Negative electrode active material and method for producing the same

Abstract

Provided is a negative electrode active material that can increase capacity while maintaining battery characteristics. [Solution] A negative electrode active material having negative electrode active material particles, the negative electrode active material particles including a porous carbon structure as a base material, amorphous low-valent nano-silicon oxide dispersed inside the porous carbon structure, the negative electrode active material particles having a part or all of their surfaces covered with a carbon-based layer separate from the porous carbon base material, Si-C bonds present in at least a part of the negative electrode active material particles, and the negative electrode active material containing, in a surface layer portion extending to a depth of 50 nm from the surface of the negative electrode active material particles, the low-valent nano-silicon oxide having a different Si phase structure from the low-valent nano-silicon oxide contained in a bulk portion extending to a depth of more than 50 nm from the surface of the negative electrode active material particles.

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 cycle characteristics, the molar ratio of oxygen to silicon in the negative electrode active material is set to 0.1 to 1.2, and the active material is controlled within a range where the difference between the maximum value and the minimum value of the molar ratio near the active material and current collector interface is 0.4 or less (see, for example, Patent Document 7). Further, in order to improve the battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 8). Further, in order to improve cycle characteristics, a hydrophobic layer such as a silane compound is formed on the surface layer of the silicon material (see, for example, Patent Document 9).

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

[0011] Further, Hitachi Maxell started shipping a rectangular secondary battery for smartphones employing a nanosilicon composite in June 2010 in 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). Further, Kapaklis has proposed a disproportionation structure that is separated 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, wherein the negative electrode active material particles include a porous carbon structure as a base material, and amorphous low-valent nano-silicon oxide is dispersed inside the porous carbon structure, and the negative electrode active material particles have a part or all of their surfaces covered with a carbon-based layer other than the porous carbon base material, and Si-C bonds are present in at least a part of the negative electrode active material particles, and the negative electrode active material particles contain, in a surface layer portion extending to a depth of 50 nm from the surface of the negative electrode active material particles, the low-valent nano-silicon oxide having a different Si phase structure from the low-valent nano-silicon oxide contained in a bulk portion extending more than 50 nm deep from the surface of the negative electrode active material particles.

[0023] The negative electrode active material of the present invention has amorphous low-valence nanosilicon oxide dispersed within a porous carbon structure, thereby reducing the adverse effects of expansion of the internal low-valence nanosilicon oxide due to the presence of the porous carbon structure. Furthermore, the presence of Si-C bonds in at least a portion of the negative electrode active material particles and the inclusion of low-valence nanosilicon oxide in the surface layer, which has a different Si phase structure from the low-valence nanosilicon oxide contained in the bulk portion of the negative electrode active material particles, can stabilize the structure and improve water resistance. As a result, the battery characteristics of a battery using a negative electrode active material having these properties can be improved.

[0024] In this case, the negative electrode active material particles 29 In the spectrum obtained by Si-MAS-NMR, it is preferable that a peak derived from an Si-C bond is observed in the range of 0 to -30 ppm, and a peak derived from Si or a low-valence Si oxide is observed in the vicinity of -40 to 80 ppm.

[0025] The presence of these peaks can more reliably indicate the presence of Si-C bonds and low-valence nano-silicon oxides.

[0026] In addition, in the total electron yield spectrum obtained from the XANES Si-edge spectrum of the negative electrode active material particles, Si 0+ or Si 1+ The peaks originating from Si and those present near 1845 eV and 1847.5 eV 2+ ,Si 3+ It is preferable that a peak derived from

[0027] The presence of these peaks can more reliably indicate the presence of low-valence nano-silicon oxide.

[0028] In addition, it is preferable that the fluorescent X-ray spectrum obtained from the XANES Si-kedge spectrum of the negative electrode active material particles has a peak attributable to a Si-C bond and at least one peak attributable to a low-valence oxide, both of which are obtained near 1846.5 eV.

[0029] The presence of these peaks can more reliably indicate the presence of Si-C bonds and low-valence nano-silicon oxides.

[0030] Furthermore, the low-valent nano silicon oxide is preferably substantially in a composite state of zero valence, monovalent, divalent, trivalent, and tetravalent.

[0031] By using such silicon oxide, it is possible to maintain a lower irreversible capacity than with general SiO.

[0032] It is also preferable that active sites exist inside the pores of the porous carbon constituting the negative electrode active material particles, and that some of the active sites are modified with OH groups.

[0033] In this way, the presence of active sites and modification with OH groups makes it easier for O-Si bonds to exist.

[0034] Furthermore, the interface between the carbon layer other than the porous carbon base material and the low-valence nano silicon oxide preferably has a C=O or CO bond.

[0035] By having such a C═O or CO bond, the adhesion of the carbon layer can be strengthened.

[0036] The grain size of zero-valent Si constituting the low-valent nano silicon oxide, calculated from the peak measured by X-ray diffraction measurement of the negative electrode active material particles using Scherrer's equation, is preferably in the range of 0.8 nm to 5 nm. The low-valent nano silicon oxide of the negative electrode active material particles is preferably substantially amorphous.

[0037] It is preferable that the negative electrode active material particles have such a grain size of zero-valent Si that is substantially amorphous, that is, the low-valent nano silicon oxide particles are substantially amorphous.

[0038] The amount of oxygen contained in the negative electrode active material particles as a whole is preferably in the range of 0.3 wt % to 8 wt %.

[0039] Such an amount of oxygen makes it possible to increase the battery capacity.

