Negative electrode for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery

Abstract

The present application provides a kind of negative electrode for reducing the initial resistance of non-aqueous electrolyte secondary battery and achieving the improvement of durability.The negative electrode for non-aqueous electrolyte secondary battery disclosed herein has a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, and the negative electrode active material layer at least contains a first negative electrode active material and a second negative electrode active material.The first negative electrode active material is composed of agglomerates of flaky graphite coated with low-crystallinity carbon on at least part of the surface, and the interlayer distance of graphite of the low-crystallinity carbon based on the electron diffraction image obtained by transmission electron microscopy is 0.3375 nm.The second negative electrode active material is natural graphite or artificial graphite with an interlayer distance of graphite based on the electron diffraction image obtained by transmission electron microscopy of 0.3375 nm.Here, the mass ratio of the first negative electrode active material to the second negative electrode active material is 50:50 to 90:10.

Description

Technical Field

[0001] This invention relates to a negative electrode for a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. Background Technology

[0002] Compared with existing batteries, non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries are lightweight and have high energy density, and therefore are preferred for use as high-output power sources for vehicles or power sources for personal computers and mobile terminals.

[0003] A typical negative electrode of this type of non-aqueous electrolyte secondary battery consists of a layer of negative electrode active material formed on a sheet-like negative electrode current collector. The negative electrode active material, which is the main component of this layer, is a compound capable of reversibly adsorbing and releasing chemical species (lithium ions in lithium-ion secondary batteries) that act as charge carriers. Various carbon materials, such as spherical graphite, are used. Preferably, this spherical graphite is graphite processed into a spherical shape by applying stress to flake-shaped graphite.

[0004] Regarding technologies related to spherical graphite, examples include Patent Documents 1 and 2. Patent Document 1 discloses a composite carbon material formed by coating a carbonaceous or graphitic material onto a pressurized carbon material to mitigate particle expansion during charging and discharging. Patent Document 2 discloses a method for manufacturing a negative electrode material for lithium-ion secondary batteries to improve high-speed characteristics. This method involves immersing a graphite granule with internal voids in a metal alkoxide solution to form a metal hydrate inside the graphite granule, and then coating the surface of the graphite granule with carbon.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2014-67636

[0008] Patent Document 2: Japanese Patent Application Publication No. 2017-54815 Summary of the Invention

[0009] However, based on the results of careful research conducted by the inventors, it has been found that the methods in Patent Documents 1 and 2 are insufficient in uniformly coating the interior of the spherical graphite (typically the individual flake graphite pieces constituting the spherical graphite), and the expansion suppression effect of the flake graphite inside the graphite is still inadequate. Furthermore, it has been found that because the edge surfaces of the flake graphite are not effectively coated, these edge surfaces react excessively with the non-aqueous electrolyte, increasing the resistance of the negative electrode. In addition, even when the flake graphite pieces present inside the spherical graphite are uniformly coated, the initial resistance of the negative electrode is high when only the spherical graphite is used as the negative electrode active material. Therefore, there is a need for a negative electrode that can both reduce resistance and improve durability.

[0010] The present invention was made in view of the above circumstances, and its main objective is to provide a negative electrode that reduces the initial resistance of a non-aqueous electrolyte secondary battery and improves its durability. Another objective is to provide a non-aqueous electrolyte secondary battery having this negative electrode.

[0011] To achieve the above objectives, a negative electrode for a non-aqueous electrolyte secondary battery, as disclosed herein, is provided. The disclosed negative electrode comprises a negative electrode current collector and a layer of negative electrode active material formed on the negative electrode current collector. The aforementioned negative electrode active material layer comprises at least a first negative electrode active material and a second negative electrode active material. The first negative electrode active material is composed of an aggregate of flake-like graphite, at least a portion of which is coated with low-crystallinity carbon. The interlayer distance of the low-crystallinity carbon, based on an electron diffraction image obtained using a transmission electron microscope, is [insert value here]. The second negative electrode active material mentioned above is based on the graphite interlayer distance obtained using the electron diffraction image obtained by the transmission electron microscope. The graphite is either natural or artificial. Here, the mass ratio of the first negative electrode active material to the second negative electrode active material is 50:50 to 90:10.

[0012] According to the above configuration, the interior of the first negative electrode active material is uniformly coated, and the volume expansion during charging and discharging is suppressed in each of the flake-like graphite constituting the first negative electrode active material. Furthermore, since the edge surfaces of the flake-like graphite are appropriately coated, the increase in negative electrode resistance due to excessive reaction with non-aqueous electrolytes is suppressed. In addition, by using a second negative electrode active material with a graphite interlayer spacing within a specified range, ion diffusion is improved, and the initial resistance value of the negative electrode is also suppressed. Therefore, it is possible to achieve a negative electrode that reduces the initial resistance of non-aqueous electrolyte secondary batteries and improves durability.

[0013] In one preferred embodiment of the negative electrode disclosed herein, the aforementioned negative electrode active material layer exhibits a peak P on the side with a relatively large pore size in the Log differential pore volume distribution measured by mercury intrusion porosimetry. L It has a peak P on the side with a relatively small pore size. S The aforementioned peak P L The peak area L occupied by the peak P mentioned above S The ratio of the peak area S occupied by L to S (L / S) is 10.5 to 11.5.

[0014] Based on the above configuration, the voids that can exist inside the negative electrode active material and the voids that can exist between the particles of the negative electrode active material can be appropriately adjusted, which can achieve a higher level of reduction in resistance and improvement in durability of non-aqueous electrolyte secondary batteries.

[0015] To achieve the other objectives mentioned above, a non-aqueous electrolyte secondary battery disclosed herein is provided. In a preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, a non-aqueous electrolyte secondary battery is provided, comprising: an electrode body having a positive electrode and a negative electrode, and a non-aqueous electrolyte. The negative electrode is characterized by having the negative electrode described above. Furthermore, in yet another preferred embodiment, the non-aqueous electrolyte comprises an oxalic acid complex compound and / or a carbonate as a film-forming agent.

[0016] By using a negative electrode with the aforementioned characteristics as the negative electrode in a non-aqueous electrolyte secondary battery, the initial resistance is reduced, and the rate of resistance increase is suppressed. Furthermore, by appropriately including a film-forming agent capable of forming an SEI (Solid Electrolyte Interface) film in the non-aqueous electrolyte, the rate of resistance increase can be further suppressed. Attached Figure Description

[0017] Figure 1 This is a schematic diagram illustrating one embodiment of a lithium-ion secondary battery.

[0018] Figure 2 This is a schematic diagram illustrating the configuration of a wound electrode body of a lithium-ion secondary battery according to one embodiment.

