Negative electrode for secondary batteries and secondary batteries

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

[Technical Field]

[0001] This disclosure relates to a negative electrode for a secondary battery and a secondary battery. [Background technology]

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

[0003] [Patent Document 1] Japanese Patent Publication No. 2007-73334 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Patent Document 1 primarily aims to provide a lithium secondary battery that can improve initial charge-discharge efficiency and further enhance charge-discharge cycle characteristics. However, further improvements to the negative electrode are required to improve the cycle characteristics of a secondary battery having a silicon (Si) negative electrode. [Means for solving the problem]

[0005] One aspect of this disclosure relates to a negative electrode for a secondary battery, comprising 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 the difference between the upper limit Rmax and the lower limit Rmin in the 1σ interval of the distribution of the probability of presence of Si elements in the thickness direction of the negative electrode composite layer in the surface direction of the negative electrode composite layer, Rmax-Rmin, is 20% or less.

[0006] Another aspect of this disclosure relates to a secondary battery comprising a positive electrode, the negative electrode, and a non-aqueous electrolyte. [Effects of the Invention]

[0007] According to this disclosure, the cycle characteristics of a secondary battery having a silicon (Si)-containing negative electrode can be improved.

[0008] Novel features of the present invention are described in the appended claims, but the present invention, both in terms of structure and content, and 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. [Brief explanation of the drawing]

[0009] [Figure 1] This is a partially cutaway perspective view of a non-aqueous secondary battery according to one embodiment of the present disclosure. [Figure 2] This is an illustrative diagram showing a method for determining the distribution of the probability of Si element presence in the thickness direction of the negative electrode composite layer in the surface direction of the negative electrode composite layer. [Modes for carrying out the invention]

[0010] The following describes embodiments of the negative electrode for secondary batteries and secondary batteries relating to this disclosure 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 of numerical values ​​relating to specific physical properties or conditions are given as examples, either of the given lower limits and either of the given upper limits can be arbitrarily combined as long as the lower limit is not greater than or equal to 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.

[0011] 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.

[0012] The negative electrode for a secondary battery according to this disclosure comprises 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 usually a layer (including a film or membrane) composed of a negative electrode composite material. The negative electrode composite material contains a negative electrode active material as an essential component. The negative electrode active material may be any material that reversibly exhibits capacity through the intercalation and release of lithium ions. The negative electrode active material contains carbon particles and Si-containing particles. The negative electrode active material may contain materials other than carbon particles and Si-containing particles, but the sum of carbon particles and Si-containing particles is the main component. For example, 50% or more by mass or 60% or more by mass (e.g., 70% or more by mass or 80% or more by mass) of the negative electrode active material may be carbon particles and Si-containing particles.

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

[0014] 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. Si-containing particles may be elemental Si, Si alloys, Si compounds, etc., but they may also be composite materials containing a lithium ion conducting phase and a silicon phase (e.g., silicon particles) dispersed within the lithium ion conducting phase. In composite materials, it is desirable that the silicon phase be as fine as possible. Composite materials are suitable for providing high capacity to the negative electrode while suppressing direct contact between the silicon phase and the electrolyte or non-aqueous electrolyte.

[0015] Here, in the distribution of the probability of existence R of Si elements in the thickness direction of the negative electrode composite layer in the surface direction of the negative electrode composite layer, the difference between the upper limit Rmax and the lower limit Rmin in the 1σ interval, Rmax-Rmin, is 20% or less. For example, by mixing Si-containing particles with carbon particles as uniformly as possible, and dispersing fine silicon phase as uniformly as possible in the lithium ion conducting phase, Rmax-Rmin can be controlled to 20% or less, and even to 18% or less or 17% or less.

[0016] The existence probability R of Si element in the thickness direction of the negative electrode mixture layer may be obtained as the ratio of the total length LB of portions intersecting Si element among straight lines parallel to the thickness direction of the negative electrode mixture to the thickness T of the negative electrode mixture layer (R=LB / T). The distribution of the existence probability R in the plane direction of the negative electrode mixture layer may be measured on one or more cross-sections obtained by cutting the negative electrode mixture layer along the thickness direction of the negative electrode together with the negative electrode current collector. Specifically, the distribution of the existence probability R can be obtained by the following method.

