Nonaqueous electrolyte secondary battery and negative electrode for use in same

Abstract

This nonaqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, a nonaqueous electrolyte, and a separator. The negative electrode is provided with a negative electrode current collector and a negative electrode active material layer supported by the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, a binder, a thickener, and an additive. The additive is a polymer material having a melting point or a thermal decomposition temperature of 200-500°C. When the thickness of the negative electrode active material layer is denoted as T, the existence probability Pb of the polymer material existing in a region from the surface of the negative electrode current collector to 0.5T in the negative electrode active material layer and the existence probability Pt of the polymer material existing in a region from a position at 0.5T from the surface of the negative electrode current collector to the outermost surface in the negative electrode active material layer satisfy 1<Pb / Pt.

Description

Non-aqueous electrolyte secondary battery and negative electrode used therein CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2023-177855, filed on October 13, 2023, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a non-aqueous electrolyte secondary battery and a negative electrode using the same.

[0003] Patent Document 1 describes a non-aqueous electrolyte battery including a positive electrode having a positive electrode mixture layer containing a positive electrode active material, a negative electrode having a negative electrode mixture layer containing a negative electrode active material, and a non-aqueous electrolyte. ６ H ４ -SO ２ -, -C ６ H ４ -S-, -C ６ H ４ The paper proposes a nonaqueous electrolyte battery, characterized in that a polymer obtained by polymerizing monomer molecules having at least one of the unit structures of -O- is added to the positive electrode mixture layer and / or the negative electrode mixture layer.

[0004] Patent Document 2 proposes "a negative electrode for a lithium secondary battery comprising: a lithium metal layer; and a protective layer of a multilayer structure formed on the lithium metal layer; wherein the protective layer includes a first protective layer containing a carbon nanotube-ionically conductive polymer composite material; and a second protective layer containing a carbon nanotube-electrically conductive polymer composite material."

[0005] Japanese Patent Application Laid-Open No. 2003-007303 Japanese Patent Application Laid-Open No. 2019-506715

[0006] The higher the energy density of a secondary battery, the more safety is required. In particular, reducing heat generation due to internal short circuits is important. When a battery has an internal short circuit, even if the short circuit is small, the separator melts due to Joule heat generated by the short circuit current, forming a larger short circuit. As the short circuit expands and the short circuit current increases, the battery temperature rises at an accelerated rate. The higher the energy density of a secondary battery, the greater the Joule heat generated by the short circuit current.

[0007] One aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, the negative electrode active material layer including a negative electrode active material, a binder, a thickener, and an additive, wherein the additive is a polymer material having a melting point or a thermal decomposition temperature of 200°C or higher and 500°C or lower, and wherein, when a thickness of the negative electrode active material layer is T, an existence probability Pb ​​of the polymer material present in a region of the negative electrode active material layer from a surface of the negative electrode current collector to 0.5T and an existence probability Pt of the polymer material present in a region of the negative electrode active material layer from a position 0.5T from the surface of the negative electrode current collector to the outermost surface satisfy the relationship 1<Pb / Pt.

[0008] Another aspect of the present disclosure relates to a negative electrode for a secondary battery, including: a negative electrode current collector; and a negative electrode active material layer supported on the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material, a binder, a thickener, and an additive, and the additive is a polymer material having a melting point or a thermal decomposition temperature of 200°C or higher and 500°C or lower, and wherein, when a thickness of the negative electrode active material layer is T, an existence probability Pb ​​of the polymer material present in a region of the negative electrode active material layer from a surface of the negative electrode current collector to 0.5 T, and an existence probability Pt of the polymer material present in a region of the negative electrode active material layer from a position 0.5 T from the surface of the negative electrode current collector to the outermost surface satisfy the relationship 1<Pb / Pt.

[0009] According to the present disclosure, it is possible to achieve both high safety and good battery performance. 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.

[0010] 1 is a longitudinal cross-sectional view schematically illustrating an internal structure of a secondary battery according to an embodiment of the present disclosure.

[0011] The following describes embodiments of the present disclosure 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 expression "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.

[0012] Non-aqueous electrolyte secondary batteries include lithium ion secondary batteries that use a material that reversibly absorbs and releases at least lithium ions as a negative electrode active material, lithium secondary batteries in which lithium metal precipitates at the negative electrode during charging and dissolves during discharging, and solid-state batteries that contain a gel electrolyte or a solid electrolyte.

[0013] The non-aqueous electrolyte secondary battery according to the present disclosure includes a positive electrode, a negative electrode, and a non-aqueous electrolyte, and a separator is usually disposed between the positive electrode and the negative electrode.

[0014] A. Negative Electrode The negative electrode includes a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector. The negative electrode active material layer is formed on the surface of the negative electrode current collector. The negative electrode active material layer may also be referred to as a negative electrode mixture layer because it is made of a negative electrode mixture.

[0015] [Negative electrode current collector] The negative electrode current collector is made of a sheet-like conductive material. As the negative electrode current collector, a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet) is used.

[0016] [Negative Electrode Active Material Layer] The negative electrode active material layer is supported on one or both surfaces of the negative electrode current collector. The negative electrode active material layer contains a negative electrode active material, a binder, a thickener, and an additive, and may further contain a conductive additive. The additive is a polymer material having a melting point or thermal decomposition temperature of 200°C or higher and 500°C or lower (hereinafter, the "melting point or thermal decomposition temperature" may be collectively referred to as the "diffusion temperature").

[0017] The polymer material is preferably dispersed in the cross section of the negative electrode active material layer, forming a plurality of island-like regions. The island-like regions are spot-like regions where the polymer material is present, and can also be referred to as particulate regions when viewed macroscopically. The island-like regions may be regions composed only of the polymer material (P), or may be aggregate regions composed of the polymer material (P) and materials other than the polymer material (P) (such as a negative electrode active material or a conductive additive). The shape of the island-like regions is not particularly limited, but may be amorphous. The polymer material may be an insulating material or a mixture of an insulating material and a conductive material. Hereinafter, such a polymer material will also be referred to as "polymer material (P)."

[0018] The polymer material (P) has the effect of suppressing an increase in the negative electrode resistance at room temperature and increasing the negative electrode resistance at high temperatures when an internal short circuit occurs in the battery, thereby achieving both high safety and good battery performance.

[0019] Specifically, the polymer material (P) has the function of partially blocking the conductive path when an internal short circuit occurs, the short-circuit current increases, and the battery temperature rises. The polymer material (P) maintains an island shape at room temperature and causes little increase in the negative electrode resistance. On the other hand, when an internal short circuit occurs, the polymer material (P) melts before the battery temperature rises excessively. The molten polymer material (P) diffuses within the negative electrode active material layer. The polymer material (P) diffused within the negative electrode active material layer penetrates between particles of the negative electrode active material, between particles of the negative electrode active material and the conductive additive, and between particles of the conductive additive, thereby partially blocking the conductive path.

[0020] Furthermore, when the polymer material (P) decomposes, the distance between particles of the negative electrode active material increases, thereby increasing the negative electrode resistance, and as a result, the short-circuit current is suppressed.

