Negative electrode for secondary battery, method for manufacturing negative electrode, and secondary battery using negative electrode

Abstract

The present invention relates to a negative electrode for a secondary battery, a method for manufacturing the negative electrode, and a secondary battery using the negative electrode. Provided is a negative electrode which contains S-C composite particles and graphite particles, and which is capable of suppressing capacity deterioration during repeated charging and discharging of a secondary battery. A negative electrode of a secondary battery according to the present disclosure includes 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 S-C composite particles as a negative electrode active material. The S-C composite particles have a porous skeleton made of carbon and Si-containing particles located in pores of the porous skeleton. A coating containing LiF is formed on at least a part of the outer surface of the Si-C composite particles.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a negative electrode of a secondary battery and a manufacturing method thereof. The present disclosure also relates to a secondary battery using the negative electrode. BACKGROUND

[0002] In recent years, secondary batteries are suitably used for portable power sources of personal computers, mobile terminals, and the like, vehicle drive power sources of electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like.

[0003] In the vehicle drive power source use, particularly the drive power source use of BEVs, from the viewpoint of extending the cruising distance of the vehicle, it is desired that the secondary battery be further high in capacity. As a negative electrode active material high in capacity, Si-C composite particles are known, and it is known that the secondary battery can be made high in capacity by the Si-C composite particles (for example, refer to Japanese Patent Application Publication No. 2015-38862 and Japanese Patent Application Publication No. 2019-522886). In Japanese Patent Application Publication No. 2015-38862, a technology is disclosed in which Si-C composite particles and graphite particles such as natural graphite are used together as negative electrode active materials. In Japanese Patent Application Publication No. 2019-522886, as a negative electrode active material, composite particles in which Si-LiF mixed particles in which Si is coated with LiF are dispersed in a carbon phase composed of a carbon material are disclosed. SUMMARY

[0004] However, in the related art, a secondary battery using a negative electrode in which Si-C composite particles and graphite particles are used together has a problem of low cycle characteristics, specifically, a problem of large capacity deterioration when repeatedly charging and discharging the secondary battery.

[0005] In view of the above-described actual circumstances, an object of the present disclosure is to provide a negative electrode containing Si-C composite particles and graphite particles, that is, a negative electrode capable of suppressing capacity deterioration when repeatedly charging and discharging a secondary battery.

[0006] The negative electrode of the secondary battery of the present disclosure includes 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 particles have a porous skeleton made of carbon and Si-containing particles located inside pores of the porous skeleton. A coating layer containing LiF is formed on at least a part of the outer surface of the Si-C composite particles.

[0007] According to such a configuration, it is possible to provide a negative electrode containing Si-C composite particles and graphite particles, that is, a negative electrode capable of suppressing capacity deterioration when repeatedly charging and discharging a secondary battery.

[0008] From another aspect, a method of manufacturing a negative electrode of a secondary battery of the present disclosure includes: a step of preparing Si-C composite particles having a porous skeleton of carbon, Si-containing particles located inside pores of the porous skeleton, and a coating layer containing LiF formed on at least a part of an outer surface; a step of mixing the Si-C composite particles and graphite particles in a dispersion medium to prepare a negative electrode paste; a step of applying the negative electrode paste to a negative electrode current collector; and a step of drying the applied 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 liquid in which particles having the porous skeleton of carbon and the Si-containing particles located inside pores of the porous skeleton are dispersed in water or a water-soluble organic solvent to generate LiF.

[0009] According to the negative electrode obtained by adopting such a configuration, excellent capacity deterioration resistance at the time of repeated charge and discharge of a secondary battery can be imparted.

[0010] From another aspect, a secondary battery of the present disclosure includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the above-described negative electrode.

[0011] According to such a configuration, a secondary battery having excellent capacity deterioration resistance at the time of repeated charge and discharge can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 A cross-sectional view schematically showing a configuration of a first embodiment of a negative electrode of a secondary battery of the present disclosure.

[0013] Figure 2 A cross-sectional view schematically showing a configuration of Si-C composite particles contained in a negative electrode active material layer of the negative electrode of Figure 1

[0014] Figure 3 A cross-sectional view schematically showing particles contained in a negative electrode active material layer of a second embodiment of a negative electrode of a secondary battery of the present disclosure.

[0015] Figure 4 A cross-sectional view schematically showing a configuration of a lithium-ion secondary battery constructed using a negative electrode of a secondary battery of the present disclosure.

[0016] Figure 5 A schematic exploded view showing a configuration of a wound electrode body of the lithium-ion secondary battery of Figure 4 DETAILED DESCRIPTION

[0017] ​​Embodiments relating to the present disclosure will be described below with reference to the accompanying drawings. Note that matters not mentioned in this specification, i.e., matters required for implementation of the present disclosure, can be understood as design matters of one skilled in the art based on the existing technology in the field. The present disclosure can be implemented based on the content disclosed in this specification and technical common sense in the field. In addition, in the following drawings, the same reference numerals are used for components that serve the same function, and the components are described. In addition, the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. Note that in this specification, a numerical range expressed as "A to B" includes A and B.

[0018] Note that in this specification, a "secondary battery" refers to a power storage device that can be repeatedly charged and discharged. In this specification, a "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as charge carriers and that performs charge and discharge by movement of charges associated with lithium ions between positive and negative electrodes.

[0019] The negative electrode of the present disclosure is used for a secondary battery, preferably a lithium-ion secondary battery. As an example of the negative electrode of the present disclosure, reference is made to Figure 1 The negative electrode relating to the first embodiment is specifically described.

[0020] [First Embodiment]

[0021] Figure 1 A cross-sectional view of the negative electrode 60 of an example relating to the first embodiment is schematically shown, which is a cross-sectional view along the thickness direction and the width direction. Figure 1 The negative electrode 60 relating to the first embodiment shown is a negative electrode of a lithium-ion secondary battery.

[0022] As illustrated, 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 the negative electrode current collector 62 and the negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 can be provided on only one surface of the negative electrode current collector 62, or can be provided on both surfaces of the negative electrode current collector 62 as illustrated. The negative electrode active material layer 64 is preferably provided on both surfaces of the negative electrode current collector 62.

[0023] As illustrated, a negative electrode active material layer non-formed portion 62a in which the negative electrode active material layer 64 is not provided can be provided at one end portion in the width direction of the negative electrode 60. In the negative electrode active material layer non-formed portion 62a, the negative electrode current collector 62 is exposed, and the negative electrode active material layer non-formed portion 62a can function as a current collecting portion. However, the configuration for current collection from the negative electrode 60 is not limited thereto.

