Negative electrode for secondary battery, and secondary battery

Abstract

This negative electrode for a secondary battery comprises: a negative electrode mixture layer that includes a negative electrode active material; and a negative electrode current collector that supports the negative electrode mixture layer. The negative electrode active material includes graphite and a silicon-including material. The silicon-including material includes a first composite material, and the first composite material includes a carbon phase and a silicon phase that is dispersed in the carbon phase. The negative electrode mixture layer has a first region on the negative electrode current collector side and a second region on the negative electrode surface side. The content ratio M1 of the silicon-including material to the total of the graphite and the silicon-including material in the first region is greater than the content ratio M2 of the silicon-including material to the total of the graphite and the silicon-including material in the second region. The average particle diameter R1 of the graphite in the first region is greater than the average particle diameter R2 of the graphite in the second region.

Description

Negative electrode for secondary batteries and secondary batteries Cross-reference of related applications

[0001] This disclosure claims priority rights to Japanese Patent Application No. 2025-028008, filed with the Japan Patent Office on 25 February 2025, and the entirety of the said patent application is incorporated herein by reference.

[0002] This disclosure relates to a negative electrode for a secondary battery and a secondary battery.

[0003] Secondary batteries equipped with non-aqueous electrolytes, particularly lithium-ion batteries, possess high voltage and high energy density, and are widely used as power sources for small electronic devices such as portable devices.

[0004] Patent Document 1 proposes a negative electrode for a secondary battery comprising a current collector, a first negative electrode active material layer formed on the current collector and containing a first active material, and a second negative electrode active material layer formed on the first negative electrode active material layer and containing a second active material, wherein the second active material is a bimodal active material consisting of small particles and large particles with different particle sizes, the particle size (D2) of the second active material is smaller than the particle size (D1) of the first active material, and the particle size of the second active material is the average particle size of the small and large particles.

[0005] Japanese Patent Publication No. 2022-74044

[0006] In recent years, there has been a growing demand for improvements in the rapid charge / discharge characteristics and cycle performance of secondary batteries.

[0007] In view of the foregoing, one aspect of the present disclosure relates to a negative electrode for a secondary battery, comprising a negative electrode composite layer containing a negative electrode active material and a negative electrode current collector supporting the negative electrode composite layer, wherein the negative electrode active material comprises graphite and a silicon-containing material, the silicon-containing material comprises a first composite material, the first composite material comprises a carbon phase and a silicon phase dispersed within the carbon phase, the negative electrode composite layer has a first region on the negative electrode current collector side and a second region on the surface side of the negative electrode, the content ratio M1 of the silicon-containing material to the sum of the graphite and the silicon-containing material in the first region is greater than the content ratio M2 of the silicon-containing material to the sum of the graphite and the silicon-containing material in the second region, and the average particle size R1 of the graphite in the first region is greater than the average particle size R2 of the graphite in the second region.

[0008] Another aspect of this disclosure relates to a secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is the negative electrode for a secondary battery described above.

[0009] According to this disclosure, the rapid charge-discharge characteristics and cycle characteristics of secondary batteries can be improved. Novel features of the present invention are described in the appended claims, but the present invention, in conjunction with other objects and features of the present invention, will be better understood by the following detailed description in conjunction with the drawings, both in terms of structure and content.

[0010] This is a schematic cross-sectional view showing an example of a negative electrode for a secondary battery according to one embodiment of the present disclosure. This is a schematic perspective view showing a portion of a secondary battery according to one embodiment of the present disclosure with a section cut out.

[0011] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be given as examples, but other numerical values ​​and materials may be applied as long as the effects of this disclosure are obtained. In this specification, the description "numerical value A to numerical value B" includes numerical value A and numerical value B, and can be read as "greater than or equal to numerical value A and less than or equal to numerical value B". In the following description, when lower and upper limits are given as examples for numerical values ​​of specific physical properties or conditions, either of the given lower limits and either of the given upper limits can be arbitrarily combined, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, one of them may be selected and used alone, or two or more may be used in combination.

[0012] Furthermore, this disclosure encompasses any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims. In other words, any combination of matters described in two or more claims, which may be arbitrarily selected from the multiple claims set forth in the attached claims, is possible, provided that no technical inconsistency arises.

[0013] (Negative electrode for secondary battery) The negative electrode for a secondary battery according to the embodiment of the present disclosure comprises a negative electrode composite layer containing a negative electrode active material and a negative electrode current collector supporting the negative electrode composite layer. The negative electrode active material contains graphite and a silicon-containing material (hereinafter also referred to as a Si-containing material), the silicon-containing material contains a first composite material, and the first composite material (hereinafter also referred to as SiC) contains a carbon phase and a silicon phase dispersed within the carbon phase. The negative electrode composite layer has a first region on the negative electrode current collector side and a second region on the surface side of the negative electrode (opposite to the negative electrode current collector). The content ratio M1 of the silicon-containing material to the total of graphite and silicon-containing material in the first region is greater than the content ratio M2 of the silicon-containing material to the total of graphite and silicon-containing material in the second region. The average particle size R1 of graphite in the first region is greater than the average particle size R2 of graphite in the second region. Hereinafter, graphite with an average particle size R1 may be referred to as large graphite particles. Graphite with an average particle size R2 may be referred to as small graphite particles. The average particle size of graphite is the median diameter based on volume, and is the particle size (D50) at which the cumulative distribution reaches 50% based on volume.

[0014] The silicon-containing material may consist only of the first composite material, or it may include other materials other than the first composite material (for example, a second composite material, a third composite material, etc.). The first region and the second region may each contain the first composite material and graphite as negative electrode active materials. The second composite material (hereinafter also referred to as the silicate-based composite material) includes a silicate phase and a silicon phase dispersed within the silicate phase. The third composite material (hereinafter also referred to as SiO) includes a silicon oxide phase and a silicon phase dispersed within the silicon oxide phase.

[0015] The content ratio of the first composite material to the total silicon-containing material in the negative electrode composite layer may be 50% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more. The content ratio of the first composite material to the total silicon-containing material in the first region (or second region) may be within the same range as described above. The content ratio of the first composite material to the total silicon-containing material in the first region may be approximately the same as, or different from, the content ratio of the first composite material to the total silicon-containing material in the second region.

[0016] By using a negative electrode for a secondary battery that satisfies the above configuration, the cycle characteristics and rapid charge / discharge characteristics of the secondary battery can be improved. For example, the cycle retention rate in rapid charge / discharge cycles can be significantly increased.

[0017] Si-containing materials offer advantages over graphite in terms of achieving high energy density. In the first region of the negative electrode composite layer on the negative electrode current collector side, high capacity can be achieved by including large graphite particles and increasing the proportion of Si-containing material. Although Si-containing materials expand and contract more during charging and discharging than graphite, in the first region, the presence of soft large graphite particles surrounding the Si-containing material alleviates the stress generated in the negative electrode composite layer due to the expansion and contraction of the Si-containing material during charging and discharging, ensuring good adhesion between the negative electrode composite layer and the negative electrode current collector. As a result, the cycle characteristics of the secondary battery are improved. The aforementioned large particles can act as a buffer within the negative electrode composite layer, mitigating the effects of the expansion and contraction of the Si-containing material during charging and discharging. Large graphite particles tend to be softer (more easily deformed) than small graphite particles. This is thought to be due to the larger particle size and the greater number of voids contained within the large particles.

