Negative electrode active material, mixed negative electrode active material, and method for producing negative electrode active material

Abstract

The present invention is a negative electrode active substance that includes negative electrode active substance particles. The negative electrode active substance is characterized in that the negative electrode active substance particles contain silicon oxide particles that are coated with a carbon layer, the position of a peak arising from the G-band on a Raman spectrum obtained by Raman spectroscopy for at least a portion of the carbon layer being above 1590 cm-1 but not above 1597 cm-1. The present invention can thereby provide a negative electrode active substance that can improve cycle characteristics when used in a secondary battery.

Description

[Technical Field]

[0001] The present invention relates to a negative electrode active material, a mixed negative electrode active material, and a method for producing the negative electrode active material. [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 highly anticipated because they can be easily miniaturized 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 main raw material for the negative electrode active material, the negative electrode active material expands and contracts during charging and discharging, making it prone to cracking, mainly near the surface of the negative electrode active material. 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 made of the electrolyte decomposition product is formed on the new surface, consuming the electrolyte. This can lead to a decrease in cycle performance.

[0007] To date, various studies have been conducted on negative electrode materials and electrode configurations for lithium-ion secondary batteries that use silicon materials as the main material 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 obtain high input / output characteristics, an active material containing silicon and oxygen is prepared, and an active material layer with a high oxygen ratio near the current collector is formed (see, for example, Patent Document 3). 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 a high temperature (see, for example, Patent Document 6). Further, in order to improve the cycle characteristics, the molar ratio of oxygen to silicon in the negative electrode active material is set to 0.1 to 1.2, and the active material is controlled within a range where the difference between the maximum value and the minimum value of the molar ratio near the interface between the active material and the current collector is 0.4 or less (see, for example, Patent Document 7). Further, in order to improve the battery load characteristics, a metal oxide containing lithium is used (see, for example, Patent Document 8). Further, in order to improve the cycle characteristics, a hydrophobic layer such as a silane compound is formed on the surface layer of the silicon material (see, for example, Patent Document 9). Further, in order to improve the cycle characteristics, silicon oxide is used, and conductivity is imparted by forming a graphite film on its surface layer (see, for example, Patent Document 10). In Patent Document 10, regarding the shift value obtained from the Raman spectrum of the graphite film, broad peaks appear at 1330 cm -1 and 1580 cm -1 , and the intensity ratio I 1330 / I 1580 is such that 1.5 < I 1330 / I 1580 < 3. Further, in order to improve the high battery capacity and cycle characteristics, particles having a silicon microcrystalline phase dispersed in silicon dioxide are used (see, for example, Patent Document 11). Further, in order to improve the overcharge and 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).

[0010] Further, there is proposed a negative electrode material for a non-aqueous electrolyte secondary battery including conductive powder in which the surface of particles of a silicon-based active material is coated with a conductive carbon film. In the Raman spectrum of the conductive carbon film, a g band is observed in the vicinity where the Raman shift is 1580 to 1590 cm -1 (see Patent Document 13).

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

[0012] As mentioned above, in recent years, small electronic devices such as mobile terminals have become more powerful and multifunctional, and this has led to a demand for increased battery capacity for lithium-ion secondary batteries, which are their main power sources. One approach to solving this problem is the development of lithium-ion secondary batteries with anodes that use silicon materials as the primary material. Furthermore, lithium-ion secondary batteries using silicon materials are expected to have cycle characteristics similar to those of lithium-ion secondary batteries that use carbon-based active materials. However, no anode active materials have been proposed that exhibit cycle stability equivalent to that of lithium-ion secondary batteries that use carbon-based active materials.

[0013] The present invention has been made in view of the above-mentioned problems, and aims to provide a negative electrode active material that can improve cycle characteristics when used as a negative electrode active material for a secondary battery, and a mixed negative electrode active material containing this negative electrode active material, and also aims to provide a method for producing the negative electrode active material of the present invention that can improve cycle characteristics. [Means for solving the problem]

[0014] In order to solve the above problems, the present invention provides a negative electrode active material including negative electrode active material particles, the negative electrode active material particles contain silicon oxide particles coated with a carbon layer, In the Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer, the peak position derived from the G band is 1590 cm -1 Over 1597cm -1 The present invention provides a negative electrode active material characterized by being in the following range:

[0015] The peak position derived from the G band obtained from such Raman spectroscopy is 1590 cm -1 Over 1597cm -1 In silicon oxide particles coated with a carbon layer within the following range, the carbon layer has a small number of graphene layers, and the number of multilayer graphene edges that cause side reactions with the electrolyte is small, so the formation of an SEI (Solid-Liquid Interface) film can be suppressed. As a result, the negative electrode active material of the present invention can improve the cycle characteristics when used as the negative electrode active material of a secondary battery.

[0016] Furthermore, since the negative electrode active material of the present invention contains silicon oxide particles, it can exhibit high capacity.

[0017] The negative electrode active material particles preferably have a crystallite size of 1.5 nm or less, which is obtained by Scherrer analysis of a peak attributable to the Si(111) crystal plane in an X-ray diffraction spectrum after waveform separation obtained by X-ray diffraction using Cu-Kα radiation.

[0018] Thus, a negative electrode active material having a crystallite size of 1.5 nm or less on the Si(111) crystal plane, as determined from the X-ray diffraction spectrum after waveform separation, has a composition with a lower proportion of crystalline Si, thereby suppressing side reactions with the electrolyte. As a result, such a negative electrode active material can further improve the cycle characteristics when used as a negative electrode active material in a secondary battery.

[0019] In addition, in an open circuit potential curve obtained by measuring the open circuit voltage of a test battery equipped with a test electrode containing the negative electrode active material and lithium metal as a counter electrode, the average potential of the test electrode in the charge state of the test battery range of 10% to 20% is 0.4 V vs. Li / Li + It is preferable that this is equal to or greater than this.

[0020] In this way, in the open circuit potential (OCP) curve of the test electrode of the test battery prepared using the negative electrode active material, the average potential of the test electrode when the state of charge (SOC) of the test battery is in the range of 10% to 20% is 0.4 V vs. Li / Li + In the above-mentioned negative electrode active materials, the proportion of the SiO component in the silicon oxide particles disproportionated into Si and SiO2 is sufficiently small, facilitating Li insertion. This improves diffusibility within the SiO bulk, thereby improving battery performance. Furthermore, since the proportion of crystallized Si in the Si component is low, side reactions with the electrolyte are suppressed. As a result, such negative electrode active materials can further improve cycle performance when used as negative electrode active materials in secondary batteries.

[0021] In addition, in a time-of-flight secondary ion mass spectrometry spectrum of at least a part of the carbon layer, positive secondary ions, C x H y O z It is particularly preferable that the intensity of the peak attributed to (x is 8 or more and 42 or less, y is 5 or more and 65 or less, and z is 1 or more and 5 or less) is 0.1 or more relative to the intensity of the peak attributed to Si.

[0022] In the present invention, the carbon layer surface of the silicon oxide particles was analyzed by TOF-SIMS. x H y O z When the intensity of the peaks attributed to Si (x is 8 to 42, y is 5 to 65, and z is 1 to 5) is in the range of 0.1 or more relative to the intensity of the peaks attributed to Si, the binder occupancy rate is appropriate, and the amount of binder that substantially covers the negative electrode active material particles is appropriate. Furthermore, with such a negative electrode active material, contact between the multilayer graphene edges and the electrolyte can be reduced, thereby suppressing side reactions with the electrolyte. As a result, when such a negative electrode active material is used as the negative electrode active material of a secondary battery, the cycle characteristics can be further improved.

[0023] Furthermore, the negative electrode active material particles preferably have a C1s peak position of 284.3 eV or less in an X-ray photoelectron spectrum obtained by X-ray photoelectron spectroscopy.

