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

Abstract

Provided are a negative electrode for a secondary battery, a method for manufacturing the negative electrode, and a secondary battery using the negative electrode. Provided is a technique for suppressing swelling of a negative electrode containing silicon-containing particles during repeated charging and discharging of the negative electrode. In a negative electrode of a secondary battery according to the present disclosure, a negative electrode active material layer includes a first layer on a negative electrode current collector side and a second layer on a surface layer side. The first layer includes first silicon-containing particles. The second layer includes second silicon-containing particles. When the expansion rate S1 of the first silicon-containing particles after charging A prior to charging A is 1, the expansion rate S2 of the second silicon-containing particles after charging A prior to charging A is greater than 0.3 and 0.9 or less. The first silicon-containing particles and the second silicon-containing particles include LiF coatings. A first peak intensity ratio of LiF for the first silicon-containing particles is greater than a second peak intensity ratio of LiF for the second silicon-containing particles (14). The difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.050.

Description

Technical Field

[0001] This disclosure relates to a negative electrode of 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, rechargeable batteries have been successfully used in portable power supplies for personal computers and mobile terminals, as well as in vehicle power supplies for battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] In applications for vehicle power supply, particularly for battery electric vehicles (BEVs), there is a desire for further increases in the capacity of secondary batteries, from the perspective of extending the vehicle's driving range. Silicon-containing particles are known as high-capacity negative electrode active materials, and it is known that high-capacity secondary batteries can be achieved using silicon-containing particles. Patent Document 1 discloses a negative electrode for a non-aqueous electrolyte secondary battery composed of a negative electrode, a positive electrode, and a non-aqueous electrolyte with lithium-ion conductivity. The negative electrode has a current collector and a negative electrode active material. The negative electrode active material has at least two types of active material composite particles with different particle size distributions, containing elemental silicon, silicon compounds, and carbon. The specific surface area of ​​the active material composite particles is 5 m². 2 / g or more and 50m 2 / g or less. The announcement describes how this configuration can improve charge-discharge cycle characteristics, suppress rapid capacity reduction during charge-discharge cycles, and improve high-speed characteristics.

[0004] Patent Document 2 discloses a composite particle comprising carbon material, silicon, and lithium fluoride (LiF), consisting of a carbon phase made of carbon material and a Si-LiF mixed particle. The Si-LiF mixed particle is dispersed uniformly or non-uniformly within the carbon phase. This publication describes how using a negative electrode containing this composite particle can improve the battery's lifespan characteristics.

[0005] Patent document 3 discloses a multilayer electrode comprising a conductive layer, a first composite electrode layer, and a second composite electrode layer in sequence. 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 material different from the main component of the active material in the second composite electrode layer. This publication describes how this configuration can improve both the battery's capacity retention and cycle life.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2010-33830

[0009] Patent Document 2: Japanese Patent Publication No. 2019-522886

[0010] Patent Document 3: Japanese Patent Publication No. 2015-537347 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] However, silicon-containing particles exhibit significant expansion and contraction during charging and discharging. In secondary batteries using silicon-containing particles, repeated charging and discharging causes the negative electrode to expand repeatedly, leading to increased internal stress. Therefore, it is desirable to develop a negative electrode with minimal expansion during repeated charging and discharging.

[0013] In view of the above actual situation, the purpose of this disclosure is to suppress the swelling of the negative electrode containing silicon particles during repeated charging and discharging.

[0014] Methods for solving problems

[0015] The negative electrode of the secondary battery disclosed herein includes 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 includes a first layer located on the side of the negative electrode current collector and a second layer located on the surface side. The first layer contains first silicon-containing particles as negative electrode active materials. The second layer contains second silicon-containing particles as negative electrode active materials. When the expansion rate S1 of the first silicon-containing particles after charging A relative to before charging A is set to 1, the expansion rate S2 of the second silicon-containing particles after charging A relative to before charging A is greater than 0.3 and less than 0.9. Herein, charging A is charging at 25°C with a constant current of 0.01C until 4.2V, followed by constant voltage charging until the current value becomes 0.005C. Both the first and second silicon-containing particles include 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 energy spectrum of the first silicon-containing particles, 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 energy spectrum of the second silicon-containing particle 14 is defined as the second peak intensity ratio. The first peak intensity ratio is larger than the second peak intensity ratio. The difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.050. According to this configuration, for a negative electrode containing silicon particles, the swelling of the negative electrode during repeated charge and discharge can be suppressed.

[0016] From another perspective, the method for manufacturing the negative electrode of the secondary battery disclosed herein includes: preparing a first paste comprising first silicon-containing particles as a negative electrode active material and a dispersion medium; preparing a second paste comprising second silicon-containing particles as a negative electrode active material and a dispersion medium; coating and drying the first paste onto a negative electrode current collector to form a dried film of the first paste; coating and drying the second paste onto the dried film of the first paste to form a dried film of the second paste; and pressing the dried film of the first paste and the dried film of the second paste together. When the expansion rate S1 of the first silicon-containing particles relative to before charge A and after charge A is set to 1, the expansion rate S2 of the second silicon-containing particles relative to before charge A and after charge A is greater than 0.3 and less than 0.9. Herein, charge A is a charge performed at 25°C using a constant current of 0.01C until 4.2V, followed by a constant voltage charge until the current becomes 0.005C. Both the first and second silicon-containing particles include 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 silicon-containing particle (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 silicon-containing particle 14 is defined as the second peak intensity ratio. The first peak intensity ratio is greater than the second peak intensity ratio. The difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.050. According to this configuration, for a negative electrode containing silicon particles, the swelling of the negative electrode during repeated charge and discharge can be suppressed. According to this configuration, a negative electrode containing silicon particles can be provided, which is a negative electrode that suppresses swelling during repeated charge and discharge.

[0017] From another perspective, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is as described above. According to this configuration, for a negative electrode containing silicon particles, it is possible to suppress the swelling of the negative electrode during repeated charging and discharging. Attached Figure Description

[0018] Figure 1 A cross-sectional view schematically illustrating the configuration of the negative electrode of a secondary battery according to one embodiment of the present disclosure.

[0019] Figure 2 To illustrate Figure 1 A cross-sectional view of the composition of particles of the negative electrode active material contained in the negative electrode active material layer.

[0020] Figure 3 A cross-sectional view schematically illustrating the structure of a lithium-ion secondary battery constructed using the negative electrode of a battery according to an embodiment of the present disclosure.

[0021] Figure 4 To indicate Figure 3 A schematic exploded view of the structure of the wound electrode body of a lithium-ion secondary battery. Detailed Implementation

[0022] The embodiments disclosed herein will be described below with reference to the accompanying drawings. It should be noted that any matters necessary for the implementation of this disclosure that are not mentioned in this specification can be grasped by those skilled in the art based on prior art in this field. This disclosure can be implemented based on the content disclosed in this specification and common technical knowledge in this field. Furthermore, in the following drawings, components and parts that perform the same function are indicated by the same reference numerals. Additionally, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships. It should be noted that in this specification, the numerical range denoted as "A to B" includes both A and B.

[0023] It should be noted that in this specification, the term "secondary battery" refers to an energy storage device capable of repeated charging and discharging. Furthermore, in this specification, the term "lithium-ion secondary battery" refers to a secondary battery that utilizes lithium ions as charge carriers and achieves charging and discharging through the movement of the accompanying charge between the positive and negative electrodes.

[0024] The negative electrode disclosed herein is used in secondary batteries, preferably in lithium-ion secondary batteries. (Refer to...) Figure 1 An embodiment of the negative electrode disclosed herein will be described in detail. Figure 1 The cross-sectional view shown schematically is 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. Figure 1 The negative electrode 60 shown in this embodiment is the negative electrode of a lithium-ion secondary battery.

[0025] 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 disposed on the negative electrode current collector 62. The negative electrode active material layer 64 may be disposed on only one side of the negative electrode current collector 62, or it may be disposed on both sides of the negative electrode current collector 62 as shown in the example. Preferably, the negative electrode active material layer 64 is disposed on both sides of the negative electrode current collector 62.

[0026] As shown in the example, a non-formed portion 62a of the negative electrode active material layer 64, where no negative electrode active material layer 64 is formed, can be provided at one end of the negative electrode 60 in the width direction. In the non-formed portion 62a, the negative electrode current collector 62 is exposed, and the non-formed portion 62a can function as a current collector. However, the configuration for collecting current from the negative electrode 60 is not limited to this.

