Negative electrode active material, negative electrode and lithium ion secondary battery

Abstract

To provide an anode active material capable of improving cycle characteristics while maintaining high initial efficiency when used as an anode active material for an anode electrode of a secondary battery.SOLUTION: The present invention relates to an anode active material consisting of monoxide silicon particles which are covered by carbon coating and doped with lithium. In a volume-based distribution measured by a laser diffraction particle size distribution measurement device, regarding the monoxide silicon particles, an integrated value of the relative amount of particles of which the particle diameter is equal to or less than 1 μm is equal to or less than 1%, an integrated value of the relative amount of particles of which the particle diameter is equal to or less than 5 μm is equal to or less than 20%, and a cumulative 50% diameter D50 satisfies 6.0 μm≤D50≤15.0 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material, a negative electrode, and a lithium ion secondary battery. [Background technology]

[0002] In recent years, small electronic devices such as mobile terminals have become widespread, and there is a strong demand for further miniaturization, weight reduction, and longer life. In response to such market demands, development of secondary batteries that are small, lightweight, and capable of achieving high energy density has been progressing. The application of these secondary batteries is being considered not only for small electronic devices but also for large electronic devices such as automobiles and power storage systems such as those for homes.

[0003] Among these, lithium-ion secondary batteries are becoming increasingly popular because they are easy to make small and have a high capacity, and can provide a higher energy density than lead batteries and nickel-cadmium batteries.

[0004] The lithium ion secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte solution, and the negative electrode contains a negative electrode active material involved in charge / discharge reactions.

[0005] While carbon-based active materials are widely used as negative electrode active materials, recent market demands have called for further improvements in battery capacity. To improve battery capacity, the use of silicon as a negative electrode active material is being considered. This is because the theoretical capacity of silicon (4199 mAh / g) is more than 10 times greater than that of graphite (372 mAh / g), and a significant improvement in battery capacity can be expected. The development of silicon materials as negative electrode active materials is being considered not only for silicon itself, but also for alloys and compounds such as oxides. Furthermore, the shape of the active material is being considered, ranging from the standard coated type for carbon-based active materials to an integrated type in which the material is deposited directly on the current collector.

[0006] However, when silicon is used as the silicon-based negative electrode active material, the negative electrode active material expands and contracts during charging and discharging, making it prone to cracking, mainly near the surface of the negative electrode active material. Furthermore, ionic substances are generated within the active material, making it prone to cracking. When the surface of the negative electrode active material cracks, a new surface is created, increasing the reactive area of ​​the active material. At this time, the electrolyte decomposes on the new surface, and a coating made of the electrolyte decomposition product is formed on the new surface, consuming the electrolyte. This can lead to a deterioration in cycle performance.

[0007] To date, various studies have been conducted on negative electrode active materials and electrode configurations for lithium ion secondary batteries that are primarily made of silicon-based materials in order to improve the initial efficiency and cycle characteristics of the battery.

[0008] Specifically, to achieve good cycle characteristics and high safety, silicon and amorphous silicon dioxide are simultaneously deposited using a vapor phase method (see, for example, Patent Document 1). Furthermore, to achieve high battery capacity and safety, a carbon material (electron conductor) is provided on the surface layer of silicon oxide particles (see, for example, Patent Document 2). Furthermore, to improve cycle characteristics and obtain high input / output characteristics, an active material containing silicon and oxygen is prepared, and an active material layer with a high oxygen ratio near the current collector is formed (see, for example, Patent Document 3). Furthermore, to improve cycle characteristics, oxygen is incorporated into the silicon active material, and the silicon active material is formed so that the average oxygen content is 40 at% or less and the oxygen content is higher near the current collector (see, for example, Patent Document 4).

[0009] In addition, in order to improve the initial charge / discharge efficiency, Si phase, SiO2, M y Nanocomposites containing SiO O metal oxides are used (see, for example, Patent Document 5). x(0.8 ≤ x ≤ 1.5, particle size range = 1 μm to 50 μm) is mixed with a carbon material and fired at a high temperature (see, for example, Patent Document 6). Further, in order to improve cycle characteristics, the molar ratio of oxygen to silicon in the negative electrode active material is set to 0.1 to 1.2, and the active material is controlled within a range where the difference between the maximum value and the minimum value of the molar ratio near the interface between the active material and the current collector is 0.4 or less (see, for example, Patent Document 7). Further, in order to improve the battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 8). Further, in order to improve cycle characteristics, a hydrophobic layer such as a silane compound is formed on the surface layer of the silicon material (see, for example, Patent Document 9).

[0010] Also, in order to improve cycle characteristics, silicon oxide is used, and conductivity is imparted by forming a graphite film on its surface layer (see, for example, Patent Document 10). In Patent Document 10, regarding the shift value obtained from the Raman spectrum of the graphite film, broad peaks appear at 1330 cm -1 and 1580 cm -1 and their intensity ratio I 1330 / I 1580 is such that 1.5 < I 1330 / I 1580 < 3. Also, in order to improve high battery capacity and cycle characteristics, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used (see, for example, Patent Document 11). Further, in order to improve overcharge and overdischarge characteristics, silicon oxide in which the atomic ratio of silicon to oxygen is controlled to 1:y (0 < y < 2) is used (see, for example, Patent Document 12).