[0024] The shape of the negative electrode current collector 62 is a foil shape (or a sheet shape) in the illustrated example, but is not limited thereto. The negative electrode current collector 62 can be various shapes such as a rod shape, a plate shape, a mesh shape, and the like. As the material of the negative electrode current collector 62, a metal having good electrical conductivity (for example, copper, nickel, titanium, stainless steel, and the like) can be used as with conventional lithium-ion secondary batteries, and copper is preferable. A copper foil is particularly preferable as the negative electrode current collector 62.

[0025] The size of the negative electrode current collector 62 is not particularly limited and can be appropriately determined in accordance with the battery design. In the case where a copper foil is used as the negative electrode current collector 62, the thickness thereof is not particularly limited and is, for example, 5 μm or more and 35 μm or less, and is preferably 6 μm or more and 20 μm or less.

[0026] The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, at least graphite particles and Si-C composite particles are used.

[0027] The graphite constituting the graphite particles can be natural graphite or artificial graphite, and the graphite can be amorphous carbon-coated graphite in a form in which amorphous carbon material is coated.

[0028] The shape of the graphite particles is not particularly limited and can be a scale shape, a spherical shape, or the like. The graphite particles are preferably spheroidized graphite particles. In the case where the graphite particles are spherical, the circularity of the graphite particles is preferably 0.85 to 1, more preferably 0.88 to 1, and further preferably 0.90 to 1.

[0029] Note that the "circularity" in the present specification refers to the ratio of the circumference of a true 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 true circle having the same area as the projected area of the particle / circumference of the particle projection image). Thus, the closer the circularity is to 1, the closer the particle projection image is to a true circle, and the closer the particle is to a true sphere. The circularity can be calculated, for example, by using a commercially available static automatic image analysis device, calculating the average value of the circularity for 100 or more particles, and thus can be calculated.

[0030] The average particle diameter (D50) of the graphite particles is not particularly limited. The average particle diameter (D50) of the graphite particles is, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and further preferably 12 μm to 20 μm.

[0031] Note that the "average particle diameter (D50)" in the present specification refers to the median diameter (D50) and refers to the particle diameter corresponding to 50% by volume of the cumulative frequency from the fine particle side in the particle size distribution based on the volume basis of the laser diffraction scattering method. The average particle diameter (D50) can be calculated using a commercially available particle size distribution measuring device of the laser diffraction scattering type or the like.

[0032] The proportion of graphite particles relative 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.

[0033] For the Si-C composite particles used in this disclosure, Figure 2 A detailed explanation. Figure 2 This is a schematic cross-sectional view of an example of Si-C composite particles. Figure 2 The Si-C composite particle 10 shown 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 coated with a LiF-containing coating 13.

[0034] The carbon material constituting the porous framework 11 is, for example, a carbon precursor (such as petroleum pitch, coal pitch, phenolic resin, etc.) carbide, graphite, hard carbon, soft carbon, etc.

[0035] The porous framework 11 has a plurality of pores 11a. The distribution and amount of the pores 11a in the porous framework 11 are not limited to the example shown in the figure, and can be the same as those in known porous carbon materials.

[0036] 12 containing Si particles, for example, Si, Si oxide (SiO) x ), Si nitride (SiN) x ), Si carbide (SiC) x It is composed of, etc. The Si-containing particles 12 are preferably composed of Si, and Si oxide (SiO2). x It consists of at least one of the following: The oxygen content in the Si-containing particles 12 is preferably 10% by mass or less.

[0037] Si-containing particles 12 enter the pores 11a of the porous framework 11. In other words, the Si-containing particles 12 are located within the pores 11a. The porous framework 11 functions to suppress the expansion of the Si-containing particles 12 during secondary battery charging. Therefore, by having the Si-containing particles 12 located within the pores 11a, the porous framework 11 can mitigate the stress caused by the expansion of the Si-containing particles 12 during secondary battery charging.

[0038] exist Figure 2 In the example shown, there are pores 11a into which Si particles 12 have not yet entered, but Si particles 12 can enter all of the pores 11a. However, since the stress caused by the expansion of Si particles 12 during secondary battery charging can be alleviated by utilizing the pores 11a, it is advantageous to have pores 11a into which Si particles 12 have not yet entered in the porous framework 11.

[0039] exist Figure 2In the example shown, one Si-containing particle 12 has entered one of the pores 11a of the porous framework 11. However, two or more Si-containing particles 12 can enter one of the pores 11a.

[0040] In addition, in the pores 11a into which the Si-containing particles 12 have entered, the Si-containing particles 12 can completely fill the pores 11a, or gaps can be left in the pores 11a. In the case where gaps are left in the pores 11a, the stress generated due to expansion of the Si-containing particles 12 at the time of charging of the lithium-ion secondary battery can be alleviated using the gaps.

[0041] The average particle diameter of the Si-containing particles is, for example, 50 nm or less, and can be 5 nm to 50 nm. Note that the "average particle diameter of the Si-containing particles" can be obtained as described below. First, the negative electrode active material layer 64 is subjected to FIB (focused ion beam) processing to produce a sample for scanning transmission electron microscope (STEM) observation. Then, the sample is subjected to elemental analysis using EDX elemental mapping, and a BF image (bright field image) and a HAADF image (high angle annular dark field image) are obtained. The diameter of the Si-containing particles can be obtained from the contrast and shape obtained using the BF image and the HAADF image. The diameters of 10 or more Si-containing particles selected at random are obtained, and the average value thereof is set as the "average particle diameter of the Si-containing particles" herein.

[0042] The Si content ratio in the Si-C composite particles 10 is not particularly limited, and when the Si content ratio is high, the secondary battery can be made high in capacity, but on the other hand, the expansion amount of the Si-C composite particles 10 increases. Therefore, the Si content ratio in the Si-C composite particles 10 is preferably 20 mass% to 80 mass%, and more preferably 25 mass% to 75 mass%. Note that the Si content ratio in the Si-C composite particles 10 can be obtained by high-frequency inductively coupled plasma (ICP) emission spectrometry. Note that the Si content ratio in the Si-C composite particles 10 is the mass ratio of Si with respect to the total mass of the porous framework 11 and the Si-containing particles 12. Therefore, the Si mass ratio is obtained excluding the mass of the coating 13.

[0043] The coating 13 containing LiF is formed on at least a part of the outer surface of the Si-C composite particles 10. Therefore, the coating 13 containing LiF is formed on at least the outer surface of the porous framework 11.