[0027] In the example shown, the negative electrode current collector 62 is in the shape of a foil (or sheet), but is not limited to this. The negative electrode current collector 62 can be in various shapes such as rod-shaped, plate-shaped, or sieve-shaped. As for the material of the negative electrode current collector 62, similar to existing lithium-ion secondary batteries, a metal with good conductivity (such as copper, nickel, titanium, stainless steel, etc.) can be used, with copper being preferred. Copper foil is particularly preferred as the negative electrode current collector 62.

[0028] There is no particular limitation on the size of the negative electrode current collector 62, which can be appropriately determined according to the battery design. When copper foil is used as the negative electrode current collector 62, there is no particular limitation on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 6 μm or more and 20 μm or less.

[0029] However, if the 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 may be the main cause of the swelling of the negative electrode active material layer. If the negative electrode active material layer swells, the thickness of the negative electrode increases, which may, for example, become the main cause of the deterioration of battery performance, and is therefore undesirable. In the negative electrode active material layer, comparing the negative electrode current collector side and the surface side (the side opposite to the negative electrode current collector), a stronger swelling tendency is found on the surface side during repeated charging and discharging of the secondary battery. Therefore, the inventors believe that by making the negative electrode active material layer a multilayer structure and arranging a negative electrode active material layer with a smaller expansion rate on the surface side of the negative electrode active material layer, it is possible to suppress the swelling of the negative electrode during repeated charging and discharging of the secondary battery.

[0030] In Si-containing negative electrode active materials, for example, the volume changes significantly with the charging and discharging of a secondary battery. Therefore, due to the expansion during charging of the secondary battery, the SEI film present on the surface of the negative electrode active material is sometimes damaged. If the SEI film is damaged, the surface of the negative electrode active material is exposed, and in this exposed portion, the non-aqueous electrolyte decomposes, leading to the formation of another SEI film. If the secondary battery is repeatedly charged and discharged, the damage to the SEI film and the formation of the SEI film in the damaged portion (the exposed portion of the negative electrode active material) occur repeatedly. Thus, during repeated charging and discharging, unevenness in the reaction and stress within the negative electrode active material layer may become a major cause of the aforementioned negative electrode swelling. Therefore, the inventors considered providing a LiF film on the surface of the Si-containing negative electrode active material.

[0031] like Figure 1 As shown, the negative electrode active material layer 64 has a multi-layer structure. The negative electrode active material layer 64 here has a first layer 64a and a second layer 64b. Figure 1As shown, the first layer 64a is located on the side of the negative electrode current collector 62. The second layer 64b is located on the surface side (here, the opposite side of the negative electrode current collector 62). Furthermore, the negative electrode active material layer 64 may further have layers other than the first layer 64a and the second layer 64b, without significantly impairing 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, which mixes the components of these layers.

[0032] The negative electrode active material layer 64 contains a negative electrode active material. For it, use... Figure 2 illustrate. Figure 2 To indicate Figure 1 The diagram shows a schematic cross-sectional view of the particles of the negative electrode active material contained in the negative electrode active material layer 64. Furthermore, Figure 2 For illustrative purposes only; therefore, the number, distribution, etc., of particles are not limited to... Figure 2 The content shown.

[0033] like Figure 2 As shown, the first layer 64a contains first silicon-containing particles 12 as the negative electrode active material. The second layer 64b contains second silicon-containing particles 14 as the negative electrode active material.

[0034] In this embodiment, the expansion rate S2 of the second silicon-containing particle 14 is smaller than the expansion rate S1 of the first silicon-containing particle 12. When the expansion rate S1 of the first silicon-containing particle 12 is set to 1, the expansion rate S2 of the second silicon-containing particle 14 is, for example, greater than 0.3 and less than 0.9, and preferably 0.4 to 0.8 from the viewpoint of achieving the effect of the technology disclosed herein. Here, expansion rates S1 and S2 refer to the volume expansion rates of the first silicon-containing particle 12 or the second silicon-containing particle 14 relative to the time before charging A and after charging A. The expansion rates S1 and S2 can be appropriately changed, for example, by varying the Si content ratio in the first silicon-containing particle 12 and the second silicon-containing particle 14, the amount of porosity in the carbon substrate, etc.

[0035] Charging A here refers to charging at a constant current of 0.01C at 25°C until 4.2V, followed by constant voltage charging until the current reaches 0.005C. Charging A is implemented, for example, for an evaluation secondary battery (e.g., a lithium-ion secondary battery) including a negative electrode containing only first or second silicon particles as the negative electrode active material. During charging A, the evaluation secondary battery is preferably in a state of, for example, 0% SOC (state of charge). Although not particularly limited, charging A is preferably performed, for example, before the initial charging of the evaluation secondary battery. Furthermore, this evaluation secondary battery, except that the negative electrode active material layer contains only first or second silicon particles as the negative electrode active material, may have the same characteristics as the lithium-ion secondary battery 100 (see reference 100). Figure 3 They have the same composition.

[0036] The expansion rate S1 can be obtained, for example, as described below. First, a cross-section along the thickness direction of the negative electrode active material layer containing only the first silicon-containing particles as the negative electrode active material is processed for observation using an electron microscope (SEM). This processing is performed using a cross-section polishing (CP) machine under conditions of 4 kV voltage and 8 hours processing time. Next, an SEM image of the processed surface is obtained. The magnification at this time can be set, for example, to 1000x to 4000x. Next, an evaluation secondary battery including the negative electrode active material layer is constructed, and it is charged A. Next, the evaluation secondary battery is disassembled, and the negative electrode is removed. Next, SEM observation of the processed surface, for which an SEM image was obtained before charging A, is performed, and an SEM image after charging A is obtained. Then, using image analysis software (e.g., "ImageJ"), the area P1 of 100 first silicon-containing particles is randomly measured from the SEM image before charging A, and then the area P2 of the same first silicon-containing particles is measured from the SEM image after charging A. Then, for each particle, use the following equation (A):

[0037] Expansion rate (%) = [√{(Area P2) / (Area P1)}] 3 Calculate the expansion rate by multiplying by 100 (A). Calculate the arithmetic mean of the expansion rates of the 100 particles obtained here, and set it as the expansion rate S1 of the first silicon-containing particle.

[0038] The expansion rate S1 is, for example, 200% or more, preferably 400% or less. There is no particular limitation on the expansion rate, but the expansion rate S1 may be 210% or more, or 380% or less, 360% or less, or 350% or less.

[0039] To determine the expansion ratio S2, firstly, a cross-section along the thickness direction of the negative electrode active material layer containing only the second silicon-containing particles as the negative electrode active material is fabricated for SEM observation. For example, the expansion ratio S2 is determined by randomly measuring the area Q1 of 100 second silicon-containing particles from the SEM image before charging A, and then measuring the area Q2 of the same second silicon-containing particles from the SEM image after charging A. Then, for each particle, the following formula (B) is used:

[0040] Expansion rate (%) = [√{(Area Q2) / (Area Q1)}] 3 Calculate the expansion ratio by multiplying by 100(B). Calculate the arithmetic mean of the expansion ratios of the 100 particles obtained here, and denote it as the expansion ratio S2 of the second silicon-containing particle. It should be noted that the steps for calculating the expansion ratio S2 are the same as those for calculating the expansion ratio S1. Therefore, the explanation of the steps for calculating the expansion ratio S2 is omitted as appropriate.

[0041] The expansion rate S2 is, for example, 100% or more, preferably less than 200%. Although there is no particular limitation, the expansion rate S2 may be 110% or more, 120% or more, or 190% or less, or 180% or less.

[0042] The average particle size (D50) of the first silicon-containing particle 12 and the average particle size (D50) of the second silicon-containing particle 14 are not particularly limited, for example, they are 0.5 μm to 25 μm, preferably 1 μm to 20 μm, and more preferably 2 μm to 15 μm. The average particle size (D50) of the first silicon-containing particle 12 and the average particle size (D50) of the second silicon-containing particle 14 may be the same or different.

[0043] It should be noted that the term "average particle size (D50)" in this specification refers to the median diameter (D50), which, in a volume-based particle size distribution based on laser diffraction and scattering, is the particle size corresponding to 50% by volume of the cumulative frequency from the side of the smallest particle. The average particle size (D50) can be determined using commercially available laser diffraction and scattering particle size distribution measuring devices.