[0011] Also, in the lithium ion secondary battery using silicon oxide, Hitachi Maxell started shipping a rectangular secondary battery for smartphones employing a nanosilicon composite in June 2010 (see, for example, Non-Patent Document 1). The silicon oxide proposed by Hohl is Si 0+ ~Si 4+It is a composite material with various oxidation states (Non-Patent Document 2). Kapaklis has also proposed a disproportionated structure in which silicon oxide is separated into Si and SiO2 by applying a thermal load (Non-Patent Document 3). Miyachi et al. have focused on Si and SiO2, which contribute to charge and discharge among silicon oxides with a disproportionated structure (Non-Patent Document 4).

[0012] To improve cycle performance, silicon oxide powders with specified particle size distributions (mode diameter, D50, and D90) are used (see, for example, Patent Document 13). In the particle size distribution, D50 indicates the cumulative 50% diameter, D90 indicates the cumulative 90% diameter, and the same applies to other values). To improve cycle performance, silicon oxide powders that have been subjected to wet classification with water after ball milling are specified for D90, D90 / D10, and the amount of fine particles of 1 μm or less (see, for example, Patent Document 14). To improve initial discharge capacity and cycle performance, silicon oxide powders with specified D50 / D10 of the silicon oxide powder before carbon coating formation and the BET specific surface area of ​​the negative electrode active material after carbon coating formation are used (see, for example, Patent Document 15).

[0013] Yamada et al. have proposed the following reaction formula between silicon oxide and Li (Non-Patent Document 5): 2SiO(Si+SiO2) + 6.85Li + + 6.85e - → 1.4Li 3.75 Si + 0.4Li4SiO4 + 0.2SiO2 In the reaction formula, Si and SiO2, which make up silicon oxide, react with Li, and are separated into Li silicide, Li silicate, and some unreacted SiO2.

[0014] The Li silicate produced here is irreversible, and once formed, it is a stable substance that does not release Li. The capacity per mass calculated from this reaction formula is close to the experimental value, and is recognized as the reaction mechanism of silicon oxide. Kim et al. also considered the irreversible component that accompanies the charge and discharge of silicon oxide, Li silicate, as Li4SiO4, 7 Li-MAS-NMR and 29This was identified using Si-MAS-NMR (Non-Patent Document 6). This irreversible capacity is the weakest point of silicon oxides, and improvement is required. Kim et al. have therefore used a Li pre-doping method to form Li silicate in advance, which significantly improved the initial efficiency of the battery and created a negative electrode that can withstand practical use (Non-Patent Document 7).

[0015] Furthermore, instead of doping the electrode with Li, they have proposed a method of treating the powder, which has resulted in an improvement in irreversible capacity (Patent Document 16). [Prior art documents] [Patent documents]

[0016] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-185127 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-042806 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-164954 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-114454 [Patent Document 5] Japanese Patent Application Laid-Open No. 2009-070825 [Patent Document 6] Japanese Patent Application Laid-Open No. 2008-282819 [Patent Document 7] Japanese Patent Application Laid-Open No. 2008-251369 [Patent Document 8] Japanese Patent Application Laid-Open No. 2008-177346 [Patent Document 9] Japanese Patent Application Laid-Open No. 2007-234255 [Patent Document 10] Japanese Patent Application Laid-Open No. 2009-212074 [Patent Document 11] Japanese Patent Application Laid-Open No. 2009-205950 [Patent Document 12] Japanese Patent Application Publication No. 06-325765 [Patent Document 13] Japanese Patent Application Laid-Open No. 2015-149171 [License 14] Special Announcement No. 2011-65934 [License 15] International Publication No. 2012 / 077268 [License 16] Special Announcement No. 2015-156355 [Non-licensed literature]

[0017] [Non-licensed Document 1] Battery Industry Association Official Paper "Denchi", May 1, 2013, page 10 [Non-licensed Document 2] A. Hohl, T. Wieder, PA van Aken, TE Weirich, G. Denninger, M. Vidal, S. Oswald, C. Deneke, J. Mayer, and H. Fuess: J. Non-Cryst. Solids, 320, (2003), 255. [Non-licensed Document 3] V. Kapaklis, J. Non-Crystalline Solids, 354 (2008) 612 [Non-licensed Document 4] Mariko Miyachi, Hironori Yamamoto, and Hidemasa Kawai, J. Electrochem. Soc. 2007 volume 154, issue 4, A376-A380 [Non-licensed Document 5] M. Yamada, A. Inaba, A. Ueda, K. Matsumoto, T. Iwasaki, T. Ohzuku, J. Electrochem. Soc., 159, A1630 (2012) [Non-licensed Document 6] Taeahn Kim, Sangjin Park, and Seung M. Oh, J. Electrochem. Soc. volume 154, (2007), A1112-A1117. [Non-Patent Document 7] Hye Jin Kim, Sunghun Choi, Seung Jong Lee, Myung Won Seo, Jae Goo Lee, Erhan Deniz, Yong Ju Lee, Eun Kyung Kim, and Jang Wook Choi,. Nano Lett. 2016, 16, 282-288. Summary of the Invention [Problem to be solved by the invention]

[0018] Lithium-ion secondary batteries using silicon-based materials with high charge / discharge capacities are desired to have high capacity, high initial efficiency, and cycle characteristics that are durable enough for practical use. Among silicon-based materials, silicon oxide has the potential to achieve superior cycle characteristics, and its initial efficiency has also been significantly improved by doping with lithium. However, even with the various technologies related to negative electrode active materials described above, the decomposition of the electrolyte on the surface of the negative electrode active material during repeated charge / discharge cycles has been insufficient, and cycle characteristics that are durable enough for practical use have not been achieved.

