Negative electrode active material and method for manufacturing the same

Abstract

To provide a negative electrode active material that can increase capacity while maintaining battery characteristics.SOLUTION: In a negative electrode active material having negative electrode active material particles, the negative electrode active material particles include a porous carbon structure having branched pores formed by an alkali activation treatment therein, and amorphous low-valent nano silicon oxide is dispersed in a network structure within 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 producing the same. [Background technology]

[0002] In recent years, small electronic devices such as mobile terminals have become widespread, and there is a strong demand for further miniaturization, weight reduction, and longer life. In response to such market demands, development of secondary batteries that are small, lightweight, and capable of achieving high energy density has been progressing. The application of these secondary batteries is being considered not only for small electronic devices but also for large electronic devices such as automobiles and power storage systems such as those for homes.

[0003] Among these, lithium-ion secondary batteries are attracting great attention because they can be easily made small and have a high capacity, and can provide a higher energy density than lead batteries and nickel-cadmium batteries.

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

[0005] While carbon-based active materials are widely used as negative electrode active materials, recent market demands have called for further improvements in battery capacity. To improve battery capacity, the use of silicon as a negative electrode active material is being considered. This is because the theoretical capacity of silicon (4199 mAh / g) is more than 10 times greater than that of graphite (372 mAh / g), and a significant improvement in battery capacity can be expected. The development of silicon materials as negative electrode active materials is being considered not only for silicon itself, but also for alloys and compounds such as oxides. Furthermore, the form of the active material is being considered, ranging from the standard coated type used for carbon-based active materials to an integrated type deposited directly on the current collector.

[0006] However, when silicon is used as the primary raw material for the negative electrode active material, the negative electrode active material expands and contracts during charging and discharging, making it prone to cracking, primarily near the surface of the negative electrode active material. Furthermore, ionic substances are generated within the active material, making it prone to cracking. When the surface layer of the negative electrode active material cracks, a new surface is created, increasing the reactive area of ​​the active material. At this time, the electrolyte decomposes on the new surface, and a coating of the electrolyte decomposition product is formed on the new surface, consuming the electrolyte. This can lead to a deterioration in cycle performance.

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

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

[0009] In addition, in order to improve the initial charge / discharge efficiency, Si phase, SiO2, M y Nanocomposites containing SiO O metal oxides are used (see, for example, Patent Document 5). x(0.8 ≤ x ≤ 1.5, particle size range = 1 μm to 50 μm) is mixed with a carbon material and fired at 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 over-discharge 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 proposed a disproportionation structure that is divided into Si and SiO2 by applying a heat load to silicon oxide (Non-Patent Document 3).

[0012] Miyachi et al. have focused on Si and SiO2, which 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, which make up silicon oxide, react with Li, and are separated into Li silicide, Li silicate, and some unreacted SiO2.

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

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

[0015] On the other hand, the price of Li metal used for Li doping fluctuates greatly depending on the market situation, and there are many challenges when it comes to industrialization. Therefore, CVD-Si-C, which uses silane gas in porous carbon to generate nanosilicon inside, has achieved a higher energy density than Li-doped SiO (Patent Documents 14 and 15). [Prior art documents] [Patent documents]

[0016] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-185127 [Patent Document 2] Japanese Patent Application Laid-Open No. 2002-042806 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-164954 [Patent Document 4] Japanese Patent Application Laid-Open No. 2006-114454 [Patent Document 5] Japanese Patent Application Laid-Open No. 2009-070825 [Patent Document 6] Japanese Patent Application Laid-Open No. 2008-282819 [Patent Document 7] Japanese Patent Application Laid-Open No. 2008-251369 [Patent Document 8] Japanese Patent Application Laid-Open No. 2008-177346 [Patent Document 9] Japanese Patent Application Laid-Open No. 2007-234255 [Patent Document 10] Japanese Patent Application Laid-Open No. 2009-212074 [Patent Document 11] Japanese Patent Application Laid-Open No. 2009-205950 [Patent Document 12] Japanese Patent Application Publication No. 06-325765 [Patent Document 13] Japanese Patent Application Laid-Open No. 2015-156355 [Patent Document 14] U.S. Patent No. 10,608,254 [Patent Document 15] U.S. Patent No. 11,165,054 [Non-patent literature]

[0017] [Non-Patent Document 1] Battery Association of Japan's official journal "Denchi," May 1, 2010, page 10

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

[0018] As mentioned above, in recent years, small electronic devices such as mobile terminals have become more powerful and multifunctional, and the lithium-ion secondary batteries that serve as their main power sources are being required to have increased battery capacity. One method for solving this problem is to develop a lithium-ion secondary battery with a negative electrode that uses silicon material as the main material.

[0019] Lithium-ion secondary batteries using silicon materials are also expected to have initial charge-discharge and cycle characteristics comparable to those of lithium-ion secondary batteries using carbon-based active materials. Therefore, by using silicon oxides modified by Li insertion and partial desorption as anode active materials, cycle characteristics and initial charge-discharge characteristics have been improved. Recently, by incorporating Li into silicon oxides in advance to form Li silicates, the irreversible capacity, a drawback of silicon oxides, has been reduced, and these batteries have actually begun to be marketed. Even when prototype batteries were fabricated using Li-SiO-C (Non-Patent Document 8), which uses Li in silicon oxide and replaces 100% of the carbon anode material, the capacity improvement was only in the high 20% range compared to carbon anode materials. This means that further improvements in battery capacity are required, considering the increasing performance of small electronic devices (e.g., 5G) and the increased driving range of electric vehicles.

[0020] Therefore, CVD-Si-C, which has a low irreversible capacity, was developed, but it was found that the reaction between Si and the electrolyte resulted in insufficient fast charging and battery cycle characteristics.

[0021] The present invention has been made in view of the above problems, and an object of the present invention is to provide a negative electrode active material that can increase the capacity while maintaining the battery characteristics. [Means for solving the problem]

[0022] In order to solve the above problems, the present invention provides a negative electrode active material having negative electrode active material particles, characterized in that the negative electrode active material particles contain a porous carbon structure having branched pores formed by alkali activation treatment therein, and amorphous low-valent nano silicon oxide is dispersed in a network structure inside the porous carbon structure.

[0023] The negative electrode active material of the present invention has amorphous low-valence nanosilicon oxide dispersed inside a porous carbon structure, and the presence of the porous carbon structure can reduce the adverse effects caused by the expansion of the low-valence nanosilicon oxide inside. Furthermore, because the porous carbon structure has branched pores due to alkali activation treatment, the Si portion deposited on it becomes thinner during the manufacturing process of the negative electrode active material, and not only can post-oxidation be carried out smoothly, but the oxidation process is also easier to control, making it easier to control the valence of the nanosilicon oxide. Therefore, Si-O bonds can be present (especially throughout), and as described above, amorphous low-valence nanosilicon oxide is dispersed in a network structure. When Si-O bonds are present in this way (especially throughout), Li 15 Si4 is less likely to form, making it more suitable for high-speed charging.

[0024] In this case, when XAFS measurements are performed on a pre-charged negative electrode removed from a secondary battery having a negative electrode containing the negative electrode active material before charging, and on a post-charged negative electrode removed from the secondary battery after charging the pre-charged secondary battery, in the Si K absorption edge XANES spectra obtained from the XAFS measurements of the pre-charged and post-charged negative electrodes, the peak attributable to a tetracoordinated silicon compound in the energy range of 1846 eV to 1850 eV is higher in the pre-charged negative electrode than in the post-charged negative electrode, The XANES spectrum of the negative electrode after charging preferably has a peak attributable to a hexacoordinated silicon compound in the energy range of 1851 eV to 1855 eV.

