Negative electrode active material and manufacturing method for the same

Abstract

To provide a negative electrode active material capable of improving battery cycle characteristics and increasing capacity.SOLUTION: A negative electrode active material having negative electrode active material particles is characterized by the following. The negative electrode active material particles include a porous carbon structure. An amorphous low-valent nano-silicon oxide is dispersed inside the porous carbon structure. The low-valent nano-silicon oxide contains SiOx: x<1.0. At least a portion of the surface layer part of the low-valent nano-silicon oxide exposed on the surface of the porous carbon structure is coated with at least one of a metal oxide and a metal hydroxide.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 long life are strongly demanded. In response to such market demands, the development of secondary batteries that are particularly small, lightweight, and capable of obtaining a high energy density has been promoted. The application of this secondary battery is being studied 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 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 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, while further improvement in battery capacity is required from recent market demands. In order to improve the 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 materials as negative electrode active materials is being studied not only for elemental silicon but also for compounds typified by alloys and oxides. In addition, the shape of the active material is being 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 prone to cracking mainly in the vicinity of 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 a substance 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 electrolytic solution occurs on the new surface, and a film that is a decomposition product of the electrolytic solution is formed on the new surface, consuming the electrolytic solution. For this reason, the cycle characteristics are likely to deteriorate.

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

[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 (electron conductive material) 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, in order to improve the first charge / discharge efficiency, a nano composite containing Si phase, SiO2, and M y O metal oxide is used (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 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 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 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] Also, 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. Also, 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] 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 some 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 problematic aspect of silicon oxide and improvement is required. Therefore, Kim et al. used the Li pre-doping method to form Li silicate in advance to significantly improve the initial efficiency as a battery 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 to achieve improvement in the irreversible capacity (Patent Document 13).

[0015] On the other hand, the Li metal used for Li doping has extremely volatile upper and lower price limits depending on market conditions, and there are many problems when considered for industrialization. Therefore, by using silane gas in porous carbon to generate 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 promoted to have higher performance and more functions, and there is a demand for an increase in the battery capacity of lithium-ion secondary batteries, which are their main power sources. As one method of solving this problem, the development of a lithium-ion secondary battery having a negative electrode made mainly of a silicon material is desired.

[0019] In addition, lithium-ion secondary batteries using silicon materials are desired to have initial charge / discharge characteristics and cycle characteristics that are almost equivalent to those of lithium-ion secondary batteries using carbon-based active materials. Therefore, by using 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 silicon oxide is used, and by previously containing Li to generate Li silicate, the irreversible capacity, which is a demerit of silicon oxide, is reduced, and it has actually started to be put on the market. Even when a battery is prototyped by replacing 100% of the carbon negative electrode material with Li-SiO-C (Non-Patent Document 8) using Li for this silicon oxide, the capacity improvement only remains 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 when considering higher performance of small electronic devices (such as 5G) and increased driving range of electric vehicles.

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

[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 nanosilicon oxide is dispersed inside the structure of the porous carbon. The low-valence nanosilicon oxide includes SiOx where x < 1.0, and at least a part of the surface layer portion of the low-valence nanosilicon oxide exposed on the surface of the structure of the porous carbon is coated with at least one of a metal oxide and a metal hydroxide.

[0023] Since the negative electrode active material of the present invention has amorphous low-valence nanosilicon oxide dispersed inside the structure of the porous carbon, the adverse effect due to the expansion of the internal low-valence nanosilicon oxide can be reduced by the presence of the structure of the porous carbon. 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 at least a part of the surface layer portion of the low-valence nanosilicon oxide exposed on the surface of the structure of the porous carbon is coated with at least one of a metal oxide and a metal hydroxide, the decomposition of the electrolyte is suppressed. 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, the metal oxide and the metal hydroxide preferably contain at least one element among aluminum, magnesium, titanium, zirconium, calcium, and niobium.

[0025] If the metal oxide and the metal hydroxide contain such elements, the decomposition of the electrolyte can be surely suppressed, and the cycle characteristics can be improved.

[0026] Further, it is preferable that the thickness of the metal oxide and the metal hydroxide is 0.1 nm or more and 10 nm or less.

[0027] With metal oxides and metal hydroxides having such thicknesses, decomposition of the electrolytic solution can be more effectively suppressed, and cycle characteristics can be further improved.