[0021] (Average Area) The average area S of the island regions is 200 μm 2 The average area S of the island regions may be 200 μm or less. 2 When the average area S of the island regions is 120 μm or less, the dispersibility of the polymer material (P) is high, and the polymer material (P) can be sufficiently diffused in a wide region in the negative electrode active material layer at high temperatures, thereby significantly increasing the resistance. 2It may be less than 60 μm 2 It may be less than 50 μm 2 It may be less than 30 μm 2 The lower limit of the average area S of the island regions is not particularly limited, but may be, for example, 10 μm 2 More than that is fine.

[0022] Here, the average area S of the island regions is the area obtained by dividing the total area St of the island regions included in the observation region of the cross section of the negative electrode active material layer at each temperature by the number N of island regions included in the observation region. However, the size of the observation region is set to a size that allows at least 30 island regions to be observed. The height of the observation region may be, for example, the same as the thickness (T) of the negative electrode active material layer, or may be 80% or more of the thickness. The length (W) of the observation region in the surface direction of the negative electrode active material layer may be 200 μm to 500 μm. In this case, the size of the observation region is T×W (μm ２ ) becomes.

[0023] The average area S will have the same value regardless of the season if measured at room temperature (e.g., 15°C to 40°C). Hereinafter, unless otherwise specified, the dimensions of the island regions refer to the dimensions at room temperature (e.g., 15°C to 40°C).

[0024] (Average Perimeter) Next, the average perimeter of the island regions is desirably 60 μm or less. When the average perimeter of the island regions is 60 μm or less, the size of the multiple island regions is considered to be sufficiently small. When the average perimeter is sufficiently small, the dispersibility of the polymer material (P) is sufficiently high, and the polymer material (P) can diffuse over a wide region in the negative electrode active material layer at high temperatures due to the occurrence of an internal short circuit. Therefore, the negative electrode resistance can be significantly increased. The average perimeter of the island regions may be 60 μm or less, 40 μm or less, 30 μm or less, or 20 μm or less. The lower limit of the average perimeter of the island regions is not particularly limited, but may be, for example, 1 μm or more.

[0025] Here, the average perimeter of the island-like regions is the total perimeter of the island-like regions included in the observation region of the cross section of the negative electrode active material layer divided by the number of island-like regions included in the observation region, N. However, the size of the observation region is the same as that when determining the average area of ​​the island-like regions.

[0026] (Aggregated Particle Area) Next, the average area of ​​the three largest island regions in the negative electrode active material layer (hereinafter also referred to as "aggregated particle area") was 900 μm ２ The area of ​​the agglomerated particles in the island region is preferably 900 μm or less. 2 When the area of ​​the agglomerated particles in the island region is 800 μm or less, it can be said that the dispersibility of the polymer material (P) is sufficiently high. Therefore, the polymer material (P) can be diffused over a wide region in the negative electrode active material layer at high temperatures due to the occurrence of an internal short circuit, and the resistance can be significantly increased. 2 It may be less than 700 μm 2 It may be less than 400 μm 2 The lower limit of the aggregated particle area of ​​the island region is not particularly limited, but may be, for example, 100 μm 2 The three island-like regions with the largest area are considered to be aggregation regions of the polymer material (P) or aggregation regions of the polymer material (P) with the negative electrode active material or the conductive additive. By sufficiently reducing the area of ​​such aggregation regions, the dispersibility of the polymer material (P) can be significantly improved.

[0027] Here, the agglomerated particle area of ​​an island region is defined as the total area of ​​the largest, second largest, and third largest island regions included in an observation region of a cross section of the negative electrode active material layer, divided by 3. The size of the observation region is the same as when determining the average area of ​​the island regions. However, the agglomerated particle area of ​​an island region is determined as the average value of three or more agglomerated particle areas determined in at least three different observation regions (cross sections).

[0028] The average area, average perimeter, and aggregate particle area of ​​the island regions can be measured as follows.

[0029] (1) Preparation of a Cross Section of the Negative Electrode Active Material Layer First, a negative electrode to be measured is prepared. Next, the negative electrode active material 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 active material layer and cured. For example, a cross section sample of the negative electrode active material layer is obtained by a CP (cross section polisher) method, an FIB (focused ion beam) method, or the like.

[0030] 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 amount of electricity in a fully charged battery. 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.

[0031] (2) Imaging of a cross section of the negative electrode active material layer with a scanning electron microscope (SEM) Next, the cross section sample of the negative electrode active material layer is observed with an SEM. The SEM observation is performed at a magnification of, for example, 100 to 500 times. The SEM image is taken so that a region having a length of 200 μm or more (preferably 300 μm or more) in the surface direction of the negative electrode active material layer is observed.

[0032] (3) Elemental Analysis Elemental analysis is performed using EDX (energy dispersive X-ray spectroscopy) or EPMA (electron probe microanalyzer) using an SEM image of a cross-sectional sample. Components derived from the polymer material (P) (e.g., elemental sulfur) are extracted from the cross-sectional analysis data to obtain a mapping image of the polymer material (P). By analyzing the mapping image using image analysis software, the average area, average perimeter, and aggregate particle area of ​​the island regions can be calculated. For example, when EDX analysis is performed on each region divided into 256 x 256 observation areas, a mapping image can be obtained by binarizing the region so that components derived from the polymer material (P) (e.g., elemental sulfur) are black and the other regions are white. When a set of two or more vertically, horizontally, or diagonally adjacent black regions is defined as an island region, the length of the outline surrounding the set is the perimeter of the island region. When a single black region is defined as an island region, the length of the outline surrounding that single black region is the perimeter of the island region, and the area enclosed by each outline is the area of ​​the island region.

[0033] (4) Analysis Conditions The following are the desirable SEM-EPMA measurement conditions for analyzing a cross-section sample of the negative electrode active material layer. The processing device used is a JEOL IB-19520CCP (cross-section polisher). The processing conditions are an acceleration voltage of 6 kV, a current value of 150 to 180 μA, and a vacuum of 5×10 -４ ~2 x 10 －３ Let it be Pa.

[0034] The measurement equipment used was a JEOL JSM-7900F (field emission scanning electron microscope) and a JEOL XM-86030 (EPMA detector). The acceleration voltage during analysis was 8 kV, and the SEM acceleration voltage was 2 kV. The electron beam current was 5 × 10 －８ A, and the observation magnification is 300. The measurement time is 30 ms.

[0035] In the binarization process, areas where the X-ray detection count of components (e.g., elemental sulfur) derived from the polymer material (P) detected under the above conditions is 50 cps or more and the signal / noise intensity ratio is 1.5 or more are binarized as black, and other areas are binarized as white.

[0036] [Polymer Material (P)] Desirable polymer materials (P) will now be described by way of example.

[0037] The melting point or thermal decomposition temperature (diffusion temperature) of the polymer material (P) is 200°C or higher and 500°C or lower. The negative electrode active material layer usually contains a polymer material that functions as a binder in addition to the polymer material (P). However, the binder has stronger binding properties than the polymer material (P). The binder does not need to form island regions. In other words, the negative electrode active material layer may contain a polymer material that forms island regions and a polymer material that does not form island regions. The binder may also be a polymer that does not melt or thermally decompose at a diffusion temperature of 200°C or higher and 500°C or lower. If the diffusion temperature of the polymer material (P) is lower than 200°C, the polymer material (P) may melt or decompose during normal battery use or during the manufacturing process, leading to deterioration of battery performance. If the diffusion temperature of the polymer material (P) exceeds 500°C, the diffusion rate of the polymer material (P) decreases as the temperature increases, and therefore a sufficient resistance increase effect cannot be obtained.