[0025] Thus, during charging, the tetravalent Si peak in the range of 1846 eV to 1850 eV decreases, and a hexavalent Si peak in the range of 1851 eV to 1855 eV appears. In other words, the tetracoordinated silicon compound undergoes a structural change during charging, transforming into a hexacoordinated silicon compound (stishovite). The hexacoordinated silicon compound has a substantially high resistance, leading to Li deposition.

[0026] In this case, it is preferable that the XANES spectrum of the negative electrode before charging has peaks at energies near 1845.5 eV and 1847.5 eV.

[0027] The peak near 1845.5 eV represents a divalent Si peak, and the peak near 1847.5 eV represents a tetravalent or trivalent Si peak containing defects.

[0028] The negative electrode active material is solid 29 It is preferable that the maximum value is in the range of −81 to −95 ppm when measured by Si-CP / MAS-NMR.

[0029] Such negative electrode active material particles are negative electrode active material particles that contain a sufficient amount of SiOSi structure, and therefore can maintain a lower irreversible capacity than general SiO.

[0030] In this case, the negative electrode active material particles preferably have an SiOSi structure in which oxygen is bonded to a Si radical.

[0031] In this way, by having an SiOSi structure in which oxygen is bonded to Si radicals, decomposition of the electrolyte can be effectively suppressed.

[0032] Furthermore, the low-valence nano silicon oxide is preferably substantially in a composite state of zero valence, monovalence and divalence, with divalence being the most predominant.

[0033] In this way, the low-valent nano silicon oxide is substantially in a composite state of zero valence, monovalence and divalence, and further, the divalence is substantially predominant, so that a lower irreversible capacity can be achieved.

[0034] Furthermore, the low-valence nanosilicon oxide dispersed in the porous carbon structure may have x increasing from the center to the surface layer of the porous carbon structure. Cut.

[0035] In this way, in the present invention, because low-valent nano silicon oxide is dispersed in the porous carbon structure, x tends to increase (the oxygen content ratio increases) from the center of the porous carbon structure to the surface during production. Because the silicon oxidation ratio is high at the surface, decomposition of the electrolyte is more effectively suppressed, while the silicon oxidation ratio is low inside the active material, allowing for a greater increase in battery capacity.

[0036] The proportion of the porous carbon structure in the entire negative electrode active material particles is preferably 45% by mass or more and 64% by mass or less.

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

[0038] Furthermore, the grain size of zero-valent Si constituting the low-valent nano silicon oxide, calculated using the Scherrer equation from the peak measured by X-ray diffraction measurement of the negative electrode active material particles, is preferably in the range of 1 nm to 5 nm.

[0039] Such a material having a grain size of zero-valent Si that is substantially amorphous is preferred.

[0040] The porous carbon structure is predominantly Type I in the IUPAC classification, and its surface area is 1325 m 2 / g or more, pore volume is 0.95 cm3 / g or more is preferable.

[0041] The porous carbon structure, which has the above IUPAC classification, surface area, and pore volume, can be used to efficiently incorporate a large amount of low-valent nano-silicon oxide into the negative electrode active material. In particular, the IUPAC type I structure facilitates the formation of Si-O bonds after deposition.

[0042] The present invention also provides a method for producing a negative electrode active material having negative electrode active material particles, comprising: preparing a porous carbon structure having branched pores therein by a manufacturing method using an alkali activation treatment; flowing nitrogen gas containing moisture through the porous carbon structure; a step of flowing the moisture-containing nitrogen gas and then flowing monosilane gas into the porous carbon structure under heating, thereby depositing silicon inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; After the cooling, the temperature of the material in which silicon is deposited inside the porous carbon structure is adjusted to be maintained at 50° C. or less, and oxygen diluted with nitrogen gas is introduced into the material in which silicon is deposited inside the porous carbon structure, thereby converting at least a portion of the silicon into low-valence nano-silicon oxide. and thereby producing negative electrode active material particles in which amorphous low-valent nano silicon oxide is dispersed in a network structure inside the porous carbon structure.

[0043] With this method for producing a negative electrode active material, it is possible to simply and efficiently produce a negative electrode active material in which amorphous low-valent nanosilicon oxide containing a sufficient amount of SiOSi structure is dispersed in a network structure inside a porous carbon structure having branched pores therein, as described above. [Effects of the Invention]

[0044] The negative electrode active material of the present invention has amorphous low-valence nanosilicon oxide dispersed inside a porous carbon structure, and therefore, when the low-valence nanosilicon oxide inside expands, the presence of the porous carbon structure can reduce the adverse effects of the expansion. Moreover, in the porous carbon structure having branched pores inside, amorphous low-valence nano silicon oxide is dispersed in the network structure inside, in other words, Si-O bonds exist (especially throughout), and Li 15 It is less likely to form Si4 and is more suitable for high-speed charging. Furthermore, the Si—O bond can suppress the decomposition of the electrolyte, making it possible to reduce the SEI that accumulates on the surface layer. As a result, a negative electrode using the negative electrode active material of the present invention has high initial efficiency, high capacity, high input characteristics, and high cycle characteristics, and is also suitable for high-speed charging.

[0045] Furthermore, the method for producing a negative electrode active material of the present invention can simply and efficiently produce a negative electrode active material in which amorphous low-valent nanosilicon oxide containing a sufficient amount of SiOSi structure is dispersed in a network structure inside the porous carbon structure having branched pores therein. [Brief explanation of the drawings]

[0046] [Figure 1] 1 is a cross-sectional view showing the structure of a negative electrode containing a negative electrode active material of the present invention. [Figure 2] FIG. 1 is an exploded view showing a configuration example (laminate film type) of a lithium ion secondary battery including the negative electrode active material of the present invention. [Figure 3] FIG. 1 is a flow chart showing an example of a method for producing a negative electrode active material according to the present invention. [Figure 4] 1 is a cross-sectional TEM photograph of the negative electrode active material of Example 1. [Figure 5] 1 is a photograph showing the appearance of the negative electrode after charge and discharge in Example 1. [Figure 6]1 is a cross-sectional TEM photograph of the negative electrode active material of Comparative Example 1. [Figure 7] 1 shows the results of XAFS measurement (XANES spectrum) analysis of the negative electrodes in Example 1 and Comparative Example 1 before and after charging. [Figure 8] 29Si-CP / MAS-NMR spectrum of the negative electrode active material of Comparative Example 1. [Figure 9] 1 is a photograph showing the appearance of the negative electrode of Comparative Example 1 after charge and discharge. [Figure 10] 29Si-CP / MAS-NMR spectrum of the negative electrode active material of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0047] Hereinafter, the present invention will be described in detail with reference to the preferred embodiments, but the present invention is not limited to these.

[0048] As mentioned above, one method for increasing the battery capacity of lithium-ion secondary batteries is to use a negative electrode with a carbon structure mainly made of low-valent nano-silicon oxide. Lithium-ion secondary batteries using this active material are expected to have high battery capacity while exhibiting battery characteristics similar to those of lithium-ion secondary batteries using carbon-based active materials.

[0049] Therefore, the present inventors conducted extensive research to obtain a negative electrode active material that, when used as a negative electrode of a secondary battery, can achieve high cycle characteristics, improve initial charge-discharge characteristics, and increase battery capacity, and have arrived at the present invention.

[0050] In particular, the CVD-Si-C disclosed in Patent Documents 14 and 15 has an issue of excessive reactivity with the electrolyte. In this invention, in order to suppress such a reaction with the electrolyte, the Si portion is converted to a Si-O phase and the siloxane bond specific to silicon oxide is used, thereby developing an anode active material that not only significantly suppresses the reactive decomposition of the electrolyte, but also ensures high-speed charging because the Si-O material with siloxane bonds has good Li-acceptance.

