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, 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, and the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer.

Description

[Technical Field]

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

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

[0003] Among these, lithium-ion secondary batteries are highly anticipated because they are easy to miniaturize and increase capacity, and they can achieve a higher energy density than lead-acid batteries and nickel-cadmium batteries.

[0004] The lithium-ion secondary battery described above comprises an electrolyte along with a positive electrode, a negative electrode, and a separator, and the negative electrode contains a negative electrode active material that is involved in the charge and discharge reaction.

[0005] While carbon-based active materials are widely used as negative electrode active materials, recent market demands require further improvements in battery capacity. To improve battery capacity, the use of silicon as a negative electrode active material is being considered. This is because the theoretical capacity of silicon (4199 mAh / g) is more than 10 times greater than that of graphite (372 mAh / g), thus promising a significant increase in battery capacity. Development of silicon materials as negative electrode active materials involves not only pure silicon but also compounds such as alloys and oxides. Furthermore, the shape of the active material is being considered, ranging from the standard coated type for carbon-based active materials to an integrated type 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 material of Si 0+ ~Si 4+ and has various oxidation states (Non-Patent Document 2). Also, 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 that contribute to charge and discharge among silicon oxides having disproportionated structures (Non-Patent Document 4), and Yamada et al. have proposed the following reaction formula between silicon oxide and Li (Non-Patent Document 5). 2SiO(Si + SiO2) + 6.85Li + + 6.85e - → 1.4Li 3.75 Si + 0.4Li4SiO4+ 0.2SiO2 In the reaction formula, Si and SiO2 that constitute the silicon oxide react with Li and are divided into Li silicide, Li silicate, and a part of unreacted SiO2.

[0013] The Li silicate generated here is generally said to be irreversible and a stable substance that does not release Li after being formed once. The capacity per mass calculated from this reaction formula has a value close to the experimental value and is recognized as the reaction mechanism of silicon oxide. Kim et al. identified the irreversible component associated with the charge and discharge of silicon oxide, Li silicate as Li4SiO4, 7 using Li-MAS-NMR and 29 Si-MAS-NMR (Non-Patent Document 6).

[0014] This irreversible capacity is the most disadvantageous part of silicon oxide and improvement is required. Therefore, Kim et al. have significantly improved the initial efficiency as a battery using the Li pre-doping method for forming Li silicate in advance and have produced a negative electrode that can withstand actual use (Non-Patent Document 7). In addition, a method of treating powder instead of doping the electrode with Li has been proposed to achieve improvement of the irreversible capacity (Patent Document 13).

[0015] On the other hand, the Li metal used for Li doping has extremely large upper and lower limits of price depending on the market situation, and there are many problems when considered as industrialization. Therefore, by using silane gas in porous carbon and generating nano-silicon inside, CVD-Si-C can achieve a higher energy density than Li-doped SiO (Patent Documents 14 and 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 Initiative] [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, and the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer.

[0028] The negative electrode active material of the present invention 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 (a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer), 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, 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. Moreover, the Si-C bonds can significantly improve water resistance. As a result, it is possible to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0029] Furthermore, the low-valent nanosilicon oxide is 29 In the spectrum obtained by Si-MAS-NMR measurement, it is preferable that the peak height (C+) of the first Si-C bond region and the peak height (C++) of the second Si-C bond region satisfy the relationship (C+) > (C++).

[0030] The first Si-C bond with the porous carbon structure is established throughout the material, while the second Si-C bond with the carbon-based coating layer is formed relatively close to the surface, mainly due to gas supplied from an external source (e.g., hydrocarbon gas). In this case, if the relationship (C+) > (C++) is satisfied (i.e., the region of the first Si-C bond with the porous carbon structure established throughout the material is larger than the region of the second Si-C bond with the carbon-based coating layer formed closer to the surface), then the optimal bond balance in terms of Li acceptance is achieved. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0031] Furthermore, it is preferable that the negative electrode active material of the present invention has acidic groups inside the pores of the porous carbon structure.

[0032] If a material has such acidic groups, these acidic groups can react with the gas used to deposit silicon oxides containing low-valence nanosilicon oxides (e.g., silane gas), and some of them can be transformed into compounds having Si-C bonds. As a result, Si-C bonds (the first Si-C bonds mentioned above) with the porous carbon structure are reliably established throughout. Consequently, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0033] Furthermore, it is preferable that the negative electrode active material of the present invention contains a polydimethylsiloxane structure.

[0034] A material containing such a polydimethylsiloxane structure is a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0035] Furthermore, it is preferable that the low-valence nanosilicon oxide of the negative electrode active material of the present invention has hydrogen bonds.

[0036] Low-valence nanosilicon oxides with such hydrogen bonds are suitable negative electrode active materials that can improve cycle characteristics while maintaining high battery capacity.

[0037] Furthermore, the low-valent nanosilicon oxide is 29 In the spectrum obtained by Si-MAS-NMR measurement, it is preferable that the peak heights are (A) in the highly regular region, (B) in the amorphous region, and (C) in the Si-C bond region, and that (A) / (B) is in the range of 0.85 to 1.08.

[0038] If a material possesses these characteristics, the ratio of highly ordered regions to amorphous regions is appropriate, resulting in a stable structure, easier reaction control, and significantly improved Li diffusion. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0039] Furthermore, the low-valent nanosilicon oxide is29 In the spectrum obtained by Si-MAS-NMR measurement, it is preferable that the spectrum has peak heights in the highly regular region (A), the amorphous region (B), and the Si-C bond region (C), and that the relationship (B) ≥ (C) is satisfied.

