A negative electrode for a secondary battery, a method for manufacturing the negative electrode, and a secondary battery using the negative electrode

Abstract

The present invention provides a negative electrode containing Si-C composite particles and graphite particles, which can suppress capacity degradation when a secondary battery is repeatedly charged and discharged. [Solution] The negative electrode of the secondary battery of this disclosure comprises a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer contains graphite particles and Si-C composite particles as the negative electrode active material. The Si-C composite particles have a porous carbon skeleton and Si-containing particles located within the pores of the porous skeleton. A coating containing LiF is formed on at least a portion of the outer surface of the Si-C composite particles.

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode for a secondary battery and a method for manufacturing the same. This disclosure also relates to a secondary battery using the said negative electrode. [Background technology]

[0002] In recent years, secondary batteries have been suitably used as portable power sources for personal computers and mobile devices, as well as power sources for vehicle propulsion systems such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] For vehicle power supply applications, particularly for BEVs, there is a demand for higher capacity secondary batteries from the viewpoint of extending the vehicle's driving range. Si-C composite particles are known as high-capacity negative electrode active materials, and it is known that secondary batteries can be made higher capacity using Si-C composite particles (see, for example, Patent Documents 1 and 2). Patent Document 1 discloses a technology that uses Si-C composite particles and graphite particles such as natural graphite in combination as negative electrode active materials. Patent Document 2 discloses composite particles as negative electrode active materials in which Si-LiF mixed particles, in which Si is coated with LiF, are dispersed in a carbon phase made of a carbon material. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-38862 [Patent Document 2] Special Publication No. 2019-522886 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, conventional secondary batteries using a negative electrode that combines Si-C composite particles and graphite particles suffer from a problem of poor cycle characteristics, specifically, a significant degradation in capacity when the secondary battery is repeatedly charged and discharged.

[0006] In view of the above circumstances, the present disclosure aims to provide a negative electrode containing Si-C composite particles and graphite particles that can suppress capacity degradation when a secondary battery is repeatedly charged and discharged. [Means for solving the problem]

[0007] The negative electrode of the secondary battery of this disclosure comprises a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer contains graphite particles and Si-C composite particles as the negative electrode active material. The Si-C composite particles have a porous carbon framework and Si-containing particles located within the pores of the porous framework. A coating containing LiF is formed on at least a portion of the outer surface of the Si-C composite particles.

[0008] With this configuration, it is possible to provide a negative electrode containing Si-C composite particles and graphite particles that can suppress capacity degradation when a secondary battery is repeatedly charged and discharged.

[0009] From another perspective, the method for manufacturing a negative electrode of a secondary battery according to the present disclosure comprises the steps of: preparing Si-C composite particles having a porous carbon framework, Si-containing particles located in the pores of the porous framework, and a coating containing LiF formed on at least a portion of the outer surface; mixing the Si-C composite particles and graphite particles in a dispersion medium to prepare a negative electrode paste; coating the negative electrode paste onto a negative electrode current collector; and drying the coated negative electrode paste. The step of preparing the Si-C composite particles includes reacting a water-soluble lithium salt and a fluorinating agent in a dispersion in which the particles having the porous carbon framework and the Si-containing particles located in the pores of the porous framework are dispersed in water or a water-soluble organic solvent to produce LiF.

[0010] The negative electrode obtained by this configuration can provide the secondary battery with excellent resistance to capacity degradation when repeatedly charged and discharged.

[0011] From another perspective, the secondary battery of this disclosure comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode described above.

[0012] This configuration makes it possible to provide a secondary battery with excellent resistance to capacity degradation when repeatedly charged and discharged. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic cross-sectional view showing the configuration of the first embodiment of the negative electrode of the secondary battery of the present disclosure. [Figure 2] This is a schematic cross-sectional view showing the composition of Si-C composite particles contained in the negative electrode active material layer of the negative electrode in Figure 1. [Figure 3] This is a schematic cross-sectional view showing particles contained in the negative electrode active material layer of a second embodiment of the negative electrode of the secondary battery of the present disclosure. [Figure 4] This is a schematic cross-sectional view showing the configuration of a lithium-ion secondary battery constructed using the negative electrode of the secondary battery of this disclosure. [Figure 5] Figure 4 is a schematic exploded view showing the configuration of the wound electrode body of a lithium-ion secondary battery. [Modes for carrying out the invention]

[0014] Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings. Matters not mentioned in this specification but necessary for the implementation of the present disclosure can be understood as the design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and the common technical knowledge in the relevant field. In the following drawings, members and parts having the same function are denoted by the same reference numerals for description. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships. In this specification, the numerical range expressed as "A to B" includes A and B.

[0015] In this specification, the "secondary battery" refers to a power storage device that can be repeatedly charged and discharged. Also, in this specification, the "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes

[0016] The negative electrode of the present disclosure is used in a secondary battery, and preferably used in a lithium-ion secondary battery. As an example of the negative electrode of the present disclosure, the negative electrode according to the first embodiment will be specifically described with reference to FIG. 1.

[0017] 〔First Embodiment〕 FIG. 1 is a cross-sectional view schematically showing an example of the negative electrode 60 according to the first embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to the first embodiment shown in FIG. 1 is the negative electrode of a lithium-ion secondary battery.

[0018] As shown in the figure, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided only on one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0019] As shown in the illustrated example, a portion 62a without a negative electrode active material layer 64 may be provided at one end of the negative electrode 60 in the width direction. In the portion 62a without a negative electrode active material layer, the negative electrode current collector 62 is exposed, and the portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0020] The shape of the negative electrode current collector 62 is foil-like (or sheet-like) in the illustrated example, but is not limited thereto. The negative electrode current collector 62 may take various forms such as rod-like, plate-like, or mesh-like. As with conventional lithium-ion secondary batteries, a metal with good conductivity (e.g., copper, nickel, titanium, stainless steel, etc.) can be used as the material for the negative electrode current collector 62, and copper is particularly preferred. Copper foil is especially preferred as the negative electrode current collector 62.