[0028] Further, it is preferable that the low-valence nanosilicon oxide is substantially in a composite state of 0 valence, 1 valence, and 2 valence.

[0029] Thus, when the low-valence nanosilicon oxide is substantially in a composite state of 0 valence, 1 valence, and 2 valence, a lower irreversible capacity can be achieved.

[0030] Further, 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.

[0031] Thus, 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, decomposition of the electrolytic solution 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.

[0032] Further, the grain size of 0-valent Si constituting the low-valence nanosilicon 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.

[0033] Those having such a grain size of 0-valent Si, which is substantially an amorphous structure, are preferable.

[0034] Further, the porous carbon structure is predominantly of type I in the IUPAC classification, and its surface area is 1400 m2 1 cm / g or more, the pore volume is preferably 2 1 cm / g or more.

[0035] Since the porous carbon structure has such an IUPAC classification, surface area, and pore volume, it is possible to obtain a negative electrode active material that efficiently contains a larger amount of low-valence nanosilicon oxide. In particular, the type I structure in the IUPAC classification enables smooth formation of Si-O bonds after deposition.

[0036] The present invention also provides a method for producing a negative electrode active material having negative electrode active material particles, comprising the steps of preparing a porous carbon structure, flowing monosilane gas under heating onto the porous carbon structure to deposit silicon inside the porous carbon structure, cooling the material having silicon deposited inside the porous carbon structure to 50°C or lower, introducing oxygen diluted with nitrogen gas into the material having silicon deposited inside the porous carbon structure while maintaining the temperature of the material having silicon deposited inside the porous carbon structure at 50°C or lower so as to change at least a part of the silicon into low-valence nanosilicon oxide, and coating at least a part of the surface layer portion of the low-valence nanosilicon oxide with at least one of a metal oxide and a metal hydroxide.

[0037] 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 inside the porous carbon structure and at least a part of the surface layer portion of the low-valence nanosilicon oxide is coated with at least one of a metal oxide and a metal hydroxide can be produced simply and efficiently.

[0038] Further, it is preferable to coat at least a part of the surface layer of the low-valence nanosilicon oxide by forming at least one of the metal oxide and the metal hydroxide by hydrolysis and dehydration condensation of the metal alkoxide.

[0039] By using such hydrolysis and dehydration condensation of the metal alkoxide, the metal oxide and the metal hydroxide can be easily formed on the surface layer of the low-valence nanosilicon oxide.

Advantages of the Invention

[0040] Since the negative electrode active material of the present invention has an amorphous low-valence nanosilicon oxide dispersed inside the structure of the porous carbon, the presence of the structure of the 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 solution, so that it is possible to reduce the SEI deposited on the surface layer portion. Further, since at least a part of the surface layer of the low-valence nanosilicon oxide exposed on the surface of the structure of the porous carbon is coated with at least one of the metal oxide and the metal hydroxide, the decomposition of the electrolyte solution is suppressed. As a result, excessive decomposition of the electrolyte solution can be suppressed, and the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

[0041] 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 an amorphous low-valence nanosilicon oxide is dispersed inside the structure of the porous carbon and at least a part of the surface layer of the low-valence nanosilicon oxide is coated with at least one of the metal oxide and the metal hydroxide.

Brief Description of the Drawings

[0042]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0043] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited thereto.

[0044] As described above, as one method for increasing the battery capacity of a lithium-ion secondary battery, it has been considered to use, as the negative electrode of a lithium-ion secondary battery, a negative electrode using a low-valence nanosilicon oxide as a main material in a carbon structure. 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.

[0045] 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 arrived at the present invention.

[0046] In particular, in the case of CVD-Si-C as disclosed in Patent Documents 14 and 15, there was a problem 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 that a negative electrode active material capable of ensuring high-speed charging has been developed. In addition to suppressing the decomposition reaction of the electrolytic solution in the SiOx phase, at least a part of the surface layer portion of the low-valence nanosilicon oxide exposed on the surface of the porous carbon structure is coated with at least one of a metal oxide and a metal hydroxide, thereby developing a negative electrode active material that further suppresses the decomposition of the electrolytic solution.

[0047] [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, 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. The low-valence nanosilicon oxide contains SiOx: x < 1.0, and at least a part of the surface layer portion of the low-valence nanosilicon oxide exposed on the surface of the porous carbon structure is coated with at least one of a metal oxide and a metal hydroxide.

