Negative electrode active material and method for manufacturing the same

Abstract

This invention provides a negative electrode active material, which is a negative electrode active material having particles. The particles are characterized in that they comprise a porous carbon structure as a matrix, the porous carbon structure having Si-C bonds internally and amorphous low-valent silicon oxide nanoparticles dispersed on its surface, and at least a portion of the low-valent silicon oxide nanoparticles containing tetravalent silicon oxide capable of intercalating and deintercalating Li. Thus, a negative electrode active material is provided that reduces the generation of ionic substances, suppresses excessive electrolyte decomposition, and consequently improves battery cycle characteristics.

Description

Technical Field

[0001] This invention relates to a negative electrode active material and its manufacturing method. Background Technology

[0002] In recent years, small electronic devices, such as mobile terminals, have become widely used, creating a strong demand for further miniaturization, weight reduction, and longer lifespan. In response to this market demand, the development of a particularly small, lightweight, and high-energy-density rechargeable battery has been promoted. The applications of this rechargeable battery are not limited to small electronic devices; its application in large electronic devices such as automobiles and in energy storage systems such as buildings is also under research.

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

[0004] The aforementioned lithium-ion secondary battery has a positive electrode, a negative electrode, and a separator, as well as an electrolyte. The negative electrode contains negative electrode active materials related to the charge and discharge reaction.

[0005] Carbon-based active materials have been widely used as anode active materials, but recent market demands require further increases in battery capacity. To improve battery capacity, silicon is being researched as anode active material. This is because silicon's theoretical capacity (4199 mAh / g) is more than 10 times greater than that of graphite (372 mAh / g), thus a significant increase in battery capacity is expected. In developing silicon materials as anode active materials, not only elemental silicon has been studied, but also alloys and compounds, such as oxides. Furthermore, regarding the shape of the active material, for carbon-based active materials, studies have been conducted on everything from standard coated types to monolithic types directly deposited on the current collector.

[0006] However, if silicon is used as the main raw material for the negative electrode active material, it is prone to cracking, primarily near the surface, due to the expansion and contraction that occurs during charging and discharging. Furthermore, ionic substances are generated within the active material, making it more susceptible to breakage. If the surface of the negative electrode active material cracks, a new surface (also called a new nascent surface) is created, increasing the reaction area. At this point, electrolyte decomposition occurs on the new surface, forming a film of electrolyte decomposition products, leading to electrolyte consumption. Consequently, cycle performance tends to decrease.

[0007] To date, various studies have been conducted on the negative electrode active material and electrode structure for lithium-ion secondary batteries, with silicon as the main material, in order to improve the initial efficiency and cycle characteristics of batteries.

[0008] Specifically, to achieve good cycle characteristics and high safety, a vapor-phase method is used to simultaneously deposit silicon and amorphous silicon dioxide (see, for example, Patent Document 1). Furthermore, to obtain high battery capacity and safety, a carbon material (conductive material) is disposed on the surface of the silicon oxide particles (see, for example, Patent Document 2). Further, to improve cycle characteristics and simultaneously obtain high input / output characteristics, an active material containing silicon and oxygen is fabricated, and an active material layer with a high oxygen ratio near the current collector is formed (see, for example, Patent Document 3). Moreover, to improve cycle characteristics, oxygen is contained in the silicon active material, and it is formed in a manner where the average oxygen content is 40 at% or less, with an increased oxygen content near the current collector (see, for example, Patent Document 4).

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

[0010] Furthermore, to improve cycling characteristics, silicon oxide is used, and a graphite coating is formed on its surface, thereby imparting conductivity (see, for example, Patent Document 10). In Patent Document 10, regarding the shift value obtained from the RAMAN (Raman) spectrum associated with the graphite coating, at 1330 cm⁻¹... -1 and 1580cm -1 Broad peaks appear, and the intensity of these peaks is greater than that of I. 1330 / I 1580 1.5 < I 1330 / I 1580 <3. Furthermore, in order to achieve high battery capacity and improve cycle characteristics, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used (for example, see Patent Document 11). In addition, in order to improve overcharge and over-discharge characteristics, silicon oxides with the atomic ratio of silicon to oxygen controlled at 1:y (0 < y < 2) are used (for example, see Patent Document 12).

[0011] Furthermore, as a lithium-ion secondary battery using silicon oxide, Hitachi Maxell began shipping square secondary batteries for smartphones employing nano-silicon composites in June 2010 (see, for example, Non-Patent Literature 1). The silicon oxide proposed by Hohl is Si... 0+ ~Si 4+ The composite material exhibits a wide variety of oxidation states (Non-Patent Document 2). Furthermore, Kapaklis proposed a disproportionated structure that separates silicon oxide into Si and SiO2 by applying a thermal load (Non-Patent Document 3).

[0012] Miyachi et al. focused on Si and SiO2 in silicon oxide with disproportionation structure that facilitate charging and discharging (Non-Patent Document 4), while Yamada et al. proposed the following reaction formula of silicon oxide with 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, which constitutes silicon oxide, reacts with SiO2 and Li to separate into Li silicide and Li silicate, and a portion of unreacted SiO2.

[0013] The Li silicate formed here is generally considered irreversible; once formed, it does not release Li and is a stable substance. The capacity per unit mass calculated using this reaction equation has a value close to experimental values, and the reaction mechanism as a silicon oxide is well known. Kim et al. used... 7 Li-MAS-NMR or 29 Si-MAS-NMR identified the irreversible component accompanying the charge and discharge of silicon oxide, namely Li silicate, as Li4SiO4 (Non-Patent Document 6).

[0014] This irreversible capacity is the biggest weakness of silicon oxide batteries and needs to be improved. To address this, Kim et al. used a Li pre-doping method to pre-form Li silicates, significantly improving the initial efficiency of the battery and fabricating a negative electrode that can withstand practical use (Non-Patent Document 7). Furthermore, a method that improves irreversible capacity by processing powder, rather than Li doping the electrode, was proposed (Patent Document 13).

[0015] However, the price of Li metal used in Li doping fluctuates wildly with market conditions, posing numerous technical challenges for industrialization. To address this, CVD-Si-C, which generates nano-silicon internally using silane gas on porous carbon, has successfully achieved an energy density higher than that of Li-doped SiO (Patent Documents 14, 15).

[0016] In addition, in view of the problem that excessive silicon is deposited on the surface and the resistivity is increased due to the small pore size of porous carbon materials, a composite particle containing no SiC (silicon carbide) or with a very low SiC content was proposed to reduce the resistivity (Patent Document 16). Existing technical documents Patent documents

[0017] Patent Document 1: Japanese Patent Application Publication No. 2001-185127 Patent Document 2: Japanese Patent Application Publication No. 2002-042806 Patent Document 3: Japanese Patent Application Publication No. 2006-164954 Patent Document 4: Japanese Patent Application Publication No. 2006-114454 Patent Document 5: Japanese Patent Application Publication No. 2009-070825 Patent Document 6: Japanese Patent Application Publication No. 2008-282819 Patent Document 7: Japanese Patent Application Publication No. 2008-251369 Patent Document 8: Japanese Patent Application Publication No. 2008-177346 Patent Document 9: Japanese Patent Application Publication No. 2007-234255 Patent Document 10: Japanese Patent Application Publication No. 2009-212074 Patent Document 11: Japanese Patent Application Publication No. 2009-205950 Patent Document 12: Japanese Patent Application Publication No. 06-325765 Patent Document 13: Japanese Patent Application 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 Non-patent literature

[0018] Non-patent document 1: Battery Manufacturers Association official paper "でんち" dated May 1, 2012, page 10 Non-Patent Literature 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-patent literature 3: V. Kapaklis, J. Non-Crystalline Solids, 354 (2008) 612 Non-patent literature 4: Mariko Miyachi, Hironori Yamamoto, and Hidemasa Kawai, J. Electrochem. Soc. 2007, volume 154, issue 4, A376-A380 Non-patent literature 5: M. Yamada, A. Inaba, A. Ueda, K. Matsumoto, T. Iwasaki, T. Ohzuku, J. Electrochem. Soc., 159, A1630 (2012) Non-patent literature 6: Taeahn Kim, Sangjin Park, and Seung M. Oh, J. Electrochem. Soc. volume 154, (2007), A1112-A1117. Non-patent literature 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 forefront of the development of automotive battery packs, P96-111, Published by Seiko, issued on November 27, 2020 Summary of the Invention (a) Technical problems to be solved

[0019] As mentioned above, in recent years, the performance and multifunctionality of small electronic devices, such as mobile terminals, have been continuously improved, requiring increased battery capacity of lithium-ion secondary batteries, which serve as their main power source. As a solution to this problem, it is desirable to develop a lithium-ion secondary battery that incorporates silicon as the primary material for the negative electrode.

