Non-aqueous electrolyte secondary battery

Abstract

This disclosed non-aqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode collector and a negative electrode mixture layer that is disposed on the negative electrode collector and includes a negative electrode active substance. The negative electrode mixture layer includes graphite particles as the negative electrode active material. When the negative electrode mixture layer is divided into two equal parts in a thickness direction, namely an inner layer on the negative electrode collector side and an outer layer on an outer surface side, the mean value of the internal void ratio of the graphite particles present in the outer layer is less than the mean value of the internal void ratio of the graphite particles present in the inner layer. The non-aqueous electrolyte includes a heterocyclic compound. A heterocyclic ring in the heterocyclic compound includes nitrogen and sulfur. The heterocyclic compound has an electron-withdrawing group R that includes oxygen and / or nitrogen.

Description

Nonaqueous electrolyte secondary battery

[0001] The present disclosure relates to a non-aqueous electrolyte secondary battery.

[0002] A nonaqueous electrolyte secondary battery, such as a lithium-ion secondary battery, includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. In nonaqueous electrolyte secondary batteries, metals such as copper and iron used in the battery materials may dissolve and become metal ions. The dissolved metal ions may precipitate on the negative electrode. When metal ions precipitate on the negative electrode, the battery's characteristics (e.g., voltage) tend to deteriorate. Furthermore, the precipitation of metal ions also increases the internal resistance of the nonaqueous electrolyte secondary battery.

[0003] Claim 1 of Patent Document 1 (WO 2023 / 145896) describes "a non-aqueous electrolyte for a non-aqueous electrolyte battery, comprising a non-aqueous solvent, an electrolyte salt, and a heterocyclic compound comprising at least one electron-withdrawing group R and a heterocyclic ring, wherein the electron-withdrawing group R contains oxygen and / or nitrogen, and the heterocyclic ring contains nitrogen and sulfur."

[0004] International Publication No. 2023 / 145896

[0005] Currently, there is a demand for reducing the internal resistance of non-aqueous electrolyte secondary batteries. In this situation, one of the objects of the present disclosure is to provide a non-aqueous electrolyte secondary battery with low internal resistance.

[0006] One aspect of the present disclosure relates to a non-aqueous electrolyte secondary battery including a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector and including a negative electrode active material, the negative electrode mixture layer including graphite particles as the negative electrode active material, and when the negative electrode mixture layer is equally divided in a thickness direction into an inner layer on the negative electrode current collector side and an outer layer on an outer surface side, the average value of internal porosity of the graphite particles present in the outer layer is smaller than the average value of internal porosity of the graphite particles present in the inner layer, and the non-aqueous electrolyte includes a heterocyclic compound, a heterocycle in the heterocyclic compound includes nitrogen and sulfur, and the heterocyclic compound has an electron-withdrawing group R including oxygen and / or nitrogen.

[0007] According to the present disclosure, a non-aqueous electrolyte secondary battery with low internal resistance can be obtained. 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.

[0008] The present disclosure also relates to a non-aqueous electrolyte secondary battery, ...

[0009] 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 other materials may be applied as long as the invention of the present disclosure can be implemented. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or greater 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 greater than the upper limit. In the following description, when examples of components or methods are listed, only one of the listed examples may be used, or multiple of the listed examples may be used in combination, unless otherwise specified.

[0010] (Non-aqueous electrolyte secondary battery) The non-aqueous electrolyte secondary battery according to this embodiment may be referred to as a "nonaqueous electrolyte secondary battery (B)" or a "secondary battery (B)" hereinafter. The secondary battery (B) includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector and containing a negative electrode active material. The negative electrode mixture layer contains graphite particles as the negative electrode active material. When the negative electrode mixture layer is divided into two equal halves in the thickness direction into an inner layer on the negative electrode current collector side and an outer layer on the outer surface side, the average internal porosity VAout of the graphite particles present in the outer layer is smaller than the average internal porosity VAinn of the graphite particles present in the inner layer. The non-aqueous electrolyte includes a heterocyclic compound. The heterocycle in the heterocyclic compound contains nitrogen and sulfur. The heterocyclic compound has an electron-withdrawing group R containing oxygen and / or nitrogen. Hereinafter, this heterocyclic compound may be referred to as "heterocyclic compound (C)" or "compound (C)."

