Negative electrode active material and method for producing the same

Abstract

The present invention provides a negative electrode active material that can reduce the generation of ionic substances and suppress excessive decomposition of the electrolyte, thereby improving the battery cycle characteristics. [Solution] A negative electrode active material having negative electrode active material particles, the negative electrode active material particles including a porous carbon structure as a base material, the porous carbon structure having Si-C bonds inside and amorphous low-valent nano-silicon oxide dispersed in the surface layer, and at least a portion of the low-valent nano-silicon oxide having tetravalent silicon oxide capable of absorbing 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 life. In response to such market demands, development of secondary batteries that are small, lightweight, and capable of achieving high energy density has been progressing. The application of these secondary batteries is being considered not only for small electronic devices but also for large electronic devices such as automobiles and power storage systems such as those for homes.

[0003] Among these, lithium-ion secondary batteries are attracting great attention because they can be easily made small and have a high capacity, and can provide a higher energy density than lead batteries and nickel-cadmium batteries.

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

[0005] While carbon-based active materials are widely used as negative electrode active materials, recent market demands have called for further improvements in battery capacity. To improve battery capacity, the use of silicon as a negative electrode active material is being considered. This is because the theoretical capacity of silicon (4199 mAh / g) is more than 10 times greater than that of graphite (372 mAh / g), and a significant improvement in battery capacity can be expected. The development of silicon materials as negative electrode active materials is being considered not only for silicon itself, but also for alloys and compounds such as oxides. Furthermore, the form of the active material is being considered, ranging from the standard coated type used for carbon-based active materials to an integrated type deposited directly 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. Furthermore, ionic substances are generated within the active material, making it prone to cracking. When the surface layer of the negative electrode active material cracks, a new surface (also called a new surface) is created, increasing the reactive area of ​​the active material. At this time, the electrolyte decomposes on the new surface, and a coating made of the electrolyte decomposition product is formed on the new surface, consuming the electrolyte. This can lead to a decrease in cycle performance.

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

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

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

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

[0011] Further, a lithium ion secondary battery using silicon oxide was shipped by Hitachi Maxell in June 2010 as a rectangular secondary battery for smartphones employing a nanosilicon composite (see, for example, Non-Patent Document 1). The silicon oxide proposed by Hohl is a composite material of Si 0+ ~Si 4+ and has various oxidation states (Non-Patent Document 2). Also, Kapaklis has proposed a disproportionation structure that is divided into Si and SiO2 by applying a heat load to silicon oxide (Non-Patent Document 3).

[0012] Miyachi et al. have focused on Si and SiO2, which contribute to charge and discharge among silicon oxides with disproportionated structures (Non-Patent Document 4), and Yamada et al. have proposed the following reaction formula between silicon oxide and Li (Non-Patent Document 5): 2SiO(Si+SiO2) + 6.85Li + + 6.85e - → 1.4Li 3.75 Si + 0.4Li4SiO4 + 0.2SiO2 In the reaction formula, Si and SiO2, which make up silicon oxide, react with Li, and are separated into Li silicide, Li silicate, and some unreacted SiO2.

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

[0014] This irreversible capacity is the weakest point of silicon oxides, and improvement is needed. Kim et al. have therefore used a Li pre-doping method to form Li silicate in advance, significantly improving the initial efficiency of the battery and producing 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, which has improved the irreversible capacity (Patent Document 13).

[0015] On the other hand, the price of Li metal used for Li doping fluctuates greatly depending on the market situation, and there are many challenges when it comes to industrialization. Therefore, CVD-Si-C, which uses silane gas in porous carbon to generate nanosilicon inside, has achieved a higher energy density than Li-doped SiO (Patent Documents 14 and 15).

[0016] Furthermore, focusing on the problem that the small pore diameter of porous carbon materials causes excessive silicon to precipitate on the surface, resulting in high electrical resistivity, composite particles that do not contain SiC (silicon carbide) or have an extremely low SiC content have been proposed in order to reduce electrical resistivity (Patent Document 16). [Prior art documents] [Patent documents]

[0017] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-185127 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-042806 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-164954 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-114454 [Patent Document 5] Japanese Patent Application Laid-Open No. 2009-070825 [Patent Document 6] Japanese Patent Application Laid-Open No. 2008-282819 [Patent Document 7] Japanese Patent Application Laid-Open No. 2008-251369 [Patent Document 8] Japanese Patent Application Laid-Open No. 2008-177346 [Patent Document 9] Japanese Patent Application Laid-Open No. 2007-234255 [Patent Document 10] Japanese Patent Application Laid-Open No. 2009-212074 [Patent Document 11] Japanese Patent Application Laid-Open No. 2009-205950 [Patent Document 12] Japanese Patent Application Publication No. 06-325765 [Patent Document 13] Japanese Patent Application Laid-Open No. 2015-156355 [Patent Document 14] U.S. Patent No. 10,608,254 [Patent Document 15] U.S. Patent No. 11,165,054 [License 16] WO2024 / 142699 [Non-licensed literature]

[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 Frontline of Automotive Lithium-ion Battery Development, P96-111, CMC Publishing, published November 27, 2020 Summary of the Invention [Problem to be solved by the invention]

[0019] As mentioned above, in recent years, small electronic devices such as mobile terminals have become more powerful and multifunctional, and the lithium-ion secondary batteries that serve as their main power sources are being required to have increased battery capacity. One method for solving this problem is to develop a lithium-ion secondary battery with a negative electrode that uses silicon material as the main material.

[0020] Lithium-ion secondary batteries using silicon materials are also expected to have initial charge-discharge and cycle characteristics comparable to those of lithium-ion secondary batteries using carbon-based active materials. Therefore, by using silicon oxide modified by Li insertion and partial desorption as the anode active material, cycle characteristics and initial charge-discharge characteristics have been improved. Recently, by incorporating Li into silicon oxide in advance to form Li silicate, the irreversible capacity, a drawback of silicon oxide, has been reduced, and this has actually begun to be commercialized. Even when prototype batteries were fabricated using Li-SiO-C (Non-Patent Document 8), which uses Li in silicon oxide and replaces 100% of the carbon anode material, the capacity improvement was only in the high 20% range compared to carbon anode materials. This means that further improvements in battery capacity are required, considering the increasing performance of small electronic devices (e.g., 5G) and the increased driving range of electric vehicles.

[0021] Therefore, CVD-Si-C, which has a low irreversible capacity, was developed, but it is known that the reaction between Si and the electrolyte results in insufficient fast charging and battery cycle characteristics.

[0022] In addition, Li-Si compounds (e.g., Li 15 Si4) exhibits ionic properties and reduces diffusibility. As mentioned above, the formation of such ionic substances makes the negative electrode active material more susceptible to cracking. When the surface layer of the negative electrode active material cracks, a new surface (a new surface) is created, increasing the reactive area of ​​the active material. At this time, the electrolyte decomposes on the new surface, and a film of the electrolyte decomposition product is formed on the new surface, further consuming the electrolyte. Repeated charging and discharging in this state results in the electrolyte being decomposed and consumed each time a new surface is created, resulting in a problem of reduced cycle performance.

