Negative electrode for secondary battery, and secondary battery

Abstract

This negative electrode for a secondary battery comprises a negative electrode mixture layer including a negative electrode current collector and a negative electrode active material, wherein: the negative electrode active material contains carbon particles and Si-containing particles; and in a distribution, in the plane direction of the negative electrode mixture layer, of the existence probability of Si in the thickness direction of the negative electrode mixture layer, the difference Rmax−Rmin between the upper limit value Rmax and the lower limit value Rmin within the 1σ interval is at most 20%.

Description

Negative electrode for secondary battery and secondary battery

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

[0002] Patent Document 1 proposes a lithium secondary battery including a negative electrode in which a negative electrode composite layer containing a silicon-containing material as a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder is sintered and disposed on the surface of a negative electrode current collector, a positive electrode, and a non-aqueous electrolyte, wherein the average particle size of the negative electrode active material before charging is regulated to between 5.0 μm and 15.0 μm, a graphite material is used as the negative electrode conductive agent, and the average particle size of this graphite material is regulated to between 2.5 μm and 15.0 μm, and the amount of the graphite material added to the negative electrode active material is regulated to between 3% by mass and 20% by mass, respectively, and the theoretical electrical capacity ratio of the positive electrode to the negative electrode is 1.0 or less.

[0003] JP 2007-73334 A

[0004] The main object of Patent Document 1 is to provide a lithium secondary battery that can improve the initial charge-discharge efficiency and further improve the charge-discharge cycle characteristics. However, in order to improve the cycle characteristics of a secondary battery having a negative electrode containing silicon (Si), further improvement of the negative electrode is required.

[0005] One aspect of the present disclosure relates to a negative electrode for a secondary battery, including a negative electrode current collector and a negative electrode composite layer containing a negative electrode active material, wherein the negative electrode active material includes carbon particles and Si-containing particles, and in a distribution of the presence probability of Si element in a thickness direction of the negative electrode composite layer in a plane direction of the negative electrode composite layer, a difference (Rmax - Rmin) between an upper limit value Rmax and a lower limit value Rmin of a 1σ interval is 20% or less.

[0006] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, the above-described negative electrode, and a non-aqueous electrolyte.

[0007] According to the present disclosure, it is possible to improve the cycle characteristics of a secondary battery having a negative electrode containing silicon (Si).

[0008] The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0009] 1 is a partially cutaway perspective view of a nonaqueous secondary battery according to an embodiment of the present disclosure, and FIG. 2 is a conceptual diagram illustrating a method for determining the distribution of the presence probability of Si element in the thickness direction of the negative electrode mixture layer in the plane direction of the negative electrode mixture layer.

[0010] Below, embodiments of a secondary battery negative electrode and a secondary battery according to the present disclosure will be described using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits of numerical values ​​related to specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more of the materials may be used in combination.

[0011] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0012] The negative electrode for a secondary battery according to the present disclosure includes a negative electrode current collector and a negative electrode composite layer containing a negative electrode active material. The negative electrode current collector is composed of a sheet-like conductive material. The negative electrode composite layer is supported on one or both surfaces of the negative electrode current collector. The negative electrode composite layer is typically a layer (including a membrane or film) composed of a negative electrode composite. The negative electrode composite contains a negative electrode active material as an essential component. The negative electrode active material may be any material that reversibly exhibits capacity by absorbing and releasing lithium ions. The negative electrode active material includes carbon particles and Si-containing particles. The negative electrode active material may contain materials other than the carbon particles and Si-containing particles, but the total of the carbon particles and the Si-containing particles is the main component. For example, 50% by mass or more or 60% by mass or more (e.g., 70% by mass or more or 80% by mass or more) of the negative electrode active material may be carbon particles and Si-containing particles.

[0013] The carbon particles may be crystalline or amorphous, or may have both crystalline and amorphous regions. The carbon particles may be conductive. Examples of crystalline carbon particles A include graphite and a composite of graphite and amorphous carbon. The graphite may be natural graphite or artificial graphite. The amorphous carbon may be hard carbon, soft carbon, or other. Note that graphite refers to carbon particles having an average interplanar spacing d002 of the (002) plane of 0.340 nm or less as measured by X-ray diffraction. In the case of amorphous carbon, only a halo pattern is usually observed in X-ray diffraction (XRD). In the case of amorphous carbon, peaks attributable to the (101) plane and the (100) plane are usually not observed.

[0014] The Si-containing particles are particles containing silicon (Si), and typically, a portion of the Si-containing particles is composed of a silicon phase that exhibits electrochemical capacity. The Si-containing particles may be simple Si, a Si alloy, a Si compound, or the like, but may also be a composite material containing a lithium ion conductive phase and a silicon phase (e.g., silicon particles) dispersed in the lithium ion conductive phase. In the composite material, it is desirable that the silicon phase be as fine as possible. The composite material is suitable for imparting high capacity to the negative electrode while suppressing direct contact between the silicon phase and the electrolyte or non-aqueous electrolyte.

[0015] In the distribution of the presence probability R of Si element in the thickness direction of the negative electrode composite layer in the plane direction of the negative electrode composite layer, the difference between the upper limit value Rmax and the lower limit value Rmin of the 1σ interval (Rmax - Rmin) is 20% or less. For example, by mixing the Si-containing particles with the carbon particles as uniformly as possible and dispersing a fine silicon phase as uniformly as possible in the lithium ion conductive phase, it becomes possible to control Rmax - Rmin to 20% or less, or even to 18% or less or 17% or less.

