Negative electrode active material and method for producing the same

Abstract

The present invention provides a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. [Solution] A negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure, wherein amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, and at least a portion of the surface of the negative electrode active material includes a carbon-based coating layer made of a carbon-based material different from the porous carbon structure, wherein the low-valence nanosilicon oxide has a Si-C bond between the porous carbon structure and at least one of the carbon-based coating layer, and the porous carbon structure is found in the 1590 cm⁻¹ spectrum obtained from infrared spectroscopy analysis. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 A negative electrode active material characterized by having an infrared light transmittance of 99.5% or less.

Description

Technical Field

[0001] The present invention relates to a negative electrode active material and a method for producing the same.

Background Art

[0002] In recent years, small electronic devices typified by mobile terminals have become widely popular, and further miniaturization, weight reduction, and long life are strongly demanded. In response to such market demands, development of secondary batteries that are particularly small, lightweight, and capable of obtaining a high energy density has been promoted. This secondary battery is not limited to small electronic devices, and its application to large electronic devices typified by automobiles and power storage systems typified by houses is also being studied.

[0003] Among them, lithium-ion secondary batteries are easily miniaturized and have increased capacity, and also have a higher energy density than lead batteries and nickel-cadmium batteries, so they are highly expected.

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

[0005] As this negative electrode active material, carbon-based active materials are widely used, while further improvement in battery capacity is required from recent market demands. In order to improve battery capacity, use of silicon as a negative electrode active material has been studied. This is because the theoretical capacity of silicon (4199 mAh / g) is more than 10 times larger than the theoretical capacity of graphite (372 mAh / g), so a significant improvement in battery capacity can be expected. Development of silicon materials as negative electrode active materials has been studied not only for elemental silicon but also for compounds typified by alloys and oxides. In addition, the shape of the active material has been studied from the standard coating type for carbon-based active materials to an integrated type that is directly deposited on the current collector.

[0006] However, when silicon is used as the main raw material for the 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. In addition, ionic substances are generated inside the active material, making the negative electrode active material more susceptible to cracking. When the surface of the negative electrode active material cracks, a new surface is created, increasing the reaction area of ​​the active material. At this time, a decomposition reaction of the electrolyte occurs on the new surface, and a film of electrolyte decomposition products is formed on the new surface, thus consuming the electrolyte. As a result, the cycle characteristics tend to deteriorate.

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

[0008] Specifically, to obtain good cycle characteristics and high safety, silicon and amorphous silicon dioxide are deposited simultaneously using a vapor phase method (see, for example, Patent Document 1). In addition, to obtain high battery capacity and safety, a carbon material (electron conductive material) is provided on the surface of the silicon oxide particles (see, for example, Patent Document 2). Furthermore, in order 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). In addition, in order to improve cycle characteristics, oxygen is incorporated into the silicon active material, and it 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] Furthermore, to improve the initial charge-discharge efficiency, Si phase, SiO2, M y A nanocomposite containing a metal oxide is used (see, for example, Patent Document 5). In addition, to improve the cycle characteristics, SiO x(0.8 ≤ x ≤ 1.5, particle size range = 1 μm to 50 μm) is mixed with a carbon material and fired at 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] Further, 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 the intensity ratio I 1330 / I 1580 is such that 1.5 < I 1330 / I 1580 < 3. Further, 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, a 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] Further, a lithium ion secondary battery using silicon oxide was shipped by Hitachi Maxell in June 2010 for a rectangular secondary battery for smartphones that employs a nanosilicon composite (see, for example, Non-Patent Document 1). The silicon oxide proposed by Hohl is a composite of Si 0+ ~Si 4+ and has various oxidation states (Non-Patent Document 2). Further, Kapaklis has proposed a disproportionation structure that is divided into Si and SiO2 by applying a heat load to silicon oxide (Non-Patent Document 3).

[0012] Miyachi et al. have focused on Si and SiO2, which contribute to charging and discharging among silicon oxides having a disproportionate structure (Non-Patent Literature 4), and Yamada et al. have proposed the following reaction equation between silicon oxide and Li (Non-Patent Literature 5). 2SiO(Si+SiO2) + 6.85Li + + 6.85e - → 1.4Li 3.75 Si + 0.4Li4SiO4 + 0.2SiO2 In the reaction equation, the silicon oxide is composed of Si and SiO2, which react with Li to form Li silicide, Li silicate, and some unreacted SiO2.

[0013] The Li silicate produced here is irreversible and is generally considered a stable substance that does not release Li once formed. The volume per unit mass calculated from this reaction equation is close to experimental values ​​and is recognized as a reaction mechanism for silicon oxides. Kim et al. referred to the irreversible component of silicon oxide charging and discharging, Li silicate, as Li4SiO4. 7 Li-MAS-NMR and 29 Identification is performed using Si-MAS-NMR (Non-Patent Document 6).

[0014] This irreversible capacity is the weakest point of silicon oxides, and improvement is needed. Therefore, Kim et al. have used a Li pre-doping method, which involves forming Li silicate in advance, to significantly improve the initial efficiency of the battery and create a negative electrode that can withstand practical use (Non-Patent Document 7). They have also proposed a method of treating the powder rather than doping the electrode with Li, achieving improvement in irreversible capacity (Patent Document 13).

[0015] On the other hand, the price of Li metal used for Li-doping fluctuates wildly depending on market conditions, presenting many challenges when considered for industrialization. Therefore, CVD-Si-C, which uses silane gas on porous carbon to generate nanosilicon inside, has been able to achieve a higher energy density than Li-doped SiO (Patent Documents 14, 15).

[0016] Furthermore, focusing on the problem that the small pore size of porous carbon materials leads to the deposition of an excess amount of silicon (Si) on the surface, resulting in high electrical resistivity, composite particles that do not contain SiC (silicon carbide) or have an extremely low SiC content have been proposed to reduce electrical resistivity (Patent Document 16).

[0017] Furthermore, SiC, as commonly referred to, is produced by using a fluidized bed tank, injecting silane gas and carrier gas from the bottom in a heated atmosphere, depositing silicon inside porous carbon, and then performing a carbon coating using hydrocarbon gas or coal tar pitch after an oxidation process. At this time, adsorption thermal decomposition is likely to occur at least 400°C (effectively 450-500°C) (Patent Documents 17 and 18).

[0018] In this case, Si-O bonds exist in the outermost layer, and general SiC can be produced by performing carbon coating CVD with hydrocarbon gas on top of them. However, if there are not enough Si-O bonds, Si-C crystals are formed, which is said to reduce battery capacity (Patent Documents 19 and 20).

[0019] Furthermore, in the oxidation process, it is believed that by performing the oxidation treatment using dry air with a low dew point (specifically -40°C), the silicon oxide reacts with the dangling bonds but does not react with the Si-H bonds (Non-Patent Documents 9 and 10).

[0020] Furthermore, while many Si-C compounds have a polydimethylsiloxane structure, some also possess Si-H bonds. It has been reported that Si-H bonds detach at around 450°C (Non-Patent Literature 11). [Prior art documents] [Patent Documents]

[0021] [Patent Document 1] Japanese Patent Publication No. 2001-185127 [Patent Document 2] Japanese Patent Publication No. 2002-042806 [Patent Document 3] Japanese Patent Publication No. 2006-164954 [Patent Document 4] Japanese Patent Publication No. 2006-114454 [Patent Document 5] Japanese Patent Publication No. 2009-070825 [Patent Document 6] Japanese Patent Publication No. 2008-282819 [Patent Document 7] Japanese Patent Publication No. 2008-251369 [Patent Document 8] Japanese Patent Publication No. 2008-177346 [Patent Document 9] Japanese Patent Publication No. 2007-234255 [Patent Document 10] Japanese Patent Publication No. 2009-212074 [Patent Document 11] Japanese Patent Publication No. 2009-205950 [Patent Document 12] Japanese Patent Application Publication No. 06-325765 [Patent Document 13] Japanese Patent Publication No. 2015-156355 [Patent Document 14] U.S. Patent No. 10,608,254 [Patent Document 15] U.S. Patent No. 11,165,054 [Patent Document 16] WO2024 / 142699 [Patent Document 17] U.S. Patent No. 10,454,103 [Patent Document 18] U.S. Patent No. 10,714,744 [Patent Document 19] WO2024 / 161756 [Patent Document 20] U.S. Patent No. 12,046,744 [Non-patent literature]

[0022] [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-licensed 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. [Non-Patent Document 8] The Cutting Edge of Automotive Lithium-ion Battery Development, pp. 96-111, CMC Publishing, November 27, 2020. [Non-Patent Document 9] Vacuum, Vol. 33, No. 11, 1990, pp. 854-860: Deactivation of Si single crystal surfaces by hydrogen termination. [Non-Patent Document 10] Surface Science Vol.19, No.3, pp.173-178, 1998 Theory of the oxidative process on Si surfaces [Non-Patent Document 11] C. Michael Greenlief, Michael Armstrong, “Hydrogen desorption from Si: How does this relate to film growth?” (J. Vac. Sci. Technol. B, Vol. 13, No. 4, Jul / Aug 1995) [Overview of the project] [Problems that the invention aims to solve]

[0023] As mentioned above, in recent years, small electronic devices such as mobile terminals have become more high-performance and multi-functional, and there is a need for increased battery capacity in the lithium-ion secondary batteries that are their main power source. One way to solve this problem is to develop a lithium-ion secondary battery in which the negative electrode is made mainly of silica material.

[0024] Furthermore, lithium-ion secondary batteries using silica materials are desired to have initial charge-discharge characteristics and cycle characteristics close to those of lithium-ion secondary batteries using carbon-based active materials. Therefore, cycle characteristics and initial charge-discharge characteristics have been improved by using silicon oxide modified by insertion and partial desorption of Li as the negative electrode active material. Recently, by mainly using silicon oxide and pre-containing Li to produce Li silicate, the irreversible capacity, which is a disadvantage of silicon oxide, has been reduced, and such products have actually begun to be marketed. Even when a prototype battery using this silicon oxide with Li, Li-SiO-C (Non-Patent Literature 8), is fabricated to replace 100% of the carbon negative electrode material, the capacity improvement compared to the carbon negative electrode material is limited to the high 20% range. This means that further improvements in battery capacity are required when considering the high performance of small electronic devices (5G, etc.) and the increased driving range of electric vehicles.

[0025] Therefore, CVD-Si-C, which has a low irreversible capacity, was developed, but it has been found that its fast charging capability and battery cycle characteristics are insufficient due to the reaction between Si and the electrolyte.

