Negative electrode for secondary battery, method for manufacturing the same, and secondary battery

Abstract

A negative electrode for a secondary battery includes a negative electrode mixture layer having a negative electrode active material, a first layer covering at least a portion of a surface of the negative electrode active material, and a second layer covering at least a portion of a surface of the first layer, the first layer including a carbon material, and the second layer including an organosilicon.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a negative electrode for a secondary battery and a manufacturing method thereof. BACKGROUND

[0002] A nonaqueous electrolyte secondary battery, particularly a lithium ion secondary battery, is expected as a power source for small-sized domestic uses, power storage devices, and electric vehicles, because it has a high voltage and a high energy density. In the process of seeking a high energy density of the battery, a silicon (Si)-containing material that is alloyed with lithium is expected as a negative electrode active material having a high theoretical capacity density.

[0003] Patent Literature 1 discloses an electrode having a current collector and an active material layer, the aforementioned active material layer having an active material, a film containing organic silicon, a conductive aid, and a binder, the aforementioned active material having a granular shape, and the aforementioned film containing organic silicon covering at least a part of the aforementioned active material. In addition, a manufacturing method of an electrode is disclosed, which includes the following steps: a step of forming a mixed solution having an active material, organic silicon, and a first solvent; a step of ejecting the aforementioned mixed solution from a nozzle to evaporate the first solvent, thereby forming an active material having a coating film; a step of forming a paste having the aforementioned active material having a coating film, a conductive aid, a binder, and a second solvent; and a step of applying the aforementioned paste to a current collector to evaporate the second solvent, thereby forming an active material layer.

[0004] PRIOR ART DOCUMENTS

[0005] PATENT LITERATURE

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2016-81922 SUMMARY

[0007] Patent Literature 1 aims to provide an electrode and a manufacturing method thereof in which a decomposition reaction of an electrolyte solution generated as a side reaction of charge and discharge is suppressed to a minimum limit, and the cycle characteristics are improved. However, according to the proposal of Patent Literature 1, it is difficult to stably improve the capacity maintenance rate in charge and discharge cycles.

[0008] In view of the above, one aspect of the present disclosure relates to a negative electrode for a secondary battery, which includes a negative electrode mixture layer having a negative electrode active material, a first layer covering at least a part of a surface of the aforementioned negative electrode active material, and a second layer covering at least a part of a surface of the aforementioned first layer, the aforementioned first layer including a carbon material, and the aforementioned second layer including organic silicon.

[0009] Another aspect of the present disclosure relates to a method for manufacturing a negative electrode for a secondary battery, including: a step of preparing a slurry including a negative electrode active material, a first layer covering at least a part of a surface of the negative electrode active material, and a dispersion medium, the first layer including a carbon material; a step of applying the slurry to a negative electrode current collector, and volatilizing the dispersion medium, thereby forming a coating film; a step of preparing a solution including an organosilicon and a solvent; and a step of bringing the coating film into contact with the solution, and volatilizing the solvent, thereby covering at least a part of a surface of the first layer with a second layer, the second layer including the organosilicon.

[0010] Still another aspect of the present disclosure relates to a secondary battery including the above-described negative electrode for a secondary battery, a positive electrode, and an electrolyte solution.

[0011] According to the present disclosure, the capacity retention rate in charge-discharge cycles of a secondary battery is stably improved. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1A FIG. 1 is a top view showing a part of the structure of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure.

[0013] Figure 1B FIG. 4 is a cross-sectional view along the line X-X' of the nonaqueous secondary battery shown in FIG. 3. Figure 1A

[0014] Figure 2 FIG. 6 is a graph showing the method for producing a negative electrode for performance evaluation.

[0015] Figure 3 FIG. 8 is a graph showing the relationship between the number of charge-discharge cycles and the capacity retention rate of a secondary battery, Al to A4 corresponding to Examples 1 to 4, and Bl corresponding to Comparative Example 1.

[0016] Figure 4A FIG. 10 is a cross-sectional TEM photograph of a Si-containing material included in the negative electrode used in the battery A4 of Example 1.

[0017] Figure 4B FIG. 12 is a graph showing the spectrum of the Si-L edge in TEM-EELS analysis.

[0018] Figure 4C FIG. 14 is a graph showing the spectrum of the C-K edge in TEM-EELS analysis.

[0019] Figure 4D FIG. 16 is a graph showing the spectrum of the O-L edge in TEM-EELS analysis. DETAILED DESCRIPTION

[0020] ​The negative electrode for a secondary battery of an embodiment of the present disclosure includes, for example, a negative electrode mixture layer formed on a negative electrode current collector, the negative electrode mixture layer having a negative electrode active material, a first layer covering at least a portion of a surface of the negative electrode active material, and a second layer covering at least a portion of a surface of the first layer. The first layer includes a carbon material, and the second layer includes an organosilicon. That is, the first layer is a base layer, and the second layer covers at least a portion of the surface of the negative electrode active material through the first layer. The negative electrode active material is typically a particulate material. At this time, the surface of the negative electrode active material refers to a surface of a particle of the negative electrode active material.

[0021] The first layer can cover the entire surface of the negative electrode active material as long as it covers a portion of the surface of the negative electrode active material. In the case where the first layer covers only a portion of the surface of the negative electrode active material, the second layer can cover at least a portion of the surface of the negative electrode active material that is not covered by the first layer as well as the surface of the first layer.

[0022] The carbon material that forms the first layer (hereinafter, also referred to as a first carbon material) has electrical conductivity, imparts electrical conductivity to the negative electrode active material, or improves the electrical conductivity of the negative electrode active material. In addition, the first layer suppresses a side reaction caused by contact of an electrolyte with the negative electrode active material. The first carbon material can have a graphite structure or can be amorphous carbon.

[0023] From the viewpoint of obtaining good electrical conductivity and a high side reaction suppression effect, it is desirable for the first carbon material to cover at least a portion of the surface of the negative electrode active material in a film form. In the case where the first layer is formed in a film form, amorphous carbon is more suitable. The first carbon material can be used alone or in combination with a plurality of types.

[0024] The second layer can include only an organosilicon or can include an organosilicon and a material other than an organosilicon. For example, the second layer can include an organosilicon and a carbon material. In this case, the boundary between the first layer and the second layer does not need to be clearly determined. For example, in analysis in a prescribed depth direction, the amount of detection of the organosilicon gradually decreases the deeper it is from the surface of the second layer, and the first layer formed substantially of a carbon material can be detected deeper. The organosilicon that forms the second layer suppresses a side reaction caused by contact of an electrolyte with the negative electrode active material. Among others, it is considered to be strong in resistance to HF, an impurity generated in an electrolyte. In addition, the affinity of the organosilicon to the first carbon material is excellent, and the organosilicon is suitable for covering the first layer. In addition, the organosilicon is suitable for forming the second layer in a film form.

