Negative electrode active material and method for producing same

Abstract

The present invention provides a negative electrode active material which has negative electrode active material particles, the negative electrode active material being characterized in that: the negative electrode active material particles each include a porous carbon structure; amorphous low valence nano silicon oxide is dispersed inside the porous carbon structure; the low valence nano silicon oxide includes the states where SiOx:x < 1.0 is satisfied; and the low valence nano silicon oxide has an average particle diameter of 50 nm or less as determined by image processing of a cross-sectional TEM image thereof. As a result, a negative electrode active material which is capable of increasing the capacity while maintaining the battery characteristics is provided.

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 proposed a disproportionated structure in which silicon oxide is separated into Si and SiO2 by applying a thermal load (Non-Patent Document 3).

[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 above (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 + 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 Journal "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, wherein the negative electrode active material particles include a porous carbon structure, and amorphous low-valence nano-silicon oxide is dispersed inside the porous carbon structure, and the low-valence nano-silicon oxide includes each state of SiOx:x<1.0, and the low-valence nano-silicon oxide has an average particle size of 50 nm or less as determined by image processing of a cross-sectional TEM image.

[0023] In the negative electrode active material of the present invention, 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 caused by the expansion of the low-valence nano silicon oxide inside. ４＋is an irreversible component, but the negative electrode active material of the present invention includes each state of SiOx:x<1.0, so that it can maintain a lower irreversible capacity than general SiO. Furthermore, since the Si—O bond can suppress the decomposition of the electrolyte, it is possible to reduce the SEI (Solid Electrolyte Interphase) that accumulates in the surface layer. Furthermore, since the particle size of the low-valence nanosilicon oxide (particle size determined by image processing of a cross-sectional TEM image of the low-valence nanosilicon oxide) is an average of 50 nm or less, it is easy to form an appropriate amount of Si—O bond.

[0024] In this case, the low valence nano silicon oxide is preferably substantially in a composite state of zero valence, monovalence and divalence.

[0025] In this way, the low-valence nano silicon oxide is substantially in a composite state of zero valence, monovalence and divalence, and therefore can have a lower irreversible capacity.

[0026] The negative electrode active material particles are solid ２９ It is preferable that, when measured by Si-MAS-NMR, no hydrogen is detected near the low valence nano silicon oxide.

[0027] In this way, solid ２９ If the negative electrode active material particles do not detect hydrogen in the vicinity of the low-valence nanosilicon oxide when measured by Si-MAS-NMR, the irreversible capacity can be further reduced.

[0028] Furthermore, the low-valence nanosilicon oxide dispersed in the porous carbon structure may have x increasing from the center of the porous carbon structure toward the surface layer.

[0029] 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.

[0030] The porous carbon structure preferably has carbon-carbon double bonds at least in part.

[0031] In this way, the porous carbon structure has carbon-carbon double bonds, which makes the structure rigid and flexible, and further effectively reduces the adverse effects of expansion of the low-valence nano silicon oxide inside.

[0032] The proportion of the porous carbon structure in the entire negative electrode active material particles is preferably 38% by mass or more and 63% by mass or less.

[0033] 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.

[0034] The low valence nano silicon oxide is a solid ２９ In Si-MAS-NMR measurement, it is preferable that the compound has a broad peak with a peak top at about −87 ppm.

[0035] If the material has such a peak, it can be more reliably determined that the material contains SiOx:x<1.0.

[0036] 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.

[0037] Such a material having a grain size of zero-valent Si that is substantially amorphous is preferred.

[0038] The negative electrode active material particles preferably have a G / D ratio in the range of 0.85 to 1.15 as determined by Raman spectroscopy, and a Si / G ratio of 0.25 or more and 0.5 or less.

[0039] If the G / D ratio is within the above range, the material is neither too hard nor too soft. If the ratio is within the above range, it is possible to prevent cracking during pressing or cracking due to volume change during Li absorption by low-valence nanosilicon. Furthermore, if the Si / G ratio is within the above range, the area of ​​the low-valence nanosilicon oxide portion visible on the particle surface is more appropriate. If the ratio is within the above range, it is possible to ensure high-speed charging while suppressing deterioration.

