A pitch powder for use in the manufacture of negative electrode materials for secondary batteries

Abstract

The present invention relates to a pitch powder for use in the manufacture of negative electrode materials for secondary batteries, said pitch powder having properties which are advantageous in that the composite powder produced using said pitch powder has a low BET specific surface area and, once used in the negative electrode of a battery, allows to achieve a long cycle life.

Description

[0001] A PITCH POWDER FOR USE IN THE MANUFACTURE OF NEGATIVE ELECTRODE MATERIALS FOR SECONDARY BATTERIES

[0002] TECHNICAL FIELD AND BACKGROUND

[0003] The present invention relates to a pitch powder for use in the manufacture of negative electrode materials for secondary batteries and to a method for producing a silicon-based composite using said pitch powder.

[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.

[0008] 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-basedparticles, 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] To solve the above-mentioned drawbacks, composite powders are usually used. In these composite powders, nano-sized silicon-based particles are mixed with at least one component suitable to protect the silicon-based particles from electrolyte decomposition and to accommodate volume changes. Such a component may be a carbon-based material, preferably forming a matrix. In the literature, pitch powders have proven to be a suitable carbon precursor to form silicon-carbon composites. The thermal behavior of pitch powders has been studied in the literature, for example in Slovak et al., J. Anal. Appl. Pyrolysis 72 (2004) 249-252 and in Alcahiz-Monge et al., Fuel 80 (2001) 41-48. However, these documents do not address the influence of specific physico-chemical properties of pitch powders in the framework of manufacturing silicon-carbon composites used as negative electrode materials for secondary batteries, in particular they do not address the influence of those parameters on the BET surface area and the electrochemical performance of the silicon-carbon composite materials. It is indeed well known in the art of negative electrode materials, certainly for silicon-based composites, that it is important for the composite powder to have a low BET specific surface area, todecrease the surface of electrochemically active particles in contact with the electrolyte, in order to limit the SEI formation, which consumes lithium, and thus to limit the loss in cycle life of a battery containing such a composite powder. By low BET specific surface it is here meant a BET value at least inferior to 5.0 m2 / g.

[0013] Such silicon-carbon composites made using pitch powders are for example disclosed in US 2019 / 0260020 (Yamamoto) and US 2018 / 0013137 (Put et al.).

[0014] However, they do not correlate the physico-chemical properties of the pitch powder with the BET surface area or the electrochemical performance of the composite materials.

[0015] It is therefore an objective of the present invention to provide a pitch powder for use in the manufacture of negative electrode materials for secondary batteries that allows achieving negative electrode materials having a low BET specific surface area, e.g. a BET value lower than 5.0 m2 / g, preferably lower than 4.0 m2 / g, particularly preferably lower than 3.0 m2 / g, said negative electrode materials which once used in the negative electrode of a battery, are advantageous in that they lead to a high capacity combined to a long cycle life, which is essential, in particular for the batteries of the electric vehicles.

[0016] SUMMARY OF THE INVENTION

[0017] This objective is achieved by providing a pitch powder according to the invention, said pitch powder, which once used to produce a negative electrode active material, used in the negative electrode of a battery, allows to achieve a long cycle life, while keeping a high specific capacity, as demonstrated in Examples El to E3 compared to Counterexample CE1. The invention also concerns a method for producing a silicon-based composite, using the pitch powder according to the invention.

[0018] DETAILED DESCRIPTION

[0019] 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 willbecome apparent from consideration of the following detailed description and accompanying drawings.

[0020] It is important to mention that, in order to determine the parameters of the pitch powder having a significant effect on the BET specific surface area of the composite powders produced using said pitch powder, all other parameters are kept as similar as possible for every example produced. In particular, the particle size distribution, e.g. dlO, d50, d90, known for having a large impact on the BET specific surface area of powders, of all samples are as similar as possible.

[0021] In a first aspect, the present invention concerns a pitch powder for use in the manufacture of negative electrode materials for secondary batteries, having a temperature of maximum rate of weight loss Tmax in thermogravimetric analysis (TGA), characterized by an absolute local maximum in the derivative thermogravimetry curve (DTG), at least equal to 440°C, preferably at least equal to 445°C, when measured according to the procedure described in the section "Thermogravimetric Analysis". The Tmax corresponds to an inflection point in the thermogravimetric curve (TG). The Tmax is preferably at least equal to 450°C, more preferably at least equal to 455°C, even more preferably at least equal to 460°C and particularly preferably at least equal to 465°C. As presented in Figure 1, the technical effect of the Tmax of the pitch powder being at least equal to 440°C is a low BET specific surface for the silicon-based composite produced using that pitch powder, preferably a BET value lower than 5.0 m2 / g, more preferably lower than 4.0 m2 / g, particularly preferably lower than 3.0 m2 / g. The inventors, without being bound by theory, believe that this may be due to a pitch powder which polymerizes slower during the heat treatment step and therefore remains in a viscous state for a longer time, which allows the pitch to better coat the particles of silicon-based composite before fully decomposing into carbon. In particular the pitch powder according to the invention is better able to penetrate the porosity at the surface of the composite powder's particles and thereby to reduce its BET specific surface. The inventors have discovered that it is preferable for the pitch powder according to the invention to have a weight loss in TGA that lasts long, i.e. that starts at a relatively low temperature, typically between 250 and 350°C and ends at a relatively high temperature, typically between 500 and 600°C. It is also preferable that most ofthe weight loss occurs in a later stage, i.e. at higher temperatures and less in the early stage, i.e. at lower temperatures, which typically results in asymmetrical TG and DTG curves.