[0026] The present invention has been made in view of the above-mentioned problems, and aims to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. [Means for solving the problem]

[0027] To solve the above problems, the present invention provides a negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure, amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, and at least a portion of the surface of the negative electrode active material includes a carbon-based coating layer made of a carbon-based material different from the porous carbon structure, the low-valence nanosilicon oxide has a Si-C bond between the porous carbon structure and at least one of the carbon-based coating layer, and the porous carbon structure is present in the 1590 cm⁻¹ spectrum obtained from infrared spectroscopy analysis. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1The present invention provides a negative electrode active material characterized by having an infrared light transmittance of 99.5% or less.

[0028] The anode active material of the present invention, firstly, contains low-valence nanosilicon oxide, which reduces the generation of ionic substances and enhances the diffusivity of Li. Furthermore, because the low-valence nanosilicon oxide has Si-C bonds, the internal Si-C bonds have a strong bond with the surface Si, which suppresses the electronic breakup of the Si portion during charging and discharging, resulting in a stable structure. Therefore, it is electrochemically stable during charging and discharging, and decomposition during charging and discharging is less likely to occur. Next, the porous carbon structure has an infrared absorption peak due to the absorption of aromatic kernels, and its transmittance is 99.5% or less, so the porous carbon structure is sufficiently carbonized. As a result, the amount of undecomposed compounds is reduced, preventing Li from being trapped, suppressing the increase in irreversible capacity, and improving efficiency. In addition, the increase in electronically conductive aromatic compounds improves electronic conductivity and cycle characteristics. As a result, it is possible to provide an anode active material that can improve cycle characteristics while maintaining high battery capacity.

[0029] Furthermore, the porous carbon structure is 1590 cm -1 It is preferable that the infrared light transmittance is 99.0% or less.

[0030] Thus, the porous carbon structure exhibits lower permeability, resulting in more thorough carbonization. Consequently, it becomes a suitable negative electrode active material capable of improving cycle characteristics while maintaining high battery capacity.

[0031] Furthermore, the porous carbon structure is found in the 1100 cm⁻¹ spectrum obtained from Raman spectroscopy analysis. -1 More than 1200cm -1 It is preferable that the peaks are within the following range.

[0032] A negative electrode active material with such a spectrum has pores of an appropriate size within the porous carbon structure, allowing gas to easily reach the pores. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0033] Furthermore, the peaks in the spectrum obtained from the Raman spectroscopy analysis are peaks originating from C=O bonds or CO bonds, and it is preferable that the porous carbon structure contains at least one of C=O bonds or CO bonds.

[0034] If a material contains C=O or CO bonds internally, for example, when it reacts with silane gas, a CO-Si moiety is formed. This O-Si bond exhibits excellent Li diffusion properties, which can improve fast charging capabilities. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0035] Furthermore, in the spectrum obtained from Raman spectroscopy, it is preferable that the porous carbon structure has a G / D ratio of less than 1.0 (ratio of peaks originating from the G band to peaks originating from the D band) and a 2D / D ratio of less than 0.1 (ratio of peaks originating from the 2D band to peaks originating from the D band), while the negative electrode active material particles have a G / D ratio of 1.0 or higher.

[0036] Thus, if the G / D ratio of the porous carbon structure is less than 1.0 and the 2D / D ratio is less than 0.1, and the G / D ratio of the negative electrode active material particles is 1.0 or higher, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. In particular, negative electrode active material particles with a G / D ratio of 1.0 or higher have good Li acceptance on the surface of the negative electrode active material, improving cycle characteristics during fast charging.

[0037] Here, the G / D ratio is an index used to evaluate crystallinity in Raman spectroscopy, representing the ratio of the intensity of the G band to the D band. A larger G / D ratio indicates better crystallinity.

[0038] Furthermore, in the spectrum obtained from Raman spectroscopy, it is preferable that the porous carbon structure has active sites within the pores of the porous carbon structure in the G band, and that some of the active sites are modified with OH groups or COOH groups.

[0039] Such a configuration results in a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. First, if the active sites inside the pores of the porous carbon structure are modified with OH groups, then, for example, a reaction with silane gas will easily disperse silane decomposition products as amorphous low-valence nanosilicon oxides inside the porous carbon structure. Second, if the active sites inside the pores of the porous carbon structure are modified with COOH groups, then, for example, a reaction with silane gas will partially transform not only the low-valence nanosilicon oxides but also compounds having Si-C bonds. Therefore, it is easy and preferable for the negative electrode active material to have the desired low-valence nanosilicon oxides and Si-C bonds.

[0040] Furthermore, it is preferable that the low-valent nanosilicon oxide is substantially in a composite state of 0, 1, 2, and 3 valent states.

[0041] In this composite state, in addition to achieving a lower irreversible capacity, by adjusting the distribution of zero, mono, di, and trivalent states, it is possible to more effectively realize low-valence nanosilicon oxides that absorb and desorb Li. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0042] Furthermore, the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction of the negative electrode active material particles, is preferably in the range of 0.8 nm to 5 nm.

[0043] Such materials with a grain size of zero-valent Si, which is essentially an amorphous structure, are preferred. If the grain size is too large (if the amount of zero-valent Si increases and crystallization progresses too much), the true density increases, which improves discharge capacity and initial efficiency but deteriorates cycle characteristics. This can be prevented and a balance can be achieved. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0044] Furthermore, it is preferable that the low-valent nanosilicon oxide is substantially amorphous.

[0045] Such substantially amorphous materials are preferred.

[0046] Furthermore, the total amount of oxygen contained in the negative electrode active material is preferably in the range of 0.3 wt% or more and 3 wt% or less.

[0047] Within this oxygen content range, it is possible to prevent excessive Si-C bond formation during acetylene CVD, which reduces battery capacity, from occurring due to insufficient oxygen, while also preventing an increase in irreversible capacity due to excessive oxygen. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0048] Furthermore, in order to solve the above problems, the present invention provides a method for producing a negative electrode active material having negative electrode active material particles, wherein the spectrum obtained from infrared spectroscopy is measured at 1590 cm⁻¹. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1The present invention provides a method for producing a negative electrode active material, comprising the steps of: preparing a porous carbon structure having an infrared light transmittance of 99.5% or less; incorporating COOH groups, OH groups, and CxHy groups into the active sites of the porous carbon structure; flowing monosilane gas to deposit silicon oxide bonded to oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds; oxidizing at least a portion of the silicon present on the surface of the negative electrode active material in a reduced-pressure atmosphere after the step of depositing the silicon oxide; and depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas after the step of oxidation in a reduced-pressure atmosphere.

[0049] The present invention provides a method for producing a negative electrode active material that involves preparing a porous carbon structure having an infrared absorption peak due to the absorption of aromatic kernels and a transmittance of 99.5% or less. This allows the porous carbon structure to be sufficiently carbonized. As a result, the amount of undegraded compounds is reduced, preventing Li from being trapped, suppressing an increase in irreversible capacity, and improving efficiency. Furthermore, the increase in electronically conductive aromatic compounds improves electronic conductivity and cycle characteristics. As a result, the present invention provides a method for producing a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0050] Furthermore, in the step of depositing the silicon oxide, it is preferable that the monosilane gas forms the silicon via silylene by a monomolecular thermal decomposition reaction in the gas phase.

[0051] If silicon is formed by such a single-molecule thermal decomposition reaction in the gas phase, compared to adsorption thermal decomposition reactions, the silicon does not adhere strongly to the porous carbon. After at least one charge-discharge cycle, it becomes fluid enough to allow extraction of silicon from the pore surface, which helps improve the diffusivity of Li. As a result, a suitable method for producing a negative electrode active material can be provided that improves cycle characteristics while maintaining high battery capacity. [Effects of the Invention]

[0052] According to the anode active material of the present invention, firstly, because it contains low-valence nanosilicon oxide, the generation of ionic substances can be reduced, and the diffusivity of Li can be increased. Furthermore, because the low-valence nanosilicon oxide has Si-C bonds, the internal Si-C bonds have a strong bond with the Si in the surface layer, which suppresses the electronic breakup of the Si portion during charging and discharging, and stabilizes the structure. As a result, it is electrochemically stable during charging and discharging, and decomposition during charging and discharging is less likely to occur. Next, the porous carbon structure has an infrared absorption peak due to the absorption of aromatic kernels, and the transmittance is 99.5% or less, so the porous carbon structure is sufficiently carbonized. This reduces the amount of undecomposed compounds, prevents Li from being trapped, suppresses the increase in irreversible capacity, and improves efficiency. In addition, the increase in electronically conductive aromatic compounds improves electronic conductivity and improves cycle characteristics. As a result, it is possible to provide an anode active material that can improve cycle characteristics while maintaining a high battery capacity.

[0053] The present invention provides a method for producing a negative electrode active material that involves preparing a porous carbon structure having an infrared absorption peak due to the absorption of aromatic kernels and a transmittance of 99.5% or less. This allows the porous carbon structure to be sufficiently carbonized. As a result, the amount of undegraded compounds is reduced, preventing Li from being trapped, suppressing an increase in irreversible capacity, and improving efficiency. Furthermore, the increase in electronically conductive aromatic compounds improves electronic conductivity and enhances cycle characteristics. As a result, the present invention provides a method for producing a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. [Brief explanation of the drawing]

[0054] [Figure 1] This is a cross-sectional view showing the configuration of a negative electrode containing the negative electrode active material of the present invention. [Figure 2] This is an exploded view showing an example of the configuration (laminated film type) of a lithium-ion secondary battery containing the negative electrode active material of the present invention. [Figure 3] This is a flowchart illustrating an example of a method for producing the negative electrode active material of the present invention. [Figure 4] This is the infrared spectroscopic spectrum of the porous carbon structure of Example 1-1. [Figure 5] This is the Raman spectroscopy spectrum of the porous carbon structure of Example 1-1. [Figure 6] This is an enlarged view of the Raman spectroscopy spectrum of the porous carbon structure of Example 1-1. [Figure 7] This is a characteristic diagram of the ESR before CVD in Example 1-1. [Figure 8] This is a characteristic diagram of the ESR after CVD in Example 1-1. [Figure 9] This is a characteristic diagram of the amount of dangling bond before and after CVD in Example 1-1. [Modes for carrying out the invention]

[0055] The following describes embodiments of the present invention, but the present invention is not limited thereto.

