Negative electrode active material and method for producing same

Abstract

The present invention is a negative electrode active material that has negative electrode active material particles, wherein the negative electrode active material particles contain a porous carbon structure having branch-shaped pores formed therein by an alkali activation treatment, and an amorphous low-valence nanosilicon oxide is dispersed in a network structure in the interior of the porous carbon structure. This provides a negative electrode active material that can increase the capacity while maintaining the battery characteristics.

Description

Negative electrode active material and method for producing the same

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

[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 led to a demand for further improvements in battery capacity. To improve battery capacity, the use of silicon as a negative electrode active material has been 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 active material shape is being considered, ranging from the standard coated type for carbon-based active materials to an integrated type in which the material is deposited directly on the current collector.

[0006] However, when silicon is used as the primary 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, primarily 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 is created, increasing the reactive area of ​​the active material. At this time, the electrolyte decomposes on the new surface, and a coating of the electrolyte decomposition product is formed on the new surface, consuming the electrolyte. This can lead to a deterioration 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 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, SiO ２ , M ｙ In order to improve cycle characteristics, nanocomposites containing SiO2 and 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 a high temperature (see, for example, Patent Document 6). 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 so that the difference between the maximum and minimum molar ratios near the interface between the active material and the current collector is 0.4 or less (see, for example, Patent Document 7). To improve battery load characteristics, a lithium-containing metal oxide is used (see, for example, Patent Document 8). To improve cycle characteristics, a hydrophobic layer such as a silane compound is formed on the surface of the silicon material (see, for example, Patent Document 9).

[0010] In addition, in order to improve cycle characteristics, silicon oxide is used and a graphite coating is formed on the surface thereof to impart conductivity (see, for example, Patent Document 10). In Patent Document 10, the shift value obtained from the RAMAN spectrum of the graphite coating is 1330 cm －１ and 1580 cm －１ A broad peak appears at the peak intensity ratio I １３３０ / I １５８０ is 1.5<I １３３０ / I １５８０ <3. In order to achieve high battery capacity and improved cycle characteristics, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used (see, for example, Patent Document 11). In addition, in order to improve overcharge and overdischarge characteristics, silicon oxide is used in which the atomic ratio of silicon to oxygen is controlled to 1:y (0<y<2) (see, for example, Patent Document 12).

[0011] In addition, Hitachi Maxell began shipping lithium ion secondary batteries using silicon oxide in June 2010, which are rectangular secondary batteries for smartphones that use nanosilicon composites (see, for example, Non-Patent Document 1). ０＋ ~Si ４＋ It is a composite material with various oxidation states (Non-Patent Document 2). Kapaklis also reported that by applying a thermal load to silicon oxide, Si and SiO ２ proposed a disproportionated structure that is divided into

[0012] Miyachi et al. have reported that among silicon oxides with disproportionated structures, Si and SiO contribute to charge and discharge. ２ have focused on the reaction between silicon oxide and Li (Non-Patent Document 4), and Yamada et al. have proposed the following reaction formula for silicon oxide and Li (Non-Patent Document 5): 2SiO(Si + SiO ２ ) + 6.85Li ＋ + 6.85e － → 1.4 Li ３．７５ Si + 0.4Li ４ SiO ４ + 0.2SiO ２ In the reaction formula, silicon oxide is composed of Si and SiO ２ reacts with Li to form Li silicide, Li silicate, and some unreacted SiO ２ Divided into two.

[0013] The Li silicate produced here is irreversible, and once formed, it is generally said to be a stable substance that does not release Li. 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. ４ SiO ４ As, ７ Li-MAS-NMR 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 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 powder treatment method instead of Li doping the electrode, 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 issues when it comes to industrialization. Therefore, CVD-Si-C, which uses silane gas to create porous carbon and generates nanosilicon inside, has achieved a higher energy density than Li-doped SiO (Patent Documents 14 and 15).

[0016] JP 2001-185127 A JP 2002-042806 A JP 2006-164954 A JP 2006-114454 A JP 2009-070825 A JP 2008-282819 A JP 2008-251369 A JP 2008-177346 A JP 2007-234255 A JP 2009-212074 A JP 2009-205950 A JP 06-325765 A JP 2015-156355 A U.S. Pat. No. 10,608,254 U.S. Pat. No. 11,165,054

[0017] Battery Industry Association Newsletter "Denchi" May 1, 2010 issue, p. 10 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.V. Kapaklis, J. Non-Crystalline Solids, 354 (2008) 612Mariko Miyachi, Hironori Yamamoto, and Hidemasa Kawai, J. Electrochem. Soc. 2007 volume 154, issue 4, A376-A380M. Yamada, A. Inaba, A. Ueda, K. Matsumoto, T. Iwasaki, T. Ohzuku, J. Electrochem. Soc., 159, A1630 (2012) Taeahn Kim, Sangjin Park, and Seung M. Oh, J. Electrochem. Soc. volume 154, (2007), A1112-A1117. 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. Frontiers of Development of Automotive Lithium-ion Batteries, pp. 96-111, CMC Publishing, published November 27, 2020.

[0018] 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 required to have larger battery capacities. One method to solve this problem is to develop lithium-ion secondary batteries with a negative electrode that uses silicon as the main material.

[0019] Furthermore, lithium-ion secondary batteries using silicon materials are expected to have initial charge-discharge characteristics 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 deintercalation as the negative electrode active material, cycle characteristics and initial charge-discharge characteristics have been improved. Recently, by incorporating Li in advance and forming Li silicate, primarily silicon oxide, the irreversible capacity, a disadvantage of silicon oxide, has been reduced, and this has actually begun to be commercialized. Even when a prototype battery was fabricated by replacing 100% of this silicon oxide with Li—SiO—C (Non-Patent Document 8), which uses Li, with carbon anode material, the capacity improvement was only in the high 20% range compared to carbon anode material. This means that further improvements in battery capacity are required when considering the increasing performance of small electronic devices (5G, etc.) and the improvement in driving range of electric vehicles.

[0020] Therefore, CVD-Si-C, which has a low irreversible capacity, has been developed, but it is known that the reaction between Si and the electrolyte results in insufficient high-speed charging and battery cycle characteristics.

[0021] The present invention has been made in view of the above problems, and an object of the present invention is to provide a negative electrode active material that can increase the capacity while maintaining the battery characteristics.

[0022] In order to solve the above problems, the present invention provides a negative electrode active material having negative electrode active material particles, characterized in that the negative electrode active material particles contain a porous carbon structure having branched pores formed by alkali activation treatment therein, and amorphous low-valent nano silicon oxide is dispersed in a network structure inside the porous carbon structure.