[0024] Thus, by having the C1s peak position of 284.3 eV or less in the XPS spectrum, contact between the multilayer graphene edges and the electrolyte at the carbon layer surface can be reduced, and side reactions with the electrolyte can be suppressed. As a result, such a negative electrode active material can further improve the cycle characteristics when used as the negative electrode active material of a secondary battery.

[0025] The negative electrode active material particles preferably have a median diameter of 4.5 μm or more and 15 μm or less.

[0026] A median diameter of 4.5 μm or more can prevent an increase in the irreversible capacity of the battery due to an increase in the surface area per mass, while a median diameter of 15 μm or less makes the particles less likely to crack and thus less likely to expose new surfaces.

[0027] It is also preferable that the proportion of particles having a particle diameter of 1 μm or less in the negative electrode active material particles is 2.5% or less on a volume basis.

[0028] When the proportion of particles having a particle diameter of 1 μm or less in the negative electrode active material particles is 2.5% or less by volume, an increase in side reactions with the electrolyte due to an increase in surface area per mass can be suppressed, and the cycle characteristics can be further improved when the negative electrode active material is used as a negative electrode active material for a secondary battery.

[0029] In addition, the present invention provides a negative electrode active material according to the present invention, Carbon-based active material The present invention provides a mixed negative electrode active material comprising:

[0030] In a negative electrode prepared using the mixed negative electrode active material of the present invention, the electrical resistance of the negative electrode active material layer is reduced and expansion stress caused by charging can be alleviated, resulting in further improved cycle characteristics when used as a negative electrode active material for a secondary battery.

[0031] The present invention also provides a method for producing a negative electrode active material including negative electrode active material particles containing silicon oxide particles coated with a carbon layer, the method comprising: preparing silicon oxide particles; a step of coating the silicon oxide particles with a carbon layer by a pyrolysis chemical vapor deposition method using a hydrocarbon gas at a temperature of 790°C or less to prepare negative electrode active material particles; In the Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer from the produced negative electrode active material particles, the peak position due to the G band is 1590 cm -1 Over 1597cm-1 a step of selecting negative electrode active material particles including the carbon layer that fall within the following range; a step of producing a negative electrode active material using the selected negative electrode active material particles; The present invention provides a method for producing a negative electrode active material, comprising:

[0032] The method for producing a negative electrode active material of the present invention makes it possible to produce a negative electrode active material that exhibits good cycle characteristics when used as a negative electrode active material for a secondary battery. [Effects of the Invention]

[0033] As described above, when the negative electrode active material of the present invention is used as a negative electrode active material for a secondary battery, high capacity and good cycle characteristics can be obtained.

[0034] Furthermore, when the mixed negative electrode active material of the present invention is used as a negative electrode active material for a secondary battery, even better cycle characteristics can be obtained.

[0035] Furthermore, the method for producing a negative electrode active material of the present invention makes it possible to produce a negative electrode active material that exhibits good cycle characteristics when used as a negative electrode active material for a secondary battery. [Brief explanation of the drawings]

[0036] [Figure 1] 1 is a Raman spectrum of a carbon layer in an example of the negative electrode active material of the present invention. [Figure 2] 1 is a schematic cross-sectional view of an example of a negative electrode containing the negative electrode active material of the present invention. [Figure 3] 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 4] 1 shows Raman spectra of carbon layers of negative electrode active materials of Example 1, Comparative Example 1, and Comparative Example 4. [Figure 5] 1 is a part of the XRD spectrum after waveform separation of the negative electrode active material of Example 1. [Figure 6]1 shows open circuit potential curves of test batteries fabricated using the negative electrode active materials of Example 1, Comparative Example 1, and Comparative Example 4. [Figure 7] 1 is a graph comparing the peak intensities of main positive secondary ions in the TOF-SIMS spectra of the surfaces of the negative electrode active materials of Example 1, Comparative Example 1, and Comparative Example 4. [Figure 8] 1 shows a portion of the XPS spectra of the negative electrode active materials of Example 1, Example 5, and Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0037] As mentioned above, one method for increasing the battery capacity of lithium-ion secondary batteries is to use a silicon-based anode as the anode of the lithium-ion secondary battery. However, no anode active material has been proposed that has the same cycle characteristics as lithium-ion secondary batteries that use carbon-based active materials.

[0038] Therefore, the present inventors have conducted extensive research to obtain a negative electrode active material that, when used in a secondary battery, provides high battery capacity and good cycle characteristics. As a result, they have found a negative electrode active material containing silicon oxide particles and a carbon layer covering the particles, and in the Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer, the peak position due to the G band is 1590 cm -1 Over 1597cm -1 The present inventors have found that a negative electrode active material within the following range can exhibit good cycle characteristics when used as a negative electrode active material for a secondary battery, and have arrived at the present invention.

[0039] That is, the present invention provides a negative electrode active material containing negative electrode active material particles, the negative electrode active material particles contain silicon oxide particles coated with a carbon layer, In the Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer, the peak position derived from the G band is 1590 cm -1 Over 1597cm -1 The negative electrode active material is characterized by being in the following range.

[0040] In addition, the present invention provides a negative electrode active material according to the present invention, Carbon-based active material The present invention also provides a mixed negative electrode active material comprising:

[0041] The present invention also provides a method for producing a negative electrode active material including negative electrode active material particles containing silicon oxide particles coated with a carbon layer, the method comprising: preparing silicon oxide particles; a step of coating the silicon oxide particles with a carbon layer by a pyrolysis chemical vapor deposition method using a hydrocarbon gas at a temperature of 790°C or less to prepare negative electrode active material particles; In the Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer from the produced negative electrode active material particles, the peak position due to the G band is 1590 cm -1 Over 1597cm -1 a step of selecting negative electrode active material particles including the carbon layer that fall within the following range; a step of producing a negative electrode active material using the selected negative electrode active material particles; Also provided is a method for producing a negative electrode active material, comprising:

[0042] The present invention will be described in detail below, but the present invention is not limited thereto.

[0043] [Negative electrode active material] The negative electrode active material of the present invention includes negative electrode active material particles, the negative electrode active material particles containing silicon oxide particles coated with a carbon layer, and the carbon layer has a peak position at 1590 cm due to a G band in a Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer. -1 Over 1597cm -1 It is characterized by being in the following range:

[0044] Such a negative electrode active material of the present invention contains negative electrode active material particles (also referred to as silicon-based active material particles) containing silicon oxide particles coated with a carbon layer, and therefore can improve battery capacity.

[0045] Furthermore, in silicon oxide particles coated with a carbon layer in which the peak position derived from the G band in the Raman spectrum obtained by Raman spectroscopy is within the above range, the number of graphene layers in the carbon layer is small. The reason for this is thought to be as follows, without wishing to be bound by theory.

[0046] The G band, one of the characteristic peaks of graphene, changes under the influence of strain and can be used as an indicator of the number of graphene layers. Specifically, the G band indicates the optical vibration of adjacent carbon atoms. In ordinary graphite, where layers are stacked, the G band peak position is 1582 cm due to the influence of interphase. -1 As the number of graphene layers decreases, the interphase effect disappears and the graphene approaches a single layer. As a result, the peak position of the G band shifts to the higher frequency side (G band upshift).

[0047] Fig. 1 shows the Raman spectrum of the carbon layer in one example of the negative electrode active material of the present invention. The peak position due to the G band in the Raman spectrum shown in Fig. 1 is 1593.5 cm -1 is.

[0048] In the negative electrode active material of the present invention, the peak position due to the G band in the Raman spectrum of the carbon layer is 1590 cm -1 Over 1597cm -1 Since the carbon layer has a small number of graphene layers and there are few edges of multilayer graphene that cause side reactions with the electrolyte, the formation of an SEI film can be suppressed. Therefore, when the negative electrode active material of the present invention is used in the negative electrode of a secondary battery, the cycle characteristics can be improved.

[0049] Therefore, when the negative electrode active material of the present invention is used in the negative electrode of a secondary battery, particularly a non-aqueous electrolyte battery such as a lithium ion secondary battery, it can achieve high battery capacity as well as excellent cycle characteristics.

