Anode active material, anode containing the anode active material, non-aqueous electrolyte secondary battery using the anode active material, and method for producing the anode active material.

Abstract

The present invention provides a negative electrode active material that reduces the generation of ionic substances, suppresses excessive decomposition of the electrolyte, and as a result improves battery cycle characteristics. [Solution] A negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure which is a matrix material, the porous carbon structure has Si-C bonds inside, amorphous low-valence nanosilicon oxide dispersed in the surface layer, and at least a portion of the low-valence nanosilicon oxide has a tetravalent silicon oxide capable of adsorbing and desorbing Li.

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 a positive electrode, a negative electrode, a separator, and an electrolyte, 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 (also called a newly formed 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 active material - current collector interface is 0.4 or less (see, for example, Patent Document 7). Further, in order to improve the battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 8). Further, in order to improve cycle characteristics, a hydrophobic layer such as a silane compound is formed on the surface layer of the silicon material (see, for example, Patent Document 9).

[0010] Also, in order to improve cycle characteristics, silicon oxide is used, and conductivity is imparted by forming a graphite film on its surface layer (see, for example, Patent Document 10). In Patent Document 10, regarding the shift value obtained from the RAMAN spectrum of the graphite film, a broad peak appears 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. Also, in order to improve high battery capacity and cycle characteristics, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used (see, for example, Patent Document 11). Further, in order to improve overcharge and overdischarge characteristics, 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] Also, a lithium - ion secondary battery using silicon oxide was such that Hitachi Maxell started shipping a rectangular secondary battery for smartphones employing a nanosilicon composite in June 2010 (see, for example, Non - Patent Document 1). The silicon oxide proposed by Hohl is a composite 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 the silicon oxide (Non - Patent Document 3).

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

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

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

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

[0016] Furthermore, focusing on the problem that an excessive amount of silicon is deposited on the surface due to the small pore diameter of the porous carbon material, resulting in an increase in the electrical resistivity, composite particles containing no SiC (silicon carbide) or having an extremely low SiC content have been proposed to reduce the electrical resistivity (Patent Document 16).

Prior Art Documents

Patent Documents

[0017]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Patent Document 9

Patent Document 10

Patent Document 11

Patent Document 12

Patent Document 13

Patent Document 14

Patent Document 15

[0018] [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. [Overview of the project] [Problems that the invention aims to solve]

[0019] 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.

[0020] 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.

[0021] 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.

[0022] Also, generally speaking, Li-Si compounds (for example, Li 15 Si4 exhibits ionic properties, reducing its diffusivity. When such ionic substances are generated, the negative electrode active material becomes more prone to cracking, as mentioned above. When the surface layer 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, further consuming the electrolyte. If charging and discharging are repeated in this state, the electrolyte decomposes and is consumed each time a new surface is created, resulting in a problem of reduced cycle performance.

[0023] The present invention has been made in view of the above-mentioned problems, and aims to provide a negative electrode active material that can reduce the generation of ionic substances, suppress excessive decomposition of the electrolyte, and as a result improve battery cycle characteristics. [Means for solving the problem]

[0024] 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 as a base material, the porous carbon structure has Si-C bonds inside, amorphous low-valence nanosilicon oxide is dispersed in the surface layer, and at least a portion of the low-valence nanosilicon oxide has a tetravalent silicon oxide capable of intercalating and deintercalating Li.

[0025] In the negative electrode active material of the present invention, a portion of the low-valence nanosilicon oxide dispersed in the surface layer contains tetravalent silicon oxide capable of intercalating and deintercalating Li. First, since the low-valence nanosilicon oxide has high Li diffusivity, the low-valence nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the formation of new surfaces. Furthermore, during charging and discharging, the tetravalent silicon oxide intercalates and deintercalates Li in the surface layer, repeatedly decomposing and generating Li silicate. As a result, excessive decomposition of the electrolyte can be suppressed, and consequently, a negative electrode active material capable of improving battery cycle characteristics can be provided. Moreover, the internal Si-C bond has a strong bond with the Si in the surface layer, which suppresses electronic disconnection of the Si portion during charging and discharging, thus stabilizing the structure.

[0026] Furthermore, it is preferable that the low-valence nanosilicon oxide absorbs Li during the Li absorption and desorption process.

[0027] If the low-valent nanosilicon oxide dispersed in the surface layer absorbs Li in this way, the tetravalent silicon oxide present in some of the low-valent nanosilicon oxide can reliably decompose and generate Li silicate in the surface layer.

[0028] Also, 29 Preferably, the spectrum obtained by Si-MAS-NMR measurement has a peak originating from the Si-C bond in the range of 0 to -30 ppm and a peak originating from Si or the low-valent nanosilicon oxide in the range of -40 to -80 ppm.

[0029] With a spectrum like this, the tetravalent silicon oxide present in some of the low-valence nanosilicon oxides dispersed in the surface layer can reliably decompose and generate Li silicate. Furthermore, the internal Si-C bonds firmly bind to the Si in the surface layer, suppressing electronic disconnection of the Si portion during charging and discharging, thereby reliably stabilizing the structure.

[0030] Furthermore, it is preferable that active sites exist within the pores of the porous carbon structure, and that some of these active sites are modified with OH groups.

[0031] With such a pore structure, for example, heat treatment can be performed to create an exhaust gas containing a large amount of CO. Further reaction with silane gas in a pressurized atmosphere allows silane decomposition products to be dispersed as amorphous, low-valence nanosilicon oxides inside the porous carbon structure, particularly in the surface layer, making it easy to produce the desired negative electrode active material.

