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

Abstract

To provide a technology that suppresses the decrease in capacity retention rate when a negative electrode containing Si-containing particles undergoes repeated charging and discharging. [Solution] In the negative electrode of the secondary battery of this disclosure, the negative electrode active material layer includes first Si-containing particles and second Si-containing particles. When the expansion rate S2 of the second Si-containing particles after charging A compared to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.9. The first Si-containing particles and the second Si-containing particles are provided with a LiF coating. The first peak intensity ratio is defined as 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. The second peak intensity ratio is defined as 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. The second peak intensity ratio is greater than the first peak intensity ratio.

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 active material for an electrochemical element. This negative electrode active material contains a carbon material, silicon, and lithium fluoride (LiF). The negative electrode active material consists of a carbon phase made of the carbon material and Si-LiF mixed particles. The Si-LiF mixed particles are dispersed in the carbon phase in a uniform or non-uniform distribution. The publication states that by using a negative electrode active material with such a configuration, initial efficiency can be increased, volume expansion can be suppressed, irreversible phenomena can be reduced, and conductivity can be improved, as well as uniform dispersion of active material particles within the electrode can be achieved. Furthermore, it states that the battery life characteristics are improved in a battery containing a negative electrode with such a configuration. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Special Publication No. 2019-522886 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, Si-containing particles have the property of expanding and contracting significantly during charging and discharging. Secondary batteries using Si-containing particles have the problem that the negative electrode expands and contracts repeatedly during repeated charging and discharging, leading to a decrease in cycle performance. Therefore, there is a need to develop a negative electrode that suppresses the decrease in capacity retention rate during repeated charging and discharging.

[0006] In view of the above circumstances, this disclosure aims to suppress the decrease in capacity retention rate when a negative electrode containing Si particles is subjected to repeated charging and discharging. [Means for solving the problem]

[0007] The negative electrode of the secondary battery of this disclosure comprises a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is supported by the negative electrode current collector. The negative electrode active material layer contains first Si-containing particles and second Si-containing particles as negative electrode active material. When the expansion rate S2 of the second Si-containing particles after charge A compared to before charge A is set to 1, the expansion rate S1 of the first Si-containing particles after charge A compared to before charge A is greater than 0.3 and less than or equal to 0.9. Charging A is a constant current charge performed at a current value of 0.01C up to 4.2V in a 25°C environment, followed by constant voltage charging until the current value is 0.005C. The first Si-containing particles are coated with a LiF film. The second Si-containing particles are coated with a LiF film. In the XPS spectrum of the first Si-containing particles 12 measured by XPS, the ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF is defined as the first peak intensity ratio. In the XPS spectrum of the second Si-containing particle 14, 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. In this case, the second peak intensity ratio is greater than the first peak intensity ratio. With this configuration, it is possible to suppress the decrease in capacity retention rate when the negative electrode containing Si-containing particles is repeatedly charged and discharged.

[0008] From another perspective, the method for manufacturing the negative electrode of the secondary battery of this disclosure includes the steps of: preparing first Si-containing particles and second Si-containing particles as negative electrode active material; mixing the first Si-containing particles and second Si-containing particles in a dispersion medium to prepare a negative electrode paste; coating the negative electrode paste onto a negative electrode current collector; and drying the coated negative electrode paste. When the expansion rate S2 of the second Si-containing particles after charging A compared to before charging A is 1, the expansion rate S1 of the first Si-containing particles after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.9. Charging A is a constant current charge performed at a current value of 0.01C up to 4.2V in a 25°C environment, followed by constant voltage charging until the current value is 0.005C. The first Si-containing particles are coated with a LiF film. The second Si-containing particles are coated with a LiF film. In the XPS spectrum of the first Si-containing particle 12, measured by XPS, the ratio of the peak intensity of F for LiF to the peak intensity of F other than LiF is defined as the first peak intensity ratio. In the XPS spectrum of the second Si-containing particle 14, 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. In this case, the second peak intensity ratio is greater than the first peak intensity ratio. With this configuration, it is possible to provide a negative electrode containing Si-containing particles that can suppress the decrease in capacity retention rate when repeated charging and discharging is performed.

[0009] 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 decrease in capacity retention rate when the negative electrode containing Si particles is repeatedly charged and discharged. [Brief explanation of the drawing]

[0010] [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 particles contained in the negative electrode active material layer of the negative electrode. [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] Figure 3 is a schematic exploded view showing the configuration of the wound electrode body of a lithium-ion secondary battery. [Modes for carrying out the invention]

[0011] Embodiments relating to this disclosure will be described below with reference to the drawings. Matters not mentioned herein but necessary for the implementation of this disclosure can be understood as design matters for those skilled in the art based on prior art. This disclosure can be implemented based on the contents disclosed herein and common technical knowledge in the art. In the following drawings, members and parts that perform the same function are denoted by the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect actual dimensional relationships. In this specification, the numerical range expressed as "A~B" includes A and B.