[0018] In the second region on the surface side of the negative electrode composite layer (negative electrode), the proportion of hard graphite particles is large, and appropriate voids are formed between the active material particles, ensuring many diffusion pathways for the electrolyte. As a result, the permeability (liquid absorption) of the electrolyte in the negative electrode is improved, and the rapid charge-discharge characteristics of the secondary battery are improved.

[0019] By combining a silicon-containing material including a first composite material (SiC) with a negative electrode composite layer having a first region and a second region, a synergistic effect of improving rapid charge / discharge characteristics and cycle characteristics can be obtained.

[0020] In the case of the first composite material (SiC), the presence of the carbon phase makes it easier to relieve stress caused by the expansion and contraction of the silicon phase, and also suppresses the expansion of the negative electrode. Combined with the formation of the first region, this results in a significant improvement in cycle characteristics. In the case of the first composite material (SiC), particle cracking (refinement) of the composite material is easily suppressed, and combined with the formation of the second region, this results in a significant improvement in rapid charge and discharge characteristics. Furthermore, in the case of the first composite material (SiC), the irreversible capacity is smaller and the charge and discharge efficiency is higher compared to the third composite material (SiO).

[0021] The first region is the region on the negative electrode current collector side of the negative electrode composite layer, and the second region is the surface side of the negative electrode (the region on the opposite side of the negative electrode current collector in the negative electrode composite layer). The negative electrode composite layer may have a two-layer structure consisting of the first region and the second region. The negative electrode composite layer may also have a third region between the first and second regions, and may have a three-layer structure consisting of the first region, the third region, and the second region. In this case, the content ratio M3 of silicon-containing material to the total of graphite and silicon-containing material in the third region may be smaller than M1 and larger than M2. The average particle size R3 of graphite in the third region may be smaller than R1 and larger than R2. Furthermore, in the third region, M3 may be approximately the same as M2, and R3 may be approximately the same as R1. In the third region, M3 may be approximately the same as M1, and R3 may be approximately the same as R2.

[0022] The second region does not necessarily have to contain silicon-containing material, but from the viewpoint of increasing capacity, it is preferable that both the first and second regions contain silicon-containing material. Note that "substantially free of silicon-containing material" in the second region means that silicon is below the detection limit in the EDX analysis of the cross-section of the negative electrode (second region).

[0023] From the viewpoint of increasing capacity and improving cycle characteristics, it is preferable that both the first and second regions contain the first composite material as a silicon-containing material. It is also preferable that both the first and second regions contain only the first composite material as a silicon-containing material. It is preferable that the content ratio of the first composite material to the total of the first composite material and graphite in the first region is greater than the content ratio of the first composite material to the total of the first composite material and graphite in the second region.

[0024] The ratio of average particle size R2 to average particle size R1, R2 / R1, may be 0.9 or less, 0.8 or less, 0.75 or less, or 0.7 or less. R2 / R1 may be 0.1 or more, 0.2 or more, 0.25 or more, or 0.3 or more. R2 / R1 may be 0.1 or more, 0.9 or less, 0.25 or more, or 0.75 or less. The average particle size R of the graphite contained in the negative electrode composite layer is, for example, 1 μm or more and 30 μm or less.

[0025] The content ratio M1 may be 1.5 times or more the content ratio M2, or 2 times or more. In this case, the second region contains silicon-containing material. Also, the content ratio M1 may be 4 times or less the content ratio M2.

[0026] Si-containing materials have a higher capacity density compared to graphite. Graphite expands and contracts less during charging and discharging compared to Si-containing materials. From the viewpoint of increasing capacity, the content ratio M of silicon-containing material to the total of graphite and silicon-containing material in the negative electrode composite layer may be 0.1 mass% or more, 1 mass% or more, or 5 mass% or less. From the viewpoint of improving cycle characteristics, the content ratio M of silicon-containing material to the total of graphite and silicon-containing material in the negative electrode composite layer may be 40 mass% or less, 35 mass% or less, or 30 mass% or less.

[0027] From the viewpoint of increasing capacity and improving cycle characteristics, the content ratio M of silicon-containing material to the total of graphite and silicon-containing material in the negative electrode composite layer may be 0.1% by mass or more and 40% by mass or less, or 1% by mass or more and 30% by mass or less.

[0028] The ratio of the thickness T1 of the first region to the thickness T of the negative electrode composite layer, T1 / T, may be 1 / 4 or more, 1 / 3 or more, or 1 / 2 or more. Also, T1 / T may be 3 / 4 or less, 2 / 3 or less, or 1 / 2 or less.

[0029] The ratio of the thickness T2 of the second region to the thickness T of the negative electrode composite layer, T2 / T, may be 1 / 4 or more, 1 / 3 or more, or 1 / 2 or more. Also, T2 / T may be 3 / 4 or less, 2 / 3 or less, or 1 / 2 or less.

[0030] The ratio of the thickness T2 of the second region to the thickness T1 of the first region, T2 / T1, may be 1 / 2 or more and 2 or less, or 4 / 5 or more and 5 / 4 or less.

[0031] The thickness T of the negative electrode composite layer (thickness per side) is, for example, 50 μm or more and 200 μm or less.

[0032] The first and second regions of the negative electrode composite layer can be identified by performing elemental mapping analysis using energy-dispersive X-ray (EDX) with cross-sectional images of the negative electrode obtained by scanning electron microscopy (SEM). For example, the first and second regions can be identified based on the silicon map, and M1 / M2 can be determined. Next, R2 / R1 can be determined for the graphite in the first and second regions using the average particle size measurement method described later.

[0033] Furthermore, M1 / M2 may be determined from the mixing ratio of graphite and silicon-containing materials included in the first negative electrode slurry for forming the first region and the second negative electrode slurry for forming the second region, as described later.

[0034] The average particle size R1 of graphite can be obtained by observing a cross-section of the first region in the negative electrode composite layer using a SEM, determining the equivalent circle diameter of 80 to 100 arbitrary graphite particles, and then determining the particle size at a cumulative 50% in the volume-based particle size distribution obtained from these equivalent circle diameters. The equivalent circle diameter is the diameter of a circle having the same area as the area of ​​the graphite particles observed in the cross-sectional image of the first region by SEM. Alternatively, the average particle size R1 may be determined by measuring the graphite contained in the first negative electrode slurry for forming the first region, as described later, using the laser diffraction scattering method. For example, the "LA-750" manufactured by HORIBA, Ltd. can be used as the measuring device. The average particle size R2 of graphite can also be determined in the same manner as above, using a cross-sectional image of the second region in the negative electrode composite layer, or using graphite contained in the second negative electrode slurry for forming the second region, as described later.

[0035] A negative electrode composite layer having a first region and a second region may be formed, for example, using a first negative electrode slurry for forming the first region and a second negative electrode slurry for forming the second region. The first negative electrode slurry includes the constituent material of the first region and a dispersion medium. The second negative electrode slurry includes the constituent material of the second region and a dispersion medium. The dispersion medium can be water, an organic solvent (for example, N-methyl-2-pyrrolidone), etc. Specifically, the first negative electrode slurry may be applied to the surface of the negative electrode current collector and the first coating film may be dried to form a first layer (first region), and the second negative electrode slurry may be applied to the surface of the first layer (first region) and the second coating film may be dried to form a second layer (second region). If necessary, the laminate of the first and second coating films after drying may be rolled. Drying of the first coating film may be performed together with drying of the second coating film after applying the second negative electrode slurry on top of the first coating film.