[0022] It is preferable for the pitch powder according to the invention to have no local maximum in the derivative thermogravimetry curve (DTG) at a temperature below 440°C, preferably having no local maximum at a temperature below 435°C, when measured according to the procedure described in the section "Thermogravimetric Analysis". A pitch powder having one or more local maximum(s) in the derivative thermogravimetry curve (DTG) at a temperature below 440°C possesses at least one volatile fraction, evaporation or decomposing at a lower temperature , which is not preferred to achieve the objective mentioned earlier. It may also be an indication that the pitch powder is not very pure, or contains some additives.

[0023] In another embodiment according to the first aspect of the invention, the pitch powder has a temperature at which the massive weight loss ends, also called Tf, as defined in the section "Thermogravimetric Analysis", at least equal to 480°C, preferably at least equal to 490°C in thermogravimetric analysis (TGA), when measured according to the procedure described in the section "Thermogravimetric Analysis". The Tf is more preferably at least equal to 495°C and even more preferably at least equal to 500°C. As presented in Figure 2, the technical effect of the Tf of the pitch powder being at least equal to 480°C is a low BET specific surface for the silicon-based composite produced using that pitch powder, preferably a BET value lower than 5.0 m2 / g, more preferably lower than 4.0 m2 / g, particularly preferably lower than 3.0 m2 / g. The inventors, without being bound by theory, believe that the beneficial effect of this delayed pyrolysis of the pitch powder may result in a better coverage of the composite powder's particles by the pitch and a better closing of the surface's porosities, before the pitch gets fully decomposed into carbon, thereby resulting in a lower BET specific surface, preferably lower than 5.0 m2 / g, more preferably lower than 4.0 m2 / g, particularly preferably lower than 3.0 m2 / g.

[0024] In another embodiment according to the first aspect of the invention, the pitch powder preferably has a softening point at least equal to 180°C. When the softening point is lower than 180°C, during the heat-treatment step, in the processof producing composite powders, the pitch might soften too fast, become too fluid and flow well before the carbonization begins. The consequence is that the pitch might migrate away from the silicon particles and that the embedding in the matrix might be less uniform and efficient, thereby possibly leading to lower performance of the composite powder in a battery. Furthermore, in case the embedding of the silicon nanoparticles in the matrix would be less uniform and efficient, more silicon surface would be exposed and therefore the BET surface area of such a composite powder might be higher than 5.0 m2 / g, which is not desired.

[0025] Preferably the pitch powder has a softening point at least equal to 200°C, more preferably at least equal to 220°C, even more preferably at least equal to 240°C and particularly preferably at least equal to 260°C.

[0026] In yet another embodiment according to the first aspect of the invention, the pitch powder comprises petroleum pitch and preferably consists of petroleum pitch.

[0027] There are two types of pitch powders commonly available: petroleum pitch and coal tar pitch. The inventors have realized that when a majority of petroleum pitch is used and preferably when only petroleum pitch is used, the resulting composite powder has a lower BET specific value, preferably lower than 5.0 m2 / g, more preferably lower than 4.0 m2 / g, particularly preferably lower than 3.0 m2 / g and hence that the performance in a battery is improved. This may be due to the fact that the coal tar pitch becomes fluid slower and solidifies faster than the petroleum pitch, which as already explained above, is not preferred and does not result in an efficient coating of the composite powder's particles, an efficient coating of the surface's pores and a low BET specific surface.

[0028] In another embodiment according to the first aspect of the invention, the pitch powder has a toluene insoluble fraction TI, measured according to the ISO 6376 standard, expressed in weight percent (wt%), wherein the TI is at least equal to 20 wt%, preferably at least equal to 25 wt%, and at most equal to 50 wt%, preferably at most equal to 45 wt%. The toluene insoluble fraction TI is more preferably at least equal to 30 wt%, even more preferably at least equal to 35 wt% and particularly preferably at least equal to 40 wt%.