[0020] Furthermore, it is desired that lithium-ion secondary batteries using silicon materials possess initial charge-discharge characteristics and cycle characteristics approximately equivalent to those of lithium-ion secondary batteries using carbon-based active materials. To date, modified silicon oxides, utilizing Li intercalation and partial deintercalation, have been used as anode active materials to improve cycle characteristics and initial charge-discharge characteristics. In recent years, by pre-containing Li with silicon oxide as the main component to generate Li silicates, the irreversible capacity, a drawback of silicon oxides, has been reduced, and related products have begun to enter the market. However, even in the prototype battery where 100% of the carbon anode material was replaced with Li-SiO-C (Non-Patent Document 8), which uses Li in silicon oxide, the capacity only increased by about 25-29% compared to the carbon anode material. This indicates a need for further increases in battery capacity when considering the increasing performance of small electronic devices (5G, etc.) and improving the driving range of electric vehicles.

[0021] In response, although CVD-Si-C with a small irreversible capacity has been developed, its high-speed charging and battery cycle characteristics are known to be insufficient due to the reaction between Si and the electrolyte.

[0022] In addition, Li-Si compounds (e.g., Li 15 Si4 typically exhibits ionicity and reduced diffusivity. If such an ionic substance is generated, as mentioned earlier, the negative electrode active material becomes easily broken. If the surface of the negative electrode active material breaks down, a new surface (newly formed surface) is created, increasing the reaction area of ​​the active material. At this point, due to the decomposition reaction of the electrolyte on the new surface and the formation of a film of electrolyte decomposition products, the electrolyte is further consumed. If repeated charging and discharging occurs under these conditions, the electrolyte will decompose and be consumed every time a new surface is formed, resulting in a decrease in cycle performance.

[0023] The present invention was made in view of the above-mentioned problems, and its object is to provide a negative electrode active material that reduces the generation of ionic substances, inhibits excessive decomposition of electrolyte, and thereby improves the battery cycle characteristics. (II) Technical Solution

[0024] To solve the above-mentioned technical problems, the present invention provides a negative electrode active material, which is a negative electrode active material having negative electrode active material particles, characterized in that the negative electrode active material particles contain a porous carbon structure as a matrix, the porous carbon structure having Si-C bonds inside, amorphous low-valent silicon oxide dispersed on the surface, and at least a portion of the low-valent silicon oxide having tetravalent silicon oxide capable of intercalating and deintercalating Li.

[0025] In the case of the negative electrode active material of the present invention, a portion of the low-valent nano-silicon oxide dispersed in the surface layer contains tetravalent silicon oxide capable of intercalating and deintercalating Li. Therefore, firstly, because the low-valent nano-silicon oxide has high Li diffusivity, the low-valent nano-silicon oxide dispersed in the surface layer can reduce the generation of ionic substances and suppress the formation of new surfaces. Simultaneously, during charge and discharge, the tetravalent silicon oxide can intercalate and deintercalate Li in the surface layer, thereby repeatedly decomposing and generating Li silicates. Through these methods, excessive decomposition of the electrolyte can be suppressed, resulting in a negative electrode active material that improves battery cycle characteristics. Furthermore, the internal Si-C bonds are firmly bonded to the Si in the surface layer, thereby suppressing the interruption of electron conduction in the Si portion during charge and discharge, resulting in structural stability.

[0026] Furthermore, the low-cost nano-silicon oxide preferably incorporates Li during the Li insertion and extraction process.

[0027] If the low-valent nano-silicon oxide dispersed in the surface layer is embedded with Li in this way, then the tetravalent silicon oxide present in a portion of the low-valent nano-silicon oxide can reliably decompose and generate Li silicates in the surface layer.

[0028] In addition, preferred method 29 The spectrum obtained by Si-MAS-NMR measurement has peaks from the Si-C bonds in the range of 0 to -30 ppm, and peaks from Si or the low-valent nano-silicon oxide in the range of -40 to -80 ppm.

[0029] If the negative electrode active material has such a spectrum, then the decomposition and generation of Li silicates can be reliably achieved through the tetravalent silicon oxide, which is present in a portion of the low-valent nano-silicon oxide dispersed in the surface layer. Furthermore, the internal Si-C bonds are strongly bonded to the Si in the surface layer, which can suppress the interruption of electron conduction in the Si portion during charging and discharging, thus reliably stabilizing the structure.

[0030] Furthermore, it is preferable that active sites exist inside the pores of the porous carbon structure, and a portion of the active sites are modified with OH.

[0031] If the internal structure is such a porous structure, it will become a state in which a large amount of CO is contained in the exhaust gas during heat treatment, for example. By further reacting it with silane gas under a pressurized atmosphere, the silane decomposition products can be made into amorphous low-valence nano-silicon oxides and dispersed in the interior of the porous carbon structure, especially the inner surface layer, so that the desired negative electrode active material can be easily manufactured.

[0032] Furthermore, the low-cost nano-silicon oxide preferably has other carbon layers that are different from the porous carbon structure that serves as the matrix, and the interfaces of the other carbon layers are formed with C=O bonds or CO bonds.

[0033] If the interface forms with C=O or CO bonds, it is considered that a compound similar to Si-OC has been generated, which can enhance the adhesion of the carbon layer and achieve a robust structure, such as maintaining integrity even when subjected to high-speed shear during slurry preparation.

[0034] Furthermore, it is preferable that the grain size of the 0-valent Si constituting the low-valent nano-silicon oxide, as determined by the peaks obtained from X-ray diffraction measurements of the negative electrode active material particles and calculated using the Scherrer formula, is in the range of 0.8 nm to 5 nm.

[0035] Low-valence nano-silicon oxides with grain size of 0-valent Si, which is considered to be substantially amorphous, are preferred.

[0036] Furthermore, the 0-valent Si is preferably substantially amorphous.

[0037] The preferred material is 0-valent Si, which is essentially amorphous.

[0038] Furthermore, it is preferred that the oxygen content of the negative electrode active material as a whole is in the range of 0.5wt% or more and 5wt% or less.

[0039] If the oxygen content is within this range, it can prevent the excessive formation of Si-C bonds during acetylene CVD due to insufficient oxygen content, which would lead to a decrease in battery capacity. At the same time, it can also prevent the irreversible increase in capacity due to excessive oxygen content.

[0040] Furthermore, to solve the above-mentioned technical problems, the present invention provides a method for manufacturing a negative electrode active material, characterized by comprising the following steps: a step of preparing a porous carbon structure; a step of making the active sites of the porous carbon structure contain OH groups and CxHy groups; a step of introducing silane gas under heating and in a state where the exhaust gas contains carbon monoxide gas or carbon dioxide gas containing oxygen atoms from the active sites, so as to deposit silicon oxide bonded to the oxygen atoms inside the porous carbon structure and make the silicon oxide contain dangling bonds; a step of oxidizing at least a portion of Si present on the surface of the porous carbon structure under a pressurized atmosphere after the step of depositing the silicon oxide; and a step of depositing a carbon layer using a hydrocarbon gas at a temperature below 580 degrees Celsius after the step of oxidation under the pressurized atmosphere.

[0041] If the method for manufacturing the negative electrode active material of the present invention is adopted, Si-C bonds are formed inside the porous carbon structure, and amorphous low-valent silicon oxide nanoparticles are dispersed on the surface. Oxidation under a pressurized atmosphere generates tetravalent silicon oxides capable of intercalating and deintercalating Li in a portion of the low-valent silicon oxides. Therefore, in the negative electrode using the above-mentioned negative electrode active material, the low-valent silicon oxides have high Li diffusivity, thus reducing the generation of ionic substances and suppressing the formation of new surfaces by dispersing the low-valent silicon oxides on the surface. Simultaneously, during charge and discharge, the tetravalent silicon oxides can intercalate and deintercalate Li in the surface, repeatedly decomposing and generating Li silicates. Through these methods, excessive decomposition of the electrolyte can be suppressed, resulting in a negative electrode active material that improves battery cycle characteristics. Furthermore, the internal Si-C bonds are firmly bonded to the Si in the surface, thereby suppressing the interruption of electron conduction in the Si portion during charge and discharge, resulting in structural stability. If the method for manufacturing the negative electrode active material of the present invention is adopted, the above-mentioned negative electrode active material can be manufactured easily and efficiently. In particular, it can provide a method for manufacturing a negative electrode active material that reduces the generation of ionic substances, inhibits excessive decomposition of electrolyte, and thereby improves the battery cycle characteristics. (III) Beneficial Effects

[0042] In the negative electrode active material of the present invention, a portion of the low-valent nano-silicon oxide dispersed in the surface layer contains tetravalent silicon oxide capable of intercalating and deintercalating Li. Therefore, firstly, because the low-valent nano-silicon oxide has high Li diffusivity, the low-valent nano-silicon oxide dispersed in the surface layer can reduce the generation of ionic substances and suppress the formation of new surfaces. Simultaneously, during charge and discharge, the tetravalent silicon oxide can intercalate and deintercalate Li in the surface layer, thereby repeatedly decomposing and generating Li silicates. Through these methods, excessive decomposition of the electrolyte can be suppressed, resulting in a negative electrode active material that improves battery cycle characteristics. Furthermore, the internal Si-C bonds are firmly bonded to the Si in the surface layer, thereby suppressing the interruption of electron conduction in the Si portion during charge and discharge, resulting in structural stability.