[0050] On the other hand, the peak position due to the G band is 1590 cm -1 If the thickness is less than 1 / 2, the multilayer graphene has many edges, which can cause side reactions with the electrolyte, resulting in poor cycle characteristics.

[0051] Silicon oxide particles can be, for example, those having the general formula SiO x The silicon oxide material may be a silicon compound having a composition expressed as 0.8≦x≦1.2. In this composition, it is preferable that x is close to 1, because this results in high cycle performance. Note that the composition of the silicon oxide particles in the present invention does not necessarily mean 100% purity, and may contain trace amounts of impurity elements.

[0052] The negative electrode active material particles preferably have a crystallite size of 1.5 nm or less, which is obtained by Scherrer analysis of the peak attributable to the Si(111) crystal plane in an X-ray diffraction (XRD) spectrum after waveform separation obtained by X-ray diffraction using Cu-Kα rays.

[0053] Thus, a negative electrode active material having a crystallite size of 1.5 nm or less on the Si(111) crystal plane, as determined from the waveform-separated XRD spectrum, has a composition with a lower proportion of crystalline Si, thereby suppressing side reactions with the electrolyte. As a result, such a negative electrode active material can further improve the cycle characteristics when used as a negative electrode active material in a secondary battery.

[0054] The lower limit of the crystallite size of the Si(111) crystal plane obtained from the XRD spectrum after waveform separation is not particularly limited, but is, for example, 0.9 nm or more. Although the lower limit is set to 0.9 nm or more because Scherrer analysis cannot analyze crystallites smaller than 0.9 nm, an amorphous structure is actually preferable.

[0055] In addition, in the open circuit potential (OCP) curve obtained by measuring the open circuit voltage of a test battery equipped with a test electrode containing a negative electrode active material and lithium metal as a counter electrode, the average potential of the test electrode in the range of the state of charge (SOC) of the test battery from 10% to 20% is 0.4 V vs. Li / Li + It is preferable that this is equal to or greater than this.

[0056] In silicon oxide particles, SiO has a high potential, while disproportionated Si has a low potential. Therefore, the smaller the proportion of disproportionated Si, the higher the average potential of the test electrode in the OCP curve.

[0057] In the OCP curve of the test electrode of the test battery made using the negative electrode active material, the average potential of the test electrode when the state of charge (SOC) of the test battery is in the range of 10% to 20% is 0.4 V vs. Li / Li + In the above-mentioned negative electrode active material, the proportion of the SiO component in the silicon oxide particles disproportionated into Si and SiO2 is sufficiently small. Such a negative electrode active material is easy to intercalate Li. This improves diffusibility within the SiO bulk, thereby improving battery characteristics. In addition, since the proportion of crystallized Si in the Si component is low, side reactions with the electrolyte are suppressed. As a result, such a negative electrode active material can further improve the cycle characteristics when used as a negative electrode active material for secondary batteries. The upper limit of the average potential of the above test electrode is not particularly limited, but in principle, it is possible to achieve a value of 0.61 V vs. Li / Li. + can be an upper limit.

[0058] For example, when the degree of mixing of crystalline Si and amorphous Si is high, the diffusivity of Li ions in the crystalline Si and amorphous Si regions differs, resulting in distorted expansion behavior of the silicon oxide particles and increased cracking of the negative electrode active material particles. On the other hand, the negative electrode active material of this preferred embodiment has a low mixing of Si regions with different crystallinity, as described above, so the negative electrode active material particles are less likely to crack due to expansion and are less likely to deteriorate even with repeated charge and discharge. As a result, the negative electrode active material of this preferred embodiment can further suppress side reactions between the battery electrolyte and newly formed surfaces of the negative electrode active material particles caused by cracking, thereby further improving the cycle performance when used as a negative electrode active material for secondary batteries.

[0059] In the negative electrode active material containing silicon oxide particles coated with a carbon layer of the present invention, positive secondary ions C are detected in a time-of-flight secondary ion mass spectrometry (TOF-SIMS) spectrum of at least a part of the carbon layer. x H y O z It is preferable that the intensity of the peak attributed to (x is 8 or more and 42 or less, y is 5 or more and 65 or less, and z is 1 or more and 5 or less) is 0.1 or more relative to the intensity of the peak attributed to Si.

[0060] The positive secondary ions C in the TOF-SIMS spectrum x H y O z If multiple peaks attributed to C are observed, x H y O z It is preferable that the maximum peak intensity among the peaks of the fragments represented by the formula (1) is 0.1 or more relative to the intensity of the peak attributed to Si.

[0061] In this preferred embodiment of the negative electrode active material, the binder occupancy rate is appropriate, so that the amount of binder that substantially covers the negative electrode active material particles is appropriate. Furthermore, contact between the multilayer graphene edges and the electrolyte can be reduced, thereby suppressing side reactions with the electrolyte. As a result, when the negative electrode active material of this preferred embodiment is used as the negative electrode active material of a secondary battery, the cycle characteristics can be further improved. C by TOF-SIMS x H y O z The upper limit of the peak intensity attributed to is not particularly limited, but may be, for example, 4.72 relative to the intensity of the peak attributed to Si.

[0062] Furthermore, the negative electrode active material particles preferably have a C1s peak position of 284.3 eV or less in an X-ray photoelectron spectrum obtained by X-ray photoelectron spectroscopy. A C1s peak position of 284.3 eV or less in an XPS spectrum indicates a small number of graphene edges. Carbon atoms at the graphene edge portions are oxidized by exposure to the atmosphere, etc., and oxygen atoms or hydroxyl groups are bonded thereto, thereby attracting electrons and increasing the C1s binding energy. On the other hand, in this preferred embodiment of the negative electrode active material, the small number of graphene edges reduces the binding energy. As a result, contact between the multilayer graphene edges and the electrolyte on the carbon layer surface can be reduced, thereby suppressing side reactions with the electrolyte. As a result, such a negative electrode active material can further improve the cycle characteristics when used as a negative electrode active material for secondary batteries. The C1s peak position in the XPS spectrum is more preferably 283.9 eV or more and 284.3 eV or less.

[0063] In this case, it is preferable that the median diameter of the negative electrode active material particles (D50: particle diameter at 50% cumulative volume) is 4.5 μm or more and 15 μm or less. This is because a median diameter within the above range facilitates the absorption and desorption of lithium ions during charge and discharge, and the particles are less likely to crack. If the median diameter is 4.5 μm or more, the surface area per mass can be reduced, improving cycle characteristics. On the other hand, by setting the median diameter to 15 μm or less, the particles are less likely to crack, making it less likely that new surfaces will appear.

[0064] Furthermore, if the proportion of particles with a particle diameter of 1 μm or less in the negative electrode active material particles is 2.5% or less by volume, the specific surface area is reduced, thereby suppressing side reactions with the electrolyte and improving cycle characteristics. It is desirable that the proportion of particles with a particle diameter of 1 μm or less in the negative electrode active material particles be substantially 0%. The proportion of particles with a particle diameter of 1 μm or less in the negative electrode active material particles can be measured, for example, using a laser diffraction particle size distribution analyzer (Shimadzu SALD-3100) with a refractive index of 3.90-0.01i.

[0065] [Mixed negative electrode active material] The mixed negative electrode active material of the present invention contains the negative electrode active material of the present invention and a carbon-based active material.

[0066] In a negative electrode prepared using the mixed negative electrode active material of the present invention, the electrical resistance of the negative electrode active material layer is reduced and expansion stress caused by charging can be alleviated, resulting in further improved cycle characteristics when used as a negative electrode active material for a secondary battery.

[0067] Examples of carbonaceous active materials that can be used include pyrolytic carbons, cokes, glassy carbon fibers, fired organic polymer compounds, and carbon blacks.