[0023] The negative electrode active material of the present invention has amorphous low-valence nanosilicon oxide dispersed within a porous carbon structure. The presence of the porous carbon structure can reduce the adverse effects of expansion of the internal low-valence nanosilicon oxide. Furthermore, because the porous carbon structure has branched pores due to alkali activation treatment, the Si portion deposited thereon becomes thinner during the manufacturing process of the negative electrode active material. This not only facilitates smooth post-oxidation, but also makes it easier to control the oxidation process, making it easier to control the valence of the nanosilicon oxide. Therefore, Si—O bonds can be present (particularly throughout), resulting in amorphous low-valence nanosilicon oxide dispersed in a network structure as described above. When Si—O bonds are present (particularly throughout), Li １５ Si ４ Therefore, it can be said that it is more suitable for high-speed charging.

[0024] In this case, when XAFS measurements are performed on a pre-charged anode removed from a secondary battery having a anode containing the anode active material before charging, and on a post-charged anode removed from the secondary battery after charging the pre-charged secondary battery, the Si K-absorption edge XANES spectra obtained from the XAFS measurements of the anode before and after charging show that the peak attributable to a tetracoordinated silicon compound in the energy range of 1846 eV to 1850 eV is higher in the anode before charging than in the anode after charging, and the XANES spectrum of the anode after charging preferably has a peak attributable to a hexacoordinated silicon compound in the energy range of 1851 eV to 1855 eV.

[0025] Thus, during charging, the tetravalent Si peak in the range of 1846 eV to 1850 eV decreases, and a hexavalent Si peak in the range of 1851 eV to 1855 eV appears. In other words, the tetracoordinated silicon compound undergoes a structural change during charging, changing into a hexacoordinated silicon compound (stishovite). The hexacoordinated silicon compound has a substantially high resistance, leading to Li deposition.

[0026] In this case, it is preferable that the XANES spectrum of the negative electrode before charging has peaks at energies near 1845.5 eV and 1847.5 eV.

[0027] The peak near 1845.5 eV represents a divalent Si peak, and the peak near 1847.5 eV represents a tetravalent or trivalent Si peak containing defects.

[0028] The negative electrode active material is solid ２９ It is preferable that the maximum value is in the range of -81 to -95 ppm when measured by Si-CP / MAS-NMR.

[0029] Such negative electrode active material particles are negative electrode active material particles that contain a sufficient amount of SiOSi structure, and therefore can maintain a lower irreversible capacity than general SiO.

[0030] In this case, the negative electrode active material particles preferably have a SiOSi structure in which oxygen is bonded to a Si radical.

[0031] In this way, by having a SiOSi structure in which oxygen is bonded to a Si radical, decomposition of the electrolyte can be effectively suppressed.

[0032] Furthermore, the low-valence nano silicon oxide is preferably substantially in a composite state of zero valence, monovalence and divalence, with divalence being the most predominant.

[0033] In this way, the low-valence nanosilicon oxide is substantially in a composite state of zero valence, monovalence, and divalence, and further, the divalence is substantially predominant, so that a lower irreversible capacity can be achieved.

[0034] Furthermore, the low-valence nano silicon oxide dispersed in the porous carbon structure may have x increasing from the center to the surface layer of the porous carbon structure.

[0035] In this way, in the present invention, because low-valent nano silicon oxide is dispersed in the porous carbon structure, x tends to increase (the oxygen content ratio increases) from the center of the porous carbon structure to the surface during production. Because the silicon oxidation ratio is high at the surface, decomposition of the electrolyte is more effectively suppressed, while the silicon oxidation ratio is low inside the active material, allowing for a greater increase in battery capacity.

[0036] The proportion of the porous carbon structure in the entire negative electrode active material particles is preferably 45% by mass or more and 64% by mass or less.

[0037] Such a ratio makes it possible to effectively ensure both the battery capacity and the volume change of the low-valence nanosilicon oxide due to the porous carbon structure.

[0038] 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 1 nm to 5 nm.

[0039] Such a material having a grain size of zero-valent Si that is substantially amorphous is preferred.

[0040] The structure of the porous carbon is predominantly Type I in the IUPAC classification, and its surface area is 1325 m ２ / g or more, pore volume is 0.95 cm ３ / g or more is preferred.

[0041] The porous carbon structure having the above IUPAC classification, surface area, and pore volume allows for a negative electrode active material that efficiently contains a large amount of low-valence nanosilicon oxide. In particular, the IUPAC type I structure allows for smooth Si—O bond formation after deposition.

[0042] The present invention also provides a method for producing a negative electrode active material having negative electrode active material particles, the method comprising the steps of: preparing a porous carbon structure having branched pores therein by a production method using an alkali activation treatment; flowing moisture-containing nitrogen gas through the porous carbon structure; and, after the moisture-containing nitrogen gas has been flowed, flowing monosilane gas through the porous carbon structure under heating to deposit silicon inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; and, after the cooling, introducing oxygen diluted with nitrogen gas into the material with silicon deposited inside the porous carbon structure while adjusting the temperature of the material with silicon deposited inside the porous carbon structure to maintain it at 50°C or less, thereby converting at least a portion of the silicon into low-valence nanosilicon oxide, thereby providing a method for producing a negative electrode active material, the method comprising the steps of:

[0043] With such a method for producing a negative electrode active material, it is possible to simply and efficiently produce a negative electrode active material in which amorphous low-valent nanosilicon oxide containing a sufficient amount of SiOSi structure is dispersed in a network structure inside a porous carbon structure having branched pores therein, as described above.

[0044] The negative electrode active material of the present invention has amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, and therefore, when the low-valence nanosilicon oxide expands, the presence of the porous carbon structure can reduce the adverse effects of the expansion. Moreover, in the porous carbon structure having branched pores inside, amorphous low-valence nanosilicon oxide is dispersed in a network structure inside, in other words, Si—O bonds exist (particularly throughout), and Li １５ Si ４The Si—O bond is less likely to form, making it more suitable for high-speed charging. Furthermore, the Si—O bond can suppress decomposition of the electrolyte, making it possible to reduce the SEI that accumulates in the surface layer. As a result, a negative electrode using the negative electrode active material of the present invention can achieve high initial efficiency, high capacity, high input characteristics, and high cycle characteristics. Moreover, it is suitable for high-speed charging.

[0045] Furthermore, the method for producing a negative electrode active material of the present invention can simply and efficiently produce a negative electrode active material in which amorphous low-valent nanosilicon oxide containing a sufficient amount of SiOSi structure is dispersed in a network structure inside the porous carbon structure having branched pores therein.

[0046] FIG. 1 is a cross-sectional view showing the configuration of a negative electrode containing the negative electrode active material of the present invention. FIG. 2 is an exploded view showing an example of the configuration of a lithium ion secondary battery (laminate film type) containing the negative electrode active material of the present invention. FIG. 3 is a flow chart showing an example of a method for producing the negative electrode active material of the present invention. FIG. 4 is a cross-sectional TEM photograph of the negative electrode active material of Example 1. FIG. 5 is an external view photograph of the negative electrode after charge and discharge in Example 1. FIG. 6 is a cross-sectional TEM photograph of the negative electrode active material of Comparative Example 1. FIG. 7 is an analysis result of XAFS measurement (XANES spectrum) before and after charging of the negative electrodes in Example 1 and Comparative Example 1. ２９ 1 is a Si-CP / MAS-NMR spectrum; 2 is a photograph of the appearance of the negative electrode after charge and discharge in Comparative Example 1; ２９ This is a Si-CP / MAS-NMR spectrum.