[0040] The present invention also provides a method for producing a negative electrode active material having negative electrode active material particles, the method comprising the steps of: preparing a porous carbon structure as a base material; incorporating OH groups into active sites contained in the porous carbon structure; placing the porous carbon structure in a heating vessel; heating the porous carbon structure in the heating vessel and placing it under a gas flow; and confirming that carbon monoxide gas or carbon dioxide gas derived from oxygen atoms contained in the active sites is detected in an exhaust step under the heating; and and a step of depositing a silicon oxide in a state containing dangling bonds by flowing monosilane gas into a porous carbon and decomposing the silane gas in a positive pressure atmosphere of 25 kPa to 80 kPa, thereby depositing the silicon oxide partially bonded to the oxygen atoms of the porous carbon inside the porous carbon structure; a step of oxidizing at least a portion of Si contained in the silicon oxide present in the surface layer of the porous carbon among the deposited silicon oxide; and a step of depositing a carbon layer at 600°C or less using a hydrocarbon gas after the oxidation.

[0041] With this method for producing a negative electrode active material, as described above, amorphous low-valent nanosilicon oxide is dispersed inside the porous carbon structure, Si-C bonds are present, and a negative electrode active material containing low-valent nanosilicon oxide with a controlled phase structure can be produced simply and efficiently. [Effects of the Invention]

[0042] The negative electrode active material of the present invention has amorphous low-valence nanosilicon oxide dispersed within a porous carbon structure, thereby reducing the adverse effects of expansion of the internal low-valence nanosilicon oxide due to the presence of the porous carbon structure. Furthermore, the presence of Si-C bonds in at least a portion of the negative electrode active material particles and the inclusion of low-valence nanosilicon oxide in the surface layer, which has a different Si phase structure from the low-valence nanosilicon oxide contained in the bulk portion of the negative electrode active material particles, can stabilize the structure and improve water resistance. As a result, the battery characteristics of a battery using a negative electrode active material having these properties can be improved.

[0043] In addition, the method for producing a negative electrode active material of the present invention can simply and efficiently produce a negative electrode active material containing low-valent nano-silicon oxide with a controlled phase structure, in which amorphous low-valent nano-silicon oxide is dispersed inside a porous carbon structure, and Si-C bonds are present. [Brief explanation of the drawings]

[0044] [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. DETAILED DESCRIPTION OF THE INVENTION

[0045] Hereinafter, the present invention will be described in detail with reference to the preferred embodiments, but the present invention is not limited to these.

[0046] 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.

[0047] 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.

[0048] The negative electrode active material of the present invention is a negative electrode active material having negative electrode active material particles, the negative electrode active material particles comprising a porous carbon structure as a base material, and amorphous low-valent nano-silicon oxide dispersed within the porous carbon structure, the negative electrode active material particles having a part or all of their surfaces covered with a carbon-based layer other than the porous carbon base material, at least a part of the negative electrode active material particles having an Si-C bond, and the negative electrode active material particles comprising, in a surface layer portion extending to a depth of 50 nm from the surface of the negative electrode active material particles, the low-valent nano-silicon oxide having a different Si phase structure from the low-valent nano-silicon oxide contained in a bulk portion extending more than 50 nm from the surface of the negative electrode active material particles.

[0049] A method for producing such a negative electrode active material will be described later.

[0050] [Negative electrode active material of the present invention] First, the negative electrode active material of the present invention will be described. The negative electrode active material 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 of expansion of the low-valence nanosilicon oxide inside.

[0051] Furthermore, by including a low-valence nanosilicon oxide having a Si-C bond in at least a portion of the negative electrode active material particles and a low-valence nanosilicon oxide having a different Si phase structure from the low-valence nanosilicon oxide contained in the bulk portion of the negative electrode active material particles in the surface layer portion, the structure can be stabilized and water resistance can be improved, resulting in good battery characteristics of a battery using a negative electrode active material having these properties.

[0052] In addition, general SiO is Si 4+ However, the negative electrode active material of the present invention contains amorphous low-valent nanosilicon oxide, which allows it to maintain a lower irreversible capacity than SiO. Furthermore, the Si-O bond can suppress decomposition of the electrolyte, thereby reducing the solid electrolyte interphase (SEI) that accumulates on the surface. In the present invention, it is sufficient that the amorphous low-valent nanosilicon oxide is dispersed inside the porous carbon structure, and some of the nanosilicon oxide may be crystalline.

[0053] 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.

[0054] Furthermore, in the negative electrode active material of the present invention, the presence of low valence nano silicon oxide and the presence of Si—C bonds can be measured by the following measurement method.

[0055] First, the negative electrode active material particles 29 In the spectrum obtained by Si-MAS-NMR, it is preferable that a peak derived from Si-C bonds is obtained in the range of 0 to -30 ppm, and a peak derived from Si or low-valent Si oxide is obtained in the range of -40 to 80 ppm. The presence of these peaks can more reliably indicate the presence of Si-C bonds and low-valent nano-silicon oxides.

[0056] Furthermore, in the negative electrode active material of the present invention, the low-valent nanosilicon oxide contained in the bulk portion, which is a range deeper than 50 nm from the surface of the negative electrode active material particle (low-valent nanosilicon oxide in the bulk portion), and the low-valent nanosilicon oxide contained in the surface portion, which is a range up to a depth of 50 nm from the surface of the negative electrode active material particle (low-valent nanosilicon oxide in the surface portion), are required to have different Si phase structures.

