Composite powder for use in battery negative electrodes, method for making said composite powder, and battery containing said composite powder

Abstract

The present invention relates to a composite powder for use in a battery anode, the composite powder comprising composite particles comprising a carbonaceous matrix material having silicon-based particles embedded therein and carbon nanotubes, the surfaces of the composite particles being at least partially covered by the carbon nanotubes.

Description

[Technical Field]

[0001] The present invention relates to a composite powder suitable for use in a battery negative electrode, a method for making the composite powder, and a battery including the composite powder. [Background technology]

[0002] Lithium-ion (Li-ion) batteries are currently the most powerful batteries available and have become the standard for portable electronic devices. Furthermore, these batteries are rapidly gaining popularity in other industries, such as automotive and electrical storage. The realization of the benefits of these batteries is that they combine good power performance with high energy density.

[0003] Lithium-ion batteries typically include a number of so-called lithium-ion cells, which include a positive electrode, also called a cathode, a negative electrode, also called an anode, and a separator immersed in an electrolyte. The lithium-ion cells most frequently used in portable applications are developed using an electrochemically active material such as lithium cobalt oxide or lithium nickel manganese cobalt oxide for the cathode and natural or artificial graphite for the anode.

[0004] It is known that one of the key limiting factors affecting battery performance, especially the energy density, is the active material in the anode. Therefore, the use of electrochemically active materials, including silicon, in the negative electrode has been investigated over the past few years to improve energy density.

[0005] In the art, the performance of batteries that contain silicon-based materials is generally quantified by the so-called cycle life of full cells, which is defined as the number of times or cycles that a cell that contains such materials can be charged and discharged until it reaches 70% of its initial discharge capacity.Therefore, most research on silicon-based materials focuses on improving this cycle life. Summary of the Invention [Problem to be solved by the invention]

[0006] A drawback of using silicon-based materials in anodes is their large volume expansion during charging, which can reach up to 300% when lithium ions are fully incorporated into the active material of the anode, for example, by alloying or intercalation, a process often called lithiation. The large volume expansion of silicon-based materials during lithium incorporation can induce stresses in the silicon-based particles, which in turn can lead to mechanical degradation of the silicon material. The repeated mechanical degradation of silicon-based materials, which is periodically repeated during charging and discharging of lithium-ion batteries, can reduce the battery's life to unacceptable levels.

[0007] Another adverse effect associated with silicon-based materials is the formation of a thick SEI (solid electrolyte interface) on the anode. The SEI is a complex reaction product of the electrolyte and lithium, resulting in a loss of lithium availability for electrochemical reactions and therefore a decrease in cycling performance, i.e., capacity loss per charge / discharge cycle. A thick SEI can further increase the battery's electrical resistance, thereby limiting its ability to charge and discharge at high currents.

[0008] In principle, SEI formation is a self-terminating process that stops as soon as a "passivation layer" forms on the surface of the silicon-based material. However, due to the volume expansion of silicon-based particles, both the silicon-based particles and the SEI can be damaged during discharge (lithiation) and recharge (delithiation), thereby liberating new silicon surfaces and leading to a new start of SEI formation.

[0009] To overcome the above drawbacks, composite powders are commonly used. In these composite powders, nano-sized silicon-based particles are mixed with at least one component suitable for protecting the silicon-based particles from electrolytic decomposition and accommodating volume changes. Such a component may be a carbon-based material, preferably forming a matrix.

[0010] Composite powders usually additionally contain graphite particles to adjust their specific capacity to a practical level of 500mAh / g to 2000mAh / g.

[0011] Despite the use of such composite powders, there is still room for improving the performance of batteries containing silicon-based materials. In particular, existing composite powders do not allow achieving both high capacity and long cycle life, which are essential, especially for batteries in electric vehicles.

[0012] It is an object of the present invention to provide a composite powder comprising composite particles, the composite particles comprising a carbon matrix material having silicon-based particles embedded therein, which is advantageous in that when used in a negative electrode of a battery, the composite powder allows for achieving high capacity combined with long cycle life. [Means for solving the problem]

[0013] This object is achieved by providing a composite powder according to the present invention, which, once used in the negative electrode of a battery, makes it possible to achieve a long cycle life while maintaining a high specific capacity, as demonstrated in Examples 1 to 11 compared to Control Example 1.

[0014] In the following detailed description, preferred embodiments are described in detail to enable the 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. On the contrary, the invention includes numerous alternatives, modifications, and equivalents that will become apparent in light of the following detailed description and the accompanying drawings.

[0015] In a first aspect, the present invention relates to a composite powder for use in a battery anode, the composite powder comprising: (i) composite particles comprising a carbonaceous matrix material having silicon-based particles embedded therein; and (ii) carbon nanotubes, wherein the surfaces of the composite particles are at least partially coated by the carbon nanotubes.

[0016] The carbon nanotubes may be single-walled carbon nanotubes (SWNTs), double-walled carbon nanotubes (DWNTs), multi-walled carbon nanotubes (MWNTs), or a mixture thereof.

[0017] The composite particles preferably comprise a carbon matrix material having silicon nanoparticles embedded therein and carbon nanotubes attached to the surface of the composite particles.

[0018] By "carbonaceous matrix material and silicon-based particles embedded therein," it is meant that the composite particles, which include these, are, on average, larger in size than the silicon-based particles. The composite particles are typically micrometric in size, while the silicon-based particles are typically nanometric in size. It also means that the surfaces of the silicon-based particles are covered with the carbonaceous matrix material over at least 50% of their surfaces, and preferably the silicon-based particles are completely covered with the carbon matrix material, to ensure adequate protection against reaction with the electrolyte during cycling. In other words, the silicon-based particles and the carbon matrix material are not simply mixed together, as this would not provide adequate coverage of the surfaces of the silicon-based particles. This can be visually confirmed based on analysis of one or more SEM images of the cross-sections of the composite particles including the silicon-based particles.

[0019] As previously mentioned, a negative effect associated with silicon is that a thick SEI (solid electrolyte interface) can form on the anode, especially on silicon-based particles. Because silicon-based particles are subject to large volume fluctuations during the lithiation / delithiation process in the battery, the already formed SEI can break down again, resulting in continuous lithium consumption and a dramatic decrease in the battery's cycle life. Protecting the surface of the silicon-based particles, at least in part, with a carbonaceous matrix material is an efficient solution to the continuous formation of the SEI and the loss of cycle life.

[0020] The silicon-based particles embedded in the carbonaceous matrix material either form agglomerates smaller than 1 μm in size or do not form agglomerates at all, and therefore preferably only contact each other and / or the carbonaceous matrix material.