[0044] like Figure 2 As shown, the first silicon-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 silicon-containing particle. The coating 122 is disposed on at least a portion of the surface of the core particle 121. Figure 2 As shown, the second silicon-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 silicon-containing particle. The coating 142 is disposed on at least a portion of the surface of the core particle 141. The silicon-containing particles constituting the core particle 121 and the core particle 141 may contain Si, for example, Si particles, Si oxide particles, Si-C composite particles, etc. The core particle 121 and the core particle 141 may both be Si-C composite particles. The Si-C composite particles may, for example, have carbon domains and Si domains.

[0045] Carbon domains can be, for example, carbides of carbon precursors (e.g., petroleum asphalt, coal tar pitch, phenolic resins, etc.); graphite, etc. Preferably, the carbon domains constitute a carbon matrix. The Si-C composite particles can be, for example, particles containing Si domains dispersed within a carbon matrix. The Si-C composite particles can, for example, have multiple Si domains within the carbon matrix. In this case, the carbon matrix can mitigate the volume changes caused by the expansion and contraction of the Si domains, which is advantageous.

[0046] Si-containing domains contain Si, for example, Si, Si oxide (SiO2). x ), Si nitride (SiN) x ), Si carbide (SiC) x It is composed of, etc. The Si-containing domains are preferably composed of Si, and Si oxide (SiO2). xIt consists of at least one of the following: Si domains can be nanoparticles. The oxygen content in the Si domains is preferably less than 10% by mass.

[0047] The average particle size of the Si domains is, for example, less than 50 nm, and can be between 5 nm and 50 nm. It should be noted that the "average particle size of the Si domains" can be determined as follows: First, the negative electrode active material layer 64 is processed using FIB (focused ion beam) to prepare a sample for observation by a scanning transmission electron microscope (STEM). Then, the sample is subjected to elemental analysis using EDX surface scanning, obtaining BF (bright field image) and HAADF (high angle scattering annular dark field image). The diameter of the Si domains can be determined from the contrast and shape obtained using the BF and HAADF images. The diameters of arbitrarily selected 10 or more Si domains are determined, and their average value is set as the "average particle size of the Si domains" here.

[0048] Si-C composite particles, for example, have 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 has a size that is contained within the carbon substrate, and if the carbon substrate has voids, it may have a size that is contained within the voids. From this viewpoint, the Si contained within the carbon substrate may be Si-containing nanoparticles (e.g., Si nanoparticles, Si oxide nanoparticles, Si nitride nanoparticles, Si carbide nanoparticles, etc.) with an average particle size (here, the average particle size of Si-containing domains) 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 even mitigating volume changes of the first silicon-containing particle 12 and the second silicon-containing particle 14, the Si-C composite particles preferably comprise 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 another form, the Si-C composite particles may comprise carbon particles and Si-containing nanoparticles attached to the surface of the carbon particles, and may comprise: particles containing Si and carbon nanoparticles attached to the surface of the Si-containing particles.

[0049] Coatings 122 and 142 contain LiF. The presence of coatings 122 and 142 can be confirmed, for example, by X-ray photoelectron spectroscopy (XPS) measurement of the first silicon-containing particle 12 and the second silicon-containing particle 14. The proportion of LiF in coating 122 (LiF concentration) corresponds to 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 silicon-containing particle 12 measured by XPS. The proportion of LiF in coating 142 (LiF concentration) corresponds to 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 silicon-containing particle 14 measured by XPS. Furthermore, in the XPS spectrum measured by XPS, the peak of F in LiF appears at 683 eV to 686 eV. The peaks of F outside of LiF are F peaks outside the range of 683 eV to 686 eV, especially those that can appear in the range of 687 eV to 690 eV.

[0050] The first peak intensity ratio is defined as the ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS energy spectrum of the first silicon-containing particle 12, measured by XPS. The second peak intensity ratio is defined as the ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS energy spectrum of the second silicon-containing particle 14. The first peak intensity ratio is greater than the second peak intensity ratio. In this embodiment, "the first peak intensity ratio is greater than the second peak intensity ratio" means that the difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.050. The difference between the first peak intensity ratio and the second peak intensity ratio is preferably 0.075 or more, more preferably 0.100 or more, and even more preferably 0.125 or more. While not particularly limited, from the viewpoint of achieving appropriate conductivity in the negative electrode 60, the difference between the first peak intensity ratio and the second peak intensity ratio is approximately 0.750 or less, for example, 0.600 or less, preferably 0.500 or less, more preferably 0.450 or less, and even more preferably 0.425 or less.

[0051] The first peak intensity ratio can be approximately 0.450 to 1.00. For example, the first peak intensity ratio is 0.460 or higher, preferably 0.480 or higher, more preferably 0.500 or higher. For example, the first peak intensity ratio is 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 can be approximately 0.200 or higher and less than 0.450. For example, the second peak intensity ratio is 0.250 or higher, preferably 0.250 or higher, more preferably 0.300 or higher. For example, the second peak intensity ratio is 0.420 or lower, preferably 0.400 or lower, more preferably 0.380 or lower.

[0052] The coating 122 on the surface of nuclear particle 121 and the coating 142 on the surface of nuclear particle 141 can also be confirmed, for example, by SEM observation. The coating 122 on the surface of nuclear particle 121 has a coverage rate of, for example, 20% or more, preferably 50% or more, more preferably 80% or more, further preferably 90% or more, and the closer to 100%, the better. The coating 142 on the surface of nuclear particle 141 has a coverage rate of, for example, 20% or more, preferably 50% or more, more preferably 80% or more, further preferably 90% or more, and the closer to 100%, the better. There is no particular limitation, but the coating 122 on the surface of nuclear particle 121 in the first silicon-containing particle 12 may be, for example, greater than the coating 142 on the surface of nuclear particle 141 in the second silicon-containing particle 14. Furthermore, the coverage rate of coating 122 or coating 142 can be obtained as follows: Obtain SEM images of cross-sections of the first silicon-containing particle 12 or the second silicon-containing particle 14. In this image, the ratio (percentage) of the total length of the coating 122 or the coating 142 relative to the circumference of the nucleus 121 or the nucleus 141 is calculated. For any five or more first silicon-containing particles 12 or second silicon-containing particles 14 selected arbitrarily, this ratio is calculated, and the average value is set as the coating rate.

[0053] The proportion of Si in the first silicon-containing particle 12 may be the same as or different from the proportion of Si in the second silicon-containing particle 14. Although there is no particular limitation, the proportion of Si in the first silicon-containing particle 12 and the proportion of Si in the second silicon-containing particle 14 may be approximately 20% to 80% by mass.

[0054] It should be noted that the first silicon-containing particle 12 and the second silicon-containing particle 14 can be manufactured according to known methods. Furthermore, various methods for manufacturing particles of Si-C composite materials are known (for example, see Japanese Patent Application Publication No. 2015-38862, International Publication No. 2014 / 046144, and other prior art documents listed in that international publication).

[0055] like Figure 2 As shown, the first layer 64a further includes first graphite particles 16 as the negative electrode active material. The second layer 64b further includes second graphite particles 18 as the negative electrode active material. Both the first graphite particles 16 and the second graphite particles 18 can be graphite particles that are substantially free of Si. The Si content in both the first graphite particles 16 and the second graphite particles 18 is 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, for example, using conventionally known methods such as ICP analysis.

[0056] The graphite constituting the first graphite particle 16 and the second graphite particle 18 can be natural graphite or artificial graphite, and can be amorphous carbon-coated graphite in the form of graphite coated with amorphous carbon material.

[0057] The shape of the first graphite particle 16 and the second graphite particle 18 is not particularly limited, and can be flake-like, spherical, etc. The first graphite particle 16 and the second graphite particle 18 are preferably spherical graphite particles. When the first graphite particle 16 and the second graphite particle 18 are spherical, the roundness of the first graphite particle 16 and the second graphite particle 18 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0058] It should be noted that the term "circularity" in this specification refers to the ratio of the circumference of a true circle having the same area as the projected area of ​​the particle to the circumference of the particle's projected image (i.e., circularity = circumference of a true circle having the same area as the projected area of ​​the particle / circumference of the particle's projected image). Therefore, the closer the circularity is to 1, the closer the particle's projected image is to a true circle, and the closer the particle is to a true sphere. Circularity can be determined, for example, by using a commercially available static automatic image analysis device to calculate the circularity of more than 100 particles and then calculating its average value.