[0057] The different phases of such low-valent nano-silicon oxides can be identified by XANES measurements. Specifically, the total electron yield (TEY) method obtained from the XANES Si-k edge spectrum reveals the surface layer up to 50 nm, while the X-ray fluorescence (PFY) method can detect information about the bulk interior at a depth of approximately 5 μm.

[0058] The XANES (X-ray Absorption Near-Edge Structure) measurement can be carried out, for example, under the following conditions. Aichi Synchrotron Light Center, BL6N1 line used The sample was processed in an Ar atmosphere, stored in a transfer vessel, and connected to the BL6N1 line under the following conditions: Acceleration energy: 1.2 GeV Accumulated current: 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 bent cylindrical mirror Upstream slit opening: 10.0mm horizontal x 3.0mm vertical Beam size: 2.0mm horizontal x 1.0mm vertical Incident angle to sample: 45 degrees (incident angle 45 degrees), to measure fluorescence yield simultaneously Energy calibration: The peak position at the K2SO4 SK edge is calibrated to 2481.70 eV Measurement method: Total electron yield method by measuring sample current, X-ray fluorescence ·I0 measurement method: XANES measurement Au-mesh Cu-mesh during EXAFS measurement

[0059] In addition, in the total electron yield spectrum obtained from the XANES Si-kedge spectrum of the negative electrode active material particles, Si 0+ or Si 1+ The peaks originating from Si and those present near 1845 eV and 1847.5 eV 2+ ,Si 3+ The presence of these peaks can more reliably indicate the presence of low-valence nano silicon oxide.

[0060] Furthermore, the X-ray fluorescence spectrum obtained from the XANES Si-edge spectrum of the negative electrode active material particles preferably has a peak attributable to the Si-C bond and at least one peak attributable to the low-valent oxide, both of which are obtained near 1846.5 eV. The presence of these peaks can more reliably indicate the presence of the Si-C bond and the low-valent nano-silicon oxide.

[0061] In addition, in the negative electrode active material of the present invention, the low-valence nanosilicon oxide is preferably in a composite state of substantially zero-, mono-, di-, tri-, and tetravalent atoms. Because tetravalent Si constituting SiO is an irreversible component, in the negative electrode active material of the present invention, the low-valence nanosilicon oxide is preferably in a composite state of substantially zero-, mono-, di-, tri-, and tetravalent atoms, as described above. In particular, it is preferable that this low-valence nanosilicon oxide is substantially predominantly mono- to divalent. By making SiOx, which is divalent or less, predominant, the irreversible capacity is larger than that of silicon alone, but a lower irreversible capacity than that of general SiO can be maintained.

[0062] In the negative electrode active material of the present invention, it is preferable that active sites exist within the pores of the porous carbon constituting the negative electrode active material particles, and that some of the active sites are modified with OH groups. The presence of active sites and modification with OH groups facilitates the formation of O-Si bonds.

[0063] In the negative electrode active material of the present invention, the interface between the carbon layer other than the porous carbon matrix and the low-valent nanosilicon oxide preferably has a C=O or C-O bond, which can enhance the adhesion of the carbon layer.

[0064] 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 0.8 nm to 5 nm. Such a grain size of zero-valent Si that is substantially amorphous is preferable.

[0065] 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”

[0066] The valence of low-valence nano silicon oxide can be quantified by NMR (nuclear magnetic resonance) and XPS (X-ray photoelectron spectroscopy).

[0067] 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℃

[0068] 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.

[0069] In the negative electrode active material of the present invention, the amount of oxygen contained in the negative electrode active material particles as a whole is preferably in the range of 0.3 wt% to 8 wt%. Such an oxygen amount can increase the battery capacity.

[0070] <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.

[0071] [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.

[0072] [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).

[0073] 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.

[0074] 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.

[0075] [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.

[0076] [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.

[0077] First, a method for producing a negative electrode active material contained in a negative electrode will be described. The method for producing a negative electrode active material of the present invention is a method for producing a negative electrode active material having negative electrode active material particles, and includes the steps of preparing a porous carbon structure as a base material, incorporating OH groups into active sites contained in the porous carbon structure, placing the porous carbon structure in a heating vessel, heating the porous carbon structure in the heating vessel and placing it under a gas flow, and confirming that carbon monoxide gas or carbon dioxide gas derived from oxygen atoms contained in the active sites is detected in an exhaust step under the heating, and then, in a state where the carbon monoxide gas or carbon dioxide gas derived from oxygen atoms is detected in the exhaust step, The method for producing a negative electrode active material includes the steps of: flowing monosilane gas into the heating container and decomposing the silane gas in a positive pressure atmosphere of 25 kPa to 80 kPa, thereby depositing silicon oxide in a state containing dangling bonds and depositing silicon oxide partially bonded to the oxygen atoms of the porous carbon inside the structure of the porous carbon; oxidizing at least a portion of Si contained in the silicon oxide present in the surface layer of the porous carbon among the deposited silicon oxide; and, after the oxidation, depositing a carbon layer at 600°C or less using a hydrocarbon gas.

[0078] Conventionally, particles of silicon deposited inside porous carbon (Si-C), as they are commonly referred to, are produced using a fluidized bed tank in a heated atmosphere by spraying silane gas and carrier gas from the bottom to deposit silicon inside the porous carbon, which is then subjected to an oxidation process and then coated with carbon using hydrocarbon gas or coal tar pitch.

[0079] In this case, with such a conventional method, at least a part of the Si produced by the reaction of silane gas has a Si-H bond and is deposited in the Si-Si state. Furthermore, the material obtained in the oxidation process is SiO2, so Si 0+ and Si 4+ The composite is formed inside the porous carbon. SiO2 is present near the outermost layer, and ordinary SiC can be produced by applying carbon coating CVD to the top of it using hydrocarbon gas.