[0048] Such a negative electrode active material has amorphous low-valence nanosilicon oxide dispersed inside the porous carbon structure. Therefore, due to the presence of the porous carbon structure, the adverse effects caused by the expansion of the internal low-valence nanosilicon oxide can be reduced. Also, general SiO is Si 4+Although it becomes an irreversible component, in the negative electrode active material of the present invention, since it includes each state of SiOx: x < 1.0, 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 solution, it becomes possible to reduce the SEI (Solid Electrolyte Interphase) deposited on the surface layer portion. Further, at least a part of the surface layer portion of the low-valence number nanosilicon oxide exposed on the surface of the porous carbon structure is coated with at least one of a metal oxide and a metal hydroxide, whereby the decomposition of the electrolyte solution is suppressed. As a result, excessive decomposition of the electrolyte solution can be suppressed, and the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

[0049] That is, the negative electrode active material of the present invention is Si-Ox oxidized with a siloxane bond in the Si phase in order to suppress the decomposition reaction of the electrolyte solution, which is insufficient for CVD-Si-C produced 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. Also, since the Si-O bond can suppress the decomposition of the electrolyte solution, it becomes possible to reduce the SEI (Solid Electrolyte Interphase) deposited on the surface layer portion of CVD-Si-C. Since the main site contributing to the main charge and discharge of this material is the low-valence number nanosilicon oxide, it can be defined as CVD-SiOx-C with respect to CVD-Si-C. In this way, the produced active material can have a high energy density and high-rate chargeability while maintaining the cycle characteristics of the battery. Further, at least a part of the surface layer portion of the low-valence number nanosilicon oxide exposed on the surface of the porous carbon structure is coated with at least one of a metal oxide and a metal hydroxide, whereby the decomposition of the electrolyte solution is suppressed. As a result, excessive decomposition of the electrolyte solution can be suppressed, and the cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

[0050] The grain size of the low-valence nano silicon 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 Tecnai G2 F20 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.

[0051] When the particle size of the low-valence nano silicon oxide increases, it becomes difficult to form Si-O bonds. As will be described later, in the method for producing a 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 into SiO2. Or at that time, the inside of the phase may be in a state of Si similar to CVD-Si-C described above. In addition, there is also a part of zero-valent amorphous Si inside the low-valence nano silicon oxide grains, 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. On the contrary, when the crystallinity is high, the electrolyte decomposition reaction peculiar to Si is promoted, so the cycle characteristics deteriorate.

[0052] Further, the metal oxide and the metal hydroxide preferably contain at least one element among aluminum, magnesium, titanium, zirconium, calcium, and niobium.

[0053] If it is a metal oxide and a metal hydroxide containing such an element, the decomposition of the electrolyte can be surely suppressed, and the cycle characteristics can be improved.

[0054] Further, the thickness of the metal oxide and the metal hydroxide is preferably 0.1 nm or more and 10 nm or less.

[0055] For metal oxides and metal hydroxides with such thickness, the decomposition of the electrolyte can be further suppressed, and the cycle characteristics can be further improved.

[0056] In addition, in the negative electrode active material of the present invention, it is preferable that the low-valence nanosilicon oxide is in a substantially composite state of 0 valence, 1 valence, and 2 valence. Thus, by having the low-valence nanosilicon oxide in a substantially composite state of 0 valence, 1 valence, and 2 valence, a lower irreversible capacity can be achieved.

[0057] The negative electrode active material of the present invention has a low-valence nanosilicon 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 nanosilicon oxide is dominated by compounds with a valence of 1 to 2, and the low-valence nanosilicon oxide phase existing inside the pores may contain a microcrystalline phase of 0-valent Si.

[0058] The valence of the low-valence nanosilicon oxide can be quantified by NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0059] 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: 4mm HR-MAS rotor 50 μL, · Sample rotation speed: 10 kHz, · Measurement environmental temperature: 25 °C

[0060] 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: 100 μm, ·Ar ion gun sputtering conditions: 0.5 kV, 2 mm × 2 mm.

[0061] In the negative electrode active material of the present invention, the grain size of zero-valent Si constituting the low-valence nanosilicon oxide calculated using the Scherrer equation 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.

[0062] 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 · Equipment: 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”

[0063] In addition, in the negative electrode active material of the present invention, it is preferable that the low-valence nano silicon 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, in order to disperse the low-valence nano silicon oxide 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, while the oxidation ratio of silicon is small inside the active material, so that the battery capacity can be further increased.