[0059] There are no particular limitations on the average particle size (D50) of the first graphite particle 16 and the average particle size (D50) of the second graphite particle 18. The average particle size (D50) of the first graphite particle 16 and the average particle size (D50) of the second graphite particle 18 are each, 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.

[0060] The first graphite particle 16 and the second graphite particle 18 can be the same graphite particle or different graphite particles. It is preferable to use the same graphite particle as the first graphite particle 16 and the second graphite particle 18.

[0061] When the first layer 64a contains first graphite particles 16, and the total of the first silicon-containing particles 12 and the first graphite particles 16 is set to 100% by mass, the proportion of the first silicon-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. Furthermore, the first graphite particles 16 are used, for example, to improve the conductivity of the negative electrode active material layer 64, and their presence is not necessary. In another embodiment, the first layer 64a may not contain the first graphite particles 16.

[0062] The negative electrode active material contained in the first layer 64a may consist only of the first silicon-containing particles 12 and the first graphite particles 16, or only of the first silicon-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 are not equivalent to the first silicon-containing particles 12 and the first graphite particles 16. In this case, the proportion of other negative electrode active materials is preferably less than 10% by mass of the total amount of negative electrode active materials contained in the first layer 64a.

[0063] When the second layer 64b contains second graphite particles 18, and the total of the second silicon-containing particles 14 and the second graphite particles 18 is set to 100% by mass, the proportion of the second silicon-containing particles 14 is, for example, 10% to 60% by mass, preferably 15% to 50% by mass, and more preferably 20% to 40% by mass. Furthermore, the second graphite particles 18 are used, for example, to improve the conductivity of the negative electrode active material layer 64, and their presence is not essential. In another embodiment, the second layer 64b may not contain the second graphite particles 18.

[0064] The negative electrode active material contained in the second layer 64b may be only the second silicon-containing particles 14 and the second graphite particles 18, or only the second silicon-containing particles 14. However, as long as the effect of the technology disclosed herein can be achieved, the second layer 64b may contain other negative electrode active materials that are not equivalent to the second silicon-containing particles 14 and the second graphite particles 18. In this case, the proportion of other negative electrode active materials is preferably less than 10% by mass of the total amount of negative electrode active materials contained in the second layer 64b.

[0065] In the negative electrode active material layer 64, the thickness of the first layer 64a (T) A ) and the thickness of the second layer 64b (T) B The ratio of (T) A :T B The ratio is approximately 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.

[0066] The negative electrode active material layer 64 may contain components other than the negative electrode active material; examples include adhesives and conductive materials. As adhesives, examples include styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and polyvinylidene fluoride (PVDF). CMC also functions as a tackifier. Examples of conductive materials include carbon black such as acetylene black, carbon fibers, and carbon nanotubes (CNTs). CNTs are preferred. When using CNTs as conductive materials, the negative electrode active material layer 64 may contain a CNT dispersant.

[0067] 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, 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, 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, more preferably 0.05% by mass or more and 1% by mass or less.

[0068] 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, 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, 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, more preferably 0.05% by mass or more and 1% by mass or less.

[0069] There is no particular limitation on the thickness of the negative electrode active material layer 64, for example, it is 10 μm or more and 400 μm or less, preferably 20 μm or more and 300 μm or less.

[0070] There is no particular limitation on the density of the negative electrode active material layer 64, for example, it can be 0.7 g / cm³. 3 The above, preferably 1.0 g / cm³ 3 The above, more preferably 1.2 g / cm³ 3 That's all. On the other hand, the density of the negative electrode active material layer 64 is, for example, 2.3 g / cm³. 3 The following can be 2.0 g / cm³ 3 the following.

[0071] 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 non-forming portion 62a of the negative electrode active material layer. This insulating layer may contain, for example, an insulating inorganic filler.

[0072] As described above, the negative electrode 60 for a secondary battery includes 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 includes a first layer 64a located on the side of the negative electrode current collector 62 and a second layer 64b located on the surface side. The first layer 64a contains first silicon-containing particles 12 as negative electrode active material. The second layer 64b contains second silicon-containing particles 14 as negative electrode active material. When the expansion rate S1 of the first silicon-containing particles 12 relative to before charging A and after charging A is set to 1, the expansion rate S2 of the second silicon-containing particles 14 relative to before charging A and after charging A is greater than 0.3 and less than 0.9. Herein, charging A is charging at 25°C with a constant current of 0.01C until 4.2V, and then charging with a constant voltage until the current becomes 0.005C.

[0073] Both the first silicon-containing particle 12 and the second silicon-containing particle 14 include a LiF coating (here, the LiF coating contained in coating 122 or coating 142). 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 silicon-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 silicon-containing particle 14 is defined as the second peak intensity ratio. The first peak intensity ratio is greater than the second peak intensity ratio. The difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.050.

[0074] The negative electrode 60 includes a first silicon-containing particle 12 with a larger expansion rate and a second silicon-containing particle 14 with a smaller expansion rate. The negative electrode active material layer 64 includes a first layer 64a on the negative electrode current collector 62 side and a second layer 64b on the surface side. During repeated charging and discharging, the expansion on the surface side of the negative electrode active material layer 64 becomes greater. If the second silicon-containing particles 14 with a smaller expansion rate are arranged in the second layer 64b on the surface side, the particles can move more easily in the second layer 64b. Therefore, by utilizing the movement of particles during the expansion and contraction caused by charging and discharging, stress can be mitigated, and the expansion of the negative electrode can be suppressed. Furthermore, in the negative electrode 60, the first layer 64a includes the first silicon-containing particle 12 with a larger expansion rate. The expansion rate S2 of the second silicon-containing particle 14 is set to be greater than 0.3 and less than 0.9 relative to the expansion rate S1 of the first silicon-containing particle 12. This makes the expansion and contraction of the entire negative electrode active material layer 64 more uniform during charging and discharging. Therefore, it can suppress localized reactions and stress concentrations in the negative electrode active material layer 64, and suppress the swelling of the negative electrode 60 during repeated charging and discharging.

[0075] In addition, the first silicon-containing particles 12 and the second silicon-containing particles 14 have a LiF coating on their surfaces. The LiF coating can follow the expansion and contraction of the silicon-containing particles, thus suppressing the exposure of the particle surface due to expansion and contraction. Specifically, the first peak intensity ratio of the first silicon-containing particle 12 is greater than the second peak intensity ratio of the second silicon-containing particle 14. In other words, the first silicon-containing particle 12, with its larger expansion rate, has a larger amount of LiF coating formed on its surface compared to the second silicon-containing particle 14, which has a smaller expansion rate. Therefore, the exposure of the particle surface caused by the aforementioned expansion and contraction can be suppressed, and localized reactions and stress concentrations in the negative electrode active material layer 64 can be suppressed. Thus, the swelling of the negative electrode 60 during repeated charge and discharge can be suppressed.

[0076] The first peak intensity ratio can be 0.450 to 1.00. Therefore, during repeated charge and discharge, the formation of exposed portions on the surfaces of the first silicon-containing particle 12 and the second silicon-containing particle 14 can be appropriately suppressed. Furthermore, excessive increase in resistance in the negative electrode 60 due to the formation of the LiF film can be suppressed. Furthermore, the tendency of silicon-containing particles to expand easily due to side reactions in the LiF film can be suppressed.

[0077] The intensity ratio of the second peak can be greater than 0.200 and less than 0.450. Therefore, during repeated charge and discharge, the formation of exposed portions on the surfaces of the first silicon-containing particle 12 and the second silicon-containing particle 14 can be appropriately suppressed.

[0078] The first silicon-containing particle 12 may comprise: a Si-C composite particle (here, core particle 121) having a carbon substrate and Si contained within the carbon substrate, and a LiF film (here, the LiF film contained in film 122) disposed on at least a portion of the surface of the Si-C composite particle (here, core particle 121). The second silicon-containing particle 14 may comprise: a Si-C composite particle (here, core particle 141) having a carbon substrate and Si contained within the carbon substrate, and a LiF film (here, the LiF film contained in film 142) disposed on at least a portion of the surface of the Si-C composite particle (here, core particle 141). Thus, the expansion of Si within the carbon substrate can be mitigated, thereby suppressing the expansion of the first silicon-containing particle 12 and the second silicon-containing particle 14. Therefore, the swelling of the negative electrode 60 during repeated charge and discharge can be suppressed.