[0017] (1) Photographing a cross-section of a negative electrode with a scanning electron microscope (SEM) First, a negative electrode to be measured is prepared. Next, the negative electrode mixture layer and the negative electrode current collector are simultaneously cut along the thickness direction of the negative electrode to form a cross-section. In this case, the negative electrode mixture layer may be filled with a thermosetting resin and cured. For example, the above cross-section of the negative electrode is obtained by a CP (cross-section polisher) method, an FIB (focused ion beam) method, or the like. The cross-section sample is observed with a SEM. Observation by SEM is performed at a low magnification (e.g., 200 times to 1000 times). An SEM image is captured such that a region having a length of 300 µm or more (preferably 400 µm or more) in the plane direction of the negative electrode mixture layer is observed.

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

[0019] (2) Elemental analysis by energy dispersive X-ray spectroscopy (EDX) Elemental analysis by EDX is performed using the SEM image of the cross-section of the negative electrode.

[0020] (3) Creating a map of silicon phases By extracting components derived from the Si element from EDX analysis data of the cross-section of the negative electrode, a Si element map is obtained. This allows for the counting of all Si-derived components in the negative electrode composite layer.

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

[0022] When the probability of existence R is measured at a predetermined pitch along the plane direction in the Si element map, the distribution of the probability of existence R in that plane direction can be obtained. If the length of the negative electrode composite layer in the plane direction of the SEM image is, for example, 300 μm, and the predetermined pitch is 5 μm, then 300 / 5 = 60 probability of existence R values ​​can be obtained, and the distribution of these 60 probability of existence R values ​​can be plotted.

[0023] Figure 2 conceptually shows 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 cut simultaneously. In Figure 2, the Si element map is represented as a two-dimensional image with the thickness direction of the negative electrode composite layer 162 as the vertical axis and an arbitrary plane direction as the horizontal axis. Multiple straight lines L are straight lines parallel to the thickness direction of the negative electrode composite layer 162 and are drawn along the plane direction at a predetermined pitch. Since each straight line L may have a portion that intersects 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 portion that intersects with Si for each straight line L to the thickness T of the negative electrode composite layer can be obtained as the probability of Si existence R. In the illustrated example, 24 straight lines are drawn, so 24 probability Rs can be obtained, and the distribution of 24 probability Rs can be drawn.

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

[0025] The 1σ interval is the interval from μ-σ to μ+σ of the distribution function, where the x-axis is the probability of existence R and the y-axis is the frequency, where μ is the mean of the probability of existence R and σ is the standard deviation of the distribution function.

[0026] To keep Rmax-Rmin below 20%, 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 becomes to uniformly mix them. The affinity between carbon particle A and Si-containing particle B can be evaluated, for example, using the amount of DBP (dibutyl phthalate) supplied as an indicator. Specifically, the ratio of the amount of DBP supplied by Si-containing particles (DBPB) to the amount of DBP supplied by carbon particles (DBPA): DBPB / DBPA may be 1.0 or less. The DBPB / DBPA ratio may be between 0.9 and 1.0, or between 0.95 and less than 1.0 (for example, 0.99 or less).

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

[0028] In the negative electrode active material, the carbon particle content may be 70% by mass or more and 95% by mass or less, and the Si-containing particle content may be 5% by mass or more and 30% by mass or less. When seeking a good balance between good cycle characteristics and high capacity, the Si-containing particle content 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 greater emphasis is placed on improving cycle characteristics, the Si-containing particle content 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 a first particle having a carbon phase and a silicon phase dispersed within the carbon phase. The first particle is a particle of a composite material of the carbon phase and the silicon phase. The carbon phase may be composed of amorphous carbon, for example. The carbon phase does not have to contain crystalline carbon. In this case, the profile measured by X-ray diffraction does not show peaks attributed to crystalline carbon, but a halo pattern attributed to amorphous carbon is observed. In the first particle, the silicon phase content can be arbitrarily changed, making it easy to design a high-capacity anode.