[0025] The organic silicon refers to a polymer or oligomer having a main chain including at least one (preferably, a plurality of) siloxane bond (i.e., Si-O-Si bond). The organic silicon can be linear, can have a branched structure, and can be cyclic. In addition, the organic silicon can have a functional group at at least one side chain, at least one terminal, or the like. The kind of the organic silicon is not particularly limited, and can be, for example, an organic silicon resin, a silicone oil, an organic silicon rubber, a silicone gel, a silicone grease, or the like. The organic silicon can be used alone, or can be used in combination of a plurality of kinds.

[0026] Since the negative electrode active material swells and shrinks greatly during charge and discharge, it is assumed that if the surface of the negative electrode active material is covered only by the first carbon material, the first carbon material is easily peeled off from the surface of the negative electrode active material. In addition, it is assumed that even if the surface of the negative electrode active material is covered by the first carbon material through the layer including the organic silicon after the formation of the base layer including the organic silicon, the first carbon material is easily peeled off from the surface of the negative electrode active material. As the peeling of the first carbon material proceeds, the conduction path in the negative electrode mixture layer deteriorates. In addition, the side reaction of the negative electrode active material with the electrolyte is promoted. As a result thereof, the capacity maintenance rate decreases.

[0027] On the other hand, as proposed in the present disclosure, if at least a part of the surface of the negative electrode active material is covered by the first layer including the first carbon material, and at least a part of the surface of the first layer is covered by the second layer including the organic silicon, the peeling of the first layer is significantly suppressed. That is, the conduction path in the negative electrode mixture layer is not easily deteriorated, the side reaction of the negative electrode active material with the electrolyte is suppressed, and the decrease in the capacity maintenance rate is suppressed. That is, the second layer including the organic silicon not only suppresses the side reaction of the electrolyte with the negative electrode active material, but also has an effect of suppressing the peeling of the first layer. Furthermore, since the organic silicon has high elasticity and is stable to reversible elastic deformation, even if the negative electrode active material swells and shrinks repeatedly due to repeated charge and discharge cycles, it is not easily damaged. In addition, an effect of pressing the first layer toward the surface of the negative electrode active material by the elastic force of the organic silicon can also be expected.

[0028] The organic silicon can be represented by General Formula A: {(R1) a SiO (4-a) / 2} c Here, R1 is a substituted or unsubstituted organic group having a carbon number of 1 to 8. a is a rational number, and c is an integer, satisfying 0≤a≤2 and 1≤c. The substituted or unsubstituted organic group includes a hydrocarbon group having a substituent (or a functional group) and a hydrocarbon group not having a substituent (or a functional group), and the like. Specifically, R1 can be at least one selected from the group consisting of an alkyl group, a vinyl group, an alkoxy group, an aryl group, an aryloxy group, a ketone group, a carboxyl group, and an ester group.

[0029] The organic silicon is desired to have a hydrophobic group. The hydrophobic group is excellent in affinity with the 1st carbon material, and thus the binding force of the 1st layer to the 2nd layer is improved. In addition, the hydrophobic group suppresses the approach of impurities such as HF and polar solvents in the electrolyte to the negative electrode active material, and thus it becomes easy to further reduce side reactions.

[0030] As the hydrophobic group, an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a ketone group, an ester group, or the like is preferable. The aryl group (Ar) forms an Ar-Si bond. The alkyl group (Alk) forms an Alk-Si bond. The aryloxy group (Ar-O) forms an Ar-O-Si bond. The alkoxy group (Alk-O) forms an Alk-O-Si bond. The ketone group (Alk-CO-Alk) forms a Si-Alk-CO-Alk bond or a Si-CO-Alk bond. The ester group (Alk-CO-OAlk) forms a Si-Alk-CO-OAlk bond or an Alk-CO-O-Si bond.

[0031] As the aryl group, a phenyl group, a benzyl group, a toluyl group, or the like can be given. As the alkyl group, an alkyl group having a carbon number of 1 to 8 can be given. As the aryloxy group, a phenoxy group, or the like can be given. As the alkoxy group, an oxyalkyl group having a carbon number of 1 to 8 can be given. As the ester group, a condensed group of an alcohol having a carbon number of 1 to 8 and a carboxylic acid having a carbon number of 1 to 8 can be given. Among them, a phenyl group, a methyl group, an ethyl group, or the like easily improves the adhesion of the 1st layer to the 2nd layer due to hydrophobic interaction, and is low in resistance, and is also high in the effect of suppressing side reactions of the electrolyte.

[0032] That is, in General Formula A, R1may include at least one selected from the group consisting of an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a ketone group, and an ester group, each of which is hydrophobic. Among them, R1is desired to be an alkyl group, and the alkyl group is preferably at least one selected from the group consisting of a methyl group and an ethyl group.

[0033] The weight average molecular weight (Mw) of the organic silicon can be, for example, 80 or more and 10,000 or less, can be 80 or more and 1,000 or less, can be 500 or more and 3,000 or less, or can be 100 or more and 1,000 or less.

[0034] The organosilicon can have at least one selected from the group consisting of a silanol group (Si-OH) and an alkoxysilyl group (Si-OR2). Here, R2is a substituted or unsubstituted organic group having 1 to 8 carbon atoms. The substituted or unsubstituted organic group includes a hydrocarbon group having a substituent and a hydrocarbon group having no substituent, and the like. R2may include, for example, at least one selected from the group consisting of an alkyl group, an alkoxy group, an aryl group, an aryloxy group, a ketone group, and an ester group. Among them, R2is desirably an alkyl group, and the alkyl group is preferably at least one selected from the group consisting of a methyl group and an ethyl group. The silanol group, for example, increases the adhesion resulting from the hydrophilic interaction of the negative electrode active material having a hydrophilic group on the surface with the second layer. Although the first layer is interposed between the negative electrode active material and the second layer, the first layer does not necessarily cover the entire surface of the negative electrode active material, and thus the affinity of the negative electrode active material with the second layer is also important. The alkoxysilyl group can exhibit the same hydrophilic interaction because it is likely to generate a silanol group at the time of forming the second layer.

[0035] The organosilicon can have a crosslinked structure in which one silicon atom is bonded to two or more oxygen atoms. Such an organosilicon having a crosslinked structure is high in strength, high in binding force with the first layer, and excellent in durability. Note that the organosilicon can include a mono-oxygen structure (hereinafter, also referred to as an R3SiO unit) in which one Si atom is bonded to two O atoms (excluding the O of a hydroxyl group), a di-oxygen structure (hereinafter, also referred to as an R2SiO2 unit) in which one Si atom is bonded to one O atom (excluding the O of a hydroxyl group), a tri-oxygen structure (hereinafter, also referred to as an RSiO3 unit) in which one Si atom is bonded to three O atoms (excluding the O of a hydroxyl group), and a tetra-oxygen structure (hereinafter, also referred to as an SiO4 unit) in which one Si atom is bonded to four O atoms (excluding the O of a hydroxyl group).

[0036] The proportion of the R2SiO2 unit in all the Si atoms contained in the molecule is the largest. In addition, the ratio of the RSiO3 unit to the R2SiO2 unit contained in the molecule, RSiO3 unit / R2SiO2 unit (hereinafter, also referred to as a T / D ratio), is, for example, greater than 0, and is preferably 0.15 or more. It is considered that the greater the T / D ratio, the higher the strength of the second layer, the higher the binding force of the second layer with the first layer, and the higher the durability of the second layer.