[0032] Furthermore, it is preferable that the low-valence nanosilicon oxide has a carbon layer separate from the porous carbon structure which is the base material, and that the interface of the separate carbon layer has a C=O bond or a CO bond formed thereon.

[0033] If such C=O or CO bonds are formed, it is thought that compounds similar to Si-OC are being produced, which can strengthen the adhesion of the carbon layer and enable the creation of a robust structure, such as maintaining integrity even when high-speed shear is applied during slurry preparation.

[0034] 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.

[0035] Such a grain size of zero-valent Si, which is substantially amorphous, is preferred.

[0036] Furthermore, it is preferable that the zero-valent Si is substantially amorphous.

[0037] Such substantially amorphous materials are preferred.

[0038] Furthermore, the total amount of oxygen contained in the negative electrode active material is preferably in the range of 0.5 wt% or more and 5 wt% or less.

[0039] Within this range of oxygen levels, it is possible to prevent excessive Si-C bond formation during acetylene CVD, which would reduce battery capacity, from occurring due to insufficient oxygen, while also preventing an irreversible increase in capacity due to excessive oxygen levels.

[0040] Furthermore, in order to solve the above problems, the present invention provides a method for producing a negative electrode active material, comprising the steps of: preparing a porous carbon structure; including OH groups and CxHy groups in the active sites of the porous carbon structure; flowing monosilane gas under heating, with carbon monoxide gas or carbon dioxide gas derived from the oxygen atoms of the active sites contained 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; oxidizing at least a portion of the Si present on the surface layer of the porous carbon structure in a pressurized atmosphere after the step of depositing the silicon oxide; and depositing a carbon layer at 580 degrees or less using a hydrocarbon gas after the step of oxidation in a pressurized atmosphere.

[0041] In the present invention's method for producing a negative electrode active material, Si-C bonds are formed inside a porous carbon structure, amorphous low-valence nanosilicon oxide is dispersed in the surface layer, and oxidation in a pressurized atmosphere generates tetravalent silicon oxide capable of intercalating and deintercalating Li in a portion of the low-valence nanosilicon oxide. Therefore, in a negative electrode electrode using such a negative electrode active material, the low-valence nanosilicon oxide has high Li diffusivity, so the low-valence nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the formation of new surfaces. Furthermore, during charging and discharging, the tetravalent silicon oxide intercalates and deintercalates Li in the surface layer, repeatedly decomposing and generating Li silicate. As a result, excessive decomposition of the electrolyte can be suppressed, and a negative electrode active material capable of improving battery cycle characteristics can be provided. Moreover, the internal Si-C bonds have a strong bond with the Si in the surface layer, which suppresses electronic disconnection of the Si portion during charging and discharging, thus stabilizing the structure. The present invention provides a method for producing negative electrode active materials that can be manufactured simply and efficiently. In particular, it reduces the generation of ionic substances, suppresses excessive decomposition of the electrolyte, and as a result provides a method for producing negative electrode active materials that can improve battery cycle characteristics. [Effects of the Invention]

[0042] In the negative electrode active material of the present invention, a portion of the low-valence nanosilicon oxide dispersed in the surface layer contains tetravalent silicon oxide capable of intercalating and deintercalating Li. First, since the low-valence nanosilicon oxide has high Li diffusivity, the low-valence nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the formation of new surfaces. Furthermore, during charging and discharging, the tetravalent silicon oxide intercalates and deintercalates Li in the surface layer, repeatedly decomposing and generating Li silicate. As a result, excessive decomposition of the electrolyte can be suppressed, and consequently, a negative electrode active material capable of improving battery cycle characteristics can be provided. Moreover, the internal Si-C bond has a strong bond with the Si in the surface layer, which suppresses electronic disconnection of the Si portion during charging and discharging, thus stabilizing the structure.

[0043] In the method for producing the negative electrode active material of the present invention, Si-C bonds are formed inside a porous carbon structure, amorphous low-valence nanosilicon oxide is dispersed in the surface layer, and oxidation in a pressurized atmosphere generates tetravalent silicon oxide capable of intercalating and deintercalating Li in a portion of the low-valence nanosilicon oxide. Therefore, in a negative electrode electrode using such a negative electrode active material, since the low-valence nanosilicon oxide has high Li diffusivity, the generation of ionic substances is reduced by the low-valence nanosilicon oxide dispersed in the surface layer, suppressing the generation of new surfaces. Furthermore, during charging and discharging, the tetravalent silicon oxide intercalates and deintercalates Li in the surface layer, repeatedly decomposing and generating Li silicate. As a result, excessive decomposition of the electrolyte can be suppressed, and consequently, a negative electrode active material capable of improving battery cycle characteristics can be provided. Moreover, the internal Si-C bonds have a strong bond with the Si in the surface layer, which suppresses electronic disconnection of the Si portion during charging and discharging, thus stabilizing the structure. The present invention provides a method for producing negative electrode active materials that can be manufactured simply and efficiently. In particular, it reduces the generation of ionic substances, suppresses excessive decomposition of the electrolyte, and as a result provides a method for producing negative electrode active materials that can improve battery cycle characteristics. [Brief explanation of the drawing]

[0044] [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 the XANES spectrum of the surface layer of the negative electrode active material during charging. [Figure 4] This is the XANES spectrum of the surface layer of the negative electrode active material during discharge. [Figure 5] This is the spectrum obtained by 29Si-MAS-NMR measurement of the negative electrode active material. [Figure 6] This is the spectrum obtained by conventional 29Si-MAS-NMR measurement of a negative electrode active material. [Figure 7]This is a flowchart illustrating an example of a method for producing the negative electrode active material of the present invention. [Figure 8] These are the XANES spectra of the surface layer of the negative electrode active material in Example 1 and Comparative Example 1. [Modes for carrying out the invention]

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

[0046] 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.