[0036] Figure 1 is a schematic cross-sectional view showing an example of a negative electrode for a secondary battery according to one embodiment of the present disclosure. In Figure 1, T is the thickness of the negative electrode composite layer 12, T1 is the thickness of the first region 12a, and T2 is the thickness of the second region 12b. However, the negative electrode for a secondary battery according to the embodiment of the present disclosure is not limited thereto.

[0037] The negative electrode 10 comprises a negative electrode composite layer 12 containing a negative electrode active material and a negative electrode current collector 11 supporting the negative electrode composite layer 12. The negative electrode composite layer 12 is formed on both main surfaces of the sheet-like negative electrode current collector 11. The negative electrode active material contains graphite and a silicon-containing material, and the silicon-containing material includes a first composite material. The first composite material includes a carbon phase and a silicon phase dispersed within the carbon phase. The negative electrode composite layer 12 has a first region 12a on the negative electrode current collector 11 side and a second region 12b on the surface side of the negative electrode 10. The content ratio M1 of the silicon-containing material to the total of graphite and silicon-containing material in the first region 12a is greater than the content ratio M2 of the silicon-containing material to the total of graphite and silicon-containing material in the second region 12b. The average particle size R1 of graphite in the first region 12a is greater than the average particle size R2 of graphite in the second region 12b.

[0038] (Secondary Battery) The secondary battery according to the embodiment of this disclosure comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. This negative electrode is the negative electrode for secondary batteries described above. The secondary battery will be described in detail below.

[0039] [Negative Electrode] The negative electrode comprises a negative electrode current collector and a negative electrode composite layer supported on the surface of the negative electrode current collector. The negative electrode composite layer contains a negative electrode active material as an essential component, and may also contain other components besides the negative electrode active material. Other components besides the negative electrode active material include binders, conductive materials, and thickeners. The other components may be used individually or in combination of two or more types.

[0040] The negative electrode active material includes graphite and a Si-containing material.

[0041] Natural graphite, artificial graphite, etc., can be used as the graphite. From the viewpoint of improving the adhesion between the negative electrode composite layer and the negative electrode current collector, it is preferable that the graphite in the first region contains at least natural graphite. The graphite may also partially contain amorphous carbon, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). The average particle size (D50) of the graphite is, for example, 1 μm or more and 30 μm or less.

[0042] Graphite is a carbon material in which a graphite-type crystal structure is well-developed. The interplanar spacing d002 of the (002) plane of graphite particles, as measured by X-ray diffraction, may be, for example, 0.340 nm or less, or 0.3354 nm or more and 0.340 nm or less. The crystallite size Lc(002) of graphite particles, as measured by X-ray diffraction, may be, for example, 5 nm or more, or 5 nm or more and 300 nm or less, or 10 nm or more and 200 nm or less. The crystallite size Lc(002) is measured, for example, by the Scherrer method. When the interplanar spacing d002 and crystallite size Lc(002) of the (002) plane of graphite particles are within the above ranges, high capacity is easily obtained.

[0043] Examples of silicon-containing materials include elemental silicon, silicon-containing alloys, and silicon-containing composite materials. For example, a silicon-containing alloy contains silicon (Si) and at least one element selected from the group consisting of tin (Sn), nickel (Ni), iron (Fe), copper (Cu), titanium (Ti), manganese (Mn), and aluminum (Al). The silicon-containing material is particulate, and the average particle size (D50) of the silicon-containing material is, for example, 1 μm or more and 25 μm or less.

[0044] A composite material comprises an ion-conducting phase (matrix) and a silicon phase (particulate Si phase) dispersed within the ion-conducting phase. Examples of composite materials include a first composite material in which the ion-conducting phase is a carbon phase, a second composite material in which the ion-conducting phase is a silicate phase, and a third composite material in which the ion-conducting phase is a silicon oxide phase. The ion-conducting phase is also called the matrix phase, and the silicon phase is also called the domain phase. In other words, a silicon-containing material may be a composite material having a sea-island structure. In such a composite material, the silicon contained in the silicon phase reversibly forms an alloy with lithium. Therefore, such a composite material can also reversibly intercept and release lithium ions.

[0045] The negative electrode composite layer contains at least a first composite material as a silicon-containing material. The first region of the negative electrode composite layer contains at least the first composite material. The second region of the negative electrode composite layer preferably contains the first composite material, but may not substantially contain the first composite material. The first composite material comprises a silicon phase and a carbon phase in which the silicon phase is dispersed. The carbon phase functions as an ion-conducting phase and has lithium-ion conductivity.

[0046] The silicon phase preferably includes at least one selected from the group consisting of elemental silicon and silicon alloys. Examples of silicon alloys include silicon-tin alloys, silicon-lithium alloys, and silicon-germanium alloys.

[0047] The carbon phase may contain crystalline carbon (graphite) or amorphous carbon with low crystallinity (i.e., amorphous carbon). Amorphous carbon may be, for example, poorly graphitizable carbon (hard carbon), easily graphitizable carbon (soft carbon), or other types. Preferably, the carbon phase consists of at least one of hard carbon and soft carbon. Both hard carbon and soft carbon have a graphite-like structure microscopically, and the random arrangement of these structures makes the whole amorphous. Therefore, when the carbon phase consists of at least one of hard carbon and soft carbon, such a carbon phase can exhibit good conductivity due to the graphite-like structure described above, and exhibits the characteristic of small expansion and contraction during charging and discharging. Consequently, when a silicon-containing material contains such a carbon phase, the non-aqueous electrolyte secondary battery exhibits high cycle characteristics.

[0048] As a carbon source, for example, sugars and water-soluble resins can be used. As a carbon source, for example, carboxymethylcellulose (CMC), polyvinylpyrrolidone, cellulose, sucrose, etc. may be used.

[0049] The first composite material can be obtained, for example, by crushing and mixing a carbon source and silicon, which are the raw materials, in a ball mill or other agitator to obtain a mixture, and then firing this mixture under pressure in an inert atmosphere. Alternatively, the first composite material may be obtained by heating the above mixture to a predetermined temperature, causing the silicon to neck in the mixture to obtain a sintered body, and then crushing this sintered body.

[0050] In the first composite material, the silicon phase content ratio MSi is preferably 10% by mass or more and 80% by mass or less. MSi may be 30% by mass or more, or 40% by mass or more. MSi may be 70% by mass or less, or 60% by mass or less. By having MSi within the above numerical range, the first composite material can sufficiently absorb lithium ions while suppressing excessive expansion. This makes it possible to further achieve both high capacity and improved cycle characteristics.

[0051] In the first composite material, the carbon phase content ratio MC is preferably 10% by mass or more and 60% by mass or less. MC may be 35% by mass or more, or 40% by mass or more. MC may be 57% by mass or less, or 55% by mass or less. By having MC within the above numerical range, the first composite material can sufficiently absorb lithium ions while suppressing excessive expansion. This allows for a more complete balance between increased capacity and improved cycle characteristics.