[0040] The structure of the porous carbon is predominantly Type I in the IUPAC classification, and its surface area is 1400 m ２ / g or more, pore volume is 1 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; depositing silicon inside the porous carbon structure by flowing monosilane gas through the porous carbon structure under heating; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; and, after the cooling, converting at least a portion of the silicon into low-valence nanosilicon oxide by 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 50°C or less, thereby producing a negative electrode active material particle having amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure.

[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 is dispersed inside a porous carbon structure, as described above.

[0044] 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 mitigate the adverse effects of expansion of the internal low-valence nanosilicon oxide. Since the SiOx:x<1.0 state is included, a lower irreversible capacity than that of conventional SiO can be maintained. Furthermore, the Si—O bond can suppress decomposition of the electrolyte, thereby reducing the amount of SEI deposited on the surface. Furthermore, the particle size of the low-valence nanosilicon oxide (particle size determined by image processing of a cross-sectional TEM image of the low-valence nanosilicon oxide) is an average of 50 nm or less, which facilitates the formation of an adequate amount of Si—O bond. 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.

[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 is dispersed inside the porous carbon structure.

[0046] 1 is a cross-sectional view showing the structure of a negative electrode containing the negative electrode active material of the present invention; FIG. 2 is an exploded view showing a structural example (laminated film type) of a lithium ion secondary battery containing the negative electrode active material of the present invention; FIG. 3 is a cross-sectional TEM photograph of an example of the negative electrode active material of the present invention; FIG. 4 is a flow chart showing an example of a method for producing the negative electrode active material of the present invention; ２９ 1 is a graph showing the change in the depth direction measurement of the O1s region in the XPS measurement of the negative electrode active material in Example 1. 2 is a graph showing the results of the Si2p region in the XPS measurement of the negative electrode active materials in Example 1, Comparative Example 1, and Comparative Example 3. 3. Example of the negative electrode active material of the present invention ２９ Si-MAS-NMR spectrum and ２９1 shows Si-CP / MAS-NMR spectra of Example 1 and the control example. １３ 12 is a C-MAS-NMR spectrum. The TEM-EDX measurement results for the negative electrode active material of Example 1 are shown, with the upper figure being a cross-sectional TEM photograph showing the measurement location, and the lower figure being a graph showing the relationship between the distance from the surface and the mass % of the elements. The X-ray diffraction spectra for Examples 1, 2, 15, and 16. The graph showing the charge-discharge characteristics of Example 1, Comparative Example 1, and Example 11. The graph is a partial enlarged view of FIG. 12.

[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 developing an anode active material that not only significantly suppresses the reactive decomposition of the electrolyte, but also ensures high-speed charging because the Si-O material with siloxane bonds has good Li acceptability.

[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, the negative electrode active material particles including a porous carbon structure, amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, the low-valence nanosilicon oxide including each state of SiOx:x<1.0, and the low-valence nanosilicon oxide having an average particle size of 50 nm or less as determined by image processing of a cross-sectional TEM image.

[0052] Since this negative electrode active material has amorphous low-valence nanosilicon oxide dispersed within the 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, since it includes the SiOx:x<1.0 state, it can maintain a lower irreversible capacity than general SiO. Furthermore, the Si—O bond can suppress decomposition of the electrolyte, thereby reducing the SEI that accumulates in the surface layer. Furthermore, since the particle size of the low-valence nanosilicon oxide (particle size determined by image processing of a cross-sectional TEM image of the low-valence nanosilicon oxide) is an average of 50 nm or less, it is easy to ensure that the Si—O bond is appropriately formed. Note that, in the present invention, it is sufficient that amorphous low-valence nanosilicon oxide is dispersed within the porous carbon structure, and some of the nanosilicon oxide may be crystalline.

[0053] That is, the negative electrode active material of the present invention is 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. However, since the tetravalent Si constituting SiO is an irreversible component, by making SiOx of divalent or lower predominant, the irreversible capacity is larger than that of silicon alone, but it can maintain a lower irreversible capacity than general SiO. Furthermore, since the Si-O bond can suppress the decomposition of the electrolyte, it is possible to reduce the SEI that accumulates on the surface layer of the CVD-Si-C. Since the part that primarily contributes to charge and discharge is a low-valent nanosilicon oxide, this material can be defined as CVD-SiOx-C in contrast to CVD-Si-C. In this way, the active material produced can have high energy density and high-speed charging while maintaining the cycle characteristics of the battery.

