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

Abstract

Providing a technology to suppress the expansion of a negative electrode containing Si-containing particles when repeated charging and discharging is performed. [Solution] In the negative electrode of the secondary battery of this disclosure, the negative electrode active material layer comprises a first layer on the negative electrode current collector side and a second layer on the surface side. The first layer contains first Si-containing particles. The second layer contains second Si-containing particles. The Si content in the second Si-containing particles is smaller than the Si content in the first Si-containing particles. Both the first Si-containing particles and the second Si-containing particles are coated with a LiF film. The second peak intensity ratio with respect to LiF of the second Si-containing particles is smaller than the first peak intensity ratio with respect to LiF of the first Si-containing particles. The difference between the first peak intensity ratio and the second peak intensity ratio is 0.050 to 1.00.

Description

[Technical Field]

[0001] This disclosure relates to a negative electrode for a secondary battery and a method for manufacturing the negative electrode. This disclosure also relates to a secondary battery using the 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 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-containing particles are known as high-capacity negative electrode active materials, and it is known that secondary batteries can be made higher capacity using Si-containing particles. Patent Document 1 discloses a negative electrode used in a non-aqueous electrolyte secondary battery, comprising a negative electrode, a positive electrode, and a lithium-ion conductive non-aqueous electrolyte. The negative electrode has a current collector and a negative electrode active material. The negative electrode active material has active material composite particles containing at least two types of elemental silicon, silicon compounds, and carbon with different particle size distributions. The specific surface area of ​​the active material composite particles is 5 m². 2 / g or more 50m 2 The value is less than / g. The publication states that this configuration can improve charge-discharge cycle characteristics, suppress rapid capacity degradation during charge-discharge cycles, and improve high-rate characteristics.

[0004] The composite particles disclosed in Patent Document 2 include a carbon material, silicon, and lithium fluoride (LiF), and consist of a carbon phase made of the carbon material and Si-LiF mixed particles. The Si-LiF mixed particles are dispersed in a uniform or non-uniform distribution within the carbon phase. The publication states that a negative electrode containing such composite particles can improve the lifespan characteristics of a battery.

[0005] Patent Document 3 discloses a multilayer electrode comprising a conductive layer, a first composite electrode layer, and a second composite electrode layer in that order. Each composite electrode layer contains a particulate material suitable for use as an active material in a metal-ion battery, and a binder. The main component of the active material in the first composite electrode layer is a different material from the main component of the active material in the second composite electrode layer. The publication states that this configuration can improve both the capacity retention rate and cycle life of the battery. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2010-33830 [Patent Document 2] Special Publication No. 2019-522886 [Patent Document 3] Special Publication No. 2015-537347 [Overview of the project] [Problems that the invention aims to solve]

[0007] Incidentally, Si-containing particles have the property of expanding and contracting significantly during charging and discharging. In secondary batteries using Si-containing particles, there is a problem that when charging and discharging are repeated, the negative electrode expands and contracts, causing the internal stress to increase. Therefore, there is a need to develop a negative electrode that expands less when repeatedly charging and discharging.

[0008] In view of the above circumstances, this disclosure aims to suppress the swelling of a negative electrode containing Si particles when it undergoes repeated charging and discharging. [Means for solving the problem]

[0009] The negative electrode of the secondary battery of this disclosure comprises a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is supported by the negative electrode current collector. The negative electrode active material layer comprises a first layer located on the negative electrode current collector side and a second layer located on the surface side. The first layer contains first Si-containing particles as negative electrode active material. The second layer 64b contains second Si-containing particles as negative electrode active material. The Si content in the second Si-containing particles is smaller than the Si content in the first Si-containing particles. Both the first Si-containing particles and the second Si-containing particles are coated with LiF. 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 first Si-containing particles, measured by XPS, is defined as the first peak intensity ratio. 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 second Si-containing particles is defined as the second peak intensity ratio. The second peak intensity ratio is smaller than the first peak intensity ratio. The difference between the first and second peak intensity ratios is between 0.050 and 1.00. With this configuration, swelling of the negative electrode containing Si particles can be suppressed when repeated charging and discharging is performed.

[0010] From another perspective, the method for manufacturing the negative electrode of the secondary battery of this disclosure includes: preparing a first paste comprising first Si-containing particles as a negative electrode active material and a dispersion medium; preparing a second paste comprising second Si-containing particles as a negative electrode active material and a dispersion medium; applying the first paste onto a negative electrode current collector and drying it to form a dry film of the first paste; applying the second paste onto the dry film of the first paste and drying it to form a dry film of the second paste; and pressing the dry film of the first paste and the dry film of the second paste together. The Si content in the second Si-containing particles is smaller than the Si content in the first Si-containing particles. Both the first Si-containing particles and the second Si-containing particles are provided with a LiF coating. 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 first Si-containing particles, measured by XPS, is defined as the first peak intensity ratio. In the XPS spectrum of the second Si-containing particle, the ratio of the peak intensity of F for LiF to the peak intensity of F other than LiF is defined as the second peak intensity ratio. The second peak intensity ratio is smaller than the first peak intensity ratio. The difference between the first peak intensity ratio and the second peak intensity ratio is between 0.050 and 1.00. With this configuration, it is possible to provide a negative electrode containing Si-containing particles in which swelling during repeated charging and discharging is suppressed.

[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. With this configuration, it is possible to suppress the swelling of the negative electrode when repeated charging and discharging is performed on a negative electrode containing Si particles. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic cross-sectional view showing the configuration of the negative electrode of a secondary battery according to one embodiment of the present disclosure. [Figure 2] Figure 1 is a schematic cross-sectional view showing the composition of the particles of the negative electrode active material contained in the negative electrode active material layer. [Figure 3] This is a schematic cross-sectional view showing the configuration of a lithium-ion secondary battery constructed using the negative electrode of a secondary battery according to one embodiment of the present disclosure. [Figure 4] It is a schematic exploded view showing the configuration of the wound electrode body of the lithium-ion secondary battery of FIG. 3.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings. Note that matters not mentioned in this specification but necessary for the implementation of the present disclosure can be grasped as 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. Also, in the following drawings, members and parts having the same function are denoted by the same reference numerals for explanation. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships. Note that the numerical range expressed as "A to B" in this specification includes A and B.

[0014] Note that 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.

[0015] The negative electrode disclosed herein is used in a secondary battery, and preferably used in a lithium-ion secondary battery. One embodiment of the negative electrode disclosed herein will be specifically described with reference to FIG. 1. FIG. 1 is a cross-sectional view schematically showing an example of the negative electrode 60 according to the present embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to the present embodiment shown in FIG. 1 is the negative electrode of a lithium-ion secondary battery.