[0021] The dimensions of the negative electrode current collector 62 are not particularly limited and can be determined as appropriate according to the battery design. When copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 6 μm or more and 20 μm or less.

[0022] The negative electrode active material layer 64 contains a negative electrode active material. At least graphite particles and Si-C composite particles are used as the negative electrode active material.

[0023] The graphite constituting the graphite particles may be natural graphite or artificial graphite, or it may be amorphous carbon-coated graphite in which the graphite is coated with an amorphous carbon material.

[0024] The shape of the graphite particles is not particularly limited and may be flaky, spherical, or the like. The graphite particles are preferably spheroidized graphite particles. When the graphite particles are spherical, the circularity of the graphite particles is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0025] In this specification, "circularity" refers to the ratio of the circumference of a perfect circle having the same area as the projected area of ​​the particle to the circumference of the particle projection image (i.e., circularity = circumference of a perfect circle having the same area as the projected area of ​​the particle / circumference of the particle projection image). Therefore, the closer the circularity is to 1, the closer the particle projection image is to a perfect circle, and the closer the particle is to a perfect sphere. Circularity can be determined, for example, by using a commercially available static automatic image analyzer to determine the circularity of 100 or more particles and calculating the average value.

[0026] The average particle size (D50) of the graphite particles is not particularly limited. For example, the average particle size (D50) of the graphite particles is 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

[0027] In this specification, "average particle size (D50)" refers to the median diameter (D50), which is the particle size corresponding to the cumulative frequency of 50% by volume from the smaller particle size side in a volume-based particle size distribution based on laser diffraction and scattering methods. The average particle size (D50) can be determined using commercially available laser diffraction and scattering type particle size distribution analyzers.

[0028] The proportion of graphite particles to the total of graphite particles and Si-C composite particles is preferably 40% to 90% by mass, more preferably 45% to 85% by mass, and even more preferably 50% to 80% by mass.

[0029] The Si-C composite particles used in this disclosure will be described in detail with reference to Figure 2. Figure 2 is a schematic cross-sectional view of an example of Si-C composite particles. The Si-C composite particle 10 shown in Figure 2 has a porous carbon framework 11 and Si-containing particles 12. At least a portion of the outer surface of the Si-C composite particle 10 is covered with a coating 13 containing LiF.

[0030] The carbon material constituting the porous framework 11 is, for example, a carbonized form of a carbon precursor (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite; hard carbon; soft carbon, etc.

[0031] The porous framework 11 has a plurality of pores 11a. The distribution and amount of pores 11a in the porous framework 11 are not limited to the illustrated example and may be the same as those of known porous carbon materials.

[0032] Si-containing particles 12 are, for example, Si, Si oxide (SiO x ), Si nitride (SiNx), Si carbide (SiCx), etc. Si-containing particles 12 are preferably composed of Si and Si oxide (SiO x It is composed of at least one of the following. The oxygen content in the Si-containing particles 12 is preferably 10% by mass or less.

[0033] The Si-containing particles 12 are embedded within the pores 11a of the porous framework 11. In other words, the Si-containing particles 12 are located within these pores 11a. The porous framework 11 functions to suppress the expansion of the Si-containing particles 12 during secondary battery charging. Therefore, because the Si-containing particles 12 are located within the pores 11a, the stress caused by the expansion of the Si-containing particles 12 during secondary battery charging can be mitigated by the porous framework 11.

[0034] In the example shown in Figure 2, there are voids 11a that do not contain Si-containing particles 12, but all voids 11a may contain Si-containing particles 12. However, it is advantageous for the porous framework 11 to have voids 11a that do not contain Si-containing particles 12, as the stress caused by the expansion of Si-containing particles 12 during secondary battery charging can be relieved by the voids 11a.

[0035] In the example shown in Figure 2, one Si-containing particle 12 is embedded in one void 11a of the porous framework 11. However, two or more Si-containing particles 12 may be embedded in one void 11a.

[0036] Furthermore, in the void 11a into which the Si-containing particles 12 are embedded, the Si-containing particles 12 may completely fill the void 11a, or there may be gaps remaining within the void 11a. If gaps remain within the void 11a, the stress caused by the expansion of the Si-containing particles 12 during charging of the lithium-ion secondary battery can be relieved by these gaps.

[0037] The average particle diameter of the Si-containing particles is, for example, 50 nm or less, and may be between 5 nm and 50 nm. The "average particle diameter of Si-containing particles" can be determined as follows: First, the negative electrode active material layer 64 is processed with FIB (Focused Ion Beam) to prepare a sample for scanning transmission electron microscopy (STEM) observation. Then, the sample is subjected to elemental analysis by EDX elemental mapping, and BF images (bright-field images) and HAADF images (high-angle scattering annular dark-field images) are obtained. The diameter of the Si-containing particles can be determined from the contrast and shape obtained from the BF and HAADF images. The diameters of 10 or more arbitrarily selected Si-containing particles are determined, and their average value is taken as the "average particle diameter of Si-containing particles" here.

[0038] The Si content in the Si-C composite particles 10 is not particularly limited, but a higher Si content allows for a higher capacity secondary battery, while increasing the expansion amount of the Si-C composite particles 10. For this reason, the Si content in the Si-C composite particles 10 is preferably 20% to 80% by mass, and more preferably 25% to 75% by mass. The Si content in the Si-C composite particles 10 can be determined by radio frequency inductively coupled plasma (ICP) emission spectroscopy. This Si content in the Si-C composite particles 10 is the mass ratio of Si to the total mass of the porous framework 11 and the Si-containing particles 12. Therefore, this Si mass ratio is calculated without including the mass of the coating 13.

[0039] A coating 13 containing LiF is formed on at least a portion of the outer surface of the Si-C composite particle 10. Therefore, the LiF-containing coating 13 is formed on at least the outer surface of the porous framework 11.

[0040] It is known that during the initial charging of a secondary battery, an SEI film is formed on the surface of the negative electrode active material by the decomposition of the non-aqueous electrolyte. According to the inventors' research, the following is the reason why the capacity of a secondary battery deteriorates when it is repeatedly charged and discharged in the conventional technology.