[0054] The grain size of the low-oxide 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) processing device without being exposed to the atmosphere. The FIB processing device 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.

[0055] In the present invention, as described above, it is preferable that the low-valence nano silicon oxide has grains of substantially 50 nm or less and contains monovalent or divalent oxide compounds as a structure. The Si-O bond can suppress the decomposition of the electrolyte. In addition, the diffusibility of Li is high, which is suitable for high-speed charging. If the grain size is larger than 50 nm, it is difficult to form Si-O bonds, and for example, only the surface layer of the grain is oxidized to form SiO ２ If the capacitance becomes too high, it will become a resistance component and will no longer be able to guarantee high-speed charging.

[0056] A cross-sectional TEM photograph of an example of the negative electrode active material of the present invention is shown in Figure 3. The white areas are low-oxide nanosilicon oxides, and their particle sizes are an average of 50 nm or less.

[0057] As the particle size of the low-oxide nanosilicon oxide increases, it becomes difficult to form Si—O bonds. As will be described later, in the method for producing the negative electrode active material of the present invention, specifically, ４ In the process of decomposing silicon dioxide and creating amorphous silicon, the silicon dioxide reacts with oxygen intentionally to create Si-O bonds. Therefore, if the particle size is large, the oxygen impregnation is poor, and for example, a concentration distribution of O occurs in the low-valence silicon oxide phase, resulting in SiO ２ The crystallinity may be reduced. Or, at that time, the interior of the phase may become in the Si state, as in the above-mentioned CVD-Si-C. Furthermore, some zero-valent amorphous Si is present inside these low-valent nanosilicon oxide grains, and it is desirable for this to have low crystallinity. Lowering the crystallinity increases the irreversible capacity, but has the advantage of improving Li acceptance. High crystallinity accelerates the electrolyte decomposition reaction specific to Si, resulting in poor cycle characteristics.

[0058] 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.

[0059] 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"

[0060] In addition, in the negative electrode active material of the present invention, the low valence nano silicon oxide is preferably substantially in a composite state of 0 valence, 1 valence, and 2 valences. In this way, by the low valence nano silicon oxide being substantially in a composite state of 0 valence, 1 valence, and 2 valences, it is possible to achieve a lower irreversible capacity.

[0061] The negative electrode active material of the present invention has a low-valence nano-silicon oxide phase inside a porous carbon material, and the ratio of silicon to oxygen constituting this low-valence silicon compound is SiO ｘ It is preferable that the ratio of the valence ratio of the low-valent nano silicon oxide to the valence ratio of the low-valent nano silicon oxide is ≦0.7. In addition, the low-valent nano silicon oxide is predominantly a monovalent or divalent compound, and the low-valent nano silicon oxide phase present inside the porous structure may contain a microcrystalline phase of Si with a valence of 0.

[0062] The valence of the low-valence nano silicon oxide can be quantified by NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0063] The NMR measurement 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.

[0064] 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.

[0065] In the negative electrode active material of the present invention, the negative electrode active material particles are solid ２９ When measured by Si-MAS-NMR, it is preferable that hydrogen is not detected near the low-valence nano-silicon oxide. This is because if the high-valence part contains hydrogen as a defect, it is likely to become irreversible capacity. ２９When hydrogen is not detected near the low-valence nanosilicon oxide when measured by Si-MAS-NMR, the irreversible capacity can be further reduced. In the negative electrode active material of the present invention, the low-valence portion is considered to have an amorphous structure containing hydrogen.

[0066] FIG. 8 shows a solid-state image of an example of the negative electrode active material of the present invention. ２９ Si-MAS-NMR measurement; ２９ The results of Si-CP / MAS-NMR measurements are shown below. ２９ In Si-MAS-NMR measurement, a broad peak with a peak top at around -87 ppm is observed. ２９ In the Si-CP / MAS-NMR measurement, the peak width is slightly narrower, and the region with the highest magnetic field is not detected by the CP / MAS method. This region can be said to be an SiOx structure with no hydrogen (H) atoms nearby (the CP method １ From H nucleus ２９ In order to detect the magnetization transfer to the Si nucleus, １ No H nucleus ２９ (Si nuclei are not detected.) The low wavenumber region is dominated by amorphous Si, but １ The presence of H has been detected by the CP / MAS method.