[0012] In this specification, "secondary battery" refers to an energy storage device that can be repeatedly charged and discharged. Furthermore, in this specification, "lithium-ion secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and achieves charging and discharging through the transfer of charge associated with lithium ions between the positive and negative electrodes.

[0013] The negative electrode disclosed herein is used in secondary batteries, and preferably in lithium-ion secondary batteries. One embodiment of the negative electrode disclosed herein will be described with reference to Figure 1. Figure 1 is a schematic cross-sectional view showing an example of the negative electrode 60 according to this embodiment, and is a cross-sectional view along the thickness direction and the width direction. The negative electrode 60 according to this embodiment shown in Figure 1 is the negative electrode of a lithium-ion secondary battery.

[0014] As shown in the figure, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 supported by the negative electrode current collector 62. In other words, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 provided on the negative electrode current collector 62. The negative electrode active material layer 64 may be provided only on one side of the negative electrode current collector 62, or may be provided on both sides of the negative electrode current collector 62 as in the illustrated example. It is preferable that the negative electrode active material layer 64 is provided on both sides of the negative electrode current collector 62.

[0015] As in the illustrated example, a non-formed portion 62a of the negative electrode active material layer where the negative electrode active material layer 64 is not provided may be provided at one end in the width direction of the negative electrode 60. In the non-formed portion 62a of the negative electrode active material layer, the negative electrode current collector 62 is exposed, and the non-formed portion 62a of the negative electrode active material layer can function as a current collecting portion. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0016] In the illustrated example, the shape of the negative electrode current collector 62 is in the form of a foil (or sheet), but it is not limited to this. The negative electrode current collector 62 may be in various forms such as a rod shape, a plate shape, a mesh shape, etc. As the material of the negative electrode current collector 62, a metal with good conductivity (for example, copper, nickel, titanium, stainless steel, etc.) can be used as in a conventional lithium ion secondary battery, and among them, copper is preferable. As the negative electrode current collector 62, a copper foil is particularly preferable.

[0017] The dimensions of the negative electrode current collector 62 are not particularly limited and may be appropriately determined according to the battery design. When a copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but for example, it is 5 μm or more and 35 μm or less, and preferably 6 μm or more and 20 μm or less. <000009​​​​​In the case of a negative electrode active material containing Si, for example, the volume change accompanying charge and discharge of a secondary battery is large. Therefore, due to the expansion when the secondary battery is charged, the SEI film existing on the surface of the negative electrode active material may be damaged. When the SEI film is damaged, the surface of the negative electrode active material is exposed, and decomposition of the non-aqueous electrolyte occurs at this exposed portion, and the SEI film is formed. When the charge and discharge of the secondary battery are repeated, this damage to the SEI film and the formation of the SEI film at the damaged portion (the exposed portion of the negative electrode active material) repeatedly occur. This can be one of the reasons for the decrease in the capacity retention rate of the secondary battery when charge and discharge are repeated.

[0020] Therefore, the inventor conceived of providing a LiF film in advance on the surface of the negative electrode active material containing Si. The LiF film has, for example, the same function as the SEI film, while being able to follow the volume change of the negative electrode active material. Therefore, the inventor considered that by providing a LiF film on the surface of the negative electrode active material in advance, even when the charge and discharge of the secondary battery are repeated, damage to the LiF film is less likely to occur, and thus exposure of the surface of the negative electrode active material can be suppressed. The inventor considered that thereby, formation of the SEI film at the exposed portion of this negative electrode active material can be suppressed, and consequently, a decrease in the capacity retention rate of the secondary battery can be suppressed.

[0021] Furthermore, the inventor considered that, for example, even when the negative electrode active material expands and contracts during charge and discharge of the secondary battery, by maintaining the conductive path in the negative electrode active material layer in a suitable state, a decrease in the capacity retention rate of the secondary battery can be better suppressed. Therefore, the inventor decided to include two types of negative electrode active materials (here, both are negative electrode active materials containing Si) having different expansion rates during charging in the negative electrode active material layer. When two types of negative electrode active materials having different expansion rates are included in the negative electrode active material layer, the filling property of the negative electrode active material layer can be enhanced even when both expand and contract during charge and discharge of the secondary battery. The inventor considered that by forming a suitable amount of LiF film on the surface of each of the two types of negative electrode active materials having different expansion rates during charging, a decrease in the capacity retention rate can be better suppressed even when the charge and discharge of the secondary battery are repeated.