[0040] If a material possesses these characteristics, the ratio of amorphous regions to Si-C bonded regions is appropriate. This prevents the decrease in battery capacity that can occur if there is too much Si-C bonded region. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0041] Furthermore, it is preferable that the low-valent nanosilicon oxide is substantially in a composite state of 0, 1, 2, and 3 valent states.

[0042] 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.

[0043] 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.

[0044] A material having a grain size of zero-valent Si, which is essentially an amorphous structure, is preferred. If the grain size is too large (if the amount of zero-valent Si increases and crystallization progresses too much), the true density becomes too high, which is a factor in the deterioration of cycle characteristics, but this can be prevented. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining a high battery capacity.

[0045] Furthermore, it is preferable that the zero-valent Si is substantially amorphous.

[0046] Such substantially amorphous materials are preferred.

[0047] 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.

[0048] 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.

[0049] 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, comprising the steps of: preparing a porous carbon structure; including COOH groups, OH groups, and CxHy groups in the active sites of the porous carbon structure; and, under heating, flowing monosilane gas while carbon monoxide gas or carbon dioxide gas originating from the oxygen atoms of the active sites is contained in the exhaust gas, thereby depositing silicon oxide bonded to the oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds, and methylsiloxane bonds are imparted via acidic groups, and inside the porous carbon structure The present invention provides a method for producing a negative electrode active material, comprising the steps of: including amorphous low-valence nanosilicon oxide dispersed in a portion thereof, and generating a first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure; after the step of depositing the silicon oxide, oxidizing at least a portion of the silicon present on the surface layer of the porous carbon structure in a reduced-pressure atmosphere; and after the step of oxidation in a reduced-pressure atmosphere, depositing a carbon-based coating layer at 530-600 degrees Celsius using a hydrocarbon gas, thereby generating a second Si-C bond between the low-valence nanosilicon oxide and the carbon-based coating layer.

[0050] In the present invention's method for producing the negative electrode active material, first, since the deposited silicon oxide includes low-valence nanosilicon oxide, the generation of ionic substances can be reduced, and the diffusivity of Li can be enhanced. Furthermore, the active sites of the porous carbon structure contain COOH groups (carboxyl groups), which are typical acidic groups. These COOH groups can react with monosilane gas to partially transform not only into composite oxides but also into compounds having Si-C bonds. Therefore, the first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure can be reliably formed. Moreover, by depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas, the second Si-C bond between the low-valence nanosilicon oxide and the carbon-based coating layer can be reliably formed. In this way, since the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer, the internal Si-C bond has a strong bond with the Si in the surface layer, which suppresses the electronic disconnection of the Si portion during charging and discharging, and stabilizes the structure. Therefore, it is electrochemically stable during charging and discharging, and decomposition during charging and discharging is unlikely to occur. Furthermore, the Si-C bond can significantly improve water resistance. As a result, it is possible to provide a method for manufacturing a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0051] 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.

[0052] 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]

[0053] The negative electrode 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, since the low-valence nanosilicon oxide has Si-C bonds (a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer), the internal Si-C bonds have a strong bond with the Si in the surface layer, which suppresses the electronic disconnection of the Si 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. Moreover, the Si-C bonds can significantly improve water resistance. As a result, it is possible to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0054] According to the method for producing the negative electrode active material of the present invention, first, since the silicon oxide to be deposited contains low-valence nanosilicon oxide, the generation of ionic substances can be reduced, and the diffusivity of Li can be increased. In addition, the active sites of the porous carbon structure contain COOH groups (carboxyl groups), which are typical acidic groups. These COOH groups can react with monosilane gas to partially change into compounds having Si-C bonds, not just composite oxides. Therefore, the first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure can be reliably formed. Furthermore, by depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas, the second Si-C bond between the low-valence nanosilicon oxide and the carbon-based coating layer can be reliably formed. In this way, since the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer, the internal Si-C bond has 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. Therefore, it is electrochemically stable during charging and discharging, and decomposition during charging and discharging is unlikely to occur. Furthermore, the Si-C bond can significantly improve water resistance. As a result, it is possible to provide a method for manufacturing a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. [Brief explanation of the drawing]

[0055] [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 29Si-MAS-NMR spectrum of Example 2. [Figure 5] This is the 29Si-MAS-NMR spectrum of Example 3. [Figure 6] This is the 29Si-MAS-NMR spectrum of Example 4. [Figure 7] This is the 29Si-MAS-NMR spectrum of Example 5. [Figure 8] This is the 29Si-MAS-NMR spectrum of Comparative Example 1. [Figure 9] This is the 29Si-MAS-NMR spectrum of Comparative Example 2. [Figure 10] These are the 1H-MAS-NMR spectra of Examples 2 and 6. [Modes for carrying out the invention]

[0056] The following describes embodiments of the present invention, but the present invention is not limited thereto.

[0057] 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.

[0058] 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.

[0059] In particular, we discovered that low-valence nanosilicon oxides, by having a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer, can improve cycle characteristics while maintaining high battery capacity, thus completing the present invention.

[0060] 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, 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, and the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer.