[0047] Therefore, the present inventors diligently conducted research to obtain a negative electrode active material that, when used as the negative electrode of a secondary battery, can improve initial charge-discharge characteristics while obtaining high cycle characteristics and increasing battery capacity, leading to the present invention.

[0048] In particular, Li-Si compounds (e.g., Li 15 Si4 exhibits ionic properties, and its reduced diffusivity makes the negative electrode active material prone to cracking. When the surface layer of the negative electrode active material cracks, a new surface (new surface) is created, and repeated charging and discharging causes the electrolyte to decompose and be excessively consumed each time a new surface is created. In this invention, in order to reduce the generation of ionic substances and suppress excessive decomposition of the electrolyte, a negative electrode active material was developed that has Si-C bonds inside a porous carbon structure, amorphous low-valence nanosilicon oxide dispersed in the surface layer, and a portion of the low-valence nanosilicon oxide contains tetravalent silicon oxide.

[0049] [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 which is a matrix material, and the porous carbon structure has Si-C bonds inside, amorphous low-valence nanosilicon oxide dispersed in the surface layer, and at least a portion of the low-valence nanosilicon oxide has a tetravalent silicon oxide capable of adsorbing and desorbing Li.

[0050] With such a negative electrode active material, some of the low-valence nanosilicon oxide dispersed in the surface layer contains tetravalent silicon oxide capable of intercalating and deintercalating Li. Since the low-valence nanosilicon oxide has high Li diffusivity, the low-valence nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the formation of new surfaces. Furthermore, during charging and discharging, the tetravalent silicon oxide intercalates and deintercalates Li in the surface layer, repeatedly decomposing and generating Li silicate. As a result, excessive decomposition of the electrolyte can be suppressed, and consequently, a negative electrode active material capable of improving battery cycle characteristics can be provided. Moreover, the internal Si-C bond has a strong bond with the Si in the surface layer, which suppresses electronic disconnection of the Si portion during charging and discharging, thus stabilizing the structure.

[0051] Furthermore, although not particularly limited, it is preferable that the low-valent nanosilicon oxide absorbs Li during the Li absorption and desorption process.

[0052] If the low-valent nanosilicon oxide dispersed in the surface layer absorbs Li in this way, the tetravalent silicon oxide present in some of the low-valent nanosilicon oxide can reliably decompose and generate Li silicate in the surface layer.

[0053] To explain the process of lithium absorption and desorption (charging and discharging), please refer to the diagram.

[0054] 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.

[0055] Figures 3 and 4 show the XANES spectra of the negative electrode in Figure 1 and the negative electrode active material applied to the lithium-ion secondary battery in Figure 2. Figure 3 shows the XANES spectrum of the surface layer (up to 50 nm from the surface) during charging, and Figure 4 shows the XANES spectrum of the surface layer during discharge.

[0056] Figure 3 shows the spectrum during charging of the surface layer. Starting from the initial state before charging (Initial in the figure), the SOC increases in the direction of the arrows in the figure as charging progresses to SOC 20% (20% charge; here, SOC (State of Charge) represents the charging state), SOC 50%, and SOC 100% (full charge).

[0057] Initially, a peak for tetravalent silicon oxide (tetravalent Si oxide in the figure) can be seen, but it can be observed that it changes to a peak for Li silicate upon charging (Li absorption). Generally, tetravalent silicon oxide changes to Li4SiO4, but it is thought that Li absorption leads to a lower-valent Li silicate, in this case closer to Li2SiO3.

[0058] Figure 4 shows the spectrum during discharge of the surface layer. From SOC 100% (fully charged), the spectrum decreases in the direction of the arrows in the figure as the discharge progresses to DOD 20% (20% discharge; here, DOD (Depth of Discharge) is the ratio of the discharge amount to the discharge capacity, i.e., the depth of discharge), DOD 50%, DOD 70%, and DOD 100%.

[0059] In this way, the surface layer repeatedly decomposes and generates Li silicate during charging and discharging, suppressing excessive decomposition of the electrolyte and enabling high cycle performance.

[0060] Furthermore, XANES (X-ray Absorption Near-Edge Structure) measurements can be performed, for example, under the following conditions. • Aichi Synchrotron Radiation Center, using BL6N1 line • Samples were prepared in an Ar atmosphere, airless, stored in a transfer vessel, and connected to the BL6N1 line for measurement under the following conditions. • Acceleration energy: 1.2 GeV • Accumulated current value: 300mA • Monochromatization conditions: White X-rays from a bending magnet are monochromatized using a two-crystal spectrometer and used for measurement. • Light focusing conditions: Light focusing in both vertical and horizontal directions using a Ni-coated bent cylindrical mirror. • Upstream slit opening: 10.0 mm horizontally x 3.0 mm vertically • Beam size: 2.0mm horizontally x 1.0mm vertically • Incidence angle to the sample: 45 degrees (incidence angle 45 degrees), so fluorescence yield can be measured simultaneously. • Energy calibration: Calibration of the peak position at the SK end of K2SO4 to 2481.70 eV. • Measurement method: Total electron yield method by measuring sample current, X-ray fluorescence ·I0 measurement method: XANES measurement Au-mesh Cu-mesh during EXAFS measurement

[0061] Also, although not specifically limited, 29 The spectrum obtained by Si-MAS-NMR measurement preferably has peaks originating from Si-C bonds in the range of 0 to -30 ppm and peaks originating from Si or low-valent nanosilicon oxides in the range of -40 to -80 ppm.