[0022] The negative electrode active material layer 64 contains a negative electrode active material. For example, first Si-containing particles and second Si-containing particles are used as the negative electrode active material. This will be explained using Figure 2. Figure 2 is a schematic cross-sectional view showing the particles contained in the negative electrode active material layer 64 shown in Figure 1. As shown in Figure 2, the negative electrode active material layer 64 contains first Si-containing particles 12 and second Si-containing particles 14. 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.

[0023] As shown in Figure 2, the first Si-containing particle 12 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, disposed on at least a portion of the surface of the core particle 121. As shown in Figure 2, the second Si-containing particle 14 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, disposed 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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. The F peaks other than LiF are F peaks outside the range of 683eV to 686eV, and may appear particularly in the range of 687eV to 690eV.

[0029] Here, 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 12 measured by XPS is defined as the first peak intensity ratio. 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 14 is defined as the second peak intensity ratio. Here, the second peak intensity ratio is greater than the first peak intensity ratio. The second peak intensity ratio is, for example, 1.1 to 3 times, preferably 1.2 to 2.5 times, the first peak intensity ratio.

[0030] The first peak intensity ratio is, for example, 0.200 or higher, preferably 0.250 or higher, and more preferably 0.300 or higher. On the other hand, the first peak intensity ratio is, for example, less than 0.450, preferably 0.400 or lower, and more preferably 0.380 or lower. The second peak intensity ratio is, for example, 0.450 or higher, preferably 0.480 or higher, and more preferably 0.500 or higher. On the other hand, the second peak intensity ratio is generally 1.00 or lower, 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.

[0031] The presence of the coating 122 on the core particle 121 and the presence of the coating 142 on the core particle 141 can be confirmed, for example, by electron microscope 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 142 on the surface of the core particle 141 may be greater than, for example, the coverage rate of the coating 122 on the surface of the core particle 121. The coverage rate of the coating 122 or the coating 142 can be determined as follows: Obtain an SEM observation image of the cross-section of the first Si-containing particle 12 or the second Si-containing particle 14. In the image, 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.

[0032] When the expansion rate S2 of the second Si-containing particle 14 is set to 1, the expansion rate S1 of the first Si-containing particle 12 is, for example, greater than 0.3 and less than or equal to 0.9, and from the viewpoint of realizing the effects of the technology disclosed herein, 0.4 to 0.8 is preferred. Here, the expansion rate S1 and the expansion rate S2 refer to the volume expansion rate after charging A compared to before charging A. The expansion rate S1 and the expansion rate S2 can be appropriately changed, for example, by changing the Si content ratio in the first Si-containing particle 12 and the second Si-containing particle 14, the amount of voids in the carbon material, etc.

[0033] Charging A, in this context, is a constant current charge performed at a current value of 0.01C up to 4.2V in a 25°C environment, followed by constant voltage charging until the current value becomes 0.005C. Charging A is performed on an evaluation secondary battery (e.g., a lithium-ion secondary battery) equipped with a negative electrode containing only first Si-containing particles or second Si-containing particles as the negative electrode active material. When performing Charging A, it is preferable that the evaluation secondary battery is in a state of charge (SOC) of 0%. Although not particularly limited, it is preferable that Charging A is performed, for example, before the initial charging of the evaluation secondary battery. Such an evaluation secondary battery may have the same configuration as lithium-ion secondary battery 100 (see Figure 3), except that the negative electrode active material layer contains only first Si-containing particles or second Si-containing particles as the negative electrode active material.

[0034] The expansion rate S1 can be determined, for example, as follows. First, a cross-section along the thickness direction of the negative electrode active material layer containing only first Si-containing particles as the negative electrode active material is processed for observation with an electron microscope (SEM). This processing is carried out using a cross-section polisher (CP) under the conditions of a voltage of 4kV and a processing time of 8 hours. Next, an SEM image of the processed surface is obtained. The observation magnification at this time should be set to, for example, 1000x to 4000x. Next, an evaluation secondary battery equipped with this negative electrode active material layer is constructed and charged A is performed on it. Next, the evaluation secondary battery is disassembled and the negative electrode is removed. Next, an SEM observation is performed on the processed surface for which an SEM image was obtained before charging A, and an SEM image is obtained after charging A. Then, using image analysis software (for example, "ImageJ"), the area P1 of 100 first Si-containing particles is measured randomly from the SEM image before charging A, and the area P2 of the same first Si-containing particles is measured from the SEM image after charging A. Then, for each particle, the following equation (A): Expansion rate (%) = [√{(Area P2) / (Area P1)}] 3 ×100 (A) The expansion coefficient is calculated using [a specific method / tool]. The arithmetic mean of the expansion coefficients of the 100 particles obtained here is calculated and designated as the expansion coefficient S1 of the first Si-containing particle.

[0035] The expansion rate S1 is, for example, smaller than the expansion rate S2. The expansion rate S1 is, for example, 100% or more, and preferably less than 200%. Although not particularly limited, the expansion rate S1 may be 110% or more, 120% or more, 190% or less, or 180% or less.