[0016] The presence probability R of Si elements in the thickness direction of the negative electrode composite layer can be calculated as the ratio (R = LB / T) of the total length LB of the portions of lines parallel to the thickness direction of the negative electrode composite that intersect with the Si element to the thickness T of the negative electrode composite layer. The distribution of the presence probability R in the surface direction of the negative electrode composite layer can be measured at one or more cross sections obtained by cutting the negative electrode composite layer together with the negative electrode current collector along the thickness direction of the negative electrode. Specifically, the distribution of the presence probability R can be obtained by the following method.

[0017] (1) Imaging a cross section of a negative electrode using a scanning electron microscope (SEM) First, a negative electrode to be measured is prepared. Next, the negative electrode composite layer and the negative electrode current collector are simultaneously cut along the thickness direction of the negative electrode to form a cross section. At this time, a thermosetting resin may be filled into the negative electrode composite layer and cured. For example, the cross section of the negative electrode is obtained using a CP (cross section polisher) method, an FIB (focused ion beam) method, or the like. The cross section sample is observed using an SEM. Observation using an SEM is performed at a low magnification (e.g., 200x to 1000x). The SEM image is taken so that a region of 300 μm or more (preferably 400 μm or more) in length in the surface direction of the negative electrode composite layer is observed.

[0018] The negative electrode to be measured is taken from a secondary battery with a depth of discharge (DOD) of 90% or more. Depth of discharge (DOD) is the ratio of the amount of discharged electricity to the rated amount of electricity of a battery in a fully charged state. Note that the amount of electricity charged (i.e., the fully charged amount) when a battery in a fully discharged state (DOD = 100%) is charged to a fully charged state (SOC = 100%, DOD = 0%) corresponds to the rated capacity. The voltage of a battery in a fully charged state corresponds to the end-of-charge voltage. The voltage of a battery in a fully discharged state corresponds to the end-of-charge voltage.

[0019] (2) Elemental Analysis by Energy Dispersive X-ray Spectroscopy (EDX) Elemental analysis by EDX is performed using an SEM image of the cross section of the negative electrode.

[0020] (3) Creating a map of the silicon phase: A Si element map is obtained by extracting components derived from the Si element from EDX analysis data of the cross section of the negative electrode. This makes it possible to count all components derived from the Si element in the negative electrode composite layer.

[0021] (4) Measurement of the Distribution of the Existence Probability R The obtained Si element map is expressed as a two-dimensional image defined by the thickness direction of the negative electrode composite and an arbitrary surface direction. In such a Si element map, when multiple lines parallel to the thickness direction of the negative electrode composite layer are drawn along the surface direction, for example, at a pitch of 5 μm, each line may have a portion intersecting with the Si element (a component derived from the Si element). The ratio R (= LB / T) of the total length LB of the portions of each line intersecting with the Si element to the thickness T of the negative electrode composite layer indicates the existence probability R of the Si element at an arbitrary point in the surface direction of the negative electrode composite layer.

[0022] Measuring the existence probability R at a predetermined pitch along the surface direction in the Si element map gives a distribution of the existence probability R in the surface direction. If the length of the negative electrode composite layer in the SEM image in the surface direction is, for example, 300 μm and the predetermined pitch is 5 μm, 300 / 5=60 existence probabilities R are obtained, and a distribution of 60 existence probabilities R can be depicted.

[0023] FIG. 2 conceptually illustrates an example of a cross section of the negative electrode 16 when the negative electrode current collector 161 and the negative electrode composite layer 162 are simultaneously cut. In FIG. 2 , the Si element map is expressed as a two-dimensional image with the thickness direction of the negative electrode composite layer 162 as the vertical axis and an arbitrary surface direction as the horizontal axis. Each of the multiple straight lines L is parallel to the thickness direction of the negative electrode composite layer 162 and is drawn at a predetermined pitch along the surface direction. Since each straight line L may have a portion intersecting with Si elements (components derived from Si elements) dispersed in the negative electrode composite layer 162, the ratio R (= LB / T) of the total length LB of the portions intersecting with Si for each straight line L to the thickness T of the negative electrode composite layer is calculated as the presence probability R of Si. In the illustrated example, 24 straight lines are drawn, resulting in 24 presence probabilities R, allowing the distribution of the 24 presence probabilities R to be depicted.

[0024] In the obtained distribution, the smaller the difference between the upper limit value Rmax and the lower limit value Rmin in the 1σ interval (Rmax - Rmin), the more uniformly Si is present in the surface direction of the negative electrode, and the more uniform the degree of expansion and contraction due to charge and discharge of the negative electrode becomes, which is advantageous for charge and discharge cycle characteristics. Specifically, when Rmax - Rmin is 20% or less, the charge and discharge cycle characteristics are significantly improved. At this time, it is considered that the Si-containing particles and carbon particles are fairly uniformly mixed.

[0025] The 1σ interval is the interval from μ-σ to μ+σ of the distribution function, with the horizontal axis representing the existence probability R and the vertical axis representing the frequency, where μ is the average of the existence probability R. σ is the standard deviation of the distribution function.

