Negative electrode active material and method for manufacturing the same

Abstract

To provide a negative electrode active material that can improve battery cycle characteristics and increase a capacity.SOLUTION: There is provided a negative electrode active material having negative electrode active material particles. The negative electrode active material particles each include a porous carbon structure. An amorphous low valence nano silicon oxide is dispersed inside the porous carbon structure. A carbon composite containing silicon as a core is dispersed in at least a surface layer portion of the inside of the porous carbon structure. The low valence nano silicon oxide contains SiOx:x<1.0, and the low valence nano silicon oxide is dispersed in a deep layer portion that is deeper than at least the carbon composite containing the silicon as a core among the inside of the porous carbon structure.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] In recent years, small electronic devices typified by mobile terminals have become widely popular, and further miniaturization, weight reduction, and extended lifespan are strongly demanded. In response to such market requirements, the development of secondary batteries that are particularly small, lightweight, and capable of achieving high energy density has been underway. The application of this secondary battery is being considered not only for small electronic devices but also for large electronic devices typified by automobiles and for power storage systems typified by houses.

[0003] Among them, lithium-ion secondary batteries are highly anticipated because they are easily miniaturized and have increased capacity, and can achieve higher energy density than lead-acid batteries and nickel-cadmium batteries.

[0004] The above lithium-ion secondary battery includes a positive electrode, a negative electrode, and an electrolytic solution together with 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. On the other hand, further improvement of battery capacity is required due to recent market demands. In order to improve battery capacity, the 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. The development of silicon-based 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 integrated type that is directly deposited on the current collector.

[0006] However, when silicon is used as the main raw material for the negative electrode active material, the negative electrode active material expands and contracts during charge and discharge, so it is likely to crack mainly near the surface layer of the negative electrode active material. In addition, ionic substances are generated inside the active material, making the negative electrode active material prone to cracking. When the surface layer of the negative electrode active material cracks, a new surface is thereby generated, increasing the reaction area of the active material. At this time, the decomposition reaction of the electrolyte occurs on the new surface, and a film that is a decomposition product of the electrolyte is formed on the new surface, consuming the electrolyte. For this reason, the cycle characteristics are likely to deteriorate.

[0007] So far, various studies have been conducted on the negative electrode active material materials and electrode configurations for lithium-ion secondary batteries with silicon materials as the main materials in order to improve the initial battery efficiency and cycle characteristics.

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

[0009] In addition, a nano-composite containing Si phase, SiO2, and M y O metal oxide is used to improve the first charge-discharge efficiency (see, for example, Patent Document 5). Also, for improving 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 high temperature (see, for example, Patent Document 6). Further, in order to improve cycle characteristics, the molar ratio of oxygen to silicon in the negative electrode active material is set to 0.1 to 1.2, and the active material is controlled within a range where the difference between the maximum value and the minimum value of the molar ratio near the interface between the active material and the current collector is 0.4 or less (see, for example, Patent Document 7). Further, in order to improve the battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 8). Further, in order to improve cycle characteristics, a hydrophobic layer such as a silane compound is formed on the surface layer of the silicon material (see, for example, Patent Document 9).

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

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

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

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

[0014] This irreversible capacity is the most disadvantageous part of silicon oxide and improvement is required. Therefore, Kim et al. have significantly improved the initial efficiency as a battery using the Li pre-doping method of forming Li silicate in advance and produced a negative electrode that can withstand actual use (Non-Patent Document 7). In addition, a method of treating the powder instead of doping the electrode with Li has also been proposed, realizing the improvement of the irreversible capacity (Patent Document 13).

[0015] On the other hand, the Li metal used for Li doping has extremely large upper and lower limits of price depending on the market situation, and there are many problems when considered as industrialization. Therefore, by using silane gas in porous carbon and generating nano-silicon inside, CVD-Si-C can achieve a higher energy density than Li-doped SiO (Patent Documents 14 and 15).

Prior Art Documents

Patent Documents

[0016]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

Patent Document 9

Patent Document 10

Patent Document 11

Patent Document 12

Patent Document 13

Patent Document 14

Patent Document 15

Non-Patent Documents

[0017]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

Summary of the Invention

Problems to be Solved by the Invention

[0018] As described above, in recent years, small electronic devices typified by mobile terminals have been advanced in high performance and multifunctionality, and an increase in battery capacity has been demanded for lithium-ion secondary batteries which are their main power sources. As one method for solving this problem, development of a lithium-ion secondary battery having a negative electrode made mainly of a silicon material has been desired.

[0019] In addition, lithium-ion secondary batteries using a silicon material are desired to have initial charge / discharge characteristics and cycle characteristics that are nearly equivalent to those of lithium-ion secondary batteries using a carbon-based active material. Therefore, by using a silicon oxide modified by insertion and partial desorption of Li as a negative electrode active material, cycle characteristics and initial charge / discharge characteristics have been improved. These days, mainly composed of silicon oxide, by previously containing Li and generating Li silicate, the irreversible capacity which is a demerit of silicon oxide has been reduced, and actually, it has started to be put on the market. Even when a battery was prototyped by replacing 100% of a carbon negative electrode material with Li-SiO-C (Non-Patent Document 8) using Li for this silicon oxide, the capacity improvement only remained in the latter half of the 20% range compared to the carbon negative electrode material. This means that further improvement in battery capacity is required in consideration of the high performance improvement (such as 5G) of small electronic devices and the improvement in driving range of electric vehicles.

[0020] Therefore, CVD-Si-C with little irreversible capacity has been developed, but it has been found that the fast chargeability and battery cycle characteristics are insufficient due to the reaction between Si and the electrolytic solution.

[0021] The present invention has been made in view of the above problems, and an object thereof is to provide a negative electrode active material capable of improving battery cycle characteristics and increasing capacity.

Means for Solving the Problems

[0022] In order to solve the above problems, the present invention provides a negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a structure of porous carbon, and amorphous low-valence nano silicon oxide is dispersed inside the structure of the porous carbon. A carbon composite with silicon as a core is dispersed in at least the surface layer portion inside the structure of the porous carbon. The low-valence nano silicon oxide includes SiOx where x < 1.0, and the low-valence nano silicon oxide is dispersed in at least a deeper layer portion inside the structure of the porous carbon than the carbon composite with silicon as a core.

[0023] Since the negative electrode active material of the present invention has amorphous low-valence nano silicon oxide dispersed inside the structure of porous carbon, the presence of the structure of porous carbon can reduce the adverse effects caused by the expansion of the internal low-valence nano silicon oxide. Also, in general SiO, Si 4+ becomes an irreversible component, but in the negative electrode active material of the present invention, since each state of SiOx where x < 1.0 is included, an irreversible capacity lower than that of general SiO can be maintained. Also, since the Si-O bond can suppress the decomposition of the electrolyte, it is possible to reduce the SEI (Solid Electrolyte Interphase) deposited on the surface layer portion. Further, since the low-valence nano silicon oxide is dispersed in the deeper layer portion inside the structure of the porous carbon and the carbon composite with silicon as a core is dispersed in the surface layer portion, the low-valence nano silicon oxide exposed on the surface layer portion can be inactivated. As a result, excessive decomposition of the electrolyte can be suppressed, and the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

[0024] Further, it is preferable that at least one of carbon black, acetylene black, and carbon nanotubes is included in the carbon composite with silicon as a core.