[0041] Si-C composite particles (especially Si-containing particles) have a very high expansion rate when recharged. Therefore, when a rechargeable battery is charged, the expansion can destroy the SEI film formed on the surface of the negative electrode active material (i.e., Si-C composite particles). When the SEI film is destroyed, the surface of the negative electrode active material is exposed, and decomposition of the non-aqueous electrolyte occurs in this exposed area, causing the SEI film to be regenerated. As the rechargeable battery is repeatedly charged and discharged, this destruction and re-formation of the SEI film occurs repeatedly, leading to an irreversible decrease in battery capacity.

[0042] In contrast, in this embodiment, the Si-C composite particles 10 have a coating 13 containing LiF on their outer surface. This LiF-containing coating 13 has a similar function to the SEI film, while being able to follow the expansion of the Si-C composite particles 10. As a result, the destruction of the coating 13 on the Si-C composite particles 10 is suppressed, and the re-formation of the SEI film becomes less likely. Consequently, the irreversible decrease in battery capacity mentioned above can be suppressed.

[0043] Therefore, a higher coverage rate of the Si-C composite particles 10 by the LiF-containing coating 13 results in a greater suppression of capacity degradation during repeated charging and discharging. Thus, the coverage rate is preferably 20% or more, more preferably 50% or more, even more preferably 80% or more, and particularly preferably 100%. Accordingly, as shown in Figure 2, it is particularly preferable that the LiF-containing coating 13 forms a layer that covers the entire outer surface of the Si-C composite particles 10. The coverage rate can be determined as follows: Obtain an electron microscope image of the cross-section of the Si-C composite particles 10. In the image, calculate the ratio (percentage) of the total length of the coating to the perimeter of the porous framework 11. Calculate this ratio for five or more arbitrarily selected Si-C composite particles 10, and use the average value as the coverage rate.

[0044] In the coating 13 containing LiF, a higher proportion of LiF is preferable. The proportion of LiF in the coating 13 (in other words, the LiF concentration) corresponds to the ratio of the peak intensity of LiF's F to the peak intensity of F other than LiF in the XPS spectrum of the Si-C composite particle 10 measured by X-ray photoelectron spectroscopy (XPS). The ratio of the peak intensity of LiF's F to the peak intensity of F other than LiF is usually 0.200 or higher, preferably 0.250 or higher, more preferably 0.300 or higher, and even more preferably 0.350 or higher. In the XPS spectrum, the peak of LiF's F appears in the range of 683eV to 686eV. Peaks of F other than LiF are F peaks outside the range of 683eV to 686eV, and may particularly appear in the range of 687eV to 690eV.

[0045] In the example shown in Figure 2, the Si-C composite particles 10 are spherical, but are not limited to this. The Si-C composite particles 10 may have an amorphous shape or other configuration.

[0046] The average particle size (D50) of the Si-C composite particles 10 is not particularly limited. For example, the average particle size (D50) of the Si-C composite particles 10 is 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 4 μm to 10 μm.

[0047] The proportion of Si-C composite particles 10 to the total of graphite particles and Si-C composite particles 10 is preferably 10% to 60% by mass, more preferably 15% to 55% by mass, and even more preferably 20% to 50% by mass.

[0048] The negative electrode active material may consist only of graphite particles and Si-C composite particles 10. Alternatively, the negative electrode active material may contain negative electrode active materials other than graphite particles and Si-C composite particles 10, to the extent that it does not significantly impair the effects of the present disclosure. For example, it may further contain particles of Si-C composite material without a coating.

[0049] The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as binders and conductive materials. Examples of binders include styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a thickening agent. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are particularly preferred. When CNTs are used as the conductive material, the negative electrode active material layer 64 may contain a CNT dispersant.

[0050] The content of the negative electrode active material in the negative electrode active material layer 64 (i.e., relative to the total mass of the negative electrode active material layer 64) is preferably 90% by mass or more, and more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.5% by mass or more and 5% by mass or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01% by mass or more and 3% by mass or less, and more preferably 0.05% by mass or more and 1% by mass or less.

[0051] The thickness of the negative electrode active material layer 64 is not particularly limited, but is, for example, 10 μm or more and 400 μm or less, and preferably 20 μm or more and 300 μm or less.

[0052] The density of the negative electrode active material layer 64 is not particularly limited, but for example, 0.7 g / cm³ 3 The above is preferable, preferably 1.0 g / cm³. 3 The above is more preferable, or 1.2 g / cm³. 3 That concludes the explanation. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm³. 3 The following is the case: 2.0 g / cm³ 3 The following is acceptable:

[0053] The negative electrode 60 may include components other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) adjacent to the negative electrode active material layer 64 may be provided on the portion 62a where the negative electrode active material layer is not formed. This insulating layer may contain, for example, an insulating inorganic filler.

[0054] Next, a preferred method for manufacturing the negative electrode 60 will be described. The preferred method for manufacturing the negative electrode 60 comprises the steps of: preparing Si-C composite particles 10 having a porous carbon skeleton 11, Si-containing particles 12 located in the pores 11a of the porous skeleton 11, and a coating 13 containing LiF formed on at least a part of the outer surface (hereinafter also referred to as the "Si-C composite particle preparation step"); mixing the Si-C composite particles 10 and graphite particles in a dispersion medium to prepare a negative electrode paste (hereinafter also referred to as the "paste preparation step"); coating the negative electrode paste onto a negative electrode current collector (hereinafter also referred to as the "coating step"); and drying the coated negative electrode paste (hereinafter also referred to as the "drying step"). The Si-C composite particle preparation step includes reacting a water-soluble lithium salt and a fluorinating agent in a dispersion in which particles having a porous carbon skeleton 11 and Si-containing particles 12 located in the pores 11a of the porous skeleton 11 are dispersed in water or a water-soluble organic solvent to produce LiF. The following describes each step of the manufacturing method in detail.

[0055] In this specification, "paste" refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, and includes so-called "slurry," "ink," etc.