[0016] As shown in the figure, the negative electrode 60 comprises 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 comprises 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 on only one side of the negative electrode current collector 62, or it 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] Incidentally, when a secondary battery is repeatedly charged and discharged, the negative electrode active material in the negative electrode active material layer containing Si repeatedly expands and contracts. This repeated expansion and contraction can cause the negative electrode active material layer to swell. When the negative electrode active material layer swells, the thickness of the negative electrode increases, which can, for example, reduce battery performance, and is therefore undesirable. In the negative electrode active material layer, when comparing the negative electrode current collector side and the surface side (opposite the negative electrode current collector), a stronger tendency to swell is observed on the surface side when the secondary battery is repeatedly charged and discharged. Therefore, the inventors of the present invention considered that by making the negative electrode active material layer a multilayer structure and arranging a negative electrode active material layer containing a relatively small Si content on the surface side of the negative electrode active material layer, it would be possible to suppress the swelling of the negative electrode when the secondary battery is repeatedly charged and discharged.

[0021] In negative electrode active materials containing Si, for example, the volume change associated with charging and discharging of secondary batteries is large. Therefore, expansion during charging of the secondary battery can damage the SEI film present on the surface of the negative electrode active material. When the SEI film is damaged, the surface of the negative electrode active material is exposed, and decomposition of the non-aqueous electrolyte occurs in this exposed area, leading to the formation of another SEI film. As the secondary battery is repeatedly charged and discharged, this damage to the SEI film and the formation of a new SEI film in the damaged area (the exposed part of the negative electrode active material) occur repeatedly. As a result, when charging and discharging are repeated, non-uniformity in reactions, stress, etc. occurs in the negative electrode active material layer, which can be a factor in the expansion of the negative electrode. Therefore, the inventors considered providing a LiF film on the surface of the negative electrode active material containing Si.

[0022] As shown in Figure 1, the negative electrode active material layer 64 has a multilayer structure. The negative electrode active material layer 64 here has a first layer 64a and a second layer 64b. As shown in Figure 1, the first layer 64a is located on the negative electrode current collector 62 side. The second layer 64b is located on the surface side (here, on the side opposite to the negative electrode current collector 62). The negative electrode active material layer 64 may further have layers other than the first layer 64a and the second layer 64b, as long as this does not significantly impede the effects of the present invention. For example, the negative electrode active material layer 64 may have an intermediate layer between the first layer 64a and the second layer 64b in which the components of these layers are mixed.

[0023] The negative electrode active material layer 64 contains the negative electrode active material. This will be explained using Figure 2. Figure 2 is a schematic cross-sectional view showing the particles of the negative electrode active material contained in the negative electrode active material layer 64 shown in Figure 1. Note that Figure 2 is a schematic diagram, and therefore the number and distribution of particles are not limited to those shown in Figure 2.

[0024] As shown in Figure 2, the first layer 64a contains first Si-containing particles 12 as a negative electrode active material. The second layer 64b contains second Si-containing particles 14 as a negative electrode active material.

[0025] In this embodiment, the Si content (S2) in the second Si-containing particle 14 is smaller than the Si content (S1) in the first Si-containing particle 12. The Si content (S1) and Si content (S2) are not particularly limited as long as the effects of the present invention can be realized. From the viewpoint of better realizing the effects of the present invention, the Si content (S2) relative to the Si content (S1) is preferably 0.3 to 0.9, and more preferably 0.4 to 0.8.

[0026] From the viewpoint of achieving high capacity for the lithium-ion secondary battery 100, the Si content (S1) is generally 30% by mass or more, for example 35% by mass or more, preferably 40% by mass or more, more preferably 45% by mass or more, and even more preferably 55% by mass or more. On the other hand, from the viewpoint of making the expansion and contraction of the first layer 64a appropriate, the Si content (S1) is preferably 80% by mass or less, and more preferably 75% by mass or less. From the viewpoint of suppressing the expansion and contraction of the second layer 64b and suppressing the swelling of the negative electrode 60 when charging and discharging is repeated, the Si content (S2) is generally 70% by mass or less, for example 65% by mass or less, preferably 60% by mass or less, more preferably 55% by mass or less, even more preferably 50% by mass or less, and particularly preferably 45% by mass or less. On the other hand, from the viewpoint of achieving high capacity for the lithium-ion secondary battery 100, the Si content (S2) is preferably 20% by mass or more, and more preferably 25% by mass or more.

[0027] The average particle diameter (D50) of the first Si-containing particles 12 and the average particle diameter (D50) of the second Si-containing particles 14 are not particularly limited, and are, for example, 0.5 μm to 25 μm, preferably 1 μm to 20 μm, and more preferably 2 μm to 15 μm. The average particle diameter (D50) of the first Si-containing particles 12 and the average particle diameter (D50) of the second Si-containing particles 14 may be the same or different.

[0028] 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.

[0029] As shown in Figure 2, the first Si-containing particle 12 contained in the first layer 64a has a core particle 121 and a coating 122. The core particle 121 is, for example, a Si-containing particle. The coating 122 is, in this case, located on at least a portion of the surface of the core particle 121. As shown in Figure 2, the second Si-containing particle 14 contained in the second layer 64b has a core particle 141 and a coating 142. The core particle 141 is, for example, a Si-containing particle. The coating 142 is, in this case, located on at least a portion of the surface of the core particle 141. The Si-containing particles constituting the core particle 121 and the core particle 141 only need to contain Si, and may be, for example, Si particles, Si oxide particles, Si-C composite particles, etc. It is preferable that both the core particle 121 and the core particle 141 are Si-C composite particles. The Si-C composite particle has, for example, a carbon domain and a Si-containing domain.

[0030] Carbon domains are, for example, carbonized carbon precursors (e.g., petroleum pitch, coal pitch, phenolic resin, etc.); graphite, etc. Preferably, the carbon domains constitute a carbon matrix. Si-C composite particles are, for example, particles in which Si-containing domains are dispersed within a carbon matrix. Si-C composite particles may, for example, have multiple Si-containing domains within the carbon matrix. In this case, the carbon matrix is ​​advantageous because it can mitigate volume changes due to the expansion and contraction of the Si-containing domains.

[0031] The Si-containing domain contains Si, for example, Si, Si oxide (SiO x ), Si nitride (SiN x ), Si carbide (SiC x It is composed of the following. The Si-containing domain is preferably Si and Si oxide (SiO x It is composed of at least one of the following. The Si-containing domain may be a nanoparticle. The oxygen content in the Si-containing domain is preferably 10% by mass or less.