[0029] As presented in Figure 3, the technical effect of the toluene insoluble fraction TI of the pitch powder being from 20 wt% to 50 wt% is a low BET specific surface for thesilicon-based composite produced using that pitch powder, preferably a BET value lower than 5.0 m2 / g, more preferably lower than 4.0 m2 / g, particularly preferably lower than 3.0 m2 / g. Pitch powders having a toluene insoluble fraction TI lower than 20 wt%, preferably lower than 25 wt%, when heated above their softening point, become very fluid and are therefore not suited to coat the composite powder's particles efficiently. On the contrary, pitch powders having a toluene insoluble fraction TI higher than 50 wt%, preferably higher than 45 wt%, when heated above their softening point, remain very viscous and therefore are not suited to coat the composite powder's particles efficiently, leaving a large part of the surface porosity open, which leads to a high BET specific surface area, typically above 5.0 m2 / g.

[0030] Likewise, the pitch powder according to the invention has a toluene insoluble fraction TI, measured according to the ISO 6376 standard, and a quinoline insoluble fraction QI, measured according to the ASTM D2318-20 standard, both expressed in percent (wt%), wherein the difference TI - QI is at least equal to 20%, preferably at least equal to 25% and at most equal to 50%, preferably at most equal to 45%. As presented in Figure 4, the technical effect of the difference TI -QI of the pitch powder being from 20 wt% to 50 wt% is a low BET specific surface for the silicon-based composite produced using that pitch powder, preferably a BET value lower than 5.0 m2 / g, more preferably lower than 4.0 m2 / g, particularly preferably lower than 3.0 m2 / g. Similarly to what was explained above, pitch powders having a difference TI - QI lower than 20 wt% are likely to become very fluid when heated, whereas pitch powders having a difference TI - QI higher than 50 wt% are likely to be too viscous when heated. In both cases those pitch powders are likely not able to coat the composite powder's particles efficiently and are therefore not preferred.

[0031] In another embodiment according to the first aspect of the invention, the pitch powder according the invention has a carbon residue at 900°C at least equal to 55%, preferably at least equal to 60%, more preferably at least equal to 65% and particularly preferably at least equal to 70%, when measured according to the procedure described in the section "Thermogravimetric Analysis". As presented in Figure 5, the technical effect of the carbon residue at 900°C of the pitch powder being at least equal to 55%, preferably at least equal to 60%, more preferably atleast equal to 65% and particularly preferably at least equal to 70%, is a low BET specific surface for the silicon-based composite produced using that pitch powder, preferably a BET specific surface lower than 5.0 m2 / g, more preferably a BET specific surface lower than 4.0 m2 / g and particularly preferably a BET specific surface lower than 3.0 m2 / g. The inventors, without being bound by theory, believe that a pitch powder having a carbon residue at 900°C lower than 55% might result in an insufficient coating of the composite particles surface's pores and therefore in a high BET specific surface for the composite powder, typically a BET specific surface higher than 5.0 m2 / g. Additionally, a pitch powder having a carbon residue at 900°C lower than 55% would lead to higher quantities of pitch being used during the synthesis of the silicon-based composite powder to obtain the same carbon content, which is not economically advantageous and therefore not preferred.

[0032] Preferably, the pitch powder according the invention has a carbon residue at 900°C at most equal to 75%. Pitch powders having a too high carbon residue might be difficult to process when producing silicon-based composite powders.

[0033] In yet another embodiment according to the first aspect of the invention, the pitch powder has a hydrogen content H and a carbon content C, both expressed in number of molecules, wherein the ratio H / C is at most equal to 0.75, preferably at most equal to 0.70. As presented in Figure 6, the technical effect of the H / C ratio of the pitch powder being at most equal to 0.75, preferably at most equal to 0.70, is a low BET specific surface for the silicon-based composite produced using that pitch powder, preferably a BET specific surface lower than 5.0 m2 / g, more preferably a BET specific surface lower than 4.0 m2 / g and particularly preferably a BET specific surface lower than 3.0 m2 / g. Preferably, the H / C ratio of the pitch powder is at least equal to 0.60, preferably at least equal to 0.65. It is assumed that pitch powders having a H / C ratio higher than 0.75 contain more aliphatic compounds that might stabilize the polymerization, which results in a delayed pyrolysis, which is not desired. On the other hand, pitch powders having a H / C ratio lower than 0.60 will likely be too viscous, resulting in the inability to properly coat the composite powder's particles and close their surface's porosities and hence will likely result in a composite powder having a too high BET specific surface, typically superior to 5.0 m2 / g.The pitch powder according to the invention has preferably all of the following characteristics:

[0034] (a) a temperature of maximum rate of weight loss Tmax in a TGA thermogravimetric analysis, characterized by a local maximum in the DTG derivative thermogravimetry curve, at least equal to 445°C, and

[0035] (b) a temperature Tf at which the massive weight loss ends in a TGA thermogravimetric analysis at least equal to 490°C, and

[0036] (c) a toluene insoluble fraction TI, expressed in weight percent (wt%), wherein the TI is at least equal to 25 wt% and at most equal to 45 wt%, and

[0037] (d) a quinoline insoluble fraction QI, wherein the difference TI - QI, expressed in weight percent (%), is at least equal to 25 wt% and at most equal to 45 wt%, and (e) a carbon residue at 900°C at least equal to 60 wt%, and

[0038] (f) a hydrogen content H and a carbon content C, both expressed in number of molecules, wherein the ratio H / C is at least equal to 0.65 and at most equal to 0.75.