[0047] Hereinafter, the present invention will be described in detail with reference to the preferred embodiments, but the present invention is not limited to these.

[0048] 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 nanosilicon 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.

[0049] 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.

[0050] In particular, the CVD-Si-C disclosed in Patent Documents 14 and 15 has an issue of excessive reactivity with the electrolyte. In the present invention, in order to suppress such a reaction with the electrolyte, the Si portion is converted to a Si-O phase, and the siloxane bond specific to silicon oxide is used, thereby not only significantly suppressing the reactive decomposition of the electrolyte, but also ensuring high-speed charging because the Si-O material with siloxane bonds has good Li acceptance.

[0051] [Negative electrode active material of the present invention] The negative electrode active material of the present invention is a negative electrode active material having negative electrode active material particles, characterized in that the negative electrode active material particles contain a porous carbon structure having branched pores formed by an alkali activation treatment therein, and amorphous low-valent nano silicon oxide is dispersed in a network structure inside the porous carbon structure.

[0052] In this negative electrode active material, amorphous low-valence nano-silicon oxide is dispersed inside the porous carbon structure, and the presence of the porous carbon structure can reduce the adverse effects of the expansion of the internal low-valence nano-silicon oxide. Moreover, in the porous carbon structure having branched pores inside due to alkali activation treatment, amorphous low-valence nano-silicon oxide is dispersed in a network structure inside, in other words, Si-O bonds are present (especially throughout). When a large amount of Si-Si is present here, when it reacts with Li, the Si-Si in the surface layer becomes an ionic substance close to an insulator, and the Li-O bond is formed. １５ Si ４ Therefore, it is not suitable for high-speed charging. However, when Si—O bonds exist (especially when they exist throughout the entire structure) as in the present invention, Li １５ Si ４is less likely to be formed, making it more suitable for high-speed charging. Furthermore, the Si—O bond can suppress the decomposition of the electrolyte, making it possible to reduce the SEI that accumulates in the surface layer. Note that in the present invention, it is sufficient that the amorphous low-valent nanosilicon oxide is dispersed in a network structure inside the porous carbon structure, and some of the nanosilicon oxide may be crystalline.

[0053] Now, let's explain the branched pores. First, conventional products have pores with a diameter of 2-3 nm (average) as well as pores with a diameter of 10-30 nm (solid pores), with the 2-3 nm pores acting as bypasses connecting the 10-30 nm pores. On the other hand, branched pores do not have pores with a diameter of 10-30 nm, and instead have pores with a diameter of 2-3 nm extending radially (branch-like) into the carbon, with the total length being in the micrometer range from the particle surface to the interior.

[0054] The solid state material used to measure the properties of the negative electrode active material of the present invention ２９ We will explain Si-CP / MAS-NMR. As is well known, CP / MAS stands for Cross Polarization Magic Angle Spinning. ２９ Si-CP / MAS-NMR is a solid-state NMR measurement method that combines cross polarization (CP) and magic angle spinning (MAS). MAS method without cross polarization (CP) can be directly ２９ CP / MAS is a method of exciting and detecting Si nuclei. １ H nucleus is excited, and then ２９ Magnetization was transferred to the Si nucleus. ２９ This method detects only the Si nuclei. Therefore, it does not cause magnetization transfer in the vicinity. １ No H nucleus ２９ As a result, Si nuclei having Si-H groups, Si-OH groups, or Si-O groups are detected with high sensitivity because their magnetization transfers with a high probability. On the other hand, parts with only Si-Si bonds and SiO ２ Only bonds are detected. ２９ Since Si-CP / MAS-NMR is such a measurement method, it is a selective detection method. ２９When negative electrode active material particles in which amorphous low-valence nano silicon oxide is dispersed inside a porous carbon structure are measured by Si-CP / MAS-NMR, if the maximum value is in the range of -81 to -95 ppm, it can be said that the SiOSi structure is sufficiently contained.

[0055] In this way, the negative electrode active material particles of the present invention can have a SiOSi structure in which oxygen is bonded to a Si radical.

[0056] solid ２９ The Si-CP / MAS-NMR measurement can be carried out, for example, under the following conditions: ２９ Si-CP / MAS-NMR measurement conditions: Apparatus: Bruker AVANCE700; Detector: 4 mmφ CPMS solid probe; Rotor: 4 mmφ zirconia; Cap: KEL-F; ２９ Si resonance frequency: 139.1 MHz Pulse sequence: CP / MAS Contact time: 5 ms Delay time: 5.0 sec MAS speed: 9 kHz Number of accumulations: 16,000 Measurement temperature: room temperature Chemical shift external standard: hexamethylcyclotrisiloxane -9.66 ppm

[0057] As described below, the negative electrode active material of the present invention can be produced as a Si-Ox material containing siloxane bonds in the Si phase to suppress the decomposition reaction of the electrolyte, which is insufficient in CVD-Si-C produced from general silane gas. The purpose of intentionally creating Si-O bonds is that while Si-Si bonds promote the decomposition of the electrolyte, Si-O bonds react more slowly with the electrolyte than Si-Si bonds. Furthermore, since Si-O bonds can suppress the decomposition of the electrolyte, it is possible to reduce the SEI that accumulates in the surface layer of the CVD-Si-C. As a result, battery cycle characteristics are improved. Furthermore, while the grain boundaries of grains containing Si-Si bonds reduce Li diffusivity, Si-O bonds improve Li diffusivity and improve high-speed charging. The material of the present invention can be defined as CVD-SiOx-C in contrast to CVD-Si-C because the portion that primarily contributes to charge and discharge is a low-valent nanosilicon oxide. The active material thus produced can have high energy density and high-speed charging properties while maintaining the cycle characteristics of the battery.

[0058] In addition, since the tetravalent Si constituting SiO is an irreversible component, in the negative electrode active material of the present invention, it is preferable that the low-valence nanosilicon oxide is substantially in a composite state of zero valence, monovalent, and divalent. In particular, it is preferable that this low-valence nanosilicon oxide is substantially most predominantly divalent. By making SiOx of divalent or less predominant, the irreversible capacity is larger than that of silicon alone, but it is possible to maintain a lower irreversible capacity than general SiO.

[0059] In the negative electrode active material of the present invention, 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 1 nm to 5 nm. Such a grain size of zero-valent Si that is substantially amorphous is preferable.