[0080] In contrast, the present invention utilizes the active sites of porous carbon to incorporate OH groups into the porous carbon structure. When heat treatment is performed in this state, CO is detected in the exhaust gas. Normally, CO gas is not emitted when the exhaust gas is completely dried, but when the exhaust gas contains a large amount of CO and is reacted with silane gas in a pressurized atmosphere, silane decomposition products are deposited on the porous carbon as low-valent oxides. At this time, CO gas continues to be emitted from inside the porous pores, so it is important to use a hydrogen carrier in a pressurized atmosphere to distribute silane gas throughout the interior.

[0081] In a general fluidized bed, the pressure loss is about 10 kPa, but it is known that the reaction of the present invention gradually starts from about 25 kPa. Low valence silicon oxide has high diffusivity of Li. Generally, compounds of Si and Li have high diffusivity of Li. 15 While Si4 is produced, which exhibits ionic properties and reduced diffusibility, low-valence oxides can reduce the production of ionic substances. Furthermore, by increasing the pressure during the reaction when depositing silicon oxide, it is possible to create many dangling bonds. These dangling bonds are highly reactive and can be carbonized at temperatures lower than the general decomposition temperature of hydrocarbon gas (acetylene gas in this case). In addition, some of the Si has Si-C bonds, which not only stabilizes the structure but also significantly improves water resistance.

[0082] In the present invention, silicon oxide can be deposited deep into the pores of porous carbon by feeding silane gas into the pores under a pressure ranging from 25 kPa to 80 kPa. While the reaction rate tends to be slower than in conventional methods, productivity does not deteriorate more than necessary as long as the pressure range is 80 kPa or less. This allows silicon oxide, which has a lower true density than activated carbon, to be deposited, resulting in a negative electrode active material with sufficient capacity as negative electrode active material particles.

[0083] To increase the pressure, the internal pressure is adjusted by adjusting the flow rate of silane introduced and the degree of valve opening on the exhaust side. A flow vessel is generally at around 10 kPa, but pressure control becomes difficult above 25 kPa, so in addition to adjusting the opening valve as described above, a vent valve is also provided. For example, if you want to run the reaction at 50 kPa, the internal pressure can be adjusted by operating the vent valve in the 50 + 5 kPa range. From a productivity perspective, this is a significant deterioration, but using this method makes it possible to distribute the gas to every detail.

[0084] Furthermore, when the porous carbon contains OH groups in its pores, it reacts with silane gas to generate CO-Si moieties. This O-Si bond has excellent diffusivity for Li, improving high-speed charging. Furthermore, when heat is applied during subsequent CVD with hydrocarbon gas, Si is less likely to crystallize, and the amorphous state can be maintained.

[0085] Furthermore, increasing the pressure during the reaction during silicon oxide deposition allows for the presence of many dangling bonds. These dangling bonds are highly reactive, and can be carbonized at temperatures (e.g., below 530°C) lower than the general decomposition temperature of hydrocarbon gases (e.g., acetylene gas).

[0086] That is, the method includes a step of depositing silicon oxide in a state in which dangling bonds are included, and further, after the silicon oxide is deposited, a step of depositing a carbon layer using a hydrocarbon gas.

[0087] Furthermore, by heat-treating the OH groups, CO gas can be detected in the exhaust gas. Silane gas is introduced after removing the moisture contained in the porous carbon, but CO gas continues to be emitted during this process. It is important to introduce silane gas and allow the reaction to proceed in this state. If silane gas is introduced without gas detection, O-Si is unlikely to be produced. Furthermore, after the silane decomposition reaction, the silane is in a highly active state. If oxygen is introduced in this state, it will become SiO2 (tetravalent), and the low-valence state cannot be maintained. Therefore, in the present invention, as described above, it is necessary to begin flowing monosilane gas into the heating vessel in a state in which carbon monoxide gas or carbon dioxide gas derived from oxygen atoms can be detected during the exhaust process, and then decompose the silane gas in a positive pressure atmosphere of 25 kPa to 80 kPa to deposit silicon oxide.

[0088] The carbon CVD coating deposited on the silicon compound near the surface uses some of the oxygen component of the silicon to generate C=O or CO compounds. At this time, it is thought that a compound close to Si-OC is generated, which can strengthen the adhesion of the carbon layer, so soundness is maintained even when high-speed shear is applied during slurry production. Furthermore, the state of Si in the surface layer is dominated by low-valence oxides, and Si 0+ There are almost none.

[0089] With this method for producing a negative electrode active material, as described above, amorphous low-valent nanosilicon oxide is dispersed inside the porous carbon structure, Si-C bonds are present, and a negative electrode active material containing low-valent nanosilicon oxide with a controlled phase structure can be produced simply and efficiently.

[0090] Each step of the method for producing the negative electrode active material will be described with reference to steps S1 to S7 in FIG.

[0091] First, a porous carbon structure is prepared as a base material (step S1). The porous carbon structure prepared here is predominantly type I in the IUPAC classification, and its surface area is 1400 m 2 / g or more, pore volume is 1cm 3 / g or more is preferable. By using such IUPAC classification, surface area, and pore volume, silicon deposition can be performed efficiently in large 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

[0092] Porous carbon usable in the present invention may be derived from biological sources, resins, or petroleum, with biological or resin sources being preferred.

[0093] Next, OH groups are incorporated into the active sites contained in the porous carbon structure prepared in step S1 (step S2). In this process, the porous carbon structure is exposed to air (gas containing moisture), thereby allowing OH groups to be incorporated into the active sites contained in the porous carbon structure.