[0037] As a preferable specific example of the organosilicon, an organosilicon having a polydimethylsiloxane skeleton as a main chain can be given. In the polydimethylsiloxane skeleton, a part of the methyl groups is optionally substituted with a phenyl group and / or a hydroxyl group. In addition, a part of the silicon atoms constituting the main chain is an RSiO3 unit or an SiO4 unit, and preferably has a branched structure.

[0038] The negative electrode active material contains, for example, a Si-containing material. The Si-containing material is a material that swells and shrinks greatly during charge and discharge. That is, although the Si-containing material can be expected to have high capacity, on the contrary, it has the disadvantage of easily inducing side reactions in the negative electrode active material, easily reducing the capacity maintenance rate in charge and discharge cycles. On the other hand, in the case where the surface of the Si-containing material is covered with the first layer, and further in the case where the surface of the Si-containing material is covered with the second layer containing organic silicon through the first layer, the effect of suppressing side reactions easily becomes apparent, and the above-mentioned disadvantage can be greatly alleviated.

[0039] The content of the Si-containing material contained in the negative electrode mixture layer may, for example, be 1 to 30 mass%. In this case, the Si-containing material and a negative electrode active material other than the Si-containing material can be used in combination. Thereby, the overall volume change of the negative electrode mixture layer can be greatly alleviated. The content of the Si-containing material in the negative electrode mixture layer may, for example, be 15 mass% or less. The content of the Si-containing material contained in the negative electrode mixture layer can be 1 mass% or more and 15 mass% or less, or 5 mass% or more and 10 mass% or less.

[0040] (Negative electrode active material)

[0041] Hereinafter, the content of the negative electrode active material will be further described. The negative electrode active material refers to a material that is contained in a negative electrode and that is electrochemically capable of occluding and releasing lithium ions. Here, the negative electrode active material contains at least a Si-containing material.

[0042] The Si-containing material may, for example, contain a lithium ion conductive phase and Si particles dispersed in the lithium ion conductive phase. As the lithium ion conductive phase, at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase may, for example, be used. Such a composite material of the lithium ion conductive phase and the Si particles is suitable for suppressing direct contact of the Si particles with an electrolyte solution and for giving a negative electrode with high capacity. As the Si-containing material, at least one selected from the group consisting of a first composite material, a second composite material, and a third composite material described below may, for example, be used.

[0043] <First composite material>

[0044] The first composite material contains a silicon oxide phase and first Si particles dispersed in the silicon oxide phase. The first composite material has the advantages of being highly stable and having a small volume change in the Si-containing material. It is considered that the high stability is due to the small particle diameter of the first Si particles dispersed in the silicon oxide phase, which makes it difficult to perform deep charging. The silicon oxide phase has a tendency to have a large irreversible capacity in the Si-containing material because it has many sites that irreversibly trap lithium ions. The first composite material can have a larger irreversible capacity than the second composite material. It is considered that the trapping of lithium ions by the silicon oxide phase improves the stability of the structure of the first composite material, and also contributes to the suppression of volume change.

[0045] The first composite material can be obtained, for example, by heating and subjecting a silicon oxide to a disproportionation reaction in a non-oxidizing atmosphere with a non-active gas such as argon. In the disproportionation reaction, Si crystallites are uniformly generated in the silicon oxide phase. The size of the Si particles generated by the disproportionation reaction is small, and the average particle diameter can be, for example, less than 100 nm, or can be in the range of 5 nm to 50 nm. The main component (for example, 95 to 100% by mass) of the silicon oxide phase is silicon dioxide. That is, the first composite material can include a SiO2 phase and first Si particles dispersed in the SiO2 phase. In this case, the entire first composite material can be represented by the general formula SiOx. x The value of x can be in the range of 0 < x < 2, and can be, for example, 0.9 ≤ x ≤ 1.1, or x = 1.

[0046] The average particle diameter of the first composite material can be, for example, 1 to 20 μm, and is preferably 5 to 12 μm. When the particle diameter is in the above range, stress due to volume changes of the Si-containing material caused by charging and discharging is easily alleviated, and good cycle characteristics are easily obtained.

[0047] <Second Composite Material>

[0048] The second composite material includes a silicate phase and second Si particles dispersed in the silicate phase. The silicate phase can include, for example, at least one element selected from the group consisting of Group 1 elements and Group 2 elements of the long-form periodic table. As the Group 1 elements of the long-form periodic table and the Group 2 elements of the long-form periodic table, for example, lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and the like can be used. As other elements, aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), titanium (Ti), and the like can be included. Among these, a silicate phase including lithium (hereinafter, also referred to as a lithium silicate phase) is preferable in terms of small irreversible capacity and high initial charging and discharging efficiency. That is, the second composite material can include a lithium silicate phase and second Si particles dispersed in the lithium silicate phase. The second composite material including a lithium silicate phase and second Si particles dispersed in the lithium silicate phase is also referred to as LSX.

[0049] The lithium silicate phase can be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and can also contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. In this case, it is advantageous in terms of stability and lithium ion conductivity. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si is, for example, greater than 0 and less than 4. As elements other than Li, Si, and O contained in the lithium silicate phase, for example, iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), aluminum (Al), and the like can be given.

[0050] The lithium silicate phase can have a composition represented by the formula: Li 2z SiO 2+z (0 < z < 2). From the viewpoints of stability, ease of production, lithium ion conductivity, and the like, z preferably satisfies the relationship 0 < z < 1, and more preferably z = 1 / 2.

[0051] <3rd composite material>

[0052] The 3rd composite material contains a carbon phase and 3rd Si particles dispersed in the carbon phase (hereinafter, the 3rd composite material is also referred to as Si-C material). The carbon phase can be composed of, for example, amorphous carbon. The amorphous carbon can be, for example, hard carbon, soft carbon, or amorphous carbon other than these. The amorphous carbon can be obtained, for example, by sintering a carbon source in an inactive atmosphere, and pulverizing the obtained sintered body. The Si-C material can be obtained, for example, by mixing a carbon source and Si particles, stirring the mixture while breaking the mixture in a stirrer such as a ball mill, and then calcining the mixture in an inactive atmosphere. As the carbon source, for example, carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, a saccharide such as sucrose, a water-soluble resin, or the like can be used. When the carbon source is mixed with the Si particles, for example, the carbon source and the Si particles can be dispersed in a dispersion medium such as alcohol.

[0053] The content of the 2nd Si particles contained in the 2nd composite material and the content of the 3rd Si particles contained in the 3rd composite material can each independently be, for example, 40% by weight or more and 80% by weight or less. Thereby, it is easy to achieve both high capacity of the battery and improvement of the cycle characteristics. Unlike the 1st composite material, in which the manufacturing method is limited, in the 2nd and 3rd composite materials, the contents of the 2nd and 3rd Si particles can be arbitrarily changed, and thus, it is easy to design a high-capacity negative electrode. By using the 1st composite material in combination with the 2nd and / or 3rd composite material, and controlling the distribution of these materials in the negative electrode mixture layer, even if the utilization rate of Si is increased, it is possible to achieve both good cycle characteristics and high capacity.