[0056] As mentioned above, one method for increasing the battery capacity of lithium-ion secondary batteries is to use a negative electrode made primarily of low-valence nanosilicon oxide in a carbon structure. A lithium-ion secondary battery using this active material is desired to exhibit battery characteristics close to those of lithium-ion secondary batteries using carbon-based active materials, while also achieving a high battery capacity.

[0057] Therefore, the inventors diligently conducted research to obtain a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity when used as the negative electrode of a secondary battery.

[0058] In particular, regarding the structure of porous carbon contained in the negative electrode active material particles, analysis focusing on the spectrum obtained from infrared spectroscopy revealed that 1590 cm⁻¹ -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel.-1 We discovered that if the infrared light transmittance is 99.5% or less, it is possible to improve cycle characteristics while maintaining high battery capacity, and thus completed the present invention.

[0059] In other words, the present invention relates to a negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure, wherein amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, and at least a portion of the surface of the negative electrode active material includes a carbon-based coating layer made of a carbon-based material different from the porous carbon structure, wherein the low-valence nanosilicon oxide has a Si-C bond between the porous carbon structure and at least one of the carbon-based coating layer, and the porous carbon structure is present in the 1590 cm⁻¹ spectrum obtained from infrared spectroscopy analysis. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 This negative electrode active material is characterized by having an infrared light transmittance of 99.5% or less.

[0060] Furthermore, the present invention relates to a method for producing a negative electrode active material having negative electrode active material particles, wherein the spectrum obtained from infrared spectroscopy is measured at 1590 cm⁻¹. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 A method for producing a negative electrode active material, comprising the steps of: preparing a porous carbon structure having an infrared light transmittance of 99.5% or less; incorporating COOH groups, OH groups, and CxHy groups into the active sites of the porous carbon structure; flowing monosilane gas to deposit silicon oxide bonded to oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds; oxidizing at least a portion of the silicon present on the surface of the negative electrode active material in a reduced-pressure atmosphere after the step of depositing the silicon oxide; and depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas after the step of oxidation in a reduced-pressure atmosphere.

[0061] The following will provide a detailed explanation with reference to the drawings.

[0062] Figure 1 is a cross-sectional view of the negative electrode containing the negative electrode active material of the present invention, and Figure 2 is an example of the configuration of a lithium-ion secondary battery (laminated film type) containing the negative electrode active material of the present invention. Details of Figures 1 and 2 will be described later.

[0063] [The negative electrode active material of the present invention] The present invention relates to a negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure, wherein amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, and at least a portion of the surface of the negative electrode active material includes a carbon-based coating layer made of a carbon-based material different from the porous carbon structure, wherein the low-valence nanosilicon oxide has a Si-C bond between at least one of the porous carbon structure and the carbon-based coating layer, and the porous carbon structure is present in the 1590 cm⁻¹ spectrum obtained from infrared spectroscopy analysis. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 This negative electrode active material is characterized by having an infrared light transmittance of 99.5% or less.

[0064] The anode active material of the present invention, firstly, contains low-valence nanosilicon oxide, which reduces the generation of ionic substances and enhances the diffusivity of Li. Furthermore, because the low-valence nanosilicon oxide has Si-C bonds, the internal Si-C bonds have a strong bond with the surface Si, which suppresses the electronic breakup of the Si portion during charging and discharging, resulting in a stable structure. Therefore, it is electrochemically stable during charging and discharging, and decomposition during charging and discharging is less likely to occur. Next, the porous carbon structure has an infrared absorption peak due to the absorption of aromatic kernels, and its transmittance is 99.5% or less, so the porous carbon structure is sufficiently carbonized. As a result, the amount of undecomposed compounds is reduced, preventing Li from being trapped, suppressing the increase in irreversible capacity, and improving efficiency. In addition, the increase in electronically conductive aromatic compounds improves electronic conductivity and cycle characteristics. As a result, it is possible to provide an anode active material that can improve cycle characteristics while maintaining high battery capacity.

[0065] Infrared spectroscopy can be performed using, for example, a Fourier transform infrared spectrophotometer (JASCO FT / IR-4X).

[0066] Furthermore, although not specifically limited, porous carbon structures are 1590 cm². -1 It is preferable that the infrared light transmittance is 99.0% or less.

[0067] Thus, the porous carbon structure exhibits lower permeability, resulting in more thorough carbonization. Consequently, it becomes a suitable negative electrode active material capable of improving cycle characteristics while maintaining high battery capacity.

[0068] Furthermore, although not particularly limited, porous carbon structures are defined by the 1100 cm⁻¹ spectrum obtained from Raman spectroscopy. -1 More than 1200cm -1 It is preferable that the peaks are within the following range.

[0069] A negative electrode active material with such a spectrum has pores of an appropriate size within the porous carbon structure, allowing gas to easily reach the pores. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0070] Furthermore, although not particularly limited, the peaks in the spectrum obtained from Raman spectroscopy are preferably peaks originating from C=O bonds or CO bonds, and the porous carbon structure preferably contains at least one of C=O bonds or CO bonds.

[0071] If a material contains C=O or CO bonds internally, for example, when it reacts with silane gas, a CO-Si moiety is formed. This O-Si bond exhibits excellent Li diffusion properties, which can improve fast charging capabilities. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0072] Furthermore, although not particularly limited, it is preferable that in the spectrum obtained from Raman spectroscopy, the porous carbon structure has a G / D ratio of less than 1.0 (ratio of peaks originating from the G band to peaks originating from the D band) and a 2D / D ratio of less than 0.1 (ratio of peaks originating from the 2D band to peaks originating from the D band), while the negative electrode active material particles have a G / D ratio of 1.0 or higher.

[0073] Thus, if the G / D ratio of the porous carbon structure is less than 1.0 and the 2D / D ratio is less than 0.1, and the G / D ratio of the negative electrode active material particles is 1.0 or higher, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. In particular, negative electrode active material particles with a G / D ratio of 1.0 or higher have good Li acceptance on the surface of the negative electrode active material, improving cycle characteristics during fast charging.

[0074] Here, the G / D ratio is an index used to evaluate crystallinity in Raman spectroscopy, representing the ratio of the intensity of the G band to the D band. A larger G / D ratio indicates better crystallinity.

[0075] Furthermore, although not particularly limited, in the spectrum obtained from Raman spectroscopy, it is preferable that the porous carbon structure has active sites inside the pores of the G-band porous carbon structure, and that some of the active sites are modified with OH groups or COOH groups.

[0076] Such a configuration results in a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. First, if the active sites inside the pores of the porous carbon structure are modified with OH groups, then, for example, a reaction with silane gas will easily disperse silane decomposition products as amorphous low-valence nanosilicon oxides inside the porous carbon structure. Second, if the active sites inside the pores of the porous carbon structure are modified with COOH groups, then, for example, a reaction with silane gas will partially transform not only the low-valence nanosilicon oxides but also compounds having Si-C bonds. Therefore, it is easy and preferable for the negative electrode active material to have the desired low-valence nanosilicon oxides and Si-C bonds.

[0077] Furthermore, although not particularly limited, it is preferable that the low-valent nanosilicon oxide is substantially in a composite state of 0, 1, 2, and 3 valent states.

[0078] In this composite state, in addition to achieving a lower irreversible capacity, by adjusting the distribution of zero, mono, di, and trivalent states, it is possible to more effectively realize low-valence nanosilicon oxides that absorb and desorb Li. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0079] The valency of low-valency nanosilicon oxides can be quantified using NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0080] NMR (nuclear magnetic resonance) measurements can be performed, for example, under the following conditions. 29 Si MAS NMR (Magic Angle Rotation Nuclear Magnetic Resonance) • Equipment: Bruker 700NMR spectrometer, • Probe: 4mm HR-MAS rotor, 50μL • Sample rotation speed: 10 kHz, ·Measurement environment temperature: 25℃

[0081] XPS can be measured under the following conditions, for example. XPS ·Equipment: X-ray photoelectron spectrometer, • X-ray source: Monochromatic Al Kα rays, • X-ray spot diameter: 100 μm Ar ion gun sputtering conditions: 0.5kV 2mm x 2mm.

[0082] Furthermore, although not particularly limited, the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction of the negative electrode active material particles, is preferably in the range of 0.8 nm to 5 nm.

[0083] Such materials with a grain size of zero-valent Si, which is essentially an amorphous structure, are preferred. If the grain size is too large (if the amount of zero-valent Si increases and crystallization progresses too much), the true density increases, which improves discharge capacity and initial efficiency but deteriorates cycle characteristics. This can be prevented and a balance can be achieved. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0084] The grain size of low-valent nanosilicon oxides can be confirmed by TEM-EDX. The conditions for this can be as follows: The negative electrode active material is processed on both sides using a focused ion beam (FIB) system in an air-free environment. The FIB system used is a SIINT XVision200DB with an acceleration voltage of 30kV. TEM observation is performed using an FEI TecnaiG2F20 with an acceleration voltage of 200kV, and EDX is performed using an EDAX r-TEM with an acceleration voltage of 200kV.

[0085] Crystallite size can be calculated using XRD under the following conditions, for example. For broad peaks, the analysis can be performed using the TOPAS software under the following conditions, for example. XRD measurement • Equipment: Bruker D2 PHASER ·X-ray source:Cu • Divergence slit: 0.5° • Incident solar angle: 4° • Solar receiving side: 4° Calculation of crystallite size • Analysis software: DIFFRAC.TOPAS • Analysis method: Peak fitting method · Emission Profile: CuKa5.lam • Function: FP (First Principle) function • Refinement Option: Select "Caluculate Error" and "Use Extrapolation".

[0086] Furthermore, although not particularly limited, it is preferable that the low-valent nanosilicon oxide is substantially amorphous.

[0087] Such substantially amorphous materials are preferred.

[0088] Furthermore, although not particularly limited, the total amount of oxygen contained in the negative electrode active material is preferably in the range of 0.3 wt% or more and 3 wt% or less.

[0089] Within this oxygen content range, it is possible to prevent excessive Si-C bond formation during acetylene CVD, which reduces battery capacity, from occurring due to insufficient oxygen, while also preventing an increase in irreversible capacity due to excessive oxygen. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0090] Furthermore, the negative electrode active material of the present invention may also include a silicon-cored carbon composite within a porous carbon structure, and this carbon composite may contain at least one of carbon black, acetylene black, and carbon nanotubes.