[0032] The average particle diameter of the Si-containing domains is, for example, 50 nm or less, and may be between 5 nm and 50 nm. The "average particle diameter of the Si-containing domains" 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 domains 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 domains are determined, and their average value is taken as the "average particle diameter of the Si-containing domains" here.

[0033] Si-C composite particles include, for example, a carbon substrate and Si contained within the carbon substrate. The carbon substrate preferably has voids, and more preferably is a porous carbon substrate. The Si contained within the carbon substrate is, for example, of a size that can be contained within the carbon substrate, and if the carbon substrate has voids, it is preferable that it is of a size that can be contained within those voids. From this viewpoint, the Si contained within the carbon substrate is preferably Si-containing nanoparticles (e.g., Si nanoparticles, Si oxide nanoparticles, Si nitride nanoparticles, Si carbide nanoparticles, etc.) with an average particle diameter (here, the average particle diameter of the Si-containing domain) of approximately 1 nm to 300 nm (preferably 1 nm to 200 nm, more preferably 1 nm to 100 nm). From the viewpoint of suppressing volume changes of Si and thereby mitigating volume changes between the first Si-containing particle 12 and the second Si-containing particle 14, it is preferable that the Si-C composite particles include particles having a carbon substrate with voids (more preferably a porous carbon substrate) and Si (e.g., Si-containing nanoparticles) disposed within the voids. Alternatively, in other forms, the Si-C composite particles may include carbon particles and Si-containing nanoparticles attached to the surface of the carbon particles, or may include Si-containing particles and carbon nanoparticles attached to the surface of the Si-containing particles.

[0034] Coatings 122 and 142, in this context, contain LiF. The presence of coatings 122 and 142 can be confirmed, for example, by X-ray photoelectron spectroscopy (XPS) measurements of the first Si-containing particle 12 and the second Si-containing particle 14. The LiF content (LiF concentration) in coating 122 corresponds to the ratio of the peak intensity of LiF's F to the peak intensity of other F components in the XPS spectrum of the first Si-containing particle 12, measured by XPS. The LiF content (LiF concentration) in coating 142 corresponds to the ratio of the peak intensity of LiF's F to the peak intensity of other F components in the XPS spectrum of the second Si-containing particle 14, measured by XPS. In the XPS spectrum measured by XPS, the LiF's F peak appears at 683 eV to 686 eV. Other F peaks besides LiF are F peaks outside the 683eV-686eV range, and may appear particularly in the 687eV-690eV range.

[0035] Here, the ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-containing particle 12 measured by XPS is defined as the first peak intensity ratio. The ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS spectrum of the second Si-containing particle 14 is defined as the second peak intensity ratio. Here, the second peak intensity ratio is smaller than the first peak intensity ratio. The difference between the first peak intensity ratio and the second peak intensity ratio is, for example, 0.050 or more, preferably 0.075 or more, more preferably 0.100 or more, and even more preferably 0.125 or more. On the other hand, the difference between the first peak intensity ratio and the second peak intensity ratio is generally 1.00 or less, for example 0.750 or less, preferably 0.600 or less, more preferably 0.500 or less, and even more preferably 0.450 or less.

[0036] The first peak intensity ratio is preferably approximately 0.450 to 1.00. The first peak intensity ratio is, for example, 0.460 or higher, preferably 0.480 or higher, and more preferably 0.500 or higher. The first peak intensity ratio is, for example, 0.900 or lower, preferably 0.850 or lower, more preferably 0.800 or lower, and even more preferably 0.750 or lower. The second peak intensity ratio is preferably approximately 0.200 or higher and less than 0.450. The second peak intensity ratio is, for example, 0.250 or higher, preferably 0.275 or higher, and more preferably 0.300 or higher. The second peak intensity ratio is, for example, 0.420 or lower, preferably 0.400 or lower, and more preferably 0.380 or lower.

[0037] The presence of the coating 122 on the surface of the core particle 121 and the presence of the coating 142 on the surface of the core particle 141 can be confirmed, for example, by SEM observation. The coverage rate of the coating 122 on the surface of the core particle 121 is, for example, 20% or more, preferably 50% or more, more preferably 80% or more, even more preferably 90% or more, and the closer to 100%, the better. The coverage rate of the coating 142 on the surface of the core particle 141 is, for example, 20% or more, preferably 50% or more, more preferably 80% or more, even more preferably 90% or more, and the closer to 100%, the better. Although not particularly limited, the coverage rate of the coating 122 on the surface of the core particle 121 in the first Si-containing particle 12 may be greater than, for example, the coverage rate of the coating 142 on the surface of the core particle 141 in the second Si-containing particle 14. The coverage rate of the coating 122 or the coating 142 can be determined as follows. SEM images of the cross-section of the first Si-containing particle 12 or the second Si-containing particle 14 are obtained. In these images, the ratio (percentage) of the total length of the coating 122 or coating 142 to the perimeter of the core particle 121 or core particle 141 is calculated. This ratio is calculated for five or more arbitrarily selected first Si-containing particles 12 or second Si-containing particles 14, and the average value is taken as the coverage rate.

[0038] The first Si-containing particles 12 and the second Si-containing particles 14 can be manufactured according to known methods. Various manufacturing methods for Si-C composite material particles are known (see, for example, Japanese Patent Publication No. 2015-38862, International Publication No. 2014 / 046144, and the prior art documents cited in said International Publication).

[0039] As shown in Figure 2, the first layer 64a further contains first graphite particles 16 as a negative electrode active material. The second layer 64b further contains second graphite particles 18 as a negative electrode active material. It is preferable that both the first graphite particles 16 and the second graphite particles 18 are substantially Si-free graphite particles. The Si content in the first graphite particles 16 and the Si content in the second graphite particles 18 are both approximately 10% by mass or less, for example 7% by mass or less, preferably 5% by mass or less, more preferably 3% by mass or less, even more preferably 1% by mass or less or 0.5% by mass or less, and the closer to 0% by mass, the better. The Si content can be calculated by conventionally known methods such as ICP analysis.

[0040] The graphite constituting the first graphite particles 16 and the second graphite particles 18 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.

[0041] The shape of the first graphite particles 16 and the second graphite particles 18 is not particularly limited and may be flaky, spherical, or the like. The first graphite particles 16 and the second graphite particles 18 are preferably spheroidized graphite particles. When the first graphite particles 16 and the second graphite particles 18 are spherical, the circularity of the first graphite particles 16 and the second graphite particles 18 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0042] 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.