[0019] Figure 3 This is a schematic diagram illustrating the first negative electrode active material of one embodiment.

[0020] Symbol Explanation

[0021] 10 First negative electrode active material

[0022] 12. Flake graphite

[0023] 14 Low-crystallinity carbon

[0024] 16 gaps

[0025] 20. Winded electrode body

[0026] 30 Battery casing

[0027] 32 Safety valve

[0028] 42 Positive extremes

[0029] 42a Positive Current Collector

[0030] 44 Negative extremes

[0031] 44a Negative Current Collector

[0032] 50 Positive Electrode

[0033] 52 Positive current collector

[0034] 54 Positive electrode active material layer

[0035] 56 Positive current collector exposed portion

[0036] 60 Negative electrode

[0037] 62 Negative current collector

[0038] 64 Negative Electrode Active Material Layer

[0039] 66. Exposed portion of negative current collector

[0040] 70 isolation components

[0041] 100 Lithium-ion Secondary Battery Detailed Implementation

[0042] Hereinafter, preferred embodiments of the technology disclosed herein will be described with appropriate reference to the accompanying drawings. It should be noted that matters other than those specifically mentioned in this specification, and matters necessary for implementation (e.g., the general structure and construction process of non-aqueous electrolyte secondary batteries), can be grasped by those skilled in the art based on existing technology in the field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the field. Furthermore, in the following drawings, components or parts that perform the same function are labeled with the same symbols, and repeated descriptions are sometimes omitted or simplified. Also, dimensional relationships (length, width, thickness, etc.) do not necessarily reflect actual dimensional relationships.

[0043] It should be noted that in this specification, the expression "A~B (where A and B are arbitrary values)" refers to the range A~B.

[0044] In this manual, "non-aqueous electrolyte secondary battery" refers to a general energy storage device that uses a non-aqueous electrolyte as the electrolyte and can be repeatedly charged and discharged. It is a term that includes storage batteries and other energy storage components such as double-layer capacitors. "Electrode active material (i.e., positive electrode active material or negative electrode active material)" refers to a compound that can reversibly adsorb and release chemical species (lithium ions in lithium-ion secondary batteries) that act as charge carriers.

[0045] The negative electrode for the non-aqueous electrolyte secondary battery disclosed herein comprises a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode active material layer includes a first negative electrode active material and a second negative electrode active material. Without being particularly limiting, the technology disclosed herein will be described in detail below as an example, using a lithium-ion secondary battery having a flat, wound electrode body and a non-aqueous electrolyte as an example.

[0046] Figure 1The lithium-ion secondary battery 100 shown is constructed by housing a flat, wound electrode 20 and a non-aqueous electrolyte (not shown) within a sealable, box-shaped battery casing 30. The battery casing 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, as well as a thin-walled safety valve 32 designed to release internal pressure when the internal pressure of the battery casing 30 rises above a predetermined level. Additionally, the battery casing 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive terminal 42 is electrically connected to a positive current collector 42a. The negative terminal 44 is electrically connected to a negative current collector 44a. The battery casing 30 is preferably made of a high-strength, lightweight, and thermally conductive metallic material; examples of such metallic materials include aluminum and stainless steel.

[0047] The wound electrode body 20 typically has a shape in which a strip-shaped positive electrode 50 and a strip-shaped negative electrode 60 are overlapped and wound along their long sides by a strip-shaped separator 70. The positive electrode 50 has a configuration in which a positive electrode active material layer 54 is formed on one or both sides of the strip-shaped positive electrode current collector 52 along its long side. The negative electrode 60 has a configuration in which a negative electrode active material layer 64 is formed on one or both sides of the strip-shaped negative electrode current collector 62 along its long side. A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are respectively bonded to the positive electrode current collector exposed portion 56 (i.e., the portion where the positive electrode active material layer 54 is not formed and the positive electrode current collector 52 is exposed) and the negative electrode current collector exposed portion 66 (i.e., the portion where the negative electrode active material layer 64 is not formed and the negative electrode current collector 62 is exposed) which are formed in a manner that protrudes outward from both ends in the winding axis direction of the wound electrode body 20.

[0048] The positive electrode 50 has a positive electrode active material layer 54 on the elongated sheet-shaped positive electrode current collector 52. Examples of materials that can be used as the positive electrode current collector 52 include metals with good conductivity such as aluminum, nickel, titanium, and stainless steel. Aluminum (e.g., aluminum foil) is particularly preferred. The thickness of the positive electrode current collector 52 is not particularly limited, but is, for example, 5 μm to 35 μm, preferably 7 μm to 20 μm.

[0049] The positive electrode active material contained in the positive electrode active material layer 54 is not particularly limited, and one or more positive electrode active materials that have been commonly used as positive electrode active materials in non-aqueous electrolyte secondary batteries, especially lithium-ion secondary batteries, can be used. For example, lithium composite oxides and lithium transition metal phosphate compounds (e.g., LiFePO4) are preferred as positive electrode active materials. Examples of lithium composite oxides include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, and lithium nickel manganese composite oxides (e.g., LiNi). 0.5 Mn 1.5 O4), lithium nickel manganese cobalt composite oxides (e.g., LiNi) 1 / 3 Co1 / 3 Mn 1 / 3 O2), etc.

[0050] The average particle size of the positive electrode active material is not particularly limited, and can be approximately 0.5 μm to 50 μm, typically 1 μm to 20 μm. It should be noted that in this specification, "average particle size" refers to the particle size (D0) equivalent to 50% of the cumulative frequency from the smallest particle side in a particle size distribution based on a general laser diffraction-light scattering method. 50 (also known as median particle size).

[0051] The positive electrode active material layer 54 may contain substances other than the positive electrode active material, such as conductive materials and adhesives. As conductive materials, carbon black such as acetylene black (AB) and other carbon materials (e.g., graphite) are preferred. As adhesives, fluorine-based adhesives such as polyvinylidene fluoride (PVdF) and polytetrafluoroethylene (PTFE), and rubber-based adhesives such as styrene-butadiene rubber (SBR) are preferred. Furthermore, the positive electrode active material layer 54 may also contain materials other than those mentioned above (e.g., various additives) as long as it does not impair the effects of the present invention.

[0052] From an energy density perspective, the content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the ratio of the positive electrode active material to the total mass of the positive electrode active material layer) is preferably approximately 60% by mass or more. More preferably, it is 75% to 99% by mass, and even more preferably 80% to 95% by mass. Furthermore, the content of the conductive material in the positive electrode active material layer 54 is preferably 1% to 10% by mass, and more preferably 1% to 8% by mass. The content of the binder in the positive electrode active material layer 54 is preferably 0.5% to 5% by mass, and more preferably 1% to 3% by mass. Additionally, when various additives such as thickeners are included, the content of additives in the positive electrode active material layer 54 is preferably 7% by mass or less, and more preferably 5% by mass or less.