[0019] The reason for this is considered to be as follows: For example, in Patent Document 14, wet classification with water is performed to obtain SiO x Using powder, D90 / D10 and the amount of fine powder of 1 μm or less are specified, but it is not possible to make a relative evaluation of the initial efficiency, cycle characteristics, and particle size distribution.

[0020] In addition, in Patent Document 15, SiO (obtained by wet classification with water in the same manner as in Patent Document 14) xThe D50 and D50 / D10 of the powder are specified. However, the examples only describe the difference in initial discharge capacity among relatively small particle sizes with a D50 of 4.41 to 5.35 μm, and state that there is no difference in cycle characteristics (10 cycles) between the examples with a D50 of approximately 5 μm and the comparative examples with a D50 of 9.36 μm. Generally, coin battery evaluation is unsuitable for long-term cycle tests exceeding 50 cycles because the Li positive electrode deteriorates with repeated charge-discharge cycles. Therefore, to evaluate the deterioration of the negative electrode due to the reaction between the electrolyte and the negative electrode active material, charge-discharge cycles exceeding 100 cycles are required, and this must be performed in a full cell using a positive electrode active material such as LCO (lithium cobalt oxide).

[0021] Furthermore, fine powders with a particle size of 1 μm or less have a large specific surface area, which is a major factor in deteriorating cycle characteristics. However, for powders with a particle size distribution that contains almost no fine powders of 1 μm or less, further improvement of cycle characteristics requires the specification of particle sizes greater than 1 μm.

[0022] The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a negative electrode active material that can improve cycle characteristics while maintaining high initial efficiency when used as a negative electrode active material in a negative electrode of a secondary battery. [Means for solving the problem]

[0023] In order to achieve the above object, the present invention provides an anode active material comprising silicon monoxide particles coated with a carbon film and doped with lithium, wherein the silicon monoxide particles have, in a volume-based distribution measured with a laser diffraction particle size distribution analyzer, a cumulative value of the relative particle amount having a particle diameter of 1 μm or less of 1% or less, a cumulative value of the relative particle amount having a particle diameter of 5 μm or less of 20% or less, and a cumulative 50% diameter D50 that satisfies 6.0 μm≦D50≦15.0 μm.

[0024] The negative electrode active material of the present invention is an negative electrode active material made of lithium-doped silicon monoxide particles, and therefore can maintain high initial efficiency. In addition, because it has the above-described particle size distribution, reaction between the particle surfaces of the negative electrode active material and the electrolyte is suppressed in a long-term cycle test in which charging and discharging are repeated, and the cycle characteristics of the battery can be significantly improved.

[0025] In this case, it is preferable that the silicon monoxide particles have a particle size of 5 μm or less, and the cumulative relative particle amount is 10% or less.

[0026] Furthermore, the silicon monoxide particles preferably have a cumulative 99.9% particle diameter D99.9 that satisfies the relationship 18.0 μm≦D99.9≦50.0 μm.

[0027] A negative electrode active material containing silicon monoxide having such a particle size distribution can more effectively suppress the reaction between the particle surface and the electrolyte, thereby significantly improving the cycle characteristics of the battery.

[0028] In the negative electrode active material of the present invention, it is preferable that at least a part of the lithium in the silicon monoxide particles coated with the carbon film and doped with lithium is present as Li2SiO3.

[0029] In such a negative electrode active material, the doped lithium exists as Li2SiO3, and therefore when it is made into a battery, excellent charge / discharge characteristics and cycle characteristics can be obtained.

[0030] Furthermore, in the negative electrode active material of the present invention, it is preferable that the silicon monoxide particles coated with the carbon film and doped with lithium have a peak attributable to a Si(111) crystal plane obtained by X-ray diffraction using Cu-Kα radiation before charging and discharging the negative electrode active material, the crystallite size corresponding to the crystal plane is 5.0 nm or less, and the ratio A / B of the intensity A of the peak attributable to the Si(111) crystal plane to the intensity B of the peak attributable to the Li2SiO3(111) crystal plane satisfies 0.5≦A / B≦1.0.

[0031] A negative electrode active material in which A / B is in this range has an appropriate amount of lithium doping, and when made into a battery, excellent charge / discharge characteristics and cycle characteristics can be obtained.

[0032] The true density of the carbon-coated and lithium-doped silicon monoxide particles is preferably greater than 2.3 g / cc and less than 2.4 g / cc.

[0033] If the true density is within the above range, the reaction rate between silicon monoxide and doped lithium is within a desired range, and the negative electrode active material has Li appropriately inserted into the silicon monoxide particles, thereby further improving the cycle characteristics.

[0034] The present invention also provides a negative electrode comprising the above-described negative electrode active material.

[0035] When such a negative electrode is incorporated into a secondary battery, in a long-term cycle test in which charging and discharging are repeated, the reaction between the particle surface of the negative electrode active material and the electrolyte is suppressed, and the cycle characteristics of the battery can be significantly improved while maintaining excellent initial battery characteristics.

[0036] The present invention also provides a lithium ion secondary battery comprising the above-mentioned negative electrode, a positive electrode, a separator, and an electrolyte.

