Mainly amorphous silicon particles and use of such particles as an anode active material in a lithium ion secondary battery

Abstract

The present invention relates to a method for producing primarily amorphous silicon particles, the produced particles, and secondary electrochemical cells utilizing the particles as the active material in the negative electrode of a secondary electrochemical cell, wherein the primarily amorphous silicon particles have the formula Si (1-x) M x wherein 0.005≦x<0.05, and M is at least one substitution element selected from C, N, or a combination thereof, and the particles, when subjected to XRD analysis using non-monochromated CuKα radiation, exhibit one peak at about 28° and one peak at about 52°, both peaks having a full width at half maximum of at least 5° using Gaussian peak fitting.

Description

Technical Field

[0001] The present invention mainly relates to a method for producing amorphous silicon particles, the produced particles, and a secondary electrochemical cell that uses the particles as an active material for the negative electrode of the secondary electrochemical cell.

Background Art

[0002] Currently, in a society powered by fossil fuel energy, with the strong increase in the use of renewable energy and electricity in many fields, it is necessary to achieve the goals of the Paris Agreement under the United Nations Framework Convention on Climate Change. An important part in achieving these goals is access to rechargeable batteries with excellent specific energy.

[0003] Lithium has a relatively very low density of 0.534 g / cm 3 and also has a high standard reduction potential of -3.045 V for the half-reaction Li + + e - → Li 0 This makes lithium an attractive candidate for manufacturing electrochemical cells with specific energy. However, secondary (rechargeable) electrochemical cells with a negative electrode of metallic lithium have been shown to suffer from the persistent problem of dendrite formation during charging, which tends to short-circuit the electrochemical cell after several charge-discharge cycles.

[0004] The dendrite problem has been solved by applying a negative electrode that can store lithium atoms reversibly by intercalation. Such batteries are known as lithium-ion secondary batteries (LIBs). The electrochemical properties of LIBs are directly affected by the physical and chemical properties of the active material of the negative electrode. The selection and preparation of the material, as well as appropriate structural modifications and designs of the active material, both affect the battery performance [Non-Patent Document 1]. An important issue in this regard is that when the battery is charged, lithium atoms are surely and reversibly stored at a high volume density, and then when discharged in a number of consecutive charge-discharge cycles, lithium is converted into ions (Li+ ) was to find, and still is, an active material that can be released as

[0005] Currently, most commercially available LIBs use graphite as the active material for the negative electrode. Graphite can accommodate / fill one lithium ion per six carbon atoms by intercalation with little shape deformation and has a theoretical specific energy of 372 mAh / g. Commercially available secondary LIBs with graphite anodes typically achieve a specific energy of 100 - 200 Wh / kg and, for example, make a medium-sized electric vehicle battery weigh several hundred kilograms. This level of specific energy is probably insufficient to achieve the goals of the Paris Agreement.

[0006] One strategy for improving the specific energy of LIBs is to find a material with a higher lithium ion storage capacity than graphite used as the active material for the negative electrode. One interesting and much studied candidate in this regard is silicon, because of its high ability to store lithium atoms by diffusion and alloying. At typical ambient temperatures, the most lithiated phase of silicon is Li 3.75 Si [Non-Patent Document 1]. The silicon negative electrode also has the advantage of being able to provide an attractive working potential that reduces safety concerns regarding lithium precipitation during cell overcharge [Non-Patent Document 2].

[0007] Lithiation and delithiation of silicon each cause a huge volume expansion and contraction in the silicon material. In the most lithiated state, Li 3.75In Si, the silicon material has a volume about 320% larger than in its delithiated state. The relatively large volume fluctuations associated with lithiation and delithiation cycles can cause cracking and / or dissipation of the silicon electrode and / or repeated formation of the solid electrolyte interface (SEI) layer, leading to various problems regarding the performance of LIBs, such as loss of electrical contact, disappearance of active material within the electrode, ineffective electron transfer, etc. [Non-Patent Documents 1, 2].

[0008] Since nanoscale Si particles can better accommodate volume fluctuations, the nanostructuring of silicon materials has been studied as a solution to overcome the volume expansion problem [Non-Patent Document 2]. The use of nanoscale particles in electrodes has been demonstrated to provide electrodes with outstanding properties due to the small particle size resulting in effects such as improved electrical conductivity, improved mechanical and optical properties, etc. [Non-Patent Document 1]. Furthermore, because nanosized particles have a very high surface area to volume ratio, anodes with nanosized active materials can provide excellent charge-discharge capacities due to the highly available surface for lithium ion adsorption / desorption [Non-Patent Document 1].

[0009] The interatomic distance between silicon atoms increases as lithium ions are accommodated (lithiated), and as a result, the particles can expand up to 320% of their original volume. In crystalline silicon, the expansion generates large anisotropic stresses within the electrode material, leading to an increase in fragmentation and disintegration of the silicon material. This anisotropic stress has been found to decrease in many anode structures when the silicon material itself is amorphous [Non-Patent Document 5].

[0010] In LIBs with liquid electrolytes, a solid electrolyte interface (SEI) is often formed during the first lithiation. The formation of the SEI layer irreversibly consumes lithium and shows an irreversible capacity loss for the electrochemical cell [Non-Patent Document 1]. Therefore, it is advantageous to form a stable SEI layer to limit the SEI-induced loss of lithium for the first lithiation / charging of the cell. To avoid direct contact between silicon and the electrolyte, coating the silicon surface with a suitable element can obtain a stable SEI layer [Non-Patent Document 1], but it has been demonstrated that when cracks occur, unprotected surfaces appear.

[0011] Carbon has been studied and utilized as a coating material and / or as a composite material with silicon in LIBs having nanostructured silicon as the active material in the negative electrode. Many silicon-carbon structures have been reported in the literature, ranging from simple mixtures of silicon to complex shapes of silicon with graphene or graphite. These complex structures exhibit excellent cycle performance and capacity, but suffer from the need for a large number of charge-discharge cycles to reach high Coulombic efficiency and require a complex multi-step synthesis process to scale up to commercial production levels [Non-Patent Document 2].

