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

Abstract

The present disclosure relates to a negative electrode for a secondary battery, a method for manufacturing the negative electrode, and a secondary battery using the negative electrode. Provided is a technique for suppressing a decrease in the capacity retention rate during repeated charging and discharging for a negative electrode containing Si particles. In a negative electrode of a secondary battery according to the present disclosure, a negative electrode active material layer contains first S-i-containing particles and second S-i-containing particles. When the expansion rate S2 of the second Si-containing particles after charging A before charging A is 1, the expansion rate S1 of the first Si-containing particles after charging A before charging A is greater than 0.3 and 0.9 or less. The first S i-containing particles and the second S i-containing particles each include a LiF coating film. The ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF in the XPS spectrum of the first S i-containing particles is set as a first peak intensity ratio. A second peak intensity ratio is defined as the ratio between the peak intensity of F of LiF and the peak intensity of F other than LiF in the XPS spectrum of the second S i-containing particles. The second peak intensity ratio is greater than the first peak intensity ratio.

Description

Technical Field

[0001] This disclosure relates to a negative electrode 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, secondary batteries have become suitable for use as portable power sources for personal computers, mobile terminals, electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs).

[0003] In applications for vehicle power supplies, particularly for battery electric vehicles (BEVs), there is a desire to further increase the capacity of secondary batteries from the viewpoint of extending the vehicle's driving range. As a high-capacity negative electrode active material, Si-containing particles are known, and it is known that secondary batteries can achieve high capacity based on Si-containing particles. Patent Document 1 discloses a negative electrode for electrochemical elements. This negative electrode active material comprises carbon material, silicon, and lithium fluoride (LiF). The negative electrode active material is composed of a carbon phase containing carbon material and Si-LiF mixed particles. The Si-LiF mixed particles are dispersed in the carbon phase in a uniform or non-uniform distribution. This publication describes how, by using the negative electrode active material with the above-described structure, initial efficiency can be improved, volume expansion can be suppressed, irreversible phenomena can be reduced, and conductivity can be improved, as well as the uniform dispersion of active material particles within the electrode can be achieved. Furthermore, in batteries containing a negative electrode with this structure, the battery life characteristics are improved.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Publication No. 2019-522886 Summary of the Invention

[0007] The problem to be solved by the present invention

[0008] However, Si-containing particles exhibit significant expansion and contraction during charge and discharge. In secondary batteries using Si-containing particles, the negative electrode repeatedly expands and contracts during repeated charge and discharge cycles, leading to a decrease in cycle performance. Therefore, it is desirable to develop a negative electrode that suppresses the decrease in capacity retention during repeated charge and discharge cycles.

[0009] In view of the above actual situation, the purpose of this disclosure is to suppress the decrease in capacity retention rate during repeated charge and discharge for anodes containing Si particles.

[0010] Methods for solving problems

[0011] 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 contains first Si-containing particles and second Si-containing particles as negative electrode active materials. When the expansion rate S2 of the second Si-containing particles after charging A relative to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles after charging A relative to before charging A is greater than 0.3 and less than 0.9. In charging A, the battery is charged at a constant current of 0.01C to 4.2V at 25°C, and then charged at a constant voltage until the current value becomes 0.005C. The first Si-containing particles include a LiF coating. The second Si-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 Si-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 Si-containing particles is defined as the second peak intensity ratio. At this point, the intensity ratio of the second peak is greater than that of the first peak. Based on this configuration, for anodes containing Si particles, the decrease in capacity retention during repeated charge and discharge cycles can be suppressed.

[0012] From another perspective, the method for manufacturing the negative electrode of the secondary battery disclosed herein includes: a step of preparing first Si-containing particles and second Si-containing particles as negative electrode active materials; a step of mixing the first Si-containing particles and second Si-containing particles in a dispersion medium to prepare a negative electrode paste; a step of coating the negative electrode paste onto a negative electrode current collector; and a step of drying the coated negative electrode paste. When the expansion rate S2 of the second Si-containing particles after charging A relative to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles after charging A relative to before charging A is greater than 0.3 and less than 0.9. Charging A is a constant current charging at 0.01C to 4.2V at 25°C, followed by constant voltage charging to a current value of 0.005C. The first Si-containing particles include a LiF coating. The second Si-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 Si-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 Si-containing electrode is defined as the second peak intensity ratio. In this case, the second peak intensity ratio is larger than the first peak intensity ratio. Based on this configuration, for anodes containing Si particles, the decrease in capacity retention during repeated charge-discharge cycles can be suppressed.

[0013] On the other hand, the secondary battery disclosed herein includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode is as described above. With this configuration, for a negative electrode containing Si particles, it is possible to suppress the decrease in capacity retention during repeated charge and discharge cycles. Attached Figure Description

[0014] Figure 1 A cross-sectional view illustrating the configuration of the negative electrode of a secondary battery according to an embodiment of the present disclosure.

[0015] Figure 2 To illustrate Figure 1 A cross-sectional view of the particles contained in the negative electrode active material layer.

[0016] Figure 3 This is a schematic diagram illustrating the structure of a lithium-ion secondary battery constructed using the negative electrode of a secondary battery according to an embodiment of the present disclosure.

[0017] Figure 4 To show Figure 3 A schematic exploded view of the structure of the wound electrode body of a lithium-ion secondary battery.

[0018] Symbol Explanation

[0019] 12 First Si-containing particles

[0020] 14 Second Si-containing particles

[0021] 16 graphite particles

[0022] 20-wound electrode body

[0023] 30 Battery Casing

[0024] 36 safety valves

[0025] 42 positive extremes

[0026] 42a Positive Current Collector

[0027] 44 Negative Extremes

[0028] 44a negative current collector

[0029] 50 positive electrode plates (positive electrode)

[0030] 52 Positive Current Collector

[0031] 52a Non-forming part of the positive electrode active material layer

[0032] 54 Positive Electrode Active Material Layer

[0033] 60 negative electrode plate (negative electrode)

[0034] 62 negative current collector

[0035] 62a Non-forming part of negative electrode active material layer

[0036] 64 negative electrode active material layer

[0037] 70-segment plate (segment)

[0038] 100 Lithium-ion Secondary Battery Detailed Implementation

[0039] The embodiments of this disclosure will be described below with reference to the accompanying drawings. Furthermore, matters not mentioned in this specification, i.e., matters necessary for the implementation of this disclosure, can be grasped by those skilled in the art based on prior art. This disclosure can be implemented based on the content disclosed in this specification and common technical knowledge in the field. Additionally, in the following drawings, components and parts that perform the same function are labeled with the same reference numerals. Furthermore, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships. Also, in this specification, the numerical range denoted as "A to B" includes both A and B.