[0037] In such a lithium-ion secondary battery, in a long-term cycle test in which charging and discharging are repeated, the reaction between the particle surface of the negative electrode active material and the electrolyte is suppressed, and the cycle characteristics of the battery can be significantly improved while maintaining excellent initial battery characteristics. [Effects of the Invention]

[0038] The negative electrode active material of the present invention has the above-described range of powder properties, and therefore, in a long-term cycle test in which charging and discharging are repeated, the reaction between the silicon monoxide particle surfaces and the electrolyte is suppressed, and the long-term cycle characteristics can be significantly improved while maintaining excellent initial battery characteristics. [Brief explanation of the drawings]

[0039] [Figure 1] 1 is a frequency integrated graph of the relative particle amounts of silicon monoxide particles in Examples 1, 2, and 4 and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0040] The inventors, taking into consideration that the deterioration of the negative electrode in long-term cycle characteristics is largely due to the decomposition of the electrolyte on the particle surface of the negative electrode active material, conducted extensive research into controlling the particle size of the small-diameter region of the monosilicon oxide particles that form the base material of the negative electrode active material. As a result, they discovered that with a specific particle size distribution and powder properties, it is possible to significantly improve cycle characteristics while maintaining initial charge-discharge characteristics, and thus arrived at the present invention.

[0041] Hereinafter, the present invention will be described in detail with reference to the preferred embodiments, but the present invention is not limited to these.

[0042] <Negative electrode active material> The negative electrode active material of the present invention is a negative electrode active material comprising silicon monoxide particles coated with a carbon film and doped with lithium. Furthermore, the silicon monoxide particles satisfy the following conditions in a volumetric distribution measured with a laser diffraction particle size distribution analyzer: The cumulative relative particle amount of particles with a particle diameter of 1 μm or less is 1% or less. The cumulative relative particle amount of particles with a particle size of 5 μm or less is 20% or less. Cumulative 50% diameter D50 is 6.0μm≦D50≦15.0μm.

[0043] More preferably, the silicon monoxide particles contained in the negative electrode active material of the present invention have a particle size of 5 μm or less, and the cumulative relative particle amount of the silicon monoxide particles contained in the negative electrode active material of the present invention is 10% or less.Furthermore, the silicon monoxide particles contained in the negative electrode active material of the present invention preferably satisfy the following relationship: 18.0 μm≦D99.9≦50.0 μm, where D99.9 is the cumulative 99.9% particle size.

[0044] Silicon oxide is a general term for amorphous silicon oxide, and silicon oxide before disproportionation is represented by the general formula SiOx (0.5≦x≦1.6). This silicon oxide can be obtained, for example, by heating a mixture of silicon dioxide and metallic silicon to produce silicon monoxide gas, which is then cooled and precipitated, to give silicon monoxide (SiO) where x is 1 or close to 1. For example, 0.9≦x≦1.1.

[0045] To achieve the specific particle size range of the present invention, silicon monoxide particles can be adjusted appropriately by processes such as pulverization and classification. Well-known pulverization equipment can be used. For example, a ball mill or media agitator mill, which pulverizes the material by moving pulverization media such as balls or beads and utilizing the impact, friction, and compression forces generated by the kinetic energy of the pulverization media; a roller mill, which pulverizes the material by utilizing the compression force of rollers; a jet mill, which pulverizes the material by colliding it with a lining material or with each other at high speed and utilizing the resulting impact force; a hammer mill, pin mill, or disk mill, which pulverizes the material by utilizing the impact force generated by the rotation of a rotor equipped with hammers, blades, pins, etc.; a colloid mill, which utilizes shear force; or a high-pressure wet opposed collision disperser such as the "Ultimizer" manufactured by Sugino Machine. Both wet and dry pulverization methods can be used. While pulverization alone results in a broad particle size distribution, dry classification, wet classification, or sieving classification can be used to further refine the particle size distribution after pulverization. Dry classification primarily uses an airflow, and the processes of dispersion, separation (separation of fine particles from coarse particles), collection (separation of solids from gas), and discharge are carried out sequentially or simultaneously. To prevent a decrease in classification efficiency due to interference between particles, particle shape, airflow turbulence, velocity distribution, static electricity, and the like, pretreatment (adjustment of moisture, dispersibility, humidity, etc.) is carried out before classification, and the moisture and oxygen concentration of the airflow used are adjusted. Furthermore, in dry classification systems with an integrated classifier, such as a cyclone, pulverization and classification are carried out simultaneously, making it possible to achieve the desired particle size distribution. When achieving the particle size distribution of the silicon monoxide particles of the present invention, it is preferable to perform jet mill pulverization so as not to increase the BET specific surface area of ​​the silicon monoxide particles, and then cut off the small particle size side by airflow classification using a classifier.

[0046] The silicon monoxide particles have a cumulative 50% diameter D50 of 6.0 to 15.0 μm, preferably 7.5 to 15.0 μm, as described above. The silicon monoxide particles also have a cumulative 99.9% diameter D99.9 of 18.0 to 50.0 μm, more preferably 25.0 to 50.0 μm. On the other hand, if D99.9 is 50.0 μm or less, expansion and contraction of coarse particles due to charge and discharge can be suppressed, reducing the risk of loss of conductive paths within the negative electrode active material layer. Furthermore, damage to the separator by pressing or the like after electrode fabrication can be prevented.

[0047] The particle size distribution of the silicon monoxide particles can be confirmed using a laser diffraction particle size distribution measuring device, and can be determined, for example, under the following conditions. Equipment: Shimadzu SALD-3100 Refractive index: 2.05-0.00i

[0048] The BET specific surface area of ​​the silicon monoxide particles serving as the substrate (a value measured by the BET single-point method using the amount of N2 gas adsorption) is 1.0 to 10 m 2 / g is preferred, and 1.0 to 3.5m 2 / g is more preferable. 2 By making the BET specific surface area 1.0 m / g or less, the reaction area between the electrolyte and silicon monoxide particles is reduced, and when used as a negative electrode active material for lithium ion secondary batteries, the cycle characteristics can be significantly improved. 2 The reason for this is that the BET specific surface area is preferably 1.0 m / g or more for silicon monoxide particles having a size of 6.0 μm≦D50≦15.0 μm. 2 This is because it is industrially difficult to produce silicon monoxide particles with a particle size of less than 1 / g.