[0053] The negative electrode 60 has a negative electrode active material layer 64 on the elongated sheet-shaped negative electrode current collector 62. The negative electrode current collector 62 is made of a metallic material with good conductivity, such as copper, copper-based alloys, nickel, titanium, or stainless steel. Copper (e.g., copper foil) is particularly preferred. The thickness of the negative electrode current collector 62 can be approximately 5 μm to 20 μm, and preferably 8 μm to 15 μm.

[0054] The negative electrode active material layer 64 includes at least a first negative electrode active material and a second negative electrode active material. For example... Figure 3As shown, the first negative electrode active material 10 is composed of aggregates of flake graphite 12, at least a portion of which is coated with low-crystallinity carbon 14. Voids 16 (intraparticle voids) exist between the flake graphite 12 coated with low-crystallinity carbon 14. The shape of the first negative electrode active material 10 is typically preferably approximately spherical. In a preferred embodiment, it is a generally spherical shape formed by applying stress to the aforementioned flake graphite 12. In other words, the first negative electrode active material 10 can be so-called spherical graphite.

[0055] It should be noted that the term "approximately spherical shape" in this specification includes terms such as spherical, rugby ball-shaped, and polyhedral, and refers to a shape with an average aspect ratio (the ratio of the length along the major axis to the length along the minor axis in the smallest rectangle circumscribed by the particle) of approximately 1 to 2, for example, 1 to 1.5. The aforementioned aspect ratio can be obtained by taking the arithmetic mean of the length along the major axis relative to the length along the minor axis in the smallest rectangle circumscribed by at least 50 active material particles selected from multiple (e.g., more than two) observation images using an electron microscope.

[0056] The low-crystallinity carbon 14 coats at least a portion of the surface of the flake graphite 12. It is sufficient that at least a portion of the surface of the flake graphite 12 is coated with the low-crystallinity carbon 14; preferably, the entire surface of the flake graphite 12 can be coated. The average thickness of the coating of the low-crystallinity carbon 14 is not particularly limited, and can be, for example, 1 nm to 50 nm, or 5 nm to 40 nm.

[0057] It should be noted that the above average thickness can be the arithmetic mean of the thickness of more than 50 sites selected in multiple (e.g., more than 2) observation images using an electron microscope (e.g., a transmission electron microscope).

[0058] Flake graphite 12 is a type of scaly graphite found in natural graphite, exhibiting a thin, flake-like (plate-like) appearance. Flake graphite 12 can be considered the most crystalline graphite among various graphite materials. For example, in typical complete graphite crystals, the interlayer distance (lattice spacing of the d(002) plane) based on X-ray diffraction is... The closer the graphite layers are The more developed the crystallinity of graphite, the higher the theoretical value of its discharge capacity tends to be.

[0059] The morphology of flake graphite 12 is not particularly limited; for example, the average particle size can be 1 μm to 100 μm, or 5 μm to 50 μm. The true density of flake graphite 12 can be 2 g / cm³. 3 ~3g / cm 3 It can also be 2.1 g / cm³. 3 ~2.8g / cm3 Such flake-like graphite 12 can be prepared by purchasing commercially available products.

[0060] Low-crystallinity carbon-14 graphite has low crystallinity; typically, the interlayer spacing (lattice plane spacing of the d(002) plane) is [missing information]. The interlayer spacing of graphite can be The above can also be used for The above can also be used for The above. The interlayer distance of graphite is less than... At that time, the low-crystallinity carbon 14 disclosed herein, due to its excessively high crystallinity, becomes rigid (i.e., lacks flexibility), and tends to form a distinct interface with the flake graphite 12. Therefore, there is a concern that the low-crystallinity carbon 14 may not be able to follow the volume expansion of the flake graphite 12 during charging and discharging and may peel off. Consequently, the expansion suppression effect and resistance increase suppression effect of the negative electrode may decrease, making it less desirable.

[0061] On the other hand, the interlayer spacing of graphite in low-crystallinity carbon-14 can be The following can also be The following can also be used for Below. The interlayer distance of graphite exceeds... At this point, the van der Waals forces become insufficient to affect the graphite layers, making it difficult to manufacture such carbon materials. Furthermore, although it is possible to form irregularly shaped materials with distances exceeding [a certain value], [the remaining distance is insufficient]. The graphite interlayer spacing is randomly oriented carbon material, but π conjugation is not well developed, therefore, the conductivity is low, and it is not preferred as a battery material (negative electrode active material).

[0062] The term "interlayer distance of graphite (lattice spacing of the d(002) plane)" in this specification can be determined based on electron diffraction images obtained using a transmission electron microscope (TEM or STEM). Specifically, multiple (e.g., more than 5) electron diffraction images of the negative electrode active material are obtained using a transmission electron microscope. The magnification of the transmission electron microscope is not particularly limited as long as electron diffraction images can be obtained, for example, it can be 2 million times or more. In these electron diffraction images, the distances (interlayer distances) between the layers with lattice patterns from the graphite crystal are measured, and the average value of this value is calculated, thereby determining the "interlayer distance of graphite (lattice spacing of the d(002) plane)" in this specification.

[0063] It should be noted that the interlayer distance (lattice plane spacing of the d(002) plane) of highly crystalline graphite (such as general natural graphite, artificial graphite, etc.) can be determined by X-ray diffraction. However, in the techniques disclosed herein, apart from the low crystallinity of low-crystalline carbon, the coating of low-crystalline carbon is only a few nm to tens of nm, making it difficult to calculate the interlayer distance of graphite by X-ray diffraction. Therefore, the value calculated using the above-described measurement method is used as the interlayer distance of graphite in this specification.

[0064] The first negative electrode active material 10, which is composed of a condensate of coated flake graphite 12 having such low crystallinity carbon 14, can be made, for example, as follows.

[0065] First, prepare graphitic materials (typically flake graphite) and precursors of low-crystallinity carbon. Examples of precursors for low-crystallinity carbon include various bituminous materials (e.g., petroleum asphalt, coal tar pitch, petroleum naphtha, etc.) or organic polymers (e.g., phenolic resins, cellulose resins, polyvinyl alcohol, polyamide resins, etc.) that can be carbonized. One type can be used alone, or two or more can be used in combination.