[0051] [Negative electrode active material of the present invention] The negative electrode active material of the present invention is a negative electrode active material having negative electrode active material particles, characterized in that the negative electrode active material particles contain a porous carbon structure having branched pores formed by alkali activation treatment therein, and amorphous low-valent nanosilicon oxide is dispersed in a network structure inside the porous carbon structure.

[0052] Such a negative electrode active material has amorphous low-valence nano-silicon oxide dispersed inside a porous carbon structure, and the presence of the porous carbon structure can reduce the adverse effects caused by the expansion of the low-valence nano-silicon oxide inside. Moreover, in the porous carbon structure with branched pores due to alkali activation treatment inside, amorphous low-valence nano silicon oxide is dispersed in a network structure inside, in other words, Si-O bonds exist (especially exist throughout). If there is a lot of Si-Si here, when it reacts with Li, the Si-Si in the surface layer becomes an ionic substance close to an insulator, and 15 However, when Si-O bonds exist (especially when they exist throughout the entire structure) as in the present invention, Li 15 It is less likely to form Si4 and is more suitable for high-speed charging. Furthermore, the Si—O bond can suppress the decomposition of the electrolyte, making it possible to reduce the SEI that accumulates on the surface layer. In the present invention, it is sufficient that the amorphous low-valent nanosilicon oxide is dispersed in a network structure inside the porous carbon structure, and some of the nanosilicon oxide may be crystalline.

[0053] Let us explain the branched pores mentioned above. First, conventional products have pores with a diameter of 2-3 nm (average) as well as pores with a diameter of 10-30 nm (solid pores), with the 2-3 nm pores acting as bypasses connecting the 10-30 nm pores. On the other hand, branched pores do not have pores with a diameter of 10-30 nm, but instead have pores with a diameter of 2-3 nm extending radially (branch-like) into the carbon, with the total length being in the micrometer range from the particle surface to the interior.

[0054] The solid state material used to measure the properties of the negative electrode active material of the present invention 29 We will explain Si-CP / MAS-NMR. CP / MAS is an abbreviation for Cross Polarization Magic Angle Spinning, as is well known. 29 Si-CP / MAS-NMR is a solid-state NMR measurement method that combines cross polarization (CP) and magic angle spinning (MAS). MAS method without cross polarization (CP) can be directly 29 This is a method of exciting and detecting Si nuclei. On the other hand, CP / MAS is 1 H nuclei are excited, and then 29 Magnetization was transferred to the Si nucleus. 29 This method detects only Si nuclei. Therefore, it is difficult to detect the magnetization transfer in the vicinity. 1 No H nuclei 29 Si nuclei are not detected. As a result, Si nuclei with Si-H groups, Si-OH groups, or Si-O groups have a high probability of magnetization transfer, so they are detected with high sensitivity. On the other hand, parts with only Si-Si bonds or only SiO2 bonds are not detected. Solid 29 Since Si-CP / MAS-NMR is a measurement method of this kind, it is a selective detection method. 29 When negative electrode active material particles in which amorphous low-valent nano silicon oxide is dispersed inside a porous carbon structure are measured using Si-CP / MAS-NMR, if the maximum value is in the range of -81 to -95 ppm, it can be said that the SiOSi structure is sufficiently contained.

[0055] In this way, the negative electrode active material particles of the present invention can have an SiOSi structure in which oxygen is bonded to a Si radical.

[0056] solid 29 The Si-CP / MAS-NMR measurement can be carried out, for example, under the following conditions. solid 29 Si-CP / MAS-NMR measurement conditions Equipment: Bruker AVANCE700 Detector: 4mmφ CPMS solid probe Rotor: 4mmφ zirconia Cap: KEL-F · 29 Si resonance frequency: 139.1MHz Pulse sequence: CP / MAS ·Contact time: 5ms Delay time: 5.0 seconds ·MAS speed: 9kHz Number of times: 16,000 ·Measurement temperature: room temperature Chemical shift external standard: hexamethylcyclotrisiloxane -9.66 ppm

[0057] As described below, the anode active material of the present invention can be manufactured as a Si-Ox material containing siloxane bonds in the Si phase to suppress the electrolyte decomposition reaction, which is insufficient in CVD-Si-C produced from general silane gas. The purpose of intentionally creating Si-O bonds is that Si-Si bonds promote electrolyte decomposition, while Si-O bonds react with the electrolyte more slowly than Si-Si bonds. Furthermore, because Si-O bonds can suppress electrolyte decomposition, it is possible to reduce the SEI deposit on the surface layer of CVD-Si-C. As a result, battery cycle characteristics are improved. Furthermore, grain boundaries with Si-Si bonds reduce Li diffusivity, while Si-O bonds improve Li diffusivity and improve high-speed charging. Since the portion primarily responsible for charge and discharge is a low-valence nanosilicon oxide, the material of the present invention can be defined as CVD-SiOx-C, as opposed to CVD-Si-C. The active material thus manufactured can maintain battery cycle characteristics while exhibiting high energy density and high-speed charging.

[0058] Furthermore, since the tetravalent Si constituting SiO is an irreversible component, in the negative electrode active material of the present invention, it is preferable that the low-valence nanosilicon oxide is substantially in a composite state of zero, monovalent, and divalent. In particular, it is preferable that this low-valence nanosilicon oxide is substantially most predominantly divalent. By making SiOx of divalent or less predominant, the irreversible capacity is larger than that of silicon alone, but it is possible to maintain a lower irreversible capacity than that of general SiO.

[0059] In the negative electrode active material of the present invention, the grain size of zero-valent Si constituting the low-valent nano silicon oxide, calculated by using the Scherrer equation from the peak measured by X-ray diffraction measurement of the negative electrode active material particles, is preferably in the range of 1 nm to 5 nm. Such a grain size of zero-valent Si that is substantially amorphous is preferable.

[0060] Calculation of the crystallite size by XRD can be carried out, for example, under the following conditions: For broad peaks, analysis can be carried out using analysis software TOPAS, for example, under the following conditions: XRD measurement Equipment: Bruker D2 Phaser ·X-ray source:Cu Divergence slit: 0.5° Incident solar: 4° Receiver side solar: 4° Calculation of crystallite size Analysis software: DIFFRAC.TOPAS Analysis method: Peak fitting method Emission Profile: CuKa5.lam Function: FP (First Principle) function Refinement Option: Select “Calculate Error” or “Use Extrapolation”

[0061] The valence of low-valence nano silicon oxide can be quantified by NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0062] The NMR measurement for measuring the valence of the low-valence nano silicon oxide can be carried out, for example, under the following conditions. 29 Si MAS NMR (Magic Angle Spinning Nuclear Magnetic Resonance) Equipment: Bruker 700 NMR spectrometer, Probe: 4mm HR-MAS rotor 50μL, Sample rotation speed: 10kHz, ·Measurement environment temperature: 25℃

[0063] The XPS measurement can be carried out, for example, under the following conditions. XPS ·Equipment: X-ray photoelectron spectrometer, X-ray source: monochromated Al Kα radiation, X-ray spot diameter: 100μm, Ar ion gun sputtering conditions: 0.5kV, 2mm x 2mm.

[0064] Furthermore, in the negative electrode active material of the present invention, it is preferable that the low-valent nanosilicon oxide dispersed in the porous carbon structure has x increasing from the center to the surface of the porous carbon structure. In the present invention, since the low-valent nanosilicon oxide is dispersed in the porous carbon structure, x tends to increase (the oxygen content increases) from the center to the surface of the porous carbon structure during production. Since the silicon oxidation rate is high at the surface, decomposition of the electrolyte is more effectively suppressed, while the silicon oxidation rate is low inside the active material, thereby further increasing the battery capacity.