[0091] Furthermore, porous carbon can be derived from bio-based sources, resin-based sources, or petroleum-based sources, with bio-based or resin-based sources being preferred.

[0092] <Non-aqueous electrolyte secondary battery negative electrode> Next, the configuration of the negative electrode for a non-aqueous electrolyte secondary battery (hereinafter also referred to as the "negative electrode") containing the negative electrode active material of the present invention will be described.

[0093] [Composition of the negative electrode] Figure 1 shows a cross-sectional view of a negative electrode containing the negative electrode active material of the present invention. As shown in Figure 1, the negative electrode 10 has a negative electrode active material layer 12 on a negative electrode current collector 11. This negative electrode active material layer 12 may be provided on both sides of the negative electrode current collector 11, or on only one side. Furthermore, in the negative electrode of the non-aqueous electrolyte secondary battery of the present invention, the negative electrode current collector 11 may be omitted.

[0094] [Negative electrode current collector] The negative electrode current collector 11 is made of a material that has excellent conductivity and high mechanical strength. Examples of conductive materials that can be used for the negative electrode current collector 11 include copper (Cu) and nickel (Ni). It is preferable that this conductive material does not form intermetallic compounds with lithium (Li).

[0095] The negative electrode current collector 11 preferably contains carbon (C) and sulfur (S) in addition to the main element. This is because it improves the physical strength of the negative electrode current collector. In particular, if the active material layer expands during charging, the presence of the above elements in the current collector has the effect of suppressing electrode deformation, including the current collector. The content of the above elements is not particularly limited, but it is preferable that each be 100 ppm by mass or less. This is because a higher deformation suppression effect can be obtained. Such deformation suppression effect can further improve cycle characteristics.

[0096] Furthermore, the surface of the negative electrode current collector 11 is preferably roughened, and more preferably, the ten-point average surface roughness Rz is 1.5 μm or more and 5 μm or less. The roughened negative electrode current collector is, for example, a metal foil that has been electrolytically treated, embossed, or chemically etched.

[0097] [Negative electrode active material layer] The negative electrode active material layer 12 may contain multiple types of negative electrode active materials, such as carbon-based active materials, in addition to silicon-based active material particles. Furthermore, for battery design purposes, it may also contain other materials such as thickeners (also called "binding agents" or "binders") and conductive additives.

[0098] [The present invention concerning a negative electrode active material and a method for producing a negative electrode] Next, an example of the anode active material of the present invention and a method for manufacturing an anode using the same will be described.

[0099] First, we will explain the manufacturing method for the negative electrode active material contained in the negative electrode.

[0100] The present invention relates to a method for producing a negative electrode active material having negative electrode active material particles, wherein the spectrum obtained from infrared spectroscopy is measured at 1590 cm⁻¹. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 A method for producing a negative electrode active material, characterized by comprising the steps of: preparing a porous carbon structure having an infrared light transmittance of 99.5% or less; incorporating COOH groups, OH groups, and CxHy groups into the active sites of the porous carbon structure; flowing monosilane gas to deposit silicon oxide bonded to oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds; oxidizing at least a portion of the silicon present on the surface of the negative electrode active material in a reduced-pressure atmosphere after the silicon oxide deposit step; and depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas after the oxidation in a reduced-pressure atmosphere step.

[0101] The present invention provides a method for producing a negative electrode active material that involves preparing a porous carbon structure having an infrared absorption peak due to the absorption of aromatic kernels and a transmittance of 99.5% or less. This allows the porous carbon structure to be sufficiently carbonized. As a result, the amount of undegraded compounds is reduced, preventing Li from being trapped, suppressing an increase in irreversible capacity, and improving efficiency. Furthermore, the increase in electronically conductive aromatic compounds improves electronic conductivity and cycle characteristics. As a result, the present invention provides a method for producing a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0102] Furthermore, although not particularly limited, it is preferable that the step of depositing silicon oxide involves a monosilane gas forming silicon via silylene through a monomolecular thermal decomposition reaction in the gas phase.

[0103] If silicon is formed by such a single-molecule thermal decomposition reaction in the gas phase, compared to adsorption thermal decomposition reactions, the silicon does not adhere strongly to the porous carbon. After at least one charge-discharge cycle, it becomes fluid enough to allow extraction of silicon from the pore surface, which helps improve the diffusivity of Li. As a result, a suitable method for producing a negative electrode active material can be provided that improves cycle characteristics while maintaining high battery capacity.

[0104] Here, we will explain the manufacturing process of SiC. Generally speaking, SiC is produced by using a fluidized bed tank, injecting silane gas and carrier gas from the bottom in a heated atmosphere, depositing silicon inside porous carbon, and then performing an oxidation process, followed by carbon coating using hydrocarbon gas or coal tar pitch.

[0105] In this case, with this method, the silicon (Si) produced by the reaction of silane gas will have at least a portion of Si-H bonds and will be deposited in a Si-Si state.

[0106] Furthermore, adsorption thermal decomposition is more likely to occur at temperatures of at least 400°C (effectively 450-500°C) (Patent Documents 17 and 18), and the resulting silicon, although amorphous, has a coordination number greater than 3.3.

[0107] The coordination number is calculated by taking an EXAFS scan using the 6N line at the Aichi Synchrotron Radiation Center, which is capable of performing XANES and EXAFS measurements as shown below, and then fitting the scan using Feff (a theoretical calculation software for XAFS based on multiple scattering theory).

[0108] Furthermore, XANES (X-ray Absorption Near-Edge Structure) measurements can be performed, for example, under the following conditions. • Aichi Synchrotron Radiation Center, using BL6N1 line • Samples were prepared in an Ar atmosphere, airless, stored in a transfer vessel, and connected to the BL6N1 line for measurement under the following conditions. • Acceleration energy: 1.2 GeV • Accumulated current value: 300mA • Monochromatization conditions: White X-rays from a bending magnet are monochromatized using a two-crystal spectrometer and used for measurement. • Light focusing conditions: Light focusing in both vertical and horizontal directions using a Ni-coated bent cylindrical mirror. • Upstream slit opening: 10.0 mm horizontally x 3.0 mm vertically • Beam size: 2.0mm horizontally x 1.0mm vertically • Incidence angle to the sample: 45 degrees (incidence angle 45 degrees), so fluorescence yield can be measured simultaneously. • Energy calibration: Calibration of the peak position at the SK end of K2SO4 to 2481.70 eV. • Measurement method: Total electron yield method by measuring sample current, X-ray fluorescence ·I0 measurement method: XANES measurement Au-mesh Cu-mesh during EXAFS measurement

[0109] Furthermore, the substance obtained in the oxidation process is SiO2, so Si 0+ and Si 4+ The composite material is formed within porous carbon.

[0110] In this case, Si-O bonds exist in the outermost layer, and general SiC can be produced by performing carbon coating CVD with hydrocarbon gas on top of them. However, if there are not enough Si-O bonds, Si-C crystals will form, reducing the battery capacity (Patent Documents 19 and 20).

[0111] In the method for producing the negative electrode active material of the present invention, the active sites of the porous carbon structure are utilized to incorporate acidic groups such as carboxyl groups (COOH groups). These acidic groups can react with silane gas to partially transform not only into complex oxides but also into compounds having Si-C bonds.

[0112] Furthermore, OH groups are incorporated into the porous carbon structure. When heat treatment is performed in this state, CO or CO2 is detected in the exhaust gas. Normally, when completely dried, no CO gas is emitted, but by reacting it with silane gas in a pressurized atmosphere while the exhaust gas contains a large amount of CO and CO2, silane decomposition products are deposited on the porous carbon structure as low-valence nanosilicon oxides. At this time, since CO and CO2 gases continue to be emitted from inside the pores of the porous carbon structure, it is important to use a hydrogen carrier in a pressurized atmosphere to allow the silane gas to penetrate to the interior. In a typical fluidized bed, the pressure drop is about 10 kPaG, but it has been found that this reaction gradually starts from about 25 kPaG. In this specification, gauge pressure is denoted as PaG and absolute pressure as PaA.

[0113] Furthermore, although not specifically limited, by introducing silane gas into the pores under a pressure range of 30 kPa to 80 kPa (gauge pressure), it becomes possible to deposit silicon oxide deep within the pores. This has been shown to allow for more efficient decomposition of the silane gas on the inner surface of the pores.

[0114] Furthermore, when porous carbon containing C=O or CO bonds is reacted with silane gas, a CO-Si moiety is formed. This O-Si bond exhibits excellent Li diffusion properties, which can improve rapid charging capabilities. The presence of this C=O or CO bond is evident in the Raman spectrum at 1100 cm⁻¹. -1 More than 1200cm -1 This can be confirmed by the presence of peaks (shoulders) within the following range.

[0115] At this time, CO-Si moieties can also be formed by reacting porous carbon with silane gas while the OH groups are present in the pores. Furthermore, when heat is applied during subsequent CVD with hydrocarbon gas, Si crystallization is difficult, and the amorphous state can be maintained.

[0116] Furthermore, low-valent nanosilicon oxides deposited on porous carbon structures exhibit high Li diffusivity. Generally, compounds of Si and Li are Li 15While Si4 is generated, it exhibits ionic properties and reduces diffusivity. In contrast, the low-valence nanosilicon oxide obtained by the method for producing the negative electrode active material of the present invention can reduce the generation of ionic substances.

[0117] Furthermore, it has been found that low-valence nanosilicon oxides with Si-C bonds are electrochemically stable during charging and discharging, and are less prone to decomposition during these processes.

[0118] Next, typical silane decomposition reactions, such as those used in semiconductors (polysilicon), primarily involve adsorption-based thermal decomposition, requiring thermal energy of approximately 400 degrees Celsius or higher.

[0119] However, in a pressurized region, for example, by impregnating the silane gas inside the pores and leaving active sites on the porous carbon, in a state where CO and CO2 gas coexist, monosilane undergoes single-molecule thermal decomposition (gas-phase thermal decomposition) in the gas phase at low temperatures below 380°C, producing silylene, and then hydrogen is removed as a single atom, ultimately forming silicon. Incidentally, even when estimated using the Boltzmann factor, decomposition does not occur below 380°C. In this temperature range, there exists a reaction pathway where the energy barrier allowing hydrogen detachment is irreversible, or where the equilibrium is heavily skewed towards the product side.

[0120] For example, in a pressurized atmosphere, monosilane and silylene mixed with hydrogen inside the pores are thought to have a lower activation energy, leading to a single-molecule thermal decomposition (gas-phase thermal decomposition) reaction. Furthermore, the dangling bonds present in porous carbon are also thought to contribute to the decomposition.