[0068] Furthermore, in the mixed negative electrode active material of the present invention, the mass ratio of the silicon-based negative electrode active material to the total mass of the negative electrode active material of the present invention (silicon-based negative electrode active material) and the carbon-based active material is preferably 6 mass% or more. If the mass ratio of the silicon-based negative electrode active material to the total mass of the silicon-based negative electrode active material and the carbon-based active material is 6 mass% or more, it becomes possible to reliably improve the battery capacity.

[0069] <Non-aqueous electrolyte secondary battery negative electrode> Next, a negative electrode for a non-aqueous electrolyte secondary battery containing the negative electrode active material of the present invention will be described. Fig. 2 is a cross-sectional view showing an example of the configuration of a negative electrode for a non-aqueous electrolyte secondary battery (hereinafter also referred to as "negative electrode").

[0070] [Negative electrode composition] As shown in Fig. 2, the negative electrode 10 has a negative electrode active material layer 12 on a negative electrode current collector 11. The negative electrode active material layer 12 may be provided on both sides or only one side of the negative electrode current collector 11. Furthermore, if the negative electrode active material of the present invention is used, the negative electrode current collector 11 may be omitted.

[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 may or may not be roughened. A roughened negative electrode current collector is, for example, a metal foil that has been subjected to electrolytic treatment, embossing treatment, or chemical etching treatment. A non-roughened negative electrode current collector is, for example, a rolled metal foil.

[0074] [Negative electrode active material layer] The negative electrode active material layer 12 contains the negative electrode active material of the present invention that can absorb and release lithium ions, and may further contain other materials such as a negative electrode binder and a conductive additive from the viewpoint of battery design.

[0075] Furthermore, the negative electrode active material layer 12 may contain a mixed negative electrode active material containing the above-described negative electrode active material of the present invention (silicon-based negative electrode active material) and a carbon-based active material, i.e., the mixed negative electrode active material of the present invention, which reduces the electrical resistance of the negative electrode active material layer 12 and also makes it possible to alleviate expansion stress caused by charging.

[0076] The negative electrode binder contained in the negative electrode active material layer 12 may be, for example, one or more of polymer materials, synthetic rubber, etc. Examples of polymer materials include polyvinylidene fluoride, polyimide, polyamideimide, aramid, polyacrylic acid, lithium polyacrylate, carboxymethyl cellulose, etc. Examples of synthetic rubber include styrene butadiene rubber, fluorine-based rubber, ethylene propylene diene, etc.

[0077] As the negative electrode conductive assistant, for example, one or more of carbon materials such as carbon black, acetylene black, graphite, Ketjen black, carbon nanotubes, and carbon nanofibers can be used.

[0078] The negative electrode active material layer 12 is formed by, for example, a coating method, in which the negative electrode active material particles are mixed with the binder and, if necessary, a conductive additive and a carbon material, and then dispersed in an organic solvent, water, or the like, and then coated.

[0079] A specific example of a method for producing the negative electrode 10 by the coating method will be described below.

[0080] First, the negative electrode active material of the present invention is mixed with other materials such as a negative electrode binder and a conductive additive to form a negative electrode mixture, and then an organic solvent or water is added to form a slurry.

[0081] Next, the above slurry is applied to the surface of the negative electrode current collector 11 and dried to form the negative electrode active material layer 12. At this time, hot pressing or the like may be carried out as necessary.

[0082] In this manner, the negative electrode 10 can be produced.

[0083] [Method of manufacturing negative electrode active material] The negative electrode active material of the present invention can be produced, for example, by the method for producing the negative electrode active material of the present invention described below.

[0084] The method for producing a negative electrode active material of the present invention is a method for producing a negative electrode active material including negative electrode active material particles containing silicon oxide particles coated with a carbon layer, the method comprising: preparing silicon oxide particles; a step of coating the silicon oxide particles with a carbon layer by a pyrolysis chemical vapor deposition method using a hydrocarbon gas at a temperature of 790°C or less to prepare negative electrode active material particles; In the Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer from the produced negative electrode active material particles, the peak position due to the G band is 1590 cm -1 Over 1597cm -1 a step of selecting negative electrode active material particles including the carbon layer that fall within the following range; a step of producing a negative electrode active material using the selected negative electrode active material particles; The method for producing a negative electrode active material is characterized by comprising the steps of:

[0085] Each step will be described in more detail below.

[0086] (Step of Producing Silicon Oxide Particles) First, silicon oxide particles can be produced by pulverizing deposits obtained by vapor deposition into particles. Vapor deposition is a method that includes a step of heating a raw material that generates silicon oxide gas to generate silicon oxide gas, and a step of solidifying and depositing the generated silicon oxide gas on an adsorption plate.

[0087] The crystallite size obtained by Scherrer analysis of the peak due to the Si(111) crystal plane after waveform separation in the XRD spectrum of silicon oxide particles can be easily controlled, for example, by adjusting the temperature of the deposition chamber in the heating furnace, the degree of vacuum, the deposition rate of the silicon compound film on the adsorption plate, and the deposition distance, which is the distance between the raw material and the adsorption plate. The temperature of the deposition chamber needs to be changed taking into account the deposition rate, radiant heat, the degree of vacuum in the deposition chamber, and the deposition distance. The silicon oxide of the present invention can be easily obtained, particularly when the temperature of the deposition chamber is lowered, by reducing the deposition rate, radiant heat, and the degree of vacuum. In other words, the conditions for obtaining the above-mentioned XRD spectrum can be determined in advance. Note that the conditions required to obtain the above-mentioned silicon oxide particles of the present invention, such as the temperature of the deposition chamber, the degree of vacuum, the deposition rate, and the deposition distance, each vary depending on the structure of the heating furnace.

[0088] More specifically, silicon oxide gas-generating raw materials are heated in the presence of an inert gas to generate silicon oxide gas. Taking into consideration the presence of oxygen on the surface of the metal silicon powder and trace amounts of oxygen in the reactor, the raw materials are preferably mixed in a molar ratio of 0.8<metal silicon powder / silicon dioxide powder<1.3.

[0089] The generated silicon oxide gas is solidified and deposited on the adsorption plate. Next, the temperature inside the reactor is lowered to 100°C or less, and the silicon oxide deposit is removed and pulverized using a ball mill, jet mill, or the like to produce a powder. The powder thus obtained may be classified. In the present invention, the particle size distribution of the silicon compound particles can be adjusted during the pulverization and classification steps. Silicon compound particles can be produced in this manner.

[0090] (Step of coating silicon oxide particles with a carbon layer to prepare negative electrode active material particles) Next, a carbon layer is formed on the surface of the silicon oxide particles. The carbon layer is formed by pyrolysis chemical vapor deposition (pyrolysis CVD) using hydrocarbon gas at a temperature of 790°C or less. The method for forming the carbon layer by pyrolysis CVD is described below.

[0091] First, silicon oxide particles are placed in a furnace. Next, hydrocarbon gas is introduced into the furnace, and the temperature inside the furnace is increased. The decomposition temperature is preferably 500°C or higher and 790°C or lower, more preferably 500°C or higher and 650°C or lower. By setting the decomposition temperature to 790°C or lower, unintended disproportionation of the silicon compound particles can be suppressed. Furthermore, by setting the decomposition temperature to 500°C or higher, production costs can be reduced and a carbon layer can be industrially formed on the surface of the silicon compound particles.

[0092] The hydrocarbon gas used as the raw material for the carbon layer is not particularly limited, but may be C n H m In the composition, it is desirable that n≦3. When n≦3, the production cost can be reduced and the physical properties of the decomposition products can be improved.

[0093] As the hydrocarbon gas that is the raw material for the carbon layer, it is more preferable to use a hydrocarbon gas having an unsaturated bond, and it is particularly preferable to use acetylene.

[0094] By using acetylene as the hydrocarbon gas that is the raw material for the carbon layer, thermal decomposition can be carried out reliably in a short time, even at temperatures of 790°C or less using thermal decomposition CVD, making industrial production possible while suppressing the generation of by-products such as tar and soot.