[0012] From Sourice et al. (2016) [Non-Patent Document 2], a method for producing nanoscale amorphous silicon particles with a carbon shell / coating is known by a two-step laser pyrolysis process in which a gas stream of silane diluted in an inert gas enters a first reaction zone irradiated by a CO2 laser to decompose the silane gas into amorphous silicon core particles. Ethylene is then added to the gas having the formed silicon core particles, and this mixture is passed through a second reaction zone and irradiated by a CO2 laser to decompose ethylene to form a carbon shell deposited on the silicon core particles. Amorphous silicon particles with a carbon coating have been found to have excellent specific capacity and high charge-discharge cycle ability.

[0013] Patent Document 1 has the formula SiC xDisclosed is an active material for the negative electrode of a LIB containing a silicon-containing compound, where x is from 0.05 to 1.5, and the carbon concentration in the material follows the relationship A ≦ B, where A is the molar concentration ratio of carbon to silicon at the center of the active material, and B is the molar concentration ratio of carbon to silicon on / at the surface region of the active material. This document informs that carbon may be covalently bonded to silicon, and further, the silicon-containing compound may be particulate and have an amorphous molecular structure. Furthermore, paragraph

[0030] of Patent Document 1 describes that if the carbon content in the active material is too low, that is, when x is less than 0.05, the active material may deteriorate due to cracks.

[0014] From Patent Document 2, an active material for the negative electrode of a rechargeable LIB containing an amorphous silicon-based compound of the formula SiA x H y is known, where A is any of carbon, nitrogen, or a combination thereof, x > 0, y > 0, and 0.1 ≦ x + y ≦ 1.5. The active material may be particulate and coated with a carbon layer. The active material can be produced by a sputtering process using hydrogen gas and targets of Si and C, or by a plasma method using hydrogen gas, silane gas, and nitrogen gas.

[0015] Patent Document 3 discloses a reactor and method for producing crystalline or amorphous silicon particles by chemical vapor deposition of a silicon-containing precursor on seed particles in a heated and rapidly rotating reactor space. The silicon-containing gas may be diluted in a carrier gas or may be any of SiH4, Si2H6, SiHCl3, or a mixture thereof. The carrier gas can be one of hydrogen, nitrogen, or argon. SiO x SiC x SiN xBy introducing a second precursor gas, liquid, substance, etc., such as a combination of carbon, oxygen, or nitrogen with silicon, amorphous carbon, graphite, low-crystalline carbon, or a short-range ordered graphene structure, etc., an outer layer of a second substance having a lower silicon content than the core substance of the particles can be provided to the formed silicon particles.

[0016] Wang et al (2013) [Non-Patent Document 6] is one of several research groups that have disclosed the formation of secondary particles from carbon precursors such as silicon nanoparticles and pitch, and the use of such particles in Li-ion batteries. The secondary particles reduce the interfacial region between silicon and the electrolyte, thereby reducing the formation of the solid electrolyte interface (SEI) known to consume both lithium and the electrolyte, thereby gradually reducing the capacity of the battery. The formation of the (SEI) in the first cycle is quantified by measuring the Coulombic efficiency (CE) of the first lithiation cycle, and furthermore, the thickness and quality of the SEI can be estimated using XPS. It has been demonstrated by Jeff Dahn (1995) [Non-Patent Document 7] that the CE of carbon formed from pitch or sugar is improved if carbonization can occur at >500 °C, preferably >700 °C, more preferably >800 °C for at least 2 hours. In Escamilla-Perez et al. (2019) [Non-Patent Document 8], it was pyrolyzed at 900 °C for 3 hours.

[0017] Patent Document 4 discloses extremely pure nanoparticulate amorphous silicon powder that can preferably be alloyed with an electron donor and / or an electron acceptor. Furthermore, a method for producing the silicon powder and the use of a reactor for producing the silicon powder are disclosed. The silicon powder of the invention can preferably be used for the production of semiconductor starting materials, semiconductors, thermocouples for energy recovery from waste heat, particularly thermocouples stable at high temperatures.

[0018] Patent Document 5 discloses silicon nanoparticles and a method for manufacturing the same. This document particularly relates to silicon nanoparticles having an amorphous or non-crystalline phase by containing silicon as an active ingredient and having an excessive amount of atomic P or atomic B inside / outside the nanoparticles beyond the doping limit, and a method for manufacturing the nanoparticles. The manufactured silicon nanoparticles have a secondary phase of an amorphous or non-crystalline phase that acts as a shock absorber for volume expansion and contraction occurring during charge and discharge of silicon, thereby improving the charge-discharge cycle (life) of a secondary battery using the silicon nanoparticles as an anode material.

Prior Art Documents

Patent Documents

[0019]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Non-Patent Documents

[0020]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

Summary of the Invention

Problems to be Solved by the Invention

[0021] The main object of the present invention is to provide a method for producing mainly amorphous silicon-containing particles suitable for use as an active material in the negative electrode of a rechargeable lithium-ion electrochemical cell.

[0022] A further object of the present invention is to provide mainly amorphous silicon-containing particles produced by the above method.

[0023] A further object of the present invention is to provide an anode material mainly comprising amorphous silicon-containing particles.

[0024] A further object of the present invention is to provide an electrochemical secondary battery having a negative electrode mainly comprising amorphous silicon-containing particles.