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

[0024] In order to solve the above-mentioned 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-valent nano-silicon oxide is dispersed in a surface layer portion, and at least a portion of the low-valent nano-silicon oxide contains tetravalent silicon oxide capable of absorbing and desorbing Li.

[0025] The negative electrode active material of the present invention contains tetravalent silicon oxide capable of absorbing and desorbing Li in a portion of the low-valent nanosilicon oxide dispersed in the surface layer. Because the low-valent nanosilicon oxide has high Li diffusivity, the low-valent nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the generation of new surfaces. During charging and discharging, the tetravalent silicon oxide absorbs and desorbs Li in the surface layer, allowing for repeated decomposition and generation of Li silicate. This suppresses excessive decomposition of the electrolyte, thereby providing a negative electrode active material capable of improving battery cycle performance. Furthermore, the internal Si-C bond strongly bonds with the surface Si, thereby suppressing electronic discontinuity in the Si portion during charging and discharging, resulting in a stable structure.

[0026] Furthermore, the low-valent nano silicon oxide preferably absorbs Li during the Li absorption-desorption process.

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

[0028] Also, 29 The spectrum obtained by Si-MAS-NMR measurement preferably has a peak derived from the Si-C bond in the range of 0 to -30 ppm and a peak derived from Si or the low-valence nano-silicon oxide in the range of -40 to -80 ppm.

[0029] With such a spectrum, the tetravalent silicon oxide present in some of the low-valent nano-silicon oxide dispersed in the surface layer can reliably decompose and generate Li silicate. Furthermore, the internal Si-C bonds form strong bonds with the Si in the surface layer, making it possible to suppress electronic disconnection in the Si portion during charging and discharging, thereby reliably stabilizing the structure.

[0030] It is also preferable that active sites exist inside the pores of the porous carbon structure, and that some of the active sites are modified with OH.

[0031] With such an internal pore structure, for example, heat treatment can be performed to make the exhaust gas contain a large amount of CO. Further, by reacting the exhaust gas with silane gas in a pressurized atmosphere, the silane decomposition product can be dispersed as amorphous low-valent nano-silicon oxide within the porous carbon structure, particularly in the surface layer, and the desired negative electrode active material can be easily produced.

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

[0033] If such C=O or CO bonds are formed, it is believed that a compound similar to Si-OC is produced, which can strengthen the adhesion of the carbon layer and realize a robust structure that maintains its integrity even when high-speed shear is applied during slurry production, for example.

[0034] Furthermore, the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using the Scherrer equation from the peak measured by X-ray diffraction measurement of the negative electrode active material particles, is preferably in the range of 0.8 nm to 5 nm.

[0035] Such a material having a grain size of zero-valent Si that is substantially amorphous is preferred.

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

[0037] Such a substantially amorphous material is preferred.

[0038] The amount of oxygen contained in the negative electrode active material as a whole is preferably in the range of 0.5 wt % to 5 wt %.

[0039] This range of oxygen content can prevent a decrease in battery capacity due to excessive generation of Si-C bonds during acetylene CVD caused by an excessively low oxygen content, and can also prevent an increase in irreversible capacity caused by an excessively high oxygen content.

[0040] Furthermore, in order to solve the above-mentioned problems, the present invention provides a method for producing an anode active material, the method comprising the steps of: preparing a porous carbon structure; incorporating OH groups and CxHy groups into active sites of the porous carbon structure; flowing monosilane gas under heating in an exhaust gas containing carbon monoxide gas or carbon dioxide gas derived from oxygen atoms at the active sites, thereby depositing silicon oxide bonded to the oxygen atoms inside the porous carbon structure, the silicon oxide containing dangling bonds; oxidizing at least a portion of Si present in a surface layer of the porous carbon structure in a pressurized atmosphere after the silicon oxide depositing step; and depositing a carbon layer at 580°C or less using a hydrocarbon gas after the oxidation step in a pressurized atmosphere.

[0041] According to the method for producing the negative electrode active material of the present invention, Si-C bonds are formed within the porous carbon structure, amorphous low-valent nanosilicon oxide is dispersed in the surface layer, and oxidation in a pressurized atmosphere produces tetravalent silicon oxide capable of absorbing and desorbing Li in some of the low-valent nanosilicon oxide. Therefore, in a negative electrode using such a negative electrode active material, the low-valent nanosilicon oxide has high Li diffusivity, so the low-valent nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the generation of new surfaces. During charging and discharging, the tetravalent silicon oxide can absorb and desorb Li in the surface layer, repeatedly decomposing and generating Li silicate. This suppresses excessive decomposition of the electrolyte, thereby providing a negative electrode active material that can improve battery cycle performance. Furthermore, the internal Si-C bonds form strong bonds with the surface Si, thereby suppressing electronic discontinuity in the Si portion during charging and discharging, resulting in a stable structure. The method for producing a negative electrode active material of the present invention can produce such a negative electrode active material simply and efficiently, and in particular can reduce the production of ionic substances and suppress excessive decomposition of the electrolyte, thereby providing a method for producing a negative electrode active material that can improve battery cycle characteristics. [Effects of the Invention]

[0042] The negative electrode active material of the present invention contains tetravalent silicon oxide capable of absorbing and desorbing Li in a portion of the low-valent nanosilicon oxide dispersed in the surface layer. Because the low-valent nanosilicon oxide has high Li diffusivity, the low-valent nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the generation of new surfaces. During charging and discharging, the tetravalent silicon oxide absorbs and desorbs Li in the surface layer, allowing for repeated decomposition and generation of Li silicate. This suppresses excessive decomposition of the electrolyte, thereby providing a negative electrode active material capable of improving battery cycle performance. Furthermore, the internal Si-C bond strongly bonds with the surface Si, thereby suppressing electronic discontinuity in the Si portion during charging and discharging, resulting in a stable structure.

[0043] The method for producing a negative electrode active material of the present invention forms Si-C bonds within the porous carbon structure, disperses amorphous low-valent nanosilicon oxide in the surface layer, and oxidizes the low-valent nanosilicon oxide in a pressurized atmosphere to produce tetravalent silicon oxide capable of absorbing and desorbing Li in some of the low-valent nanosilicon oxide. Therefore, in a negative electrode using such a negative electrode active material, the low-valent nanosilicon oxide has high Li diffusivity, so the low-valent nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the generation of new surfaces. During charging and discharging, the tetravalent silicon oxide absorbs and desorbs Li in the surface layer, allowing repeated decomposition and generation of Li silicate. This suppresses excessive decomposition of the electrolyte, thereby providing a negative electrode active material that can improve battery cycle performance. Furthermore, the internal Si-C bonds form strong bonds with the surface Si, thereby suppressing electronic discontinuity in the Si portion during charging and discharging, resulting in a stable structure. The method for producing a negative electrode active material of the present invention can produce such a negative electrode active material simply and efficiently, and in particular can reduce the production of ionic substances and suppress excessive decomposition of the electrolyte, thereby providing a method for producing a negative electrode active material that can improve battery cycle characteristics. [Brief explanation of the drawings]

[0044] [Figure 1] 1 is a cross-sectional view showing the structure of a negative electrode containing a negative electrode active material of the present invention. [Figure 2] FIG. 1 is an exploded view showing a configuration example (laminate film type) of a lithium ion secondary battery including the negative electrode active material of the present invention. [Figure 3] 1 shows a XANES spectrum of the surface layer of the negative electrode active material during charging. [Figure 4] 1 shows a XANES spectrum of the surface layer of the negative electrode active material during discharge. [Figure 5] This is a spectrum obtained by 29Si-MAS-NMR measurement of the negative electrode active material. [Figure 6] This is a spectrum obtained by 29Si-MAS-NMR measurement of a conventional negative electrode active material. [Figure 7]FIG. 1 is a flow chart showing an example of a method for producing a negative electrode active material according to the present invention. [Figure 8] 1 shows XANES spectra of the surface layer portions of the negative electrode active materials of Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0045] Hereinafter, the present invention will be described in detail with reference to the preferred embodiments, but the present invention is not limited to these.