[0030] Amorphous carbon can be hard carbon, soft carbon, or any other type. Amorphous carbon can be obtained, for example, by sintering a carbon source in an inert atmosphere and then crushing the resulting sintered body.

[0031] The first particle can be obtained, for example, by mixing a carbon source and Si particles, milling the mixture with a stirrer such as a ball mill, and then calcining the mixture in an inert atmosphere. As the carbon source, for example, petroleum resins such as coal pitch, petroleum pitch, and tar, carboxymethylcellulose (CMC), polyvinylpyrrolidone, cellulose, sugars such as sucrose, and water-soluble resins can be used. When mixing the carbon source and Si particles, for example, the carbon source and Si particles may be dispersed in a dispersion medium such as alcohol. After drying the milled mixture, the carbon phase is formed by heating it 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. The carbon phase formed in this way is amorphous carbon that does not contain crystalline carbon.

[0032] The silicon phase dispersed within the carbon phase is typically composed of multiple crystallites. The crystallite size of the silicon phase is, for example, 500 nm or less, and may also be 30 nm or less. There is no particular lower limit to the crystallite size of the silicon phase, but it is, for example, 5 nm or more. The crystallite size is calculated from the full width at half maximum of the diffraction peaks attributed to the Si(111) plane in the X-ray diffraction (XRD) pattern of the silicon phase using Scherrer's formula.

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

[0034] The silicon phase content in the first particle can be measured by Si-NMR. The preferred measurement conditions for Si-NMR are shown below.

[0035] Measurement device: Varian Solid State Nuclear Magnetic Resonance Spectrometer (INOVA-400) Probe: Varian 7mm CPMAS-2 MAS: 4.2kHz MAS speed: 4kHz Pulse: DD (45° pulse + 1H signal acquisition time decoupler) Repeat time: 1200 sec Observation width: 100kHz Observation center: around -100 ppm Signal acquisition time: 0.05 sec Total count: 560 Sample amount: 207.6 mg

[0036] The average particle size of the first particle can be, for example, 1 to 20 μm, and preferably 2 to 12 μm. Within this particle size range, stress due to volume changes of the first particle during charging and discharging is easily relieved, making it easier to obtain good cycle characteristics.

[0037] The average particle size of the first particle refers to the particle size at which the integrated volume value in the particle size distribution measured by laser diffraction scattering method becomes 50% (volume-average particle size). For the measuring device, for example, the "LA-750" manufactured by HORIBA, Ltd. can be used.

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

[0039] The Si-containing particles may also 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. In the second particles, the content of the silicon phase can be arbitrarily changed, making it easy to design high-capacity negative electrodes.

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

[0041] The lithium silicate phase only needs to 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 case 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 may be represented by the formula: Li 2z SiO 2+z (0<z<2) may have the composition represented by this formula. From the viewpoints of stability, ease of production, lithium ion conductivity, and the like, z preferably satisfies the relationship 0<z<1, and z=1 / 2 is more preferable.

[0043] The silicon phase dispersed within the silicate phase is typically composed of multiple crystallites. The crystallite size of the silicon phase is, for example, 500 nm or less, and may also be 30 nm or less. The lower limit of the silicon phase crystallite size is not particularly limited, but is, for example, 5 nm or more. The crystallite size can be measured in accordance with the first particle.

[0044] The silicon phase content in the second particle is, for example, 40% by mass or more, but may also be 45% by mass or more, 50% by mass or more, or 65% by mass or more. Alternatively, the silicon phase content in the second particle may be, for example, 80% by mass or less, but may also be 75% by mass or less, 70% by mass or less, or 65% by mass or less. Within the range in which the upper and lower limits are arbitrarily selected from the above, it becomes easier to achieve both high battery capacity and improved cycle characteristics. The silicon phase content in the second particle can be measured in the same manner as the first particle.