[0036] To determine the expansion coefficient S2, first, a cross-section along the thickness direction of the negative electrode active material layer containing only secondary Si-containing particles is prepared for observation by SEM. The expansion coefficient S2 is determined, for example, by measuring the area Q1 of 100 random secondary Si-containing particles from the SEM observation image before charge A, and then measuring the area Q2 of the same secondary Si-containing particles from the SEM observation image after charge A. Then, for each particle, the following equation (B) is used: Expansion rate (%)=[√{(Area Q2) / (Area Q1)}] 3 ×100 (B) The expansion coefficient is calculated using [a specific method]. The arithmetic mean of the expansion coefficients of the 100 particles obtained here is calculated and designated as the expansion coefficient S2 of the second Si-containing particle. Note that the procedure for determining the expansion coefficient S2 may be the same as the procedure for determining the expansion coefficient S1. Therefore, the explanation of the procedure for determining the expansion coefficient S2 is omitted as appropriate.

[0037] The expansion rate S2 is, for example, greater than the expansion rate S1. The expansion rate S2 is, for example, 200% or more, and preferably 400% or less. Although not particularly limited, the expansion rate S2 may be 210% or more, or 380% or less, 360% or less, or 350% or less.

[0038] 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.

[0039] The Si content in the first Si-containing particle 12 and the Si content in the second Si-containing particle 14 may be the same or different. While not particularly limited, it is preferable that the Si content in the first Si-containing particle 12 and the Si content in the second Si-containing particle 14 be approximately 20% by mass to 80% by mass.

[0040] The mass ratio of the first Si-containing particles 12 and the second Si-containing particles 14 in the negative electrode active material layer 64 may be the same or different. The mass ratio (first Si-containing particles 12: second Si-containing particles 14) is not particularly limited as long as the effects of the disclosed technology are obtained, and is, for example, 5:95 to 95:5. The mass ratio (first Si-containing particles 12: second Si-containing particles 14) is preferably 10:90 to 90:10, and more preferably 15:85 to 85:15. When the mass ratio of the first Si-containing particles 12 and the second Si-containing particles 14 in the negative electrode active material layer 64 are different, it is preferable that the mass ratio of the second Si-containing particles 14 be relatively small from the viewpoint of more favorably realizing the effects of the disclosed technology. In this case, when the total of the first Si-containing particles 12 and the second Si-containing particles 14 is 100% by mass, the proportion of the second Si-containing particles 14 is preferably 5% to 40% by mass, and more preferably 10% to 30% by mass.

[0041] 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).

[0042] As shown in Figure 3, the negative electrode active material layer 64 may contain graphite particles 16 as the negative electrode active material, from the viewpoint of improving conductivity at the negative electrode 60. The graphite particles 16 are, for example, substantially free of Si. The Si content in the graphite particles 16 is generally 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, for example, by conventionally known methods such as ICP analysis.

[0043] The graphite constituting the graphite particles 16 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.

[0044] The shape of the graphite particles 16 is not particularly limited and may be flaky, spherical, or the like. Preferably, the graphite particles 16 are spheroidized graphite particles. When the graphite particles 16 are spherical, the circularity of the graphite particles 16 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0045] 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.

[0046] The average particle size (D50) of the graphite particles 16 is not particularly limited. The average particle size (D50) of the graphite particles 16 is, 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.

[0047] The ratio of graphite particles 16 to the total of first Si-containing particles 12, second Si-containing particles 14, and graphite particles 16 is preferably 40% to 90% by mass, more preferably 45% to 85% by mass, and even more preferably 50% to 80% by mass. The ratio of the total of first Si-containing particles 12 and second Si-containing particles 14 to the total of first Si-containing particles 12, second Si-containing particles 14, and graphite particles 16 is preferably 10% to 60% by mass, more preferably 15% to 55% by mass, and even more preferably 20% to 50% by mass.

[0048] The negative electrode active material 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.

[0049] The content of the negative electrode active material in the negative electrode active material layer 64 (i.e., relative to the total mass of the negative electrode active material layer 64) is preferably 90% by mass or more, and more preferably 95% by mass or more. The content of the binder in the negative electrode active material layer 64 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.

[0050] 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.

[0051] 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.

[0052] 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 contains first Si-containing particles 12 and second Si-containing particles 14 as negative electrode active material. When the expansion rate S2 of the second Si-containing particles 14 after charging A compared to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles 12 after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.9. Charging A is a constant current charge performed at a current value of 0.01C up to 4.2V in a 25°C environment, followed by constant voltage charging until the current value becomes 0.005C.

[0053] 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. In this case, the second peak intensity ratio is greater than the first peak intensity ratio.