[0094] The active sites present inside the porous carbon structure are generated by the activation treatment. As will be described later, the presence of these active sites can be confirmed by the results of the heat treatment after OH group treatment, as they appear in the exhaust gas as carbon dioxide and carbon monoxide.

[0095] Next, the porous carbon structure is placed in a heating vessel (step S3). Note that step S2 may be performed after step S3. That is, after the porous carbon structure is placed in the heating vessel, OH groups may be incorporated into the active sites contained in the porous carbon structure.

[0096] Next, the porous carbon structure is heated in a heating vessel and placed under a gas flow, and it is confirmed that carbon monoxide gas or carbon dioxide gas derived from the oxygen atoms contained in the active sites is detected during the evacuation process under this heating condition (step S4).

[0097] More specifically, step S4 can be performed as follows, but is not limited to this. First, the porous carbon structure is placed in a reaction vessel and heated to approximately 150°C while nitrogen gas is flowing through it. At this time, a trap is installed in the exhaust gas section to ensure that moisture is not completely discharged. At the same time, a gas detector is used to check the CO or CO2 gas concentration. Typically, the gas concentration in a state where moisture is not being released is approximately 8000 ppm.

[0098] Next, step S5 is performed in a state where carbon monoxide gas or carbon dioxide gas derived from oxygen atoms is detected in the exhaust process. In this state, monosilane gas is started to flow into the heating vessel, and the silane gas is decomposed in a positive pressure atmosphere of 25 kPa to 80 kPa. This causes silicon oxide to deposit inside the porous carbon structure. At this time, silicon oxide partially bonded to the oxygen atoms of the porous carbon is deposited.

[0099] More specifically, step S5 can be performed as follows, but is not limited to this. First, the heating vessel is heated to 400°C to 500°C. In this case, the porous carbon is typically heated to 380°C or below. When the internal temperature reaches 350°C, silane gas is introduced from the bottom to deposit silicon inside the porous carbon. The pore volume of the porous carbon is estimated in advance from pore distribution measurement, and a silane amount of 0.9 times the pore volume is introduced, preferably controlling the reaction rate at 90%. Furthermore, the pressure inside the vessel fluctuates, but is averaged between 25 kPa and 80 kPa. Deposition at pressures above 80 kPa may result in a denser film, but considering the durability of the equipment, it is stopped at 80 kPa. This silicon deposition can be performed by flowing monosilane gas at, for example, approximately 400°C to 500°C. The deposition time can be, for example, 30 minutes to 10 hours.

[0100] Next, at least a portion of the Si contained in the deposited silicon oxide present in the surface layer of the porous carbon is oxidized (step S6).

[0101] More specifically, this step S6 can be performed as follows, but is not limited to this. After the reaction in step S5, the temperature of the material is temporarily lowered to room temperature, and oxygen diluted with nitrogen is introduced to increase the weight of the oxygen by about 0.5 wt%. At this time, it is important to use a stirrer to remove heat from the powder. Rapid oxidation will cause SiO2, which will deteriorate the properties.

[0102] This oxidation process allows the formation of a negative electrode active material structure having different Si phase structures, with the low-valent nanosilicon oxide contained in the bulk portion, which is a range deeper than 50 nm from the surface of the negative electrode active material particle (low-valent nanosilicon oxide in the bulk portion), and the low-valent nanosilicon oxide contained in the surface portion, which is a range up to a depth of 50 nm from the surface of the negative electrode active material particle (low-valent nanosilicon oxide in the surface portion).

[0103] The surface state can also be changed by adjusting it as follows. For example, at the end of the silane reaction (step S5), the ratio of silane to hydrogen inside the heating vessel can be reduced from 10% to 3%, which increases the amount of monohydride. Controlling the state of monohydride incorporated inside the bulk facilitates the change in valence during oxidation treatment. Silicon parts formed with a high silane ratio are relatively easy to convert to SiO2. However, this changes with deheating, pressure, etc., so fine adjustments are important.

[0104] Next, after oxidation, a carbon layer is deposited at 600°C or less using hydrocarbon gas (step S7). After the reaction in step S6, when the internal temperature drops to a low temperature (for example, 530 to 600°C) in a nitrogen atmosphere, acetylene gas is introduced to form a surface carbon film. The pressure at this time can be 10,000 Pa, and the process can be carried out for 8 hours.

[0105] The resulting material was analyzed by XRD, and the crystallite size of the Si was calculated using the Scherrer equation, resulting in a typical value of 0.8 nm. However, this is merely a calculation, and the material is considered essentially amorphous. The reaction is carried out in a reduced pressure atmosphere, allowing acetylene gas to be distributed throughout the Si filled inside the pores. This reacts with the Si-H bonds that did not reach a low valence oxidation state, forming Si-C bonds, resulting in a stable material.

[0106] The oxidation step in step S6 is preferably carried out under reduced pressure. This oxidation step promotes oxidation of the area near the surface. While the oxygen component contained in the porous carbon is used inside the bulk, the area near the surface has a low amount of oxygen, so oxygen must be added from the outside.

[0107] Furthermore, some of the Si-O compounds formed in the oxidation process in step S6 above form compounds during the C-CVD process in step S7 (confirmed by XPS), thereby improving the adhesion between the carbon coating and the Si portion.

[0108] <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.

[0109] [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.

[0110] 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.

[0111] 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.

[0112] 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.

[0113] 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.