[0040] It should be noted that in this manual, "secondary battery" refers to an energy storage device capable of repeated charging and discharging. Furthermore, in this manual, "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.

[0041] The negative electrode disclosed herein is used in secondary batteries and is suitable for use in lithium-ion secondary batteries. For one embodiment of the negative electrode disclosed herein, see [reference needed]. Figure 1 Please explain in detail. Figure 1 The cross-sectional view shown is schematically an example of the negative electrode 60 of 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.

[0042] 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.

[0043] 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.

[0044] The negative electrode current collector 62 is shown in the example as 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 mesh-shaped. As with conventional lithium-ion secondary batteries, a metal with good conductivity (such as copper, nickel, titanium, stainless steel, etc.) can be used as the material for the negative electrode current collector 62, with copper being preferred. Copper foil is particularly preferred as the negative electrode current collector 62.

[0045] 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.

[0046] However, it is known that during the initial charging of a secondary battery, the non-aqueous electrolyte decomposes, forming an SEI film on the surface of the negative electrode active material. The inventors have discovered that, in the prior art, the capacity retention rate of a secondary battery decreases during repeated charge-discharge cycles as follows.

[0047] In silicon-containing negative electrode active materials, such as those used in secondary batteries, there is significant volume change during charge and discharge. Therefore, due to the expansion during charging, the SEI film on the surface of the negative electrode active material is damaged. When the SEI film is damaged, the surface of the negative electrode active material is exposed, and the non-aqueous electrolyte decomposes in the exposed portion, forming another SEI film. With repeated charge and discharge cycles of the secondary battery, this damage to the SEI film and the formation of another SEI film in the damaged portion (the exposed portion of the negative electrode active material) occur repeatedly. This can be one reason for the decrease in capacity retention of the secondary battery during repeated charge and discharge cycles.

[0048] Therefore, the inventors envision pre-forming a LiF film on the surface of a Si-containing negative electrode active material. The LiF film, for example, has the same function as an SEI film, and on the other hand, can follow the volume change of the negative electrode active material. Therefore, the inventors believe that by pre-forming a LiF film on the surface of the negative electrode active material, the LiF film is unlikely to be damaged even with repeated charging and discharging of the secondary battery, thus suppressing the exposure of the negative electrode active material's surface. The inventors believe that this can suppress the formation of an SEI film on the exposed portion of the negative electrode active material, thereby suppressing the decrease in the capacity retention rate of the secondary battery.

[0049] Furthermore, the inventors believe that, for example, even when the negative electrode active material expands and contracts during the charging and discharging of the secondary battery, maintaining the conductive pathways in the negative electrode active material layer in an appropriate state can better suppress the decrease in the capacity retention rate of the secondary battery. Therefore, the inventors have incorporated two negative electrode active materials (both containing Si) with different expansion rates during charging into the negative electrode active material layer. When the negative electrode active material layer contains two negative electrode active materials with different expansion rates, the filling capacity of the negative electrode active material layer can be improved even when both expand and contract during the charging and discharging of the secondary battery. The inventors believe that, for two negative electrode active materials with different expansion rates during charging, by forming an appropriate amount of LiF film on their respective surfaces, the decrease in capacity retention rate can be better suppressed even under repeated charging and discharging of the secondary battery.

[0050] The negative electrode active material layer 64 contains a negative electrode active material. For example, a first Si-containing particle and a second Si-containing particle are used as the negative electrode active material. For this purpose, [the following is used]... Figure 2 Please provide an explanation. Figure 2 To show Figure 1 A schematic cross-sectional view of the particles of the negative electrode active material contained in the negative electrode active material layer 64 is shown. Figure 2 As shown, the negative electrode active material layer 64 includes a first Si-containing particle 12 and a second Si-containing particle 14. This will be explained further. Figure 2 For illustrative purposes only; therefore, the number, distribution, etc., of particles are not limited to... Figure 2 The content shown.

[0051] like Figure 2 As shown, the first Si-containing particle 12 has a core particle 121 and a coating 122. The core particle 121 is, for example, a Si-containing particle. Here, the coating 122 is disposed on at least a portion of the surface of the core particle 121. Figure 2 As shown, the second Si-containing particle 14 has a core particle 141 and a coating 142. The core particle 141 is, for example, a Si-containing particle. Here, the coating 142 is disposed on at least a portion of the surface of the core particle 141. The Si-containing particles constituting the core particle 121 and the core particle 141 can be Si particles, such as Si particles, Si oxide particles, Si-C composite particles, etc. Both the core particle 121 and the core particle 141 can be Si-C composite particles. Si-C composite particles, for example, have carbon domains or Si domains.

[0052] 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, it is advantageous because the carbon matrix can mitigate volume changes caused by the expansion and contraction of the Si domains.

[0053] 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). x It 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.

[0054] The average particle size of the Si domains is, for example, less than 50 nm, and can be between 5 nm and 50 nm. 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 create a sample for observation using a scanning transmission electron microscope (STEM). Then, elemental analysis of the sample is performed using EDX elemental mapping, obtaining BF (bright field) and HAADF (high angle scattering annular dark field) images. 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.

[0055] 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 can be contained within the carbon substrate, and if the carbon substrate has voids, it may have a size that can be contained within the voids. From this viewpoint, the Si contained within the carbon substrate can 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 thus mitigating volume changes of the first Si-containing particle 12 and the second Si-containing particle 14, the Si-C composite particles preferably comprise a void-containing carbon substrate (more preferably a porous carbon substrate) and Si particles (e.g., Si-containing nanoparticles) disposed within the voids. Alternatively, in other ways, the Si-C composite particles may contain carbon particles and Si-containing nanoparticles attached to the surface of the carbon particles, or they may contain Si-containing particles and carbon nanoparticles attached to the surface of the Si-containing particles.