[0046] As mentioned above, one method for increasing the battery capacity of lithium-ion secondary batteries is to use a negative electrode with a carbon structure mainly made of low-valent nano-silicon oxide. Lithium-ion secondary batteries using this active material are expected to have high battery capacity while exhibiting battery characteristics similar to those of lithium-ion secondary batteries using carbon-based active materials.

[0047] Therefore, the present inventors conducted extensive research to obtain a negative electrode active material that, when used as a negative electrode of a secondary battery, can achieve high cycle characteristics, improve initial charge-discharge characteristics, and increase battery capacity, and have arrived at the present invention.

[0048] In particular, Li-Si compounds (e.g., Li 15 Si4) exhibits ionic properties, and its reduced diffusibility makes the negative electrode active material more susceptible to cracking. However, 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 consumed excessively 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, we have developed a negative electrode active material that has Si-C bonds inside the porous carbon structure, has amorphous low-valent nanosilicon oxide dispersed in the surface layer, and has tetravalent silicon oxide as part of the low-valent nanosilicon oxide.

[0049] [Negative electrode active material of the present invention] The present invention provides a negative electrode active material having negative electrode active material particles, the negative electrode active material particles including a porous carbon structure as a base material, the porous carbon structure having Si-C bonds therein and amorphous low-valent nano-silicon oxide dispersed in a surface layer portion, and at least a portion of the low-valent nano-silicon oxide having tetravalent silicon oxide capable of absorbing and desorbing Li.

[0050] In this negative electrode active material, a portion of the low-valent nanosilicon oxide dispersed in the surface layer contains tetravalent silicon oxide capable of absorbing and desorbing Li. First, because the low-valent nanosilicon oxide has high Li diffusivity, the low-valent nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the generation of new surfaces. Furthermore, during charging and discharging, the tetravalent silicon oxide absorbs and desorbs Li in the surface layer, allowing for repeated decomposition and generation of Li silicate. This suppresses excessive decomposition of the electrolyte, resulting in a negative electrode active material that can improve battery cycle performance. Furthermore, the internal Si-C bond forms a strong bond with the surface Si, suppressing electronic discontinuity in the Si portion during charging and discharging, thereby stabilizing the structure.

[0051] Furthermore, although not particularly limited, the low-valence nano-silicon oxide is preferably one that absorbs Li during the Li absorption-desorption process.

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

[0053] Here, to explain the state of Li absorption and desorption (charge and discharge), reference will be made to the drawings.

[0054] Fig. 1 is a cross-sectional view of a negative electrode containing the negative electrode active material of the present invention, and Fig. 2 is a structural example (laminate film type) of a lithium ion secondary battery containing the negative electrode active material of the present invention. Details of Figs. 1 and 2 will be described later.

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

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

[0057] In the initial state, a peak for tetravalent silicon oxide (tetravalent Si oxide in the figure) can be seen, but it can be seen that upon charging (Li absorption), this changes to a peak for Li silicate. Generally, tetravalent silicon oxide changes to Li4SiO4, but it is thought that Li absorption changes it to a state close to low-valent Li silicate, in this case Li2SiO3.

[0058] Figure 4 shows the spectrum of the surface layer during discharge, decreasing in the direction of the arrows as the discharge progresses from SOC 100% (fully charged) to DOD 20% (20% discharge, where DOD (Depth of Discharge) represents 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 characteristics.

[0060] The XANES (X-ray Absorption Near-Edge Structure) measurement can be carried out, for example, under the following conditions. Aichi Synchrotron Light Center, BL6N1 line used The sample was processed in an Ar atmosphere, stored in a transfer vessel, and connected to the BL6N1 line under the following conditions: Acceleration energy: 1.2 GeV Accumulated current: 300mA Monochromatization conditions: White X-rays from a bending magnet are monochromatized using a double crystal monochromator and used for measurement. Focusing conditions: Focusing in the vertical and horizontal directions using a Ni-coated bent cylindrical mirror Upstream slit opening: 10.0mm horizontal x 3.0mm vertical Beam size: 2.0mm horizontal x 1.0mm vertical Incident angle to sample: 45 degrees (incident angle 45 degrees), to measure fluorescence yield simultaneously Energy calibration: The peak position at the K2SO4 SK edge is calibrated 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] In addition, although not particularly limited, 29 The spectrum obtained by Si-MAS-NMR measurement preferably has a peak derived from the Si-C bond in the range of 0 to -30 ppm and a peak derived from Si or low-valence nano-silicon oxide in the range of -40 to -80 ppm.

[0062] With such a spectrum, the tetravalent silicon oxide present in some of the low-valent nano-silicon oxide dispersed in the surface layer can reliably decompose and generate Li silicate. Furthermore, the internal Si-C bonds form strong bonds with the Si in the surface layer, making it possible to suppress electronic disconnection in the Si portion during charging and discharging, thereby reliably stabilizing the structure.

[0063] For reference, FIG. 5 shows the negative electrode active material of the present invention. 29An example of a spectrum obtained by Si-MAS-NMR measurement is shown below. It has a peak derived from the Si-C bond (Si-C in the figure) in the range of 0 to -30 ppm, and a peak derived from Si or low-valence nano-silicon oxide (amorphous Si in the figure) in the range of -40 to -80 ppm.

[0064] Also, FIG. 6 shows the results of a conventional negative electrode active material without Si-C bonds (such as in Patent Document 16). 29 The spectrum obtained by Si-MAS-NMR measurement is shown below. Compared to Figure 5, no peaks due to Si-C bonds are observed.

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

[0066] NMR measurement can be carried out, for example, under the following conditions. 29 Si MAS NMR (Magic Angle Spinning Nuclear Magnetic Resonance) Equipment: Bruker 700 NMR spectrometer, Probe: 4mm HR-MAS rotor 50μL, Sample rotation speed: 10kHz, ·Measurement environment temperature: 25℃

[0067] The XPS measurement can be carried out, for example, under the following conditions. XPS ·Equipment: X-ray photoelectron spectrometer, X-ray source: monochromated Al Kα radiation, X-ray spot diameter: 100μm, Ar ion gun sputtering conditions: 0.5kV, 2mm x 2mm.

[0068] Although not particularly limited, the low-valent nano silicon oxide is preferably in a composite state of substantially zero valence, monovalent, divalent and trivalent.