[0043] In the manufacturing method of the negative electrode active material of the present invention, Si-C bonds are formed inside the porous carbon structure, and amorphous low-valent silicon oxide nanoparticles are dispersed on the surface. Oxidation under a pressurized atmosphere generates tetravalent silicon oxides capable of intercalating and deintercalating Li in a portion of the low-valent silicon oxides. Therefore, in the negative electrode using the above-mentioned negative electrode active material, the low-valent silicon oxides have high Li diffusivity, thus reducing the generation of ionic substances and suppressing the formation of new surfaces by dispersing the low-valent silicon oxides on the surface. Simultaneously, during charge and discharge, the tetravalent silicon oxides can intercalate and deintercalate Li in the surface, repeatedly decomposing and generating Li silicates. Through these methods, excessive decomposition of the electrolyte can be suppressed, resulting in a negative electrode active material that improves battery cycle characteristics. Furthermore, the internal Si-C bonds are firmly bonded to the Si in the surface, thereby suppressing the interruption of electron conduction in the Si portion during charge and discharge, resulting in structural stability. If the method for manufacturing the negative electrode active material of the present invention is adopted, the above-mentioned negative electrode active material can be manufactured easily and efficiently. In particular, it can provide a method for manufacturing a negative electrode active material that reduces the generation of ionic substances, inhibits excessive decomposition of electrolyte, and thereby improves the battery cycle characteristics. Attached Figure Description

[0044] Figure 1 A cross-sectional view showing the structure of the negative electrode containing the negative electrode active material of the present invention. Figure 2 An exploded view showing a configuration example (laminated film type) of a lithium-ion secondary battery containing the negative electrode active material of the present invention. Figure 3 The XANES spectrum of the surface layer of the negative electrode active material during charging. Figure 4 The XANES spectrum of the surface layer of the negative electrode active material during discharge is shown. Figure 5For the passage of negative electrode active material 29 The spectrum obtained by Si-MAS-NMR measurement. Figure 6 For the existing negative electrode active material 29 The spectrum obtained by Si-MAS-NMR measurement. Figure 7 A flowchart illustrating an example of a method for manufacturing the negative electrode active material of the present invention. Figure 8 The XANES spectra of the surface layer of the negative electrode active material of Example 1 and Comparative Example 1 are shown. Detailed Implementation

[0045] The following describes embodiments of the present invention, but the present invention is not limited to these embodiments.

[0046] As mentioned earlier, as a method to increase the capacity of lithium-ion secondary batteries, an investigation was conducted on using low-cost nano-silicon oxide as the main material in a carbon structure as the negative electrode for lithium-ion secondary batteries. It is expected that lithium-ion secondary batteries using this active material will exhibit battery characteristics approximately equivalent to those using carbon-based active materials, but with higher capacity.

[0047] In order to obtain a negative electrode active material that can achieve high cycle characteristics, improve initial charge-discharge characteristics, and increase battery capacity when used as the negative electrode of a secondary battery, the inventors of this application have conducted careful research and completed this invention.

[0048] Especially Li-Si compounds (e.g., Li 15 Si4 exhibits ionicity and reduced diffusivity, leading to the following problems: the negative electrode active material becomes prone to breakage. When the surface of the negative electrode active material breaks, a new surface (newly formed surface) is generated. If repeated charging and discharging occur, the electrolyte will decompose and be excessively consumed each time a new surface is generated. In this invention, to reduce the generation of ionic substances and suppress excessive decomposition of the electrolyte, a negative electrode active material was developed. This material has Si-C bonds inside a porous carbon structure, amorphous low-valent nano-silicon oxides dispersed on the surface, and a portion of the low-valent nano-silicon oxides contains tetravalent silicon oxides.

[0049] [The negative electrode active material of the present invention] The present invention relates to a negative electrode active material, which is a negative electrode active material having negative electrode active material particles. The negative electrode active material particles contain a porous carbon structure as a matrix. The porous carbon structure has Si-C bonds inside and amorphous low-valent nano-silicon oxides are scattered on the surface. At least a portion of the low-valent nano-silicon oxides has tetravalent silicon oxides that can be inserted and extracted with Li.

[0050] In such a negative electrode active material, a portion of the low-valent nano-silicon oxide dispersed in the surface layer contains tetravalent silicon oxide capable of intercalating and deintercalating Li. Therefore, due to the high diffusivity of the low-valent nano-silicon oxide for Li, the formation of ionic substances and the inhibition of new surface formation can be reduced by the low-valent nano-silicon oxide dispersed in the surface layer. Simultaneously, during charge and discharge, the tetravalent silicon oxide can repeatedly decompose and generate Li silicates by intercalating and deintercalating Li in the surface layer. Through these methods, excessive decomposition of the electrolyte can be suppressed, resulting in a negative electrode active material that improves battery cycle characteristics. Furthermore, the internal Si-C bonds are firmly bonded to the Si in the surface layer, thereby suppressing the interruption of electron conduction in the Si portion during charge and discharge, resulting in structural stability.

[0051] In addition, although not specifically limited, it is preferred to use low-cost nano-silicon oxides to insert Li during the Li insertion and deintercalation process.

[0052] If the low-valent nano-silicon oxide dispersed in the surface layer is embedded with Li in this way, then the tetravalent silicon oxide present in a portion of the low-valent nano-silicon oxide can reliably decompose and generate Li silicates in the surface layer.

[0053] Here, the insertion and extraction (charging and discharging) of Li is explained with reference to the accompanying drawings.

[0054] Figure 1 This is a cross-sectional view of the negative electrode containing the negative electrode active material of the present invention. Figure 2 Example of a lithium-ion secondary battery comprising the negative electrode active material of the present invention (laminated film type). Figure 1 , Figure 2 The details are as follows.

[0055] Figures 3-4 For application Figure 1 negative electrode and Figure 2 XANES spectrum of the negative electrode active material of lithium-ion secondary batteries. Figure 3 The XANES spectrum of the surface layer (within 50 nm of the surface) during charging is shown. Figure 4 The XANES spectrum of the surface layer during discharge is shown.

[0056] Figure 3 The spectrum of the surface layer during charging is shown. As the charge progresses from the initial state before charging (Initial in the figure) to SOC 20% (20% charge, where SOC (State of charge) represents the charging state), SOC 50%, and SOC 100% (fully charged), the spectrum increases along the direction of the arrows in the figure.

[0057] Initially, although a peak for tetravalent silicon oxide (tetravalent Si oxide in the figure) could be observed, it is known that this peak changes to a Li silicate peak due to charging (Li intercalation). Normally, tetravalent silicon oxide would change to Li4SiO4, but it is believed that due to Li intercalation, it becomes a state similar to low-valent Li silicates, in which case it becomes a state similar to Li2SiO3.

[0058] also, Figure 4 The spectrum of the surface layer during discharge is shown in the figure. As the discharge progresses from SOC 100% (fully charged) to DOD 20% (20% discharge, where DOD (Depth of Discharge) represents the ratio of discharge amount to discharge capacity, i.e., the depth of discharge), DOD 50%, DOD 70%, and DOD 100%, the spectrum decreases along the direction of the arrows in the figure.

[0059] Thus, by repeatedly decomposing and generating Li silicate on the surface layer during charging and discharging, excessive decomposition of the electrolyte is suppressed, making high cycle performance possible.

[0060] In addition, XANES (X-ray Absorption Near-Edge Structure) measurements can be performed, for example, under the following conditions. • Using the BL6N1 line of the Aichi Synchrotron Radiation Center The sample was prepared under an argon atmosphere and in a non-atmospheric environment, placed in a transfer vessel, connected to the BL6N1 line, and measured under the following conditions. • Acceleration Energy: 1.2 GeV • Cumulative current value: 300mA Monochromatic conditions: White X-rays from a bending magnet are monochromated using a dual-crystal spectrometer and then used in the measurement. • Concentration conditions: Concentrate light in both vertical and horizontal directions using a Ni-plated curved cylindrical mirror. • Upstream slit opening: 10.0mm horizontally × 3.0mm vertically • Beam size: 2.0mm horizontally × 1.0mm vertically • Incident angle of the sample: 45 degrees (incident angle 45 degrees) in order to simultaneously determine the fluorescence yield. • Energy correction: The peak position at the SK end of K2SO4 will be corrected to 2481.70 eV. • Measurement methods: All-electron yield method achieved by measuring sample current, fluorescence X-ray • I0 (zero-sequence current) measurement method: Au-mesh during XANES measurement Cu-mesh during EXAFS measurement

[0061] Furthermore, although not specifically limited, preference is given to those who pass the test. 29 The spectra obtained by Si-MAS-NMR measurement have peaks from Si-C bonds in the range of 0 to -30 ppm, and peaks from Si or low-valent nano-silicon oxides in the range of -40 to -80 ppm.