[0095] (Step of sorting negative electrode active material particles) Next, after the above-mentioned pyrolysis CVD method is performed on silicon oxide particles coated with a carbon layer, a Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer is obtained, and the peak position due to the G band is 1590 cm -1 Over 1597cm-1 Negative electrode active material particles containing a carbon layer that falls within the following range are selected: In this selection process, silicon oxide particles coated with a carbon layer in a state that is actually used as a negative electrode active material are measured by Raman spectroscopy, and particles whose obtained spectrum satisfies the above conditions are selected.

[0096] The selection of silicon compound particles to be coated with a carbon layer does not necessarily need to be carried out every time a negative electrode active material is produced. -1 Over 1597cm -1 Once the manufacturing conditions that give the following range are found and selected, silicon oxide particles coated with a carbon layer can be manufactured under the same conditions as the selected conditions.

[0097] (Process for producing negative electrode active material) Next, the negative electrode active material selected as above is used to manufacture a negative electrode active material, which can provide the negative electrode active material of the present invention.

[0098] <Lithium-ion secondary battery> Next, a lithium ion secondary battery containing the negative electrode active material of the present invention will be described, taking a laminate film type lithium ion secondary battery as a specific example.

[0099] [Configuration of laminated film type lithium-ion secondary battery] The laminate film type lithium ion secondary battery 20 shown in FIG. 3 mainly comprises a wound electrode body 21 housed inside a sheet-like exterior member 25. This wound body has a separator between the positive and negative electrodes and is wound around. 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 22 is attached to the positive electrode, and a negative electrode lead 23 is attached to the negative electrode. The outermost periphery of the electrode body 21 is protected by protective tape.

[0100] The positive and negative electrode leads 22 and 23 are, for example, led out in one direction from the inside to the outside of the exterior member 25. The positive electrode lead 22 is formed of a conductive material such as aluminum, and the negative electrode lead 23 is formed of a conductive material such as nickel or copper.

[0101] The exterior member 25 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 21. 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.

[0102] An adhesive film 24 is inserted between the exterior member 25 and the positive and negative electrode leads to prevent outside air from entering, and is made of, for example, polyethylene, polypropylene, or polyolefin resin.

[0103] [Positive electrode] 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.

[0104] The positive electrode current collector is made of a conductive material such as aluminum.

[0105] 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 binder, a conductive additive, a dispersant, etc. Depending on the design, the binder and the conductive additive may be the same as those for the negative electrode binder and the negative electrode conductive additive already described.

[0106] As the positive electrode material, a lithium-containing compound is desirable. Examples of this lithium-containing compound include composite oxides composed of lithium and transition metal elements, or phosphate compounds having lithium and transition metal elements. Among these positive electrode materials, compounds containing at least one of nickel, iron, manganese, and cobalt are preferred. As these chemical formulas, for example, Li x M1O2 or Li y M2PO4. In the formula, M1 and M2 represent at least one transition metal element. The values of x and y represent different values 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.

[0107] 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), etc. 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)), etc. By using these positive electrode materials, a high battery capacity can be obtained, and excellent cycle characteristics can also be obtained.

[0108] [Negative Electrode] The negative electrode has the same configuration as the negative electrode 10 for a lithium-ion secondary battery shown in the above figure 2 . For example, it has negative electrode active material layers 12 on both sides of the current collector 11. It is preferable that the negative electrode charging capacity is larger than the electric capacity obtained from the positive electrode active material agent (charging capacity as a battery). This is because precipitation of lithium metal on the negative electrode can be suppressed.

[0109] The positive electrode active material layer is provided on a portion of both sides of the positive electrode current collector, and the negative electrode active material layer is also 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.

[0110] The non-opposing region, i.e., 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. Therefore, the state of the negative electrode active material layer is maintained as it was immediately after formation. This allows the composition of the negative electrode active material to be accurately determined with good reproducibility, regardless of whether charging and discharging are performed.

[0111] [Separator] The separator separates the positive and negative electrodes, preventing current short-circuiting due to 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 laminated structure in which two or more types of porous film are laminated. Examples of synthetic resins include polytetrafluoroethylene, polypropylene, and polyethylene.

[0112] [Electrolyte] At least a portion of the positive electrode active material layer, at least a portion of the negative electrode active material layer, and / or the separator are impregnated with a liquid electrolyte (electrolytic solution). This electrolytic solution contains an electrolyte salt dissolved in a solvent and may contain other materials such as additives.

[0113] 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.

[0114] 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).

[0115] 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.

[0116] 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.

[0117] 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 unsaturated carbon-bond cyclic carbonate include vinylene carbonate and vinylethylene carbonate.

[0118] 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.

[0119] 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.

[0120] 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).

[0121] 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.

[0122] [Manufacturing method of laminated film type secondary battery] In the present invention, a negative electrode can be produced using the negative electrode active material produced by the above-described method for producing a negative electrode active material of the present invention, and a lithium ion secondary battery can be produced using the produced negative electrode.

[0123] An example of a method for manufacturing a laminate film type secondary battery will be described below with reference to FIG.

[0124] First, a positive electrode is fabricated using the above-described positive electrode material. First, a positive electrode active material is mixed with, if necessary, a binder, a conductive additive, etc. to form a positive electrode mixture, which is then dispersed in an organic solvent to form a positive electrode mixture slurry. Next, the mixture slurry is applied to a positive electrode current collector using a coating device such as a knife roll or a die coater having a die head, and then dried with hot air to obtain a positive electrode active material layer. Finally, the positive electrode active material layer is compression-molded using a roll press or the like. At this time, heating may be performed, and heating or compression may be repeated multiple times.

[0125] Next, using the same procedure as in the above-described method for producing the negative electrode, a negative electrode active material layer is formed on the negative electrode current collector to produce the negative electrode.

[0126] When fabricating the positive and negative electrodes, active material layers are formed on both sides of the positive and negative electrode current collectors, respectively, and the lengths of the active material coatings on both sides of each electrode may be offset (see Figure 2).

[0127] Next, the electrolyte is prepared. Next, a positive electrode lead 22 is attached to the positive electrode current collector by ultrasonic welding or the like, and a negative electrode lead 23 is attached to the negative electrode current collector (see FIG. 3). Next, the positive electrode and the negative electrode are laminated or wound with a separator interposed therebetween to prepare a wound electrode body 21, and a protective tape is adhered to the outermost periphery. Next, the wound body is molded into a flat shape. Next, the wound electrode body is sandwiched between folded film-like exterior members 25, and then the insulating portions of the exterior members are bonded together by a heat fusion method, and the wound electrode body is encapsulated with only one side open. An adhesive film is inserted between the positive electrode lead and the negative electrode lead and the exterior member. A predetermined amount of the prepared electrolyte is poured into the open portions, and vacuum impregnation is performed. After impregnation, the open portions are bonded by a vacuum heat fusion method.

[0128] In this manner, the laminate film type lithium ion secondary battery 20 can be manufactured. [Example]

[0129] EXAMPLES The present invention will be specifically explained below using examples and comparative examples, but the present invention is not limited to these.

[0130] Example 1 The laminate film type lithium ion secondary battery 20 shown in FIG. 3 was fabricated by the following procedure.

[0131] [Preparation of positive electrode] First, the positive electrode was fabricated using the following procedure. The positive electrode active material was a lithium nickel cobalt composite oxide (LiNi 0.7 Co 0.25 Al 0.05 A positive electrode mixture was prepared by mixing 95% by mass of lithium nickel cobalt aluminum composite oxide (NCA), 2.5% by mass of a positive electrode conductive additive, and 2.5% by mass of a positive electrode binder (polyvinylidene fluoride: PVDF). The positive electrode mixture was then dispersed in an organic solvent (N-methyl-2-pyrrolidone: NMP) to form a paste-like slurry. The slurry was then applied to both sides of a positive electrode current collector using a coating device with a die head and dried using a hot air dryer. A positive electrode current collector with a thickness of 15 μm was used. Finally, compression molding was performed using a roll press. This resulted in a positive electrode.

