A silicon-containing suspension for use in the manufacture of negative electrode materials for secondary batteries and a method for producing such a suspension

Abstract

The present invention relates to a silicon-containing suspension for use in the manufacture of negative electrode materials for secondary batteries, to a method for producing such a suspension and to a composite powder obtainable from such a suspension.

Description

[0001] A SILICON-CONTAINING SUSPENSION FOR USE IN THE MANUFACTURE OF NEGATIVE ELECTRODE MATERIALS FOR SECONDARY BATTERIES AND A METHOD FOR PRODUCING SUCH A SUSPENSION

[0002] TECHNICAL FIELD AND BACKGROUND

[0003] The present invention relates to a silicon-containing suspension for use in the manufacture of negative electrode materials for secondary batteries, to a method for producing such a suspension and to a composite powder obtainable from such a suspension.

[0004] Lithium ion (Li-ion) batteries are currently the best performing batteries and already became the standard for portable electronic devices. In addition, these batteries now rapidly gain ground in other industries such as automotive and electrical storage. Enabling advantages of such batteries are a high-energy density combined with a good power performance.

[0005] A Li-ion battery typically contains a number of so-called Li-ion cells, which in turn contain a positive electrode, also called cathode, a negative electrode, also called anode, and a separator which are immersed in an electrolyte. The most frequently used Li-ion cells for portable applications are developed using electrochemically active materials such as lithium cobalt oxide or lithium nickel manganese cobalt oxide for the cathode and a natural or artificial graphite for the anode.

[0006] It is known that one of the important limitative factors influencing a battery's performance and in particular a battery's energy density is the active material in the anode. Therefore, to improve the energy density, the use of electrochemically active materials comprising silicon, in the negative electrode, has been investigated over the past years.

[0007] In the art, the performance of a battery containing silicon-based materials is generally quantified by a so-called cycle life of a full-cell, which is defined as the number of times or cycles that a cell comprising such material can be charged and discharged until it reaches 70% of its initial discharge capacity. Most works on silicon-based materials are therefore focused on improving said cycle life. A drawback of using a silicon-based material in an anode is its large volume expansion during charging, which is as high as 300% when the lithium ions are fully incorporated, e.g. by alloying or insertion, in the anode's active material - a process often called lithiation. The large volume expansion of the silicon-based materials during lithium incorporation may induce stresses in the silicon-based particles, which in turn could lead to a mechanical degradation of the silicon material. Repeated periodically during charging and discharging of the Li-ion battery, the repetitive mechanical degradation of the silicon-based material may reduce the life of a battery to an unacceptable level.

[0008] Further, a negative effect associated with silicon-based materials is that a thick SEI, a Solid-Electrolyte Interface, may be formed on the anode. A SEI is a complex reaction product of the electrolyte and lithium, which leads to a loss of lithium availability for electrochemical reactions and therefore to a poor cycle performance, which is the capacity loss per charging-discharging cycle. A thick SEI may further increase the electrical resistance of a battery and thereby limit its ability to charge and discharge at high currents.

[0009] In principle, the SEI formation is a self-terminating process that stops as soon as a 'passivation layer' has formed on the surface of the silicon-based material. However, because of the volume expansion of silicon-based particles, both silicon- based particles and the SEI may be damaged during discharging (lithiation) and recharging (delithiation), thereby freeing new silicon surface and leading to a new onset of SEI formation.

[0010] To solve the above-mentioned drawbacks, the silicon-based materials typically contain nano-sized silicon-based particles, which may be mixed with at least one carbonaceous material suitable to protect the silicon-based particles from electrolyte decomposition and to accommodate volume changes. Such a carbonaceous material is preferably forming a matrix in which the nano-sized silicon-based particles are embedded and well dispersed.

[0011] However, producing nano-sized silicon-based particles, which are not agglomerated and which can easily be dispersed in a carbonaceous matrix is not straight forward. One way to produce such nano-sized silicon-based particles consists in milling a silicon powder in a liquid, thereby forming a suspension, until the desired particle size distribution of the silicon-based particles is obtained. Since the silicon-based particles get very small during the process, with average sizes below 100 nm, the viscosity of the suspension increases proportionally, which leads to high power consumptions of the mills, clogging issues and therefore an increased throughput time. The viscosity of the suspension inside the mill may even be so high that reaching the desired particle size distribution of the silicon-based particles is impossible because the mill can no longer operate.

[0012] US2003 / 0246167 discloses a mixture of silicon particles, at least one carbon compound and optionally at least one dispersant. CN114566637 discloses a dispersion comprising a silicon powder in an organic solvent, a dispersant and a carbon material. US2022 / 0209215 discloses a dispersion comprising silica, a dispersant and a liquid medium.

[0013] It is an objective of the present invention to provide a silicon-containing suspension, which is stable and easy to manufacture, and which once used in the manufacturing of a composite powder is advantageous in that the battery containing such a powder as negative electrode active material achieves a high capacity combined to a long cycle life. A composite powder obtainable from the silicon-containing suspension also forms part of the invention.

[0014] SUMMARY OF THE INVENTION

[0015] This objective is achieved by providing a silicon-containing suspension according to the invention, said silicon-containing suspension being stable and easy to manufacture, and which once used in the manufacturing of a composite powder is advantageous in that the battery containing such a powder as negative electrode active material achieves a high capacity combined to a long cycle life, as demonstrated in Examples 1 to 11 compared to Counterexamples 1 and 2. A composite powder obtainable from the silicon-containing suspension also forms part of the invention.

[0016] DETAILED DESCRIPTION

[0017] In the following detailed description, preferred embodiments are described in detail to enable practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. To the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description and accompanying drawings. Preferred embodiments of the different aspects of the invention can be combined with one another, unless it is specifically mentioned otherwise.

[0018] Milling, in particular bead milling is a process of grinding and dispersing materials into fine particles. Small beads, typically having diameters of 0.05 mm to 0.5 mm, are used as grinding media. The process is called wet milling as the particles are grinded and dispersed into a liquid. The suspension consists of a powder comprising particles, a liquid and optionally additives which can be dispersing agents and / or rheology modifiers such as viscosity-reducing agents. The beads are placed into a chamber with the sample. The beads and the sample are agitated by a rotating shaft called the rotor. The rotor is so designed to accelerate the beads to a maximum velocity. Using this kinetic energy the beads collide and crush the particles in the sample, reducing their size. This process is continued until the desired particle size is reached. The beads are kept in the milling chamber using either a sieve, a gap or a dynamic classifier. This allows the suspension to recirculate through the mill, whilst the beads are kept in the milling chamber. The amount of energy required to break down the particles to a specific particle size is called the specific energy. The specific energy is calculated by multiplying the time elapsed with the total amount of net power consumed by the mill, divided by the mass of solids in the suspension expressed in tons. The particle size of the milled particles depends on the size and composition of the beads, the amount of beads used, the milling time, the milling speed and the ductility and viscosity of the suspension.

[0019] The first aspect of the present invention concerns a silicon-containing suspension for use in the manufacture of negative electrode materials for secondary batteries, the suspension comprising at least a liquid, silicon-based particles and a viscosityreducing agent, wherein the content of silicon-based particles is from 10 weight% to 60 weight% relative to the total weight of the suspension and the content of the viscosity reducing agent is at least equal to 0.01 weight% relative to the content of the silicon-based particles. The liquid and the viscosity-reducing agent need to be two different products, otherwise the suspension would comprise either way too much viscosity-reducing agent, or not enough liquid to reach the required viscosity. The content of the viscosity-reducing agent is preferably at least equal to 0.02 weight%, more preferably at least equal to 0.05 weight%, even more preferably at least equal to 0.10 weight%, particularly preferably at least equal to 0.20 weight%, even more particularly preferably at least equal to 0.50 weight% and utterly preferably at least equal to 1.00 weight%, relative to the content of silicon-based particles. The content of the viscosity-reducing agent is preferably not more than 4.00 weight%, more preferably not more than 3.00 weight%, even more preferably not more than 2.00 weight%, particularly preferably not more than 1.50 weight% and utterly preferably not more than 1.00 weight%, relative to the content of silicon-based particles. The content of the viscosity-reducing agent is preferably from 0.01 weight% to 4.00 weight%, more preferably from 0.02 weight% to 3.00 weight%, even more preferably from 0.05 weight% to 2.00 weight% and particularly preferably from 0.10 weight% to 1.50 weight%, relative to the content of silicon-based particles, although these are merely preferred ranges and other combinations of lower and upper limits are possible.

[0020] During the milling of a suspension comprising silicon-based particles, the viscosity increases with the decrease in particle size. This can result in suspensions reaching a viscosity that high that it becomes impossible to proceed with the milling, because, for example, the milling media, or beads, get stuck in the highly viscous suspension and the sieves of the mill get clogged. The consequence is that it becomes impossible to reach the desired particle size distribution of the silicon- based particles, or that it requires a throughput time which is incompatible with an industrial application. Likewise, when the milled silicon-based particles are not sufficiently well dispersed, they have a tendency to re-agglomerate, with the consequence that the particle size does not decrease anymore and that the desired particle size distribution of the silicon-based particles is not reached.