[0121] Furthermore, by mixing hydrogen or an inert gas into the silane and controlling the reaction of silylene returning to silane and the frequency of contact with dangling bonds, strain is generated during decomposition, making it possible to control the coordination number to 3.3 or less.

[0122] In particular, when the coordination number falls below 3, the form of Si that reacts with Li has a high theoretical capacity. 22 Si5 can be achieved.

[0123] In general, silicon reacts with lithium to produce a reaction product of lithium. 15 It is Si4, Li 15 The theoretical capacity of Si4 is Li 22 It has a lower pH than Si5 and exhibits ionic properties.

[0124] Furthermore, silicon deposited by gas-phase thermal decomposition and increased reaction pressure does not adhere as strongly to porous carbon as silicon deposited by adsorption thermal decomposition. After at least one charge-discharge cycle, it becomes fluid enough to allow extraction of silicon from the pore surface. This helps to improve the diffusivity of Li.

[0125] Furthermore, increasing the pressure during silicon oxide deposition and reaction makes it possible to create a larger number of dangling bonds.

[0126] As described above, it is preferable to use a pressurized atmosphere during silicon oxide deposition. However, it is not the only option.

[0127] These dangling bonds and Si-H bonds are highly reactive, allowing for carbonization at temperatures lower than the typical decomposition temperature of hydrocarbon gases (in this case, acetylene gas). (At the same temperature, generally only tar components are produced.)

[0128] Furthermore, at this stage, some of the Si possesses Si-C bonds, which not only stabilize the structure but also significantly improve water resistance. What is needed here is a Si-C bond, which is different from crystalline SiC in the semiconductor field.

[0129] However, since Si-C bonds deactivate Si, they cannot be formed in large quantities. Therefore, before introducing hydrocarbon gas, the Si-H portion of the outermost layer is converted to low-valent silicon oxides (monovalent to trivalent), and then the reaction with hydrocarbon gas is carried out to suppress the formation of Si-C crystals.

[0130] Incidentally, silicon oxides with Si-C bonds produced using hydrocarbon gases and silicon oxides with Si-C bonds derived from porous carbon structures have the same structure.

[0131] The decomposition reaction of hydrocarbon gases is not particularly limited, but it is preferable to carry it out in a reduced pressure atmosphere of 10,000 PaA or less. The carbon components decomposed under these conditions contain many dangling bonds and form CH bond states on the outermost layer. When forming a slurry for use in a battery, these bonds create repulsive forces with the binder, making it possible to ensure a stable dispersion state for a long period of time.

[0132] <An example of each step in the manufacturing method of the negative electrode active material> Referring to Figure 3, an example of each step in the method for producing the negative electrode active material will be explained.

[0133] The present invention relates to a method for producing a negative electrode active material, and the spectrum obtained from infrared spectroscopy is measured at 1590 cm⁻¹. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 The method includes the steps of: preparing a porous carbon structure having an infrared light transmittance of 99.5% or less (Step S1); incorporating COOH groups, OH groups, and CxHy groups into the active sites of the porous carbon structure (Step S2); flowing monosilane gas to deposit silicon oxide bonded to oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds (Step S3); oxidizing at least a portion of the silicon present on the surface of the negative electrode active material in a reduced-pressure atmosphere after the silicon oxide deposit step (Step S4); and depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas after the oxidation in a reduced-pressure atmosphere (Step S5).

[0134] (Step S1) First, the spectrum obtained from infrared spectroscopy is at 1590 cm⁻¹. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1This step involves preparing a porous carbon structure in which the infrared light transmittance is 99.5% or less.

[0135] The porous carbon structure is not particularly limited, but it is preferable to prepare it in one of the following two ways, for example.

[0136] (1) Using a 10 μm phenolic resin, heat treatment is performed at 600°C for 1 hour under vacuum. Next, potassium hydroxide and carbide are mixed in a 5:1 ratio and activated at 820°C. After the activation treatment, potassium is deactivated with water, and then hydrochloric acid (9) and oxalic acid (1) are added to adjust the pH to 7. After filtration, the mixture is washed with water. The extracted porous carbon is dried at 60°C under vacuum.

[0137] (2) Heat treatment is performed at 600°C for 1 hour under vacuum to produce charcoal. It is then pulverized to a median diameter of 10 μm, and porous carbon is produced in the same manner as in (1) above.

[0138] In addition, infrared spectroscopy can be performed using, for example, a Fourier transform infrared spectrophotometer (JASCO FT / IR-4X).

[0139] (Step S2) This step involves incorporating COOH groups, OH groups, and CxHy groups into the active sites of the porous carbon structure prepared in step S1.

[0140] Porous carbon is placed in a reaction vessel and heated to 100°C while nitrogen gas is flowed through it. At the same time, a trap is installed in the exhaust gas section to ensure that no moisture is completely discharged. Simultaneously, a gas detector is used to check the CO and CO2 gas concentrations. The gas concentration when no moisture is released should be approximately 7000-8000 ppm.

[0141] (Step S3) The step involves flowing monosilane gas to deposit silicon oxide bonded to oxygen atoms inside a porous carbon structure, wherein the silicon oxide contains dangling bonds.

[0142] The container is heated. When the internal temperature reaches 370°C, silane gas is introduced from the bottom to deposit silicon inside the porous carbon. (The pore volume of the porous carbon is estimated in advance from pore distribution measurement, and the amount of silane flowed is 0.9 times the pore volume ⇒ the reaction rate is controlled to 90%).

[0143] At this time, although the pressure inside the container is variable, it is preferable to average it out to a range of 25kPaG to 80kPaG. Deposition at pressures above 80kPaG may result in a denser film, but considering the durability of the equipment, it is stopped at 80kPaG (in reality, it is variable and has been confirmed to rise up to a maximum of 95kPaG). At this point, hydrogen gas equal to the amount of silane is flowed from the side for the purpose of stirring the powder.

[0144] (Step S4) This step involves oxidizing at least a portion of the silicon present on the surface of the negative electrode active material in a reduced-pressure atmosphere.

[0145] After the reaction, the material temperature is lowered to room temperature, and under reduced pressure, nitrogen is introduced to form a nitrogen layer on the exposed surfaces, including the inside of the pores. Next, oxygen diluted with nitrogen is introduced to increase the weight of the oxygen by approximately 0.3 wt% (the moisture content of the introduced air is 100 ppm).

[0146] During this process, it is important to stir the powder and allow it to cool slowly. Rapid oxidation leads to SiO2 formation, which degrades the properties of the powder. Therefore, pre-pressurizing with nitrogen helps to create a nitrogen adsorption layer, which suppresses rapid oxidation.

[0147] Furthermore, by maintaining an oxidation atmosphere at atmospheric pressure to reduced pressure and controlling the sample temperature to below 50°C as an exothermic reaction, low-valence nanosilicon oxides can be formed. In this case, including the oxygen derived from the O component adsorbed inside the porous carbon structure, an oxygen content of approximately 1 wt% can be achieved in the entire negative electrode active material.

[0148] (Step S5) This step involves depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas.

[0149] The material is heated to an internal temperature of 530-600°C under a nitrogen atmosphere, and acetylene gas is introduced to form a carbon-based coating layer on the surface. The process is carried out at 8000 PaA for 9 hours. The resulting material is analyzed by XRD, and the Si crystallite size is calculated using Scherrer's equation. While the result is 0.8 nm, this is a calculated result, and the material is considered substantially amorphous. (Here, the standard acetylene reaction temperature is set to 600°C.)

[0150] During this process, the reaction is carried out under reduced pressure to allow acetylene gas to permeate the Si particles filling the pores. This reaction then causes the Si-H bonds that did not reach a low-valent oxidation state to react, thereby obtaining Si-C bonds and creating a stable material.

[0151] Here, the oxidation process described in step S3 is carried out in a reduced pressure atmosphere. The system is first brought to a Full Vacuum (FV) state, then repressurized with nitrogen, and then reduced to -20 kPaG. Oxygen-containing nitrogen is then introduced to -5 kPaG or near atmospheric pressure. The system becomes reduced pressure once the oxygen is consumed. This promotes oxidation of the parts near the surface, while the bulk interior utilizes the oxygen component present in the porous carbon structure described in step S1. Since the parts near the surface have less oxygen, it is necessary to add oxygen from the outside.

[0152] Furthermore, some of the Si-O compound formed in step S3 undergoes compound formation during the C-CVD process in step S5 (confirmed by XPS), thereby improving the adhesion between the carbon-based coating layer and the Si portion.

[0153] As described above, steps S1 to S5 provide a negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure, amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, and at least a portion of the surface of the negative electrode active material includes a carbon-based coating layer made of a carbon-based material different from the porous carbon structure, the low-valence nanosilicon oxide has a Si-C bond between at least one of the porous carbon structure and the carbon-based coating layer, and the porous carbon structure is found in the 1590 cm⁻¹ spectrum obtained from infrared spectroscopy. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 A negative electrode active material can be manufactured that is characterized by having an infrared light transmittance of 99.5% or less.

[0154] <Lithium-ion rechargeable battery> Next, as a specific example of a non-aqueous electrolyte secondary battery using the negative electrode active material of the present invention described above, a laminate film type lithium-ion secondary battery will be explained.

[0155] [Configuration of a laminate film type rechargeable battery] The laminate film type lithium-ion secondary battery 30 shown in Figure 2 mainly consists of a wound electrode body 31 housed inside a sheet-like outer casing member 35. This wound electrode body 31 has a separator between the positive and negative electrodes and is wound around them. There are also cases where a laminate is housed inside with a separator between the positive and negative electrodes. In both electrode bodies, a positive electrode lead 32 is attached to the positive electrode and a negative electrode lead 33 is attached to the negative electrode. The outermost part of the electrode body is protected by protective tape.

[0156] The positive and negative electrode leads 32 and 33 are led out in one direction, for example, from the inside to the outside of the outer casing member 35. The positive electrode lead 32 is made of a conductive material such as aluminum, and the negative electrode lead 33 is made of a conductive material such as nickel or copper.

[0157] The exterior component 35 is, for example, a laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order. In this laminate film, the outer edges of the fusion layers of two films are fused together or bonded together with an adhesive so that the fusion layer faces the electrode body 31. The fusion part is, for example, a film such as polyethylene or polypropylene, and the metal part is, for example, aluminum foil. The protective layer is, for example, nylon.