[0021] Silicon-based particles can have any shape, e.g., substantially spherical, but can also be irregular, rod-shaped, plate-shaped, etc. In silicon-based particles, silicon is present predominantly as silicon metal, to which small amounts of other elements are added to improve properties, or the particles can contain some impurities such as oxygen or trace metals. When considering a representative number of silicon-based particles, e.g., 10 or more separate silicon-based particles, the average silicon content in these silicon-based particles is 70 wt.% or more, preferably 80 wt.% or more, and more preferably 90 wt.% or more, based on the total weight of the silicon-based particles. This can be determined, for example, by elemental mapping analysis of a cross-section of a composite particle containing multiple cross-sections of silicon-based particles using a high-resolution FEG-SEM microscope. Furthermore, silicon-based particles typically have a surface layer with an average molar composition SiOx, where 0≦x<2, preferably 0≦x<1.

[0022] By "the surface of the composite particle is at least partially covered by carbon nanotubes" is meant, for example, the micrometers of the surface of the composite particle when considering a photograph of the surface of a representative number of composite particles, e.g., 10 or more distinct composite particles, taken with an electron microscope, e.g., a scanning electron microscope (SEM) or a transmission electron microscope (TEM). 2 This means that at least one carbon nanotube can be observed on average per μm of the composite particle surface. 2 Preferably, at least 5 carbon nanotubes, more preferably at least 10 carbon nanotubes, particularly preferably at least 20 carbon nanotubes, even more preferably at least 50 carbon nanotubes, and most preferably at least 100 carbon nanotubes can be observed on average per sample.

[0023] The present inventors believe that the presence of carbon nanotubes on the surface of the composite particles is beneficial in several aspects. First, the presence of carbon nanotubes increases the electrical conductivity of the composite particles, which is essential for achieving both good cycle life and high-rate capability (fast charging capability) in a battery. While other materials, such as carbon layer coatings or graphene sheets, can be used for this purpose, carbon nanotubes have the advantage of having excellent adhesion to the surface and remaining on the surface of the composite particles, despite enduring significant volume expansion / contraction during the charge / discharge cycles of the battery. This is not the case, for example, with carbon layer coatings. The coating is subjected to mechanical stress induced by the continuous volume expansion / contraction cycles, leading to the formation of cracks and ultimately to the coating's failure. The high electrical conductivity of the powder induced by the presence of carbon nanotubes on the surface of the composite particles ensures the high-rate capability of batteries containing the powder. The carbon nanotubes at the surface of the composite particles are preferably single-walled carbon nanotubes (SWNTs), as this allows for higher conductivity values ​​to be achieved, or similar conductivity values ​​at lower contents, compared to double-walled carbon nanotubes (DWNTs) or multi-walled carbon nanotubes (MWNTs) at similar contents. Therefore, the use of single-walled carbon nanotubes (SWNTs) is advantageous in that the carbon nanotube content in the composite powder can be reduced, which is beneficial for both the cost of the composite powder and its specific capacity in the battery.

[0024] Second, the carbon nanotubes on the surface of the composite particles act as spacers between the particles, thus preventing the composite particles from agglomerating into agglomerated powders. Without such spacers, the agglomerated powders may require mechanical processing, such as a grinding process, for use in battery anodes, which can result in weakening the integrity of the carbonaceous matrix material and ultimately reduce the cycle life of batteries containing such agglomerated powders.

[0025] The higher the coverage of the composite particle surface with carbon nanotubes, the stronger the aforementioned effects, i.e., improved conductivity and deagglomerated powder. Furthermore, the carbon nanotubes forming a coating layer on the surface of the composite particle are also believed to have a beneficial effect on the formation of a stable, conformal SEI layer that better resists the strong mechanical deformation induced by volume expansion / contraction cycles and therefore can lead to improved cycle life when used as an anode material in a battery.

[0026] In another preferred embodiment according to the first aspect of the present invention, at least 50% of the carbon nanotubes present in the composite powder are located on the surface of the composite particles. As already mentioned above, the technical effect is achieved by the presence of the carbon nanotubes on the surface of the composite particles. The carbon nanotubes present in other parts of the composite particles, for example in the carbonaceous matrix material, do not contribute to undesirable technical effects. Therefore, it is preferred that the majority of the carbon nanotubes present in the composite powder are located on the surface of the composite particles. It is preferred that at least 60%, more preferably at least 70%, even more preferably at least 80%, and particularly preferably at least 90% of the carbon nanotubes present in the composite powder are located on the surface of the composite particles.

[0027] The location of carbon nanotubes can be assessed, for example, by visual observation with or without the aid of an image analysis program of scanning electron microscope (SEM) or transmission electron microscope (TEM) photographs. In particular, SEM and / or TEM photographs of cross sections of composite particles can be used to detect the presence of carbon nanotubes within the core of the composite particles, e.g., embedded in a carbonaceous matrix material.

[0028] In another preferred embodiment according to the first aspect of the present invention, the inventors have determined that the observable μm of the surface of the composite particle 2It has been determined that a positive effect on performance can already be obtained at a concentration of only one carbon nanotube per average. When considering a photograph of the surface of a representative number of composite particles, for example, 10 or more individual composite particles, is taken with an electron microscope, for example, a scanning electron microscope (SEM) or a transmission electron microscope (TEM). Preferably, the micrometers of the composite particle surface are 2 An average of at least 5 carbon nanotubes can be observed per sample, more preferably at least 10 carbon nanotubes, even more preferably at least 20 carbon nanotubes, particularly preferably at least 50 carbon nanotubes, and most preferably at least 100 carbon nanotubes.

[0029] Alternatively, the carbon nanotube content in the composite powder is at least 0.02 weight percent (wt%) based on the total weight of the composite powder. The carbon nanotube content in the composite powder is preferably at least 0.05 wt%, more preferably at least 0.10 wt%, even more preferably at least 0.20 wt%, particularly preferably at least 0.30 wt%, and most preferably at least 0.50 wt%, based on the total weight of the composite powder. On the other hand, carbon nanotubes can be as small as several hundred meters. 2 Since they have a very high specific surface area with a BET value of 1000 / g, it is preferable to limit their amount in the composite powder in order to limit the specific surface area of ​​the composite powder. In order to limit the amount of lithium irreversibly consumed in the formation of the SEI layer and thus improve the initial coulombic efficiency (first CE) of the battery containing such composite powder, the specific surface area of ​​the composite powder is increased to a maximum of 10 m. 2 / g, with a maximum of 8m 2 It is more preferable to limit the value to / g, with a maximum of 6m 2It is particularly preferred to limit the carbon nanotube content to a value of 1 / g, which limits the surface area of ​​the electrochemically active particles that come into contact with the electrolyte in the battery. The inventors have determined that the carbon nanotube content in the composite powder of the present invention should preferably be at most 4.0 wt. % based on the total weight of the composite powder. This is because a content greater than 4.0 wt. % can result in the composite powder having an excessively high specific surface area, resulting in the aforementioned adverse effects. The carbon nanotube content in the composite powder of the present invention is preferably at most 3.0 wt. %, even more preferably at most 2.0 wt. %, particularly preferably at most 1.0 wt. %, more preferably at most 0.80 wt. %, and most preferably at most 0.60 wt. % based on the total weight of the composite powder.