[0067] In addition, in the negative electrode active material of the present invention, the low valence nano silicon oxide is a solid, as shown in FIG. ２９ In Si-MAS-NMR measurement, it is preferable that the material has a broad peak with a peak top at around −87 ppm. If the material has such a peak, it can be more reliably determined that the material contains SiOx:x<1.0.

[0068] In addition, in the negative electrode active material of the present invention, it is preferable that the x of the low-valent nanosilicon oxide dispersed in the porous carbon structure increases from the center to the surface of the porous carbon structure. In the present invention, since the low-valent 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 is more effectively suppressed, while the silicon oxidation rate is low inside the active material, thereby increasing the battery capacity.

[0069] In the negative electrode active material of the present invention, it is preferable that the porous carbon structure has at least a carbon-carbon double bond. By having the carbon-carbon double bond in the porous carbon structure, the structure can be made rigid and flexible, and the adverse effects of expansion of the low-valence nanosilicon oxide inside can be more effectively reduced. Although commercially available porous carbon structures can be used, it is preferable to select an appropriate porous carbon structure. In the negative electrode active material of the present invention, the porous carbon structure having the carbon-carbon double bond is １３ This can be confirmed by C-MAS-NMR spectrum. Note that the porous carbon structure is preferably rigid and flexible, and therefore does not preferably contain SiC.

[0070] The negative electrode active material particles preferably have a G / D ratio obtained by Raman spectroscopy in the range of 0.85 to 1.15 and an Si / G ratio of 0.25 or more and 0.5 or less. The G / D ratio is the ratio of the G-band peak intensity I G , which is the intensity of the peak derived from the G band, to the D-band peak intensity I D , which is the intensity of the peak derived from the D band, in Raman spectroscopy. The Si / G ratio is the intensity ratio of the Si peak intensity I Si , which is the intensity of the peak derived from Si, to the G-band peak intensity I G , which is the intensity of the peak derived from the G band, in Raman spectroscopy.

[0071] If the carbon particles are too hard, they will crack during pressing, but if they are too soft, they will crack due to expansion when silicon absorbs Li, so there is an optimal range. Furthermore, if the area of ​​the low-valence silicon oxide portion visible on the particle surface is large, the reaction area with the electrolyte is large and they are prone to deterioration relatively quickly, but if it is narrow, the Li-accepting area becomes small and high-speed charging performance deteriorates. An optimal value for the Si / G ratio serves as a guideline for these. That is, if the G / D ratio is within the above range, the carbon particles are neither too hard nor too soft. Within the above range, cracking during pressing and cracking due to volume changes during Li absorption by low-valence nanosilicon can be prevented. Furthermore, if the Si / G ratio is within the above range, the area of ​​the low-valence nanosilicon oxide portion visible on the particle surface is more appropriate. Within the above range, high-speed charging performance can be ensured while suppressing deterioration.

[0072] <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.

[0073] [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.

[0074] [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).

[0075] 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.

[0076] 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.

[0077] [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, 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.

[0078] [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.

[0079] First, a method for producing a negative electrode active material contained in a negative electrode will be described. The method for producing a negative electrode active material having negative electrode active material particles includes the steps of preparing a porous carbon structure, depositing silicon inside the porous carbon structure by flowing monosilane gas through the porous carbon structure under heating, cooling the material with silicon deposited inside the porous carbon structure to 50°C or less, and, after cooling, converting at least a portion of the silicon into low-valence nanosilicon oxide by 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 50°C or less, thereby producing negative electrode active material particles with amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure.

[0080] Each step of the method for producing the negative electrode active material will be described with reference to steps S1 to S4 in FIG.

[0081] First, a porous carbon structure is prepared (Step S1). The porous carbon structure prepared here preferably has carbon-carbon double bonds at least in part. 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 1 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

[0082] Next, monosilane gas is passed through the porous carbon structure prepared in step S1 under heating, thereby depositing silicon inside the porous carbon structure (step S2).