Means for Solving the Problems

[0025] The present invention is based on the discovery that by heating a mixture of a silicon precursor gas and a relatively small amount of a substitution element precursor gas to a temperature at which the gas decomposes and reacts to form solid particles, silicon particles mainly having an amorphous structure are produced, and these silicon particles do not contain the trace elements and do not convert to a crystalline phase at a temperature at which a similar particle sample heated for the same time cannot withstand without a measurable portion converting to the crystalline phase, that is, they are heat resistant. The ability of the silicon particles to maintain an amorphous state at a relatively high temperature is advantageous from an economic point of view of the process by enabling a higher yield rate and a high resistance to heat treatment during subsequent manufacturing processes. Further, it is also advantageous from the viewpoint of battery performance when using the particles as an active material in the negative electrode of a secondary lithium ion battery. This is because nanoscale amorphous Si particles have high resistance to volume changes associated with lithiation and delithiation, and provide a negative electrode with improved cycle performance.

[0026] Therefore, in a first aspect, the present invention relates to a method for producing mainly amorphous silicon-containing particles composed of a compound of the formula Si (1-x) M x where 0.005 ≦ x < 0.05, M is at least one substitution element selected from C, N, or a combination thereof, and when the particles are subjected to XRD analysis using non-monochromatized CuKα radiation, show one peak at about 28° and one peak at about 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used. The method comprises Form a homogeneous gas mixture of a first precursor gas of a silicon-containing compound and at least one second precursor gas of a compound containing a substitution element M, Inject the homogeneous gas mixture of the first and second precursor gases into a reactor space, where the precursor gases react to form particles by being heated to a temperature within the range of 700 to 900 °C, Collect the particles and cool them to a temperature within the range from ambient temperature to about 350 °C, The relative amounts of the first and second precursor gases are adapted such that the formed particles obtain an atomic ratio M:Si within the range of [0.005, 0.05). This is the method.

[0027] In an exemplary embodiment, the homogeneous gas mixture may further contain additional inert gases such as hydrogen, nitrogen, argon, neon, helium, and other gases. These can be used to affect heating, cooling, particle formation rate, and mass transport, and do not leave chemical impurities in the final particle product. Heating of the precursor gases in the reaction chamber can be achieved by convection, conduction, radiation, laser, mixing with warmer gases, and other known methods.

[0028] The particles produced according to the first aspect of the present invention are mainly amorphous silicon-containing particles, The particles have the formula Si (i-x) M x and consist of a compound where 0.005 ≤ x < 0.05 and M is at least one substitution element selected from C, N, or a combination thereof, When the particles are subjected to XRD analysis using non-monochromatized CuKα radiation, they show one peak at about 28° and one peak at about 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used.

[0029] In a second aspect, the present invention relates to a negative electrode of a lithium-ion secondary electrochemical cell, the negative electrode comprising at least one particulate active material, a binder material, It has a current collector substrate, The at least one particulate active material is embedded in the binder material to form an anode mass layer on the current collector substrate, forming an anode mass. The at least one or one of the particulate active materials is a negative electrode, characterized in that it is mainly amorphous silicon-containing particles produced by the method according to the first aspect of the present invention.

[0030] The chemical formula Si used here (1-x) M x (0.005 ≦ x < 0.05) should be interpreted and understood according to the IUPAC Recommendations 2005, IR-11.3.2, "Phases with Variable Composition" of inorganic chemical nomenclature. That is, the formula defines a single (phase) compound having a composition that varies alone or partially by isovalent substitution of Si atoms with M atoms by an amount defined by the variable "x". Therefore, the term "silicon-containing particles" used here means that the particles are made from a silicon-dominant phase containing alloying elements dispersed in the molecular structure of the silicon phase. The terms "phase" and "molecular structure" may be unclear in the context of amorphous materials, but the important feature is that the M atoms are chemically bonded and dispersed as in an alloy, and a plurality of adjacent particles closest and next closest to a typical M atom are Si atoms.

[0031] The reaction rate in the gas reaction for forming the particles from the precursor gas varies greatly depending on which gas is used as the first and / or the second precursor gas, and on the reaction temperature at which the particles are formed. The atomic ratio M:Si in the precursor gas may deviate significantly from the atomic ratio M:Si in the produced particles. Therefore, the term "the relative amounts of the first and the second precursor gases are adapted so that the formed particles have an atomic ratio M:Si within the range of..." used herein means that the relative amounts of the first and the second precursor gases, which are mixed and homogenized, are adjusted so that the obtained particles have the intended atomic ratio when the precursor gas mixture is heated to the intended reaction temperature and reacts to form particles.

[0032] For forming the intended particles to the relative amounts of the first and the second precursor gases and adaptation the act of is within the scope of ordinary skill of those skilled in the art. For example, the adaptation of the relative amounts of the precursor gases for a given first and second precursor gas and the intended reaction temperature can be obtained prior to the manufacturing stage by determining The change is the gas mixture and the reaction temperature used with the relative quantity to to be determined simply perform a trial-and-error test thereby.

[0033] Alternatively, the atomic ratio M:Si in the formed particles is monitored / determined by analyzing the exhaust gas exiting the reactor in a mass spectrometer, determining how much of the supplied first and second precursor gases have reacted / been consumed in the reactor, and then indirectly determining the relative amounts of M and Si in the formed particles. For example, adjusting the flow rates of the first and second precursor gases injected into the reactor, measuring the composition of the exhaust gas exiting the reactor using a mass spectrometer to determine the rate at which the injected first and second precursor gases are converted into particles, using this information to estimate the atomic ratio M:Si in the formed particles, and adjusting the supply rates of the first and second precursor gases to obtain the intended atomic ratio M:Si in the generated particles. Chemical formula Si (1-x) M x (0.005 ≦ x < 0.05) corresponds to an M:Si atomic ratio within the range of [0.005025, 0.0526). In one exemplary embodiment, the atomic ratio M:Si in the formed particles is within the range of [0.005, 0.05], preferably within the range of [0.01, 0.04], preferably within the range of [0.01, 0.03], and most preferably within the range of [0.01, 0.02]. These atomic ratios generally correspond to predominantly amorphous silicon of the formula Si (1-x) M x where 0.005 ≦ x < 0.05, preferably 0.01 ≦ x ≦ 0.04, more preferably 0.01 ≦ x ≦ 0.03, and most preferably 0.01 ≦ x ≦ 0.02. M is a substitutional element selected from C or N.