[0069] In such a composite state, not only can the irreversible capacity be lowered, but by adjusting the distribution of zero-valent, monovalent, divalent, and trivalent atoms, it is possible to realize a low-valent nanosilicon oxide that absorbs and desorbs Li more effectively.

[0070] The low-valent nano silicon oxide phase present inside the porous carbon structure may contain a zero-valent Si microcrystalline phase.

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

[0072] With such an internal pore structure, for example, heat treatment can be performed to make the exhaust gas contain a large amount of CO. Further, by reacting the exhaust gas with silane gas in a pressurized atmosphere, the silane decomposition product can be dispersed as amorphous low-valent nano-silicon oxide within the porous carbon structure, particularly in the surface layer, and the desired negative electrode active material can be easily produced.

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

[0074] If such C=O or CO bonds are formed, it is believed that a compound similar to Si-OC is produced, which can strengthen the adhesion of the carbon layer and realize a robust structure that maintains its integrity even when high-speed shear is applied during slurry production, for example.

[0075] Furthermore, although not particularly limited, the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using the Scherrer equation from the peak measured by X-ray diffraction measurement of the negative electrode active material particles, is preferably in the range of 0.8 nm to 5 nm.

[0076] Such a material having a grain size of zero-valent Si that is substantially amorphous is preferred.

[0077] Calculation of the crystallite size by XRD can be carried out, for example, under the following conditions: For broad peaks, analysis can be carried out using analysis software TOPAS, for example, under the following conditions: XRD measurement Equipment: Bruker D2 Phaser ·X-ray source:Cu Divergence slit: 0.5° Incident solar: 4° Receiver side solar: 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 “Calculate Error” or “Use Extrapolation”

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

[0079] Although not particularly limited, the amount of oxygen contained in the entire negative electrode active material is preferably in the range of 0.5 wt % to 5 wt %.

[0080] This range of oxygen content can prevent a decrease in battery capacity due to excessive generation of Si-C bonds during acetylene CVD caused by an excessively low oxygen content, and can also prevent an increase in irreversible capacity caused by an excessively high oxygen content.

[0081] The grain size of the low-valent nanosilicon oxide can be confirmed by TEM-EDX. The conditions for this can be as follows: The negative electrode active material is processed on both cross sections using a focused ion beam (FIB) machine without being exposed to the atmosphere. The FIB machine is a SIINT XVision 200DB with an acceleration voltage of 30 kV. TEM observation is performed using an FEI Tecnai G2F20 with an acceleration voltage of 200 kV, and EDX is performed using an EDAX r-TEM with an acceleration voltage of 200 kV.

[0082] Furthermore, the negative electrode active material of the present invention may contain a carbon composite having a silicon core inside the porous carbon structure, and this carbon composite may contain at least one of carbon black, acetylene black, and carbon nanotubes.

[0083] Porous carbon may be derived from biological sources, resins, or petroleum, with biological or resin sources being preferred.

[0084] <Non-aqueous electrolyte secondary battery negative electrode> Next, the configuration of a 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] [Negative electrode composition] Fig. 1 shows a cross-sectional view of a negative electrode containing the negative electrode active material of the present invention. As shown in Fig. 1, a 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 or only one side of the negative electrode current collector 11. Furthermore, the negative electrode of the nonaqueous electrolyte secondary battery of the present invention does not necessarily need to have the negative electrode current collector 11.

[0086] [Negative electrode current collector] The negative electrode current collector 11 is made of a highly conductive material that has excellent 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 an intermetallic compound with lithium (Li).

[0087] The negative electrode current collector 11 preferably 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 an active material layer that expands during charging is included, if the current collector contains the above elements, deformation of the electrode including the current collector is suppressed. The contents of the above contained elements are not particularly limited, but are preferably 100 mass ppm or less each. This is because a higher deformation suppression effect can be obtained. Such a deformation suppression effect can further improve cycle characteristics.

[0088] The surface of the negative electrode current collector 11 is preferably roughened, and the ten-point height of the surface roughness Rz is preferably 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 subjected to an electrolytic treatment, an embossing treatment, or a chemical etching treatment.

[0089] [Negative electrode active material layer] The negative electrode active material layer 12 may contain multiple types of negative electrode active materials, such as silicon-based active material particles and carbon-based active materials, etc. Furthermore, depending on the battery design, it may also contain other materials such as a thickener (also called a "binding agent" or "binder") and a conductive additive.

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

[0091] First, a method for producing the negative electrode active material contained in the negative electrode will be described.

[0092] The method for producing an anode active material of the present invention includes the steps of: preparing a porous carbon structure; incorporating OH groups and CxHy groups into active sites of the porous carbon structure; flowing monosilane gas under heating in an exhaust gas containing carbon monoxide gas or carbon dioxide gas derived from oxygen atoms at the active sites, thereby depositing silicon oxide bonded to the oxygen atoms inside the porous carbon structure, the silicon oxide containing dangling bonds; oxidizing at least a portion of Si present in a surface layer of the porous carbon structure in a pressurized atmosphere after the silicon oxide depositing step; and depositing a carbon layer at 580°C or less using a hydrocarbon gas after the oxidation step in a pressurized atmosphere.

[0093] According to the method for producing the negative electrode active material of the present invention, Si-C bonds are formed within the porous carbon structure, amorphous low-valent nanosilicon oxide is dispersed in the surface layer, and oxidation in a pressurized atmosphere produces tetravalent silicon oxide capable of absorbing and desorbing Li in some of the low-valent nanosilicon oxide. Therefore, in a negative electrode using such a negative electrode active material, the low-valent nanosilicon oxide has high Li diffusivity, so the low-valent nanosilicon oxide dispersed in the surface layer reduces the generation of ionic substances and suppresses the generation of new surfaces. During charging and discharging, the tetravalent silicon oxide can absorb and desorb Li in the surface layer, repeatedly decomposing and generating Li silicate. This suppresses excessive decomposition of the electrolyte, thereby providing a negative electrode active material that can improve battery cycle performance. Furthermore, the internal Si-C bonds form strong bonds with the surface Si, thereby suppressing electronic discontinuity in the Si portion during charging and discharging, resulting in a stable structure. The method for producing a negative electrode active material of the present invention can produce such a negative electrode active material simply and efficiently, and in particular can reduce the production of ionic substances and suppress excessive decomposition of the electrolyte, thereby providing a method for producing a negative electrode active material that can improve battery cycle characteristics.

[0094] Here we will explain how to manufacture SiC. Generally speaking, SiC is produced by using a fluidized bed tank in a heated atmosphere, spraying silane gas and carrier gas from the bottom to deposit silicon inside the porous carbon, and then going through an oxidation process to coat it with carbon using hydrocarbon gas or coal tar pitch.

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

[0096] In addition, since SiO2 is present near the outermost layer, a typical SiC active material can be produced by performing carbon coating CVD on the top of it using hydrocarbon gas.

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

[0098] When heat treatment is carried out while the OH group is present, CO is detected in the exhaust gas. Normally, when the material is completely dried, CO gas is not emitted, but when the exhaust gas contains a large amount of CO and is reacted with silane gas in a pressurized atmosphere, the silane decomposition products are deposited on the porous carbon as low-valent oxides. At this time, CO gas continues to be emitted from inside the porous pores, so it is important to use a hydrogen carrier in a pressurized atmosphere to distribute the silane gas throughout the interior.