[0083] After step S1 and before step S2, 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 pressure with nitrogen and heat the chamber to about 350 to 450°C using an external heater while 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, nucleation for silicon deposition in step S2 and removal of hydrogen and water attached to the porous carbon structure can be performed, thereby more reliably carrying out silicon deposition in step S2.

[0084] The silicon deposition in step S2 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.

[0085] Next, the material with silicon deposited inside the porous carbon structure is cooled to 50° C. or less (step S3). In this step, it is preferable to cool the material while flowing nitrogen gas. This cooling can be performed to room temperature, for example.

[0086] After the cooling (step S3), 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 S4). This step allows the formation of Si—O bonds. It is more preferable to maintain the temperature at 35°C or less.

[0087] The dilution of oxygen with nitrogen can be, for example, 5 to 50 times, preferably 10 to 30 times, and typically 20 times.

[0088] Furthermore, in step S4, 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 S4, 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 S4 is preferably set to, for example, 25°C or higher, and more preferably 30°C or higher.

[0089] The oxidation time in step S4 (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.

[0090] 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.

[0091] The material is then removed from the storage container. Through the above steps, negative electrode active material particles can be produced in which amorphous low-valent nano-silicon oxide is dispersed inside the porous carbon structure.

[0092] 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 38 mass % or more and 63 mass % or less.

[0093] <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.

[0094] [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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] The positive electrode current collector is made of a conductive material such as aluminum.

[0100] 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.

[0101] 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 0.05≦x≦1.10 and 0.05≦y≦1.10.

[0102] 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).

[0103] 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.

[0104] [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.

[0105] 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.

[0106] 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.

[0107] [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.

[0108] [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.

[0109] 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.

[0110] 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).

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] 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.

[0120] A negative electrode active material was produced as follows. First, a negative electrode active material having a surface area (BET specific surface area) of 2483 m ２ / g, pore volume 1.35 cm ３A porous carbon material (porous carbon structure) with a particle size (D50) of 11 μm and an IUPAC classification type I was prepared. This porous carbon material was stored 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, the temperature was increased to 415 ° C., monosilane gas was flowed, and deposition was carried out for 4 hours. Thereafter, the material was cooled to room temperature while flowing nitrogen gas. After lowering the temperature 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 gas. After flowing for 60 minutes, the material was removed from the storage container and used as a 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, NMR analysis, and Raman spectroscopic analysis.

[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 ６The 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 cycles of charge and discharge 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 1000 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.

[0131] The results of each measurement are shown in Table 1. Table 1 also shows the results of Comparative Examples 1 to 3 and Examples 2 to 17, which will be described later.

[0132]

[0133] Comparative Example 1 A porous carbon structure similar to that of Example 1 was prepared. Then, SiH ４ Using a gas, amorphous silicon was formed from the surface layer of the porous carbon structure particle to a location near the center at 415°C. Since Si-H bonds were present in this state, the temperature was raised to 435°C to stabilize the Si-Si. The sample was then cooled to room temperature and exposed to the atmosphere before being removed. Using this method, it is possible to prototype a material that does not contain low-valent silicon oxide compounds (unlike the present invention). Various measurements were performed in the same manner as in Example 1, and the results are shown in Table 1. Although the capacity and initial coulombic efficiency were high, the reactivity with the electrolyte was high, resulting in a deterioration in battery cycle characteristics, and in particular in high-speed charging due to Li deposition.

[0134] (Comparative Example 2) A negative electrode active material was produced in the same manner as in Example 1, except that the same gas as in Comparative Example 1 was decomposed at 435°C to enlarge the SiOx phase. Various measurements were performed in the same manner as in Example 1, and the results are shown in Table 1. Analysis of cross-sectional TEM images revealed that the particle size of the low-valence nanosilicon oxide was 55 nm. The SiOx phase also contained zero-valence Si, which increased the probability of contact with the electrolyte, resulting in a deterioration in battery characteristics.

[0135] (Comparative Example 3) After amorphous silicon was formed and cooled in the same manner as in Comparative Example 1, dry air was introduced and the material temperature rose to 55°C. Various measurements were carried out in the same manner as in Example 1 and the results are shown in Table 1. In particular, the Li acceptance deteriorated, and Li precipitated during high-speed charging.