[0066] Next, the graphitic material is mixed with a precursor of low-crystallinity carbon. The precursor is melted by calcining the mixed powder at the softening point of the low-crystallinity carbon precursor (e.g., above 300°C) and then adhered to the surface of the graphitic material. By calcining the powder with the precursor attached in an inert gas atmosphere at a higher temperature (e.g., 600°C to 1000°C), it is possible to obtain carbon powder coated with the low-crystallinity carbon precursor on at least a portion of the surface of the graphitic material.

[0067] The carbon powder obtained above is granulated while rotating, so that each powder (particle) is firmly and tightly bound together, thereby obtaining spherical graphite particles (first negative electrode active material 10). There are no particular limitations on the granulation apparatus, as long as it has a mechanism that allows particle rotation and collisions between individual particles. Examples include ball mills, bead mills, mixing systems manufactured by Nara Machinery Manufacturing Co., Ltd., Nobilta manufactured by Hosokawa Micron, FM mixers manufactured by Nippon Coke Industry Co., Ltd., and COMPOSI.

[0068] Generally, in graphite, the highly reactive facets, known as edge faces (typically the ends of the graphite's basal surface), are well-developed. In spherical graphite, these edge faces are typically folded or pleated. This allows for relatively low suppression of battery capacity reduction and increased resistance caused by reactions between the edge faces and non-aqueous electrolytes (typically reductive decomposition reactions). Furthermore, spheroidization reduces the orientation of the graphite, enabling homogenization of conductivity within the negative electrode active material layer 64.

[0069] In addition, in the technology disclosed herein, at least a portion of the surface of the flake graphite 12 constituting the first negative electrode active material 10 is coated with low-crystallinity carbon 14. This results in a situation where not only the surface of the first negative electrode active material 10, but also the interior of the first negative electrode active material 10 (in other words, the edge surfaces of each flake graphite 12 constituting the first negative electrode active material 10) are appropriately coated. Therefore, excessive reaction between the edge surfaces and the non-aqueous electrolyte can be more appropriately suppressed within the first negative electrode active material 10. Furthermore, as described above, by spherizing the flake graphite 12 after coating it with low-crystallinity carbon 14, the voids 16 within the first negative electrode active material 10 can be reduced. The reduced voids 16 increase the bonding force between the low-crystallinity carbon 14, thereby increasing the overall adhesion of the first negative electrode active material 10. Therefore, the expansion of the negative electrode 60 can be suppressed. Furthermore, by reducing the voids 16 within the first negative electrode active material 10, conductivity is improved compared to the past. Based on the above configuration, it is possible to reduce the resistance and improve the durability of non-aqueous electrolyte secondary batteries.

[0070] The second negative electrode active material is typically based on the interlayer distance of graphite (lattice spacing of the d(002) plane) obtained from electron diffraction images using a transmission electron microscope. Natural or synthetic graphite. As the second negative electrode active material, a material that has been processed (crushed, spherically shaped, etc.) from various types of graphite, such as natural or synthetic graphite, into particles (spherical shape) is preferred. For example, the second negative electrode active material can be a material obtained by spheroidizing flake graphite. As a method for processing various types of graphite into particles, existing known methods can be used without particular limitation. Since such natural or synthetic graphite is readily available at a lower cost, it is also preferred from a cost perspective.

[0071] The average particle size of the second negative electrode active material is not particularly limited, and can be approximately 1 μm to 30 μm, for example, 5 μm to 15 μm.

[0072] The interlayer distance (lattice spacing of the d(002) plane) of the second negative electrode active material, based on the electron diffraction image obtained using a transmission electron microscope, is: Graphite interlayer distance less than At this point, the lithium-ion insertion path narrows, and the reaction resistance involved in ion insertion can be significantly reduced. Furthermore, the interlayer distance of graphite exceeds... At that time, there is a tendency for the graphite interlayer distance to widen and the volume of the second negative electrode active material to increase from before charging, thus raising concerns about a decrease in the energy density of the negative electrode 60. The second negative electrode active material, by having the graphite interlayer distance described above (i.e., high graphite crystallinity), can become a material with high discharge capacity. Furthermore, due to the high crystallinity of graphite, it can be in a state where electron conductivity and ion transport within the active material are appropriately achieved (a state of high ion diffusivity).

[0073] As the second negative electrode active material, there is no particular limitation on the interlayer distance of graphite as long as it falls within the aforementioned range. This second negative electrode active material can be manufactured using existing known methods or prepared by purchasing commercially available products.

[0074] The mass ratio of the first negative electrode active material 10 to the second negative electrode active material in the negative electrode active material layer 64 is typically 50:50 to 90:10. When the first negative electrode active material 10 is less than 50% by mass, the expansion suppression effect and resistance increase suppression effect of the negative electrode 60 due to the coating with the aforementioned low-crystallinity carbon 14 become lower, thus reducing the durability of the non-aqueous electrolyte secondary battery. On the other hand, when the first negative electrode active material 10 exceeds 90% by mass, the proportion of the first negative electrode active material 10 with fewer pores becomes too high, making it difficult to obtain the resistance reduction effect brought about by the second negative electrode active material with high ion diffusivity, resulting in a higher resistance value (especially the initial resistance value). Therefore, by having the first and second negative electrode active materials in the above-mentioned range of a mass ratio, a non-aqueous electrolyte secondary battery with improved durability and reduced initial resistance can be achieved.

[0075] In addition to the aforementioned negative electrode active material, the negative electrode active material layer 64 may also include, as needed, materials that can be used as components of the negative electrode active material layer in general non-aqueous electrolyte secondary batteries. Examples of such materials include adhesives and various additives. As an adhesive, styrene-butadiene rubber (SBR) can be used, for example. Furthermore, various additives such as thickeners, dispersants, and conductive agents can be used appropriately. For example, as a thickener, carboxymethyl cellulose (CMC) and methyl cellulose (MC) can be used appropriately.

[0076] From the viewpoint of energy density, the content of the negative electrode active material in the negative electrode active material layer 64 is preferably approximately 60% by mass or more. More preferably, it is 90% to 99% by mass, and even more preferably 95% to 99% by mass. Furthermore, when a binder is used, the content of the binder in the negative electrode active material layer 64 is preferably 1% to 10% by mass, and more preferably 1% to 5% by mass. When a thickener is used, the content of the thickener in the negative electrode active material layer 64 is preferably 1% to 10% by mass, and more preferably 1% to 5% by mass.

[0077] The negative electrode active material layer 64 of the negative electrode 60 used in the non-aqueous electrolyte secondary battery disclosed here has a peak P located on the side with a relatively large pore size in the Log differential pore volume distribution determined by mercury intrusion porosimetry. L And the peak P located on the side with a relatively small pore size S Peak P L The peak area L and peak P S The ratio of the peak area S occupied by the peak area S (L / S) is preferably 10.5 to 12.0, more preferably 10.5 to 11.5.

