Negative active material

Abstract

The present invention provides a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. [Solution] The porous carbon structure before the low-valence nanosilicon oxide is dispersed has a BET specific surface area of ​​1500 m². 2 / g or more, pore volume of 1.2 cm³ 3 The adsorption average pore diameter is 1 nm to 50 nm, and the low-valence nanosilicon oxide is 1 / g or more. 29 The spectrum obtained by Si-MAS-NMR measurement has peaks originating from Si-C bonds in the range of 0 to -30 ppm, the volume ratio of carbonaceous material in the low-valence nanosilicon oxide and porous carbon structure is 1.4 to 2.7, the coating layer consists of carbonaceous material different from that of the porous carbon structure, or decomposition products or polymers of organosilicon compounds, and covers the pore openings, and the volume of voids inside the coating layer in the negative electrode active material particles (cm³) 3 ( / g) is the total volume (cm²) within the coating layer. 3 A negative electrode active material characterized by being 38-55% of ( / g).

Description

Technical Field

[0001] The present invention relates to a negative electrode active material.

Background Art

[0002] In recent years, small electronic devices typified by mobile terminals have been widely spread, and further miniaturization, weight reduction, and long life are strongly demanded. In response to such market demands, development of secondary batteries that are particularly small, lightweight, and capable of obtaining a high energy density has been promoted. This secondary battery is not limited to small electronic devices, and its application to large electronic devices typified by automobiles and power storage systems typified by houses is also being studied.

[0003] Among them, lithium ion secondary batteries are easy to miniaturize and increase in capacity, and are highly expected because they can obtain a higher energy density than lead batteries and nickel cadmium batteries.

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

[0005] As this negative electrode active material, carbon-based active materials are widely used, while further improvement in battery capacity is required from recent market demands. In order to improve the battery capacity, use of silicon as a negative electrode active material has been studied. This is because the theoretical capacity of silicon (4199 mAh / g) is more than 10 times larger than the theoretical capacity of graphite (372 mAh / g), so a significant improvement in battery capacity can be expected. Development of silicon materials as negative electrode active materials has been studied not only for elemental silicon but also for compounds typified by alloys and oxides. In addition, the shape of the active material has been studied from the standard coating type for carbon-based active materials to an integral type that is directly deposited on a current collector.

[0006] However, when silicon is used as the main raw material for the negative electrode active material, the negative electrode active material expands and contracts during charging and discharging, making it prone to cracking, mainly near the surface of the negative electrode active material. In addition, ionic substances are generated inside the active material, making the negative electrode active material more susceptible to cracking. When the surface of the negative electrode active material cracks, a new surface is created, increasing the reaction area of ​​the active material. At this time, a decomposition reaction of the electrolyte occurs on the new surface, and a film of electrolyte decomposition products is formed on the new surface, thus consuming the electrolyte. As a result, the cycle characteristics tend to deteriorate.

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

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

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

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

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

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

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

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

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

[0016] Furthermore, focusing on the problem that the small pore size of porous carbon materials leads to the deposition of an excess amount of silicon (Si) on the surface, resulting in high electrical resistivity, composite particles that do not contain SiC (silicon carbide) or have an extremely low SiC content have been proposed to reduce electrical resistivity (Patent Document 16).

[0017] Furthermore, SiC, as commonly referred to, is produced by using a fluidized bed tank, injecting silane gas and carrier gas from the bottom in a heated atmosphere, depositing silicon inside porous carbon, and then performing a carbon coating using hydrocarbon gas or coal tar pitch after an oxidation process. At this time, adsorption thermal decomposition is likely to occur at least 400°C (effectively 450-500°C) (Patent Documents 17 and 18).

[0018] In this case, Si-O bonds exist in the outermost layer, and general SiC can be produced by performing carbon coating CVD with hydrocarbon gas on top of them. However, if there are not enough Si-O bonds, Si-C crystals are formed, which is said to reduce battery capacity (Patent Documents 19 and 20).

[0019] Furthermore, in the oxidation process, it is believed that by performing the oxidation treatment using dry air with a low dew point (specifically -40°C), the silicon oxide reacts with the dangling bonds but does not react with the Si-H bonds (Non-Patent Documents 9 and 10).

[0020] Furthermore, while many Si-C compounds have a polydimethylsiloxane structure, some also possess Si-H bonds. It has been reported that Si-H bonds detach at around 450°C (Non-Patent Literature 11). [Prior art documents] [Patent Documents]

[0021] [Patent Document 1] Japanese Patent Publication No. 2001-185127 [Patent Document 2] Japanese Patent Publication No. 2002-042806 [Patent Document 3] Japanese Patent Publication No. 2006-164954 [Patent Document 4] Japanese Patent Publication No. 2006-114454 [Patent Document 5] Japanese Patent Publication No. 2009-070825 [Patent Document 6] Japanese Patent Publication No. 2008-282819 [Patent Document 7] Japanese Patent Publication No. 2008-251369 [Patent Document 8] Japanese Patent Publication No. 2008-177346 [Patent Document 9] Japanese Patent Publication No. 2007-234255 [Patent Document 10] Japanese Patent Publication No. 2009-212074 [Patent Document 11] Japanese Patent Publication No. 2009-205950 [Patent Document 12] Japanese Patent Application Publication No. 06-325765 [Patent Document 13] Japanese Patent Publication No. 2015-156355 [Patent Document 14] U.S. Patent No. 10,608,254 [Patent Document 15] U.S. Patent No. 11,165,054 [Patent Document 16] WO2024 / 142699 [Patent Document 17] U.S. Patent No. 10,454,103 [Patent Document 18] U.S. Patent No. 10,714,744 [Patent Document 19] WO2024 / 161756 [Patent Document 20] U.S. Patent No. 12,046,744 [Non-patent literature]

[0022] [Non-licensed Document 1] Battery Industry Association Official Paper "Denchi", May 1, 2013, page 10 [Non-licensed Document 2] A. Hohl, T. Wieder, PA van Aken, TE Weirich, G. Denninger, M. Vidal, S. Oswald, C. Deneke, J. Mayer, and H. Fuess : J. Non-Cryst. Solids, 320, (2003), 255. [Non-licensed Document 3] V. Kapaklis, J. Non-Crystalline Solids, 354 (2008) 612 [Non-licensed Document 4] Mariko Miyachi, Hironori Yamamoto, and Hidemasa Kawai, J. Electrochem. Soc. 2007 volume 154, issue 4, A376-A380 [Non-licensed Document 5] M. Yamada, A. Inaba, A. Ueda, K. Matsumoto, T. Iwasaki, T. Ohzuku, J. Electrochem. Soc., 159, A1630 (2012) [Non-licensed Document 6] Taeahn Kim, Sangjin Park, and Seung M. Oh, J. Electrochem. Soc. volume 154, (2007), A1112-A1117. [Non-licensed Document 7] Hye Jin Kim, Sunghun Choi, Seung Jong Lee, Myung Won Seo, Jae Goo Lee, Erhan Deniz, Yong Ju Lee, Eun Kyung Kim, and Jang Wook Choi,. Nano Lett. 2016, 16, 282-288. [Non-Patent Document 8] The Cutting Edge of Automotive Lithium-ion Battery Development, pp. 96-111, CMC Publishing, November 27, 2020. [Non-Patent Document 9] Vacuum, Vol. 33, No. 11, 1990, pp. 854-860: Deactivation of Si single crystal surfaces by hydrogen termination. [Non-Patent Document 10] Surface Science Vol.19, No.3, pp.173-178, 1998 Theory of the oxidative process on Si surfaces [Non-Patent Document 11] C. Michael Greenlief, Michael Armstrong, “Hydrogen desorption from Si: How does this relate to film growth?” (J. Vac. Sci. Technol. B, Vol. 13, No. 4, Jul / Aug 1995) [Overview of the project] [Problems that the invention aims to solve]

[0023] As mentioned above, in recent years, small electronic devices such as mobile terminals have become more high-performance and multi-functional, and there is a need for increased battery capacity in the lithium-ion secondary batteries that are their main power source. One way to solve this problem is to develop a lithium-ion secondary battery in which the negative electrode is made mainly of silica material.

[0024] Furthermore, lithium-ion secondary batteries using silica materials are desired to have initial charge-discharge characteristics and cycle characteristics close to those of lithium-ion secondary batteries using carbon-based active materials. Therefore, cycle characteristics and initial charge-discharge characteristics have been improved by using silicon oxide modified by insertion and partial desorption of Li as the negative electrode active material. Recently, by mainly using silicon oxide and pre-containing Li to produce Li silicate, the irreversible capacity, which is a disadvantage of silicon oxide, has been reduced, and such products have actually begun to be marketed. Even when a prototype battery using this silicon oxide with Li, Li-SiO-C (Non-Patent Literature 8), is fabricated to replace 100% of the carbon negative electrode material, the capacity improvement compared to the carbon negative electrode material is limited to the high 20% range. This means that further improvements in battery capacity are required when considering the high performance of small electronic devices (5G, etc.) and the increased driving range of electric vehicles.

[0025] Therefore, CVD-Si-C, which has a low irreversible capacity, was developed, but it has been found that its fast charging capability and battery cycle characteristics are insufficient due to the reaction between Si and the electrolyte.