[0065] Furthermore, when XAFS measurements are performed on a pre-charged negative electrode removed from a secondary battery having a negative electrode containing the negative electrode active material before charging, and on a post-charged negative electrode removed from the secondary battery after charging the pre-charged secondary battery, the Si K-absorption edge XANES spectra obtained from the XAFS measurements of the negative electrode before and after charging preferably have a peak attributable to a tetracoordinated silicon compound in the energy range of 1846 eV to 1850 eV higher in the negative electrode before charging than in the negative electrode after charging, and the XANES spectrum of the post-charged negative electrode preferably has a peak attributable to a hexacoordinated silicon compound in the energy range of 1851 eV to 1855 eV.

[0066] In this way, the tetracoordinated silicon compound undergoes a structural change during charging, and changes into a hexacoordinated silicon compound (stishovite). In particular, when lithium hexafluorophosphate (LiPF6) is contained as the electrolyte salt in the electrolyte of a secondary battery described below, the hexacoordinated silicon compound (stishovite) contains P.

[0067] In this case, it is preferable that the XANES spectrum of the negative electrode before charging has peaks at energies near 1845.5 eV and 1847.5 eV. The peak near 1845.5 eV is a divalent Si peak, and the peak near 1847.5 eV is a tetravalent or trivalent Si peak containing defects.

[0068] Furthermore, the valence of the Si component in a high-valence silicon compound can be confirmed by a spectrum in the XANES (X-ray absorption near edge structure) region of an XAFS (X-ray absorption fine structure) measurement. For example, it can be performed under the following conditions. XAFS measurements Si K-edge (K absorption edge) Measurement facility: BL6N1 at Aichi Synchrotron Light Center, Acceleration energy: 1.2 GeV, Accumulated current value: 300mA, Monochromatization conditions: White X-rays from a bending magnet are monochromatized using a double crystal monochromator and used for measurement. Focusing conditions: Focusing in the vertical and horizontal directions using a Ni-coated bend cylindrical mirror Upstream slit opening: 7.0mm horizontal x 3.0mm vertical Beam size: 2.0mm horizontal x 1.0mm vertical Incident angle to the sample: normal incidence (incident angle 0 degrees) Energy calibration: The peak position at the K2SO4 SK edge is calibrated to 2481.70 eV Measurement method: Total electron yield method by measuring the sample current ·I0 measurement method: XANES measurement Au-mesh Vacuum level of measuring chamber: 5×10 -7 Pa · Sample environment: The transfer vessel is set up without exposure to the atmosphere.

[0069] <Non-aqueous electrolyte secondary battery negative electrode> Next, the configuration of a negative electrode for a non-aqueous electrolyte secondary battery (hereinafter also referred to as "negative electrode") containing the negative electrode active material of the present invention will be described.

[0070] [Negative electrode composition] Fig. 1 shows a cross-sectional view of a negative electrode containing the negative electrode active material of the present invention. As shown in Fig. 1, a negative electrode 10 has a negative electrode active material layer 12 on a negative electrode current collector 11. This negative electrode active material layer 12 may be provided on both sides or only one side of the negative electrode current collector 11. Furthermore, the negative electrode of the nonaqueous electrolyte secondary battery of the present invention does not necessarily need to have the negative electrode current collector 11.

[0071] [Negative electrode current collector] The negative electrode current collector 11 is made of a highly conductive material that has excellent mechanical strength. Examples of conductive materials that can be used for the negative electrode current collector 11 include copper (Cu) and nickel (Ni). It is preferable that this conductive material does not form an intermetallic compound with lithium (Li).

[0072] The negative electrode current collector 11 preferably contains carbon (C) and sulfur (S) in addition to the main elements. This is because the physical strength of the negative electrode current collector is improved. In particular, when an active material layer that expands during charging is included, if the current collector contains the above elements, deformation of the electrode including the current collector is suppressed. The contents of the above contained elements are not particularly limited, but are preferably 100 mass ppm or less each. This is because a higher deformation suppression effect can be obtained. Such a deformation suppression effect can further improve cycle characteristics.

[0073] The surface of the negative electrode current collector 11 is preferably roughened, and the ten-point height of the surface roughness Rz is preferably 1.5 μm or more and 5 μm or less. The roughened negative electrode current collector is, for example, a metal foil that has been subjected to an electrolytic treatment, an embossing treatment, or a chemical etching treatment.

[0074] [Negative electrode active material layer] The negative electrode active material layer 12 may contain multiple types of negative electrode active materials, such as carbon-based active materials, in addition to the silicon-based active material particles of the present invention. Furthermore, depending on the battery design, other materials such as a thickener (also called a "binding agent" or "binder") and a conductive additive may be included.

[0075] [Method of manufacturing negative electrode active material and negative electrode] Next, an example of a method for manufacturing the negative electrode active material of the nonaqueous electrolyte secondary battery of the present invention and a negative electrode using the same will be described.

[0076] First, a method for producing the negative electrode active material contained in the negative electrode will be described. The method for producing a negative electrode active material of the present invention is a method for producing a negative electrode active material having negative electrode active material particles, and includes the steps of: preparing a porous carbon structure having branched pores therein by a production method using an alkali activation treatment; flowing moisture-containing nitrogen gas through the porous carbon structure; and, after the moisture-containing nitrogen gas has been flowed, flowing monosilane gas through the porous carbon structure under heating to deposit silicon inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; and, after the cooling, introducing oxygen diluted with nitrogen gas into the material with silicon deposited inside the porous carbon structure while adjusting the temperature of the material with silicon deposited inside the porous carbon structure to maintain it at 50°C or less, thereby converting at least a portion of the silicon into low-valence nanosilicon oxide, thereby producing a negative electrode active material particle having amorphous low-valence nanosilicon oxide dispersed in a network structure inside the porous carbon structure.

[0077] First, each step of an example of a method for producing a negative electrode active material will be described with reference to steps S11 to S15 in FIG.

[0078] First, a porous carbon structure having branched pores therein is prepared by a manufacturing method using an alkali activation treatment (step S11). First, select a material with few real pores. This can be achieved by steam activation of coconut shells, for example, or by changing the carbonization temperature of the substrate. Regarding real pores, the carbonization state of the raw material (substrate) can be appropriately controlled, and then the pores can be adjusted to become real through the subsequent activation process. The prepared material is then subjected to an alkali activation treatment. For example, this can be done by mixing an alkali metal hydroxide with the prepared material and heating it. The heating temperature can be, for example, about 1000°C. Examples of alkali metal hydroxides include, but are not limited to, KOH, NaOH, LiOH, RbOH, and CsOH (especially KOH). To explain a more specific example of the alkali activation procedure, for example, three times the weight of KOH is added to carbonized coconut shells (or other materials such as resin) or charcoal, and the temperature is raised to 1000°C at 10°C / min, maintained for two hours, and then cooled to room temperature. The mixture is then subjected to ultrasonic cleaning in pure water for two hours and cleaning with a stirrer for three hours to remove potassium, yielding the porous carbon structure described above. By using such an alkali activation treatment, it is possible to adjust the amount of branched pores generated inside, and a porous carbon structure having such branched pores can be manufactured. The number of solid pores can be changed by activation treatment (steam activation treatment, alkali activation treatment, etc.), but as described above, by using alkali activation treatment as in the present invention, it is possible to form branched pores in which pores with a diameter of 2-3 nm (average) are formed radially, without having solid pores (pores of 10-30 nm). Regarding pores with a diameter of 2-3 nm (average), if the same raw material (e.g., coconut shell) is used, the surface area of ​​the pores can be increased by, for example, steam activation treatment to 1500 m. 2 / g, whereas alkali activation treatment resulted in 2000m 2 / g or more. In other words, the alkali activation treatment can increase the solubility by about 33% or more compared to the steam activation treatment.

