Powder for use in negative electrode of battery, method for preparing such powder, and battery comprising such powder

Abstract

To provide a stable electrochemically active powder comprising (i) particles, the particles comprising a matrix material and silicon-based sub-particles embedded in the matrix material, and (ii) sulfur, i.e., a powder which is used once in a negative electrode in a Li-ion battery and is advantageous in capability of achieving a high capacity combined with a long cycle life.SOLUTION: A powder suitable for use in a negative electrode of a battery comprises particles. The particles comprise a matrix material, and silicon-based sub-particles embedded in the matrix material. The matrix material comprises a carbonaceous material. The powder further comprises sulfur. The sulfur content by weight in the powder is at least 0.1% of the content of the carbonaceous material by weight in the powder, and at most 1% of the content of the carbonaceous material by weight in the powder.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to powders for use in battery negative electrodes, methods for preparing such powders, and batteries containing such powders. [Background technology]

[0002] Lithium-ion (Li-ion) batteries are currently the most powerful batteries and have already become the standard for portable electronic devices. In addition, these batteries have already penetrated and are rapidly expanding in other industries, such as automotive and energy storage. The advantage of using such batteries is their high energy density combined with good power performance.

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

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

[0005] In the art, the performance of batteries containing Si-based electrochemically active powders is generally quantified by the so-called full cell cycle life, which is defined as the number of times or cycles that a cell containing such material can be charged and discharged until it reaches 80% of its initial discharge capacity. Therefore, most research on silicon-based electrochemically active powders focuses on improving this cycle life.

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

[0007] Another adverse effect associated with silicon is that a thick SEI, or solid electrolyte interface, can form on the anode. The SEI is a complex reaction product between the electrolyte and lithium, which leads to poor cycling performance due to a loss of lithium availability for electrochemical reactions, resulting in a loss of capacity per charge-discharge cycle. Furthermore, a thick SEI can further increase the battery's electrical resistance, thereby limiting its ability to discharge and charge at high currents.

[0008] SEI formation is a self-terminating process based on the principle that it stops as soon as a "passivation layer" forms on the surface of a 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 freeing up new silicon surfaces and initiating new SEI formation.

[0009] To overcome the above drawbacks, composite powders are typically 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 electrolyte decomposition and for accommodating volume changes. Such a component may be a carbon-based material, preferably forming a matrix.

[0010] Such composite powders are described, for example, in U.S. Patent No. 10,964,940, which discloses a particulate material consisting of composite particles, the composite particles including a porous carbon skeleton and a plurality of nanoscale domains of elemental silicon located within the pores of the porous carbon skeleton. International Publication No. 2020 / 129879 discloses an anode mixture for an all-solid-state lithium-ion battery, including an anode material and a solid electrolyte, the anode material including a composite material (A) containing silicon-containing particles and a carbonaceous material, and one or more components (B) selected from a carbonaceous material and graphite. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] U.S. Patent No. 10,964,940 [Patent Document 2] International Publication No. 2020 / 129879 Summary of the Invention [Problem to be solved by the invention]

[0012] Despite the use of such composite powders, there is still room for improvement in the performance of batteries containing Si-based electrochemically active powders. In particular, existing composite powders cannot achieve both the high capacity and long cycle life required for electric vehicle batteries.

[0013] It is an object of the present invention to provide a stable electrochemically active powder comprising (i) particles, i.e., particles comprising a matrix material and silicon-based subparticles embedded in the matrix material, and (ii) sulfur, which powder is advantageous in that it can achieve high capacity combined with long cycle life once used in the negative electrode of a Li-ion battery. [Means for solving the problem]

[0014] This object is achieved by providing a powder according to embodiment 1, which once used in an anode of a Li-ion battery can achieve a higher initial coulombic efficiency (CE) and a higher average coulombic efficiency, as demonstrated in Examples 1-4, compared to Comparative Examples 1-4.

[0015] The present invention relates to the following embodiments.

[0016] [Embodiment 1] In a first aspect, the present invention relates to a powder comprising particles, the particles comprising a matrix material and silicon-based subparticles embedded in the matrix material, the matrix material comprising a carbonaceous material, the powder further comprising sulfur, wherein the content by weight of sulfur in the powder is at least 0.1% of the content by weight of the carbonaceous material and at most 1% of the content by weight of the carbonaceous material.

[0017] Preferably, the sulfur content by weight in the powder is at most 0.8% by weight of the carbonaceous material content, more preferably at most 0.6% by weight of the carbonaceous material content.

[0018] Preferably, at least 50% by weight of the matrix material is a carbonaceous material, more preferably at least 70% by weight of the matrix material is a carbonaceous material, and most preferably at least 90% by weight of the matrix material is a carbonaceous material.

[0019] Preferably, the silicon-based subparticles are embedded in the carbonaceous material.

[0020] "The particles comprise a matrix material and silicon-based subparticles embedded in the matrix material" means that the particles comprised in the powder are, on average, larger in size than the silicon-based subparticles because they comprise the silicon-based subparticles. The particles are typically micrometer-sized, while the silicon-based subparticles are typically nanometer-sized.

[0021] "Silicon-based subparticles embedded in a matrix material" means that the silicon-based subparticles are fixed in and surrounded by the matrix material. The silicon-based subparticles are mostly, and preferably entirely, covered by the matrix material. Thus, in the powder according to embodiment 1, the silicon-based subparticles preferably only contact each other and / or the matrix material.