[0056] In the Si-C composite particle preparation process, particles (hereinafter also referred to as "uncoated particles") are prepared, each having a porous carbon framework 11 and Si-containing particles 12 located within the pores 11a of the porous framework 11. Such uncoated particles are known (see, for example, Japanese Patent Publication No. 2015-38862, International Publication No. 2014 / 046144, etc.). Therefore, uncoated particles can be prepared by manufacturing them according to known methods.

[0057] Next, in a dispersion in which uncoated particles are dispersed in water or a water-soluble organic solvent, a water-soluble lithium salt and a fluorinating agent are reacted to produce LiF.

[0058] As a water-soluble organic solvent, for example, alcohols such as ethanol can be used. As a water-soluble lithium salt, for example, lithium acetate (dihydrate), lithium carbonate, lithium nitrate, lithium chloride, lithium hydroxide, etc. can be used, with lithium acetate (dihydrate) being preferred. As a fluorinating agent, hydrofluoric acid, ammonium fluoride, acidic ammonium fluoride, etc. can be used, with ammonium fluoride being preferred.

[0059] In this procedure, for example, first, an aqueous solution of a water-soluble lithium salt, a dispersion of uncoated particles in water or a water-soluble organic solvent, and an aqueous solution of a fluorinating agent are prepared. The aqueous solution of the water-soluble lithium salt and the dispersion are mixed to prepare a mixture. Under stirring, the aqueous solution of the fluorinating agent is added to the mixture. This causes the water-soluble lithium salt and the fluorinating agent to react and produce LiF. This yields a reaction solution containing LiF. The uncoated particles are present in this reaction solution.

[0060] The reaction conditions for the water-soluble lithium salt and fluorinating agent may be the same as those for known LiF synthesis using water-soluble lithium salt and fluorinating agent. For example, the reaction can be carried out at room temperature (i.e., 25°C ± 10°C) or under heating. The reaction time may be appropriately determined depending on the concentrations of the water-soluble lithium salt and fluorinating agent in the reaction solution, the desired content of LiF in the coating 13, etc. The longer the reaction time, the higher the content of LiF in the coating 13.

[0061] Uncoated particles are recovered from the reaction solution with the reaction solution containing LiF adhering to their surface. By drying the uncoated particles with the LiF-containing reaction solution adhering to them, LiF can be attached to the surface of the uncoated particles, thereby obtaining Si-C composite particles 10 having a LiF-containing coating 13. To adjust the amount of coating, the reaction solution containing LiF may be diluted with water or the like.

[0062] The paste preparation process can be carried out by mixing graphite particles, Si-C composite particles 10, and optional components (e.g., binders) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method.

[0063] The coating process can be carried out according to known methods. Specifically, for example, the coating process can be carried out by coating the obtained negative electrode paste onto the negative electrode current collector 62 using a coating device such as a gravure coater, comma coater, slit coater, or die coater.

[0064] The drying process can be carried out according to a known method. Specifically, for example, the negative electrode active material layer 64 is formed by removing the dispersion medium from the negative electrode current collector 62 coated with the negative electrode paste using a drying apparatus such as a drying oven. This allows the drying process to be carried out. The drying temperature and drying time can be appropriately determined according to the solid content concentration of the negative electrode paste and are not particularly limited. The drying temperature is, for example, 60°C to 200°C, preferably 70°C to 150°C. The drying time is, for example, 10 seconds to 30 minutes, preferably 30 seconds to 10 minutes.

[0065] After the drying process, a further step of pressing the negative electrode active material layer 64 may be performed. The pressing process can be carried out according to a known method. Specifically, the pressing process can be performed by applying pressure to the formed negative electrode active material layer 64 using a roller press or the like. The pressing process can densely pack the graphite particles and Si-C composite particles 10 contained in the negative electrode active material layer 64. In this way, the negative electrode 60 can be obtained.

[0066] In the first embodiment, one type of Si-C composite particle is used as the Si-C composite particle. However, two or more types of Si-C composite particles may be used. Therefore, as another example of the negative electrode of this disclosure, the negative electrode of the second embodiment will be described.

[0067] [Second Embodiment] Figure 3 is a schematic cross-sectional view showing the particles contained in the negative electrode active material layer 64 of the negative electrode of the second embodiment. The negative electrode active material layer 64 contains graphite particles 14, first Si-C composite particles 110, and second Si-C composite particles 120. The second embodiment differs from the first embodiment in that the first Si-C composite particles 110 and the second Si-C composite particles 120 are used as the Si-C composite particles 10. The same aspects as the first embodiment will not be explained. Note that Figure 3 is a schematic diagram, and the particle packing state is not limited to that shown example.

[0068] In the second embodiment, the Si content (S1) in the first Si-C composite particle 110 is lower than the Si content (S2) in the second Si-C composite particle 120. The Si content (S1) in the first Si-C composite particle 110 and the Si content (S2) in the second Si-C composite particle 120 are not particularly limited as long as this relationship is satisfied. If these Si content ratios are too low, the effect of improving cycle characteristics will be small, and the effect of increasing the capacity of the secondary battery may be reduced. On the other hand, if these Si content ratios are too high, the volume change due to expansion / contraction of the first Si-C composite particle 110 and the second Si-C composite particle 120 may become too large when the secondary battery is repeatedly charged and discharged.

[0069] Therefore, the Si content (S1) in the first Si-C composite particle 110 is preferably 20% to 55% by mass, and more preferably 25% to 45% by mass. The Si content (S2) in the second Si-C composite particle 120 is preferably 45% to 80% by mass, and more preferably 55% to 75% by mass. The Si content (S1) and Si content (S2) can be determined by radio frequency inductively coupled plasma (ICP) emission spectroscopy.

[0070] Furthermore, the ratio (S1 / S2) of the Si content (S1) in the first Si-C composite particle 110 to the Si content (S2) in the second Si-C composite particle 120 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and even more preferably 0.40 to 0.75.