[0079] The porous carbon structure prepared here preferably has at least a carbon-carbon double bond in a part thereof. The porous carbon structure prepared here is predominantly of type I in the IUPAC classification, and its surface area is 1325 m 2 / g or more, pore volume is 0.95 cm 3 / g or more is preferable. By using such IUPAC classification, surface area, and pore volume, silicon deposition can be carried out more efficiently in larger quantities. Furthermore, the IUPAC classification, surface area, and pore volume can be measured using the following methods. The specific surface area / pore size distribution was measured using a Shimadzu Tristar II Plus by a constant volume method based on gas adsorption. The conditions were as follows: Gas used: Nitrogen Environment: Liquid nitrogen Pressure operating range: P / P0 Adsorption 0~0.998 Desorption 0.998~0.10 Pretreatment: Vacuum 200℃ 1 hour

[0080] Next, nitrogen gas containing moisture is flowed through the porous carbon structure prepared in step S11 (step S12). This allows OH groups to be attached inside the pores, which facilitates the oxidation process in step S15, which will be described later. Because the pores in the prepared porous carbon structure are branched, not only does this facilitate the oxidation process in step S15, which will be described later, but it also makes the oxidation process easier to control. Furthermore, by controlling this oxidation process, the valence of the nano-silicon oxide can be controlled.

[0081] After step S11 and before step S12, the porous carbon structure is preferably placed in a vacuum container and evacuated. The degree of vacuum can be, for example, but is not limited to, about -100 kPa. After evacuating, the pressure is restored with nitrogen, and the container is preferably heated to about 350 to 450°C using an external heater while the nitrogen is flowing. This heating can be performed for 5 minutes to 1 hour. By performing such evacuation and preheating in the presence of nitrogen, nuclei for silicon deposition in step S13 (described later) can be formed and hydrogen and water attached to the porous carbon structure can be removed, thereby more reliably carrying out silicon deposition in step S13.

[0082] Next, monosilane gas is made to flow into the porous carbon structure after step S12 while heating, thereby depositing silicon inside the porous carbon structure (step S13). The silicon deposition in step S13 can be performed by flowing monosilane gas at, for example, about 400 to 500° C. The deposition time can be, for example, 30 minutes to 10 hours.

[0083] Next, the material with silicon deposited inside the porous carbon structure is cooled to 50°C or less (step S14). In this step, it is preferable to cool the material while flowing nitrogen gas. This cooling can be performed to room temperature, for example.

[0084] After the cooling (step S14), the temperature of the material containing silicon deposited inside the porous carbon structure is adjusted to maintain 50°C or less. Then, oxygen diluted with nitrogen gas is introduced into the material containing silicon deposited inside the porous carbon structure, converting at least a portion of the silicon into low-valence nano-silicon oxide (step S15). This step allows the formation of Si-O bonds. It is more preferable to maintain the temperature at 35°C or less.

[0085] The dilution of oxygen with nitrogen can be, for example, 5 to 50 times, preferably 10 to 30 times. This dilution degree can typically be 20 times.

[0086] Furthermore, if the internal temperature rises in step S15 and exceeds 50°C, silicon dioxide will be formed in some parts, which is undesirable for a negative electrode active material. Therefore, in step S15, it is necessary to adjust the temperature of the material so that it remains at 50°C or lower. Furthermore, the material temperature in step S15 is preferably set to, for example, 25°C or higher, and more preferably 30°C or higher, in order to facilitate the oxidation reaction and the formation of Si-O bonds.

[0087] The oxidation time in step S15 (flow time of oxygen diluted with nitrogen gas) can be, for example, 30 minutes to 5 hours, preferably 1 hour to 3 hours. After the flow of oxygen diluted with nitrogen gas, the flow can be switched to nitrogen gas for further cooling. The flow of nitrogen gas only can be, for example, 30 minutes to 2 hours.

[0088] In the oxidation by the flow of oxygen diluted with nitrogen gas, it is preferable to adjust the low-valence nanosilicon oxide dispersed in the porous carbon structure so that x increases from the center to the surface of the porous carbon structure. Note that in the porous carbon structure, the pore structure tends to have a large cross-sectional area at the particle surface and a smaller cross-sectional area toward the particle interior. Therefore, by performing oxidation by the flow of oxygen diluted with nitrogen gas as in the present invention, x naturally tends to increase from the center to the surface of the porous carbon structure.

[0089] The material is then removed from the storage container. Through the above steps, it is possible to produce negative electrode active material particles in which amorphous low-valent nano silicon oxide is dispersed in a network structure in a porous carbon structure having branched pores inside. In the above steps, the negative electrode active material particles are solidified by appropriately setting the manufacturing conditions. 29 When measured by Si-CP / MAS-NMR, it is possible to produce a compound having a maximum value in the range of -81 to -95 ppm.

[0090] When producing the negative electrode active material in this manner, it is preferable to adjust the amount of silicon deposition and the degree of oxidation so that the proportion of the porous carbon structure in the entire negative electrode active material particles is 45% by mass or more and 64% by mass or less.

[0091] <Lithium-ion secondary battery> Next, a laminate film type lithium ion secondary battery will be described as a specific example of a nonaqueous electrolyte secondary battery using the above-described negative electrode active material of the present invention.

[0092] [Configuration of laminated film type secondary battery] The laminate film type lithium ion secondary battery 30 shown in FIG. 2 mainly comprises a wound electrode body 31 housed inside a sheet-like exterior member 35. This wound electrode body 31 has a separator between the positive and negative electrodes and is wound. There are also cases where a separator is placed between the positive and negative electrodes and a laminate is housed. In either type of electrode body, a positive electrode lead 32 is attached to the positive electrode, and a negative electrode lead 33 is attached to the negative electrode. The outermost periphery of the electrode body is protected by protective tape.

[0093] The positive and negative electrode leads 32, 33 are led out in one direction, for example, from the inside to the outside of the exterior member 35. The positive electrode lead 32 is formed of a conductive material such as aluminum, and the negative electrode lead 33 is formed of a conductive material such as nickel or copper.

[0094] The exterior member 35 is, for example, a laminate film in which a fusion layer, a metal layer, and a surface protection layer are laminated in this order, and the outer peripheral edges of the fusion layers of the two films of this laminate film are fused together or attached with an adhesive or the like so that the fusion layer faces the electrode body 31. The fusion part is, for example, a film such as polyethylene or polypropylene, and the metal part is, for example, aluminum foil. The protection layer is, for example, nylon or the like.

[0095] An adhesive 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 polyolefin resin.

[0096] The positive electrode has a positive electrode active material layer on one or both sides of a positive electrode current collector, similar to the negative electrode 10 in FIG.

[0097] The positive electrode current collector is made of a conductive material such as aluminum.

[0098] 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 aid, a dispersant, etc. 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 described above, for example.

[0099] 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, compounds having at least one or more of nickel, iron, manganese, and cobalt are 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 battery charge and discharge state, but are generally represented by 0.05 ≦ x ≦ 1.10 and 0.05 ≦ y ≦ 1.10.

[0100] Examples of the composite oxide having lithium and a transition metal element include 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) and lithium nickel cobalt manganese composite oxide (NCM).

[0101] Examples of the phosphate compound having lithium and a transition metal element include 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 positive electrode materials, a high battery capacity can be obtained, and excellent cycle characteristics can also be obtained.

[0102] [Negative electrode] The negative electrode has a configuration similar to that of the lithium-ion secondary battery negative electrode 10 shown in Fig. 1, and has, for example, negative electrode active material layers on both sides of a current collector. This negative electrode preferably has a negative electrode charge capacity greater than the electrical capacity (charge capacity as a battery) obtained from the positive electrode active material. This can suppress the deposition of lithium metal on the negative electrode.