[0064] <Negative electrode for non-aqueous electrolyte secondary battery> 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.

[0065] [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 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.

[0066] [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).

[0067] 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, it has the 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.

[0068] Also, the surface of the negative electrode current collector 11 is preferably roughened. 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.

[0069] [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, for battery design, it may also contain other materials such as a thickening agent (also referred to as a "binder" or "adhesive") and a conductive auxiliary agent.

[0070] [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.

[0071] First, a method for manufacturing a negative electrode active material initially contained in a negative electrode will be described. The method for manufacturing a negative electrode active material of the present invention is a method for manufacturing a negative electrode active material having negative electrode active material particles, including 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, cooling the material with silicon deposited inside the porous carbon structure to 50°C or lower, introducing oxygen diluted with nitrogen gas into the material with silicon deposited inside the porous carbon structure while adjusting to maintain the temperature of the material with silicon deposited inside the porous carbon structure at 50°C or lower after cooling to change at least a part of the silicon into a low-valence nanosilicon oxide, and coating at least a part of the surface layer portion of the low-valence nanosilicon oxide with at least one of a metal oxide and a metal hydroxide.

[0072] Referring to Steps S1 to S5 in FIG. 3, each step of the method for manufacturing a negative electrode active material will be described.

[0073] First, a porous carbon structure is prepared (Step S1). The porous carbon structure prepared here preferably has carbon-carbon double bonds at least in part.

[0074] Also, the porous carbon structure prepared here is predominantly of 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, more efficient deposition of silicon can be achieved. Regarding the IUPAC classification, surface area, and pore volume, the following measurement methods can be used. · Measure the specific surface area / pore distribution by the constant volume method based on the gas adsorption method using Shimadzu TriStar II Plus. The conditions are as follows. · Gas used: Nitrogen · Environment: Under liquid nitrogen · Pressure operating range: P / P0 · Adsorption 0 to 0.998 · Desorption 0.998 to 0.10 · Pretreatment: Vacuum at 200 °C for 1 hour

[0075] 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).

[0076] Note that after step S1 and before step S2, it is preferable to store the porous carbon structure in a vacuum container 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 to about 350 to 450 °C using an external heater 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.

[0077] The silicon deposition in step S2 can be performed, for example, by flowing monosilane gas at about 400 °C to 500 °C. The deposition time can be, for example, 30 minutes to 10 hours.

[0078] Next, the material with silicon deposited inside the porous carbon structure is cooled to 50 °C or lower (step S3). In this step, for example, it is preferable to cool while flowing nitrogen gas. In this cooling, for example, it can be cooled to room temperature.

[0079] After the above cooling (step S3), next, while adjusting so that the temperature of the material in which silicon is deposited inside the porous carbon structure is maintained at 50°C or lower, oxygen diluted with nitrogen gas is introduced into the material in which silicon is deposited inside the porous carbon structure, whereby at least a part of the silicon is changed to a low-valence nano silicon oxide (step S4). 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.

[0080] 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.

[0081] Also, in this step S4, 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 S4, it is necessary to adjust so that the temperature of the material is maintained at 50°C or lower. Further, the material temperature in step S4 is preferably, for example, 25°C or higher and more preferably 30°C or higher in order to facilitate the progress of the oxidation reaction and the formation of the Si—O bond.

[0082] The oxidation time (flow time of oxygen diluted with nitrogen gas) in this step S4 can be, for example, 30 minutes or longer and 5 hours or shorter, preferably 1 hour or longer and 3 hours or shorter. Further, after the flow of oxygen diluted with nitrogen gas, the flow can be switched to the flow of nitrogen gas for further cooling. The flow of only nitrogen gas can be, for example, 30 minutes or longer and 2 hours or shorter.

[0083] In the oxidation by the flow of oxygen diluted with the nitrogen gas, it is preferable that the low-valence nanosilicon 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 easily becomes the case where x increases from the center to the surface layer of the porous carbon structure naturally.

[0084] After step S4, next, at least a part of the surface layer portion of the low-valence nanosilicon oxide is coated with at least one of a metal oxide and a metal hydroxide (step S5). The method of this coating is not particularly limited, but it is preferable to coat at least a part of the surface layer portion of the low-valence nanosilicon oxide by forming at least one of a metal oxide and a metal hydroxide by hydrolysis and dehydration condensation of a metal alkoxide on at least a part of the surface layer portion of the low-valence nanosilicon oxide.