[0062] If the negative electrode active material has such a spectrum, then the decomposition and generation of Li silicates can be reliably achieved through the tetravalent silicon oxide, which is present in a portion of the low-valent nano-silicon oxide dispersed in the surface layer. Furthermore, the internal Si-C bonds are firmly bonded to the Si in the surface layer, which can suppress the interruption of electron conduction in the Si portion during charging and discharging, thus reliably stabilizing the structure.

[0063] Figure 5 The diagram illustrates the process of the negative electrode active material of the present invention. 29 An example of a spectrum obtained by Si-MAS-NMR measurement is provided for reference. Peaks originating from Si-C bonds are present in the range of 0 to -30 ppm (Si-C in the figure), and peaks originating from Si or low-valent nano-silicon oxides (amorphous Si in the figure) are present in the range of -40 to -80 ppm.

[0064] also, Figure 6 The paper illustrates the use of existing (e.g., patent document 16) negative electrode active materials that do not have Si-C bonds. 29 The spectrum obtained by Si-MAS-NMR measurement. Compared to Figure 5 No peaks from Si-C bonds were observed.

[0065] The valence of low-cost nano-silicon oxides can be quantified using NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0066] NMR measurements can be performed, for example, under the following conditions. 29 Si MAS NMR (Magic Angle Rotation Nuclear Magnetic Resonance) • Apparatus: A 700 NMR spectrometer manufactured by Bruker. • Probe: 4mm HR-MAS rotor, 50μL • Sample rotation speed: 10kHz • Measured ambient temperature: 25℃

[0067] XPS measurements can be performed, for example, under the following conditions. XPS • Apparatus: 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×2mm.

[0068] Furthermore, although not specifically limited, it is preferred that the low-cost nano-silicon oxide is essentially a composite state of 0, 1, 2, and 3 valences.

[0069] In such a composite state, in addition to reducing the irreversible capacity, it is also possible to more effectively achieve the insertion and extraction of Li by low-valent nano-silicon oxide by adjusting the ratio of 0-valent, 1-valent, 2-valent, and 3-valent oxides.

[0070] In addition, the low-valence nano-silicon oxide phase present inside the porous carbon structure can contain a 0-valence microcrystalline phase of Si.

[0071] Furthermore, although not specifically limited, it is preferred that active sites exist inside the pores of the porous carbon structure, and that a portion of the active sites are modified with OH.

[0072] If the internal structure is such a porous structure, it will become a state in which a large amount of CO is contained in the exhaust gas during heat treatment, for example. By further reacting it with silane gas under a pressurized atmosphere, the silane decomposition products can be made into amorphous low-valence nano-silicon oxides and dispersed in the interior of the porous carbon structure, especially the inner surface layer, so that the desired negative electrode active material can be easily manufactured.

[0073] Furthermore, although not specifically limited, it is preferred that the low-cost nano-silicon oxide has other carbon layers that are different from the porous carbon structure that serves as the matrix, and that the interfaces of the other carbon layers have C=O bonds or CO bonds.

[0074] If the interface forms with C=O or CO bonds, it is considered that a compound similar to Si-OC has been generated, which can enhance the adhesion of the carbon layer and achieve a robust structure, such as maintaining integrity even when subjected to high-speed shear during slurry preparation.

[0075] Furthermore, although not specifically limited, it is preferred that the grain size of the 0-valent Si constituting the low-valent nano-silicon oxide, as determined by measuring the peaks of the negative electrode active material particles by X-ray diffraction and calculated using the Scherrer formula, is in the range of 0.8 nm to 5 nm.

[0076] Low-valence nano-silicon oxides with grain size of 0-valent Si, which is considered to be substantially amorphous, are preferred.

[0077] Crystallite size calculations via XRD can be performed, for example, under the following conditions. For broad peaks, analysis software TOPAS can be used, for example, under the following conditions. XRD measurement • Device: D2 PHASER manufactured by Bruker X-ray source: Cu • Diverging slit: 0.5° • Incident side Sola slit: 4° • Light-receiving side of the Sola slit: 4° Calculation of crystallite size • Analysis software: DIFFRAC, TOPAS • Analysis method: Peak fitting method • Emission Profile: Cu Ka5.lam • Function: FP (First Principle) function • Refinement Option: Select "Calculate Error" and "Use Extrapolation"

[0078] Furthermore, although not specifically limited, it is preferred that Si with zero valence is essentially amorphous.

[0079] In addition, although not specifically limited, it is preferred that the oxygen content of the negative electrode active material as a whole is in the range of 0.5wt% or more and 5wt% or less.

[0080] If the oxygen content is within this range, it can prevent the excessive formation of Si-C bonds during acetylene CVD due to insufficient oxygen content, which would lead to a decrease in battery capacity. At the same time, it can also prevent the excessive oxygen content from causing an irreversible increase in capacity.

[0081] Furthermore, the grain size of low-valent silicon oxide nanoparticles can be confirmed using TEM-EDX. The conditions for this are set as follows: A focused ion beam (FIB) apparatus is used to process the negative electrode active material across two cross sections without exposure to the atmosphere. The FIB apparatus is an XVision 200DB manufactured by SII NanoTechnology Inc., with an accelerating voltage of 30 kV. TEM observation is performed using a Tecnai G2 F20 manufactured by FEI Company Japan Ltd., with an accelerating voltage of 200 kV. EDX is performed using an r-TEM manufactured by EDAX Inc., with an accelerating voltage of 200 kV.

[0082] Furthermore, in the negative electrode active material of the present invention, a carbon complex with silicon as the core can also be included inside the porous carbon structure, such that the carbon complex contains at least one of carbon black, acetylene black, and carbon nanotubes.

[0083] In addition, porous carbon is derived from biological sources, resins, and petroleum, with biological sources or resins being preferred.

[0084] <Negative electrode for non-aqueous electrolyte secondary batteries> Next, the structure of a negative electrode (hereinafter also referred to as "negative electrode") for a non-aqueous electrolyte secondary battery containing the negative electrode active material of the present invention will be described.

[0085] [Composition of the negative electrode] Figure 1 A cross-sectional view showing the negative electrode containing the negative electrode active material of the present invention. For example... Figure 1 As shown, the negative electrode 10 has a negative electrode active material layer 12 on the negative electrode current collector 11. This negative electrode active material layer 12 can be disposed on both sides of the negative electrode current collector 11 or only on one side. Furthermore, the negative electrode current collector 11 may not be present in the negative electrode of the non-aqueous electrolyte secondary battery of the present invention.

[0086] [Negative current collector] The negative electrode current collector 11 is made of a material with excellent conductivity and excellent mechanical strength. Examples of conductive materials that can be used for the negative electrode current collector 11 include copper (Cu) or nickel (Ni). Preferably, this conductive material is one that does not form intermetallic compounds with lithium (Li).

[0087] The preferred negative electrode current collector 11 contains carbon (C) and sulfur (S) in addition to the main elements. This is because the physical strength of the negative electrode current collector is improved. In particular, when there is an active material layer that expands during charging, the inclusion of the above-mentioned elements in the current collector helps to suppress deformation of the electrode containing the current collector. The content of the above-mentioned elements is not particularly limited, but it is preferred that each is 100 ppm by mass or less. This is because a higher effect of suppressing deformation can be obtained. Through such an effect of suppressing deformation, the cycle characteristics can be further improved.

[0088] Furthermore, the surface of the preferred negative current collector 11 is roughened, with a desired ten-point average roughness Rz of 1.5 μm or more and 5 μm or less. The roughened negative current collector is, for example, a metal foil that has undergone electrolytic treatment, embossing treatment, or chemical etching treatment.

[0089] [Negative electrode active material layer] In addition to silicon-based active material particles, the negative electrode active material layer 12 may also contain various types of negative electrode active materials, such as carbon-based active materials. Furthermore, from a battery design perspective, it may also contain other materials such as thickeners (also known as "binders" or "adhesives") or conductive additives.

[0090] [The negative electrode active material and the method for manufacturing the negative electrode of the present invention] Next, an example of the negative electrode active material of the present invention and a method for manufacturing a negative electrode using the negative electrode active material will be described.

[0091] First, the manufacturing method of the negative electrode active material contained in the negative electrode will be explained.

[0092] The method for manufacturing the negative electrode active material of the present invention is characterized by comprising the following steps: a step of preparing a porous carbon structure; a step of making the active sites of the porous carbon structure contain OH groups and CxHy groups; a step of introducing silane gas under heating and in a state where the exhaust gas contains carbon monoxide gas or carbon dioxide gas containing oxygen atoms from the active sites, so as to deposit silicon oxide bonded to the oxygen atoms inside the porous carbon structure and make the silicon oxide contain dangling bonds; a step of oxidizing at least a portion of Si present on the surface of the porous carbon structure under a pressurized atmosphere after the step of depositing the silicon oxide; and a step of depositing a carbon layer using a hydrocarbon gas at a temperature below 580 degrees Celsius after the step of oxidation under the pressurized atmosphere.