[0078] As will be discussed in detail later, the total volume of the voids generated between particles in the negative electrode active material layer 64 (in other words, the voids outside the particles) can be used as the peak P with a relatively large pore size in the Log differential pore volume distribution. L The area L occupied is used to determine this. Additionally, the total volume of the voids within each particle in the negative electrode active material layer 64 (in other words, the voids within the particles) can be used as the peak P with a relatively small pore size in the Log differential pore volume distribution. S The area S occupied is used to determine the peak P. L The peak area L and peak P S A higher peak area ratio (L / S) indicates smaller intraparticle voids in each particle (negative electrode active material). Based on the inventors' dedicated research, by adjusting the balance between intraparticle and extraparticle voids within the aforementioned range, it is possible to reduce the resistance and improve the durability of non-aqueous electrolyte secondary batteries. The balance between intraparticle and extraparticle voids can be adjusted, for example, by the properties of each negative electrode active material and the mixing ratio of the first and second negative electrode active materials.

[0079] The volume of the pores (fine holes) in the aforementioned negative electrode active material layer 64 can be measured, for example, using a mercury porosimeter. A mercury porosimeter is a device for measuring the pore distribution of a porous body using the mercury porosimeter method. For example, multiple samples are cut from the negative electrode 60, and the volume of the pores in the negative electrode active material layer 64 of each sample is measured using a mercury porosimeter. In the mercury porosimeter method, the sample is first evacuated and immersed in mercury. In this state, if the pressure applied to the mercury increases, the mercury gradually penetrates into small spaces (e.g., the pores in the negative electrode active material layer 64). Then, the volume of the pores in the negative electrode active material layer 64 can be determined based on the relationship between the amount of mercury penetrating into the negative electrode active material layer 64 and the pressure applied to the mercury.

[0080] The total volume of the external voids in the negative electrode active material layer 64 and the total volume of the internal voids in the negative electrode active material layer 64 can be determined, for example, by using a mercury porosimeter as described above to measure the pore volume distribution. In the technique disclosed herein, the Log differential pore volume distribution (a graph showing the relationship between the Log differential pore volume and the pore size) measured using a mercury porosimeter has a peak P located on the side with a relatively large pore size. L And the peak P located on the side with a relatively small pore size S Here, it is speculated that the peak P with a relatively large pore size is... L This is caused by the voids (external voids) generated between particles (typically between the negative electrode active materials themselves) in the negative electrode active material layer 64. At this time, the total volume of the external voids in the negative electrode active material layer 64 can be used as the peak P with a relatively large pore size in the Log differential pore volume distribution. L The peak area L (cumulative pore volume) is used to determine the peak P. Although there is no specific limitation, the integral value of the pore volume in the range of Log pore diameter of about 1μm to 20μm (e.g., 1μm to 10μm) can be used as the peak P. L The peak area L occupied. Furthermore, it is speculated that the peak P, with its relatively small pore size... S This is caused by the voids (intra-particle voids) generated within each particle (typically, negative electrode active material particles) in the negative electrode active material layer 64. At this time, the total volume of the intra-particle voids in the negative electrode active material layer 64 can be used as the peak P with a relatively small pore size in the Log differential pore volume distribution. S The peak area S (cumulative pore volume) is used to determine the peak P. Although there is no specific limitation, the integral value of the pore volume in the range of Log pore diameter of about 0.01 μm to 1 μm can be used as the peak P. S The peak area S it occupies.

[0081] That is, peak P can be used L The peak area L and peak P SThe ratio of the peak area S occupied by each peak (L / S) is used to determine the ratio of the total volume of the external voids to the total volume of the internal voids of the particle. This is achieved by considering the two peaks P... L P S The location where the inter-log fine pore size becomes the minimum is set to have two peaks P. L P S The boundary is sufficient. Additionally, the minimum value of the Log pore size is a broad peak (e.g., no 0.01cm). 3 When the difference is greater than / g, as long as there is no 0.01cm in the extracted peak. 3 The region with a difference of / g or more, and based on peak P L Half-width and peak P S The boundary H can be determined by the ratio of the half-width to the half-width.

[0082] As the separator 70, it is sufficient to insulate the positive electrode active material layer 54 from the negative electrode active material layer 64. The separator 70 may have a non-aqueous electrolyte retention function and a shut-off function. Preferred examples of the separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Among these, the shut-off temperature of polyolefin-based porous sheets such as PE and PP is 80°C to 140°C, which is sufficiently lower than the battery's heat resistance temperature (typically above 200°C), thus enabling them to perform the shut-off function at appropriate times, and is therefore preferred. This porous sheet can be a single-layer structure or a multi-layer structure with two or more layers (e.g., a three-layer structure with PP layers stacked on both sides of a PE layer). The separator 70 may be provided with a heat-resistant layer (HRL).

[0083] The thickness of the isolator 70 is not particularly limited, but is generally 10 μm or more (typically 15 μm or more, e.g., 20 μm or more), preferably 40 μm or less (typically 35 μm or less, e.g., 30 μm or less). With the average thickness of the isolator 70 within the above range, ion permeability becomes better, and micro-short circuits (leakage currents) are less likely to occur. Therefore, it is possible to achieve a higher level of both input / output density and durability.

[0084] When the separator 70 has a heat-resistant layer (HRL), it can maintain its shape without softening or melting (allowing for slight deformation) even when a micro-short circuit occurs and the battery reaches a high temperature (typically above 150°C, for example above 200°C). The heat-resistant layer typically includes inorganic fillers and a binder. Examples of inorganic fillers include alumina, boehmite, silica, titanium dioxide, magnesium oxide, zirconium oxide, boron nitride, and aluminum nitride. Examples of binders include various materials exemplified as those used in the positive electrode active material layer 54 and the negative electrode active material layer 64.

[0085] The thickness of the heat-resistant layer is not particularly limited, and can generally be 1 μm to 10 μm, for example, 2 μm to 8 μm. A heat-resistant layer with a thickness within this range can adequately prevent internal short circuits and provide high short-circuit protection. The heat-resistant layer can be formed on the surface of the separator 70 opposite the positive electrode 50 and / or on the surface opposite the negative electrode 60. In a preferred embodiment, the heat-resistant layer is formed on the surface of the separator 70 opposite the positive electrode 50. This ensures that the non-aqueous electrolyte is properly retained at the interface with the separator 70, thereby improving the output characteristics of the secondary battery.

[0086] As a non-aqueous electrolyte, typically, a liquid electrolyte (non-aqueous electrolyte) obtained by dissolving or dispersing a supporting salt (e.g., lithium salt, sodium salt, magnesium salt, etc.; lithium salt in lithium-ion secondary batteries) in a non-aqueous solvent can be used. Alternatively, a non-aqueous electrolyte can be obtained by adding a polymer to it to form a solid (typically a so-called gel) non-aqueous electrolyte.