[0038] (Average particle diameter (D50)) The polymer material (P) is in a powder state before being mixed into the negative electrode mixture. The average particle diameter (D50) of the particles of the polymer material (P) constituting the powder is desirably 50 μm or less. When the average particle diameter (D50) is 50 μm or less, it becomes easier to control the aforementioned average area, average perimeter, agglomerated particle area, Voronoi variance value, etc. within desired numerical ranges. The average particle diameter (D50) is more desirably 40 μm or less, and even more desirably 30 μm or less. The lower limit of the average particle diameter (D50) is not particularly limited, but may be, for example, 1 μm or more.

[0039] Here, the average particle diameter (D50) of the polymer material (P) refers to the median diameter at which the cumulative volume is 50% in the volume-based particle size distribution of the polymer material (P). The volume-based particle size distribution of the polymer material (P) can be measured, for example, using a laser diffraction / scattering particle size distribution measuring device. The sample of the polymer material (P) to be measured may be prepared by separating it from the negative electrode active material layer, or, if the raw polymer material (P) before being mixed into the negative electrode active material layer is available, the raw polymer material (P) may be used.

[0040] (BET specific surface area) The BET specific surface area of ​​the polymer material (P) is 10 m ２ / g or less. The BET specific surface area is influenced by the particle size of the polymer material (P), but is also greatly influenced by the state of the particles themselves. For example, when the particle surface has many irregularities or when the particles themselves are porous, the BET specific surface area will be large even if the average particle size (D50) is small. ２ / g or less, it is considered that the particle surface of the polymer material (P) has few irregularities, or the particles themselves do not contain many voids. In this case, it is presumed that a strong expansion stress acts on the polymer material (P) when heated, thereby enhancing the diffusibility when heated. ２ / g or less, and 2m ２ / g or less, and 1.8m ２ / g or less.

[0041] Here, the BET specific surface area of ​​the polymer material (P) is measured by a gas adsorption method (BET single-point method). Nitrogen gas is used as the gas. Details of the BET method may conform to JIS R1626. A sample of the polymer material (P) to be measured may be prepared by separating it from the negative electrode active material layer, or, if the raw polymer material (P) before being mixed into the negative electrode active material layer is available, the raw polymer material (P) may be used.

[0042] (Weight-average molecular weight (Mw)) The weight-average molecular weight (Mw) of the polymer material (P) preferably satisfies Mw<50,000, and more preferably satisfies 5,000≦Mw<50,000. When the weight-average molecular weight (Mw) satisfies Mw<50,000 (and even 5,000≦Mw<50,000), the melting rate of the polymer material (P) during heating is increased, dispersibility is improved, and thermal decomposition is facilitated, thereby enhancing the effect of increasing the resistance of the negative electrode active material layer. Mw is more preferably 30,000 or less, even more preferably 20,000 or less, and may be 15,000 or less.

[0043] The weight average molecular weight (Mw) of the polymeric material (P) can be measured, for example, by ultra-high temperature gel permeation chromatography (GPC), where Mw is the weight average molecular weight based on polystyrene.

[0044] The measurement conditions for ultra-high temperature GPC are shown below. The measurement device used is an Ultra-High Temperature GPC SSC-7110. One TSKguard column HHR(S)HT and two TSKgel GMHHR-H(S)HT (7.8 mm ID * 30 cm) columns are used. A differential refractometer (RI detector) is used as the detector. Sample pretreatment involves weighing a sample of polymer material (P), adding a predetermined amount of 1-chloronaphthalene (1-CN) as a solvent, and dissolving by heating at 250°C for 1 hour. Then, hot filtration is performed using a PTFE filter with a pore size of 0.5 μm. The sample injection volume is 500 μL of a solution with a sample concentration of 2 mg / mL. The flow rate is 1.0 mL / min, and the measurement temperature is set to 210°C for the column and 250°C for the pre-oven. Polystyrene is used as the molecular weight standard.

[0045] The low molecular weight side of the obtained data (peak) may partially overlap with the blank derived from the GPC eluent, but the entire peak including this overlap is analyzed to calculate the weight average molecular weight.

[0046] (Specific Examples of Polymer Material (P)) Preferred examples of the polymer material (P) will be described below. Among the polymer materials (P), a polymer material (P) having a main chain containing a phenylene group and a sulfide group (hereinafter also referred to as a "polymer material (PS)") has high chemical stability and is unlikely to bond with other materials, and therefore does not excessively coat the particles of the negative electrode active material in the negative electrode active material layer, making it easy to form island regions. Furthermore, the polymer material (PS) undergoes a large change in state when melted at or above its melting point, and can exhibit high diffusibility in the negative electrode active material layer. When the polymer material (PS) is used, sulfur atoms can be used as a component derived from the polymer material (P) in elemental analysis such as SEM-EDX and SEM-EPMA.

[0047] The main chain of the polymer material (PS) may be, for example, of the formula:

[0048]

[0049] (n is any integer). In polymers, n is typically 20 or more, or 30 or more. Such a simple structure has high chemical stability and hardly causes side reactions in the negative electrode active material layer. In addition, the polymer material (PS) having the structure represented by the above formula is typically a linear polymer without a crosslinked structure, has a high degree of freedom when melted, and also has high diffusibility in the negative electrode active material layer after melting. The polymer material (PS) having such a structure is highly insulating and is thought to have a significant effect of blocking conductive paths when it penetrates between particles of the negative electrode active material, between particles of the negative electrode active material and the conductive additive, and between particles of the conductive additive.

[0050] A more specific example of the polymer material (PS) is polyphenylene sulfide (hereinafter also referred to as "PPS").

[0051] The polymer material (P) may contain, by mass, 10 ppm to 1000 ppm, or even 10 ppm to 500 ppm of chloride ions. The amount of chloride ions contained in the polymer material (P) can be measured, for example, by peeling the negative electrode active material layer from the negative electrode, separating the polymer material (P) from the negative electrode active material layer, and analyzing it by ion chromatography using ion-exchanged water as a solvent.

[0052] The content of the polymer material (P) in the negative electrode active material layer may be, for example, 0.01% by mass to 5% by mass, 0.1% by mass to 5% by mass, 0.3% by mass to 3% by mass, or 0.5% by mass to 2% by mass.

[0053] The polymer material (P) may be unevenly distributed on the outermost surface side (i.e., the separator side) of the negative electrode active material layer. Internal short circuits in batteries often begin to expand on the separator side of the negative electrode active material layer. Internal short circuits occur, for example, when conductive foreign matter penetrates the separator. By unevenly distributing the polymer material (P) on the separator side of the negative electrode active material layer, a high-resistance region can be formed on the separator side of the negative electrode active material layer in the early stages after a short circuit occurs. This makes it possible to suppress the expansion of the short-circuited portion.

[0054] More specifically, when the thickness of the negative electrode active material layer is T, the existence probability Pb ​​of the polymer material (P) present in the region of the negative electrode active material layer from the surface of the negative electrode current collector to 0.5 T and the existence probability Pt of the polymer material (P) present in the region of the negative electrode active material layer from a position 0.5 T from the surface of the negative electrode current collector to the outermost surface (i.e., the surface of the negative electrode active material layer on the separator side) may satisfy, for example, 1 < Pt / Pb, or may satisfy 1.2 ≦ Pt / Pb, or may satisfy 2 ≦ Pt / Pb.