[0052] The presence of both a silicon phase and a carbon phase in the first composite material can be confirmed by SEM images of its cross-section. Furthermore, by performing elemental mapping analysis using EDX on the cross-section of the particles in the first composite material, both quantitative and qualitative analysis of both the silicon phase and the carbon phase can be performed. In other words, both the silicon phase content ratio (MSi) and the carbon phase content ratio (MC) can be measured in the first composite material.

[0053] In the first composite material, the average particle size of the silicon phase is preferably 1 nm or more and 500 nm or less, and more preferably 1 nm or more and 30 nm or less. That is, it is preferable that the silicon phase has a sufficiently small average particle size. By having a sufficiently small average particle size of the silicon phase in this way, it is possible to suppress excessive expansion and contraction of the silicon phase due to the intercalation and release of lithium ions. As a result, the silicon phase can be refined, and thus it is possible to further achieve both high capacity and improved cycle characteristics. The average particle size of the silicon phase can be measured using a cross-sectional image of the first composite material obtained by TEM. The average particle size of the silicon phase is the particle size of 50% of the cumulative total (D50) in the volume-based particle size distribution obtained using the maximum diameter obtained for any 100 silicon phases.

[0054] In the first composite material, the carbon phase may be formed from a porous carbon material. In this case, the first composite material may be a first composite material having a carbon phase formed from a porous carbon material and a silicon phase dispersed within the carbon phase.

[0055] A porous carbon material can be any porous carbon material having multiple pores, but a carbon material having many micropores and mesopores with a pore diameter of 1 nm to 10 nm is preferred. Micropores and mesopores are suitable for arranging nano-sized silicon particles inside them. The more micropores and mesopores a porous carbon material has, the easier it is to arrange more silicon particles inside the pores. The volume ratio of micropores and mesopores to the total pore volume of the porous carbon material is preferably 50% or more, more preferably 70% or more, and most preferably 90% or more. Pores are classified into micropores (<2 nm), mesopores (2 nm to 50 nm), and macropores (>50 nm) according to their pore diameter. The volume ratio of micropores and mesopores to the total pore volume can be determined from the pore diameter distribution of the porous carbon material.

[0056] As the porous carbon material, for example, activated carbon can be used. Alternatively, the porous carbon material may be hard carbon or soft carbon, as described above. It is preferable that the porous carbon material has many mesopores. A commercially available porous carbon material having many mesopores is, for example, Knobel®, a porous carbon manufactured by Toyo Tanso Co., Ltd. It is preferable that the porous carbon material is at least one of hard carbon and soft carbon. In this case, the carbon phase is formed by at least one of hard carbon and soft carbon.

[0057] The first composite material having a carbon phase formed by a porous carbon material and a silicon phase dispersed within this carbon phase can be obtained by (i) preparing a precursor composite including a porous carbon material and siloxanes arranged in a plurality of pores of the porous carbon material, and (ii) contacting the precursor composite with magnesium vapor to reduce the siloxanes arranged in the pores to silicon.

[0058] In step (i), a siloxane can be produced from an organosilicon compound within multiple pores of a porous carbon material.

[0059] For example, a porous carbon material can be mixed with an organosilicon compound having a hydrolyzable functional group to hydrolyze the functional group and obtain a hydrolysis product. This hydrolysis product can then be subjected to dehydration condensation to obtain a precursor composite containing the porous carbon material and a siloxane. When the functional group is hydrolyzed, a hydroxyl group (-OH) is generated. Dehydration condensation of the hydroxyl groups between molecules of the hydrolysis product forms a siloxane bond (Si-O-Si), yielding a siloxane compound.

[0060] The conditions for carrying out the hydrolysis and dehydration condensation reactions of organosilicon compounds are not particularly limited. The hydrolysis and dehydration condensation reactions may proceed in the liquid phase or in the gas phase.

[0061] When hydrolysis and dehydration condensation reactions are carried out in the liquid phase, for example, a dispersion containing a porous carbon material, an organosilicon compound, an organic solvent, and water can be prepared, and the above-mentioned functional groups of the organosilicon compound can be hydrolyzed by stirring the dispersion at room temperature to 80°C. When preparing the dispersion, it is preferable to mix the porous carbon material, the organic solvent, and water, allow the mixture of the organic solvent and water to thoroughly permeate the pores of the porous carbon material, and then add the organosilicon compound. After that, the organic solvent and the silicon compound remaining in the organic solvent are removed by centrifugation, filtration, etc., and the solid content is recovered.

[0062] For example, toluene can be used as the organic solvent. The organic solvent is not particularly limited as long as it has high affinity for the porous carbon material and can dissolve a small amount of water. In the dispersion, the amount of organosilicon compound relative to the porous carbon material should be selected according to the amount of silicon phase to be contained in the pores. In the dispersion, the amount of organic solvent relative to the porous carbon material is not particularly limited, and should be an amount that sufficiently wets the surface of the porous carbon material with the organic solvent. In the dispersion, the amount of water relative to the porous carbon material is also not particularly limited, and should be an amount that sufficiently penetrates the pores of the porous carbon material with water molecules.

[0063] Next, the recovered solids are heated or dried to promote the dehydration condensation reaction of the hydrolysis products and generate siloxanes. For example, heating the dried solids under reduced pressure causes the hydrolysis products to undergo dehydration condensation, thereby accelerating the siloxane formation reaction.

[0064] When hydrolysis and dehydration condensation reactions are carried out in the gas phase, for example, a porous carbon material can be exposed to vapor and water vapor of an organosilicon compound having hydrolyzable functional groups, allowing molecules of the organosilicon compound and water molecules to penetrate into the pores of the porous carbon material, and then the porous carbon material can be heated. This heating promotes the hydrolysis reaction of the above-mentioned functional groups of the organosilicon compound within the pores of the porous carbon material to obtain a hydrolysis product, and then the dehydration condensation reaction can be promoted in this hydrolysis product.

[0065] For example, in the gas phase method, first, a porous carbon material is placed in a reaction chamber under reduced pressure, and an organosilicon compound vapor is brought into contact with the porous carbon material in the reaction chamber, thereby allowing the organosilicon compound to penetrate into the pores of the porous carbon material. Next, the organosilicon compound is exhausted from the reaction chamber, water vapor is introduced into the reaction chamber under reduced pressure, and water molecules are allowed to penetrate into the pores of the porous carbon material. In addition to performing such an operation one or more times, heating is appropriately performed, whereby a precursor composite including the porous carbon material and siloxane can be obtained.

[0066] The type of the organosilicon compound is not particularly limited, but from the viewpoint of being easily reduced to silicon, silicon compounds having hydrolyzable functional groups such as alkoxysilanes and chlorosilanes are preferred. Among these, alkoxysilanes are preferred because of their high stability and ease of handling. The alkoxysilane may include a Si-C bond (an alkyl group directly bonded to a silicon atom), or may not include a Si-C bond.

[0067] Examples of alkoxysilanes that do not include a Si-C bond include tetramethyl orthosilicate (Si(OCH ３ ) ４ ), tetraethyl orthosilicate (Si(OC ２ H ５ ) ４） , tetrapropyl orthosilicate (Si(OC ３ H ７ ) ４ ), tetrabutyl orthosilicate (Si(OC ４ H ９ ) ４ ), and the like.