[0087] As the supporting salt, existing supporting salts used in non-aqueous electrolyte secondary batteries can be used without particular restriction. Examples include lithium salts such as LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, and LiC(CF3SO2)3. LiPF6 is preferred. The concentration of the supporting salt is, for example, 0.7 mol / L to 1.3 mol / L.

[0088] As non-aqueous solvents, carbonates, esters, ethers, nitriles, sulfones, lactones, and other non-aqueous solvents can be used without particular restrictions. Specifically, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene monofluorocarbonate (MFEC), ethylene difluorocarbonate (DFEC), methyl monofluorodifluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC) are preferred non-aqueous solvents. One such non-aqueous solvent can be used alone, or two or more can be used in appropriate combinations.

[0089] In a preferred embodiment, the non-aqueous electrolyte contains a film-forming agent. Typically, various compounds that decompose at a lower voltage than other components of the non-aqueous electrolyte (e.g., non-aqueous solvents) and form an SEI film on the surface of the electrode active material (typically the negative electrode active material) can be used as film-forming agents. It should be noted that the film-forming agent typically decomposes on the surface of the electrode active material during the initial charging phase; therefore, in the secondary battery after the initial charge and discharge, the film-forming agent does not retain its original form.

[0090] Preferred examples of film-forming agents include oxalate complexes and / or carbonates. Examples of oxalate complexes include (oxalate)borates such as lithium bis(oxalate)borate (LiBOB), fluoro(oxalate)borates such as lithium difluoro(oxalate)borate, and (oxalate)phosphates such as lithium tri(oxalate)phosphate. Examples of carbonates include vinylene carbonate (VC), vinyl ethyl carbonate, methyl phenyl carbonate, and fluoroethylene carbonate. The non-aqueous electrolyte may contain only one of the above-mentioned film-forming agents, or it may contain two or more. LiBOB is particularly suitable for forming an SEI film and is therefore preferred. From the viewpoint of appropriately suppressing solvent decomposition, the concentration of the film-forming agent in the non-aqueous electrolyte may be 0.1% by mass or more, or 0.3% by mass or more. On the other hand, from the viewpoint of suppressing residual film-forming agent after the initial charge and discharge, it is preferable to have 3% by mass or less, more preferably 2% by mass or less, and even more preferably 1.5% by mass or less. It should be noted that, in addition to the film-forming agent, the non-aqueous electrolyte may also contain various additives such as overcharge inhibitors and freeze inhibitors.

[0091] In the technology disclosed herein, the edge surfaces of the flake-like graphite 12 constituting the first negative electrode active material 10 are coated with low-crystallinity carbon 14. Therefore, even if a non-aqueous electrolyte penetrates into the interior of the first negative electrode active material 10, the reaction between the edge surfaces and the non-aqueous electrolyte is suppressed within the interior of the first negative electrode active material 10, inhibiting excessive SEI film formation. This suppresses the increase in negative electrode resistance caused by excessive SEI film formation, contributing to improved durability of the non-aqueous electrolyte secondary battery.

[0092] The non-aqueous electrolyte secondary battery constructed as described above can be used for various applications. The non-aqueous electrolyte secondary battery disclosed herein is characterized by improved durability and reduced initial resistance. Therefore, it can be appropriately used as a power source (drive power supply) for motors in vehicles such as plug-in hybrid electric vehicles (PHEVs), hybrid electric vehicles (HEVs), and battery electric vehicles (BEVs). It should be noted that the aforementioned non-aqueous electrolyte secondary battery can typically be used in the form of a battery pack consisting of multiple batteries connected in series and / or in parallel.

[0093] The following describes embodiments relating to the negative electrode disclosed herein, but it is not intended to limit the technology disclosed herein to what is shown in these embodiments.

[0094] <Preparation of the first negative electrode active material>

[0095] (Negative electrode active material A)

[0096] Flake graphite (manufactured by Ito Graphite Industry Co., Ltd., CNP15) and coke pitch as a precursor of low-crystallinity carbon were prepared. 100 parts by weight of flake graphite and 5 parts by weight of coke pitch were mixed using a roller mixer to obtain a mixed powder. This mixed powder was placed in an alumina crucible and calcined in a tube furnace at 300°C for 1 hour under a nitrogen atmosphere. Then, under a nitrogen atmosphere, the temperature was increased to 1000°C for 1 hour (calcination treatment). This yielded carbon powder with at least a portion of the surface of the flake graphite coated with low-crystallinity carbon. This carbon powder was fed into a ball mill and granulated while rotating until it became spherical graphite particles. This was thus obtained as the first negative electrode active material, negative electrode active material A.

[0097] (Negative electrode active material B)

[0098] By calcining the material at a temperature different from that of negative electrode active material A, negative electrode active material B is obtained. Specifically, a mixture of 100 parts by mass of flake graphite and 5 parts by mass of coke pitch is calcined in a tubular furnace at 300°C for 1 hour under a nitrogen atmosphere, followed by calcination at 800°C for 1 hour. Otherwise, negative electrode active material B is obtained in the same manner as negative electrode active material A.

[0099] (Negative electrode active material C)

[0100] By modifying the precursor of low-crystallinity carbon, negative electrode active material C is obtained. Specifically, 100 parts by mass of flake graphite and 5 parts by mass of polyvinyl alcohol are prepared, and negative electrode active material C is obtained in the same manner as negative electrode active material A.

[0101] (Negative electrode active material D)

[0102] By modifying the precursor of low-crystallinity carbon, negative electrode active material D is obtained. Specifically, 100 parts by mass of flake graphite and 5 parts by mass of phenolic resin are prepared, and negative electrode active material D is obtained in the same manner as negative electrode active material A.

[0103] (Negative electrode active material E)

[0104] Natural graphite (manufactured by Ito Graphite Industry Co., Ltd., SG-BH8) and coke pitch as a precursor of low-crystallinity carbon were prepared. 100 parts by weight of spherical natural graphite and 5 parts by weight of coke pitch were mixed using a roller mixer to obtain a mixed powder. This mixed powder was placed in an alumina crucible and calcined in a tubular furnace at 300°C for 1 hour under a nitrogen atmosphere. Then, the temperature was increased to 1000°C for 1 hour under a nitrogen atmosphere (calcination treatment). This yielded a negative electrode active material E with low-crystallinity carbon coated on the surface of the spherical natural graphite.