[0054] In other words, in the negative electrode 60, the negative electrode active material layer 64 contains first Si-containing particles 12 with a relatively low expansion rate and second Si-containing particles 14 with a relatively high expansion rate. The expansion rate of the first Si-containing particles 12 is set to be greater than 0.3 and less than or equal to 0.9 relative to the expansion rate of the second Si-containing particles 14. This suppresses a decrease in the packing density of the negative electrode active material in the negative electrode active material layer 64 even when the negative electrode active material expands and contracts with charging and discharging, thereby maintaining the conductive path in a suitable state. In addition, both the first Si-containing particles 12 and the second Si-containing particles 14 have a LiF coating. The second peak intensity ratio in the second Si-containing particles 14, which have a relatively high expansion rate, is greater than the first peak intensity ratio in the first Si-containing particles 12, which have a relatively low expansion rate. In other words, the amount of LiF coating is greater in the second Si-containing particles 14, which have a higher expansion rate than the first Si-containing particles 12, than in the first Si-containing particles 12. By applying a LiF coating in accordance with the expansion rate, for example, surface exposure due to expansion can be appropriately suppressed. Therefore, with this configuration, the decrease in capacity retention rate when repeated charging and discharging is suppressed.

[0055] The first 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 first Si-containing particles 12 due to expansion during charging.

[0056] The second peak intensity ratio may be between 0.450 and 1.00. This effectively suppresses the formation of exposed areas on the surface of the second Si-containing particles 14 due to expansion during charging. Furthermore, it suppresses the excessive increase in resistance at the negative electrode 60 due to the formation of the LiF film.

[0057] The first Si-containing particle 12 may include a Si-C composite particle (here, 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 portion of the surface of the Si-C composite particle (here, core particle 121). The second Si-containing particle 14 may include a Si-C composite particle (here, 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 portion of the surface of the Si-C composite particle (here, 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.

[0058] The negative electrode active material layer 64 may further contain graphite particles that are substantially free of Si as the negative electrode active material. This can improve the conductivity of the negative electrode active material layer 64.

[0059] The negative electrode 60 can be manufactured, for example, by the following manufacturing method. The manufacturing method for the negative electrode 60 includes, for example, a step of preparing first Si-containing particles 12 and second Si-containing particles 14 (hereinafter also referred to as the "preparation step"); a step of mixing the first Si-containing particles 12, the second Si-containing particles 14, and graphite particles 16 as needed in a dispersion medium to prepare a negative electrode paste (hereinafter also referred to as the "paste preparation step"); a step of coating the negative electrode paste onto a negative electrode current collector (hereinafter also referred to as the "coating step"); and a step of drying the coated negative electrode paste (hereinafter also referred to as the "drying step"). By implementing such a manufacturing method, it is possible to provide a negative electrode 60 that can suppress the decrease in capacity retention rate when repeated charging and discharging is performed.

[0060] 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.

[0061] In the preparation step, a first Si-containing particle 12 comprising a Si-containing particle as a core particle 121 and a coating 122, and a second Si-containing particle 14 comprising a Si-containing particle as a core particle 141 and a coating 142 are prepared. The Si-containing particle as core particle 121 and the Si-containing particle as core particle 141 are, for example, Si-C composite particles. Si-C composite particles can be prepared, for example, using known methods as described above.

[0062] Next, the obtained core particles 121 and core particles 141 are dispersed in water or a water-soluble organic solvent, and the Si-C composite particles 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 core particle 121 and a coating 142 on the surface of core particle 141.

[0063] 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.

[0064] In this operation, for example, first, an aqueous solution of a water-soluble lithium salt, a dispersion of core particles 121 or core particles 141 in water or a water-soluble organic solvent, and an aqueous solution of a fluorinating agent are prepared. The aqueous solution of the water-soluble lithium salt and the dispersion are mixed to prepare a mixture. Under stirring, the aqueous solution of the fluorinating agent is added to the mixture. This causes the water-soluble lithium salt and the fluorinating agent to react and produce LiF. This yields a reaction solution containing LiF. In this reaction solution, the surface of core particles 121 or core particles 141 can be brought into contact with LiF.

[0065] The reaction conditions for the water-soluble lithium salt and fluorinating agent may be the same as those for the known synthesis of LiF using a 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 ratio of LiF in coatings 122 and 142, etc. The longer the reaction time, the higher the content ratio of LiF in coatings 122 and 142.

[0066] Core particles 121 or core particles 141 are recovered from the reaction solution with the reaction solution containing LiF adhering to their surfaces. By drying the core particles 121 or core particles 141 with the reaction solution containing LiF adhering to them, LiF can be attached to the surface of the core particles 121 or core particles 141, thereby obtaining first Si-containing particles 12 or second Si-containing particles 14 having a LiF-containing coating 122 or coating 142. The reaction solution containing LiF may be diluted with water or the like to adjust the amount of LiF.