[0071] The mass ratio of the first Si-C composite particles 110 to the second Si-C composite particles 120 is not particularly limited as long as the effects of this disclosure are obtained. In order to obtain a better packing state of the first Si-C composite particles 110 to the second Si-C composite particles 120, the mass ratio of the first Si-C composite particles 110 to the second Si-C composite particles 120 is preferably 40:60 to 90:10, more preferably 45:55 to 85:15, and even more preferably 55:45 to 80:20.

[0072] The first Si-C composite particle 110 and the second Si-C composite particle 120 each have a coating containing LiF on a portion of their outer surface. In the XPS spectrum of the first Si-C composite particle 110, measured by XPS, the ratio of the peak intensity of LiF's F to the peak intensity of F other than LiF (P1) is smaller than the ratio of the peak intensity of LiF's F to the peak intensity of F other than LiF (P2) in the XPS spectrum of the second Si-C composite particle 120.

[0073] The peak intensity ratio (P1) of the first Si-C composite particle 110 and the peak intensity ratio (P2) of the second Si-C composite particle 120 are not particularly limited as long as this relationship is satisfied. The peak intensity ratio (P1) of the first Si-C composite particle 110 is preferably 0.250 or more and less than 0.500, and more preferably 0.300 or more and 0.450 or less. The peak intensity ratio (P2) of the second Si-C composite particle 120 is preferably 0.500 or more and 0.800 or less, and more preferably 0.550 or more and 0.700 or less.

[0074] The ratio (P1 / P2) of the peak intensity ratio (P1) of the first Si-C composite particle 110 to the peak intensity ratio (P2) of the second Si-C composite particle 120 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and even more preferably 0.40 to 0.75.

[0075] In this second embodiment, the Si-C composite particles 10 consist of a first Si-C composite particle 110 having a relatively low Si content and a relatively low LiF concentration in the coating, and a second Si-C composite particle 120 having a relatively high Si content and a relatively high LiF concentration in the coating. Here, the amount of expansion of the Si-C composite particles during secondary battery charging increases as the Si content increases. On the other hand, Si-C composite particles with a high Si content can greatly contribute to increasing battery capacity. By forming a LiF coating of sufficient concentration on the second Si-C composite particle 120 with a high Si content, the breakdown of the coating on the second Si-C composite particle 120 can be suitably suppressed. Furthermore, with the first Si-C composite particle 110 with a low Si content, the breakdown of the coating can be sufficiently suppressed even if the LiF concentration in the coating is low. By mixing these two types of particles, the capacity degradation when the secondary battery is repeatedly charged and discharged can be highly suppressed. Therefore, the negative electrode of the second embodiment can suppress capacity degradation when the secondary battery is repeatedly charged and discharged more effectively than the negative electrode of the first embodiment.

[0076] The average particle diameter (D50) of the first Si-C composite particle 110 and the second Si-C composite particle 120 may be the same or different.

[0077] In the second embodiment, the negative electrode active material may contain negative electrode active materials other than graphite particles, first Si-C composite particles 110, and second Si-C composite particles 120, to the extent that the effects of the present disclosure are not significantly impaired.

[0078] The negative electrode of this disclosure described above can provide a secondary battery with excellent resistance to capacity degradation when repeatedly charged and discharged. Furthermore, since the negative electrode of this disclosure uses a negative electrode active material containing Si, the capacity of the secondary battery can be increased. Therefore, a secondary battery using the negative electrode of this disclosure has high capacity and excellent cycle characteristics.

[0079] Therefore, from another perspective, the secondary battery of the present disclosure comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode of the present disclosure described above. Hereinafter, an embodiment of the secondary battery of the present disclosure will be described with reference to Figures 4 and 5, using a lithium-ion secondary battery as an example. The following example configuration is a flat rectangular lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0080] The lithium-ion secondary battery 100 shown in Figure 4 is a sealed lithium-ion secondary battery 100 constructed by housing a flat-shaped wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat rectangular battery case (i.e., outer container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 is also provided with an inlet (not shown) for injecting the non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. As the material of the battery case 30, for example, a lightweight metal material with good thermal conductivity such as aluminum is used.

[0081] As shown in Figures 4 and 5, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are superimposed on each other via two elongated separator sheets 70 and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (here, both sides) of an elongated negative electrode current collector 62. The portion 52a where the positive electrode active material layer is not formed (i.e., the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the portion 62a where the negative electrode active material layer is not formed (i.e., the portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed) are formed to protrude outward from both ends of the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction perpendicular to the longitudinal direction). The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are joined to the positive electrode active material layer not formed portion 52a and the negative electrode active material layer not formed portion 62a, respectively.

[0082] As the positive electrode current collector 52 constituting the positive electrode sheet 50, a known positive electrode current collector used in lithium-ion secondary batteries may be used, and examples include a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive electrode current collector 52.

[0083] The dimensions of the positive electrode current collector 52 are not particularly limited and can be determined as appropriate according to the battery design. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, and preferably 7 μm or more and 20 μm or less.

[0084] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition of positive electrode active material used in lithium-ion secondary batteries may be used. Specifically, for example, lithium composite oxides, lithium transition metal phosphate compounds, etc., can be used as the positive electrode active material. The crystal structure of the positive electrode active material is not particularly limited and may be a layered structure, spinel structure, olivine structure, etc.

[0085] Preferably, lithium transition metal composite oxides are lithium transition metal composite oxides containing at least one of Ni, Co, and Mn as a transition metal element. Specific examples include lithium nickel-based composite oxides, lithium cobalt-based composite oxides, lithium manganese-based composite oxides, lithium nickel-manganese-based composite oxides, lithium nickel-cobalt-manganese-based composite oxides, lithium nickel-cobalt-aluminum-based composite oxides, and lithium iron-nickel-manganese-based composite oxides.

[0086] In this specification, "lithium nickel cobalt manganese composite oxide" is a term that encompasses not only oxides whose constituent elements are Li, Ni, Co, Mn, and O, but also oxides that contain one or more additive elements other than these. Examples of such additive elements include transition metal elements and main group metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additive elements may also be metalloid elements such as B, C, Si, and P, or nonmetallic elements such as S, F, Cl, Br, and I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, lithium iron nickel manganese composite oxide, etc.