[0103] The positive electrode active material layer is provided on a portion of both sides of the positive electrode current collector, and similarly, the negative electrode active material layer is provided on a portion of both sides of the negative electrode current collector. In this case, for example, the negative electrode active material layer provided on the negative electrode current collector has a region where the opposing positive electrode active material layer is not present. This is for the purpose of designing a stable battery.

[0104] The region where the negative electrode active material layer and the positive electrode active material layer do not face each other is hardly affected by charging and discharging, and therefore the state of the negative electrode active material layer is maintained as it was immediately after formation, allowing the composition of the negative electrode active material to be accurately determined with good reproducibility, regardless of whether charging and discharging are performed.

[0105] [Separator] The separator separates the positive and negative electrodes, preventing current short circuits caused by contact between the electrodes while allowing lithium ions to pass through. This separator is formed of a porous film made of, for example, synthetic resin or ceramic, and may have a laminate structure in which two or more types of porous film are laminated. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, and polyethylene.

[0106] [Electrolyte] At least a portion of the active material layer or the separator is impregnated with a liquid electrolyte (electrolytic solution), which is a solution of an electrolyte salt dissolved in a solvent and may contain other materials such as additives.

[0107] The solvent can be, for example, a non-aqueous solvent. Examples of non-aqueous solvents include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, 1,2-dimethoxyethane, and tetrahydrofuran. Among these, it is desirable to use at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate. This is because better properties can be obtained. In this case, more advantageous properties can be obtained by combining a high-viscosity solvent such as ethylene carbonate or propylene carbonate with a low-viscosity solvent such as dimethyl carbonate, ethyl methyl carbonate, or diethyl carbonate. This is because the dissociation property and ion mobility of the electrolyte salt are improved.

[0108] When an alloy-based negative electrode is used, it is particularly desirable to use a solvent containing at least one of halogenated chain carbonates or halogenated cyclic carbonates. This allows a stable coating to be formed on the surface of the negative electrode active material during charge and discharge, particularly during charging. Here, the halogenated chain carbonate is a chain carbonate having a halogen as a constituent element (i.e., at least one hydrogen atom is substituted with a halogen). Furthermore, the halogenated cyclic carbonate is a cyclic carbonate having a halogen as a constituent element (i.e., at least one hydrogen atom is substituted with a halogen).

[0109] Although the type of halogen is not particularly limited, fluorine is preferred because it forms a better coating than other halogens. Furthermore, the more halogens there are, the more desirable they are because the resulting coating is more stable and the decomposition reaction of the electrolyte is reduced.

[0110] Examples of halogenated chain carbonates include fluoromethyl methyl carbonate, difluoromethyl methyl carbonate, etc. Examples of halogenated cyclic carbonates include 4-fluoro-1,3-dioxolan-2-one, 4,5-difluoro-1,3-dioxolan-2-one, etc.

[0111] It is preferable that the solvent additive contains an unsaturated carbon-bond cyclic carbonate. This is because a stable coating is formed on the negative electrode surface during charge and discharge, and the decomposition reaction of the electrolyte can be suppressed. Examples of the unsaturated carbon-bond cyclic carbonate include vinylene carbonate and vinylethylene carbonate.

[0112] It is also preferable that the solvent additive contains sultone (cyclic sulfonic acid ester), as this improves the chemical stability of the battery. Examples of sultones include propane sultone and propene sultone.

[0113] Furthermore, it is preferable that the solvent contains an acid anhydride, because this improves the chemical stability of the electrolyte solution. Examples of acid anhydrides include propanedisulfonic acid anhydride.

[0114] The electrolyte salt may include, for example, one or more light metal salts such as lithium salts, for example, lithium hexafluorophosphate (LiPF6) and lithium tetrafluoroborate (LiBF4).

[0115] The content of the electrolyte salt is preferably 0.5 mol / kg or more and 2.5 mol / kg or less relative to the solvent, because this allows high ionic conductivity to be obtained. [Example]

[0116] EXAMPLES The present invention will be explained in more detail below by showing examples and comparative examples of the present invention, but the present invention is not limited to these examples.

[0117] Example 1 A negative electrode active material was prepared by the following procedure, and further a laminate film type lithium ion secondary battery 30 shown in FIG. 2 was fabricated.

[0118] The negative electrode active material was manufactured as follows. First, a carbon material with few actual pores was prepared by steam activation (1000°C) to make it porous (mesopores). The prepared material was then subjected to alkali activation as follows, resulting in a surface area (BET specific surface area) of 1940 m. 2 / g, pore volume 1.00 cm 3 A porous carbon material (porous carbon structure) of IUPAC type I, with a particle size (D50) of 11 μm and a molecular weight of 10 ... [Alkaline activation treatment] (Process Procedure) The prepared material with few pores was mixed with alkali metal hydroxide (KOH) and heated to 1000°C. Specifically, three times the weight of the material was added with KOH, and the temperature was raised to 1000°C at 10°C / min and held at that temperature for two hours. The material was then cooled to room temperature. The material was then ultrasonically cleaned in pure water for two hours and washed in a stirrer for three hours to remove the potassium.

[0119] This porous carbon material was placed in a vacuum container and evacuated to -90 kPa. Next, the pressure was restored with nitrogen and heated to 400°C using an external heater while nitrogen was flowing. After 30 minutes of heating, moisture-containing nitrogen gas was introduced.

[0120] After the above flow, the temperature was raised to 415 ° C, monosilane gas was flowed, and deposition was carried out for 4 hours. Then, nitrogen gas was flowed and the mixture was cooled to room temperature. After the temperature was lowered to 25 ° C, oxygen diluted 20 times with nitrogen was introduced, and the material temperature was adjusted to 50 ° C or less to form Si-O bonds. Next, oxygen-containing nitrogen was flowed for 2 hours, and when the material temperature reached 30 ° C or less, the gas was switched to nitrogen. After flowing for 60 minutes, the material was removed from the storage container and used as the negative electrode active material.

[0121] [Measurement of negative electrode active material] The negative electrode active material prepared as described above was subjected to TEM-EDX, XRD analysis, and NMR (solid state) analysis. 29 Si-CP / MAS-NMR was performed.

[0122] [Preparation of negative electrode] The negative electrode active material (active material containing CVD-SiOx-C) prepared as described above, graphite, conductive additive 1 (carbon nanotubes, CNT), conductive additive 2 (carbon microparticles with a median diameter of approximately 50 nm), sodium polyacrylate, and carboxymethyl cellulose (hereinafter referred to as CMC) were mixed in a dry mass ratio of 9.3:83.7:1:1:4:1, and then diluted with pure water to form a negative electrode mixture slurry.

[0123] The negative electrode current collector was a 15 μm-thick electrolytic copper foil. This electrolytic copper foil contained carbon and sulfur at concentrations of 70 mass ppm each. Finally, the negative electrode mixture slurry was applied to the negative electrode current collector and dried at 100°C in a vacuum atmosphere for 1 hour. After drying, the deposition amount of the negative electrode active material layer per unit area on one side of the negative electrode (also referred to as area density) was 7.0 mg / cm. 2 It was.

[0124] [Assembling a coin battery for testing] Next, the solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed, and then an electrolyte salt (lithium hexafluorophosphate: LiPF6) was dissolved to prepare an electrolyte solution. In this case, the solvent composition was EC:DMC = 30:70 by volume, and the content of the electrolyte salt was 1 mol / kg relative to the solvent. As additives, vinylene carbonate (VC) and fluoroethylene carbonate (FEC) were added in amounts of 1.0 mass% and 2.0 mass%, respectively.

[0125] Next, a coin battery was assembled as follows: First, a 1 mm thick Li foil was punched out to a diameter of 16 mm and attached to an aluminum clad.

