the crucible towards the upper opening according to a rising solidification front (solid / liquid interface).

[0118] 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 included: • between 1500°C, preferably 1450°C at the start of the directional solidification stage, and • 1410°C at the end of the directional solidification stage, the tem silicon melting temperature 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).

[0119] Advantageously, the temperature at the surface of the silicon in the liquid state is kept 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).

[0120] Indeed, the encapsulation of silicon in the liquid state induces a risk of cracking of the ingot.

[0121] To reduce the risks of encapsulation, a thermal gradient is applied between the bottom 11 and the upper opening of the crucible 1 throughout the directional solidification step.

[0122] 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 block of silicon obtained after trimming, particularly in the production of Li-ion battery anodes.

[0123] 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 rate between 2 and 6 cm / h (unlike a directional solidification furnace for solar where the solidification rate 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 crucible 1).

[0124] 3.1.4.3. Optional sub-steps

[0125] 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 1.5 L / min, preferably of the order of 1 L / min for a concentration of carbon-based compound of 100%.

[0126] The injection of a carbon-based gas makes it possible to contaminate the silicon 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.

[0127] 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%.

[0128] At the end of the directional solidification step 104, a silicon ingot in the solid state is obtained.

[0129] 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 experienced during cooling (the faster the ingot is cooled, the greater the number of dislocations generated). Of course, excessively rapid cooling, which would lead to the formation of dislocations, is still avoided. uration of the graphite ingot and / or crucible. The aim will therefore be to cool 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 omitted.

[0130] A step of demolding and trimming the silicon ingot is then carried out to obtain a block of silicon of purity 3N+.

[0131] During trimming, the upper portion of the silicon ingot is cut to a height between 70 and 90% of the total height of the block. This portion of the silicon ingot contains the majority of impurities (this portion corresponding to the last volume of silicon having been solidified). The greater the thickness of the trimmed silicon layer, the purer the remaining silicon block (lower part of the ingot).

[0132] The weight of the remaining lower portion is between 400 kg and 2,000 kg. The concentrations of this remaining lower portion in impurities are: • [Fe] < 100 ppmw, • [Al]<150ppmw, • [Ca]<50ppmw, • [Ni]<20ppmw, • [Cu]<20ppmw, • [O]< lOOOppmw, • [N] <50 ppmw.

[0133] The remaining lower portion thus obtained can be sprayed during the spraying phase 20.

[0134] 3.2. Spraying phase

[0135] The spraying phase 20 makes it possible to obtain a powder of micrometric particles of silicon of purity 3N+ which can be used for the production of Li-ion battery anodes.

[0136] The spraying phase 20 comprises: • a step of crushing the silicon block to obtain silicon granules of 3N+ purity 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 micrometric and submicron sizes (between 0.5 and 20 pm), for example to sizes less than 10 pm and preferably less than 5 pm.

[0137] 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 block. or silicon bricks in 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.

[0138] The crushing step is conventionally known to those skilled in the art and will not be described in more detail below.

[0139] 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.

[0140] This technique consists of placing the 3N+ purity silicon granules 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 through jet nozzles. The size reduction is accomplished through collisions between the particles by the transmission of energy.

[0141] The ground 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 (for example 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.

[0142] 3.2.1. Conclusions

[0143] The main disadvantages of silicon Li-ion battery anodes concern: • their manufacturing cost, and • their resistance to cycling.

[0144] The method described above provides a solution to these two problems.

[0145] 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 cost of producing 3N+ quality silicon that can be used for the manufacture of lithium-ion battery anodes.

[0146] 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.

[0147] The reader will have understood that numerous modifications can be made to the invention described above without materially departing from the new teachings and advantages described here.

Claims

Claims

1. A method of manufacturing silicon of 3N purity or higher, the method comprising a phase of forming (10) a silicon block by directional solidification in a directional solidification furnace including a crucible, said forming phase comprising: • 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, characterized in that 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.

2. Method according to claim 1, 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.

3. Method according to any one of claims 1 or 2, in which the germinating agent comprises a nitride-based fluid medium including particles of a 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 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 of a size between 1 and 200 microns extend in projection, • 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 particles with a size between 1 and 200 microns.

4. Method according to the preceding claim, in which the concentration of particles in the fluid medium is between 20 and 60% by weight of the fluid medium.

5. Method according to any one of claims 1 or 2, 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 placing the powder at the bottom of the crucible to obtain a layer of particles of a thickness between 1 and 10 millimeters.

6. Method according to any one of claims 3 to 5, in which the particles of size between 1 and 200 microns are chosen from: • silicon-based particles such as SiC particles, SiO particles, SiO2 particles, Si3N4 particles, • alumina A12O3 particles, • ceramic particles.

7. A method according to any one of claims 1 or 2, wherein the germinating agent comprises at least one silicon wafer 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 of arranging said and at least one wafer at the bottom of the crucible.

8. Method according to any one of claims 1 to 7, in which during the injection sub-step: • the gas is injected at a flow rate of 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 a further sub-step injection of 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 of between 0.5 and 150 L / min, and • the concentration of the gas in carbon-based compound 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. Manufacturing method according to any one of claims 1 to 11, which further comprises a phase of pulverizing (20) the silicon block to obtain a silicon powder.

13. A manufacturing method according to claim 12, wherein the pulverizing step comprises a mechanical grinding step, such as fluidized bed grinding to reduce the size of the silicon particles to micron and submicron sizes.

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