[0061] Furthermore, the present invention relates to a method for producing a negative electrode active material having negative electrode active material particles, comprising the steps of: preparing a porous carbon structure; incorporating COOH groups, OH groups, and CxHy groups into the active sites of the porous carbon structure; and, under heating, flowing monosilane gas while carbon monoxide gas or carbon dioxide gas originating from the oxygen atoms of the active sites is present in the exhaust gas, thereby depositing silicon oxide bonded to the oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds, is given methylsiloxane bonds via acidic groups, and is dispersed inside the porous carbon structure. The method for producing a negative electrode active material is characterized by comprising the steps of: including amorphous low-valence nanosilicon oxide, generating a first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure; after the step of depositing the silicon oxide, oxidizing at least a portion of the silicon present on the surface layer of the porous carbon structure in a reduced-pressure atmosphere; and after the step of oxidation in a reduced-pressure atmosphere, depositing a carbon-based coating layer at 530-600 degrees Celsius using a hydrocarbon gas, thereby generating a second Si-C bond between the low-valence nanosilicon oxide and the carbon-based coating layer.

[0062] The following will provide a detailed explanation with reference to the drawings.

[0063] 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.

[0064] [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, and the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer.

[0065] The negative electrode active material of the present invention 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 (a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer), 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, 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. Moreover, the Si-C bonds can significantly improve water resistance. As a result, it is possible to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0066] Furthermore, although not specifically limited, low-valent nanosilicon oxides are 29 In the spectrum obtained by Si-MAS-NMR measurement, it is preferable that the peak height (C+) of the first Si-C bond region and the peak height (C++) of the second Si-C bond region satisfy the relationship (C+) > (C++).

[0067] The first Si-C bond with the porous carbon structure is established throughout the material, while the second Si-C bond with the carbon-based coating layer is formed relatively close to the surface, mainly due to gas supplied from an external source (e.g., hydrocarbon gas). In this case, if the relationship (C+) > (C++) is satisfied (i.e., the region of the first Si-C bond with the porous carbon structure established throughout the material is larger than the region of the second Si-C bond with the carbon-based coating layer formed closer to the surface), then the optimal bond balance in terms of Li acceptance is achieved. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0068] 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℃

[0069] Furthermore, although not particularly limited, it is preferable that the porous carbon structure has acidic groups inside its pores.

[0070] If a material has such acidic groups, these acidic groups can react with the gas used to deposit silicon oxides containing low-valence nanosilicon oxides (e.g., silane gas), and some of them can be transformed into compounds having Si-C bonds. As a result, Si-C bonds (the first Si-C bonds mentioned above) with the porous carbon structure are reliably established throughout. Consequently, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0071] Here, the method for incorporating acidic groups involves heat-treating the resin and coconut shell at 600°C for 1 hour, carbonizing them with nitrogen flow, and then performing an alkaline activation treatment using KOH. In particular, the COOH group is formed by neutralizing with HCl to remove the K component, neutralizing to pH 7, washing with pure water, and then washing with an aqueous solution containing 1% of either formic acid, acetic acid, or oxalic acid. The OH group is formed by applying heat at 45°C during the washing with pure water. Furthermore, the CxHy group is formed by heat-treating at 600°C, with 50 minutes of nitrogen flow and the remaining 10 minutes of heat treatment in a sealed state.

[0072] Furthermore, acidic groups can be observed, for example, in the secondary ions produced by TOF-SIMS.

[0073] Furthermore, although not particularly limited, it is preferable that it contains a polydimethylsiloxane structure.

[0074] A material containing such a polydimethylsiloxane structure is a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0075] Furthermore, although not particularly limited, it is preferable that the low-valent nanosilicon oxide has hydrogen bonds.

[0076] Low-valence nanosilicon oxides with such hydrogen bonds are suitable negative electrode active materials that can improve cycle characteristics while maintaining high battery capacity.

[0077] Here, regarding hydrogen bonds, for example, 1 It can be observed using the H-MAS-NMR spectrum.

[0078] Furthermore, although not specifically limited, low-valent nanosilicon oxides are 29In the spectrum obtained by Si-MAS-NMR measurement, the peak heights are (A) in the highly regular region, (B) in the amorphous region, and (C) in the Si-C bond region, and (A) / (B) is preferably in the range of 0.8 to 1.2, and particularly preferably in the range of 0.85 to 1.08. However, it is not limited to this range.

[0079] If the above (A) / (B) has properties in the range of 0.85 to 1.08, the ratio of the highly regular region to the amorphous region is appropriate, the structure is stabilized, reaction control becomes easier, and the diffusivity of Li can be greatly improved. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0080] At this time, analysis of the spectrum obtained by the XAFS method using the Feff method revealed a coordination number in the range of 2. This suggests that the amorphous region is in a lower coordination number state, which significantly improves the diffusivity of Li.

[0081] Furthermore, if there are too many low coordination number states, the structure becomes unstable and active, making it difficult to control subsequent oxidation and hydrocarbon gas reactions. Therefore, we believe that a balance between the highly regular region and the amorphous region (low coordination number region) is important.

[0082] Furthermore, although not specifically limited, low-valent nanosilicon oxides are 29 In the spectrum obtained by Si-MAS-NMR measurement, it is preferable that the spectrum has peak heights in the highly regular region (A), the amorphous region (B), and the Si-C bond region (C), and that the relationship (B) ≥ (C) is satisfied.

[0083] If a material possesses these characteristics, the ratio of amorphous regions to Si-C bonded regions is appropriate. This prevents the decrease in battery capacity that can occur if there is too much Si-C bonded region. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0084] 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.

[0085] 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.

[0086] The valency of low-valency nanosilicon oxides can be quantified using the aforementioned NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0087] 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.

[0088] 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.

[0089] A material having a grain size of zero-valent Si, which is essentially an amorphous structure, is preferred. If the grain size is too large (if the amount of zero-valent Si increases and crystallization progresses too much), the true density becomes too high, which is a factor in the deterioration of cycle characteristics, but this can be prevented. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining a high battery capacity.