[0045] The average particle size of the second particle can be, for example, 1 to 20 μm, and preferably 5 to 12 μm. Within this particle size range, stress due to volume changes in the second particle during charging and discharging is easily relieved, making it easier to obtain good cycle characteristics. The average particle size of the second particle can be measured in the same way as the first particle.

[0046] The first and second particles are superior in that they have low irreversible capacity. This is because the carbon phase and silicate phase have fewer sites that irreversibly trap lithium ions. By using the first and even the second particles, excellent charge and discharge efficiency can be obtained. This effect is particularly noticeable in the initial stages of charging and discharging.

[0047] The composition of the first and 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, dried, and then 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 elemental analysis of the observed particles can then be performed using an Auger electron spectroscopy (AES) analyzer (acceleration voltage 10kV, beam current 10nA).

[0048] The carbon particles may include two or more types of particles with different internal porosities. In this case, it becomes easier to control the structural strength of the negative electrode composite layer and the fluidity of the electrolyte or non-aqueous electrolyte within the negative electrode composite layer, thereby improving the cycle characteristics. The carbon particles may include, for example, a third particle with an internal porosity of 5% or less and a fourth particle with an internal porosity of 8% or more and 20% or less.

[0049] The internal porosity of 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 area of ​​each is determined. The ratio of the area of ​​the void portion to the sum of the area of ​​the carbon portion and the area of ​​the void portion is the internal porosity of the particle. However, the internal porosity of the particle is the average value obtained from 10 or more of a given carbon particle. When determining the internal porosity of the third particle, the average value of the internal porosity of 10 or more third particles is calculated. When determining the internal porosity of the fourth particle, the average value of the internal porosity of 10 or more fourth particles is calculated. However, carbon particles with an internal porosity of 6.5% or less are classified as third particles, and carbon particles with an internal porosity exceeding 6.5% are classified as fourth particles.

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

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

[0052] From the viewpoint of the fluidity of the electrolyte or non-aqueous electrolyte, the content of the third particle 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 composite layer is divided into two regions of the same thickness in the thickness direction, it is desirable that the third particles be present in a larger quantity in the second region, which is further from the negative electrode current collector, than in the first region, which is closer to the negative electrode current collector. The third particles are dense and strong, and are suitable for forming sufficient gaps in the surface layer of the negative electrode composite layer, where the negative electrode active material is easily crushed. By including a relatively large amount of third particles in the second region, it becomes easier to improve the fluidity of the electrolyte or non-aqueous electrolyte within the negative electrode composite layer. On the other hand, the fourth particles have a large internal porosity, making them suitable for forming sufficient gaps in the deeper layers of the negative electrode composite layer. By including a relatively large amount of fourth particles in the first region, it becomes easier to further improve the fluidity of the electrolyte or non-aqueous electrolyte within the negative electrode composite layer.

[0054] If the third particle is included in greater quantities in the second region than in the first region, and sufficient gaps are formed in the surface layer of the negative electrode composite material layer, then if the negative electrode composite material layer expands significantly locally due to charging and discharging of the secondary battery, the surface layer is likely to be strained and its strength is likely to deteriorate. On the other hand, in this embodiment, the difference between the upper limit Rmax and the lower limit Rmin of the 1σ interval of the probability of existence R (Rmax-Rmin) is small and limited, so the degree of expansion and contraction due to charging and discharging of the negative electrode tends to be uniform. Therefore, the strength of the surface layer is easily maintained.

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

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

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

[0058] [Negative electrode] As previously described, the negative electrode comprises a negative electrode composite layer containing a negative electrode active material and a negative electrode current collector. The negative electrode composite layer can be formed by coating the surface of the negative electrode current collector with a negative electrode slurry, which is obtained by dispersing the components of the negative electrode composite in a dispersion medium, and drying it. The dried coating may be rolled if necessary.

[0059] The negative electrode composite material may contain a negative electrode active material as an essential component, and may also contain binders, thickeners, conductive agents, etc., as optional components.

[0060] (Negative electrode active material) As previously described, the negative electrode active material includes carbon particles and Si-containing particles.