[0026] The present invention has been made in view of the above-mentioned problems, and aims to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. [Means for solving the problem]

[0027] To solve the above problems, the present invention provides a negative electrode active material having negative electrode active material particles comprising a porous carbon structure, amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, and a coating layer, wherein the porous carbon structure before the dispersion of the low-valence nanosilicon oxide has a BET specific surface area of ​​1500 m² as measured by gas adsorption method. 2 The pore volume measured by the BJH method is 1.2 cm³ or more, and the pore volume is 1.2 cm³ or more. 3 The low-valence nanosilicon oxide is greater than or equal to / g, and the average adsorption pore diameter calculated from the BET specific surface area and the pore volume is between 1 nm and 50 nm. 29The spectrum obtained by Si-MAS-NMR measurement has a peak originating from Si-C bonds in the range of 0 to -30 ppm, the volume ratio of the low-valence nanosilicon oxide to the carbonaceous material of the porous carbon structure is 1.4 or more and 2.7 or less, the coating layer consists of a carbonaceous material different from that of the porous carbon structure, or a decomposition product or polymer of an organosilicon compound, and covers the pore openings of the porous carbon structure, and the volume of voids inside the coating layer (cm³) in the negative electrode active material particles is 3 ( / g) is the total volume (cm²) within the coating layer. 3 The present invention provides a negative electrode active material characterized by being 38-55% of ( / g).

[0028] The negative electrode active material of the present invention has, firstly, a relatively large BET specific surface area and pore volume, so that the average adsorption pore diameter is within an appropriate range, and secondly, the volume ratio of carbonaceous material in the low-valence nanosilicon oxide and porous carbon structure (hereinafter also referred to as the Si / C volume ratio) is within an appropriate range, and thirdly, the ratio of the volume of voids in the coating layer to the total volume of the coating layer is within an appropriate range.

[0029] The battery capacity produced by SiC mainly depends on the amount of silicon deposited in the porous carbon structure, but if the Si / C volume ratio is 1.4 or higher, a sufficient amount of silicon can be secured, and high capacity can be expected. On the other hand, silicon undergoes a large volume expansion when it accepts lithium ions during charging, but if the Si / C volume ratio is 2.7 or lower, cracking or chipping due to internal stress caused by volume expansion during charging does not occur.

[0030] Furthermore, if the ratio of the volume of voids within the coating layer to the total volume within the coating layer is 38-55%, sufficient voids are secured. As a result, the stress associated with silicon expansion during charging is dispersed by these voids, further suppressing particle cracking and chipping. Consequently, the charge-discharge cycle characteristics can be improved.

[0031] As a result, it is possible to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0032] Furthermore, the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction of the negative electrode active material particles, is preferably in the range of 0.8 nm to 5 nm.

[0033] A material having a grain size of zero-valent Si, which is essentially an amorphous structure, is preferred. If the grain size is too large (if the amount of zero-valent Si increases and crystallization progresses too much), the true density becomes too high, which is a factor in the deterioration of cycle characteristics, but this can be prevented. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining a high battery capacity.

[0034] Furthermore, the porous carbon structure has a BET specific surface area of ​​2300 m². 2 / g or more, and the pore volume is 1.2 cm³ or more. 3 It is preferable that the amount is 1 / g or more, and the average pore diameter of the adsorption is 1 nm or more and 3 nm or less.

[0035] Such porous carbon structures have a relatively small volume per pore and a high BET specific surface area due to the pores extending deep into the particle. Therefore, there is less exposed surface area of ​​the pores where silicon is deposited, and the coating layer can cover the exposed pores under relatively mild conditions, protecting the active silicon. In addition, the surface area-to-volume ratio of silicon deposited in the pores is improved, and the stress associated with silicon expansion during charging is dispersed, further suppressing particle cracking and chipping. As a result, it becomes a more suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0036] Furthermore, the porous carbon structure has a BET specific surface area of ​​1500 m². 2 / g or more 2300m 2 Less than / g, and the pore volume is 1.3cm³ 3It is above / g, the average adsorption pore diameter is 3 nm or more and 50 nm or less, and at the maximum value of dV / d(logD) of the Log differential pore volume distribution calculated by the BJH method during the measurement of the pore volume, if the maximum value in the pore diameter range of 1 to 3 nm is A, the maximum value in the pore diameter range of 3 to 50 nm is B, and the maximum value in the region where the pore diameter is larger than 50 nm is C, it is preferable that B / A > 0.2, A > C and B > C.

[0037] Such a porous carbon structure has a relatively large volume per pore and is a porous carbon structure with a large pore diameter. Therefore, the pore volume per particle volume (volume calculated from the particle diameter) is large, it is easy to secure voids with respect to the deposition amount of silicon, and it is possible to realize an improvement in the Si / C volume ratio and the securing of voids at a high level. As a result, it becomes a more suitable negative electrode active material that can improve the cycle characteristics while maintaining a high battery capacity.

[0038] Further, the coating layer is carbonaceous, and in the ID / IG ratio which is the peak intensity ratio of the G band and D band of carbon calculated from the Raman spectrum obtained by Raman spectroscopic analysis, if the ID / IG ratio of the carbonaceous of the porous carbon structure is D and the ID / IG ratio of the carbonaceous of the coating layer is E, it is preferable that 1.20 < D, 1.0 < E < 1.25, and 0.75 < E / D < 0.97.

[0039] Here, the ID / IG ratio is an index for evaluating crystallinity in Raman spectroscopy, and the larger the ID / IG ratio, the better the crystallinity.

[0040] Particularly, for negative electrode active material particles including a porous carbon structure and a coating layer having an ID / IG ratio of 1 or more such as D and E, the acceptability of Li on the surface of the negative electrode active material is good, and the cycle characteristics during high-speed charging are improved. As a result, it becomes a more suitable negative electrode active material that can improve the cycle characteristics while maintaining a high battery capacity.

[0041] Further, it is preferable that the coating layer has a thickness of 20 nm or less.

[0042] A coating layer of this thickness makes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0043] Furthermore, the coating layer is carbonaceous, and the amount of silicon atoms bonded to Si-C is preferably 8.1% to 14.3% of the amount of silicon atoms present as amorphous silicon in the negative electrode active material.

[0044] With this proportion of silicon atoms, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. [Effects of the Invention]

[0045] According to the anode active material of the present invention, firstly, it has a relatively large BET specific surface area and pore volume, so that the average adsorption pore diameter is within an appropriate range, and secondly, the volume ratio of carbonaceous material in the low-valence nanosilicon oxide and porous carbon structure (hereinafter also referred to as the Si / C volume ratio) is within an appropriate range, and thirdly, the ratio of the volume of voids in the coating layer to the total volume in the coating layer is within an appropriate range.

[0046] The battery capacity produced by SiC mainly depends on the amount of silicon deposited in the porous carbon structure, but if the Si / C volume ratio is 1.4 or higher, a sufficient amount of silicon can be secured, and high capacity can be expected. On the other hand, silicon undergoes a large volume expansion when it accepts lithium ions during charging, but if the Si / C volume ratio is 2.7 or lower, cracking or chipping due to internal stress caused by volume expansion during charging does not occur.

[0047] Furthermore, if the ratio of the volume of voids within the coating layer to the total volume within the coating layer is 38-55%, sufficient voids are secured. As a result, the stress associated with silicon expansion during charging is dispersed by these voids, further suppressing particle cracking and chipping. Consequently, the charge-discharge cycle characteristics can be improved.

[0048] As a result, it is possible to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity. [Brief explanation of the drawing]

[0049] [Figure 1] This is a cross-sectional view showing the configuration of a negative electrode containing the negative electrode active material of the present invention. [Figure 2] This is an exploded view showing an example of the configuration (laminated film type) of a lithium-ion secondary battery containing the negative electrode active material of the present invention. [Figure 3] This is the 29Si-MAS-NMR spectrum of the negative electrode active material. [Figure 4] This is the logarithmic differential pore volume distribution for Comparative Example 1. [Figure 5] This is the logarithmic differential pore volume distribution for Example 2. [Modes for carrying out the invention]

[0050] The following describes embodiments of the present invention, but the present invention is not limited thereto.

[0051] As mentioned above, one method for increasing the battery capacity of lithium-ion secondary batteries is to use a negative electrode made primarily of low-valence nanosilicon oxide in a carbon structure. A lithium-ion secondary battery using this active material is desired to exhibit battery characteristics close to those of lithium-ion secondary batteries using carbon-based active materials, while also achieving a high battery capacity.

[0052] Therefore, the inventors diligently conducted research to obtain a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity when used as the negative electrode of a secondary battery.

[0053] In particular, we have discovered that if a negative electrode active material is provided that has a relatively large BET specific surface area and pore volume, the average adsorption pore diameter is within an appropriate range, the volume ratio of carbonaceous material in the low-valence nanosilicon oxide and porous carbon structure (hereinafter also referred to as the Si / C volume ratio) is within an appropriate range, and the ratio of the volume of voids within the coating layer to the total volume within the coating layer is within an appropriate range, then it is possible to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity, and thus the present invention has been completed.

[0054] In other words, the present invention relates to a negative electrode active material having negative electrode active material particles comprising a porous carbon structure, amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, and a coating layer, wherein the porous carbon structure before the dispersion of the low-valence nanosilicon oxide has a BET specific surface area of ​​1500 m² as measured by gas adsorption method. 2 The pore volume measured by the BJH method is 1.2 cm³ or more, and the pore volume is 1.2 cm³ or more. 3 The low-valence nanosilicon oxide is greater than or equal to / g, and the average adsorption pore diameter calculated from the BET specific surface area and the pore volume is between 1 nm and 50 nm. 29 The spectrum obtained by Si-MAS-NMR measurement has a peak originating from Si-C bonds in the range of 0 to -30 ppm, the volume ratio of the low-valence nanosilicon oxide to the carbonaceous material of the porous carbon structure is 1.4 or more and 2.7 or less, the coating layer consists of a carbonaceous material different from that of the porous carbon structure, or a decomposition product or polymer of an organosilicon compound, and covers the pore openings of the porous carbon structure, and the volume of voids inside the coating layer (cm³) in the negative electrode active material particles is 3 ( / g) is the total volume (cm²) within the coating layer. 3 This negative electrode active material is characterized by being 38-55% of the total weight ( / g).