[0090] 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.

[0091] 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".

[0092] Furthermore, although not particularly limited, it is preferable that the zero-valent Si is substantially amorphous.

[0093] Such substantially amorphous materials are preferred.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] Furthermore, porous carbon can be derived from bio-based, resin-based, or petroleum-based sources, with bio-based or resin-based sources being preferred.

[0098] <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 "negative electrode") containing the negative electrode active material of the present invention will be described.

[0099] [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.

[0100] [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).

[0101] 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.

[0102] 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.

[0103] [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.

[0104] [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.

[0105] First, we will explain the manufacturing method for the negative electrode active material contained in the negative electrode.

[0106] The present invention relates to a method for producing a negative electrode active material having negative electrode active material particles, comprising the steps of: preparing a porous carbon structure; incorporating COOH groups, OH groups, and CxHy groups into the active sites of the porous carbon structure; and, under heating, flowing monosilane gas while carbon monoxide gas or carbon dioxide gas originating from oxygen atoms at the active sites is present in the exhaust gas, thereby depositing silicon oxide bonded to oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds, and methylsiloxane bonds are imparted via acidic groups, inside the porous carbon structure The method for producing a negative electrode active material is characterized by comprising the steps of: including amorphous low-valence nanosilicon oxide dispersed in a porous carbon structure to form a first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure; after depositing the silicon oxide, oxidizing at least a portion of the silicon present on the surface of the porous carbon structure in a reduced-pressure atmosphere; and after the oxidation in a reduced-pressure atmosphere, depositing a carbon-based coating layer at 530-600 degrees Celsius using a hydrocarbon gas to form a second Si-C bond between the low-valence nanosilicon oxide and the carbon-based coating layer.

[0107] In the present invention's method for producing the negative electrode active material, first, since the deposited silicon oxide includes low-valence nanosilicon oxide, the generation of ionic substances can be reduced, and the diffusivity of Li can be enhanced. Furthermore, the active sites of the porous carbon structure contain COOH groups (carboxyl groups), which are typical acidic groups. These COOH groups can react with monosilane gas to partially transform not only into composite oxides but also into compounds having Si-C bonds. Therefore, the first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure can be reliably formed. Moreover, by depositing a carbon-based coating layer at 530-600 degrees Celsius using hydrocarbon gas, the second Si-C bond between the low-valence nanosilicon oxide and the carbon-based coating layer can be reliably formed. In this way, since the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer, the internal Si-C bond has a strong bond with the Si in the surface layer, which suppresses the electronic disconnection of the Si portion during charging and discharging, and stabilizes the structure. Therefore, it is electrochemically stable during charging and discharging, and decomposition during charging and discharging is unlikely to occur. Furthermore, the Si-C bond can significantly improve water resistance. As a result, it is possible to provide a method for manufacturing a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] Furthermore, the substance obtained in the oxidation process is SiO2, so Si 0+ and Si 4+ The composite material is formed within porous carbon.

[0114] 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).

[0115] 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.

[0116] 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.

[0117] Furthermore, low-valent nanosilicon oxides deposited on porous carbon structures exhibit high Li diffusivity. Generally, compounds of Si and Li are Li 15 While 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] 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.

[0125] 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.

[0126] Furthermore, increasing the pressure during silicon oxide deposition and reaction makes it possible to create a larger number of dangling bonds.

[0127] As described above, it is preferable to use a pressurized atmosphere during silicon oxide deposition. However, it is not the only option.

[0128] 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.)

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] Si-C bonds are 29In Si-MAS-NMR measurements, it is desirable that the peak height of the first Si-C bond region (C+) originating from the porous carbon structure and the peak height of the second Si-C bond region (C++) of the carbon-based coating layer (derived from hydrocarbon gas) be (C+) > (C++).

[0134] Si-C bonds derived from hydrocarbon gases are formed relatively close to the surface, whereas Si-C bonds derived from porous carbon are established throughout the entire structure. If (C+) < (C++), the surface Si portion becomes covered with Si-C bonds, worsening the acceptance of Li. Therefore, the optimal bond balance is (C+) > (C++), and achieving this relationship improves the acceptance of Li.

[0135] While many Si-C compounds have a polydimethylsiloxane structure, some also possess Si-H bonds. Although it has been reported that Si-H bonds detach from around 450°C (Non-Patent Literature 11), the invention is not limited to this temperature. In the embodiments of the present invention, it is preferable that the silane decomposition reaction is carried out at a temperature lower than that, so that a polydimethylsiloxane structure is formed with a certain amount of Si-H bonds remaining.

[0136] On the other hand, if the reaction temperature is increased, H is released, and a Si dangling bond is formed. If even higher temperatures are applied, for example around 600°C, the Si dangling bond changes to a Si-Si bond, becoming stable, but Si grains grow, and Li diffusion deteriorates. Also, the acidic group is released, making it difficult to form a polydimethylsiloxane structure.

[0137] If the reaction proceeds in the range of 400-600°C, with H detaching from Si-H, the formed Si dangling bond is active. When the adjacent Si-H is in a healthy state, the Si dangling bond cannot bond with the adjacent Si, so it actively decomposes the hydrocarbon gas, making it easier for crystalline SiC to form.

[0138] For the reasons stated above, although not particularly limited, it is preferable to form the polydimethylsiloxane structure in a temperature range where Si-H bonds are less likely to dissociate (for example, below 400°C), in which case some Si-H bonds will remain inside the bulk material.