[0044] It is known that at the time of initial charging of a secondary battery, an SEI film is formed on the surface of the negative electrode active material by decomposition of the nonaqueous electrolyte. According to the research of the present inventors, it was found that the reason for capacity deterioration at the time of repeated charging and discharging of a secondary battery in the prior art is as described below.

[0045] The expansion ratio of the Si-C composite particle (particularly, the Si-containing particle) at the time of charging of the secondary battery is very large. Therefore, at the time of charging of the secondary battery, sometimes the SEI film formed on the surface of the negative electrode active material (i.e., the Si-C composite particle) is destroyed due to the expansion. If the SEI film is destroyed, the surface of the negative electrode active material is exposed, and at the exposed portion, decomposition of the nonaqueous electrolyte occurs, and the SEI film is generated again. At the time of repeated charging and discharging of the secondary battery, the destruction and the re-formation of the SEI film repeatedly occur, and the battery capacity is irreversibly reduced.

[0046] In the present embodiment, however, the Si-C composite particle 10 has the coating 13 containing LiF on the outer surface thereof. The coating 13 containing LiF has the same function as the SEI film, and on the other hand, is able to follow the expansion of the Si-C composite particle 10. As a result, the destruction of the coating 13 possessed by the Si-C composite particle 10 is suppressed, and the re-formation of the SEI film is difficult to occur. As a result, the above-mentioned irreversible reduction in the battery capacity can be suppressed.

[0047] Therefore, the Si-C composite particle 10 having the coating 13 containing LiF has a large coating ratio, and the suppression effect of the capacity deterioration at the time of repeated charging and discharging is increased. Therefore, the coating ratio is preferably 20% or more, more preferably 50% or more, further preferably 80% or more, and particularly preferably 100%. Therefore, as shown in FIG. 1, it is particularly preferable that the coating 13 containing LiF form a layer covering the entire outer surface of the Si-C composite particle 10. Note that the coating ratio can be obtained as described below. An electron microscope image of the cross section of the Si-C composite particle 10 is obtained. In the image, the proportion (percentage) of the total length of the coating with respect to the circumference of the porous skeleton 11 is calculated. For five or more Si-C composite particles 10 selected at random, the proportion is calculated, and the average thereof is set as the coating ratio. Figure 2

[0048] In the coating 13 containing LiF, it is preferable that the proportion of LiF contained be large. In terms of the proportion of LiF contained in the coating 13 (in other words, the concentration of 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 F corresponding to LiF to the peak intensity of F other than LiF. The ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF is usually 0.200 or more, preferably 0.250 or more, more preferably 0.300 or more, and further preferably 0.350 or more. Note that in the XPS spectrum, the peak of F of LiF appears at 683 eV to 686 eV. The peak of F other than LiF is a peak of F outside the range of 683 eV to 686 eV, and particularly can appear at 687 eV to 690 eV.

[0049] In the coating 13 containing LiF, it is preferable that the proportion of LiF contained be large. In terms of the proportion of LiF contained in the coating 13 (in other words, the concentration of 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 F corresponding to LiF to the peak intensity of F other than LiF. The ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF is usually 0.200 or more, preferably 0.250 or more, more preferably 0.300 or more, and further preferably 0.350 or more. Note that in the XPS spectrum, the peak of F of LiF appears at 683 eV to 686 eV. The peak of F other than LiF is a peak of F outside the range of 683 eV to 686 eV, and particularly can appear at 687 eV to 690 eV. Figure 2 ​In the illustrated example, the Si-C composite particles 10 are spherical, but are not limited thereto. The Si-C composite particles 10 can be amorphous or the like.

[0050] The average particle diameter (D50) of the Si-C composite particles 10 is not particularly limited. The average particle diameter (D50) of the Si-C composite particles 10 is, for example, 1 μm to 15 μm, preferably 2 μm to 10 μm, and more preferably 4 μm to 10 μm.

[0051] The content ratio of the Si-C composite particles 10 with respect to the total of the graphite particles and the Si-C composite particles 10 is preferably 10 mass% to 60 mass%, more preferably 15 mass% to 55 mass%, and further preferably 20 mass% to 50 mass%.

[0052] The negative electrode active material can be composed only of the graphite particles and the Si-C composite particles 10. Alternatively, in the negative electrode active material, a negative electrode active material other than the graphite particles and the Si-C composite particles 10 can be further contained, within a range that does not significantly impair the effects of the present disclosure. For example, a particle of a Si-C composite material that does not have a coating can be further contained.

[0053] The negative electrode active material layer 64 can contain components other than the negative electrode active material, and as examples thereof, a binder, a conductive material, and the like can be listed. As the binder, for example, styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride (PVdF), and the like can be used. The CMC also functions as a tackifier. As examples of the conductive material, carbon black such as acetylene black, carbon fibers, carbon nanotubes (CNTs), and the like can be listed. Among these, the CNTs are preferred. As the conductive material, in the case where the CNTs are used, the negative electrode active material layer 64 can contain a dispersant of the CNTs.

[0054] The content of the negative electrode active material in the negative electrode active material layer 64 (i.e., with respect to the total mass of the negative electrode active material layer 64) is preferably 90 mass% or more, and more preferably 95 mass% or more. The content of the binder in the negative electrode active material layer is preferably 0.1 mass% or more and 8 mass% or less, and more preferably 0.5 mass% or more and 5 mass% or less. The content of the conductive material in the negative electrode active material layer 64 is preferably 0.01 mass% or more and 3 mass% or less, and more preferably 0.05 mass% or more and 1 mass% or less.

[0055] The thickness of the negative electrode active material layer 64 is not particularly limited, and is, for example, 10 μm or more and 400 μm or less, and is preferably 20 μm or more and 300 μm or less.

[0056] The density of the negative electrode active material layer 64 is not particularly limited, and is, for example, 0.7 g / cm 3 The above is preferably 1.0 g / cm3 More preferably, the density is 1.2 g / cm3or more 3 More preferably, the density is 1.2 g / cm3or more 3 More preferably, the density is 1.2 g / cm3or more 3 More preferably, the density is 1.2 g / cm3or more

[0057] The negative electrode 60 can include members other than the negative electrode current collector 62 and the negative electrode active material layer 64. For example, an insulating layer (not shown) can be provided on the negative electrode active material layer non-forming portion 62a so as to be adjacent to the negative electrode active material layer 64. The insulating layer contains, for example, an inorganic filler having insulating properties and the like.