[0021] These technical issues are solved by the presence of a viscosity-reducing agent, or rheology modifier, in the suspension, which works by altering the intermolecular forces within the liquid medium, reducing its resistance to flow, as explained in Perspective Chapter: Additives for Viscosity Reduction of Heavy Oils, Garcia- Mayorga J.C., Escobar- Barrios V, 2025. Viscosity-reducing agents have a different operation mode compared to dispersing agents, or dispersants, in that they do not need to adsorb onto particle's surfaces, or form monolayers to stabilize the suspension and prevent agglomeration, as explained in Rheology modifiers and pigment dispersants, J.B. Clarke, Surface Application of Paper Chemicals, 1997, pp. 109-128. When dispersing agents are used, the monolayer formed creates repulsive forces between particles, ensuring uniform distribution within the medium. The primary function of dispersing agents is to maintain the stability of solid particles in a liquid, thereby preventing sedimentation and flocculation. A dispersing agent, or dispersant, facilitates the dispersion and stabilization of solid particles in a liquid medium by lowering the interfacial tension between two components, as explained in Dispersing agents, Encyclopedia of Surface and Colloid Science second edition, CRC Press, 2006, pp. 1-10.

[0022] Conversely, the viscosity-reducing agents modify the overall viscosity of the liquid, making it easier to handle and apply. In the present invention no monolayer is formed around the silicon-based particles. Instead, it is based on the contraction of the electric double layer in the presence of multivalent ions. Due to this contraction, more free solvent is available to flow between the particles, thereby reducing the viscosity. Thus, in the present invention, the intermolecular forces are reduced within the liquid medium, in accordance with the definition of a viscosity reducing agent. In order to obtain a monolayer, as it is the case for dispersing agents and given the size of our silicon-based particles, it would be necessary to add an agent at a concentration of more than 5.0 weight% relative to the content of silicon-based particles, which, as previously mentioned, is not the case here. An additional advantage of viscosity-reducing agents compared to dispersing agents or dispersants is therefore that viscosity-reducing agents are efficient at lower concentrations, inferior or equal to 4.0 weight% and down to less or equal to 1.0 weight % relative to the content of silicon-based particles, which means that the process requires a lower quantity of agents and is therefore cheaper.

[0023] Thanks to the presence of the viscosity-reducing agent, both the re-agglomeration of milled silicon-based particles and the dramatic increase of the viscosity are avoided and the desired particle size distribution of the silicon-based particles is reached in an acceptable throughput time.

[0024] The suspension according to the invention has a viscosity i measured at a sheer rate of 1 s’1and a viscosity iooo measured at a sheer rate of 1000 s’1, wherein the ratio i / Viooo is lower than 200, preferably lower than 100, more preferably lower than 70, even more preferably lower than 50, particularly preferably lower than 30, more particularly preferably lower than 20 and utterly preferably lower than 10. It is preferable to have a suspension with a ratio i / iooo lower than the values mentioned above, to keep the suspension easily processable, both in a mill and in further steps, such as the spray drying step or the preparation of a negative electrode material, for example a composite powder. When the ratio Vi / Viooo is higher than the values mentioned above, the suspension becomes too viscous and issues, such as a too high power consumptions of the mills, clogging of the mills' sieves and difficulties to spray dry the suspension, appear.

[0025] For the same reasons, it is preferable for the suspension according to the invention to have a viscosity iooo at a sheer rate of 1000 s’1lower than 500 mPa.s, preferably lower than 400 mPa.s, even more preferably lower than 300 mPa.s, particularly preferably lower than 200 mPa.s, even more particularly preferably lower than 100 mPa.s and utterly preferably lower than 50 mPa.s. The viscosity values are measured according to a method described in the Analytical Methods section.

[0026] In a preferred embodiment according to the first aspect of the invention, the viscosity-reducing agent contained in the suspension comprises an ionic compound, i.e. an ionic assembly of positively charged ions (or cations) and negatively charged ions (or anions). Once added to the suspension, a dissociation reaction occurs, causing the ionic compound to separate into positively charged ions (or cations) and negatively charged ions (or anions) and therefore the suspension comprises, in particular, negatively charged ions (or anions). The surface of the silicon-based particles in the suspension comprises silanol groups Si-OH. Without being bond by theory, the inventors believe that the negatively charged ions interact with the silanol groups through hydrogen bonds. This likely triggers the presence of a negative charge on the surface of the silicon-based particles, which causes the silicon-based particles to repel one another electrostatically and therefore to align better with the flow of the liquid ; hence, as the viscosity is a measure of the resistance to flow, the viscosity of the suspension lowers. Preferably, the viscosityreducing agent is an ionic compound.

[0027] In another preferred embodiment according to the first aspect of the invention, the ionic compound comprises phosphate ions PO43'. In that case, the ionic compound is preferably a metal phosphate powder M3PO4 or a mixture of metal phosphate powders M3PO4, M being a metal selected from a list of one or more of Li, Na and K. In that case, the suspension comprises phosphate ions PO43'. The advantage of metal phosphate salts over other sources of phosphates is that they are widely available, relatively cheap and easy and safe to process. Furthermore, steric hindrance of metal phosphates is much more limited than, for example, for polyphosphates, or phosphate esters, since those are larger molecules and thus their size can limit their ability to interact with the silicon-based particles. Additionally, for polymeric phosphates, a solvent compatible with the polymer chain would be necessary for the viscosity-reducing agent to get dispersed in the suspension, otherwise it would likely simply float at the surface of the suspension, thereby having no effect at all on the viscosity of the suspension. Preferably, the viscosity-reducing agent is a metal phosphate powder M3PO4 or a mixture of metal phosphate powders M3PO4, M being a metal selected from a list of one or more of Li, Na and K. Particularly preferably, the viscosity-reducing agent is lithium phosphate U3PO4. Metal phosphate powders have the advantage of being abundant, relatively cheap and above all of being compatible with battery applications. Lithium phosphate has the additional advantages of the lithium ions not being dead weight in the final composite powder, once used in the negative electrode of a lithium-ion battery. Furthermore, sodium and potassium are larger ions than lithium and might thus expand the double layer more, thereby leading to a reduced effect on the reduction of the viscosity, making them slightly less preferred.

[0028] The inventors have discovered that, even at small concentrations, the presence of phosphorous, in particular in the form of phosphates, in the viscosity-reducing agent, and therefore in the suspension, had the surprising effect of maintaining the viscosity of the suspension, during the milling operations, at a very low level. Likewise, the presence of phosphorous, in particular in the form of phosphates, in the viscosity-reducing agent, and therefore in the suspension, increases the stability of the dispersion and allows for processing suspensions with high contents of silicon-based particles, up to 60 weight% relative to the total weight of the suspension. The content of phosphorous in the suspension is preferably at least 50 ppm, more preferably at least 100 ppm, even more preferably at least 200 ppm, particularly preferably at least 300 ppm, even more particularly preferably at least 500 ppm and utterly preferably at least 1000 ppm, relative to the total weight of the suspension. Other viscosity-reducing agents which are efficient in maintaining the viscosity of the suspension, during the milling operations, at a low level are one of the following compounds or a combination thereof: benzoic acid, terephthalic acid, benzyl alcohol and benzylamine. However, when tested, the efficiency was lower than with agents comprising an ionic compound, such as for example metal phosphates or metal sulfates M2SO4, M being a metal selected from a list of one or more of Li, Na and K, and in particular the efficiency was lower than with agents comprising phosphates.

[0029] The silicon in the suspension is present as silicon-based particles and more preferably as silicon particles. The silicon-based particles (or silicon particles) may have any shape, e.g. substantially spherical but also irregularly shaped, rod-shaped, plateshaped, etc. In the silicon-based particles (or silicon particles), the silicon is present in its majority as silicon "metal", to which minor amounts of other elements may have been added to improve properties, or which may contain some impurities, such as oxygen or traces of metals. When considering a representative number of silicon- based particles, for example not less than 10 distinct silicon-based particles, the average silicon content in such a silicon-based particle is 70 weight % or more, preferably 80 weight % or more, and more preferably 90 weight % or more with respect to the total weight of the silicon-based particle. Silicon-based particles having a silicon content lower than 70 weight %, preferably lower than 80 weight %, will have a too low specific capacity once integrated in a battery as negative electrode material, or part of it, and therefore should be avoided. The content of silicon in a silicon-based particle can for example be determined by an elemental mapping analysis of a cross-section of a composite particle, comprising numerous crosssections of silicon-based particles, using a high resolution FEG-SEM microscope. For the avoidance of doubt, "silicon" here needs to be understood as the chemical element silicon (Si), independent from its oxidation state. This applies to the entire document.

[0030] Furthermore, the silicon-based particles typically have a surface layer with an average molar composition SiOxwith 0<x<2, and preferably 0<x<l.