[0055] The following will provide a detailed explanation with reference to the drawings.

[0056] Figure 1 is a cross-sectional view of the negative electrode containing the negative electrode active material of the present invention, and Figure 2 is an example of the configuration of a lithium-ion secondary battery (laminated film type) containing the negative electrode active material of the present invention. Details of Figures 1 and 2 will be described later.

[0057] [The negative electrode active material of the present invention] The present invention provides a negative electrode active material having negative electrode active material particles comprising a porous carbon structure, amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, and a coating layer, wherein the porous carbon structure before the dispersion of the low-valence nanosilicon oxide has a BET specific surface area of ​​1500 m² as measured by gas adsorption method. 2 The pore volume measured by the BJH method is 1.2 cm³ or more, and the pore volume is 1.2 cm³ or more. 3 Low-valence nanosilicon oxides are those with a concentration of 1 nm or more per gram, and an average adsorption pore diameter calculated from the BET specific surface area and pore volume of 1 nm to 50 nm. 29 In the spectrum obtained by Si-MAS-NMR measurement, there is a peak originating from Si-C bonds in the range of 0 to -30 ppm, the volume ratio of low-valence nanosilicon oxide to carbonaceous material in the porous carbon structure is 1.4 to 2.7, the coating layer consists of carbonaceous material different from that of the porous carbon structure, or decomposition products or polymers of organosilicon compounds, and covers the pore openings of the porous carbon structure, and the volume of voids inside the coating layer in the negative electrode active material particles (cm³) 3 ( / g) is the total volume (cm²) within the coating layer. 3 This negative electrode active material is characterized by being 38-55% of the total weight ( / g).

[0058] Here, the porous carbon structure of the present invention has limitations on BET specific surface area, pore volume, and adsorption average pore diameter, but these are limitations on the porous carbon structure before low-valent nanosilicon oxide is dispersed.

[0059] The negative electrode active material of the present invention has, firstly, a relatively large BET specific surface area and pore volume, so that the average adsorption pore diameter is within an appropriate range, and secondly, the volume ratio of carbonaceous material in the low-valence nanosilicon oxide and porous carbon structure (hereinafter also referred to as the Si / C volume ratio) is within an appropriate range, and thirdly, the ratio of the volume of voids in the coating layer to the total volume of the coating layer is within an appropriate range.

[0060] The battery capacity produced by SiC mainly depends on the amount of silicon deposited in the porous carbon structure, but if the Si / C volume ratio is 1.4 or higher, a sufficient amount of silicon can be secured, and high capacity can be expected. On the other hand, silicon undergoes a large volume expansion when it accepts lithium ions during charging, but if the Si / C volume ratio is 2.7 or lower, cracking or chipping due to internal stress caused by volume expansion during charging does not occur.

[0061] Furthermore, if the ratio of the volume of voids within the coating layer to the total volume within the coating layer is 38-55%, sufficient voids are secured. As a result, the stress associated with silicon expansion during charging is dispersed by these voids, further suppressing particle cracking and chipping. Consequently, the charge-discharge cycle characteristics can be improved.

[0062] As a result, it is possible to provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0063] The BET specific surface area measured here by gas adsorption is not particularly limited, but can be measured as specific surface area / pore distribution using a constant volume method based on gas adsorption, for example, with a Shimadzu Tristar II Plus. The conditions are as follows: • Gas used: Nitrogen • Environment: Under liquid nitrogen • Pressure operating range: P / P0 Adsorption 0~0.998 Detachable 0.998~0.10 Pre-treatment: Vacuum, 200°C, 1 hour

[0064] Furthermore, the pore volume (cm³) of porous carbon structures 3The amount (per g) can be measured by the BJH method. Then, the average adsorption pore diameter can be calculated from the BET specific surface area and pore volume.

[0065] Furthermore, amorphous low-valence nanosilicon oxides in the negative electrode active material (for example, true density 2.27 [g / cm³) 3 [cm³] and the volume ratio of carbonaceous material in the porous carbon structure (Si / C), and the volume of voids in the coating layer per unit mass. 3 [g] is the total volume [cm²] of the coating layer 3 The proportion of [g / cm³] to the true density of porous carbon is [g / cm³]. 3 ], and pore volume [cm³] measured by the BJH method 3 [ / g] is calculated from the carbon content combustion analysis (weight ratio of Si to C) of porous carbon after silicon deposition. That is, it is as follows:

[0066] Volume of carbonaceous material derived from porous carbon structures per unit mass [cm³] 3 / g] = 1 / (True density of porous carbon structure)

[0067] Volume of amorphous low-valence nanosilicon oxide per unit mass [cm³] 3 / g] = 1 / ((Weight ratio of Si to C) × (True density of amorphous low-valent nanosilicon oxide))

[0068] Volume ratio of carbonaceous material (Si / C) in low-valence nanosilicon oxide and porous carbon structures. = (Volume of amorphous low-valence nanosilicon oxide per unit mass) / (Volume of carbonaceous material derived from porous carbon structures per unit mass)

[0069] Total volume within the coating layer per unit mass [cm²] 3 / g] = 1 / (True density of porous carbon structure) + (Pore volume of porous carbon structure)

[0070] Volume of voids in the coating layer per unit mass [cm³] 3 / g] = (Pore volume of porous carbon structure) - (Volume of amorphous low-valence nanosilicon oxide per unit mass) - (Pore volume of coated particles)

[0071] Next, the negative electrode active material of the present invention is shown in the upper part of Figure 3. 29 An example of a spectrum obtained by Si-MAS-NMR measurement is shown. In this invention, it is necessary to have a peak originating from the Si-C bond (SiC in the figure) in the range of 0 to -30 ppm. In addition, although not particularly limited, a peak originating from Si or low-valent nanosilicon oxide can be seen in the range of -40 to -80 ppm.

[0072] Furthermore, the lower part of Figure 3 shows a conventional negative electrode active material without Si-C bonds (for example, as in Patent Document 16). 29 The spectrum obtained by Si-MAS-NMR measurement is shown. Compared to the upper part of Figure 3, which is an example of the present invention, the lower part of Figure 3 does not show any peaks originating from the Si-C bond. Only peaks originating from Si are visible in the lower part of Figure 3.

[0073] NMR (nuclear magnetic resonance) measurements can be performed, for example, under the following conditions. 29 Si MAS NMR (Magic Angle Rotation Nuclear Magnetic Resonance) • Equipment: Bruker 700NMR spectrometer, • Probe: 4mm HR-MAS rotor, 50μL • Sample rotation speed: 10 kHz, ·Measurement environment temperature: 25℃

[0074] Furthermore, although not particularly limited, the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction of the negative electrode active material particles, is preferably in the range of 0.8 nm to 5 nm.

[0075] A material having a grain size of zero-valent Si, which is essentially an amorphous structure, is preferred. If the grain size is too large (if the amount of zero-valent Si increases and crystallization progresses too much), the true density becomes too high, which is a factor in the deterioration of cycle characteristics, but this can be prevented. As a result, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining a high battery capacity.

[0076] The valency of low-valency nanosilicon oxides can be quantified using the aforementioned NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0077] XPS can be measured under the following conditions, for example. XPS ·Equipment: X-ray photoelectron spectrometer, • X-ray source: Monochromatic Al Kα rays, • X-ray spot diameter: 100 μm Ar ion gun sputtering conditions: 0.5kV 2mm x 2mm.

[0078] Furthermore, the grain size of low-valent nanosilicon oxides can be confirmed by TEM-EDX. The conditions for this can be as follows: The negative electrode active material is processed on both sides using a focused ion beam (FIB) system in an air-free state. The FIB system is a SIINT XVision200DB with an acceleration voltage of 30kV. TEM observation is performed using an FEI TecnaiG2F20 with an acceleration voltage of 200kV, and EDX is performed using an EDAX r-TEM with an acceleration voltage of 200kV.

[0079] Alternatively, the obtained material can be analyzed by XRD, and the crystallite size of Si can be calculated using Scherrer's formula. If the calculated result is 0.8 nm, this is a calculated result and the material is considered to be substantially amorphous.

[0080] Crystallite size can be calculated using XRD under the following conditions, for example. For broad peaks, the analysis can be performed using the TOPAS software under the following conditions, for example. XRD measurement • Equipment: Bruker D2 PHASER ·X-ray source:Cu • Divergence slit: 0.5° • Incident solar angle: 4° • Solar receiving side: 4° Calculation of crystallite size • Analysis software: DIFFRAC.TOPAS • Analysis method: Peak fitting method · Emission Profile: CuKa5.lam • Function: FP (First Principle) function • Refinement Option: Select "Caluculate Error" and "Use Extrapolation".

[0081] Furthermore, although not specifically limited, porous carbon structures have a BET specific surface area of ​​2300 m². 2 A concentration of 1.2 cm³ or more, and a pore volume of 1.2 cm³ or more. 3 It is preferable that the amount is 1 / g or more, and the average adsorption pore diameter is 1 nm to 3 nm.