[0068] Examples of alkoxysilanes that include a Si-C bond include methyltriethoxysilane (CH ３ Si(OC ２ H ５ ) ３ ), 3-aminopropylmethyldiethoxysilane (H ２ N(CH ２ ) ３ Si(CH ３ )(OC ２ H ５ ) ２) are some examples.

[0069] Since Si-C bonds are difficult to break at temperatures below 700°C, they may remain intact during the reduction reaction in step (ii). Therefore, it is more preferable that the alkoxysilane does not contain Si-C bonds. By using an alkoxysilane that does not contain Si-C bonds, Si-C bond-free siloxanes can be obtained with high efficiency. Si-C-free siloxanes are readily reduced to silicon, allowing for high-efficiency silicon production.

[0070] In step (ii) of reducing siloxane to silicon, the precursor composite obtained in step (i) should be heated in a non-oxidizing atmosphere containing magnesium vapor. When siloxane is reduced with magnesium vapor, nano-sized silicon particles (nano-silicon particles) are generated. This forms a silicon phase within the pores of the porous carbon material. Magnesium oxide, silicon oxide, and silicon carbide are also generated. This allows the silicon phase formed by nano-silicon particles, silicon oxide, and silicon carbide to be arranged within the pores of the porous carbon material.

[0071] Silicon oxide and silicon carbide have the function of shielding the silicon phase from the non-aqueous electrolyte. Even if a gap forms between the silicon phase and the carbon phase within the pores, or if the silicon phase is damaged and an active surface of the silicon phase is formed, contact between the active surface of the silicon phase and the non-aqueous electrolyte is suppressed by silicon oxide and silicon carbide. As a result, the formation of SEI is suppressed, and the decrease in capacity of the non-aqueous electrolyte secondary battery can be further suppressed.

[0072] Silicon oxide may exist in close contact with the silicon phase, or it may exist as an integral part of the silicon phase. Silicon oxide may be particulate, or it may be a matrix. For example, the silicon phase may be present in a matrix formed by silicon oxide. From the viewpoint of relieving stress due to the expansion and contraction of the silicon phase, at least a portion of the silicon oxide may be located inside the silicon phase. Silicon carbide may exist in close contact with the porous carbon material or the silicon phase, or it may exist as an integral part of the porous material or the silicon phase. Silicon carbide may be particulate, or it may be a film interposed between the porous carbon material and the silicon phase. Silicon carbide may form a matrix together with silicon oxide. From the viewpoint of relieving stress due to the expansion and contraction of the silicon phase, at least a portion of the silicon carbide may be located inside the silicon phase.

[0073] A non-oxidizing atmosphere can be a vacuum, reduced pressure atmosphere, or inert gas atmosphere. Inert gases such as argon, noble gases, or nitrogen can be used. In an oxidizing atmosphere, the generated magnesium vapor is oxidized to magnesium oxide (MgO) or magnesium dioxide (MgO). ２ Because of the formation of siloxanes, the reduction of siloxanes may not proceed sufficiently.

[0074] The melting point of magnesium is 650°C. Near its melting point, magnesium has an extremely high vapor pressure (372 Pa). When the precursor complex obtained in step (i) is heated in the presence of magnesium vapor, the chemical reaction represented by the following formula (1) proceeds, and the siloxane is reduced to silicon. At this time, magnesium oxide (MgO) is also produced. The higher the vapor pressure of magnesium, the more efficient the reduction reaction becomes.

[0075] 2Mg + -(O-Si-O)- → 2MgO + Si...(1)

[0076] To reduce siloxanes disposed within the pores of a porous carbon material with magnesium vapor, the precursor complex obtained in step (i) is mixed with magnesium, and the mixture is heated. The mixture of the precursor complex and magnesium can be heated in a container made of graphite, stainless steel, or the like. The form of magnesium mixed with the precursor complex is not particularly limited, as long as it can produce magnesium vapor. For example, magnesium in the form of powder, particles, ribbons, rods, pellets, etc., can be used.

[0077] The temperature at which the mixture of the precursor complex and magnesium is heated is not particularly limited, but the higher the temperature, the higher the vapor pressure of magnesium. However, at temperatures significantly above the melting point of magnesium, aggregation of molten magnesium occurs, reducing the surface area of ​​magnesium that can evaporate. The heating temperature is preferably, for example, near the melting point of magnesium.

[0078] The amount of magnesium relative to the precursor composite should be appropriately selected so that most of the magnesium can be vaporized and the inclusion of metallic magnesium in the first composite material is reduced.

[0079] In step (ii), it is preferable to remove the magnesium oxide disposed within the pores of the porous carbon material. Removal of magnesium oxide can be carried out, for example, by dissolving the magnesium oxide using an aqueous solution containing an acid or ammonium salt, thereby eluting the magnesium oxide from within the pores. Magnesium oxide, which is formed near the melting point of magnesium, dissolves readily in an aqueous solution containing an acid or ammonium salt. By removing magnesium oxide from within the pores, the space that mitigates the expansion of silicon particles within the pores can be increased. This significantly reduces the stress applied to the porous carbon material.

[0080] As the acid, inorganic acids such as sulfuric acid, boric acid, phosphoric acid, hydrochloric acid, and nitric acid may be used, or organic acids such as acetic acid, oxalic acid, succinic acid, and malonic acid may be used. Examples of ammonium salts include ammonium chloride.

[0081] The density of the first composite material is 1.5 g / cm³. ３ Above, 2.3g / cm ３ Preferably, it is 1.6 g / cm³. ３ Above, 2.2g / cm ３ It is more preferable that the following conditions are met: The first composite material preferably has a relatively low density. By having a density within the above range, the first composite material has sufficient voids and becomes soft. This suppresses particle cracking of the first composite material due to the expansion and contraction of the silicon phase, and suppresses the deterioration of cycle characteristics associated with such particle cracking. The density of the first composite material can be measured by the gas displacement method using a gas displacement pycnometer. That is, the density is the density measured by the gas displacement method. For example, the density of the first composite material can be measured using an Accupic II 1345TC-10CC (manufactured by Shimadzu Corporation) as the measuring device and helium as the gas type.

[0082] The silicate phase of the second composite material includes, for example, at least one selected from the group consisting of alkali metal elements and Group 2 elements of the long-period periodic table. The lithium silicate phase is, for example, given the formula: Li ２ｚ SiO ２＋ｚ The composition may be represented by (0 < z < 2). z may be 1 / 2 or 1. Composite particles in which silicon particles are dispersed in a silicate phase can be obtained, for example, by grinding a mixture of silicate and raw silicon while stirring with a ball mill or the like to make it into fine particles, and then heat-treating the mixture in an inert atmosphere. The silicon oxide phase may be, for example, SiO2 containing 95% by mass or more of silicon dioxide. ２ It is a phase. SiO ２ The third composite material in which the silicon phase is dispersed within the phase is SiO ｘ This is expressed as follows, where x is, for example, 0.8 ≤ x ≤ 1.6.