[0031] The content of oxygen in the silicon-based particles is, in average, preferably not more than 20 wt%, preferably not more than 15 wt%, more preferably not more than 10 wt%, even more preferably not more than 8 wt%, particularly preferably not more than 6 wt% and utterly preferably not more than 4 wt%, relative to the total mass of a silicon-based particle. Silicon-based particles having a too high oxygen content would suffer from an additional irreversible consumption of lithium by the formation of lithium silicate (LizSiOs, Li4SiO4) during the first lithiation of the powder, once used as negative electrode material in a battery, thus increasing the initial irreversible capacity loss of said battery.

[0032] The silicon content in the suspension is at least 5 weight%, preferably at least 10 weight%, more preferably at least 15 weight%, particularly preferably at least 20 weight% and utterly preferably more than 30 weight% relative to the total weight of the suspension. The silicon content in the suspension is preferably at most 60 weight%, more preferably at most 50 weight% and even more preferably at most 40 weight%, relative to the total weight of the suspension.

[0033] The silicon contained in the suspension according to the invention, is present as silicon-based particles characterized by a volume-based distribution of sizes having a dvgo, wherein the dvgo is lower than 200 nm, preferably lower than 170 nm, even more preferably lower than 150 nm, particularly preferably lower than 120 nm and utterly preferably lower than 100 nm. For the sake of clarity, a dvgo of 200 nm for example, would here mean that 90% in volume of the silicon-based particles have a size smaller than 200 nm and that 10% in volume of the silicon-based particles have a size larger than 200 nm. The volume-based size distribution of silicon-based particles is based on a Disc Centrifuge (CPS) particle size measurement, as described in the Analytical Methods section. Silicon-based particles having a volume-based size distribution with a dvgo larger than 200 nm are more subject to fractures during their lithiation, causing a dramatic reduction of the cycle life of a battery containing such silicon-based particles in the negative electrode.

[0034] It is considered that the dvgo value is not affected by the process of making a composite powder, which means that the dvgo value of the silicon-based particles used as precursor in the process is the same as the dvgo value of the silicon-based particles comprised in the composite powder.

[0035] In yet another embodiment according to the first aspect of the invention, the liquid contained in the suspension according to the invention, is polar. A polar liquid is a liquid containing polar molecules, in which at least two atoms do not share electrons equally in a covalent bond. An electric dipole forms, with part of the polar molecule carrying a slight positive charge and the other part carrying a slight negative charge. Examples of polar liquids are water, ammonia, ethanol and more generally alcohols. Silicon-based particles having a dvgo lower than 200 nm tend to disperse stably in polar liquids, such as ethanol or other alcohols, but they aggregate when non-polar liquids such as cyclohexane are used or even added to the polar liquids. This behaviour is typically understood using the Derjaguin- Landau-Verwey-Overbeek (DLVO) theory, i.e. decreased electrostatic repulsion and increased Van der Waals attraction between colloidal particles in a non-polar liquid. Since aggregation of silicon-based particles in the suspension is to be avoided, polar liquids are preferred.

[0036] In another preferred embodiment according to the first aspect of the invention, the polar liquid contained in the suspension according to the invention, is an alcohol or a mixture of alcohols. Examples of alcohols are ethanol, propanol, isopropanol, isobutanol (IBU) and isopropyl alcohol (IPA). The polar liquid is particularly preferably isobutanol (IBU) and / or isopropyl alcohol (IPA). IBU and IPA are advantageous in that they have a high auto-ignition temperature, their safety and health hazards are not too constraining for an industrial use and they are commercially available in large quantities. Their boiling point is also not too high and they are thus relatively easy to dry / evaporate. Furthermore, alcohols with longer chains typically become less polar while the length of the chain increases, and thus the stabilization effect on the suspension become less effective.

[0037] In yet another embodiment according to the first aspect of the invention, the suspension may additionally comprise a carbon precursor material capable of fully decomposing into carbon at a temperature above 600°C or above 800°C.

[0038] The advantage of having the carbon precursor material mixed directly in the suspension together with the silicon-based particles is to obtain an homogeneous suspension with an intimate mixture of both elements, which will, after heat treatment of the dried suspension, result in composite particles, comprising silicon- based particles embedded in a carbonaceous matrix, with no or nearly no silicon- based particles exposed to the air and thus, once used in a battery, to the electrolyte. Less exposure of the silicon-based particles to the electrolyte results in less electrolyte decomposition, thus less SEI formation and consequently a longer cycle life. Alternatively, the suspension could be dried before the carbon precursor material is added, in a dry state, but the intimate contact between the silicon-based particles and the carbon precursor material would not be obtained and after heat treatment, a substantial proportion of the silicon-based particles comprised in the composite particles would be exposed to the air, which would result in a battery having a shorter cycle life, for the reasons previously given.

[0039] The carbon precursor material in the suspension may be any type of carbon precursor, but preferably is one, or a mixture of several of the following materials: polyvinyl alcohol (PVA), polyvinyl chloride (PVC), sucrose, coal-tar pitch, petroleum pitch, lignin, and a resin. Preferably, the carbon precursor decomposes into soft carbon when heated at a temperature above 600°C. The carbon precursor, when heated at a temperature above 600°C, preferably has a carbon yield at least equal to 40 wt% - meaning that 40 wt% of the carbon precursor has decomposed into carbon and 60 wt% of the carbon precursor has decomposed into gases - more preferably at least equal to 50 wt% and particularly preferably at least equal to 60 wt% .

[0040] The ratio of the content of carbon precursor material to the content of silicon-based particles, both relative to the total weight of the suspension, is at most equal to 3.0, preferably at most equal to 2.5, more preferably at most equal to 2.0, even more preferably at most equal to 1.8, particularly preferably at most equal to 1.5, more particularly preferably at most equal to 1.2 and utmost preferably at most equal to 1.0. The carbonaceous matrix material obtained after a heat treatment of the carbon precursor above 600°C, or above 800°C, typically having a specific capacity of 200-300 mAh / g, i.e. at least 10 times lower than the specific capacity of silicon, it is preferable to keep the weight ratio "carbon precursor / silicon-based particles" as low as possible, but still high enough to have a full embedment and a full coverage of the silicon-based particles in the carbonaceous matrix material.

[0041] This in order to avoid a direct contact between the silicon-based particles and the liquid electrolyte, triggering the formation of an unstable SEI layer and a decrease of the cycle life of a battery comprising such a composite powder. Preferably, the ratio of the content of carbon precursor material to the content of silicon-based particles, both relative to the total weight of the suspension, is at least equal to 0.5, preferably at least equal to 0.8 and even more preferably at least equal to 1.0. Preferably, the ratio of the content of carbon precursor material to the content of silicon-based particles, both relative to the total weight of the suspension, is from 0.5 to 3.0, more preferably from 0.8 to 2.5 and particularly preferably from 1.0 to 2.0, however these are merely preferred ranges and other combinations of lower and upper limits are possible.

[0042] The second aspect of the present invention concerns a method for producing a silicon-containing suspension, preferably the suspension according to the first aspect of the invention, the method comprising the steps of: a) providing a silicon-based powder comprising silicon-based particles having a volume-based distribution of sizes with a dvgo higher than 500 nm, preferably higher than 1 pm, more preferably higher than 2 pm, even more preferably higher than 5 pm and ultimately preferably higher than 10 pm, but preferably lower than 20 pm, a viscosity-reducing agent and a liquid and loading them into a mill such as to obtain a mixture, b) milling the mixture of step a) until the volume-based distribution of sizes of the silicon-based particles has a dvgo lower than 200 nm, preferably lower than 170 nm, even more preferably lower than 150 nm, particularly preferably lower than 120 nm and particularly preferably lower than 100 nm.

[0043] The viscosity-reducing agent in step a) preferably comprises an ionic compound and more preferably phosphates PO43'. Even more preferably, the viscosityreducing agent in step a) is M3PO4, M being a metal selected from a list of one or more of Li, Na and K. The liquid in step a) is preferably a polar liquid, more preferably an alcohol, and even more preferably isobutanol (IBU) and / or isopropyl alcohol (IPA).

[0044] The mills in step a) are preferably equipped with a ceramic inner lining, to avoid, or at least limit the iron contamination that may result from the use of a standard steel-based mill, with a transfer of iron from the mill to the suspension and later to the negative electrode material obtained from the suspension. Iron is considered to be a contaminant that can lead to reduced electrochemical performances when present in the negative electrode of a battery.

[0045] The milling in step b) is typically done with beads having a diameter from 0.05 mm to 0.5 mm, preferably from 0.1 mm to 0.4 mm and more preferably from 0.1 mm to 0.3 mm. The beads are preferably made of zirconium oxide ZrOz, more preferably of yttrium-stabilized zirconium oxide (or zirconia), but may be in a different material, as long as they do not lead to an undesired contamination of the suspension, for example an iron contamination.