[0022] The silicon-based subparticles may have any shape, e.g., substantially spherical, but may also have irregular shapes, rods, plates, etc. In the silicon-based subparticles, the silicon is present predominantly as silicon metal, to which small amounts of other elements may be added to improve properties, or the silicon may contain some impurities such as oxygen or trace metals. Taking into account all elements other than oxygen, the average silicon content in such silicon-based subparticles is preferably 80 wt. % or more, more preferably 90 wt. % or more, based on the total weight of the silicon-based subparticles.

[0023] Without being bound by theory, the inventors believe that the presence of sulfur in the powder allows for the creation of bridges between small graphite domains of the carbonaceous material contained in the matrix material, thereby increasing the elasticity of the carbonaceous material and therefore the matrix material. Thanks to its elastic properties, the matrix material can better accommodate the expansion / contraction of the silicon-based subparticles during the charge / discharge of the battery, thereby reducing the risk of fracture of the matrix material, i.e., the risk of exposure of the silicon-based subparticles to the electrolyte, which could lead to the formation of an additional solid electrolyte interface (SEI) and consequently a decrease in the first coulombic efficiency and the average coulombic efficiency.

[0024] The weight content of sulfur in the powder should be 0.1% or more of the weight content of the carbonaceous material, because if the sulfur content is too low, the desired technical effect of increasing the elasticity of the carbonaceous material from the matrix cannot be achieved. Similarly, the weight content of sulfur in the powder should be 1% or less of the weight content of the carbonaceous material, preferably 0.8% or less of the weight content of the carbonaceous material, and more preferably 0.6% or less of the weight content of the carbonaceous material. If the sulfur content is too high, the carbonaceous material from the matrix becomes too elastic and therefore deforms too much, especially during battery charging (i.e., lithiation of the silicon-based subparticles). This can lead to unacceptable expansion of the negative electrode, which can cause both reduced cycle life and safety issues if the anode expands more than allowed by the battery casing. Furthermore, because sulfur is electrochemically inert, it is best to limit its content to the level necessary to achieve the technical effect in order to keep the specific capacity of the powder as high as possible.

[0025] The content of carbonaceous material in the matrix material of the powder can be measured by conventional techniques or calculated based on the specific volume of the powder. Examples of such calculations are provided in the "Analytical Methods" section.

[0026] Preferably, the powder also has a silicon content A and a carbon content B, both expressed in weight percent (wt%, wt%), where 10 wt%≦A≦60 wt% and 30 wt%≦B≦89 wt%. Too low a silicon content and / or too high a carbon content will result in an anode material with too low a specific capacity, which is undesirable for industrial applications. Too high a silicon content will result in too much volume expansion during cycling, which is undesirable primarily for safety reasons. Too low a carbon content will be insufficient to completely coat the silicon-based subparticles, causing a reaction between the surface of the silicon-based subparticles and the electrolyte, resulting in the formation of an additional SEI layer and a decrease in battery performance.

[0027] [Embodiment 2] In a second embodiment according to embodiment 1, the carbonaceous material includes graphite domains, and the graphite domains have a 2θ angle of 26° to 27°. Cu Maximum intensity I C The powder has an average size of less than 10 nm as determined by the Scherrer equation applied to the powder X-ray diffraction peak assigned to C(002).

[0028] Preferably, the graphite domains have an average size of less than 5 nm, more preferably less than 3 nm, and most preferably less than 2 nm. Graphite domains with an average size of less than 10 nm, preferably less than 5 nm, more preferably less than 3 nm, and most preferably less than 2 nm, result in powders with higher electronic conductivity than graphite domains with a size of 10 nm or greater, and are therefore preferred. Furthermore, as already mentioned above, the presence of sulfur in the powder causes the formation of crosslinks between the small graphite domains of the carbonaceous material contained in the matrix, thereby increasing the elasticity of the carbonaceous material and, therefore, the matrix material. Therefore, the smaller the average size of the graphite domains of the carbonaceous material contained in the matrix, the more crosslinks are formed, making the matrix material more elastic, which, as already mentioned above, leads to an increase in the first coulombic efficiency and the average coulombic efficiency. In other words, there is a synergistic effect between sulfur and graphite domains with a size of less than 10 nm, preferably less than 5 nm, more preferably less than 3 nm, and even more preferably less than 2 nm.

[0029] The Scherrer equation (P. Scherrer, Göttinger Nachricheten 2, 98 (1918)) is a well-known equation for calculating the size of ordered (crystalline) domains from X-ray diffraction data. To avoid instrument-to-instrument variations, standardized samples can be used for calibration.

[0030] The presence or absence of graphitic domains in a matrix material and their average size can be assessed, for example, based on transmission electron microscopy (TEM) analysis. Examples of such analyses are provided in the "Analytical Methods" section.

[0031] [Embodiment 3] In a third embodiment according to embodiment 1 or 2, the powder has a particle size of 0.005 cm as determined by nitrogen adsorption / desorption measurements. 3 / g. Preferably, the powder has a total specific volume of porosity of less than 0.003 cm 3 More preferably, the powder has a porosity of less than 0.002 cm 3 / g. Ideally, the powder has no / no porosity at all.

[0032] High porosity increases the volumetric capacity (mAh / cm 3 It is advantageous to have a powder with low or no porosity, since this would lower the specific capacitance (units of specific capacitance, or Ah / I), which would defeat the purpose of obtaining a powder with a high specific capacity. Furthermore, the formation of bridges between small graphitic domains of the carbonaceous material contained in the matrix is ​​promoted when the matrix material is dense, i.e., when the matrix material, and therefore the powder, has low or no porosity.