[0132] [Preparation of negative electrode] Next, a negative electrode was prepared in the following manner.

[0133] [Preparation of negative electrode active material] First, the negative electrode active material was prepared as follows: A mixture of silicon metal and silicon dioxide was introduced into a reactor, vaporized in a vacuum atmosphere of 10 Pa, and deposited on an adsorption plate. After sufficient cooling, the deposit was removed and pulverized in a ball mill. The SiO of the silicon oxide particles obtained in this way was x The value of x was 1.0. Subsequently, the particle size of the silicon oxide particles was adjusted by classification.

[0134] The silicon oxide particles were then introduced into a pyrolysis chemical vapor deposition furnace. Next, acetylene gas was introduced into the furnace as a hydrocarbon gas, and the temperature inside the furnace was increased. In Example 1, pyrolysis chemical vapor deposition (pyrolysis CVD) was performed at 630°C for 14 hours, thereby coating the surfaces of the silicon oxide particles with a carbon material (carbon layer). This resulted in the production of negative electrode active material particles.

[0135] Here, the Raman spectrum of the carbon layer covering the silicon oxide particles in the negative electrode active material particles after pyrolysis CVD was measured by Raman spectroscopy. The Raman spectrum of the carbon layer can be measured using, for example, a Horiba XploRa Raman microscope. The G-band peak intensity Ig and other values in the measured Raman spectrum can be calculated using analysis software such as LabSpec. The measurement conditions in Example 1 were as follows. ·Device name: HORIBA XploRA Plus Olympus BX41 Microscope Laser (532nm)

[0136] The measured Raman spectrum is shown by a thick solid line in Figure 4. The peak position due to the G band in the Raman spectrum of the carbon layer coating the silicon oxide particles of Example 1 shown in Figure 4 is 1593.5 cm -1 Thus, the peak position was 1590 cm -1 Over 1597cm -1 By keeping the thickness within the range below, the number of graphene layers in the carbon layer is small, and there are fewer edges of multi-layer graphene that can cause side reactions with the electrolyte, which makes it possible to suppress the formation of an SEI film. This fact will be demonstrated by the results shown later.

[0137] Based on the measurement results of the Raman spectroscopy described above, the negative electrode active material particles produced as described above were analyzed to determine whether the peak position of the G band in the Raman spectrum of the carbon layer was 1590 cm -1 Over 1597cm -1 The negative electrode active material particles containing a carbon layer in the following range were selected and used as the negative electrode active material of Example 1.

[0138] [Analysis of negative electrode active material] [X-ray diffraction] Furthermore, the peak intensity due to the Si(111) crystal plane in the X-ray diffraction spectrum of the silicon oxide particles can be calculated by XRD peak analysis. In Example 1, measurements were carried out under the following conditions. X-ray diffraction device (XRD) Equipment: Bruker D2 Phaser ·X-ray source:Cu Voltage: 30kV ·Current: 10mA Step width: 0.05° ·Holding time: 1 second Divergence slit: 0.5° Incident solar: 4° Receiver side solar: 4° Calculation of peak intensity Software: DIFFRAC.EVA

[0139] The crystallite size, obtained by Scherrer analysis of the peaks due to the Si(111) crystal plane after waveform separation in the X-ray diffraction spectrum, can be calculated by XRD peak analysis. The XRD spectrum before waveform separation is shown in the upper part of Figure 5, and the XRD spectrum after waveform separation is shown in the lower part of Figure 5. The conditions for calculating the crystallite size due to the Si(111) crystal plane using the Scherrer equation were as follows: Calculation of crystallite size Analysis software: DIFFRAC.TOPAS Analysis method: Peak fitting method Emission Profile: CuKa5.lam Function: FP (First Principle) function Refinement Option: Calculate Error”, “Use Extrapolation”

[0140] The crystallite size of the negative electrode active material of Example 1 obtained from the peak due to the Si(111) crystal plane was 1.14 nm.

[0141] [Open circuit potential curve measurement] Furthermore, a coin battery was fabricated as a test battery using the negative electrode active material of Example 1, and the open circuit voltage (OCV) was measured. The open circuit potential (OCP) curve calculated from this measurement is shown by the solid line in Figure 6. From this curve, the average potential of the test electrode was calculated when the state of charge of the test battery was in the range of 10% to 20%.

[0142] The coin battery used as the test battery was fabricated as follows.

[0143] First, a 1 mm thick Li foil was punched into a disk with a diameter of 16 mm, which was then attached to an aluminum clad to serve as the counter electrode.

[0144] Furthermore, a negative electrode was produced using the negative electrode active material of Example 1 in the same procedure as that for producing the negative electrode of Example 1, which will be described in detail below.

[0145] The resulting negative electrode was punched into a disk with a diameter of 15 mm to prepare a test electrode. This test electrode was placed opposite a Li foil attached to an aluminum clad with a separator in between, and after injecting the electrolyte, a 2032 coin battery was fabricated.

[0146] The open circuit voltage measurement and the calculation of the open circuit potential curve were carried out under the following conditions.

[0147] First, the coin battery was charged at 5% SOC with the fully charged capacity set to 100, and the open circuit voltage was measured. The current density was 0.2 mA / cm. 2 In CC / CV, the open circuit time was 5 hours and the voltage after 5 hours was plotted. SOC 100% is essentially 0.2mA / cm 2 The current values were as follows, but were stopped at the actually measured current value of 0.1 mA.

[0148] [Time-of-flight secondary ion mass spectrometry] Furthermore, time-of-flight secondary ion mass spectrometry was performed on the carbon layer of the negative electrode active material of Example 1 to obtain a TOF-SIMS spectrum. The measurement conditions were as follows. The measurement was performed twice. TOF-SIMS (time-of-flight secondary ion mass spectrometry) Instrument: TOF.SIMS 5 (ION-TOF), Secondary ion polarity: positive, ·Mass range (m / z): 0~1500, Raster size: 300μm□, Number of scans: 16 scans, Number of pixels (per side): 256 pixels, Measured vacuum (before sample introduction): 4×10 -7 Below Pa, Primary ion species: Bi3 ++ , Primary ion acceleration voltage: 30kV, Pulse width: 12.5ns, Bunching: Yes (high mass resolution measurement) Charge neutralization: None, ·Rear stage acceleration: 9.5kV.

[0149] The peak intensities (count values, normalized values, and average values of normalized values) of the main positive secondary ions in the measured TOF-SIMS spectrum are shown in Table 1 below. In Table 1, the normalized values are shown to three significant digits. The peak intensities (normalized values) of the positive secondary ions in the measured TOF-SIMS spectrum are shown in Figure 7. In Figure 7, C x H y O z The fragment represented by (x is 8 to 42, y is 5 to 65, and z is 1 to 5) is enclosed in a box.

[0150] [Table 1]

[0151] As shown in Table 1 and FIG. 7, in the TOF-SIMS spectrum of the carbon layer of the negative electrode active material of Example 1, the intensities of the peaks attributable to the positive secondary ions C4H7 and C7H7, relative to the intensity of the peak attributable to Si, were 241.4 for C4H7 and 23.2 for C7H7.

[0152] In addition, in the time-of-flight secondary ion mass spectrometry spectrum, the positive secondary ions C x H y O z The intensity of the peaks (x is 8 to 42, y is 5 to 65, and z is 1 to 5) attributed to Si is 0.91 for C8H5O3 and 0.92 for C 15 H 23 O is 2.91, C 24 H 49 O2 is 2.18, C 35 H 62 O3 is 0.70, C 35 H 65 The O4 was 2.84.

[0153] [Analysis by X-ray photoelectron spectroscopy] The bond energy of the carbon layer of the negative electrode active material of Example 1 was also quantified by XPS (X-ray photoelectron spectroscopy). The XPS measurement was carried out 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.