[0054] The 2nd and 3rd composite materials are excellent in terms of small irreversible capacity. This is because the sites of the silicate phase and the carbon phase that irreversibly trap lithium ions are small. By using the 2nd and / or 3rd composite material, excellent charge-discharge efficiency can thus be obtained. This effect is particularly significant at the initial stage of charge-discharge.

[0055] The average particle diameter of the 2nd and 3rd Si particles is, independently, for example, 500 nm or less, can be 400 nm or less, and can be 200 nm or less. The 2nd Si particles have such a large average particle diameter that the capacity of the 2nd and 3rd composite materials can be easily increased. On the other hand, if it is 500 nm or less, the volume change of the 2nd and 3rd Si particles at the time of charge-discharge becomes small, and the structural stability of the 2nd and 3rd composite materials improves. Here, the average particle diameter of the 2nd and 3rd Si particles is a value after at least the initial charge.

[0056] The Si particles dispersed in the silicate phase and / or the carbon phase are generally composed of a plurality of crystallites. The crystallite size of the Si particles is, for example, preferably 30 nm or less. In this case, the volume change caused by the expansion and contraction of the Si particles accompanying charge-discharge can be minimized, and thus the cycle characteristics can be further improved. For example, when the Si particles contract, pores are formed around the Si particles, the isolation of the Si particles caused by the reduction in the contact of the Si particles with their surroundings is suppressed, and the decrease in the charge-discharge efficiency is suppressed. The lower limit of the crystallite size of the Si particles is not particularly limited, and is, for example, 5 nm or more. The crystallite size can be 10 nm or more and 30 nm or less, and can be 15 nm or more and 25 nm or less.

[0057] The average particle diameter of the 2nd and 3rd composite materials is, independently, for example, 1 to 20 μm, and can be 5 to 12 μm. When the particle diameter is within the above range, the stress caused by the volume change of the Si-containing material accompanying charge-discharge can be easily alleviated, and good cycle characteristics can be easily obtained.

[0058] The content of the Si particles contained in each Si-containing material (each composite material) can be measured by Si-NMR. Hereinafter, the ideal measurement conditions of Si-NMR are shown.

[0059] Measurement device: solid nuclear magnetic resonance spectrometer (INOVA-400) manufactured by Varian

[0060] Probe: Varian 7 mm CPMAS-2

[0061] MAS: 4.2 kHz

[0062] MAS speed: 4 kHz

[0063] Pulse: DD (45° pulse + signal collection time 1H decoupling)

[0064] Repeat time: 1200 seconds

[0065] Observation amplitude: 100kHz

[0066] Observation center: around -100ppm

[0067] Signal acquisition time: 0.05 seconds

[0068] Total number of times: 560

[0069] Sample volume: 207.6 mg

[0070] The average particle size of Si particles in each Si-containing material (each composite material) can be determined by cross-sectional SEM (scanning electron microscope) images of the Si-containing material. Specifically, the average particle size of Si particles is calculated by averaging the maximum diameters of any 100 individual Si particles.

[0071] The crystallite size of each Si particle is calculated from the half-width of the diffraction peaks attributable to the Si(111) plane in the X-ray diffraction (XRD) pattern of the Si particle, according to the Scherer formula.

[0072] The average particle size of each Si-containing material (each composite material) refers to the particle size (volume average particle size) that accounts for 50% of the total volume in the particle size distribution measured by laser diffraction scattering. For example, the "LA-750" manufactured by Horiba Corporation can be used in the measuring apparatus.

[0073] It should be noted that the composition of the second composite material can be analyzed, for example, using the following methods.

[0074] The battery was disassembled, the negative electrode was removed, cleaned with a non-aqueous solvent such as ethylene carbonate, and dried. The cross-section of the negative electrode composite layer was then machined using a cross-section polishing machine (CP) to obtain a sample. A field emission scanning electron microscope (FE-SEM) was used to obtain reflected electron images of the sample cross-section, allowing observation of the cross-section of the second composite material. Auger electron spectroscopy (AES) was used to perform qualitative and quantitative elemental analysis on the observed second composite material (accelerating voltage 10 kV, beam current 10 nA).

[0075] For example, if it is Li 2z SiO 2+z The ratio of 2z to (2+z) can be determined from the lithium (Li) content and oxygen (O) content of the LSX shown.

[0076] Note that, in the cross-sectional observation and analysis of the above-described sample, in order to prevent diffusion of Li, a carbon sample stage can be used to fix the sample. In order to not deteriorate the cross section of the sample, a transfer container that does not expose the sample to the atmosphere can be used to maintain the sample during transfer.

[0077] <Carbon material as negative electrode active material>

[0078] The negative electrode mixture layer can contain a carbon material (hereinafter, also referred to as a second carbon material) as a negative electrode active material. The second carbon material has a smaller degree of expansion and shrinkage during charge and discharge than the Si-containing material, and thus, by being used in combination with the Si-containing material, it is easy to improve the cycle characteristics of the battery. The content of the second carbon material in the negative electrode active material can be, for example, 70% by mass or more and 99% by mass or less, can be 85% by mass or more and 99% by mass or less, or can be 90% by mass or more and 95% by mass or less. Thereby, it becomes easy to balance high capacity and better cycle characteristics.

[0079] As the second carbon material, for example, graphite, easily graphitizable carbon (soft carbon), and difficultly graphitizable carbon (hard carbon) can be exemplified. Among these, graphite is preferable because it has excellent stability during charge and discharge and also has a small irreversible capacity. Graphite refers to a material having a graphite-type crystal structure, and for example, includes natural graphite, artificial graphite, graphitized mesocarbon microbeads, and the like. The second carbon material can be used alone or in combination with two or more.

[0080] Note that the carbon phase of the lithium ion conductive layer as the third composite material is not included in the second carbon material nor the first carbon material.

[0081] (First layer)

[0082] Hereinafter, the first layer is further described. As described above, the first carbon material forming the first layer has an effect of imparting conductivity to the negative electrode active material or improving the conductivity of the negative electrode active material, and an effect of suppressing direct contact of the electrolyte with the negative electrode active material. Thereby, the first layer can be selectively formed to cover a component of the negative electrode active material that has poor conductivity or a component of the negative electrode active material that is likely to induce a side reaction with the electrolyte. In the case where, for example, the Si-containing material and the second carbon material are used in combination as the negative electrode active material, at least a part of the surface of the Si-containing material that is likely to induce a side reaction with the electrolyte can be selectively covered by the first layer, and the step of forming the first layer on the surface of the second carbon material can be omitted.

[0083] In the Si-containing material, the lithium ion conductive phase of the first composite material (SiO x ) and the second composite material (LSX or the like) lacks electron conductivity, and thus, it is desirable to form the first layer having conductivity on the surface of the first composite material and the second composite material.