[0033] The porosity of a powder can be measured by nitrogen adsorption / desorption measurements. The fact that a powder is not porous can be confirmed by microscopic observation (using SEM or TEM) of one or more cross-sections of the powder particles. High-density particles should be considered non-porous even if they contain a small number of irregularly distributed pores (fewer than 10 per cross-sectional image at 50,000x magnification), as these are simply an undesirable result of the thermal decomposition of the carbon precursor used to form the matrix material.

[0034] [Embodiment 4] In a fourth embodiment according to embodiment 1 or 2, the carbonaceous material is soft carbon. The matrix material may also consist of 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 contrast to hard carbon, which cannot be graphitized.

[0035] Soft carbon exhibits higher electronic conductivity compared to hard carbon and is therefore desirable. Furthermore, due to the disordered assembly of small graphitic domains that results in the presence of nanovoids in the matrix material, the volume expansion of particles comprising a matrix material that comprises mostly soft carbon is reduced during anodic lithiation compared to particles comprising a matrix material that comprises mostly graphite or graphene.

[0036] [Embodiment 5] In a fifth embodiment according to any one of the first to fourth embodiments, at least 80% by weight of the sulfur contained in the powder is present in the matrix material, preferably at least 90% by weight of the sulfur contained in the powder is present in the matrix material.

[0037] In other words, less than 20% by weight, preferably less than 10% by weight, of the sulfur contained in the powder is present outside the matrix material. It is preferable that all of the sulfur contained in the powder is present in the matrix material, but migration of some of the sulfur into the silicon particles cannot be excluded.

[0038] As already explained above, the technical effect resulting from the presence of sulfur is a matrix material with increased elasticity. Even if the technical effect can still be achieved with a reduced sulfur content, it is preferred that the majority of the sulfur, at least 80% by weight, preferably at least 90% by weight, is present in the matrix material. Even more specifically, the technical effect is expected to be fully maximized if the sulfur is contained in soft carbon contained in the matrix material.

[0039] [Embodiment 6] In a sixth embodiment according to any one of the first to fifth embodiments, the silicon-based subparticles have d NS 50, and the d NS 50 is 40 nm or more and 150 nm or less.

[0040] The number-based particle size distribution is based on visual analysis, with or without the aid of an image analysis program, of the minimum number of silicon-based subparticles contained in the powder. This minimum number of silicon-based subparticles is at least 1000 particles. Examples of measuring the number-based fraction of particles are provided in the "Analytical Methods" section.

[0041] For clarity, let's say d = 100 nm. NS 50 means that 50% by number of at least 1000 silicon-based subparticles have a size smaller than 100 nm and 50% by number of at least 1000 silicon-based subparticles have a size greater than 100 nm.

[0042] d less than 40nm NS Silicon-based subparticles having a number-based particle size distribution of 50 are very difficult to disperse efficiently in the matrix material, which can reduce the electronic conductivity of the powder.

[0043] d over 150nm NS Silicon-based subparticles with a number-based particle size distribution having a value of 50 are more prone to fragmentation during their lithiation, causing a dramatic reduction in the cycle life of batteries containing such powders.

[0044] d NS 50 is believed to be unaffected by the process for making the powder, which is consistent with the d of the silicon-based powder used as a precursor in the process. NS The 50 value is the d of silicon-based subparticles contained in the powder. NS This means it is the same as the 50 value.

[0045] [Embodiment 7] In a seventh embodiment according to any one of the first to sixth embodiments, the silicon-based subparticles have a silicon weight content of at least 80% by weight. Preferably, the silicon-based subparticles have a silicon weight content of at least 90% by weight. Preferably, the silicon-based subparticles contain no elements other than Si and O to avoid excessively reducing the specific capacity of the silicon-based subparticles. The silicon-based subparticles primarily contribute to the specific capacity of the powder, and it is preferred that their own capacity is as high as possible; therefore, it is preferred that the silicon content is as high as possible, in this case at least 80% by weight, preferably at least 90% by weight.

[0046] [Embodiment 8] In an eighth embodiment according to any one of the first to seventh embodiments, the powder has a silicon content A and an oxygen content C, both expressed in weight percent (wt%), where C≦0.3×A, preferably C≦0.2×A, more preferably C≦0.1×A.

[0047] Powders with too high an oxygen content suffer from additional irreversible lithium consumption due to the formation of lithium oxide (LiO) during the initial lithiation of the powder, thereby increasing the initial irreversible capacity loss of batteries containing such powders.

[0048] [Embodiment 9] In a ninth embodiment according to any one of the first to eighth embodiments, the powder has a density of at most 10 m 2 / g, preferably at most 5m 2 / g BET surface area.

[0049] To limit the formation of a lithium-consuming solid electrolyte interphase (SEI) and the resulting irreversible loss of capacity in batteries containing such powders, it is preferable for the powder to have a low BET specific surface area, reducing the surface of the electrochemically active particles in contact with the electrolyte.

[0050] [Embodiment 10] In a tenth embodiment according to any one of the first to ninth embodiments, the powder further comprises graphite particles.

[0051] In particular, the graphite particles are not embedded in the matrix material. This can be visually confirmed based on the analysis of one or more SEM images of the powder cross-section. The fact that the graphite particles are not embedded in the matrix material is beneficial for at least two reasons: (i) only the silicon-based subparticles need to be coated by the matrix material, and therefore less matrix material with high irreversible capacity and low specific capacity is required, and (ii) particles comprising the matrix material with the silicon-based subparticles embedded therein are smaller than when the matrix material also comprises graphite particles, leading to reduced volume expansion of the particles upon lithiation during battery cycling.