[0087] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0088] These positive electrode active materials may be used individually or in combination of two or more. Lithium nickel cobalt manganese composite oxides are particularly preferred as positive electrode active materials due to their excellent properties, such as initial resistance characteristics.

[0089] The average particle size (D50) of the positive electrode active material is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 15 μm or less.

[0090] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as trilithium phosphate, conductive material, binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB); carbon fibers such as vapor-processed carbon fiber (VGCF) and carbon nanotubes (CNT); and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF), etc.

[0091] The content of positive electrode active material in the positive electrode active material layer 54 (i.e., the content of positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 85% by mass or more and 99% by mass or less. The content of trilithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 15% by mass or less, and more preferably 0.2% by mass or more and 10% by mass or less. The content of conductive material in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less. The content of binder in the positive electrode active material layer 54 is not particularly limited, but is preferably 0.4% by mass or more and 15% by mass or less, and more preferably 0.5% by mass or more and 10% by mass or less.

[0092] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is usually 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is usually 400 μm or less, and preferably 300 μm or less.

[0093] As the negative electrode sheet 60, the negative electrode of the present disclosure described above (for example, the negative electrode of the first embodiment or the negative electrode of the second embodiment described above) is used.

[0094] Examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such porous sheets may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). A heat-resistant layer (HRL) may be provided on the surface of the separator 70.

[0095] The thickness of the separator 70 is not particularly limited, but is, for example, 5 μm to 50 μm, and preferably 10 μm to 30 μm. The air permeability obtained by the Gurley test method of the separator 70 is not particularly limited, but is preferably 350 seconds / 100 cc or less.

[0096] Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents commonly used in lithium-ion secondary battery electrolytes, such as carbonates, ethers, esters, nitriles, sulfones, and lactones, can be used without particular limitation. Among these, carbonates are preferred, with specific examples including ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC). Such non-aqueous solvents can be used individually or in appropriate combinations of two or more. For example, the non-aqueous solvent may consist solely of carbonates. As another example, non-aqueous solvents include carbonates and esters such as methyl acetate.

[0097] Suitable supporting salts include lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI), preferably LiPF6. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0098] Furthermore, the above-mentioned non-aqueous electrolyte may contain components other than those described above, such as film-forming agents like vinylene carbonate (VC) and oxalat complexes; gas-generating agents like biphenyl (BP) and cyclohexylbenzene (CHB); thickeners; and other additives, as long as they do not significantly impair the effects of the present disclosure.

[0099] The lithium-ion secondary battery 100 exhibits suppressed capacity degradation during repeated charging and discharging, and also boasts high capacity. The lithium-ion secondary battery 100 can be used in a variety of applications. Suitable applications include power supplies for vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Furthermore, the lithium-ion secondary battery 100 can be used as a storage battery for small-scale power storage devices. The lithium-ion secondary battery 100 can also typically be used in the form of a battery pack, consisting of multiple batteries connected in series and / or parallel.

[0100] The above describes a rectangular lithium-ion secondary battery 100 equipped with a flattened wound electrode body 20 as an example. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries equipped with a stacked electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are stacked alternately). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, laminated case type lithium-ion secondary batteries, and the like.

[0101] Furthermore, according to known methods, the lithium-ion secondary battery 100 can also be configured as an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0102] Furthermore, while the negative electrode of this disclosure is suitable as a negative electrode for lithium-ion secondary batteries, it can be constructed and used as a negative electrode for other secondary batteries, and other secondary batteries can be constructed according to known methods.

[0103] Hereinafter, embodiments of the present disclosure will be described in detail, but the present disclosure is not intended to be limited to those shown in such embodiments.

[0104] <Production of Si-C composite particles having a coating containing LiF> Uncoated particles having a porous carbon skeleton and Si-containing particles that had entered into the pores thereof were prepared. The powder of the uncoated particles was dispersed in ethyl alcohol while being ultrasonicated for 30 minutes. Lithium acetate was dissolved in deionized water to prepare an aqueous lithium acetate solution. The dispersion of the uncoated particles was added to this aqueous solution while stirring for 30 minutes. An aqueous ammonium fluoride solution was added thereto while stirring for a predetermined time to react lithium acetate and ammonium fluoride. Thereby, a reaction solution containing LiF was obtained. After diluting the reaction solution, it was filtered. The filtrate was dried to obtain Si-C composite particles having a coating containing LiF.

[0105] When the XPS spectrum was measured for the obtained Si-C composite particles, a peak of F of Li-F (about 684.8 eV) and a peak of Li of Li-F (about 55.1 eV) were observed, and it was confirmed that LiF was contained. In addition, the ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF (hereinafter, also referred to as "LiF intensity ratio") was determined. The peak of F of LiF appeared at 683 eV to 686 eV, and the peak of F other than LiF appeared at 687 eV to 690 eV.

[0106] Following the above procedure, Si-C composite particles (A) with a Si content of 40% by mass and a LiF intensity ratio of 0.372, Si-C composite particles (B) with a Si content of 65% by mass and a LiF intensity ratio of 0.619, Si-C composite particles (C) with a Si content of 40% by mass and a LiF intensity ratio of 0.621, and Si-C composite particles (D) with a Si content of 65% by mass and a LiF intensity ratio of 0.373 were obtained. In the production of Si-C composite particles (A) and (D), the addition time of the ammonium fluoride aqueous solution (i.e., reaction time) was 60 minutes, and in the production of Si-C composite particles (B) and (C), the addition time of the ammonium fluoride aqueous solution was 120 minutes. The Si content in the Si-C composite particles was measured using a commercially available ICP analyzer.

[0107] <Fabrication of the negative electrode> [Example 1] As the negative electrode active material, graphite particles with an average particle size (D50) of 15 μm and the above-mentioned Si-C composite particles (A) were prepared.

[0108] A dispersion of single-walled carbon nanotubes (SWCNTs) as a conductive material was prepared. Carboxymethylcellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared as binders.