[0034] X-ray diffraction (XRD) (when applied to particulate materials, also shown as powder X-ray diffraction (PXD) in the literature) gives different diffraction patterns for crystalline and amorphous materials, respectively. Crystalline materials tend to give sharp peaks, Bragg peaks, in XRD measurements due to their high degree of order and symmetry in their atomic structure. In the case of crystalline silicon materials, XRD analysis typically gives sharp peaks at 28.4°, 47.4°, and 56.1° in the measured diffraction pattern. In comparison, amorphous materials lacking the long-range order characteristics of crystalline molecular structures typically give significantly "fuzzier" broader peaks in the measured diffraction pattern. Amorphous silicon typically gives rounded peaks at 28° and 52°. These rounded peaks can be fit using a Gaussian fit to reduce noise and obtain well-defined values for the peak maximum and width. Such fitting can be done by a skilled XRD operator.

[0035] Also, to distinguish between crystalline and amorphous materials, the "sharpness" of the peaks can be utilized. The typical full width at half maximum (FWHM) of the XRD peaks of crystalline silicon is less than 2°, while the FHWM of amorphous silicon is typically greater than 5° when measured with a diffractometer using non-monochromatized CuKα radiation and the measurement noise is reduced using a Gaussian fit. The full width at half maximum (FWHM) is the width of the peak curve measured between points on the y-axis that are half of the maximum amplitude of the peak curve (after subtracting the background signal and / or the signal from the sample holder). A sample containing both amorphous and crystalline silicon gives an XRD analysis diffraction pattern showing both sharp Bragg peaks typical of the crystalline phase and broader Gaussian peaks typical of the amorphous phase. The diffraction pattern can be used to estimate the crystalline component of the sample from the ratio of the area under the broad amorphous peak and above the Bragg peak to the total area of the broad and Bragg peaks. A linear background should be subtracted from the calculated values prior to calculation, as shown in Figure 6.

[0036] The angles and angular tolerances in XRD analysis as used herein refer to the use of a diffractometer with non-monochromatized CuKα radiation. This is because this radiation has a wavelength of 1.5406 Å that is both of high intensity and well-suited to the interatomic distances in crystalline solids, making the analysis sensitive to the presence of crystalline phases in silicon particles. XRD analysis using a diffractometer with CuKα radiation is a natural choice for the same reason and is thus the most widely used method in XRD analysis and is well-known and learned by those skilled in the art. Other diffractometers using radiation of other wavelengths can give different angles and angular tolerances. However, those skilled in the art will know how to convert these values from one radiation source to another. The particles produced by the method according to the first aspect of the present invention have been shown by X-ray diffraction (XRD) analysis to have a predominantly amorphous molecular structure, as can be seen from FIG. 2. This figure shows a graph representing the diffraction patterns measured for three particle samples produced by the method according to the first aspect of the present invention. The diffraction patterns measured for all three samples show one peak at approximately 28° and one peak at approximately 52°, and both peaks (for all samples) have a FHWM of around 5° or greater when estimated using Gaussian peak fitting. XRD analysis of the Si (i-x) M x (0.005 ≦ x < 0.05) particles shows that they predominantly have an amorphous molecular structure. Without being bound by theory, substituting Si atoms with a substitutional element M, such as a carbon atom, in the molecular structure of the silicon host material causes a "collapse" in the silicon molecules and prevents the gas reaction process from forming a silicon phase having any long-range order in the molecular structure characteristics of crystalline silicon and / or prevents a rearrangement process that could also occur at the same temperature.

[0037] Amorphous materials (see, for example, Non-Patent Document 4) have some internal structure that provides short-range order on the atomic length scale due to the nature of the chemical bonds. This internal structure may be considered to be composed of interconnected structural blocks. These blocks may or may not be similar to the basic structural units found in the corresponding crystal phase, that is, they may or may not provide a material with very small crystal-like domains. Furthermore, in the case of very small crystals, surface relaxation and interface effects distort the atomic arrangement and reduce the structural order. Even the most advanced structural characterization techniques such as X-ray diffraction and transmission electron microscopy have difficulty distinguishing between amorphous and crystal structures on these length scales.

[0038] Therefore, since it is difficult to determine by structural characterization techniques whether the silicon material of the particles produced according to the first aspect of the present invention is completely amorphous or contains small crystal domains on the atomic length scale, the term "predominantly amorphous" as used herein encompasses silicon materials having a 100% amorphous molecular structure to silicon materials containing very small crystal domains (substantially undetectable by XRD analysis) on the atomic length scale. Also, even when the material contains very small microcrystals, typically microcrystals less than 1 nm, the advantages of the amorphous material at the anode (i.e., less directional stress and faster charging) are maintained, so it is reasonable to consider that atoms with distorted nearest-neighbor distances due to grain boundaries constitute a mass fraction similar to that of atoms with crystalline order for all nearest-neighbor atoms. The predominantly amorphous silicon-containing particles produced according to the first aspect of the present invention are compounds of the formula Si (i-) M x where 0.005 ≦ x < 0.5, M is a substitution element selected from C or N, and when subjected to XRD analysis using non-monochromatized CuKα radiation, shows one peak at about 28° and one peak at about 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used, and includes any particles containing such compounds.