[0139] Referring to Figure 3, an example of each step in the method for producing the negative electrode active material will be explained.

[0140] The present invention provides a method for producing a negative electrode active material, comprising the steps of: preparing a porous carbon structure (step S1); incorporating COOH groups, OH groups, and CxHy groups into the active sites of the porous carbon structure (step S2); and, under heating, flowing monosilane gas while carbon monoxide gas or carbon dioxide gas originating from oxygen atoms at the active sites is present in the exhaust gas, thereby depositing silicon oxide bonded to oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds, and methylsiloxane bonds are imparted via acidic groups, and the silicon oxide is dispersed inside the porous carbon structure. The process includes: a step (S3) of including a morphous low-valence nanosilicon oxide and generating a first Si-C bond between the low-valence nanosilicon oxide and a porous carbon structure; a step (S4) of oxidizing at least a portion of the silicon present on the surface of the porous carbon structure in a reduced-pressure atmosphere after the silicon oxide deposition step; and a step (S5) of depositing a carbon-based coating layer at 530-600 degrees Celsius using a hydrocarbon gas after the oxidation step in a reduced-pressure atmosphere, thereby generating a second Si-C bond between the low-valence nanosilicon oxide and the carbon-based coating layer.

[0141] (Step S1) First, there is the step of preparing a porous carbon structure.

[0142] The porous carbon structure is not particularly limited, but it is preferable to prepare it in one of the following two ways, for example.

[0143] (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.

[0144] (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.

[0145] (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.

[0146] 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.

[0147] (Step S3) The step involves flowing monosilane gas through a heated exhaust gas containing carbon monoxide or carbon dioxide gas originating from oxygen atoms at the active sites, thereby depositing silicon oxide bonded to oxygen atoms inside a porous carbon structure. The silicon oxide is provided with dangling bonds, to which methylsiloxane bonds are imparted via acidic groups, and to contain amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, thereby generating a first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure.

[0148] 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%).

[0149] 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.

[0150] (Step S4) This step involves oxidizing at least a portion of the silicon present on the surface of a porous carbon structure in a reduced-pressure atmosphere.

[0151] 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).

[0152] 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.

[0153] 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.

[0154] (Step S5) The step involves depositing a carbon-based coating layer at 530-600 degrees Celsius using a hydrocarbon gas, thereby generating a second Si-C bond between the low-valent nanosilicon oxide and the carbon-based coating layer.

[0155] The material is heated to an internal temperature of 530-600 degrees Celsius 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 degrees Celsius.)

[0156] At this time, the reaction is carried out under reduced pressure to allow acetylene gas to permeate the Si filling the pores. By reacting with the Si-H bonds that did not reach a low-valent oxidation state, Si-C bonds are obtained, resulting in a stable material.

[0157] 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.

[0158] 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.

[0159] 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 image using Feff (a theoretical calculation software for XAFS based on multiple scattering theory).

[0160] 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 airless environment under an Ar atmosphere, 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

[0161] Furthermore, dimethylsiloxane can be evaluated by observing the positive and negative secondary ions in TOF-SIMS. Specifically, 73 SiC3H9+, 147 Si2OC5H 15 +, 207 Si3O3C5H 15 + These signals are detected. This allows us to demonstrate that a portion of the Si-C bond is a structure related to the negative electrode active material of the present invention.

[0162] As described above, by steps S1 to S5, a negative electrode active material having negative electrode active material particles can be manufactured, 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, and the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer.

[0163] <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.

[0164] [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.

[0165] 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.

[0166] 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.

[0167] 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.

[0168] 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.

[0169] The positive electrode current collector is formed from a conductive material such as aluminum.

[0170] 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.

[0171] 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.

[0172] 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.

[0173] 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.

[0174] [Negative electrode] The negative electrode has the same configuration as the negative electrode 10 for a lithium ion secondary battery shown 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.

[0175] 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 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 the opposing positive electrode active material layer does not exist. This is for performing a stable battery design.

[0176] 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 investigated with good reproducibility without depending on the presence or absence of charge and discharge.

[0177] [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.

[0178] [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.

[0179] 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.

[0180] 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).

[0181] 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.

[0182] 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.

[0183] 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.

[0184] 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.

[0185] 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.

[0186] 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).

[0187] 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]

[0188] The present invention will be described more specifically below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0189] 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.

[0190] 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.

[0191] (Comparative Example 1) [Step S1] Preparation of porous carbon structure In the cleaning process for the porous carbon, only hydrochloric acid was used for acid treatment, resulting in a pH of 7. The carbon was then washed with water three times and dried overnight at 60 degrees Celsius. [Step S2] None [Step S3] Deposition of silicon oxide In the drying process prior to the silane reaction, after confirming that the sample temperature was 370°C while CO gas was not detected (below the detection limit), silane was introduced to form silicon oxide inside the pores. NMR confirmed that by introducing silane gas and carrying out the reaction in the absence of acidic groups, Si-C bonds derived from porous carbon were not formed. [Step S4] Oxidation For the oxidation process during extraction, a method was used in which oxidation occurred after nitrogen adsorption. [Step S5] None

[0192] [Measurement of negative electrode active material] The negative electrode active material prepared as described above was subjected to TEM-EDX, XRD analysis, Raman spectroscopy, and other analyses.

[0193] [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.

[0194] 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.

[0195] [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.

[0196] 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.

[0197] 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.

[0198] [Initial Efficiency Measurement] 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.

[0199] 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.