[0136] (Examples 2 to 11) Negative electrode active materials were produced in the same manner as in Example 1, except that porous carbon structures with different properties were used as the first prepared porous carbon structures, as shown in Table 1. In addition, evaluations were performed in the same manner as in Example 1. From the results of Examples 2 to 11, it was found that, considering the balance between capacity, efficiency, etc., the pore volume was 1 cm ３ / g or more, BET specific surface area 1400m ２ / g or more, and IUPAC classification is considered to be type I.

[0137] (Examples 12 to 16) A negative electrode active material was produced using the same porous carbon structure as in Example 1, but under the following conditions: First, the temperature was raised to 415°C, and monosilane gas was introduced and deposited for 4 hours. Then, the temperature was raised to 415°C, and Si ０＋ The grain size was grown. Various measurements were performed in the same manner as in Example 1, and the results are shown in Table 1. As a result, the closer the structure was to an amorphous structure, the lower the capacity, but the faster chargeability and battery cycle characteristics were improved. From the results of Examples 12 to 16, the closer the structure was to an amorphous structure, the lower the capacity, but the faster chargeability and battery cycle characteristics were improved.

[0138] Example 17 A negative electrode active material was produced by oxidizing a porous carbon structure similar to that of Example 1 using half the amount of diluted oxygen as in Example 1. Various measurements were carried out in the same manner as in Example 1, and the results are shown in Table 1. As a result, defects were observed in the SiOx region.

[0139] [Explanation of each figure] FIG. 5 shows the solid-state negative electrode active material of Example 1 and the solid-state negative electrode active material of each control example. ２９The graph shows the results of Si-MAS-NMR measurements. As control examples, the graph shows the NMR measurements of three types of amorphous silicon: amorphous silicon itself, carbon-coated SiOx particles (SiO-C), and SiO-C made from silicone that was heat-treated at 1000°C under argon. The negative electrode active material of Example 1 shows a peak of amorphous silicon and a large SiOx region.

[0140] Fig. 6 shows the change in the depth direction measurement of the O1s region in the XPS measurement of the negative electrode active material in Example 1. As can be seen from Fig. 6, the O1s peak decreases in the depth direction from the surface, and it is clear that the oxygen concentration decreases from the surface toward the deeper layers.

[0141] FIG. 7 shows a graph illustrating the results of the Si2p region in the XPS measurements of the negative electrode active materials in Example 1, Comparative Example 1, and Comparative Example 3. In Example 1, a peak is observed at 99 to 100 eV, indicating a high concentration of low-valence nanosilicon oxide (SiOx) components. In Comparative Example 1, a peak is observed at 98.5 eV, indicating a high concentration of Si components. In Comparative Example 3, in addition to the peak at 99 to 100 eV, a shoulder peak is observed at 103 eV, indicating a high concentration of SiO ２ It is clear that it contains many ingredients.

[0142] FIG. 9 shows the results of Example 1 and a control example in which SiO—C made from silicone was heat-treated at 1000° C. under argon. １３ The C-MAS-NMR spectrum is shown. In Example 1, a peak due to the carbon-carbon double bond is observed, as well as peaks due to the spinning side bands of the carbon-carbon double bond.

[0143] Figure 10 shows the results of TEM-EDX measurement of the negative electrode active material of Example 1. The upper figure is a cross-sectional TEM photograph, and the straight lines indicate the measurement points. The lower figure is a graph showing the relationship between the distance from the surface (on the straight lines in the upper figure) and the mass percentage of the elements. As can be seen from Figure 10, the oxygen concentration decreases from the surface of the negative electrode active material particles toward the deeper layers.

[0144] Figure 11 shows the X-ray diffraction spectra of Examples 1, 2, 15, and 16. All of these show a peak near 2θ = 28°, and the silicon crystallite size was calculated from this peak. The same applies to the other Examples and Comparative Examples.

[0145] Fig. 12 shows a graph illustrating the charge-discharge characteristics of Example 1, Comparative Example 1, and Example 11. Fig. 13 is a partially enlarged view of Fig. 12. As shown in Figs. 12 and 13, Comparative Example 1 shows the presence of crystalline Si, while Example 1 suggests the presence of SiOx.