[0043] The average particle diameter (D50) of the first graphite particles 16 and the average particle diameter (D50) of the second graphite particles 18 are not particularly limited. The average particle diameter (D50) of the first graphite particles 16 and the average particle diameter (D50) of the second graphite particles 18 are, for example, 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.

[0044] The first graphite particles 16 and the second graphite particles 18 may be the same graphite particles, or different graphite particles may be used. It is preferable to use the same graphite particles for the first graphite particles 16 and the second graphite particles 18.

[0045] In the first layer 64a, if the first graphite particles 16 are included, when the total of the first Si-containing particles 12 and the first graphite particles 16 is taken as 100% by mass, the proportion of the first Si-containing particles 12 is, for example, 5% to 60% by mass, preferably 10% to 50% by mass, and more preferably 15% to 40% by mass. The first graphite particles 16 are used, for example, to improve the conductivity of the negative electrode active material layer 64, and their inclusion is not essential. In other embodiments, the first graphite particles 16 may not be included in the first layer 64a.

[0046] The negative electrode active material contained in the first layer 64a may be only the first Si-containing particles 12 and the first graphite particles 16, or may be only the first Si-containing particles 12. However, as long as the effects of the technology disclosed herein can be achieved, the first layer 64a may contain other negative electrode active materials that do not correspond to either the first Si-containing particles 12 or the first graphite particles 16. In this case, the proportion of the other negative electrode active material is preferably 10% by mass or less of the total amount of the negative electrode active material contained in the first layer 64a.

[0047] In the second layer 64b, when the second graphite particles 18 are contained, when the total of the second Si-containing particles 14 and the second graphite particles 18 is 100% by mass, the proportion of the second Si-containing particles 14 is, for example, 10% by mass to 60% by mass, preferably 15% by mass to 50% by mass, and more preferably 20% by mass to 40% by mass. The second graphite particles 18 are used, for example, for the purpose of improving the conductivity of the negative electrode active material layer 64, and their inclusion is not essential. In other embodiments, in the second layer 64b, the second graphite particles 18 may not be contained.

[0048] The negative electrode active material contained in the second layer 64b may be only the second Si-containing particles 14 and the second graphite particles 18, or may be only the second Si-containing particles 14. However, as long as the effects of the technology disclosed herein can be achieved, the second layer 64b may contain other negative electrode active materials that do not correspond to either the second Si-containing particles 14 or the second graphite particles 18. In this case, the proportion of the other negative electrode active material is preferably 10% by mass or less of the total amount of the negative electrode active material contained in the second layer 64b.

[0049] In the negative electrode active material layer 64, the thickness (T A ) of the first layer 64a and the thickness (T B ) of the second layer 64b, the ratio (T A : T B ) is generally 5:95 to 95:5, for example 10:90 to 90:10, preferably 20:80 to 80:20, more preferably 30:70 to 70:30, and even more preferably 40:60 to 60:40.

[0050] 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.

[0051] The content of the negative electrode active material in the first layer 64a (i.e., relative to the total mass of the first layer 64a) 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.

[0052] The content of the negative electrode active material in the second layer 64b (i.e., relative to the total mass of the second layer 64b) 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.

[0053] 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.

[0054] 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. 3The 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:

[0055] 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.

[0056] As described above, the negative electrode 60 for the secondary battery comprises a negative electrode current collector 62 and a negative electrode active material layer 64. The negative electrode active material layer 64 is supported by the negative electrode current collector 62. The negative electrode active material layer 64 comprises a first layer 64a located on the negative electrode current collector 62 side and a second layer 64b located on the surface side. The first layer 64a contains first Si-containing particles 12 as negative electrode active material. The second layer 64b contains second Si-containing particles 14 as negative electrode active material. The Si content ratio (S2) in the second Si-containing particles 14 is smaller than the Si content ratio (S1) in the first Si-containing particles 12.

[0057] The first Si-containing particle 12 is equipped with a LiF coating (here, the LiF coating contained in coating 122). The second Si-containing particle 14 is equipped with a LiF coating (here, the LiF coating contained in coating 142). 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 first Si-containing particle 12, measured by XPS, is defined as the first peak intensity ratio. 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 second Si-containing particle 14 is defined as the second peak intensity ratio. The second peak intensity ratio is smaller than the first peak intensity ratio. The difference between the first peak intensity ratio and the second peak intensity ratio is between 0.050 and 1.00.

[0058] The negative electrode 60 contains first Si-containing particles 12 with a relatively high Si content and second Si-containing particles 14 with a relatively low Si content. In the negative electrode 60, the negative electrode active material layer 64 comprises a first layer 64a on the negative electrode current collector 62 side and a second layer 64b on the surface side. When charging and discharging are repeated, the expansion on the surface side of the negative electrode active material layer 64 becomes relatively larger. In Si-containing particles, for example, the higher the Si content in the Si-containing particles, the greater the degree of expansion and contraction during charging and discharging. Conversely, the lower the Si content in the Si-containing particles, the smaller the degree of expansion and contraction during charging and discharging. Therefore, by placing second Si-containing particles 14 with a relatively low Si content in the second layer 64b on the surface side, the particles can be made to move more easily in the second layer 64b. As a result, the movement of particles during expansion and contraction due to charging and discharging can relieve stress and suppress the expansion of the negative electrode. Furthermore, in the negative electrode 60, the first layer 64a contains first Si-containing particles 12, which have a relatively high Si content. As a result, the degree of expansion and contraction of the first layer 64a in the negative electrode 60 is increased. This makes it possible to make the degree of expansion and contraction of the entire negative electrode active material layer 64 more uniform during charging and discharging. Therefore, it is possible to suppress localized concentration of reactions, stresses, etc. in the negative electrode active material layer 64, and to suppress swelling of the negative electrode 60 when charging and discharging is repeated.

[0059] In addition, the first Si-containing particles 12 and the second Si-containing particles 14 are provided with a LiF coating on their surfaces. The LiF coating can follow the expansion and contraction of the Si-containing particles, thereby suppressing the exposure of the particle surface due to expansion and contraction. Here, the second peak intensity ratio of the second Si-containing particles 14 is smaller than the first peak intensity ratio of the first Si-containing particles 12. The difference between the first peak intensity ratio and the second peak intensity ratio is adjusted to 0.050 to 1.00. In other words, the first Si-containing particles 12 and the second Si-containing particles 14 are provided with a LiF coating according to their Si content. This suppresses the exposure of the particle surface due to the aforementioned expansion and contraction in each Si-containing particle, and suppresses the concentration of local reactions, stresses, etc. in the negative electrode active material layer 64. Therefore, swelling of the negative electrode 60 when repeated charging and discharging is performed can be suppressed.