[0061] For example, a resin material can be used as the binder for the negative electrode. Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, vinyl resins, and rubber-like materials (e.g., styrene-butadiene copolymer (SBR)). The binder may be used alone or in combination of two or more types.

[0062] Examples of thickening agents include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethylcellulose (CMC) and its modified forms, and methylcellulose. A single thickening agent may be used alone, or two or more may be used in combination.

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

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

[0065] For example, a metal foil may be used as the negative electrode current collector. The negative electrode current collector may be porous. Examples of materials 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 for example, it may be 1 to 50 μm, or 5 to 30 μm.

[0066] [Positive electrode] The positive electrode contains a positive electrode active material. The positive electrode typically comprises a positive electrode current collector and a layered positive electrode composite material (hereinafter referred to as the "positive electrode composite material layer") held by the positive electrode current collector. The positive electrode composite material layer can be formed by coating a positive electrode slurry, which is obtained by dispersing the components of the positive electrode composite material in a dispersion medium, onto the surface of the positive electrode current collector and drying it. The coating film after drying may be rolled if necessary. The positive electrode composite material contains a positive electrode active material as an essential component and may contain binders, thickeners, etc., as optional components.

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

[0068] The composite oxide N may, for example, be a lithium transition metal composite oxide having a layered rock salt structure and containing Ni and at least one element 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 element selected from the group consisting of Co, Mn, and Al, in which the proportion of Ni among the metal elements other than Li is 80 atomic percent or more, will also be referred to as "composite oxide HN". The proportion of composite oxide HN in the composite oxide N used as a positive electrode active material is, for example, 90 mass% or more, may be 95 mass% or more, or may be 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 composite oxide HN with a high Ni content. From the viewpoint of reducing manufacturing costs, a lower Co content is desirable, but from the viewpoint of improving durability, it is desirable to include Co in the composite oxide HN. Composite oxide HN with a low Co content (or no Co at all) may also contain Mn and Al.

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

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

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

[0073] The composite oxide HN is, for example, represented by the formula: Li α Ni (1-x1-x2-y-z) 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 the inclusion of 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 representing the atomic ratio of lithium satisfies, for example, 0.95≦α≦1.05. α increases and decreases with charge and discharge. In (2+β) representing the atomic ratio of oxygen, β satisfies -0.05≦β≦0.05.

[0075] The atomic ratio of Ni, 1-x1-x2-yz (=v), is, for example, 0.8 or greater, may be 0.85 or greater, or 0.90 or greater, or 0.95 or greater. Also, v may be 0.98 or less, or 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 element M, z, is, for example, 0≦z≦0.10.

[0076] 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 included in the composite oxide HN, it is thought that the surface structure of the composite oxide HN is stabilized, resistance is reduced, and metal elution is further suppressed. Element M is more effective when it is concentrated near the particle surface of the composite oxide HN.

[0077] For example, a resin material can be used as the binder for the positive electrode. Examples of binders include fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, and vinyl resins. The binder may be used alone or in combination of two or more types.

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

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

[0080] For example, a metal foil may be used as the positive electrode current collector. The positive electrode current collector may be porous. Examples of porous current collectors include nets, perforated sheets, and expanded metal. 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 for example, it may be 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 undergoes ionic dissociation in the electrolyte. The solute may include, for example, a lithium salt. Components of the electrolyte other than the solvent and solute are additives. Various additives may be included in the electrolyte.

[0082] The solvent used can be an aqueous or non-aqueous solvent. 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 (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of linear carbonate esters 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 linear carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). The non-aqueous solvent may be used alone or in combination of two or more types.

[0083] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10Lithium salts of fluorine-containing acids (such as LiPF6, LiPF2O2, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (such as LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(C4F9SO2), LiN(C2F5SO2)2, etc.), lithium halides (such as LiCl, LiBr, LiI, etc.) can be used. Lithium salts may be used individually or in combination of two or more types.