[0011] When impurity metal ions are present in a non-aqueous electrolyte, they precipitate on the negative electrode, increasing the internal resistance of the battery. Compound (C) contained in the non-aqueous electrolyte of secondary battery (B) captures impurity metal ions in the non-aqueous electrolyte. As a result, the internal resistance of the battery can be reduced. However, further reductions in internal resistance are required to improve battery performance. As a result of investigations, the present inventors have newly discovered that the internal resistance of a battery can be significantly reduced by combining the use of compound (C) with controlling the distribution of graphite with different internal porosities (distribution in the negative electrode mixture layer). The present disclosure is based on this new finding.

[0012] In the negative electrode mixture layer of the secondary battery (B), the average value VAout of the internal porosity of the graphite particles present in the outer layer is smaller than the average value VAinn of the internal porosity of the graphite particles present in the inner layer. Compound (C) acts on the negative electrode to reduce the internal resistance of the battery. However, the presence of compound (C) in the non-aqueous electrolyte may result in significant formation of a coating on the surface of the negative electrode in contact with the non-aqueous electrolyte (non-aqueous electrolytic solution). On the other hand, in the above configuration, the graphite particles present in the outer layer have fewer voids, making it easier for the non-aqueous electrolyte to penetrate into the negative electrode. Therefore, the formation of the coating can be prevented from concentrating on the surface of the negative electrode, and the coating is formed uniformly inside the negative electrode. As a result, it is believed that the internal resistance of the battery can be significantly reduced.

[0013] When metals contained in a battery are exposed to a positive electrode potential, metal ions may dissolve from the metal into the nonaqueous electrolyte. These metal ions dissolved in the nonaqueous electrolyte migrate from the positive electrode side to the negative electrode side and precipitate on the negative electrode side. As this dissolution-precipitation reaction progresses, the precipitated metal grows into a dendrite-like structure, resulting in a decrease in the characteristics (e.g., voltage) of the nonaqueous electrolyte secondary battery. Therefore, it is important to suppress the deterioration of characteristics due to the dissolution and precipitation of metals and the like in nonaqueous electrolyte secondary batteries. Because the nonaqueous electrolyte of the present disclosure contains a heterocyclic compound (C), the deterioration of characteristics due to the dissolution-precipitation reaction of metals is significantly suppressed.

[0014] The heterocyclic compound (C) captures metal ions (e.g., copper ions) in the non-aqueous electrolyte using its heterocycle (H), suppressing the reductive precipitation reaction of the metal ions at the negative electrode. In addition, the electron-withdrawing group R contained in the heterocyclic compound (C) has the effect of enhancing the ability of the heterocycle (H) to capture metal ions. Furthermore, it is believed that the electron-withdrawing group R itself also captures metal ions in the non-aqueous electrolyte and suppresses the reductive precipitation reaction of the metal ions at the negative electrode. In this way, the metal ion capturing ability of the heterocyclic compound (C) is enhanced, and the heterocyclic compound (C) has multiple different atomic groups capable of capturing metal ions. As a result, a large number of metal ions are efficiently captured, and as a result, the reductive precipitation of metal ions is significantly suppressed.

[0015] Generally, metal ions can exist in a number of different valences in a non-aqueous electrolyte. For example, copper ions exist in a non-aqueous electrolyte as Cu ＋ and Cu ２＋ The heterocyclic compound (C) can exist with two different valences, each with different electron-accepting properties. When there is only one type of atomic group capable of forming a coordinate bond with a metal ion, it is easy to form a coordinate bond with one of the metal ions with different valences (e.g., a monovalent copper ion), but it may be difficult to form a coordinate bond with the other ion (e.g., a divalent copper ion). In this case, it is difficult to capture all of the metal ions generated in the battery. In contrast, by using a heterocyclic compound (C) having two or more different atomic groups that are easy to coordinate depending on the valence of the metal ion, highly efficient capture of metal ions is possible.