[0052] However, there may be some contact between both types of particles located on their outer surfaces, which is more preferable to ensure good electronic conductivity of the powder and therefore high rate capability of batteries containing the powder.

[0053] The graphite particles act as spacers between the particles comprising the matrix material having the silicon-based subparticles embedded therein, thus preventing these particles comprising the matrix material from agglomerating and turning into an agglomerated powder. In the absence of such spacers, the agglomerated powder may require mechanical processing, such as a grinding process, for use in a battery negative electrode, which may result in weakening of the integrity of the matrix material and ultimately lead to lower performance of batteries comprising such agglomerated powder.

[0054] The presence of graphite particles in the powder can be determined, for example, by X-ray diffraction analysis, the method of which is described in the "Analytical Methods" section.

[0055] Preferably, the powder comprises graphite particles and not graphene particles, as graphene particles typically have a much higher specific surface area and are therefore expected to significantly increase the formation of an SEI layer during cycling, thereby reducing the performance of batteries containing such powders containing graphene particles, especially in the early cycles.

[0056] [Embodiment 11] In an eleventh embodiment, the present invention also relates to a method for preparing any of the powder variants defined above, said method comprising the following steps:

[0057] In step A, a powder containing a carbon precursor, a powder containing silicon-based particles, and a powder containing sulfur are provided.

[0058] In step B, a powder containing a carbon precursor and a powder containing sulfur are mixed, and the resulting mixture is heated to and maintained at a temperature above the softening point of the powder containing carbon precursor while mixing and flowing the mixture, thereby ensuring good dispersion of the powder containing sulfur within the flow of carbon precursor.

[0059] In step C, the powder comprising silicon-based particles is added to the mixture obtained in step B under constant mixing at a temperature still above the softening point of the powder comprising the carbon precursor, which ensures good dispersion of the powder comprising silicon-based particles within the stream already containing the powder comprising sulfur.

[0060] In step D, the mixture obtained in step C is cooled to room temperature and then pulverized.

[0061] In step E, a heat treatment of the powder obtained in step D is carried out in an oxygen-free atmosphere at a temperature at least equal to 1000° C. Examples of oxygen-free atmospheres are flowing argon or flowing nitrogen.

[0062] Additional steps may include final grinding and / or sieving of the powder obtained in step E after cooling to room temperature.

[0063] Preferably, the powder comprising silicon-based particles has a volume particle size distribution with a dvs50 value of at most 200 nm. This is preferred for powders comprising silicon-based particles that are easily dispersed during step C, and for powders obtained at the end of step E that comprise uniformly distributed silicon-based subparticles.

[0064] [Embodiment 12] In a twelfth embodiment according to embodiment 11, the mixture obtained in step B has a sulfur content by weight at least equal to 0.06% by weight and at most equal to 0.65% by weight, which is preferred to obtain a powder according to embodiment 1.

[0065] [Embodiment 13] In a thirteenth embodiment according to embodiment 11 or 12, the carbon precursor is converted into soft carbon during the heat treatment of step E. It is important that the temperature at which the heat treatment is carried out is at least equal to 1000°C in order to completely convert the carbon precursor into soft carbon. The temperature at which the heat treatment is carried out is preferably not higher than 1100°C, to prevent the possible formation of silicon carbide and the formation of graphitic domains of the carbonaceous material with an average size larger than 10 nm contained in the matrix.

[0066] [Embodiment 14] In a fourteenth embodiment according to any one of embodiments 11 to 13, the powder comprising a carbon precursor is petroleum pitch. Petroleum pitch is advantageous in that it has a relatively high carbon yield of about 65 wt % upon calcination. When calcined at a temperature of at least 1000°C, petroleum pitch converts to soft carbon.

[0067] [Embodiment 15] In a fifteenth embodiment, the present invention finally relates to a battery comprising a powder according to any one of the first to tenth embodiments. DETAILED DESCRIPTION OF THE INVENTION

[0068] In the following detailed description, preferred embodiments are described in detail to facilitate the practice of the present invention. While the present invention has been described with reference to these specific preferred embodiments, it will be understood that the present invention is not limited to these preferred embodiments. To the contrary, the present invention encompasses numerous alternatives, modifications, and equivalents, as will become apparent from consideration of the following detailed description.

[0069] [Analysis method used] Measurement of Si content The Si content of the powders of the examples and comparative examples is measured by X-ray fluorescence (XRF) using an energy dispersive spectrometer, which has a random experimental error of ±0.3 wt% Si.

[0070] When it is necessary to measure the Si content of a particular particle containing Si-based subparticles, or the Si content of the Si-based subparticles themselves, it may be difficult to measure the silicon content by XRF. In this case, analysis by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) may be preferred. This allows the silicon content of a given particle or subparticle to be measured. Analysis of 10 particles or subparticles is sufficient to obtain an average silicon content value.

[0071] Oxygen content measurement The oxygen content of the powders in the examples and comparative examples is measured by the following method using an oxygen-nitrogen analyzer (Leco TC600): A sample of the powder to be analyzed is placed in a sealed tin capsule, which is then placed in a nickel basket. The basket is placed in a graphite crucible and heated to over 2000°C under helium as a carrier gas. This melts the sample, and oxygen reacts with the graphite from the crucible until it becomes CO or CO gas. These gases are introduced into an infrared measuring cell. The observed signal is recalculated into the oxygen content.