[0060] Calculation of crystallite size by XRD can be performed, for example, under the following conditions. For broad peaks, analysis software TOPAS can be used, for example, under the following conditions. XRD measurement: Apparatus: Bruker D2 PHASER X-ray source: Cu Divergence slit: 0.5° Incident side solar: 4° Receiving 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" and "Use Extrapolation"

[0061] The valence of the low-valence nano silicon oxide can be quantified by NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0062] The NMR measurement for measuring the valence of the low-valence nano silicon oxide can be carried out, for example, under the following conditions. ２９ Si MAS NMR (Magic Angle Spinning Nuclear Magnetic Resonance) Apparatus: Bruker 700 NMR spectrometer, Probe: 4 mm HR-MAS rotor 50 μL, Sample rotation speed: 10 kHz, Measurement environment temperature: 25° C.

[0063] The XPS measurement can be carried out, for example, under the following conditions: XPS Apparatus: X-ray photoelectron spectrometer, X-ray source: monochromated Al Kα ray, X-ray spot diameter: 100 μm, Ar ion gun sputtering conditions: 0.5 kV 2 mm×2 mm.

[0064] In addition, in the negative electrode active material of the present invention, it is preferable that the low-valence nanosilicon oxide dispersed in the porous carbon structure has x increasing from the center to the surface of the porous carbon structure. In the present invention, since the low-valence nanosilicon oxide is dispersed in the porous carbon structure, x tends to increase (the oxygen content increases) from the center to the surface of the porous carbon structure during production. Because the silicon oxidation rate is high at the surface, decomposition of the electrolyte can be more effectively suppressed, while the silicon oxidation rate is low inside the active material, thereby increasing the battery capacity.

[0065] Furthermore, when XAFS measurements are performed on a pre-charged anode removed from a secondary battery having a negative electrode containing the anode active material before charging, and on a post-charged anode removed from the secondary battery after charging the pre-charged secondary battery, the Si K-absorption edge XANES spectra obtained from the XAFS measurements of the anode before and after charging show that the peak derived from a tetracoordinated silicon compound in the energy range of 1846 eV to 1850 eV is higher in the anode before charging than in the anode after charging, and the XANES spectrum of the anode after charging preferably has a peak derived from a hexacoordinated silicon compound in the energy range of 1851 eV to 1855 eV.

[0066] In this way, the tetracoordinated silicon compound undergoes a structural change during charging, and changes into a hexacoordinated silicon compound (stishovite). In particular, lithium hexafluorophosphate (LiPF ) is used as an electrolyte salt in the electrolyte solution of a secondary battery, which will be described later. ６ ), the hexacoordinated silicon compound (stishovite) contains P.

[0067] In this case, the XANES spectrum of the negative electrode before charging preferably has peaks at energies near 1845.5 eV and 1847.5 eV. The peak near 1845.5 eV is a peak of divalent Si, and the peak near 1847.5 eV is a peak of tetravalent or trivalent Si including vacancies.

[0068] The valence of the Si component in a high-valent silicon compound can be confirmed by a spectrum in the XANES (X-ray absorption near edge structure) region of an XAFS (X-ray absorption fine structure) measurement. For example, this can be performed under the following conditions. XAFS measurement Si K-edge (K absorption edge) Measurement facility: BL6N1 at the Aichi Synchrotron Light Center, Acceleration energy: 1.2 GeV, Accumulation current: 300 mA, 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 bend cylindrical mirror. Upstream slit opening: 7.0 mm horizontal x 3.0 mm vertical. Beam size: 2.0 mm horizontal x 1.0 mm vertical. Angle of incidence on the sample: Normal incidence (incident angle 0 degrees). Energy calibration: K ２ SO ４ The peak position at the S-K edge of the sample is calibrated to 2481.70 eV. Measurement method: Total electron yield method by measuring the sample current. I0 measurement method: Au-mesh during XANES measurement. Degree of vacuum in the measurement chamber: 5 x 10 －７ Pa Sample environment: The transfer vessel was set without exposure to the atmosphere.

[0069] <Negative Electrode for Non-Aqueous Electrolyte Secondary Battery> 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.

[0070] [Configuration of Negative Electrode] 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, 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 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 have to have the negative electrode current collector 11.

[0071] [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). This conductive material is preferably a material that does not form an intermetallic compound with lithium (Li).

[0072] 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, it is effective in suppressing deformation of the electrode including the current collector. 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.

[0073] The surface of the negative electrode current collector 11 is preferably roughened, and the ten-point average roughness Rz of the surface 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.

[0074] [Negative Electrode Active Material Layer] The negative electrode active material layer 12 may contain multiple types of negative electrode active materials, such as a carbon-based active material, in addition to the silicon-based active material particles of the present invention. Furthermore, in terms of battery design, it may also contain other materials such as a thickener (also referred to as a "binding agent" or "binder") and a conductive additive.

[0075] [Negative Electrode Active Material and Method for Producing Negative Electrode] Next, an example of a method for producing a negative electrode active material for a nonaqueous electrolyte secondary battery of the present invention and a negative electrode using the same will be described.

[0076] First, a method for producing the negative electrode active material contained in the negative electrode will be described. The method for producing a negative electrode active material of the present invention is a method for producing a negative electrode active material having negative electrode active material particles, and includes the steps of: preparing a porous carbon structure having branched pores therein by a production method using an alkali activation treatment; flowing moisture-containing nitrogen gas through the porous carbon structure; and, after the moisture-containing nitrogen gas has been flowed, flowing monosilane gas through the porous carbon structure under heating to deposit silicon inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; and, after the cooling, introducing oxygen diluted with nitrogen gas into the material with silicon deposited inside the porous carbon structure while adjusting the temperature of the material with silicon deposited inside the porous carbon structure to maintain it at 50°C or less, thereby converting at least a portion of the silicon into low-valence nanosilicon oxide, thereby producing a negative electrode active material particle having amorphous low-valence nanosilicon oxide dispersed in a network structure inside the porous carbon structure.

[0077] First, each step of an example of a method for producing a negative electrode active material will be described with reference to steps S11 to S15 in FIG.

[0078] First, a porous carbon structure having branched pores therein is prepared by a manufacturing method using an alkali activation treatment (step S11). First, a material with few solid pores is selected. This can be prepared, for example, by subjecting coconut shells to steam activation treatment or by changing the carbonization temperature of the substrate. The solid pores can be adjusted by appropriately controlling the carbonization state of the raw material (substrate) and then adjusting them to solid pores through a subsequent activation treatment. The prepared material is then subjected to an alkali activation treatment. For example, this can be achieved by mixing an alkali metal hydroxide with the prepared material and heating it. The heating temperature can be, for example, about 1000°C. Examples of alkali metal hydroxides include, but are not limited to, KOH, NaOH, LiOH, RbOH, and CsOH (especially KOH). To explain a more specific example of the alkali activation procedure, for example, a three-fold weight amount of KOH is added to carbonized coconut shells (or other materials such as resin) or charcoal, and the mixture is heated to 1,000°C at 10°C / min, maintained at that temperature for two hours, and then cooled to room temperature. The mixture is then subjected to ultrasonic cleaning with pure water for two hours and cleaning with a stirrer for three hours to remove potassium, thereby obtaining the porous carbon structure described above. Using this alkali activation treatment, it is possible to adjust the formation of branched pores within the structure, and a porous carbon structure having these branched pores can be manufactured. While the number of solid pores can be varied by activation treatment (steam activation treatment, alkali activation treatment, etc.), the alkali activation treatment of the present invention can form branched pores, which are radially arranged with pores having a diameter of 2-3 nm (average), without any solid pores (10-30 nm), as described above. For pores with a diameter of 2-3 nm (average), if the same raw material (e.g., coconut shell) is used, the surface area of ​​the pores can be increased to 1500 m by steam activation treatment, for example. ２ / g, whereas alkali activation treatment resulted in 2000m ２ That is, in the case of alkali activation treatment, the carbon black can be increased by about 33% or more compared to the case of steam activation treatment.