[0055] On the other hand, the polymer material (P) may be unevenly distributed on the negative electrode current collector side, rather than on the outermost surface side (i.e., the separator side) of the negative electrode active material layer. The short-circuit current of the battery tends to flow largely on the negative electrode current collector side of the negative electrode active material layer, which has the lowest resistance. By unevenly distributing the polymer material (P) on the negative electrode current collector side of the negative electrode active material layer, the low-resistance region of the negative electrode active material layer can be selectively made high-resistance. Therefore, an increase in short-circuit current can be suppressed.

[0056] More specifically, when the thickness of the negative electrode active material layer is T, the existence probability Pb ​​of the polymer material (P) present in the region of the negative electrode active material layer from the surface of the negative electrode current collector to 0.5T and the existence probability Pt of the polymer material (P) present in the region of the negative electrode active material layer from a position 0.5T from the surface of the negative electrode current collector to the outermost surface (i.e., the surface of the negative electrode active material layer on the separator side) may, for example, satisfy 1<Pb / Pt, or may satisfy 1.2≦Pb / Pt, or may satisfy 2≦Pb / Pt. Note that, for example, when 0.97≦Pb / Pt≦1.03, it can be considered that Pb=Pt substantially.

[0057] Hereinafter, the region of the negative electrode active material layer from the surface of the negative electrode current collector to 0.5 T will be referred to as the "lower layer region," and the region of the negative electrode active material layer from the position 0.5 T from the surface of the negative electrode current collector to the outermost surface will also be referred to as the "upper layer region." That is, when the negative electrode active material layer is equally divided into two regions in the thickness direction, the region closer to the negative electrode current collector is the lower layer region, and the region farther from the negative electrode current collector is the upper layer region. The lower layer region and the upper layer region have the same thickness.

[0058] When 1<Pt / Pb is satisfied, the "observation region" may be set in a region as close to the center as possible in the thickness direction of the "upper layer region," including the center, and the height of the "observation region" may be the same as the thickness (T / 2) of the upper layer region. On the other hand, when 1<Pb / Pt is satisfied, the "observation region" may be set in a region as close to the center as possible in the thickness direction of the "lower layer region," including the center, and the height of the "observation region" may be the same as the thickness (T / 2) of the lower layer region. Furthermore, when the above-described uneven distribution of the polymer material (P) is not substantially observed, the "observation region" may be set in a region as close to the center as possible in the thickness T of the entire negative electrode active material layer, including the center (a point 0.5T from the surface of the negative electrode current collector).

[0059] As described above, when the polymer material (P) is unevenly distributed in one of the "lower layer region" and the "upper layer region," the content of the polymer material (P) contained in the other region can be made very small. In other words, a small amount of the polymer material (P) can efficiently increase the resistance of the negative electrode active material layer during heating, thereby suppressing the expansion of short-circuited portions and the increase in short-circuit current. Such a configuration is desirable in that it makes it easier to maintain a high capacity of the negative electrode.

[0060] Pb and Pt can be measured by analyzing a cross-sectional sample of the negative electrode obtained by the above-mentioned method using SEM and EDX (or EPMA). Specifically, a mapping image of the polymer material (P) obtained from EDX analysis data of the cross-sectional sample is prepared. In the mapping images of the lower layer region and the upper layer region, the area occupied by components derived from the polymer material (P) (e.g., elemental sulfur) is measured as the area of ​​the polymer material (P). The proportions of the area of ​​the polymer material (P) to the area of ​​the lower layer region and the upper layer region are then regarded as Pb and Pt, respectively. Pb and Pt are measured in a rectangular observation region defined by a length of 300 μm in the surface direction of the negative electrode active material layer and a thickness T of the negative electrode active material layer. Pb and Pt may each be an average value of Pb and Pt determined in multiple (e.g., three or more) observation regions.

[0061] The polymer material (P) may be unevenly distributed on the outermost surface side (i.e., the separator side) of the negative electrode active material layer, for example, in a region up to a depth of 20 μm from the outermost surface, or may be unevenly distributed in a region up to a depth of 1 μm from the outermost surface. In this case, when the thickness of the negative electrode active material layer is T, the existence probability Pb(q) of the polymer material (P) existing in the region from the surface of the negative electrode current collector to 0.25 T in the negative electrode active material layer and the existence probability Pt(q) of the polymer material (P) existing in the region from a position 0.75 T from the surface of the negative electrode current collector to the outermost surface (i.e., the separator-side surface of the negative electrode active material layer) may satisfy 5≦Pt(q) / Pb(q)≦250.

[0062] <Method for Manufacturing Negative Electrode> Next, an example of a method for manufacturing a negative electrode according to an embodiment of the present disclosure will be described. The method includes the steps of: (I) preparing a negative electrode slurry containing a negative electrode active material, a binder, a thickener, a polymer material (P), and a dispersion medium; (II) preparing a negative electrode current collector; (III) applying the negative electrode slurry to the surface of the negative electrode current collector to form a coating film; (IV) drying the coating film to form an unrolled layer; and (V) rolling the unrolled layer to form a negative electrode active material layer.

[0063] (I) Slurry Preparation Step The negative electrode slurry is prepared by mixing a negative electrode mixture containing a negative electrode active material, a binder, a thickener, and a polymer material (P) with a liquid dispersion medium and dispersing the mixture in the dispersion medium. The negative electrode mixture may further contain other components (e.g., a conductive additive). As the liquid dispersion medium, water or an organic solvent having excellent affinity with the binder may be used.

[0064] The average area, average perimeter, Voronoi variance, aggregate particle area, etc. of the island regions formed by the polymer material (P) can be controlled by the dispersion state of the polymer material (P) in the negative electrode slurry. The dispersibility of the polymer material (P) tends to be significantly improved by pre-dry mixing with the negative electrode active material. However, the preparation method of the negative electrode slurry is not particularly limited as long as the desired dispersion state of the polymer material (P) can be obtained. For example, the polymer material (P) may be pre-wet mixed with the conductive additive, and then the negative electrode active material may be added to the mixture and further mixed.

[0065] The dispersion medium is not particularly limited, but examples thereof include water, alcohols such as ethanol, ethers such as tetrahydrofuran, amides such as dimethylformamide, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0066] As the binder, it is desirable to use a resin material such as styrene-butadiene rubber, but there are no particular limitations as long as it has a high binding strength and is stable within the negative electrode active material layer.

[0067] (II) Step of Preparing a Negative Electrode Current Collector As the negative electrode current collector, a sheet-like conductive material (metal foil, mesh, net, punched sheet, etc.) is used. Among these, metal foil is preferred. 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.

[0068] (III) Step of forming a coating film of negative electrode slurry The coating film is formed by applying the negative electrode slurry to the surface of the negative electrode current collector. As an apparatus for applying the negative electrode slurry, for example, a bar coater, a gravure coater, a blade coater, a roll coater, a comma coater, a die coater, a lip coater, or the like is used. When the polymer material (P) is unevenly distributed in the lower layer region or the upper layer region of the negative electrode active material layer, a plurality of negative electrode slurries having different contents of the polymer material (P) may be prepared, and two or more layers of these slurries may be applied one on top of the other to vary the concentration of the polymer material (P) in the thickness direction.