[0026] In order to make Rmax-Rmin 20% or less, it is effective to increase the affinity between carbon particles and Si-containing particles. The higher the affinity between carbon particles and Si-containing particles, the easier it is to uniformly mix the carbon particles and Si-containing particles. The affinity between carbon particles A and Si-containing particles B can be evaluated, for example, using the DBP (dibutyl phthalate) oil supply amount as an index. Specifically, the ratio of the DBP oil supply amount (DBPB) of the Si-containing particles to the DBP oil supply amount (DBPA) of the carbon particles: DBPB / DBPA may be 1.0 or less. The DBPB / DBPA ratio may be 0.9 or more and 1.0 or less, or 0.95 or more and less than 1.0 (e.g., 0.99 or less).

[0027] The DBP oil supply amount can be measured using a measuring device (for example, S-500 manufactured by Asahi Research Institute Co., Ltd.) in accordance with JIS K6217-4 (ISO4656).

[0028] In the negative electrode active material, the content of carbon particles may be 70% by mass or more and 95% by mass or less, and the content of Si-containing particles may be 5% by mass or more and 30% by mass or less. When attempting to achieve a good balance between good cycle characteristics and high capacity, the content of Si-containing particles in the negative electrode active material may be, for example, 0.5% by mass or more, 1% by mass or more, or 2% by mass or more. When more importance is placed on improving cycle characteristics, the content of Si-containing particles in the negative electrode active material may be, for example, 20% by mass or less, 15% by mass or less, or 10% by mass or less.

[0029] The Si-containing particles may include first particles having a carbon phase and a silicon phase dispersed within the carbon phase. The first particles are particles of a composite material of a carbon phase and a silicon phase. The carbon phase may be composed of, for example, amorphous carbon. The carbon phase may not contain crystalline carbon. In this case, a profile measured by X-ray diffraction does not show a peak attributed to crystalline carbon, but shows a halo pattern attributed to amorphous carbon. Since the content of the silicon phase in the first particles can be arbitrarily changed, it is easy to design a high-capacity negative electrode.

[0030] The amorphous carbon may be, for example, hard carbon, soft carbon, or other types of carbon. Amorphous carbon can be obtained, for example, by sintering a carbon source in an inert atmosphere and pulverizing the resulting sintered body.

[0031] The first particles can be obtained, for example, by mixing a carbon source with Si particles, milling the mixture with a mixer such as a ball mill, and then firing the mixture in an inert atmosphere. Examples of carbon sources that can be used include petroleum resins such as coal pitch, petroleum pitch, and tar, sugars such as carboxymethyl cellulose (CMC), polyvinylpyrrolidone, cellulose, and sucrose, and water-soluble resins. When mixing the carbon source with the Si particles, the carbon source and the Si particles may be dispersed in a dispersion medium such as alcohol. After drying the milled mixture, the mixture is heated in an inert gas atmosphere, for example, at a temperature of 600°C or higher and 1000°C or lower, to carbonize the carbon source and form a carbon phase. The carbon phase formed in this manner is amorphous carbon, not containing crystalline carbon.

[0032] The silicon phase dispersed in the carbon phase is usually composed of multiple crystallites. The crystallite size of the silicon phase is, for example, 500 nm or less, and may be 30 nm or less. The lower limit of the crystallite size of the silicon phase is not particularly limited, but is, for example, 5 nm or more. The crystallite size is calculated by the Scherrer equation from the half-width of the diffraction peak assigned to the Si (111) plane in the X-ray diffraction (XRD) pattern of the silicon phase.

[0033] The content of the silicon phase contained in the first particles is, for example, 35% by mass or more, 45% by mass or more, 50% by mass or more, or 65% by mass or more. The content of the silicon phase contained in the first particles is, for example, 80% by mass or less, 75% by mass or less, 70% by mass or less, or 65% by mass or less. When the upper and lower limits are arbitrarily selected from the above range, it becomes easier to achieve both high battery capacity and improved cycle characteristics.

[0034] The content of the silicon phase contained in the primary particles can be measured by Si-NMR. Desirable measurement conditions for Si-NMR are shown below.

[0035] Measurement equipment: Varian solid-state nuclear magnetic resonance spectrometer (INOVA-400) Probe: Varian 7 mm CPMAS-2 MAS: 4.2 kHz MAS speed: 4 kHz Pulse: DD (45° pulse + signal acquisition time 1 H decoupled) Repetition time: 1200 sec Observation width: 100 kHz Observation center: around -100 ppm Signal acquisition time: 0.05 sec Number of accumulations: 560 Sample amount: 207.6 mg

[0036] The average particle size of the primary particles may be, for example, 1 to 20 μm, and preferably 2 to 12 μm. Within this particle size range, stress caused by volumetric changes of the primary particles during charge and discharge can be easily alleviated, and good cycle characteristics can be easily obtained.

[0037] The average particle size of the primary particles means a particle size (volume average particle size) at which the volume cumulative value is 50% in a particle size distribution measured by a laser diffraction scattering method. As a measuring device, for example, "LA-750" manufactured by Horiba Ltd. can be used.

[0038] The average particle size of the primary particles may be determined from a cross-sectional sample of the negative electrode formed to obtain an SEM image. The equivalent circle diameters of the cross sections of 10 or more particles B1 are determined, and the average of these diameters is determined as the average particle size. Here, the equivalent circle diameter refers to the diameter of a circle having the same area as the area of ​​a particle observed in the cross section of the negative electrode.