[0039] In a second aspect, the present invention concerns a method for producing a silicon-based composite for use as a negative electrode material for secondary batteries, the method comprising the steps of:

[0040] a) providing a silicon source and a pitch powder according to the first aspect of the invention,

[0041] b) mixing the silicon source and the pitch powder according to the first aspect of the invention in a liquid to form a dispersion

[0042] c) drying the dispersion to obtain an intermediate powder

[0043] d) heating up the intermediate powder in an oxygen-free atmosphere at a temperature between 800°C and 1100°C to obtain a silicon-based composite.

[0044] Prior to step b) the pitch powder according to the first aspect of the invention can be dispersed in a liquid, preferably the same liquid as in step b), as a pre-mixing step, in order to facilitate the mixing of step b).

[0045] The residual liquid content in the intermediate powder of step c) is preferably at most 1.0 weight%, more preferably at most 0.5 weight% and particularly preferably at most 0.3 weight%, relative to the total weight of the powder. Thelower 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.

[0046] 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.

[0047] In step d), oxygen-free atmospheres that may be used are for example nitrogen or argon. The temperature at which the powder is heated is preferably comprised between 850°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.

[0048] A final crushing step of the heat-treated material obtained in step d) may be performed. It 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.

[0049] Between step c) and step d) shear forces may be applied to the intermediate powder, in order to improve the physical contact between the silicon-based powders and the pitch powder and to ensure an optimal dispersion of the silicon-based particles in the carbonaceous matrix material obtained after step d). Shear forces may be applied, for example using a mixer or an extruder.

[0050] Carbon additives, such as graphite, graphene, carbon nanotubes or a mixture thereof, may be added to the powder obtained at step c), i.e. before the heattreatment. The carbon additives act as a spacer between the particles of the powder obtained at step c), 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.

[0051] The silicon source in step a) is preferably a slurry comprising silicon-based particles, characterized by a volume-based distribution of sizes having a dgo, wherein the dgo 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 dgo 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 dgo 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.

[0052] It is considered that the dgo 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 dgo value of the silicon-based particles comprised in the composite powder.

[0053] Furthermore, the silicon-based particles may have any shape, e.g. substantially spherical but also irregularly shaped, rod-shaped, plate-shaped, etc. In the silicon-based 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. This can for example be determined by an elemental mapping analysis of a crosssection of a composite particle, comprising numerous cross-sections of silicon-based particles, using a high resolution FEG-SEM microscope. Furthermore, the silicon-based particles typically have a surface layer comprising SiOxwith 0<x<2, and preferably 0<x<l.

[0054] The liquid in step b) is preferably an alcohol or a mixture of alcohols, more preferably comprising isopropyl alcohol (IPA) or isobutanol (IBU). Examples of alcohols are ethanol, propanol, isopropanol, isobutanol (IBU) and isopropyl alcohol (IPA). The 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.

[0055] 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.

[0056] In the dispersion of step b) the weight content of pitch powder to the weight content of silicon, both referring to the dry content, is preferably from 0.5 to 3.0, more preferably from 0.8 to 2.5, even more preferably from 1.0 to 2.0, particularly preferably from 0.8 to 1.8 and even more particularly preferably from 0.8 to 1.5. The carbonaceous matrix material obtained after a heat treatment of the pitch powder above 800 or 900°C typically having a specific capacity of 200-300 mAh / h, i.e. at least 10 times lower than the specific capacity of silicon, it is preferable to keep the weight ratio "pitch powder / silicon" 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, in particular of the particles surface's pores to result in a low BET specific surface for the composite powder, in particular in a BET specific surface area lower than 5.0 m2 / g. This coverage is also meant 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.

[0057] In a third aspect, the present invention also concerns the composite powder for use as a negative electrode material for secondary batteries, obtainable from the method according to the second aspect of the invention, previously described. Asappreciated by the skilled person all embodiments directed to the method according to the second aspect of the invention apply mutatis mutandis to the composite powder obtainable from the method according to the invention.

[0058] After step d) of said method, a composite powder is obtained. The 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.

[0059] 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.