[0079] The porous carbon structure prepared here preferably has at least a carbon-carbon double bond in a part thereof. The porous carbon structure prepared here is predominantly Type I in the IUPAC classification, and its surface area is 1325 m ２ / g or more, pore volume is 0.95 cm ３ / g or more is preferable. By using such IUPAC classification, surface area, and pore volume, silicon deposition can be performed efficiently in larger quantities. Furthermore, the following measurement methods can be used for the IUPAC classification, surface area, and pore volume. - Shimadzu Corporation Tristar II Plus is used to measure the specific surface area / pore distribution by a constant volume method based on the gas adsorption method. The conditions are as follows: - Gas used: Nitrogen - Environment: Under liquid nitrogen - Pressure operating range: P / P0 - Adsorption 0 to 0.998 - Desorption 0.998 to 0.10 - Pretreatment: Vacuum 200°C 1 hour

[0080] Next, nitrogen gas containing moisture is passed through the porous carbon structure prepared in step S11 (step S12). This allows OH groups to be attached to the inside of the pores, which allows for smooth oxidation in step S15, which will be described later. Because the pores in the prepared porous carbon structure are branched, not only can the oxidation in step S15, which will be described later, be smooth, but the oxidation process is also easy to control. Furthermore, by controlling the oxidation process, the valence of the nanosilicon oxide can be controlled.

[0081] After step S11 and before step S12, it is preferable to place the porous carbon structure in a vacuum container and evacuate it. The degree of vacuum can be, for example, but is not limited to, about -100 kPa. After evacuating, it is preferable to restore the pressure with nitrogen and heat the chamber to about 350 to 450°C using an external heater while the nitrogen is flowing. This heating can be performed for 5 minutes to 1 hour. By performing such evacuation and preheating in the presence of nitrogen, nuclei for silicon deposition in step S13 (described later) can be formed and hydrogen and water attached to the porous carbon structure can be removed, thereby more reliably carrying out silicon deposition in step S13.

[0082] Next, monosilane gas is flowed into the porous carbon structure after step S12 while heating, thereby depositing silicon inside the porous carbon structure (step S13). The silicon deposition in step S13 can be performed by flowing monosilane gas at, for example, about 400° C. to 500° C. The deposition time can be, for example, 30 minutes to 10 hours.

[0083] Next, the material with silicon deposited inside the porous carbon structure is cooled to 50°C or less (step S14). In this step, it is preferable to cool the material while flowing nitrogen gas. This cooling can be performed, for example, to room temperature (15°C, preferably 20°C).

[0084] After the cooling (step S14), the temperature of the material with silicon deposited inside the porous carbon structure is adjusted to maintain 50°C or less. Then, oxygen diluted with nitrogen gas is introduced into the material with silicon deposited inside the porous carbon structure, converting at least a portion of the silicon into low-valence nano-silicon oxide (step S15). This step allows the formation of Si—O bonds. It is more preferable to maintain the temperature at 35°C or less.

[0085] The dilution of oxygen with nitrogen can be, for example, 5 to 50 times, preferably 10 to 30 times, and typically 20 times.

[0086] Furthermore, in step S15, if the internal temperature rises and exceeds 50°C, silicon dioxide will be formed in some parts, which is undesirable as a negative electrode active material. Therefore, in step S15, it is necessary to adjust the temperature of the material so that it remains at 50°C or lower. Furthermore, in order to facilitate the oxidation reaction and the formation of Si—O bonds, the material temperature in step S15 is preferably set to, for example, 25°C or higher, and more preferably 30°C or higher.

[0087] The oxidation time in step S15 (flow time of oxygen diluted with nitrogen gas) can be, for example, 30 minutes to 5 hours, preferably 1 hour to 3 hours. After the flow of oxygen diluted with nitrogen gas, the flow can be switched to nitrogen gas for further cooling. The flow of nitrogen gas alone can be, for example, 30 minutes to 2 hours.

[0088] In the oxidation by the flow of oxygen diluted with nitrogen gas, it is preferable to adjust the low-valence nanosilicon oxide dispersed in the porous carbon structure so that x increases from the center to the surface of the porous carbon structure. Note that in the porous carbon structure, the pore structure tends to have a large cross-sectional area at the particle surface and a smaller cross-sectional area toward the inside of the particle. Therefore, by performing oxidation by the flow of oxygen diluted with nitrogen gas as in the present invention, x naturally tends to increase from the center to the surface of the porous carbon structure.

[0089] The material is then removed from the storage container. Through the above steps, it is possible to produce negative electrode active material particles in which amorphous low-valent nano silicon oxide is dispersed in a network structure in a porous carbon structure having branched pores inside. In the above steps, the negative electrode active material particles are solidified by appropriately setting the manufacturing conditions. ２９ It is possible to prepare a compound having a maximum value in the range of −81 to −95 ppm when measured by Si-CP / MAS-NMR.

[0090] When producing the negative electrode active material produced in this manner, it is preferable to adjust the amount of silicon deposition and the degree of oxidation so that the proportion of the porous carbon structure in the entire negative electrode active material particles is 45 mass % or more and 64 mass % or less.

[0091] <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.

[0092] [Configuration of Laminate 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 between the positive and negative electrodes and a laminate is housed. In either 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.

[0093] The positive and negative electrode leads 32, 33 are, for example, led out in one direction 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.

[0094] 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 portion is, for example, a film such as polyethylene or polypropylene, and the metal portion is, for example, aluminum foil. The protection layer is, for example, nylon or the like.

[0095] An adhesive film 34 is inserted between the exterior member 35 and the positive and negative electrode leads to prevent outside air from entering, and is made of, for example, polyethylene, polypropylene, or polyolefin resin.

[0096] 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. 1, for example.

[0097] The positive electrode current collector is made of a conductive material such as aluminum.

[0098] 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. Depending on the design, the positive electrode binder and the positive electrode conductive additive may be similar in detail to, for example, the negative electrode binder and the negative electrode conductive additive already described.