[0025] With such a carbon composite, the low-valence nano silicon oxide exposed on the surface layer portion can be more surely inactivated.

[0026] Further, it is preferable that the low-valence nano silicon oxide is substantially in a composite state of 0 valence, 1 valence, and 2 valence.

[0027] Thus, when the low-valence nano silicon oxide is substantially in a composite state of 0 valence, 1 valence, and 2 valence, a lower irreversible capacity can be achieved.

[0028] Further, the grain size of 0-valent Si constituting the low-valence nano silicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction measurement of the negative electrode active material particles, is preferably in the range of 1 nm to 5 nm.

[0029] Those having such a grain size of 0-valent Si with a substantially amorphous structure are preferable.

[0030] Further, the structure of the porous carbon is mainly of type I in the IUPAC classification, and its surface area is 1400 m 2 / g or more, and the pore volume is 1 cm 2 / g or more, which is preferable.

[0031] When the structure of the porous carbon has such IUPAC classification, surface area, and pore volume, a negative electrode active material containing a larger amount of low-valence nano silicon oxide efficiently can be obtained. In particular, the type I structure in the IUPAC classification can smoothly generate Si-O bonds after deposition.

[0032] Further, it is preferable that the low-valence nano silicon oxide dispersed in the structure of the porous carbon increases from the center to the surface layer of the structure of the porous carbon as x increases.

[0033] Thus, in the present invention, in order to disperse a low-valence nano silicon oxide in the porous carbon structure, it is likely that from the center to the surface layer of the porous carbon structure during manufacturing, x increases (the oxygen composition ratio increases). Since the oxidation ratio of silicon is large on the surface, the decomposition of the electrolyte can be more effectively suppressed, while since the oxidation ratio of silicon is small inside the active material, the battery capacity can be increased more.

[0034] Moreover, the present invention is a method for manufacturing a negative electrode active material having negative electrode active material particles, comprising the steps of preparing a porous carbon structure, flowing monosilane gas under heating to the porous carbon structure to deposit silicon inside the porous carbon structure, and after the deposition of silicon by the monosilane gas, flowing the following general formula (1) under heating to the porous carbon structure Si(R 1 ) l (R 2 ) m (R 3 ) 4-l-m (1) (In general formula (1), R 1 is an alkyl group having 1 to 20 carbon atoms, R 2 is an alkenyl group having 2 to 20 carbon atoms, R 3is an alkynyl group having 2 to 20 carbon atoms. Also, l and m each independently represent an integer of 0 to 4.) By introducing an organosilicon compound represented by, a carbon composite with silicon as a nucleus is deposited on at least the surface layer portion inside the structure of the porous carbon, and after the deposition of the carbon composite with silicon as a nucleus, the structure of the porous carbon is cooled to 50°C or lower, and after the cooling, oxygen diluted with nitrogen gas is introduced into the structure of the porous carbon in a state where the temperature of the structure of the porous carbon is adjusted to maintain 50°C or lower, so that at least a part of the silicon inside the structure of the porous carbon is changed to a low-valence nanosilicon oxide, thereby dispersing the carbon composite with silicon as a nucleus in at least the surface layer portion inside the structure of the porous carbon, and providing a method for producing a negative electrode active material, characterized in that negative electrode active material particles in which the low-valence nanosilicon oxide is dispersed are produced in at least a deeper layer portion than the carbon composite with silicon as a nucleus inside the structure of the porous carbon.

[0035] With such a method for producing a negative electrode active material, as described above, a negative electrode active material in which amorphous low-valence nanosilicon oxide is dispersed can be produced simply and efficiently in at least a deeper layer portion than the carbon composite with silicon as a nucleus inside the structure of the porous carbon.

Effects of the Invention

[0036] Since the negative electrode active material of the present invention has amorphous low-valence nanosilicon oxide dispersed inside the structure of porous carbon, the presence of the structure of porous carbon can reduce the adverse effects caused by expansion when the internal low-valence nanosilicon oxide expands. Since it contains SiOx: x < 1.0, an irreversible capacity lower than that of general SiO can be maintained. In addition, the Si-O bond can suppress the decomposition of the electrolyte, so it is possible to reduce the SEI deposited on the surface layer part. Further, since the low-valence nanosilicon oxide is dispersed in the deep part inside the structure of the porous carbon, and the carbon composite with silicon as the core is dispersed in the surface layer part, the low-valence nanosilicon oxide exposed on the surface layer part can be inactivated. As a result, excessive decomposition of the electrolyte can be suppressed, and the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved. Therefore, the negative electrode using the negative electrode active material of the present invention can obtain high initial efficiency, high capacity, high input characteristics, and high cycle characteristics.

[0037] In addition, the method for producing the negative electrode active material of the present invention can easily and efficiently produce a negative electrode active material in which amorphous low-valence nanosilicon oxide is dispersed in at least a deeper part than the carbon composite with silicon as the core inside the structure of the porous carbon.

Brief Description of the Drawings

[0038]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0039] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited thereto.

[0040] As described above, as one method for increasing the battery capacity of a lithium-ion secondary battery, using a negative electrode in which a low-valence nanosilicon oxide is used as a main material in a carbon structure as the negative electrode of the lithium-ion secondary battery has been considered. A lithium-ion secondary battery using this active material is desired to have a high battery capacity while showing battery characteristics almost equivalent to those of a lithium-ion secondary battery using a carbon-based active material.

[0041] Therefore, the present inventors have conducted intensive studies to obtain a negative electrode active material that can improve the initial charge-discharge characteristics and increase the battery capacity while obtaining high cycle characteristics when used as the negative electrode of a secondary battery, and have reached the present invention.

[0042] In particular, in CVD-Si-C as disclosed in Patent Documents 14 and 15, a problem was that the reaction with the electrolytic solution was too large. In the present invention, in order to suppress such a reaction with the electrolytic solution, the Si part is converted into a Si-O phase, and by using the siloxane bond peculiar to silicon oxide, not only is the reaction decomposition of the electrolytic solution significantly suppressed, but also the Si-O material having a siloxane bond has good Li acceptability, so a negative electrode active material that can ensure fast charging has been developed. In addition to suppressing the decomposition reaction of the electrolytic solution in the SiOx phase, a negative electrode active material has been developed that inactivates the low-valence nanosilicon oxide exposed on the surface layer portion by a carbon composite having silicon dispersed in the surface layer portion, thereby further suppressing the excessive decomposition of the electrolytic solution.

[0043] [Negative Electrode Active Material of the Present Invention] The negative electrode active material of the present invention is a negative electrode active material having negative electrode active material particles. The negative electrode active material particles include a structure of porous carbon, and an amorphous low-valence nanosilicon oxide is dispersed inside the structure of the porous carbon. Among the interior of the structure of the porous carbon, at least in the surface layer portion, a carbon composite with silicon as the core is dispersed. The low-valence nanosilicon oxide includes SiOx where x < 1.0, and the low-valence nanosilicon oxide is dispersed in at least a deeper layer portion than the carbon composite with silicon as the core inside the structure of the porous carbon. It is a negative electrode active material characterized by this.