[0060] 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.In another embodiment according to the third 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 third 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 dgo, the dgo 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 dgo 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 dgo 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.In another embodiment according to the third aspect of the invention, the composite particles have a volume-based particle size distribution having a DIO, a D50 and a D90, with 1 pm < DIO < 10 pm, 5 pm < D50 < 25 pm and 10 pm < D90 < 40 pm.

[0063] 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.

[0064] 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.

[0065] In yet another embodiment according to the third aspect of the invention, the composite powder according to the invention might further comprise 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.

[0066] 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.

[0067] Alternatively, the composite powder may also comprise exfoliated graphite particles, expanded graphite particles and / or graphene nanoplatelets, all alsopreferably not being fully embedded, or not at all embedded in the carbonaceous matrix material, for the same reasons as provided above.

[0068] In another embodiment according to the third 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 (LizSiOs, 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.

[0069] The composite powder according to the invention has a silicon content at least equal to 40 weight percent (wt%), preferably at least equal to 45 wt% and particularly preferably at least 50 wt%. The composite powder according to the invention also has a silicon content at most equal to 80 weight percent, preferably at most equal to 70 wt%, relative to the total weight of the powder. 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, therefore with higher silicon contents. Preferably, said specific capacity should be at least equal to 1400 mAh / g, more preferably at least equal to 1600 mAh / g and even more 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 to limit the specific capacity of thecomposite powder to at most 2400 mAh / g, more preferably at most 2200 mAh / g and even more preferably at most 2000 mAh / g.

[0070] In another embodiment according to the third aspect of the invention, the composite powder has a BET specific surface area which is less than 5 m2 / g. It is important for the composite powder to have a low BET specific surface area, to decrease the surface of electrochemically active particles in contact with the electrolyte, in order to limit the SEI formation, which consumes lithium, and thus to limit the loss in cycle life of a battery containing such a composite powder. The lower BET is obtained by removing the spacer particles from the manufacturing process of the composite powder. The BET specific surface area is preferably less than 4 m2 / g, more preferably less than 3 m2 / g and particularly preferably less than 2 m2 / g.

[0071] In another embodiment according to the third aspect of the invention, the composite powder also has a carbon content relative to the total weight of the powder, whereby the ratio of the carbon content to the silicon content is between 0.50 and 1.00. The ratio of the carbon content to the silicon content is at least 0.50, preferably at least 0.60, more preferably at least 0.70 and even more preferably at least 0.80. The ratio of the carbon content to the silicon content is at most 0.90, preferably at most 0.80. The ratio of the carbon content to the silicon content is preferably between 0.60 and 0.90.

[0072] When the ratio of the carbon content to the silicon content in the composite powder is lower than 0.50, the carbonaceous matrix material is not present in an amount sufficient to adequately cover the silicon-based particles, therefore leading to an increased electrolyte decomposition at the surface of the silicon-based particles and thus to an increased SEI formation. When the ratio of the carbon content to the silicon content in the composite powder is higher than 1.00, the specific capacity of the composite powder is too low, since the carbon forming the matrix material has a specific capacity more than 10 times lower than the silicon specific capacity.

[0073] Furthermore, since the carbonaceous matrix material triggers a high irreversible capacity (low coulombic efficiency at first cycle), it is preferable for the composite powder to contain only the amount of carbonaceous matrix material that is necessary to cover the silicon-based particles.BRIEF DESCRIPTION OF THE FIGURES

[0074] Figure 1: Evolution of the BET specific surface area of the composite powder in function of the temperature of maximum rate of weight loss Tmax of the pitch powder used for its synthesis.

[0075] Figure 2: Evolution of the BET specific surface area of the composite powder in function of the temperature of end of the massive weight loss Tf of the pitch powder used for its synthesis.

[0076] Figure 3: Evolution of the BET specific surface area of the composite powder in function of the toluene insoluble fraction TI of the pitch powder used for its synthesis.

[0077] Figure 4: Evolution of the BET specific surface area of the composite powder in function of the difference of the toluene insoluble fraction and the quinoline insoluble fraction TI-QI of the pitch powder used for its synthesis.

[0078] Figure 5: Evolution of the BET specific surface area of the composite powder in function of the carbon residue at 900°C of the pitch powder used for its synthesis.

[0079] Figure 6: Evolution of the BET specific surface area of the composite powder in function of the hydrogen over carbon weight ratio of the pitch powder used for its synthesis.

[0080] Figure 7: Example of data treatment of a TG / DTG curve, illustrating the temperature of maximum rate of weight loss (Peak temperature = Tmax) and the temperature of end of weight loss (End temperature = Tf).