[0085] By using such hydrolysis and dehydration condensation of a metal alkoxide, a metal oxide and a metal hydroxide can be easily formed on the surface layer portion of the low-valence nanosilicon oxide.

[0086] Furthermore, as a specific example of the forming method, diethyl ether in which 1.0 to 7.0% by mass of aluminum-i-propoxide is dissolved with respect to the porous carbon material is added to a storage container, heated at 400°C for 2 hours in a state where nitrogen is flowing, and then cooled to room temperature. After that, if the material is taken out from the storage container, aluminum oxide and / or aluminum hydroxide can be easily formed. Note that it may be added directly to the storage container without dissolving it in diethyl ether.

[0087] Subsequently, the material is taken out from the storage container. Through the above steps, amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, and at least a part of the surface layer of the low-valence nanosilicon oxide is coated with at least one of a metal oxide and a metal hydroxide, so that negative electrode active material particles can be produced.

[0088] 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 proportion 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.

[0089] <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.

[0090] [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.

[0091] The positive and negative electrode leads 32 and 33 are led out, for example, in one direction from the inside of the exterior member 35 to the outside. The positive electrode lead 32 is formed of a conductive material such as aluminum, and the negative electrode lead 33 is formed of a conductive material such as nickel or copper.

[0092] The exterior member 35 is, for example, a laminated film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order. The outer peripheral edge portions of the fusion layers of the two films are fused or bonded together with an adhesive or the like so that the fusion layer of the laminated film faces the electrode body 31. The fusion part is, for example, a film such as polyethylene or polypropylene, and the metal part is an aluminum foil or the like. The protective layer is, for example, nylon or the like.

[0093] A close-contact film 34 is inserted between the exterior member 35 and the positive and negative electrode leads to prevent outside air from entering. This material is, for example, polyethylene, polypropylene, or a polyolefin resin.

[0094] The positive electrode has, for example, 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 FIG. 1.

[0095] The positive electrode current collector is formed of a conductive material such as aluminum or the like.

[0096] The positive electrode active material layer contains any one or two or more kinds 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 assistant, and a dispersant according to the design. In this case, the details regarding the positive electrode binder and the positive electrode conductive assistant are the same as those of the negative electrode binder and the negative electrode conductive assistant described above, for example.

[0097] 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, Li x M1O2 or Li y M2PO4. In the formula, M1 and M2 represent at least one or more transition metal elements. The values of x and y represent different values 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.

[0098] 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), lithium nickel cobalt composite oxide, and the like. Examples of the lithium nickel cobalt composite oxide include lithium nickel cobalt aluminum composite oxide (NCA), lithium nickel cobalt manganese composite oxide (NCM), and the like.

[0099] 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)), and the like. By using these cathode materials, a high battery capacity can be obtained, and excellent cycle characteristics can also be obtained.

[0100] [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, it has a negative electrode active material layer 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.

[0101] 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.

[0102] 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 and the like can be accurately examined with good reproducibility without depending on the presence or absence of charge and discharge.

[0103] [Separator] The separator separates 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 synthetic resin or ceramic, and may have a laminated structure in which two or more porous membranes are laminated. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, polyethylene, and the like.

[0104] [Electrolyte solution] 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.

[0105] As the solvent, for example, a non - aqueous solvent can be used. Examples of non - aqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, 1,2 - dimethoxyethane, 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 of the electrolyte salt and the ion mobility are improved.

[0106] 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 (at least one hydrogen is substituted by a halogen). Further, 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).

[0107] 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.

[0108] 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.

[0109] As a solvent additive, it is preferable to contain 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.

[0110] Also, as a solvent additive, it is preferable to contain sultone (cyclic sulfonic acid ester). This is because the chemical stability of the battery is improved. Examples of sultone include propane sultone, propene sultone.

[0111] Furthermore, the solvent preferably contains an acid anhydride, because the chemical stability of the electrolyte solution is improved. Examples of the acid anhydride include propane disulfonic anhydride.

[0112] 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.

[0113] 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

[0114] 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.

[0115] (Example 1) The negative electrode active material was produced by the following procedure, and further, the laminate film type lithium ion secondary battery 30 shown in FIG. 2 was produced.