[0049] Methods for imparting conductivity to the silicon monoxide particles and improving battery characteristics include mixing the silicon monoxide particles with conductive particles such as graphite, coating the surfaces of the composite particles with a carbon film, and combining both of these methods. Among these, the negative electrode active material of the present invention uses coated particles in which the surfaces of silicon monoxide particles are coated with a carbon film. Chemical vapor deposition (CVD) is a suitable method for coating with a carbon film.

[0050] An example of a chemical vapor deposition (CVD) method is to form a carbon coating by chemically depositing carbon onto silicon monoxide particles in an organic gas atmosphere that can be thermally decomposed to produce carbon at a temperature range of 600 to 1,200°C.

[0051] Chemical vapor deposition (CVD) can be performed under both atmospheric pressure and reduced pressure, and reduced pressure can be exemplified by a reduced pressure of 50 to 30,000 Pa. Furthermore, commonly known devices such as batch furnaces, continuous furnaces such as rotary kilns and roller hearth kilns, and fluidized beds can be used as the apparatus used in the carbon coating formation process. In particular, when the deposition apparatus is a batch furnace in which particles are left stationary, performing the deposition under reduced pressure allows for a more uniform carbon coating, thereby improving battery characteristics.

[0052] Various organic substances, such as those listed below, can be used as carbon sources to form carbon coatings by chemical vapor deposition. However, the pyrolysis temperature, deposition rate, and properties of the carbon coating formed after deposition can vary significantly depending on the substance used. Substances with high deposition rates often result in insufficient uniformity of the carbon coating on the surface. On the other hand, if high temperatures are required for decomposition, the silicon crystals in the silicon monoxide particles to be coated may grow too large during deposition at high temperatures, potentially resulting in reduced discharge efficiency and cycle characteristics. Therefore, the temperature range for the CVD is preferably 950°C or lower, and most preferably 850°C or lower.

[0053] Examples of raw materials for organic gases that can be thermally decomposed to produce carbon include hydrocarbons such as methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, and hexane; aromatic hydrocarbons with one to three rings such as benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, and phenanthrene; and gas diesel, creosote oil, anthracene oil, and naphtha cracking tar oil obtained in a tar distillation process. These may be used alone or in combination of two or more. From an economical standpoint, it is preferable to use a hydrocarbon gas having a CxHy composition.

[0054] The amount of carbon coating is preferably 1.0% by mass or more and 5.0% by mass or less based on the total amount of the carbon-coated particles. Although it depends on the particles to be coated, a carbon coating amount of 1.0% by mass or more can generally maintain sufficient conductivity. Furthermore, a carbon coating amount of 5.0% by mass or less can maintain an appropriate carbon content in the negative electrode active material without making it too high, ensuring sufficient charge / discharge capacity when used as a negative electrode active material for a lithium-ion secondary battery.

[0055] Next, the carbon-coated silicon monoxide particles are doped with Li to produce negative electrode active material particles containing Li-doped silicon monoxide particles. The silicon monoxide particles are modified by Li doping, resulting in the formation of Li compounds within the silicon monoxide particles. Li doping is preferably performed by a redox method to prevent coarsening of the Si crystallite size in the silicon monoxide particles. This Li doping reduces the irreversible capacity of the silicon monoxide particles, contributing to improved initial efficiency. Furthermore, it is preferable to dope Li so that at least a portion of the lithium exists as Li2SiO3. Li2SiO3 has high water resistance among silicates, which stabilizes the negative electrode slurry. The type of lithium silicate can be adjusted by adjusting the conditions of the Li doping process.

[0056] In the modification by the oxidation-reduction method, for example, lithium can be inserted by first immersing silicon monoxide particles in Solution A, which is prepared by dissolving lithium in an ether solvent. This Solution A may further contain a polycyclic aromatic compound or a linear polyphenylene compound. After lithium insertion, active lithium can be desorbed from the silicon monoxide particles by immersing the silicon monoxide particles in Solution B, which contains a polycyclic aromatic compound or its derivative. The solvent for Solution B may be, for example, an ether-based solvent, a ketone-based solvent, an ester-based solvent, an alcohol-based solvent, an amine-based solvent, or a mixture thereof. Alternatively, after immersion in Solution A, the resulting silicon monoxide particles may be heat-treated under an inert gas. The heat treatment can stabilize the Li compound. The heat treatment temperature is preferably 450 to 700°C, more preferably 500 to 650°C. Subsequently, the silicon monoxide particles may be washed with alcohol, alkaline water containing lithium carbonate, a weak acid, pure water, or the like.

[0057] Examples of ether solvents that can be used in solution A include diethyl ether, tert-butyl methyl ether, tetrahydrofuran, dioxane, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and mixtures thereof. Among these, tetrahydrofuran, dioxane, and 1,2-dimethoxyethane are particularly preferred. These solvents are preferably dehydrated and deoxygenated.

[0058] Furthermore, the polycyclic aromatic compound contained in solution A may be one or more of naphthalene, anthracene, phenanthrene, naphthacene, pentacene, pyrene, picene, triphenylene, coronene, chrysene, and derivatives thereof, and the linear polyphenylene compound may be one or more of biphenyl, terphenyl, and derivatives thereof.