[0084] The lithium salt concentration in the electrolyte may be between 1 mol / liter and 2 mol / liter, or between 1 mol / liter and 1.5 mol / liter. By controlling the lithium salt concentration within the above range, an electrolyte with 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 and negative electrodes. The separator should have 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.

[0086] One example of a non-aqueous secondary battery structure is one in which an electrode group, consisting of a positive electrode and a negative electrode wound around a separator, is housed together with an electrolyte in an outer casing. However, the structure is not limited to this, and other forms of electrode groups may be used. For example, a stacked electrode group in which the positive electrode and negative electrode are stacked with a separator in between may also be used. The form of the secondary battery is also not limited; for example, it may be cylindrical, prismatic, coin-type, button-type, laminate-type, etc.

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

[0088] The secondary battery 10 comprises an electrode group 18, an electrolyte (not shown), and a bottomed cylindrical battery case 22 that houses these. A sealing body 11 is crimped and fixed to the opening of the battery case 22 via a gasket 21. This seals the inside of the battery. The sealing body 11 comprises 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, which is led out from the positive electrode plate 15, is connected to the metal plate 13. Therefore, the valve body 12 functions as an external terminal of the positive electrode. A negative electrode lead 16a, which is led out from the negative electrode plate 16, is connected to the inner surface of the bottom of the battery case 22. An annular groove 22a is formed near the open end of the battery case 22. A first insulating plate 23 is positioned between one end face of the electrode group 18 and the annular groove 22a. A second insulating plate 24 is positioned 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 in between.

[0089] 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.

[0090] Example 1 A non-aqueous secondary battery was fabricated and evaluated using the following procedure. (1) Preparation of the positive electrode LiNi 0.91 Co 0.04 Al 0.05 O2 was used. A positive electrode slurry was obtained by mixing 100 parts by mass of 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 the foil was rolled to form a positive electrode composite layer on both sides of the aluminum foil, thereby obtaining a positive electrode.

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

[0092] A second negative electrode slurry was prepared by mixing 90 parts by mass of second carbon particles A, 10 parts by mass of Si-containing particles B, 1 part by mass of 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 porosity of 12.8%.

[0093] Silicon-containing particle B is a first particle B1 having a carbon phase and a silicon phase dispersed within the carbon phase. Multiple lots with different average particle sizes and silicon phase content were prepared using roughly the following procedure.

[0094] The first particle B1 was prepared by mixing pitch, a carbon source, with coarse Si particles, milling the mixture in a ball mill, and then calcining the mixture in an inert atmosphere. The crystallite size of the silicon phase (silicon particles) dispersed in the carbon phase was determined to be 200 nm or less by the method described above. XRD analysis revealed that the carbon phase consisted of amorphous carbon without crystalline carbon.

[0095] The ratio of DBP supply amount (DBPB) of Si-containing particle B (first particle B1) to DBP supply amount (DBPA) of first carbon particle A (third particle A1:fourth particle 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, which served as the negative electrode current collector, and the coating was dried. Then, the first negative electrode slurry was applied on top of 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 the negative electrode. The thickness of the negative electrode composite layer was 81 μm on one side, and the coating thickness of the first and second negative electrode slurry was the same.

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

[0098] (3) Preparation of electrolyte An electrolyte was prepared by dissolving LiPF6 in a mixed solvent of FEC, EC, EMC, and DMC (FEC:EC:EMC:DMC = 10:5:5:80 (volume ratio)). The concentration of LiPF6 in the electrolyte was 1.3 mol / L.

[0099] (4) Fabrication of lithium-ion secondary batteries 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. The positive and negative electrodes were wound together with a polyethylene separator to create an electrode body. After vacuum drying the electrode body at 105°C for 2 hours, it was housed in a bottomed cylindrical battery case that also served as the negative electrode terminal. An iron case was used for the battery case. Next, electrolyte was injected into the battery case, and the opening of the battery case was closed using a metal sealing body that also served as the positive electrode terminal. At this time, a resin gasket was interposed between the sealing body and the opening end of the battery case. In this way, a 21700 type cylindrical lithium-ion secondary battery (battery X1) was manufactured. In Table 1, X1 to X3 represent the batteries of Examples 1 to 3, and Y1 to Y2 represent the batteries of Comparative Examples 1 and 2.

