Negative electrode active material and method for producing same

Abstract

The present invention is a negative electrode active material comprising negative electrode active material particles. The negative electrode active material is characterized in that: the negative electrode active material particles include porous carbon structures; silicon and a silicon oxide are dispersed in the interiors of the porous carbon structures; a low-valent nanosilicon oxide in an amorphous state is dispersed in at least surface-layer portions of the interiors of the porous carbon structures; the low-valent nanosilicon oxide is SiOx, where x is less than 1.0; and nanosilicon with a crystalline structure is dispersed at least in deep layer portions of the interiors of the porous carbon structures, such portions being deeper than where the low-valent nanosilicon oxide is dispersed. Due to this configuration, it is possible to provide a negative electrode active material that can increase capacity while maintaining 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 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 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, wherein the negative electrode active material particles include a porous carbon structure, silicon and silicon oxide are dispersed inside the porous carbon structure, amorphous low-valence nanosilicon oxide is dispersed at least in the surface layer portion of the inside of the porous carbon structure, the low-valence nanosilicon oxide has SiOx:x<1.0, and nanosilicon having a crystalline structure is dispersed at least in the deeper portion of the inside of the porous carbon structure than the low-valence nanosilicon oxide.

[0023] The negative electrode active material of the present invention has silicon and silicon oxide dispersed within the porous carbon structure, and the presence of the porous carbon structure can reduce the adverse effects of expansion of the silicon and silicon oxide within the porous carbon structure. Furthermore, in order to reduce irreversible capacity, a porous carbon structure having a crystalline silicon structure within the particle and containing a low-valent silicon oxide near the particle surface is used as the negative electrode active material, thereby achieving both the effects of using a SiOx phase that suppresses the decomposition reaction of the electrolyte and a crystalline silicon phase that can reduce irreversible capacity.

[0024] In this case, when the distance from the particle center to the surface layer of the negative electrode active material particle is t, it is preferable that the nanosilicon having the crystalline structure is present in a range of 1 / 2t to 2 / 3t from the particle center.

[0025] With such a distribution, in the negative electrode active material, the component exposed to the electrolyte portion can be more reliably made into a low-valent nanosilicon oxide, while a large amount of nanosilicon having a crystalline structure with a small amount of irreversible components can be arranged in the deep layer portion not exposed to the electrolyte portion.

[0026] Furthermore, the nanosilicon having the crystalline structure may contain chlorine at least in part.

[0027] The chlorine contained in nanosilicon having such a crystal structure may improve the diffusibility of Li.

[0028] Furthermore, the nanosilicon having the crystalline structure is preferably a polycrystalline structure.

[0029] Such polycrystalline nanosilicon is easy to form and has a small irreversible capacity.

[0030] Furthermore, it is preferable that 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.

[0031] Such a low-valence nano-silicon oxide is likely to have an appropriate amount of Si—O bonds formed.

[0032] In this case, the low-valent nano silicon oxide is preferably substantially in a monovalent to divalent state.

[0033] In this way, the low-valence nano silicon oxide is substantially in a monovalent to divalent state, and therefore can have a lower irreversible capacity.

[0034] The porous carbon structure preferably has carbon-carbon double bonds at least in part.

[0035] 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.

[0036] 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.

[0037] With such a ratio, it is possible to effectively ensure both the battery capacity and the volume change of the low-valence nano silicon oxide due to the porous carbon structure.

[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 an 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 ２ / g or more is preferred.

[0041] When the porous carbon structure has such an IUPAC classification, surface area, and pore volume, it is possible to obtain a negative electrode active material that efficiently contains a large amount of low-valence nanosilicon oxide.

[0042] The present invention also provides a method for producing a negative electrode active material having negative electrode active material particles, comprising the steps of: preparing a porous carbon structure; flowing trichlorosilane gas through the porous carbon structure under heating to deposit silicon derived from the trichlorosilane gas inside the porous carbon structure; after the silicon deposition by the trichlorosilane gas, further flowing silane gas through the porous carbon structure under heating to deposit silicon derived from the silane gas inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50° C. or less; and a step of converting at least a portion of the silicon into low-valence nanosilicon oxide by introducing oxygen diluted with nitrogen gas into the material having silicon deposited inside the porous carbon structure while adjusting the temperature of the material to maintain it at 50°C or below, thereby dispersing amorphous low-valence nanosilicon oxide in at least the surface layer portion of the porous carbon structure, and producing negative electrode active material particles in which nanosilicon having a crystalline structure is dispersed in at least a portion of the porous carbon structure deeper than the low-valence nanosilicon oxide.