[0081] ANALYTICAL METHODS USED

[0082] Thermogravimetric analysis (TGA)

[0083] The thermogravimetric analysis (TGA) is performed using a Thermal Analyzer (Netzsch STA 449 F5). The following parameters are used:

[0084] -Crucible: AI2O3 85pl, open-Furnace: SiC S

[0085] -Sample carrier: DSC / TG Std S

[0086] -Sample size: 15-20 mg sample (<210pm)

[0087] -Inert gas: Nitrogen

[0088] -Inert gas flow: 50ml / min

[0089] -Protective flow: 20ml / min

[0090] -Heating rate: 7K / min

[0091] -Holding temperature 1 : 25°C

[0092] -Holding time 1 : 60 min

[0093] -Holding temperature 2: 900°C

[0094] -Holding time 2: 30 min

[0095] Once the analysis is complete, the measurement file is opened with a software capable of treating the data, for example here the Netzsch Proteus software. The TG curve (top curve in Figure 7) is smoothened to remove the noise (adaptive smoothing: 14). The obtained curve is used to determine the onset temperature (Ti) and the temperature of the end of the massive weight loss (Tf), as shown in Figure 7. As it can be seen, the temperature of end of weight loss (Tf) corresponds to the intersection between the tangent to the inflection point of the TG curve and the tangent to the end of the TG curve. Likewise, the onset temperature (Ti) corresponds to the tangent to the inflection point and the tangent to the start of the TG curve.

[0096] The first derivative of the TG curve, called DTG curve (bottom curve in Figure 7) is calculated by the software and the maximum rate of weight loss, expressed in % / min is determined. The temperature at which the maximum rate of weight loss occurs is the Tmax. This temperature is the same as the temperature of infliction point on the TG curve.

[0097] The residual mass is calculated between 100°C and 900°C to determine the residual mass, or carbon residue, at 900°C, expressed in %.

[0098] An example of such data treatment is presented in Figure 7.

[0099] Determination of the H / C ratio

[0100] The H / C ratio is determine via a CHNS analysis, using an elemental analyzer (ThermoFisher Scientific Thermo Electron Flash EA111). The calibration isperformed using BBOT ((2,5-bis (5-tert-butylbenzoxazol-2-yl) thiophene)). The total carbon, hydrogen, nitrogen and sulfur contents of the samples are then determined.

[0101] Determination of the volume-based distribution of silicon-based particles

[0102] The volume-based distribution of silicon-based particles is determined by the following method, using a disc centrifuge (CPS) particle size measurements. First, the sample is prepared by diluting the suspension comprising the silicon-based particles 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.

[0103] 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.

[0104] Halocarbon 1.8 oil in Halocarbon 1.8 oil in total spinfluid # injection #ml injected

[0105] BGA (10 wt%) BGA (5 wt%) volume (ml) 1 1,60 0,00 1,600 1,600 2 1,40 0,20 1,600 3,200 3 1,20 0,40 1,600 4,800 4 1,00 0,60 1,600 6,400 5 0,80 0,80 1,600 8,000 6 0,60 1,00 1,600 9,600 7 0,40 1,20 1,600 11,200 8 0,20 1,40 1,600 12,800 9 0,00 1,60 1,600 14,400

[0106]

[0107] After completion of the gradient, the disc is allowed to spin for 45min in order to stabilize the gradient.

[0108] 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 thestandard 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 dgo value is then determined.

[0109] Determination of the silicon content

[0110] 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.

[0111] Determination of the oxygen content

[0112] 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.

[0113] Determination of the carbon content

[0114] The carbon content of the composite powders is determined by the following method, using a Leco CS230 carbon-sulfur analyzer. The sample is melted in a constant oxygen flow in a ceramic crucible in a high frequency furnace. The carbon in the sample reacts with the oxygen gas and leaves the crucible as CO or CO2. After conversion of an eventual presence of CO into CO2, all produced CO2 is finally detected by an infrared detector. The signal is finally converted into a carbon content.

[0115] Determination of the specific surface area (BET)

[0116] The specific surface area of the composite powders is measured with the Brunauer-Emmett-Teller (BET) method using a Micromeritics Tristar 3000. 2g of the powder to be analyzed is first dried in an oven at 120°C for 2 hours, followed byNz purging. Then the powder is degassed in vacuum at 120°C for 1 hour prior to the measurement, in order to remove adsorbed species.

[0117] Determination of the electrochemical performance

[0118] The electrochemical performance of the composite powders in the examples and the counterexamples is determined by the following method.

[0119] 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.

[0120] 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.

[0121] 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".

[0122] • Cycle 1:

[0123] o Rest 6h

[0124] o CC lithiation to 10 mV at C / 10, then CV lithiation until C / 100 o Rest 5 min

[0125] o CC delithiation to 1.5 V at C / 10

[0126] o Rest 5 minThe capacity obtained for the delithiation at cycle 1 is the specific capacity of the composite powder.

[0127] 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.