[0100] (5) Evaluation <Charge-discharge cycle> The completed battery underwent 50 charge-discharge cycles under the following conditions. The degradation rate (difference between C0 and C50) after 50 cycles was calculated, with the initial discharge capacity C0 set to 100%.

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

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

[0103] Example 2 Battery X2 was fabricated in the same manner as in Example 1, except that the amount of DBP supplied to Si-containing particle B (first particle B1) and the upper and lower limits Rmax and Rmin of the 1σ interval of the distribution of the probability of existence R were changed by changing Si-containing particle B (first particle B1).

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

[0105] Comparative Example 1 Instead of the Si-containing particle B (first particle B1), a second particle B2 comprising a lithium silicate phase (Li2Si2O5) and a silicon phase dispersed within the lithium silicate phase was used. Battery Y1 was fabricated in the same manner as in Example 1, except as described above.

[0106] Comparative Example 2 Battery Y2 was fabricated in the same manner as in Example 1, except that the amount of DBP supplied to Si-containing particle B (first particle B1) and the upper limit Rmax and lower limit Rmin of the 1σ interval of the distribution of the probability of existence R were changed by changing Si-containing particle B (first particle B1).

[0107] [Table 1]

[0108] Example 4 Battery X4 was fabricated in the same manner as in Example 2, except that the content of Si-containing particles B (first particle 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 Rmax and lower limit Rmin of the 1σ interval of the distribution of the probability of existence R.

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

[0110] [Table 2] [Industrial applicability]

[0111] The secondary battery equipped with the negative electrode described herein is suitable for use as a main power source for mobile communication devices, portable electronic devices, and in-vehicle power sources, but its applications are not limited to these.

[0112] 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. [Explanation of Symbols]

[0113] 10: Secondary battery, 11: Sealing body, 12: Valve body, 13: Metal plate, 14: Insulating material, 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. It comprises a negative electrode current collector and a negative electrode composite layer containing a negative electrode active material, The negative electrode active material comprises carbon particles and Si-containing particles. In the distribution of the probability of Si element presence in the thickness direction of the negative electrode composite layer in the surface direction of the negative electrode composite layer, the difference between the upper limit Rmax and the lower limit Rmin in the 1σ interval: Rmax - Rmin is 20% or less. The Si-containing particles include the first particle, The first particle comprises a carbon phase and a silicon phase dispersed within the carbon phase. A negative electrode for a secondary battery, wherein the ratio of the DBP supply amount (DBPB) of the Si-containing particles to the DBP supply amount (DBPA) of the carbon particles (DBPB / DBPA) is 0.9 or more and 1.0 or less.

2. In the aforementioned negative electrode active material, The carbon particle content is 70% by mass or more and 95% by mass or less. The negative electrode for a secondary battery according to claim 1, wherein the content of the Si-containing particles is 5% by mass or more and 30% by mass or less.

3. The Si-containing particles include the second particle, The negative electrode for a secondary battery according to claim 1 or 2, wherein the second particle comprises a silicate phase and a silicon phase dispersed within the silicate phase.

4. The carbon particles include a third particle and a fourth particle, The internal porosity of the third particle is 5% or less. The porosity of the four particles is 8% or more and 20% or less. The negative electrode for a secondary battery according to claim 1 or 2, wherein the carbon particles have a content of the third particle of 10% by mass or more and 80% by mass or less.

5. When the negative electrode composite layer is divided into a first region and a second region of the same thickness in the thickness direction, The negative electrode for a secondary battery according to claim 4, wherein the third particle is present in a greater quantity in the second region, which is further from the negative electrode current collector, than in the first region, which is closer to the negative electrode current collector.

6. Equipped with a positive electrode, a negative electrode, and a non-aqueous electrolyte, A secondary battery wherein the negative electrode is the negative electrode for a secondary battery described in claim 1 or 2.

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