[0067] The paste preparation process can be carried out by mixing the first Si-containing particles 12, the second Si-containing particles 14, graphite particles 16 if necessary, and optional components (e.g., binder) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method.

[0068] The coating process can be carried out according to known methods. For example, the coating process can be performed by coating the obtained negative electrode paste onto the negative electrode current collector 62 using a coating device such as a gravure coater, comma coater, slit coater, or die coater.

[0069] The drying process can be carried out according to a known method. For example, the negative electrode active material layer 64 can be formed by removing the dispersion medium from the negative electrode current collector 62 coated with the negative electrode paste using a drying apparatus such as a drying oven. This allows the drying process to be carried out. The drying temperature and drying time can be appropriately determined according to the solid content concentration of the negative electrode paste and are not particularly limited. The drying temperature is, for example, 60°C to 200°C, preferably 70°C to 150°C. The drying time is, for example, 10 seconds to 30 minutes, preferably 30 seconds to 10 minutes.

[0070] After the drying process, a further step of pressing the negative electrode active material layer 64 may be performed. The pressing process can be carried out according to a known method. For example, the pressing process can be performed by applying pressure to the formed negative electrode active material layer 64 using a roller press or the like. The pressing process can densely pack the first Si-containing particles 12, the second Si-containing particles 14, and, if necessary, the graphite particles 16 contained in the negative electrode active material layer 64. In this way, the negative electrode 60 can be obtained.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] The negative electrode sheet 60 used is the negative electrode 60 described above.

[0086] 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.

[0087] 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.

[0088] A non-aqueous electrolyte contains, for example, 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, and specific examples include 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. As an example, the non-aqueous solvent may consist solely of carbonates. As another example, non-aqueous solvents include carbonates and esters such as methyl acetate.

[0089] 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.

[0090] 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.

[0091] 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 be used, for example, in the form of a battery pack formed by connecting a plurality of them in series and / or in parallel.

[0092] As described above, as an example, the square lithium-ion secondary battery 100 including the flat wound electrode body 20 has been described. However, the lithium-ion secondary battery can also be configured as a lithium-ion secondary battery including a 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] <Production of Si-C Composite Particles Having a Film Containing LiF> Si-C composite particles were prepared as core particles. These Si-C composite particles consisted of a porous carbon substrate and Si nanoparticles arranged within the pores of the carbon substrate. The Si-C composite particles were dispersed in ethyl alcohol while being sonicated for 30 minutes. A lithium acetate aqueous solution was prepared by dissolving lithium acetate in deionized water. The dispersion of Si-C composite particles was added to this aqueous solution while stirring for 30 minutes. Then, an aqueous solution of ammonium fluoride was added while stirring for a predetermined time to react the lithium acetate with ammonium fluoride. This yielded a reaction solution containing LiF. After diluting the reaction solution, it was filtered. The filtrate was dried to obtain Si-C composite particles having a LiF-containing coating.

[0097] XPS spectra were measured on the obtained Si-C composite particles, revealing peaks for Li-F (approximately 684.8 eV) and Li-F (approximately 55.1 eV), confirming the presence of LiF. The ratio of the peak intensity of LiF's F to the peak intensity of other F components (hereinafter also referred to as the "LiF intensity ratio") was also determined. The LiF's F peak appeared between 683 eV and 686 eV, while the peaks of other F components appeared between 687 eV and 690 eV.

[0098] Si-containing particles (A) to (J) were obtained by the above procedure. Si-containing particle (A) had an expansion coefficient of 153% and a LiF intensity ratio of 0.362. Si-containing particle (B) had an expansion coefficient of 258% and a LiF intensity ratio of 0.629. Si-containing particle (C) had an expansion coefficient of 126% and a LiF intensity ratio of 0.308. Si-containing particle (D) had an expansion coefficient of 339% and a LiF intensity ratio of 0.729. Si-containing particle (E) had an expansion coefficient of 173% and a LiF intensity ratio of 0.373. Si-containing particle (F) had an expansion coefficient of 228% and a LiF intensity ratio of 0.514. Si-containing particle (G) had an expansion coefficient of 121% and a LiF intensity ratio of 0.301. Si-containing particle (H) had an expansion coefficient of 416% and a LiF intensity ratio of 1.280. Si-containing particle (I) had an expansion coefficient of 183% and a LiF intensity ratio of 0.382. Si-containing particle (J) had an expansion coefficient of 187% and a LiF intensity ratio of 0.502. The LiF intensity ratio of each Si-containing particle was varied by appropriately changing the addition time of the ammonium fluoride aqueous solution (i.e., the reaction time). Table 1 shows the presence or absence of a LiF coating and the LiF intensity ratio of the Si-containing particles in each example.