[0072] Determination of carbon content The carbon content of the powders in the examples and comparative examples is measured by the following method using a carbon-sulfur analyzer (Leco CS230). The sample is melted in a ceramic crucible in a high-frequency furnace with a constant oxygen flow. The carbon in the sample reacts with the oxygen gas and leaves the crucible as CO or CO2. After the final conversion of the CO present to CO2, all the CO2 produced is detected by an infrared detector. The signal obtained is finally converted into the carbon content.

[0073] Determination of sulfur content The sulfur content of the powders in the examples and comparative examples is measured by the following method using a carbon-sulfur analyzer (Leco CS230). Sulfur in the sample reacts with oxygen gas and leaves the crucible as SO2. All the SO2 produced is detected by an infrared detector. The resulting signal is finally converted into sulfur content.

[0074] Determination of specific surface area (BET) The specific surface area of ​​the powder is measured by the Brunauer-Emmett-Teller (BET) method using a Micromeritics Tristar 3000. First, 2 g of the powder to be analyzed is dried in an oven at 120°C for 2 hours, followed by N2 purging. The powder is then degassed under vacuum at 120°C for 1 hour prior to measurement to remove adsorbed species.

[0075] Determination of total specific volume of porosity Nitrogen adsorption / desorption analysis (Micromeritics Tristar 3020) is used to determine the total specific volume of porosity of the powders in the examples and comparative examples by the following method: The powder is introduced into a sample tube and preparation (heating, vacuum or N2 gas flushing) is carried out to remove all foreign molecules from the powder surface and from the sample tube.

[0076] It is then cooled to liquid N temperature where N adsorption occurs on the powder particles. This adsorption occurs at a relative pressure (P / P) of 0.10 to 0.99. o) after which the relative pressure drops, resulting in N2 desorption on the powder particles. This is measured at a relative pressure (P / P) of 0.99 to 0.10. o ) is measured. In this way, the BJH pore size distribution curve is obtained. Finally, the total specific volume of porosity is calculated.

[0077] Electrochemical performance determination The electrochemical performance of the powders in the examples and comparative examples is measured by the following method.

[0078] The powder to be evaluated is sieved using a 45 μm sieve and mixed with carbon black, carbon fiber, and sodium carboxymethylcellulose binder (2.5 wt%) in water. The ratio used is 89 parts by weight of the powder to be evaluated / 1 part by weight of carbon black (C65) / 2 parts by weight of carbon fiber (VGCF) and 8 parts by weight of carboxymethylcellulose (CMC). These components are mixed in a Pulverisette 7 planetary ball mill at 250 rpm for 30 minutes.

[0079] Copper foil washed with ethanol was used as a current collector. A 200 μm thick layer of the mixed components was coated onto the copper foil. The coated copper foil was then dried in a vacuum at 70 °C for 45 minutes. A 1.27 cm thick layer was removed from the dried, coated copper foil. 2 Circles were punched out and used as electrodes in coin cells using lithium metal as the counter electrode. The electrolyte was 1 M LiPF6 dissolved in EC / DEC 1 / 1 + 2% VC + 10% FEC solvent.

[0080] All coin cells are cycled using a high precision battery tester (Maccor 4000 series) using the procedure below, where "CC" stands for "constant current" and "CV" stands for "constant voltage." ● Cycle 1: ○ 6-hour break CC lithiation up to 10mV at C / 10, then CV lithiation up to C / 100 ○ 5 minute break ○ CC delithiation up to 1.5V at C / 10 ○ 5 minute break ● From Cycle 2 onwards: CC lithiation to 10mV at C / 2, then CV lithiation to C / 50 ○ 5 minute break ○ CC delithiation up to 1.2V at C / 2 ○ 5 minute break

[0081] The Coulombic Efficiency (CE) of a coin cell is the ratio of the capacity upon delithiation to the capacity upon lithiation in a given cycle, calculated for the initial cycle and subsequent cycles. The initial cycle is the most important in terms of Coulombic Efficiency, since the SEI formation reaction has a significant impact on CE. Typically, for silicon-based powders, the Coulombic Efficiency in 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 very significant. The goal is to achieve a CE of at least 90% in the initial cycle.

[0082] Even though the CE typically rises significantly to above 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 remaining 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).

[0083] For a cell containing a negative electrode powder with a specific capacity of 840±20 mAh / g, the goal is to reach a coulombic efficiency (CE) of at least 90% on the initial cycles and an average CE of at least 99.7% from cycle 5 to cycle 50.

[0084] Measurement of powder volume particle size distribution The volume particle size distribution of the powder is determined by Laser Diffraction Sympatec (Sympatec-Helos / BFS-Magic 1812) according to the user's instructions. The following settings are used for the measurement: Dispergen System: Sympatec-Rodos-M Dispersion machine:Sympatec-Vibri 1227 Lens: R2 (range 0.45 to 87.5 μm) Dispersion: 3 bar pressurized air Optical density: 3~12% Start / Stop: 2% Time axis: 100ms Supply rate: 80% Aperture: 1.0mm

[0085] Note that the feed rate and aperture settings can be varied as a function of optical density.

[0086] The volumetric particle size distribution of the powder containing silicon-based particles is then determined using the method described above. VS 10, d VS 50 and d VS Calculate the 90 value.

[0087] Determination of number-based particle size distribution The number-based particle size distribution of the silicon-based subparticles is determined by electron microscopy (SEM or TEM) of a cross section of the powder, combined with image analysis.

[0088] To do this, a cross-section of the powder is prepared according to the procedure detailed below, comprising a plurality of cross-sections of particles of matrix material, each of which comprises a plurality of cross-sections of silicon-based sub-particles.