[0099] In a typical fluidized bed, the pressure loss is about 10 kPa, but it is known that this reaction begins gradually from about 25 kPa.

[0100] Low valence oxides have high diffusivity of Li. Generally, compounds of Si and Li have high diffusivity of Li. 15 Although Si4 is generated, it exhibits ionic properties and its diffusibility is reduced. On the other hand, low-valence oxides can reduce the generation of ionic substances.

[0101] Furthermore, increasing the pressure during the silicon oxide deposition reaction allows for the creation of many dangling bonds, which are highly reactive and can be carbonized at temperatures lower than the general 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 is possible to suppress the Si-C bond that occurs during the subsequent acetylene CVD. The tetravalent silicon oxide that makes up this silicon oxide is generally an irreversible component, but by reducing the size of the formed nanoparticles and oxidizing it while slowly heating it in a pressurized atmosphere, it becomes an oxide that can absorb and desorb Li.

[0103] Incidentally, during the Li absorption / desorption process, tetravalent silicon oxide generally changes to Li4SiO4, but it is thought that it becomes a state closer to a lower valent Li silicate, in this case Li2SiO3.

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

[0105] Here, if there is an Si-C bond inside the bulk (inside the porous carbon structure), the structure becomes stable.

[0106] Furthermore, in the method for producing a negative electrode active material of the present invention, CxHy groups are attached to the inside of the porous carbon.

[0107] This is because the acid washing after activation treatment is carried out with the aim of removing metal components present on the surface, and washing is carried out without allowing the metal components to penetrate deep into the pores, which means that hydrocarbon components generated during the activation treatment remain in the pores.

[0108] Furthermore, the silane reaction is carried out at temperatures below 400°C, but subsequent heat treatment at 600°C causes a reaction with Si-H, creating Si-C bonds within the bulk. Therefore, by forming strong bonds with the Si deposited on the surface of the porous carbon pores, it is possible to suppress electronic discontinuity in the Si portion that occurs during charging and discharging.

[0109] The oxidation treatment is carried out using oxygen gas at a concentration of 1% in nitrogen, pressurized to a 20 kPa atmosphere (pressure is expressed as pressure (gauge pressure) relative to atmospheric pressure of 101.33 kPa).

[0110] The amount of oxygen is estimated from the amount of gas added. A load cell is attached to the oxidation tank, weight increase is confirmed, and oxidation is stopped at a predetermined amount. The range where there is generally no weight increase is about 3 wt% oxygen.

[0111] The carbon CVD coating that deposits on the silicon compound near the surface uses some of the oxygen contained in the silicon to generate C=O or CO compounds. It is believed that at this time, a compound similar to Si-OC is generated, which can strengthen the adhesion of the carbon layer, so that soundness is maintained even when high-speed shear is applied during slurry production.

[0112] Each step of the method for producing the negative electrode active material will be described with reference to FIG.

[0113] The method for producing a negative electrode active material of the present invention includes 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); depositing silicon oxide bonded to oxygen atoms inside the porous carbon structure by flowing monosilane gas under heating in a state in which carbon monoxide gas or carbon dioxide gas derived from oxygen atoms at the active sites is contained in the exhaust gas; depositing silicon oxide bonded to oxygen atoms inside the porous carbon structure, the silicon oxide containing dangling bonds (Step S3); oxidizing at least a portion of Si present in the surface layer of the porous carbon structure in a pressurized atmosphere after Step S3 of depositing silicon oxide; and depositing a carbon layer using hydrocarbon gas at 580°C or less after Step S4 of oxidation in a pressurized atmosphere.

[0114] (Step S1) First, a porous carbon structure is prepared.

[0115] The porous carbon structure prepared here preferably has carbon-carbon double bonds at least in part.

[0116] The porous carbon structure prepared here is predominantly type I in the IUPAC classification, and its surface area is 1400 m 2 / g or more, pore volume is 1cm 3 / g or more is preferable. By using such IUPAC classification, surface area, and pore volume, silicon deposition can be carried out more efficiently in larger quantities. Furthermore, the IUPAC classification, surface area, and pore volume can be measured using the following methods. The specific surface area / pore size distribution was measured using a Shimadzu Tristar II Plus by a constant volume method based on gas adsorption. The conditions were as follows: Gas used: Nitrogen Environment: Liquid nitrogen Pressure operating range: P / P0 Adsorption 0~0.998 Desorption 0.998~0.10 Pretreatment: Vacuum 200℃ 1 hour

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

[0118] The porous carbon is placed in a reaction vessel and heated to 150°C while nitrogen gas is flowing through it. At this time, a trap is installed in the exhaust gas section to ensure that no moisture is being discharged. At the same time, a gas detector is used to check the CO gas concentration. The gas concentration when no moisture is being discharged should be approximately 3000 ppm.

[0119] (Step S3) Next, in a step where monosilane gas is flowed under heating in a state where carbon monoxide gas or carbon dioxide gas derived from oxygen atoms at the active sites is contained in the exhaust gas, silicon oxide bonded to oxygen atoms is deposited inside the porous carbon structure, and the silicon oxide contains dangling bonds.

[0120] The container is heated to 410°C. When the internal temperature reaches 350°C, silane gas is introduced from the bottom, depositing silicon inside the porous carbon. (The pore volume of the porous carbon is estimated in advance from pore distribution measurements, and the amount of silane introduced is 0.9 times the pore volume, controlling the reaction rate at 90%.)

[0121] At this time, the pressure inside the container fluctuates, but is averaged between 25kPa and 80kPa. Deposition at a pressure 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 to a maximum of 95kPa).

[0122] (Step S4) This is a step in which at least a portion of the Si present in the surface layer of the porous carbon structure is oxidized in a pressurized atmosphere.

[0123] The temperature of the material is first lowered to room temperature, and oxygen diluted with nitrogen is introduced to increase the weight by approximately 3.0 wt%. At this time, it is important to stir the powder and remove heat. Rapid oxidation here will cause SiO2 formation, which will deteriorate the properties. Therefore, it is preferable to carry out the reaction slowly in a pressurized atmosphere.

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

[0125] In addition, this oxidation process is carried out under pressure, which causes a change in the valence of Si, resulting in low valence oxidation and a tetravalent state in which Li can be absorbed and desorbed.

[0126] Some of the Si-O compounds formed here will form new compounds during the next C-CVD process (confirmed by XPS), which improves the adhesion between the carbon coating and the Si part.

[0127] The material is heated to an internal temperature of 600°C in a nitrogen atmosphere, causing the Si-H bond to react with the CxHy group present inside the porous carbon pores, generating an Si-C bond.

[0128] (Step S5) This is a step in which a carbon layer is deposited using hydrocarbon gas at temperatures below 580°C.

[0129] The temperature is lowered to below 580°C, and acetylene gas is introduced as a hydrocarbon gas to form a carbon layer (surface carbon film). The treatment 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 the Scherrer equation to be 0.8 nm, but this is a calculated result and the material is considered to be essentially amorphous.

[0130] At this time, the reaction is carried out in a reduced pressure atmosphere, allowing acetylene gas to spread throughout the Si filled inside the pores. Here, acetylene reacts with the Si-H bonds that did not reach a low valence oxidation state, forming Si-C bonds, resulting in a stable material.