[0030] The content of carbon nanotubes in the composite powder is preferably in the range of 0.02% by weight to 4.0% by weight, more preferably in the range of 0.05% by weight to 3.0% by weight, even more preferably in the range of 0.05% by weight to 2.0% by weight, and particularly preferably in the range of 0.05% by weight to 1.0% by weight, relative to the total weight of the composite powder.

[0031] In another embodiment according to the first aspect of the present invention, when used as an anode material in a battery, the composite powder has a specific capacity of at least 600 mAh / g, more preferably at least 800 mAh / g, even more preferably at least 1000 mAh / g, and particularly preferably at least 1200 mAh / g. Due to the need to transition from thermal vehicles to clean vehicles, particularly electric vehicles, a key goal for user acceptance is achieving a driving range of at least 500-600 km. Because the size and weight of battery packs cannot be scaled indefinitely, it is necessary to produce batteries with higher energy densities and, therefore, anode materials with higher specific capacities. However, because a higher specific capacity also means greater expansion and greater mechanical deformation during charge-discharge cycles, it is preferable to limit the specific capacity of the composite powder to 2600 mAh / g, preferably 2400 mAh / g, more preferably 2200 mAh / g, and particularly preferably 2000 mAh / g.

[0032] In yet another embodiment according to the first aspect of the present invention, the composite powder according to the present invention has a conductivity of at least 3.0 S / cm, preferably at least 4.0 S / cm, when measured at room temperature and under a pressure of 40 MPa. As already explained, the anode material must have high conductivity, which in the present invention is induced by the presence of carbon nanotubes on the surface of the composite particles. The conductivity can be measured, for example, by the methods described in the "Analytical Methods" section of this document. Similarly, the composite powder according to the present invention has a conductivity of at least 1.5 S / cm, preferably at least 2.0 S / cm, when measured under a pressure of 15 MPa; at least 2.5 S / cm, preferably at least 3.5 S / cm, when measured under a pressure of 30 MPa; at least 3.5 S / cm, preferably at least 4.5 S / cm, when measured under a pressure of 50 MPa; and at least 4.0 S / cm, preferably at least 5.0 S / cm, when measured under a pressure of 60 MPa, all of which values ​​are measured at room temperature. It is even more preferred that the composite powder according to the invention has, always at room temperature, a conductivity equal to at least 3.0 S / cm when measured under a pressure of 15 MPa, at least 4.5 S / cm when measured under a pressure of 30 MPa, at least 5.0 S / cm when measured under a pressure of 40 MPa, at least 5.5 S / cm when measured under a pressure of 50 MPa, and at least 6.0 S / cm when measured under a pressure of 60 MPa.

[0033] In another embodiment according to the first aspect of the present invention, the carbonaceous matrix material comprised in the composite powder according to the present invention is soft carbon, which, in contrast to hard carbon that is not graphitizable, corresponds to an arrangement of small irregular graphitic domains that can be converted to graphite upon heating at a temperature of 3000°C.

[0034] Soft carbon exhibits higher electronic conductivity than hard carbon and is therefore preferred. Furthermore, due to its irregular collection of small graphite domains, which leads to the presence of nanovoids in the matrix material, the volume expansion of particles containing a matrix material primarily composed of soft carbon during anode lithiation is reduced compared to particles containing a matrix material primarily composed of graphite or graphene. This reduced volume expansion leads to an extended battery cycle life.

[0035] In another embodiment according to the first aspect of the present invention, the silicon-based particles included in the composite powder according to the present invention have a number-based size distribution with a d50 of 20 nm or more and 150 nm or less. The number-based size distribution is based on a visual analysis, with or without an image analysis program, of the minimum number of silicon-based particles included in the composite powder. This minimum number of silicon-based particles is at least 1000 particles. An example of determining the number-based distribution of Si-based particles is provided in the Analytical Methods section.

[0036] For clarity, for example, a d50 of 100 nm herein means that 50% of the number of at least 1000 silicon-based particles have a size smaller than 100 nm and 50% of the number of at least 1000 silicon-based particles have a size greater than 100 nm.

[0037] Silicon-based particles with a number-based size distribution with a d50 of less than 20 nm are very difficult to disperse efficiently within the carbon matrix material, which can reduce the electronic conductivity of the powder.

[0038] Silicon-based particles with a number-based size distribution having a d50 greater than 150 nm tend to fracture during their lithiation, causing a dramatic decrease in the cycle life of batteries containing such composite powders.

[0039] The d50 is considered to be unaffected by the process of making the composite powder, which means that the d50 value of the silicon-based powder used as a precursor in the process is the same as the d50 value of the silicon-based particles contained in the composite powder.

[0040] In another embodiment of the first aspect of the invention, the composite particles have volume-based particle size distributions D10, D50 and D90, where 1 μm≦D10≦10 μm, 5 μm≦D50≦25 μm and 10 μm≦D90≦40 μm.

[0041] For clarity, for example, a D50 of 15 μm means here that 50% of the volume of the composite particles have a size smaller than 15 μm and 50% of the volume of the composite particles have a size larger than 15 μm.

[0042] Particles of matrix material having a volume-based size distribution with a D50 smaller than 5 μm may have too high a specific surface and therefore increase the surface area for reaction with the electrolyte and the formation of an SEI, which is disadvantageous for the reasons mentioned above.Particles of matrix material having a volume-based size distribution with a D50 larger than 25 μm are prone to fracture during lithium uptake due to their size, and therefore reduce the cycle life of batteries containing such particles.

[0043] In another embodiment according to the first aspect of the present invention, the weight ratio of the carbonaceous matrix material to silicon in the composite powder is at most 2.0, preferably at most 1.8, more preferably at most 1.6, even more preferably at most 1.4, even more preferably at most 1.2, particularly preferably at most 1.0, and most preferably at most 0.8. Because the carbonaceous matrix material induces high irreversible capacity (low coulombic efficiency on the first cycle) and low specific capacity, the composite powder preferably contains only the amount of carbonaceous matrix material necessary to cover the silicon-based particles. Furthermore, a minimum amount of carbonaceous matrix material is required to cover the silicon-based particles, and therefore the weight ratio of the carbonaceous matrix material to silicon in the composite powder is at least 0.5, preferably at least 0.6, more preferably at least 0.7, and even more preferably at least 0.8.

[0044] In yet another embodiment of the first aspect of the present invention, the composite powder according to the present invention further comprises other carbonaceous materials, such as graphite particles or graphene particles. Using a combination of carbon nanotubes and graphene particles, or graphite particles, or graphene particles and graphite particles, instead of carbon nanotubes alone, has at least two advantages. First, the presence of graphene particles facilitates the manufacturing process of the composite powder, and in particular, the milling step can be carried out with lower energy, which reduces the risk of damaging the structure of the composite particles. Second, carbon nanotubes are expensive, and therefore, it is economically advantageous to replace some of them with graphene particles, or graphite particles, or a combination of graphene particles and graphite particles, especially if the battery performance remains unchanged.