[0039] The particles according to the first aspect of the present invention are for at least some substitution elements that have been observed to exhibit the counterintuitive property of being more heat resistant, i.e., with a lower content of substitution element M, the crystallization temperature increases. Experiments conducted by the inventors have shown that amorphous Si produced from silane and ethene exposed to a longer heat treatment (in this case 2 hours) 0.96 C 0.04 The particles remained amorphous up to 800 °C but showed that crystallization started somewhere between 800 °C and 820 °C. However, Si 0.92 C 0.08 particles produced by exactly the same process show such a transition at temperatures between 780 and 800 °C. X-ray characterization of these two materials after different heat exposures is shown in FIGS. 3 and 4. For comparison, pure amorphous silicon particles (Si) of the same size fully crystallize at temperatures far below 780 °C, as shown in FIG. 5. Without being bound by theory, it is possible to speculate that crystallization is governed by carbon or carbon vacancy migration and that this can explain the counterintuitive results. The XRD peak that first appears around 35° is usually associated with the SiC crystal phase, so it is clear that an increase in the amount of C can lead to an increased likelihood of SiC crystal formation.

[0040] Another beneficial property of the particles according to the first and second aspects of the present invention is that a lower content of substitution element M is observed (when M is carbon), which gives an increase in lithium mobility and a decrease in electrical resistivity. Thus, the particles according to the present invention have the advantage of obtaining a relatively high temperature tolerance / crystallization temperature at a low substitution level with a small / negligible negative impact on the capacity and transport properties of the active material / particles.

[0041] A further beneficial property of the particles according to the first and second aspects of the present invention is that a lower content of substitution element M usually means a higher charge capacity for the material, because none of the substitution elements M are known to accept as much lithium as silicon.

[0042] As used herein, the term "first precursor gas of silicon-containing compound" means any silicon-containing compound that is in a gaseous state and reacts at the intended reaction temperature to form Si particles. Examples of suitable first precursor gases include, but are not limited to, silane (SiH4), disilane (Si2H6), and trichlorosilane (HCl3Si), or mixtures thereof. Similarly, as used herein, the term "second precursor gas of compound containing substitution element M" means any compound containing substitution element M that is in a gaseous state, participates in a gas-phase reaction, and when heated to the intended reaction temperature, means any compound in which M atoms are incorporated into the molecular structure of the formed Si particles. Examples of suitable second precursor gases include, but are not limited to, alkanes, alkenes, alkynes, aromatic compounds, or hydrides of N, hydrogen cyanide, and mixtures thereof.

[0043] A particularly preferred embodiment of the precursor gas, i.e., a homogeneous gas mixture of a gaseous silicon and hydrogen compound and a gaseous substitution element M and hydrogen compound, is either silane (SiH4) or disilane (Si2H6) mixed with a hydrocarbon gas selected from one of methane (CH4), ethane (C2H6), propane (C3H8), ethene (C2H4), ethyne (C2H2), and mixtures thereof.

[0044] The notation of intervals used herein follows the international standard ISO80000-2, where the square brackets "[" and "]" indicate the boundaries of a closed interval, and the round brackets "(" and ")" indicate the boundaries of an open interval. For example, [a, b] is a closed interval that includes every real number from a (inclusive) to b (inclusive).

Number

Number

[0045] The production yield in a gas-phase reaction process, defined as the ratio of the mass of the precursor gas supplied to the reactor to the mass of the generated particles, has been shown to depend on process parameters such as the concentration of the precursor gas in the condensation zone, the reaction temperature in the condensation zone, and / or the residence time of the condensed gas in the condensation zone. Generally, the higher the reaction temperature, the higher the degree of dissociation of the precursor gas, and thus the higher the production yield. Therefore, since the mainly amorphous silicon-containing particles of the present invention have been observed to maintain their mainly amorphous structure at a temperature considerably higher, generally about 50 °C higher, than the temperature at which amorphous (pure) silicon particles are observed to convert to crystalline silicon, the method according to the first aspect of the present invention has the advantage of significantly improving the production yield compared to the production of (pure) silicon particles without compromising on the suitable amorphous structure. Raising the decomposition temperature of the homogeneous mixture of the first and second precursor gases from 750 °C to 800 °C gives an increase of 20 percentage points in the production yield. Since silicon hydride gases such as silane and disilane are expensive, this feature gives a great economic advantage to the method according to the present invention.

[0046] Therefore, in an exemplary embodiment of the present invention according to the first aspect of the present invention, preferably, a homogeneous gas mixture can be injected and heated to a temperature within the range of 740 to 850 °C, preferably within the range of 780 to 830 °C, most preferably within the range of 790 to 820 °C, where the maximum allowable temperature depends on the residence time before the particles are cooled to a temperature at which crystallization cannot occur. The exact temperature limit can be established for a specific reactor geometry and residence time through trial-and-error experiments.

[0047] A further advantage of the relatively high reaction temperature is that the dissociation reaction of the precursor gas is driven better towards complete dissociation, and thus hydrogen is more effectively expelled from the condensed phase. This is advantageous since hydrogen in the active material of the anode can cause an irreversible loss of the capacity of the electrochemical cell by the irreversible formation of lithium hydride.

[0048] The particle size of particles produced by nucleation and vapor phase growth depends on the precursor gas concentration in the condensation zone. Generally, the higher the precursor gas concentration, the larger the particles formed. To enable the production of smaller particles, the precursor gas can be diluted in an inert gas such as hydrogen gas. Selecting hydrogen to dilute the precursor gas has the advantage of avoiding the supply of "foreign" elements into the formation process and thus producing ultra-high purity particles. Alternatively, an inert gas such as a noble gas may be used to dilute the precursor gas. Any inert gas, i.e., a gas that does not chemically react with the precursor gas or silicon particles, can be used for dilution purposes.