[0200] [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.

[0201] 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.

[0202] 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).

[0203] The results of each measurement are shown in Table 1. Table 1 also includes the results for Comparative Example 2 and Examples 1 to 17, which are described later.

[0204] [Table 1]

[0205] Comparative Example 1 showed battery capacity and cycle characteristics similar to conventional batteries, and lithium deposition occurred during 4C charging. These less-than-ideal results are thought to be due to the absence of Si-C bonds in the silicon oxide.

[0206] (Comparative Example 2) [Step S1] Preparation of porous carbon structure The porous carbon was cleaned using an acidic solution containing oxalic acid. It was then washed once with water and dried overnight at 60 degrees Celsius. [Step S2] None [Step S3] Deposition of silicon oxide During the drying process prior to the silane reaction, the product temperature was confirmed to be 350 degrees Celsius when 8500 ppm of CO was detected in the exhaust gas, and then silane gas was introduced and the reaction was carried out. [Step S4] Oxidation The same method as in Comparative Example 1 was used for nitrogen adsorption and oxidation. [Step S5] None

[0207] In Comparative Example 2, the phase near the particle surface was only oxidized. Furthermore, the oxygen layer had poorer Li-acceptance than the Si-C bond, resulting in somewhat unfavorable 4C charging performance. However, no Li precipitation occurred. While this represents a significant improvement compared to Comparative Example 1, this is thought to be due to the presence of amorphous, low-valence nanosilicon oxide dispersed within the porous carbon structure, and the existence of Si-C bonds between the low-valence nanosilicon oxide and the porous carbon structure. Furthermore, while the poor lithium acceptance was reflected in the cycle characteristics, it was within an acceptable range.

[0208] (Example 1) Unless otherwise described, the methods described above for the negative electrode active material and negative electrode are followed. [Step S1] Preparation of porous carbon structure 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. [Step S2] COOH groups, OH groups, and CxHy groups were added to the active sites of the porous carbon structure. [Step S3] Deposition of silicon oxide In the drying process prior to the silane reaction, silane gas was introduced when the CO gas concentration detected in the exhaust gas was 8500 ppm. At this time, the reaction temperature was lowered to 355 degrees Celsius to achieve a more amorphous state. The average pressure was 50 kPaG. [Step S4] Oxidation The material temperature is first lowered to room temperature, then the atmosphere is reduced to low pressure, and nitrogen is introduced to form a nitrogen layer on the exposed surface, including the inside of the pores. Next, oxygen diluted with nitrogen is introduced, increasing the weight of oxygen by approximately 0.3 wt% (the moisture content of the introduced air is 100 ppm). Including the oxygen derived from the O component adsorbed inside the porous carbon structure, the total amount of oxygen in the negative electrode active material becomes 1 wt%. [Step S5] Deposition of carbon-based coating layer After the oxidation treatment was completed, acetylene gas was used for 9 hours to form Si-C bonds, mainly in the phase near the surface.

[0209] As can be seen in Table 1, Example 1 shows improved Li acceptance, significantly improved fast charging, and improved cycle maintenance. This is thought to be because the silicon oxide contains amorphous low-valence nanosilicon oxide dispersed within the porous carbon structure, and the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer.

[0210] (Example 2) The only difference from Example 1 is step S3. In step S3, during the drying process before the silane reaction, silane gas was introduced when the CO gas concentration detected from the exhaust gas was 8500 ppm, and the reaction was carried out at 370 degrees Celsius, which is higher than in Example 1.

[0211] Table 1 shows that the regularity of the silicon is higher compared to Example 1 (the ratio of the peak height A in the highly regular region to the peak height B in the amorphous region, A / B, is higher), but productivity can also be ensured due to the higher temperature. Specifically, a decomposition rate more than three times higher than in Example 1 was obtained.

[0212] (Example 3) The only difference from Examples 1 and 2 is step S3. To increase the regularity, the reaction was carried out at 380 degrees.

[0213] As a result, the decomposition rate became 1.3 times higher than that of Example 2. However, when decomposing at a temperature higher than this, the adsorption thermal decomposition mode also begins to coexist. When the adsorption thermal decomposition mode occurs, Si begins to precipitate on the surface of the porous carbon, which may deteriorate the battery characteristics. Therefore, it is preferable to keep the temperature below 380 degrees in Example 3.

[0214] (Example 4) The difference from Example 1 is only in step S3. The reaction was carried out with silane gas in a state where the CO gas concentration was detected at 10000 ppm.

[0215] As a result, due to the fact that the ratio of the Si-C bond and the amorphous Si part was close (the peak height B of the amorphous region and the peak height C of the Si-C bond region were of the same degree: B = C), the charge-discharge characteristics of the battery were significantly improved. However, since the Si-C bond increased, the capacity decreased slightly.

[0216] (Example 5) The differences from Example 1 are in steps S3 and S5, and the difference from Example 4 is only in step S5. In addition to reacting silane gas in a state where the CO gas concentration was detected at 10000 ppm in step S3, in the Si-C bond reaction using acetylene gas in step S5, by performing heat treatment for 14 hours, the Si-C bond of a phase closer to the surface layer was increased (at this time C > B).

[0217] As a result, the battery characteristics were improved, but the capacity decreased compared to Example 4.

[0218] (Example 6) The difference from Example 1 is that heat treatment was performed at 600 degrees for 2 hours before step S5 to reduce the Si-H bond. The Si-H bond can be confirmed by NMR.