[0059] The polycyclic aromatic compound contained in solution B may be one or more of naphthalene, anthracene, phenanthrene, naphthacene, pentacene, pyrene, picene, triphenylene, coronene, chrysene, and derivatives thereof.

[0060] As the ether solvent for solution B, diethyl ether, tert-butyl methyl ether, tetrahydrofuran, dioxane, 1,2-dimethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and the like can be used.

[0061] As the ketone solvent, acetone, acetophenone, etc. can be used.

[0062] Examples of the ester solvent that can be used include methyl formate, methyl acetate, ethyl acetate, propyl acetate, and isopropyl acetate.

[0063] As the alcohol-based solvent, methanol, ethanol, propanol, isopropyl alcohol, and the like can be used.

[0064] As the amine solvent, methylamine, ethylamine, ethylenediamine, etc. can be used.

[0065] According to the present invention, a negative electrode can be produced using the above-mentioned negative electrode active material in which silicon monoxide particles are coated with a carbon film and doped with lithium, and a lithium ion secondary battery can be produced.

[0066] In a negative electrode active material made of silicon monoxide particles coated with a carbon film and doped with lithium, the Si crystallite size and the crystallinity of lithium silicate can be confirmed, for example, using the following XRD apparatus. XRD: Bruker D8 ADVANCE The X-ray source is Cu Kα radiation, and a Ni filter is used, and measurements are made from 10 to 40° with an output of 40 kV / 40 mA, a slit width of 0.3°, a step width of 0.008°, and a counting time of 0.15 seconds per step.

[0067] The Si crystallite size of the negative electrode active material composed of carbon-coated, lithium-doped silicon monoxide particles is determined by the Scherrer method from the half-width of the Si(111) peak appearing near 2θ = 28.4 ± 0.3° in powder XRD measurements. It is preferably 5 nm or less, and amorphous. The smaller the Si crystallite size, the more suppressed the disproportionation of silicon monoxide during charge and discharge, resulting in better cycle performance. Furthermore, the Si(111) peak intensity A and the Li2SiO3(111) peak intensity B appearing near 2θ = 26.5 ± 0.3° in the negative electrode active material vary depending on the amount of lithium doping and the heat treatment temperature after lithium doping. The ratio A / B is preferably 0.5 ≤ A / B ≤ 1.0. Negative electrode active materials within this range have an appropriate amount of lithium doping and heat treatment temperature, resulting in excellent charge / discharge and cycle performance when fabricated into batteries.

[0068] Furthermore, the true density of the silicon monoxide particles coated with a carbon film and doped with lithium in the present invention is preferably greater than 2.3 g / cc and less than 2.4 g / cc. This true density varies depending on the degree of lithium doping into the silicon monoxide particles. If the true density is within the above range, the reaction rate between silicon monoxide and doped lithium is within the desired range, and the negative electrode active material has Li appropriately inserted into the silicon monoxide particles. This further improves cycle performance.

[0069] [Negative electrode] When preparing a negative electrode using the above-described negative electrode active material, a conductive agent such as carbon or graphite can be further added. The type of conductive agent is not particularly limited, and any conductive material that does not decompose or deteriorate in the resulting battery can be used. Specifically, metal particles or fibers of Al, Ti, Fe, Ni, Cu, Zn, Ag, Sn, Si, etc., or graphite such as natural graphite, artificial graphite, various coke particles, mesophase carbon, vapor-grown carbon fiber, pitch-based carbon fiber, PAN-based carbon fiber, and various resin-fired bodies can be used.

[0070] The following method is an example of a method for preparing a negative electrode. The above-mentioned negative electrode active material, optionally a conductive agent, and other additives such as a binder such as carboxymethyl cellulose (hereinafter referred to as CMC), and a solvent such as an organic solvent or water are kneaded to form a paste mixture, and this mixture is then applied to a current collector sheet. In this case, any material typically used as a current collector for a negative electrode, such as copper foil or nickel foil, can be used as the current collector, without any particular restrictions on thickness or surface treatment. The method for forming the mixture into a sheet is not particularly limited, and known methods can be used.

[0071] [Lithium-ion secondary battery] The lithium ion secondary battery of the present invention is a lithium ion secondary battery having at least a positive electrode, a negative electrode, and a lithium ion conductive non-aqueous electrolyte, and the negative electrode active material of the present invention is used in the negative electrode. Other materials such as the positive electrode, electrolyte, and separator, as well as the battery shape, are not particularly limited and may be known. As described above, the negative electrode active material of the present invention has good battery characteristics (charge / discharge capacity and cycle characteristics) when used as a negative electrode active material for a lithium ion secondary battery, and is particularly excellent in cycle durability.

[0072] The positive electrode active material may be an oxide of a transition metal such as LiCoO2, LiNiO2, LiMn2O4, V2O5, MnO2, TiS2, or MoS2, lithium, or a chalcogen compound.

[0073] As the electrolyte, for example, a non-aqueous solution containing a lithium salt such as lithium hexafluorophosphate or lithium perchlorate is used. As the non-aqueous solvent, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethoxyethane, γ-butyrolactone, 2-methyltetrahydrofuran, etc. are used alone or in combination. In addition, various other non-aqueous electrolytes and solid electrolytes can also be used. [Example]

[0074] EXAMPLES The present invention will be explained in more detail below by showing examples and comparative examples of the present invention, but the present invention is not limited to these examples.