[0056] Here, coatings 122 and 142 contain LiF. The presence of coatings 122 and 142 can be confirmed, for example, by measuring the first Si-containing particles 12 and the second Si-containing particles 14 using X-ray photoelectron spectrometry (XPS). Regarding the LiF content ratio (LiF concentration) in coating 122, it is the ratio of the peak intensity of F corresponding to LiF to the peak intensity of F other than LiF in the XPS energy spectrum of the first Si-containing particles 12 measured by XPS. Regarding the LiF content (LiF concentration) in coating 142, it is the ratio of the peak intensity of F corresponding to LiF to the peak intensity of F other than LiF in the XPS energy spectrum of the second Si-containing particles 14 measured by XPS. Note that in the XPS energy spectrum measured by XPS, the peak of F of LiF appears at 683 eV to 686 eV. The peaks of F other than 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.

[0057] Here, the ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS energy spectrum of the first Si-particle-12 sample 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 Si-particle-14 sample is defined as the second peak intensity ratio. Here, the second peak intensity ratio is larger than the first peak intensity ratio. The second peak intensity ratio is, for example, 1.1 to 3 times the first peak intensity ratio, preferably 1.2 to 2.5 times.

[0058] The first peak intensity ratio is, for example, 0.200 or more, preferably 0.250 or more, and more preferably 0.300 or more. On the other hand, the first peak intensity ratio is, for example, less than 0.450, preferably 0.400 or less, and more preferably 0.380 or less. The second peak intensity ratio is, for example, 0.450 or more, preferably 0.480 or more, and more preferably 0.500 or more. On the other hand, the second peak intensity ratio is approximately 1.00 or less, for example, 0.900 or less, preferably 0.850 or less, more preferably 0.800 or less, and even more preferably 0.750 or less.

[0059] The presence of a coating 122 on the nucleus particle 121 and a coating 142 on the nucleus particle 141 can be confirmed, for example, by SEM observation. The coating 122 coverage on the surface of the nucleus particle 121 is, 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 coverage on the surface of the nucleus particle 141 is, 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. Although not particularly limited, the coating 142 coverage on the surface of the nucleus particle 141 can, for example, be greater than the coating 122 coverage on the surface of the nucleus particle 121. It should be noted that the coating 122 or coating 142 can be determined as follows: Obtain an SEM image of the cross-section of the first Si-containing particle 12 or the second Si-containing particle 14. In this image, the ratio (percentage) of the total length of the coating 122 or coating 142 to the perimeter of the nucleus 121 or nucleus 141 is calculated. For any five or more first Si-containing particles 12 or second Si-containing particles 14 selected arbitrarily, this ratio is calculated, and the average of these ratios is taken as the coating rate.

[0060] When the expansion rate S2 of the second Si-containing particle 14 is set to 1, the expansion rate S1 of the first Si-containing particle 12 is, for example, greater than 0.3 and less than 0.9, and preferably 0.4 to 0.8 from the viewpoint of achieving the technical effects disclosed herein. Expansion rates S1 and S2 here refer to the volume expansion rates after charging A relative to before charging A. Expansion rates S1 and S2 can be appropriately changed, for example, by altering the Si content ratio in the first Si-containing particle 12 and the second Si-containing particle 14, the amount of porosity in the carbon substrate, etc.

[0061] Here, charging A refers to charging at 25°C with a constant current of 0.01C to 4.2V, followed by constant voltage charging to 0.005C. For example, charging A is performed on an evaluation secondary battery (e.g., a lithium-ion secondary battery) having a negative electrode containing only first or second Si-containing particles as the negative electrode active material. During charging A, the evaluation secondary battery is preferably in a state of 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 Si-containing 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.

[0062] The expansion rate S1 can be calculated, for example, as follows. First, a cross-section along the thickness direction of the negative electrode active material layer containing only the first Si-containing particles as the negative electrode active material is processed for observation in a electron microscope (SEM). This processing is carried out using a cross-section polishing machine under conditions of 4 kV voltage and 8 hours of 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 having this 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 with the SEM image 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 Si-containing particles is randomly measured from the SEM image before charging A, and then the area P2 of the same first Si-containing particles is measured for the SEM image after charging A. For each particle, the expansion rate is calculated using the following formula (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 Si-containing particle.

[0063] Expansion rate (%) = [√{(Area P2) / (Area P1)}] 3 ×100(A)

[0064] The expansion rate S1 is, for example, smaller than the expansion rate S2. The expansion rate S1 is, for example, 100% or more, preferably less than 200%. Although not particularly limited, the expansion rate S1 may be 110% or more, 120% or more, or 190% or less, or 180% or less.

[0065] 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 Si-containing particles as the negative electrode active material, is processed for observation in a SEM. Regarding the expansion ratio S2, for example, the area Q1 of 100 second Si-containing particles is randomly measured from the SEM image before charging A, and then the area Q2 of the same second Si-containing particles is measured for the SEM image after charging A. Then, for each particle, the expansion ratio is calculated using the following formula (B). The arithmetic mean of the expansion ratios of the 100 particles obtained here is taken as the expansion ratio S2 of the second Si-containing particles. Furthermore, the steps for determining the expansion ratio S2 can be the same as those for determining the expansion ratio S1. Therefore, the explanation of the steps for determining the expansion ratio S2 is appropriately omitted.

[0066] Expansion rate (%) = [√{(Area Q2) / (Area Q1)}] 3 ×100(B)

[0067] The expansion rate S2 is, for example, greater than the expansion rate S1. The expansion rate S2 is, for example, 200% or more, preferably 400% or less. Although not particularly limited, the expansion rate S2 may be 210% or more, or 380% or less, 360% or less, or 350% or less.

[0068] The average particle size (D50) of the first Si-containing particles 12 and the average particle size (D50) of the second Si-containing particles 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 Si-containing particles 12 and the average particle size (D50) of the second Si-containing particles 14 may be the same or different.