[0089] 500 mg of the 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 1-inch diameter sample was allowed to dry for at least 8 hours. It was then first mechanically polished using a Struers Tegramin-30 to reach a maximum thickness of 5 mm, 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. Finally, a carbon coating was applied to this polished surface by carbon sputtering using a Cressington 208 carbon coater for 12 seconds to obtain the sample, also called a "cross section," that would be analyzed by SEM.

[0090] The prepared cross-sections were then scanned using an EDS detector Xflash 5030-127 (30 mm) from Bruker. 2 The analysis is carried out using a JEOL FEG-SEM JSM-7600F equipped with a 127 eV (127 eV) detector. The signal from this detector is processed with a Bruker Quantax 800 EDS system.

[0091] A voltage of 15 kV is applied at a working distance of a few millimeters to generate a magnified image. When pricing optical microscope images, images of backscattered electrons are reported.

[0092] The size of a silicon-based subparticle is considered to correspond to the maximum linear distance between two points on the periphery of an individual cross-section of that particle.

[0093] To illustrate, but not limit, the measurement of the number-based particle size distribution of silicon-based subparticles, an SEM procedure is provided below. 1. Obtain multiple SEM images of a cross section of a powder containing particles of matrix material with silicon-based subparticles dispersed within. 2. Adjust the image contrast and brightness settings to easily visualize the cross-sections of the matrix material particles and silicon-based sub-particles. Due to their different chemical compositions, particles and sub-particles can be easily distinguished by their brightness differences. 3. Using suitable image analysis software, select at least 1000 distinct cross-sections of silicon-based sub-particles from one or more of the acquired SEM image(s) that do not overlap with another cross-section of the silicon-based sub-particles. These distinct cross-sections of silicon-based sub-particles can be selected from one or more cross-sections of a powder comprising particles of matrix material and silicon-based sub-particles. 4. For each of at least 1000 individual cross-sections of silicon-based sub-particles, measure the size of the individual cross-section of the silicon-based sub-particle using suitable image analysis software.

[0094] Then, the d of the number-based particle size distribution of the silicon-based subparticles determined using the above method NS 10, d NS 50 and d NS These number-based particle size distributions can be easily converted to weight-based or volume-based particle size distributions using well-known mathematical formulas.

[0095] Measurement of graphitic domain size The size of the graphite domains contained in the carbonaceous material can be determined by TEM analysis of a cross section of the powder obtained as described above.

[0096] However, the preferred method is powder X-ray diffraction (XRD) analysis, using the following method:

[0097] For identification of this compound, a scan of at least approximately 2 cm was performed using the ICDD database, PDF-4+, using CuKα1 and CuKα2 radiation, λ = 0.15418 nm, a step size of 0.017° 2θ, and a scan rate of 34 min (2064 s). 3The XRD measurements of the powders are carried out on a Panalytical'X Pert Pro system, measuring at 2θ between 5° and 90° on the plane of the powder material.

[0098] 2θ between 26° and 27° Cu The XRD peak with a maximum at 2θ corresponds to the (002) reflection of graphitic carbon, resulting from the diffraction of X-rays from the interplanar graphene layers. The background is first subtracted from the raw XRD data. Then, the 2θ at half maximum intensity on the left and right of the C(002) peak is calculated. Cu The Full Width at Half Maximum (FWHM) value is determined by the ratio of these two 2θ Cu The FWHM value is usually measured using a program provided with the X-ray diffractometer. Manual calculations can also be used.

[0099] The average size of the graphitic domains is finally calculated by applying the Scherrer equation to the C(002) peak using the just measured FWHM value, the instrument's X-ray wavelength and the position of the C(002) peak.

[0100] Determination of carbonaceous material content in powder matrix material If it is difficult to directly measure the carbonaceous material content in the matrix material of the powder using well-known physicochemical analytical techniques, the following mathematical method can also be used to calculate this content.

[0101] Two powders will be used as examples for the application of this method. The first powder (Ex1) has the following contents by weight: 20.0% silicon (Si), 1.6% oxygen (O), 0.4% sulfur (S), and 78% carbon (I), and has an average delithiation capacity of 795 mAh / g measured in the first cycle of three identical coin cells using the method described above. The carbonaceous material, containing Si-based particles and graphitic domains with an average size of less than 10 nm, is observed by TEM, and the average size of the domains is determined by applying the Scherrer equation as described above. No graphitic particles or other materials with graphitic domains larger than 10 nm are observed.

[0102] The second powder (Ex2) has the following contents by weight: 20.0% Si, 1.7% O, 0.3% S, and 78% C, and an average delithiation capacity of 820 mAh / g measured on the first cycle of three identical coin cells using the method described above. Both Si-based particles and carbonaceous material containing graphitic domains with an average size of less than 10 nm and graphitic particles not embedded in the matrix material are observed by combining TEM and XRD analysis.

[0103] Carbonaceous materials with graphite domains smaller than 10 nm, and soft carbons in particular, are known to typically have a specific capacity of about 250 mAh / g as anode materials. Graphite particles are also known to have a capacity of about 350 mAh / g as anode materials. For silicon, a specific capacity of 3000 mAh / g is used to account for the irreversible capacity loss that occurs during the first cycle.