[0114] The positive electrode current collector is made of a conductive material such as aluminum.

[0115] The positive electrode active material layer contains one or more positive electrode materials capable of absorbing and releasing lithium ions, and may contain other materials such as a positive electrode binder, a positive electrode conductive additive, a dispersant, etc. In this case, details regarding the positive electrode binder and the positive electrode conductive additive are the same as, for example, the negative electrode binder and the negative electrode conductive additive already described.

[0116] A lithium-containing compound is desirable as a positive electrode material. Examples of the lithium-containing compound include a composite oxide made of lithium and a transition metal element, or a phosphate compound containing lithium and a transition metal element. Among these positive electrode materials, compounds containing at least one of nickel, iron, manganese, and cobalt are preferred. Their chemical formulas include, for example, Lix M1O2 or Li y It is represented by M2PO4. In the formula, M1 and M2 represent at least one or more transition metal elements. The values of x and y vary depending on the charge and discharge state of the battery, but generally are represented by 0.05 ≦ x ≦ 1.10 and 0.05 ≦ y ≦ 1.10.

[0117] 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, for example, lithium nickel cobalt aluminum composite oxide (NCA), lithium nickel cobalt manganese composite oxide (NCM) and the like.

[0118] 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 positive electrode materials, a high battery capacity can be obtained and excellent cycle characteristics can also be obtained.

[0119] [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, the negative electrode has negative electrode active material layers 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.

[0120] 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.

[0121] 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.

[0122] [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.

[0123] [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.

[0124] 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.

[0125] 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).

[0126] 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.

[0127] 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.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] 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).

[0132] 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]

[0133] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the examples and comparative examples, a negative electrode active material was prepared according to the following procedure, and a laminate film-type lithium ion secondary battery 30 shown in FIG. 2 was then prepared.

[0134] (Comparative Example 1) A negative electrode active material was produced as follows. First, a surface area (BET specific surface area) of 1940 m 2 / g, pore volume 1.00 cm 3A porous carbon material (porous carbon structure) of IUPAC type I, with a molecular weight of 1 / g and a particle size (D50) of 9 μm, was prepared. Next, this porous carbon material was placed in air to incorporate OH groups. This porous carbon material was then placed in a vacuum chamber, purged with nitrogen, and heated to 380°C using an external heater while nitrogen was flowing. At this time, CO gas and CO2 gas were not detected (below the detection limit). With no carbon monoxide or carbon dioxide gases detected during the evacuation process, monosilane gas was started to flow into the heating chamber. Furthermore, with OH groups contained in the pores of the porous carbon, silane gas was flowed while controlling the pressure inside the chamber to 50 kPa to deposit silicon. During drying prior to the silane reaction, amorphous silicon was formed within the bulk by the silane decomposition reaction (380°C) without detecting CO gas (below the detection limit).

[0135] The reaction was completed when the pore volume reached a filling rate of 98%, after which the material was cooled and returned to room temperature. Oxygen diluted with nitrogen (1% concentration) was then flowed through the material for 48 hours to gently oxidize it. The high oxygen concentration caused an exothermic reaction with the highly active powder, resulting not only in the formation of SiO2 but also in the transformation into a high BET material. The material was then heated to an internal temperature of 580-600°C in a nitrogen atmosphere, and acetylene gas was introduced at 10,000 Pa for 8 hours to form a surface carbon film.

[0136] [Calculation of crystallite size] The crystallite size of the negative electrode active material particles was calculated by XRD measurement using the above-mentioned D2 PHASER manufactured by Bruker.

[0137] [Measurement of different phases of low-valence nano-silicon oxide] XANES measurements revealed that the low-valent nano-silicon oxide in the bulk of the negative electrode active material particles and the low-valent nano-silicon oxide in the surface layer have different Si phase structures. Specifically, the total electron yield (TEY) method obtained from the XANES Si-k edge spectrum shows the surface layer up to 50 nm, while the X-ray fluorescence (PFY) method can detect information about the interior of the bulk at a depth of approximately 5 μm.

[0138] [Measurement of the number of silicon dangling bonds] The number of silicon dangling bonds was measured using ESR.

[0139] [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.

[0140] 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.

[0141] [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.

[0142] 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.

[0143] 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.

[0144] [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.

[0145] 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.

[0146] [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.

[0147] 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 at 1000 cycles. Charging and discharging were performed at 0.7C and 0.5C, respectively. The charging voltage was 4.3V, the discharge cut-off voltage was 2.5V, and the charge cut-off rate was 0.07C.

[0148] [Slurry gas generation measurement] The negative electrode active material was mixed with pure water in a weight ratio of 1:4 to prepare a slurry. The prepared slurry was sealed in an airtight aluminum laminated bag, and the amount of gas generated from the slurry was measured 7 days after the slurry preparation. The amount of gas generated was calculated using a hydrometer from the difference in specific gravity between the time of slurry preparation and 7 days later.

[0149] The results of each measurement are shown in Tables 1 and 2. Tables 1 and 2 also show examples and comparative examples described below.

[0150] [Table 1]

[0151] [Table 2]

[0152] (Comparative Example 2) In Comparative Example 2, the oxidation process was carried out for 72 hours, and multiple valence states were formed so as not to form tetravalent (SiO2), as compared to Comparative Example 1. Looking at the NMR results, there was no significant difference between Comparative Examples 1 and 2, but the oxidized state was confirmed from the XANES results.

[0153] (Comparative Example 3) In Comparative Example 3, in addition to Comparative Example 1, acetylene CVD was performed at 580° C. to form Si—C bonds. If a sufficient SiO 2 layer is formed, Si—C crystallization does not proceed.