[0083] In composite materials, the degree of expansion and contraction during charging and discharging is smaller compared to pure silicon, and particle cracking of the active material and the resulting deterioration of cycle characteristics are suppressed. Furthermore, the first composite material (SiC) and the second composite material (silicate-based composite material) have lower irreversible capacity and higher charge-discharge efficiency compared to the third composite material (SiO). In addition, the carbon phase can develop capacity through a Faraday reaction with lithium ions, which is advantageous in terms of increasing capacity.

[0084] From the viewpoint of improving conductivity, at least a portion of the surface of the composite material particles may be coated with a conductive layer. The conductive layer contains a conductive material such as conductive carbon. The amount of the conductive layer is, for example, 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the total of the composite material particles and the conductive layer. It is preferable that the thickness of the conductive layer is thin enough not to substantially affect the average particle size of the composite material particles. Composite material particles having a conductive layer on their surface can be obtained, for example, by mixing coal pitch or the like with the composite material particles and heat-treating them in an inert atmosphere.

[0085] Examples of binders include resin materials such as fluororesins like polytetrafluoroethylene and polyvinylidene fluoride (PVDF); polyolefin resins like polyethylene and polypropylene; polyamide resins like aramid resins; polyimide resins like polyimide and polyamideimide; acrylic resins like polyacrylic acid, methyl polyacrylate, and ethylene-acrylic acid copolymers; vinyl resins like polyacrylonitrile and polyvinyl acetate; polyvinylpyrrolidone; polyethersulfone; and rubber-like materials like styrene-butadiene copolymer rubber (SBR). A single binder may be used alone, or two or more may be used in combination.

[0086] Examples of conductive materials include carbon compounds such as acetylene black; conductive fibers such as carbon fibers and metal fibers; and metal powders such as aluminum. Conductive materials may be used individually or in combination of two or more types.

[0087] Examples of thickening agents include carboxymethylcellulose (CMC) and its modified forms (including salts such as Na salts), cellulose derivatives such as methylcellulose (such as cellulose ethers); and saponified polymers having vinyl acetate units, such as polyvinyl alcohol. The thickening agent may be used alone or in combination of two or more types.

[0088] As the negative electrode current collector, non-porous conductive substrates (such as metal foil) or porous conductive substrates (such as mesh, net, or perforated sheet) are used. Examples of materials for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys. The thickness of the negative electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.

[0089] The negative electrode composite layer can be formed, for example, by applying a negative electrode slurry containing a negative electrode composite and a dispersion medium to the surface of the negative electrode current collector and drying it. The dried coating may be rolled if necessary. The negative electrode composite layer may be formed on one surface of the sheet-like negative electrode current collector, or on both surfaces.

[0090] [Positive Electrode] The positive electrode comprises, for example, a positive electrode current collector and a positive electrode composite layer supported on the surface of the positive electrode current collector. The positive electrode composite layer can be formed by coating a positive electrode slurry, obtained by dispersing the positive electrode composite in a dispersion medium, onto the surface of the positive electrode current collector and drying it. The dried coating may be rolled if necessary. The positive electrode composite layer may be formed on one surface of the sheet-like positive electrode current collector, or on both surfaces. The positive electrode composite contains a positive electrode active material as an essential component, and may also contain binders, conductive materials, etc., as optional components. NMP or the like can be used as the dispersion medium for the positive electrode slurry.

[0091] As the positive electrode active material, for example, a composite oxide containing lithium and a transition metal such as Ni, Co, or Mn can be used. For example, Li ａ CoO ２ Li ａ NiO ２ Li ａ MnO ２ Li ａ Co ｂ Ni１－ｂ O ２ Li ａ Co ｂ Me １－ｂ O ｃ Li ａ Ni １－ｂ Me ｂ O ｃ Li ａ Mn ２ O ４ Li ａ Mn ２－ｂ Me ｂ O ４ LiMePO ４ Li ２ MeP.O. ４ F (where Me is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B) is an example. Here, 0 < a ≤ 1.2, 0 < b ≤ 0.9, and 2.0 ≤ c ≤ 2.3. Note that the value of a, which indicates the molar ratio of lithium, increases or decreases with charging and discharging.

[0092] Among them, Li ａ Ni ｂ M １－ｂ O ２ Lithium nickel composite oxides represented by (M being at least one selected from the group consisting of Mn, Co, and Al, with 0 < a ≤ 1.2 and 0.3 ≤ b ≤ 1) are preferred. From the viewpoint of increasing capacity, it is more preferable that 0.85 ≤ b ≤ 1 is satisfied. From the viewpoint of crystal structure stability, Li ａ Ni ｂ Co ｃ Al ｄ O ２ (0 < a ≤ 1.2, 0.85 ≤ b < 1, 0 < c < 0.15, 0 < d ≤ 0.1, b + c + d = 1) is even more preferable.

[0093] The same materials as those exemplified for the negative electrode can be used as the binder and conductive material. Graphite such as natural graphite or artificial graphite may be used as the conductive material.

[0094] The shape and thickness of the positive electrode current collector can be selected from the same shape and range as the negative electrode current collector. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0095] [Non-aqueous electrolytes] Non-aqueous electrolytes are ionic conductive (e.g., lithium ion conductive). Non-aqueous electrolytes may be liquid electrolytes (electrolytes) or gel electrolytes. Liquid non-aqueous electrolytes (non-aqueous electrolytes) contain a solvent (non-aqueous solvent) and a solute dissolved in the solvent. An example of a solute is a lithium salt. Various additives may be added to non-aqueous electrolytes. The concentration of lithium salt in the non-aqueous electrolyte is, for example, 0.5 mol / L or more and 2 mol / L or less.

[0096] A gel-like non-aqueous electrolyte comprises, for example, a lithium salt, a non-aqueous solvent, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. Examples of polymer materials include fluororesins, acrylic resins, polyether resins, and polyethylene oxide.

[0097] Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate and ethylene carbonate. A small amount of cyclic carbonate esters having unsaturated bonds, such as vinylene carbonate, or cyclic carbonate esters having fluorine atoms, such as fluoroethylene carbonate, may be included in the non-aqueous electrolyte. Examples of linear carbonate esters include diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate. Examples of cyclic carboxylic acid esters include γ-butyrolactone and γ-valerolactone. Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.

[0098] Examples of the lithium salt include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, borates, imide salts, and the like. Examples of the borates include lithium bis(1,2-benzenediolate(2−)-O,O′)borate, lithium bis(2,3-naphthalenediolate(2−)-O,O′)borate, lithium bis(2,2′-biphenyldiolate(2−)-O,O′)borate, and lithium bis(5-fluoro-2-olate-1-benzenesulfonic acid-O,O′)borate. Examples of the imide salts include lithium bis(fluorosulfonyl)imide (LiN(FSO ２ ) ２ ), lithium bis(trifluoromethanesulfonyl)imide (LiN(CF ３ SO ２ ) ２ ), lithium trifluoromethanesulfonyl nonafluorobutanesulfonyl imide (LiN(CF ３ SO ２ )(C ４ F ９ SO ２ )), and lithium bis(pentafluoroethanesulfonyl)imide (LiN(C ２ F ５ SO ２ ) ２ )). One type of the lithium salt may be used alone, or two or more types thereof may be used in combination.

[0099] [Separator] It is generally desirable to interpose a separator between the positive and negative electrodes. The separator has high ion permeability and appropriate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc., can be used. As the material of the separator, polyolefins such as polypropylene and polyethylene are preferred.