[0058] Next, a preferred production method of the negative electrode 60 will be described. The preferred production method of the negative electrode 60 includes a step of preparing the Si-C composite particle 10 having the carbon-made porous skeleton 11, the Si-containing particle 12 positioned inside the void 11a of the porous skeleton 11, and the coating 13 containing LiF formed on at least a part of the outer surface (hereinafter also referred to as "Si-C composite particle preparation step"); a step of mixing the Si-C composite particle 10 and the graphite particle in a dispersion medium, preparing a negative electrode paste (hereinafter also referred to as "paste preparation step"); a step of applying the negative electrode paste on the negative electrode current collector (hereinafter also referred to as "application step"); and a step of drying the applied negative electrode paste (hereinafter also referred to as "drying step"). The Si-C composite particle preparation step includes reacting a water-soluble lithium salt and a fluorinating agent in a dispersion liquid in which the particle having the carbon-made porous skeleton 11 and the Si-containing particle 12 positioned inside the void 11a of the porous skeleton 11 are dispersed in water or a water-soluble organic solvent, to generate LiF. Each step of the production method will be described in detail below.

[0059] Note that, in the present specification, the "paste" refers to a mixture in which a part or all of solid components are dispersed in a dispersion medium, including so-called "slurry", "ink", and the like.

[0060] In the Si-C composite particle preparation step, the particle having the carbon-made porous skeleton 11 and the Si-containing particle 12 positioned inside the void 11a of the porous skeleton 11 (hereinafter also referred to as "uncoated particle") is prepared. Such an uncoated particle is known (for example, refer to Japanese Patent Application Publication No. 2015-38862, International Publication No. 2014 / 046144, and the like). Therefore, the uncoated particle can be produced and prepared according to the known method.

[0061] Next, the water-soluble lithium salt and the fluorinating agent are reacted in a dispersion liquid in which the uncoated particle is dispersed in water or a water-soluble organic solvent, to generate LiF.

[0062] As the water-soluble organic solvent, for example, an alcohol such as ethanol, and the like can be used. As the water-soluble lithium salt, for example, lithium acetate (dihydrate), lithium carbonate, lithium nitrate, lithium chloride, lithium hydroxide, and the like can be used, and lithium acetate (dihydrate) is preferable. As the fluorinating agent, hydrofluoric acid, ammonium fluoride, acidic ammonium fluoride, and the like can be used, and ammonium fluoride is preferable.

[0063] In this operation, for example, first, an aqueous solution of a water-soluble lithium salt, a dispersion liquid in which the non-coated particles are dispersed 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 liquid are mixed to make a mixed liquid. The aqueous solution of the fluorinating agent is added to the mixed liquid with stirring. Thereby, the water-soluble lithium salt and the fluorinating agent are reacted to generate LiF. Thereby, a reaction liquid containing LiF is obtained. The non-coated particles are present in the reaction liquid.

[0064] The reaction conditions of the water-soluble lithium salt and the fluorinating agent can be the same as the known synthesis conditions of LiF using a water-soluble lithium salt and a fluorinating agent. For example, the reaction can be performed at room temperature (i.e., 25°C ± 10°C), or can be performed with heating. In addition, the reaction time can be appropriately determined depending on the concentration of the water-soluble lithium salt and the fluorinating agent in the reaction liquid, the desired content ratio of LiF in the coating 13, and the like. The longer the reaction time, the higher the content ratio of LiF in the coating 13.

[0065] The non-coated particles are recovered from the reaction liquid in a state in which the reaction liquid in which LiF is generated is attached to the surface, and by drying the non-coated particles to which the reaction liquid in which LiF is generated is attached, it is possible to cause LiF to be attached to the surface of the non-coated particles, and it is possible to obtain the Si-C composite particles 10 having the coating 13 containing LiF. In order to adjust the amount of coating, the reaction liquid in which LiF is generated can be diluted with water or the like.

[0066] The paste preparation step can be performed according to a known method by mixing the graphite particles, the Si-C composite particles 10, and optional components (e.g., a binder, and the like) with a dispersion medium (e.g., water) using a known mixing device, a stirring device, or the like.

[0067] The coating step can be performed according to a known method. Specifically, for example, the coating step can be performed by coating the obtained negative electrode paste on the negative electrode current collector 62 using a gravure coater, a comma coater, a slot coater, a die coater, or the like.

[0068] The drying process can be performed according to a known method. Specifically, for example, the above dispersion medium is removed by drying the negative electrode current collector 62 coated with the negative electrode paste using a drying furnace or the like drying device, thereby forming the negative electrode active material layer 64. Thus, the drying process can be performed. The drying temperature and the 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 or higher and 200°C or lower, and is preferably 70°C or higher and 150°C or lower. The drying time is, for example, 10 seconds or longer and 30 minutes or less, and is preferably 30 seconds or longer and 10 minutes or less.

[0069] After the drying process, a process of pressing the negative electrode active material layer 64 can be further performed. The pressing process can be performed according to a known method. Specifically, for the negative electrode active material layer 64 formed as described above, a pressure is applied using a roll press or the like, thereby the pressing process can be performed. With the pressing process, the graphite particles and the Si-C composite particles 10 contained in the negative electrode active material layer 64 can be densely packed. As described above, the negative electrode 60 can be obtained.

[0070] In the first embodiment, as the Si-C composite particles, one kind of Si-C composite particles was used. However, two or more kinds of Si-C composite particles can be used. Thus, as another example of the negative electrode of the present disclosure, the negative electrode of the second embodiment is described.

[0071] [Second Embodiment]

[0072] Figure 3 A schematic cross-sectional view of the particles contained in the negative electrode active material layer 64 of the negative electrode of the second embodiment is shown. The negative electrode active material layer 64 contains the graphite particles 14, the first Si-C composite particles 110, and the second Si-C composite particles 120. In the second embodiment, as the Si-C composite particles 10, the first Si-C composite particles 110 and the second Si-C composite particles 120 are used, which is different from the first embodiment in this point. For the aspects common to the first embodiment, the description is omitted. Further, Figure 3 The packing state of the particles is not limited to the illustrated example for the schematic view.

[0073] In the second embodiment, the Si content ratio (S1) in the first Si-C composite particles 110 is lower than the Si content ratio (S2) in the second Si-C composite particles 120. As for the Si content ratio (S1) in the first Si-C composite particles 110 and the Si content ratio (S2) in the second Si-C composite particles 120, there is no particular limitation as long as the relationship is satisfied. If these Si content ratios are too low, the cycle characteristic improvement effect can possibly become small, and the high capacity effect of the secondary battery can possibly decrease. On the other hand, if these Si content ratios are too high, the volume change due to the expansion / contraction of the first Si-C composite particles 110 and the second Si-C composite particles 120 can possibly become large at the time of repeating charge and discharge of the secondary battery.