[0105] (Negative electrode active material F)

[0106] By calcining the material at a temperature different from that of negative electrode active material A, negative electrode active material F is obtained. Specifically, a mixture of 100 parts by mass of flake graphite and 5 parts by mass of coke pitch is calcined in a tubular furnace under a nitrogen atmosphere at 300°C for 1 hour and then at 2800°C for 1 hour. Otherwise, negative electrode active material F is obtained in the same manner as negative electrode active material A.

[0107] <Determination of interlayer distance of graphite>

[0108] The interlayer distance of graphite in the obtained negative electrode active materials A to F was measured. First, each negative electrode active material was observed using a transmission electron microscope (JEM-ARM200F, TEM). Cross-sectional images of the negative electrode active materials were obtained to confirm whether there was a coating of low-crystallinity carbon inside the active material. The results are shown in Table 1.

[0109] Next, the interlayer distance of graphite with low crystalline carbon was determined by TEM-based electron diffraction. Specifically, multiple (5) electron diffraction images of each negative electrode active material were obtained using transmission electron microscopy, and the distances between the layers with lattice patterns from the graphite crystals (interlayer distances) were measured. The interlayer distances of the low-crystallinity carbon in each active material were calculated by averaging these interlayer distances. The results are shown in Table 1. It should be noted that the TEM observation conditions were set at an accelerating voltage of 200 kV and a magnification of more than 2 million times to enable the observation of the electron diffraction images.

[0110] <Preparation of the second negative electrode active material>

[0111] (Negative electrode active material G)

[0112] As the second negative electrode active material, a natural graphite negative electrode active material G (manufactured by Ito Graphite Industry Co., Ltd., SG-BH8) is prepared as the negative electrode active material. The interlayer distance of the graphite in this negative electrode active material G is...

[0113] (Negative electrode active material H)

[0114] As the second negative electrode active material, a negative electrode active material H (manufactured by Ito Graphite Industry Co., Ltd., AG.B.) is prepared as the artificial graphite negative electrode active material H. The interlayer distance of the graphite in this negative electrode active material H is...

[0115] 1. Research on overlying layers

[0116] (Example 1)

[0117] Negative active material A, used as the first negative electrode active material, and negative active material G, used as the second negative electrode active material, were mixed at a mass ratio of 50:50. This mixture of negative active materials, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed with deionized water as a solvent at a mass ratio of 99:0.5:0.5 to prepare a negative electrode paste. This negative electrode paste was coated on both sides of a strip of copper foil (10 μm thick) serving as the negative electrode current collector. After drying, it was pressed using a roller press to produce a sheet-like negative electrode.

[0118] (Examples 2-4)

[0119] The first negative electrode active material was modified as shown in Table 1. Otherwise, the negative electrodes of Examples 2 to 4 were prepared in the same manner as in Example 1.

[0120] (Example 5)

[0121] The negative electrode of Example 5 was prepared in the same manner as in Example 1, except that the negative electrode active material H was used as the second negative electrode active material.

[0122] (Comparative Example 1)

[0123] The negative electrode of Comparative Example 1 was prepared in the same manner as in Example 1, except that the negative electrode was made using negative electrode active material E as the first negative electrode active material and without the second active material.

[0124] (Comparative Example 2)

[0125] The negative electrode of Comparative Example 2 was prepared in the same manner as in Example 1, except that the negative electrode active material F was used as the first negative electrode active material.

[0126] <Determination of the pore distribution of the negative electrode>

[0127] The pore size distribution of the negative electrode in each example was determined using a mercury porosimeter (Auto Pore, Micromeritics). From the obtained Log differential pore volume distribution, the peak P with the relatively large pore size was determined. L The peak area L (cumulative pore volume) and the peak P with relatively small pore size. S The peak area S (cumulative pore volume) is used to calculate the ratio of peak area L to peak area S (L / S). The results are shown in Table 1.

[0128] <Evaluation of the fabrication of lithium-ion secondary batteries>

[0129] LiNi will be used as the positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3O2 (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are mixed with N-methylpyrrolidone (NMP) as a solvent in a mass ratio of 92:5:3 to prepare a positive electrode paste. This positive electrode paste is coated onto both sides of a strip of aluminum foil (15 μm thick) serving as the positive electrode current collector. After drying, it is pressed using a roller press to produce a sheet-like positive electrode.

[0130] As a separator, a heat-resistant layer (HRL) is prepared on the side of a porous polyolefin sheet with a three-layer structure of PP / PE / PP that is opposite to the positive electrode.

[0131] The prepared sheet-shaped negative electrode (Examples 1-5 and Comparative Examples 1 and 2) and the sheet-shaped positive electrode are stacked together through a prepared separator, and wound in the longitudinal direction to form a wound electrode body. Next, electrode terminals are installed on the positive and negative electrode sheets of the prepared wound electrode body by welding, and the body is housed in a battery casing with an injection port.

[0132] As the non-aqueous electrolyte, a non-aqueous electrolyte was prepared by dissolving LiPF6, the supporting salt, in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at a volume ratio of 30:30:40 at a concentration of 1.0 mol / L. The non-aqueous electrolyte was injected through the injection port of the battery casing, and the injection port was hermetically sealed using a cap. After charging, the batteries were aged at 60°C to obtain the evaluation lithium-ion secondary batteries for each example.

[0133] <Determination of the expansion rate of the negative electrode>

[0134] The thickness of the negative electrode (i.e., the pressed negative electrode) in each example before constructing the evaluation lithium-ion secondary battery was measured using a contact micrometer. The thickness of the negative electrode at three locations was measured, and the average value was taken as the average thickness of the negative electrode before charging. Next, a coin cell was fabricated by sequentially stacking the negative electrode, separator, and lithium metal. The coin cell was charged to 80% of the specific capacity of the negative electrode active material (SOC 80%). The charged coin cell was disassembled, and the average thickness of the negative electrode after charging was calculated in the same way as described above. The expansion rate of the negative electrode was calculated using the following formula: Expansion rate of negative electrode = (Average thickness of negative electrode after charging / Average thickness of negative electrode before charging) × 100. The results are shown in Table 1.

[0135] <Determination of the rate of increase in resistance after storage>

[0136] The lithium-ion secondary batteries used for evaluation (Examples 1-5 and Comparative Examples 1 and 2) were adjusted to a SOC of 60%. They were placed in an environment of -10°C and charged at a current of 15C for 2 seconds. The resistance value was calculated by dividing the voltage difference before and after charging by the charging current, and this was taken as the initial resistance value. The lithium-ion secondary batteries used for evaluation were adjusted to a SOC of 80% and stored at 70°C for 30 days. The resistance value of the stored batteries was calculated in the same way as the initial resistance value, and the resistance increase rate after storage was calculated using the following formula: Resistance increase rate = (Resistance value after storage test / Initial resistance value) × 100. It should be noted that the results shown in Table 1 are the resistance increase rates when the initial resistance value for each example is set to 100.