[0082] Such porous carbon structures have relatively large pore volumes, but with relatively small volumes per pore and high BET specific surface area due to the pores extending deep into the particles. Therefore, there is less exposed surface area of ​​the pores where silicon is deposited, and the coating layer can cover the exposed pores under relatively mild conditions, protecting the active silicon. In addition, the surface area-to-volume ratio of silicon deposited within the pores is improved, and the stress associated with silicon expansion during charging is dispersed, further suppressing particle cracking and chipping. As a result, it becomes a more suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0083] Furthermore, although not specifically limited, porous carbon structures have a BET specific surface area of ​​1500 m². 2 / g or more 2300m 2 Less than / g and pore volume of 1.3cm³ 3 / g or more, the average adsorption pore diameter is 3 nm or more and 50 nm or less, and when calculating the pore volume by the BJH method during the measurement of the pore volume, at the maximum value of dV / d(logD) of the Log differential pore volume distribution, let the maximum value in the pore diameter range of 1 to 3 nm be A, the maximum value in the pore diameter range of 3 to 50 nm be B, and the maximum value in the region where the pore diameter is larger than 50 nm be C. Then, it is preferable that B / A > 0.2, and A > C and B > C.

[0084] Such a porous carbon structure has a relatively large pore volume, and among those with a relatively large pore volume, it is a porous carbon structure with a relatively large volume per pore and a large pore diameter. Therefore, the pore volume per particle volume (volume calculated from the particle diameter) is large, it is easy to secure voids with respect to the deposition amount of silicon, and it is possible to realize an improvement in the Si / C volume ratio and the securing of voids at a high level. As a result, it becomes a more suitable negative electrode active material that can improve the cycle characteristics while maintaining a high battery capacity.

[0085] Also, without particular limitation, the coating layer is carbonaceous. When the ID / IG ratio, which is the peak intensity ratio of the G band and D band of carbon calculated from the Raman spectrum obtained by Raman spectroscopic analysis, is D for the carbonaceous ID / IG ratio of the porous carbon structure and E for the carbonaceous ID / IG ratio of the coating layer, it is preferable that 1.20 < D, 1.0 < E < 1.25, and 0.75 < E / D < 0.97. [[ID=IO]]

[0086] Here, the ID / IG ratio is an index for evaluating crystallinity in Raman spectroscopy, and the larger the ID / IG ratio, the better the crystallinity.

[0087] Particularly, for negative electrode active material particles containing a porous carbon structure and a coating layer having an ID / IG ratio of 1.0 or more such as D and E, the acceptability of Li on the surface of the negative electrode active material is good, and the cycle characteristics during high-speed charging are improved. As a result, it becomes a more suitable negative electrode active material that can improve the cycle characteristics while maintaining a high battery capacity.

[0088] Note that the D band is 1300 - 1460 cm It should be noted that there seems to be an incomplete expression in "Note that the D band is 1300~1460cm " in the original text. The translation is presented based on the existing content as accurately as possible.-1 Those with a peak in the G band are 1500-1660 cm. -1 It has a peak at [location].

[0089] Furthermore, Raman spectroscopy can be performed, for example, under the following conditions. • Device: HORIBA XploRA Plus • Laser wavelength: 532nm

[0090] Furthermore, although not particularly limited, the thickness of the coating layer is preferably 20 nm or less.

[0091] A coating layer of this thickness makes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0092] The method for measuring the thickness of the coating layer is not particularly limited, but the thickness of the coating layer can be confirmed by cutting the obtained material with a FIB (Focused Ion Beam) and observing the cross-section of the material with a TEM image.

[0093] Furthermore, although not particularly limited, the coating layer is carbonaceous, and the amount of silicon atoms bonded to Si-C is preferably 8.1% to 14.3% of the amount of silicon atoms present as amorphous silicon in the negative electrode active material.

[0094] With this proportion of silicon atoms, it becomes a suitable negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0095] The ratio of silicon atoms in the Si-C bonded state to the amount of silicon atoms present as amorphous silicon in the negative electrode active material can be calculated using the following method.

[0096] The individual capacity of porous carbon structures before silicon deposition as the negative electrode active material, the individual capacity of particles after silicon deposition (and before the coating process), and the individual capacity of negative electrode active material particles after the coating process are measured by charge-discharge evaluation using a coin cell. Since Si-C formed by the coating process does not have charging capacity, The ratio of states of silicon atoms (Si-C) / (Amorphous Si) =[(Volume of individual silicon after deposition)-(Volume of individual silicon after coating process)] / [(Volume of individual silicon after deposition)-(Volume of individual porous carbon structure)] It can be calculated by [method].

[0097] Furthermore, the negative electrode active material of the present invention may also include a silicon-cored carbon composite within a porous carbon structure, and this carbon composite may contain at least one of carbon black, acetylene black, and carbon nanotubes.

[0098] Furthermore, porous carbon can be derived from bio-based, resin-based, or petroleum-based sources, with bio-based or resin-based sources being preferred.

[0099] <Non-aqueous electrolyte secondary battery negative electrode> Next, the configuration of the negative electrode for a non-aqueous electrolyte secondary battery (hereinafter also referred to as "negative electrode") containing the negative electrode active material of the present invention will be described.

[0100] [Composition of the negative electrode] Figure 1 shows a cross-sectional view of a negative electrode containing the negative electrode active material of the present invention. As shown in Figure 1, the negative electrode 10 has a negative electrode active material layer 12 on a negative electrode current collector 11. This negative electrode active material layer 12 may be provided on both sides of the negative electrode current collector 11, or on only one side. Furthermore, in the negative electrode of the non-aqueous electrolyte secondary battery of the present invention, the negative electrode current collector 11 may be omitted.

[0101] [Negative electrode current collector] The negative electrode current collector 11 is made of a material that has excellent conductivity and high mechanical strength. Examples of conductive materials that can be used for the negative electrode current collector 11 include copper (Cu) and nickel (Ni). It is preferable that this conductive material does not form intermetallic compounds with lithium (Li).

[0102] The negative electrode current collector 11 preferably contains carbon (C) and sulfur (S) in addition to the main element. This is because it improves the physical strength of the negative electrode current collector. In particular, if the active material layer expands during charging, the presence of the above elements in the current collector has the effect of suppressing electrode deformation, including the current collector. The content of the above elements is not particularly limited, but it is preferable that each be 100 ppm by mass or less. This is because a higher deformation suppression effect can be obtained. Such deformation suppression effect can further improve cycle characteristics.

[0103] Furthermore, the surface of the negative electrode current collector 11 is preferably roughened, and more preferably, the ten-point average surface roughness Rz is 1.5 μm or more and 5 μm or less. The roughened negative electrode current collector is, for example, a metal foil that has been electrolytically treated, embossed, or chemically etched.

[0104] [Negative electrode active material layer] The negative electrode active material layer 12 may contain multiple types of negative electrode active materials, such as carbon-based active materials, in addition to silicon-based active material particles. Furthermore, for battery design purposes, it may also contain other materials such as thickeners (also called "binding agents" or "binders") and conductive additives.

[0105] [The present invention concerning a negative electrode active material and a method for producing a negative electrode] Next, an example of the anode active material of the present invention and a method for manufacturing an anode using the same will be described.

[0106] Typically, SiC is manufactured using a fluidized bed tank, under a heated atmosphere, by injecting silane gas and a carrier gas from the bottom to deposit silicon inside porous carbon. This process involves oxidation, followed by carbon coating using hydrocarbon gas or coal tar pitch.

[0107] In this case, with this method, the silicon (Si) produced by the reaction of silane gas will have at least a portion of Si-H bonds and will be deposited in a Si-Si state.

[0108] Furthermore, adsorption thermal decomposition is more likely to occur at temperatures of at least 400°C (effectively 450-500°C) (Patent Documents 17 and 18), and the resulting silicon, although amorphous, has a coordination number greater than 3.3.

[0109] Furthermore, the substance obtained in the oxidation process is SiO2, so Si 0+ and Si 4+ The composite material is formed within porous carbon.

[0110] In this case, Si-O bonds exist in the outermost layer, and general SiC can be produced by performing carbon coating CVD with hydrocarbon gas on top of them. However, if there are not enough Si-O bonds, Si-C crystals will form, reducing the battery capacity (Patent Documents 19 and 20).

[0111] Generally, the battery capacity produced by SiC depends mainly on the amount of silicon deposited in the porous carbon, and higher capacity can be expected by increasing the Si / C volume ratio within the particles. On the other hand, silicon expands significantly in volume when it accepts lithium ions during charging, and SiC particles with a high Si / C volume ratio are prone to cracking and chipping due to internal stress associated with volume expansion during charging and discharging, thus degrading the charge-discharge cycle characteristics. Therefore, it is necessary to keep the Si / C volume ratio within an appropriate range. In this invention, by setting the volume ratio of Si (low-valence nanosilicon oxide) to C (carbonaceous material of the porous carbon structure) to 1.4 or more and 2.7 or less, high capacity can be achieved, and cracking and chipping due to internal stress associated with volume expansion during charging can be prevented.

[0112] In the method for producing the negative electrode active material of the present invention, the active sites of the porous carbon structure are utilized to incorporate acidic groups such as carboxyl groups (COOH groups). These acidic groups can react with silane gas to partially transform not only into complex oxides but also into compounds having Si-C bonds.

[0113] Here, the presence or absence of Si-C bond formation is, 29 This can be confirmed by checking whether there is a peak in the 0 to -30 ppm range in the Si-MAS-NMR spectrum. As mentioned above (as shown in the upper part of Figure 3), the present invention has a peak in the 0 to -30 ppm range, indicating the presence of a Si-C bond.