[0154] (Examples 1 and 2) The same procedure as in Comparative Example 1 was followed, except that the silane decomposition reaction was carried out under conditions where approximately 7500 ppm of CO gas was detected during drying prior to the silane reaction. The decomposition reaction could be carried out at temperatures between 350 and 360°C, below the decomposition temperature of silane. This is because the chamber was maintained at a positive pressure, allowing silane to be introduced into the pores. In addition, the molecular structure was distorted in the presence of CO gas, lowering the activation energy and accelerating the reaction rate. This enabled low-temperature decomposition, and the oxygen components attached to the porous carbon were captured during decomposition to form Si-O bonds. After the reaction, the chamber was returned to room temperature, and the same oxidation treatment as in Comparative Example 1 was performed to slightly enrich the Si surface layer in oxygen. Subsequently, CVD was carried out using acetylene gas at 540°C (Example 1) and 580°C (Example 2). This resulted in partial oxygen exchange at the interface between the oxygen-rich layer and the carbon layer, forming C=O or CO bonds, thereby ensuring adhesion.

[0155] Additionally, acetylene CVD is performed under reduced pressure of 10,000 Pa, allowing the acetylene to penetrate and react inside the porous carbon. The reaction breaks the bond in the Si-H portion that has been produced, creating an Si-C bond. This makes it possible to suppress the generation of hydrogen gas when the material is made into a slurry. The higher the CVD temperature, the more Si-C bonds are created.

[0156] In addition, the silicon part of the surface layer is in a low valence oxidation state, Si 0+ The surface layer is almost completely devoid of water (OH, to be precise), which is quite different from the bulk structure. - ) and the absence of any substance that reacts with the slurry also improves the stability of the slurry.

[0157] Examples 3 to 7 Similar to Example 2, however, a heat treatment was performed after the oxidation treatment to crystallize some of the Si. The aim of this was to reduce the Si-H bonds, but no significant reduction was observed. The Si-H bonds were confirmed by CP / MAS-NMR. Higher crystallinity leads to poorer Li acceptance and slightly worse battery performance. However, considering that the powder becomes easier to handle once the temperature is raised, crystallites of 5 nm or less are desirable. Battery performance is best achieved when the material is substantially amorphous (0.8 nm is calculated using the Scherrer equation and is considered to be substantially amorphous).

[0158] (Examples 8 to 13) The same procedure as in Example 2 was repeated, except that the amount of oxidation was varied. When the amount of oxygen was low, many Si-C bonds tended to be formed during acetylene CVD, resulting in a slight decrease in battery capacity. The amount of oxidation that did not significantly decrease was 0.3 wt% or higher. Furthermore, if the amount of oxygen was too high, SiO2 began to form, so there was an optimal range for the amount of oxygen.

[0159] (Examples 14 to 17) As in Example 2, the pressure during silane gas introduction was varied. If the pressure during silane gas introduction is low, the silane gas does not reach the depths of the pores, resulting in voids. This reduces the ability to fill silicon, resulting in a decrease in battery capacity. Furthermore, silicon decomposed and deposited in a pressurized atmosphere contains a large number of dangling bonds, but this is reduced when the pressure is low. The dangling bonds are highly active, allowing carbon to be decomposed at low temperatures during subsequent carbon CVD.

[0160] To increase the pressure, the internal pressure is adjusted by adjusting the silane flow rate and the valve opening on the exhaust side. A flow vessel is generally around 10 kPa, but pressure control becomes difficult above 25 kPa, so in addition to adjusting the opening valve as described above, a vent valve is also provided. For example, if you want to run the reaction at 50 kPa, the internal pressure can be adjusted by operating the vent valve in the 50 + 5 kPa range. From a productivity perspective, this is a significant deterioration, but using this method makes it possible to distribute the gas to every detail.

[0161] 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 including a porous carbon structure as a base material, and amorphous low-valence nano-silicon oxide dispersed within the porous carbon structure, the negative electrode active material particles having a part or all of their surfaces covered with a carbon-based layer separate from the porous carbon base material, at least a part of the negative electrode active material particles having an Si-C bond, and the negative electrode active material particles including, in a surface layer portion extending to a depth of 50 nm from the surface of the negative electrode active material particles, the low-valence nano-silicon oxide having a different Si phase structure from the low-valence nano-silicon oxide contained in a bulk portion extending more than 50 nm from the surface of the negative electrode active material particles. [2]: The negative electrode active material particles 29 The negative electrode active material of [1] above, in which a peak derived from a Si-C bond is observed in the range of 0 to -30 ppm and a peak derived from Si or a low-valent Si oxide is observed in the range of -40 to 80 ppm in a spectrum obtained by Si-MAS-NMR. [3]: In the total electron yield spectrum obtained from the XANES Si-edge spectrum of the negative electrode active material particles, Si 0+ or Si 1+ The peaks originating from Si and those present near 1845 eV and 1847.5 eV 2+ ,Si 3+ The negative electrode active material according to the above [1] or [2], wherein a peak derived from [4]: The negative electrode active material according to any one of [1] to [3] above, wherein the negative electrode active material has a peak attributable to a Si-C bond and at least one peak attributable to a low-valent oxide, both of which are observed in the vicinity of 1846.5 eV in a fluorescent X-ray spectrum obtained from an XANES Si-edge spectrum of the negative electrode active material particles. [5]: The negative electrode active material according to any one of [1] to [4], wherein the low-valence nanosilicon oxide is substantially in a composite state of zero valence, monovalent, divalent, trivalent, and tetravalent. [6]: The negative electrode active material according to any one of [1] to [5], wherein active sites are present inside the pores of the porous carbon constituting the negative electrode active material particles, and some of the active sites are modified with OH groups. [7]: The negative electrode active material according to any one of [1] to [6] above, wherein the interface between the carbon layer other than the porous carbon base material and the low-valence nanosilicon oxide has a C=O or CO bond. [8]: The negative electrode active material according to any one of [1] to [7], 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 0.8 nm to 5 nm. [9]: The negative electrode active material according to any one of [1] to [8], wherein the low-valent nanosilicon oxide of the negative electrode active material particles is substantially amorphous.