[0046] In a preferred embodiment according to the second aspect of the invention, a step of dry jet-milling of a silicon-based powder can occur before step a). The advantage of jet-milling the silicon-based powder first is a faster reduction of the size of the silicon-based particles. This way, a coarser and therefore cheaper silicon-based powder may be used as precursor. For example, a silicon-based power comprising silicon-based particles having a volume-based distribution of sizes with a dvgo larger than 100 pm may be used. During the step of jet-milling, the dV9o might be reduced to a value as small as a few microns, typically from 0.5 to 20 pm and preferably from 5 to 15 pm. This jet-milled powder is then used as precursor in step a) of the method described above. Reducing the size of a silicon-based powder with a dvgo larger than 100 pm, purely by wet-milling, without this initial step of dry jet-milling would require a much longer processing time.

[0047] In another preferred embodiment according to the second aspect of the invention, step b) may be done in two separate consecutive sub-steps, b) 1) and b)2). Instead of a 1-step milling with beads having a typical diameter from 0.05 mm to 0.5 mm, a 2-step milling or cascade milling may be used. In sub-step b)l), the mixture of step a) is first milled with slightly larger beads having a diameter from 0.2 mm to 0.5 mm, preferably from 0.2 mm to 0.4 mm and more preferably from 0.2 mm to 0.3 mm, until the volume-based distribution of sizes of the silicon-based particles has a dvgo between 200 nm and 500 nm, preferably between 200 nm and 400 nm and more preferably between 200 nm and 300 nm. Then, in sub-step b)2), the mixture of step b)l) is further milled with smaller beads having a diameter of at most 0.2 mm, preferably of at most 0.1 mm, until the volume-based distribution of sizes of the silicon-based particles has a dV9o lower than 200 nm, preferably lower than 150 nm and more preferably lower than 100 nm. The advantage of cascade milling is that the use of larger beads in the first step b)l) allows for a more efficient breaking of the large silicon-based particles and that the use of smaller beads in the step b)2) allows for a faster reduction of the size of the "already premilled" silicon-based particles, in particular in the last part of the milling process, going from a dvgo of 200 nm down to a dvgo of 100 nm or less, which is the longest part of the milling process. Since the beads are smaller, there are more of them per unit of volume and therefore the probability of impact increases and the size reduction occurs faster. In summary, the advantage of cascade milling is that the beginning of the milling curve is optimized, while avoiding operational issues such as sieve clogging, since larger sieves can be used in combination with the larger beads. The drawback of cascade milling, however, is that the beads need to be changed between sub-steps b)l and b)2, which requires some time, but less than the time gained from the faster reduction of the size of the particles.

[0048] The third aspect of the present invention concerns a method for producing a composite powder, for use as a negative electrode material for secondary batteries, comprising the steps of: a) providing a suspension according to the first aspect of the invention, comprising a carbon precursor material capable of fully decomposing into carbon at a temperature above 600°C, b) removing the liquid from the suspension of step a) until a powder with a residual liquid content of at most 5.0 weight%, relative to the total weight of the powder, is obtained, c) heating up the powder of step b) at a temperature above 600°C and below 1100°C, under an oxygen-free atmosphere, to obtain a composite powder.

[0049] The residual liquid content in the powder of step b) is preferably at most 3.0 weight%, more preferably at most 2.0 weight%, even more preferably 1.0 weight%, particularly preferably at most 0.5 weight% and even more particularly preferably at most 0.3 weight%, relative to the total weight of the powder. The lower the residual liquid content, the dryer the powder and the better the composite powder will perform in a battery. A too high residual liquid content will result in a sticky, agglomerated powder, which will be more difficult to process further and which will eventually result in a composite powder having lower performances in a battery.

[0050] The type of drying equipment used to remove the liquid from the suspension is not particularly limiting, it may for example be a spray dryer, a screw dryer, a paddle dryer or a thin film dryer. In step c), oxygen-free atmospheres that may be used are for example nitrogen or argon. The temperature at which the powder A is heated is preferably comprised between 800°C and 1050°C and more preferably comprised between 900°C and 1000°C. At those temperatures the formation of silicon carbide, which is not desired since it is an electrochemically inactive compound, is limited.

[0051] An optional step may be performed before step c). The powder of step b) is heated at a temperature above the softening point of the carbon precursor material, for example above 400°C, to obtain a mixture. The mixture is then mixed under high shear to ensure that the silicon-based particles and the carbon precursor material are intimately mixed before the final heat treatment step takes place. The quality of the coating of the carbonaceous matrix material around the silicon-based particles in the composite powder will be improved and less silicon-based particles will be exposed to the air.

[0052] Optionally, a final step d) of crushing the heat-treated material of step c) can be performed. The crushing step should be done at energies as low as possible, to deagglomerate the composite particles, without damaging their structure, which would for example expose the surface of silicon-based particles to contacts with the liquid electrolyte in a battery, followed by the consequences already described earlier.

[0053] Carbon additives, such as graphite, graphene, carbon nanotubes or a mixture thereof, may be added to the powder obtained at step b), or to the mixture obtained after the optional step, but before the final heat-treatment step takes place. The carbon additives act as a spacer between the particles of the powder obtained at step b), thereby preventing an agglomeration of said particles into an agglomerated powder, in particular during the heat-treatment step. In the absence of such a spacer, the agglomerated powder, in order to be used in the negative electrode of a battery, may require an intensive mechanical treatment, such as an intensive grinding step, which may result in a weakening of the integrity of the carbonaceous matrix material and eventually in a reduced cycle life of a battery comprising such an agglomerated powder. With the addition of carbon additives, a soft crushing step is sufficient to obtain deagglomerated particles. The fourth aspect of the present invention concerns a composite powder for use as a negative electrode material for secondary batteries, preferably a composite powder obtainable from the method according to the third aspect of the invention, previously described. As appreciated by the skilled person all embodiments directed to the method according to the third aspect of the invention apply mutatis mutandis to the composite powder obtainable from the method according to the invention.

[0054] The composite powder comprises particles comprising a carbonaceous matrix material with silicon-based particles embedded therein. This means that the composite particles, are on average larger in size than the silicon-based particles, since they comprise these latter. The composite particles are typically of micrometric size, while the silicon-based particles are typically of nanometric size. It is also meant that the surface of the silicon-based particles is covered with the carbonaceous matrix material for at least 50% of the surface and preferably that the silicon-based particles are completely covered with the carbonaceous matrix material, to ensure a proper protection against the reaction with the electrolyte during cycling. In other words, the silicon-based particles and the carbonaceous matrix material are not just mixed together, since a proper coverage of the surface of the silicon-based particles cannot be obtained that way. This can be visually confirmed based on the analysis of one or several SEM images of cross-sections of composite particles, comprising the silicon-based particles.

[0055] As already mentioned, a negative effect associated with silicon is that a thick SEI, a Solid-Electrolyte Interface, may be formed on the anode, in particular on the silicon-based particles. Since the silicon-based particles are affected by a large volume variation during the lithiation / delithiation process in the battery, the SEI which has already formed might break again, leading to a continuous consumption of lithium and thereby to a dramatic drop of the cycle life of the battery. Protecting the surface of the silicon-based particles with the carbonaceous matrix material, at least partially, is an efficient solution against the continuous formation of the SEI and the loss in cycle life.

[0056] The ratio of carbon content (expressed in weight%) resulting from the carbonization of the carbon precursor material, forming the matrix material, over the silicon content (expressed in weight%) in the composite powder is at least equal to 0.5, preferably at least equal to 0.8, more preferably at least equal to 1.0, even more preferably at least equal to 1.2 and particularly preferably at least equal to 1.5. A too low ratio would translate into silicon-based particles not being sufficiently embedded in the carbonaceous matrix material and therefore being at least partially exposed to contacts with the electrolyte, once used in the negative electrode of a battery, with the negative consequences already mentioned earlier. The ratio of carbon content (expressed in weight%) resulting from the carbonization of the carbon precursor material, forming the matrix material, over the silicon content (expressed in weight%) in the composite powder is at most 3.0, preferably at most 2.5 and more preferably at most 2.0. A too high ratio would translate into an excessive amount of carbonaceous matrix material, resulting in a lower specific capacity of the composite powder and a lower initial coulombic efficiency for the battery comprising such a composite powder.

[0057] The silicon-based particles embedded in the carbonaceous matrix material either form agglomerates of a size smaller than 1 pm or do not for agglomerates at all. Hence, the silicon-based particles are preferably in contact only with each other and / or with the carbonaceous matrix material.

[0058] Since the composite powder is obtained from a suspension that may comprise phosphates, the composite powder may comprise phosphorous, despite the heat treatment step. The content of phosphorous in the composite powder is preferably at least 50 ppm, more preferably at least 100 ppm, even more preferably at least 200 ppm, particularly preferably at least 300 ppm, even more particularly preferably at least 500 ppm and utterly preferably at least 1000 ppm, relative to the total weight of the composite powder.