[0060] The Si content ratio (S2) in the second Si-containing particle 14 relative to the Si content ratio (S1) in the first Si-containing particle 12 may be between 0.3 and 0.9. By setting the Si content ratio (S1) and Si content ratio (S2) within this range, the degree of expansion and contraction of the first layer 64a and the degree of expansion and contraction of the second layer 64b during charging and discharging can be optimized. This makes it possible to better suppress the swelling of the negative electrode 60 when charging and discharging is repeated.

[0061] The Si content (S1) in the first Si-containing particle 12 may be 30% to 80% by mass. The Si content (S2) in the second Si-containing particle 14 may be 20% to 70% by mass. This makes it possible to increase the capacity of the secondary battery 100 while suppressing the swelling of the negative electrode 60 when repeated charging and discharging occurs.

[0062] The first peak intensity ratio may be 0.450 to 1.00. This effectively suppresses the occurrence of exposed areas on the surface of the first Si-containing particles 12 when charging and discharging is repeated. Furthermore, it suppresses the excessive increase in resistance at the negative electrode 60 due to the thickness of the LiF coating. In addition, it suppresses the tendency of Si-containing particles to expand due to side reactions in the LiF coating.

[0063] The second peak intensity ratio may be 0.200 or more and less than 0.450. This effectively suppresses the occurrence of exposed areas on the surface of the second Si-containing particles 14 when charging and discharging is repeated.

[0064] The first Si-containing particle 12 may include a Si-C composite particle (here, a core particle 121) having a carbon substrate and Si contained inside the carbon substrate, and a LiF coating (here, a LiF coating included in coating 122) disposed on at least a part of the surface of the Si-C composite particle (here, a core particle 121). The second Si-containing particle 14 may include a Si-C composite particle (here, a core particle 141) having a carbon substrate and Si contained inside the carbon substrate, and a LiF coating (here, a LiF coating included in coating 142) disposed on at least a part of the surface of the Si-C composite particle (here, a core particle 141). This makes it possible to mitigate the expansion of Si within the carbon substrate, thereby suppressing the expansion of the first Si-containing particle 12 and the second Si-containing particle 14. Therefore, it is possible to suppress the swelling of the negative electrode 60 when charging and discharging are repeated.

[0065] The first layer 64a may further contain first graphite particles 16 that are substantially free of Si as a negative electrode active material. The second layer 64b may further contain second graphite particles 18 that are substantially free of Si as a negative electrode active material. This improves the conductivity in both the first layer 64a and the second layer 64b.

[0066] The negative electrode 60 is manufactured by a manufacturing method that includes, for example, a first preparation step, a second preparation step, a first coating step, a first drying step, a second coating step, a second drying step, and a pressing step.

[0067] The first preparation step is to prepare a first paste, for example, comprising first Si-containing particles 12 as a negative electrode active material and a dispersion medium. The first preparation step can be carried out by mixing the first Si-containing particles 12, optionally first graphite particles 16, and optional components (e.g., binder, conductive material, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method. 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.

[0068] The first preparation step includes, for example, preparing the first Si-containing particles 12. In preparing the first Si-containing particles 12, for example, first prepare Si-containing particles as core particles 121. The Si-containing particles as core particles 121 are, for example, Si-C composite particles. Si-C composite particles can be prepared, for example, using known methods as described above.

[0069] Next, the obtained core particles 121 are dispersed in water or a water-soluble organic solvent, and the core particles 121 are reacted with a water-soluble lithium salt and a fluorinating agent to produce LiF. The LiF produced here forms a coating 122 on the surface of the core particles 121.

[0070] As a water-soluble organic solvent, for example, alcohols such as ethanol (ethyl alcohol) 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.

[0071] In this operation, for example, first, an aqueous solution of a water-soluble lithium salt, a dispersion of core particles 121 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 make 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. In this reaction solution, the surface of the core particles 121 and the LiF can be brought into contact.

[0072] 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 122, etc. The longer the reaction time, the higher the content of LiF in the coating 122.

[0073] The core particles 121 are recovered from the reaction solution with the reaction solution containing LiF adhering to its surface. By drying the core particles 121 with the LiF-containing reaction solution adhering to them, LiF can be attached to the surface of the core particles 121, and a first Si-containing particle 12 having a LiF-containing coating 122 can be obtained. To adjust the amount of LiF, the reaction solution containing LiF may be diluted with water or the like.

[0074] The second preparation step is a step of preparing a second paste, for example, comprising a second Si-containing particle 14 as a negative electrode active material and a dispersion medium. The second preparation step can be carried out by mixing the second Si-containing particle 14, optionally a second graphite particle 18, and optional components (e.g., binder, conductive material, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method.

[0075] The second preparation step includes, for example, preparing the second Si-containing particles 14. The second Si-containing particles 14 can be prepared by using Si-C composite particles as core particles 141, along with other materials and procedures used when preparing the first Si-containing particles 12, and by appropriately changing the conditions.

[0076] The first coating step is, for example, the step of applying the first paste onto the negative electrode current collector 62. The first coating step can be carried out according to a known method. For example, the first coating step can be performed by applying the obtained first paste onto the negative electrode current collector 62 using a coating device such as a gravure coater, comma coater, slit coater, or die coater.

[0077] The first drying step is, for example, a step of drying the first paste applied to the negative electrode current collector 62 to form a dried film of the first paste. The first drying step can be carried out according to a known method. For example, a dried film of the first paste can be formed by removing the dispersion medium from the negative electrode current collector 62 to which the first paste has been applied using a drying apparatus such as a drying oven. This allows the first drying step to be carried out. The drying temperature and drying time can be appropriately determined according to the solid content concentration of the first 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.

[0078] The second coating step is, for example, the step of applying the second paste onto the dried film of the first paste. The second coating step can be carried out according to a known method. For example, the second coating step can be performed by applying the obtained second paste onto the dried film of the first paste using the apparatus described above.

[0079] The second drying step is, for example, the step of drying the second paste applied to the dried film of the first paste to form a dried film of the second paste. The second drying step can be carried out according to a known method. The apparatus and drying conditions used in the second drying step are as described in the description of the first drying step.

[0080] The pressing process is, for example, a process of pressing the dried film of the first paste and the dried film of the second paste together. The pressing process can be carried out according to a known method. For example, the pressing process can be carried out by applying pressure to the dried film of the first paste and the dried film of the second paste using a roller press or the like. The pressing process can densely pack the first Si-containing particles 12, the second Si-containing particles 14, and, if necessary, the first graphite particles 16 and the second graphite particles 18. In this way, a negative electrode 60 can be obtained that comprises a negative electrode active material layer 64 having a first layer 64a and a second layer 64b.