[0099] In addition, two types of Si-C composite particles, Si-containing particles (X) and Si-containing particles (Y), which do not have a LiF coating, were prepared. Si-containing particles (X) had an expansion coefficient of 153%. Si-containing particles (Y) had an expansion coefficient of 258%.

[0100] <Measurement of expansion rate> The expansion rate of the Si-containing particles was obtained by the following procedure. An anode with the same configuration as in this example, except that only each Si-containing particle was used as the anode active material, was fabricated using the same procedure as in this example. A cross-section along the thickness direction of this anode was processed for SEM observation, and an SEM image of the processed surface was obtained. The observation magnification at this time was 1,500x. Next, a measurement battery, which is a lithium-ion secondary battery with the same configuration as in this example except that this anode was used, was fabricated using the same procedure as in this example. The measurement battery was placed in a 25°C environment and charged with a constant current of 0.01C up to 4.2V, and then charged with a constant voltage until the current value was 0.005C. After that, the measurement battery was disassembled and the anode was removed. Then, the processed surface, from which an SEM image was obtained before charging, was observed with an SEM, and an SEM image was obtained after charging. Then, using the image analysis software "ImageJ", 100 first particles were randomly selected from the SEM image before charging, and the area A1 of the selected particles was measured. Next, the area A2 of the same first particle was measured in the SEM image after charging. Then, for each particle, the following equation (R): Expansion rate (%)=[√{(Area A2) / (Area A1)}] 3 ×100 (R) The expansion coefficient was calculated using [a specific method / tool]. The arithmetic mean of the expansion coefficients of the 100 particles obtained was calculated and used as the expansion coefficient for the Si-containing particles.

[0101] <Fabrication of the negative electrode> [Example 1] As negative electrode active materials, the above-mentioned Si-containing particles (A) as the first particles, Si-containing particles (B) as the second particles, and graphite particles with an average particle diameter (D50) of 15 μm were prepared. These graphite particles were substantially Si-free. A dispersion of single-walled carbon nanotubes (SWCNTs) was prepared as a conductive material. Carboxymethylcellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared as binders.

[0102] The materials described above were mixed with water as a solvent to prepare a negative electrode paste, with a mass ratio of graphite particles / first particle / second particle / SWCNT / CMC / PAA / SBR = 65 / 28 / 7 / 0.1 / 1 / 1 / 1.5.

[0103] The negative electrode paste was prepared by performing the following two steps. In the first step, the first and second particles, paste-like SWCNT (solid content 2%), and a dispersion medium were first placed in a kneader and dispersed and mixed at 3000 rpm using a disperser to prepare the first paste. In the second step, graphite particles, CMC, and PAA were dry-mixed using a stirring granulator. The first paste, the mixed powder obtained by dry mixing, and the dispersion medium (water) were then 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 way, the negative electrode paste was prepared.

[0104] The prepared negative electrode paste was applied to the surface of a 10 μm thick copper foil and dried to form a negative electrode active material layer. After roll pressing the negative electrode active material layer, the resulting sheet was processed to a predetermined size to obtain the negative electrode sheet of this example.

[0105] [Example 2] Si-containing particles (C) were used as the first particles. Si-containing particles (D) were used as the second particles. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0106] [Example 3] Si-containing particles (E) were used as the first particles. Si-containing particles (F) were used as the second particles. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0107] [Comparative Example 1] Si-containing particles (X) were used as the first particles. Si-containing particles (Y) were used as the second particles. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0108] [Comparative Example 2] As the first particles, Si-containing particles (G) were used. As the second particles, Si-containing particles (H) were used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0109] [Comparative Example 3] As the first particles, Si-containing particles (I) were used. As the second particles, Si-containing particles (J) were used. Otherwise, the same materials and procedures as in Example 1 were used to obtain the negative electrode sheet of this example.

[0110] [Fabrication of Lithium-Ion Secondary Battery for Evaluation] LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed with N-methylpyrrolidone (NMP) at a mass ratio of NCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. This paste was applied to the surface of an aluminum foil with a thickness of 15 μm and dried to form a positive electrode active material layer. After roll-pressing the positive electrode active material layer, the obtained sheet was processed into a predetermined size to obtain a positive electrode sheet.

[0111] A separator made of porous polyolefin was prepared. Leads were attached to each of the above-prepared negative electrode sheet and positive electrode sheet, and they were laminated through the separator to fabricate an electrode body. This was housed together with a non-aqueous electrolyte in a case made of an aluminum laminate film. The non-aqueous electrolyte used was a mixture solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 15:5:40:40, in which LiPF6 as a supporting salt was dissolved at a concentration of 1.0 mol / L. Then, the case was sealed to obtain a lithium-ion secondary battery for evaluation.