[0043] With this 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 and nanosilicon having a crystalline structure are dispersed inside a porous carbon structure, as described above.

[0044] The negative electrode active material of the present invention has silicon and silicon oxide dispersed within a porous carbon structure, and the presence of the porous carbon structure can reduce the adverse effects of expansion of the silicon and silicon oxide within the porous carbon structure. Furthermore, to reduce irreversible capacity, a porous carbon structure having a crystalline silicon structure within the particle and containing a low-valence silicon oxide near the particle surface is used as the negative electrode active material. This allows for the combined use of a SiOx phase that suppresses the decomposition reaction of the electrolyte and a crystalline silicon phase that can reduce irreversible capacity, thereby achieving both of these effects. As a result, a negative electrode using the negative electrode active material of the present invention exhibits 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 and nanosilicon having a crystalline structure are dispersed inside the porous carbon structure.

[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 Raman spectrum of the negative electrode active material of Example 9. Fig. 5 is an XPS spectrum of the negative electrode active materials of Example 9 and Comparative Example 4.

[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, as one method for increasing the battery capacity of lithium-ion secondary batteries, we investigated the use of a negative electrode with a carbon structure mainly composed of low-valent nanosilicon oxide / crystalline nanosilicon as the negative electrode of lithium-ion secondary batteries. 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. Therefore, in the present invention, in order to reduce irreversible capacity, a porous carbon structure is used as the negative electrode active material, which has a silicon structure with a crystalline structure inside the particle and contains low-valent silicon oxide in the area near the particle surface, thereby developing a negative electrode active material that combines a SiOx phase that suppresses the decomposition reaction of the electrolyte and a polycrystalline silicon phase that can reduce irreversible capacity.

[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, silicon and silicon oxide dispersed inside the porous carbon structure, amorphous low-valence nanosilicon oxide dispersed at least in a surface layer portion of the porous carbon structure, the low-valence nanosilicon oxide having SiOx:x<1.0, and nanosilicon having a crystalline structure dispersed at least in a layer deeper than the low-valence nanosilicon oxide inside the porous carbon structure.

[0052] Since such a negative electrode active material has silicon and silicon oxide dispersed within the porous carbon structure, the presence of the porous carbon structure can reduce the adverse effects of expansion of the silicon and silicon oxide within. Furthermore, in order to reduce irreversible capacity, a porous carbon structure having a silicon structure with a crystalline structure within the particle and containing low-valent silicon oxide near the particle surface is used as the negative electrode active material, thereby achieving both the effects of using a SiOx phase that suppresses the decomposition reaction of the electrolyte and a crystalline silicon phase that can reduce irreversible capacity.

[0053] That is, the negative electrode active material of the present invention is a Si-Ox material having siloxane bonds in the Si phase present near the surface in order 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 becomes an irreversible component, by making it a divalent or less SiOx, 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 (Solid Electrolyte Interphase) that accumulates in the surface layer of the CVD-Si-C. Furthermore, as the particle approaches the center, the Si-O structure changes to a crystalline structure. Since the center does not directly come into contact with the electrolyte, the presence of many Si-Si bonds is not a problem. Furthermore, since there are no or few Si-O bonds, the irreversible components are small. In this way, the produced active material can have a high energy density while maintaining the cycle characteristics of the battery.

[0054] The ratio of the nanosilicon phase having a crystalline structure to the low-valence nanosilicon oxide phase is preferably such that the polycrystalline nanosilicon phase has a width corresponding to the longest distance possible when a line is drawn from the center of the negative electrode active material particle to the surface layer, because polycrystalline nanosilicon has a low irreversible capacity. However, if polycrystalline nanosilicon is present in a region close to the surface layer of the negative electrode active material particle, the decomposition reaction of the electrolytic solution progresses, resulting in a loss of reversible Li. In addition, the crystalline phase has grain boundaries, which deteriorates Li diffusibility. As a result, high-speed charging is not achieved, leading to Li precipitation. In addition, Li-Si compounds exhibit ionic properties. Li １５ Si ４ is a typical example, but because it is hard and brittle, the silicide cracks and the newly formed surface is exposed to the electrolyte. Polycrystalline nanosilicon is not preferred because it affects the decomposition reaction of the electrolyte when exposed to the electrolyte. Therefore, more preferably, when the distance from the particle center to the surface layer of the negative electrode active material particle is t, nanosilicon having a crystalline structure is preferably present in a range of 1 / 2t to 2 / 3t from the particle center. It is preferable that a low-valence nanosilicon oxide phase is mainly present from the periphery to the surface layer.