[0044] Such a negative electrode active material has an amorphous low-valence nanosilicon oxide dispersed inside the structure of the porous carbon. Therefore, due to the presence of the structure of the porous carbon, the adverse effects caused by the expansion of the internal low-valence nanosilicon oxide can be reduced. Also, general SiO has Si 4+ as an irreversible component, but in the negative electrode active material of the present invention, since it includes each state of SiOx where x < 1.0, an irreversible capacity lower than that of general SiO can be maintained. Also, the Si-O bond can suppress the decomposition of the electrolytic solution, so it becomes possible to reduce the SEI (Solid Electrolyte Interphase) deposited on the surface layer portion. Further, since the low-valence nanosilicon oxide is dispersed in the deeper layer portion inside the structure of the porous carbon and the carbon composite with silicon as the core is dispersed in the surface layer portion, the low-valence nanosilicon oxide exposed on the surface layer portion can be inactivated. As a result, excessive decomposition of the electrolytic solution can be suppressed, and the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

[0045] That is, the negative electrode active material of the present invention is Si-Ox with siloxane bonds in the Si phase in order to suppress the decomposition reaction of the electrolytic solution, which is insufficient for CVD-Si-C generated from general silane gas. However, since the tetravalent Si constituting SiO becomes an irreversible component, by using SiOx with a valence of two or less, although the irreversible capacity is larger than that of silicon alone, an irreversible capacity lower than that of general SiO can be maintained. In addition, since the Si-O bond can suppress the decomposition of the electrolytic solution, it is possible to reduce the SEI (Solid Electrolyte Interphase) deposited on the surface layer of CVD-Si-C. Since the main site contributing to the main charge and discharge of this material is the low-valence nanosilicon oxide, it can be defined as CVD-SiOx-C with respect to CVD-Si-C. Thus, the prepared active material can have a high energy density and high-rate chargeability while maintaining the cycle characteristics of the battery. Furthermore, due to the dispersion of the carbon composite with silicon as the core in the surface layer portion, the low-valence nanosilicon oxide exposed on the surface layer portion can be inactivated. As a result, excessive decomposition of the electrolytic solution can be suppressed, and in particular, the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

[0046] The grain size of the low-valence nanosilicon oxide can be confirmed by TEM-EDX. The conditions at this time can be as follows. The negative electrode active material is processed into a cross-section in a non-atmospheric exposure state by a focused ion beam processing apparatus (FIB). The FIB processing apparatus is XVision200DB manufactured by SIINT, and the acceleration voltage is 30 kV. The TEM observation is TecnaiG2F20 manufactured by FEI, the acceleration voltage is 200 kV, and the EDX is r-TEM manufactured by EDAX, and the acceleration voltage is 200 kV.

[0047] When the particle size of the low-valence nanosilicon oxide increases, it becomes difficult to form Si-O bonds. As will be described later, in the method for producing the negative electrode active material of the present invention, specifically, in the process of decomposing SiH4 to produce amorphous Si, it reacts with oxygen intentionally to form Si-O bonds. Therefore, when the particle size is large, the invasiveness of oxygen is poor, for example, a concentration distribution of O occurs in the low-valence silicon oxide phase, and it may be converted to SiO2. Or at that time, the inside of the phase may be in the state of Si similar to the above-mentioned CVD-Si-C. Also, inside the low-valence nanosilicon oxide grains, there is also a part of zero-valent amorphous Si, and it is desirable that the crystallinity is lower. Lowering the crystallinity has the advantage that although the irreversible capacity increases, the Li acceptance property improves. Conversely, when the crystallinity is high, the electrolyte decomposition reaction peculiar to Si is promoted, so the cycle characteristics deteriorate.

[0048] Further, in the negative electrode active material of the present invention, it is preferable that at least one of carbon black, acetylene black, and carbon nanotubes is included in the carbon composite with silicon as the core.

[0049] With such a carbon composite, it is possible to more reliably inactivate the low-valence nanosilicon oxide exposed on the surface layer portion.

[0050] The carbon composite containing carbon black with silicon as the core present inside or on the surface layer of the porous carbon structure plays a role in maintaining the contact between the negative electrode active material particles and the porous carbon, leading to an improvement in cycle characteristics. Also, the carbon composite containing acetylene black or carbon nanotubes with silicon as the core present on the surface layer of the porous carbon structure suppresses the excessive decomposition of the electrolyte by inactivating particularly the low-valence silicon oxide exposed on the surface layer of the porous carbon. Therefore, the cycle characteristics are excellent.

[0051] In addition, in the negative electrode active material of the present invention, it is preferable that the low-valence nano silicon oxide is substantially in a composite state of 0 valence, 1 valence, and 2 valence. Thus, since the low-valence nano silicon oxide is substantially in a composite state of 0 valence, 1 valence, and 2 valence, a lower irreversible capacity can be achieved.

[0052] The negative electrode active material of the present invention has a low-valence nano silicon oxide phase inside the porous carbon material, and the ratio of silicon to oxygen constituting this low-valence silicon compound is SiO x :x<1.0 is preferably included, and more preferably x≦0.7. Further, the low-valence nano silicon oxide is dominated by compounds with a valence of 1 to 2, and the low-valence nano silicon oxide phase present inside the pores may contain a microcrystalline phase of 0 valence of Si.

[0053] The valence of the low-valence nano silicon oxide can be quantified by NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0054] The NMR measurement can be performed, for example, under the following conditions. 29 Si MAS NMR (magic angle spinning nuclear magnetic resonance) · Apparatus: Bruker 700 NMR spectrometer, · Probe: 4mmHR-MAS rotor 50μL, · Sample rotation speed: 10kHz, · Measurement environmental temperature: 25°C

[0055] The XPS measurement can be performed, for example, under the following conditions. XPS · Apparatus: X-ray photoelectron spectrometer, · X-ray source: Monochromatized Al Kα ray, · X-ray spot diameter: 100μm, · Ar ion gun sputtering conditions: 0.5kV 2mm×2mm.

[0056] In the negative electrode active material of the present invention, the grain size of zero-valent Si constituting the low-valence nanosilicon oxide, which is calculated using Scherrer's formula from the peaks measured by X-ray diffraction measurement of the negative electrode active material particles, is preferably in the range of 1 nm to 5 nm. It is preferable to have such a grain size of zero-valent Si having a substantially amorphous structure.

[0057] The calculation of the crystallite size by XRD can be performed, for example, under the following conditions. For broad peaks, it can be performed, for example, under the following conditions using the analysis software TOPAS. XRD measurement · Apparatus: D2 PHASER manufactured by Bruker · X-ray source: Cu · Divergence slit: 0.5° · Incident side Soller: 4° · Receiving side Soller: 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”

[0058] Further, in the negative electrode active material of the present invention, it is preferable that the low-valence nanosilicon oxide dispersed in the porous carbon structure increases from the center to the surface layer of the porous carbon structure as x increases. In the present invention, since the low-valence nanosilicon oxide is dispersed in the porous carbon structure, it tends to increase from the center to the surface layer of the porous carbon structure during production (the oxygen composition ratio increases). Since the oxidation ratio of silicon is large on the surface, the decomposition of the electrolyte can be more effectively suppressed, and since the oxidation ratio of silicon is small inside the active material, the battery capacity can be further increased.

[0059] <Negative electrode for non-aqueous electrolyte secondary battery> Next, the configuration of the negative electrode for a non-aqueous electrolyte secondary battery containing the negative electrode active material of the present invention (hereinafter also referred to as "negative electrode") will be described.