[0016] The average value VAout of the internal porosity of the graphite particles present in the outer layer is smaller than the average value VAinn of the internal porosity of the graphite particles present in the inner layer. The ratio VAout / VAinn of VAout to VAinn is equal to or greater than 0 and less than 1. The ratio VAout / VAinn may be 0.75 or less, 0.5 or less, or 0.25 or less.

[0017] The negative electrode mixture layer may satisfy the following conditions (1) to (3). By satisfying the following conditions (1) to (3), it is possible to make the average value VAout of the internal porosity of the graphite particles present in the outer layer smaller than the average value VAinn of the internal porosity of the graphite particles present in the inner layer. (1) The graphite particles (negative electrode active material) include first graphite particles and second graphite particles. (2) The internal porosity Vr1 of the first graphite particles is smaller than the internal porosity Vr2 of the second graphite particles. (3) The content R1out of the first graphite particles in the outer layer is larger than the content R1inn of the first graphite particles in the inner layer.

[0018] Regarding condition (1), the total proportion of the first graphite particles and the second graphite particles in the graphite particles may be 90 mass% or more, or the graphite particles may be composed only of the first graphite particles and the second graphite particles.

[0019] Regarding condition (2), the internal porosity Vr1 of the first graphite particles may be 1% or more, or 3% or more, and may be 15% or less, 10% or less, 5% or less, or 3% or less. The internal porosity Vr2 of the second graphite particles may be 8% or more, or 15% or more, and may be 30% or less, 25% or less, or 15% or less. In a preferred example, the internal porosity of the first graphite particles is 5% or less, and the internal porosity of the second graphite particles is in the range of 8 to 20%.

[0020] Regarding condition (3), the ratio R1inn / R1out of the content R1inn of the first graphite particles in the inner layer to the content R1out of the first graphite particles in the outer layer is equal to or greater than 0 and less than 1. The ratio R1inn / R1out may be equal to or less than 0.5, or equal to or less than 0.3.

[0021] The negative electrode mixture layer may satisfy the following condition (4) in addition to the conditions (1) to (3): (4) The content rate R2out of the second graphite particles in the outer layer is equal to or less than the content rate R2inn of the second graphite particles in the inner layer.

[0022] Regarding condition (4), the ratio R2out / R2inn of the content rate R2out of the second graphite particles in the outer layer to the content rate R2inn of the second graphite particles in the inner layer is equal to or greater than 0 and less than 1. The ratio R2out / R2inn may be equal to or less than 0.5, or equal to or less than 0.3.

[0023] The negative electrode mixture layer may satisfy the following conditions (1') to (3'). By satisfying the following conditions (1') to (3'), it is possible to make the average value VAout of the internal porosity of the graphite particles present in the outer layer smaller than the average value VAinn of the internal porosity of the graphite particles present in the inner layer. (1') The graphite particles (negative electrode active material) contain graphite particles A and graphite particles B. (2') The internal porosity of the graphite particles A is 5% or less, and the internal porosity of the graphite particles B is 8% or more and 20% or less. (3') The content RAout of the graphite particles A in the outer layer is larger than the content RAinn of the graphite particles A in the inner layer.

[0024] The negative electrode mixture layer may satisfy the following condition (4') in addition to the following conditions (1') to (3'): (4') The content RBout of the graphite particles B in the outer layer is equal to or less than the content RBinn of the graphite particles B in the inner layer.

[0025] Regarding the condition (3′), the ratio RAinn / RAout of the content RAinn of graphite particles A in the inner layer to the content RAinn of graphite particles A in the outer layer is equal to or greater than 0 and less than 1. The ratio RAinn / RAout may be 0.5 or less, or 0.3 or less.