[0158] An adhesive film 34 is inserted between the outer casing member 35 and the positive and negative electrode leads to prevent outside air from entering. This material can be, for example, polyethylene, polypropylene, or polyolefin resin.

[0159] The positive electrode, for example, has a positive electrode active material layer on both sides or one side of the positive electrode current collector, similar to the negative electrode 10 in Figure 1.

[0160] The positive electrode current collector is formed from a conductive material such as aluminum.

[0161] The positive electrode active material layer contains one or more positive electrode materials capable of intercalating and deintercalating lithium ions, and may also contain other materials such as positive electrode binders, positive electrode conductive additives, and dispersants, depending on the design. In this case, the details regarding the positive electrode binder and positive electrode conductive additive are the same as those for the negative electrode binder and negative electrode conductive additive already described, for example.

[0162] Lithium-containing compounds are preferred as the cathode material. Examples of lithium-containing compounds include composite oxides composed of lithium and transition metal elements, or phosphoric acid compounds having lithium and transition metal elements. Among these cathode materials, compounds containing at least one of nickel, iron, manganese, and cobalt are preferred. Examples of these chemical formulas include Li x M1O2 or Li y It is represented as M2PO4. In the formula, M1 and M2 represent at least one transition metal element. The values ​​of x and y vary depending on the battery charge and discharge state, but are generally given by 0.05 ≤ x ≤ 1.10 and 0.05 ≤ y ≤ 1.10.

[0163] Examples of the composite oxide having lithium and a transition metal element include, for example, lithium cobalt composite oxide (Li x CoO2), lithium nickel composite oxide (Li x NiO2), lithium nickel cobalt composite oxide, and the like. Examples of the lithium nickel cobalt composite oxide include lithium nickel cobalt aluminum composite oxide (NCA), lithium nickel cobalt manganese composite oxide (NCM), and the like.

[0164] Examples of the phosphate compound having lithium and a transition metal element include, for example, lithium iron phosphate compound (LiFePO4) or lithium iron manganese phosphate compound (LiFe 1-u Mn u PO4 (0 < u < 1)), and the like. By using these positive electrode materials, a high battery capacity can be obtained, and excellent cycle characteristics can also be obtained.

[0165] [Negative electrode] The negative electrode has the same configuration as the negative electrode 10 for a lithium ion secondary battery in FIG. 1 described above. For example, the negative electrode has negative electrode active material layers on both sides of a current collector. It is preferable that the negative electrode charging capacity is larger than the electric capacity (charging capacity as a battery) obtained from the positive electrode active material agent. Thereby, precipitation of lithium metal on the negative electrode can be suppressed.

[0166] The positive electrode active material layer is provided on a part of both sides of the positive electrode current collector, and similarly, the negative electrode active material layer is also provided on a part of both sides of the negative electrode current collector. In this case, for example, the negative electrode active material layer provided on the negative electrode current collector is provided with a region where there is no opposing positive electrode active material layer. This is for performing a stable battery design.

[0167] In the region where the above negative electrode active material layer and positive electrode active material layer do not face each other, it is hardly affected by charge and discharge. Therefore, the state of the negative electrode active material layer is maintained as it is immediately after formation, and thereby the composition of the negative electrode active material and the like can be accurately examined with good reproducibility without depending on the presence or absence of charge and discharge.

[0168] [Separator] A separator separates the positive and negative electrodes, preventing current short circuits caused by contact between the two electrodes while allowing lithium ions to pass through. This separator is formed from a porous membrane made of, for example, synthetic resin or ceramic, and may have a laminated structure in which two or more porous membranes are stacked. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, and polyethylene.

[0169] [Electrolyte] At least a portion of the active material layer, or the separator, is impregnated with a liquid electrolyte. This electrolyte contains an electrolyte salt dissolved in a solvent and may also contain other materials such as additives.

[0170] For example, non-aqueous solvents can be used as solvents. Examples of non-aqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, 1,2-dimethoxyethane, or tetrahydrofuran. Among these, it is desirable to use at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or ethylmethyl carbonate, as this will yield better properties. In this case, even more advantageous properties can be obtained by combining high-viscosity solvents such as ethylene carbonate and propylene carbonate with low-viscosity solvents such as dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate. This is because the dissociability and ion mobility of the electrolyte salt are improved.

[0171] When using an alloy-based negative electrode, it is particularly desirable to include at least one of the following as a solvent: a halogenated linear carbonate ester or a halogenated cyclic carbonate ester. This allows for the formation of a stable film on the surface of the negative electrode active material during charging and discharging, especially during charging. Here, a halogenated linear carbonate ester is a linear carbonate ester having halogen as a constituent element (at least one hydrogen atom is substituted by halogen). A halogenated cyclic carbonate ester is a cyclic carbonate ester having halogen as a constituent element (i.e., at least one hydrogen atom is substituted by halogen).

[0172] While there are no particular limitations on the type of halogen, fluorine is preferred because it forms a better quality film than other halogens. Furthermore, a higher number of halogens is desirable because it results in a more stable film and reduces the decomposition reaction of the electrolyte.

[0173] Examples of halogenated chain carbonate esters include fluoromethylmethyl carbonate and difluoromethylmethyl carbonate. Examples of halogenated cyclic carbonate esters include 4-fluoro-1,3-dioxolan-2-one and 4,5-difluoro-1,3-dioxolan-2-one.

[0174] It is preferable that the solvent additive contains an unsaturated carbon-bonded cyclic carbonate ester. This is because a stable film is formed on the negative electrode surface during charging and discharging, which suppresses the decomposition reaction of the electrolyte. Examples of unsaturated carbon-bonded cyclic carbonate esters include vinylene carbonate or vinylethylene carbonate.

[0175] Furthermore, it is preferable to include a sultone (cyclic sulfonic acid ester) as a solvent additive, as this improves the chemical stability of the battery. Examples of sultones include propanesultone and propenesultone.

[0176] Furthermore, the solvent preferably contains an acid anhydride, as this improves the chemical stability of the electrolyte. Examples of acid anhydrides include propanedisulfonic acid anhydride.

[0177] The electrolyte salt may contain one or more light metal salts, such as lithium salts. Examples of lithium salts include lithium hexafluoride phosphate (LiPF6) and lithium tetrafluoroborate (LiBF4).

[0178] The electrolyte salt content is preferably 0.5 mol / kg to 2.5 mol / kg relative to the solvent, because this allows for high ionic conductivity. [Examples]

[0179] The present invention will be described more specifically below with reference to examples and comparative examples of the present invention, but the present invention is not limited to these examples. Furthermore, to avoid redundancy, explanations of the steps of the above-mentioned <Example of each step in the method for producing the negative electrode active material> and Figure 3 may be omitted.

[0180] First, the negative electrode active material was prepared using the following procedure, and then the laminate film type lithium-ion secondary battery 30 shown in Figure 2 was fabricated.

[0181] A common specification for both the examples and comparative examples is that the porous carbon structure is 1845m 2 The surface area per gram, pore volume of 0.93 cc / g, and particle size D50 = 7.9 μm were used. KOH was used for the activation treatment.

[0182] (Comparative Example 1-1) In step S1 of Figure 3, to prepare the porous carbon structure, the phenolic resin was subjected to vacuum heat treatment at 550°C and activation at 800°C.

[0183] [Measurement of negative electrode active material] The negative electrode active material prepared as described above was subjected to infrared spectroscopy, Raman spectroscopy, XAFS, ESR, etc.

[0184] [Fabrication of the negative electrode] The negative electrode active material prepared as described above, graphite, conductive additive 1 (carbon nanotubes, CNTs), conductive additive 2 (carbon nanoparticles with a median diameter of approximately 50 nm), sodium polyacrylate, and carboxymethylcellulose (hereinafter referred to as CMC) were mixed in a dry mass ratio of 9.3:83.7:1:1:4:1, and then diluted with pure water to obtain a negative electrode mixture slurry.

[0185] Furthermore, a 15 μm thick electrolytic copper foil was used as the negative electrode current collector. This electrolytic copper foil contained carbon and sulfur at concentrations of 70 ppm by mass each. Finally, the negative electrode mixture slurry was applied to the negative electrode current collector and dried in a vacuum atmosphere at 100°C for 1 hour. After drying, the amount of negative electrode active material deposited per unit area on one side of the negative electrode (also called area density) was 7.0 mg / cm³. 2 That was the case.

[0186] [Assembly of a coin cell battery for testing] Next, the solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed, and then the electrolyte salt (lithium hexafluoride phosphate: LiPF6) was dissolved to prepare the electrolyte. In this case, the solvent composition was set to a volume ratio of EC:DMC = 30:70, and the electrolyte salt content was 1 mol / kg relative to the solvent. As additives, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added in amounts of 1.0% by mass and 2.0% by mass, respectively.

[0187] Next, the coin cell was assembled as follows: First, a 1mm thick lithium foil was punched out to a diameter of 16mm and attached to the aluminum cladding.

[0188] Next, the negative electrode obtained earlier was punched out to a diameter of 15 mm, and this was placed opposite a Li foil attached to an aluminum cladding via a separator. After injecting the electrolyte, a 2032 coin cell was fabricated.

[0189] [Measurement of initial efficiency] The initial efficiency was measured under the following conditions. First, the coin cell battery prepared for the initial efficiency test was charged in CCCV mode with a charge rate equivalent to 0.03C (initial charge). The CV was 0V and the termination current was 0.04mA. Next, CC discharge (initial discharge) was performed with a discharge rate of 0.03C and a discharge termination voltage of 1.0V.

[0190] When investigating the initial charge-discharge characteristics, the initial efficiency (sometimes referred to as initial efficiency below) was calculated. The initial efficiency was calculated using the formula: Initial Efficiency (%) = (Initial Discharge Capacity / Initial Charge Capacity) × 100.

[0191] [Manufacturing and evaluation of lithium-ion secondary batteries] Based on the initial data obtained, the cathode was designed so that the utilization rate of the negative electrode was 95%. The utilization rate was calculated from the capacities of the positive and negative electrodes obtained with the counter electrode Li, based on the following formula. Utilization rate = (Positive electrode capacity - Negative electrode loss) / (Negative electrode capacity - Negative electrode loss) × 100 Based on this design, lithium-ion secondary batteries (as shown in Figure 2) for both the examples and comparative examples were manufactured. Battery evaluation was performed on each of the lithium-ion secondary batteries for both the examples and comparative examples.