[0084] The thickness of the first layer is, for example, suitably 2 nm or more and 100 nm or less, but can be appropriately controlled depending on the use of the secondary battery, the kind of the negative electrode active material, the kind of the first carbon material, the structure. Note that the thickness of the first layer can be measured by analyzing the cross section of the negative electrode active material on which the first layer is formed, using a cross-sectional SEM (scanning electron microscope) photograph, TEM-EELS, or the like. The thickness of the first layer can be measured at any three points, and the average value thereof is taken.

[0085] The method of forming the first layer is not particularly limited. For example, the negative electrode active material that is the object of forming the first layer can be mixed with the raw material of the first carbon material, and the mixture obtained is subjected to calcination in a non-active atmosphere at a high temperature to carbonize the raw material of the first carbon material, thereby forming the first layer. In the case where the calcination product is aggregated, the calcination product is moderately pulverized, and the use of a sieve can control the particle size distribution to be desired. As the raw material of the first carbon material, pitch, tar, an organic polymer, or the like can be used. In addition, the first layer can be formed by a vapor phase method such as a chemical vapor deposition method (CVD method), a vacuum evaporation method, or the like. In addition, the first carbon material can be mixed with the negative electrode active material that is the object of forming the first layer using a mixing device using a medium such as a ball mill.

[0086] (Second Layer)

[0087] Hereinafter, the second layer is further described. As described above, the second layer has the effects of suppressing the side reaction of the electrolyte with the negative electrode active material and suppressing the peeling of the first layer. The second layer is formed in such a manner as to cover at least a part of the first layer on the negative electrode active material having at least the first layer.

[0088] The content of the organosilicon contained in the negative electrode mixture layer is, for example, 0.01 mass% or more and 1 mass% or less, and is preferably 0.05 mass% or more and 0.5 mass% or less. When in this range, a sufficient amount of the second layer of low resistance can be easily formed on the surface of the negative electrode active material.

[0089] The thickness of the second layer is, for example, suitably 2 nm or more and 20 nm or less, and is more preferably 5 nm or more and 10 nm or less. The thickness of the second layer can be measured by analyzing the cross section of the negative electrode active material on which the second layer is formed, using a cross-sectional SEM (scanning electron microscope) photograph, TEM-EELS, or the like. The thickness of the second layer can be measured at any three points, and the average value thereof is taken.

[0090] The method of forming the second layer is not particularly limited, and for example, it is sufficient to contact the negative electrode active material (a negative electrode active material including at least the first layer) that is the object of forming the second layer with the organosilicon. At this time, the organosilicon having a reactive functional group can be used as the organosilicon, and the functional group present on the negative electrode active material or the first layer surface is reacted with the reactive functional group to form a chemical bond. Alternatively, the negative electrode active material that is the object of forming the second layer can be mixed with a raw material of the organosilicon, and the raw material of the organosilicon is reacted in the obtained mixture to produce the organosilicon. When the negative electrode active material that is the object of forming the second layer is contacted or mixed with the organosilicon or the raw material thereof, a solution prepared by mixing the organosilicon or the raw material thereof with a solvent can be used instead of the organosilicon or the raw material thereof. In this case, the solvent can be volatilized at an appropriate timing.

[0091] As described above, since the step of forming the second layer is simple, the entire negative electrode active material contained in the negative electrode mixture layer can be targeted to form the second layer. For example, after forming the negative electrode mixture layer, the second layer can be formed on the entire negative electrode active material contained in the negative electrode mixture layer non-selectively by subjecting the negative electrode mixture layer to a prescribed step.

[0092] [Negative electrode]

[0093] Hereinafter, the negative electrode will be further described. The negative electrode, for example, has a sheet-shaped negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector. The negative electrode mixture layer, for example, can be formed by coating a negative electrode slurry in which a negative electrode mixture including a negative electrode active material is dispersed in a dispersion medium on the surface of the negative electrode current collector and drying. The coating film after drying can be calendered as needed. The negative electrode mixture layer can be formed on one surface of the negative electrode current collector or on both surfaces.

[0094] The negative electrode mixture includes a negative electrode active material as an essential component, and as an arbitrary component, a binder, a conductive aid, a thickening agent, and the like can be included.

[0095] As the negative electrode current collector, a non-porous conductive substrate (metal foil or the like), a porous conductive substrate (sieve body, mesh body, punched sheet, or the like) can be used. As the material of the negative electrode current collector, stainless steel, nickel, nickel alloy, copper, copper alloy, and the like can be exemplified.

[0096] As the binder, resin materials such as polytetrafluoroethylene, polyvinylidene fluoride (PVDF), and the like fluororesin; polyethylene, polypropylene, and the like polyolefin resin; aromatic polyamide resin, and the like polyamide resin; polyimide, polyamide-imide, and the like polyimide resin; polyacrylic acid, polymethyl acrylate, ethylene-acrylic acid copolymer, and the like acrylic resin; polyacrylonitrile, polyvinyl acetate, and the like vinyl resin; polyvinylpyrrolidone; polyether sulfone; styrene-butadiene copolymer rubber (SBR), and the like rubbery material, and the like can be exemplified. They can be used alone as one kind, or can be used in combination of two or more kinds.

[0097] As the conductive aid, for example, carbon black such as acetylene black, carbon nanotube (hereinafter, also referred to as CNT), metal fiber, carbon fluoride, metal powder, zinc oxide, potassium titanate, and the like conductive whisker, conductive metal oxide such as titanium oxide, organic conductive material such as phenylene derivative, and the like can be exemplified. They can be used alone as one kind, or can be used in combination of two or more kinds.

[0098] Among the conductive aids, CNT is suitable as the conductive aid used in combination with the Si-containing material. CNT is fibrous, and thus, when the Si-containing material that has swelled by charging shrinks by discharging, a pore is formed around the Si-containing material, and thus, the contact between the Si-containing material and the carbon material can be effectively ensured.

[0099] Hereinafter, an example of the manufacturing method of the negative electrode will be described.

[0100] (i) First, a negative electrode slurry in which a negative electrode mixture is dispersed in a dispersion medium is prepared. The negative electrode mixture contains a negative electrode active material having a first layer, the first layer covering at least a part of the surface of the negative electrode active material, and the first layer containing a first carbon material. The negative electrode active material having the first layer can be formed, for example, by mixing a negative electrode active material that is an object of forming the first layer and a raw material (pitch or the like) of the first carbon material, and by baking the mixture at a high temperature in an inactive atmosphere.

[0101] (ii) Next, the negative electrode slurry is applied to a negative electrode current collector, and the dispersion medium is volatilized, thereby forming a coating film. The method of applying the negative electrode slurry to the negative electrode current collector is not particularly limited. The dried coating film after the volatilization of the dispersion medium can be calendered. Note that the calendering can be performed after the formation of a second layer that covers at least a part of the surface of the first layer, in accordance with the steps described below.

[0102] (iii) A solution containing silicone (hereinafter, referred to as silicone solution) is prepared. The silicone solution can be obtained by dissolving silicone in a solvent. The concentration of the silicone in the silicone solution is, for example, 0.5 to 10% by mass.