[0131] In addition, Si-O bonds are formed in the outermost layer, and the temperature has not yet reached the decomposition temperature of acetylene (or the rate is very slow), so a carbon layer is not actively formed. On the other hand, many Si-H bonds remain inside where Si is precipitated, and react with these to form Si-C bonds.

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

[0133] [Configuration of laminated film type secondary battery] The laminate film type lithium ion secondary battery 30 shown in FIG. 2 mainly comprises a wound electrode body 31 housed inside a sheet-like exterior member 35. This wound electrode body 31 has a separator between the positive and negative electrodes and is wound. There are also cases where a separator is placed between the positive and negative electrodes and a laminate is housed. In either type of electrode body, 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 periphery of the electrode body is protected by protective tape.

[0134] The positive and negative electrode leads 32, 33 are led out in one direction, for example, from the inside to the outside of the exterior member 35. The positive electrode lead 32 is formed of a conductive material such as aluminum, and the negative electrode lead 33 is formed of a conductive material such as nickel or copper.

[0135] The exterior member 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, and the outer peripheral edges of the fusion layers of the two films of this laminate film are fused together or attached with an adhesive or the like 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 protection layer is, for example, nylon or the like.

[0136] An adhesive film 34 is inserted between the exterior member 35 and the positive and negative electrode leads to prevent outside air from entering. This material is, for example, polyethylene, polypropylene, or polyolefin resin.

[0137] The positive electrode has a positive electrode active material layer on one or both sides of a positive electrode current collector, similar to the negative electrode 10 in FIG.

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

[0139] The positive electrode active material layer contains one or more positive electrode materials capable of absorbing and releasing lithium ions, and may contain other materials such as a positive electrode binder, a positive electrode conductive additive, a dispersant, etc. In this case, details regarding the positive electrode binder and the positive electrode conductive additive are the same as, for example, the negative electrode binder and the negative electrode conductive additive already described.

[0140] A lithium-containing compound is desirable as a positive electrode material. Examples of the lithium-containing compound include a composite oxide made of lithium and a transition metal element, or a phosphate compound containing lithium and a transition metal element. Among these positive electrode materials, compounds containing at least one of nickel, iron, manganese, and cobalt are preferred. Their chemical formulas include, for example, Li x M1O2 or Li y It is represented by M2PO4, where M1 and M2 represent at least one transition metal element. The values ​​of x and y vary depending on the charge / discharge state of the battery, but are generally 0.05≦x≦1.10 and 0.05≦y≦1.10.

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

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

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

[0144] The positive electrode active material layer is provided on a part of both sides of the positive electrode current collector, and similarly, the negative electrode active material layer is also provided on a part of both sides of the negative electrode current collector. In this case, for example, the negative electrode active material layer provided on the negative electrode current collector has a region where the opposing positive electrode active material layer does not exist. This is for performing a stable battery design.

[0145] In the region where the above negative electrode active material layer and positive electrode active material layer do not face each other, it is hardly affected by charge and discharge. Therefore, the state of the negative electrode active material layer is maintained as it is immediately after formation, and thereby the composition of the negative electrode active material etc. can be accurately examined with good reproducibility without depending on the presence or absence of charge and discharge.

[0146] [Separator] The separator separates the positive and negative electrodes, preventing current short circuits caused by contact between the electrodes while allowing lithium ions to pass through. This separator is formed of a porous film made of, for example, synthetic resin or ceramic, and may have a laminate structure in which two or more types of porous film are laminated. 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 (electrolytic solution). This electrolytic solution contains an electrolyte salt dissolved in a solvent and may contain other materials such as additives.

[0148] The solvent can be, for example, a non-aqueous solvent. Examples of non-aqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, 1,2-dimethoxyethane, and tetrahydrofuran. Among these, it is desirable to use at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. This is because better properties can be obtained. In this case, more advantageous properties can be obtained by combining a high-viscosity solvent such as ethylene carbonate or propylene carbonate with a low-viscosity solvent such as dimethyl carbonate, ethyl methyl carbonate, or diethyl carbonate. This is because the dissociation property and ion mobility of the electrolyte salt are improved.

[0149] When an alloy-based negative electrode is used, it is particularly desirable to use a solvent containing at least one of halogenated chain carbonates or halogenated cyclic carbonates. This allows a stable coating to be formed on the surface of the negative electrode active material during charge and discharge, particularly during charging. Here, the halogenated chain carbonate is a chain carbonate having a halogen as a constituent element (i.e., at least one hydrogen atom is substituted with a halogen). Furthermore, the halogenated cyclic carbonate is a cyclic carbonate having a halogen as a constituent element (i.e., at least one hydrogen atom is substituted with a halogen).

[0150] Although the type of halogen is not particularly limited, fluorine is preferred because it forms a better coating than other halogens. Furthermore, the more halogens there are, the more desirable they are because the resulting coating is more stable and the decomposition reaction of the electrolyte is reduced.

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

[0152] It is preferable that the solvent additive contains an unsaturated carbon-bond cyclic carbonate. This is because a stable coating is formed on the negative electrode surface during charge and discharge, and the decomposition reaction of the electrolyte can be suppressed. Examples of the unsaturated carbon-bond cyclic carbonate include vinylene carbonate and vinylethylene carbonate.

[0153] It is also preferable that the solvent additive contains sultone (cyclic sulfonic acid ester), as this improves the chemical stability of the battery. Examples of sultones include propane sultone and propene sultone.

[0154] Furthermore, it is preferable that the solvent contains an acid anhydride, because this improves the chemical stability of the electrolyte solution. Examples of acid anhydrides include propanedisulfonic acid anhydride.

[0155] The electrolyte salt may include, for example, one or more light metal salts such as lithium salts, for example, lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4).

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

[0157] EXAMPLES The present invention will be explained in more detail below by showing examples and comparative examples of the present invention, but the present invention is not limited to these examples.

[0158] Example 1 A negative electrode active material was prepared according to the following procedure, and further a laminate film type lithium ion secondary battery 30 shown in FIG. 2 was prepared.

[0159] (Step S1: Preparing a porous carbon structure) First, the surface area (BET specific surface area) is 1940m 2 / g, pore volume 1cm 3 A porous carbon material (porous carbon structure) having a molecular weight of 1 / g, a particle size (D50) of 8 μm, and an IUPAC classification type I was prepared. This porous carbon material was placed in a vacuum container, and a vacuum was drawn to −90 kPa.

[0160] (Step S2: Including OH groups and CxHy groups at the active points of the porous carbon structure) Next, the pressure was restored with nitrogen and the mixture was heated to 150°C while nitrogen was flowing. At this time, a trap was installed in the exhaust gas section to confirm that moisture was not completely discharged. At the same time, a gas detector was used to check the CO gas concentration. This confirmed that the active sites of the porous carbon structure contained OH groups and CxHy groups.