[0112] [Cycle Performance Evaluation] Each of the lithium-ion secondary batteries prepared for evaluation was placed in a 25°C environment. 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. Next, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.4C up to 2.5V. The discharge capacity at this time was measured to determine the initial capacity.

[0113] The above charge-discharge cycle was repeated 200 times, with each cycle being considered one. The discharge capacity after 200 cycles was determined using the same method as the initial capacity. As an indicator of cycle characteristics, the capacity retention rate (%) was calculated using the formula: (discharge capacity after 200 charge-discharge cycles / initial capacity) × 100. The initial capacity was the discharge capacity of the first cycle of the above charge-discharge process. A capacity retention rate closer to 100% is considered good, and a rate of 80% or higher was evaluated as suppressing the decrease in capacity retention rate after charge-discharge cycles.

[0114] [Table 1]

[0115] As shown in Table 1, in Examples 1 to 3, the decrease in capacity retention rate when charging and discharging was repeated was suppressed. In Examples 1 to 3, as described above, when the expansion rate of the second particle after charging A compared to before charging A was set to 1, the expansion rate of the first particle after charging A compared to before charging A was greater than 0.3 and less than or equal to 0.9. The first particle and the second particle were equipped with a LiF coating. 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 greater 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.

[0116] 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.

[0117] 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 [8]. [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 aforementioned negative electrode active material layer contains first Si-containing particles and second Si-containing particles as the negative electrode active material. Under a 25°C environment, after constant current charging at a current value of 0.01C up to 4.2V, and then constant voltage charging until the current value is 0.005C, the expansion rate S2 of the second Si-containing particles after charging A compared to before charging A is set to 1. Then, the expansion rate S1 of the first Si-containing particles after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.9. 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 has a second peak intensity ratio that is greater than the first peak intensity ratio. [2] The negative electrode according to [1], wherein the first peak intensity ratio is 0.200 or more and less than 0.450. [3] The negative electrode according to [1] or [2], wherein the second peak intensity ratio is 0.450 or more and 1.00 or less. [4] The negative electrode according to any one of [1] to [3], 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. [5] The negative electrode according to any one of [1] to [4], wherein the negative electrode active material layer further comprises substantially Si-free graphite particles as the negative electrode active material. [6] A method for manufacturing the negative electrode of a secondary battery, A step to prepare first Si-containing particles and second Si-containing particles as negative electrode active materials. A step of preparing a negative electrode paste by mixing the first Si-containing particles and the second Si-containing particles in a dispersion medium. The process involves coating the negative electrode paste onto the negative electrode current collector, and A step of drying the coated negative electrode paste, It includes, Under a 25°C environment, after constant current charging at a current value of 0.01C up to 4.2V, and then constant voltage charging until the current value is 0.005C, the expansion rate S2 of the second Si-containing particles after charging A compared to before charging A is set to 1. Then, the expansion rate S1 of the first Si-containing particles after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.9. 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 greater than the first peak intensity ratio. [7] In the preparation process described above, 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 [6], comprising preparing the first Si-containing particles and the second Si-containing particles. [8] 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 [1]. [Explanation of Symbols]

[0118] 12 1st Si-containing particles 14 2nd Si-containing particles 16 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 aforementioned negative electrode active material layer contains first Si-containing particles and second Si-containing particles as the negative electrode active material. Under a 25°C environment, constant current charging is performed at a current value of 0.01C up to 4.2V, followed by constant voltage charging until the current value reaches 0.005C. When the expansion rate S2 of the second Si-containing particles after charging A compared to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.

9. 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 has a second peak intensity ratio that is greater than the first peak intensity ratio.

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

450.

3. The negative electrode according to claim 2, wherein the second peak intensity ratio is 0.450 or more and 1.00 or less.

4. The negative electrode according to any one of claims 1 to 3, 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.

5. The negative electrode according to any one of claims 1 to 3, wherein the negative electrode active material layer further contains substantially Si-free graphite particles as the negative electrode active material.

6. A method for manufacturing the negative electrode of a secondary battery, A step of preparing first Si-containing particles and second Si-containing particles as negative electrode active materials. A step of preparing a negative electrode paste by mixing the first Si-containing particles and the second Si-containing particles in a dispersion medium. The process involves coating the negative electrode paste onto the negative electrode current collector, and A step of drying the coated negative electrode paste, It includes, Under a 25°C environment, constant current charging is performed at a current value of 0.01C up to 4.2V, followed by constant voltage charging until the current value reaches 0.005C. When the expansion rate S2 of the second Si-containing particles after charging A compared to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles after charging A compared to before charging A is greater than 0.3 and less than or equal to 0.

9. 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 greater than the first peak intensity ratio.

7. In the aforementioned preparation process, 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 6, comprising preparing the first Si-containing particles and the second Si-containing particles.

8. 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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