[0069] The Si content in the first Si-containing particle 12 and the Si content in the second Si-containing particle 14 may be the same or different. Although not particularly limited, the Si content in the first Si-containing particle 12 and the Si content in the second Si-containing particle 14 may be approximately 20% to 80% by mass.

[0070] The mass ratio of the first Si-containing particles 12 to the second Si-containing particles 14 in the negative electrode active material layer 64 can be the same or different. Regarding the mass ratio (first Si-containing particles 12: second Si-containing particles 14), there is no particular limitation as long as the effect of the technology disclosed herein is obtained, for example, it is 5:95 to 95:5. This mass ratio (first Si-containing particles 12: second Si-containing particles 14) is preferably 10:90 to 90:10, more preferably 15:85 to 85:15. When the mass ratio of the first Si-containing particles 12 to the second Si-containing particles 14 in the negative electrode active material layer 64 is different, from the viewpoint of better achieving the effect of the technology disclosed herein, it is preferable that the mass ratio of the second Si-containing particles 14 is relatively small. In this case, when the total of the first Si-containing particles 12 and the second Si-containing particles 14 is 100% by mass, the proportion of the second Si-containing particles 14 is preferably 5% to 40% by mass, more preferably 10% to 30% by mass.

[0071] It should be noted that the first Si-containing particle 12 and the second Si-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).

[0072] like Figure 3 As shown, from the viewpoint of improving the conductivity of the negative electrode 60, the negative electrode active material layer 64 may also contain graphite particles 16 as the negative electrode active material. The graphite particles 16 may, for example, be graphite particles that are substantially free of Si. The Si content in the graphite particles 16 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 using conventionally known methods such as ICP analysis.

[0073] The graphite constituting the graphite particles 16 can be natural graphite, artificial graphite, or graphite coated with amorphous carbon material.

[0074] The shape of the graphite particles 16 is not particularly limited and can be flake-like, spherical, etc. The graphite particles 16 are preferably spherical graphite particles. When the graphite particles 16 are spherical, the sphericity of the graphite particles 16 is preferably 0.85 to 1, more preferably 0.88 to 1, and even more preferably 0.90 to 1.

[0075] To clarify, in this specification, "circularity" refers to the ratio of the circumference of a circle with the same area as the particle's projected area to the circumference of the particle's projected image (i.e., circularity = circumference of a circle with the same area as the particle's projected area / 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 perfect circle, and the closer the particle is to a perfect sphere. Circularity can be calculated, for example, by using a commercially available static automatic image analysis device to determine the circularity of more than 100 particles and calculating its average value.

[0076] There is no particular limitation on the average particle size (D50) of the graphite particles 16. The average particle size (D50) of the graphite particles 16 is, for example, 1 μm to 30 μm, preferably 5 μm to 25 μm, more preferably 10 μm to 23 μm, and even more preferably 12 μm to 20 μm.

[0077] The proportion of graphite particles 16 relative to the total of the first Si-containing particles 12, the second Si-containing particles 14, and the graphite particles 16 is preferably 40% to 90% by mass, more preferably 45% to 85% by mass, and even more preferably 50% to 80% by mass. The proportion of the first Si-containing particles 12 and the second Si-containing particles 14 relative to the total of the first Si-containing particles 12, the second Si-containing particles 14, and the graphite particles 16 is preferably 10% to 60% by mass, more preferably 15% to 55% by mass, and even more preferably 20% to 50% by mass.

[0078] 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 is also used 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 the conductive material, the negative electrode active material layer 64 may contain a CNT dispersant.

[0079] The content of negative electrode active material in the negative electrode active material layer 64 (i.e., relative to the total mass of the active material layer 64) is preferably 90% by mass or more, more preferably 95% by mass or more. The content of binder in the negative electrode active material layer 64 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 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.

[0080] 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.

[0081] 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.

[0082] 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 contains first Si-containing particles 12 and second Si-containing particles 14 as negative electrode active materials. When the expansion rate S2 of the second Si-containing particles 14 after charging A relative to before charging A is set to 1, the expansion rate S1 of the first Si-containing particles 12 after charging A relative to before charging A is greater than 0.3 and less than 0.9. Charging A is a constant current charging at 0.01C to 4.2V at 25°C, followed by constant voltage charging to a current value of 0.005C.

[0083] The first Si-containing particle 12 has a LiF coating (here, the LiF coating contained in coating 122). The second Si-containing particle 14 has a LiF coating (here, the LiF coating contained in 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 Si-containing particle 12, measured by XPS, is defined as the first peak intensity ratio. The ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS spectrum of the second Si-containing particle 14 is defined as the second peak intensity ratio. In this case, the second peak intensity ratio is larger than the first peak intensity ratio.

[0084] In other words, in the negative electrode 60, the negative electrode active material layer 64 includes a first Si-containing particle 12 with a relatively small expansion rate and a second Si-containing particle 14 with a relatively large expansion rate. The expansion rate of the first Si-containing particle 12 relative to the expansion rate of the second Si-containing particle 14 is set to be greater than 0.3 and less than 0.9. Therefore, even with the expansion and contraction of the negative electrode active material during charge and discharge, the decrease in the filling capacity of the negative electrode active material in the negative electrode active material layer 64 can be suppressed, thus maintaining the conductive path in an appropriate state. Furthermore, both the first Si-containing particle 12 and the second Si-containing particle 14 include a LiF coating. The second peak intensity ratio of the second Si-containing particle 14 with a relatively large expansion rate is greater than the first peak intensity ratio of the first Si-containing particle 12 with a relatively small expansion rate. In other words, in the second Si-containing particle 14 with a larger expansion rate than the first Si-containing particle 12, the amount of LiF coating is greater than that of the first Si-containing particle 12. By setting the LiF coating according to the expansion rate, for example, surface exposure caused by expansion can be appropriately suppressed. Therefore, according to this configuration, the decrease in capacity retention rate during repeated charge and discharge can be suppressed.

[0085] The first peak intensity ratio can be greater than 0.200 and less than 0.450. This can appropriately suppress the formation of exposed portions on the surface of the first Si-particle-containing 12 due to expansion during charging.