[0069] (IV) Step of Drying the Coating Film Next, the coating film is dried to volatilize the dispersion medium, thereby forming an unrolled coating film. For example, when the coating film is dried at a temperature of 150°C or higher, or even 180°C or higher, the polymer material (P) is likely to be unevenly distributed on the separator side of the negative electrode active material layer rather than on the negative electrode current collector side.

[0070] (V) Rolling Step of Unrolled Layer Next, the unrolled layer is rolled to form a negative electrode active material layer. The conditions for rolling are not particularly limited. However, the density of the negative electrode active material in the negative electrode active material layer is, for example, 1.0 g / cm ３ 2.0g / cm or more ３ or less, and 1.5 g / cm ３ 1.7g / cm or more ３ The following is also acceptable.

[0071] B. Secondary Battery A nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure includes the above-described negative electrode, positive electrode, nonaqueous electrolyte, and a separator interposed between the negative electrode and the positive electrode. The nonaqueous electrolyte secondary battery may be a liquid secondary battery containing an electrolytic solution as the electrolyte, or may be an all-solid-state secondary battery containing a solid electrolyte as the electrolyte. The configuration of the secondary battery will be specifically described below using a lithium-ion secondary battery as an example.

[0072] [Negative Electrode] The negative electrode has the above-described characteristics. The negative electrode active material layer is composed of a negative electrode mixture. The negative electrode mixture contains, as essential components, a negative electrode active material, a binder, a thickener, and a polymer material (P), and may contain optional components. The optional components may include a conductive additive, etc.

[0073] The negative electrode active material includes a material that electrochemically absorbs and releases lithium ions. As the material that electrochemically absorbs and releases lithium ions, at least one material selected from the group consisting of carbon materials and silicon-containing materials is preferred. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Among these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of silicon-containing materials include elemental silicon, silicon and SiO ２ composite materials of silicon and lithium silicate, composite materials of silicon and carbon, etc.

[0074] When a carbon material and a silicon-containing material are used in combination, the content of the silicon-containing material relative to the total of the carbon material and the silicon-containing material is, for example, 1 mass % or more and 100 mass % or less, and may be 3 mass % or more and 15 mass % or less.

[0075] The average particle size (D50) of the carbon material particles is, for example, 5 μm to 30 μm, or may be 10 μm to 20 μm. The average particle size (D50) of the silicon-containing material is, for example, 3 μm to 18 μm, or may be 6 μm to 15 μm. Here, the average particle size (D50) refers to the median diameter at which the cumulative volume is 50% in a volume-based particle size distribution.

[0076] The average particle size of the negative electrode active material may be measured from a cross-sectional sample of the negative electrode active material layer described above. An SEM image of the cross section may be taken so that 10 or more particles of the negative electrode active material are observed, and the diameters of the circles having the same area as the cross sections of the 10 or more negative electrode active material particles may be determined by image processing, and the average value of these diameters may be used as the average particle size.

[0077] Examples of the conductive additive include carbon nanotubes (CNT), carbon fibers other than CNT, and conductive particles (for example, carbon black).

[0078] The thickener may be, for example, a water-soluble polymer, such as carboxymethyl cellulose (CMC) and its modified products (including salts such as Na salts), cellulose derivatives such as methyl cellulose (cellulose ethers, etc.), saponified polymers having vinyl acetate units such as polyvinyl alcohol, and polyethers (polyalkylene oxides such as polyethylene oxide, etc.).

[0079] The thickness of the negative electrode active material layer is not particularly limited, but may be, for example, 50 μm to 200 μm, or 75 μm to 150 μm. A single negative electrode active material layer may be formed by a plurality of layers having different morphologies. For example, two or more layers containing negative electrode active materials with different average particle sizes may be stacked, or two or more layers containing negative electrode active materials of different types or compositions may be stacked.

[0080] [Positive Electrode] The positive electrode comprises a positive electrode current collector and a positive electrode active material layer carried on one or both surfaces of the positive electrode current collector.

[0081] As the positive electrode current collector, a sheet-like conductive material (metal foil, mesh, net, punched sheet, etc.) is used. Among these, metal foil is preferred. Examples of the material for the positive electrode current collector include stainless steel, aluminum, aluminum alloy, 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.

[0082] The positive electrode active material layer is composed of a positive electrode mixture and may be referred to as a positive electrode mixture layer. The positive electrode mixture layer is in the form of a membrane or film. The positive electrode mixture contains particles of a positive electrode active material as an essential component, and may contain optional components such as a binder and a conductive additive.

[0083] The positive electrode mixture layer can be formed, for example, by applying a positive electrode slurry, in which the positive electrode mixture is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying the applied film. The dried coating film may be rolled as necessary.

[0084] The thickness of the positive electrode active material layer is not particularly limited, but may be, for example, 50 μm to 150 μm, or 75 μm to 125 μm. A single positive electrode active material layer may be formed by a plurality of layers having different morphologies. For example, two or more layers containing positive electrode active materials with different average particle sizes may be stacked, or two or more layers containing positive electrode active materials of different types or compositions may be stacked.

[0085] The density of the positive electrode active material in the positive electrode active material layer is, for example, 3.3 g / cm ３ 4.0g / cm or more ３ or less, and 3.5 g / cm ３ 4.0g / cm or more ３ The following is also acceptable.

[0086] The average particle size (D50) of the positive electrode active material particles is, for example, 1 μm to 50 μm, and may be 5 μm to 25 μm. Here, the average particle size (D50) refers to the median diameter at which the cumulative volume is 50% in the volume-based particle size distribution.

[0087] The positive electrode active material may contain a lithium-containing transition metal oxide. From the viewpoint of increasing capacity, the lithium-containing transition metal oxide preferably contains lithium nickel oxide (composite oxide N) containing lithium and Ni and having a layered rock salt crystal structure. The proportion of the composite oxide N in the positive electrode active material may be, for example, 70 mass % or more, 90 mass % or more, or even 95 mass % or more. The proportion of Ni in the metal elements other than Li contained in the composite oxide N may be 50 atomic % or more.

[0088] The composite oxide N is, for example, a compound represented by the formula (1): Li α Niｘ１ M1 ｘ２ M2 （１－ｘ１－ｘ２） O ２＋β Here, element M1 is at least one selected from the group consisting of V, Co, and Mn. Element M2 is at least one selected from the group consisting of Mg, Al, Ca, Ti, Cu, Zn, and Nb. However, formula (1) satisfies the following conditions: 0.95≦α≦1.05, −0.05≦β≦0.05, 0.5≦x1<1, 0≦x2≦0.5, 0<1−x1−x2≦0.5. α increases or decreases with charge and discharge.

[0089] The composite oxide N contains Ni and may contain at least one element selected from the group consisting of Co, Mn, and Al as the element M1 and the element M2. Co, Mn, and Al contribute to stabilizing the crystal structure of the composite oxide N.

[0090] From the viewpoint of reducing costs and increasing capacity, the proportion of Co in the metal elements other than Li contained in the composite oxide N is preferably 0 atomic % or more and 20 atomic % or less, and more preferably 0 atomic % or more and 15 atomic % or less.