[0045] Preferably, the graphite and / or graphene particles are not completely embedded in the carbonaceous matrix material, and more preferably, are not embedded at all in the carbonaceous matrix material. This can be visually confirmed based on analysis of one or more SEM images of the cross section of the composite particle. The fact that the graphite and / or graphene particles are not completely embedded in the carbonaceous matrix material is beneficial because only the silicon-based particles need to be covered by the carbonaceous matrix material, thus requiring less carbonaceous matrix material with high irreversible capacity and low specific capacity.

[0046] Alternatively, the composite powder may also include exfoliated graphite particles, expanded graphite particles, and / or graphene nanoplatelets, all preferably not fully embedded or not embedded at all in the matrix material, for the same reasons as above.

[0047] In another embodiment according to the first aspect of the present invention, the composite powder according to the present invention has a silicon content C, expressed in weight percent (wt%), of at least 15 wt%, preferably at least 20 wt%, more preferably at least 25 wt%, and particularly preferably at least 30 wt%, based on the total weight of the composite powder. Composite powders with a silicon content of less than 15 wt% will not meet the requirements in terms of the specific capacity of the composite powder, i.e., at least 600 mAh / g. Preferably, the composite powder has a silicon content of at most 70 wt%, more preferably at most 60 wt%. If the silicon content is too high, the volume expansion of the silicon-based particles during lithium incorporation may be too great, which may induce stress in the silicon-based particles, leading to mechanical degradation of the negative electrode and, consequently, to an unacceptable shortening of the service life of batteries containing such composite powders.

[0048] In another embodiment according to the first aspect of the present invention, the composite powder according to the present invention has an oxygen content D, expressed in weight percent (wt%), which satisfies D≦0.20×C, where C is the silicon content of the composite powder. In other words, the oxygen content in the composite powder is 20% by weight or less of the silicon content in the composite powder. Composite powders with too high an oxygen content suffer from additional irreversible lithium consumption due to the formation of lithium silicates (LiSiO, LiSiO) during the initial lithiation of the powder, thus increasing the initial irreversible capacity loss of batteries containing such composite powders. Preferably, the oxygen content satisfies D≦0.15×C, more preferably D≦0.10×C.

[0049] In yet another embodiment of the first aspect of the present invention, the composite powder of the present invention comprises crystalline silicon carbide (SiC), wherein the ratio of the area of ​​the X-ray diffraction (XRD) peak attributable to SiC having a maximum between 35.0° and 36.0° in 2θ to the area of ​​the X-ray diffraction peak attributable to Si having a maximum between 28.0° and 29.0° in 2θ (the ratio "SiC / Si") is at most 0.15, preferably at most 0.12, more preferably at most 0.10, even more preferably at most 0.08, and most preferably at most 0.06, as measured with a copper anticathode generating Kα1 and Kα2 X-rays at a wavelength of 0.15418 nm. Because silicon carbide is electrochemically inert, its presence in the composite powder is undesirable, or at least its content is preferably limited. Silicon carbide is formed during heat treatment of the "silicon / carbon precursor / carbon nanotube mixture," and the silicon carbide content increases with increasing heat treatment temperature. This is why the heat treatment temperature should be kept as low as possible, preferably at a maximum of 990° C., and why there should be only one heat treatment step.

[0050] In a second aspect, the present invention relates to a method for producing a composite powder, the method comprising the steps of: a. providing a silicon-based powder comprising silicon-based particles; b. mixing the silicon-based powder with a carbon precursor powder capable of decomposing into carbon when heated at a temperature above 900°C to obtain mixture A; c. Dispersing mixture A in a solvent containing carbon nanotubes to obtain mixture B; d. drying mixture B at a temperature below the decomposition temperature of the carbon precursor to obtain powder A; e. heating powder A in an oxygen-free atmosphere at a temperature above 900 ° C to obtain powder B; f. grinding and sieving powder B to obtain the final composite powder; The silicon-based powder provided in step a may be produced by any means, such as, for example, dry milling, wet milling, plasma synthesis, laser pyrolysis, hot wall reactor synthesis, and the like.

[0051] The carbon precursor powder mixed in step b. can be any type of carbon precursor, such as one or a mixture of polyvinyl alcohol (PVA), polyvinyl chloride (PVC), sucrose, coal tar pitch, petroleum pitch, lignin, and resin materials. The carbon precursor preferably decomposes into soft carbon when heated at a temperature above 900°C. The carbon precursor preferably has a carbon yield of at least 40% by weight when heated at a temperature above 900°C, meaning that 40% by weight of the carbon precursor is decomposed into carbon, and 60% by weight of the carbon precursor is decomposed into gas, more preferably at least 50% by weight, and particularly preferably at least 60% by weight.

[0052] The weight ratio "carbon precursor / silicon" is at most 2.0, preferably at most 1.8, more preferably at most 1.6, even more preferably at most 1.4, particularly preferably at most 1.2, and most preferably at most 1.0. Here, "silicon" should be understood as the chemical element silicon, regardless of its oxidation state. The carbonaceous matrix material obtained after heat treatment of the carbon precursor above 900 °C typically has a specific capacity of 200-300 mAh / h, i.e., a ratio at least 10 times lower than the specific capacity of silicon. However, it is preferable to keep the weight ratio "carbon precursor / silicon" as low as possible, but high enough to ensure complete embedding and complete coverage of the silicon-based particles in the carbonaceous matrix material. This avoids direct contact between the silicon-based particles and the liquid electrolyte, which can lead to the formation of an unstable SEI layer and a decrease in the cycle life of batteries containing such composite powders.

[0053] In step c., the solvent in which mixture A is dispersed together with the carbon nanotubes is preferably the same solvent that the carbon nanotubes are stored in. This solvent is preferably water or an alcohol such as ethanol, as it has the advantages of being environmentally friendly, non-toxic, and inexpensive.

[0054] The carbon nanotubes used in step c preferably have an average diameter in the range of 1.0 nm to 5.0 nm, more preferably 1.5 nm to 4.0 nm. The carbon nanotubes are preferably 300 to 1500 nm. 2 / g, more preferably 500 to 1000m 2 / g. Furthermore, carbon nanotubes have a specific surface area in the range of I / g. G / I D The ratio, which corresponds to the intensity of the G peak divided by the intensity of the D peak, is at least 5, more preferably at least 10, even more preferably at least 20, and especially preferably at least 30. G / I DThe ratio is a good indicator of the amount of defects in the carbon material; the higher the ratio, the lower the amount of defects and the better the quality of the carbon nanotubes.

[0055] It should be noted that all properties of the carbon nanotubes used in the method for producing the composite powder according to the present invention and described above remain unchanged during the production of the composite powder and therefore also apply to the carbon nanotubes contained in the final composite powder.