[0039] The Si-containing particles may include second particles having a silicate phase and a silicon phase dispersed within the silicate phase. The second particles are particles of a composite material of the silicate phase and the silicon phase. The silicon phase content of the second particles can be arbitrarily changed, making it easy to design a high-capacity negative electrode.

[0040] The silicate phase may contain, for example, at least one element selected from the group consisting of Group 1 and Group 2 elements of the long-form periodic table. Examples of Group 1 and Group 2 elements of the long-form periodic table include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Other elements may include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), and titanium (Ti). Among these, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferred due to its small irreversible capacity and high initial charge / discharge efficiency. That is, the second particles may contain a lithium silicate phase and a silicon phase dispersed within the lithium silicate phase.

[0041] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may 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. This 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. Examples of elements other than Li, Si, and O that may be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0042] The lithium silicate phase has the formula: Li ２ｚ SiO ２＋ｚ It may have a composition expressed as (0<z<2). From the viewpoints of stability, ease of preparation, lithium ion conductivity, etc., z preferably satisfies the relationship 0<z<1, and more preferably z=1 / 2.

[0043] The silicon phase dispersed in the silicate phase is usually composed of a plurality of crystallites. The crystallite size of the silicon phase is, for example, 500 nm or less, and may be 30 nm or less. The lower limit of the crystallite size of the silicon phase is not particularly limited, but is, for example, 5 nm or more. The crystallite size can be measured based on the primary particles.

[0044] The content of the silicon phase contained in the second particles is, for example, 40% by mass or more, 45% by mass or more, 50% by mass or more, or 65% by mass or more. The content of the silicon phase contained in the second particles is, for example, 80% by mass or less, 75% by mass or less, 70% by mass or less, or 65% by mass or less. When the upper and lower limits are arbitrarily selected from the above range, it becomes easier to achieve both high battery capacity and improved cycle characteristics. The content of the silicon phase contained in the second particles can be measured in accordance with the first particles.

[0045] The average particle size of the secondary particles may be, for example, 1 to 20 μm, and preferably 5 to 12 μm. Within this particle size range, stress caused by volume changes of the secondary particles during charge and discharge can be easily alleviated, making it easier to obtain good cycle characteristics. The average particle size of the secondary particles can be measured in accordance with the average particle size of the primary particles.

[0046] The first and second particles are advantageous in that they have a small irreversible capacity. This is because the carbon phase and silicate phase have few sites for irreversibly trapping lithium ions. The use of the first and second particles results in excellent charge-discharge efficiency. This effect is particularly noticeable in the early stages of charge-discharge.

[0047] The compositions of the first particles and the second particles can be analyzed, for example, by the following method. First, the battery is disassembled, the negative electrode is removed, washed with a non-aqueous solvent such as ethylene carbonate, and dried. After that, the cross section of the negative electrode composite layer is processed using a cross-section polisher (CP) to obtain a sample. A backscattered electron image of the sample cross section is obtained using a field emission scanning electron microscope (FE-SEM), and the cross section is observed. Qualitative and quantitative analysis of elements of the observed particles can be performed using an Auger electron spectroscopy (AES) analyzer (accelerating voltage 10 kV, beam current 10 nA).

[0048] The carbon particles may include two or more types of particles having different internal porosities. In this case, it is easy to control the structural strength of the negative electrode mixture layer and the liquid circulation of the electrolyte or nonaqueous electrolyte within the negative electrode mixture layer, and it is easy to improve the cycle characteristics. The carbon particles may include, for example, third particles having an internal porosity of 5% or less and fourth particles having an internal porosity of 8% or more and 20% or less.

[0049] The intraparticle porosity of the carbon particles (third and fourth particles) may be determined from a cross-sectional sample of the negative electrode formed to obtain an SEM image. In the cross-sectional image of the carbon particles, the carbon portion and the void portion are distinguished by image processing, and the areas of each are determined. The ratio of the area of ​​the void portion to the total area of ​​the carbon portion and the void portion is the intraparticle porosity. However, the intraparticle porosity is an average value obtained from 10 or more specified carbon particles. When determining the intraparticle porosity of the third particles, the average value of the intraparticle porosities of 10 or more third particles is determined. When determining the intraparticle porosity of the fourth particles, the average value of the intraparticle porosities of 10 or more fourth particles is determined. However, carbon particles with an intraparticle porosity of 6.5% or less are classified as third particles, and carbon particles with an intraparticle porosity of more than 6.5% are classified as fourth particles.

[0050] The third particles may be crystalline, amorphous, or may have both crystalline and amorphous regions. The third particles may be, for example, graphite, hard carbon, or soft carbon. The average particle size of the third particles may be, for example, 5 to 50 μm, and preferably 12 to 20 μm. The average particle size of the third particles can be measured in accordance with the first particles.

[0051] The fourth particles may be crystalline or amorphous, or may have both crystalline and amorphous regions. The fourth particles may be, for example, graphite, hard carbon, or soft carbon. The average particle size of the fourth particles may be, for example, 5 to 50 μm, and preferably 15 to 30 μm. The average particle size of the fourth particles can be measured in accordance with the first particles. Note that at least one of the third particles and the fourth particles may contain amorphous carbon, but from the viewpoint of increasing capacity and improving cycle characteristics, it is desirable that both the third particles and the fourth particles be graphite.