[0093] If the method for manufacturing the negative electrode active material of the present invention is adopted, Si-C bonds are formed inside the porous carbon structure, and amorphous low-valent silicon oxide nanoparticles are dispersed on the surface. Oxidation is performed under a pressurized atmosphere, and tetravalent silicon oxide capable of intercalating and deintercalating Li is generated in a portion of the low-valent silicon oxide nanoparticles. Therefore, in the negative electrode using such a negative electrode active material, the low-valent silicon oxide nanoparticles have high Li diffusivity. Thus, the low-valent silicon oxide nanoparticles dispersed on the surface reduce the generation of ionic substances and suppress the formation of new surfaces. Simultaneously, during charge and discharge, the tetravalent silicon oxide can intercalate and deintercalate Li on the surface, thereby repeatedly decomposing and generating Li silicates. Through these methods, excessive decomposition of the electrolyte can be suppressed, resulting in a negative electrode active material that improves battery cycle characteristics. Furthermore, the internal Si-C bonds are firmly bonded to the Si on the surface, thereby suppressing the interruption of electron conduction in the Si portion during charge and discharge, resulting in structural stability. If the method for manufacturing the negative electrode active material of the present invention is adopted, the above-mentioned negative electrode active material can be manufactured easily and efficiently. In particular, it can provide a method for manufacturing a negative electrode active material that reduces the generation of ionic substances, inhibits excessive decomposition of electrolyte, and thereby improves the battery cycle characteristics.

[0094] Here, the manufacturing process of SiC will be explained. Generally speaking, SiC is manufactured by the following method: using a fluidized bed, silane gas and carrier gas are ejected from the bottom under a heated atmosphere to deposit silicon inside porous carbon, followed by an oxidation process, and carbon coating using hydrocarbon gases or coal tar pitch, etc.

[0095] At this point, at least a portion of the Si generated from the reaction of silane gas has Si-H bonds and is deposited simultaneously in a Si-Si state. Furthermore, since the substance obtained in the oxidation process is SiO2, Si... 0+ With Si 4+ The complex is formed in porous carbon.

[0096] Furthermore, since SiO2 is present near the outermost layer, conventional SiC active materials can be manufactured by carbon coating CVD on its upper part using hydrocarbon gases.

[0097] Compared to existing methods, the method for manufacturing the negative electrode active material of the present invention utilizes the active sites of porous carbon to contain OH groups.

[0098] If heat treatment is performed in a state containing OH groups, CO will be detected in the exhaust gas. Normally, complete drying does not generate CO gas, but by reacting with silane gas in a pressurized atmosphere with a high CO content in the exhaust gas, silane decomposition products will deposit as low-valence oxides on the porous carbon. Since CO gas is continuously generated from within the porous pores, it is crucial to use hydrogen as a carrier gas under pressurized atmosphere to ensure sufficient diffusion of the silane gas into the interior.

[0099] The pressure drop of a typical fluidized bed is around 10 kPa, but it is known that this reaction begins to proceed gradually at around 25 kPa.

[0100] Low-valence oxides have high diffusivity for Li. Typically, compounds of Si and Li will form Li₂. 15 Si₄, but it exhibits ionic properties and reduced diffusivity. In contrast, low-valence oxides can reduce the formation of substances that exhibit ionic properties.

[0101] Furthermore, during silicon oxide deposition, increasing the reaction pressure allows for the presence of a large number of dangling bonds. These dangling bonds are highly reactive and can be carbonized at temperatures lower than the typical decomposition temperatures of hydrocarbon gases (such as acetylene).

[0102] To address this, oxidation under a pressurized atmosphere converts the silicon oxide into divalent and tetravalent silicon oxides, thereby suppressing the Si-C bonds generated during subsequent acetylene CVD. The tetravalent silicon oxides constituting these oxides are typically irreversible, but by nano-particle size reduction and simultaneous slow heating under a pressurized atmosphere while oxidation, they become oxides capable of intercalating and deintercalating Li.

[0103] In addition, during the insertion and extraction of Li, tetravalent silicon oxide usually changes to Li4SiO4, but it is considered to become a state close to the lower-valent Li silicate, which in this case becomes a state close to Li2SiO3.

[0104] High cycling performance is made possible by generating Li silicate on the surface to suppress excessive decomposition of the electrolyte.

[0105] Here, if there are Si-C bonds inside the bulk phase (the interior of the porous carbon structure), the structure is stable.

[0106] Furthermore, the method for manufacturing the negative electrode active material of the present invention allows CxHy groups to attach to the interior of porous carbon.

[0107] After pickling, which aims to remove metallic components present on the surface and avoid acid penetration into the depth of the pores, hydrocarbon components generated during the activation process remain in the pores.

[0108] Furthermore, the silane reaction occurs below 400 degrees Celsius. If a heat treatment of 600 degrees Celsius is subsequently performed, it will react with Si-H to form Si-C bonds within the bulk phase. Therefore, by firmly bonding with the Si deposited on the surface of the porous carbon pores, the interruption of electron conduction in the Si pores during charging and discharging can be suppressed.

[0109] Regarding the oxidation treatment method, an atmosphere with 1% oxygen in nitrogen and pressurized to 20 kPa is used for oxidation treatment (in addition, the pressure is recorded as 101.33 kPa relative to atmospheric pressure (gauge pressure)).

[0110] The oxygen content is estimated based on the amount of gas introduced. A load cell is installed in the oxidation vessel to confirm the weight increase, and oxidation is stopped once the specified oxidation amount is reached. The range where there is essentially no weight increase, in terms of oxygen content, is approximately 3 wt%.

[0111] Carbon CVD coatings deposited near the surface of silicon compounds utilize a portion of the oxygen content inherent in silicon to generate C=O or CO compounds. At this point, compounds similar to Si-OC are considered to be formed, which enhance the adhesion of the carbon layer, thus maintaining its integrity even under high-speed shearing during slurry preparation.

[0112] Reference Figure 7 The steps of the manufacturing method for the negative electrode active material are explained.

[0113] The method for manufacturing the negative electrode active material of the present invention includes: a step of preparing a porous carbon structure (step S1); a step of making the active sites of the porous carbon structure contain OH groups and CxHy groups (step S2); a step of introducing silane gas under heating and in a state where the exhaust gas contains carbon monoxide gas or carbon dioxide gas containing oxygen atoms from the active sites, so that silicon oxide bonded with oxygen atoms is deposited inside the porous carbon structure and the silicon oxide is in a state containing dangling bonds (step S3); after step S3 of depositing silicon oxide, a step of oxidizing at least a portion of Si present on the surface of the porous carbon structure under a pressurized atmosphere (step S4); and a step of depositing a carbon layer using a hydrocarbon gas at a temperature below 580 degrees Celsius after step S4 of oxidation under a pressurized atmosphere (step S5).

[0114] (Step S1) The first step is to prepare porous carbon structures.

[0115] The porous carbon structure prepared herein preferably has carbon-carbon double bonds in at least a portion.

[0116] Furthermore, the porous carbon structure prepared here is predominantly Type I in the IUPAC classification, with a preferred surface area of ​​1400 m². 2 / g or more, pore volume of 1cm 3 / g or more. By setting the classification, surface area, and pore volume based on the above IUPAC classification, silicon deposition can be performed in larger quantities and more efficiently. Furthermore, the IUPAC classification, surface area, and pore volume can be measured using the methods described below. • Specific surface area / pore distribution was determined using a TriStar II Plus manufactured by SHIMADZU CORPORATION via a constant volume method based on gas adsorption. Conditions are as follows. • Gas used: Nitrogen • Environment: Under liquid nitrogen • Operating pressure range: P / P0 • Adsorption 0~0.998 • Desorption 0.998~0.10 • Pretreatment at 200℃ for 1 hour

[0117] (Step S2) The next step is to make the active sites of the porous carbon structure prepared in step S1 include OH groups and CxHy groups.

[0118] Porous carbon was added to the reaction vessel, and the mixture was heated to 150 degrees Celsius while nitrogen was being introduced. At this point, a trap was installed in the exhaust section to ensure that moisture was not completely removed. Simultaneously, a gas detector was used to monitor the CO gas concentration. The gas concentration, at which no moisture was detected, was set to approximately 3000 ppm.

[0119] (Step S3) The next step involves introducing silane gas while the gas is heated and in the state of exhaust containing carbon monoxide or carbon dioxide gas containing oxygen atoms from active sites, so that silicon oxide bonded to oxygen atoms is deposited inside the porous carbon structure and the silicon oxide is in a state containing dangling bonds.

[0120] Heat the container to 410 degrees Celsius. When the internal temperature reaches 350 degrees Celsius, introduce silane gas from the bottom to deposit silicon inside the porous carbon (estimate the pore volume of the porous carbon beforehand by measuring the pore distribution, and introduce silane at a ratio of pore volume / 0.9 → control the reaction rate at 90%).