[0062] With a spectrum like this, the tetravalent silicon oxide present in some of the low-valence nanosilicon oxides dispersed in the surface layer can reliably decompose and generate Li silicate. Furthermore, the internal Si-C bonds firmly bind to the Si in the surface layer, suppressing electronic disconnection of the Si portion during charging and discharging, thereby reliably stabilizing the structure.

[0063] For reference, Figure 5 shows the negative electrode active material of the present invention. 29An example of a spectrum obtained by Si-MAS-NMR measurement is shown. It has peaks originating from Si-C bonds in the range of 0 to -30 ppm (Si-C in the figure) and peaks originating from Si or low-valent nanosilicon oxides in the range of -40 to -80 ppm (amorphous Si in the figure).

[0064] Figure 6 also shows a conventional negative electrode active material without Si-C bonds (for example, as in Patent Document 16). 29 The spectrum obtained by Si-MAS-NMR measurement is shown. Compared to Figure 5, no peaks originating from the Si-C bond are visible.

[0065] The valency of low-valency nanosilicon 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) • Equipment: Bruker 700NMR spectrometer, • Probe: 4mm HR-MAS rotor, 50μL • Sample rotation speed: 10 kHz, ·Measurement environment temperature: 25℃

[0067] 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.

[0068] 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.

[0069] In this composite state, in addition to achieving a lower irreversible capacity, by adjusting the distribution of zero, mono, di, and trivalent elements, it is possible to more effectively realize low-valence nanosilicon oxides that absorb and desorb Li.

[0070] Furthermore, the low-valent nanosilicon oxide phase present inside the porous carbon structure may also contain a microcrystalline phase of Si with a zero valency.

[0071] Furthermore, although not particularly limited, it is preferable that active sites exist within the pores of the porous carbon structure, and that some of the active sites are modified with OH groups.

[0072] With such a pore structure, for example, heat treatment can be performed to create an exhaust gas containing a large amount of CO. Further reaction with silane gas in a pressurized atmosphere allows silane decomposition products to be dispersed as amorphous, low-valence nanosilicon oxides inside the porous carbon structure, particularly in the surface layer, making it easy to produce the desired negative electrode active material.

[0073] Furthermore, although not particularly limited, it is preferable that the low-valent nanosilicon oxide has a carbon layer separate from the porous carbon structure of the matrix material, and that the interface of the separate carbon layer has a C=O bond or a CO bond formed thereon.

[0074] If such C=O or CO bonds are formed, it is thought that compounds similar to Si-OC are being produced, which can strengthen the adhesion of the carbon layer and enable the creation of a robust structure, such as maintaining integrity even when high-speed shear is applied during slurry preparation.

[0075] 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.

[0076] Such a grain size of zero-valent Si, which is substantially amorphous, is preferred.

[0077] 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".

[0078] Furthermore, although not particularly limited, it is preferable that the zero-valent Si is substantially amorphous.

[0079] Furthermore, although not particularly limited, the total amount of oxygen contained in the negative electrode active material is preferably in the range of 0.5 wt% or more and 5 wt% or less.

[0080] Within this range of oxygen levels, it is possible to prevent excessive Si-C bond formation during acetylene CVD, which would reduce battery capacity, from occurring due to insufficient oxygen, while also preventing an irreversible increase in capacity due to excessive oxygen levels.

[0081] 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.

[0082] 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.

[0083] Furthermore, porous carbon can be derived from bio-based, resin-based, or petroleum-based sources, with bio-based or resin-based sources being preferred.

[0084] <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.

[0085] [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.

[0086] [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).

[0087] 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.

[0088] 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.

[0089] [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.

[0090] [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.

[0091] First, we will explain the manufacturing method for the negative electrode active material contained in the negative electrode.

[0092] The present invention provides a method for producing a negative electrode active material, comprising the steps of: preparing a porous carbon structure; including OH groups and CxHy groups in the active sites of the porous carbon structure; flowing monosilane gas through the porous carbon structure under heating, with carbon monoxide gas or carbon dioxide gas derived 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; oxidizing at least a portion of the Si present on the surface layer of the porous carbon structure in a pressurized atmosphere after the step of depositing the silicon oxide; and depositing a carbon layer at 580 degrees Celsius or lower using a hydrocarbon gas after the step of oxidation in a pressurized atmosphere.

[0093] In the present invention's method for producing a negative electrode active material, Si-C bonds are formed inside a porous carbon structure, amorphous low-valence nanosilicon oxide is dispersed in the surface layer, and oxidation in a pressurized atmosphere generates tetravalent silicon oxide capable of intercalating and deintercalating Li in a portion of the low-valence nanosilicon oxide. Therefore, in a negative electrode electrode using such a negative electrode active material, the low-valence nanosilicon oxide has high Li diffusivity, so the low-valence nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the formation of new surfaces. Furthermore, during charging and discharging, the tetravalent silicon oxide intercalates and deintercalates Li in the surface layer, repeatedly decomposing and generating Li silicate. As a result, excessive decomposition of the electrolyte can be suppressed, and a negative electrode active material capable of improving battery cycle characteristics can be provided. Moreover, the internal Si-C bonds have a strong bond with the Si in the surface layer, which suppresses electronic disconnection of the Si portion during charging and discharging, thus stabilizing the structure. The present invention provides a method for producing negative electrode active materials that can be manufactured simply and efficiently. In particular, it reduces the generation of ionic substances, suppresses excessive decomposition of the electrolyte, and as a result provides a method for producing negative electrode active materials that can improve battery cycle characteristics.