[0075] [Example 1] First, a mixture of silicon dioxide and metallic silicon was heated to produce silicon monoxide gas, which was then cooled and precipitated to obtain silicon monoxide (SiOx) with an x ​​of approximately 1.0. Next, this SiOx (x = 1.0) was roughly crushed using a jaw crusher (Maekawa Industries Co., Ltd.) and further ground in a ball mill (Makino Co., Ltd.) for 15 minutes to obtain silicon monoxide particles with a D50 of 100 μm. These particles were then finely pulverized using a jet mill (KJ800, Kurimoto Iron Works Co., Ltd.) under conditions of a compressed air pressure of 0.58 MPa and a classifier rotation speed of 3,500 rpm, and then collected in a cyclone. When these particles were measured using a laser diffraction particle size distribution analyzer (Shimadzu SALD-3100) under the condition of a refractive index of 2.05-0.00i, the cumulative value of the relative particle amount for particle diameters of 1 μm or less was 0.0%, the cumulative value of the relative particle amount for particle diameters of 5 μm or less was 20.0%, the D50 was 7.9 μm, the D99.9 was 28.1 μm, and the BET specific surface area was 2.9 m 2 / g of silicon monoxide particles.

[0076] The particles were placed on a tray to form a powder layer 10 mm thick and placed in a batch-type heating furnace. The furnace was then heated to 850°C at a heating rate of 200°C / hr while reducing the pressure inside the furnace using an oil-sealed rotary vacuum pump. After reaching 850°C, propane was vented into the furnace at 0.3 L / min, and the carbon coating treatment was carried out for 12 hours. After the propane supply was stopped, the furnace was cooled and the collected agglomerates were crushed to obtain black particles. The black particles were conductive particles with a carbon coating amount (mass of carbon coating relative to the total mass of the black particles) of 2.8% by mass. Next, silicon monoxide particles were doped with lithium by oxidation-reduction using a solvent with a moisture content reduced to 50 ppm by mass, and then modified by heat treatment at 600°C in an inert gas atmosphere.

[0077] The resulting negative electrode active material had a D50 of 7.9 μm and a true density of 2.34 g / cc. Furthermore, when the negative electrode active material was measured by X-ray diffraction using Cu-Kα radiation, it had a peak due to the Si(111) crystal plane, and the crystallite size corresponding to the Si(111) crystal plane was 5.0 nm. Furthermore, in the X-ray diffraction measurement, the ratio A / B of the peak intensity A due to the Si(111) crystal plane to the peak intensity B due to the Li2SiO3(111) crystal plane was 0.55.

[0078] <Battery evaluation> Next, a battery using the obtained carbon coated particles as a negative electrode active material was evaluated by the following method.

[0079] First, the negative electrode active material, graphite, conductive additive 1 (carbon nanotubes, CNT), conductive additive 2 (carbon microparticles with a median diameter of approximately 50 nm), sodium polyacrylate, and CMC were mixed in a dry mass ratio of 9.3:83.7:1:1:4:1, and then diluted with pure water to form a negative electrode mixture slurry.

[0080] This slurry was applied to a copper foil having a thickness of 15 μm and dried in a vacuum atmosphere at 100° C. for 1 hour. After drying, the deposition amount of the negative electrode active material layer per unit area on one side of the negative electrode (also referred to as area density) was 7.0 mg / cm 2 It was.

[0081] Next, the solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed, and then an electrolyte salt (lithium hexafluorophosphate: LiPF6) was dissolved to prepare an electrolyte solution. In this case, the solvent composition was EC:DMC = 30:70 in volume ratio, and the content of the electrolyte salt was 1 mol / kg relative to the solvent. As additives, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added in amounts of 1.0 mass% and 2.0 mass%, respectively.

[0082] Next, a coin battery was assembled as follows.

[0083] First, a 1mm thick Li foil was punched out to a diameter of 16mm and attached to an aluminum clad. The resulting electrode was punched out to a diameter of 15mm, and placed opposite a Li counter electrode with a separator between them. After injecting the electrolyte, a 2032 coin battery was fabricated.

[0084] The initial efficiency was measured under the following conditions. First, the charge rate was equivalent to 0.03C. Charging was performed in CCCV mode. CV was set to 0V and the cut-off current was 0.04mA. The discharge rate was also 0.03C, the discharge voltage was 1.2V, and CC discharge was performed.

[0085] When examining the initial charge-discharge characteristics, the initial efficiency (hereinafter sometimes referred to as initial efficiency) was calculated using the formula: initial efficiency (%) = (initial discharge capacity / initial charge capacity) × 100.

[0086] Based on the initial data obtained, a counter cathode was designed and the battery was evaluated. The cathode active material was lithium cobalt oxide (LCO).

[0087] The cycle characteristics were investigated as follows. First, to stabilize the battery, two charge / discharge cycles were performed at 0.2C in an atmosphere of 25°C, and the discharge capacity at the second cycle was measured. The battery cycle characteristics were calculated from the discharge capacity at the third cycle, and the battery test was stopped when the discharge capacity retention rate reached 70%. Charge / discharge was performed at 0.7C and 0.5C. The charge voltage was 4.3V, the discharge cut-off voltage was 2.5V, and the charge cut-off rate was 0.07C.

[0088] [Examples 2 to 6, Comparative Examples 1 to 4] The same SiOx (x = 1.0) as in Example 1 was similarly ball milled to a D50 of 100 μm. In the subsequent jet milling process, the rotation speed of the classifier, the grinding pressure, and the atmosphere were controlled to produce silicon monoxide particles having the powder properties shown in Table 1. In the same manner as in Example 1, the silicon monoxide particles were coated with a carbon film and doped with lithium to produce conductive particles. A negative electrode was produced using the conductive particles obtained in this manner, and a battery evaluation was performed.