[0114] Furthermore, OH groups are incorporated into the porous carbon structure. When heat treatment is performed in this state, CO or CO2 is detected in the exhaust gas. Normally, when completely dried, no CO gas is emitted, but by reacting it with silane gas in a pressurized atmosphere while the exhaust gas contains a large amount of CO and CO2, silane decomposition products are deposited on the porous carbon structure as low-valence nanosilicon oxides. At this time, since CO and CO2 gases continue to be emitted from inside the pores of the porous carbon structure, it is important to use a hydrogen carrier in a pressurized atmosphere to allow the silane gas to penetrate to the interior. In a typical fluidized bed, the pressure drop is about 10 kPaG, but it has been found that this reaction gradually starts from about 25 kPaG. In this specification, gauge pressure is denoted as PaG and absolute pressure as PaA.

[0115] Furthermore, low-valent nanosilicon oxides deposited on porous carbon structures exhibit high Li diffusivity. Generally, compounds of Si and Li are Li 15 While Si4 is generated, it exhibits ionic properties and reduces diffusivity. In contrast, the low-valence nanosilicon oxide obtained by the method for producing the negative electrode active material of the present invention can reduce the generation of ionic substances.

[0116] Furthermore, it has been found that low-valence nanosilicon oxides with Si-C bonds are electrochemically stable during charging and discharging, and are less prone to decomposition during these processes.

[0117] Next, typical silane decomposition reactions, such as those used in semiconductors (polysilicon), primarily involve adsorption-based thermal decomposition, requiring thermal energy of approximately 400 degrees Celsius or higher.

[0118] However, in a pressurized region, for example, by impregnating the silane gas inside the pores and leaving active sites on the porous carbon, in a state where CO and CO2 gas coexist, monosilane undergoes single-molecule thermal decomposition (gas-phase thermal decomposition) in the gas phase at low temperatures below 380°C, producing silylene, and then hydrogen is removed as a single atom, ultimately forming silicon. Incidentally, even when estimated using the Boltzmann factor, decomposition does not occur below 380°C. In this temperature range, there exists a reaction pathway where the energy barrier allowing hydrogen detachment is irreversible, or where the equilibrium is heavily skewed towards the product side.

[0119] For example, in a pressurized atmosphere, monosilane and silylene mixed with hydrogen inside the pores are thought to have a lower activation energy, leading to a single-molecule thermal decomposition (gas-phase thermal decomposition) reaction. Furthermore, the dangling bonds present in porous carbon are also thought to contribute to the decomposition.

[0120] Furthermore, by mixing hydrogen or an inert gas into the silane and controlling the reaction of silylene returning to silane and the frequency of contact with dangling bonds, strain is generated during decomposition, making it possible to control the coordination number to 3.3 or less.

[0121] In particular, when the coordination number falls below 3, the form of Si that reacts with Li has a high theoretical capacity. 22 Si5 can be achieved.

[0122] In general, silicon reacts with lithium to produce Li 15 It is Si4, Li 15 The theoretical capacity of Si4 is Li 22 It has a lower pH than Si5 and exhibits ionic properties.

[0123] Furthermore, silicon deposited by gas-phase thermal decomposition and increased reaction pressure does not adhere as strongly to porous carbon as silicon deposited by adsorption thermal decomposition. After at least one charge-discharge cycle, it becomes fluid enough to allow extraction of silicon from the pore surface. This helps to improve the diffusivity of Li.

[0124] Furthermore, this method involves using porous carbon with a large pore volume as a substrate, depositing silicon inside it, and completing the silicon deposit while leaving a certain percentage of voids within the particles relative to the total volume of the particles.

[0125] By using a porous carbon structure with a large pore volume as the base material, the proportion of carbon in the particle density of the SiC composite decreases, improving the Si / C volume ratio, and thus increasing the gravimetric energy density of the SiC composite.

[0126] Furthermore, when silicon is deposited onto a porous carbon structure with a relatively large pore volume, a larger amount of unused space (voids) remains within the particles compared to SiC particles made using ordinary porous carbon with equivalent gravimetric energy density. This remaining space within the particles mitigates the volume expansion of silicon that occurs during charging, suppressing cracking and chipping of the particles due to internal stress.

[0127] Two types of porous carbon structures with large pore volumes suitable for this application can be considered: <1> A porous carbon structure with a small volume per pore and high specific surface area, where the pores extend deep into the particle. <2> Examples include porous carbon structures with a large volume per pore and large pore diameters.

[0128] <1> SiC using a porous carbon structure has fewer exposed pores where silicon is deposited, making it possible to cover the exposed pores under relatively mild conditions and protect the active silicon. Furthermore, by improving the surface area-to-volume ratio of silicon deposited within the pores, stress associated with silicon expansion during charging is dispersed, further suppressing particle cracking and chipping.

[0129] <2> SiC using a porous carbon structure has a large pore volume per particle volume (volume calculated from particle size), making it easy to secure voids relative to the amount of silicon deposited, and enabling a high level of improvement in the Si / C volume ratio and void securing.

[0130] Next, we will explain three particularly important steps in the production of the negative electrode active material: preparation of porous carbon, silicon deposition, and coating of the negative electrode active material.

[0131] <1. Preparation of porous carbon> As the first step, we will describe the preparation of porous carbon.

[0132] (Carbon-1) For typical examples A phenolic resin with a particle size of 10 μm is used and heat-treated at 600 degrees Celsius for 1 hour under a nitrogen atmosphere. Next, potassium hydroxide and carbide are mixed in a 5:1 ratio and activated at 700 degrees Celsius. After the activation treatment, the potassium is deactivated with water, then hydrochloric acid (9) and oxalic acid (1) are added to adjust the pH to 7. • After filtration, wash with water. The extracted porous carbon structure is dried at 60 degrees Celsius under vacuum. For example, the specific surface area of ​​the porous carbon structure obtained by this method is 2511 m². 2 / g, pore volume is 1.43 cm³ 3 The ratio B / A of the maximum value B in the 3-50 nm range to the maximum value A in the pore diameter (also called pore diameter) range was 0.18 in the semi-logarithmic graph of the log differential pore volume distribution calculated from pore volume measurement by the BJH method. <1> This corresponds to a porous carbon structure with a small volume per pore and a high specific surface area, where the pores extend deep into the particle.

[0133] (Carbon-2) For typical applications A phenolic resin with a particle size of 10 μm is used and heat-treated at 600 degrees Celsius for 1 hour under a nitrogen atmosphere. Next, the activation treatment is performed at 800 degrees Celsius under a carbon dioxide atmosphere. For example, the specific surface area of ​​the porous carbon structure obtained by this method is 2035 m². 2 / g, pore volume is 1.82 cm³ 3 The value is / g. Furthermore, in the semi-logarithmic graph of the log differential pore volume distribution calculated from pore volume measurement by the BJH method, the ratio B / A of the maximum value B in the 3-50 nm range to the maximum value A in the 1-3 nm range of dV / d(logD) was 0.42. This is the same as above. <2> This corresponds to a porous carbon structure with a large volume per pore and a large pore diameter.

[0134] (Carbon-3) for comparative example A phenolic resin with a particle size of 10 μm is used and heat-treated at 600 degrees Celsius for 1 hour under a nitrogen atmosphere. Next, the activation treatment is performed at 800 degrees Celsius under a steam atmosphere. For example, the specific surface area of ​​the porous carbon structure obtained by this method is 1850 m². 2 / g, pore volume is 0.9 cm³ 3 B / A = 0.09 is calculated from the semi-logarithmic graph of the differential pore volume distribution / g, Log, and the above <1> , <2> Neither applies.

[0135] <2. Silicon Deposition> As the second step, we will explain silicon deposition.

[0136] The porous carbon structure is placed in a reaction vessel and heated to 150 degrees Celsius while nitrogen gas is flowed through it. At that time, a trap is installed in the exhaust gas section to ensure that no moisture is completely discharged. • Simultaneously, the CO gas concentration will be checked using a gas detector. The gas concentration in the absence of moisture will be approximately 8000 ppm.

[0137] Next, the container is heated. When the internal temperature reaches 350 degrees Celsius, silane gas is introduced from the bottom to deposit silicon inside the porous carbon structure (at this time, the Si / C ratio is estimated to be between 1.4 and 2.7 based on the true density and pore volume of the porous carbon structure, and the corresponding amount of silane is flowed). • To intentionally create voids, the usual 90% reaction rate cannot be used. The optimal reaction rate obtained by varying the total flow rate of silane gas will be used in the examples described later.

[0138] Before introducing silane gas, hydrogen gas is introduced to create a hydrogen atmosphere inside the container, and the silane gas concentration is controlled to 20% or less. This controls the degree of formation of sealed voids (closed pores) formed by silicon deposits within the pores.

[0139] • At this time, although the pressure inside the container is variable, it is averaged to be in the range of 25kPa to 80kPa. Deposition at pressures above 80kPa may result in denser Si grains, but considering the durability of the equipment, it is stopped at 80kPa (in reality, it is variable and has been confirmed to rise up to a maximum of 95kPa).

[0140] After the reaction, the material temperature is lowered to room temperature, and oxygen diluted with nitrogen is introduced to increase the weight of the oxygen. During this process, it is important to stir the powder and remove heat. Rapid oxidation will lead to SiO2 formation, which will degrade the properties.

[0141] <3. Coating of the negative electrode active material> As the third step, we will explain the coating of the negative electrode active material.