[0094] 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.

[0095] In this case, the Si produced by the reaction of silane gas has at least a portion of Si-H bonds and is deposited in a Si-Si state. Furthermore, the substance obtained in the oxidation process is SiO2, so Si 0+ and Si 4+ The composite material is formed within porous carbon.

[0096] Furthermore, since SiO2 is present in the outermost layer, general SiC active materials can be produced by performing carbon coating CVD with hydrocarbon gas on the upper part of the layer.

[0097] In contrast to these conventional methods, the present invention's method for producing a negative electrode active material utilizes the active sites of porous carbon to incorporate OH groups.

[0098] When heat treatment is performed with OH groups present, CO is detected in the exhaust gas. Normally, CO gas does not appear when the material is completely dried, but by reacting it with silane gas in a pressurized atmosphere while the exhaust gas contains a large amount of CO, silane decomposition products are deposited on porous carbon as low-valent oxides. At this time, since CO gas continues to escape from inside the porous pores, it is important to use a hydrogen carrier in a pressurized atmosphere to allow the silane gas to penetrate to the interior.

[0099] While typical fluidized beds experience a pressure drop of around 10 kPa, this reaction is known to gradually begin from around 25 kPa.

[0100] Low-valent oxides have high lithium diffusion. Generally, compounds of Si and Li contain Li 15 While Si4 is produced, it exhibits ionic properties and reduced diffusivity. In contrast, low-valent oxides can reduce the formation of ionic substances.

[0101] Furthermore, by increasing the pressure during silicon oxide deposition and reaction, it becomes possible to create a large number of dangling bonds. These dangling bonds are highly reactive and can be carbonized at temperatures lower than the typical decomposition temperature of hydrocarbon gases (e.g., acetylene gas).

[0102] Therefore, by oxidizing it in a pressurized atmosphere and converting it into divalent and tetravalent silicon oxides, it becomes possible to suppress the formation of Si-C bonds during subsequent acetylene CVD. Although the tetravalent silicon oxide that makes up this silicon oxide is generally an irreversible component, by reducing the size of the formed particles to nanoparticles and oxidizing them slowly while removing heat in a pressurized atmosphere, it becomes an oxide that can absorb and desorb Li.

[0103] Incidentally, during the lithium absorption and desorption process, tetravalent silicon oxides generally change to Li4SiO4, but it is thought that they become closer to a lower-valent lithium silicate, in this case Li2SiO3.

[0104] The formation of Li-silicate on the surface suppresses excessive decomposition of the electrolyte, enabling high cycle performance.

[0105] In this case, the structure becomes stable when Si-C bonds are present inside the bulk (inside the porous carbon structure).

[0106] Furthermore, the method for producing the negative electrode active material of the present invention involves attaching CxHy groups to the inside of porous carbon.

[0107] This method involves acid washing after the activation treatment with the aim of removing metal components present on the surface, without penetrating deep into the pores. As a result, hydrocarbon components generated during the activation treatment remain in the pores.

[0108] Furthermore, although the silane reaction is carried out at temperatures below 400 degrees Celsius, subsequent heat treatment at 600 degrees Celsius causes a reaction with Si-H, resulting in the formation of Si-C bonds within the bulk material. Therefore, by having a strong bond with the Si deposited on the surface of the porous carbon pores, it becomes possible to suppress the electronic disconnection of the Si portion during charging and discharging.

[0109] Regarding the oxidation treatment method, oxygen gas at a concentration of 1% in nitrogen is used, and the oxidation treatment is performed under a pressurized atmosphere of 20 kPa (note that the pressure is expressed as gauge pressure relative to atmospheric pressure of 101.33 kPa).

[0110] The amount of oxygen is estimated from the amount of gas introduced. A load cell is attached to the oxidation tank, and the weight increase is checked. Oxidation is stopped at the predetermined oxidation level. Generally, the range in which there is no weight increase corresponds to an oxygen content of approximately 3 wt%.

[0111] The carbon CVD coating deposited on the silicon compound near the surface uses some of the oxygen component of silicon to produce C=O or CO compounds. At this time, it is thought that compounds similar to Si-OC are produced, which can strengthen the adhesion of the carbon layer, thus maintaining integrity even when high-speed shear is applied during slurry preparation.

[0112] The steps for the method of producing the negative electrode active material will be explained with reference to Figure 7.

[0113] 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 OH groups and CxHy groups into the active sites of the porous carbon structure (step S2); flowing monosilane gas under heating, with carbon monoxide gas or carbon dioxide gas derived from oxygen atoms at the active sites present in the exhaust gas, to deposit silicon oxide bonded to oxygen atoms inside the porous carbon structure, wherein the silicon oxide contains dangling bonds (step S3); oxidizing at least a portion of the Si present on the surface layer of the porous carbon structure in a pressurized atmosphere after step S3 (step S4); and depositing a carbon layer at 580 degrees or less using hydrocarbon gas after step S4 (step S5).

[0114] (Step S1) First, there is the step of preparing a porous carbon structure.

[0115] The porous carbon structure prepared here preferably has at least a portion of a carbon-carbon double bond.