[0089] [Example 7] A negative electrode active material was prepared in the same manner as in Example 1, except that silicon monoxide particles with a D50 adjusted to 7.9 μm, the same as in Example 1, were used and the heat treatment temperature after lithium doping was set to 630°C. A negative electrode was prepared and a battery evaluation was performed.

[0090] [Example 8] A negative electrode active material was prepared in the same manner as in Example 1, except that silicon monoxide particles with a D50 adjusted to 7.9 μm, the same as in Example 1, were used and the heat treatment temperature after lithium doping was set to 700°C. A negative electrode was prepared and a battery was evaluated.

[0091] [Comparative Examples 5 to 7] In Comparative Examples 5 to 7, negative electrodes were prepared from the silicon monoxide particles coated with the carbon film obtained in Examples 1, 2, and 4, respectively, without being subjected to lithium doping treatment, and batteries were evaluated.

[0092] [Example 9] A negative electrode active material was prepared in the same manner as in Example 1, except that silicon monoxide particles with a D50 adjusted to 7.9 μm, the same as in Example 1, were used and the heat treatment temperature after lithium doping was set to 730°C. A negative electrode was prepared and a battery evaluation was performed.

[0093] [Example 10] A negative electrode active material was prepared in the same manner as in Example 1, except that silicon monoxide particles with a D50 adjusted to 7.9 μm, the same as in Example 1, were used and the heat treatment temperature after lithium doping was set to 800°C. A negative electrode was prepared and a battery evaluation was performed.

[0094] The powder properties and battery characteristics of the examples and comparative examples are shown in Table 1. Figure 1 shows a frequency integrated graph of the relative particle amounts of silicon monoxide particles in Examples 1, 2, and 4 and Comparative Example 2. As shown in Figure 1, Examples 1, 2, and 4 satisfy the particle size distribution of the present invention, but Comparative Example 2 does not satisfy the particle size distribution range of the present invention.

[0095] [Table 1]

[0096] Table 1 confirms that Examples 1 to 10 are lithium ion secondary batteries with significantly improved cycle characteristics while maintaining the initial charge-discharge characteristics compared to Comparative Examples 1 to 7. In particular, when the relative amount of particles with a particle diameter of 5 μm or less was 10% or less in Examples 2 to 4 and 6, extremely excellent cycle characteristics were obtained. In contrast, Comparative Examples 1 and 2 did not achieve cycle characteristics comparable to those of the Examples because the relative amount of particles with a particle diameter of 5 μm or less was high. Furthermore, in Comparative Examples 3 and 4, it was confirmed that the cycle characteristics were degraded due to the negative electrode active material made of silicon monoxide particles with a particle size distribution in which D99.9 exceeded 50 μm. It is presumed that the coarse particles expanded and contracted during charge and discharge, causing the loss of conductive paths. Furthermore, Comparative Examples 5 to 7 did not undergo lithium doping, and therefore exhibited good cycle characteristics but a decreased initial efficiency. Furthermore, in Examples 9 and 10, the initial efficiency was high, but the Si crystallite size exceeded 5 nm due to the high heat treatment temperature after lithium doping. Therefore, the cycle characteristics were degraded compared to Examples 1 to 8. However, in Examples 9 and 10, the results were better than those in Comparative Examples 1-4.

[0097] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and that provides similar effects is included within the technical scope of the present invention.

Claims

1. A negative electrode active material comprising silicon monoxide particles coated with a carbon film and doped with lithium, The silicon monoxide particles have a volume-based distribution measured by a laser diffraction particle size distribution measuring device, The cumulative value of the relative particle amount of particles with a particle diameter of 1 μm or less is 1% or less, The cumulative value of the relative particle amount of particles with a particle diameter of 5 μm or less is 20% or less, Cumulative 50% diameter D50 is 6.0 μm≦D50≦15.0 μm Fulfilling The silicon monoxide particles have a cumulative 99.9% particle diameter D99.9, and the cumulative 99.9% particle diameter D99.9 satisfies the relationship 18.0 μm≦D99.9≦50.0 μm.

2. 2. The negative electrode active material according to claim 1, wherein the silicon monoxide particles have a particle diameter of 5 μm or less, and the cumulative relative particle amount of the silicon monoxide particles is 10% or less.

3. The silicon monoxide particles coated with the carbon film and doped with lithium have at least a part of the lithium in the silicon monoxide particles. ２ SiO ３ 3. The negative electrode active material according to claim 1, wherein the negative electrode active material is present as follows:

4. The silicon monoxide particles coated with the carbon film and doped with lithium have a peak attributable to a Si(111) crystal plane obtained by X-ray diffraction using Cu-Kα radiation before charging and discharging the negative electrode active material, and the crystallite size corresponding to the crystal plane is 5.0 nm or less, and Li ２ SiO ３ 4. The negative electrode active material according to claim 1, wherein a ratio A / B of an intensity A of a peak due to the Si(111) crystal plane to an intensity B of a peak due to the (111) crystal plane satisfies 0.5≦A / B≦1.

0.

5. 5. The negative electrode active material according to claim 1, wherein the true density of the silicon monoxide particles coated with the carbon film and doped with lithium is greater than 2.3 g / cc and less than 2.4 g / cc.

6. A negative electrode comprising the negative electrode active material according to claim 1 .

7. A lithium ion secondary battery comprising the negative electrode according to claim 6, a positive electrode, a separator, and an electrolyte.

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