[0142] (Coating Method 1: CVD) The material is heated to an internal temperature of 530-600 degrees Celsius in a nitrogen atmosphere, and acetylene gas is introduced to form a carbonaceous film on the surface as a coating layer, distinct from the porous carbon structure. The treatment is performed at 10,000 Pa for 8 hours (however, when treating porous carbon structures with wide pore openings (Carbon-2), the treatment is performed at 15,000 Pa for 4 hours).

[0143] As a preliminary step, the material is repressurized to a relative pressure of 0.2 (p / p0) with nitrogen, and then oxygen diluted with nitrogen is introduced to perform oxidation. This allows the amorphous low-valent nanosilicon oxide near the surface of the pores to oxidize more rapidly than the low-valent nanosilicon oxide deep within the pores. Subsequently, the formation of a carbonaceous film on the surface by acetylene gas starts from the Si-O bonds formed by oxidation and then forms Si-C bonds. As a result, film formation by acetylene gas proceeds intensively near the pore entrance (called the pore opening). Consequently, a carbonaceous film is formed on the surface of the pore opening, effectively sealing the voids within the pores.

[0144] (Coating Method 2: Silane Coupling Agent Coating) The device is heated to an internal temperature of 450 degrees Celsius in a nitrogen atmosphere, and evaporated tetramethylsilane (or tetravinylsilane) is introduced to form a film on the surface over a period of 2 hours, consisting of decomposition products or polymers of organosilicon compounds.

[0145] (Coating method 3: Coating by spray coating of pitch solution) • A toluene solution of pitch is used to coat the surface of the porous carbon structure after silicon deposition under a nitrogen atmosphere using a fluidized bed microparticle coating apparatus (egFD-MP-micro, manufactured by Powrec). The obtained coated particles are heat-treated at 500°C under a nitrogen atmosphere to form a carbon coating layer with a thickness of approximately 5 nm.

[0146] During the coating of the negative electrode active material, the reaction is carried out under reduced pressure to allow acetylene gas to permeate the Si filling the pores. Here, the Si-H bonds that did not reach a low-valence oxidation state react with the acetylene gas to form Si-C bonds, resulting in a stable material.

[0147] Furthermore, if the amount of Si-C bonding is excessive, the amorphous silicon formed by silicon deposition will be consumed, which may reduce the individual capacity and increase the resistance during charging and discharging. Conversely, if the amount of Si-C bonding is insufficient, stabilization will be inadequate, which may lead to a decrease in cycle characteristics due to charging and discharging. Therefore, it is preferable to appropriately adjust the amount of Si-C bonding, and for this purpose, as in the present invention, low-valent nanosilicon oxide is used. 29 The Si-MAS-NMR spectrum must have peaks originating from Si-C bonds in the 0 to -30 ppm range, the volume ratio of carbonaceous material in the low-valence nanosilicon oxide to porous carbon structure must be between 1.4 and 2.7, and the volume of voids inside the coating layer in the negative electrode active material particles must be between 38 and 55% of the total volume inside the coating layer.

[0148] <Lithium-ion rechargeable battery> Next, as a specific example of a non-aqueous electrolyte secondary battery using the negative electrode active material of the present invention described above, a laminate film type lithium-ion secondary battery will be explained.

[0149] [Configuration of a laminate film type rechargeable battery] The laminate film type lithium-ion secondary battery 30 shown in Figure 2 mainly consists of a wound electrode body 31 housed inside a sheet-like outer casing member 35. This wound electrode body 31 has a separator between the positive and negative electrodes and is wound around them. There are also cases where a laminate is housed inside with a separator between the positive and negative electrodes. In both electrode bodies, a positive electrode lead 32 is attached to the positive electrode and a negative electrode lead 33 is attached to the negative electrode. The outermost part of the electrode body is protected by protective tape.

[0150] The positive and negative electrode leads 32 and 33 are led out in one direction, for example, from the inside to the outside of the outer casing member 35. The positive electrode lead 32 is made of a conductive material such as aluminum, and the negative electrode lead 33 is made of a conductive material such as nickel or copper.

[0151] The exterior component 35 is, for example, a laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order. In this laminate film, the outer edges of the fusion layers of two films are fused together or bonded together with an adhesive so that the fusion layer faces the electrode body 31. The fusion part is, for example, a film such as polyethylene or polypropylene, and the metal part is, for example, aluminum foil. The protective layer is, for example, nylon.

[0152] An adhesive film 34 is inserted between the outer casing member 35 and the positive and negative electrode leads to prevent outside air from entering. This material can be, for example, polyethylene, polypropylene, or polyolefin resin.

[0153] The positive electrode, for example, has a positive electrode active material layer on both sides or one side of the positive electrode current collector, similar to the negative electrode 10 in Figure 1.

[0154] The positive electrode current collector is formed from a conductive material such as aluminum.

[0155] The positive electrode active material layer contains one or more positive electrode materials capable of intercalating and deintercalating lithium ions, and may also contain other materials such as positive electrode binders, positive electrode conductive additives, and dispersants, depending on the design. In this case, the details regarding the positive electrode binder and positive electrode conductive additive are the same as those for the negative electrode binder and negative electrode conductive additive already described, for example.

[0156] Lithium-containing compounds are preferred as the cathode material. Examples of lithium-containing compounds include composite oxides composed of lithium and transition metal elements, or phosphoric acid compounds having lithium and transition metal elements. Among these cathode materials, compounds containing at least one of nickel, iron, manganese, and cobalt are preferred. Examples of these chemical formulas include Li x M1O2 or Li y It is represented as M2PO4. In the formula, M1 and M2 represent at least one transition metal element. The values ​​of x and y vary depending on the battery charge and discharge state, but are generally given by 0.05 ≤ x ≤ 1.10 and 0.05 ≤ y ≤ 1.10.

[0157] Examples of the composite oxide having lithium and a transition metal element include, for example, lithium cobalt composite oxide (Li x CoO2), lithium nickel composite oxide (Li x NiO2), and lithium nickel cobalt composite oxide. Examples of the lithium nickel cobalt composite oxide include, for example, lithium nickel cobalt aluminum composite oxide (NCA) and lithium nickel cobalt manganese composite oxide (NCM).

[0158] Examples of the phosphate compound having lithium and a transition metal element include, for example, lithium iron phosphate compound (LiFePO4) or lithium iron manganese phosphate compound (LiFe 1-u Mn u PO4 (0 < u < 1)). By using these positive electrode materials, a high battery capacity can be obtained, and excellent cycle characteristics can also be obtained.

[0159] [Negative electrode] The negative electrode has the same configuration as the negative electrode 10 for a lithium ion secondary battery shown in FIG. 1 described above. For example, negative electrode active material layers are provided on both sides of a current collector. It is preferable that the negative electrode charging capacity is larger than the electric capacity (charging capacity as a battery) obtained from the positive electrode active material agent. Thereby, precipitation of lithium metal on the negative electrode can be suppressed.

[0160] The positive electrode active material layer is provided on a part of both sides of the positive electrode current collector, and similarly, the negative electrode active material layer is also provided on a part of both sides of the negative electrode current collector. In this case, for example, the negative electrode active material layer provided on the negative electrode current collector has a region where the opposing positive electrode active material layer does not exist. This is for performing a stable battery design.

[0161] In the region where the above negative electrode active material layer and positive electrode active material layer do not face each other, it is hardly affected by charge and discharge. Therefore, the state of the negative electrode active material layer is maintained as it is immediately after formation, and thereby the composition of the negative electrode active material can be accurately examined with good reproducibility without depending on the presence or absence of charge and discharge.

[0162] [Separator] A separator separates the positive and negative electrodes, preventing current short circuits caused by contact between the two electrodes while allowing lithium ions to pass through. This separator is formed from a porous membrane made of, for example, synthetic resin or ceramic, and may have a laminated structure in which two or more porous membranes are stacked. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, and polyethylene.

[0163] [Electrolyte] At least a portion of the active material layer, or the separator, is impregnated with a liquid electrolyte. This electrolyte contains an electrolyte salt dissolved in a solvent and may also contain other materials such as additives.

[0164] For example, non-aqueous solvents can be used as solvents. Examples of non-aqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, 1,2-dimethoxyethane, or tetrahydrofuran. Among these, it is desirable to use at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, or ethylmethyl carbonate, as this will yield better properties. In this case, even more advantageous properties can be obtained by combining high-viscosity solvents such as ethylene carbonate and propylene carbonate with low-viscosity solvents such as dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate. This is because the dissociability and ion mobility of the electrolyte salt are improved.

[0165] When using an alloy-based negative electrode, it is particularly desirable to include at least one of the following as a solvent: a halogenated linear carbonate ester or a halogenated cyclic carbonate ester. This allows for the formation of a stable film on the surface of the negative electrode active material during charging and discharging, especially during charging. Here, a halogenated linear carbonate ester is a linear carbonate ester having halogen as a constituent element (at least one hydrogen atom is substituted by halogen). A halogenated cyclic carbonate ester is a cyclic carbonate ester having halogen as a constituent element (i.e., at least one hydrogen atom is substituted by halogen).

[0166] While there are no particular limitations on the type of halogen, fluorine is preferred because it forms a better quality film than other halogens. Furthermore, a higher number of halogens is desirable because it results in a more stable film and reduces the decomposition reaction of the electrolyte.

[0167] Examples of halogenated chain carbonate esters include fluoromethylmethyl carbonate and difluoromethylmethyl carbonate. Examples of halogenated cyclic carbonate esters include 4-fluoro-1,3-dioxolan-2-one and 4,5-difluoro-1,3-dioxolan-2-one.