[0116] Furthermore, the porous carbon structure prepared here is predominantly Type I according to the IUPAC classification, and its surface area is 1400 m². 2 / g or more, pore volume 1cm³ 3 It is preferable that the value is 1 / g or more. By using such IUPAC classification, surface area, and pore volume, silicon can be deposited more efficiently in larger quantities. Furthermore, the following measurement methods can be used for IUPAC classification, surface area, and pore volume. The specific surface area / pore size distribution will be measured using a constant-volume method based on gas adsorption with Shimadzu Tristar II Plus. The conditions are as follows: • Gas used: Nitrogen • Environment: Under liquid nitrogen • Pressure operating range: P / P0 Adsorption 0~0.998 Detachable 0.998~0.10 Pre-treatment: Vacuum, 200°C, 1 hour

[0117] (Step S2) Next is the step of incorporating OH groups and CxHy groups into the active sites of the porous carbon structure prepared in step S1.

[0118] Porous carbon is placed in a reaction vessel and heated to 150 degrees Celsius 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, the CO gas concentration is checked using a gas detector. The gas concentration when no moisture is released should be approximately 3000 ppm.

[0119] (Step S3) Next, under heating conditions, monosilane gas is flowed through the exhaust gas while carbon monoxide gas or carbon dioxide gas originating from the oxygen atoms at the active sites is present, causing silicon oxide bonded to oxygen atoms to deposit inside the porous carbon structure, resulting in the silicon oxide containing dangling bonds.

[0120] The container is heated to 410 degrees Celsius. When the internal temperature reaches 350 degrees Celsius, 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%.)

[0121] At this time, although the pressure inside the container fluctuates, it is averaged to be in the range of 25kPa to 80kPa. Deposition at pressures above 80kPa may result in a denser film, but considering the durability of the device, it is stopped at 80kPa (in reality, it fluctuates and has been confirmed to rise up to a maximum of 95kPa).

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

[0123] The material temperature is first lowered to room temperature, and oxygen diluted with nitrogen is introduced to increase the weight of the oxygen by approximately 3.0 wt%. During this process, it is important to stir the powder and allow for slow heating. Rapid oxidation at this stage will lead to SiO2 formation, which will degrade the properties. Therefore, it is preferable to carry out the reaction slowly in a pressurized atmosphere.

[0124] Specifically, an oxidation treatment is performed using oxygen gas at a concentration of 1% in nitrogen, pressurized to a 20 kPa atmosphere.

[0125] Furthermore, this oxidation process is carried out under pressure, which causes a change in the valence state of Si, resulting in a state where low-valence oxidation and tetravalent Li can be intercalated and deintercalated.

[0126] Some of the Si-O compounds formed here will form further compounds during the subsequent C-CVD process (confirmed by XPS). This improves the adhesion between the carbon coating and the Si portion.

[0127] The material is heated to an internal temperature of 600 degrees Celsius in a nitrogen atmosphere, causing the Si-H bonds to react with the CxHy groups present inside the porous carbon pores to generate Si-C bonds.

[0128] (Step S5) This step involves depositing a carbon layer at temperatures below 580 degrees Celsius using hydrocarbon gas.

[0129] The temperature is lowered to below 580 degrees Celsius, and acetylene gas is introduced as a hydrocarbon gas to form a carbon layer (surface carbon film). The process is carried out at 10,000 Pa for 8 hours. The obtained material is analyzed by XRD, and the crystallite size of Si is calculated using Scherrer's equation to be 0.8 nm. However, this is a calculated result, and the material is considered to be amorphous in substance.

[0130] 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.

[0131] Furthermore, Si-O bonds are formed in the outermost layer, and since the decomposition temperature for acetylene has not been reached (or the rate is very slow), active carbon layer formation does not occur. On the other hand, many Si-H bonds remain in the interior where Si is precipitated, and these react to form Si-C bonds.

[0132] <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.

[0133] [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.

[0134] 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.

[0135] 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.

[0136] 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.

[0137] 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.

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

[0139] 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.

[0140] 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.

[0141] Examples of composite oxides having lithium and a transition metal element include lithium cobalt composite oxide (Li x CoO2), lithium nickel composite oxide (Li xExamples include NiO2, lithium nickel cobalt composite oxides, etc. Examples of lithium nickel cobalt composite oxides include lithium nickel cobalt aluminum composite oxide (NCA), lithium nickel cobalt manganese composite oxide (NCM), etc.

[0142] Examples of phosphate compounds containing lithium and transition metal elements include, for example, lithium iron phosphate compound (LiFePO4) or lithium iron manganese phosphate compound (LiFe 1-u Mn u PO4 (0 < u < 1)), etc. By using these positive electrode materials, high battery capacity can be obtained, and excellent cycle characteristics can also be obtained.

[0143] [Negative electrode] The negative electrode has the same structure as the negative electrode 10 for a lithium-ion secondary battery shown in FIG. 1 above. For example, it has negative electrode active material layers on both sides of the current collector. It is preferable that the negative electrode charging capacity is larger than the electric capacity obtained from the positive electrode active material agent (charging capacity as a battery). Thereby, precipitation of lithium metal on the negative electrode can be suppressed.

[0144] The positive electrode active material layer is provided on a part of both sides of the positive electrode current collector. Similarly, the negative electrode active material layer is also provided on a part of both sides of the negative electrode current collector. In this case, for example, the negative electrode active material layer provided on the negative electrode current collector has a region where there is no opposing positive electrode active material layer. This is for performing a stable battery design.

[0145] In the region where the above-mentioned 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, etc., can be accurately investigated with good reproducibility without depending on the presence or absence of charge and discharge.

[0146] [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.

[0147] [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.

[0148] 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.

[0149] 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).

[0150] 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.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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).

[0156] 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]

[0157] 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.

[0158] (Example 1) The negative electrode active material was prepared using the following procedure, and then a laminate film type lithium-ion secondary battery 30, as shown in Figure 2, was fabricated.