[0055] Furthermore, in the negative electrode active material of the present invention, it is preferable that the nanosilicon having a crystalline structure contains at least a portion of chlorine (Cl atoms). This is derived from chlorosilane gas, and from the perspective that it can become an irreversible component, it is better to have as little as possible. However, if chlorine is completely absent, when the formed crystalline silicon occludes Li, the diffusibility of Li decreases, so by leaving chlorine in a manner similar to a defect, the diffusibility of Li is greatly improved. In the absence of chlorine, when Li is inserted into the Si-Si crystalline structure, a Li concentration distribution is likely to occur, resulting in distorted expansion, which limits the charging path and makes it difficult to ensure high-speed charging. However, if there is too much chlorine, it becomes LiCl, an irreversible component, so it is desirable to have 100 ppm or less of the nanosilicon portion.

[0056] In the negative electrode active material of the present invention, the nanosilicon having a crystalline structure is preferably a polycrystalline structure, since such nanosilicon having a polycrystalline structure is easy to form and has a small irreversible capacity.

[0057] Furthermore, in the negative electrode active material of the present invention, the low-valent nanosilicon oxide preferably has an average particle size of 50 nm or less, as determined by image processing of a cross-sectional TEM image. 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 exposure to the atmosphere. The FIB processing device is a SIINT XVision 200DB with an acceleration voltage of 30 kV. TEM observation is performed using a 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.

[0058] The Si structure near the surface can be confirmed by XPS.

[0059] In the present invention, the low-valence silicon oxide phase in the surface layer is preferably 50 nm or less on average when observed by cross-sectional TEM. This is because such a particle size facilitates the formation of Si—O bonds. As will be described later, SiH ４In 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 At that time, the interior of the phase may become Si, similar to CVD-Si-C.

[0060] In addition, in the negative electrode active material of the present invention, the low-valence nanosilicon oxide is preferably substantially in a monovalent to divalent state. In this way, by having the low-valence nanosilicon oxide in a substantially monovalent and divalent composite state, a lower irreversible capacity can be achieved. In particular, it is desirable to reduce zero valence as much as possible, and also to reduce trivalent or higher valences. In such a state, when the state has Si-O bonds, decomposition of the electrolyte can be suppressed.

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

[0062] 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.

[0063] 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.

[0064] In the negative electrode active material of the present invention, the proportion of the porous carbon structure in the total negative electrode active material particles is preferably 38% by mass or more and 63% by mass or less, which effectively ensures both the battery capacity and the mitigation of the volume change of the internal silicon and silicon oxide by the porous carbon structure.

[0065] 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.

[0066] 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.

[0067] <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.

[0068] [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.

[0069] [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).

[0070] 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.

[0071] 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.

[0072] [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.

[0073] [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.

[0074] 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 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, flowing trichlorosilane gas through the porous carbon structure under heating to deposit silicon derived from the trichlorosilane gas inside the porous carbon structure, and after the silicon deposition by the trichlorosilane gas, further flowing silane gas through the porous carbon structure under heating to deposit silicon derived from the silane gas inside the porous carbon structure, and cooling the material with silicon deposited inside the porous carbon structure to 50° C. or less, and after the cooling, and a step of converting at least a portion of the silicon into low-valence nanosilicon oxide by introducing oxygen diluted with nitrogen gas into the material having silicon deposited inside the porous carbon structure while adjusting the temperature of the material having silicon deposited inside the porous carbon structure to maintain it at 50°C or below, thereby dispersing amorphous low-valence nanosilicon oxide in at least the surface portion of the porous carbon structure, and producing negative electrode active material particles in which nanosilicon having a crystalline structure is dispersed at least in a portion of the porous carbon structure deeper than the low-valence nanosilicon oxide.

[0075] Each step of the method for producing the negative electrode active material will be described with reference to steps S1 to S5 in FIG.