[0079] The first layer 64a may further include first graphite particles 16 that are substantially free of Si as the negative electrode active material. The second layer 64b may further include second graphite particles 18 that are substantially free of Si as the negative electrode active material. Thus, conductivity can be improved in the first layer 64a and the second layer 64b.

[0080] The negative electrode 60 is manufactured, for example, by a manufacturing method including 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.

[0081] The first preparation step is, for example, a step of preparing a first paste comprising first silicon-containing particles as a negative electrode active material and a dispersion medium. This first preparation step can be performed by mixing the first silicon-containing particles 12, first graphite particles 16 (if necessary), and optional components (e.g., binders, conductive materials, etc.) with a dispersion medium (e.g., water) using a known mixing apparatus or stirring apparatus, according to a known method. It should be noted that, in this specification, the term "paste" refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, including terms such as "slurry" or "ink."

[0082] The first preparation step includes, for example, preparing a first silicon-containing particle 12. In preparing the first silicon-containing particle 12, for example, firstly, silicon-containing particles as core particles 121 are prepared. The silicon-containing particles as core particles 121 are, for example, Si-C composite particles. Si-C composite particles, for example, as described above, can be prepared using known methods.

[0083] Next, while the obtained nuclei 121 are dispersed in water or a water-soluble organic solvent, a water-soluble lithium salt and a fluorinating agent are reacted in the nuclei 121 to generate LiF. The LiF generated here forms a film 122 on the surface of the nuclei 121.

[0084] As a water-soluble organic solvent, alcohols such as ethanol (ethyl alcohol) can be used. As a water-soluble lithium salt, 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.

[0085] In this operation, for example, firstly, an aqueous solution of a water-soluble lithium salt, a dispersion in which the nucleus particles 121 are dispersed in water or a water-soluble organic solvent, and an aqueous solution of a fluorinating agent are prepared. The aqueous solution of the water-soluble lithium salt and the dispersion are mixed to prepare a mixture. The aqueous solution of the fluorinating agent is added to the mixture under stirring. This causes the water-soluble lithium salt and the fluorinating agent to react and generate LiF. Thus, a reaction solution containing LiF is obtained. In this reaction solution, the surface of the nucleus particles 121 can be brought into contact with LiF.

[0086] The reaction conditions for the water-soluble lithium salt and fluorinating agent can be the same as those for the known synthesis of LiF 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. Furthermore, the reaction time can be appropriately determined based on the concentrations of the water-soluble lithium salt and fluorinating agent in the reaction solution, the desired proportion of LiF in the coating 122, etc. A longer reaction time results in a higher proportion of LiF in the coating 122.

[0087] By recovering the nuclei 121 from the reaction solution while the reaction solution containing LiF is attached to its surface, and drying the nuclei 121 attached to the reaction solution containing LiF, LiF can be attached to the surface of the nuclei 121, thereby obtaining first silicon-containing particles 12 having a coating 122 containing LiF. To adjust the amount of LiF, the reaction solution containing LiF can be diluted with water or the like.

[0088] The second preparation step is, for example, a step of preparing a second paste comprising second silicon-containing particles 14 as a negative electrode active material and a dispersion medium. The second preparation step can be carried out by mixing the second silicon-containing particles 14, second graphite particles 18 as needed, and optional components (e.g., adhesives, conductive materials, etc.) with a dispersion medium (e.g., water) using a known mixing device, stirring device, etc., according to a known method.

[0089] The second preparation step includes, for example, preparing a second silicon-containing particle 14. The preparation of the second silicon-containing particle 14 can be carried out by using Si-C composite particles as the core particle 141, while employing other materials and steps used in preparing the first silicon-containing particle 12, and by appropriately changing the conditions.

[0090] The first coating process is, for example, the process of coating a first paste onto the negative current collector 62. The first coating process can be performed according to known methods. For example, the first coating process can be performed by coating the obtained first paste onto the negative current collector 62 using a coating device such as a gravure coater, a notched wheel coater, a slot coater, or a molding coater.

[0091] The first drying step is, for example, a step of drying a first paste coated on the negative electrode current collector 62 to form a dry film of the first paste. The first drying step can be performed using known methods. For example, by using a drying apparatus such as a drying oven to remove the dispersion medium from the negative electrode current collector 62 coated with the first paste, a dry film of the first paste is formed. Thus, the first drying step can be performed. The drying temperature and drying time can be appropriately determined according to the concentration of the solid components of the first paste and are not particularly limited. The drying temperature is, for example, 60°C or higher and 200°C or lower, preferably 70°C or higher and 150°C or lower. The drying time is, for example, 10 seconds or higher and 30 minutes or lower, preferably 30 seconds or higher and 10 minutes or lower.

[0092] The second coating step is, for example, the process of coating a second paste onto the dried film of a first paste. The second coating step can be performed using known methods. For example, the second coating step can be performed by coating the obtained second paste onto the dried film of the first paste using the aforementioned apparatus.

[0093] The second drying step is, for example, a step of drying a second paste coated on a dry film of the first paste to form a dry film of the second paste. The second drying step can be carried out according to known methods. The apparatus used in the second drying step and the drying conditions are as described in the description of the first drying step.

[0094] 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. The pressing process can be performed according to known methods. For example, pressure can be applied to the dried film of the first paste and the dried film of the second paste using a roller press, thereby enabling the pressing process. By employing the pressing process, the first silicon-containing particles 12, the second silicon-containing particles 14, and, as needed, the first graphite particles 16 and the second graphite particles 18 can be tightly filled. As described above, a negative electrode 60 including a negative electrode active material layer 64 having a first layer 64a and a second layer 64b can be obtained.

[0095] According to the negative electrode 60 of this embodiment, the swelling of the negative electrode 60 during repeated charging and discharging of the secondary battery can be suppressed. Furthermore, since the negative electrode 60 of this embodiment uses a negative electrode active material containing Si, the secondary battery can achieve high capacity.

[0096] Therefore, from another perspective, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the negative electrode 60 described in the above-described embodiment. The following describes an embodiment of the secondary battery disclosed herein, using a lithium-ion secondary battery as an example. Figure 3 and Figure 4 The following is an example of a flat, square lithium-ion secondary battery with a flat, wound electrode body and a flat, battery casing.

[0097] Figure 3 The lithium-ion secondary battery 100 shown is a sealed lithium-ion secondary battery constructed by housing a flat, wound electrode body 20 and a non-aqueous electrolyte (not shown) within a flat, square battery casing (i.e., outer container) 30. The battery casing 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin-walled safety valve 36 designed to release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, an injection port (not shown) for injecting the non-aqueous electrolyte is provided in the battery casing 30. The positive terminal 42 is electrically connected to a positive current collector 42a. The negative terminal 44 is electrically connected to a negative current collector 44a. The battery casing 30 is made of a lightweight, thermally conductive metal material such as aluminum.

[0098] 20 winding electrode bodies Figure 3 and Figure 4 As shown, the positive electrode 50 and negative electrode 60 are formed by overlapping and winding two elongated separator sheets 70 in the longitudinal direction. The positive electrode 50 has a configuration in which a positive active material layer 54 is formed on one or both (in this case, both) sides of the elongated positive current collector 52 in the longitudinal direction. The negative electrode 60 has a configuration in which a negative active material layer 64 is formed on one or both (in this case, both) sides of the elongated negative current collector 62 in the longitudinal direction. The non-formed portions 52a of the positive active material layer (i.e., the portions where the positive current collector 52 is exposed without the formation of the positive active material layer 54) and the non-formed portions 62a of the negative active material layer (i.e., the portions where the negative current collector 62 is exposed without the formation of the negative active material layer 64) are formed in a manner that overflows outward from both ends in the winding axis direction of the winding electrode body 20 (i.e., the sheet width direction orthogonal to the aforementioned longitudinal direction). Positive current collector plate 42a and negative current collector plate 44a are respectively bonded to the non-formed portion 52a of the positive active material layer and the non-formed portion 62a of the negative active material layer.

[0099] As the positive current collector 52 constituting the positive electrode sheet 50, a known positive current collector for lithium-ion secondary batteries can be used. Examples of such current collectors include sheets or foils made of metals with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.). Aluminum foil is preferred as the positive current collector 52.