[0052] From the viewpoint of the liquid circulation of the electrolytic solution or the non-aqueous electrolyte, the content of the third particles in the carbon particles may be 10% by mass or more and 80% by mass or less, or 10% by mass or more and 50% by mass or less, or 10% by mass or more and 30% by mass or less.

[0053] When the negative electrode mixture layer is divided into a first region and a second region of the same thickness in the thickness direction, it is desirable that the third particles be contained in a greater amount in the second region farther from the negative electrode current collector than in the first region closer to the negative electrode current collector. The third particles are dense and strong, and are suitable for forming sufficient gaps in the surface portion of the negative electrode mixture layer where the negative electrode active material is easily crushed. By containing a relatively large amount of the third particles in the second region, it becomes easier to improve the liquid circulation of the electrolyte solution or non-aqueous electrolyte in the negative electrode mixture layer. On the other hand, the fourth particles have a high internal porosity and are therefore suitable for forming sufficient gaps in the deeper portion of the negative electrode mixture layer. By containing a relatively large amount of the fourth particles in the first region, it becomes easier to further improve the liquid circulation of the electrolyte solution or non-aqueous electrolyte in the negative electrode mixture layer.

[0054] When the second region contains a larger amount of third particles than the first region, forming sufficient gaps in the surface layer portion of the negative electrode composite layer, significant local expansion of the negative electrode composite layer due to charging and discharging of the secondary battery is thought to cause distortion of the surface layer portion and lead to deterioration in strength. On the other hand, in this embodiment, the difference between the upper limit value Rmax and the lower limit value Rmin of the 1σ interval of the existence probability R (Rmax - Rmin) is limited to a small value, so that the degree of expansion and contraction due to charging and discharging of the negative electrode is likely to be uniform. Therefore, the strength of the surface layer portion is likely to be maintained.

[0055] The mass ratio of the third particles to the total of the third particles and the fourth particles in the second region may be, for example, 20 mass% or more and 80 mass% or less, or 30 mass% or more and 60 mass% or less, or 35 mass% or more and 50 mass% or less.

[0056] The mass ratio of the third particles to the total of the third particles and the fourth particles in the first region may be, for example, 0 mass% or more and 40 mass% or less, or 0 mass% or more and 30 mass% or less, or 0 mass% or more and 20 mass% or less.

[0057] Next, a secondary battery according to an embodiment of the present disclosure will be described in detail. The secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode is the same as the negative electrode described above. The positive electrode and the negative electrode are arranged to face each other with a separator interposed therebetween.

[0058] [Negative Electrode] As described above, the negative electrode comprises a negative electrode mixture layer containing a negative electrode active material and a negative electrode current collector. The negative electrode mixture layer can be formed by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied film. The dried coating film may be rolled, if necessary.

[0059] The negative electrode mixture contains a negative electrode active material as an essential component, and may contain a binder, a thickener, a conductive agent, and the like as optional components.

[0060] (Negative Electrode Active Material) As described above, the negative electrode active material contains carbon particles and Si-containing particles.

[0061] As the binder for the negative electrode, for example, a resin material is used. Examples of the binder include fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, vinyl resin, and rubber-like material (for example, styrene butadiene copolymer (SBR)). One type of binder may be used alone, or two or more types may be used in combination.

[0062] Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified forms, methyl cellulose, etc. One type of thickener may be used alone, or two or more types may be used in combination.

[0063] Examples of conductive materials include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0064] The dispersion medium used in the negative electrode slurry is not particularly limited, but examples thereof include water, alcohol, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0065] The negative electrode current collector may be, for example, a metal foil. The negative electrode current collector may be porous. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0066] [Positive Electrode] The positive electrode contains a positive electrode active material. The positive electrode usually includes a positive electrode current collector and a layer of positive electrode composite material (hereinafter referred to as a "positive electrode composite layer") held on the positive electrode current collector. The positive electrode composite layer can be formed by applying a positive electrode slurry, in which the components of the positive electrode composite material are dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the slurry. The dried coating film may be rolled as necessary. The positive electrode composite material contains a positive electrode active material as an essential component and may contain a binder, a thickener, etc. as optional components.

[0067] (Positive Electrode Active Material) The positive electrode active material may be any material that can be used as a positive electrode active material for a non-aqueous secondary battery (particularly a lithium ion secondary battery), but from the viewpoint of increasing capacity, it contains a lithium transition metal composite oxide (composite oxide N) containing at least nickel as a transition metal. The proportion of the composite oxide N in the positive electrode active material is, for example, 70 mass % or more, or may be 90 mass % or more, or may be 95 mass % or more.

[0068] The composite oxide N may be, for example, a lithium transition metal composite oxide having a layered rock salt structure and containing Ni and at least one selected from the group consisting of Co, Mn, and Al. Hereinafter, a lithium transition metal composite oxide having a layered rock salt structure and containing Ni and at least one selected from the group consisting of Co, Mn, and Al, in which the proportion of Ni among metal elements other than Li is 80 atomic % or more, will also be referred to as a "composite oxide HN." The proportion of the composite oxide HN in the composite oxide N used as the positive electrode active material is, for example, 90 mass % or more, or may be 95 mass % or more, or even 100%. The higher the proportion of Ni, the more lithium ions can be extracted from the composite oxide HN during charging, thereby increasing the capacity.