[0081] According to the negative electrode 60 of this embodiment, swelling of the negative electrode 60 when the secondary battery is repeatedly charged and discharged can be suppressed. Furthermore, since the negative electrode 60 of this embodiment uses a negative electrode active material containing Si, the capacity of the secondary battery can be increased.

[0082] Therefore, from another perspective, the secondary battery disclosed herein comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 according to the above-described embodiment. Hereinafter, an embodiment of the secondary battery disclosed herein will be described with reference to Figures 3 and 4, using a lithium-ion secondary battery as an example. The following configuration example is a flat rectangular lithium-ion secondary battery having a flat wound electrode body and a flat battery case.

[0083] The lithium-ion secondary battery 100 shown in Figure 3 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.

[0084] As shown in Figures 3 and 4, 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] The negative electrode sheet 60 used is the negative electrode 60 described above.

[0097] 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.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power sources for driving mounted on vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc. Also, the lithium-ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium-ion secondary battery 100 can typically also be used in the form of a battery pack formed by connecting a plurality of them in series and / or in parallel.

[0103] As described above, the square lithium-ion secondary battery 100 including the flat wound electrode body 20 has been explained as an example. However, the lithium-ion secondary battery can also be configured as a lithium-ion secondary battery including a laminated electrode body (that is, an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). Also, the lithium-ion secondary battery can be configured as a cylindrical lithium-ion secondary battery, a laminated case type lithium-ion secondary battery, or the like.

[0104] Also, according to a known method, the lithium-ion secondary battery 100 can be configured as an all-solid-state lithium-ion secondary battery using a solid electrolyte instead of a non-aqueous electrolyte.

[0105] Also, the negative electrode 60 according to the present embodiment is suitable for the negative electrode of a lithium-ion secondary battery, but can be constructed and used as the negative electrode of other secondary batteries, and the other secondary batteries can be configured according to a known method.

[0106] Hereinafter, examples related to the present disclosure will be described in detail, but the present disclosure is not intended to be limited to those shown in such examples. Unless otherwise specified, “%” regarding content (content ratio) is based on mass.

[0107] <Production of Si-C Composite Particles Having a Film Containing LiF> Si-C composite particles (A) to (H) were prepared as core particles. The Si-C composite particles (A) to (H) prepared here were particles having a porous carbon substrate and Si nanoparticles arranged in the pores of the carbon substrate. Si-C composite particle (A) was a particle with a Si content of 60%. Si-C composite particle (B) was a particle with a Si content of 40%. Si-C composite particle (C) was a particle with a Si content of 55%. Si-C composite particle (D) was a particle with a Si content of 45%. Si-C composite particle (E) was a particle with a Si content of 70%. Si-C composite particle (F) was a particle with a Si content of 30%. Si-C composite particle (G) was a particle with a Si content of 80%. Si-C composite particle (H) was a particle with a Si content of 20%.

[0108] [Example 1] Two particles, a first particle and a second particle, were prepared as negative electrode active materials. For the preparation of the first particle, a Si-C composite particle (A) was prepared as the core particle. The core particle was dispersed in ethyl alcohol while sonicating for 30 minutes. A lithium acetate aqueous solution was prepared by dissolving lithium acetate in deionized water. To this aqueous solution, the dispersion of Si-C composite particles was added while stirring for 30 minutes. Then, an aqueous solution of ammonium fluoride was added while stirring for 120 minutes to react the lithium acetate with ammonium fluoride. This obtained a reaction solution containing LiF. After diluting the reaction solution, it was filtered. The filtrate was dried to form a LiF-containing film on the Si-C composite particle (A). The procedure for preparing the second particle was the same as that for the first particle, except that a Si-C composite particle (B) was used as the core particle, and the addition time of the aqueous ammonium fluoride solution (i.e., the reaction time) was set to 60 minutes.

[0109] XPS spectra were measured for the obtained first and second particles. The presence of LiF was confirmed by observing peaks for F (approximately 684.8 eV) and Li (approximately 55.1 eV). For the first particle, the ratio of the peak intensity of F from LiF to the peak intensity of F from other elements (hereinafter also referred to as the "first LiF intensity ratio") was determined. For the second particle, the ratio of the peak intensity of F from LiF to the peak intensity of F from other elements (hereinafter also referred to as the "second LiF intensity ratio") was determined. The results are shown in the corresponding columns of Table 1. The peak of F from LiF appeared between 683 eV and 686 eV, while the peak of F from other elements appeared between 687 eV and 690 eV.

[0110] Furthermore, graphite particles with an average particle size (D50) of 15 μm were prepared as the negative electrode active material. These graphite particles were substantially Si-free. A dispersion of single-walled carbon nanotubes (SWCNTs) was prepared as the conductive material. Carboxymethylcellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared as binders.

[0111] The materials described above were kneaded with water as a solvent in a mass ratio of graphite particles / first particle / SWCNT / CMC / PAA / SBR = 80 / 20 / 0.1 / 1 / 1 / 1.5 to prepare the first paste according to the following procedure. The materials described above were kneaded with water as a solvent in a mass ratio of graphite particles / second particle / SWCNT / CMC / PAA / SBR = 70 / 30 / 0.1 / 1 / 1 / 1.5 to prepare the second paste according to the following procedure.

[0112] The first particles, SWCNT dispersion, and dispersion medium were mixed using a disperser at a rotational speed of 3000 rpm. Graphite particles, CMC, and PAA were dry-mixed using a stirring granulator. The resulting dry mixture was then solid-kneaded with the mixture containing the first particles. The solid content at this stage was 65%. Next, SBR and additional dispersion medium were added to the mixture obtained by solid-kneading and diluted to obtain the first paste.

[0113] The second particle, SWCNT dispersion, and dispersion medium were mixed using a disperser at a rotational speed of 3000 rpm. Graphite particles, CMC, and PAA were dry-mixed using a stirring granulator. The resulting dry mixture was then solid-kneaded with the mixture containing the second particle. The solid content at this stage was 65%. Next, SBR and additional dispersion medium were added to the mixture obtained by solid-kneading and diluted to obtain the second paste.

[0114] A first paste was applied to the surface of a 10 μm thick copper foil and dried to form a dried film of the first paste. Next, a second paste was applied onto the dried film of the first paste and dried to form a dried film of the second paste. Here, the amount of the second paste applied was set so that when the thickness of the negative electrode active material layer (here, the total thickness of the dried films of the first and second pastes) was 100, the thickness of the dried film of the second paste was 50. After the negative electrode active material layer was roll-pressed, the resulting sheet was processed to a predetermined size to obtain a negative electrode sheet.