[0091] The proportion of Mn in the metal elements other than Li may be 10 atomic % or less, or 5 atomic % or less, or 1 atomic % or more, or 3 atomic % or more, or 5 atomic % or more.

[0092] 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, or 3 atomic % or more, or 5 atomic % or more.

[0093] As the binder contained as an optional component in the positive electrode active material layer, it is desirable to use a fluororesin, hydrogenated nitrile butadiene rubber, or the like, but there are no particular limitations as long as it has high binding strength and is stable within the positive electrode active material layer. Examples of fluororesins include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), ethylene-tetrafluoroethylene copolymer (ETFE), vinylidene fluoride-hexafluoropropylene copolymer, etc. Among these, PVDF is preferred.

[0094] Examples of conductive additives that may be optionally contained in the positive electrode active material layer include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black and graphite).

[0095] [Separator] The separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. Examples of the separator include a microporous thin film, a woven fabric, and a nonwoven fabric. The separator is made of a polyolefin such as polypropylene or polyethylene.

[0096] The thickness of the separator may be, for example, 5 μm to 25 μm.

[0097] The separator may have a substrate layer and a heat-resistant layer laminated on the substrate layer. The substrate layer may be made of a microporous thin film, a woven fabric, a nonwoven fabric, or the like. The heat-resistant layer may be provided on at least one surface of the substrate layer. It is desirable that the heat-resistant layer be provided at least on the positive electrode side of the separator.

[0098] Here, the melting point or thermal decomposition temperature (diffusion temperature) of the polymer material (P) is desirably equal to or higher than the melting point of the substrate layer. A thermoplastic resin having a melting point is usually used as the material for the substrate layer. Among these, polyolefins such as polypropylene and polyethylene are preferably used. A substrate layer made of a thermoplastic resin is generally provided with a shutdown function that closes the voids in the substrate layer when the temperature exceeds a predetermined temperature. In other words, until the shutdown function is activated, current is allowed to flow through the battery, and the battery is required to have sufficient performance as a power source. If the diffusion temperature of the polymer material (P) is lower than the melting point of the substrate layer, the polymer material (P) may increase the resistance of the negative electrode active material layer, potentially making it impossible to ensure sufficient performance as a power source.

[0099] The heat-resistant layer may contain an inorganic oxide filler as a main component (e.g., 80% by mass or more of the heat-resistant layer) or a heat-resistant resin as a main component (e.g., 40% by mass or more of the heat-resistant layer). The heat-resistant resin may be a polyamide resin such as aromatic polyamide (aramid), a polyimide resin, or a polyamide-imide resin.

[0100] The thickness of the substrate layer may be, for example, 0% to 100% of the thickness of the separator, for example, 5 μm to 20 μm. When the thickness of the substrate layer is 0% (no substrate layer), a separator composed only of a heat-resistant layer may be used.

[0101] The thickness of the heat-resistant layer may be, for example, 0% to 100% of the thickness of the separator, for example, 0 μm to 15 μm.

[0102] [Non-aqueous electrolyte] The non-aqueous electrolyte of the lithium ion secondary battery has lithium ion conductivity. The non-aqueous electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0103] As the solid electrolyte, for example, a solid or gel-like polymer electrolyte, an inorganic solid electrolyte, etc. can be used. As the inorganic solid electrolyte, a material known in all-solid-state lithium ion secondary batteries, etc. (for example, an oxide-based solid electrolyte, a sulfide-based solid electrolyte, a halogen-based solid electrolyte, etc.) can be used. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. As the matrix polymer, for example, a polymer material that absorbs the non-aqueous solvent and gels is used. As the polymer material, for example, a fluororesin, an acrylic resin, a polyether resin, etc. can be used.

[0104] The electrolyte solution contains a non-aqueous solvent and an electrolyte salt. In the case of a lithium ion secondary battery, the electrolyte salt contains at least a lithium salt. The concentration of the lithium salt in the electrolyte solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The non-aqueous electrolyte solution may contain known additives.

[0105] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, and cyclic carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC) and ethylene carbonate (EC). 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). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0106] 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, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2 etc.), lithium salts of fluorine-containing acid imides (LiN(SO ２ F) ２, LiN(CFSO)2, LiN(CFSO)(CFSO), LiN(CFSO)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc. The lithium salt may be used alone or in combination of two or more.

[0107] An example of the structure of a 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 exterior body together with an electrolyte solution. However, this is not limited thereto, and other forms of electrode groups may also be applied. 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.

[0108] The structure of the 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.

[0109] 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.

[0110] (Additional Notes) The above description discloses the following technology: (Technology 1) A non-aqueous electrolyte secondary battery comprising: a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, wherein the negative electrode active material layer contains a negative electrode active material, a binder, a thickener, and an additive, wherein the additive is a polymer material having a melting point or a thermal decomposition temperature of 200°C or higher and 500°C or lower, wherein, when a thickness of the negative electrode active material layer is T, an existence probability Pb ​​of the polymer material present in a region of the negative electrode active material layer from the surface of the negative electrode current collector to 0.5T, and an existence probability Pt of the polymer material present in a region of the negative electrode active material layer from a position 0.5T from the surface of the negative electrode current collector to the outermost surface satisfy the relationship 1<Pb / Pt. (Technology 2) The nonaqueous electrolyte secondary battery according to Technology 1, wherein the polymer material has an average particle size of 50 μm or less. (Technology 3) The polymer material has a BET specific surface area of ​​10 m ２ / g or less. (Technology 4) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 3, wherein the weight average molecular weight (Mw) of the polymer material satisfies Mw<50,000. (Technology 5) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 4, wherein the polymer material has a main chain containing a phenylene group and a sulfide group. (Technology 6) The main chain has a formula: (Technology 7) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 5, wherein the polymer material comprises a repeating structure represented by the formula (n is any integer). (Technology 8) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 7, wherein the polymer material comprises 10 ppm to 1000 ppm by mass of chloride ions. (Technology 9) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 8, wherein the polymer material is dispersed in the cross section of the negative electrode active material layer, forming a plurality of island regions, and the island regions have an average perimeter of 60 μm or less. (Technology 10) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 8, wherein the polymer material is dispersed in the cross section of the negative electrode active material layer, forming a plurality of island regions, and the average area of ​​the three largest island regions is 900 μm or less. ２ The nonaqueous electrolyte secondary battery according to any one of the first to ninth aspects of the present invention is characterized in that the polymer material is dispersed in the cross section of the negative electrode active material layer to form a plurality of island regions, and the average area of ​​the island regions is 200 μm 2The nonaqueous electrolyte secondary battery according to any one of Techniques 1 to 10, wherein: (Technology 12) The nonaqueous electrolyte secondary battery according to any one of Techniques 1 to 11, wherein a content of the polymer material contained in the negative electrode active material layer is 0.01% by mass or more and 5% by mass or less; (Technology 13) The nonaqueous electrolyte secondary battery according to any one of Techniques 1 to 12, wherein the negative electrode active material includes at least one material selected from the group consisting of a carbon material and a silicon-containing material; (Technology 14) The nonaqueous electrolyte secondary battery according to any one of Techniques 1 to 13, wherein the separator has a substrate layer and a heat-resistant layer laminated on the substrate layer, and wherein the melting point or thermal decomposition temperature of the polymer material is equal to or higher than the melting point of the substrate layer. (Technology 15) A negative electrode for a secondary battery, comprising: a negative electrode current collector; and a negative electrode active material layer supported on the negative electrode current collector, wherein the negative electrode active material layer contains a negative electrode active material, a binder, a thickener, and an additive, wherein the additive is a polymer material having a melting point or a thermal decomposition temperature of 200°C or higher and 500°C or lower, and wherein, when a thickness of the negative electrode active material layer is T, an existence probability Pb ​​of the polymer material present in a region of the negative electrode active material layer from a surface of the negative electrode current collector to 0.5T, and an existence probability Pt of the polymer material present in a region of the negative electrode active material layer from a position 0.5T from the surface of the negative electrode current collector to the outermost surface satisfy 1<Pb / Pt.