[0060] [Configuration of negative electrode] Figure 1 shows a cross-sectional view of the 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 the negative electrode current collector 11. This negative electrode active material layer 12 may be provided on both sides or only on one side of the negative electrode current collector 11. Furthermore, in the negative electrode of the non-aqueous electrolyte secondary battery of the present invention, the negative electrode current collector 11 may not be provided.

[0061] [Negative electrode current collector] The negative electrode current collector 11 is composed of an excellent conductive material and has high mechanical strength. Examples of the conductive material that can be used for the negative electrode current collector 11 include copper (Cu) and nickel (Ni). This conductive material is preferably a material that does not form an intermetallic compound with lithium (Li).

[0062] The negative electrode current collector 11 preferably contains carbon (C) and sulfur (S) in addition to the main element. This is to improve the physical strength of the negative electrode current collector. In particular, when there is an active material layer that expands during charging, if the current collector contains the above elements, there is an effect of suppressing electrode deformation including the current collector. The content of the above-mentioned contained elements is not particularly limited, but among them, it is preferably 100 mass ppm or less respectively. This is because a higher deformation suppression effect can be obtained. Such a deformation suppression effect can further improve the cycle characteristics.

[0063] Also, the surface of the negative electrode current collector 11 is preferably roughened, and desirably, the ten-point average roughness Rz of the surface 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.

[0064] [Negative electrode active material layer] The negative electrode active material layer 12 may contain a plurality of types of negative electrode active materials such as carbon-based active materials in addition to silicon-based active material particles. Further, in terms of battery design, it may also contain other materials such as a thickening agent (also referred to as a "binder" or "adhesive") and a conductive aid.

[0065] [Negative Electrode Active Material and Method for Manufacturing Negative Electrode] Subsequently, an example of the negative electrode active material of the non-aqueous electrolyte secondary battery of the present invention and a method for manufacturing a negative electrode using the same will be described.

[0066] First, a method for manufacturing the negative electrode active material contained in the negative electrode will be described. The method for manufacturing the negative electrode active material of the present invention is a method for manufacturing a negative electrode active material having negative electrode active material particles, which includes a step of preparing a porous carbon structure, a step of depositing silicon inside the porous carbon structure by flowing monosilane gas under heating with respect to the porous carbon structure, and after the deposition of silicon by the monosilane gas, the following general formula (1) is applied to the porous carbon structure under heating Si(R 1 ) l (R 2 ) m (R 3 ) 4-l-m (1) (In the general formula (1), R 1 is an alkyl group having 1 to 20 carbon atoms, R 2 is an alkenyl group having 2 to 20 carbon atoms, R 3is an alkynyl group having 2 to 20 carbon atoms. Also, l and m each independently represent an integer of 0 to 4. By introducing an organosilicon compound represented by), a carbon composite with silicon as a nucleus is deposited on at least the surface layer portion inside the structure of the porous carbon, and after the deposition of the carbon composite with silicon as a nucleus, the structure of the porous carbon is cooled to 50 °C or lower, and after the cooling, oxygen diluted with nitrogen gas is introduced into the structure of the porous carbon in a state where the temperature of the structure of the porous carbon is adjusted to maintain 50 °C or lower, so that at least a part of the silicon inside the structure of the porous carbon is changed to a low-valence nano silicon oxide, and thereby, the carbon composite with silicon as a nucleus is dispersed in at least the surface layer portion inside the structure of the porous carbon, and negative electrode active material particles in which the low-valence nano silicon oxide is dispersed are produced in at least a deeper layer portion than the carbon composite with silicon as a nucleus inside the structure of the porous carbon. A method for producing a negative electrode active material is characterized by this.

[0067] Referring to Steps S1 to S5 in FIG. 3, each step of the method for producing a negative electrode active material will be described.

[0068] First, a structure of porous carbon is prepared (Step S1). The structure of porous carbon prepared here preferably has a carbon-carbon double bond in at least a part.

[0069] Also, the structure of porous carbon prepared here is dominated by Type I in the IUPAC classification, and its surface area is 1400 m 2 / g or more, and the pore volume is preferably 1 cm 3 / g or more. By setting such classification according to the IUPAC classification, surface area, and pore volume, the deposition of silicon can be carried out more efficiently in a larger amount. Also, for the IUPAC classification, surface area, and pore volume, the following measurement methods can be used. · Using a Shimadzu TriStar II Plus, the specific surface area / pore distribution is measured by the constant volume method based on the gas adsorption method. The conditions are as follows. · Gas used: Nitrogen · Environment: Under liquid nitrogen · Pressure operating range: P / P0 · Adsorption 0 - 0.998 · Desorption 0.998 - 0.10 · Pretreatment: Vacuum at 200 °C for 1 hour

[0070] Next, by flowing monosilane gas under heating to the porous carbon structure prepared in step S1, silicon derived from the monosilane gas is deposited inside the porous carbon structure (step S2).

[0071] Note that after step S1 and before step S2, it is preferable to store the porous carbon structure in a vacuum vessel and perform evacuation. The degree of vacuum can be, for example, up to about -100 kPa, but is not limited thereto. Further, after evacuation, it is preferable to repressurize with nitrogen and heat using an external heater to about 350 - 450 °C in a state where nitrogen is flowing. This heating can be performed for 5 minutes to 1 hour. By performing such preheating under evacuation and in the presence of nitrogen, nucleation for silicon deposition in step S2 and removal of hydrogen and water adhering to the porous carbon structure can be performed, so that silicon deposition in step S2 can be performed more reliably.

[0072] The silicon deposition in step S2 can be performed, for example, by flowing monosilane gas at about 400 °C - 500 °C. The deposition time can be, for example, 30 minutes to 10 hours.

[0073] After the deposition of silicon by the above monosilane gas (step S2), further, under heating, the following general formula (1) is applied to the porous carbon structure Si(R 1 ) l (R 2 ) m (R 3 ) 4-l-m (1) (In general formula (1), R 1 is an alkyl group having 1 to 20 carbon atoms, and R2 is an alkenyl group having 2 to 20 carbon atoms, and R 3 is an alkynyl group having 2 to 20 carbon atoms. Also, l and m each independently represent an integer of 0 to 4. By introducing an organosilicon compound represented by the formula (), a carbon composite with silicon as a core is deposited inside the structure of the porous carbon (step S3).

[0074] In the general formula (1), R 1 is an alkyl group having 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms, more preferably 1 to 4 carbon atoms.

[0075] R 1 Specific examples of the alkyl group of include linear alkyl groups such as methyl group, ethyl group, n-propyl group, n-butyl group, n-pentyl group, n-hexyl group, n-heptyl group, n-octyl group, n-nonyl group, n-decyl group, n-undecyl group, n-dodecyl group; branched alkyl groups such as isopropyl group, isobutyl group, sec-butyl group, tert-butyl group, isopentyl group, neopentyl group, isohexyl group, isoheptyl group, isooctyl group, tert-octyl group, isononyl group, isodecyl group, isoundecyl group, etc.

[0076] Among these, from the viewpoint of ensuring the thermal decomposability of the organosilicon compound, a methyl group, an ethyl group, and an n-propyl group are preferable.

[0077] In the general formula (1), R 2 is an alkenyl group having 2 to 20 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 2 to 5 carbon atoms.