[0219] In FIG. 10 1Referring to the H-MAS-NMR spectrum, the Si-H bond peak in Example 6 is smaller compared to Example 2, but it does not disappear completely. However, if heat treatment is performed at an even higher temperature than in Example 6, the Si-H bond will disappear completely, but in that case, it may lead to unwanted Si crystal growth and degrade the battery performance.

[0220] Furthermore, while the presence of Si-H bonds improves Li diffusion, it worsens the handling properties of the slurry. A storage life of one week is preferred for the slurry. In the state of Example 2, it is 48 hours, but reducing it to the state of Example 6 ensures one week, which is preferable.

[0221] (Example 7) The only difference from Example 1 is step S3. During the silane decomposition reaction, the average pressure was set to 80 kPaG.

[0222] As a result, we confirmed that while density improved and the amount of highly regular Si increased (A / B ratio became larger), Li acceptance decreased compared to Example 1 (cycle characteristics decreased).

[0223] (Example 8) The only difference from Example 1 is step S4. Regarding oxidation, the moisture content of the introduced air was changed to 500 ppm. In the presence of moisture, hydrogen is released from the back bond of the Si-H bond, resulting in the formation of an SiO2 bond.

[0224] We confirmed that the formation of SiO2 bonds makes it difficult to form Si-C bonds, thus degrading battery performance.

[0225] (Examples 9-11) The difference from Example 1 is that before step S5, heat treatment was performed at 620°C, 640°C, and 660°C for 2 hours to promote Si crystal growth.

[0226] As crystal growth progressed, the grain size of zero-valent Si increased, leading to a decrease in battery performance. Furthermore, in Example 11, the Si-H bond disappeared.

[0227] (Examples 12-16) The only difference from Example 1 is step S4. The total amount of oxygen contained in the negative electrode active material was changed.

[0228] When the oxygen level was low, there was a tendency for more Si-C bonds to be formed during acetylene CVD in step S5, resulting in a slight decrease in battery capacity compared to Example 1. The oxygen level (lower limit) at which the battery capacity did not drop significantly was 0.3 wt% or higher.

[0229] Furthermore, too much oxygen increases irreversible capacity. The maximum oxygen level (upper limit) that does not significantly reduce battery capacity was 3 wt% or less.

[0230] (Example 17) The only difference from Example 1 is step S1. A porous carbon structure is subjected to physical activation treatment using carbon dioxide, resulting in a surface area of ​​1840 m². 2 The prototype was prepared using a sample size of 0.95 cc / g, a pore volume of 0.95 cc / g, and a particle size of 8.2 μm.

[0231] In the case of physical activation, the mesopore size increases slightly, improving lithium acceptance, but the increased reaction area slightly reduces battery cycle characteristics.

[0232] Further details will be provided below using diagrams.

[0233] Figures 4-9 show the results of Examples 2-5 and Comparative Examples 1 and 2. 29 These are spectra obtained by Si-MAS-NMR measurement. In Figure 4, "Si-C," "Amorphous Si," and "Si" indicate the peak heights of the Si-C bond region (C), the amorphous region (B), and the highly regular region (A), respectively. By comparing the figures, the differences between (A) and (B), and between (B) and (C), can be easily confirmed. For reference, the (A) / (B) values ​​are indicated above the figures.

[0234] Particularly, focusing on the peak height (C) of the Si-C bond region, it can be easily understood that in Comparative Example 1 (Fig. 8) there is no Si-C bond, in Comparative Example 2 (Fig. 9) there is only the first Si-C bond with the porous carbon structure, and in Example 5 (Fig. 7) there is an excessive second Si-C bond in the surface layer.

[0235] In addition, an explanation will be added regarding the peak height (C+) of the first Si-C bond region and the peak height (C++) of the second Si-C bond region. Referring to Figs. 4 to 9, the peak of the Si-C bond region appears at -18 ppm on the horizontal axis of the spectrum obtained by Si-MAS-NMR measurement. Here, since Comparative Example 2 (Fig. 9) does not perform Step S5, there is the first Si-C bond with the porous carbon structure, but there is no second Si-C bond with the carbon-based coating layer. Therefore, the peak height at -18 ppm in Comparative Example 2 (Fig. 9) represents the peak height (C+) of the first Si-C bond region. 29 On the other hand, Examples 2 to 5 (Figs. 4 to 7) perform Step S5, so they have both the first Si-C bond with the porous carbon structure and the second Si-C bond with the carbon-based coating layer. Thus, the peak height at -18 ppm in Examples 2 to 5 (Figs. 4 to 7) is considered to be the sum of the peak height (C+) of the first Si-C bond region and the peak height (C++) of the second Si-C bond region. Therefore, the difference between the peak height at -18 ppm in each of Examples 2 to 5 (Figs. 4 to 7) and the peak height at -18 ppm in Comparative Example 2 (Fig. x) represents the peak height (C++) of each second Si-C bond region. Referring to Table 1, in Examples 2 to 4 (Figs. 4 to 6), (C+) > (C++), and in Example 5 (Fig. 7), (C+) < (C++). Also, considering from Table 1 that Examples 2 to 4 have a higher specific capacity than Example 5, it is preferable that (C+) > (C++).

[0236] This specification includes the following aspects.