[0099] The positive electrode material is preferably a lithium-containing compound. 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 are, for example, Li ｘ M １ O ２ Or Li ｙ M ２ P.O. ４ In the formula, M １ , M ２ represents 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 expressed as 0.05≦x≦1.10 and 0.05≦y≦1.10.

[0100] Examples of composite oxides containing lithium and transition metal elements include lithium cobalt composite oxide (Li ｘ CoO ２ ), lithium nickel composite oxide (Li ｘ NiO ２ ), lithium nickel cobalt composite oxide, etc. Examples of the lithium nickel cobalt composite oxide include lithium nickel cobalt aluminum composite oxide (NCA) and lithium nickel cobalt manganese composite oxide (NCM).

[0101] Examples of phosphate compounds containing lithium and a transition metal element include lithium iron phosphate compounds (LiFePO ４ ) or lithium iron manganese phosphate compound (LiFe １－ｕ Mn ｕ P.O. ４ (0<u<1)). By using these positive electrode materials, it is possible to obtain a high battery capacity and also excellent cycle characteristics.

[0102] [Negative Electrode] The negative electrode has a configuration similar to that of the lithium-ion secondary battery negative electrode 10 shown in Fig. 1, and has, for example, negative electrode active material layers on both sides of a current collector. This negative electrode preferably has a negative electrode charge capacity greater than the electrical capacity (charge capacity as a battery) obtained from the positive electrode active material. This can suppress the deposition of lithium metal on the negative electrode.

[0103] The positive electrode active material layer is provided on a portion of both sides of the positive electrode current collector, and similarly, the negative electrode active material layer is provided on a portion 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 is not present. This is for the purpose of designing a stable battery.

[0104] The region where the negative electrode active material layer and the positive electrode active material layer do not face each other is hardly affected by charging and discharging, and therefore the state of the negative electrode active material layer is maintained as it was immediately after formation, allowing the composition of the negative electrode active material to be accurately determined with good reproducibility, regardless of whether charging and discharging are performed.

[0105] [Separator] The separator separates the positive electrode and the negative electrode, prevents current short circuit due to contact between the electrodes, and allows lithium ions to pass through. This separator is formed of a porous film made of, for example, a 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.

[0106] [Electrolyte Solution] At least a portion of the active material layer or the separator is impregnated with a liquid electrolyte (electrolyte solution). This electrolyte solution contains an electrolyte salt dissolved in a solvent and may contain other materials such as additives.

[0107] 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.

[0108] When an alloy-based negative electrode is used, it is particularly desirable to use a solvent containing at least one of a halogenated chain carbonate or a halogenated cyclic carbonate. 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).

[0109] 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.

[0110] 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.

[0111] 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.

[0112] It is also preferable that the solvent additive contains sultone (cyclic sulfonic acid ester), which improves the chemical stability of the battery. Examples of sultones include propane sultone and propene sultone.

[0113] 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.

[0114] The electrolyte salt may include, for example, one or more light metal salts such as lithium salts. Examples of the lithium salt include lithium hexafluorophosphate (LiPF ６ ), lithium tetrafluoroborate (LiBF ４ ) etc.

[0115] 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 high ionic conductivity can be obtained.

[0116] 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.

[0117] 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 fabricated.

[0118] A negative electrode active material was produced as follows. First, a carbon material with few actual pores was prepared by steam activation (1000°C) to make it porous (mesopore-forming). The prepared material was then subjected to alkali activation as follows, resulting in a surface area (BET specific surface area) of 1940 m. ２ / g, pore volume 1.00 cm ３ A porous carbon material (porous carbon structure) of IUPAC type I classification with a molecular weight of 1 / g and a particle size (D50) of 11 μm was prepared. It had branched pores inside. [Alkali activation treatment] (Process procedure) The prepared material with few solid pores was mixed with alkali metal hydroxide (KOH) and heated to 1000°C. Specifically, KOH was added in an amount three times the weight of the material, and the temperature was raised to 1000°C at 10°C / min and maintained at that temperature for two hours. It was then cooled to room temperature. It was then ultrasonically cleaned in pure water for two hours and washed in a stirrer for three hours to remove potassium.

[0119] This porous carbon material was placed in a vacuum container and evacuated to -90 kPa. Next, the pressure was restored with nitrogen, and the material was heated to 400°C using an external heater while nitrogen was flowing. After 30 minutes of heating, moisture-containing nitrogen gas was introduced.

[0120] After the above flow, the temperature was raised to 415°C, monosilane gas was flowed, and deposition was carried out for 4 hours. Then, the material was cooled to room temperature while nitrogen gas was flowing. After the temperature was lowered to 25°C, oxygen diluted 20 times with nitrogen was introduced, and the material temperature was adjusted to 50°C or less to form Si-O bonds. Next, oxygen-containing nitrogen was flowed for 2 hours, and when the material temperature reached 30°C or less, the gas was switched to nitrogen. After flowing for 60 minutes, the material was removed from the storage container and used as the negative electrode active material.

[0121] [Measurement of Negative Electrode Active Material] The negative electrode active material prepared as described above was subjected to TEM-EDX, XRD analysis, and NMR analysis (solid ２９ Si-CP / MAS-NMR) was performed.

[0122] [Preparation of Negative Electrode] The negative electrode active material (active material containing CVD-SiOx-C) prepared as described above, graphite, conductive additive 1 (carbon nanotubes, CNT), conductive additive 2 (carbon fine particles having 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 prepare a negative electrode mixture slurry.

[0123] 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 for 1 hour in a vacuum atmosphere. 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.

[0124] [Assembly of Test Coin Battery] Next, solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed, and then an electrolyte salt (lithium hexafluorophosphate: LiPF ６ The electrolyte solution was prepared by dissolving EC and DMC in a solvent with a volume ratio of 30:70, and the electrolyte salt content was 1 mol / kg relative to the solvent. Vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added as additives in amounts of 1.0% by mass and 2.0% by mass, respectively.

[0125] Next, a coin battery was assembled as follows: First, a Li foil having a thickness of 1 mm was punched out to a diameter of 16 mm and attached to an aluminum clad.

[0126] Next, the previously obtained negative electrode 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 injection of an electrolyte, a 2032 coin battery was fabricated.

[0127] [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 0.03 C. The CV was 0 V and the cut-off current was 0.04 mA. Next, CC discharge (initial discharge) was performed at a discharge rate of 0.03 C and a discharge cut-off voltage of 1.2 V.

[0128] 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.

[0129] [Production of Lithium-Ion Secondary Batteries and Battery Evaluation] From the obtained initial data, a counter positive electrode was designed so that the utilization rate of the negative electrode would be 95%. The utilization rate was calculated from the capacity 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) x 100 Based on this design, lithium-ion secondary batteries (lithium-ion secondary batteries as shown in Figure 2) of the Examples and Comparative Examples were produced. Battery evaluation was performed on each of the lithium-ion secondary batteries of the Examples and Comparative Examples.