[0078] R 2Specific examples of the alkenyl group include linear alkenyl groups such as vinyl group, 1-propenyl group, 1-butenyl group, 1-pentenyl group, 1-hexenyl group, 1-heptenyl group, 1-octenyl group, 1-nonenyl group, 1-decenyl group, 1-undecenyl group, 1-dodecenyl group; and branched alkenyl groups such as isopropenyl group, 1-methyl-1-propenyl group, 2-methyl-1-propenyl group, 1-methyl-1-butenyl group, 2-methyl-1-butenyl group, 3-methyl-1-butenyl group, isohexenyl group, isoheptenyl group, isooctenyl group, isononyl group, isodecenyl group, isoundecyl group.

[0079] Among these, from the viewpoint of ensuring the thermal decomposability of the organosilicon compound, vinyl group and 1-propenyl group are preferred.

[0080] In general formula (1), R 3 represents an alkynyl group having 2 to 20 carbon atoms, preferably 2 to 10 carbon atoms, more preferably 2 to 5 carbon atoms.

[0081] R 3 Specific examples of the alkynyl group include linear alkynyl groups such as ethynyl group, 1-propynyl group, 1-butynyl group, 1-n-pentynyl group, 1-n-hexynyl group, 1-n-heptynyl group, 1-n-octynyl group, 1-n-nonynyl group, 1-n-decynyl group, 1-n-undecynyl group, 1-n-dodecynyl group; and branched alkynyl groups such as 3-methyl-1-butynyl group, 3,3-dimethyl-1-butynyl group, 3-methyl-1-pentynyl group, 4-methyl-1-pentynyl group, 3,3-dimethyl-1-pentynyl group, 3,4-methyl-1-pentynyl group, 4,4-dimethyl-1-pentynyl group.

[0082] Among these, from the viewpoint of ensuring the thermal decomposability of the organosilicon compound, ethynyl group, 1-propynyl group, and 1-butynyl group are preferred.

[0083] In the general formula (1), l and m each independently represent an integer from 0 to 4. From the viewpoint of ensuring the thermal decomposability of the organosilicon compound, in the general formula (1), it is preferable that l represents 0 to 3 and n represents 1 to 4.

[0084] The deposition of the carbon composite with silicon as the core in step S3 is not particularly limited as long as it is a method capable of introducing the organosilicon compound satisfying the general formula (1). For example, it can be carried out by flowing evaporated tetramethylsilane (TMSI).

[0085] Specific examples of the organosilicon compound satisfying the general formula (1) include tetramethylsilane, trimethylvinylsilane, dimethyldivinylsilane, methyltrivinylsilane, tetravinylsilane, trimethylethynylsilane, diethynyldimethylsilane, methyltriethynylsilane, tetraethynylsilane, ethynyltrivinylsilane, diethynyldivinylsilane, triethynylvinylsilane, tetrapropylsilane, ethynyldimethylvinylsilane, trimethyl(1-propenyl)silane, dimethyldi(1-propenyl)silane, methyltri(1-propenyl)silane, tetra(1-propenyl)silane, trimethyl(1-propynyl)silane, dimethyldi(1-propynyl)silane, methyltri(1-propynyl)silane, tetra(1-propynyl)silane, trimethyl(1-butynyl)silane, dimethyldi(1-butynyl)silane, methyltri(1-butynyl)silane, tetra(1-butynyl)silane, and the like.

[0086] The deposition time of the carbon composite with silicon as the core in step S3 is not particularly limited and may be 10 minutes or more and 10 hours or less. For example, it can be 2 hours. Here, when flowing evaporated tetramethylsilane (TMSI) for the introduction of the organosilicon compound, it can also be diluted with a carrier gas such as hydrogen. The dilution can be, for example, 5 to 50 times, and preferably 10 to 30 times. This dilution ratio can typically be 20 times.

[0087] Next, the material in which silicon and a carbon composite with silicon as the core are deposited inside the porous carbon structure is cooled to 50°C or lower (step S4). In this step, for example, it is preferable to cool while flowing nitrogen gas. With this cooling, for example, it can be cooled to room temperature.

[0088] After the above cooling (step S4), next, while adjusting so that the temperature of the material in which silicon is deposited inside the porous carbon structure remains at 50°C or lower, oxygen diluted with nitrogen gas is introduced into the material in which silicon and a carbon composite with silicon as the core are deposited inside the porous carbon structure, thereby changing at least a part of the silicon into low-valence nanosilicon oxide (step S5). By this step, an Si-O bond can be formed. Note that the temperature to be maintained here is more preferably 35°C or lower.

[0089] The dilution of oxygen with nitrogen can be, for example, 5 to 50 times, preferably 10 to 30 times. This dilution ratio can typically be 20 times.

[0090] Also, in this step S5, if the internal temperature rises and exceeds 50°C, silicon dioxide is partially generated, which is not preferable as the negative electrode active material. Therefore, in this step S5, it is necessary to adjust so that the temperature of the material remains at 50°C or lower. Further, the material temperature in step S5 is preferably, for example, 25°C or higher, more preferably 30°C or higher, in order to facilitate the progress of the oxidation reaction and the formation of Si-O bonds.

[0091] The oxidation time (flow time of oxygen diluted with nitrogen gas) in this step S5 can be, for example, 30 minutes or longer and 5 hours or shorter, preferably 1 hour or longer and 3 hours or shorter. Also, after the flow of oxygen diluted with nitrogen gas, it is possible to switch to the flow of nitrogen gas and further cool. The flow of only nitrogen gas can be, for example, 30 minutes or longer and 2 hours or shorter.

[0092] In the oxidation by the flow of oxygen diluted with the nitrogen gas, it is preferable that the low-valence nano silicon oxide dispersed in the porous carbon structure is adjusted so that x increases from the center to the surface layer of the porous carbon structure. In the porous carbon structure, the pore structure tends to have a large cross-sectional area on the particle surface and a small cross-sectional area toward the particle interior. Therefore, by performing oxidation by the flow of oxygen diluted with nitrogen gas as in the present invention, it naturally tends to increase x from the center to the surface layer of the porous carbon structure.

[0093] Thereafter, the material is taken out from the storage container. Through the above steps, a carbon composite with silicon as the core is dispersed in at least the surface layer portion inside the porous carbon structure, and negative electrode active material particles in which low-valence nano silicon oxide is dispersed can be produced in a deeper layer portion.

[0094] When manufacturing the negative electrode active material in this way, it is preferable to adjust the deposition amount of silicon and the degree of oxidation so that the ratio of the porous carbon structure in the entire negative electrode active material particles is 38% by mass or more and 63% by mass or less.

[0095] <Lithium Ion Secondary 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 described.

[0096] [Configuration of Laminate Film Type Secondary Battery] The laminate film type lithium ion secondary battery 30 shown in FIG. 2 mainly has a wound electrode body 31 housed inside a sheet-like exterior member 35. This wound electrode body 31 has a separator between the positive electrode and the negative electrode and is wound. There is also a case where a separator is provided between the positive electrode and the negative electrode and a laminate is housed. 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 peripheral portion of the electrode body is protected by a protective tape.

[0097] The positive and negative electrodes 32 and 33 are led out in one direction from the inside to the outside of the exterior member 35, for example. The positive electrode lead 32 is formed of a conductive material such as aluminum, and the negative electrode lead 33 is formed of a conductive material such as nickel or copper.

[0098] The exterior member 35 is, for example, a laminated film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order. In this laminated film, the outer peripheral edges of the fusion layers of the two films are fused or bonded together with an adhesive or the like so that the fusion layer faces the electrode body 31. The fusion part is a film such as polyethylene or polypropylene, and the metal part is aluminum foil or the like. The protective layer is, for example, nylon.