[0086] The intensity ratio of the second peak can be between 0.450 and 1.00. This allows for the appropriate suppression of exposed portions on the surface of the second Si-containing particle 14 due to expansion during charging. Furthermore, it suppresses the excessive resistance of the negative electrode 60 caused by the formation of the LiF film.

[0087] The first Si-containing particle 12 may include: 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 Si-containing particle 14 may include: 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 Si-containing particle 12 and the second Si-containing particle 14.

[0088] The negative electrode active material layer 64 may further contain graphite particles that are essentially free of Si as the negative electrode active material. This improves the conductivity of the negative electrode active material layer 64.

[0089] The negative electrode 60 can be manufactured, for example, by the following manufacturing method. The manufacturing method of the negative electrode 60 includes, for example, a step of preparing first Si-containing particles 12 and second Si-containing particles 14 (hereinafter also referred to as the "preparation step"); a step of mixing the first Si-containing particles 12, the second Si-containing particles 14, and graphite particles 16 as needed in a dispersion medium to prepare a negative electrode paste (hereinafter also referred to as the "paste preparation step"); a step of coating the negative electrode paste onto a negative electrode current collector (hereinafter also referred to as the "coating step"); and a step of drying the coated negative electrode paste (hereinafter also referred to as the "drying step"). By implementing such a manufacturing method, a negative electrode 60 capable of suppressing the decrease in capacity retention during repeated charge and discharge can be provided.

[0090] It should be noted that the term "paste" in this specification refers to a mixture in which some or all of the solid components are dispersed in a dispersion medium, including so-called "slurry," "ink," etc.

[0091] The preparation process includes, for example, preparing a first Si-containing particle 12 having Si-containing particles as a core particle 121 and a coating 122, and a second Si-containing particle 14 having Si-containing particles as a core particle 141 and a coating 142. The Si-containing particles 121 and 141 are, for example, Si-C composite particles. Regarding Si-C composite particles, as described above, known methods can be used for preparation.

[0092] Next, with the obtained nuclei 121 and 141 dispersed in water or a water-soluble organic solvent, a water-soluble lithium salt and a fluorinating agent are reacted on the Si-C composite particles to generate LiF. The LiF generated here forms a film 122 on the surface of nuclei 121 and a film 142 on the surface of nuclei 141.

[0093] 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.

[0094] In this operation, for example, an aqueous solution of a water-soluble lithium salt, a dispersion in which nuclei 121 or 141 are dispersed in water or a water-soluble organic solvent, and an aqueous solution of a fluorinating agent are first prepared. The aqueous solution of the water-soluble lithium salt and the dispersion are mixed to prepare a mixture. Under stirring, an aqueous solution of the fluorinating agent is added to the mixture. This causes the water-soluble lithium salt and the fluorinating agent to react and generate LiF. Thus, a reaction solution containing LiF is obtained. In this reaction solution, the surface of nuclei 121 or 141 can be in contact with LiF.

[0095] The reaction conditions for the water-soluble lithium salt and fluorinating agent can be the same as those for the synthesis of known LiF using water-soluble lithium salt and fluorinating agent. For example, the reaction can be carried out at room temperature (i.e., 25℃ ± 10℃) 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 membranes 122 and 142, etc. The longer the reaction time, the higher the proportion of LiF in membranes 122 and 142.

[0096] By recovering nuclei 121 or 141 from the reaction solution while they are attached to the surface of the LiF-generating reaction solution, and drying the nuclei 121 or 141 with the LiF-generating reaction solution attached, LiF can be attached to the surface of the nuclei 121 or 141, resulting in a first Si-containing particle 12 or a second Si-containing particle 14 having a LiF-containing film 122 or 142. To adjust the amount of LiF, the LiF-generating reaction solution can be diluted with water or the like.

[0097] Regarding the paste preparation process, it can be carried out by mixing the first Si-containing particles 12, the second Si-containing particles 14, the required graphite particles 16, and any component (e.g., binder) with a dispersion medium (e.g., water) using known methods and known mixing devices, stirring devices, etc.

[0098] The coating process can be performed using known methods. For example, the obtained negative electrode paste can be coated onto the negative electrode current collector 62 using coating equipment such as a gravure coating machine, a comma coating machine, a slot coating machine, or a die coating machine.

[0099] The drying process can be performed using known methods. For example, the dispersion medium can be removed from the negative electrode current collector 62 coated with negative electrode paste using a drying device such as a drying oven, forming a negative electrode active material layer 64. This allows the drying process to be performed. The drying temperature and drying time can be appropriately determined based on the solid component concentration of the negative electrode paste and are not particularly limited. The drying temperature is, for example, 60°C or higher and 200°C or lower, 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.

[0100] After the drying process, a pressing process can be further performed on the negative electrode active material layer 64. The pressing process can be carried out using known methods. For example, the pressing process can be performed by applying pressure to the negative electrode active material layer 64 formed above using a roller press or the like. Through the pressing process, the first Si-containing particles 12, the second Si-containing particles 14, and the graphite particles 16 contained in the negative electrode active material layer 64 can be tightly filled. As described above, a negative electrode 60 can be obtained.

[0101] 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 uses a lithium-ion secondary battery as an example, referring to... Figure 3 and Figure 4 An embodiment of the secondary battery disclosed herein will be described. The following configuration example is a flat, square lithium-ion secondary battery having a flat-shaped wound electrode body and a flat-shaped battery casing.

[0102] Figure 3 The lithium-ion secondary battery 100 shown is a sealed lithium-ion secondary battery 100 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 packaging 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 if the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, the battery casing 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. 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 metal material with good thermal conductivity, such as aluminum.

[0103] 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 along their length. 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 along its length. 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 along its length. 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 extending outward from both ends in the winding axis direction of the wound electrode body 20 (i.e., the sheet width direction orthogonal to the aforementioned length direction). A positive electrode current collector 42a and a negative electrode current collector 44a are respectively bonded to the non-forming portion 52a of the positive electrode active material layer and the non-forming portion 62a of the negative electrode active material layer.

[0104] As the positive current collector 52 constituting the positive electrode sheet 50, a known positive current collector used in 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.