[0111] Hereinafter, the present disclosure will be specifically described based on experimental samples, but the present disclosure is not limited to the following examples.

[0112] <Sample R1> [Preparation of Positive Electrode] The positive electrode active material was a composite oxide N (LiNi) having an average particle diameter of 13 μm. 0.85 Co 0.10 Al 0.05 O2) was used. Furthermore, an aluminum foil having a thickness of 15 μm was prepared as a positive electrode current collector.

[0113] NMP was added to a positive electrode mixture containing a positive electrode active material, acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 95:2.5:2.5, and the mixture was stirred to prepare a positive electrode slurry.

[0114] The positive electrode slurry was applied to the surface of an aluminum foil serving as a positive electrode current collector, and the coating was dried to form an unrolled layer on both sides of the aluminum foil.

[0115] Next, the unrolled layer was rolled to obtain a positive electrode active material having a density of 3.6 g / cm ３ The total thickness of the positive electrode after rolling was 160 μm.

[0116] [Fabrication of Negative Electrode] A negative electrode was fabricated using a negative electrode active material containing graphite, sodium carboxymethyl cellulose (CMC-Na), and styrene-butadiene rubber (SBR) in a mass ratio of 96:2:2, and polyphenylene sulfide (PPS) (average particle size (D50) = 10 μm, weight-average molecular weight 28,000, BET specific surface area 1.1 m). ２ Water was added to a negative electrode mixture containing 2.6 mass % of ammonium nitrate (NaCl, ZnO, ZnS, ZnO ...

[0117] Next, the negative electrode slurry was applied to the surface of a copper foil serving as a negative electrode current collector, the coating was dried, and then rolled to form negative electrode active material layers on both sides of the copper foil. At this time, the drying temperature was controlled to 50°C so that the distribution of PPS in the negative electrode active material layer would not be unevenly distributed in the lower layer region or the upper layer region, and the negative electrode active material layer was left to dry.

[0118] The density of the negative electrode active material in the negative electrode active material layer is 1.6 g / cm ３ The total thickness of the negative electrode was 170 μm.

[0119] The average area of ​​the island regions formed of the polymer material (P) determined by the above-mentioned method was 11 μm ２ It was.

[0120] [Preparation of Nonaqueous Electrolyte] A nonaqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 3:7.

[0121] [Preparation of Secondary Battery] An electrode group was prepared by attaching a tab to each electrode and spirally winding the positive and negative electrodes with a separator (14 μm thick) interposed therebetween so that the tab was located at the outermost periphery. The separator had a 10 μm-thick microporous thin film substrate layer made of polyethylene and a 4 μm-thick heat-resistant layer laminated on one side (the positive electrode side) of the substrate layer. The melting point of the microporous thin film was 150°C. The heat-resistant layer contained 20% by mass of inorganic oxide filler (alumina), with the remainder composed of aromatic polyamide (aramid). The electrode group was inserted into an exterior case made of aluminum laminate film and vacuum-dried at 105°C for 2 hours. After that, an electrolyte solution was poured in, and the opening of the exterior case was sealed to obtain secondary battery sample R1.

[0122] <<Sample R2>> A secondary battery sample R2 was produced in the same manner as sample R1, except that the content of the polymer material (P) in the negative electrode mixture was changed to 1.3 mass %.

[0123] <<Sample R3>> A secondary battery sample R3 was produced in the same manner as sample R1, except that the negative electrode mixture did not contain the polymer material (P).

[0124] Sample A1 A secondary battery sample A1 was produced in the same manner as sample B1, except that the PPS content in the negative electrode mixture in the lower layer region was 2.6 mass % and the PPS content in the negative electrode mixture in the upper layer region was 0 mass %.

[0125] For Sample A1, a negative electrode mixture slurry for the upper layer region and a negative electrode mixture slurry for the lower layer region were prepared. Only the negative electrode mixture for the lower layer region contained a polymer material (P). The negative electrode slurry for the lower layer region was then applied to the negative electrode current collector to a thickness half that of Sample R1, and then the negative electrode slurry for the upper layer region was applied to a thickness half that of Sample R1. Thereafter, the negative electrode mixture was dried in the same manner as Sample R1, and the unrolled layer was rolled to a density of 1.6 g / cm . ３ A negative electrode active material layer of 170 μm thick was formed on the negative electrode.

[0126] [Evaluation] <Safety> The following evaluations were performed on the obtained samples. (a) In an environment of 25°C, the battery sample was charged at a constant current of 0.3 It until the voltage reached 4.2 V, and then at a constant voltage of 4.2 V, the battery sample was charged at a constant voltage of 0.05 It until the current reached 0.05 It. (b) In an environment of 25°C, the tip of a round nail (diameter 2.7 mm) was contacted with the center of the battery sample charged in (a) and pierced at a speed of 1 mm / sec. Immediately after detecting a battery voltage drop (Δ50 mV) due to an internal short circuit, the piercing of the round nail was stopped. The surface temperature of the battery sample was measured 1 minute after the short circuit. The surface temperatures of each sample are shown in Table 1.

[0127] <Discharge Capacity> In an environment of 25°C, the battery sample was charged at a constant current of 0.3 It until the voltage reached 4.2 V, and then charged at a constant voltage of 4.2 V until the current reached 0.05 It. Next, the battery was discharged at a constant current of 0.3 It until the voltage reached 2.5 V. The same charge / discharge cycle was repeated twice, and the second discharge capacity was determined. Table 1 shows the discharge capacities of samples A1, R2, and R3 as relative values ​​(%) when the discharge capacity of sample R3 was taken as 100%.

[0128]

[0129] It can be seen from Table 1 that when the polymer material (P) is unevenly distributed in the lower layer region, the safety of the battery can be efficiently improved by using a smaller amount of the polymer material (P).

[0130] Up to this point, an example in which a polymer material (P) is added to the negative electrode active material layer in advance has been described. However, the manufacturing method of the negative electrode active material layer according to this embodiment is not limited to such a manufacturing method. A first layer may be formed by applying a slurry containing no negative electrode active material but a polymer material (P) to the surface of the negative electrode current collector in advance, and then a negative electrode active material layer may be formed by further applying a negative electrode slurry containing a negative electrode active material but not a polymer material (P) so as to cover the first layer. In this case, a negative electrode active material layer consisting of two layers can be obtained. Note that although the first layer does not contain a negative electrode active material, for convenience, the two layers will be collectively referred to as the negative electrode active material layer.