[0100] A secondary battery may include, for example, a wound electrode group formed by winding a positive electrode and a negative electrode with a separator in between, or a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator in between. The secondary battery may take any form, such as cylindrical, prismatic, coin-type, button-type, or laminate-type.

[0101] Hereinafter, the structure of a rectangular secondary battery will be described as an example of a secondary battery relating to this disclosure, with reference to Figure 2. Figure 2 is a schematic perspective view in which a part of a secondary battery according to one embodiment of this disclosure is cut out.

[0102] The battery comprises a bottomed rectangular battery case 4, an electrode group 1 housed within the battery case 4, and a non-aqueous electrolyte (not shown). The electrode group 1 has a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed between them to prevent direct contact. The electrode group 1 is formed by winding the negative electrode, positive electrode, and separator around a flat core and then removing the core.

[0103] One end of the negative electrode lead 3 is attached to the negative electrode current collector by welding or the like. The other end of the negative electrode lead 3 is electrically connected to the negative electrode terminal 6 provided on the sealing plate 5 via a resin insulating plate (not shown). The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. One end of the positive electrode lead 2 is attached to the positive electrode current collector by welding or the like. The other end of the positive electrode lead 2 is connected to the back surface of the sealing plate 5 via an insulating plate. That is, the positive electrode lead 2 is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The insulating plate separates the electrode group 1 from the sealing plate 5 and also separates the negative electrode lead 3 from the battery case 4. The periphery of the sealing plate 5 is fitted into the open end of the battery case 4, and the fitting portion is laser welded. In this way, the opening of the battery case 4 is sealed by the sealing plate 5. The injection hole for the non-aqueous electrolyte provided in the sealing plate 5 is closed by a seal 8.

[0104] (Supplementary Note) The following technology is disclosed by the above description. (Technology 1) A negative electrode comprising: a negative electrode mixture layer containing a negative electrode active material; and a negative electrode current collector that supports the negative electrode mixture layer, wherein the negative electrode active material includes graphite and a silicon-containing material, the silicon-containing material includes a first composite material, the first composite material includes a carbon phase and a silicon phase dispersed in the carbon phase, the negative electrode mixture layer has a first region on the negative electrode current collector side and a second region on the surface side of the negative electrode, a content ratio M1 of the silicon-containing material relative to a total of the graphite and the silicon-containing material in the first region is larger than a content ratio M2 of the silicon-containing material relative to the total of the graphite and the silicon-containing material in the second region, and an average particle diameter R1 of the graphite in the first region is larger than an average particle diameter R2 of the graphite in the second region. A negative electrode for a secondary battery. (Technology 2) The negative electrode for a secondary battery according to Technology 1, wherein a ratio of the average particle diameter R2 to the average particle diameter R1: R2 / R1 is 0.1 or more and 0.9 or less. (Technology 3) The negative electrode for a secondary battery according to Technology 1 or 2, wherein the content ratio M1 is 1.5 times or more the content ratio M2. (Technology 4) The negative electrode for a secondary battery according to any one of Technologies 1 to 3, wherein the silicon-containing material includes a second composite material, and the second composite material includes a silicate phase and a silicon phase dispersed in the silicate phase. (Technology 5) The negative electrode for a secondary battery according to any one of Technologies 1 to 4, wherein the silicon-containing material includes a third composite material, and the third composite material includes a silicon oxide phase and a silicon phase dispersed in the silicon oxide phase. (Technology 6) The negative electrode for a secondary battery according to any one of Technologies 1 to 5, wherein a content ratio of the silicon-containing material relative to a total of the graphite and the silicon-containing material in the negative electrode mixture layer is 0.1% by mass or more and 40% by mass or less. (Technology 7) The negative electrode for a secondary battery according to any one of Technologies 1 to 6, wherein in the first region, the graphite contains at least natural graphite. (Technology 8) The density of the first composite material is 1.6 g / cm ３ or more and 2.2 g / cm ３The following is a negative electrode for a secondary battery, according to any one of technologies 1 to 7. (Technology 9) A secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is the negative electrode for a secondary battery described in any one of technologies 1 to 8.

[0105] The present disclosure will be described in detail below based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0106] 《Batteries A1-A4, B1-B6》 (Preparation of the first negative electrode slurry) An appropriate amount of water was added to the first negative electrode mixture to obtain the first negative electrode slurry. The first negative electrode mixture was a mixture of the first negative electrode active material, styrene-butadiene copolymer rubber (SBR) as a binder, and carboxymethylcellulose (CMC) as a thickener. The mass ratio of the first negative electrode active material, SBR, and CMC was 100:1:1.

[0107] For the first negative electrode active material, either graphite alone or a mixture of graphite and Si-containing material was used. In the first negative electrode active material, the content ratio M1 of the Si-containing material to the total of graphite and Si-containing material was set to the values ​​shown in Table 1. The average particle size R1 of the graphite was set to the values ​​shown in Table 1. The average particle size of the Si-containing material was set to 7 μm.

[0108] For the Si-containing material, a first composite material (SiC) or a mixture of the first composite material (SiC) and a third composite material (SiO) (mass ratio SiC / SiO = 1:1) was used. Natural graphite was used for the graphite.

[0109] The density of the first composite material is 1.6 g / cm³. ３ Above, 2.2g / cm ３ It was within the following range.

[0110] (Preparation of the second negative electrode slurry) An appropriate amount of water was added to the second negative electrode mixture to obtain the second negative electrode slurry. The second negative electrode mixture used was a mixture of the second negative electrode active material, SBR, and CMC. The mass ratio of the second negative electrode active material, SBR, and CMC was 100:1:1.

[0111] For the second negative electrode active material, either graphite alone or a mixture of graphite and Si-containing material was used. In the second negative electrode active material, the Si-containing material content ratio M2 to the total of graphite and Si-containing material was set to the values ​​shown in Table 1. The average particle size R2 of the graphite was also set to the values ​​shown in Table 1. The same materials as those used for the first negative electrode active material were used for the Si-containing material and graphite.

[0112] (Fabrication of the negative electrode) The first negative electrode slurry was applied to both sides of a copper foil (10 μm thick) which served as the negative electrode current collector, and the first coating was dried. Furthermore, the second negative electrode slurry was applied on top of the first coating formed on both sides of the copper foil, and the second coating was dried. The doctor blade method was used to apply the first and second negative electrode slurries. The application amounts of the first and second negative electrode slurries were the same. The application amount of the negative electrode slurry refers to the mass of negative electrode slurry applied per unit area of ​​the negative electrode current collector.

[0113] The first and second coating films were rolled to form a first layer (first region) and a second layer (second region) on both sides of the negative electrode current collector, thereby obtaining a negative electrode. The first and second layers formed a negative electrode composite layer. By appropriately adjusting the application amounts of the first and second negative electrode slurries, the ratio of the thickness of the second layer T2 to the thickness of the first layer T1: T2 / T1 was set to the values ​​shown in Table 1. The total thickness of the first and second layers: T1 + T2 was set to 100 μm.