[0059] The composite powder, once used as anode material in a battery, has a specific capacity at least equal to 600 mAh / g. With the necessity to move from thermic vehicles to clean vehicles, in particular to electric vehicles, the main objective to get acceptance from the users is to achieve driving ranges of at least 500-600 km. Since the size and weight of battery packs cannot be extended infinitely, it is necessary to produce batteries with higher energy densities and thus to produce anode materials with higher specific capacities. Preferably, said specific capacity should be at least equal to 800 mAh / g, more preferably at least equal to 1000 mAh / g, even more preferably at least equal to 1200 mAh / g, particularly preferably at least equal to 1400 mAh / g, more particularly preferably at least equal to 1600 mAh / g and utterly preferably at least equal to 1800 mAh / g. However, because a high specific capacity also implies more swelling and more mechanical deformations during the charge / discharge cycles, it is preferable not to exceed a specific capacity of 2000 mAh / g for the composite powder.

[0060] In another embodiment according to the fourth aspect of the invention, the carbonaceous matrix material comprised in the composite powder according to the invention, is soft carbon. Soft carbon corresponds to an arrangement of small disordered graphitic domains that can be converted to graphite upon heating at a temperature of 3000°C, in opposition to hard carbon which is not graphitizable. Soft carbon shows a higher electronic conductivity compared to hard carbon and is therefore preferable. Furthermore, thanks to its disordered collection of small graphitic domains, which leads to the presence of nanovoids in the carbonaceous matrix material, the volumetric expansion of a particle comprising a carbonaceous matrix material mostly comprising soft carbon, during the lithiation of the anode, is reduced compared to a particle comprising a carbonaceous matrix material mostly comprising graphite or graphene. A reduced volumetric expansion will lead to a longer cycle life in a battery.

[0061] In another embodiment according to the fourth aspect of the invention, the silicon- based particles comprised in the composite powder according to the invention, have a number-based size distribution having a dN9o, the dN9o being lower than 200 nm, preferably lower than 150 nm, even more preferably lower than 150 nm, particularly preferably lower than 120 nm and utterly preferably lower than 100 nm. For the sake of clarity, a dN9o of 200 nm for example, would here mean that 90% in number of the silicon-based particles have a size smaller than 200 nm and that 10% in number of the silicon-based particles have a size larger than 200 nm. Silicon-based particles having a number-based size distribution with a dN9o larger than 200 nm are more subject to fractures during their lithiation, causing a dramatic reduction of the cycle life of a battery containing such silicon-based particles in the negative electrode.

[0062] The number-based size distribution is based on a visual analysis, with or without assistance of an image analysis program, of a minimum number of silicon-based particles comprised in the composite powder. This minimum number of silicon- based particles is at least 1000 particles. An example of a determination of a number-based distribution of silicon-based particles is provided in the Analytical methods section.

[0063] In another embodiment according to the fourth aspect of the invention, the composite particles have a volume-based particle size distribution having a D10, a D50 and a D90, with 1 pm < D10 < 10 pm, 5 pm < D50 < 25 pm and 10 pm < D90 < 40 pm.

[0064] For the sake of clarity, a D50 of 15 pm for example, would here mean that 50% in volume of the composite particles have a size smaller than 15 pm and that 50% in volume of the composite particles have a size larger than 15 pm.

[0065] Composite particles having a volume-based size distribution with a D50 smaller than 5 pm, may have a too high specific surface and thus increase the surface of reaction with the electrolyte and the formation of SEI, which is disadvantageous for the reasons previously explained. Composite particles material having a volumebased size distribution with a D50 larger than 25 pm, may, due to their size, be more susceptible to suffer from the formation of fractures during the lithium uptake, thus leading to a reduced cycle life of the battery containing such particles.

[0066] In yet another embodiment according to the fourth aspect of the invention, the composite powder according to the invention further comprises carbon additives, such as for example graphite particles, graphene particles, carbon nanotubes, or a mixture thereof. As already mentioned previously, the presence of carbon additives facilitates the manufacturing process of the composite powder, in particular crushing steps can be performed at lower energy, which decreases the risks of damaging the structure of the composite particles.

[0067] The particles of carbon additives are preferably not fully embedded in the carbonaceous matrix material, and more preferably not at all embedded in the carbonaceous matrix material. This can be visually confirmed based on the analysis of one or several SEM images of cross-sections of the composite particles. The fact that the particles of carbon additives are not fully, or not at all embedded in the carbonaceous matrix material is beneficial since only the silicon-based particles need to be covered by the carbonaceous matrix material, hence less carbonaceous matrix material having a high irreversible capacity and a low specific capacity is needed.

[0068] Alternatively, the composite powder may also comprise exfoliated graphite particles, expanded graphite particles and / or graphene nanoplatelets, all also preferably not being fully embedded, or not at all embedded in the carbonaceous matrix material, for the same reasons as provided above.

[0069] In another embodiment according to the fourth aspect of the invention, the composite powder according to the invention has a silicon content C and an oxygen content D, both expressed in weight percent (wt%), wherein 20 < C < 60 and D < 0.20 x C. In other words, the silicon content in the composite powder is comprised between 20 wt% and 60 wt% and the oxygen content in the composite powder is not more than 20% of the silicon content in said composite powder. A composite powder having a too high oxygen content would suffer from an additional irreversible consumption of lithium by the formation of lithium silicate (Li2SiO3, Li4SiO4) during the first lithiation of the powder, thus increasing the initial irreversible capacity loss of a battery containing such a composite powder. The silicon content in the composite powder is preferably at least 30 weight% and more preferably at least 40 weight%, relative to the total weight of the composite powder. A too low silicon content would not allow the composite powder to reach a specific capacity which is high enough to meet the targets of at least 500-600 km driving range for electric vehicles.

[0070] The fifth aspect of the present invention concerns a negative electrode for a battery, preferably a lithium-ion battery, comprising the composite powder according to the present invention, as previously defined or prepared as previously disclosed. The negative electrode typically also comprises electronically conductive additives, such as carbon black, graphite particles, graphene particles, carbon nanotubes, or a mixture thereof. The content of electronically conductive additives is comprised between 0% and 10% by weight, in particular from 0.1% to 5% by weight, relative to the total weight of the negative electrode layer (excluding the current collector).

[0071] The negative electrode typically also comprises a binder or a mixture of binders. Specific examples of binders include polysaccharides, lithium-polyacrylate (Li-PAA), sodium polyacrylate (Na-PAA), potassium polyacrylate (K-PAA), polyacrylic acid (H- PAA), sodium carboxymethyl cellulose (Na-CMC), styrene-butadiene rubber (SBR). The binder(s) is / are added to improve the cohesion of the various components of the negative electrode, its mechanical strength on the current collector or even its flexibility properties. The binder(s) represent from 1% to 15% by weight, in particular from 2% to 10% by weight, relative to the total weight of the negative electrode layer (excluding the current collector). An example of a negative electrode preparation is provided elsewhere in this document.

[0072] Finally, the last aspect of the present invention concerns a battery, preferably a lithium-ion battery, comprising a negative electrode according to the present invention, and therefore comprising a composite powder according to the present invention, as previously defined or prepared as previously disclosed. A battery according to the invention more specifically comprises a negative electrode (anode) according to the invention, a positive electrode (cathode) and an electrolyte, preferably a non-aqueous electrolyte. As examples of the positive electrode, mention may be made of the positive electrode active materials selected from LiCoO2, LiNio,eMno,2Coo,202, LiNio,sMno,iCoo,i02, LiNio,8Coo,i5Alo,os02, Lii,2Nio,2Mno,e02, LiFePC , and the like. The electrolyte may be preferably a non-aqueous electrolytic solution, a non-aqueous polymer electrolyte or even a solid electrolyte. Specific examples thereof include an organic electrolytic solution obtained by dissolving lithium salt such as UCIO4, LiPF6, LiAsF6, LiBF4, USO3CF3, CH3SO3 Li, CF3SO3U or the like into a non-aqueous solvent such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), fluoro ethylene carbonate (FEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), butylene carbonate, acetonitrile, propionitrile, dimethoxyethane, tetra hydrofuran, y-butyrolactone or the like; a gel polymer electrolyte comprising polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate or the like; and a solid polymer electrolyte comprising a polymer having an ethylene oxide bond. Moreover, an additive which causes a decomposition reaction during initial charge of the lithium ion battery may be added to the electrolytic solution. Specific examples of additives include vinylene carbonate (VC), biphenyl, propane sultone (PS), fluoro ethylene carbonate (FEC), ethylene sultone (ES) or the like. The additive amount thereof is preferably not less than 0.1% by weight and not more than 20% by weight, relative to the total weight of the electrolyte. BRIEF DESCRIPTION OF THE FIGURES

[0073] Figure 1: Microscopy picture of a silicon-containing suspension according to the invention

[0074] Figure 2: Evolution of the viscosity of a selection of suspensions at 30.0 wt% solid load, at a shear rate 1000 s’1, in function of the milling specific energy and the content of viscosity-reducing agent.

[0075] Figure 3: Evolution of the dvgo particle size of the silicon-based particles in a selection of suspensions at 30.0 wt% solid load, in function of the milling specific energy and the content of viscosity-reducing agent.