[0168] It is preferable that the solvent additive contains an unsaturated carbon-bonded cyclic carbonate ester. This is because a stable film is formed on the negative electrode surface during charging and discharging, which suppresses the decomposition reaction of the electrolyte. Examples of unsaturated carbon-bonded cyclic carbonate esters include vinylene carbonate or vinylethylene carbonate.

[0169] Furthermore, it is preferable to include a sultone (cyclic sulfonic acid ester) as a solvent additive, as this improves the chemical stability of the battery. Examples of sultones include propanesultone and propenesultone.

[0170] Furthermore, the solvent preferably contains an acid anhydride, as this improves the chemical stability of the electrolyte. Examples of acid anhydrides include propanedisulfonic acid anhydride.

[0171] The electrolyte salt may contain one or more light metal salts, such as lithium salts. Examples of lithium salts include lithium hexafluoride phosphate (LiPF6) and lithium tetrafluoroborate (LiBF4).

[0172] The electrolyte salt content is preferably 0.5 mol / kg to 2.5 mol / kg relative to the solvent, because this allows for high ionic conductivity. [Examples]

[0173] The present invention will be described more specifically below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0174] First, the negative electrode active material was prepared using the following procedure, and then the laminate film type lithium-ion secondary battery 30 shown in Figure 2 was fabricated.

[0175] (Comparative Example 1) Comparative Example 1 used (Carbon-3) as the porous carbon structure. Furthermore, amorphous silicon was formed inside the bulk by performing an adsorption thermal decomposition reaction of silane at a temperature of 430 degrees Celsius within the porous carbon structure. The coating method was Coating Method 1. CVD treatment was performed at 10,000 Pa for 8 hours.

[0176] [Measurement of negative electrode active material] The negative electrode active material prepared as described above was subjected to TEM-EDX, XRD analysis, Raman spectroscopy, and other analyses.

[0177] [Fabrication of the negative electrode] The negative electrode active material prepared as described above, graphite, conductive additive 1 (carbon nanotubes, CNTs), conductive additive 2 (carbon nanoparticles with a median diameter of approximately 50 nm), sodium polyacrylate, and carboxymethylcellulose (hereinafter referred to as CMC) were mixed in a dry mass ratio of 9.3:83.7:1:1:4:1, and then diluted with pure water to obtain a negative electrode mixture slurry.

[0178] Furthermore, a 15 μm thick electrolytic copper foil was used as the negative electrode current collector. This electrolytic copper foil contained carbon and sulfur at concentrations of 70 ppm by mass each. Finally, the negative electrode mixture slurry was applied to the negative electrode current collector and dried in a vacuum atmosphere at 100°C for 1 hour. After drying, the amount of negative electrode active material deposited per unit area on one side of the negative electrode (also called area density) was 7.0 mg / cm³. 2 That was the case.

[0179] [Assembly of a coin cell battery for testing] Next, the solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed, and then the electrolyte salt (lithium hexafluoride phosphate: LiPF6) was dissolved to prepare the electrolyte. In this case, the solvent composition was set to a volume ratio of EC:DMC = 30:70, and the electrolyte salt content was 1 mol / kg relative to the solvent. As additives, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added in amounts of 1.0% by mass and 2.0% by mass, respectively.

[0180] Next, the coin cell was assembled as follows: First, a 1mm thick lithium foil was punched out to a diameter of 16mm and attached to the aluminum cladding.

[0181] 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 cladding via a separator. After injecting the electrolyte, a 2032 coin cell was fabricated.

[0182] [Initial Efficiency Measurement] The initial efficiency was measured under the following conditions. First, the coin cell battery prepared for the initial efficiency test was charged in CCCV mode with a charge rate equivalent to 0.03C (initial charge). The CV was 0V and the termination current was 0.04mA. Next, CC discharge (initial discharge) was performed with a discharge rate of 0.03C and a discharge termination voltage of 1.0V.

[0183] When investigating the initial charge-discharge characteristics, the initial efficiency (sometimes referred to as initial efficiency below) was calculated. The initial efficiency was calculated using the formula: Initial Efficiency (%) = (Initial Discharge Capacity / Initial Charge Capacity) × 100.

[0184] [Manufacturing and evaluation of lithium-ion secondary batteries] Based on the initial data obtained, the cathode was designed so that the utilization rate of the negative electrode was 95%. The utilization rate was calculated from the capacities of the positive and negative electrodes obtained with the counter electrode Li, based on the following formula. Utilization rate = (Positive electrode capacity - Negative electrode loss) / (Negative electrode capacity - Negative electrode loss) × 100 Based on this design, lithium-ion secondary batteries (as shown in Figure 2) for both the examples and comparative examples were manufactured. Battery evaluation was performed on each of the lithium-ion secondary batteries for both the examples and comparative examples.

[0185] The cycle characteristics were investigated as follows: First, to stabilize the battery, two charge-discharge cycles were performed at 0.2C in a 25°C atmosphere, and the discharge capacity of the second cycle was measured. The battery cycle characteristics were calculated from the discharge capacity of the third cycle, and the battery test was stopped at 1000 cycles. Charging was performed at 0.7C and discharging at 0.5C. The charging voltage was 4.3V, the discharge termination voltage was 2.5V, and the charge termination rate was 0.07C.

[0186] The types of silicon-cored carbon composites were investigated by observing the porous carbon structure in the negative electrode active material particles using a scanning electron microscope (SEM).

[0187] The results of each measurement are shown in Table 1. Table 1 also includes the results for Comparative Examples 2-3 and Examples 1-8, which are described later.

[0188] [Table 1]

[0189] In Comparative Example 1, the low voids in the negative electrode active material particles resulted in a low capacity retention rate after cycle evaluation, and the low diffusivity of Li ions in the silicon deposits formed by adsorption thermal decomposition led to a large decrease in the 8C discharge capacity / 0.2C discharge capacity.

[0190] (Example 1) (Carbon-1) was used as a porous carbon structure, and silicon deposition was carried out at 380 degrees. Coating method was coating method 1. Treatment was performed by CVD at 10,000 Pa for 8 hours.

[0191] Compared to Comparative Example 1, the higher Si / C volume ratio results in a higher gravimetric energy density. Because Si grains are uniformly formed within the porous carbon structure with appropriate voids, the 1000Cy retention rate is high.

[0192] (Example 2) (Carbon-2) was used as a porous carbon structure, and silicon deposition was carried out at 380 degrees. Coating method was coating method 1. Treatment was performed by CVD at 15000 Pa for 5 hours.

[0193] Due to the larger Si / C volume ratio, the gravimetric energy density is slightly higher. The wider pore size also allows the Si grains to form more spherically.

[0194] (Comparative Example 2) (Carbon-2) was used as a porous carbon structure, and silicon was deposited by adsorption and thermal decomposition at 430°C.

[0195] Si grains grew unevenly in the deeper parts of the pores, with almost no Si grains forming in the shallower parts. Coating method 1 was used. When this was performed using CVD at 8000 Pa for 9 hours, a carbon film did not form covering the pore openings, and the coating layer collapsed to match the pores. The capacity retention rate due to cycling decreased.

[0196] (Example 3) (Carbon-1) was used as a porous carbon structure, and silicon was deposited at 380 degrees. Coating method 2 was used. Evaporated tetramethylsilane was used for coating over 2 hours.

[0197] Because the coating layer is covered with an organosilicon compound, the Raman ID / IG ratio increases. Due to the lower electronic conductivity of the coating layer compared to a carbonaceous coating layer, the initial efficiency and rate characteristics are slightly reduced. However, due to the high stability of the coating layer, the 1000Cy retention rate is improved. Because Si grains are formed uniformly within the porous carbon structure with appropriate voids, the cycle capacity retention rate is high.

[0198] (Example 4) (Carbon-2) was used as a porous carbon structure, and silicon deposition was performed at 380 degrees. The coating method was coating method 3. Coating was performed with a toluene solution of pitch followed by heat treatment. Compared to CVD coating method 1, the film thickness of the coating layer is thicker, resulting in a slight decrease in unit volume. The formation of Si-C bonds inside the pores is also reduced, resulting in a slight decrease in cycle characteristics.

[0199] (Example 5) A phenolic resin with a particle size of 10 μm is heat-treated at 600°C for 1 hour under a nitrogen atmosphere. Next, potassium hydroxide and carbide are mixed in a 7:1 ratio and activated at 900°C. Using the resulting porous carbon, silane is deposited at 380°C. Coating method 1 is used. Treatment is performed by CVD at 1200 Pa for 8 hours.

[0200] Compared to Comparative Examples 1 and 2, the larger Si / C volume ratio results in a higher gravimetric energy density. Because Si grains are uniformly formed within the porous carbon structure with appropriate voids, the 1000Cy capacity retention rate is high.

[0201] (Comparative Example 3) Silicon deposition was performed at 380°C using porous carbon with macro-diameter (carbonization of resorcinol resin + steam activation). Coating method 1 was used. Treatment was performed by CVD at 15000 Pa for 5 hours.

[0202] For the silicon deposited in the pores with a macro diameter, the pore openings were not coated and the Si grains were exposed. The initial efficiency and cycle characteristics deteriorated significantly.

[0203] (Example 6) The difference from Example 1 is that in the activation process of (Carbon-1), potassium hydroxide and carbide are mixed at a ratio of 3:1 and activated at 700. Otherwise, the same as Example 1, the obtained porous carbon structure was used, and silicon was deposited at 380 degrees. The coating method was Coating Method 1. The treatment was carried out by CVD at 10000 Pa for 8 hours.