[0126] Next, the negative electrode obtained above was punched out to a diameter of 15 mm, and this was placed opposite a Li foil attached to an aluminum clad via a separator. After injecting an electrolyte, a 2032 coin battery was fabricated.

[0127] [Measurement of initial efficiency] The initial efficiency was measured under the following conditions. First, the coin battery prepared for the initial efficiency test was charged (initial charge) in CCCV mode at a charge rate equivalent to 0.03C. The CV was 0V and the cut-off current was 0.04mA. Next, CC discharge (initial discharge) was performed at a discharge rate of 0.03C and a discharge cut-off voltage of 1.2V.

[0128] When examining the initial charge-discharge characteristics, the initial efficiency (hereinafter sometimes referred to as initial efficiency) was calculated using the formula: initial efficiency (%) = (initial discharge capacity / initial charge capacity) × 100.

[0129] [Production and evaluation of lithium-ion secondary batteries] Based on the initial data obtained, the positive electrode was designed so that the negative electrode utilization rate would be 95%. The utilization rate was calculated using the following formula from the positive and negative electrode capacities obtained with the Li counter electrode. Utilization rate = (positive electrode capacity - negative electrode loss) / (negative electrode capacity - negative electrode loss) x 100 Based on this design, lithium ion secondary batteries (lithium ion secondary batteries as shown in FIG. 2) of the examples and comparative examples were manufactured. Battery evaluation was carried out on each of the lithium ion secondary batteries of the examples and comparative examples.

[0130] The cycle characteristics were investigated as follows. First, to stabilize the battery, two charge / discharge cycles were performed at 0.2C in an atmosphere of 25°C, and the discharge capacity at the second cycle was measured. The battery cycle characteristics were calculated from the discharge capacity at the third cycle, and the battery test was stopped after 1,000 cycles. The charge / discharge cycle was performed at 0.7C and 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. The fast charge characteristics were calculated from the discharge capacity at the third cycle, and the battery test was stopped after 500 cycles. The charge / discharge cycle was performed at 4C and 0.5C. The charge cut-off voltage was 4.3V, the discharge cut-off voltage was 2.5V, and the charge cut-off rate was 0.07C.

[0131] The results of each measurement are shown in Table 1. Table 1 also shows the results of Comparative Example 1 and Examples 2 to 16, which will be described later.

[0132] [Table 1]

[0133] A cross-sectional TEM photograph of the negative electrode active material of Example 1 is shown in Figure 4. In this cross-sectional TEM photograph, the white parts are Si and the rest are carbon. It can be seen that the white lines form a mesh pattern all over the surface. The analysis results of XAFS measurements (XANES spectra) before and after charging the negative electrode in Example 1 (and Comparative Example 1 described later) are shown in Figure 7. The XANES spectra in Figure 7 show that low-valence SiOx is distributed in various states. The negative electrode active material of Example 1 29 The Si-CP / MAS-NMR spectrum is shown in Figure 10. A photograph of the appearance of the negative electrode after charge and discharge in Example 1 is shown in Figure 5. This shows the wound negative electrode in an unrolled state. As can be seen from Figure 10, the maximum value is at -91 ppm, indicating that the SiOSi structure is sufficiently contained. If there are many Si-O bonds, when it reacts with Li, Li 15 Since it is known that Si4 is not produced, the surface layer does not become highly resistive and Li does not precipitate (see Figure 5).

[0134] When a porous carbon structure with branched pores is prepared, as in Example 1 (and Examples 2-16 described below), the valence is easily controlled in the oxidation process because each wire (net diameter) in the Si portion is thin. Furthermore, the oxidation process can be further controlled by generating OH groups inside the structure using a flow of nitrogen gas containing moisture.

[0135] (Comparative Example 1) In Example 1, an alkali activation treatment was performed on a material with few actual pores (prepared by activation treatment with water vapor (1000°C)) to prepare a porous carbon structure having branched pores inside, but in Comparative Example 1, no alkali activation treatment was performed, and a porous carbon structure was prepared by only the above-mentioned activation treatment with water vapor (1000°C), which does not have branched pores inside.

[0136] Thereafter, the following steps were performed in sequence under the same conditions as in Example 1: evacuation, pressure recovery with nitrogen and heating, flow of nitrogen gas containing moisture, deposition of Si, cooling, and oxidation (introduction of oxygen diluted with nitrogen gas).Then, the sample was cooled to room temperature and exposed to the atmosphere, and then removed. Various measurements were carried out in the same manner as in Example 1, and the results are shown in Table 1. Although the capacity and initial coulombic efficiency were high, the reactivity with the electrolyte was high, resulting in a deterioration in the battery cycle characteristics, and in particular in the high-speed chargeability due to Li deposition.

[0137] A cross-sectional TEM photograph of the negative electrode active material of Comparative Example 1 is shown in FIG. 29 The Si-CP / MAS-NMR spectrum is shown in FIG. The white parts in the cross-sectional TEM photograph in Figure 6 are Si, and the rest is carbon. It can be seen that the white silicon phase exists as islands in a sea of ​​carbon (sea-island structure). Because it exists as islands, even when oxidation treatment is performed, only the surface layer of the grains is oxidized. As a result, Figure 7 shows a large separation between zero valence (before charging) and four valence (after charging). Since only the surface layer is oxidized, there are many Si-Si bonds and little SiOSi (see Figure 8). As mentioned above, in Comparative Example 1, the same oxidation treatment as in Example 1 was performed, and a flow of nitrogen gas containing moisture was also performed in advance to attach OH groups to the rear end. As a result, the surface layer of the deposited Si part was cleanly oxidized.

[0138] In addition, Fig. 9 shows a photograph of the appearance of the negative electrode after charging and discharging in Comparative Example 1. Looking at Fig. 7, Li 15 Si4 is formed on the surface. Li 15Si4 is an ionic substance and an insulator, and since it has poor permeability to Li, it is thought that Li precipitated throughout the entire surface during high-speed charging, as shown in Figure 9. Furthermore, when a sea-island structure is used, the paths connecting the islands become narrower, resulting in poor diffusion of Li. In the negative electrode of Comparative Example 1, the XANES spectrum of the bulk interior indicates this phenomenon (Li 15 The formation of Si4) could not be confirmed.

[0139] (Examples 2 to 5) The oxidation method was changed from Example 1. For example, when the degree of oxidation was to be lowered, oxygen diluted with more nitrogen was introduced to suppress the rise in internal temperature. When the degree of oxidation was to be higher, the oxygen concentration was increased within the range of an upper limit of 50°C and oxidation was carried out. 29 The Si-CP / MAS-NMR spectrum shifts to the low magnetic field side when the oxygen content is low, and shifts to the high magnetic field side when the oxygen content is high. If the shift exceeds -95 ppm, the low valence state cannot be maintained and a sample that can be evaluated in a battery cannot be produced. Therefore, in Examples 2 to 5, the range was set to -81 to -95 ppm. Among these, when the amount of oxidation was relatively small (Examples 4 and 5), the SiOSi structure decreased, and the high-speed charging performance was slightly worse than in Example 1.

[0140] (Examples 6, 7, and 13) This is an example of increasing the ratio of carbon active material in the negative electrode. This simply reduces the capacity. Since a capacity of 1300 mAh / g or more is desirable, the amount of carbon is also important.

[0141] Examples 8 to 12 A negative electrode active material was produced using the same porous carbon structure as in Example 1, but with the following conditions: First, after flowing nitrogen gas containing moisture, the temperature was raised to 415°C, and monosilane gas was introduced and deposited for 4 hours. Then, the temperature was raised to 415°C, and Si 0+ In this way, the grain size of Si, which exists in the low-valence nano-silicon oxide, was grown. 0+ The grain size of Si was changed. 0+As the crystal grain size of Si increases, the number of Si / Si grain boundaries also increases, which reduces Li diffusibility and deteriorates fast charging characteristics. Therefore, the grain size of zero-valent Si is better to be small, preferably 5 nm or less.