[0056] In step d., mixture B is dried at a temperature below 200° C., preferably below 100° C., preferably under vacuum. Drying devices that can be used for this purpose are, for example, vacuum ovens or thin-film dryers.

[0057] In step e., an oxygen-free atmosphere can be used, for example, nitrogen or argon. The temperature to which powder A is heated ranges from 900°C to 1050°C, preferably from 950°C to 990°C. These temperatures limit the formation of silicon carbide, which is an electrochemically inactive compound and therefore undesirable.

[0058] Finally, the grinding of powder B in step f. should be carried out with as low an energy as possible in order to deagglomerate the composite particles without damaging the structure, for example, to bring the surface of the silicon-based particles into contact with the liquid electrolyte in the battery, with the subsequent consequences already mentioned above.

[0059] An alternative method for producing a composite powder comprising carbon nanotubes is to start with an existing silicon-carbon composite powder, for example produced by the method described in EP 3032616, and to carry out steps c to f described in the method described herein above, namely dispersing the composite powder in a solvent containing carbon nanotubes to obtain a mixture, drying the mixture at a temperature below 200°C to obtain powder C, heating powder C at a temperature above 900°C in an oxygen-free atmosphere to obtain powder D, and finally grinding and sieving powder D to obtain said final composite powder.

[0060] However, according to this alternative method, the final composite powder undergoes two heat treatment steps: the first heat treatment step produces a composite powder to be used as a precursor, and the second heat treatment step produces the final composite powder. As previously mentioned, this induces a higher silicon carbide content in the final composite powder produced according to this alternative method, typically having a SiC / Si ratio, as measured by X-ray diffraction, greater than 0.15, compared to the composite powder produced according to the previously described method, resulting in a lower specific capacity and lower average coulombic efficiency in the battery. Furthermore, although the core steps are performed in an oxygen-free atmosphere, silicon oxidizes more with the second heat treatment than with the first, inducing a higher oxygen content in the composite powder, which adversely affects the coulombic efficiency in the first cycle (first CE). Therefore, this alternative method produces a composite powder with lower performance in the battery than the composite powder produced according to the previously described method, and is therefore undesirable.

[0061] In a third aspect, the present invention relates to a composite powder obtainable by the method according to the second aspect of the invention as described above. As will be understood by a person skilled in the art, all embodiments directed to a composite powder according to the first aspect of the invention and / or a method according to the second aspect of the invention apply mutatis mutandis to a composite powder obtainable by a method according to the invention.

[0062] In a fourth aspect, the present invention relates to a negative electrode of a battery, preferably a lithium-ion battery, comprising the composite powder according to the present invention. The negative electrode also typically comprises a conductive additive such as carbon black, graphite particles, graphene particles, carbon nanotubes, or a mixture thereof. The content of the conductive additive is 0% to 10% by weight, particularly 0.1% to 5% by weight, based on the total weight of the negative electrode layer (excluding the current collector).

[0063] It should be noted that simply adding carbon nanotubes to an anode formulation containing a composite powder that does not contain carbon nanotubes will not result in an anode containing a composite powder according to the present invention, because the carbon nanotubes will be dispersed throughout the anode rather than attached to the surface of the composite particles, as is the case in the present invention.

[0064] The negative electrode also typically contains 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), and styrene-butadiene rubber (SBR). The binder(s) are added to improve the adhesion of the various components of the negative electrode, their mechanical strength on the current collector, or even their flexibility. The binder(s) represent 1% to 15% by weight, particularly 2% to 10% by weight, based on the total weight of the negative electrode layer (excluding the current collector). Examples of negative electrode preparations are provided elsewhere in this document.

[0065] Finally, the present invention also relates to a battery, preferably a lithium-ion battery, which comprises the negative electrode according to the present invention as defined or prepared as above and previously disclosed, and thus also comprises the composite powder according to the present invention. The battery according to the present invention more specifically comprises the negative electrode (anode) according to the present invention, the positive electrode (cathode) and an electrolyte, preferably a non-aqueous electrolyte. Examples of the positive electrode include LiCoO2, LiNi 0,6 Mn 0,2 Co 0,2 O2, LiNi 0,8 Mn 0,1 Co 0,1 O2, LiNi 0,8 Co 0,15 Al 0,05 O2, Li 1,2 Ni 0,2 Mn 0,6The electrolyte may be made from a positive electrode active material selected from the group consisting of 02, LiFePO4, etc. The electrolyte may preferably be a non-aqueous electrolyte solution, a non-aqueous polymer electrolyte, or even a solid electrolyte. Specific examples include organic electrolyte solutions obtained by dissolving lithium salts such as LiClO4, LiPF6, LiAsF6, LiBF4, LiSO3CF3, CH3SO3Li, and CF3SO3Li in non-aqueous solvents such as ethylene carbonate (EC), diethyl carbonate (dec), dimethyl carbonate (DMC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), propylene carbonate (PC), butylene carbonate, acetonitrile, propionitrile, dimethoxyethane, tetrahydrofuran, and γ-butyrolactone; gel polymer electrolytes including polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, and polymethyl methacrylate; and solid polymer electrolytes including polymers with ethylene oxide bonds. Furthermore, an additive that causes a decomposition reaction during initial charging of a lithium-ion battery may be added to the electrolyte solution. Specific examples of the additive include vinylene carbonate (VC), biphenyl, propane sulfone (PS), fluoroethylene carbonate (FEC), and ethylene sulfone (ES). The amount of the additive is preferably 0.1 wt % or more and 20 wt % or less based on the total weight of the electrolyte. [Brief explanation of the drawings]

[0066] [Figure 1] Figure 1 is an SEM photograph (E5) of a composite particle according to the present invention. The enlarged image on the top right shows the presence of carbon nanotubes on the surface of the composite particle. [Figure 2] FIG. 2 is a comparison of the conductivity values ​​measured for composite powders according to the invention and not according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0067] Analytical methods used Determination of silicon content 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.

[0068] Determination of oxygen content The oxygen content of composite powders is determined using a LECO TC600 oxygen / nitrogen analyzer by the following method: The powder sample to be analyzed is placed in a closed tin capsule, which itself is placed in a nickel basket. The basket is placed in a graphite crucible and heated to over 2000°C under helium as carrier gas. The sample is then melted, and the oxygen reacts with the graphite in the crucible to form CO or CO2 gas. These gases are then introduced into an infrared measuring cell. The observed signal is then recalculated into the oxygen content.

[0069] Determination of carbon content The carbon content of composite powders is determined using the Leco CS230 Carbon-Sulfur Analyzer by the following method: The sample is melted in a ceramic crucible in a high-frequency furnace under a constant flow of oxygen. The carbon in the sample reacts with the oxygen gas and leaves the crucible as CO or CO2. After the final conversion of CO to CO2, all the CO2 produced is finally detected by an infrared detector. The signal is then converted into the carbon content.