[0161] (Step S3: Under heating, monosilane gas is passed through the exhaust gas containing carbon monoxide gas or carbon dioxide gas derived from oxygen atoms at the active sites, depositing silicon oxide bonded to oxygen atoms inside the porous carbon structure, and the silicon oxide contains dangling bonds.) During drying before the silane reaction, the silane decomposition reaction was carried out in a state where CO gas was detected at approximately 8000 ppm. At this time, the decomposition reaction was carried out in the range of 365 to 380 degrees, which was below the decomposition reaction temperature of silane.

[0162] This is thought to be because, by maintaining a positive pressure inside the vessel, silane is introduced into the pores, and in an atmosphere containing CO gas, the molecular structure is distorted, lowering the activation energy and increasing the reaction rate, enabling low-temperature decomposition. It is also thought that oxygen components attached to the porous carbon are captured during decomposition, creating Si-O bonds.

[0163] (Step S4: Oxidizing at least a portion of the Si present on the surface layer of the porous carbon structure in a pressurized atmosphere) After the reaction, the temperature was returned to room temperature and oxidation treatment was carried out in a pressurized atmosphere of 20 kPa, which resulted in the outermost layer of Si being in a low-valence oxide state and having an SiO2 structure containing defects.

[0164] After that, heat treatment was carried out at 600°C, causing the residual hydrocarbon components to react with Si-H, generating Si-C bonds inside the bulk.

[0165] (Step S5: Deposit a carbon layer at 580°C or less using hydrocarbon gas) When CVD is performed at 580°C 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 in the surface layer, Si-C bonds will not be formed in the surface layer when the acetylene gas is decomposed.

[0166] Acetylene CVD is performed under reduced pressure of 10,000 Pa, allowing it to penetrate the porous carbon and react inside the porous carbon. This reaction breaks the bond of the Si-H moiety that has been generated, generating Si-C bonds. This makes it possible to suppress the generation of hydrogen gas during the slurrying process. Note that the higher the CVD temperature, the more Si-C bonds are created.

[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 spectroscopic analysis.

[0168] [Preparation of negative electrode] The negative electrode active material prepared as described above, graphite, conductive additive 1 (carbon nanotubes, CNT), conductive additive 2 (carbon microparticles with a median diameter of approximately 50 nm), sodium polyacrylate, and carboxymethyl cellulose (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 form a negative electrode mixture slurry.

[0169] The negative electrode current collector was a 15 μm-thick electrolytic copper foil. This electrolytic copper foil contained carbon and sulfur at concentrations of 70 mass ppm each. Finally, the negative electrode mixture slurry was applied to the negative electrode current collector and dried at 100°C in a vacuum atmosphere for 1 hour. After drying, the deposition amount of the negative electrode active material layer per unit area on one side of the negative electrode (also referred to as area density) was 7.0 mg / cm. 2 It was.

[0170] [Assembling a coin battery for testing] Next, the solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed, and then an electrolyte salt (lithium hexafluorophosphate: LiPF6) was dissolved to prepare an electrolyte solution. In this case, the solvent composition was EC:DMC = 30:70 by volume, and the content of the electrolyte salt was 1 mol / kg relative to the solvent. As additives, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added in amounts of 1.0 mass% and 2.0 mass%, respectively.

[0171] Next, a coin battery was assembled as follows: First, a 1 mm thick Li foil was punched out to a diameter of 16 mm and attached to an aluminum clad.

[0172] Next, the negative electrode obtained above was punched out to a diameter of 15 mm, and this was placed opposite a Li foil attached to an aluminum clad via a separator. After injecting an electrolyte, a 2032 coin battery was fabricated.

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

[0174] When examining the initial charge-discharge characteristics, the initial efficiency (hereinafter sometimes referred to as initial efficiency) was calculated using the formula: initial efficiency (%) = (initial discharge capacity / initial charge capacity) × 100.

[0175] [Production and evaluation of lithium-ion secondary batteries] Based on the initial data obtained, the positive electrode was designed so that the negative electrode utilization rate would be 95%. The utilization rate was calculated using the following formula from the positive and negative electrode capacities obtained with the Li counter electrode. Utilization rate = (positive electrode capacity - negative electrode loss) / (negative electrode capacity - negative electrode loss) x 100 Based on this design, lithium ion secondary batteries (lithium ion secondary batteries as shown in FIG. 2) of the examples and comparative examples were manufactured. Battery evaluation was carried out on each of the lithium ion secondary batteries of 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 an atmosphere of 25°C, and the discharge capacity at the second cycle was measured. The battery cycle characteristics were calculated from the discharge capacity at the third cycle, and the battery test was stopped at 1000 cycles. Charging and discharging were performed at 0.7C and 0.5C, respectively. The charging voltage was 4.3V, the discharge cut-off voltage was 2.5V, and the charge cut-off rate was 0.07C.

[0177] The type of silicon-cored carbon composite was investigated by observing the porous carbon structure in the negative electrode active material particles using an SEM.

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

[0179] [Table 1]

[0180] (Comparative Examples 1 and 2) In Comparative Example 1, CO gas was not detected (below the detection limit) during drying before the silane reaction. At this time, heat treatment at 600°C was performed to remove the CxHy components present inside the pores of the porous carbon.

[0181] The material was then subjected to a silane reaction at a temperature of 380°C, forming amorphous silicon inside the bulk. The reaction was completed when the pore volume was filled to 98%, after which it was cooled and returned to room temperature. Oxygen diluted with nitrogen (1% concentration) was then flowed through the material for 48 hours to carry out a gentle oxidation treatment. During this time, the powder was stirred under reduced pressure (reaching -95kPa as indicated by the gauge), and the oxidation treatment was carried out while the powder was stirred, ensuring that oxygen was thoroughly distributed inside the pores. The resulting oxide became SiO2.

[0182] In Comparative Example 2, in addition to Comparative Example 1, the oxidizing atmosphere was slightly reduced to -5 kPa, and oxidation was carried out more slowly than in Comparative Example 1.

[0183] For reference, Figure 8 shows the XANES spectra of the surface layer of the negative electrode active materials of Example 1 and Comparative Example 1. In the comparative example, a structure similar to SiO2 is formed, but the XANES spectrum is shifted slightly to the lower energy side, suggesting defective SiO2. The structure of the surface layer is also similar to that of Example 1.

[0184] Then, when the same evaluation as in Example 1 was carried out, both Comparative Examples 1 and 2 had poor cycle characteristics (1000 Cy maintenance rate in Table 1) and low battery capacity (single unit capacity in Table 1), resulting in unsatisfactory results.

[0185] Examples 2 to 6 In addition to the manufacturing method of Example 1, a heat treatment was performed after the oxidation treatment to crystallize a part of the Si. 0+The grain size was 3 to 6.5 nm. It is believed that the higher the crystallinity, the worse the Li acceptance becomes, which is why the battery characteristics deteriorated slightly.

[0186] The battery characteristics were best when the material was substantially amorphous (the grain size of 0.8 nm in the table was calculated using the Scherrer formula, and is substantially amorphous).

[0187] However, taking into consideration that once the temperature is raised, the powder can be handled more easily, it is desirable to have crystallites of 5 nm or less.