[0074] Therefore, the Si content ratio (S1) in the first Si-C composite particles 110 is preferably 20% by mass to 55% by mass, more preferably 25% by mass to 45% by mass. The Si content ratio (S2) in the second Si-C composite particles 120 is preferably 45% by mass to 80% by mass, more preferably 55% by mass to 75% by mass. Further, the Si content ratio (S1) and the Si content ratio (S2) can be found by high-frequency inductively coupled plasma (ICP) emission spectrometry.

[0075] In addition, the ratio (S1 / S2) of the Si content ratio (S1) in the first Si-C composite particles 110 to the Si content ratio (S2) in the second Si-C composite particles 120 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and further preferably 0.40 to 0.75.

[0076] As for the mass ratio of the first Si-C composite particles 110 to the second Si-C composite particles 120, there is no particular limitation as long as the effect of the present disclosure can be obtained. In order to obtain a more favorable filling state of the first Si-C composite particles 110 and 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 further preferably 55:45 to 80:20.

[0077] The first Si-C composite particles 110 and the second Si-C composite particles 120 each have a coating containing LiF on at least a part of the outer surface. The ratio (P1) 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 particles 110 measured by X-ray photoelectron spectroscopy (XPS) is smaller than the ratio (P2) 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 particles 120.

[0078] The peak intensity ratio (P1) of the first Si-C composite particles 110 and the peak intensity ratio (P2) of the second Si-C composite particles 120 are not particularly limited as long as the relationship is satisfied. The peak intensity ratio (P1) of the first Si-C composite particles 110 is preferably 0.250 or greater and less than 0.500, more preferably 0.300 or greater and 0.450 or less. The peak intensity ratio (P2) of the second Si-C composite particles 120 is preferably 0.500 or greater and 0.800 or less, more preferably 0.550 or greater and 0.700 or less.

[0079] The ratio (P1 / P2) of the peak intensity ratio (P1) of the first Si-C composite particles 110 to the peak intensity ratio (P2) of the second Si-C composite particles 120 is preferably 0.10 to 0.90, more preferably 0.20 to 0.80, and further preferably 0.40 to 0.75.

[0080] In the second embodiment, as the Si-C composite particles 10, the first Si-C composite particles 110 having a low Si content and a low LiF concentration in the coating film and the second Si-C composite particles 120 having a high Si content and a high LiF concentration in the coating film are used in combination. Here, as the Si content ratio of the Si-C composite particles increases, the expansion amount during charging of the secondary battery increases. On the other hand, the Si-C composite particles having a high Si content ratio can greatly contribute to an increase in the capacity of the battery. By forming a LiF coating film of a sufficient concentration on the second Si-C composite particles 120 having a high Si content ratio, the destruction of the coating film of the second Si-C composite particles 120 can be appropriately suppressed. In addition, in the first Si-C composite particles 110 having a low Si content ratio, even if the LiF concentration of the coating film is low, the destruction of the coating film can be sufficiently suppressed. By mixing these two types of particles, the capacity deterioration during repeated charging and discharging of the secondary battery can be highly suppressed. Therefore, the negative electrode of the second embodiment can further suppress the capacity deterioration during repeated charging and discharging of the secondary battery compared to the negative electrode of the first embodiment.

[0081] The average particle diameter (D50) of the first Si-C composite particles 110 and the second Si-C composite particles 120 can be the same or different.

[0082] In the second embodiment, as the negative electrode active material, a negative electrode active material other than the graphite particles, the first Si-C composite particles 110, and the second Si-C composite particles 120 can be further contained within a range that does not significantly impair the effects of the present disclosure.

[0083] The negative electrode of the present disclosure described above can impart excellent capacity deterioration resistance when a secondary battery is repeatedly charged and discharged. In addition, in the negative electrode of the present disclosure, since a Si-containing negative electrode active material is used, the secondary battery can be made high in capacity. Therefore, the secondary battery using the negative electrode of the present disclosure is high in capacity and excellent in cycle characteristics.

[0084] Therefore, from another aspect, the secondary battery of the present disclosure includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode of the present disclosure described above. Hereinafter, a lithium ion secondary battery is exemplified, and reference is made to Figure 4 and Figure 5 An embodiment of the secondary battery of the present disclosure is described. The following configuration example is a flat square type lithium ion secondary battery having a flat square shape wound electrode body and a flat square shape battery case.

[0085] Figure 4 The lithium ion secondary battery 100 shown is a sealed type lithium ion secondary battery 100 configured by housing a flat square shape wound electrode body 20 and a nonaqueous electrolyte (not shown) in a flat square shape battery case (i.e., an outer packaging 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 provided to release the internal pressure of the battery case 30 in the case where the internal pressure of the battery case 30 rises to a prescribed level or more. In addition, the battery case 30 is provided with an injection port (not shown) for injecting the nonaqueous electrolyte. The positive electrode terminal 42 is electrically connected to the positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to the negative electrode current collector plate 44a. As the material of the battery case 30, a lightweight, thermally conductive metal material such as aluminum is used, for example.

[0086] The wound electrode body 20 is as shown in Figure 4 and Figure 5As shown, the positive electrode sheet 50 and the negative electrode sheet 60 are overlapped via two long strip-shaped separator sheets 70 and wound in the length direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed on one face or both faces (in this case, both faces) of a long strip-shaped positive electrode current collector 52 along the length direction. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed on one face or both faces (in this case, both faces) of a long strip-shaped negative electrode current collector 62 along the length direction. The positive electrode active material layer non-formed portion 52a (i.e., a portion in which the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed) and the negative electrode active material layer non-formed portion 62a (i.e., a portion in which the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed) are formed in a manner that they extend outward from both ends in the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the above-described length direction). The positive electrode current collector plate 42a and the negative electrode current collector plate 44a are respectively joined to the positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a.

[0087] As the positive electrode current collector 52 that constitutes the positive electrode sheet 50, a known positive electrode current collector for a lithium ion secondary battery can be used, and as examples thereof, a sheet or foil made of a metal (e.g., aluminum, nickel, titanium, stainless steel, etc.) having good electrical conductivity can be listed. As the positive electrode current collector 52, an aluminum foil is preferable.