[0137] [Table 1]

[0138] Table 1

[0139]

[0140] As shown in Table 1, it can be seen that Comparative Example 1 and Comparative Example 2 have high negative electrode expansion rates and high resistance increase rates after storage. Although the surface of the negative electrode active material E, which serves as the first negative electrode active material in Comparative Example 1, is coated with low-crystallinity carbon, and the interlayer distance of this low-crystallinity carbon is... However, no coating of low-crystallinity carbon was found inside the negative electrode active material E. Therefore, it is speculated that the negative electrode expansion rate and the rate of increase in resistance after storage are higher. In Comparative Example 2, in the negative electrode active material F, which is the first negative electrode active material, the interlayer distance of the low-crystallinity carbon is less than [missing information]. Therefore, it is speculated that low-crystallinity carbon is easy to peel off, and the negative electrode expansion rate and the rate of increase in resistance after storage are higher.

[0141] On the other hand, it is known that the material includes at least a first negative electrode active material and a second negative electrode active material, wherein the first negative electrode active material is composed of at least a portion of agglomerated flake graphite whose surface is coated with low-crystallinity carbon, and the interlayer distance of the low-crystallinity carbon graphite is [missing information]. The negative electrode expansion rate and the rate of increase in resistance after storage are low in Examples 1 to 5.

[0142] 2. Study on the mixing ratio of the first negative electrode active material and the second negative electrode active material

[0143] (Examples 6 and 7)

[0144] The combination of the first negative electrode active material and the second negative electrode active material was changed as shown in Table 2. Otherwise, the evaluation lithium-ion secondary batteries of Examples 6 and 7 were made in the same manner as in Example 1.

[0145] (Example 8)

[0146] The lithium-ion secondary battery for evaluation in Example 8 was prepared in the same manner as in Example 1, except that 1% by mass of LiBOB was added as an additive to the non-aqueous electrolyte.

[0147] (Compare Examples 3 and 4)

[0148] The combination of the first negative electrode active material and the second negative electrode active material was changed as shown in Table 2. Otherwise, the evaluation lithium-ion secondary batteries of Comparative Examples 3 and 4 were prepared in the same manner as in Example 1.

[0149] <Evaluation of the lithium-ion secondary batteries used in each evaluation>

[0150] For each lithium-ion secondary battery used in the evaluation (Examples 1 and 6-8, and Comparative Examples 3 and 4), the negative electrode expansion rate, resistance increase rate, and initial resistance value were determined as described above. The results are shown in Table 2. It should be noted that the resistance increase rate shown in Table 2 is the resistance increase rate when the initial resistance value of each example is set to 100. In addition, the ratio of initial resistance values ​​shown in Table 2 is the ratio of the initial resistance values ​​of each example when the initial resistance value of Example 1 is set to 100.

[0151] [Table 2]

[0152] Table 2

[0153]

[0154] As shown in Table 2, it can be seen that Comparative Example 3 has a high negative electrode expansion rate and a high rate of increase in resistance after storage. It is speculated that in Comparative Example 3, because the proportion of the first negative electrode active material is less than 50% by mass, it is difficult to achieve the effect of low-crystallinity carbon coating inside, resulting in a higher negative electrode expansion rate and a higher rate of increase in resistance after storage. It can also be seen that the initial resistance value of Comparative Example 4 is higher. It is speculated that in Comparative Example 4, because the proportion of the second negative electrode active material with high ion diffusivity is less than 10% by mass, the initial resistance is higher.

[0155] On the other hand, it can be seen that in Examples 1 and 6-8, where the mass ratio (mass ratio) of the first negative electrode active material to the second negative electrode active material is 50:50 to 90:10, the ratios of negative electrode expansion rate, resistance increase rate after storage, and initial resistance value are lower. Furthermore, the results of Example 8 show that by adding LiBOB, the resistance increase rate after storage is further reduced compared to Example 1. This is presumably because the edge surfaces of the flake-like graphite constituting the first negative electrode active material are appropriately coated with low-crystallinity carbon, thereby suppressing the excessive formation of the SEI film.

[0156] Based on the above results, a negative electrode containing a first negative electrode active material and a second negative electrode active material, with a mass ratio of the first negative electrode active material to the second negative electrode active material of 50:50 to 90:10, reduces the initial resistance of non-aqueous electrolyte secondary batteries and improves durability. The first negative electrode active material is composed of at least a portion of agglomerated flake graphite coated with low-crystallinity carbon, and the interlayer distance of the low-crystallinity carbon, based on an electron diffraction image obtained using a transmission electron microscope, is... The second negative electrode active material has an interlayer spacing of [missing information]. Natural or artificial graphite.

[0157] The specific examples of the present invention have been described in detail above, but they are merely illustrative and do not limit the scope of protection claimed. The technology described in the scope of protection includes technologies obtained by various modifications and alterations to the specific examples described above.

Claims

1. A negative electrode for a nonaqueous electrolyte secondary battery, which is a negative electrode for a nonaqueous electrolyte secondary battery, the negative electrode comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector, the negative electrode active material layer comprising at least a first negative electrode active material and a second negative electrode active material, the first negative electrode active material being composed of agglomerates of flaky graphite in which at least a part of the surfaces is coated with low-crystallinity carbon, The low-crystalline carbon has a graphite interlayer distance of 0.340 nm or more and less than 0.350 nm, based on an electron diffraction image obtained using a transmission electron microscope The second negative electrode active material is natural graphite or artificial graphite in which the interlayer distance of a graphite layer based on an electron diffraction image obtained by the transmission electron microscope is 0.335 nm or more and less than 0.340 nm. 0.335 nm or more and less than 0.340 nm. wherein the mass ratio of the first negative electrode active material to the second negative electrode active material is 50:50 to 90:

10. The negative electrode active material layer has a peak P on the side of a relatively large pore diameter in a Log differential pore volume distribution measured by a mercury porosimetry L has a peak P on the side of a relatively small pore diameter S , The peak P L The ratio of the peak area L to the peak P S The ratio of the peak area S to the peak P is 10.5 to 11.

5.

2. A nonaqueous electrolyte secondary battery comprising an electrode body having a positive electrode and a negative electrode, and a nonaqueous electrolyte, characterized in that the negative electrode according to claim 1 is provided as the negative electrode.

3. The nonaqueous electrolyte secondary battery according to claim 2, wherein the nonaqueous electrolyte contains an oxalate complex compound and / or a carbonate ester as a coating forming agent.

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