[0069] Co, Mn, and Al contribute to stabilizing the crystal structure of the composite oxide HN having a high Ni content. From the viewpoint of reducing production costs, a smaller Co content is desirable, but from the viewpoint of improving durability, it is desirable to include Co in the composite oxide HN. A composite oxide HN having a low Co content (or no Co) may contain Mn and Al.

[0070] The proportion of Co in the metal elements other than Li is preferably 10 atomic % or less, more preferably 5 atomic % or less, and the composite oxide HN may not contain Co. From the viewpoint of stabilizing the crystal structure of the composite oxide HN, the composite oxide HN may contain 1 atomic % or more or 1.5 atomic % or more (for example, 5 atomic % or more) of Co.

[0071] The ratio of Mn to the metal elements other than Li may be 10 atomic % or less, or 5 atomic % or less. The ratio of Mn to the metal elements other than Li may be 1 atomic % or more, 3 atomic % or more, or 5 atomic % or more. When limiting the range, these upper and lower limits may be combined arbitrarily.

[0072] The proportion of Al in the metal elements other than Li may be 10 atomic % or less, or 5 atomic % or less. The proportion of Al in the metal elements other than Li may be 1 atomic % or more, 3 atomic % or more, or 5 atomic % or more. When limiting the range, these upper and lower limits may be combined arbitrarily.

[0073] The composite oxide HN may be, for example, a compound represented by the formula: Li α Ni(1-x1-x2-yz)Co ｘ１ Mn ｘ２ Al ｙ M ｚ O ２＋β The element M is an element other than Li, Ni, Co, Mn, Al, and oxygen. Mn contributes to stabilizing the crystal structure of the composite oxide HN, and containing inexpensive Mn in the composite oxide HN is advantageous for cost reduction. Al contributes to stabilizing the crystal structure of the composite oxide HN.

[0074] In the above formula, α, which represents the atomic ratio of lithium, is, for example, 0.95≦α≦1.05. α increases or decreases with charge and discharge. In (2+β), which represents the atomic ratio of oxygen, β satisfies −0.05≦β≦0.05.

[0075] The atomic ratio of Ni, 1-x1-x2-y-z (=v), is, for example, 0.8 or more, or may be 0.85 or more, or 0.90 or more, or 0.95 or more. Furthermore, v may be 0.98 or less, or may be 0.95 or less. The atomic ratio of Co, x1, is, for example, 0.1 or less (0≦x1≦0.1), the atomic ratio of Mn, x2, is, for example, 0.1 or less (0≦x2≦0.1), the atomic ratio of Al, y, is, for example, 0.1 or less (0≦y≦0.1), and the atomic ratio of M, z, is, for example, 0≦z≦0.10.

[0076] The element M may be at least one selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, Sc, and Y. In particular, when at least one selected from the group consisting of Nb, Sr, and Ca is contained in the composite oxide HN, it is thought that the surface structure of the composite oxide HN is stabilized, the resistance is reduced, and metal elution is further suppressed. It is more effective if the element M is unevenly distributed in the vicinity of the particle surface of the composite oxide HN.

[0077] As the binder for the positive electrode, for example, a resin material is used. Examples of the binder include fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, vinyl resin, etc. One type of binder may be used alone, or two or more types may be used in combination.

[0078] Examples of conductive materials include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0079] The dispersion medium used in the positive electrode slurry is not particularly limited, but examples thereof include water, alcohol, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0080] The positive electrode current collector may be, for example, a metal foil. The positive electrode current collector may be porous. Examples of porous current collectors include nets, punched sheets, and expanded metals. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloys, and titanium. The thickness of the positive electrode current collector is not particularly limited, but may be, for example, 1 to 50 μm, or 5 to 30 μm.

[0081] [Electrolyte] The electrolyte contains a solvent and a solute dissolved in the solvent. The solute is an electrolyte salt that ionically dissociates in the electrolyte. The solute may include, for example, a lithium salt. Components of the electrolyte other than the solvent and the solute are additives. The electrolyte may contain various additives.

[0082] The solvent may be an aqueous solvent or a non-aqueous solvent. Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0083] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF ２ O ２ , LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(CF4F9SO2), LiN(CF5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0084] The concentration of the lithium salt in the electrolyte solution may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte solution having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0085] [Separator] It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator may be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0086] An example of the structure of a nonaqueous secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween is housed in an outer casing together with an electrolyte solution. However, this is not limited thereto, and other forms of electrode groups may also be used. For example, a stacked electrode group in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween may also be used. The shape of the secondary battery is also not limited, and may be, for example, a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0087] The structure of a nonaqueous secondary battery will be described below with reference to Fig. 1. Fig. 1 is a longitudinal cross-sectional view of a cylindrical nonaqueous secondary battery 10 that is an example of this embodiment. However, the present disclosure is not limited to the following configuration.

[0088] The secondary battery 10 includes an electrode group 18, an electrolyte (not shown), and a cylindrical battery can 22 with a bottom that accommodates these. A sealing body 11 is crimped to the opening of the battery can 22 via a gasket 21, thereby sealing the battery. The sealing body 11 includes a valve body 12, a metal plate 13, and an annular insulating member 14 interposed between the valve body 12 and the metal plate 13. The valve body 12 and the metal plate 13 are connected to each other at their respective centers. A positive electrode lead 15a extending from a positive electrode plate 15 is connected to the metal plate 13. Thus, the valve body 12 functions as an external terminal for the positive electrode. A negative electrode lead 16a extending from a negative electrode plate 16 is connected to the inner bottom surface of the battery can 22. An annular groove 22a is formed near the open end of the battery can 22. A first insulating plate 23 is disposed between one end face of the electrode group 18 and the annular groove portion 22a. A second insulating plate 24 is disposed between the other end face of the electrode group 18 and the bottom of the battery can 22. The electrode group 18 is formed by winding a positive electrode plate 15 and a negative electrode plate 16 with a separator 17 interposed therebetween.