[0237] A negative electrode active material having negative electrode active material particles, <000097> [1]: wherein the negative electrode active material particles contain a porous carbon structure, the negative electrode active material particles include a porous carbon structure, Amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure. 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 negative electrode active material is characterized in that the low-valence nanosilicon oxide has a first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer. [2]: The low-valent nanosilicon oxide is 29 The negative electrode active material according to [1] above, characterized in that, in the spectrum obtained by Si-MAS-NMR measurement, the peak height (C+) of the first Si-C bond region and the peak height (C++) of the second Si-C bond region satisfy the relationship (C+) > (C++). [3]: The negative electrode active material according to [1] or [2] above, characterized in that it has acidic groups inside the pores of the porous carbon structure. [4]: Any of the negative electrode active materials described in [1] to [3] above, characterized in that it contains a polydimethylsiloxane structure. [5]: The negative electrode active material according to [1] to [4] above, characterized in that the low-valence nanosilicon oxide has hydrogen bonds. [6]: The low-valent nanosilicon oxide is 29 In the spectrum obtained by Si-MAS-NMR measurement, there are peak heights in the highly regular region (A), the amorphous region (B), and the Si-C bond region (C). The negative electrode active material according to [1] to [5] above, characterized in that (A) / (B) is in the range of 0.85 to 1.08. [7]: The low-valent nanosilicon oxide is 29 In the spectrum obtained by Si-MAS-NMR measurement, there are peak heights in the highly regular region (A), the amorphous region (B), and the Si-C bond region (C). The negative electrode active material according to [1] to [6] above, characterized in that it satisfies the relationship (B)≧(C). [8]: The negative electrode active material according to [1] to [7] above, characterized in that the low-valence nanosilicon oxide is substantially in a composite state of 0, 1, 2, and 3 valencies. [9]: The anode active material according to [1] to [8] above, 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 anode active material particles, is in the range of 0.8 nm to 5 nm.

[10] : The negative electrode active material of [9], characterized in that the zero-valent Si is substantially amorphous.

[11] : The negative electrode active material according to [1] to

[10] above, 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.

[12] : A method for producing a negative electrode active material having negative electrode active material particles, Steps include preparing a porous carbon structure, The steps include: providing COOH groups, OH groups, and CxHy groups to the active sites of the porous carbon structure; The steps include: flowing monosilane gas under heating, with carbon monoxide gas or carbon dioxide gas originating from the oxygen atoms of the active sites present in the exhaust gas, to deposit silicon oxide bonded to the oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds, to which methylsiloxane bonds are imparted via acidic groups, and to contain amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, thereby generating a first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure; The step of depositing the silicon oxide is followed by the step of oxidizing at least a portion of the silicon present on the surface of the porous carbon structure in a reduced-pressure atmosphere. The step of oxidizing in a reduced-pressure atmosphere, followed by the step of depositing a carbon-based coating layer at 530-600 degrees Celsius using a hydrocarbon gas, wherein a second Si-C bond is formed between the low-valence nanosilicon oxide and the carbon-based coating layer. A method for producing a negative electrode active material, characterized by containing the following:

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

[12] , 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.

[0238] 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]

[0239] 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. 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 first Si-C bond with the porous carbon structure and a second Si-C bond with the carbon-based coating layer. 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 aforementioned low-valence nanosilicon oxide is ２９ The negative electrode active material according to claim 1, characterized in that, in the spectrum obtained by Si-MAS-NMR measurement, the peak height (C+) of the first Si-C bond region and the peak height (C++) of the second Si-C bond region satisfy the relationship (C+) > (C++).

3. The negative electrode active material according to claim 1, characterized in that the porous carbon structure has acidic groups inside its pores.

4. The negative electrode active material according to claim 1, characterized in that it contains a polydimethylsiloxane structure.

5. The negative electrode active material according to claim 1, characterized in that the low-valence nanosilicon oxide has hydrogen bonds.

6. The aforementioned low-valence nanosilicon oxide is ２９ In the spectrum obtained by Si-MAS-NMR measurement, there is a peak height in the highly regular region (A), a peak height in the amorphous region (B), and a peak height in the Si-C bond region (C). The negative electrode active material according to claim 1, characterized in that (A) / (B) is in the range of 0.85 to 1.

08.

7. The aforementioned low-valence nanosilicon oxide is ２９ In the spectrum obtained by Si-MAS-NMR measurement, there is a peak height in the highly regular region (A), a peak height in the amorphous region (B), and a peak height in the Si-C bond region (C). The negative electrode active material according to claim 1, characterized in that it satisfies the relationship (B) ≥ (C).

8. The negative electrode active material according to claim 1, characterized in that the zero-valent Si is amorphous.

9. 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.

10. A method for producing a negative electrode active material having negative electrode active material particles, Steps include preparing a porous carbon structure, The steps include: providing COOH groups, OH groups, and CxHy groups to the active sites of the porous carbon structure; The steps include: flowing monosilane gas under heating, with carbon monoxide gas or carbon dioxide gas originating from the oxygen atoms of the active sites present in the exhaust gas, to deposit silicon oxide bonded to the oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds, to which methylsiloxane bonds are imparted via acidic groups, and to contain amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, thereby generating a first Si-C bond between the low-valence nanosilicon oxide and the porous carbon structure; The step of depositing the silicon oxide is followed by the step of oxidizing at least a portion of the silicon present on the surface of the porous carbon structure in a reduced-pressure atmosphere. The step of oxidizing in a reduced-pressure atmosphere, followed by the step of depositing a carbon-based coating layer at 530 to 600 degrees Celsius using a hydrocarbon gas, wherein a second Si-C bond is formed between the low-valence nanosilicon oxide and the carbon-based coating layer. 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. 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 the first Si-C bond with the porous carbon structure and the second Si-C bond with the carbon-based coating layer. 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.

11. The method for producing a negative electrode active material according to claim 10, 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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