[0130] The cycle characteristics were investigated as follows. First, to stabilize the battery, two charge / discharge cycles were performed at 0.2 C 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 after 1,000 cycles. Charging and discharging were performed at 0.7 C and 0.5 C. The charging voltage was 4.3 V, the discharge cut-off voltage was 2.5 V, and the charge cut-off rate was 0.07 C. Furthermore, the fast charging characteristics were calculated from the discharge capacity at the third cycle, and the battery test was stopped after 500 cycles. Charging and discharging were performed at 4 C and 0.5 C. The charge cut-off voltage was 4.3 V, the discharge cut-off voltage was 2.5 V, and the charge cut-off rate was 0.07 C.

[0131] The results of each measurement are shown in Table 1. Table 1 also shows the results of Comparative Example 1 and Examples 2 to 16, which will be described later.

[0132]

[0133] A cross-sectional TEM photograph of the negative electrode active material of Example 1 is shown in Figure 4. In this cross-sectional TEM photograph, the white parts are Si and the rest are carbon. It can be seen that the white lines form a mesh pattern all over the surface. The analysis results of XAFS measurements (XANES spectra) before and after charging the negative electrode in Example 1 (and Comparative Example 1, which will be described later) are shown in Figure 7. The XANES spectrum in Figure 7 shows that low-valence SiOx is distributed in various states. ２９ The Si-CP / MAS-NMR spectrum is shown in Figure 10. A photograph of the appearance of the negative electrode after charge and discharge in Example 1 is shown in Figure 5. This shows the rolled negative electrode in an unrolled state. As can be seen from Figure 10, the maximum value is at -91 ppm, indicating that the SiOSi structure is sufficiently contained. If there are many Si-O bonds, when it reacts with Li, Li １５ Si ４ Since it is known that the surface layer does not become highly resistive and Li does not precipitate (see FIG. 5).

[0134] When a porous carbon structure having branched pores is prepared as in Example 1 (and Examples 2-16 described later), the valence is easily controlled in the oxidation process because each wire (net diameter) in the Si portion is thin. Furthermore, the oxidation process can be further controlled by generating OH groups inside the structure in advance using a flow of nitrogen gas containing moisture.

[0135] Comparative Example 1 In Example 1, an alkali activation treatment was performed on a material with few actual pores (prepared by activation treatment with water vapor (1000°C)) to prepare a porous carbon structure having branch-like pores inside, but in Comparative Example 1, no alkali activation treatment was performed, and a porous carbon structure was prepared by only the above-mentioned activation treatment with water vapor (1000°C) alone, which does not have branch-like pores inside.

[0136] Thereafter, under the same conditions as in Example 1, evacuation, nitrogen pressure recovery and heating, moisture-containing nitrogen gas flow, Si deposition, cooling, and an oxidation process (introduction of oxygen diluted with nitrogen gas) were carried out in that order. The sample was then cooled to room temperature and exposed to the atmosphere before being removed. Various measurements were carried out in the same manner as in Example 1, and the results are shown in Table 1. While the capacity and initial coulombic efficiency were high, the reactivity with the electrolyte was high, resulting in a deterioration in battery cycle characteristics, particularly in high-speed charging due to Li deposition.

[0137] A cross-sectional TEM photograph of the negative electrode active material of Comparative Example 1 is shown in FIG. ２９ The Si-CP / MAS-NMR spectrum is shown in Figure 8. The white areas in the cross-sectional TEM photograph of Figure 6 are Si, and the rest is carbon. It can be seen that the white silicon phase exists as islands in a sea of ​​carbon (sea-island structure). Because it exists as islands, even when oxidation treatment is performed, only the surface layer of the grains is oxidized. The results are shown in Figure 7, where a large separation is seen between zero-valent (before charging) and tetravalent (after charging). Because only the surface layer is oxidized, there are many Si-Si bonds and little SiOSi (see Figure 8). As mentioned above, in Comparative Example 1, the same oxidation treatment as in Example 1 was performed, and a flow of nitrogen gas containing moisture was also performed in advance to attach OH groups to the rearmost part. As a result, the surface layer of the deposited Si part is oxidized cleanly.

[0138] 9 shows a photograph of the appearance of the negative electrode after charge and discharge in Comparative Example 1. １５ Si ４ is formed on the surface. １５ Si ４ is an ionic substance and an insulator, and its permeability to Li is poor. Therefore, it is thought that Li precipitated throughout the entire electrode during high-speed charging, as shown in Figure 9. In addition, when a sea-island structure is used, the paths connecting the islands become narrower, which results in poor Li diffusion. In addition, in the negative electrode of Comparative Example 1, the XANES spectrum inside the bulk indicates that this phenomenon (Li １５ Si ４ It was not possible to confirm the generation phenomenon of

[0139] (Examples 2 to 5) The oxidation method was changed from Example 1. For example, when the degree of oxidation was to be lowered, oxygen diluted with more nitrogen was introduced to suppress the rise in internal temperature. When the degree of oxidation was to be higher, the oxygen concentration was increased within the range of an upper limit of 50°C and oxidation was carried out. ２９ The Si-CP / MAS-NMR spectrum shifts to the low magnetic field side when the amount of oxygen is small, and shifts to the high magnetic field side when the amount of oxygen is large. If the shift exceeds -95 ppm, the low valence state cannot be maintained and a sample that can be evaluated in a battery cannot be produced, so in Examples 2 to 5, the range was set to -81 to -95 ppm. Among these, when the amount of oxidation was relatively small (Examples 4 and 5), the SiOSi structure was reduced and the high-speed charging performance was slightly worse than in Example 1.

[0140] Examples 6, 7, and 13: These are examples in which the ratio of carbon active material in the negative electrode was increased. This simply resulted in a decrease in capacity. Since a capacity of 1300 mAh / g or more is preferred, the amount of carbon is also important.

[0141] (Examples 8 to 12) A negative electrode active material was produced using the same porous carbon structure as in Example 1, but with the following conditions changed. First, after flowing nitrogen gas containing moisture, the temperature was raised to 415°C, and monosilane gas was flowed and deposited for 4 hours. Then, the temperature was raised to 415°C, and Si ０＋ In this way, the grain size of Si, which exists in the low-valence nano-silicon oxide, was grown. ０＋ The grain size of Si was changed. ０＋ When the crystal grain size of Si increases, the number of Si / Si grain boundaries also increases, which reduces Li diffusibility and deteriorates high-speed charging characteristics. Therefore, the grain size of zero-valent Si is preferably small, and is preferably 5 nm or less.