[0070] Determination of specific surface area (BET) The specific surface area of ​​the composite powders was measured by the Brunauer-Emmett-Teller (BET) method using a Micromeritics Tristar 3000. Two grams of powder to be analyzed was first dried in an oven at 120 °C for 2 h, followed by a N2 purge. To remove adsorbed species, the powder was then degassed in vacuum at 120 °C for 1 h before measurement.

[0071] Determination of electrochemical performance The electrochemical performance of the composite powders in the examples and controls is determined by the following method.

[0072] The composite powders to be evaluated are sieved using a 45 μm sieve. Then, in the first stage, the composite powders are tested as is without dilution with graphite particles to determine their specific capacity. They are mixed with carbon black, carbon fiber, and sodium carboxymethylcellulose binder in water (2.5 wt%).

[0073] The ratio used is 89 parts by weight of composite powder / 1 part by weight of carbon black (C65) / 2 parts by weight of carbon fiber (VGCF) and 8 parts by weight of carboxymethyl cellulose (CMC). All these components are mixed in a Pulverisette 7 planetary ball mill at 250 rpm for 30 minutes.

[0074] Copper foil washed with ethanol is used as a current collector. A 200 μm thick layer of the mixed components is coated onto the copper foil. The coated copper foil is then dried in a vacuum at 70 °C for 45 minutes. The thickness of the foil is 1.27 cm. 2 Circles are punched from the dried coated copper foil and used as electrodes in coin cells with lithium metal as the counter electrode. The electrolyte is 1M LiPF6 dissolved in EC / DEC 1 / 1 + 2% VC + 10% FEC solvent.

[0075] All coin cells were 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." Cycle 1: 6 hours rest CC lithiation to 10mV at C / 10, then CV lithiation to C / 100 5 minutes rest CC delithiation to 1.5V at C / 10 5 minutes rest

[0076] The capacity obtained for delithiation in cycle 1 is the specific capacity of the composite powder.

[0077] In the second stage, the composite powder is tested at a lower capacity, i.e., after dilution with graphite particles in the mixture "composite powder + graphite." The respective weight contents of the composite powder and graphite in the mixture "composite powder + graphite" are adjusted to obtain a theoretical specific capacity of the mixture of about 550 mAh / g. For example, for a composite powder with a measured specific capacity of 1500 mAh / g and using a theoretical capacity of 350 mAh / g for graphite, the respective weight contents of the composite powder and graphite are 17.4 wt.% and 82.6 wt.%, respectively. The remaining procedures, anode formulation, cell composition, and cell assembly remain unchanged.

[0078] The cycle procedure is as follows: Cycle 1: 6 hours rest CC lithiation to 10mV at C / 10, then CV lithiation to C / 100 5 minutes rest CC delithiation to 1.5V at C / 10 5 minutes rest From cycle 2: CC lithiation to 10 mV at C / 2, then CV lithiation to C / 50 5 minutes rest CC delithiation to 1.2 V at C / 2 5 minutes rest

[0079] The Coulombic Efficiency (CE) of a coin cell is the ratio of the delithiation capacity to the lithiation capacity at a given cycle, calculated for the initial cycle and subsequent cycles. The initial cycle is the most important cycle in terms of Coulombic Efficiency, as the SEI formation reaction has a significant impact on CE. Typically, for silicon-based powders, the Coulombic Efficiency at the initial cycle can be as low as 80% (or even lower), which corresponds to an irreversible capacity loss of 20% of the coin cell, which is significant. The goal is to reach a CE of at least 90% at the initial cycle.

[0080] While CE typically increases well beyond 99% on subsequent cycles, those skilled in the art will recognize that even small differences in coulombic efficiency per cycle can have a significant cumulative effect over the hundreds or thousands of charge-discharge cycles that a battery is expected to last. As an example, a cell with an initial capacity of 1 Ah and an average CE of 99.8% will have a residual capacity of 0.8 Ah after 100 charge-discharge cycles, which is 60% higher than a cell with an average CE of 99.5% (0.5 Ah residual capacity).

[0081] The goal for the 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 cells containing a negative electrode material (i.e., a mixture of composite powder and a carbon additive such as graphite or graphene) with a specific capacity of 550±10 mAh / g.

[0082] Determination of number-based particle size distribution The number-based particle size distribution of the silicon-based particles is determined via electron microscopy analysis (SEM or TEM) of cross-sections of the composite powders combined with image analysis.

[0083] To do this, a cross-section of a composite powder comprising multiple cross-sections of composite particles, each of which comprises multiple cross-sections of silicon-based particles, is processed according to the procedures detailed herein.

[0084] 500 mg of the composite powder to be analyzed was embedded in 7 g of resin (Buehler EpoxiCure 2) consisting of a mixture of 4 parts epoxy resin (20-3430-128) and 1 part epoxy hardener (20-3432-032). The resulting sample, measuring 1" in diameter, was allowed to dry for at least 8 hours. It was then polished, first mechanically using a Struers Tegramin-30 until a maximum thickness of 5 mm was reached, and then further polished by ion beam polishing (Jeol SM-09010 cross-section polisher) at 6 kV for approximately 6 hours to obtain a polished surface. A carbon coating was finally applied to this polished surface by carbon sputtering for 12 seconds using a Cressington 208 carbon coater, obtaining the sample, also called a "cross section," that was then analyzed by SEM.

[0085] The processed cross-sections were then cut into a Bruker (30 mm 2 The analysis was carried out using a JEOL FEG-SEM JSM-7600F equipped with an EDS detector Xflash 5030-127 (127 eV). The signal from this detector was processed by a Bruker Quantax 800 EDS system.

[0086] The magnified image is generated by applying a voltage of 15 kV at a working distance of a few millimeters. The image from the backscattered electrons is reported as an additional value to the image from the optical microscope.

[0087] The size of a silicon-based particle is considered to be equivalent to the maximum linear distance between two points around the perimeter of an individual cross-section of that silicon-based particle.

[0088] To illustrate, in a non-limiting manner, the determination of number-based particle size distribution of silicon-based particles, an SEM-based procedure is provided below.

[0089] 1. Multiple SEM images are obtained of a cross section of a composite powder comprising composite particles having silicon-based particles dispersed therein. 2. Image contrast and brightness settings are adjusted for easy visualization of cross sections of composite and silicon-based particles. Due to their different chemical compositions, the difference in brightness allows for easy distinction between both types of particles. 3. At least 1000 distinct cross-sections of the silicon-based particles that do not overlap with another cross-section of the silicon-based particles are selected from one or more of the acquired SEM image(s) using suitable image analysis software. These distinct cross-sections of the silicon-based particles may be selected from one or more cross-sections of composite particles and composite powders comprising the silicon-based particles. 4. The size of individual cross-sections of the silicon-based particles is measured using appropriate image analysis software for each of at least 1000 individual cross-sections of the silicon-based particles.