[0049] In exemplary embodiments of the invention according to the first and second aspects of the invention, the predominantly amorphous silicon-containing particles have a BET surface area in the range of 10 - 250 m 2 / g, preferably in the range of 15 - 170 m 2 / g, more preferably in the range of 25 - 130 m 2 / g, and most preferably in the range of 35 - 130 m 2 / g. For the sake of simplicity, assuming that these particles are spherical or quasi-spherical and non-porous, these BET surface areas correspond (in rough estimate) to an average particle size in the range of 10 - 200 nm, preferably in the range of 15 - 150 nm, preferably in the range of 20 - 100 nm, and most preferably in the range of 20 - 70 nm. The BET determination of the particle surface area is well-known to those skilled in the art. An example of a standard that can be used to determine the BET surface area of predominantly amorphous silicon-containing particles according to the first and second aspects of the invention is ISO9277:2010.

[0050] The mainly amorphous silicon-containing particles according to the first and second aspects of the present invention are provided with a surface coating, preferably a carbon coating having a thickness of 0.05 to 3 nm, preferably 0.2 to 1 nm, which can improve surface properties, reduce the risk of fire, and promote the formation of a stable solid electrolyte interface (SEI). The present invention is not restricted to a specific coating material or method for coating the particles, and any coating and coating method known to those skilled in the art for coating silicon particles can be applied.

[0051] The relatively high heat resistance of the mainly amorphous silicon-containing particles of the present invention is advantageous in that the particles can better withstand the temperatures associated with the formation of a carbon coating - and / or the formation of composite particles containing silicon nanoparticles - by thermal decomposition without showing significant transformation into a crystalline state. The carbon-encapsulated or carbon-coated mainly amorphous silicon-containing particles of the present invention can maintain their mainly amorphous structure even after undergoing a thermal decomposition process. The quality of thermal decomposition often depends on temperature, and it is preferably set at 600 °C, 700 °C, preferably 800 °C or 900 °C in order to obtain a high-quality carbon material coating or carbon encapsulation.

[0052] The mainly amorphous structure of the silicon-containing particles of the present invention makes these particles well-suited for use as an active material in the negative electrode of a lithium-ion secondary electrochemical cell (battery). The amorphous structure is known to enable the first charge at a lower overvoltage, has higher stress resistance, and the particles have more resilience against the volume fluctuations associated with the lithiation / delithiation cycles during charge and discharge, respectively.

[0053] In a third aspect, the present invention relates to mainly amorphous silicon-containing particles, said particles consist of a compound of the formula Si (i-x) M X where 0.005 ≤ x < 0.02 and M is at least one substitution element selected from C, N, or a combination thereof, When the particles are subjected to XRD analysis using non-monochromatized CuKα rays, they show one peak at approximately 28° and one peak at approximately 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used.

[0054] As used herein, the term "active material" generally refers to the compound / substance of the electrodes (anode and cathode) that take in and give out lithium ions and electrons to generate or store energy, that is, the substance that undergoes lithiation and delithiation in the charge-discharge cycle of an electrochemical cell. More specifically, the active material of the negative electrode according to the third aspect of the present invention refers to the mainly amorphous silicon-containing particles according to the first or second aspect of the present invention.

[0055] In a secondary electrochemical cell, the chemical half-cell reactions at the electrodes switch from oxidation to reduction reactions respectively with the charge-discharge states of the charge-discharge cycle. As used herein, the term "negative electrode" is used to indicate the electrode of an electrochemical cell where the oxidation side of the chemical reaction occurs during discharge, that is, the negative electrode is the electron-generating electrode when drawing energy from the electrochemical cell. The negative electrode may sometimes be referred to as the anode in the literature. The terms anode and negative electrode can be used interchangeably herein.

[0056] The negative electrode according to the third aspect of the present invention can utilize any conductive substrate known or conceivable to those skilled in the art that is suitable for use as a current collector in the negative electrode of a lithium-ion secondary electrochemical cell. Examples of suitable conductive substrates include, but are not limited to, graphite, aluminum, or copper foils / sheets.

[0057] The negative electrode according to the third aspect of the present invention can utilize any binder material known or conceivable to those skilled in the art that is suitable for use as a binder in the negative electrode of a lithium-ion secondary electrochemical cell. Examples of suitable binders include, but are not limited to, styrene-butadiene copolymer (SBR), carboxymethyl cellulose (CMC), ethylene-propylene-diene monomer (EPDM), and polyacrylic acid (PAA).

[0058] In an exemplary embodiment, the anode mass can further include a particulate conductive filler material mixed with the particulate active material and embedded together in the binder material. The negative electrode according to the third aspect of the present invention can use any conductive filler material known or conceivable to those skilled in the art that is suitable for use as a conductive filler for the anode mass of the negative electrode of a lithium-ion secondary electrochemical cell. Examples of suitable particulate conductive filler materials include, but are not limited to, carbon isotopes such as graphene, reduced graphene oxide, elastic polymers, carbon-rich substances, carbon black, carbon nanotubes, or predominantly carbon-containing substances produced by pyrolysis of mixtures thereof.

[0059] In a fourth aspect of the present invention, the predominantly amorphous silicon-containing particles according to the first or second aspect of the present invention can be used to form composite particles through a post-manufacturing process involving thermal decomposition of a carbon-containing material comprising a large number of the predominantly amorphous silicon particles of the present invention. These composite particles can be reused in a battery electrode, and each composite particle may contain, for example, from 10 to perhaps up to 1 million of the predominantly amorphous silicon particles of the present invention and a large amount of carbon formed by heat treatment of a precursor material containing carbon atoms. This precursor material may be a large, carbon-intensive molecule such as oil or pitch. Alternatively, the precursor material may be a strongly cross-linked material such as resorcinol formaldehyde or melamine-formaldehyde, where pyrolysis can be used to form a nanoporous structure or aerogel. The pyrolysis process can be carried out at >600 °C, preferably >700 °C, more preferably >800 °C. Examples of the composite particles can have a size similar to that of the graphite particles used in today's batteries, i.e., an average cross-sectional distance of, for example, 2 - 5 microns.