[0104] The specific volume of the powder is then calculated as follows: Specific capacity powder (mAh / g)=wt% Si×3000(mAh / g)+ wt% carbonaceous material x 250 (mAh / g) + wt% graphite x 350 (mAh / g) (Equation 1) wt% Si + wt% O + wt% S + wt% carbonaceous material + wt% graphite = 1 ⇔wt% graphite = 1-wt% Si-wt% O-wt% S-wt% carbonaceous material (Equation 2)

[0105] Inserting Equation 2 into Equation 1 gives the following Equation 1: wt% carbonaceous material = (wt% Si×2650+(1-wt% O-wt% S)×350-specific capacity powder) / 100 (Formula 1)

[0106] The graphite content can then be calculated using Equation 2.

[0107] Using equations 1 and 2, the respective contents are calculated for powders Ex1 and Ex2 and are reported in Table 1.

[0108] [Table 1]

[0109] It can be noted that both powders Ex1 and Ex2 are powders according to the invention.

[0110] This mathematical method was evaluated on 20 samples with known contents of different components and was proven to have an accuracy margin of at least 10%.

[0111] Experimental Preparation of Comparative Examples and Examples [Example 1 (E1) of the present invention] To produce the powder of Example 1, a 60 kW radio frequency (RF) inductively coupled plasma (ICP) was applied, using argon as the plasma gas, into which micron-sized silicon powder precursor was injected at a rate of about 200 g / h, to first obtain a silicon-based powder by bringing the temperature (i.e., in the reaction zone) above 2000 K. In this first process step, the precursor was completely vaporized. In the second process step, a 20 Nm sieve was used to reduce the temperature of the gas to below 1600 K.3 An argon flow of 100 L / h is used as a quench gas immediately downstream of the reaction zone to induce nucleation into metallic submicron silicon powder. Finally, a passivation step is carried out at a temperature of 100°C for 5 minutes by adding 100 L / h of a N2 / O2 mixture containing 1 mol% oxygen.

[0112] The specific surface area (BET) of the obtained silicon powder was measured and found to be 81 m 2 The oxygen content of the obtained silicon powder was measured to be 7.8% by weight. The number-based particle size distribution of the silicon powder was measured to be d NS 10=59 nm, d NS 50=114nm and d NS 90=192nm.

[0113] A dry blend is then made from 200 g of petroleum-based pitch powder and 0.25 g of sulfur powder (Sigma-Aldrich, 99.98% purity). It should be noted that the sulfur content of the petroleum-based pitch powder used here was measured using the method described above and was below the detection limit of the instrument. Therefore, the contribution of the pitch powder to the sulfur content in the final powder is negligible.

[0114] The blend is heated under a nitrogen flow to a temperature of 400° C. and after a 60 minute waiting period is mixed under high shear for 30 minutes with a Cowles dissolver type mixer operating at 1000 rpm.

[0115] 100 g of silicon powder is then added to the fresh mixture, still at 400° C. The blend is heated to a temperature of 400° C. under a nitrogen flow and, after a waiting period of 60 minutes, mixed for 30 minutes under high shear with a Cowles dissolver-type mixer operating at 1000 rpm.

[0116] The mixture of silicon-based powder in pitch thus obtained is cooled to room temperature, solidified once, pulverized, and sieved through a 400 mesh sieve to produce an intermediate powder.

[0117] The intermediate powder is further subjected to a thermal post-treatment as follows: the product is placed in a quartz crucible in a tube furnace and heated to 1020°C at a heating rate of 3°C / min, held at that temperature for 2 hours, and then cooled, all under an argon atmosphere.

[0118] The calcined product is finally hand-ground in a mortar and pestle and sieved through a 325 mesh sieve to form the final powder.

[0119] The total Si content of this powder is 40.1 wt. % as determined by XRF. The oxygen, carbon, and sulfur contents of this powder are measured to be 3.4 wt. %, 56.4 wt. %, and 0.109 wt. % respectively. Since all the carbon is in the matrix material and corresponds to soft carbon with graphitic domains smaller than 10 nm, the ratio "S / carbonaceous material in matrix" is equal to 0.193%.

[0120] The specific surface area (BET) of the obtained powder was measured to be 4.8m 2 / g.

[0121] The main physicochemical properties of powder E1 are reported in Table 2.

[0122] [Example 2 (E2) and Example 3 (E3) of the present invention] To prepare the powders of Example 2 (E2) and Example 3 (E3), the same method as for the powder of Example 1 is used, except that sulfur powder in amounts of 0.45 g and 0.7 g, respectively, is used instead of 0.25 g in Example 1. The main physicochemical properties of the powders E2 and E3 thus obtained are reported in Table 2.

[0123] [Comparative Example 1 (CE1) not according to the present invention] To prepare the powder of Comparative Example 1 (CE1), the same method as for the powder of Example 1 is used, except that no sulfur powder is used. The main physicochemical properties of the powder CE1 obtained are reported in Table 2.

[0124] [Comparative Example 2 (CE2) not according to the present invention] To prepare the powder of Comparative Example 2 (CE2), the same method as for the powder of Example 1 is used, except that an amount of 0.1 g of sulfur powder is used instead of 0.25 g of Example 1. The main physicochemical properties of the powder CE2 obtained thereby are reported in Table 2.

[0125] [Comparative Example 3 (CE3) not according to the present invention] To prepare the powder of Comparative Example 3 (CE3), the same method as for the powder of Example 1 is used, except that an amount of 1.7 g of sulfur powder is used instead of 0.25 g in Example 1. The main physicochemical properties of the powder CE3 obtained thereby are reported in Table 2.