[0115] [Example 2] In preparing the first particle, a Si-C composite particle (C) was used as the core particle, and the addition time of the ammonium fluoride aqueous solution was set to 90 minutes. In preparing the second particle, a Si-C composite particle (D) was used as the core particle. Except for these differences, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0116] [Example 3] In the preparation of the first particle, a Si-C composite particle (E) was used as the core particle, and the addition time of the ammonium fluoride aqueous solution was set to 150 minutes. In the preparation of the second particle, a Si-C composite particle (F) was used as the core particle. Except for these differences, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0117] [Comparative Example 1] A negative electrode paste was prepared by kneading graphite particles / 1st particle / 2nd particle / SWCNT / CMC / PAA / SBR with water as a solvent to achieve a mass ratio of 70 / 10 / 15 / 0.1 / 1 / 1 / 1.5. The negative electrode paste was prepared by carrying out the following 1st and 2nd steps. In the 1st step, the 1st and 2nd particles, paste-like SWCNT (solid content 2%), and a dispersion medium were first dispersed and mixed at 3000 rpm using a disperser to prepare the 1st paste. In the 2nd step, graphite particles, CMC, and PAA were dry mixed using a stirring granulator. The 1st paste, the mixed powder obtained by dry mixing, and the dispersion medium (water) were kneaded together. The solid content during kneading was 65%. SBR and the dispersion medium (water) were further added to the kneaded mixture and mixed. In this manner, a negative electrode paste was prepared. 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. Except for this, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0118] [Comparative Example 2] Si-C composite particles (A) were used as the first particles. Si-C composite particles (B) were used as the second particles. No LiF coating was formed on Si-C composite particles (A) or Si-C composite particles (B). Except for this, the negative electrode sheet of this example was obtained using the same materials and procedures as in Example 1.

[0119] [Comparative Example 3] In preparing the first particle, a Si-C composite particle (B) was used as the core particle, and the ammonium fluoride aqueous solution was added for 60 minutes. In preparing the second particle, a Si-C composite particle (A) was used as the core particle, and the ammonium fluoride aqueous solution was added for 120 minutes. Except for these differences, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0120] [Comparative Example 4] As the second particle, a Si-C composite particle (B) was used. No LiF coating was formed on the Si-C composite particle (B). Except for this, the same materials and procedure as in Example 1 were used to obtain the negative electrode sheet of this example.

[0121] [Comparative Example 5] Si-C composite particles (A) were used as the first particles. No LiF coating was formed on the Si-C composite particles (A). Except for this, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0122] [Comparative Example 6] In preparing the first particle, a Si-C composite particle (G) was used as the core particle, and the addition time of the ammonium fluoride aqueous solution was 240 minutes. In preparing the second particle, a Si-C composite particle (H) was used as the core particle. Except for these differences, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0123] <Evaluation of plate expansion coefficient> The thickness of the negative electrode in each example and comparative example was measured. This thickness was defined as the initial thickness (T0). Using this negative electrode, an evaluation lithium-ion secondary battery was fabricated as follows.

[0124] LiNi as a positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode paste was prepared by mixing O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF = 100:1:1. This paste was applied to the surface of a 15 μm thick aluminum foil and dried to form a positive electrode active material layer. After roll pressing the positive electrode active material layer, the resulting sheet was processed to a predetermined size to obtain a positive electrode sheet.

[0125] A separator made of porous polyolefin was prepared. Leads were attached to the negative electrode sheet and positive electrode sheet prepared above, and the electrodes were stacked via the separator to create an electrode body. This was then placed in an aluminum laminate film case along with a non-aqueous electrolyte. The non-aqueous electrolyte was a mixed 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, with LiPF6 dissolved as a supporting salt at a concentration of 1.0 mol / L. The case was then sealed to obtain a lithium-ion secondary battery for evaluation.

[0126] Next, each of the prepared lithium-ion secondary batteries for evaluation was placed in an environment of 25°C. Each evaluation lithium-ion secondary battery was charged with a constant current of 0.4C up to 4.2V, and then charged with a constant voltage until the current value was 0.1C. Subsequently, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.4C up to 2.5V.

[0127] The above charge-discharge cycle was repeated 250 times, with one cycle being defined as the charge-discharge cycle described above. After 250 cycles, the batteries were discharged to 2.5V. Subsequently, each evaluation lithium-ion secondary battery was disassembled under an argon atmosphere, the negative electrode was cleaned by immersion in DMC, and then dried. The thickness of the negative electrode was then measured and defined as the thickness after the charge-discharge cycle (T1). The percentage increase in negative electrode thickness before and after the charge-discharge cycle was calculated using (T1 / T0-1) × 100. The results are shown in the "Percentage Increase (%)" column of Table 1. In this case, cases where the percentage increase was less than 33% are evaluated as having suppressed negative electrode swelling due to the charge-discharge cycle.

[0128] [Table 1]

[0129] As shown in Table 1, in Examples 1 to 3, the swelling of the negative electrode during charge-discharge cycles was suppressed. As described above, in Examples 1 to 3, the first particle was contained in the negative electrode current collector side layer of the negative electrode active material layer, and the second particle was contained in the surface layer side. The Si content of the second particle was smaller than that of the first particle. In the XPS spectrum of the second particle, the ratio of the peak intensity of LiF's F to the peak intensity of F other than LiF was smaller than 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 first particle, with the difference between the two being 0.050 to 1.00.

[0130] 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.

[0131] In other words, the negative electrode of the secondary battery and its manufacturing method, as well as the secondary battery, are as described in the following sections [1] to