[0076] ANALYTICAL METHODS USED

[0077] Determination of the volume-based distribution of silicon-based particles in suspensions

[0078] The volume-based distribution of silicon-based particles in suspensions is determined by the following method, using a disc centrifuge (CPS) particle size measurements. First, the sample is prepared by diluting the suspension in butylglycolacetate (BGA) until a concentration of 0.0075 wt% silicon is reached. The sample is homogenized using a vortex mixer. Before injection into the disc centrifuge equipment, the sample is treated with ultrasounds for 45 seconds in order to remove any aggregates and ensure a homogeneous dispersion.

[0079] Then, the density gradient is built by injecting 2 liquids with different densities in varying ratios into the disc centrifuge equipment. This will create layers with slightly different densities. The speed of the disc is set to 24000 rpm during the build-up of the gradient. The 2 liquids used are BGA and Halocarbon 1.8 oil. After completion of the gradient, the disc is allowed to spin for 45min in order to stabilize the gradient.

[0080] Finally, and before measuring the actual sample, a calibration is needed. This requires the injection of a standard material with a known density and particle size distribution. The standard material is purchased from the CPS equipment supplier. The calibration standard is first homogenized on a vortex mixture. 0.15 ml of the standard material is injected into the CPS equipment. Once the standard procedure is completed, 0.1 ml of the sample to be analyzed is injected and the particle size distribution of the silicon-based particles is measured. The dvgo value is then determined.

[0081] Determination of the viscosity of suspensions

[0082] The viscosity of the suspensions is measured using a rheometer, for example the Anton Paar MCR.302. The equipment is set in a cone-plate configuration using a gap of 100 pm. The wedge-shapes gap formed by the cone ensures a constant shear rate over the entire sample. The viscosity is measured using a rotational test. During the measurement, a Peltier element keeps the sample at a constant temperature of 25°C. Before starting the measurement, the sample is pre-sheared to remove any history effects and to ensure a uniform and stable state of the material before testing. The cone is rotated with different speeds generating different shear rates ranging from Is-1to 1000s'1. The measurement duration time needs to be long enough in order to ensure an equilibrium state before the shear stress is recorded. The resulting shear stress, related to the torque experienced by the device is recorded. The viscosity is the ratio of the shear stress to the shear rate. In order to compare viscosities from one sample to another, the viscosity ratio is calculated. The viscosity ratio is the quotient of the viscosity at Is-1to the viscosity at 1000s’1. It also provides a measurement of the strength of the shear thinning properties of the sample. A high quotient indicates a high shear thinning effect, a low quotient indicates a more Newtonian behaviour.

[0083] Determination of the phosphorous content in the suspensions or the composite powders

[0084] The phosphorous content in the suspensions, or the composite powders, is measured using an Inductively Coupled Plasma (ICP) analyzer, in this case an Agillent ICP-OES 5110. The suspension, or the composite powder, is digested using acids such as HCI, HNO3 and HF to ensure full dissolution of the silicon particles and of any other particles possibly present. The suspension can be heated to aid in the dissolution process. Upon complete dissolution, the content of phosphorus, and possibly of other elements in the samples, are analyzed.

[0085] Determination of the silicon content in the suspensions

[0086] The silicon content is calculated based on the weights of silicon, liquid and possibly of other materials introduced in the mill. Since there is no evaporation during the milling process, the initial silicon content calculated based on the materials introduced in the mill is similar to the silicon content in the milled suspension. Alternatively, when the suspension consists of silicon and a liquid, it is possible to weigh a certain quantity of suspension, fully evaporate the liquid and then weigh the remaining dry material. For example, for 100g of suspension, if after the evaporation of all the liquid, the remaining dry material weighs 30g, the silicon content relative to the weight of the suspension is 30 wt%.

[0087] Determination of the silicon content in the composite powders

[0088] The silicon content of the composite powders is measured by X-Ray Fluorescence (XRF) using an energy dispersive spectrometer. This method has an experimental random error of + / - 0.3 wt% Si.

[0089] Determination of the oxygen content in the composite powders

[0090] The oxygen content of the composite powders is determined by the following method, using a LECO TC600 oxygen-nitrogen analyzer. A sample of the powder to be analyzed is put in a closed tin capsule that is put itself in a nickel basket. The basket is put in a graphite crucible and heated under helium as carrier gas to above 2000°C. The sample thereby melts and oxygen reacts with the graphite from the crucible to CO or CO2 gas. These gases are guided into an infrared measuring cell. The observed signal is recalculated to an oxygen content.

[0091] Determination of the number-based oarticle size distribution of silicon-based in the com

[0092] The number-based particle size distribution of the silicon-based particles in the composite powders is determined via an electron microscopy analysis (SEM or TEM) of a cross-section of the composite powder, combined with an image analysis. To do this, a cross-section of the composite powder, comprising multiple crosssections of composite particles, each of them comprising multiple cross-sections of silicon-based particles, is prepared following the procedure detailed hereunder.

[0093] 500 mg of the composite powder to be analyzed is embedded in 7g of a resin (Buehler EpoxiCure 2) consisting of a mix of 4 parts Epoxy Resin (20-3430-128) and 1 part Epoxy Hardener (20-3432-032). The resulting sample of 1" diameter is dried during at least 8 hours. It is then polished, first mechanically using a Struers Tegramin-30 until a thickness of maximum 5 mm is reached, and then further polished by ion-beam polishing (Cross Section Polisher Jeol SM-09010) for about 6 hours at 6 kV, to obtain a polished surface. A carbon coating is finally applied on this polished surface by carbon sputtering using a Cressington 208 carbon coater for 12 seconds, to obtain the sample, also called "cross-section", that will be analyzed by SEM.

[0094] The prepared cross-section is then analyzed using a FEG-SEM JSM-7600F from JEOL equipped with an EDS detector Xflash 5030-127 from Bruker (30mm2, 127 eV). The signals from this detector are treated by the Quantax 800 EDS system from Bruker.

[0095] The enlargements are generated by applying a voltage of 15kV at a working distance of several millimeters. The images from the backscattered electrons are reported when adding value to the images from the optical microscope.

[0096] The size of a silicon-based particle is considered to be equivalent to the maximum straight-line distance between two points on the perimeter of a discrete crosssection of that silicon-based particle.

[0097] For the purpose of illustrating, in a non-limitative way, the determination of the number-based particle size distribution of silicon-based particles, a SEM-based procedure is provided below.

[0098] 1. Multiple SEM images of the cross-section of the composite powder comprising composite particles with silicon-based particles dispersed therein, are acquired.

[0099] 2. The contrast and brightness settings of the images are adjusted for an easy visualization of the cross-sections of the composite particles and the silicon- based particles. Due to their different chemical composition, the difference in brightness allows for an easy distinction between both types of particles.

[0100] 3. At least 1000 discrete cross-sections of silicon-based particles, not overlapping with another cross-section of a silicon-based particle, are selected from one or several of the acquired SEM image(s), using a suitable image analysis software. These discrete cross-sections of silicon-based particles can be selected from one or more cross-sections of the composite powder comprising the composite particles and the silicon-based particles.

[0101] 4. The size of the discrete cross-sections of the silicon-based particles are measured using a suitable image analysis software for each of the at least 1000 discrete cross-sections of silicon-based particles.

[0102] The dmo, dnso and dN9o values of the number-based particle size distribution of silicon-based particles, determined using the method described above, are then calculated. These number-based particle size distributions can be readily converted to a weight- or a volume-based particle size distribution via well-known mathematical equations.

[0103] Determination of the volume-based size distribution of the com

[0104] The volume-based particle size distribution of the composite particles is determined with a laser diffraction particle size analyzer, for example a Malvern Mastersizer 2000. The following measurement conditions are selected: compressed range; active beam length 2.4 mm; measurement range: 300 RF; 0.01 to 900 pm. The sample preparation and measurement are carried out in accordance with the manufacturer's instructions. The D10, D50 and D90 values are then determined.

[0105] Determination of the electrochemical performance of the composite powders

[0106] The electrochemical performance of the composite powders in the examples and the counterexamples is determined by the following method.

[0107] The composite powders to be evaluated are sieved using a 45 pm sieve. Then, in a first stage, the composite powders are tested as such, without any dilution with graphite particles, to determine their specific capacity. They are mixed with carbon black, carbon fibers and sodium carboxymethyl cellulose binder in water (2.5 wt%) The ratio used is 89 weight parts composite powder / 1 weight part carbon black (C65) / 2 weight parts carbon fibers (VGCF) and 8 weight parts carboxymethyl cellulose (CMC). All these components are mixed in a Pulverisette 7 planetary ball mill for 30 minutes at 250 rpm.

[0108] A copper foil cleaned with ethanol is used as current collector. A 200 pm thick layer of the mixed components is coated on the copper foil. The coated copper foil is then dried for 45 minutes in vacuum at 70°C. A 1.27 cm2circle is punched from the dried coated copper foil and used as an electrode in a coin cell using lithium metal as counter electrode. The electrolyte is IM LiPFe dissolved in EC / DEC 1 / 1 + 2% VC + 10% FEC solvents.