[0126] [Example 4 (E4) of the present invention] To produce the powder of Example 4 (E4), 20 g of the intermediate powder obtained in Example 2 was mixed with 20 g of graphite on a roller bench for 3 hours, after which the mixture was passed through a mill to deagglomerate it. Under these conditions, good mixing was obtained, but the graphite particles were not embedded in the pitch.

[0127] The resulting mixture was further subjected to a thermal post-treatment as follows: the product was placed in a quartz crucible in a tube furnace and heated to 1000°C at a heating rate of 3°C / min, held at that temperature for 2 hours, and then cooled, all under an argon atmosphere.

[0128] The calcined product was finally hand-ground in a mortar and sieved through a 325 mesh sieve to form the final composite powder. The main physicochemical properties of the resulting powder, E4, are reported in Table 2.

[0129] [Comparative Example 4 (CE4) not according to the present invention] To produce the powder of Comparative Example 4 (CE4), 20 g of the intermediate powder obtained in Comparative Example 2 was mixed with 20 g of graphite on a roller bench for 3 hours, after which the resulting mixture was passed through a mill to deagglomerate it. Under these conditions, good mixing was obtained, but the graphite particles were not embedded in the pitch.

[0130] The resulting mixture was further subjected to a thermal post-treatment as follows: the product was placed in a quartz crucible in a tube furnace and heated to 1000°C at a heating rate of 3°C / min, held at that temperature for 2 hours, and then cooled, all under an argon atmosphere.

[0131] The calcined product was finally hand-ground in a mortar and sieved through a 325 mesh sieve to form the final composite powder. The main physicochemical properties of the resulting powder, CE4, are reported in Table 2.

[0132] The specific surface area (BET value) of all powders is 3.2 to 4.8 m 2 / g.

[0133] [Table 2]

[0134] Electrochemical evaluation of the powders Powders E1-E3 and CE1-CE3 are tested in coin cells according to the procedure specified above. The cells are stopped at the end of the first cycle and the first lithiation capacity and first delithiation capacity are calculated.

[0135] Powders E1-E3 and CE1-CE3 are further diluted with graphite by blending them in a 1:1 mass ratio.

[0136] The diluted powders from the E1-E3 and CE1-CE3 powders, as well as the powders E4 and CE4, obtained thereby, are then tested in coin cells according to the procedure specified above.

[0137] The results are reported in Table 3. The first cycle coulombic efficiency and average coulombic efficiency values ​​reported here are for diluted powders of E1-E3 and CE1-CE3 powders, and for pure powders of E4 and CE6, in order to compare cells containing anode materials with similar capacities.

[0138] [Table 3]

[0139] Comparing the results, it is clear that the cells containing powders E1 to E4 according to the invention as negative electrode materials have a higher coulombic efficiency at cycle 1 and a higher average coulombic efficiency at cycles 5 to 50 compared to the cells containing powders CE1 to CE4 not according to the invention.

Claims

1. 1. A powder for use in a battery negative electrode, the powder comprising particles, the particles comprising a matrix material and silicon-based subparticles embedded in the matrix material, the matrix material comprising a carbonaceous material, the powder further comprising sulfur, wherein the weight content of sulfur in the powder is at least 0.1% of the weight content of the carbonaceous material and at most 1% of the weight content of the carbonaceous material.

2. The carbonaceous material includes graphite domains, and the graphite domains have a 2θ of 26° to 27°. Ｃｕ Maximum intensity I Ｃ 2. The powder of claim 1, having an average size of less than 10 nm as determined by the Scherrer equation applied to the X-ray diffraction peak of the powder assigned to C(002) having a crystal structure of:

3. 0.005 cm as determined by nitrogen adsorption / desorption measurements ３ 10. The powder of claim 1 having a total specific volume of porosity of less than 1000 .mu.m / g.

4. The powder of claim 1 , wherein the carbonaceous material is soft carbon.

5. 10. The powder of claim 1, wherein at least 80% by weight of the sulfur contained in the powder is present in the matrix material.

6. The silicon-based subparticles are ＮＳ 50, and the d ＮＳ 50 is 40 nm or more and 150 nm or less.

7. 10. The powder of claim 1, wherein the silicon-based subparticles have a silicon weight content that is at least 80% by weight.

8. 2. The powder of claim 1, wherein the powder has a silicon content A and an oxygen content C, both expressed in weight percent (wt%), and C≦0.3×A.

9. Maximum 10m ２ 10. The powder of claim 1 having a BET surface area of ​​0.1g / g.

10. The powder of claim 1 further comprising graphite particles.

11. 10. A method for preparing the powder of claim 1, comprising the steps of: Step A: providing a powder comprising a carbon precursor, providing a powder comprising silicon-based particles, and providing a powder comprising sulfur; a step B of mixing the powder containing the carbon precursor with the powder containing sulfur, and heating the mixture to a temperature higher than the softening point of the powder containing the carbon precursor while further mixing; a step C of adding the powder containing silicon-based particles to the mixture obtained in the step B and mixing them; Step D: cooling to room temperature followed by grinding the mixture obtained in step C; and step E, carrying out a heat treatment of the powder obtained in step D, in an oxygen-free atmosphere, at a temperature at least equal to 1000°C.

12. 12. The method according to claim 11, wherein the weight content of sulfur in the mixture of step B is at least equal to 0.06% by weight and at most equal to 0.65% by weight.

13. 12. The method of claim 11, wherein the carbon precursor converts to soft carbon upon heat treatment in step E.

14. 12. The method of claim 11, wherein the carbon precursor is a petroleum pitch.

15. A battery comprising the powder of any one of claims 1 to 10.

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