[0146] This specification includes the following aspects. [1]: A negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure, and an amorphous low-valence nano-silicon oxide is dispersed inside the porous carbon structure, and the low-valence nano-silicon oxide includes each state of SiOx:x<1.0, and the low-valence nano-silicon oxide has an average particle size of 50 nm or less as determined by image processing of a cross-sectional TEM image. [2]: The negative electrode active material of [1] above, wherein the low-valence nano-silicon oxide is substantially in a composite state of zero-valent, monovalent, and divalent. [3]: The negative electrode active material having the negative electrode active material particles in a solid state. ２９ The negative electrode active material of [1] or [2] above, in which hydrogen is not detected near the low-valence nanosilicon oxide when measured by Si-MAS-NMR. [4]: ​​The negative electrode active material of any of [1] to [3] above, in which 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. [5]: The negative electrode active material of any of [1] to [4] above, in which the porous carbon structure has a carbon-carbon double bond in at least a part thereof. [6]: The negative electrode active material of any of [1] to [5] above, in which the proportion of the porous carbon structure in the total negative electrode active material particles is 38 mass % or more and 63 mass % or less. [7]: The low-valence nanosilicon oxide is a solid ２９The negative electrode active material of any one of [1] to [6] above, which has a broad peak with a peak top at around -87 ppm in Si-MAS-NMR measurement. [8]: The negative electrode active material of any one of [1] to [7] above, 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. [9]: The negative electrode active material of any one of [1] to [8] above, wherein the negative electrode active material particles have a G / D ratio obtained by Raman spectroscopy in the range of 0.85 to 1.15, and a Si / G ratio of 0.25 or more and 0.5 or less.

[10] : The porous carbon structure is predominantly Type I in the IUPAC classification, and its surface area is 1,400 m ２ / g or more, pore volume is 1 cm ３

[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; depositing silicon inside the porous carbon structure by flowing monosilane gas through the porous carbon structure under heating; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; and, after the cooling, converting at least a portion of the silicon into low-valence nanosilicon oxide by 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 producing negative electrode active material particles having amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure.

[0147] 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.

Claims

1. A negative electrode active material having negative electrode active material particles, the negative electrode active material particles including a porous carbon structure, an amorphous low valence nano silicon oxide being dispersed inside the porous carbon structure, the low valence nano silicon oxide including each state of SiOx:x<1.0, and the low valence nano silicon oxide having an average particle diameter of 50 nm or less as determined by image processing of a cross-sectional TEM image.

2. The negative electrode active material according to claim 1, wherein the low valence nanosilicon oxide is substantially in a composite state of zero valence, monovalence and divalence.

3. The negative electrode active material particles are solidified. ２９ 2. The negative electrode active material according to claim 1, wherein when measured by Si-MAS-NMR, hydrogen is not detected in the vicinity of the low valence nanosilicon oxide.

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

5. The negative electrode active material according to claim 1, wherein the porous carbon structure has carbon-carbon double bonds in at least a portion thereof.

6. 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 38 mass % or more and 63 mass % or less.

7. The low valence nano silicon oxide is a solid ２９ 2. The negative electrode active material according to claim 1, which has a broad peak with a top at about −87 ppm in Si-MAS-NMR measurement.

8. 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 peaks measured by X-ray diffraction measurement of the negative electrode active material particles, is in the range of 1 nm to 5 nm.

9. The negative electrode active material according to claim 1, characterized in that the negative electrode active material particles have a G / D ratio, as determined by Raman spectroscopy, in the range of 0.85 to 1.15 and a Si / G ratio of 0.25 or more and 0.5 or less.

10. The structure of the porous carbon is predominantly Type I in the IUPAC classification, and its surface area is 1400 m ２ / g or more, pore volume is 1 cm ３ 2. The negative electrode active material according to claim 1, wherein the molecular weight of the negative electrode active material is 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; depositing silicon inside the porous carbon structure by flowing monosilane gas through the porous carbon structure under heating; 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 in a state in which the temperature of the material with silicon deposited inside the porous carbon structure is adjusted to be maintained at 50°C or less, thereby changing at least a part of the silicon into low valence nano silicon oxide, characterized in that a negative electrode active material particle having amorphous low valence nano silicon oxide dispersed inside the porous carbon structure is produced.

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