[0090] The d10, d50, and d90 values ​​of the number-based particle size distributions of the silicon-based particles determined using the above-described methods are then calculated. These number-based particle size distributions can be readily converted to weight-based or volume-based particle size distributions via well-known mathematical formulas.

[0091] Determination of volume-based particle size distribution The volume-based particle size distribution of the composite particles is determined using a Malvern Mastersizer 2000 laser diffraction particle size analyzer. The following measurement conditions are selected: compression range, active beam length 2.4 mm, measurement range: 300 RF, 0.01–900 μm. Sample preparation and measurements are carried out according to the manufacturer's instructions.

[0092] Determination of conductivity values The electrical conductivity of the composite powders is determined at room temperature using a four-point probe method with a Nuseyko Analytech MCP-PD51 powder resistivity measurement system. Five measurements are taken for each sample at loads of 4 kN, 8 kN, 12 kN, 16 kN, and 20 kN.

[0093] Determining the "SiC / Si" ratio The SiC / Si ratio was determined using a step size of 0.0334°2θ, a scan rate of approximately 30 min, and a scan speed of approximately 2 cm 3 The X-ray diffraction (XRD) data was determined using a Panalytical 'X Pert Pro' system with a copper anticathode generating Kα1 and Kα2 X-rays, measuring 20°-40° (2θ) on a flattened surface of the powder material, and a wavelength λ of 0.15418 nm. The latest version of the ICDD data was used to identify this compound.

[0094] The ratio "SiC / Si" corresponds to the ratio of the area of ​​the X-ray diffraction peak due to silicon carbide (SiC) having a maximum value between 35.0° and 36.0° in 2θ to the area of ​​the X-ray diffraction peak due to silicon (Si) having a maximum value between 28.0° and 29.0° in 2θ.

[0095] Experimental setup for the example Control Example 1 (CE1) not according to the invention To produce the powder of Control Example 1, a silicon-based powder was first obtained by applying a 60 kW radio frequency (RF) inductively coupled plasma (ICP) using argon as the plasma gas into which a micron-sized silicon powder precursor was injected at a rate of approximately 200 g / hr, resulting in a general temperature (i.e., in the reaction zone) of over 2000 K. In this first process step, the precursor was completely vaporized. In the second process step, a 20 Nm argon gas was added immediately downstream of the reaction zone to reduce the temperature of the gas to below 1600 K. 3 A 100 l / h flow of argon is used as a gas quench to nucleate the metallic submicron silicon powder. Finally, a passivation step is carried out at a temperature of 100 °C for 5 min by adding 100 l / h of a N2 / O2 mixture containing 1 mol% oxygen.

[0096] The specific surface area (BET) of the obtained silicon powder was 83 m 2 / g. The oxygen content of the resulting silicon powder is measured to be 8.0 wt%. The number-based particle size distribution of the silicon powder is determined to be d10=52 nm, d50=108 nm, and d90=170 nm.

[0097] A dry mixture is then made of 36 g of the resulting silicon-based powder and 60 g of a petroleum-based pitch powder with a softening point of 180° C. The mixture is fed into a twin-screw extruder operating at a temperature of 230° C. under a nitrogen flow at a feed rate of 500 g / h.

[0098] The resulting mixture of silicon-based powder with pitch is cooled to room temperature under N2, solidified, crushed, and sieved on a 400 mesh sieve to produce an intermediate powder.

[0099] 20 g of the resulting intermediate powder 1 was then placed in a quartz crucible in a tube furnace and heated to 960°C at a heating rate of 3°C / min, held at that temperature for 2 hours, and then cooled. All of this was done in an oxygen-free argon atmosphere. In the resulting product, silicon-based particles were dispersed and embedded in a matrix of soft carbon, resulting from the pyrolysis of pitch.

[0100] Finally, the heated product is ball milled with alumina balls at 300 rpm for 1 hour and sieved through a 325 mesh sieve to obtain the composite powder of Control Example 1.

[0101] The chemical compositions of the composite powders of CE1 and subsequent composite powders are summarized in Table 1.

[0102] The total Si content in this powder is determined by XPF to be 45.4 wt. % with an experimental error of ±0.3 wt. %. This corresponds to a calculated value based on a weight loss of the pitch on heating of about 35 wt. % and a small weight loss of the other components on heating. The calculated ratio of the carbon content resulting from carbonization of the pitch forming the matrix material to the silicon content in the powder is about 1.08. The oxygen content of this powder is measured to be 5.4 wt. %. The specific surface area (BET) of the resulting powder is 2.9 m 2 / g.

[0103] The volume-based particle size distribution of the resulting composite particles is D10=5.3 μm, D50=15.9 μm, and D90=24.4 μm.

[0104] Example 1 (E1) according to the invention The composite powder of Example 1 (E1) is produced starting from the same intermediate powder as the composite powder of Control Example 1 (CE1), ie the mixture of silicon-based powder with pitch.

[0105] 80 g of the intermediate powder was dispersed in water with 13.3 mg of single-walled carbon nanotubes (SWNTs) and 20.0 mg of sodium carboxymethylcellulose (CMC) used as a dispersant, and stirred for 20 min, resulting in a total solids content of approximately 40 wt% in the resulting dispersion.

[0106] The dispersion is further dried in a vacuum oven at 90° C. for 2 hours until the water has completely evaporated and a powder is obtained.

[0107] 20 g of this powder was then placed in a quartz crucible in a tube furnace and heated to 960 °C at a heating rate of 3 °C / min, held at that temperature for 2 hours, and then cooled. All of this was done under an oxygen-free argon atmosphere. Note that the initial CMC partially decomposed to carbon during the heat treatment, with a carbonization yield of approximately 20%.

[0108] In the resulting product, the silicon-based particles are dispersed and embedded in a matrix of soft carbon resulting from the pyrolysis of the pitch, with carbon nanotubes present on the surface of the particles. Finally, the heated product is ball-milled with alumina balls at 300 rpm for 1 hour and sieved through a 325-mesh sieve to obtain the composite powder of Example 1. The carbon nanotube content in the composite powder of Example 1 is 0.02 wt. % (wt. %) based on the total weight of the composite powder.

[0109] Examples 2, 3, 4, 5, 6 and 7 (E2 to E7) of the present invention The composite powders of Examples 2-7 (E2-E7) are produced using the same method as the production of the composite powder of Example 1, except for the respective amounts of carbon nanotubes and CMC involved. The carbon nanotube contents in the final composite powders of Examples 2-7 are 0.05 wt%, 0.10 wt%, 0.20 wt%, 0.50 wt%, 1.00 wt%, and 2.00 wt%, respectively, based on the total weight of the composite powder.

[0110] Example 8 (E8) according to the invention The composite powder of Example 8 (E8) was produced using the same method as that for producing the composite powder of Example 7 (E7), except that only single-walled carbon nanotubes (SWNTs) were added to the dispersion, and a small amount of a mixture of SWNTs and graphene was used. As already mentioned above, it is economically advantageous to replace some of the expensive SWNTs with graphene, especially if the battery performance remains unchanged. The chemical composition of composite powder E8 is shown in Table 1.