[0109] A negative electrode paste containing graphite particles, Si-C composite particles, SWCNTs, CMC, PAA, and SBR in a mass ratio of 65:35:0.1:1:1:1.5 was prepared by the following procedure.

[0110] Paste A was prepared by mixing Si-C composite particles, a dispersion of SWCNTs, and a dispersion medium at a rotational speed of 3000 rpm using a disperser. Graphite particles, CMC, and PAA were dry-blended using a stirring granulator. To the resulting mixed powder, Paste A and the dispersion medium were added to achieve a solid content concentration of 65% by mass, and the mixture was kneaded. SBR and the dispersion medium were then added and uniformly mixed to prepare the negative electrode paste.

[0111] The prepared negative electrode paste was applied to the surface of a 10 μm thick copper foil and dried to form a negative electrode active material layer. After roll pressing the negative electrode active material layer, the resulting sheet was processed to a predetermined size to obtain a negative electrode sheet.

[0112] [Comparative Example 1] A negative electrode sheet was obtained in the same manner as in Example 1, except that Si-C composite particles without a coating and with a Si content of 65% by mass were used as the Si-C composite particles.

[0113] [Comparative Example 2] A negative electrode sheet was obtained in the same manner as in Example 1, except that Si-C composite particles without a coating and with a Si content of 40% by mass were used as the Si-C composite particles.

[0114] [Example 2] A negative electrode sheet was obtained in the same manner as in Example 1, except that the following first Si-C composite particles and second Si-C composite particles were used as Si-C composite particles in a mass ratio of 28:7. First Si-C composite particle: The above Si-C composite particle (A) Second Si-C composite particle: The above Si-C composite particle (B)

[0115] [Example 3] A negative electrode sheet was obtained in the same manner as in Example 1, except that the following first Si-C composite particles and second Si-C composite particles were used as Si-C composite particles in a mass ratio of 28:7. First Si-C composite particle: No coating containing LiF, Si content = 40% by mass Second Si-C composite particle: The above Si-C composite particle (B)

[0116] [Example 4] A negative electrode sheet was obtained in the same manner as in Example 1, except that the following first Si-C composite particles and second Si-C composite particles were used as Si-C composite particles in a mass ratio of 28:7. First Si-C composite particle: The above Si-C composite particle (C) Second Si-C composite particle: The above Si-C composite particle (D)

[0117] <Fabrication of Lithium-Ion Secondary Battery for Evaluation> LiNi as the positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were mixed with N-methylpyrrolidone (NMP) at a mass ratio of NCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. This paste was applied to the surface of an aluminum foil with a thickness of 15 μm and dried to form a positive electrode active material layer. After roll pressing the positive electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a positive electrode sheet.

[0118] A separator made of porous polyolefin was prepared. Leads were attached to each of the above-prepared negative electrode sheet and positive electrode sheet, and they were laminated through the separator to fabricate an electrode body. This was housed together with a non-aqueous electrolyte in a case made of an aluminum laminate film. The non-aqueous electrolyte used was a mixture solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 15:5:40:40, in which LiPF6 as a supporting salt was dissolved at a concentration of 1.0 mol / L. Then, the case was sealed to obtain a lithium-ion secondary battery for evaluation.

[0119] <Cycle Performance Evaluation> Each of the above-prepared lithium-ion secondary batteries for evaluation was placed in an environment at 25°C. Each lithium-ion secondary battery for evaluation was charged at a constant current up to 4.2 V at a current value of 0.4C, and then charged at a constant voltage until the current value reached 0.1C. Next, each lithium-ion secondary battery for evaluation was discharged at a constant current to 2.5 V at a current value of 0.4C. And the discharge capacity at this time was measured to obtain the initial capacity.

[0120] The above charge-discharge cycle was repeated 200 times, with each cycle being considered one. The discharge capacity after 200 cycles was determined using the same method as for the initial capacity. As an indicator of cycle characteristics, the capacity retention rate (%) was calculated using the formula: (discharge capacity after 200 charge-discharge cycles / initial capacity) × 100. The results are shown in Table 1.

[0121] [Table 1]

[0122] As in Comparative Examples 1 and 2, when Si-C composite particles without a LiF-containing coating are used, a normal SEI film (i.e., a film derived from the decomposition products of the non-aqueous electrolyte) is formed on the surface of the Si-C composite particles when the lithium-ion secondary battery is charged. A comparison of Example 1 and Comparative Examples 1 and 2 shows that by using Si-C composite particles with a pre-applied LiF-containing coating, capacity degradation during repeated charging and discharging can be significantly suppressed. Furthermore, Examples 2 to 4 used two types of Si-C composite particles. A comparison of these shows that by using a combination of Si-C composite particles with a relatively low Si content and a relatively low LiF concentration in the coating, and Si-C composite particles with a relatively high Si content and a relatively high LiF concentration in the coating, capacity degradation can be further suppressed.

[0123] Therefore, from the above, it can be seen that the negative electrode of this disclosure can suppress capacity degradation when the secondary battery is repeatedly charged and discharged.

[0124] The specific examples of this disclosure have been described in detail above, but these are merely illustrative and do not limit the scope of the claims. The technologies described in the claims include various modifications and changes to the specific examples described above.