[0100] There is no particular limitation on the size of the positive current collector 52, which can be appropriately determined according to the battery design. When aluminum foil is used as the positive current collector 52, there is no particular limitation on its thickness, for example, it can be 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

[0101] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition for lithium-ion secondary batteries can be used. Specifically, for example, lithium composite oxides, lithium transition metal phosphate compounds, etc., can be used as the positive electrode active material. There are no particular limitations on the crystal structure of the positive electrode active material; it can be a layered structure, a spinel structure, an olivine structure, etc.

[0102] As a lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element is preferred. Specific examples include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium iron nickel manganese composite oxides.

[0103] It should be noted that in this specification, the term "lithium-nickel-cobalt-manganese composite oxide" includes oxides containing one or more additional elements besides Li, Ni, Co, Mn, and O. Examples of such additional elements include transition metals and typical metallic elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. Additionally, the added elements may be half-metals such as B, C, Si, and P, or non-metals such as S, F, Cl, Br, and I. This also applies to the aforementioned lithium-nickel composite oxides, lithium-cobalt composite oxides, lithium-manganese composite oxides, lithium-nickel-manganese composite oxides, lithium-nickel-cobalt-aluminum composite oxides, and lithium-iron-nickel-manganese composite oxides.

[0104] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0105] These positive electrode active materials can be used alone, or in combination of two or more. Lithium-nickel-cobalt-manganese composite oxides are particularly preferred as positive electrode active materials due to their superior initial resistance and other properties.

[0106] There is no particular limitation on the average particle size (D50) of the positive electrode active material, for example, it is 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.

[0107] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as lithium triphosphate, conductive materials, and binders. As conductive materials, carbon black such as acetylene black (AB), carbon fibers such as fumed carbon fiber (VGCF) and carbon nanotubes (CNT), and other carbon materials (such as graphite) are preferred. As binders, polyvinylidene fluoride (PVdF) can be used, for example.

[0108] The content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the content of the positive electrode active material relative to the total mass of the positive electrode active material layer 54) is not particularly limited, but 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 preferably 0.1% by mass or more and 15% by mass or less, 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 preferably 0.1% by mass or more and 20% by mass or less, 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 preferably 0.4% by mass or more and 15% by mass or less, more preferably 0.5% by mass or more and 10% by mass or less.

[0109] The thickness of each single side of the positive electrode active material layer 54 is not particularly limited, but is generally 10 μm or more, preferably 20 μm or more. On the other hand, the thickness is generally 400 μm or less, preferably 300 μm or less.

[0110] The negative electrode 60 described above is used as the negative electrode plate 60.

[0111] As the separator 70, examples include porous sheets (membranes) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. This porous sheet can be a single-layer structure or a multi-layered structure with two or more layers (e.g., a three-layer structure with PP layers laminated on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0112] The thickness of the separator 70 is not particularly limited, for example, it is 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less. The air permeability of the separator 70 obtained by the Gray test is not particularly limited, but preferably 350 seconds / 100cc or less.

[0113] Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). As a non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones, which are commonly used in electrolytes of general lithium-ion secondary batteries, 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), fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), fluoromethyl difluoromethyl carbonate (F-DMC), and difluorotrifluoromethyl carbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As an example, the non-aqueous solvent may contain only carbonates. As another example, the non-aqueous solvent may contain carbonates and esters such as methyl acetate.

[0114] As the supporting salt, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) are preferably used (LiPF6 is preferred). The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0115] Furthermore, the aforementioned non-aqueous electrolyte may contain components other than those mentioned above, provided that it does not significantly impair the effectiveness of this disclosure, such as film-forming agents like vinylene carbonate (VC) and oxalic acid complexes; gas generators like biphenyl (BP) and cyclohexylbenzene (CHB); and various additives such as tackifiers.

[0116] The lithium-ion secondary battery 100 can be used for various applications. Preferred applications include its use as a power source in vehicles such as battery electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Additionally, the lithium-ion secondary battery 100 can be used as a battery in small energy storage devices. The lithium-ion secondary battery 100 can typically be used even in the form of a pack of batteries connected in series and / or parallel.

[0117] The above description, as an example, describes a square lithium-ion secondary battery 100 including a flat, wound electrode body 20. However, lithium-ion secondary batteries can also be configured as lithium-ion secondary batteries including stacked electrode bodies (i.e., electrode bodies in which multiple positive electrodes and multiple negative electrodes are alternately stacked). Furthermore, lithium-ion secondary batteries can also be configured as cylindrical lithium-ion secondary batteries, stacked-casing lithium-ion secondary batteries, etc.

[0118] In addition, according to known methods, the lithium-ion secondary battery 100 can also be constructed as an all-solid-state lithium-ion secondary battery that uses a solid electrolyte instead of a non-aqueous electrolyte.

[0119] Furthermore, the negative electrode 60 involved in this embodiment is suitable as the negative electrode of a lithium-ion secondary battery, but can also be constructed and used as the negative electrode of other secondary batteries, which can be constructed according to known methods.

[0120] The following describes in detail embodiments related to this disclosure, but it is not intended to limit this disclosure to the contents shown in these embodiments.

[0121] <Fabrication of Si-C composite particles with LiF-containing films>

[0122] Si-C composite particles (A) to (J) were prepared as core particles. The Si-C composite particles (A) to (J) prepared here are particles having a porous carbon substrate and Si nanoparticles disposed within the pores of the carbon substrate. Si-C composite particle (A) has an expansion rate of 258%. Si-C composite particle (B) has an expansion rate of 153%. Si-C composite particle (C) has an expansion rate of 228%. Si-C composite particle (D) has an expansion rate of 173%. Si-C composite particle (E) has an expansion rate of 339%. Si-C composite particle (F) has an expansion rate of 126%. Si-C composite particle (G) has an expansion rate of 416%. Si-C composite particle (H) has an expansion rate of 121%. Si-C composite particle (I) has an expansion rate of 217%. Si-C composite particle (J) has an expansion rate of 176%.

[0123] <Determination of Expansion Rate>

[0124] Furthermore, the expansion rate of the nuclear particles was obtained using the following steps. As the negative electrode active material, a negative electrode with the same configuration as in Example 1 was fabricated using the same materials and steps as in Example 1, except that only one of the aforementioned nuclear particles was used. A cross-section along the thickness direction of this negative electrode was machined for SEM observation, and an SEM image of the machined surface was obtained. The magnification at this time was 1500x. Next, except that this negative electrode was used, a lithium-ion secondary battery with the same configuration as in this example was fabricated using the same steps as in this example, i.e., a test battery. The test battery was placed in an environment of 25°C and charged with a constant current of 0.01C to 4.2V, followed by constant voltage charging until the current value became 0.005C. Then, the test battery was disassembled, and the negative electrode was removed. Then, an SEM observation of the machined surface, for which an SEM image was obtained before charging, was performed, and an SEM image after charging was obtained. 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. Secondly, for the SEM images after charging, the area A2 of the same first particle was measured. Then, for each particle, the following equation (R) was used:

[0125] Expansion rate (%) = [√{(Area A2) / (Area A1)}] 3 The expansion rate is calculated by multiplying by 100(R). The arithmetic mean of the expansion rates of the 100 particles obtained here is then calculated and set as the expansion rate of the silicon-containing particles.

[0126] [Example 1]

[0127] As negative electrode active materials, first particles and second particles were prepared. In the preparation of the first particle, Si-C composite particles (A) were prepared as the core particle. The core particles were dispersed in ethanol while being sonicated for 30 minutes. Lithium acetate was dissolved in deionized water to prepare an aqueous solution of lithium acetate. The dispersion of Si-C composite particles was added to this aqueous solution while stirring for 30 minutes. An aqueous solution of ammonium fluoride was added to this solution while stirring for 60 minutes to allow the lithium acetate to react with the ammonium fluoride. This yielded a reaction solution containing LiF. After diluting the reaction solution, it was filtered. The filter was dried, and a LiF-containing film was formed on the Si-C composite particles (A). The preparation steps for the second particle were the same as for the first particle, except that Si-C composite particles (B) were used as the core particle and the ammonium fluoride aqueous solution was added for 120 minutes (i.e., the reaction time).