[0114] (Preparation of the positive electrode) An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the positive electrode mixture to obtain a positive electrode slurry. The positive electrode mixture used was a mixture of lithium-containing composite oxide, which is the positive electrode active material, graphite, which is the conductive material, and polyvinylidene fluoride (PVDF), which is the binder. LiNi ０．８８ Co ０．０９ Al ０．０３ O ２ The following was used. In the positive electrode composite material, the mass ratio of lithium-containing composite oxide, graphite, and PVDF was set to 100:1:0.9.

[0115] A positive electrode slurry was applied to both sides of an aluminum foil (15 μm thick), which served as the positive electrode current collector. The coating was dried and rolled to form a positive electrode composite layer, thereby obtaining the positive electrode. The doctor blade method was used to apply the positive electrode slurry.

[0116] (Preparation of non-aqueous electrolyte) Ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed in a volume ratio of 1:3 (EC:DMC) to form a non-aqueous solvent, to which vinylene carbonate (VC) is added, and LiPF is prepared. ６ A non-aqueous electrolyte was prepared by dissolving [the substance]. The VC content in the non-aqueous electrolyte was 5% by mass. LiPF in the non-aqueous electrolyte ６ The concentration was set to 1.5 mol / L.

[0117] (Fabrication of secondary battery) An aluminum positive electrode lead was attached to the exposed part of the positive electrode current collector of the positive electrode, and a nickel negative electrode lead was attached to the exposed part of the negative electrode current collector of the negative electrode. Then, the positive and negative electrodes were wound together with a separator in between to create a wound electrode group. A microporous polyethylene membrane was used as the separator. The electrode group was housed in a battery case. At this time, an upper insulating plate and a lower insulating plate made of resin were placed above and below the electrode group, respectively. The negative electrode lead was welded to the inner bottom surface of the battery case. The positive electrode lead was welded to a metal sealing body that also served as the positive electrode terminal. Then, a non-aqueous electrolyte was injected into the battery case, and the opening of the battery case was closed using the sealing body. At this time, a resin gasket was interposed between the opening end of the battery case and the sealing body. In this way, a cylindrical secondary battery was obtained. In Table 1, A1 to A4 are the batteries of the example, and B1 to B6 are the batteries of the comparative example.

[0118] [Evaluation] The following evaluations were performed on each of the batteries prepared as described above.

[0119] (Discharge Capacity) Under conditions of 25°C, constant current charging was performed with a current of 0.1C until the voltage reached 4.2V, and then constant voltage charging was performed with a voltage of 4.2V until the current reached 0.02C. After a 10-minute rest, constant current discharge was performed with a current of 0.1C until the voltage reached 2.5V. The discharge capacity of the negative electrode (discharge capacity per unit mass of negative electrode active material) was determined from the discharge capacity at this time and the amount of negative electrode active material packed in. The amount of negative electrode active material packed in is the total mass of Si-containing material and graphite packed in the negative electrode.

[0120] (Rapid Charge / Discharge Cycle Test) Under conditions of 25°C, constant current charging was performed with a current of 0.7C until the voltage reached 4.2V, and then constant voltage charging was performed with a voltage of 4.2V until the current reached 0.02C. After a 10-minute pause, constant current discharge was performed with a current of 0.5C until the voltage reached 2.5V. This charge / discharge cycle was considered one cycle, and 100 cycles were performed. The cycle maintenance rate was calculated as the ratio (percentage) of the discharge capacity at cycle 100 to the discharge capacity at cycle 1.

[0121] The evaluation results are shown in Table 1. In Table 1, the discharge capacity is expressed as a relative value with the discharge capacity of battery B2 set to 100. The cycle maintenance rate is expressed as a relative value with the cycle maintenance rate of battery B2 set to 100.

[0122]

[0123] In batteries A1 to A4, where R2 < R1 and M2 < M1, and where at least SiC was used in the Si-containing material, the capacity retention rate in rapid charge-discharge cycles improved, resulting in excellent rapid charge-discharge characteristics and cycle characteristics. Furthermore, in batteries A1 to A4, high discharge capacity was obtained by using SiC in the Si-containing material.

[0124] Battery A1, which uses SiC as the Si-containing material, showed a significant improvement in discharge capacity and cycle maintenance rate compared to battery B6, which uses SiO as the Si-containing material. The improvement in rapid charge / discharge characteristics and cycle characteristics by setting R2 < R1 and M2 < M1 was particularly pronounced when SiC was used as the Si-containing material.

[0125] In battery B1, the discharge capacity decreased because a Si-containing material was not used for the negative electrode active material. In battery B2, the discharge capacity improved by using a Si-containing material for the negative electrode active material, but the cycle maintenance rate decreased because the average particle size R of graphite and the content ratio M of Si-containing material were almost the same on the surface side of the negative electrode and the current collector side. In batteries B3 to B5, R1 < R2 and / or M1 < M2, resulting in a decrease in cycle maintenance rate.

[0126] The negative electrode for secondary batteries according to this disclosure is suitably used in secondary batteries that require excellent rapid charge / discharge characteristics and cycle characteristics.

[0127] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.

[0128] 1: Electrode group, 2: Positive electrode lead, 3: Negative electrode lead, 4: Battery case, 5: Sealing plate, 6: Negative electrode terminal, 7: Gasket, 8: Sealing plug, 10: Negative electrode, 11: Negative electrode current collector, 12: Negative electrode active material layer, 12a: First region, 12b: Second region

Claims

1. A negative electrode for a secondary battery comprising a negative electrode composite layer containing a negative electrode active material and a negative electrode current collector supporting the negative electrode composite layer, wherein the negative electrode active material comprises graphite and a silicon-containing material, the silicon-containing material comprises a first composite material, the first composite material comprises a carbon phase and a silicon phase dispersed within the carbon phase, the negative electrode composite layer has a first region on the negative electrode current collector side and a second region on the surface side of the negative electrode, the content ratio M1 of the silicon-containing material to the sum of the graphite and the silicon-containing material in the first region is greater than the content ratio M2 of the silicon-containing material to the sum of the graphite and the silicon-containing material in the second region, and the average particle size R1 of the graphite in the first region is greater than the average particle size R2 of the graphite in the second region.

2. The ratio of the average particle size R2 to the average particle size R1: R2 / R1 is 0.1 or more and 0.9 or less, the negative electrode for a secondary battery according to claim 1.

3. The negative electrode for a secondary battery according to claim 1, wherein the content ratio M1 is 1.5 times or more the content ratio M2.

4. The negative electrode for a secondary battery according to claim 1, wherein the silicon-containing material comprises a second composite material, the second composite material comprising a silicate phase and a silicon phase dispersed within the silicate phase.

5. The silicon-containing material comprises a third composite material, the third composite material comprising a silicon oxide phase and a silicon phase dispersed within the silicon oxide phase, the negative electrode for a secondary battery according to claim 1.

6. The negative electrode for a secondary battery according to claim 1, wherein the content ratio of the silicon-containing material to the total amount of the graphite and the silicon-containing material in the negative electrode composite layer is 0.1% by mass or more and 40% by mass or less.

7. The negative electrode for a secondary battery according to claim 1, wherein in the first region, the graphite comprises at least natural graphite.

8. The density of the first composite material is 1.6 g / cm³. ３ Above, 2.2g / cm ３ The negative electrode for a secondary battery according to claim 1 is as follows:

9. A secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode is a negative electrode for a secondary battery as described in any one of claims 1 to 8.

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