[0159] (Step S1: Prepare the porous carbon structure) First, the surface area (BET specific surface area) is 1940 m². 2 / g, pore volume 1cm³ 3 A porous carbon material (porous carbon structure) with a particle size (D50) of 8 μm and IUPAC classification type I was prepared. This porous carbon material was placed in a vacuum container and vacuumed to -90 kPa.

[0160] (Step S2: The active sites of the porous carbon structure are given OH groups and CxHy groups.) Next, the material was repressurized with nitrogen and heated to 150 degrees Celsius while nitrogen was flowing through it. At that time, a trap was installed in the exhaust gas section to ensure that no moisture was completely discharged. Simultaneously, a gas detector was used to check the CO gas concentration. This confirmed that the porous carbon structure contained OH groups and CxHy groups at its active sites.

[0161] (Step S3: Under heating, monosilane gas is flowed through the exhaust gas containing carbon monoxide or carbon dioxide gas originating from the oxygen atoms at the active sites, causing silicon oxide bonded to oxygen atoms to deposit inside the porous carbon structure, resulting in the silicon oxide containing dangling bonds.) The silane decomposition reaction was carried out with approximately 8000 ppm of CO gas detected during drying before the silane reaction. The decomposition reaction occurred within a temperature range of 365-380 degrees Celsius, which is below the temperature required for silane decomposition.

[0162] This is thought to be because controlling the pressure inside the tank to a positive pressure introduced silane into the pores, and in the presence of CO gas, the molecular structure was distorted, lowering the activation energy, which increased the reaction rate and enabled low-temperature decomposition. Furthermore, it was possible to incorporate oxygen components attached to the porous carbon during decomposition and form Si-O bonds.

[0163] (Step S4: Oxidize at least some of the Si present on the surface of the porous carbon structure in a pressurized atmosphere.) After the reaction, the mixture was returned to room temperature and oxidized under a 20 kPa pressure atmosphere to convert the outermost Si layer into a low-valent oxide state with a defect-containing SiO2 structure.

[0164] Subsequently, heat treatment was performed at 600 degrees Celsius to react the residual hydrocarbon components with Si-H, generating Si-C bonds within the bulk material.

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

[0166] Acetylene CVD is performed under reduced pressure of 10,000 Pa, allowing impregnation into the porous carbon and enabling the reaction within the porous carbon. This reaction breaks the Si-H bond and creates Si-C bond. This suppresses the generation of hydrogen gas during slurry formation. Furthermore, higher CVD temperatures result in the formation of more Si-C bonds.

[0167] [Measurement of negative electrode active material] The negative electrode active material prepared as described above was subjected to TEM-EDX, XRD analysis, and Raman spectroscopy.

[0168] [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.

[0169] 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.

[0170] [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.

[0171] 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.

[0172] 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.

[0173] [Measurement of initial efficiency] The initial efficiency was measured under the following conditions. First, the coin cell battery prepared for the initial efficiency test was charged in CCCV mode with a charge rate equivalent to 0.03C (initial charge). The CV was 0V and the termination current was 0.04mA. Next, CC discharge (initial discharge) was performed with a discharge rate of 0.03C and a discharge termination voltage of 1.0V.

[0174] 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.

[0175] [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.

[0176] 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.

[0177] 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).

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

[0179] [Table 1]

[0180] (Comparative Examples 1 and 2) In Comparative Example 1, the 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] Subsequently, a silane reaction was performed at a temperature of 380 degrees Celsius to form amorphous silicon within the bulk material. The reaction was completed when the pore volume reached 98% filling rate, and the material was then cooled to room temperature. After that, oxygen diluted with nitrogen (1% concentration) was flowed over the material for 48 hours to slowly oxidize it. During this time, the oxidation treatment was carried out under reduced pressure (reaching -95 kPa as indicated by the gauge) while stirring the powder, ensuring that oxygen was sufficiently distributed throughout the pores. The oxide formed was SiO2.

[0182] Comparative Example 2 involved the addition of a slight reduced-pressure oxidation atmosphere of -5kPa compared to Comparative Example 1, resulting in a slower oxidation process.

[0183] For reference, Figure 8 shows the XANES spectra of the surface layers of the negative electrode active materials for Example 1 and Comparative Example 1. The comparative example forms a structure similar to SiO2, but the XANES spectrum shows a slight shift to the lower energy side, suggesting that it is defective SiO2. The structure of the surface layer is similar to that of Example 1.

[0184] When the same evaluation as in Example 1 was performed, both Comparative Examples 1 and 2 showed poor cycle characteristics (1000 Cy maintenance rate in Table 1) and low battery capacity (single unit capacity in Table 1), resulting in unfavorable outcomes.

[0185] (Examples 2-6) In addition to the method of Example 1, heat treatment was performed after oxidation treatment to crystallize some of the Si. 0+The grain size ranged from 3 to 6.5 nm. As crystallinity increases, the acceptance of lithium deteriorates, which is likely why the battery performance slightly worsened.

[0186] The best battery performance was achieved when the material was essentially amorphous (the grain size of 0.8 nm in the table was calculated using Scherrer's formula, and is essentially amorphous).

[0187] However, considering that raising the temperature once improves the handling of the powder afterward, it is desirable to use crystallites of 5 nm or smaller.

[0188] (Examples 7-11) The results of Example 1 were evaluated by varying the amount of oxygen. When the amount of oxygen was low, there was a tendency for more Si-C bonds to be formed during acetylene CVD, and the cycle characteristics deteriorated slightly. At this point, the amount of oxygen that did not significantly degrade the cycle characteristics was 0.5 wt% or more. Conversely, when the amount of oxygen was too high, the excess oxygen reacted with Li, increasing the irreversible capacity and reducing the battery capacity. Therefore, the optimal range for the amount of oxygen is considered to be between 0.5 wt% and 5 wt%.