[0188] Examples 7 to 11 The oxygen content was varied compared to Example 1 and evaluated. When the oxygen content was low, many Si-C bonds tended to be generated during acetylene CVD, slightly worsening the cycle characteristics. In this case, the oxygen content at which the cycle characteristics did not deteriorate significantly was 0.5 wt% or more. Conversely, when the oxygen content was too high, the excess oxygen reacted with Li, increasing the irreversible capacity and reducing the battery capacity. Therefore, the optimal range of oxygen content is considered to be 0.5 wt% or more and 5 wt% or less.

[0189] (Examples 12 to 14) The pressure at the time of introducing silane gas was changed and evaluated in comparison with Example 1. When the pressure was low, the silane gas did not reach the depths of the pores, creating voids, which prevented silicon from being filled, resulting in a decrease in battery capacity.

[0190] Furthermore, silicon deposited by decomposition in a pressurized atmosphere contains many dangling bonds, but at lower pressures, the number of dangling bonds is reduced. Silicon with many dangling bonds is highly active, allowing carbon to be decomposed at low temperatures during subsequent carbon CVD.

[0191] To increase the pressure, the internal pressure can be adjusted by adjusting the silane flow rate and the valve opening on the exhaust side. A flow chamber is generally about 10 kPa, but pressure control becomes difficult above 30 kPa. Therefore, in addition to adjusting the opening valve, a vent valve can also be installed. For example, if you want to run the reaction at 50 kPa, you can adjust the internal pressure by operating the vent valve in the 50 + 5 kPa range.

[0192] From the viewpoint of productivity, this is a significant deterioration, but by using this method, it is possible to distribute the gas to every detail.

[0193] As described above, according to the examples of the present invention, it was shown that excessive decomposition of the electrolyte solution could be suppressed compared to the comparative examples, and as a result, it was possible to realize a negative electrode active material that can improve battery cycle characteristics and increase capacity.

[0194] The present specification includes the following aspects. [1]: A negative electrode active material having negative electrode active material particles, the negative electrode active material particles include a porous carbon structure serving as a base material, The porous carbon structure has an Si-C bond inside, amorphous low-valent nano-silicon oxide dispersed in the surface layer, and at least a portion of the low-valent nano-silicon oxide contains tetravalent silicon oxide capable of absorbing and desorbing Li. [2]: The negative electrode active material according to [1], wherein the low-valent nano-silicon oxide absorbs Li during a Li absorption-desorption process. [3]: 29 The negative electrode active material according to [1] or [2], characterized in that the spectrum obtained by Si-MAS-NMR measurement has a peak derived from the Si-C bond in the range of 0 to -30 ppm and a peak derived from Si or the low-valence nano-silicon oxide in the range of -40 to -80 ppm. [4]: beforeThe negative electrode active material according to any one of [1] to [3] above, characterized in that active sites exist inside the pores of the porous carbon structure, and some of the active sites are modified with OH. [5]: The low-valence nanosilicon oxide has a carbon layer separate from the porous carbon structure that is the base material, The negative electrode active material according to any one of [1] to [4] above, wherein a C═O bond or a CO bond is formed at the interface of the other carbon layer. [6]: The negative electrode active material according to any one of [1] to [5], characterized in that the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using the Scherrer equation from the peak measured by X-ray diffraction measurement of the negative electrode active material particles, is in the range of 0.8 nm to 5 nm. [7]: The negative electrode active material according to [6], wherein the zero-valent Si is substantially amorphous. [8]: The negative electrode active material according to any one of [1] to [7], wherein the amount of oxygen contained in the negative electrode active material as a whole 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, Providing a porous carbon structure; a step of including OH groups and CxHy groups at the active points of the porous carbon structure; a step of flowing monosilane gas under heating in a state in which carbon monoxide gas or carbon dioxide gas derived from oxygen atoms of the active sites is contained in exhaust gas, thereby depositing silicon oxide bonded to the oxygen atoms inside the porous carbon structure, the silicon oxide containing dangling bonds; a step of oxidizing at least a portion of Si present in a surface layer of the porous carbon structure in a pressurized atmosphere after the step of depositing silicon oxide; After the step of oxidizing in a pressurized atmosphere, depositing a carbon layer using a hydrocarbon gas at a temperature of 580°C or less; 2. A method for producing a negative electrode active material, comprising:

[0195] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits 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 (laminate film type), 31... Electrode body, 32...Positive electrode lead (positive electrode aluminum lead), 33...Negative electrode lead (negative electrode nickel lead), 34...adhesive film, 35...exterior component. 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 serving as a base material, The porous carbon structure has an Si-C bond therein, an amorphous low-valence nanosilicon oxide is dispersed in a surface layer portion, and at least a part of the low-valence nanosilicon oxide contains a tetravalent silicon oxide capable of absorbing and desorbing Li; The spectrum obtained by 29Si-MAS-NMR measurement has a peak derived from the Si-C bond in the range of 0 to -30 ppm and a peak derived from Si or the low-valence nanosilicon oxide in the range of -40 to -80 ppm. A negative electrode active material.

2. The negative electrode active material according to claim 1 , wherein the low-valent nano-silicon oxide absorbs Li during a Li absorption / desorption process.

3. 2. The negative electrode active material according to claim 1, wherein active sites exist inside the pores of the porous carbon structure, and some of the active sites are modified with OH.

4. the low-valence nanosilicon oxide has a carbon layer separate from the porous carbon structure that is the base material, 2. The negative electrode active material according to claim 1, wherein a C═O bond or a C—O bond is formed at the interface of the other carbon layer.

5. The negative electrode active material particles are subjected to X-ray diffraction measurement, and the grain size of zero-valent Si constituting the low-valent nanosilicon oxide calculated using the Scherrer equation from the peak measured is in the range of 0.8 nm to 5 nm. The negative electrode active material according to claim 1.

6. 6. The negative electrode active material according to claim 5, wherein the zero-valent Si is substantially amorphous.

7. 2. The negative electrode active material according to claim 1, wherein the amount of oxygen contained in the negative electrode active material as a whole is in the range of 0.5 wt % to 5 wt %.

8. A method for producing a negative electrode active material, comprising: Providing a porous carbon structure; a step of including OH groups and CxHy groups at the active points of the porous carbon structure; a step of flowing monosilane gas under heating in a state in which carbon monoxide gas or carbon dioxide gas derived from oxygen atoms of the active sites is contained in exhaust gas, thereby depositing silicon oxide bonded to the oxygen atoms inside the porous carbon structure, the silicon oxide containing dangling bonds; a step of oxidizing at least a portion of the Si present in the surface layer of the porous carbon structure in a pressurized atmosphere after the step of depositing the silicon oxide; After the step of oxidizing in a pressurized atmosphere, depositing a carbon layer using a hydrocarbon gas at a temperature of 580° C. or less; By including The negative electrode active material having negative electrode active material particles, the negative electrode active material particles contain the porous carbon structure that is a base material, The porous carbon structure has an Si-C bond inside, an amorphous low-valence nano-silicon oxide is dispersed in a surface layer portion, and at least a part of the low-valence nano-silicon oxide contains a tetravalent silicon oxide capable of absorbing and desorbing Li; A method for producing a negative electrode active material, characterized in that the spectrum obtained by 29Si-MAS-NMR measurement has a peak derived from the Si-C bond in the range of 0 to -30 ppm and a peak derived from the Si or the low-valence nanosilicon oxide in the range of -40 to -80 ppm.

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