[0121] At this point, the pressure inside the container will vary, but it will average within the range of 25 kPa to 80 kPa. Although the film may be denser when deposition is performed at a pressure above 80 kPa, considering the durability of the device, the pressure is capped at 80 kPa (in reality, there are variations, with a confirmed maximum increase to 95 kPa).

[0122] (Step S4) This step involves oxidizing at least a portion of the Si present on the surface of a porous carbon structure under a pressurized atmosphere.

[0123] First, the material temperature is lowered to room temperature, and then oxygen diluted with nitrogen is introduced to increase the oxygen content by approximately 3.0 wt%. At this point, slow heating of the powder with a stir bar is crucial. Vigorous oxidation here will cause SiO2 formation, thus deteriorating the properties. Therefore, it is preferable to carry out the reaction slowly under a pressurized atmosphere.

[0124] Specifically, an oxidation process is carried out using an atmosphere containing 1% oxygen in nitrogen and pressurized to 20 kPa.

[0125] Furthermore, the oxidation process is carried out under pressure. By carrying out the process under pressure, the oxidation state of Si changes, resulting in a state that allows for low-valence oxidation and the insertion and extraction of Li in the tetravalent state.

[0126] A portion of the Si-O compound formed here forms a compound on a portion of it during the subsequent C-CVD process (confirmed by XPS). This improves the adhesion between the carbon coating and the Si portion.

[0127] Heating to an internal temperature of 600 degrees Celsius under a nitrogen atmosphere causes the Si-H bonds to react with the CxHy groups present inside the porous carbon pores, generating Si-C bonds.

[0128] (Step S5) This step involves depositing a carbon layer using hydrocarbon gases at temperatures below 580 degrees Celsius. Furthermore, there is no specific limitation on the lower temperature limit; for example, it can be set to above 400 degrees Celsius.

[0129] The temperature was lowered to below 580 degrees Celsius, and acetylene gas, a hydrocarbon gas, was introduced to form a carbon layer (surface carbon film). The material was then treated at 10000 Pa for 8 hours. XRD analysis was performed on the obtained material, and the crystallite size of Si was calculated using the Scherrer equation, yielding a result of 0.8 nm. However, this result is calculated, and the material is considered to be essentially amorphous.

[0130] At this point, by reacting under reduced pressure, acetylene gas can be fully diffused into the Si filling the pores. Here, by reacting it with the Si-H bonds that are not in a low-valence oxidation state, Si-C bonds are formed, thereby creating a stable material.

[0131] Furthermore, Si-O bonds are formed on the outermost layer, but since the decomposition temperature of acetylene is not reached (or the rate is extremely slow), a carbon layer will not actively form. On the other hand, a large number of Si-H bonds remain inside where Si has been deposited, and these Si-H bonds react to form Si-C bonds.

[0132] <Lithium-ion secondary batteries> Next, as a specific example of a non-aqueous electrolyte secondary battery using the negative electrode active material of the present invention, a laminated film type lithium-ion secondary battery will be described.

[0133] [The structure of a laminated film secondary battery] Figure 2 The laminated film type lithium-ion secondary battery 30 shown is mainly formed by housing a wound electrode body 31 inside a sheet-like outer component 35. This wound electrode body 31 has a separator between the positive and negative electrodes and is wound. Alternatively, there may be a case where a separator is present between the positive and negative electrodes and a laminate is housed therein. In either electrode body, a positive electrode lead 32 is installed at the positive electrode and a negative electrode lead 33 is installed at the negative electrode. Protective tape is used to protect the outermost periphery of the electrode body.

[0134] The positive lead 32 and the negative lead 33 extend from the inside of the outer component 35 toward the outside in one direction. The positive lead 32 is formed of a conductive material such as aluminum, and the negative lead 33 is formed of a conductive material such as nickel or copper.

[0135] The outer component 35 is, for example, a laminated film formed by sequentially stacking a weld layer, a metal layer, and a surface protective layer. The outer peripheries of the weld layers of the two films are welded together or bonded using an adhesive or similar method, with the weld layer facing the electrode body 31. The weld portion is, for example, a film of polyethylene or polypropylene, and the metal portion is, for example, aluminum foil. The protective layer is, for example, nylon.

[0136] A sealing membrane 34 is inserted between the outer component 35 and the positive lead, and between the outer component 35 and the negative lead, to prevent the ingress of external gas. The material of the membrane is, for example, polyethylene, polypropylene, or polyolefin resin.

[0137] Positive electrode, for example, with Figure 1 The negative electrode 10 is the same, and the positive electrode current collector has a layer of positive electrode active material on both sides or one side.

[0138] The positive current collector is formed, for example, from a conductive material such as aluminum.

[0139] The positive electrode active material layer includes any one or more positive electrode materials capable of embedding and releasing lithium ions, and may include 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 binders and positive electrode conductive additives are, for example, the same as those already described for negative electrode binders and negative electrode conductive additives.

[0140] The cathode material is preferably a lithium-containing compound. Examples of such lithium-containing compounds include composite oxides composed of lithium and transition metal elements, or phosphate compounds containing lithium and transition metal elements. Among these cathode materials, compounds containing at least one of nickel, iron, manganese, and cobalt are preferred. Their chemical formulas are, for example, denoted by Li. x M1O2 or Li ｙ M2PO4 represents the formula. In the formula, M1 and M2 represent at least one or more transition metal elements. The values ​​of x and y vary depending on the battery's charge and discharge state, but are usually expressed as 0.05≤x≤1.10 and 0.05≤y≤1.10.

[0141] As a composite oxide containing lithium and a transition metal element, examples include lithium-cobalt composite oxide (Li₂O₃). x CoO2), lithium nickel composite oxide (Li x NiO2), lithium nickel cobalt composite oxides, etc. Examples of lithium nickel cobalt composite oxides include lithium nickel cobalt aluminum composite oxide (NCA) or lithium nickel cobalt manganese composite oxide (NCM).

[0142] Examples of phosphoric acid compounds containing lithium and transition metal elements include lithium iron phosphate (LiFePO4) and lithium iron manganese phosphate (LiFe). 1-u Mn u PO4 (0 < u < 1), etc. Using these cathode materials can achieve high battery capacity and excellent cycle characteristics.

[0143] [negative electrode] The negative electrode has the same characteristics as the above. Figure 1 The lithium-ion secondary battery uses the same structure as the negative electrode 10, for example, having negative electrode active material layers on both sides of the current collector. Preferably, the negative electrode charging capacity is greater than the capacity obtained from the positive electrode active material (as the battery charging capacity). This suppresses the deposition of lithium metal on the negative electrode.

[0144] The positive electrode active material layer is disposed on a portion of both sides of the positive electrode current collector, and similarly, the negative electrode active material layer is also disposed on a portion of both sides of the negative electrode current collector. In this case, for example, the negative electrode active material layer disposed on the negative electrode current collector has regions where there is no opposing positive electrode active material layer. This is for the purpose of stable battery design.

[0145] The regions where the negative electrode active material layer and the positive electrode active material layer are not opposite each other are almost unaffected by charging and discharging. Therefore, the state of the negative electrode active material layer remains in its newly formed state, thus allowing for reproducible and accurate adjustment of the composition of the negative electrode active material regardless of whether charging and discharging occurs.

[0146] [Septum] The separator isolates the positive and negative electrodes, preventing short circuits caused by contact between the two electrodes and allowing lithium ions to pass through. This separator can be formed from a porous membrane, for example, a synthetic resin or ceramic, or it can have a laminated structure composed of two or more porous membranes. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, and polyethylene.

[0147] Electrolyte At least a portion of the active material layer or the membrane is impregnated with a liquid electrolyte (electrolyte). The electrolyte contains electrolyte salts dissolved in a solvent and may also contain other materials such as additives.

[0148] Non-aqueous solvents can be used, for example. Examples of non-aqueous solvents include ethylene carbonate, propylene carbonate, butenyl carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, 1,2-dimethoxyethane, or tetrahydrofuran. Preferably, at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate is used. This is because better properties can be obtained. Furthermore, in this case, even better properties can be obtained by combining high-viscosity solvents such as ethylene carbonate and propylene carbonate with low-viscosity solvents such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate. This is because the dissociation or ion mobility of the electrolyte salt is increased.

[0149] When using an alloy-based negative electrode, a solvent comprising at least one of a halochain carbonate or a halocyclic carbonate is particularly preferred. This allows a stable coating to form on the surface of the negative electrode active material during charging and discharging, especially during charging. Here, a halochain carbonate refers to a chain carbonate having a halogen as a constituent element (at least one hydrogen atom is substituted by a halogen). Furthermore, a halocyclic carbonate refers to a cyclic carbonate having a halogen as a constituent element (i.e., at least one hydrogen atom is substituted by a halogen).

[0150] The type of halogen is not particularly limited, but fluorine is preferred. This is because it forms a coating of better quality than other halogens. Furthermore, a higher quantity of halogen is more desirable. This is because the resulting coating is more stable, and electrolyte decomposition reactions are reduced.