[0128] The cycling procedure is as follows:

[0129] • Cycle 1:

[0130] o Rest 6h

[0131] o CC lithiation to 10 mV at C / 10, then CV lithiation until C / 100 o Rest 5 min

[0132] o CC delithiation to 1.5 V at C / 10

[0133] o Rest 5 min

[0134] • From cycle 2 on:

[0135] o CC lithiation to 10 mV at C / 2, then CV lithiation until C / 50

[0136] o Rest 5 min

[0137] o CC delithiation to 1.2 V at C / 2

[0138] o Rest 5 min

[0139] 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 90% CE at the initial cycle.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).

[0140] The target in terms of average CE from cycle 5 to cycle 50 is to reach at least 99.75%, preferably at least 99,80%, and even more preferably at least 99.85% for a cell comprising a composite powder with a specific capacity of 550 ± 10 mAh / g.

[0141] Determination of the number-based particle size distribution

[0142] The number-based particle size distribution of the silicon-based particles is determined via an electron microscopy analysis (SEM or TEM) of a cross-section of the composite powder, combined with an image analysis.

[0143] 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.

[0144] 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.

[0145] 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, 127eV). The signals from this detector are treated by the Quantax 800 EDS system from Bruker.

[0146] 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.

[0147] 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.

[0148] 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.

[0149] 1. Multiple SEM images of the cross-section of the composite powder comprising composite particles with silicon-based particles dispersed therein, are acquired.

[0150] 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. 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. 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.

[0151] The dlO, d50 and d90 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 convertedto a weight- or a volume-based particle size distribution via well-known mathematical equations.

[0152] Determination of the volume-based particle size distribution of composite particles The volume-based particle size distribution of the composite particles is determined with a laser diffraction particle size analyzer 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.

[0153] EXPERIMENTAL PREPARATION OF EXAMPLES

[0154] All examples, which are composite powders, are prepared using the same method, described hereunder. The only parameter which varies is the pitch powder, all other parameters are kept as similar as possible.

[0155] First, a silicon suspension SI is prepared by wet-milling. 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.

[0156] 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 (dgo =10 pm) is then slowly added to the mill. A viscosity-reducing agent is added to the mill together with the silicon powder, in order to limit the increase in viscosity of the suspension during the milling operation, thereby limiting the risks of clogging of the sieves, increased milling time and silicon particles not reaching the desired size.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. At 40000 kWh / t, a dgo particle size of 115 nm is reached for the silicon particles comprised in the suspension. At this point the mill is stopped and the suspension SI is drained. The BET specific surface of the dried silicon particles is also measured to be 125 m2 / g and the oxygen content in the dried silicon particles is measured to be 12.6 wt%.

[0157] Then, a silicon-pitch suspension S2 is prepared by mixing the suspension SI with a pitch powder. For a batch size of 700 g and a solid load of 20 weight%, 419 g of liquid (IPA), 203 g of silicon-containing dispersion SI (at 30 wt% solid load) and 79 g of pitch powder are weighed. The ratio between the pitch powder content and the silicon-based particles content (ratio pitch / silicon), both expressed in weight%, in the suspension S2 is 1.30. 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.

[0158] The mill is then flushed with N2 in 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 S2 is drained. The solid load is determined using a dry balance.

[0159] Next, the suspension S2 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-pitch" powder is collected from the cyclone. The inlet temperature is set to 130°C and the nitrogen flow is 1052 l / h.

[0160] Afterwards, the "silicon-pitch" powder is fed under a nitrogen flow at a feed rate of 500g / h into a twin-screw extruder, operated at a temperature of 300°C. The mixture of the silicon-based powder in the carbon precursor thus obtained is cooledunder N2 to room temperature and, once solidified, pulverized and sieved on a 400-mesh sieve, to produce an intermediate powder.

[0161] The intermediate powder is 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 pitch powder. 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.

[0162] The total Si content in the composite powders is measured to be between 47.2 and 47.6 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 0.96. The oxygen content in the composite powders is measured to be between 6.7 and 6.9 wt%. The specific surface area (BET) of the obtained composite powders is reported in Table 2.

[0163] 3 Examples named E1-E3 and 1 Counterexample named CE1, i.e. 4 composite powders are prepared following the method previously described, using 4 different pitch powders, named A, C, D and E, whose physicochemical properties are reported in Table 1. All 4 pitch powders have a softening point above 180°C. The composite powder CE1 is produced using the pitch powder A and the composite powders E1-E3 are produced using the pitch powders C, D and E. The BET specific surface area values of the obtained composites are reported in Table 2. Comparing the results obtained for the composite powders CE1 and El to E3, it can be seen that the lowest BET specific surface area values are obtained for the composite powders produced using the pitch powder according to the invention.Table 1: Physicochemical properties of the pitch powders

[0164] Table 1

[0165] Pitch powder

[0166] A C D E

[0167] Tmax (°C) 438.7 448.0 470.0 445.4 Tf (°C) 473.2 499.9 502.3 498.6 TI (%) 17.2 33.2 41.0 27.8 TI-QI (° / o) 16.2 31.4 40.7 27.5 Carbon residue @900°C (%) 54.8 65.2 66.9 59.8 H / C ratio 0.77 0.69 0.67 0.74

[0168]

[0169] Table 2: BET specific surface areas of the composite powders

[0170] Table 2

[0171] Example Pitch powder used BET (m2 / g) CE1 A 5.27 El C 2.94 E2 D 2.04 E3 E 3.55

[0172]

[0173] Electrochemical evaluation of the composite powders

[0174] 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, around 1530 mAh / g.