[0099] A close contact film 34 is inserted between the exterior member 35 and the positive and negative electrode leads to prevent the intrusion of outside air. This material is, for example, polyethylene, polypropylene, or a polyolefin resin.

[0100] The positive electrode has a positive electrode active material layer on both sides or one side of the positive electrode current collector, for example, in the same manner as the negative electrode 10 in FIG. 1.

[0101] The positive electrode current collector is formed of a conductive material such as aluminum, for example.

[0102] The positive electrode active material layer contains any one or more of positive electrode materials capable of occluding and releasing lithium ions, and may contain other materials such as a positive electrode binder, a positive electrode conductive aid, and a dispersant according to the design. In this case, the details regarding the positive electrode binder and the positive electrode conductive aid are the same as those of the negative electrode binder and the negative electrode conductive aid already described, for example.

[0103] As the positive electrode material, a lithium-containing compound is desirable. Examples of this lithium-containing compound include a composite oxide composed of lithium and a transition metal element, or a phosphate compound having lithium and a transition metal element. Among these positive electrode materials, a compound having at least one or more of nickel, iron, manganese, and cobalt is preferable. As these chemical formulas, for example, Lix M1O2 or Li y It is represented by M2PO4. In the formula, M1 and M2 represent at least one transition metal element. The values of x and y are different depending on the charge and discharge state of the battery, but are generally represented by 0.05 ≦ x ≦ 1.10 and 0.05 ≦ y ≦ 1.10.

[0104] 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).

[0105] Examples of the phosphate compound having lithium and a transition metal element include, for example, lithium iron phosphate compound (LiFePO4) or lithium iron manganese phosphate compound (LiFe 1-u Mn u PO4 (0 < u < 1)), etc. By using these positive electrode materials, a high battery capacity can be obtained and excellent cycle characteristics can also be obtained.

[0106] [Negative electrode] The negative electrode has the same configuration as the negative electrode 10 for the lithium ion secondary battery shown in FIG. 1 described above. For example, it has negative electrode active material layers on both sides of the current collector. It is preferable that the negative electrode charging capacity is larger than the electric capacity obtained from the positive electrode active material agent (charging capacity as a battery). Thereby, precipitation of lithium metal on the negative electrode can be suppressed.

[0107] 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 is provided with a region where there is no opposing positive electrode active material layer. This is for performing a stable battery design.

[0108] In the region where the above-mentioned 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 thus the composition of the negative electrode active material can be accurately investigated with good reproducibility without depending on the presence or absence of charge and discharge.

[0109] [Separator] The separator isolates the positive electrode and the negative electrode, prevents current short-circuit due to contact between the two electrodes, and allows lithium ions to pass through. This separator is formed of, for example, a porous membrane made of a synthetic resin or a ceramic, and may have a laminated structure in which two or more kinds of porous membranes are laminated. Examples of the synthetic resin include polytetrafluoroethylene, polypropylene, polyethylene, and the like.

[0110] [Electrolyte] At least a part of the active material layer or the separator is impregnated with a liquid electrolyte (electrolyte solution). This electrolyte solution has an electrolyte salt dissolved in a solvent and may contain other materials such as additives.

[0111] As the solvent, for example, a non-aqueous solvent can be used. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, 1,2-dimethoxyethane, or tetrahydrofuran. Among these, it is desirable to use at least one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. This is because better characteristics can be obtained. Also in this case, by combining a high-viscosity solvent such as ethylene carbonate and propylene carbonate with a low-viscosity solvent such as dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate, more excellent characteristics can be obtained. This is because the dissociation property and ion mobility of the electrolyte salt are improved.

[0112] When using an alloy-based negative electrode, it is particularly desirable that the solvent contains at least one of a halogenated chain carbonate ester or a halogenated cyclic carbonate ester. Thereby, during charge and discharge, particularly during charging, a stable film is formed on the surface of the negative electrode active material. Here, the halogenated chain carbonate ester is a chain carbonate ester having a halogen as a constituent element (where at least one hydrogen is substituted by a halogen). Also, the halogenated cyclic carbonate ester is a cyclic carbonate ester having a halogen as a constituent element (that is, at least one hydrogen is substituted by a halogen).

[0113] The type of halogen is not particularly limited, but fluorine is preferred. This is because it forms a better-quality film than other halogens. Also, the larger the number of halogens, the more desirable. This is because the resulting film is more stable and the decomposition reaction of the electrolyte is reduced.

[0114] Examples of the halogenated chain carbonate ester include fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, etc. Examples of the halogenated cyclic carbonate ester include 4-fluoro-1,3-dioxolan-2-one, 4,5-difluoro-1,3-dioxolan-2-one, etc.

[0115] As a solvent additive, it is preferably included an unsaturated carbon-bonded cyclic carbonate ester. This is because a stable film is formed on the negative electrode surface during charge and discharge, and the decomposition reaction of the electrolyte can be suppressed. Examples of the unsaturated carbon-bonded cyclic carbonate ester include vinylene carbonate or vinyl ethylene carbonate, etc.

[0116] Also, as a solvent additive, it is preferably included sultone (cyclic sulfonic acid ester). This is because the chemical stability of the battery is improved. Examples of sultone include propane sultone, propene sultone.

[0117] Furthermore, the solvent preferably contains an acid anhydride, because the chemical stability of the electrolytic solution is improved. Examples of the acid anhydride include propane disulfonic anhydride.

[0118] The electrolyte salt can contain, for example, any one or more of light metal salts such as lithium salts. Examples of the lithium salt include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), and the like.

[0119] The content of the electrolyte salt is preferably 0.5 mol / kg or more and 2.5 mol / kg or less with respect to the solvent, because high ionic conductivity can be obtained.

Examples

[0120] Hereinafter, the present invention will be described more specifically by showing Examples and Comparative Examples of the present invention, but the present invention is not limited to these Examples.

[0121] (Example 1) The negative electrode active material was produced by the following procedure, and further, the laminated film type lithium ion secondary battery 30 shown in FIG. 2 was produced.

[0122] The negative electrode active material was produced as follows. First, the surface area (BET specific surface area) was 2483 m 2 / g, and the pore volume was 1.35 cm 3 / g, a porous carbon material (porous carbon structure) with a particle size (D50) = 11 μm and an IUPAC classification of type I was prepared. This porous carbon material was stored in a vacuum container and evacuated to -90 kPa. Next, the pressure was restored with nitrogen, and while nitrogen was flowing, it was heated to 400 °C using an external heater. After 30 minutes of heating, the temperature was raised to 415 °C, monosilane gas was flowed, and deposition was carried out for 4 hours. Next, evaporated tetramethylsilane (TMSI) was flowed and deposition was carried out for 2 hours. Then, it was cooled to room temperature while flowing nitrogen gas. After the temperature was lowered to 25 °C, oxygen diluted 20-fold with nitrogen was introduced, and the material temperature was adjusted to be 50 °C or lower to form Si-O bonds. Next, nitrogen containing oxygen was flowed for 2 hours, and when the material temperature reached 30 °C or lower, it was switched to nitrogen gas, flowed for 60 minutes, and then the material was taken out from the storage container to be used as a negative electrode active material.