[0088] The size of the positive electrode current collector 52 is not particularly limited and can be appropriately determined in accordance with the battery design. In the case where an aluminum foil is used as the positive electrode current collector 52, the thickness thereof is not particularly limited and is, for example, 5 μm or more and 35 μm or less, and is preferably 7 μm or more and 20 μm or less.

[0089] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known positive electrode active material for a lithium ion secondary battery can be used. Specifically, for example, as the positive electrode active material, a lithium complex oxide, a lithium transition metal phosphate compound, etc. can be used. The crystal structure of the positive electrode active material is not particularly limited and can be a layered structure, a spinel structure, an olivine structure, etc.

[0090] As the lithium complex oxide, a lithium transition metal complex oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferable, and as specific examples thereof, a lithium nickel-based complex oxide, a lithium cobalt-based complex oxide, a lithium manganese-based complex oxide, a lithium nickel-manganese-based complex oxide, a lithium nickel-cobalt-manganese-based complex oxide, a lithium nickel-cobalt-aluminum-based complex oxide, a lithium iron-nickel-manganese-based complex oxide, etc. can be listed.

[0091] Note that in the present specification, "lithium-nickel-cobalt-manganese composite oxide" is a term for an oxide containing one or two or more added elements other than the oxide of Li, Ni, Co, Mn, and O. Examples of the added elements include transition metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, Sn, and typical metal elements. In addition, the added elements can be semimetal elements such as B, C, Si, and P, and nonmetal elements such as S, F, Cl, Br, and I. The same applies to the above-described lithium-nickel composite oxide, lithium-cobalt composite oxide, lithium-manganese composite oxide, lithium-nickel-manganese composite oxide, lithium-nickel-cobalt-aluminum composite oxide, and lithium-iron-nickel-manganese composite oxide.

[0092] Examples of the lithium transition metal phosphate compound include lithium iron phosphate (LiFeP04), lithium manganese phosphate (LiMnP04), and lithium manganese iron phosphate.

[0093] These positive electrode active materials can be used alone or in combination of two or more. As the positive electrode active material, a lithium-nickel-cobalt-manganese composite oxide is particularly preferable in view of excellent characteristics such as initial resistance characteristics.

[0094] The average particle diameter (D50) of the positive electrode active material is not particularly limited, and 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.

[0095] The positive electrode active material layer 54 can contain components other than the positive electrode active material, such as trilithium phosphate, a conductive material, and a binder. As the conductive material, for example, carbon black such as acetylene black (AB), carbon fibers such as vapor-grown carbon fiber (VGCF) and carbon nanotube (CNT), and other carbon materials such as graphite can be preferably used. As the binder, for example, polyvinylidene fluoride (PVdF) can be used.

[0096] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material with respect to the total mass of the positive electrode active material layer 54) is not particularly limited, and is preferably 70% by mass or more, more preferably 80% by mass or more, and further preferably 85% by mass or more and 99% by mass or less. The content of lithium phosphate in the positive electrode active material layer 54 is not particularly limited, and 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 the conductive material in the positive electrode active material layer 54 is not particularly limited, and 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 the binder in the positive electrode active material layer 54 is not particularly limited, and 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.

[0097] The thickness of each surface of the positive electrode active material layer 54 is not particularly limited, and is typically 10 μm or more, and preferably 20 μm or more. On the other hand, the thickness is typically 400 μm or less, and preferably 300 μm or less.

[0098] As the negative electrode sheet 60, the negative electrode of the present disclosure described above (e.g., the negative electrode of the first embodiment or the negative electrode of the second embodiment described above) is used.

[0099] As the separator 70, for example, a porous sheet (film) composed of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, or the like can be exemplified. The porous sheet can be a single layer structure, or can be a laminated structure of two or more layers (e.g., a three-layer structure in which a PP layer is laminated on both surfaces of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0100] The thickness of the separator 70 is not particularly limited, and is, for example, 5 μm or more and 50 μm or less, and is preferably 10 μm or more and 30 μm or less. The air permeability of the separator 70 obtained by the Gurley test method is not particularly limited, and is preferably 350 seconds / 100 cc or less.

[0101] The nonaqueous electrolyte solution typically contains a nonaqueous solvent and a supporting salt (electrolyte salt). As the nonaqueous solvent, an organic solvent such as a carbonate, an ether, an ester, a nitrile, a sulfone, a lactone, or the like, which is used in the electrolyte solution of a general lithium-ion secondary battery, can be used without particular limitation. Among them, a carbonate is preferred, and as specific examples thereof, 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), trifluorodimethyl carbonate (TFDMC), and the like can be exemplified. Such a nonaqueous solvent can be used alone as one kind, or two or more kinds can be appropriately combined and used. As an example, the nonaqueous solvent contains only a carbonate. As another example, the nonaqueous solvent contains a carbonate and an ester such as methyl acetate.

[0102] As the supporting salt, for example, a lithium salt such as LiPF6, LiBF4, lithium bis(fluorosulfonyl)imide (LiFSI), or the like (preferably LiPF6) can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0103] Further, in the above nonaqueous electrolyte solution, a component other than the above-described components, such as vinylene carbonate (VC), an oxalate complex, or the like, a film forming agent; biphenyl (BP), cyclohexylbenzene (CHB), or the like, a gas generating agent; a tackifier, or the like, various additives can be contained, as long as the effects of the present disclosure are not significantly impaired.

[0104] The lithium-ion secondary battery 100 suppresses capacity deterioration at the time of repeated charge and discharge, and is high in capacity. The lithium-ion secondary battery 100 can be used for various uses. As a preferred use, a power source mounted in a vehicle such as an electric vehicle (BEV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), or the like, can be exemplified. In addition, the lithium-ion secondary battery 100 can be used as a storage battery for a small-sized power storage device or the like. The lithium-ion secondary battery 100 can also be typically used in the form of a battery pack in which a plurality of batteries are connected in series and / or in parallel.

[0105] In the above, as an example, a square lithium-ion secondary battery 100 including a flat-shaped wound electrode body 20 has been described. However, the lithium-ion secondary battery can also be configured as a lithium-ion secondary battery including a stacked electrode body (i.e., an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately stacked). In addition, the lithium-ion secondary battery can also be configured as a cylindrical lithium-ion secondary battery, a stacked case type lithium-ion secondary battery, or the like.

[0106] In addition, the lithium-ion secondary battery 100 can also be configured as an all-solid lithium-ion secondary battery in which a solid electrolyte is used instead of a nonaqueous electrolyte, according to a known method.