[0111] Example 9 (E9) according to the invention The composite powder of Example 9 (E9) is produced using the same method as the production of the composite powder of Example 5 (E5), except that multi-walled carbon nanotubes (MWNTs) are used instead of single-walled carbon nanotubes (SWNTs).

[0112] Example 10 (E10) produced using a higher amount of carbon nanotubes The composite powder of Example 10 (E10) is produced using the same method as the production of the composite powders of Examples 1 to 7 (E1 to E7), except for the amounts of carbon nanotubes and CMC involved. The carbon nanotube content in the final composite powder of Example 10 is 5.00 wt. % based on the total weight of the composite powder.

[0113] Example 11 (E11) prepared using an alternative method The composite powder of Example 11 (E11) was produced using the alternative method described above. The composite powder of Comparative Example 1 was used as the starting material. 80 g of this composite powder CE1 was dispersed in water with 0.333 g of single-walled carbon nanotubes (SWNTs) and 0.5 g of CMC used as a dispersant, and stirred for 20 minutes. The total solid content of the resulting dispersion was approximately 40 wt%.

[0114] The dispersion is further dried in a vacuum oven at 90° C. for 2 hours until the water has completely evaporated and a powder is obtained.

[0115] 20 g of this powder is then placed in a quartz crucible in a tube furnace and heated to 960°C at a heating rate of 3°C / min, held at that temperature for 2 hours, and then cooled, all in an oxygen-free argon atmosphere.

[0116] Finally, the heated product is ball milled with alumina balls at 300 rpm for 1 hour and sieved through a 325 mesh sieve to obtain the composite powder of Example 11. The carbon nanotube content in the composite powder of Example 11 is 0.5 wt% (wt%) based on the total weight of the composite powder.

[0117] Table 1: Chemical composition of composite powders E1-E11 and CE1. The "C matrix" content corresponds to the carbon content resulting from the pyrolysis of the carbon precursor (pitch) and CMC.

[0118] [Table 1]

[0119] Determination of the electrical conductivity of composite powders The conductivity of the composite powders is measured according to the method described above. The results are shown in Figure 2. For better readability of the figure, only the results obtained for selected composite powders are shown.

[0120] Electrochemical evaluation of composite powders The resulting composite powders were tested in coin cells according to the procedure described above. In the first stage, the composite powders were evaluated as such, i.e., without dilution with graphite particles, to determine their specific capacities. As shown in Table 2, all composite powders had high specific capacities ranging from 1475 mAh / g to approximately 1620 mAh / g.

[0121] For a simpler comparison of their performance, in a second step, the composite powder was mixed with graphite particles during the preparation of the electrode, and the capacity of the mixture "composite powder + graphite" reached approximately 550 ± 10 mAh / g. The results obtained for the initial and average coulombic efficiencies of coin cells containing different composite powders between cycle 5 and cycle 50 are shown in Table 2.

[0122] Comparing the results obtained for composite powders CE1 and E1-E7, we see that cells containing composite powders with carbon nanotubes achieve the best results, for the reasons previously provided. However, we also note that (i) the average coulombic efficiency increases with increasing carbon nanotube content, but reaches a plateau in performance for carbon nanotube contents in the range of 1.00 wt% to 2.00 wt%. (ii) Increasing the carbon nanotube content above 4.00 wt%, as in composite powder E10, still results in a higher average coulombic efficiency, but also leads to a decrease in specific capacity and initial coulombic efficiency, for the reasons previously provided. A better alternative is to combine carbon nanotubes and graphene, as can be seen by comparing the results obtained for composite powders E7 and E8.

[0123] [Table 2]

Claims

1. 1. A composite powder for use in a battery negative electrode, comprising: Composite particles comprising a carbonaceous matrix material having silicon-based particles embedded therein; carbon nanotubes, A composite powder, wherein the surfaces of the composite particles are at least partially covered with the carbon nanotubes.

2. 2. The composite powder of claim 1, further comprising crystalline silicon carbide SiC, wherein the ratio of the area of ​​the X-ray diffraction peak attributable to SiC having a maximum between 35.0° and 36.0° in 2θ to the area of ​​the X-ray diffraction peak attributable to silicon Si having a maximum between 28.0° and 29.0° in 2θ is at most 0.15, as measured with a copper anticathode producing Kα1 and Kα2 X-rays having a wavelength of 0.15418 nm.

3. When considering photographs of the surfaces of at least 10 separate composite particles taken with an electron microscope, the μm ２ 3. The composite powder according to claim 1, wherein an average of at least one carbon nanotube can be observed per particle.

4. 4. The composite powder according to claim 1, wherein the carbon nanotube content is at least 0.02 wt. % relative to the total weight of the composite powder.

5. 5. The composite powder according to claim 1, wherein the carbon nanotube content is at most 4.00 wt. % relative to the total weight of the composite powder.

6. 6. Composite powder according to any one of claims 1 to 5, having a silicon content of at least 15% by weight relative to the total weight of the composite powder.

7. Maximum 10m ２ The composite powder according to any one of claims 1 to 6, having a specific surface area of ​​1 / g.

8. The composite powder according to any one of claims 1 to 7, wherein the silicon-based particles have an average silicon content of 70% by weight or more.

9. 9. The composite powder of any one of claims 1 to 8, wherein the silicon-based particles are characterized by a number-based size distribution with a d50 equal to or greater than 20 nm and equal to or less than 150 nm.

10. 10. The composite powder of any one of claims 1 to 9, having an electronic conductivity of at least 3.0 S / cm when measured at room temperature and at a pressure of 40 MPa.

11. 11. The composite powder of claim 1, further comprising graphene particles, or graphite particles, or a combination of graphene and graphite particles.

12. A method for producing the composite powder according to any one of claims 1 to 11, comprising the steps of: a. providing a silicon-based powder comprising silicon-based particles; b. mixing the silicon-based powder with a carbon precursor powder capable of decomposing into carbon when heated at a temperature above 900°C to obtain mixture A; c. Dispersing mixture A in a solvent containing carbon nanotubes to obtain mixture B; d. drying mixture B at a temperature below the decomposition temperature of the carbon precursor to obtain powder A; e. heating powder A in an oxygen-free atmosphere at a temperature above 900°C to obtain powder B; f. Milling and sieving powder B to obtain the final composite powder.

13. 13. The method of claim 12, wherein the silicon-based powder and the carbon precursor are mixed in step b in a "carbon precursor / silicon" weight ratio of at least 0.5 and at most 2.

0.

14. A composite powder according to any one of claims 1 to 11 obtainable by the method according to any one of claims 12 to 13.

15. A battery comprising the composite powder of any one of claims 1 to 11 or 14.

Images