[0076] 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

[0077] Next, the porous carbon structure prepared in step S1 is heated and treated with trichlorosilane (SiHCl ３ ) gas is flowed to deposit silicon derived from the trichlorosilane gas inside the porous carbon structure (step S2).

[0078] 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, nuclei for silicon deposition in step S2 can be formed and hydrogen attached to the porous carbon structure can be removed, thereby more reliably carrying out silicon deposition in step S2.

[0079] The silicon deposition using trichlorosilane gas in step S2 can be performed by, for example, flowing trichlorosilane gas at a material temperature of 900°C to 1100°C. Trichlorosilane gas can be generated by evaporating liquid trichlorosilane. Trichlorosilane gas can also be diluted with a carrier gas such as hydrogen. The dilution can be, for example, 20 times or less. The deposition time can be, for example, 15 minutes to 10 hours.

[0080] After the silicon deposition using trichlorosilane gas (step S2), silane gas (monosilane gas, SiH ４ ) is flowed to deposit silicon derived from silane gas inside the porous carbon structure (step S3). The silicon deposition in step S3 can be performed by flowing monosilane gas at, for example, about 400°C to 500°C. The deposition time can be, for example, 15 minutes to 10 hours. The monosilane gas used here can also be diluted with a carrier gas such as hydrogen. The dilution can be, for example, 20 times or less.

[0081] Next, the material with silicon deposited inside the porous carbon structure is cooled to 50° C. or less (step S4). In this step, it is preferable to cool the material while flowing nitrogen gas. This cooling can be performed to room temperature, for example.

[0082] After the cooling (step S4), 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 S5). This step allows the formation of Si—O bonds. It is more preferable to maintain the temperature at 35°C or less.

[0083] The dilution of oxygen with nitrogen can be, for example, 5 to 50 times, preferably 10 to 30 times, and typically 20 times.

[0084] Furthermore, in step S5, 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 S5, 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 S5 is preferably set to, for example, 25°C or higher, and more preferably 30°C or higher.

[0085] The oxidation time in step S5 (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.

[0086] 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.

[0087] The material is then removed from the storage container. Through these steps, negative electrode active material particles can be produced, in which crystalline nanosilicon and amorphous low-valent nanosilicon oxide are dispersed within the porous carbon structure.

[0088] 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.

[0089] <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.

[0090] [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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] The positive electrode current collector is made of a conductive material such as aluminum.

[0096] 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.

[0097] 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.

[0098] 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).

[0099] 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.

[0100] [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.

[0101] 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.

[0102] 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.

[0103] [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.

[0104] [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.

[0105] 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.

[0106] 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).

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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.

[0116] A negative electrode active material was produced as follows. First, a surface area (BET specific surface area) of 1689 m ２ / g, pore volume 0.99 cm ３A porous carbon material (porous carbon structure) with a molecular weight of 1000 kJ / g, particle size (D50) = 8 μm, and IUPAC classification type I was prepared. 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, liquid trichlorosilane was evaporated, and a trichlorosilane-containing gas diluted 10 times with hydrogen was flowed. The material temperature was raised to 1050 °C, and polycrystalline silicon was deposited inside for 2 hours. The temperature was lowered to 415 °C, and monosilane gas was flowed under the same conditions, and deposition was continued for 2 hours. The material was then 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 1 hour, and when the material temperature reached 30°C or below, the gas was switched to nitrogen. After flowing for 30 minutes, the material was removed from the storage container and used as the negative electrode active material.

[0117] [Measurement of Negative Electrode Active Material] The negative electrode active material prepared as described above was subjected to TEM-EDX, XRD analysis, and Raman spectroscopic analysis.

[0118] [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.

[0119] 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.

[0120] [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.

[0121] 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.

[0122] 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.

[0123] [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.

[0124] 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.

[0125] [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.

[0126] 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 100 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.

[0127] The results of each measurement are shown in Table 1. Table 1 also shows the results of Comparative Examples 1 to 4 and Examples 2 to 12, which will be described later.

[0128]

[0129] In Example 1, the pore volume of the porous carbon structure was small, so the capacity was low, but it was within the usable range.