[0103] The weight-average molecular weight (Mw) of organosilicon can be, for example, 80 or more and 10,000 or less, 80 or more and 1,000 or less, 500 or more and 3,000 or less, or 100 or more and 1,000 or less. Within such ranges, the surface of the negative electrode active material is stable to reversible elastic deformation, and a low-resistance second layer can be easily formed.

[0104] (iv) Next, the coating of the negative electrode mixture formed on the negative electrode current collector is brought into contact with the organosilicon solution. For example, the coating of the negative electrode mixture and the negative electrode current collector are immersed together in the organosilicon solution, and after being lifted from the organosilicon solution, the solvent is allowed to evaporate. Thus, at least a portion of the surface of the first layer is covered by the second layer containing organosilicon. At this time, the second layer can be formed not only on the negative electrode active material having the first layer, but also on the negative electrode active material not having the first layer. Without any particular problem, it is expected to suppress the side reactions between the negative electrode active material not having the first layer and the electrolyte.

[0105] Next, a secondary battery according to an embodiment of the present disclosure will be described in detail. The secondary battery, for example, includes the aforementioned negative electrode, positive electrode, and non-aqueous electrolyte.

[0106] [positive electrode]

[0107] The positive electrode contains an electrochemically active material capable of absorbing, storing, and releasing lithium ions. For example, the positive electrode comprises a positive current collector and a positive electrode additive layer formed on the surface of the positive current collector. The positive electrode additive layer is formed by coating a positive electrode slurry containing a positive electrode additive dispersed in a dispersion medium onto the surface of the positive current collector and then drying it. The dried coating can be calendered as needed. The positive electrode additive layer can be formed on one surface or on both surfaces of the positive current collector. The positive electrode additive contains the active material as a necessary component, and as an optional component, it can include binders, conductive agents, etc.

[0108] As a positive electrode active material, lithium-containing composite oxides can be used, for example. Li can be cited as an example. a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c Li a Ni 1-b M b O c Li a Mn2O4, Li a Mn 2-b M bO4, LiMPO4, Li2MPO4F (M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, B). Here, a = 0 to 1.2, b = 0 to 0.9, c = 2.0 to 2.3. Note that the value of a, which represents the molar ratio of lithium, increases and decreases according to charge and discharge.

[0109] wherein Li a Ni b M 1-b O2 (M is at least one selected from the group consisting of Mn, Co, and Al, 0 < a ≤ 1.2, 0.3 ≤ b ≤ 1). From the viewpoint of high capacity, it is more preferable to satisfy 0.85 ≤ b ≤ 1. From the viewpoint of stability of crystal structure, it is further preferable that Li a Ni b Co c Al d O2 (0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c < 0.15, 0 < d ≤ 0.1, b + c + d = 1).

[0110] As the binder and the conductive agent, the same substances as those described with respect to the negative electrode can be used. As the conductive agent, graphite such as natural graphite and artificial graphite can be used.

[0111] The shape and the thickness of the positive electrode current collector can be selected from those of the negative electrode current collector, respectively. As the material of the positive electrode current collector, for example, stainless steel, aluminum, aluminum alloy, titanium, and the like can be exemplified.

[0112] [Electrolyte]

[0113] The electrolyte contains a solvent and an electrolyte salt. As the solvent, a nonaqueous solvent can be used, or water can be used. In the case of producing a lithium ion secondary battery, which is a representative example of a nonaqueous electrolyte secondary battery, the electrolyte salt contains at least a lithium salt.

[0114] The concentration of the lithium salt in the electrolyte is, for example, preferably 0.5 mol / L or more and 2 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ion conductivity and moderate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0115] As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylic acid ester, a chain carboxylic acid ester, or the like can be used. As the cyclic carbonate, propylene carbonate (PC), ethylene carbonate (EC), or the like can be given. As the chain carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or the like can be given. As the cyclic carboxylic acid ester, γ-butyrolactone (GBL), γ-valerolactone (GVL), or the like can be given. As the chain carboxylic acid ester, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, or the like can be given. The non-aqueous solvent can be used alone as one kind, or two or more kinds can be used in combination.

[0116] As the lithium salt, for example, LiClO4, LiBF4, LiPF6, LiAlCl4, LiSbF6, LiSCN, LiCF3SO3, LiCF3CO2, LiAsF6, LiB 10 Cl 10 , a lithium lower aliphatic carboxylate, LiCl, LiBr, LiI, a borate salt, an imide salt, or the like can be given. As the borate salt, lithium bis(1,2-phenylenedioxy(2-)-O,O')borate, lithium bis(2,3-naphthalenedioxy(2-)-O,O')borate, lithium bis(2,2'-diphenyldioxy(2-)-O,O')borate, lithium bis(5-fluoro-2-ethylenedioxy-1-phenylsulfonyl-O,O')borate, or the like can be given. As the imide salt, lithium bisfluorosulfonylimide (LiN(FSO2)2: hereinafter, also referred to as LFSI), lithium bistrifluoromethanesulfonylimide (LiN(CF3SO2)2), lithium trifluoromethanesulfonic acid nonafluorobutanesulfonylimide (LiN(CF3SO2)(C4F9SO2)), lithium bispentafluoroethanesulfonylimide (LiN(C2F5SO2)2), or the like can be given. Of these, at least one of LiPF6and LFSI is preferred. The lithium salt can be used alone as one kind, or two or more kinds can be used in combination.

[0117] [Separator]

[0118] It is generally desirable to interpose a separator between the positive electrode and the negative electrode. The separator has a high degree of ion permeability, and has a moderate mechanical strength and insulating properties. As the separator, a microporous film, a woven fabric, a nonwoven fabric, or the like can be used. As the material of the separator, a polyolefin such as polypropylene or polyethylene is preferred.

[0119] As an example of the structure of the secondary battery, a structure in which an electrode group in which a positive electrode and a negative electrode are wound with a separator interposed therebetween and an electrolyte are accommodated in an outer case can be given. A laminated electrode group in which a positive electrode and a negative electrode are laminated with a separator interposed therebetween, or the like other form of electrode group can also be applied. The secondary battery can be of any form such as a cylindrical type, a square type, a coin type, a button type, a sheet type, or a laminate type.

[0120] Hereinafter, an embodiment of the present disclosure will be described with reference to Figure 1A and Figure 1B while a nonaqueous electrolyte secondary battery of the embodiment is described. Figure 1A is a plan view showing a part of a structure of a nonaqueous electrolyte secondary battery. Figure 1B is Figure 1A a cross-sectional view taken along the line X-X' of

[0121] As shown in Figure 1A and Figure 1B , the nonaqueous electrolyte secondary battery 100 is a sheet-type battery having a positive electrode plate group 4 and a case 5 housing the positive electrode plate group 4.

[0122] The positive electrode plate group 4 has a structure in which a positive electrode 10, a separator 30, and a negative electrode 20 are sequentially stacked, and the positive electrode 10 and the negative electrode 20 face each other with the separator 30 interposed therebetween. Thus, the positive electrode plate group 4 is formed. The positive electrode plate group 4 is impregnated with a nonaqueous electrolyte.