[0131] Samples B1 to B5 Secondary battery samples B1 to B5 were prepared in the same manner as sample A1, except that the average area was changed as shown in Table 2 by changing the content of the polymer material (P) contained in the negative electrode mixture and the stirring conditions when preparing the negative electrode slurry for the lower layer region, and were evaluated in the same manner.

[0132]

[0133] In Table 2, the surface temperatures of samples B1 to B5 are significantly lower than that of sample R3, and among these, the surface temperatures of samples B1 to B4 are particularly low. ２ It is found that the following are preferred:

[0134] Samples C1 to C5 Secondary battery samples C1 to C5 were prepared in the same manner as sample A1, except that the average perimeter of the island-like regions was changed as shown in Table 3 by appropriately changing the content of the polymer material (P) contained in the negative electrode mixture and the stirring conditions when preparing the negative electrode slurry for the lower layer region, and were evaluated in the same manner.

[0135]

[0136] In Table 3, the surface temperatures of samples C1 to C5 are significantly lower than that of sample R3, and among these, the surface temperatures of samples C1 to C4 are particularly low. Therefore, it is clear that the average perimeter of the island regions is preferably 60 μm or less.

[0137] Examples D1 to D5 Secondary battery samples D1 to D5 were produced in the same manner as sample A1, except that the aggregated particle area of ​​the island regions was changed as shown in Table 4 by appropriately changing the content of the polymer material (P) contained in the negative electrode mixture and the stirring conditions when preparing the negative electrode slurry for the lower layer region, and were evaluated in the same manner.

[0138]

[0139] In Table 4, the surface temperatures of samples D1 to D5 are significantly lower than that of sample R3, and among these, the surface temperatures of samples D1 to D4 are particularly low. ２It is found that the following are preferred:

[0140] Examples E1 to E5 Secondary battery samples E1 to E5 were prepared in the same manner as sample A1, except that the content and average particle diameter (D50) of the polymer material (P) contained in the negative electrode mixture were changed as shown in Table 5, and were evaluated in the same manner.

[0141]

[0142] In Table 5, the surface temperatures of samples E1 to E5 are lower than that of sample R3, and among these, the surface temperatures of samples E1 to E4 are particularly low. Therefore, it is clear that the average particle size (D50) of the polymer material (P) is preferably 50 μm or less.

[0143] Examples F1 to F5 Secondary battery samples F1 to F5 were produced in the same manner as sample A1, except that the content of the polymer material (P) contained in the negative electrode mixture and the BET specific surface area were changed as shown in Table 6, and were evaluated in the same manner.

[0144]

[0145] In Table 6, the surface temperatures of samples F1 to F5 are significantly lower than that of sample R3, and among these, the surface temperatures of samples F1 to F4 are particularly low. Therefore, the BET specific surface area of ​​the polymer material (P) is 10 m ２ It can be seen that a value of 1 / g or less is preferable.

[0146] Examples G1 to G4 Secondary battery samples G1 to G4 were prepared in the same manner as sample A1, except that the content and weight average molecular weight Mw of the polymer material (P) contained in the negative electrode mixture were changed as shown in Table 7, and were evaluated in the same manner.

[0147]

[0148] In Table 7, the surface temperatures of Samples G1 to G4 are significantly lower than that of Sample R3, and the surface temperatures of Samples G1 to G3 are particularly low. Therefore, it is clear that the weight average molecular weight Mw of the polymer material (P) is preferably less than 50,000.

[0149] <Samples H1 to H4> The content of the polymer material (P) contained in the negative electrode mixture was changed as shown in Table 8. In addition, for samples H3 and H4, the separator was changed to a 13 μm-thick microporous thin film made of polyethylene and not having a heat-resistant layer. Other than the above, secondary battery samples H1 to H4 were produced in the same manner as sample A1 and evaluated in the same manner.

[0150]

[0151] In Table 8, the surface temperatures of samples H1 and H2 are lower than those of samples H3 and H4, which indicates that it is preferable for the separator to have a heat-resistant layer.

[0152] The nonaqueous electrolyte secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, electric vehicles, and the like.

[0153] 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.

[0154] 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 portion, 23: First insulating plate, 24: Second insulating plate

Claims

1. A non-aqueous electrolyte secondary battery comprising: a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer supported on the negative electrode current collector, wherein the negative electrode active material layer comprises a negative electrode active material, a binder, a thickener, and an additive, wherein the additive is a polymer material having a melting point or a thermal decomposition temperature of 200°C or higher and 500°C or lower, wherein, when a thickness of the negative electrode active material layer is T, an existence probability Pb ​​of the polymer material present in a region of the negative electrode active material layer from a surface of the negative electrode current collector to 0.5T, and an existence probability Pt of the polymer material present in a region of the negative electrode active material layer from a position 0.5T from the surface of the negative electrode current collector to the outermost surface satisfy 1<Pb / Pt.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the average particle size of said polymer material is 50 μm or less.

3. The BET specific surface area of ​​the polymer material is 10 m ２ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the capacitance is 0.1 to 0.5 μm.

4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the weight average molecular weight (Mw) of the polymer material satisfies Mw<50,000.

5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the polymer material has a main chain containing a phenylene group and a sulfide group.

6. The backbone has the formula:

6. The nonaqueous electrolyte secondary battery according to claim 5, comprising a repeating structure represented by the formula: (n is an arbitrary integer).

7. The nonaqueous electrolyte secondary battery according to claim 6, wherein the polymer material includes polyphenylene sulfide.

8. The nonaqueous electrolyte secondary battery according to claim 1, wherein the polymer material contains chloride ions in a mass ratio of 10 ppm to 1000 ppm.

9. The nonaqueous electrolyte secondary battery according to claim 1, wherein the polymer material is dispersed in the cross section of the negative electrode active material layer to form a plurality of island-like regions, and the average perimeter of each of the island-like regions is 60 μm or less.

10. The polymer material is dispersed in the cross section of the negative electrode active material layer to form a plurality of island regions, and the average area of ​​the three largest island regions is 900 μm ２ 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein:

11. The polymer material is dispersed in the cross section of the negative electrode active material layer to form a plurality of island regions, and the average area of ​​the island regions is 200 μm 2 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein:

12. The nonaqueous electrolyte secondary battery according to claim 1, wherein the content of the polymer material in the negative electrode active material layer is 0.01% by mass or more and 5% by mass or less.

13. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material includes at least one material selected from the group consisting of carbon materials and silicon-containing materials.

14. The nonaqueous electrolyte secondary battery according to claim 1, wherein the separator has a base material layer and a heat-resistant layer laminated on the base material layer, and the melting point or thermal decomposition temperature of the polymer material is equal to or higher than the melting point of the base material layer.

15. A negative electrode for a secondary battery, comprising: a negative electrode current collector; and a negative electrode active material layer supported on the negative electrode current collector, wherein the negative electrode active material layer contains a negative electrode active material, a binder, a thickener, and an additive, and the additive is a polymer material having a melting point or thermal decomposition temperature of 200°C or higher and 500°C or lower, and wherein, when a thickness of the negative electrode active material layer is T, an existence probability Pb ​​of the polymer material present in a region of the negative electrode active material layer from a position 0.5T from the surface of the negative electrode current collector and an existence probability Pt of the polymer material present in a region of the negative electrode active material layer from a position 0.5T from the surface of the negative electrode current collector to the outermost surface satisfy 1<Pb / Pt.