[0154] FIG. 8 shows the range of 280 eV to 290 eV of the XPS spectrum of the negative electrode active material of Example 1.

[0155] As shown in FIG. 8, the XPS spectrum of the negative electrode active material of Example 1 had a C1s peak at a binding energy position of 284.1 eV.

[0156] From the above, it is clear that in the negative electrode active material of Example 1, the negative electrode active material particles are coated with a carbon layer.

[0157] [Preparation of negative electrode] Next, the negative electrode active material of Example 1, conductive additive 1 (carbon nanotubes, CNT), conductive additive 2 (carbon fine particles with a median diameter of approximately 50 nm), styrene butadiene rubber (styrene butadiene copolymer, hereinafter referred to as SBR), and carboxymethyl cellulose (hereinafter referred to as CMC) were mixed in a dry mass ratio of 92.5:1:1:2.5:3, and then diluted with pure water to form a negative electrode mixture slurry. Note that the SBR and CMC are negative electrode binders (negative electrode binding agents).

[0158] The negative electrode current collector was an electrolytic copper foil having a thickness of 15 μm, which contained carbon and sulfur at concentrations of 70 ppm by mass each.

[0159] Finally, the negative electrode mixture slurry was applied to a negative electrode current collector and dried at 100°C for 1 hour in a vacuum atmosphere. 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 5 mg / cm. 2 It was.

[0160] The negative electrode of Example 1 was produced by the above procedure.

[0161] [Fabrication of lithium-ion secondary batteries] Next, 4-fluoro-1,3-dioxolane-2-one (FEC), ethylene carbonate (EC), and dimethyl carbonate (DMC) were mixed to prepare a solvent. An electrolyte salt (lithium hexafluorophosphate: LiPF6) was dissolved in this solvent to prepare an electrolyte solution. In this case, the composition of the solvent was volume The ratio of FEC:EC:DMC was 10:20:70, and the content of the electrolyte salt was 1.2 mol / kg relative to the solvent.

[0162] Next, a secondary battery was assembled as follows. First, an aluminum lead was ultrasonically welded to one end of the positive electrode current collector, and a nickel lead was welded to one end of the negative electrode current collector. Next, the positive electrode, separator, negative electrode, and separator were stacked in this order and wound longitudinally to obtain a wound electrode assembly. The end of the winding was fixed with PET protective tape. The separator was a laminated film (12 μm thick) consisting of a film primarily composed of porous polypropylene sandwiched between films primarily composed of porous polyethylene. Next, the electrode assembly was sandwiched between exterior members, and the outer edges, excluding one side, were heat-sealed to encase the electrode assembly inside. The exterior member was an aluminum laminate film composed of a nylon film, aluminum foil, and polypropylene film. Next, the prepared electrolyte was injected through the opening, impregnated under a vacuum atmosphere, and then heat-sealed and sealed. This produced the lithium-ion secondary battery of Example 1.

[0163] [Evaluation of cycle characteristics] Next, the cycle characteristics of the lithium ion secondary battery of Example 1 fabricated as described above were evaluated.

[0164] The cycle characteristics were investigated as follows. First, to stabilize the battery, two cycles of charge and discharge were performed at 0.2 C in an atmosphere of 25°C, and the discharge capacity at the second cycle was measured. Next, charge and discharge were performed until the total number of cycles reached 499, and the discharge capacity was measured each time. Finally, the discharge capacity at the 500th cycle obtained by 0.2 C charge and discharge was divided by the discharge capacity at the second cycle to calculate the capacity retention rate (hereinafter also referred to as cycle retention rate). For normal cycles, i.e., from the third cycle to the 499th cycle, charge and discharge were performed at 0.7 C for charge and 0.5 C for discharge.

[0165] Examples 2 to 5 In Examples 2 to 5, negative electrode active materials were produced in the same manner as in Example 1, except that the CVD temperature and time were adjusted as shown in Table 2 below, thereby adjusting the G-band peak position of the Raman spectrum as shown in Table 2, and the lithium ion secondary batteries of Examples 2 to 5 were fabricated using the produced negative electrode active materials. Note that in Example 4, the CVD temperature was adjusted by plasma CVD because the temperature was too low for thermal CVD.

[0166] (Examples 6 to 8) In Examples 6 to 8, negative electrode active materials were produced in the same manner as in Example 1, except that the median diameter of the silicon oxide particles and / or the proportion of particles having a particle diameter of 1 μm or less in the negative electrode active material particles were adjusted as shown in Table 2. 6~8 A lithium-ion secondary battery was fabricated.

[0167] (Comparative Examples 1 and 2) In Comparative Examples 1 and 2, negative electrode active materials were produced in the same manner as in Example 1, except that the CVD temperature, time, and hydrocarbon gas were adjusted as shown in Table 2, thereby adjusting the G-band peak position of the Raman spectrum as shown in Table 2. Lithium ion secondary batteries of Comparative Examples 1 and 2 were fabricated using the produced negative electrode active materials.

[0168] (Comparative Example 3) In Comparative Example 3, a negative electrode active material was produced in the same manner as in Example 1, except that the CVD temperature, time, and hydrocarbon gas, as well as the proportion of particles having a particle diameter of 1 μm or less in the negative electrode active material particles, were adjusted as shown in Table 2, and a lithium ion secondary battery of Comparative Example 3 was fabricated using the produced negative electrode active material.

[0169] Comparative Example 4 In Comparative Example 4, a negative electrode active material was produced in the same manner as in Example 1, except that the CVD temperature and the hydrocarbon gas were adjusted as shown in Table 2, thereby adjusting the G-band peak position of the Raman spectrum as shown in Table 2. A lithium ion secondary battery of Comparative Example 4 was fabricated using the produced negative electrode active material.

[0170] (Comparative Example 5) Comparative Example 5 In Comparative Example 5, a negative electrode active material was produced in the same manner as in Comparative Example 4, except that the CVD temperature and CVD time were adjusted as shown in Table 2, thereby adjusting the G band peak position of the Raman spectrum as shown in Table 2, and a lithium ion secondary battery of Comparative Example 5 was fabricated using the produced negative electrode active material.

[0171] The negative electrode active materials of Examples 2 to 8 and Comparative Examples 1 to 5 were analyzed in the same manner as in Example 1. The cycle characteristics of the lithium ion secondary batteries of these examples were also evaluated in the same manner as in Example 1. The results are shown in Tables 2 and 3 below.

[0172] [Table 2]

[0173] [Table 3]

[0174] 4 shows the Raman spectrum of the carbon layer of the negative electrode active material of Comparative Example 1 (dashed line) and Comparative Example 4 (thin solid line) in addition to the Raman spectrum of the carbon layer of the negative electrode active material of Example 1. FIG. 6 shows the OCP curve of Comparative Example 1 and Comparative Example 4 in addition to the OCP curve of Example 1.

[0175] The peak intensities (count values, normalized values, and average values of the normalized values) of main positive secondary ions in the TOF-SIMS spectra of the negative electrode active materials of Comparative Examples 1 and 2 are shown in Tables 4 and 5. Fig. 7 also compares the peak intensities of main positive secondary ions in the TOF-SIMS spectra of the negative electrode active material surfaces of Comparative Examples 1 and 4 in addition to Example 1.

[0176] [Table 4]

[0177] [Table 5]

[0178] FIG. 8 also shows the XPS spectra of the negative electrode active materials of Example 1, Example 5, and Comparative Example 1.

[0179] As is clear from Tables 2 and 3, the peak position due to the G band in the Raman spectrum of the carbon layer is 1590 cm -1 Over 1597cm -1 The negative electrode active materials of Examples 1 to 8, which are in the following ranges, have a peak position attributable to the G band of 1590 cm -1 The cycle characteristics were superior to those of Comparative Examples 1 to 5. The results show that the peak position due to the G band was 1590 cm -1 Over 1597cm -1 This is thought to be because, by keeping the number of graphene layers in the carbon layer within the range below, there are fewer edges of multilayer graphene that can cause side reactions with the electrolyte, which makes it possible to suppress the formation of an SEI film.