[0116] The negative electrode active material was produced as follows. First, the surface area (BET specific surface area) was 2433 m 2 / g, and the pore volume was 1.34 cm 3 / g, a porous carbon material (porous carbon structure) with a particle size (D50) = 11 μm and IUPAC classification type I was prepared. This porous carbon material was stored in a vacuum container and evacuated to -90 kPa. Next, it was repressurized with nitrogen and heated to 400 °C using an external heater while nitrogen was flowing. After 30 minutes of heating, the temperature was raised to 415 °C, monosilane gas was flowed, and deposition was carried out for 4 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 Si-O bond was formed by adjusting the material temperature to 50 °C or lower. 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 and flowed for 60 minutes. Subsequently, diethyl ether in which 1.0 mass% of aluminum-i-propoxide was dissolved with respect to the porous carbon material was added to the storage container, and it was heated at 400 °C for 2 hours while flowing nitrogen. After cooling to room temperature, the material was taken out from the storage container and used as a negative electrode active material.

[0117] [Fabrication of Negative Electrode] The negative electrode active material (active material containing CVD-SiOx-C) prepared as described above, graphite, conductive assistant 1 (carbon nanotube, CNT), conductive assistant 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.

[0118] 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 mass ppm, 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 It was.

[0119] [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 set to EC:DMC = 30:70 by volume ratio, and the content of the electrolyte salt was set to 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.

[0120] 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.

[0121] Next, the previously obtained negative electrode was punched out to a diameter of 15 mm, and this was faced with the Li foil attached to the aluminum clad through a separator. After injecting the electrolyte solution, a 2032 coin cell was fabricated.

[0122] [Measurement of initial efficiency] The initial efficiency was measured under the following conditions. First, for the fabricated coin cell for initial efficiency test, the charge rate was set to the equivalent of 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 discharge rate was similarly set to 0.03C, and CC discharge (initial discharge) was performed with the discharge termination voltage set to 1.2V.

[0123] When examining the initial charge-discharge characteristics, the initial efficiency (sometimes also referred to as the initial efficiency below) was calculated. The initial efficiency was calculated from the formula: Initial efficiency (%) = (Initial discharge capacity / Initial charge capacity) × 100.

[0124] [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, lithium-ion secondary batteries (lithium-ion secondary batteries as shown in FIG. 2) of each of the examples and comparative examples were manufactured. Battery evaluations were performed on the lithium-ion secondary batteries of each of the examples and comparative examples.

[0125] Regarding the cycle characteristics, they 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 a charge of 0.7C and a discharge of 0.5C. The charge voltage was 4.3V, the discharge cut-off voltage was 2.5V, and the charge cut-off rate was 0.07C.

[0126] The thickness of the aluminum oxide and / or aluminum hydroxide was examined by disassembling the battery after the cycle test and observing the structure of the porous carbon in the negative electrode active material particles with TEM.

[0127] The results of each measurement are shown in Table 1. In Table 1, Comparative Examples 1 and 2 and Examples 2 to 20, which will be described later, are also shown together.

[0128]

Table 1

[0129] (Comparative Example 1) Diethyl ether in which 1.0% by mass of aluminum-i-propoxide was dissolved was added to the porous carbon material, and a step of heating at 400°C for 2 hours in a state where nitrogen was flowing was not performed. The negative electrode active material was produced in the same manner as in Example 1. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0130] In the negative electrode active material of Comparative Example 1, the low-valence nanosilicon oxide exposed on the surface of the porous carbon structure was not coated with aluminum oxide and / or aluminum hydroxide, so the cycle characteristics deteriorated.

[0131] (Comparative Example 2) A porous carbon structure similar to that of Example 1 was prepared. Subsequently, under the same conditions as in Example 1, using monosilane gas, amorphous silicon was formed at 415 °C from the surface layer of the porous carbon structure particles to a location near the center. 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. Then, it was cooled to room temperature while flowing nitrogen gas. Subsequently, diethyl ether in which 1.0% by mass of aluminum-i-propoxide was dissolved with respect to the porous carbon material was added to the storage container, and it was heated at 400 °C for 2 hours while flowing nitrogen. After cooling to room temperature, it was cooled to room temperature, and the sample was taken out with the atmosphere open. By using this method, (different from the present invention) a material without a low-valence silicon oxide compound can be prototyped. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0132] Since the negative electrode active material of Comparative Example 2 has no low-valence silicon oxide compound, the capacity and the initial efficiency are high, but the reactivity with the electrolyte is high, and the battery cycle characteristics deteriorate.