[10] : The negative electrode active material according to any one of [1] to [9], wherein the amount of oxygen contained in the negative electrode active material particles as a whole is in the range of 0.3 wt % or more and 8 wt % or less.

[11] : A method for producing a negative electrode active material having negative electrode active material particles, the method comprising the steps of: preparing a porous carbon structure as a base material; incorporating OH groups into active sites contained in the porous carbon structure; placing the porous carbon structure in a heating vessel; heating the porous carbon structure in the heating vessel and placing it under a gas flow; and confirming that carbon monoxide gas or carbon dioxide gas derived from oxygen atoms contained in the active sites is detected in an exhaust step under the heating; and and decomposing the silane gas in a positive pressure atmosphere of 25 kPa to 80 kPa to deposit silicon oxide in a state containing dangling bonds, thereby depositing silicon oxide partially bonded to the oxygen atoms of the porous carbon inside the porous carbon structure; oxidizing at least a portion of Si contained in the silicon oxide present in the surface layer of the porous carbon among the deposited silicon oxide; and depositing a carbon layer at 600°C or less using a hydrocarbon gas after the oxidation.

[0162] The present invention is not limited to the above-described embodiments, which 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 provides similar effects is included within the technical scope of the present invention. [Explanation of symbols]

[0163] 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 material.

Claims

1. A negative electrode active material having negative electrode active material particles, the negative electrode active material particles include a porous carbon structure serving as a base material, Amorphous low-valence nano silicon oxide is dispersed inside the porous carbon structure, the negative electrode active material particles have a surface partially or entirely covered with a carbon layer separate from the porous carbon matrix; a Si—C bond is present in at least a portion of the negative electrode active material particles, The negative electrode active material is characterized in that the surface layer portion of the negative electrode active material particles is comprised of a low-valent nanosilicon oxide having a different Si phase structure from the low-valent nanosilicon oxide contained in the bulk portion, which is a depth of more than 50 nm from the surface of the negative electrode active material particles, and in that the total electron yield spectrum obtained from the XANES Si-kedge spectrum of the negative electrode active material particles shows a peak derived from Si 0+ or ​​Si 1+ near 1843 eV and peaks derived from Si 2+ and Si 3+ near 1845 eV and 1847.5 eV.

2. The negative electrode active material particles ２９ 2. The negative electrode active material according to claim 1, wherein a spectrum obtained by Si-MAS-NMR shows a peak derived from a Si—C bond in the range of 0 to −30 ppm and a peak derived from Si or a low-valence Si oxide in the vicinity of −40 to 80 ppm.

3. The negative electrode active material according to claim 1, characterized in that the negative electrode active material particles have a peak attributable to a Si-C bond and at least one peak attributable to a low-valence oxide, both of which are obtained in the vicinity of 1846.5 eV in a fluorescent X-ray spectrum obtained from a XANES Si-kedge spectrum of the negative electrode active material particles.

4. The negative electrode active material according to claim 1 , wherein the low-valent nano-silicon oxide is substantially in a complex state of zero-valent, mono-valent, di-valent, tri-valent, and tetra-valent.

5. 2. The negative electrode active material according to claim 1, wherein active sites are present inside the pores of the porous carbon constituting the negative electrode active material particles, and some of the active sites are modified with OH groups.

6. The negative electrode active material according to claim 1, wherein the interface between the carbon layer other than the porous carbon base material and the low-valence nanosilicon oxide has a C=O or C-O bond.

7. The negative electrode active material particles have a 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. The negative electrode active material according to claim 1, characterized in that the grain size is in the range of 0.8 nm to 5 nm.

8. 2. The negative electrode active material according to claim 1, wherein the amount of oxygen contained in the negative electrode active material particles as a whole is in the range of 0.3 wt % to 8 wt %.

9. A method for producing a negative electrode active material having negative electrode active material particles, comprising: A step of preparing a porous carbon structure as a base material; a step of incorporating an OH group into the active site contained in the porous carbon structure; placing the porous carbon structure in a heating vessel; a step of heating the porous carbon structure in the heating vessel and placing it under a gas flow, and confirming that carbon monoxide gas or carbon dioxide gas derived from oxygen atoms contained in the active sites is detected in an exhaust step under the heating condition; a step of flowing monosilane gas into the heating container in a state in which carbon monoxide gas or carbon dioxide gas derived from oxygen atoms is detected in an exhausting step, and decomposing the silane gas in a positive pressure atmosphere of 25 kPa or more and 80 kPa or less to deposit silicon oxide in a state containing dangling bonds, thereby depositing silicon oxide partially bonded to the oxygen atoms of the porous carbon inside a structure of the porous carbon; a step of oxidizing at least a portion of Si contained in the silicon oxide present in a surface layer of the porous carbon among the deposited silicon oxide; and a step of depositing a carbon layer at 600°C or less using a hydrocarbon gas after the oxidation.

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