[0204] Due to the reduction in the voids within the particles, the effect of improving the cycle capacity retention rate was slightly reduced.

[0205] (Example 7) The difference from Example 2 is that the carbon dioxide activation treatment process of (Carbon-2) was carried out at 700 °C.

[0206] The evaluation results were almost the same as those of Example 2.

[0207] (Example 8) The difference from Examples 2 and 7 is that the carbon dioxide activation treatment process of (Carbon-2) was carried out at 1000 °C.

[0208] The evaluation results were almost the same as those of Examples 2 and 7.

[0209] The following is further supplemented with reference to the drawings.

[0210] Figure 4 shows the Log differential pore volume distribution of Comparative Example 1, and Figure 5 shows the Log differential pore volume distribution of Example 2. As shown in Table 1, for Example 2, the BET specific surface area was 2035 m 2 / g, and the pore volume was 1.82 cm 3The ratio is / g, and the average adsorption pore diameter is 5.1 nm. In Figure 5, if A is the maximum value in the range of pore diameter from 1 to 3 nm, B is the maximum value in the range of pore diameter from 3 to 50 nm, and C is the maximum value in the region where the pore diameter is greater than 50 nm, then B / A = 0.42 > 0.2, and A > C and B > C.

[0211] Therefore, the porous carbon structure of Example 2 is a porous carbon structure with a relatively large volume per pore and a large pore diameter. As a result, the pore volume per particle volume (volume calculated from particle diameter) is large, making it easy to secure voids relative to the amount of silicon deposited, improving the Si / C volume ratio (2.14 in Table 1), and achieving a high level of void securing. As a result, it is thought that both a high battery capacity (2103 mAh / g in Table 1) and excellent cycle characteristics (1000 Cy maintenance rate of 77% in Table 1) were achieved.

[0212] As shown in Table 1, Examples 1 to 8 achieved both high battery capacity and excellent cycle characteristics compared to Comparative Examples 1 to 3. This is thought to be because the negative electrode active materials of Examples 1 to 8 of the present invention have a relatively large BET specific surface area and pore volume, resulting in an adsorbed average pore diameter within an appropriate range, and the volume ratio of carbonaceous material (Si / C volume ratio) of the low-valence nanosilicon oxide and porous carbon structure is within an appropriate range, as is the ratio of the volume of voids within the coating layer to the total volume within the coating layer. Thus, it has been shown that the present invention can provide a negative electrode active material that can improve cycle characteristics while maintaining high battery capacity.

[0213] This specification includes the following embodiments: [1]: A negative electrode active material having negative electrode active material particles comprising a porous carbon structure, amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, and a coating layer, wherein the porous carbon structure before the dispersion of the low-valence nanosilicon oxide has a BET specific surface area of ​​1500 m² as measured by gas adsorption method. 2 The pore volume measured by the BJH method is 1.2 cm³ or more, and the pore volume is 1.2 cm³ or more.3 The low-valence nanosilicon oxide is greater than or equal to / g, and the average adsorption pore diameter calculated from the BET specific surface area and the pore volume is between 1 nm and 50 nm. 29 The spectrum obtained by Si-MAS-NMR measurement has a peak originating from Si-C bonds in the range of 0 to -30 ppm, the volume ratio of the low-valence nanosilicon oxide to the carbonaceous material of the porous carbon structure is 1.4 or more and 2.7 or less, the coating layer consists of a carbonaceous material different from that of the porous carbon structure, or a decomposition product or polymer of an organosilicon compound, and covers the pore openings of the porous carbon structure, and the volume of voids inside the coating layer (cm³) in the negative electrode active material particles is 3 ( / g) is the total volume (cm²) within the coating layer. 3 A negative electrode active material characterized by being 38-55% of ( / g). [2]: The anode active material according to [1], characterized in that the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction of the anode active material particles, is in the range of 0.8 nm to 5 nm. [3]: The porous carbon structure has a BET specific surface area of ​​2300 m 2 / g or more, and the pore volume is 1.2 cm³ or more. 3 The negative electrode active material according to [1] or [2] above, characterized in that it is 1 nm or more and the average adsorption pore diameter is 1 nm or more and 3 nm or less. [4]: The porous carbon structure has a BET specific surface area of ​​1500 m 2 / g or more 2300m 2 Less than / g, and the pore volume is 1.3cm³ 3 / g or more, the average adsorption pore diameter is 3 nm or more and 50 nm or less, and at the maximum value of dV / d(logD) of the Log differential pore volume distribution calculated by the BJH method during the measurement of the pore volume, when the maximum value in the pore diameter range of 1 to 3 nm is A, the maximum value in the pore diameter range of 3 to 50 nm is B, and the maximum value in the region where the pore diameter is greater than 50 nm is C, B / A > 0.2, and A > C and B > C, and the negative electrode active material of the above [1] or the above [2]. [5]: The coating layer is carbonaceous, and in the ID / IG ratio, which is the peak intensity ratio of the G band and D band of carbon calculated from the Raman spectrum obtained by Raman spectroscopic analysis, when the ID / IG ratio of the carbonaceous material of the porous carbon structure is D and the ID / IG ratio of the carbonaceous material of the coating layer is E, 1.20 < D, 1.0 < E < 1.25, and 0.75 < E / D < 0.97, and the negative electrode active material of any one of the above [1] to the above [4]. [6]: The coating layer has a thickness of 20 nm or less, and the negative electrode active material of any one of the above [1] to the above [5]. [7]: The coating layer is carbonaceous, and the silicon atomic weight of the Si-C bond is 8.1% or more and 14.3% or less with respect to the silicon atomic weight existing as amorphous silicon in the negative electrode active material, and the negative electrode active material of any one of the above [1] to the above [6].

[0214] Note that the present invention is not limited to the above embodiments. The above embodiments are examples, and any configuration that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits the same operational effects is included in the technical scope of the present invention.

Explanation of Signs

[0215] [[ID=z16]] 10... negative electrode, 11... negative electrode current collector, 12... negative electrode active material layer, 30... lithium ion secondary battery (laminate film type), 31... electrode body, 32... positive electrode lead (positive electrode aluminum lead), 33... Negative electrode lead (negative electrode nickel lead), 34...Adhesive film, 35...Exterior components.

Claims

1. A negative electrode active material having negative electrode active material particles comprising a porous carbon structure, amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure, and a coating layer, The porous carbon structure before the dispersion of the low-valence nanosilicon oxide has a BET specific surface area of ​​1500 m² as measured by gas adsorption. ２ The pore volume is 1.2 cm³ or more, and measured by the BJH method. ３ The amount is 1 nm or more / g, and the average adsorption pore diameter calculated from the BET specific surface area and the pore volume is 1 nm or more and 50 nm or less. The aforementioned low-valence nanosilicon oxide is ２９ In the spectrum obtained by Si-MAS-NMR measurement, there is a peak originating from the Si-C bond in the range of 0 to -30 ppm. The volume ratio of the carbonaceous material in the low-valence nanosilicon oxide and the porous carbon structure is 1.4 or more and 2.7 or less. The coating layer consists of a carbonaceous material different from the porous carbon structure, or a decomposition product or polymer of an organosilicon compound, and covers the pore openings of the porous carbon structure. The volume of voids inside the coating layer (cm³) in the negative electrode active material particles is the volume of voids inside the coating layer. ３ ( / g) is the total volume within the coating layer (cm ３ A negative electrode active material characterized by being 38-55% of ( / g).

2. The negative electrode active material according to claim 1, characterized in that the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction of the negative electrode active material particles, is in the range of 0.8 nm to 5 nm.

3. The porous carbon structure has a BET specific surface area of ​​2300 m². ２ / g or more, and the pore volume is 1.2 cm³ or more. ３ The negative electrode active material according to claim 1, characterized in that it is 1 nm or more / g and the average adsorption pore diameter is 1 nm or more and 3 nm or less.

4. The structure of the porous carbon has a BET specific surface area of 1500 m ２ / g or more and less than 2300 m ２ / g, and the pore volume is 1.3 cm ３ / g or more, and the average adsorption pore diameter is 3 nm or more and 50 nm or less, The negative electrode active material according to claim 1, characterized in that, when measuring the pore volume, the maximum value of dV / d(logD) of the log differential pore volume distribution calculated by the BJH method is such that A is the maximum value in the range of pore diameters from 1 to 3 nm, B is the maximum value in the range of pore diameters from 3 to 50 nm, and C is the maximum value in the region where the pore diameter is greater than 50 nm, then B / A > 0.2, A > C, and B > C.

5. The negative electrode active material according to claim 1, wherein the coating layer is carbonaceous, and in terms of the ID / IG ratio, which is the ratio of the peak intensity of the G band and the D band of carbon calculated by the Raman spectrum obtained from Raman spectroscopy, if D is the ID / IG ratio of the carbonaceous material of the porous carbon structure and E is the ID / IG ratio of the carbonaceous material of the coating layer, then 1.20 < D, 1.0 < E < 1.25, and 0.75 < E / D < 0.

97.

6. The negative electrode active material according to claim 1, characterized in that the coating layer has a thickness of 20 nm or less.

7. The anode active material according to claim 1, characterized in that the coating layer is carbonaceous, and the amount of silicon atoms bonded to Si-C is 8.1% or more and 14.3% or less relative to the amount of silicon atoms present as amorphous silicon in the anode active material.

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