[10] . [1] Negative electrode current collector and The negative electrode active material layer supported by the negative electrode current collector, The negative electrode of a secondary battery, The negative electrode active material layer comprises a first layer located on the negative electrode current collector side and a second layer located on the surface side. The first layer contains first Si-containing particles as a negative electrode active material. The second layer contains secondary Si-containing particles as a negative electrode active material. The Si content in the second Si-containing particle is smaller than the Si content in the first Si-containing particle. Both the first Si-containing particles and the second Si-containing particles are equipped with a LiF coating. 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-containing particle measured by X-ray photoelectron spectroscopy is defined as the first peak intensity ratio. When 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-containing particle is defined as the second peak intensity ratio, The negative electrode wherein the second peak intensity ratio is smaller than the first peak intensity ratio, and the difference between the first peak intensity ratio and the second peak intensity ratio is 0.050 or more and 1.00 or less. [2] The negative electrode according to [1], wherein the Si content ratio in the second Si-containing particle is 0.3 or more and 0.9 or less relative to the Si content ratio in the first Si-containing particle. [3] The Si content in the first Si-containing particle is 30% by mass or more and 80% by mass or less, The negative electrode according to [1] or [2], wherein the Si content in the second Si-containing particle is 20% by mass or more and 70% by mass or less. [4] The negative electrode according to any one of [1] to [3], wherein the first peak intensity ratio is 0.450 or more and 1.00 or less. [5] The negative electrode according to any one of [1] to [4], wherein the second peak intensity ratio is 0.200 or more and less than 0.450. [6] The negative electrode according to any one of [1] to [5], wherein the first Si-containing particles and the second Si-containing particles each comprise a Si-C composite particle having a carbon substrate and Si contained within the carbon substrate, and the LiF coating disposed on at least a portion of the surface of the Si-C composite particle. [7] The negative electrode according to any one of [1] to [6], wherein the first layer and the second layer further contain substantially Si-free graphite particles as a negative electrode active material. [8] A method for manufacturing the negative electrode of a secondary battery, A first paste is prepared, comprising a first Si-containing particle as a negative electrode active material and a dispersion medium. A second paste is prepared, comprising a second Si-containing particle as a negative electrode active material and a dispersion medium. The first paste is applied to the negative electrode current collector and dried to form a dried film of the first paste, The second paste is applied onto the dried film of the first paste and dried to form a dried film of the second paste, Pressing the dried film of the first paste and the dried film of the second paste, It includes, The Si content in the second Si-containing particle is smaller than the Si content in the first Si-containing particle. Both the first Si-containing particles and the second Si-containing particles are equipped with a LiF coating. 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-containing particle measured by X-ray photoelectron spectroscopy is defined as the first peak intensity ratio. When 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-containing particle is defined as the second peak intensity ratio, A manufacturing method wherein the second peak intensity ratio is smaller than the first peak intensity ratio, and the difference between the first peak intensity ratio and the second peak intensity ratio is 0.050 or more and 1.00 or less. [9] Preparing the first paste includes preparing the first Si-containing particles, Preparing the second paste includes preparing the second Si-containing particles, In preparing the first Si-containing particles and the second Si-containing particles, A Si-C composite particle is prepared, which contains a carbon substrate and Si contained within the carbon substrate. By dispersing the Si-C composite particles in water or a water-soluble organic solvent, and reacting the Si-C composite particles with a water-soluble lithium salt and a fluorinating agent, LiF is produced. The manufacturing method according to [8], comprising preparing the first Si-containing particles and the second Si-containing particles.

[10] Positive electrode and, The negative electrode and, Electrolytes, A secondary battery equipped with, A secondary battery in which the negative electrode is the negative electrode described in any one of items [1] to [7]. [Explanation of Symbols]

[0132] 12 1st Si-containing particles 14 2nd Si-containing particles 16. First Graphite Particle 18. Second-order 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

Claims

1. Negative electrode current collector and The negative electrode active material layer supported by the negative electrode current collector, The negative electrode of a secondary battery, The negative electrode active material layer comprises a first layer located on the negative electrode current collector side and a second layer located on the surface side. The first layer contains first Si-containing particles as a negative electrode active material. The second layer contains second Si-containing particles as a negative electrode active material. The Si content in the second Si-containing particle is smaller than the Si content in the first Si-containing particle. The first Si-containing particles and the second Si-containing particles both have a LiF coating. The ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-containing particle measured by X-ray photoelectron spectroscopy is defined as the first peak intensity ratio. When 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-containing particle is defined as the second peak intensity ratio, The negative electrode wherein the second peak intensity ratio is smaller than the first peak intensity ratio, and the difference between the first peak intensity ratio and the second peak intensity ratio is 0.050 or more and 1.00 or less.

2. The negative electrode according to claim 1, wherein the Si content ratio in the second Si-containing particle is 0.3 or more and 0.9 or less relative to the Si content ratio in the first Si-containing particle.

3. The Si content in the first Si-containing particle is 30% by mass or more and 80% by mass or less. The negative electrode according to claim 1, wherein the Si content in the second Si-containing particle is 20% by mass or more and 70% by mass or less.

4. The negative electrode according to claim 1, wherein the first peak intensity ratio is 0.450 or more and 1.00 or less.

5. The negative electrode according to claim 1, wherein the second peak intensity ratio is 0.200 or more and less than 0.

450.

6. The negative electrode according to any one of claims 1 to 5, wherein the first Si-containing particles and the second Si-containing particles each comprise a Si-C composite particle having a carbon substrate and Si contained within the carbon substrate, and the LiF coating disposed on at least a portion of the surface of the Si-C composite particle.

7. The negative electrode according to any one of claims 1 to 5, wherein the first layer and the second layer further contain substantially Si-free graphite particles as a negative electrode active material.

8. A method for manufacturing the negative electrode of a secondary battery, A first paste is prepared, comprising a first Si-containing particle as a negative electrode active material and a dispersion medium. A second paste is prepared, comprising a second Si-containing particle as a negative electrode active material and a dispersion medium. The first paste is applied to the negative electrode current collector and dried to form a dried film of the first paste, The second paste is applied onto the dried film of the first paste and dried to form a dried film of the second paste, Pressing the dried film of the first paste and the dried film of the second paste, It includes, The Si content in the second Si-containing particle is smaller than the Si content in the first Si-containing particle. The first Si-containing particles and the second Si-containing particles both have a LiF coating. The ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-containing particle measured by X-ray photoelectron spectroscopy is defined as the first peak intensity ratio. When 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-containing particle is defined as the second peak intensity ratio, A manufacturing method wherein the second peak intensity ratio is smaller than the first peak intensity ratio, and the difference between the first peak intensity ratio and the second peak intensity ratio is 0.050 or more and 1.00 or less.

9. Preparing the first paste includes preparing the first Si-containing particles, Preparing the second paste includes preparing the second Si-containing particles, In preparing the first Si-containing particles and the second Si-containing particles, A Si-C composite particle is prepared, which contains a carbon substrate and Si contained within the carbon substrate. By dispersing the Si-C composite particles in water or a water-soluble organic solvent, and reacting the Si-C composite particles with a water-soluble lithium salt and a fluorinating agent, LiF is produced. The manufacturing method according to claim 8, comprising preparing the first Si-containing particles and the second Si-containing particles.

10. Positive electrode and, The negative electrode and, Electrolytes, A secondary battery equipped with, A secondary battery wherein the negative electrode is the negative electrode described in claim 1.

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