[0123] The positive electrode 10 includes a positive electrode active material layer la and a positive electrode current collector lb. The positive electrode active material layer la is formed on the surface of the positive electrode current collector lb.

[0124] The negative electrode 20 includes a negative electrode mixture layer 2a and a negative electrode current collector 2b. The negative electrode mixture layer 2a is formed on the surface of the negative electrode current collector 2b.

[0125] A positive electrode tab lead lc is connected to the positive electrode current collector lb, and a negative electrode tab lead 2c is connected to the negative electrode current collector 2b. The positive electrode tab lead lc and the negative electrode tab lead 2c extend to the outside of the case 5, respectively.

[0126] The positive electrode tab lead lc and the negative electrode tab lead 2c are insulated from the case 5 by an insulating tab film 6, respectively.

[0127] Hereinafter, the present disclosure will be specifically described based on Examples and Comparative Examples, but the present application is not limited to the following Examples.

[0128] (Comparative Example 1)

[0129] (1) Production of Negative Electrode

[0130] As the Si-containing material, SiOx(x = 1) (1st Si-containing material) having an average particle diameter of 5 μm was prepared. SiO x contains a SiO2 phase and Si particles (1st Si particles) dispersed in the SiO2 phase. SiO x The content of the Si particles contained in the SiO2 phase was 50 mass%. The average particle diameter of the Si particles was 20 nm.

[0131] In the SiO xA first layer containing a first carbon material is formed on the surface. The amount of the first carbon material is set to 5% by mass relative to the total mass of the Si-containing material and the first carbon material. Specifically, SiO2 is used. x The mixture is mixed with coal tar pitch and calcined at 800°C in an inert atmosphere, with SiO2 covered by a first layer containing the first carbon material. x At least a portion of the surface. Then, SiO₂ with the first layer... x Crush the material and use a sieve to adjust the average particle size to 5 μm.

[0132] A negative electrode slurry containing SiOx, acetylene black, and polyacrylamide in a mass ratio of 75:15:10, with water as the dispersion medium, was prepared. The negative electrode slurry was then coated onto one side of an electrolytic copper foil serving as the negative electrode current collector. After the coating was dried, it was rolled to produce a negative electrode sheet with a negative electrode mixture layer formed on one side of the negative electrode current collector.

[0133] Cut the negative electrode sheet into Figure 2 The shape of (a) is used to obtain the negative electrode 20 for evaluation. Figure 2 In (a), the 60mm × 40mm area functions as the negative electrode, and the 10mm × 10mm protrusion is the connection area with the tab lead 2c. Furthermore, as... Figure 2 As shown in (b), the negative electrode binder layer 2a formed on the aforementioned connection region is cut away, exposing the negative electrode current collector 2b. Then, as... Figure 2 As shown in (c), the exposed portion of the negative current collector 2b is connected to the negative electrode tab lead 2c, and the outer periphery of the negative electrode tab lead 2c is covered by the insulating tab film 6.

[0134] (2) Fabrication of the electrodes

[0135] The counter electrode is made by adhering lithium metal foil to one side of an electrolytic copper foil (current collector).

[0136] The counter electrode is cut into the same shape as the negative electrode, and the lithium metal foil formed on the same connection area as the negative electrode is peeled off to expose the current collector. Then, the exposed portion of the current collector is connected to the tab lead in the same way as the negative electrode, and a predetermined area around the outer periphery of the tab lead is covered by an insulating tab film.

[0137] (3) Preparation of non-aqueous electrolyte

[0138] A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1 mol / L in a mixed solvent of ethylene fluorocarbonate (FEC) and dimethyl carbonate (DMC) in a volume ratio of 20:80.

[0139] (4) Evaluation of the fabrication of the battery cell

[0140] Using the above-described evaluation negative electrode and counter electrode, a battery cell of a standard design capacity of 114 mAh for the negative electrode was produced. First, two pieces of the polyethylene separator (thickness 15 μm) having an aromatic polyamide coating layer were opposed to each other with the negative electrode and the counter electrode interposed therebetween so that the negative electrode mixture layer and the lithium metal foil overlapped, to obtain a jelly-roll. Next, an Al laminated film (thickness 100 μm) cut into a rectangular shape was folded in half, and the end portions on the long side were heat-sealed at 230°C to form a cylindrical shape. Thereafter, the jelly-roll produced was inserted into the cylinder from one of the short sides, and the end face of the Al laminated film was aligned with the position of the heat-fusion sealing resin of each tab lead, and heat-sealed at 230°C. Next, a non-aqueous electrolyte solution was injected from the short side of the cylinder that had not been heat-sealed, to a depth of 1.2 cm from the end face of the Al laminated film. After the injection, the non-aqueous electrolyte solution was allowed to soak into the negative electrode mixture layer under reduced pressure of 0.02 MPa for 3 minutes, and then returned to atmospheric pressure. This operation was repeated twice. Finally, the end face of the Al laminated film on the side after the injection was heat-sealed at 230°C, to obtain the evaluation battery cell B of Comparative Example 1. Note that the production of the evaluation battery cell was performed in a dry air atmosphere having a dew point of -60°C or lower. 3

[0141] (Examples 1 to 4)

[0142] In the production of the negative electrode, the negative electrode slurry was applied to the surface of the negative electrode current collector, the coating film was dried, and calendered, and then the dried coating film was immersed in a prescribed organosilicon solution together with the negative electrode current collector, after which it was taken out of the organosilicon solution and dried at 110°C for 12 hours. At this time, the second layer containing organosilicon was formed on the Si-containing material having the first layer, with the first layer interposed therebetween. Otherwise, the negative electrode was produced in the same manner as in Comparative Example 1, to produce the battery Al to A4 of Examples 1 to 4, respectively. Note that the organosilicon solution was obtained by dissolving the following organosilicon at a concentration of 2 mass% in dimethyl carbonate (DMC).

[0143] <Example 1>

[0144] Organosilicon Al: KR400 (organosilicon oligomer having only methyl groups as hydrophobic groups) manufactured by Shin-Etsu Chemical Co., Ltd.

[0145] <Example 2>

[0146] Organosilicon A2: KR300 (organosilicon polymer having methyl and phenyl groups as hydrophobic groups) manufactured by Shin-Etsu Chemical Co., Ltd.

[0147] <Example 3>

[0148] Organosilicon A3: KR311 (organosilicon polymer having methyl and phenyl groups as hydrophobic groups) manufactured by Shin-Etsu Chemical Co., Ltd.

[0149] ​Example 4

[0150] Silicone A4: KR112 (silicone polymer having methyl as a hydrophobic group) manufactured by Shin-Etsu Chemical Co., Ltd.

[0151] Note that the order of the TD ratio from the high is silicone Al > A2 > A3 > A4. All have a polydimethylsiloxane skeleton as a main chain, and in silicone A2 and A3, a part of the methyl groups is substituted with phenyl groups.