[0109] All coin-cells are cycled using a high precision battery tester (Maccor 4000 series) using the procedure described below, where "CC" stands for "constant current" and "CV" stands for "constant voltage".

[0110] • Cycle 1 : o Rest 6h o CC lithiation to 10 mV at C / 10, then CV lithiation until C / 100 o Rest 5 min o CC delithiation to 1.5 V at C / 10 o Rest 5 min

[0111] The capacity obtained for the delithiation at cycle 1 is the specific capacity of the composite powder.

[0112] In a second stage, the composite powders are tested at a lower capacity, i.e. after dilution with graphite particles in a mixture "composite powder + graphite" . The respective weight contents of composite powder and graphite in the mixture "composite powder + graphite" are adjusted such as to obtain a theoretical specific capacity for said mixture of about 550 mAh / g. For example, for a composite powder having a measured specific capacity of 1500 mAh / g and using a theoretical capacity of 350 mAh / g for the graphite, the respective weight contents of composite powder and graphite are respectively 17.4 wt% and 82.6 wt%. The rest of the procedure, anode formulation, cell composition and cell assembling, is kept unchanged.

[0113] The cycling procedure is as follows:

[0114] Cycle 1 : o Rest 6h o CC lithiation to 10 mV at C / 10, then CV lithiation until C / 100 o Rest 5 min o CC delithiation to 1.5 V at C / 10 o Rest 5 min

[0115] • From cycle 2 on: o CC lithiation to 10 mV at C / 2, then CV lithiation until C / 50 o Rest 5 min o CC delithiation to 1.2 V at C / 2 o Rest 5 min

[0116] The coulombic efficiency (CE) of the coin-cell, being the ratio of the capacity at delithiation to the capacity at lithiation at a given cycle, is calculated for the initial cycle as well as for the subsequent ones. The initial cycle is the most important one in terms of coulombic efficiency, since the reaction of SEI formation has a huge impact on the CE. Typically for a silicon-based powder the coulombic efficiency at the initial cycle can be as low as 80% (or even lower), corresponding to an irreversible capacity loss for the coin-cell of 20%, which is huge. The target is to reach at least 89.0% CE at the initial cycle and preferably at least 90.0% CE.

[0117] For the subsequent cycles even though the CE usually increases well over 99%, the skilled person will be aware that even a small difference in coulombic efficiency per cycle, will have, over the hundreds or thousands of charging-discharging cycles a battery is expected to last, a significant cumulative effect. To give an example, a cell with an initial capacity of 1 Ah having an average CE of 99.8% will, after 100 charging-discharging cycles, have a remaining capacity of 0.8 Ah, which is 60% higher than for a cell having an average CE of 99.5% (remaining capacity of 0.5 Ah).

[0118] The target in terms of average CE from cycle 5 to cycle 50 is to reach at least 99.70%, preferably at least 99,75%, and even more preferably at least 99.80% for a cell comprising a composite powder with a specific capacity of 550 ± 10 mAh / g.

[0119] EXPERIMENTAL PREPARATION OF EXAMPLES Counterexample 1 (CE1), not according to the invention

[0120] To produce the silicon-containing suspension of Counterexample 1, 0.1 mm yttrium-stabilized zirconia beads are weighed to reach a filling ratio of 80%. The filling ratio is defined as the volume of the grinding beads divided by the volume of the grinding chamber and usually ranges from 60% to 90% depending on the type of mill and rotor used. After weighing the correct amount of beads, they are introduced into the mill, a Buhler MMX1. The mill is then flushed with N2 gas in order to create an inert atmosphere. This is to avoid the creation of a potentially explosive environment (ATEX) when introducing the liquid. All equipment is also grounded.

[0121] Then, the liquid, in this case isopropyl alcohol (IPA), is added to the mill to pre-wet the beads. The amount of liquid depends on the batch size and the solid load of the suspension. In this example, the batch size is 3000 g and the solid load is 30.0 weight%. This amounts to 2100 g of IPA being introduced in the mill. At this point the mill is started to allow the solvent to mix with the beads. The mill is set to circulation mode. 900 g of silicon powder (dvgo =10 pm) is then slowly added to the mill. The mill is programmed to maintain a power output of 4 kW and the tip speed is varied accordingly. Using 0.1 mm beads, the tip speed varies between 14 m / s and 15 m / s. The milling is continued until a specific energy of 40000 kWh / t is reached. This amounts to approximately 9h of milling. During the milling process, intermediate samples are taken every 4000 kWh / t and the PSD is measured. At 40000 kWh / t, a dvgo particle size of 133 nm is reached for the silicon particles comprised in the suspension. The dvgo particle size of the silicon particles in the suspension of CE1 and of subsequent examples are summarized in Table 1. At this point the mill is stopped and the suspension is drained. The BET specific surface of the dried silicon particles is also measured to be 107 m2 / g and the oxygen content in the dried silicon particles is measured to be 10.6 wt%.

[0122] Then, the silicon-containing suspension is mixed with a carbon precursor capable of fully decomposing into carbon at a temperature above 600°C, in that case petroleum pitch. For a batch size of 700 g and a solid load of 20 weight%, 447 g of liquid (IPA), 161 g of silicon-containing dispersion (at 30 wt% solid load) and 91 g of pitch are weighed. The ratio between the pitch content and the silicon-based particles content, both expressed in weight%, in the suspension (ratio "carbon precursor / silicon-based particles") is 1.9. The components are mixed using a dissolver for 15 min at 500 rpm using a 80 mm dispersion disc. 0.3 mm yttrium- stabilized zirconia beads are then weighed to achieve a filling ratio of 83%. After weighing, the beads are added to the mill.

[0123] The mill is then flushed with N2in order to create an inert atmosphere. This to avoid the creation of an ATEX environment when introducing solvent. All equipment is also grounded. The dispersion is added to the mill and milling is initiated. The tip speed is set to 12 m / s and milling is continued in circulation mode until a specific energy of 1000 kWh / t is reached. Afterwards the mill is stopped and the suspension is drained. The solid load is determined using a dry balance.

[0124] Next, the suspension is dried using a lab spray dryer. The dispersion is sprayed using a 2-fluid nozzle into a hot nitrogen gas stream upon which the solvent quickly vaporizes and a "silicon-carbon precursor" powder is collected from the cyclone. The inlet temperature is set to 130°C and the nitrogen flow is 1052 l / h.

[0125] Afterwards, the "silicon-carbon precursor" powder (with petroleum pitch as carbon precursor) is heated to a temperature of 400°C, under a flow of nitrogen and, after a waiting period of 60 minutes, mixed for 30 minutes under high shear by means of a Cowles dissolver-type mixer operating at 1000 rpm. The mixture of the silicon- based powder in the carbon precursor thus obtained is cooled under N2to room temperature and, once solidified, pulverized and sieved on a 400-mesh sieve, to produce an intermediate powder.

[0126] 100 g of the obtained intermediate powder are then placed in a quartz crucible in a tube furnace, heated up at a heating rate of 3°C / min to 960°C, kept at that temperature for two hours and then cooled. All this is performed under an oxygen- free argon atmosphere. In the obtained composite powder, the silicon-based particles are dispersed and embedded in a matrix of soft carbon, resulting from the thermal decomposition of the carbon precursor (the petroleum pitch).

[0127] The fired product is finally ball-milled with alumina balls for 1 hour at 300 rpm and sieved over a 325-mesh sieve, to obtain the composite powder of Counterexample 1.

[0128] As reported in Table 2, the total Si content in this powder is measured to be 40.2 wt% by XRF, having an experimental error of + / - 0.3 wt%. This corresponds to a calculated value based on a weight loss of the pitch upon heating of circa 35 wt% and an insignificant weight loss upon heating of the other components. The calculated ratio of carbon content (expressed in weight%) resulting from the carbonization of the pitch, forming the matrix material, over the silicon content (expressed in weight%) in the powder is around 1.37. The oxygen content of this powder is measured to be 4.8 wt%. The specific surface area (BET) of the obtained powder is measured to be 2.9 m2 / g.

[0129] The volume-based particle size distribution of the composite particles obtained has a DIO equal to 5.3 pm, a D50 equal to 15.9 pm and a D90 equal to 24.4 pm.

[0130] Example 1 (El), according to the invention

[0131] To produce the silicon-containing suspension of Example 1, the same method as for the production of the suspension of CE1 is used, except that 0.61g of lithium phosphate (U3PO4), used as viscosity-reducing agent, is added to the mill together with the 900g of silicon powder (dvgo =10 pm). The content of viscosity reducing agent is thus 0.068 weight% relative to the content of the silicon particles.

[0132] The milling is continued until a specific energy of 40000 kWh / t is reached in about 9h of milling. At 40000 kWh / t, a dvgo particle size of 125 nm is reached for the silicon particles comprised in the suspension. At this point the mill is stopped and the suspension is drained. The BET specific surface of the dried silicon particles is also measured to be 113 m2 / g and the oxygen content in the dried silicon particles is measured to be 11.3 wt%.