[0125] In other words, the negative electrode of the secondary battery, the method for manufacturing the same, and the secondary battery of this disclosure are as described in the following sections [1] to [9]. [1] A negative electrode of a secondary battery comprising a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector, The negative electrode active material layer contains graphite particles and Si-C composite particles as negative electrode active materials. The Si-C composite particle has a porous carbon framework and Si-containing particles located within the pores of the porous framework. A negative electrode in which a coating containing LiF is formed on at least a portion of the outer surface of the Si-C composite particles. [2] The negative electrode according to item [1], wherein the content ratio of the Si-C composite particles to the total of the graphite particles and the Si-C composite particles is 10% by mass to 60% by mass. [3] The Si-C composite particles contain first Si-C composite particles and second Si-C composite particles, Both the first Si-C composite particle and the second Si-C composite particle have a coating containing LiF formed on at least a portion of their outer surface. The Si content in the first Si-C composite particle is lower than the Si content in the second Si-C composite particle. The negative electrode according to item [1] or [2], wherein the ratio of the peak intensity of F for LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-C composite particle, measured by X-ray photoelectron spectroscopy, is smaller than the ratio of the peak intensity of F for LiF to the peak intensity of F other than LiF in the XPS spectrum of the second Si-C composite particle. [4] The ratio of Si content in the first Si-C composite particle to the Si content in the second Si-C composite particle is 0.10 to 0.90, and The negative electrode according to item [3], wherein the ratio of the ratio of the peak intensity in the XPS spectrum of the first Si-C composite particle to the ratio of the peak intensity in the XPS spectrum of the second Si-C composite particle is 0.10 to 0.90. [5] The negative electrode according to item [3] or [4], wherein the Si content in the first Si-C composite particle is 20% by mass to 55% by mass, and the Si content in the second Si-C composite particle is 45% by mass to 80% by mass. [6] The negative electrode according to any one of items [3] to [5], wherein the ratio of the peak intensities in the XPS spectrum of the first Si-C composite particle is 0.250 or more and less than 0.500, and the ratio of the peak intensities in the XPS spectrum of the second Si-C composite particle is 0.500 or more and 0.800 or less. [7] The negative electrode according to any one of items [3] to [6], wherein the mass ratio of the first Si-C composite particle to the second Si-C composite particle is 40:60 to 90:10. [8] A step of preparing Si-C composite particles having a porous carbon framework, Si-containing particles located in the pores of the porous framework, and a coating containing LiF formed on at least a portion of the outer surface, A step of preparing a negative electrode paste by mixing the Si-C composite particles and graphite particles in a dispersion medium. The steps include: applying the negative electrode paste onto the negative electrode current collector, and The process of drying the coated negative electrode paste. Equipped with, A method for manufacturing a negative electrode of a secondary battery, comprising the step of preparing the Si-C composite particles, which includes reacting a water-soluble lithium salt and a fluorinating agent in a dispersion in which particles having a porous carbon framework and Si-containing particles located within the pores of the porous framework are dispersed in water or a water-soluble organic solvent to produce LiF, [9] Positive electrode, negative electrode, electrolyte, A secondary battery equipped with, A secondary battery wherein the negative electrode is the negative electrode described in any one of items [1] to [7]. [Explanation of Symbols]

[0126] 10 Si-C composite particles 11 Porous skeleton 11a Hole 12 Si-containing particles 13 Covering 14 Graphite Particles 20 Wound electrode body 30 Battery Cases 36 Safety valve 42 Positive terminal 42a Positive electrode current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode sheets (positive electrode) 52 Positive electrode current collector 52a Portion where positive electrode active material layer is not formed 54 Cathode active material layer 60 Negative electrode sheets (negative electrode) 62 Negative electrode current collector 62a Part where negative electrode active material layer is not formed 64 Negative electrode active material layer 70 Separator Sheets (Separators) 100 Lithium-ion rechargeable batteries 110 1st Si-C composite particle 120 2nd Si-C composite particles

Claims

1. A negative electrode of a secondary battery comprising a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector, The negative electrode active material layer contains graphite particles and Si-C composite particles as negative electrode active materials. The Si-C composite particle has a porous carbon framework and Si-containing particles located within the pores of the porous framework. A negative electrode in which a coating containing LiF is formed on at least a portion of the outer surface of the Si-C composite particles.

2. The negative electrode according to claim 1, wherein the content ratio of the Si-C composite particles to the total of the graphite particles and the Si-C composite particles is 10% by mass to 60% by mass.

3. The Si-C composite particles contain first Si-C composite particles and second Si-C composite particles. Both the first Si-C composite particle and the second Si-C composite particle have a coating containing LiF formed on at least a portion of their outer surface. The Si content in the first Si-C composite particle is lower than the Si content in the second Si-C composite particle. The negative electrode according to claim 1, wherein the ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-C composite particle measured by X-ray photoelectron spectroscopy is smaller than the ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF in the XPS spectrum of the second Si-C composite particle.

4. The Si content ratio in the first Si-C composite particle is 0.10 to 0.90 relative to the Si content ratio in the second Si-C composite particle, and The negative electrode according to claim 3, wherein the ratio of the ratio of the peak intensity in the XPS spectrum of the first Si-C composite particle to the ratio of the peak intensity in the XPS spectrum of the second Si-C composite particle is 0.10 to 0.

90.

5. The negative electrode according to claim 3, wherein the Si content in the first Si-C composite particle is 20% by mass to 55% by mass, and the Si content in the second Si-C composite particle is 45% by mass to 80% by mass.

6. The negative electrode according to claim 3, wherein the ratio of the peak intensities in the XPS spectrum of the first Si-C composite particle is 0.250 or more and less than 0.500, and the ratio of the peak intensities in the XPS spectrum of the second Si-C composite particle is 0.500 or more and 0.800 or less.

7. The negative electrode according to claim 3, wherein the mass ratio of the first Si-C composite particle to the second Si-C composite particle is 40:60 to 90:

10.

8. A step of preparing Si-C composite particles having a porous carbon framework, Si-containing particles located within the voids of the porous framework, and a coating containing LiF formed on at least a portion of the outer surface, A step of preparing a negative electrode paste by mixing the Si-C composite particles and graphite particles in a dispersion medium. The steps include: applying the negative electrode paste onto the negative electrode current collector, and The process of drying the coated negative electrode paste. Equipped with, A method for producing a negative electrode of a secondary battery, comprising the step of preparing the Si-C composite particles, wherein particles having a porous carbon framework and Si-containing particles located within the pores of the porous framework are dispersed in water or a water-soluble organic solvent, and a water-soluble lithium salt and a fluorinating agent are reacted to produce LiF.

9. Positive electrode, negative electrode, electrolyte, A secondary battery equipped with, A secondary battery wherein the negative electrode is the negative electrode described in claim 1.

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