[0152] Each of the batteries produced above was evaluated by the following method.

[0153] [evaluation]

[0154] The evaluation battery cell was held by a pair of 10 x 5 cm stainless steel jigs (thickness 6 mm) and pressurized and fixed at 3.2 MPa.

[0155] <1st cycle>

[0156] In a thermostat at 25°C, lithium was charged to the negative electrode at a constant current of 0.05 C (1 C is a current value at which the design capacity is discharged in 1 hour) for 2 hours, after which it was rested for 12 hours. Next, lithium was further charged to the negative electrode at a constant current of 0.05 C until the battery cell voltage was 0.01 V, after which it was rested for 20 minutes. Next, lithium was discharged from the negative electrode at a constant current of 0.05 C until the battery cell voltage was 1.5 V, after which it was rested for 20 minutes.

[0157] <2nd to 3rd cycles>

[0158] Next, lithium was charged to the negative electrode at a constant current of 0.05 C until the battery cell voltage was 0.01 V, after which it was rested for 20 minutes. Next, lithium was discharged from the negative electrode at a constant current of 0.05 C until the battery cell voltage was 1.5 V, after which it was rested for 20 minutes.

[0159] <4th to 25th cycles>

[0160] Lithium was charged to the negative electrode at a constant current of 0.3 C until the battery cell voltage was 0.01 V, after which it was rested for 20 minutes, and then lithium was discharged from the negative electrode at a constant current of 0.3 C until the battery cell voltage was 1.5 V, after which it was rested for 20 minutes, and this cycle was repeated.

[0161] The charge and discharge was performed in an environment at 25°C. At this time, the capacity retention rate until the 25th cycle when the initial discharge capacity was set to 100% is shown in Figure 3 .

[0162] It should be noted that (1 / X)It represents the current, which is (1 / X)It(A) = rated capacity (Ah) / X(h), where X represents the time used to charge or discharge the rated capacity of the electrical equipment. For example, 0.5It means that X = 2, and the current value is rated capacity (Ah) / 2(h).

[0163] according to Figure 3 It can be understood that, in the batteries A1 to A4 of Examples 1 to 4, which have a second layer containing organosilicon in a Si-containing material, superior cycle characteristics were obtained compared to battery B of Comparative Example 1, which did not have a second layer. Furthermore, a higher T / D ratio resulted in better capacity retention.

[0164] then, Figure 4A The image shows a cross-sectional SEM photograph of the Si-containing material in the negative electrode used in battery A4 of Example 1. Additionally, Figure 4B to 4D It is by Figure 3 Results of TEM-EELS analysis of regions A, designated 1-8. Figure 4B The spectrum of the Si-L edge is shown. Figure 4C The spectrum of the CK edge is shown. Figure 4D The spectrum of the OL edge is shown. In the figure, the regions indicated by numbers 1 and 2 are Si-containing materials (SiOx), the regions indicated by numbers 3 and 4 are the first carbon material forming the first layer, and the regions indicated by numbers 5 and 6 are the regions containing organosilicon. These figures confirm that a first layer containing the first carbon material and a second layer containing organosilicon are formed on the surface of the Si-containing material. It should be noted that the thickness of the first layer is approximately 20 nm, and the thickness of the second layer is approximately 5 nm.

[0165] Industrial availability

[0166] The negative electrode for secondary batteries disclosed herein is useful in secondary batteries that require high capacity as the main power source for mobile communication devices, portable electronic devices, and the like.

[0167] Explanation of reference numerals in the attached figures

[0168] 1a Positive electrode active material layer

[0169] 1b Positive current collector

[0170] 1c Positive electrode tab lead

[0171] 2a Negative electrode mixture layer

[0172] 2b Negative current collector

[0173] 2c Negative electrode tab lead

[0174] 4-plate assembly

[0175] 5 case

[0176] 6 insulating tab film

[0177] 10 positive electrode

[0178] 20 negative electrode

[0179] 30 separator

[0180] 100 nonaqueous electrolyte secondary battery

Claims

1. A negative electrode for a secondary battery, comprising a negative electrode mixture layer having a negative electrode active material, a first layer covering at least a part of a surface of the negative electrode active material, and a second layer covering at least a part of a surface of the first layer, the negative electrode active material comprising a Si-containing material, a content of the Si-containing material contained in the negative electrode mixture layer is 1 to 30 mass%, the Si-containing material comprises a lithium ion conductive phase and Si particles dispersed in the lithium ion conductive phase, the lithium ion conductive phase is at least one selected from the group consisting of a silicon oxide phase, a silicate phase, and a carbon phase, the first layer comprises a first carbon material, and the first carbon material is amorphous carbon, the second layer comprises an organosilicon, with {(R1) a SiO (4-a) / 2} c when said organosilicon is represented by R1 is a substituted or unsubstituted organic group having a carbon number of 1 to 8, the R1 comprises at least one selected from the group consisting of an alkyl group, a vinyl group, an alkoxy group, an aryl group, an aryloxy group, a ketone group, a carboxyl group, and an ester group, a is a rational number satisfying 0 ≤ a ≤ 2, c is an integer satisfying 1 ≤ c, the organosilicon has a dioxygen structure R2SiO2 unit and a trioxgen structure RSiO3 unit, a ratio of the RSiO3 unit to the R2SiO2 unit contained in a molecule of the organosilicon: RSiO3 unit / R2SiO2 unit is 0.15 or more.

2. The negative electrode for a secondary battery according to claim 1, wherein the R1 is an alkyl group, and the alkyl group is at least one selected from the group consisting of a methyl group and an ethyl group.

3. The negative electrode for a secondary battery according to claim 1 or 2, wherein the organosilicon has at least one selected from the group consisting of a silanol group and an alkoxy silyl group.

4. The negative electrode for a secondary battery according to claim 1 or 2, wherein a thickness of the second layer is 2 nm or more and 20 nm or less.

5. The negative electrode for a secondary battery according to claim 4, wherein a thickness of the second layer is 5 nm or more and 10 nm or less.

6. A manufacturing method of the negative electrode for a secondary battery according to any one of claims 1 to 5, comprising the following steps: a step of preparing a slurry comprising a negative electrode active material, a first layer covering at least a part of a surface of the negative electrode active material, and a dispersion medium, the first layer comprising a carbon material; a step of applying the slurry on a negative electrode current collector, and volatilizing the dispersion medium, thereby forming a coating film; a step of preparing a solution comprising an organosilicon and a solvent; and, a step of bringing the coating film into contact with the solution, and volatilizing the solvent, thereby covering at least a part of a surface of the first layer with a second layer, the second layer comprises the organosilicon.

7. The method for manufacturing a negative electrode for a secondary battery according to claim 6, wherein a weight average molecular weight of the organosilicon is 80 or more and 10,000 or less.

8. The method for manufacturing a negative electrode for a secondary battery according to claim 7, wherein a weight average molecular weight of the organosilicon is 1,000 or less.

9. A secondary battery comprising: the negative electrode for a secondary battery according to claim 1, a positive electrode, and an electrolyte.

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