[0133] (Examples 2 to 13) The negative electrode active material was produced in the same manner as in Example 1, except that the starting material (metal alkoxide) for forming the metal oxide and / or metal hydroxide was changed as shown in Table 1 above. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0134] (Example 14) The negative electrode active material was produced in the same manner as in Example 1, except that 1.0% by mass of aluminum-i-propoxide with respect to the porous carbon material was directly added to the storage container without dissolving it in diethyl ether, and the step of heating at 400 °C for 2 hours while flowing nitrogen was performed. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0135] In the negative electrode active materials of Examples 1 to 14, at least a part of the surface layer portion of the low-valence nanosilicon oxide exposed on the surface of the porous carbon structure is coated with a metal oxide and / or a metal hydroxide, and the reactivity with the electrolytic solution is suppressed. Therefore, the cycle characteristics of Examples 1 to 14 are superior to those of Comparative Examples 1 and 2.

[0136] (Examples 15 to 17) The negative electrode active material was produced in the same manner as in Example 1, except that the amount of aluminum-i-propoxide used was changed as shown in Table 1 above. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0137] The cycle characteristics of Examples 15 to 17 are improved compared to those of Comparative Examples 1 and 2. From the results of Example 17, it can be seen that even when the thickness of the metal oxide and / or the metal hydroxide exceeds 10 nm, the cycle characteristics are improved. On the other hand, the cycle characteristics of Example 17 are worse than those of Example 1, Example 15, and Example 16. From this result, it is considered desirable that the thickness of the metal oxide and / or the metal hydroxide be 10 nm or less.

[0138] (Example 18) For the same porous carbon structure as in 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 flowed, and deposition was carried out for 4 hours. Then, the temperature was raised to grow the grain size of Si 0+ . 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 50 °C or less to form Si-O bonds. Next, nitrogen containing oxygen was flowed for 2 hours, and when the material temperature reached 30 °C or less, it was switched to nitrogen gas and flowed for 60 minutes. Subsequently, diethyl ether in which 1.0% by mass of aluminum-i-propoxide was dissolved with respect to the porous carbon material was added to the storage container, and it was heated at 400 °C for 2 hours while flowing nitrogen. After cooling to room temperature, the material was taken out from the storage container to obtain a negative electrode active material. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0139] The cycle characteristics of Example 18 are improved compared to Comparative Example 1, but are worse than those of Example 1. Specifically, Example 18 has a larger grain size compared to Example 1, and the cycle characteristics (1000-cycle retention rate in Table 1) are slightly lower. From these results, it can be seen 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 of the negative electrode active material improve as the negative electrode active material is closer to the amorphous structure.

[0140] (Examples 19, 20) The negative electrode active material was produced in the same manner as in Example 1, except that the structure of the porous carbon prepared first was changed as shown in Table 1 above. The obtained negative electrode active material was evaluated in the same manner as in Example 1.

[0141] The cycle characteristics of Examples 19 and 20 are improved compared to Comparative Example 1, but are worse than those of Example 1. From these results, it can be considered that for the structure of the porous carbon, a pore volume of 1 cm 3 / g or more and a BET specific surface area of 1400 m 2 / g or more, and an IUPAC classification of type I is more desirable.

[0142] 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 becomes smaller in the depth direction from the surface, and it can be seen that the oxygen concentration decreases from the surface toward the deep layer side.

[0143] FIG. 5 shows the X-ray diffraction spectra of Examples 1, 2, 10, and 11. Peaks are observed near 2θ = 28° in all of these, and the crystallite size of silicon was calculated from this peak. The same is true for other examples and comparative examples.