[0105] 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.

[0106] The positive electrode active material layer 54 contains a positive electrode active material. As the positive electrode active material, a known composition of positive electrode active material used in lithium-ion secondary batteries 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.

[0107] 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.

[0108] It should be noted that in this specification, "lithium-nickel-cobalt-manganese composite oxide" is a term used to describe oxides that contain one or more additional elements besides Li, Ni, Co, Mn, and O as constituent elements. 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-metallic elements such as B, C, Si, and P, and non-metallic elements 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.

[0109] Examples of lithium transition metal phosphate compounds include lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), and lithium manganese iron phosphate.

[0110] 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 properties, such as initial resistance characteristics.

[0111] 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.

[0112] 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 (CNTs), and other carbon materials (such as graphite) are preferred. As binders, polyvinylidene fluoride (PVdF) can be used, for example.

[0113] 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.

[0114] 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.

[0115] The negative electrode 60 described above is used as the negative electrode sheet 60.

[0116] 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 stacked on both sides of a PE layer). A heat-resistant layer (HRL) can be provided on the surface of the separator 70.

[0117] 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 Grie test method is not particularly limited, but preferably 350 seconds / 100cc or less.

[0118] Non-aqueous electrolytes typically contain a non-aqueous solvent and a supporting salt (electrolyte salt). As the non-aqueous solvent, organic solvents such as carbonates, ethers, esters, nitriles, sulfones, and lactones, commonly used in electrolytes for 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), ethylene monofluorocarbonate (FEC), ethylene difluorocarbonate (DFEC), methyl monofluorodifluoromethyl carbonate (F-DMC), and dimethyl trifluorocarbonate (TFDMC). Such non-aqueous solvents can be used alone or in appropriate combinations of two or more. As one 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.

[0119] As a supporting salt, lithium salts such as LiPF6, LiBF4, and lithium bis(fluorosulfonyl)imide (LiFSI) (preferably LiPF6) can be used appropriately. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.

[0120] It should be noted that, for the aforementioned non-aqueous electrolyte, components other than those mentioned above may be included, provided that the effects of this disclosure are not significantly impaired, 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 thickeners.

[0121] The lithium-ion secondary battery 100 can be used for various applications. Suitable applications include power supplies for electric vehicles (BEVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs). Additionally, the lithium-ion secondary battery 100 can also be used as a battery for small energy storage devices. The lithium-ion secondary battery 100 can also typically be used in the form of a battery pack consisting of multiple batteries connected in series and / or in parallel.

[0122] The above description, as an example, describes a square lithium-ion secondary battery 100 including a flat, wound electrode body 20. However, the lithium-ion secondary battery disclosed herein can also be configured as a lithium-ion secondary battery including a stacked electrode body (i.e., an electrode body in which multiple positive electrodes and multiple negative electrodes are alternately stacked). Furthermore, the lithium-ion secondary battery can also be configured as a cylindrical lithium-ion secondary battery, a laminated shell type lithium-ion secondary battery, etc.

[0123] In addition, according to known methods, the lithium-ion secondary battery 100 can also be configured as an all-solid-state lithium-ion secondary battery that uses a solid electrolyte instead of a non-aqueous electrolyte.

[0124] Furthermore, the negative electrode 60 involved in this embodiment is suitable for the negative electrode of lithium-ion secondary batteries, but it can also be constructed as the negative electrode of other secondary batteries, which can be constructed according to known methods.

[0125] The following describes in detail the embodiments related to this disclosure, but it is not intended to limit this disclosure to the contents shown in the embodiments.

[0126] Fabrication of Si-C composite particles with LiF-containing coatings

[0127] Si-C composite particles were prepared as the core particles. The prepared Si-C composite particles consisted of a porous carbon substrate and Si nanoparticles disposed within the pores of the carbon substrate. The Si-C composite particles were dispersed in ethanol and sonicated for 30 minutes. Lithium acetate was dissolved in deionized water to prepare an aqueous lithium acetate solution. The dispersion of the Si-C composite particles was added to this aqueous solution while stirring for 30 minutes. An aqueous ammonium fluoride solution was added to the solution while stirring for a specified time to allow the lithium acetate and ammonium fluoride to react. This yielded a reaction solution containing LiF. The reaction solution was diluted and filtered. The filtrate was dried to obtain Si-C composite particles with a LiF-containing coating.

[0128] XPS spectra of the obtained Si-C composite particles revealed peaks for F in Li-F (approximately 684.8 eV) and Li in Li-F (approximately 55.1 eV), confirming the presence of LiF. The ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF (hereinafter referred to as the "LiF intensity ratio") was calculated. 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] Through the above steps, Si-containing particles (A) to (J) were obtained. Si-containing particle (A) has an expansion rate of 153% and a LiF intensity ratio of 0.362. Si-containing particle (B) has an expansion rate of 258% and a LiF intensity ratio of 0.629. Si-containing particle (C) has an expansion rate of 126% and a LiF intensity ratio of 0.308. Si-containing particle (D) has an expansion rate of 339% and a LiF intensity ratio of 0.729. Si-containing particle (E) has an expansion rate of 173% and a LiF intensity ratio of 0.373. Si-containing particle (F) has an expansion rate of 228% and a LiF intensity ratio of 0.514. Si-containing particle (G) has an expansion rate of 121% and a LiF intensity ratio of 0.301. Si-containing particle (H) has an expansion rate of 416% and a LiF intensity ratio of 1.280. Si-containing particles (I) have an expansion rate of 183% and a LiF strength ratio of 0.382. Si-containing particles (J) have an expansion rate of 187% and a LiF strength ratio of 0.502. It should be noted that the LiF strength ratio of each Si-containing particle is different by appropriately varying the addition time of the ammonium fluoride aqueous solution (i.e., the reaction time). Furthermore, Table 1 records the presence or absence of a LiF coating and the LiF strength ratio of the Si-containing particles in each example.

[0130] Simultaneously, Si-containing particles (X) and Si-containing particles (Y) were prepared as Si-C composite particles without a LiF coating. Si-containing particles (X) have an expansion rate of 153%. Si-containing particles (Y) have an expansion rate of 258%.