[0151] Examples of halogenated chain carbonates include fluoromethyl methyl carbonate and difluoromethyl methyl carbonate. Examples of halogenated cyclic carbonates include 4-fluoro-1,3-dioxolane-2-one and 4,5-difluoro-1,3-dioxolane-2-one.

[0152] As a solvent additive, unsaturated carbon-bonded cyclic carbonates are preferred. This is because a stable coating forms on the negative electrode surface during charging and discharging, which can suppress the decomposition reaction of the electrolyte. Examples of unsaturated carbon-bonded cyclic carbonates include vinylene carbonate or ethylene ethylene carbonate.

[0153] Furthermore, sulfonyl lactones (cyclic sulfonates) are preferably included as solvent additives. This is because the chemical stability of the battery is improved. Examples of sulfonyl lactones include propane sulfonyl lactone and propene sulfonyl lactone.

[0154] Furthermore, the solvent preferably contains an acid anhydride. This is because it improves the chemical stability of the electrolyte. Examples of acid anhydrides include malondisulfonic anhydride.

[0155] The electrolyte salt may contain any one or more light metal salts such as lithium salts. Examples of lithium salts include lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4).

[0156] The content of the electrolyte salt relative to the solvent is preferably 0.5 mol / kg or more and 2.5 mol / kg or less. This is because high ionic conductivity can be obtained. Example

[0157] The following describes the present invention in more detail with examples and comparative examples, but the present invention is not limited to these examples.

[0158] (Example 1) The negative electrode active material is prepared through the following steps, and further processing is carried out. Figure 2 The laminated film type lithium-ion secondary battery 30 is shown.

[0159] (Step S1: Prepare porous carbon structures) First, the prepared surface area (BET specific surface area) is 1940 m². 2 / g, pore volume 1cm 3 / g, a porous carbon material (porous carbon structure) with a particle size (D50) of 8μm and classified as Type I by IUPAC. The porous carbon material was placed in a vacuum container and evacuated to -90kPa.

[0160] (Step S2: Make the active sites of the porous carbon structure contain OH groups and CxHy groups) Next, nitrogen gas was used for repressurization, and the mixture was heated to 150 degrees Celsius while under nitrogen pressure. At this point, a collection device was installed in the exhaust section to confirm that moisture had not been completely removed. Additionally, a gas detector was used simultaneously to confirm the CO gas concentration. This confirmed that the active sites of the porous carbon structure included OH and CxHy groups.

[0161] (Step S3: Under heating and in the state of exhaust gas containing carbon monoxide or carbon dioxide containing oxygen atoms from active sites, silane gas is introduced, so that silicon oxide bonded with oxygen atoms is deposited inside the porous carbon structure and the silicon oxide is in a state containing dangling bonds) The silane decomposition reaction was carried out under conditions where approximately 8000 ppm of CO gas was detected during drying prior to the silane reaction. At this time, the decomposition reaction occurred in the range of 365–380 degrees Celsius, which is below the typical silane decomposition reaction temperature.

[0162] It is believed that by controlling the fluidized bed to a positive pressure, silane is introduced into the pores. At the same time, in the presence of CO gas, the molecular structure is deformed and the activation energy is lowered, thereby accelerating the reaction rate and enabling low-temperature decomposition. Furthermore, it can capture oxygen components attached to porous carbon during decomposition to form Si-O bonds.

[0163] (Step S4: Oxidize at least a portion of the Si present on the surface of the porous carbon structure under a pressurized atmosphere) After the reaction, the mixture was allowed to return to room temperature and then subjected to oxidation under a 20 kPa pressurized atmosphere, thereby transforming the outermost Si into a low-valence oxide state containing defects, forming a SiO2 structure.

[0164] Then, heat treatment at 600 degrees Celsius is performed to react the residual hydrocarbon components with Si-H, thereby generating Si-C bonds inside the bulk phase.

[0165] (Step S5: Deposit a carbon layer using hydrocarbon gas at a temperature below 580 degrees Celsius) When acetylene gas is used as a hydrocarbon gas and subjected to CVD at 580 degrees Celsius, some oxygen transfer occurs at the interface between the oxygen-rich layer and the carbon layer, forming C=O or CO bonds. If a structure with a valence similar to tetravalent exists on the surface, Si-C bonds will not be formed on the surface during the decomposition of acetylene gas.

[0166] Acetylene CVD is carried out under reduced pressure of 10000 Pa, penetrating into the porous carbon and enabling reactions within the porous carbon. This reaction involves breaking the bonds in the generated Si-H groups to form Si-C bonds. Therefore, hydrogen production can be suppressed during slurry preparation. Furthermore, the higher the CVD temperature, the more Si-C bonds are formed.

[0167] [Determination of negative electrode active materials] The negative electrode active material prepared by the above method was subjected to TEM-EDX, XRD and Raman spectroscopy analysis.

[0168] [Making the negative electrode] The negative electrode active material, graphite, conductive additive 1 (carbon nanotubes, CNT), conductive additive 2 (carbon microparticles with a median diameter of about 50 nm), sodium polyacrylate, and carboxymethyl cellulose (hereinafter referred to as CMC) prepared by the above method were mixed with a dry mass ratio of 9.3:83.7:1:1:4:1 and then diluted with pure water to prepare a negative electrode slurry.

[0169] Furthermore, an electrolytic copper foil with a thickness of 15 μm was used as the negative electrode current collector. This electrolytic copper foil contained carbon and sulfur at a concentration of 70 ppm by mass. Finally, a negative electrode slurry was coated onto the negative electrode current collector and dried at 100°C for 1 hour in a vacuum atmosphere. The deposition amount (also known as areal density) of the negative electrode active material layer per unit area on one side of the dried negative electrode was 7.0 mg / cm³. 2 .

[0170] [Assembly of the experimental button battery] Next, an electrolyte was prepared by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) as solvents and then dissolving an electrolyte salt (lithium hexafluorophosphate: LiPF6) in the mixture. In this case, the solvent composition was set to EC:DMC = 30:70 by volume, and the electrolyte salt content relative to the solvent was set to 1 mol / kg. Ethylene carbonate (VC) and fluoroethylene carbonate (FEC) were added as additives at amounts of 1.0% by mass and 2.0% by mass, respectively.

[0171] Next, the button cell is assembled as follows. First, a 1mm thick Li foil is punched into a 16mm diameter piece and attached to the aluminum clad.

[0172] Next, the previously obtained negative electrode is punched into a diameter of 15mm, so that it faces the Li foil attached to the aluminum coating through the separator. After injecting electrolyte, a 2032 button cell is made.

[0173] [Determination of initial efficiency] The initial efficiency was determined under the following conditions. First, for the button cell prepared for the initial efficiency test, the charging rate was set to 0.03C, and charging was performed in CCCV mode (first charge). CV was set to 0V and the cutoff current was set to 0.04mA. Next, the discharge rate was similarly set to 0.03C, and the discharge cutoff voltage was set to 1.0V, and CC discharge was performed (first discharge).

[0174] The initial efficiency (sometimes referred to as initial efficiency) is calculated when investigating the initial charge and discharge characteristics. The initial efficiency is calculated using the formula shown below. First-time efficiency (%) = (first-time discharge capacity / first-time charge capacity) × 100

[0175] [Manufacturing and Evaluation of Lithium-ion Secondary Batteries] Based on the initial data obtained, the positive electrode was designed to achieve a 95% utilization rate for the negative electrode. The utilization rate was calculated using the following formula, based on the capacities of the positive and negative electrodes obtained from the counter electrode Li. Utilization rate = (positive electrode capacity - negative electrode loss) / (negative electrode capacity - negative electrode loss) × 100 Based on this design, lithium-ion secondary batteries (such as those used in the embodiments and comparative examples) were manufactured respectively. Figure 2 (The lithium-ion secondary battery shown). Battery evaluation was performed on each lithium-ion secondary battery of the examples and comparative examples.

[0176] Cycle characteristics were investigated as follows. First, for battery stabilization, two charge-discharge cycles were performed at 0.2C in an atmosphere at 25°C, and the discharge capacity of the second cycle was measured. Battery cycle characteristics were calculated starting from the discharge capacity of the third cycle, and the battery test was stopped at the 1000th cycle. Charge and discharge were performed at 0.7C and 0.5C. The charging voltage was set to 4.3V, the discharge cutoff voltage to 2.5V, and the charging cutoff rate to 0.07C.

[0177] SEM was used to observe porous carbon structures in the negative electrode active material particles, thereby investigating the types of silicon-core carbon complexes.

[0178] The results of each measurement are shown in Table 1. Comparative Examples 1-2 and Examples 2-14, which will be described later, are also shown in Table 1.

[0179] [Table 1]

[0180] (Compare Examples 1 and 2) Comparative Example 1 was placed in a state where CO gas was not detected (below the detection limit) during drying before the silane reaction. At this time, the CxHy component present inside the porous carbon pores was removed by heat treatment at 600 degrees Celsius.