[0123] [Fabrication of Negative Electrode] The negative electrode active material (active material containing CVD-SiOx-C) prepared as described above, graphite, conductive aid 1 (carbon nanotubes, CNT), conductive aid 2 (carbon fine particles with a median diameter of about 50 nm), sodium polyacrylate, and carboxymethyl cellulose (hereinafter referred to as CMC) were mixed at a dry mass ratio of 9.3:83.7:1:1:4:1, and then diluted with pure water to obtain a negative electrode binder slurry.

[0124] Also, as the negative electrode current collector, an electrolytic copper foil with a thickness of 15 μm was used. This electrolytic copper foil contained carbon and sulfur at concentrations of 70 ppm by mass, respectively. Finally, the negative electrode binder slurry was applied to the negative electrode current collector and dried in a vacuum atmosphere at 100 °C for 1 hour. The deposition amount (also referred to as areal density) of the negative electrode active material layer per unit area on one side of the dried negative electrode was 7.0 mg / cm 2 was.

[0125] [Assembly of Coin Cell for Testing] Next, after mixing the solvents ethylene carbonate (EC) and dimethyl carbonate (DMC), the electrolyte salt (lithium hexafluorophosphate: LiPF6) was dissolved to prepare an electrolyte solution. In this case, the composition of the solvent was EC:DMC = 30:70 by volume ratio, and the content of the electrolyte salt was 1 mol / kg with respect 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.

[0126] Next, a coin cell was assembled as follows. First, a Li foil with a thickness of 1 mm was punched out to a diameter of 16 mm and attached to an aluminum clad.

[0127] Next, the previously obtained negative electrode was punched out to a diameter of 15 mm, and this was placed facing the Li foil attached to the aluminum clad through a separator. After injecting the electrolyte solution, a 2032 coin cell was fabricated.

[0128] [Measurement of Initial Efficiency] The initial efficiency was measured under the following conditions. First, for the fabricated coin cell for initial efficiency test, the charging rate was set to be equivalent to 0.03C, and charging (initial charge) was performed in the CCCV mode. The CV was 0V and the termination current was 0.04 mA. Next, the discharging rate was similarly set to 0.03C, and discharging (initial discharge) was performed with the discharge termination voltage set to 1.2V.

[0129] [Manufacture and Battery Evaluation of Lithium-Ion Secondary Battery] Based on the obtained initial data, the positive electrode was designed so that the utilization rate of the negative electrode would be 95%. The utilization rate was calculated based on the following formula from the capacities of the positive and negative electrodes obtained with the counter electrode Li. Utilization rate = (Positive electrode capacity - Negative electrode loss) / (Negative electrode capacity - Negative electrode loss) × 100 Based on this design, each lithium-ion secondary battery (lithium-ion secondary battery as shown in Figure 2) of the examples and comparative examples was manufactured. Battery evaluations were conducted for each lithium-ion secondary battery of the examples and comparative examples.

[0130] The cycle characteristics were examined as follows. First, for battery stabilization, charge and discharge were performed at 0.2C for 2 cycles in an atmosphere of 25°C, 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. Charge and discharge were performed at 0.7C for charging and 0.5C for discharging. The charging voltage was 4.3V, the discharge cut-off voltage was 2.5V, and the charging cut-off rate was 0.07C.

[0131] The types of carbon composites with silicon as the core were investigated by observing the structure of the porous carbon in the negative electrode active material particles using SEM.

[0132] The results of each measurement are shown in Table 1. In Table 1, Comparative Examples 1-2 and Examples 2-9, which will be described later, are also shown together.

[0133]

Table 1

[0134] (Comparative Example 1) The negative electrode active material was produced in the same manner as in Example 1, except that the evaporated tetramethylsilane (TMSI) was not passed through. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0135] (Comparative Example 2) A porous carbon structure similar to that in Example 1 was prepared. Then, under the same conditions as in Example 1, using monosilane gas, amorphous silicon was formed from the surface layer to a location near the center of the porous carbon structure particles at 415°C. In this state, since Si-H bonds are included, in order to form Si-Si, the temperature was raised to 435°C to stabilize Si-Si. Next, the evaporated tetramethylsilane (TMSI) was passed through and deposited for 2 hours. After cooling to room temperature, the sample was taken out by opening to the atmosphere. By using this method, (different from the present invention) a material without low-valence silicon oxide compounds can be prototyped. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0136] (Example 2) The negative electrode active material was produced in the same manner as in Example 1, except that tetramethylsilane (TMSI) was changed to tetravinylsilane (TVSI). The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0137] (Example 3) The negative electrode active material was produced in the same manner as in Example 1, except that tetramethylsilane (TMSI) was changed to tetraethynylsilane (TESI). The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0138] (Example 4) The negative electrode active material was produced in the same manner as in Example 1, except that tetramethylsilane (TMSI) was changed to diethynyldimethylsilane (DEDMS). The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0139] (Example 5) The negative electrode active material was produced in the same manner as in Example 1, except that tetramethylsilane (TMSI) was changed to tetrapropylsilane (TPSI). The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0140] TMSI, TVSI, and DEDMS become a carbon composite containing carbon black with silicon as the core by thermal decomposition. Among the interiors of the porous carbon structures of Examples 1, 2, and 4, a carbon composite containing carbon black with silicon as the core is dispersed at least in the surface layer portion. The carbon composite containing carbon black with silicon as the core present inside or in the surface layer portion of the porous carbon structure plays a role in maintaining the contact between the negative electrode active material particles and the porous carbon. Therefore, the cycle characteristics of Examples 1, 2, and 4 are superior to those of Comparative Example 1.

[0141] TESI and DEDMS become a carbon composite containing acetylene black with silicon as the core by thermal decomposition. Among the interiors of the porous carbon structures of Examples 3 and 4, a carbon composite containing acetylene black with silicon as the core is dispersed at least in the surface layer part. The carbon composite containing acetylene black with silicon as the core present in the surface layer part of the porous carbon structure inactivates the low-valence silicon oxide exposed on the surface layer part of the porous carbon, thereby suppressing the excessive decomposition of the electrolyte. Therefore, the cycle characteristics of Examples 3 and 4 are superior to those of Comparative Example 1.

[0142] TPSI becomes a carbon composite containing carbon nanotubes with silicon as the core by thermal decomposition. Among the interiors of the porous carbon structures of Example 5, a carbon composite containing carbon nanotubes with silicon as the core is dispersed at least in the surface layer part. The carbon composite containing carbon nanotubes with silicon as the core present in the surface layer part of the porous carbon structure inactivates the low-valence silicon oxide exposed on the surface layer part of the porous carbon, thereby suppressing the excessive decomposition of the electrolyte. Therefore, the cycle characteristics of Example 5 are superior to those of Comparative Example 1.

[0143] (Examples 6, 7) For the porous carbon structure similar to that of Example 1, the conditions were changed as follows to produce a negative electrode active material. First, the temperature was raised to 415 °C, monosilane gas was passed, and deposition was carried out for 4 hours. Then, the temperature was raised to grow the grain size of Si 0+ . Next, after lowering the temperature to 415 °C, evaporated tetramethylsilane (TMSI) was passed and deposition was carried out for 2 hours. Then, it was cooled to room temperature while flowing nitrogen gas. After lowering the temperature to 25 °C, oxygen diluted 20 times with nitrogen was introduced, and the material temperature was adjusted to be 50 °C or lower to form Si-O bonds. Next, nitrogen containing oxygen was flowed for 2 hours, and when the material temperature reached 30 °C or lower, it was switched to nitrogen gas, flowed for 60 minutes, and then the material was taken out from the storage container to obtain a negative electrode active material.