[0175] For an easier comparison of their performance, in a second stage, 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 results obtained for the initial coulombic efficiency and the averagecoulombic efficiency of the coin cells comprising the different composite powders, between cycle 5 and cycle 50, are given in Table 3.

[0176] It is noted that the pitch powders C, D and E used in Examples El to E3 fall within the preferred ranges for Tmax, Tf, TI, TI - QI, carbon residue and H / C ratio, whereas pitch A does not. Composite powders prepared with pitches C, D and E exhibit significantly lower BET specific surface area and therefore improved average coulombic efficiency compared with composite powder CE1.

[0177] Table 3: Performance of coin-cells containing composite powders E1-E3 and CE1 Table 3

[0178] Initial coulombic Average coulombic Specific efficiency for efficiency cycles 5- Pitch capacity pure "composite 50 for "composite Example powder composite powders + powders +

[0179] used powder graphite" at graphite" at (mAh / g) 550±10 mAh / g 550 ±10 mAh / g (%) (%) CE1 A 1531 91.11 99.81 El C 1533 91.09 99.89 E2 D 1535 91.22 99.92 E3 E 1532 91.15 99.84

[0180]

Claims

CLAIMS1.- A pitch powder for use in the manufacture of negative electrode materials for secondary batteries, having:(a) a temperature of maximum rate of weight loss Tmax in a TGA thermogravimetric analysis, characterized by a local maximum in the DTG derivative thermogravimetry curve, at least equal to 445°C, and(b) a temperature Tf at which the massive weight loss ends in a TGA thermogravimetric analysis at least equal to 490°C, and(c) a toluene insoluble fraction TI, expressed in weight percent (wt%), wherein the TI is at least equal to 25 wt% and at most equal to 45 wt%, and(d) a quinoline insoluble fraction QI, wherein the difference TI - QI, expressed in weight percent (%), is at least equal to 25 wt% and at most equal to 45 wt%, and (e) a carbon residue at 900°C at least equal to 60 wt%, and(f) a hydrogen content H and a carbon content C, both expressed in number of molecules, wherein the ratio H / C is at least equal to 0.65 and at most equal to 0.75.2.- The pitch powder according claim 1, having no local maximum in the DTG derivative thermogravimetry curve at a temperature below 440°C.3.- The pitch powder according to claim 1 or 2, wherein Tmax is at least 450°C.4.- The pitch powder according to any one of the preceding claims, wherein Tf is at least equal to 495°C.5.- The pitch powder according to any one of the preceding claims, wherein the carbon residue at 900 °C is at least 65wt%.6.- The pitch powder according to any one of the preceding claims, wherein the ratio H / C is at least equal to 0.65 and at most equal to 0.70.7.- The pitch powder according to any one of the preceding claims, comprising petroleum pitch.8.- The pitch powder according to any one of the preceding claims, having a softening point at least equal to 180°C.9.- A method for producing a silicon-based composite for use as a negative electrode material for secondary batteries, comprising the steps of:a) providing a silicon source and a pitch powder according to any one of the claims 1 to 8,b) mixing the silicon source and the pitch powder in a liquid to form a dispersion, c) drying the dispersion to obtain an intermediate powder, andd) heating up the intermediate powder in an oxygen-free atmosphere at a temperature between 800°C and 1100°C to obtain a silicon-based composite.10.- The method according to claim 9, wherein the silicon source in step a) is a slurry comprising silicon-based particles having a volume-based distribution of sizes with a d90 lower than 200 nm.11.- The method according to claim 9 or 10, wherein the liquid in step b) is an alcohol or a mixture of alcohols.12.- The method according to any one of claims 9 to 11, wherein in the dispersion of step b) the weight content of pitch powder to the weight content of silicon, both referring to the dry content, is from 0.5 to 3.0.13.- The method according to any one of claims 9 to 12, wherein between step c) and step d) shear forces are applied to the intermediate powder.14.- A silicon-based composite for use as a negative electrode material for secondary batteries obtainable from the method according to any one of claims 9 to 13.15.- The silicon-based composite according to claim 14, having a BET specific surface area lower than 5.0 m2 / g.

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