[0180] Comparing the results of Examples 1 to 5, it can be seen that the lower the CVD temperature, the more the G-band peak position in the Raman spectrum shifts to the higher wavenumber side. Furthermore, the shift of the G-band peak position in the Raman spectrum to the higher wavenumber side indicates that the number of graphene layers in the carbon layer is small, and it is thought that the generation of an SEI film can be suppressed because there are fewer edges of multilayer graphene that cause side reactions with the electrolyte.

[0181] Furthermore, as is clear from Tables 2 and 3, the crystallite size obtained from the peak due to the Si(111) crystal plane tended to be smaller as the CVD temperature was lower. The smaller the crystallite size, the lower the proportion of crystalline Si in the composition, which is thought to be why side reactions with the electrolyte can be suppressed.

[0182] Furthermore, as is clear from Tables 2 and 3, the lower the CVD temperature, the higher the average potential of the test electrode when the state of charge of the test battery was in the range of 10% to 20%. This is thought to be because when the average potential is high, the proportion of SiO components in the silicon oxide particles that are disproportionated into Si and SiO2 decreases, making it easier for Li to be inserted. This is thought to improve diffusibility within the SiO bulk, thereby improving battery performance. As a result, it was confirmed that the cycle capacity retention rate improved.

[0183] On the other hand, in Example 2, a long CVD process is required, and there is an optimum range for industrial production.

[0184] Furthermore, it is believed that the resistance of the electrode increased due to the increased proportion of the diamond-like structure in the carbon layer in the CVD of Example 4. As a result, it was confirmed that the characteristics of Example 4 were inferior to those of Examples 1 to 3.

[0185] In Example 5, the CVD temperature was set to 790°C, and as a result, the peak position of the G band in the Raman spectrum shifted to the lower wavenumber side. This suggests that the number of graphene layers in the carbon layer was larger than in Examples 1 to 3, making side reactions with the electrolyte more likely to occur.

[0186] In Example 5, the crystallite size was rapidly enlarged, but the peak position due to the G band in the Raman spectrum of the carbon layer was 1590 cm -1 Over 1597cm -1 It was confirmed that better cycle characteristics than those of Comparative Examples 1 to 5 could be obtained by using the following ranges.

[0187] In Example 6, it was confirmed that a smaller median diameter increases the surface area per mass, thereby increasing the irreversible capacity of the battery. In Example 7, it was confirmed that a median diameter of more than 15 μm makes the particles more susceptible to cracking, making new surfaces more likely to appear. In addition, in Example 8, it was confirmed that a large proportion of particles with a particle diameter of 1 μm or less in the negative electrode active material particles increases the surface area per mass, thereby increasing side reactions with the electrolyte. As a result, it was confirmed that the negative electrode active materials of Examples 6 to 8 exhibited deteriorated cycle characteristics compared to Example 1.

[0188] In Comparative Examples 1 to 5, the CVD temperature was increased above 790°C, resulting in a shift in the position of the peak derived from the G band to a lower wavenumber side compared to Examples 1 to 8 (Fig. 4), the crystallite size obtained from the peak derived from the Si(111) crystal plane also increased (Table 3), and the average potential of the test electrode in the charge state range of the test battery from 10% to 20% also decreased (Fig. 6). As a result, it was confirmed that the cycle characteristics deteriorated.

[0189] As is clear from FIG. 7 and Tables 4 and 5, the TOF-SIMS spectrum of the carbon layer of the negative electrode active material of Example 1 shows positive secondary ions C8H5O3, C 15 H 23 O, C 24 H 49 O2, C 35 H 62 O3 and C 35 H 65 The intensity (normalized intensity) of the peak attributed to O4 relative to the peak attributed to Si was higher than the normalized intensities of these peaks in the TOF-SIMS spectra of Comparative Examples 1 and 4. This suggests that the negative electrode active material of Example 1 was able to reduce contact between the multilayer graphene edges and the electrolyte solution more than the negative electrode active materials of Comparative Examples 1 and 4, and was therefore able to suppress side reactions with the electrolyte solution.

[0190] Furthermore, as is clear from FIG. 8, as the CVD temperature increased, the C1s peak position shifted toward higher binding energy. Furthermore, although not shown, as the CVD temperature increased, the intensity of the O1s peak increased. From these results, it can be concluded that the negative electrode active material of Comparative Example 1 had more graphene edge portions in the carbon layer than Examples 1 and 5. As a result, it is believed that in Comparative Example 1, the contact between the multilayer graphene edges and the electrolyte on the surface of the formed carbon layer could not be reduced, resulting in a side reaction with the electrolyte.

[0191] In Comparative Example 3, it was confirmed that the proportion of particles with a particle diameter of 1 μm or less in the negative electrode active material particles was high, which increased the surface area per mass and resulted in an increase in side reactions with the electrolyte, and as a result, it was confirmed that the cycle characteristics were further deteriorated.

[0192] 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.

Claims

1. A negative electrode active material containing negative electrode active material particles, the negative electrode active material particles contain silicon oxide particles coated with a carbon layer, In the Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer, the peak position derived from the G band is 1590 cm －１ Over 1597cm －１ A negative electrode active material characterized by being in the following range:

2. The negative electrode active material according to claim 1, wherein the negative electrode active material particles have a crystallite size of 1.5 nm or less, which is obtained by Scherrer analysis of a peak attributable to a Si (111) crystal plane in an X-ray diffraction spectrum after waveform separation obtained by X-ray diffraction using Cu-Kα radiation.

3. In an open circuit potential curve obtained by measuring the open circuit voltage of a test battery including a test electrode containing the negative electrode active material and lithium metal as a counter electrode, the average potential of the test electrode when the state of charge of the test battery is in the range of 10% to 20% is 0.4 V vs. Li / Li ＋ 3. The negative electrode active material according to claim 1, wherein the negative electrode active material is a material selected from the group consisting of hydroxyapatite, ...

4. In a time-of-flight secondary ion mass spectrometry spectrum of at least a portion of the carbon layer, positive secondary ions, C ｘ H ｙ O ｚ 4. The negative electrode active material according to claim 1, wherein the intensity of a peak attributed to Si (where x is 8 or more and 42 or less, y is 5 or more and 65 or less, and z is 1 or more and 5 or less) is 0.1 or more relative to the intensity of a peak attributed to Si.

5. 5. The negative electrode active material according to claim 1, wherein the negative electrode active material particles have a C1s peak position of 284.3 eV or less in an X-ray photoelectron spectrum obtained by X-ray photoelectron spectroscopy.

6. 6. The negative electrode active material according to claim 1, wherein the negative electrode active material particles have a median diameter of 4.5 μm or more and 15 μm or less.

7. 7. The negative electrode active material according to claim 1, wherein the proportion of particles having a particle diameter of 1 μm or less in the negative electrode active material particles is 2.5% or less by volume.

8. The negative electrode active material according to any one of claims 1 to 7, Carbon-based active material A mixed negative electrode active material comprising:

9. A method for producing a negative electrode active material including negative electrode active material particles containing silicon oxide particles coated with a carbon layer, comprising: preparing silicon oxide particles; a step of coating the silicon oxide particles with a carbon layer by a pyrolysis chemical vapor deposition method using a hydrocarbon gas at a temperature of 790°C or less to prepare negative electrode active material particles; In the Raman spectrum obtained by Raman spectroscopy of at least a part of the carbon layer of the prepared negative electrode active material particles, the peak position due to the G band is 1590 cm －１ Over 1597cm －１ a step of selecting negative electrode active material particles including the carbon layer that fall within the following range; a step of producing a negative electrode active material using the selected negative electrode active material particles; 2. A method for producing a negative electrode active material, comprising:

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