[0144] 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 structure of porous carbon, Amorphous low-valence nanosilicon oxide is dispersed inside the structure of the porous carbon, The low-valence nanosilicon oxide contains SiOx where x < 1.0, A negative electrode active material, characterized in that at least a part of the surface layer portion of the low-valence nanosilicon oxide exposed on the surface of the structure of the porous carbon is coated with at least one of a metal oxide and a metal hydroxide. [2]: The negative electrode active material according to [1] above, wherein the metal oxide and the metal hydroxide contain at least one element among aluminum, magnesium, titanium, zirconium, calcium, and niobium. [3]: The negative electrode active material according to [1] or [2] above, wherein the thickness of the metal oxide and the metal hydroxide is 0.1 nm or more and 10 nm or less. [4]: The negative electrode active material according to any one of [1] to [3] above, wherein the low-valence nanosilicon oxide is substantially in a composite state of 0 valence, 1 valence, and 2 valence. [5]: The negative electrode active material according to any one of [1] to [4] above, wherein the low-valence nanosilicon oxide dispersed in the structure of the porous carbon increases in x from the center to the surface layer of the structure of the porous carbon. [6]: The negative electrode active material according to any one of [1] to [5] above, wherein the grain size of 0-valent Si constituting the low-valence nanosilicon 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. [7]: The structure of the porous carbon is predominantly 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. The negative electrode active material according to any one of [1] to [6] above. [8]: A method for manufacturing a negative electrode active material having negative electrode active material particles, A step of preparing a structure of porous carbon, A step of depositing silicon inside the structure of the porous carbon by flowing monosilane gas under heating with respect to the structure of the porous carbon, A step of cooling a material in which silicon is deposited inside the structure of the porous carbon to 50°C or lower; After the cooling, while adjusting so that the temperature of the material in which silicon is deposited inside the structure of the porous carbon remains at 50°C or lower, introducing oxygen diluted with nitrogen gas into the material in which silicon is deposited inside the structure of the porous carbon, thereby changing at least a part of the silicon into a low-valence nano silicon oxide; Coating at least a part of the surface layer portion of the low-valence nano silicon oxide with at least one of a metal oxide and a metal hydroxide; A method for producing a negative electrode active material, characterized by comprising the above steps. [9]: The method for producing a negative electrode active material according to the above [8], wherein at least a part of the surface layer portion of the low-valence nano silicon oxide is coated by forming at least one of the metal oxide and the metal hydroxide by hydrolysis and dehydration condensation of a metal alkoxide.

[0145] 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 Reference Numerals

[0146] 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 member. S1, S2, S3, S4, S5... steps.

Claims

1. A negative electrode active material having negative electrode active material particles, wherein the negative electrode active material particles include a porous carbon structure, amorphous low-valence nanosilicon oxide is dispersed inside the porous carbon structure, the low-valence nanosilicon oxide includes SiOₓ: x < 1.0, and at least a part of the surface layer portion of the low-valence nanosilicon oxide exposed on the surface of the porous carbon structure is coated with at least one of a metal oxide and a metal hydroxide. A negative electrode active material characterized by this.

2. The negative electrode active material according to claim 1, wherein the metal oxide and the metal hydroxide contain at least one element among aluminum, magnesium, titanium, zirconium, calcium, and niobium.

3. The negative electrode active material according to claim 1, wherein the thickness of the metal oxide and the metal hydroxide is 0.1 nm or more and 10 nm or less.

4. The negative electrode active material according to claim 1, wherein the low-valence nanosilicon oxide is substantially in a composite state of 0 valence, 1 valence, and 2 valence.

5. The low-valence nanosilicon oxide dispersed in the porous carbon structure increases in x from the center to the surface layer of the porous carbon structure. The negative electrode active material according to claim 1, characterized by this.

6. The grain size of 0-valent Si constituting the low-valence nanosilicon 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. The negative electrode active material according to claim 1, characterized by this.

7. The porous carbon structure is predominantly 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 ２The negative electrode active material according to claim 1, characterized in that it is above / g.

8. A method for producing a negative electrode active material having negative electrode active material particles, preparing a porous carbon structure; depositing silicon inside the porous carbon structure by flowing monosilane gas under heating on the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50 ° C or lower; After the cooling, oxygen diluted with nitrogen gas is introduced into the material with silicon deposited inside the porous carbon structure while adjusting to maintain the temperature of the material with silicon deposited inside the porous carbon structure at 50 ° C or lower, thereby changing at least a part of the silicon into a low-valence nanosilicon oxide; coating at least a part of the surface layer of the low-valence nanosilicon oxide with at least one of a metal oxide and a metal hydroxide A method for producing a negative electrode active material, characterized by comprising:

9. The method for producing a negative electrode active material according to claim 8, characterized in that at least a part of the surface layer of the low-valence nanosilicon oxide is coated by forming at least one of the metal oxide and the metal hydroxide by hydrolysis and dehydration condensation of a metal alkoxide.

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