[0192] The cycle characteristics were investigated as follows: First, to stabilize the battery, two charge-discharge cycles were performed at 0.2C in a 25°C atmosphere, and the discharge capacity of the second cycle was measured. The battery cycle characteristics were calculated from the discharge capacity of the third cycle, and the battery test was stopped at 1000 cycles. Charging was performed at 0.7C and discharging at 0.5C. The charging voltage was 4.3V, the discharge termination voltage was 2.5V, and the charge termination rate was 0.07C.

[0193] The types of silicon-cored carbon composites were investigated by observing the porous carbon structure in the negative electrode active material particles using a scanning electron microscope (SEM).

[0194] The results of each measurement are shown in Table 1. Table 1 also includes the results for Comparative Example 1-2 and Examples 1-1 to 5-5, which are described later.

[0195] [Table 1]

[0196] In Comparative Example 1-1, when preparing the porous carbon structure, the vacuum heat treatment temperature of the phenolic resin was carried out at a low temperature of 550°C and the activation temperature was 800°C. As a result, the formation of aromatic compounds was suppressed, and the value obtained by infrared spectroscopy was 1590 cm⁻¹. -1 The absorption rate decreased, and the transmittance increased to 99.6%.

[0197] In other words, insufficient carbonization of the porous carbon structure made Li trapping more likely with undegraded compounds, leading to a large irreversible capacity and reduced efficiency. Furthermore, the elemental capacity was low, and the cycle characteristics were poor.

[0198] (Comparative Example 1-2) Step S1 was carried out in the same manner as in Comparative Example 1-1, with a vacuum heat treatment temperature of 550°C and an activation temperature of 800°C for the phenolic resin. Furthermore, acetylene CVD in Step S5 was omitted, and the sample was removed without a carbon-based coating layer. Upon removal, the powder temperature was checked and it had risen to 50-80°C, indicating that oxidation had progressed.

[0199] First, by lowering the heat treatment temperature in step S1, the formation of aromatic compounds is suppressed, and the 1590 cm⁻¹ obtained by infrared spectroscopy analysis is suppressed. -1 The absorption rate decreased, and the transmittance increased to 99.6% (similar to Comparative Example 1-1). In addition, because acetylene CVD in step S5 was not performed, Si-C bonds were not formed on the surface. As a result, the efficiency, single-component capacity, and cycle characteristics were all worse than those of Comparative Example 1-1.

[0200] (Example 1-1) The difference from Comparative Example 1-1 is that the temperature in step S1 was increased. A porous carbon structure was used, which was obtained by vacuum heat treatment of phenolic resin at 600°C and activation treatment at 820°C. By increasing the temperature, the porous carbon structure became sufficiently carbonized, aromatic compounds were formed, and 1590 cm³ was obtained by infrared spectroscopy. -1 The absorption rate increased, and the transmittance became low at 98.4%. As a result, Example 1-1 showed better results than Comparative Examples 1-1 and 1-2 in terms of efficiency, single-unit capacity, and cycle characteristics.

[0201] (Examples 1-2) The difference from Example 1-1 is that charcoal was used instead of phenolic resin in step S1. A porous carbon structure was used, which was obtained by vacuum heat treatment of charcoal at 600°C and activation treatment at 820°C. The porous carbon structure was sufficiently carbonized, aromatic compounds were formed, and 1590 cm² was obtained by infrared spectroscopy. -1 The transmittance was 99.3%. This transmittance is higher than the 98.4% of Example 1-1, but lower than the 99.6% of Comparative Examples 1-1 and 1-2. As a result, Example 1-2 also showed better results than Comparative Examples 1-1 and 1-2 in terms of efficiency, single-unit capacity, and cycle characteristics.

[0202] (Examples 1-3) The difference from Example 1-1 is that the temperature in step S1 was further increased. A porous carbon structure was used, which was obtained by vacuum heat treatment of phenolic resin at 650°C and activation treatment at 870°C. The porous carbon structure became sufficiently carbonized, aromatic compounds were formed, and 1590 cm² was obtained by infrared spectroscopy. -1 The transmittance was 98.0%, the lowest to date. As a result, Example 1-3 also showed better results than Comparative Examples 1-1 and 1-2 in terms of efficiency, single-unit capacity, and cycle characteristics.

[0203] (Summary so far: Comparative Examples 1-1, 1-2, Examples 1-1 to 1-3) As in Examples 1-1 to 1-3, by setting the heat treatment temperature in step S1 to 600°C or higher and the activation temperature to 820°C or higher, an aromatic compound is formed, and the spectrum obtained by infrared spectroscopy is at 1590 cm⁻¹. -1 The resulting product has an infrared absorption peak nearby, which is due to absorption by the aromatic kernel (see, for example, Example 1-1 in Figure 4; Figure 4 will be explained in more detail later), at 1590 cm⁻¹. -1 The infrared light transmittance was reduced to 99.5% or less. In this case, the amount of undegraded compounds was reduced, preventing Li from being trapped, suppressing the increase in irreversible capacity, and improving efficiency. In addition, the increase in electronically conductive aromatic compounds improved electronic conductivity and improved cycle characteristics.

[0204] Examples 1-1 to 1-3 all involve the 1590 cm⁻¹ spectrum obtained from infrared spectroscopy. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 By preparing a porous carbon structure with an infrared light transmittance of 99.5% or less, we were able to produce a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. Furthermore, to obtain such infrared spectroscopic analysis results, although not particularly limited, it is preferable to set the heat treatment temperature in step S1 to 600°C or higher and the activation temperature to 820°C or higher, and this is an extremely simple method.

[0205] (Example 2-1) The difference from Example 1-1 is that in step S3, after drying the porous carbon in a nitrogen atmosphere to remove moisture, the exhaust gas was analyzed, and silane gas was introduced and the reaction was carried out when no CO gas was detected. Subsequently, acetylene CVD treatment was performed in step 5, but the deposition rate was about 1 / 5 of that of Example 1-1. This is thought to be because the number of active sites decreased (the number of dangling bonds decreased), making it more difficult for acetylene to decompose. Furthermore, analysis of the Si-kedge using the aforementioned XAFS revealed that it was a composite of zero-valent and tetra-valent components. Therefore, it can be inferred that the Si-O bond was reduced as a result of the removal of the oxygen component from the porous carbon, or that after deposition as Si, oxygen was incorporated when exposed to the atmosphere, and some of it became SiO2.

[0206] Note that in Example 2-1, the measurement was 1590 cm. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹. -1 Since the infrared light transmittance is 98.4% (99.5% or less), the efficiency, single unit capacity, and cycle characteristics were all better than those of Comparative Examples 1-1 and 1-2. However, it was slightly inferior to Examples 1-1 to 1-3. From this, it was found that it is more preferable to introduce silane gas in step S3 while CO gas is detected in the exhaust gas and then proceed with the reaction.

[0207] (Examples 3-1 to 3-2) The difference from Example 1-1 is that in step S5, acetylene CVD was performed at atmospheric pressure instead of 8000 PaA. While the CVD rate improved, the carbon film lacked density, and the G / D ratio decreased. It is thought that the reduced acceptance of Li on the surface of the negative electrode active material may slightly decrease the cycle characteristics during fast charging. However, even so, the results were better than Comparative Examples 1-1 and 1-2 in terms of efficiency, single-unit capacity, and cycle characteristics.

[0208] (Examples 4-1 to 4-5) The difference from Example 1-1 is that before flowing acetylene gas in Step S5, heat treatment was performed in the range of 600 to 670 degrees to crystallize a part of the deposited silicon. The crystallization of silicon means an increase in the amount of zero-valent silicon. The set temperature at this time was adjusted with the intention of changing the grain size of zero-valent Si (see Table 1). After the heat treatment, the temperature was lowered and acetylene CVD was performed.

[0209] As a result, the true density increased, and although the discharge capacity and initial efficiency improved, the cycle characteristics tended to deteriorate slightly. Considering this, in order to improve the cycle characteristics, it is desirable to make the crystallinity of Si (grain size of zero-valent Si) 5 nm or less.

[0210] (Examples 5-1 to 5-5) The difference from Example 1-1 is that in Step S4, the oxidation amount, that is, the amount of oxygen contained in the negative electrode active material (oxygen concentration in Table 1) was changed. Here, if the amount of oxygen is too small, Si-C bonds tend to be generated more during acetylene CVD in Step S5, and it is considered that the battery capacity will be slightly reduced. Therefore, in order to improve the battery capacity, it is preferable that the amount of oxygen is 0.3 wt% or more. On the contrary, if the amount of oxygen is too large, the irreversible capacity tends to increase. Therefore, in order to further reduce the irreversible capacity, it is preferable that the amount of oxygen is 3 wt% or less.

[0211] Hereinafter, Example 1-1 will be further supplemented with reference to the drawings.

[0212] FIG. 4 is a spectrum of infrared spectroscopic analysis in the porous carbon structure of Example 1-1.

[0213] Here, the spectrum of infrared spectroscopic analysis was obtained using a Fourier transform infrared spectrophotometer (FT / IR-4X manufactured by JASCO). The horizontal axis in FIG. 4 represents the wave number, and the vertical axis represents the relative value of the transmission amount through the sample for measurement. Also, the spectra of the sample of Example 1-1 and blank (that is, the state without a sample, also called background) are shown. In Example 1-1, 1590 cm-1 It has an infrared absorption peak nearby due to absorption by the aromatic nucleus (the transmission amount decreases as it is absorbed by the sample, so it is convex downwards), and this 1590 cm⁻¹ -1 The infrared light transmittance was calculated to be 98.4%, as shown in Table 1.

[0214] Examples 1-1 to 5-5 above all use 1590 cm. -1 It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. -1 The infrared light transmittance was 99.5% or less. Such characteristics are necessary to improve cycle performance while maintaining high battery capacity.

[0215] Furthermore, based on the results of Examples 1-1 and 1-2 described above, 1590 cm is obtained as in Example 1-1. -1 It is more preferable that the infrared light transmittance is 99.0% or less.

[0216] Figures 5 and 6 show the Raman spectroscopy spectra of the porous carbon structure of Example 1-1, with Figure 6 being an enlarged view of Figure 5 with the horizontal axis reversed.

[0217] Raman spectroscopy can be performed, for example, under the following conditions. • Device: HORIBA XploRA Plus • Laser wavelength: 532nm

[0218] First, from Figure 6, we see 1150cm -1 It can be seen that it has a peak (shoulder).