[0128] XPS spectra were measured for the first and second particles obtained, revealing a peak for F in Li-F (approximately 684.8 eV) and a peak for Li in Li-F (approximately 55.1 eV), confirming the presence of LiF. For the first particle, the ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF was calculated (hereinafter referred to as the "first LiF intensity ratio"). For the second particle, the ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF was calculated (hereinafter referred to as the "second LiF intensity ratio"). The results are shown in the corresponding columns of Table 1. Furthermore, the peak of F in LiF appears between 683 eV and 686 eV, while the peaks of F other than LiF appear between 687 eV and 690 eV.

[0129] Furthermore, graphite particles with an average particle size (D50) of 15 μm were prepared as the negative electrode active material. These graphite particles were essentially free of Si. A dispersion of single-layer carbon nanotubes (SWCNTs) was prepared as the conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) were prepared as binders.

[0130] The above materials were mixed with water as a solvent in a mass ratio of graphite particles / first particle / SWCNT / CMC / PAA / SBR of 80 / 20 / 7 / 0.1 / 1 / 1 / 1.5, and a first paste was prepared using the following steps. The above materials were mixed with water as a solvent in a mass ratio of graphite particles / second particle / SWCNT / CMC / PAA / SBR of 70 / 30 / 0.1 / 1 / 1 / 1.5, and a second paste was prepared using the following steps.

[0131] Graphite particles, first particles, CMC, and PAA were dry-mixed using a planetary mixer. The resulting dry mixture, SWCNT dispersion, and dispersion medium were then kneaded using a planetary mixer. Subsequently, SBR and additional dispersion medium were added to the planetary mixer for dilution and mixing to obtain a first paste.

[0132] Graphite particles, second particles, CMC, and PAA were dry-mixed using a planetary mixer. The resulting dry mixture, SWCNT dispersion, and dispersion medium were then kneaded using a planetary mixer. Subsequently, SBR and additional dispersion medium were added to the planetary mixer for dilution and mixing to obtain a second paste.

[0133] 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 to the dried film of the first paste and dried to form a dried film of the second paste. The coating amount of the second paste was set such that the thickness of the negative electrode active material layer (here, the combined thickness of the dried films of the first and second pastes) was 100, and the thickness of the dried film of the second paste was 50. Then, the negative electrode active material layer was rolled and the resulting sheet was processed into a specified size to obtain a negative electrode sheet.

[0134] [Example 2]

[0135] In the preparation of the first particle, Si-C composite particles (C) were used as the core particle, and the ammonium fluoride aqueous solution was added over a time of 90 minutes. In the preparation of the second particle, Si-C composite particles (D) were used as the core particle. Otherwise, the negative electrode of this example was obtained using the same materials and procedures as in Example 1.

[0136] [Example 3]

[0137] In the preparation of the first particle, Si-C composite particles (E) were used as the core particle, and the ammonium fluoride aqueous solution was added over a time of 150 minutes. In the preparation of the second particle, Si-C composite particles (F) were used as the core particle. Otherwise, the negative electrode of this example was obtained using the same materials and procedures as in Example 1.

[0138] [Comparative Example 1]

[0139] To become graphite particles / first particles / second particles / SWCNT / CMC / PAA / SBR=

[0140] A negative electrode paste was prepared by mixing it with water as a solvent in a mass ratio of 70 / 10 / 15 / 0.1 / 1 / 1 / 1.5. The negative electrode paste was prepared by performing the following first and second steps. In the first step, first particles and second particles, a paste-like SWCNT (2% solids content), and a dispersion medium were first fed into a mixer and dispersed using a dispersioner at 3000 rpm to prepare the first paste. In the second step, graphite particles, CMC, and PAA were dry-mixed using a stirred granulator. The first paste, the mixed powder obtained by dry mixing, and the dispersion medium (water) were then solid-kneaded. The solids content during solid-kneading was 65%. SBR and the dispersion medium (water) were further added to the solid-kneaded mixture and mixed. This prepared the negative electrode paste. The prepared negative electrode paste was coated onto the surface of a 10 μm thick copper foil and dried to form a negative electrode active material layer. After rolling the negative electrode active material layer, the resulting sheet is processed into a specified size. Otherwise, the negative electrode sheet of this example was obtained using the same materials and steps as in Example 1.

[0141] [Comparative Example 2]

[0142] Si-C composite particles (A) were used as the first particle. Si-C composite particles (B) were used as the second particle. No LiF film was formed in the Si-C composite particles (A) and Si-C composite particles (B). Apart from this, the negative electrode of this example was obtained using the same materials and procedures as in Example 1.

[0143] [Comparative Example 3]

[0144] In the preparation of the first particle, Si-C composite particles (F) were used as the core particle, and the ammonium fluoride aqueous solution was added over a time of 60 minutes. In the preparation of the second particle, Si-C composite particles (A) were used as the core particle, and the ammonium fluoride aqueous solution was added over a time of 120 minutes. Apart from this, the negative electrode sheet of this example was obtained using the same materials and procedures as in Example 1.

[0145] [Comparative Example 4]

[0146] Si-C composite particles (B) were used as the second particle. No LiF film was formed in the Si-C composite particles (B). Otherwise, the negative electrode of this example was obtained using the same materials and procedures as in Example 1.

[0147] [Comparative Example 5]

[0148] Si-C composite particles (A) were used as the first particle. No LiF film was formed in the Si-C composite particles (A). Otherwise, the negative electrode of this example was obtained using the same materials and procedures as in Example 1.

[0149] [Comparative Example 6]

[0150] In the preparation of the first particle, Si-C composite particles (G) were used as the core particle, and the ammonium fluoride aqueous solution was added over a time of 240 minutes. In the preparation of the second particle, Si-C composite particles (H) were used as the core particle. Otherwise, the negative electrode of this example was obtained using the same materials and procedures as in Example 1.

[0151] [Comparative Example 7]

[0152] In the preparation of the first particle, Si-C composite particles (I) were used as the core particle, and the ammonium fluoride aqueous solution was added over a time of 60 minutes. In the preparation of the second particle, Si-C composite particles (J) were used as the core particle. Otherwise, the negative electrode sheet of this example was obtained using the same materials and procedures as in Example 1.

[0153] <Evaluation of Plate Expansion Rate>

[0154] The thickness of the negative electrode in each embodiment and each comparative example was measured. This thickness was set as the initial thickness (T0). Using this negative electrode, an evaluation lithium-ion secondary battery was fabricated as described below.

[0155] LiNi as the 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 coated onto the surface of a 15 μm thick aluminum foil and dried to form a positive electrode active material layer. The positive electrode active material layer was then rolled to obtain a sheet of specified dimensions, yielding the positive electrode sheet.

[0156] A porous polyolefin separator was prepared. Leads were attached to the prepared negative and positive electrode sheets, and the electrode body was fabricated by stacking the separators. This electrode body, along with a non-aqueous electrolyte, was housed in an aluminum laminated film casing. For the non-aqueous electrolyte, a product containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a volume ratio of 15:5:40:40, with LiPF6 dissolved as a supporting salt at a concentration of 1.0 mol / L, was used. The casing was then sealed, resulting in an evaluation lithium-ion secondary battery.

[0157] Next, the lithium-ion secondary batteries prepared above for evaluation were placed in an environment of 25°C. Each lithium-ion secondary battery was charged with a constant current of 0.4C until it reached 4.2V, and then charged with a constant voltage until the current reached 0.1C. Next, each lithium-ion secondary battery was discharged with a constant current of 0.4C until it reached 2.5V.

[0158] The above charge-discharge cycle was repeated for 250 cycles. Each evaluation lithium-ion secondary battery was disassembled under an argon atmosphere, and the negative electrode was immersed in DMC, cleaned, and dried. The thickness of the negative electrode was then measured and defined as the thickness after charge-discharge cycles (T1). The increase rate (%) of the negative electrode thickness before and after charge-discharge cycles was calculated by (T1 / T0-1)×100. The results are shown in the "Increase Rate (%)" column of Table 1. It should be noted that for examples with an increase rate of less than 33%, the evaluation was that the expansion of the negative electrode during charge-discharge cycles was suppressed.