[0107] In addition, the negative electrode of the present disclosure is suitable for a negative electrode of a lithium ion secondary battery, but can be constructed and used as a negative electrode of other secondary batteries, which can be constructed in accordance with a publicly known method.

[0108] The following describes embodiments related to the present disclosure in detail, but is not intended to limit the present disclosure to what is shown in the embodiments.

[0109] <Manufacture of Si-C composite particles having a coating containing LiF>

[0110] A non-coated particle having a porous skeleton of carbon and Si-containing particles inside the pores thereof was prepared. The powder of the non-coated particle was dispersed in ethanol and ultrasonically treated for 30 minutes. Lithium acetate was dissolved in deionized water to prepare a lithium acetate aqueous solution. The dispersion liquid of the non-coated particle was added to the aqueous solution while stirring for 30 minutes. An aqueous ammonium fluoride solution was added thereto while stirring for a prescribed time to react lithium acetate and ammonium fluoride. Thus, a reaction liquid containing LiF was obtained. The reaction liquid was diluted and filtered. The filtrate was dried to obtain Si-C composite particles having a coating containing LiF.

[0111] For the obtained Si-C composite particles, the XPS spectrum was measured, and as a result, a peak of F of Li-F (about 684.8 eV) and a peak of Li of Li-F (about 55.1 eV) were found, confirming 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 calculated. Further, 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.

[0112] By using the above-described procedure, Si-C composite particles (A) having a Si content ratio of 40 mass% and a LiF intensity ratio of 0.372, Si-C composite particles (B) having a Si content ratio of 65 mass% and a LiF intensity ratio of 0.619, Si-C composite particles (C) having a Si content ratio of 40 mass% and a LiF intensity ratio of 0.621, and Si-C composite particles (D) having a Si content ratio of 65 mass% and a LiF intensity ratio of 0.373 were obtained. Further, in the manufacture of the Si-C composite particles (A) and (D), the addition time (i.e., the reaction time) of the aqueous ammonium fluoride solution was 60 minutes, and in the manufacture of the Si-C composite particles (B) and (C), the addition time of the aqueous ammonium fluoride solution was 120 minutes. Further, the Si content ratio in the Si-C composite particles was measured using a commercially available ICP analysis device.

[0113] <Manufacture of a negative electrode>

[0114] [Example 1]

[0115] As the negative active material, graphite particles having an average particle diameter (D50) of 15 μm and the above Si-C composite particles (A) were prepared.

[0116] A dispersion liquid of single-walled carbon nanotubes (SWCNT) as a conductive material was prepared. As a binder, carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were prepared.

[0117] A negative electrode paste containing graphite particles, Si-C composite particles, SWCNT, CMC, PAA, and SBR in a mass ratio of 65:35:0.1:1:1:1.5 was prepared using the following procedure.

[0118] A paste A was prepared by mixing a dispersion liquid of Si-C composite particles, SWCNT, and a dispersion medium at a rotation speed of 3000 rpm using a disperser. Graphite particles, CMC, and PAA were dry-mixed using a stirring granulator. The above paste A and a dispersion medium were added to the resulting mixed powder so that the solid content concentration became 65 mass%, and kneading was performed. SBR and a dispersion medium were added thereto, and mixed uniformly to produce a negative electrode paste.

[0119] A negative active material layer was formed by applying the produced negative electrode paste to the surface of a copper foil having a thickness of 10 μm, and drying. The negative active material layer was roll-pressed, and the resulting sheet was processed to a prescribed size to obtain a negative electrode sheet.

[0120] [Comparative Example 1]

[0121] As the Si-C composite particles, Si-C composite particles having no coating and having a Si content ratio of 65 mass% were used, and otherwise, a negative electrode sheet was obtained in the same manner as in Example 1.

[0122] [Comparative Example 2]

[0123] As the Si-C composite particles, Si-C composite particles having no coating and having a Si content ratio of 40 mass% were used, and otherwise, a negative electrode sheet was obtained in the same manner as in Example 1.

[0124] [Example 2]

[0125] As the Si-C composite particles, a first Si-C composite particle and a second Si-C composite particle were used in a mass ratio of 28:7, and otherwise, a negative electrode sheet was obtained in the same manner as in Example 1.

[0126] First Si-C composite particle: the above Si-C composite particle (A)

[0127] Second Si-C composite particle: the above Si-C composite particle (B)

[0128] [Example 3]

[0129] As the Si-C composite particles, the following first Si-C composite particles and second Si-C composite particles were used at a mass ratio of 28:7, and otherwise, the negative electrode sheet was obtained in the same manner as in Example 1.

[0130] First Si-C composite particles: no coating containing LiF, Si content = 40 mass%

[0131] Second Si-C composite particles: the above Si-C composite particles (B)

[0132] [Example 4]

[0133] As the Si-C composite particles, the following first Si-C composite particles and second Si-C composite particles were used at a mass ratio of 28:7, and otherwise, the negative electrode sheet was obtained in the same manner as in Example 1.

[0134] First Si-C composite particles: the above Si-C composite particles (C)

[0135] Second Si-C composite particles: the above Si-C composite particles (D)

[0136] <Production of lithium ion secondary battery for evaluation>

[0137] LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 02(NCM) as the positive electrode active material powder, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were mixed at a mass ratio of NCM:AB:PVdF = 100:1:1 with N-methylpyrrolidone (NMP) to prepare a positive electrode paste. The paste was applied to the surface of an aluminum foil having a thickness of 15 μm, and dried to form a positive electrode active material layer. After the positive electrode active material layer was roll-pressed, the resulting sheet was processed to a prescribed size to obtain a positive electrode sheet.

[0138] A separator made of porous polyolefin was prepared. The above- produced negative electrode sheet and positive electrode sheet were each provided with a lead wire, and were laminated via the separator to produce an electrode body. This was housed in an aluminum laminated film-made case together with a nonaqueous electrolyte solution. As the nonaqueous electrolyte solution, an electrolyte solution obtained by dissolving LiPF6 as a supporting salt at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC), monofluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 15:5:40:40 was used. Then, the case was sealed to obtain a lithium ion secondary battery for evaluation.

[0139] <Cycle characteristic evaluation>

[0140] Each of the evaluation lithium-ion secondary batteries thus produced was left in an environment of 25°C. After each of the evaluation lithium-ion secondary batteries was subjected to constant-current charging at a current value of 0.4 C until 4.2 V, it was subjected to constant-voltage charging until the current value became 0.1 C. Subsequently, each of the evaluation lithium-ion secondary batteries was subjected to constant-current discharging at a current value of 0.4 C until 2.5 V. Then, the discharging capacity at that time was measured, and the initial capacity was calculated.