[0144] Examples 6 and 7 have a larger grain size and a slightly lower 1000-cycle retention rate compared to Example 1, indicating that a smaller grain size is more preferable. Here, since a smaller grain size is closer to the amorphous structure, it is considered that the cycle characteristics improve as the structure approaches the amorphous structure.

[0145] (Examples 8 and 9) The negative electrode active material was produced in the same manner as in Example 1, except that the surface area, total pore volume, and IUPAC classification of the porous carbon structure prepared first were changed as shown in Table 1 above. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0146] The results of Examples 8 and 9 show that the 1000-cycle retention rate is slightly lower compared to Example 1. Therefore, a pore volume of 1 cm 3 / g or more and a BET specific surface area of 1400 m 2 / g or more are more preferable, and an IUPAC classification of type I is considered more desirable.

[0147] Fig. 4 shows the change in the depth direction measurement of the O1s region in the XPS measurement of the negative electrode active material in Example 1. As can be seen from Fig. 4, the O1s peak decreases in the depth direction from the surface, indicating that the oxygen concentration decreases from the surface toward the deeper layer side.

[0148] Fig. 5 shows the X-ray diffraction spectra of Examples 1, 2, 6, and 7. Peaks are observed near 2θ = 28° in all of these, and the crystallite size of silicon was calculated from this peak. The same applies to other examples and comparative examples.

[0149] This specification includes the following aspects. [1]: A negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure, and amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, Among the interior of the porous carbon structure, at least in the surface layer portion, a carbon composite with silicon as a core is dispersed. The low-valence nano-silicon oxide contains SiOx where x < 1.0. A negative electrode active material, wherein among the interior of the porous carbon structure, the low-valence nano-silicon oxide is dispersed in at least a deeper layer portion than the carbon composite with silicon as a core. [2]: The negative electrode active material according to [1] above, wherein the carbon composite with silicon as a core contains at least one of carbon black, acetylene black, and carbon nanotubes. [3]: The negative electrode active material according to [1] or [2] above, wherein the low-valence nano-silicon oxide is in a substantially zero-valent, monovalent, or divalent composite state. [4]: The negative electrode active material according to any one of [1] to [3] above, wherein the grain size of zero-valent Si constituting the low-valence nano-silicon oxide, calculated using Scherrer's formula from the peaks measured by X-ray diffraction measurement of the negative electrode active material particles, is in the range of 1 nm to 5 nm. [5]: The porous carbon structure is predominantly of type I in the IUPAC classification, its surface area is 1400 m 2 / g or more, and the pore volume is 1 cm 2 / g or more. The negative electrode active material according to any one of [1] to [4] above. [6]: The negative electrode active material according to any one of [1] to [5] above, wherein the low-valence nano-silicon oxide dispersed in the porous carbon structure increases in x from the center to the surface layer of the porous carbon structure. [7]: A method for manufacturing a negative electrode active material having negative electrode active material particles, A step of preparing a porous carbon structure, A step of depositing silicon inside the porous carbon structure by flowing monosilane gas under heating on the porous carbon structure, After the deposition of silicon by the monosilane gas, under heating on the porous carbon structure, the following general formula (1) Si(R 1 ) l (R2 ) m (R 3 ) 4-l-m (1) (In general formula (1), R 1 is an alkyl group having 1 to 20 carbon atoms, and R 2 is an alkenyl group having 2 to 20 carbon atoms, and R 3 is an alkynyl group having 2 to 20 carbon atoms. Also, l and m each independently represent an integer from 0 to 4.) By introducing an organosilicon compound represented by the formula, depositing a carbon composite with silicon as a nucleus on at least the surface layer portion inside the structure of the porous carbon; After the deposition of the carbon composite with silicon as a nucleus, cooling the structure of the porous carbon to 50°C or lower; After the cooling, while adjusting to maintain the temperature of the structure of the porous carbon at 50°C or lower, introducing oxygen diluted with nitrogen gas into the structure of the porous carbon to change at least a part of the silicon inside the structure of the porous carbon into a low-valence nano silicon oxide; It has, thereby, dispersing the carbon composite with silicon as a nucleus in at least the surface layer portion inside the structure of the porous carbon, and producing negative electrode active material particles in which the low-valence nano silicon oxide is dispersed in at least a deeper layer portion than the carbon composite with silicon as a nucleus inside the structure of the porous carbon. A method for producing a negative electrode active material, characterized by the above.

[0150] Note that the present invention is not limited to the above embodiments. The above embodiments are examples, and any structure 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 Reference Numerals

[0151] 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 aluminum lead), 33… Negative electrode lead (negative nickel lead), 34… Adhesive film, 35… Exterior member. S1, S2, S3, S4, S5… Steps.

Claims

1. A negative electrode active material having negative electrode active material particles, The negative electrode active material particles include a porous carbon structure, Amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, a carbon composite having a silicon core is dispersed in at least a surface layer portion of the inside of the porous carbon structure, The low valence nano silicon oxide comprises SiOx:x<1.0, A negative electrode active material characterized in that the low valence nano silicon oxide is dispersed at least in a deeper portion of the inside of the porous carbon structure than the silicon-cored carbon composite.

2. 2. The negative electrode active material according to claim 1, wherein the silicon-core carbon composite contains at least one of carbon black, acetylene black, and carbon nanotubes.

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

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

5. The porous carbon structure is predominantly Type I in the IUPAC classification and has a surface area of ​​1400 m ２ / g or more, pore volume is 1 cm ２ 2. The negative electrode active material according to claim 1, wherein the molecular weight of the negative electrode active material is 1 / g or more.

6. The negative electrode active material according to claim 1, characterized in that the low valence nanosilicon oxide dispersed in the porous carbon structure has x increasing from the center of the porous carbon structure to the surface layer.

7. A method for producing a negative electrode active material having negative electrode active material particles, comprising: Providing a porous carbon structure; depositing silicon inside the porous carbon structure by flowing monosilane gas through the porous carbon structure under heating; After the deposition of silicon using the monosilane gas, the porous carbon structure is heated to form a compound represented by the following general formula (1): Si(R １ ) ｌ (R ２ ) ｍ (R ３ ) ４－ｌ－ｍ (1) (In general formula (1), R １ is an alkyl group having 1 to 20 carbon atoms; R ２ is an alkenyl group having 2 to 20 carbon atoms, R ３ is an alkynyl group having 2 to 20 carbon atoms. Furthermore, l and m each independently represent an integer of 0 to 4. depositing a carbon composite having a silicon core on at least a surface layer portion of the inside of the porous carbon structure by introducing an organosilicon compound represented by the formula: cooling the porous carbon structure to below 50° C. after deposition of the silicon-nucleated carbon composite; After the cooling, while adjusting the temperature of the porous carbon structure to be 50° C. or less, oxygen diluted with nitrogen gas is introduced into the porous carbon structure to convert at least a part of the silicon inside the porous carbon structure into low-valence nanosilicon oxide; and thereby dispersing the silicon-cored carbon composite in at least a surface layer portion of the interior of the porous carbon structure, and producing negative electrode active material particles in which the low valent nano silicon oxide is dispersed in at least a layer deeper than the silicon-cored carbon composite in the interior of the porous carbon structure.

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