[0189] (Examples 12-14) For Example 1, the evaluation was performed by varying the pressure during silane gas introduction. When the pressure was low, the silane gas could not reach the depths of the pores, creating voids. As a result, silicon could not be filled in those areas, leading to a decrease in battery capacity.

[0190] Furthermore, silicon deposited under a pressurized atmosphere contains abundant dangling bonds, but the amount of dangling bonds decreases at lower pressures. Silicon rich in dangling bonds is highly reactive and can decompose carbon at low temperatures during subsequent carbon CVD.

[0191] One way to increase the pressure is to adjust the internal pressure by adjusting the silane flow rate and the opening of the exhaust valve. Fluidized tanks are generally around 10 kPa, but pressure control becomes difficult above 30 kPa. Therefore, in addition to adjusting the opening valve as described above, a vent valve can also be provided. For example, if you want to react at 50 kPa, the vent valve can operate in the 50 + 5 kPa range to adjust the internal pressure.

[0192] From a productivity standpoint, this would be a significant step backward, but by using this method, it becomes possible to distribute the gas to even the smallest details.

[0193] As shown above, the embodiments of the present invention demonstrate that, compared to the comparative example, excessive decomposition of the electrolyte can be suppressed, resulting in improved battery cycle characteristics and the realization of a negative electrode active material capable of increasing capacity.

[0194] This specification includes the following embodiments: [1]: A negative electrode active material having negative electrode active material particles, The negative electrode active material particles include a porous carbon structure which is the base material. The anode active material is characterized in that the porous carbon structure has Si-C bonds inside, amorphous low-valence nanosilicon oxide is dispersed in the surface layer, and at least a portion of the low-valence nanosilicon oxide has a tetravalent silicon oxide capable of adsorbing and desorbing Li. [2]: The negative electrode active material of [1], characterized in that the low-valence nanosilicon oxide absorbs Li during the Li absorption and desorption process. [3]: 29 The negative electrode active material according to [1] or [2] above, characterized in that the spectrum obtained by Si-MAS-NMR measurement has a peak originating from the Si-C bond in the range of 0 to -30 ppm and a peak originating from Si or the low-valence nanosilicon oxide in the range of -40 to -80 ppm. [4]: Any of the negative electrode active materials described in [1] to [3] above, characterized in that active sites exist inside the pores of the porous carbon structure, and a portion of the active sites are modified with OH. [5]: The low-valence nanosilicon oxide has a carbon layer separate from the porous carbon structure which is the matrix material, The negative electrode active material of any of the above [1] to [4] is characterized in that the interface of the other carbon layer has a C=O bond or a CO bond formed thereon. [6]: The negative electrode active material according to any of [1] to [5] 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 negative electrode active material particles, is in the range of 0.8 nm to 5 nm. [7]: The negative electrode active material of [6], characterized in that the zero-valent Si is substantially amorphous. [8]: The negative electrode active material according to any of the above [1] to [7], characterized in that the total amount of oxygen contained in the negative electrode active material is in the range of 0.5 wt% or more and 5 wt% or less. [9]: A method for producing a negative electrode active material, Steps include preparing a porous carbon structure, The steps include: providing the active sites of the porous carbon structure with OH groups and CxHy groups; The steps include: 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; 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 in a pressurized atmosphere, Following the step of oxidation in the aforementioned pressurized atmosphere, the carbon layer is deposited using hydrocarbon gas at a temperature of 580 degrees or less. A method for producing a negative electrode active material, characterized by containing the following:

[0195] 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]

[0196] 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 which is the base material. The porous carbon structure has Si-C bonds inside, amorphous low-valence nanosilicon oxide dispersed in the surface layer, and at least a portion of the low-valence nanosilicon oxide contains tetravalent silicon oxide capable of adsorbing and desorbing Li. The aforementioned low-valence nanosilicon oxide is a negative electrode active material characterized by its ability to absorb Li during the Li absorption and desorption process.

2. The low-valence nanosilicon oxide has a carbon layer separate from the porous carbon structure which is the matrix material. The negative electrode active material according to claim 1, characterized in that the interface of the other carbon layer has a C=O bond or a C-O bond formed thereon.

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

4. The negative electrode active material according to claim 1, 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.

5. The negative electrode active material according to claim 4, characterized in that the zero-valent Si is substantially amorphous.

6. 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 0.5 wt% or more and 5 wt% or less.

7. A negative electrode comprising the negative electrode active material according to any one of claims 1 to 6.

8. A non-aqueous electrolyte secondary battery using the negative electrode active material according to any one of claims 1 to 6.

9. A method for producing a negative electrode active material, Steps include preparing a porous carbon structure, The steps include: providing the active sites of the porous carbon structure with OH groups and CxHy groups; The steps include: 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; 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 in a pressurized atmosphere, Following the step of oxidation in the aforementioned pressurized atmosphere, the carbon layer is deposited using hydrocarbon gas at a temperature of 580 degrees or less. By including, The negative electrode active material having negative electrode active material particles, The negative electrode active material particles include the porous carbon structure which is the base material. The porous carbon structure has Si-C bonds inside, amorphous low-valence nanosilicon oxide dispersed in the surface layer, and at least a portion of the low-valence nanosilicon oxide contains tetravalent silicon oxide capable of adsorbing and desorbing Li. A method for producing a negative electrode active material, characterized in that the low-valence nanosilicon oxide absorbs Li during the Li absorption and desorption process.

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