[0181] Then, a silane reaction was performed at a product temperature of 380 degrees Celsius to form amorphous silicon within the bulk phase. The reaction was terminated when the fill rate was 98% relative to the pore volume, and then the mixture was cooled and allowed to return to room temperature. Next, oxygen diluted with nitrogen (1% concentration) was introduced for 48 hours for slow oxidation. During this process, oxidation was carried out under reduced pressure (reaching a gauge pressure of -95 kPa) while stirring the powder to ensure sufficient diffusion of oxygen into the pores. The oxide was SiO2.

[0182] Comparative Example 2, based on Comparative Example 1, adjusted the oxidizing atmosphere to a slight reduced pressure of -5 kPa, and the oxidation was carried out more slowly than in Comparative Example 1.

[0183] For reference, Figure 8The XANES spectra of the surface portion of the negative electrode active material of Example 1 and Comparative Example 1 are shown. Although the Comparative Example formed a structure similar to SiO2, observation of the XANES spectrum shows that the peak is slightly shifted to the lower energy side, indicating that it is SiO2 with defects. Furthermore, the structure of the surface portion is similar to that of Example 1.

[0184] Then, the same evaluation as in Example 1 was performed, and the results showed that the cycle characteristics (retention rate after 1000 cycles in Table 1) of Comparative Examples 1 and 2 were both poor, and the battery capacity (cell capacity in Table 1) was also low, resulting in unsatisfactory results.

[0185] (Examples 2-6) Based on the manufacturing method of Example 1, a heat treatment is performed after oxidation to cause a portion of the Si to crystallize. At this time, Si... 0+ The grain size is 3~6.5nm. Since higher crystallinity leads to poorer Li acceptance, the battery performance is considered to be slightly worse.

[0186] The battery achieves its best characteristics when it is essentially amorphous (the 0.8nm grain size in the table is calculated using the Scherrer formula, and is essentially amorphous).

[0187] However, considering that the powder workability would improve once the temperature was increased, it is best to make the crystallites below 5 nm.

[0188] (Examples 7-11) The oxygen content of Example 1 was modified and evaluated. If the oxygen content was low, there was a tendency to generate a large number of Si-C bonds during acetylene CVD, resulting in a slight deterioration in cycle characteristics. In this case, an oxygen content of 0.5 wt% or higher was considered optimal to prevent a significant decrease in cycle characteristics. Conversely, if the oxygen content was too high, the excess oxygen would react with Li, increasing irreversible capacity and decreasing battery capacity. Therefore, the optimal range for oxygen content was considered to be between 0.5 wt% and 5 wt%.

[0189] (Examples 12-14) The pressure during the introduction of silane gas in Example 1 was varied and evaluated. If the pressure was too low, the silane gas could not reach the depth of the pores, creating voids that could not fill the silicon, thus reducing the battery capacity.

[0190] Furthermore, the silicon deposited under pressure is rich in dangling bonds, which decreases at lower pressures. The high dangling bond content results in very high reactivity, enabling low-temperature carbon decomposition during subsequent carbon CVD.

[0191] As a method to increase pressure, the internal pressure can be adjusted by regulating the flow rate of the introduced silane and the opening of the valve on the exhaust side. The pressure of a fluidized bed is usually around 10 kPa, and it is difficult to control the pressure if it is above 30 kPa. In addition to adjusting the aforementioned open valve, the internal pressure can be adjusted by simultaneously preparing an exhaust valve. For example, if you want to carry out the reaction at 50 kPa, you can adjust the internal pressure by operating the exhaust valve in the range of 50 + 5 kPa.

[0192] From a productivity standpoint, although this method significantly reduces productivity, it allows gas to diffuse into minute areas.

[0193] As demonstrated above, compared to the comparative example, the embodiments of the present invention can suppress excessive decomposition of the electrolyte, resulting in a negative electrode active material that can improve battery cycle characteristics and increase capacity.

[0194] This manual contains the following solutions. [1]: A negative electrode active material, which is a negative electrode active material having negative electrode active material particles, characterized in that, The negative electrode active material particles contain porous carbon structures as a matrix. The porous carbon structure has Si-C bonds inside, amorphous low-valent nano-silicon oxides scattered on the surface, and at least a portion of the low-valent nano-silicon oxides has tetravalent silicon oxides capable of intercalating and deintercalating Li. [2]: According to the negative electrode active material of [1] above, the low-valence nano silicon oxide is characterized in that it is inserted into Li during the insertion and extraction of Li. [3]: The negative electrode active material according to [1] or [2] above is characterized in that, through 29 The spectrum obtained by Si-MAS-NMR measurement has peaks from the Si-C bonds in the range of 0 to -30 ppm, and peaks from Si or the low-valent nano-silicon oxide in the range of -40 to -80 ppm. [4]: According to any one of the above [1] to [3], the active material is characterized in that there are active sites inside the pores of the porous carbon structure, and a portion of the active sites are modified with OH. [5]: According to any one of the negative electrode active materials in [1] to [4] above, the characteristic is that, The low-cost nano-silicon oxide has other carbon layers that are different from the porous carbon structure that serves as the matrix. The interfaces of the other carbon layers are formed with C=O bonds or CO bonds. [6]: According to any one of the above [1] to [5], the negative electrode active material is characterized in that the grain size of the 0-valent Si constituting the low-valent nano-silicon oxide is in the range of 0.8 nm to 5 nm, based on the peak obtained by measuring the particles of the negative electrode active material by X-ray diffraction and calculated using the Scherrer formula. [7]: The negative electrode active material according to [6] above is characterized in that the 0-valent Si is substantially amorphous. [8]: According to any one of the above [1] to [7], the negative electrode active material is characterized in that the oxygen content contained in the negative electrode active material as a whole is in the range of more than 0.5 wt% and less than 5 wt%. [9]: A method for manufacturing a negative electrode active material, characterized in that it comprises the following steps: Steps for preparing porous carbon structures; The step of making the active sites of the porous carbon structure contain OH groups and CxHy groups; The step of introducing silane gas under heating and in a state where the exhaust gas contains carbon monoxide or carbon dioxide gas containing oxygen atoms from the active sites, causing silicon oxide bonded to the oxygen atoms to be deposited inside the porous carbon structure and making the silicon oxide contain dangling bonds. Following the step of depositing the silicon oxide, the step of oxidizing at least a portion of the Si present on the surface of the porous carbon structure under a pressurized atmosphere; and Following the oxidation step under the pressurized atmosphere, a carbon layer is deposited using hydrocarbon gas at a temperature below 580 degrees Celsius.

[0195] Furthermore, this invention is not limited to the above-described embodiments. The above embodiments are illustrative examples, and any technical solution having a substantially identical structure and achieving the same effect as the technical concept described in the claims of this invention is included within the technical scope of this invention.

Claims

1. A negative electrode active material, which is a negative electrode active material having negative electrode active material particles, characterized in that, The negative electrode active material particles contain porous carbon structures as a matrix. The porous carbon structure has Si-C bonds inside, amorphous low-valent nano-silicon oxides scattered on the surface, and at least a portion of the low-valent nano-silicon oxides has tetravalent silicon oxides capable of intercalating and deintercalating Li.

2. The negative electrode active material according to claim 1, characterized in that, The low-cost nano-silicon oxide inserts Li during the Li insertion and deintercalation process.

3. The negative electrode active material according to claim 1, characterized in that, pass 29 The spectrum obtained by Si-MAS-NMR measurement has peaks from the Si-C bonds in the range of 0 to -30 ppm, and peaks from Si or the low-valent nano-silicon oxide in the range of -40 to -80 ppm.

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

5. The negative electrode active material according to claim 1, characterized in that, The low-cost nano-silicon oxide has other carbon layers that are different from the porous carbon structure that serves as the matrix. The interfaces of the other carbon layers are formed with C=O bonds or CO bonds.

6. The negative electrode active material according to claim 1, characterized in that, Based on the peaks obtained by measuring the negative electrode active material particles by X-ray diffraction, and calculated using the Scherrer formula, the grain size of the 0-valent Si constituting the low-valent nano-silicon oxide is in the range of 0.8 nm to 5 nm.

7. The negative electrode active material according to claim 6, characterized in that, The zero-valent Si is essentially amorphous.

8. The negative electrode active material according to claim 1, characterized in that, The oxygen content of the negative electrode active material is in the range of 0.5wt% to 5wt%.

9. A method for manufacturing a negative electrode active material, characterized in that, It includes the following steps: Steps for preparing porous carbon structures; The step of making the active sites of the porous carbon structure contain OH groups and CxHy groups; The step of introducing silane gas under heating and in a state where the exhaust gas contains carbon monoxide or carbon dioxide gas containing oxygen atoms from the active sites, causing silicon oxide bonded to the oxygen atoms to be deposited inside the porous carbon structure and making the silicon oxide contain dangling bonds. Following the step of depositing the silicon oxide, a step of oxidizing at least a portion of the Si present on the surface of the porous carbon structure under a pressurized atmosphere; and Following the oxidation step under the pressurized atmosphere, a carbon layer is deposited using hydrocarbon gas at a temperature below 580 degrees Celsius.

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