[0089] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0090] Example 1 A non-aqueous secondary battery was fabricated and evaluated according to the following procedure. (1) Fabrication of Positive Electrode: LiNi was used as the positive electrode active material. ０．９１ Co ０．０４ Al ０．０５ O ２ A positive electrode slurry was obtained by mixing 100 parts by mass of a positive electrode active material (average particle size 12 μm), 1 part by mass of carbon nanotubes, 1 part by mass of polyvinylidene fluoride, and an appropriate amount of NMP. Next, the positive electrode slurry was applied to both sides of an aluminum foil, the coating was dried, and then rolled to form a positive electrode composite layer on both sides of the aluminum foil, thereby obtaining a positive electrode.

[0091] (2) Preparation of Negative Electrode A first negative electrode slurry was prepared by mixing 90 parts by mass of the first carbon particles A, 10 parts by mass of the Si-containing particles B, 1 part by mass of a sodium salt of CMC (CMC-Na), 1 part by mass of SBR, and an appropriate amount of water. The first carbon particles A included 40% by mass of third particles A1 (average particle size 15 μm) with an internal particle porosity of 2.4% and 60% by mass of fourth particles A2 (average particle size 18 μm) with an internal particle porosity of 12.8%.

[0092] A second negative electrode slurry was prepared by mixing 90 parts by mass of the second carbon particles A, 10 parts by mass of the Si-containing particles B, 1 part by mass of a sodium salt of CMC (CMC-Na), 1 part by mass of SBR, and an appropriate amount of water. All (100% by mass) of the second carbon particles A were fourth particles A2 (average particle size 18 μm) with an internal particle porosity of 12.8%.

[0093] The silicon-containing particles B are first particles B1 having a carbon phase and a silicon phase dispersed within the carbon phase, and multiple lots having different average particle sizes and silicon phase contents were prepared generally according to the following procedure.

[0094] The first particles B1 were prepared by mixing pitch, a carbon source, with coarse Si particles, milling the mixture in a ball mill, and then firing the mixture in an inert atmosphere. The crystallite size of the silicon phase (silicon particles) dispersed within the carbon phase was determined by the method described above and was found to be 200 nm or less. XRD analysis revealed that the carbon phase was amorphous carbon, not containing crystalline carbon.

[0095] The ratio of the DBP oil amount (DBPB) of the Si-containing particles B (first particles B1) to the DBP oil amount (DBPA) of the first carbon particles A (third particles A1:fourth particles A2 (mass ratio) = 40:60): DBPB / DBPA was 0.93.

[0096] The second negative electrode slurry was applied to both sides of a copper foil serving as a negative electrode current collector, and the coating was dried. The first negative electrode slurry was then applied to the second negative electrode slurry coating, dried, and rolled to form a negative electrode composite layer on both sides of the copper foil, thereby obtaining a negative electrode. The thickness of the negative electrode composite layer was 81 μm on each side, and the coating thickness of the first negative electrode slurry and the second negative electrode slurry was the same.

[0097] The distribution of the Si element presence probability R in the thickness direction of the negative electrode composite layer of the obtained negative electrode was determined in the surface direction of the negative electrode composite layer, and the difference between the upper limit value Rmax and the lower limit value Rmin of the 1σ interval (Rmax - Rmin) was determined to be 18%. The pitch of the multiple straight lines drawn in the Si element map that were parallel to the thickness direction of the negative electrode composite layer was 5 μm.

[0098] (3) Preparation of Electrolyte Solution An electrolyte solution was prepared by dissolving LiPF in a mixed solvent of FEC, EC, EMC, and DMC (FEC:EC:EMC:DMC=10:5:5:80 (volume ratio)). The concentration of LiPF in the electrolyte solution was 1.3 mol / L.

[0099] (4) Preparation of Lithium-Ion Secondary Battery One end of an aluminum positive electrode lead was attached to the positive electrode. One end of a nickel negative electrode lead was attached to the negative electrode. An electrode assembly was prepared by winding the positive and negative electrodes with a polyethylene separator between them. The electrode assembly was vacuum dried at 105°C for 2 hours and then housed in a cylindrical battery case with a bottom that also served as the negative electrode terminal. An iron battery case was used. Next, an electrolyte solution was poured into the battery case, and the opening of the battery case was closed with a metal seal that also served as the positive electrode terminal. A resin gasket was interposed between the seal and the open end of the battery case. In this way, a 21700-type cylindrical lithium-ion secondary battery (Battery X1) was prepared. In Table 1, X1 to X3 represent the batteries of Examples 1 to 3, and Y1 and Y2 represent the batteries of Comparative Examples 1 and 2.

[0100] (5) Evaluation <Charge-Discharge Cycles> The completed battery was subjected to 50 charge-discharge cycles under the following conditions: The deterioration rate of the discharge capacity C50 after 50 cycles (the difference between C0 and C50) was calculated, with the initial discharge capacity C0 being 100%.