[0142] (Examples 14 to 16) Negative electrode active materials with increased surface area and pore volume were manufactured in the same manner as in Example 1, except that the carbonization conditions of the raw materials and the activation treatment temperature were changed when preparing a porous carbon structure having branched pores inside. In addition, evaluations were performed in the same manner as in Example 1. From the results of Examples 14 to 16, it was found that the capacity of the active material can be simply increased by increasing the surface area and pore volume, and considering the balance between capacity, efficiency, etc., a pore volume of 0.95 cm ３ / g or more, BET specific surface area 1325m ２ / g or more, and IUPAC type I is considered to be preferable. The larger the specific surface area, the greater the capacity of the active material. Therefore, the upper limit of these values ​​cannot be determined. For example, the surface area (BET specific surface area) should be 2400 m ２ / g, pore volume is 1.3 cm ３ / g is sufficient.

[0143] This specification includes the following aspects: [1]: A negative electrode active material having negative electrode active material particles, the negative electrode active material particles containing a porous carbon structure having branched pores formed by an alkali activation treatment therein, and amorphous low-valent nano silicon oxide dispersed in a network structure within the porous carbon structure. [2]: The negative electrode active material according to [1] above, wherein, when XAFS measurements are performed on a pre-charged negative electrode removed from a secondary battery having a negative electrode containing the negative electrode active material before charging, and on a post-charged negative electrode removed from the secondary battery after charging the pre-charged secondary battery, in the Si K-absorption edge XANES spectra obtained from the XAFS measurements of the anode before and after charging, a peak attributable to a tetracoordinated silicon compound in the energy range of 1846 eV to 1850 eV is higher in the anode before charging than in the anode after charging, and the XANES spectrum of the anode after charging has a peak attributable to a hexacoordinated silicon compound in the energy range of 1851 eV to 1855 eV. [3]: The negative electrode active material according to [2] above, wherein the XANES spectrum of the anode before charging has peaks at energies near 1845.5 eV and 1847.5 eV. [4]: The negative electrode active material is solid ２９The negative electrode active material of any one of [1] to [3] above, which has a maximum value in the range of -81 to -95 ppm when measured by Si-CP / MAS-NMR. [5]: The negative electrode active material of any one of [1] to [4] above, wherein the negative electrode active material particles have a SiOSi structure in which oxygen is bonded to a Si radical. [6]: The negative electrode active material of any one of [1] to [5] above, wherein the low-valence nanosilicon oxide is substantially in a composite state of zero-valent, monovalent, and divalent, with divalent being most predominant. [7]: The negative electrode active material of any one of [1] to [6] above, wherein the low-valence nanosilicon oxide dispersed in the porous carbon structure has x increasing from the center of the porous carbon structure to the surface layer. [8]: The negative electrode active material of any one of [1] to [7] above, wherein the proportion of the porous carbon structure in the total negative electrode active material particles is 45% by mass or more and 64% by mass or less. [9]: The negative electrode active material according to any one of [1] to [8], wherein 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 1 nm to 5 nm.

[10] : The porous carbon structure is predominantly Type I in the IUPAC classification, and its surface area is 1325 m ２ / g or more, pore volume is 0.95 cm ３The negative electrode active material according to any one of [1] to [9] above, wherein the Cr content is 0.15 or more.

[11] : A method for producing a negative electrode active material having negative electrode active material particles, comprising the steps of: preparing a porous carbon structure having branched pores therein by a production method using an alkali activation treatment; flowing nitrogen gas containing moisture into the porous carbon structure; and after the flow of the nitrogen gas containing moisture, flowing monosilane gas into the porous carbon structure under heating to deposit silicon inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; and after the cooling, introducing oxygen diluted with nitrogen gas into the material with silicon deposited inside the porous carbon structure while adjusting the temperature of the material with silicon deposited inside the porous carbon structure to maintain it at 50°C or less, thereby converting at least a portion of the silicon into low-valence nano-silicon oxide, the method for producing a negative electrode active material, characterized in that

[0144] The present invention is not limited to the above-described embodiments, which 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 provides similar effects is included within the technical scope of the present invention.

Claims

1. A negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles contain a porous carbon structure having branched pores formed by an alkali activation treatment therein, and amorphous low-valent nano silicon oxide is dispersed in a network structure within the porous carbon structure.

2. The negative electrode active material according to claim 1, wherein, when XAFS measurements are performed on a pre-charged negative electrode removed from a secondary battery having a negative electrode containing the negative electrode active material before charging, and on a post-charged negative electrode removed from the secondary battery after charging the pre-charged secondary battery, the Si K-absorption edge XANES spectra obtained from the XAFS measurements of the anode before and after charging show that the peak attributable to a tetracoordinated silicon compound in the energy range of 1846 eV to 1850 eV is higher in the anode before charging than in the anode after charging, and the XANES spectrum of the anode after charging has a peak attributable to a hexacoordinated silicon compound in the energy range of 1851 eV to 1855 eV.

3. The negative electrode active material according to claim 2, characterized in that the XANES spectrum of the negative electrode before charging has peaks at energies of approximately 1845.5 eV and 1847.5 eV.

4. The negative electrode active material is solidified ２９ 2. The negative electrode active material according to claim 1, which has a maximum value in the range of −81 to −95 ppm when measured by Si-CP / MAS-NMR.

5. The negative electrode active material according to claim 1, wherein the negative electrode active material particles have a SiOSi structure in which oxygen is bonded to a Si radical.

6. The negative electrode active material according to claim 1, wherein the low-valent nano-silicon oxide is substantially in a complex state of zero valence, monovalent, and divalent, with divalent being the most predominant.

7. The negative electrode active material according to claim 1, characterized in that the low-valent nano-silicon oxide dispersed in the porous carbon structure has x increasing from the center of the porous carbon structure to the surface layer.

8. The negative electrode active material according to claim 1, characterized in that the proportion of the porous carbon structure in the entire negative electrode active material particles is 45 mass % or more and 64 mass % or less.

9. 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 the Scherrer equation from the peak measured by X-ray diffraction measurement of the negative electrode active material particles, is in the range of 1 nm to 5 nm.

10. The structure of the porous carbon is predominantly Type I in the IUPAC classification, and its surface area is 1325 m ２ / g or more, pore volume is 0.95 cm ３ 2. The negative electrode active material according to claim 1, wherein the negative electrode active material has a molecular weight of 1 / g or more.

11. A method for producing a negative electrode active material having negative electrode active material particles, comprising the steps of: preparing a porous carbon structure having branched pores therein by a production method using an alkali activation treatment; flowing moisture-containing nitrogen gas through the porous carbon structure; and, after the moisture-containing nitrogen gas has been flowed, flowing monosilane gas through the porous carbon structure under heating to deposit silicon inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; and, after the cooling, introducing oxygen diluted with nitrogen gas into the material with silicon deposited inside the porous carbon structure while adjusting the temperature of the material with silicon deposited inside the porous carbon structure to maintain it at 50°C or less, thereby converting at least a portion of the silicon into low-valence nano-silicon oxide, the method for producing a negative electrode active material, characterized in that it produces negative electrode active material particles in which amorphous low-valence nano-silicon oxide is dispersed in a network structure inside the porous carbon structure.

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