[0142] (Examples 14 to 16) A negative electrode active material with increased surface area and pore volume was produced in the same manner as in Example 1, except that the carbonization conditions of the raw material and the activation treatment temperature were changed when preparing a porous carbon structure having branched pores therein. In addition, the same evaluations as in Example 1 were carried out. From the results of Examples 14 to 16, it is possible to simply increase the capacity of the active material by increasing the surface area and pore volume. Considering the balance between capacity and efficiency, the pore volume is 0.95 cm 3 / g or more, BET specific surface area 1325m 2 / g or more, and IUPAC classification is considered to be type I.

[0143] The present specification includes the following aspects. [1]: A negative electrode active material having negative electrode active material particles, the negative electrode active material particles contain a porous carbon structure having branched pores formed by alkali activation treatment therein; The negative electrode active material is characterized in that amorphous low-valent nano-silicon oxide is dispersed in a network structure inside the porous carbon structure. [2]: When XAFS measurements are performed on a pre-charged negative electrode removed from a secondary battery having a negative electrode containing the negative electrode active material before charging the secondary battery, and a post-charged negative electrode removed from the secondary battery after charging the pre-charged secondary battery, the negative electrode active material has a high resistance to charge. In the Si K-absorption edge XANES spectrum obtained from the XAFS measurement of the negative electrode before and after charging, the peak attributable to a tetracoordinated silicon compound in the energy range of 1846 eV to 1850 eV is higher in the negative electrode before charging than in the negative electrode after charging, The negative electrode active material of [1] above, wherein the XANES spectrum of the negative electrode after charging has a peak attributable to a hexacoordinated silicon compound in the energy range of 1851 eV to 1855 eV. [3]: The negative electrode active material according to [2] above, which has energy peaks in the vicinity of 1845.5 eV and 1847.5 eV in the XANES spectrum of the negative electrode before charging. [4]: The negative electrode active material is solid 29 The negative electrode active material according to any one of [1] to [3] above, which has a maximum value in the range of -81 to -95 ppm when measured by Si-CP / MAS-NMR. [5]: The negative electrode active material according to any one of [1] to [4], wherein the negative electrode active material particles have an SiOSi structure in which oxygen is bonded to a Si radical. [6]: The negative electrode active material according to any one of [1] to [5], wherein the low-valence nanosilicon oxide is substantially in a composite state of zero valence, monovalent, and divalent, with divalent being the most predominant. [7]: The negative electrode active material according to any one of [1] to [6], wherein the low-valent nanosilicon oxide dispersed in the porous carbon structure has x increasing from the center of the porous carbon structure toward the surface. [8]: The negative electrode active material according to any one of [1] to [7], wherein the proportion of the porous carbon structure in the entire negative electrode active material particles is 45% by mass or more and 64% by mass or less. [9]: The negative electrode active material according to any one of [1] to [8], wherein the grain size of zero-valent Si constituting the low-valent nanosilicon oxide, calculated using the Scherrer equation from the peak measured by X-ray diffraction measurement of the negative electrode active material particles, is in the range of 1 nm to 5 nm.

[10] : The porous carbon structure is predominantly type I in the IUPAC classification, and its surface area is 1325 m 2 / g or more, pore volume is 0.95 cm 3 The negative electrode active material according to any one of [1] to [9] above, wherein the ionic liquid solubility (CO solubility) is 0.1 / g or more.

[11] : A method for producing a negative electrode active material having negative electrode active material particles, comprising: preparing a porous carbon structure having branched pores therein by a manufacturing method using an alkali activation treatment; flowing nitrogen gas containing moisture through the porous carbon structure; a step of flowing the moisture-containing nitrogen gas and then flowing monosilane gas into the porous carbon structure under heating, thereby depositing silicon inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; After the cooling, the temperature of the material in which silicon is deposited inside the porous carbon structure is adjusted to be maintained at 50° C. or less, and oxygen diluted with nitrogen gas is introduced into the material in which silicon is deposited inside the porous carbon structure, thereby converting at least a portion of the silicon into low-valence nano-silicon oxide. and thereby producing negative electrode active material particles in which amorphous low-valent nano silicon oxide is dispersed in a network structure inside the porous carbon structure.

[0144] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and that exhibits similar effects is included within the technical scope of the present invention. [Explanation of symbols]

[0145] 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 component.

Claims

1. A negative electrode active material having negative electrode active material particles, the negative electrode active material particles contain a porous carbon structure having branched pores formed by alkali activation treatment therein; The negative electrode active material is characterized in that amorphous low-valent nano-silicon oxide is dispersed in a network structure inside the porous carbon structure.

2. When XAFS measurements were performed on a pre-charged negative electrode removed from a secondary battery having a negative electrode containing the negative electrode active material before charging the secondary battery, and a post-charged negative electrode removed from the secondary battery after charging the pre-charged secondary battery, the negative electrode active material contained the negative electrode active material exhibited the following properties: In the Si K-absorption edge XANES spectrum obtained from XAFS measurement of the negative electrode before and after charging, the peak attributable to a tetracoordinated silicon compound in the energy range of 1846 eV to 1850 eV is higher in the negative electrode before charging than in the negative electrode after charging, 2. The negative electrode active material according to claim 1, wherein the XANES spectrum of the negative electrode after charging has a peak attributable to a hexacoordinated silicon compound in the energy range of 1851 eV to 1855 eV.

3. 3. The negative electrode active material according to claim 2, wherein the XANES spectrum of the negative electrode before charging has peaks at energies of approximately 1845.5 eV and 1847.5 eV.

4. The negative electrode active material is solid ２９ 2. The negative electrode active material according to claim 1, which has a maximum value in the range of −81 to −95 ppm when measured by Si-CP / MAS-NMR.

5. 2. The negative electrode active material according to claim 1, wherein the negative electrode active material particles have a SiOSi structure in which oxygen is bonded to a Si radical.

6. The negative electrode active material according to claim 1, wherein the low-valent nano-silicon oxide is substantially in a complex state of zero valence, monovalent, and divalent, with divalent being the most predominant.

7. 2. The negative electrode active material according to claim 1, wherein the low-valent nanosilicon oxide dispersed in the porous carbon structure has x increasing from the center of the porous carbon structure to the surface layer.

8. 2. The negative electrode active material according to claim 1, wherein the proportion of the porous carbon structure in the entire negative electrode active material particles is 45% by mass or more and 64% by mass or less.

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

10. The structure of the porous carbon is predominantly Type I in the IUPAC classification, and its surface area is 1325 m ２ / g or more, pore volume is 0.95 cm ３ 2. The negative electrode active material according to claim 1, wherein the negative electrode active material has a molecular weight of 1 / g or more.

11. A method for producing a negative electrode active material having negative electrode active material particles, comprising: preparing a porous carbon structure having branched pores therein by a manufacturing method using an alkali activation treatment; flowing nitrogen gas containing moisture through the porous carbon structure; a step of flowing the moisture-containing nitrogen gas and then flowing monosilane gas into the porous carbon structure under heating, thereby depositing silicon inside the porous carbon structure; cooling the material with silicon deposited inside the porous carbon structure to 50°C or less; After the cooling, the temperature of the material in which silicon is deposited inside the porous carbon structure is adjusted to be maintained at 50° C. or less, and oxygen diluted with nitrogen gas is introduced into the material in which silicon is deposited inside the porous carbon structure, thereby converting at least a portion of the silicon into low-valence nano-silicon oxide. and thereby producing negative electrode active material particles in which amorphous low-valent nano silicon oxide is dispersed in a network structure inside the porous carbon structure.

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