[0159] Table 1

[0160] Table 1

[0161]

[0162] Expansion ratio ratio* = Expansion ratio of the second particle / Expansion ratio of the first particle

[0163] As shown in Table 1, in Examples 1-3, the increase in the rate of increase of the negative electrode thickness before and after charge-discharge cycles (negative electrode swelling) was suppressed. In Examples 1-3, as described above, the layer on the negative electrode current collector side of the negative electrode active material layer contains a first particle, and the layer on the surface side contains a second particle. When the expansion rate of the first particle after charge A relative to before charge A is set to 1, the expansion rate of the second particle after charge A relative to before charge A is greater than 0.3 and less than 0.9. The first particle and the second particle comprise 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 particle is greater than 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 particle, and the difference between the two is at least 0.050.

[0164] 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 technology described in the claims includes various modifications and alterations to the specific examples illustrated above.

[0165] That is, the negative electrode of the secondary battery disclosed herein, its manufacturing method, and the secondary battery are the following items [1] to [8].

[0166] [1] The negative electrode is the negative electrode of a secondary battery, which includes a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer includes a first layer located on the side of the negative electrode current collector and a second layer located on the surface side. The first layer contains first silicon-containing particles as negative electrode active material, and the second layer contains second silicon-containing particles as negative electrode active material. When the expansion rate S1 of the first silicon-containing particles after charging A at a constant current value of 0.01C to 4.2V and then charging at a constant voltage value to 0.005C before charging A at 25°C is set to 1, the expansion rate S1 of the first silicon-containing particles after charging A is set to 1. After charging A, the expansion rate S2 of the second silicon-containing particle is greater than 0.3 and less than 0.9. Both the first and second silicon-containing particles include 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 silicon-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 in LiF to the peak intensity of F other than LiF in the XPS spectrum of the second silicon-containing particle is defined as the second peak intensity ratio, the first peak intensity ratio is greater than the second peak intensity ratio, and the difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.050.

[0167] [2] According to the negative electrode described in [1], wherein the first peak intensity ratio is 0.450 or higher and 1.00 or lower.

[0168] [3] The negative electrode according to [1] or [2], wherein the second peak intensity ratio is greater than 0.200 and less than 0.450.

[0169] [4] The negative electrode according to any one of [1] to [3], wherein the first silicon-containing particle and the second silicon-containing particle comprise a Si-C composite particle having a carbon substrate and Si contained in the interior of the carbon substrate, and the LiF film disposed on at least a portion of the surface of the Si-C composite particle.

[0170] [5] The negative electrode according to any one of [1] to [4], wherein the first layer and the second layer further comprise graphite particles that are substantially free of Si as the negative electrode active material.

[0171] [6] The manufacturing method is a method for manufacturing the negative electrode of a secondary battery, comprising: preparing a first paste comprising first silicon-containing particles as a negative electrode active material and a dispersion medium; preparing a second paste comprising second silicon-containing particles as a negative electrode active material and a dispersion medium; coating and drying the first paste on a negative electrode current collector to form a dried film of the first paste; coating and drying the second paste on the dried film of the first paste to form a dried film of the second paste; and pressing the dried film of the first paste and the dried film of the second paste together, and then charging the second paste with a constant current of 0.01C at 25°C until 4.2V and then with a constant voltage until the current becomes 0.005C, followed by the first charge A. When the expansion rate S1 of a silicon-containing particle is set to 1, the expansion rate S2 of the second silicon-containing particle after charging A relative to before charging A is greater than 0.3 and less than 0.9. Both the first and second silicon-containing particles include 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 energy spectrum of the first silicon-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 in LiF to the peak intensity of F other than LiF in the XPS energy spectrum of the second silicon-containing particle is defined as the second peak intensity ratio, the first peak intensity ratio is greater than the second peak intensity ratio, and the difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.050.

[0172] [7] According to the manufacturing method described in [6], the preparation of the first paste includes the preparation of the first silicon-containing particles, the preparation of the second paste includes the preparation of the second silicon-containing particles, and in the preparation of the first silicon-containing particles and the preparation of the second silicon-containing particles, Si-C composite particles comprising a carbon substrate and Si contained within the carbon substrate are prepared, and the first silicon-containing particles and the second silicon-containing particles are prepared by reacting the Si-C composite particles with a water-soluble lithium salt and a fluorinating agent to generate LiF while the Si-C composite particles are dispersed in water or a water-soluble organic solvent.

[0173] [8] A secondary battery is a secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode is a negative electrode according to any one of [1] to [5].

[0174] Explanation of reference numerals in the attached figures

[0175] 12 First silicon-containing particles

[0176] 14 Second silicon-containing particles

[0177] 16 First Graphite Particle

[0178] 18 Second graphite particles

[0179] 20. Winded electrode body

[0180] 30 Battery casing

[0181] 36 Safety valve

[0182] 42 Positive extremes

[0183] 42a Positive Current Collector

[0184] 44 Negative extremes

[0185] 44a Negative Current Collector

[0186] 50 Positive Electrode Sheets (Positive Electrode)

[0187] 52 Positive current collector

[0188] 52a Non-forming portion of the positive electrode active material layer

[0189] 54 Positive electrode active material layer

[0190] 60 Negative electrode plate (negative electrode)

[0191] 62 Negative current collector

[0192] 62a Non-forming portion of the negative electrode active material layer

[0193] 64 Negative Electrode Active Material Layer

[0194] 70. Partition plate (partition)

[0195] 100 Lithium-ion Secondary Battery

Claims

1. A negative electrode is the negative electrode of a secondary battery comprising a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer comprises a first layer located on the side of the negative electrode current collector and a second layer located on the surface side. The first layer contains first silicon-containing particles as negative electrode active material, and the second layer contains second silicon-containing particles as negative electrode active material. When the expansion rate S1 of the first silicon-containing particles after charging A (which involves constant current charging at 0.01C to 4.2V at 25°C and then constant voltage charging to 0.005C) is set to 1, the expansion rate S1 of the first silicon-containing particles after charging A is... After charging A, the expansion rate S2 of the second silicon-containing particle is greater than 0.3 and less than 0.

9. Both the first and second silicon-containing particles include 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 silicon-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 in LiF to the peak intensity of F other than LiF in the XPS spectrum of the second silicon-containing particle is defined as the second peak intensity ratio, the first peak intensity ratio is greater than the second peak intensity ratio, and the difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.

050.

2. The negative electrode according to claim 1, wherein, The first peak intensity ratio is greater than 0.450 and less than 1.

00.

3. The negative electrode according to claim 1, wherein, The second peak intensity ratio is above 0.200 and less than 0.

450.

4. The negative electrode according to any one of claims 1 to 3, wherein, The first silicon-containing particle and the second silicon-containing particle comprise Si-C composite particles having a carbon substrate and Si contained within the carbon substrate, and the LiF film disposed on at least a portion of the surface of the Si-C composite particles.

5. The negative electrode according to any one of claims 1 to 3, wherein, The first and second layers also contain essentially Si-free graphite particles as negative electrode active materials.

6. A manufacturing method for a negative electrode of a secondary battery, comprising: preparing a first paste comprising first silicon-containing particles as a negative electrode active material and a dispersion medium; preparing a second paste comprising second silicon-containing particles as a negative electrode active material and a dispersion medium; coating and drying the first paste onto a negative electrode current collector to form a dried film of the first paste; coating and drying the second paste onto the dried film of the first paste to form a dried film of the second paste; and pressing the dried film of the first paste and the dried film of the second paste together. When the expansion rate S1 of the first silicon-containing particle after charging A (which involves constant current charging at 0.01C to 4.2V and then constant voltage charging to 0.005C at 25°C) is set to 1, the expansion rate S2 of the second silicon-containing particle after charging A is greater than 0.3 and less than 0.

9. Both the first and second silicon-containing particles include 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 silicon-containing particle, measured by X-ray photoelectron spectroscopy, 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 silicon-containing particle is defined as the second peak intensity ratio. The first peak intensity ratio is greater than the second peak intensity ratio, and the difference between the first peak intensity ratio and the second peak intensity ratio is at least 0.

050.

7. The manufacturing method according to claim 6, wherein, Preparing the first paste includes preparing the first silicon-containing particles, and preparing the second paste includes preparing the second silicon-containing particles. In preparing the first silicon-containing particles and the second silicon-containing particles, Si-C composite particles comprising a carbon substrate and Si contained within the carbon substrate are prepared by reacting the Si-C composite particles with a water-soluble lithium salt and a fluorinating agent to generate LiF while the Si-C composite particles are dispersed in water or a water-soluble organic solvent.

8. A secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode is the negative electrode according to claim 1.

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