[0130] Comparative Example 1 A porous carbon structure similar to that of Example 1 was prepared. Thereafter, the flow of trichlorosilane-containing gas was not performed, and SiH ４ Using a gas, amorphous silicon was formed from the surface layer of the porous carbon structure particle to a portion near the center by deposition at 415°C for 4 hours. Because Si-H bonds were present in this state, the temperature was increased 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-valence silicon oxide compounds (unlike the present invention). Various measurements were performed in the same manner as in Example 1 and are shown in Table 1. In Comparative Example 1, Li precipitated during high-speed charging. This means that in Comparative Example 1, the Si-O bond has capacity on the high-potential side and is excellent in Li diffusibility, but the presence of Si-Si in the particle surface layer deteriorated high-speed charging performance.

[0131] (Comparative Example 2) First, as in Comparative Example 1, SiH ４Using a gas, amorphous silicon was formed from the surface layer of the porous carbon structure particle to a portion near the center. The temperature was then cooled, and oxygen diluted with nitrogen was flowed through to convert the surface layer into SiOx. Various measurements were then performed in the same manner as in Example 1, and the results are shown in Table 1. As a result, due to the presence of many Si—O bonds, the capacity and initial efficiency were reduced compared to Example 1. Considering that an initial efficiency of 85% or more is essential, the initial efficiency was insufficient. The reason for this is that, because graphite was mixed as the negative electrode active material, using a material with low efficiency would not increase the battery capacity.

[0132] (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. As a result, the surface layer was oxidized. The valence of the surface layer was estimated from EDX of the particle cross section. Various measurements were performed in the same manner as in Example 1 and are shown in Table 1. In particular, Li acceptance deteriorated, and Li precipitated during high-speed charging.

[0133] (Comparative Example 4) First, as in Comparative Example 1, SiH ４ Using a gas, amorphous silicon was formed from the surface layer of the porous carbon structure particle to a portion near the center. The structure was then converted to Si-Si and released. Various measurements were performed in the same manner as in Example 1, and the results are shown in Table 1. There were no particular problems except for the high-speed chargeability, but Li deposition was confirmed during high-speed charge. This also indicates that the surface layer must be made of SiOx.

[0134] (Examples 2 to 11) Negative electrode active materials were produced in the same manner as in Example 1, except that porous carbon structures having different properties were used as the first porous carbon structures to be prepared, as shown in Table 1. In addition, evaluations similar to those in Example 1 were performed.

[0135] In Example 2, the porous carbon structure had a large surface area and a large pore volume, resulting in a well-balanced negative electrode active material.

[0136] In Example 3, the pore volume of the porous carbon structure was large and a large amount of SiOx was exposed on the surface, resulting in a negative electrode active material with slightly deteriorated cycle characteristics and high capacity.

[0137] In Example 4, the pores were large relative to the small surface area of ​​the porous carbon structure, and therefore the Si grains were large, and the cycle characteristics were prone to deterioration, but the negative electrode active material was within the usable range.

[0138] In Example 5, the surface area and pore volume of the porous carbon structure were both slightly small, but the negative electrode active material was within the usable range.

[0139] In Example 6, the pore volume of the porous carbon structure was small, so the capacity was somewhat small, but it was still within the usable range of the negative electrode active material.

[0140] Example 7 was a relatively well-balanced negative electrode active material.

[0141] In Example 8, the pore volume of the porous carbon structure was small, but the surface area was large, making it a usable negative electrode active material.

[0142] Example 9 was a negative electrode active material with well-balanced materials. FIG. 4 shows the Raman spectrum of the negative electrode active material of Example 9. From this spectrum, it can be seen that the G / D ratio was 1.02 and the Si / G ratio was 0.37. FIG. 5 also shows the XPS spectra (particle surface layer) of the Si2p region of the negative electrode active materials of Example 9 and Comparative Example 4. In Example 9, the peak was at 99-100 eV, indicating that there was a large amount of low-valence nanosilicon oxide (SiOx) components. In Comparative Example 4, the peak was at 98.5 eV, indicating that there was a large amount of Si components.

[0143] In Example 10, the pore volume of the porous carbon structure was small, so the capacity was small, but it was still within the usable range of the negative electrode active material.

[0144] In Example 11, the porous carbon structure had a large surface area and a large pore volume, and the negative electrode active material was made of a material that provided good characteristics.