[0131] <Determination of Expansion Rate>

[0132] The expansion rate of the Si-containing particles was obtained through the following steps. A negative electrode with the same structure as in this example was fabricated using the same steps as in this example, except that only the Si-containing particles were used as the negative electrode active material. A cross-section of this negative electrode along its thickness direction was machined for SEM observation, and an SEM image of the machined surface was obtained. The magnification at this time was 1500x. Next, a test battery with the same structure as in this example was fabricated using the same steps as in this example, except that the negative electrode was used. The test battery was placed at 25°C and charged with a constant current of 0.01C to 4.2V, followed by constant voltage charging until the current reached 0.005C. Then, the test battery was disassembled, and the negative electrode was removed. Then, an SEM image of the machined surface obtained before charging was performed, and a 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. Next, for the SEM image after charging, the area A2 of the same first particle was measured. Then, for each particle, the expansion rate was calculated using the following formula (R). The arithmetic mean of the expansion rates of the 100 particles obtained here was calculated as the expansion rate of the Si-containing particles.

[0133] Expansion rate (%) = [√{(Area A2) / (Area A1)}] 3 ×100(R)

[0134] <Making the Negative Electrode>

[0135] [Example 1]

[0136] As the negative electrode active material, the above-mentioned Si-containing particles (A) as the first particle and Si-containing particles (B) as the second particle, and graphite particles with an average particle size (D50) of 15 μm are prepared. These graphite particles are substantially free of Si. Single-layer carbon nanotubes (SWCNTs) are prepared as a dispersion of the conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene-butadiene rubber (SBR) are prepared as binders.

[0137] The above materials were mixed with water as a solvent in a mass ratio of graphite particles / first particle / second particle / SWCNT / CMC / PAA / SBR = 65 / 28 / 7 / 0.1 / 1 / 1 / 1.5 to prepare a negative electrode paste.

[0138] The negative electrode paste is produced 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 are first fed into a mixer and dispersed at 3000 rpm using a disperser, thereby producing the first paste. In the second step, graphite particles, CMC, and PAA are dry-mixed using a stirred granulator. The first paste, the mixed powder obtained by dry mixing, and the dispersion medium (water) are then thickened and kneaded. The solids content during thickening and kneading is 65%. SBR and the dispersion medium (water) are further added to the thickened and kneaded mixture and mixed. This produces the negative electrode paste.

[0139] The prepared negative electrode paste is 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 the specified size to obtain the negative electrode sheet in this example.

[0140] [Example 2]

[0141] Si-containing composite particles (C) were used as the first particles. Si-containing particles (D) were used as the second particles. Otherwise, the negative electrode of this example was obtained using the same materials and steps as in Example 1.

[0142] [Example 3]

[0143] Si-containing particles (E) were used as the first particles. Si-containing particles (F) were used as the second particles. Otherwise, the negative electrode of this example was obtained using the same materials and steps as in Example 1.

[0144] [Comparative Example 1]

[0145] Si-containing particles (X) were used as the first particles. Si-containing particles (Y) were used as the second particles. Otherwise, the negative electrode of this example was obtained using the same materials and steps as in Example 1.

[0146] [Comparative Example 2]

[0147] Si-containing particles (G) were used as the first particles. Si-containing particles (H) were used as the second particles. Otherwise, the negative electrode of this example was obtained using the same materials and steps as in Example 1.

[0148] [Comparative Example 3]

[0149] Si-containing particles (I) were used as the first particles. Si-containing particles (J) were used as the second particles. Otherwise, the negative electrode of this example was obtained using the same materials and steps as in Example 1.

[0150] <Evaluation of the fabrication of lithium-ion secondary batteries>

[0151] LiNi as the positive electrode active material powder 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are mixed with N-methylpyrrolidone (NMP) in a mass ratio of NCM:AB:PVdF = 100:1:1 to prepare a positive electrode paste. This paste is 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 is then rolled to obtain a sheet of specified dimensions, yielding the positive electrode sheet.

[0152] Prepare a separator made of porous polyolefin. Attach wires to the negative and positive electrode sheets prepared above, and fabricate the electrode body by stacking the separators. Contain this electrode body together with a non-aqueous electrolyte in a housing made of aluminum laminate. The non-aqueous electrolyte is prepared by dissolving LiPF6 (as a supporting salt) at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 15:5:40:40. Then, seal the housing to obtain an evaluation lithium-ion secondary battery.

[0153] <Cyclic Performance Evaluation>

[0154] The prepared evaluation lithium-ion secondary batteries were placed in an environment of 25°C. Each evaluation lithium-ion secondary battery was charged with a constant current of 0.4C until 4.2V, and then charged with a constant voltage until the current reached 0.1C. Next, each evaluation lithium-ion secondary battery was discharged with a constant current of 0.4C until 2.5V. The discharge capacity at this point was then measured to determine the initial capacity.

[0155] The above charge-discharge cycle is defined as one cycle, and this process is repeated for 200 cycles. The discharge capacity after 200 cycles is calculated using the same method as the initial capacity. As an indicator of cycle performance, the capacity retention rate (%) is calculated by multiplying (discharge capacity after 200 charge-discharge cycles / initial capacity) by 100%. The initial capacity is the discharge capacity of the first charge-discharge cycle described above. Furthermore, a capacity retention rate closer to 100% is better; a rate above 80% is considered to have suppressed the decrease in capacity retention after charge-discharge cycles.

[0156] [Table 1]

[0157] Table 1

[0158]

[0159] *Expansion ratio = Expansion ratio of the first particle / Expansion ratio of the second particle

[0160] As shown in Table 1, in Examples 1-3, the decrease in capacity retention rate during repeated charge-discharge cycles was suppressed. In Examples 1-3, as described above, when the expansion rate of the second particle after charge A relative to before charge A was set to 1, the expansion rate of the first particle after charge A relative to before charge A was greater than 0.3 and less than 0.9. The first and second particles 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 second 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 first particle.

[0161] 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 technical solutions with various modifications and alterations to the specific examples described above.

[0162] That is, the negative electrode of the secondary battery disclosed herein and its manufacturing method, and the secondary battery are the following items [1] to [8].