[0060] In a fifth aspect of the present invention, the predominantly amorphous silicon-containing particles according to the first and second aspects of the present invention can be used to form composite particles using graphene or graphene oxide as a conductive additive and as a protective barrier against the electrolyte. These composite particles can be reused in a battery electrode and may contain, for example, from 10 to perhaps up to 1 million of the predominantly amorphous silicon particles of the present invention and a large amount of graphene or reduced graphene oxide formed by heat treatment of a precursor material containing oxidized graphene or graphene oxide. The reduction process can be carried out at >600 °C, preferably >700 °C, more preferably >800 °C. Examples of the composite particles can have a size similar to that of the graphite particles used in today's batteries, i.e., they can have an average cross-sectional distance of 2 - 5 microns. The composite particles can further contain binders and other components that ensure the geometric stability of the composite particles in subsequent manufacturing steps.

[0061] In a sixth aspect of the present invention, the mainly amorphous silicon-containing particles according to the first and second aspects of the present invention may be used to form composite particles using an elastic binder as a barrier to the electrolyte. The composite particles may also contain a conductive additive. These composite particles can be reused in battery electrodes and may contain, for example, from 10 to perhaps up to 1 million of the mainly amorphous silicon particles of the present invention. The elastic binder can be any elastic polymer or plastic, including known elastomers such as imides, amides, silicones, styrene-butadiene rubber, nitrile rubber, and the like. Examples of the composite particles can have a size similar to that of graphite particles used in today's batteries, i.e., an average cross-sectional distance of, for example, 2 to 5 microns.

Brief Description of the Drawings

[0062]

Figure 1

Figure 2

Figure 3

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Figure 6

Mode for Carrying Out the Invention

[0063] Hereinafter, the present invention will be described in more detail by way of exemplary embodiments.

[0064] Comparative Example A homogeneous gas mixture of 33% silane diluted in hydrogen gas was preheated to about 400 °C, this gas was introduced into a decomposition reactor, and the silane gas was mixed with preheated hydrogen gas at temperatures of 710 °C, 745 °C, and 770 °C respectively, thereby producing three samples of (pure) silicon particles. The residence time in the reactor was about 1.5 seconds. The obtained silicon particles were rapidly cooled to below 300 °C and recovered by filtration.

[0065] Next, the sample particles were analyzed by XRD to investigate their atomic structures. The particles produced at 710 °C (marked as RTF1 in Figure 1) have an XRD curve typical of amorphous silicon, the particles produced at 745 °C (marked as RTF2 in Figure 1) have an XRD curve showing some formation of crystalline silicon, and the particles produced at 770 °C (marked as RTF3 in Figure 1) have an XRD curve typical of crystalline silicon.

[0066] Example 1 The mainly amorphous silicon particles according to the exemplary embodiments of the present invention can be produced as follows.

[0067] A homogeneous mixture of silane gas and ethene was preheated to about 400 °C and introduced into the reactor chamber. The homogeneous mixture of silane gas and ethene was further mixed with an inert gas (nitrogen) preheated to a temperature such that the temperature of the resulting gas mixture was 810 °C. The relative amounts of the gases in the final mixture were about 28 mol% silane, 1.5 mol% ethene, and the remainder (70 mol%) nitrogen, which gave an atomic ratio of C:Si of 0.05 in the gas mixture. However, the resulting particles had an atomic ratio of C:Si of 0.02, i.e., the particles consisted mainly of amorphous Si 0.98 C 0.02 and consisted of.

[0068] The residence time in the reactor was about 1.0 second. Thereafter, the exhaust gas and particles exiting the reactor space were rapidly cooled and collected on a filter. The particles were analyzed by XRD to investigate their atomic structure. The results are shown in Figure 2 as the curve marked R11_FA. The XRD curve is typical of silicon having an amorphous molecular structure.

[0069] Example 2 For substance R11_FB, the gas mixture was heated to 800 °C in the reactor, and in sample R11_FC, the ethene concentration was reduced to 50% to produce Si 0.99 C 0.01 In the same manner as in Example 1, except that, two more exemplary embodiments of particles were produced. These particle samples were analyzed by XRD, and the results are shown in Figure 2 as the curves marked R11_FB and R11_FC, respectively. Both curves are typical of amorphous silicon.

[0070] Example 3 Three additional embodiments of particles were produced in the same manner as in Example 1, except that the gas mixture included silane, ethene, ammonia, and nitrogen. These samples were characterized using differential scanning calorimetry, and the crystallization temperature was determined from the energy released upon crystallization. The nitrogen content, because of its low level, is not as easily measured as the carbon content, but was estimated based on linear extrapolation from samples with higher nitrogen content and by analyzing the gas consumption in the reaction to be that the particles had compositions of Si x 、C y 、N z of Si 0.984 C 0.016 N0, Si 0.992 C 0.08 N0 and Si 0.976 Co 0.012 N 0.012 .

[0071] All of these samples showed higher crystallization temperatures compared to the comparative example particles of pure silicon described above, with the sample of the estimated composition of Si 0.976 C 0.012 N 0.012 having a maximum crystallization temperature of 794 °C. The other two samples showed crystallization temperatures at least 10 °C lower, i.e., somewhat lower than 784 °C.

Claims

1. Formula Si （１－ｘ） C ｘ A method for producing mainly amorphous silicon-containing particles composed of a compound of, where 0.005 ≦ x < 0.05, and when the particles are subjected to XRD analysis using non-monochromatized CuKα rays, show one peak at about 28° and one peak at about 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used. The method comprises: forming a homogeneous gas mixture of a first precursor gas of a silicon-containing compound and at least one second precursor gas of a C-containing compound; injecting the homogeneous gas mixture of the first and the second precursor gases into a reactor space, where the precursor gases react to form particles by being heated to a temperature within the range of 700 to 900 °C; collecting the particles and cooling them to a temperature within the range from ambient temperature to 350 °C; wherein the relative amounts of the first and the second precursor gases are adapted such that the formed particles have an atomic ratio C:Si within the range of [0.005, 0.05).