[0133] Then, the silicon-containing suspension comprising a carbon precursor and the composite powder are prepared following the same procedure as for CE1.

[0134] The total Si content in the composite powder is measured to be 39.9 wt% by XR.F, having an experimental error of + / - 0.3 wt%. This corresponds to a calculated value based on a weight loss of the pitch upon heating of circa 35 wt% and an insignificant weight loss upon heating of the other components. The calculated ratio of carbon content (expressed in weight%) resulting from the carbonization of the pitch, forming the matrix material, over the silicon content (expressed in weight%) in the powder is around 1.38. The oxygen content in this composite powder is measured to be 5.1 wt%. The phosphorous content in this composite powder is measured to be 50 ppm. The specific surface area (BET) and the volume-based particle size distribution of the composite particles obtained are comparable to the ones obtained for CE1.

[0135] Examples 2, 3, 4, 5 (E2-E5), according to the invention

[0136] The silicon-containing suspensions of Examples 2 to 5 (E2-E5) are produced using the same method as for the production of the silicon-containing suspension of Example 1, except for the content of viscosity reducing agent, which is adjusted to 0.137 weight%, 0.273 weight%, 0.683 weight% and 1.370 weight% relative to the content of the silicon particles, respectively for Examples E2, E3, E4 and E5.

[0137] The composite powders are produced using the same method as for the production of the composite powders of CE1 and El. The phosphorous contents in the composite powders E2-E5 are measured and are all comprised between 110 ppm and 1020 ppm.

[0138] Examples 6 (E6)

[0139] The silicon-containing suspensions of Examples 6 (E6) is produced using the same method as for the production of the silicon-containing suspension of Example 4, except that benzylamine is used as viscosity-reducing agent instead of lithium phosphate. The composite powder is produced using the same method as for the production of the composite powders of E4. No phosphorous is detected in the composite powder E6.

[0140] Counterexample 2 (CE2), not according to the invention

[0141] The silicon-containing suspension of Counterexample 2 (CE2) is produced following the same method as for the production of the suspension of CE1, except that the quantities of silicon powder and liquid (I PA) are adjusted to obtain a solid load of 50.0 weight%. The rest is kept exactly the same. Quickly after the start, the rotor got blocked and it became impossible to proceed further, due to the too high viscosity of the suspension. The specific energy of 40000 kWh / t could not be achieved. The final suspension and the composite powder were therefore not produced.

[0142] Examples 7, 8, 9, 10. 11 (E7-E11), according to the invention

[0143] The silicon-containing suspensions of Examples 7 to 11 (E7-E11) are produced using the same method as for the production of the silicon-containing suspension of E1-E5, except that the quantities of silicon powder, viscosity-reducing agent and liquid (IPA) are adjusted to obtain a solid load of 50.0 weight%. The rest is kept exactly the same for the suspensions and the composite powders. The phosphorous contents in the composite powders E7-E10 are measured and are all comprised between 80 ppm and 1760 ppm.

[0144] It is clear from Table 1 that the presence of a viscosity-reducing agent, in particular an agent comprising phosphorous, in the suspensions, is efficient in obtaining smaller silicon-based particles, down to a dvgo of less than 100 nm, while maintaining a constant milling specific energy. It also helps avoiding production issues, in particular at high solid loads.

[0145] Electrochemical evaluation of the composite powders

[0146] The produced composite powders are tested in coin-cells according to the procedure specified above. In a first stage, the composite powders are evaluated as such, i.e. without dilution with graphite particles, to determine their specific capacities. As shown in Table 3, all composite powders have high specific capacities, comprised between around 1275 mAh / g and around 1400 mAh / g.

[0147] In a second stage, for an easier comparison of their performance, the composite powders are mixed with graphite particles during the electrode preparation, to reach a capacity of the mixture "composite powder + graphite" of around 550±10 mAh / g. The negative electrodes thereby obtained comprise around 20 wt% of composite powder and around 80 wt% of graphite particles. The results obtained for the initial coulombic efficiency and the average coulombic efficiency of the coin cells comprising the different mixtures of composite powders and graphite, between cycle 5 and cycle 50, are given in Table 3.

[0148] Comparing the results obtained for the composite powders CE1 and CE2 with the results obtained for the composite powders El to Ell, it can be seen that the best results are obtained for the cells containing composite powders according to the invention, obtained from the suspensions according to the invention, i.e. suspensions comprising a viscosity-reducing agent, in particular U3PO4. Furthermore, it can be seen that the best results are obtained for the composite powders comprising phosphorous, compared to the composite powders not comprising phosphorous, for example comparing E4 and E6.

[0149] It can also be seen that the lower the dV9o of the silicon-based particles, the better the cycle life, however with a small decrease of the initial coulombic efficiency, due to a higher oxygen content, for the reasons already mentioned earlier.

[0150] Table 1: Physico-chemical properties of the suspensions of Examples El-Ell and CE1-CE2, measured once a specific energy of 40000 kWh / t is reached.

[0151] Table 2: Physico-chemical properties of the composite powders of Examples El-Ell and CE1-CE2.

[0152] Table 3: Performance of coin-cells containing composite powders El-Ell and CE1-

[0153] CE2

Claims

CLAIMS1.- A silicon-containing suspension for use in the manufacture of negative electrode materials for secondary batteries, the suspension comprising a liquid, silicon-based particles and a viscosity-reducing agent which is different from the liquid, wherein the content of silicon-based particles is from 10 weight% to 60 weight% relative to the total weight of the suspension and the content of the viscosity reducing agent is at least equal to 0.01 weight% and at most equal to 4.00 weight% relative to the content of the silicon-based particles, wherein the average silicon content in the silicon-based particles is 70 weight % or more and preferably 80 weight % or more, wherein the silicon-based particles are characterized by a volume-based distribution of sizes having a dvgo, wherein the dvgo is lower than 200 nm, preferably lower than 150 nm and particularly preferably lower than 100 nm, the suspension having a viscosity Vi measured at a sheer rate of 1 s’1and a viscosity iooo measured at a sheer rate of 1000 s’1, wherein the ratio i / iooo is lower than 200 and preferably lower than 100.2.- The suspension according to claim 1, wherein the content of the viscosity reducing agent is at most equal to 3.00 weight%, more preferably at most equal to 2.00 weight% and particularly preferably at most 1.50 weight% relative to the content of the silicon-based particles.3.- The suspension according to claim 1 or 2, wherein the viscosity-reducing agent does not form a monolayer around the silicon-based particles.4.- The suspension according to any one of the preceding claims, wherein the viscosity-reducing agent comprises an ionic compound, preferably phosphate ions PO43’.5.- The suspension according to claim 4, wherein the ionic compound is lithium phosphate U3PO4.6.- The suspension according to any one of the preceding claims, comprising phosphorous.7.- The suspension according to any one of the preceding claims, wherein the content of silicon is at least 5 weight%, preferably at least 10 weight%, more preferably at least 15 weight%, particularly preferably at least 20 weight% and utterly preferably more than 30 weight%, relative to the total weight of the suspension.8.- The suspension according to any one of the preceding claims, wherein the liquid is polar.9.- The suspension according to claim 8, wherein the polar liquid is an alcohol or a mixture of alcohols.10.- The suspension according to claim 9, wherein the alcohol or the mixture of alcohols is isopropyl alcohol (IPA) and / or isobutanol (IBU).11.- The suspension according to any one of the preceding claims, further comprising a carbon precursor material capable of fully decomposing into carbon at a temperature above 600°C.12.- The suspension according to claim 11, wherein the content of carbon precursor material to the content of silicon-based particles, both relative to the total weight of the suspension, is from 0.5 to 3.0, preferably from 0.8 to 2.5 and more preferably from 1.0 to 2.0.13.- A method for producing a silicon-containing suspension according to any one of the preceding claims, the method comprising the steps of: a) providing a silicon-based powder comprising silicon-based particles having a volume-based distribution of sizes with a d90 higher than 500 nm, a viscosityreducing agent and a liquid and loading them into a mill such as to obtain a mixture, b) milling the mixture of step a) until the volume-based distribution of sizes of the silicon-based particles has a dvgo lower than 200 nm, preferably lower than 150 nm, and particularly preferably lower than 100 nm.14.- A method for producing a composite powder for use as a negative electrode material for secondary batteries, comprising the steps of: a) providing a suspension according to claim 11 or 12, b) removing the liquid from the suspension of step a) until a powder with a residual liquid content of at most 5.0 weight%, relative to the total weight of the powder, is obtained, c) heating up the powder of step b) at a temperature above 600°C and below 1100°C, under an oxygen-free atmosphere, to obtain a composite powder. 15.- A composite powder for use as a negative electrode material for secondary batteries obtainable from the method of claim 14.16.- The composite powder according to claim 15, comprising phosphorous. 17.- A battery comprising the composite powder according to claim 15 or 16.

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