[0163] [1] The negative electrode is a secondary battery comprising a negative current collector and a negative active material layer supported by the negative current collector, wherein the negative active material layer comprises a first Si-containing particle and a second Si-containing particle as the negative active material, and when the expansion rate S2 of the second Si-containing particle after the charging A relative to before the charging A is set to 1, the expansion rate S1 of the first Si-containing particle after the charging A relative to before the charging A is greater than 0.3 and less than 0.9, wherein the charging A is performed at a constant current value of 0.01C under a 25°C environment. After charging to 4.2V, the constant voltage is used to charge until the current value becomes 0.005C. Both the first and second Si-containing particles have a LiF coating. The ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-containing particles, 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 Si-containing particles is defined as the second peak intensity ratio. The second peak intensity ratio is larger than the first peak intensity ratio.

[0164] [2] According to the negative electrode described in [1], wherein the first peak intensity ratio is greater than or equal to 0.200 and less than 0.450.

[0165] [3] The negative electrode according to [1] or [2], wherein the second peak intensity ratio is 0.450 or higher and 1.00 or lower.

[0166] [4] The negative electrode according to any one of [1] to [3], wherein the first Si-containing particles and the second Si-containing particles 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.

[0167] [5] The negative electrode according to any one of [1] to [4], wherein the negative electrode active material layer further comprises graphite particles that are substantially free of Si as the negative electrode active material.

[0168] [6] A method for manufacturing the negative electrode of a secondary battery, comprising: a step of preparing a first Si-containing particle and a second Si-containing particle as negative electrode active materials; a step of mixing the first Si-containing particle and the second Si-containing particle in a dispersion medium to prepare a negative electrode paste; a step of coating the negative electrode paste onto a negative electrode current collector; and a step of drying the coated negative electrode paste, wherein, when the expansion rate S2 of the second Si-containing particle after the charge A relative to before the charge A is set to 1, the expansion rate S1 of the first Si-containing particle after the charge A relative to before the charge A is greater than 0.3 and less than 0.9, wherein... In the charging process A, the particles are charged to 4.2V at a constant current of 0.01C under a 25°C environment, and then charged to 0.005C under a constant voltage. Both the first and second Si-containing particles have a LiF coating. The ratio of the peak intensity of F in LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-containing particles, 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 Si-containing particles is defined as the second peak intensity ratio. The second peak intensity ratio is greater than the first peak intensity ratio.

[0169] [7] According to the manufacturing method described in [6], in the preparation step, Si-C composite particles containing a carbon substrate and Si contained in the interior of the carbon substrate are prepared. While dispersing the Si-C composite particles in water or a water-soluble organic solvent, a water-soluble lithium salt and a fluorinating agent are reacted on the Si-C composite particles to generate LiF, thereby preparing the first Si-containing particles and the second Si-containing particles.

[0170] [8] A secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode is the negative electrode described in [1].

Claims

1. A negative electrode, which is a negative electrode of a secondary battery, comprising a negative electrode current collector, and a negative electrode active material layer supported by the negative electrode current collector, wherein the negative electrode active material layer contains a first Si-containing particle and a second Si-containing particle as a negative electrode active material, the negative electrode active material layer contains a first Si-containing particle and a second Si-containing particle as a negative electrode active material, when the expansion ratio S2 of the second Si-containing particle after the charge A with respect to the second Si-containing particle before the charge A is set to 1, the expansion ratio S1 of the first Si-containing particle after the charge A with respect to the first Si-containing particle before the charge A is greater than 0.3 and 0.9 or less, in the charge A, after constant current charging to 4.2 V at 0.01 C current value in a 25°C environment, constant voltage charging is performed until the current value becomes 0.005 C, both the first Si-containing particle and the second Si-containing particle have a LiF coating film, when the ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-containing particle measured by X-ray photoelectron spectroscopy is defined as a first peak intensity ratio, and the ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF in the XPS spectrum of the second Si-containing particle is defined as a second peak intensity ratio, the second peak intensity ratio is greater than the first peak intensity ratio.

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

450.

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

4. The negative electrode according to any one of claims 1 to 3, wherein the first Si-containing particle and the second Si-containing particle include: a Si-C composite particle having a carbon base material and Si contained inside the carbon base material, and the LiF coating film disposed on at least a part of the surface of the Si-C composite particle.

5. The negative electrode according to any one of claims 1 to 3, wherein the negative electrode active material layer further contains a graphite particle substantially not containing Si as a negative electrode active material.

6. A manufacturing method of a negative electrode of a secondary battery, comprising: a step of preparing a first Si-containing particle and a second Si-containing particle as a negative electrode active material; a step of mixing the first Si-containing particle and the second Si-containing particle in a dispersion medium to prepare a negative electrode paste; a step of coating the negative electrode paste on a negative electrode current collector; and a step of drying the coated negative electrode paste, wherein when the expansion ratio S2 of the second Si-containing particle after the charge A with respect to the second Si-containing particle before the charge A is set to 1, the expansion ratio S1 of the first Si-containing particle after the charge A with respect to the first Si-containing particle before the charge A is greater than 0.3 and 0.9 or less, in the charge A, after constant current charging to 4.2 V at 0.01 C current value in a 25°C environment, constant voltage charging is performed until the current value becomes 0.005 C, both the first Si-containing particle and the second Si-containing particle have a LiF coating film, when the ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF in the XPS spectrum of the first Si-containing particle measured by X-ray photoelectron spectroscopy is defined as a first peak intensity ratio, and the ratio of the peak intensity of F of LiF to the peak intensity of F other than LiF in the XPS spectrum of the second Si-containing particle is defined as a second peak intensity ratio, the second peak intensity ratio is greater than the first peak intensity ratio.

7. The manufacturing method according to claim 6, wherein In the preparation step, Si-C composite particles containing a carbon base material and Si contained in the inside of the carbon base material are prepared, In a state in which the Si-C composite particles are dispersed in water or a water-soluble organic solvent, a water-soluble lithium salt and a fluorinating agent are allowed to react on the Si-C composite particles to produce LiF, thereby preparing first Si-containing particles and second Si-containing particles.

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

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