2. The first precursor gas is silane (SiH ４ ), disilane (Si ２ H ６ ), trichlorosilane (HCl ３ Si), or a mixture thereof, the method according to claim 1.

3. The second precursor gas is methane (CH ４ ), ethane (C ２ H ６ ), propane (C ３ H ８ ), ethene (C ２ H ４ ), ethyne (C ２ H ２ ), an alkane, an alkene, an alkyne, or a mixture thereof, and the method according to claim 1 or 2.

4. The relative amounts of the first and second precursor gases are adapted such that the formed particles have an atomic ratio C:Si within the range of 0.01 or more and 0.04 or less, according to any one of claims 1 to 3.

5. The homogeneous gas mixture of the first and the second precursor gases is preheated to a temperature within the range of 400 to 500 °C before being inserted into the reactor space and then further heated to a temperature within the range of 740 to 850 °C after injection into the reactor space, according to any one of claims 1 to 4.

6. The gas mixture further comprises hydrogen, nitrogen, a noble gas such as helium, neon, argon, or any other gas that does not chemically react with the precursor gases at the temperature at which the homogeneous gas mixture injected into the reactor space is heated, according to any one of claims 1 to 5.

7. The relative amounts of the first and the second precursor gases are determined by adjusting the flow rates of the first and the second precursor gases injected into the reactor, measuring the composition of the exhaust gas exiting the reactor using a mass spectrometer to determine the ratio of the injected first and second precursor gases that are converted into particles, using this information to estimate the atomic ratio C:Si in the formed particles, and adjusting the supply rates of the first and the second precursor gases to obtain the intended atomic ratio C:Si in the manufactured particles, according to any one of claims 1 to 6.

8. The method according to any one of claims 1 to 7 further comprises a step of depositing a carbon layer with a thickness of 0.05 to 3 nm on the surface of the condensed particles.

9. Substantially amorphous silicon-containing particles, The particles are composed of a compound of the formula Si （ｉ－ｘ） C ｘ where 0.005 ≦ x < 0.02, When the particles are subjected to XRD analysis using non-monochromatized CuKα radiation, they exhibit one peak at approximately 28° and one peak at approximately 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used. The particles are mainly amorphous silicon-containing particles.

10. The particles are composed of a compound of the formula Si （ｉ－ｘ） C ｘ and 0.01 ≦ x < 0.02, the substantially amorphous silicon-containing particles according to claim 9.

11. The particles have an average particle diameter in the range of 10 to 200 nm, and are mainly amorphous silicon-containing particles according to claim 9 or 10.

12. The particles are coated with a carbon film having a thickness of 0.05 to 3 nm, and are mainly amorphous silicon-containing particles according to any one of claims 9 to 11.

13. A negative electrode of a lithium-ion secondary electrochemical cell, the negative electrode comprising: at least one particulate active material; a binder material; a current collector substrate, and the at least one particulate active material is embedded in the binder material to form an anode mass deposited as an anode mass layer on the current collector substrate; the at least one or one of the particulate active materials is mainly an amorphous silicon-containing particle; The particles are composed of a compound of the formula Si （ｉ－ｘ） C ｘ where 0.005 ≤ x ≤ 0.04, When the particles are subjected to XRD analysis using non-monochromatized CuKα radiation, they exhibit one peak at approximately 28° and one peak at approximately 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used. The negative electrode.

14. The negative electrode according to claim 13, wherein the current collector substrate is any one of graphite, Cu, and Al foils or sheets.

15. The negative electrode according to claim 13 or 14, wherein the binder is any one of a styrene-butadiene copolymer, carboxymethyl cellulose, ethylene-propylene-diene monomer (EPDM), and polyacrylic acid (PAA).

16. The negative electrode according to any one of claims 13 to 15, wherein the anode mass further comprises a particulate conductive filler mixed with the particulate active material and embedded together in the binder material.

17. The negative electrode according to claim 16, wherein the particulate conductive filler is carbon black, a carbon nanotube, graphene, or a mixture thereof.

18. Composite particles used in a negative electrode of a lithium-ion secondary electrochemical cell, the composite particles comprising: a plurality of mainly amorphous silicon-containing particles The particles are composed of a compound of the formula Si （１－ｘ） C ｘ where 0.005 ≤ x ≤ 0.04, When the particles are subjected to XRD analysis using non-monochromatized CuKα radiation, they exhibit one peak at approximately 28° and one peak at approximately 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used. the plurality of substantially amorphous silicon-containing particles; a composite particle comprising a substantially carbon-containing material produced by thermal decomposition of a carbon-rich material.

19. A composite particle used for a negative electrode in a lithium-ion secondary electrochemical cell, wherein the composite particle is a plurality of substantially amorphous silicon-containing particles, The particles are composed of a compound of the formula Si （１－ｘ） C ｘ where 0.005 ≦ x ≦ 0.04, When the particles are subjected to XRD analysis using non-monochromatized CuKα radiation, they exhibit one peak at approximately 28° and one peak at approximately 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used. the plurality of substantially amorphous silicon-containing particles; and an elastic polymer.

20. A composite particle used for a negative electrode in a lithium-ion secondary electrochemical cell, wherein the composite particle is a plurality of substantially amorphous silicon-containing particles, The particles are composed of a compound of the formula Si （１－ｘ） C ｘ where 0.005 ≦ x ≦ 0.04, When the particles are subjected to XRD analysis using non-monochromatized CuKα radiation, they exhibit one peak at approximately 28° and one peak at approximately 52°, and both peaks have a full width at half maximum of at least 5° when Gaussian peak fitting is used. the plurality of substantially amorphous silicon-containing particles; and graphene or reduced graphene oxide.

21. Use of the composite particle according to any one of claims 18 to 20 for a negative electrode in a lithium-ion secondary electrochemical cell.

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