[0101] <Charging> The battery was charged at a constant current of 0.2 It in an environment of 25° C. until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.02 It. After constant voltage charging, the battery was allowed to rest for 20 minutes.

[0102] <Discharge> After the rest, the battery was discharged at a constant current of 0.2 It in an environment of 25°C until the voltage reached 2.5V.

[0103] Example 2 A battery X2 was produced in the same manner as in Example 1, except that the Si-containing particles B (first particles B1) were changed to change the DBP fuel supply amount of the Si-containing particles B (first particles B1) and the upper limit value Rmax and the lower limit value Rmin of the 1σ interval of the distribution of the existence probability R.

[0104] Example 3 A battery X3 was fabricated in the same manner as in Example 2, except that the upper limit value Rmax and the lower limit value Rmin of the 1σ interval of the distribution of the existence probability R were changed by changing the composite mixing time during preparation of the negative electrode slurry.

[0105] Comparative Example 1 Instead of the Si-containing particles B (first particles B1), a lithium silicate phase (Li ２ Si ２ O ５ A battery Y1 was produced in the same manner as in Example 1 except for the above.

[0106] Comparative Example 2 A battery Y2 was produced in the same manner as in Example 1, except that the Si-containing particles B (first particles B1) were changed to change the DBP oil supply amount of the Si-containing particles B (first particles B1) and the upper limit value Rmax and the lower limit value Rmin of the 1σ interval of the distribution of the existence probability R.

[0107]

[0108] Example 4 A battery X4 was produced in the same manner as in Example 2, except that the content of the Si-containing particles B (first particles B1) in the negative electrode active material was changed to 12 mass % in the first negative electrode slurry and the second negative electrode slurry, thereby changing the upper limit value Rmax and the lower limit value Rmin of the 1σ interval of the distribution of the existence probability R.

[0109] Comparative Example 3 Battery Y3 was produced in the same manner as in Example 1, except that in the first negative electrode slurry and the second negative electrode slurry, the content of the Si-containing particles B (first particles B1) in the negative electrode active material was changed to 12 mass %, and the Si-containing particles B (first particles B1) were changed, thereby changing the DBP oil supply amount of the Si-containing particles B (first particles B1) and the upper limit value Rmax and lower limit value Rmin of the 1σ interval of the distribution of the existence probability R.

[0110]

[0111] A secondary battery having a negative electrode according to the present disclosure is suitable for use as a main power source for mobile communication devices, portable electronic devices, and the like, an in-vehicle power source, but its applications are not limited to these.

[0112] While the present invention has been described in terms of presently preferred embodiments, such disclosure is not to be interpreted as limiting. Various changes and modifications will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. It is therefore intended that the appended claims be interpreted to cover all changes and modifications that do not depart from the true spirit and scope of the invention.

[0113] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating member, 15: Positive electrode plate, 15a: Positive electrode lead, 16: Negative electrode plate, 16a: Negative electrode lead, 17: Separator, 18: Electrode group, 21: Gasket, 22: Battery can, 22a: Groove, 23: First insulating plate, 24: Second insulating plate, 16: Negative electrode, 161: Negative electrode current collector, 162: Negative electrode composite layer

Claims

1. A negative electrode current collector, and a negative electrode composite material layer containing a negative electrode active material, The negative electrode active material includes carbon particles and Si-containing particles, In the distribution of the presence probability of Si element in the thickness direction of the negative electrode composite material layer in the plane direction of the negative electrode composite material layer, the difference between the upper limit value Rmax and the lower limit value Rmin of the 1σ interval: Rmax−Rmin is 20% or less. A negative electrode for a secondary battery.

2. The Si-containing particles include first particles, The first particles have a carbon phase and a silicon phase dispersed in the carbon phase. The negative electrode for a secondary battery according to claim 1.

3. The carbon phase does not contain crystalline carbon. The negative electrode for a secondary battery according to claim 2.

4. The ratio of the DBP oil absorption amount (DBPB) of the Si-containing particles to the DBP oil absorption amount (DBPA) of the carbon particles: DBPB / DBPA is 0.9 or more and 1.0 or less. The negative electrode for a secondary battery according to claim 2 or 3.

5. In the negative electrode active material, The content of the carbon particles is 70% by mass or more and 95% by mass or less, The content of the Si-containing particles is 5% by mass or more and 30% by mass or less. The negative electrode for a secondary battery according to any one of claims 1 to 3.

6. The Si-containing particles include second particles, The second particles have a silicate phase and a silicon phase dispersed in the silicate phase. The negative electrode for a secondary battery according to any one of claims 1 to 3.

7. The carbon particles include third particles and fourth particles, The internal porosity of the third particles is 5% or less, The internal porosity of the fourth particles is 8% or more and 20% or less, In the carbon particles, the content of the third particles is 10% by mass or more and 80% by mass or less. The negative electrode for a secondary battery according to any one of claims 1 to 3.

8. When the negative electrode composite material layer is divided into a first region and a second region having the same thickness in the thickness direction, The third particles are contained in a larger amount in the second region farther from the negative electrode current collector than in the first region closer to the negative electrode current collector. The negative electrode for a secondary battery according to claim 7.

9. A positive electrode, a negative electrode, and a non-aqueous electrolyte are provided, The negative electrode is the negative electrode for a secondary battery according to any one of claims 1 to 3. A secondary battery.