[0145] Example 12 A negative electrode active material was produced using a porous carbon structure similar to that of Example 9, but with the same conditions as in Example 9, except that the decomposition temperature of monosilane gas was set to 800°C. By setting the decomposition temperature of monosilane gas to 800°C, the amount of chlorine atoms present in the internal polycrystalline silicon was below the detection limit. In Example 12, a slight decrease in characteristics was observed compared to Example 9, but this was within an acceptable range.

[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, silicon and silicon oxide are dispersed within the porous carbon structure, an amorphous low-valence nanosilicon oxide is dispersed at least in a surface layer of the porous carbon structure, the low-valence nanosilicon oxide has SiOx:x<1.0, and nanosilicon having a crystalline structure is dispersed at least in a deeper layer of the porous carbon structure than the low-valence nanosilicon oxide. [2]: The negative electrode active material of [1] above, wherein the nanosilicon having a crystalline structure is present in a range of 1 / 2t to 2 / 3t from the particle center, where t is the distance from the particle center to the surface of the negative electrode active material particle. [3]: The negative electrode active material of [1] above or [2] above, wherein the nanosilicon having a crystalline structure contains chlorine at least in part. [4]: The negative electrode active material of any one of [1] to [3] above, wherein the nanosilicon having a crystalline structure has a polycrystalline structure. [5]: The negative electrode active material of any one of [1] to [4] above, wherein the low-valence nanosilicon oxide has an average particle size of 50 nm or less, as determined by image processing of a cross-sectional TEM image. [6]: The negative electrode active material of any one of [1] to [5] above, wherein the low-valence nanosilicon oxide is substantially in a monovalent to divalent state. [7]: The negative electrode active material of any one of [1] to [6] above, wherein the porous carbon structure has a carbon-carbon double bond in at least a portion thereof. [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 particles of the negative electrode active material is 38 mass % or more and 63 mass % or less. [9]: The negative electrode active material according to any one of [1] to [8], 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 of type I in the IUPAC classification, and its surface area is 1400 m ２ / g or more, pore volume is 1 cm ２

[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; flowing trichlorosilane gas through the porous carbon structure under heating to deposit silicon derived from the trichlorosilane gas inside the porous carbon structure; after the silicon deposition using the trichlorosilane gas, further flowing silane gas through the porous carbon structure under heating to deposit silicon derived from the silane gas 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. and thereby dispersing amorphous low-valence nanosilicon oxide in at least the surface layer portion of the interior of the porous carbon structure, and producing negative electrode active material particles in which nanosilicon having a crystalline structure is dispersed at least in a portion of the interior of the porous carbon structure deeper than the low-valence nanosilicon oxide.

[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, silicon and silicon oxide dispersed inside the porous carbon structure, amorphous low valence nano silicon oxide dispersed at least in a surface layer of the inside of the porous carbon structure, the low valence nano silicon oxide being SiOx:x<1.0, and nano silicon having a crystalline structure dispersed at least in a layer deeper than the low valence nano silicon oxide inside the porous carbon structure.

2. The negative electrode active material according to claim 1, characterized in that, when the distance from the center of the negative electrode active material particle to the surface layer is t, the nanosilicon having the crystalline structure is present in a range from 1 / 2t to 2 / 3t from the center of the particle.

3. The negative electrode active material according to claim 1, characterized in that the nanosilicon having a crystal structure contains chlorine at least in part.

4. The negative electrode active material according to claim 1, wherein the nanosilicon having a crystalline structure is a polycrystalline structure.

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

6. The negative electrode active material according to claim 1, wherein the low valence nanosilicon oxide is substantially in a monovalent to divalent state.

7. 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.

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

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 ２ 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; flowing trichlorosilane gas through the porous carbon structure under heating to deposit silicon derived from the trichlorosilane gas inside the porous carbon structure; after the silicon deposition by the trichlorosilane gas, further flowing silane gas through the porous carbon structure under heating to deposit silicon derived from the silane gas 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, while adjusting the temperature of the material with silicon deposited inside the porous carbon structure to be maintained at 50° C. or less, introducing oxygen diluted with nitrogen gas into the material with silicon deposited inside the porous carbon structure to convert at least a portion of the silicon into low valence nanosilicon oxide. and thereby dispersing amorphous low-valence nanosilicon oxide in at least a surface layer portion of the interior of the porous carbon structure, and producing negative electrode active material particles in which nanosilicon having a crystalline structure is dispersed at least in a layer deeper than the low-valence nanosilicon oxide within the interior of the porous carbon structure.

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