[0141] The charging and discharging described above were repeated as one cycle, and the charging and discharging were repeated for 200 cycles. The discharging capacity after 200 cycles was calculated in the same manner as the initial capacity. As an index of the cycle characteristics, the capacity retention rate (%) was calculated from (discharging capacity after 200 cycles / initial capacity) x 100. The results are shown in Table 1.

[0142] [Table 1]

[0143] Table 1

[0144]

[0145] As in Comparative Examples 1 and 2, in the case where the Si-C composite particles having no coating containing LiF were used and the lithium-ion secondary batteries were charged, a normal SEI film (i.e., a film from the decomposition product of the nonaqueous electrolyte) was formed on the surface of the Si-C composite particles. From the comparison between Example 1 and Comparative Examples 1 and 2, it was found that by using the Si-C composite particles provided with the coating containing LiF in advance, the capacity deterioration during repeated charging and discharging could be greatly suppressed. In addition, in Examples 2 to 4, two kinds of Si-C composite particles were used. From the comparison between them, it was found that by using the Si-C composite particles having a low Si content and a low LiF concentration in the film in combination with the Si-C composite particles having a high Si content and a high LiF concentration in the film, the capacity deterioration could be further suppressed.

[0146] Therefore, from the above, it was found that according to the negative electrode of the present disclosure, the capacity deterioration during repeated charging and discharging of the secondary battery can be suppressed.

[0147] The specific examples of the present disclosure are described in detail above, but these are merely examples and do not limit the claims. The technology described in the claims includes technology in which the above-described specific examples are variously modified and changed.

[0148] That is, the negative electrode of the secondary battery, the method for manufacturing the same, and the secondary battery of the present disclosure are the following items [1] to [9].

[0149] [1] A negative electrode, which is a negative electrode of a secondary battery including 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 containing graphite particles and Si-C composite particles as negative electrode active materials, the Si-C composite particles having a porous skeleton made of carbon and Si-containing particles located inside pores of the porous skeleton, a coating layer containing LiF being formed on at least a part of an outer surface of the Si-C composite particles.

[0150] [2] The negative electrode according to item [1], wherein a contained proportion of the Si-C composite particles with respect to a total of the graphite particles and the Si-C composite particles is 10 to 60 mass%.

[0151] [3] The negative electrode according to item [1] or [2], wherein the Si-C composite particles contain first Si-C composite particles and second Si-C composite particles, both of which have a coating layer containing LiF formed on at least a part of an outer surface, a Si contained proportion in the first Si-C composite particles is lower than a Si contained proportion in the second Si-C composite particles, and a ratio of a peak intensity of F of LiF to a peak intensity of F other than LiF in an XPS spectrum of the first Si-C composite particles measured by X-ray photoelectron spectroscopy is smaller than a ratio of a peak intensity of F of LiF to a peak intensity of F other than LiF in an XPS spectrum of the second Si-C composite particles.

[0152] [4] The negative electrode according to item [3], wherein the Si contained proportion in the first Si-C composite particles with respect to the Si contained proportion in the second Si-C composite particles is 0.10 to 0.90, and a ratio of the peak intensity in the XPS spectrum of the first Si-C composite particles with respect to the peak intensity in the XPS spectrum of the second Si-C composite particles is 0.10 to 0.90.

[0153] [5] The negative electrode according to item [3] or [4], wherein the Si contained proportion in the first Si-C composite particles is 20 to 55 mass%, and the Si contained proportion in the second Si-C composite particles is 45 to 80 mass%.

[0154] [6] The negative electrode according to any one of items [3] to [5], wherein the ratio of the peak intensity in the XPS spectrum of the first Si-C composite particles is 0.250 or more and less than 0.500, and the ratio of the peak intensity in the XPS spectrum of the second Si-C composite particles is 0.500 or more and 0.800 or less.

[0155] [7] The negative electrode according to any one of items [3] to [6], wherein a mass ratio of the first Si-C composite particle to the second Si-C composite particle is 40:60 to 90:10.

[0156] [8] A method for manufacturing a negative electrode of a secondary battery, comprising: a step of preparing Si-C composite particles having a porous skeleton made of carbon, Si-containing particles located inside pores of the porous skeleton, and a coating layer containing LiF formed on at least a part of an outer surface; a step of mixing the Si-C composite particles and graphite particles in a dispersion medium to prepare a negative electrode paste; a step of applying the negative electrode paste to a negative electrode current collector; and a step of drying the applied negative electrode paste, wherein the step of preparing the Si-C composite particles includes a step of reacting a water-soluble lithium salt and a fluorinating agent in a dispersion liquid in which particles having the porous skeleton made of carbon and the Si-containing particles located inside pores of the porous skeleton are dispersed in water or a water-soluble organic solvent to generate LiF.

[0157] [9] A secondary battery, which is a secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode according to any one of items [1] to [7].

Claims

1. A negative electrode, which is the 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. in, 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. At least a portion of the outer surface of the Si-C composite particles is coated with LiF.

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

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

4. The negative electrode according to claim 3, wherein, The Si content in the first Si-C composite particles is 0.10 to 0.90 relative to the Si content in the second Si-C composite particles, and... 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 first Si-C composite particle contains 20% to 55% Si by mass, and the second Si-C composite particle contains 45% to 80% Si by mass.

6. The negative electrode according to claim 3, wherein, The peak intensity ratio in the XPS spectrum of the first Si-C composite particle is greater than or equal to 0.250 and less than 0.500, and the peak intensity ratio in the XPS spectrum of the second Si-C composite particle is greater than or equal to 0.500 and less than 0.

800.

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 method for manufacturing the negative electrode of a secondary battery, including: A process for preparing Si-C composite particles having a porous carbon framework, Si-containing particles located in the pores of the porous framework, and a LiF-coated surface formed on at least a portion of the outer surface. The process of mixing the Si-C composite particles and graphite particles in a dispersion medium to prepare a negative electrode paste; The process of coating the negative electrode paste onto the negative electrode current collector; and The process of drying the coated negative electrode paste. The process of preparing the Si-C composite particles includes: dispersing the Si-containing particles, which have a porous framework made of carbon and are located in the pores of the porous framework, in a dispersion in water or a water-soluble organic solvent, and reacting the water-soluble lithium salt and fluorinating agent to generate LiF.

9. A secondary battery, which includes a positive electrode, a negative electrode, and an electrolyte, wherein... The negative electrode is the negative electrode according to claim 1.

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