[0219] Thus, 1100cm -1 More than 1200cm -1 It is preferable that the peaks are within the following range.

[0220] Next, from Figure 5, we can see the carbon-derived D band (1300-1460 cm⁻¹). -1 The peak of the strait is approximately 1330 cm. -1 G-band (1500~1660cm) -1The peak of the strait is approximately 1590 cm. -1 The peak of the 2D band is approximately 2650 cm. -1 These can be seen in each of them.

[0221] Referring to Table 1, it is preferable that the porous carbon structure has a G / D ratio (ratio of the respective peaks) of less than 1.0 and a 2D / D ratio of less than 0.1.

[0222] On the other hand, the negative electrode active material particles are preferably 1.0 or higher, referring to the G / D ratio after CVD in Table 1. If the negative electrode active material particles have a G / D ratio of 1.0 or higher, the acceptance of Li on the surface of the negative electrode active material is good, and the cycle characteristics during fast charging are improved.

[0223] Figures 7 and 8 show the characteristic ESR of Example 1-1 before and after CVD. In Figure 7, we can see the dangling voids of Si (2.0026 in the figure) and the dangling bonds of the carbon material (2.004 in the figure).

[0224] This document summarizes the measurement conditions for ESR (Electron Spin Resonance). • Equipment: Elexses E580 (manufactured by BRUKER) • Attached device: ESR900 cryostat (manufactured by OXFORD) • Main conditions [Narrow area] [Wide area] Measurement temperature 10K Same as left Central magnetic field around 3345G (same as above) Magnetic field sweep width 200G 1000G Modulation 100kHz, 2G (same as above) Microwave 9.38GHz, 0.68μW (same as above) Sweeping time: 83.89s × 2 times, 167.77s × 4 times Time constant 327.68ms (same as above) Number of data points: 1024 points, 2048 points Cavity Super-High Q (Same as above)

[0225] Figure 9 is a characteristic diagram of the amount of dangling bonds before and after CVD in Example 1-1. It was confirmed that the amount of dangling bonds of Si was large (however, after CVD, it includes dangling bonds of C, so it is for reference).

[0226] This specification includes the following aspects. [1]: A negative electrode active material having negative electrode active material particles, where the negative electrode active material particles include a porous carbon structure, amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, at least a part of the surface of the negative electrode active material includes a carbon-based coating layer made of a carbon-based material different from the porous carbon structure, the low-valence nanosilicon oxide has a Si-C bond between at least one of the porous carbon structure and the carbon-based coating layer, the porous carbon structure has an infrared absorption peak caused by the absorption of an aromatic nucleus near 1590 cm -1 and the transmittance of infrared light at 1590 cm -1 is 99.5% or less, and the negative electrode active material is characterized by this. [2]: The porous carbon structure has a transmittance of infrared light at 1590 cm -1 of 99.0% or less, and the negative electrode active material of [1] above is characterized by this. [3]: The porous carbon structure has a peak in the range of 1100 cm -1 or more and 1200 cm -1 or less in the spectrum obtained by Raman spectroscopy, and the negative electrode active material of [1] or [2] above is characterized by this. [4]: The peak in the spectrum obtained by Raman spectroscopy is a peak derived from a C=O bond or a C-O bond, and the inside of the porous carbon structure includes at least one of a C=O bond and a C-O bond, and the negative electrode active material of [3] above is characterized by this. [5]: In the spectrum obtained by Raman spectroscopy, The structure of the porous carbon has a G / D ratio, which is the ratio of the peak derived from the G band to the peak derived from the D band, less than 1.0, and a 2D / D ratio, which is the ratio of the peak derived from the 2D band to the peak derived from the D band, less than 0.1. The negative electrode active material particles are the negative electrode active materials of the above [1] to the above [4], characterized in that the G / D ratio is 1.0 or more. [6]: The negative electrode active materials of the above [1] to the above [5], characterized in that active sites exist inside the pores of the structure of the porous carbon, and a part of the active sites is modified with an OH group or a COOH group. [7]: The negative electrode active materials of the above [1] to the above [6], characterized in that the low-valence nanosilicon oxide is substantially in a composite state of 0 valence, 1 valence, 2 valence, and 3 valence. [8]: The negative electrode active materials of the above [1] to the above [7], characterized in that the grain size of 0-valent Si constituting the low-valence nanosilicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction measurement of the negative electrode active material particles, is in the range of 0.8 nm to 5 nm. [9]: The negative electrode active materials of the above [1] to the above [8], characterized in that the low-valence nanosilicon oxide is substantially amorphous.

[10] : The negative electrode active materials of the above [1] to the above [9], characterized in that the amount of oxygen contained in the entire negative electrode active material is in the range of 0.3 wt% or more and 3 wt% or less.

[11] : A method for manufacturing a negative electrode active material having negative electrode active material particles, preparing a structure of porous carbon having an infrared absorption peak due to the absorption of an aromatic nucleus near 1590 cm -1 and a transmittance of infrared light at 1590 cm -1 of 99.5% or less; introducing a COOH group, an OH group, and a CxHy group into the active sites of the structure of the porous carbon; flowing monosilane gas and depositing a silicon oxide bonded to an oxygen atom inside the structure of the porous carbon, wherein the silicon oxide is in a state containing dangling bonds; Following the step of depositing the silicon oxide, the step of oxidizing at least a portion of the silicon present on the surface of the negative electrode active material in a reduced-pressure atmosphere, Following the step of oxidation in a reduced-pressure atmosphere, the process involves depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas. A method for producing a negative electrode active material, characterized by containing the following:

[12] : The method for producing the negative electrode active material according to

[11] , characterized in that the step of depositing the silicon oxide is such that the monosilane gas forms silicon via silylene by a monomolecule thermal decomposition reaction in the gas phase.

[0227] It should be noted that the present invention is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and achieves similar effects is included within the technical scope of the present invention. [Explanation of Symbols]

[0228] 10...Negative electrode, 11...Negative electrode current collector, 12...Negative electrode active material layer, 30...Lithium-ion secondary battery (laminated film type), 31...Electrode body, 32... Positive lead (positive aluminum lead), 33... Negative electrode lead (negative electrode nickel lead), 34...Adhesive film, 35...Exterior components. S1, S2, S3, S4, S5... Steps.

Claims

1. A negative electrode active material having negative electrode active material particles, The negative electrode active material particles include a porous carbon structure, Amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure. At least a portion of the surface of the negative electrode active material includes a carbon-based coating layer made of a carbon-based material different from the porous carbon structure, The low-valence nanosilicon oxide has a Si-C bond between the porous carbon structure and at least one of the carbon-based coating layer. The porous carbon structure is found in the spectrum obtained from infrared spectroscopy at 1590 cm⁻¹. －１ It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. －１ The infrared light transmittance is 99.5% or less. The aforementioned low-valence nanosilicon oxide is in a composite state of zero, one, two, and three valencies. The negative electrode active material is characterized in that the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction of the negative electrode active material particles, is in the range of 0.8 nm to 5 nm.

2. The porous carbon structure is 1590 cm －１ The negative electrode active material according to claim 1, characterized in that its infrared light transmittance is 99.0% or less.

3. The porous carbon structure is found in the 1100 cm⁻¹ spectrum obtained from Raman spectroscopy. －１ More than 1200cm －１ The negative electrode active material according to claim 1, characterized in that it has peaks within the following range.

4. The negative electrode active material according to claim 3, characterized in that the peaks in the spectrum obtained from the Raman spectroscopy analysis are peaks originating from C=O bonds or C-O bonds, and the porous carbon structure contains at least one of C=O bonds or C-O bonds.

5. In the spectrum obtained from Raman spectroscopy, The porous carbon structure has a G / D ratio, which is the ratio of peaks originating from the G band to peaks originating from the D band, of less than 1.0, and a 2D / D ratio, which is the ratio of peaks originating from the 2D band to peaks originating from the D band, of less than 0.

1. The negative electrode active material according to claim 1, characterized in that the negative electrode active material particles have a G / D ratio of 1.0 or more.

6. The negative electrode active material according to claim 1, characterized in that active sites exist inside the pores of the porous carbon structure, and a portion of the active sites are modified with OH groups or COOH groups.

7. The negative electrode active material according to claim 1, characterized in that the low-valence nanosilicon oxide is amorphous.

8. The negative electrode active material according to claim 1, characterized in that the total amount of oxygen contained in the negative electrode active material is in the range of 0.3 wt% or more and 3 wt% or less.

9. A method for producing a negative electrode active material having negative electrode active material particles, The spectrum obtained from infrared spectroscopy is at 1590 cm⁻¹. －１ It has an infrared absorption peak nearby, at 1590 cm⁻¹, which is due to absorption by the aromatic kernel. －１ The steps include: preparing a porous carbon structure in which the transmittance of infrared light is 99.5% or less; The steps include: providing COOH groups, OH groups, and CxHy groups to the active sites of the porous carbon structure; A step of flowing monosilane gas to deposit silicon oxide bonded to oxygen atoms inside the porous carbon structure, wherein the silicon oxide is in a state containing dangling bonds, Following the step of depositing the silicon oxide, the step of oxidizing at least a portion of the silicon present on the surface of the negative electrode active material in a reduced-pressure atmosphere, Following the step of oxidation in a reduced-pressure atmosphere, the process involves depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas. By including, The negative electrode active material having the negative electrode active material particles, The negative electrode active material particles include the porous carbon structure, Amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure. At least a portion of the surface of the negative electrode active material includes the carbon-based coating layer made of a carbon-based material different from the porous carbon structure, The low-valence nanosilicon oxide has a Si-C bond between the porous carbon structure and at least one of the carbon-based coating layer. The porous carbon structure has an infrared absorption peak near 1590 cm⁻¹ in the spectrum obtained from the infrared spectroscopic analysis, which is attributed to the absorption of an aromatic nucleus, and the transmittance of infrared light at 1590 cm⁻¹ is 99.5% or less. The aforementioned low-valence nanosilicon oxide is in a composite state of zero, one, two, and three valencies. A method for producing a negative electrode active material, characterized in that the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using Scherrer's formula from peaks measured by X-ray diffraction of the negative electrode active material particles, is in the range of 0.8 nm to 5 nm.

10. The method for producing a negative electrode active material according to claim 9, characterized in that the step of depositing the silicon oxide is characterized in that the monosilane gas forms the silicon via silylene by a single-molecule thermal decomposition reaction in the gas phase.

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