[0026] Regarding the condition (4'), the ratio RBout / RBinn of the content RBout of graphite particles B in the outer layer to the content RBinn of graphite particles B in the inner layer is equal to or greater than 0 and less than 1. The ratio RBout / RBinn may be equal to or less than 0.5, or equal to or less than 0.3.

[0027] The internal porosity of the graphite particles can be measured by the following method. First, a cross section of the negative electrode mixture layer is exposed. The method for exposing the cross section is not particularly limited. In one example method, a part of the negative electrode is cut out and processed with an ion milling device (e.g., IM4000PLUS manufactured by Hitachi High-Technologies Corporation) to expose the cross section of the negative electrode mixture layer.

[0028] Next, a backscattered electron image of the cross section of the exposed negative electrode mixture layer is taken using a scanning electron microscope at a magnification of 3,000 to 5,000 times.

[0029] Next, the obtained cross-sectional image is input into a computer and binarized using image analysis software (e.g., ImageJ, manufactured by the National Institutes of Health, USA). By the binarization process, a binarized image is obtained in which particle cross sections in the cross-sectional image are converted into black and voids present in the particle cross sections are converted into white.

[0030] Next, graphite particles having a particle size of 5 μm or more and 50 μm or less are selected from the binarized image, and the area of ​​the cross section of the graphite particle and the area of ​​the internal voids present in the cross section of the graphite particle are calculated. Here, the area of ​​the cross section of the graphite particle refers to the area of ​​the region surrounded by the outer periphery of the graphite particle, i.e., the entire area of ​​the cross section of the graphite particle. Furthermore, for voids present in the cross section of the graphite particle that are 3 μm or less in width, it may be difficult to distinguish between internal and external voids in image analysis, so voids with a width of 3 μm or less are considered internal voids. Then, from the calculated area of ​​the cross section of the graphite particle and the area of ​​the internal voids in the cross section of the graphite particle, the internal porosity of the graphite particle can be calculated using the following formula: Internal porosity of graphite particle (%) = 100 × (area of ​​internal voids in the cross section of the graphite particle) / (area of ​​the cross section of the graphite particle).

[0031] The internal porosity Vr1 of the first graphite particles is determined by arithmetically averaging the internal porosities of 10 randomly selected first graphite particles. Similarly, the internal porosity Vr2 of the second graphite particles is determined by arithmetically averaging the internal porosities of 10 randomly selected second graphite particles. That is, the internal porosity Vr1 of the first graphite particles and the internal porosity Vr2 of the second graphite particles are each the average value of the internal porosities. Meanwhile, the internal porosities of graphite particles A and graphite particles B are the internal porosities of the respective particles.

[0032] The average value VAout of the internal porosity of graphite particles present in the outer layer is determined by arithmetically averaging the internal porosity of 10 graphite particles randomly selected in the outer layer. Similarly, the average value VAinn of the internal porosity of graphite particles present in the inner layer is determined by arithmetically averaging the internal porosity of 10 graphite particles randomly selected in the inner layer.

[0033] The comparison of the contents of predetermined graphite particles (first graphite particles, second graphite particles, graphite particles A, graphite particles B) in predetermined layers (inner layer, outer layer) can be made using the proportion of the cross section of the predetermined graphite particles in the cross section of the predetermined layer. For example, the comparison of the content of first graphite particles R1out with the content of first graphite particles in the inner layer R1inn can be made by comparing the proportion of the cross section of the first graphite particles in the cross section of the outer layer with the proportion of the cross section of the first graphite particles in the cross section of the inner layer. The comparison of the contents of predetermined graphite particles in predetermined layers can be made using values ​​evaluated by arbitrarily selecting 10 graphite particles in each layer.

[0034] The BET specific surface area of ​​the first graphite particles is 3.5 m ２ / g or less, and ２ It is more preferable that the BET specific surface area of ​​the first graphite particles is 3.5 m / g or less. ２ When the carbon black content is 0.05 wt. / g or less, the side reaction between the first graphite particles and the non-aqueous electrolyte can be further suppressed.

[0035] The BET specific surface area of ​​the second graphite particles is 3.5 m ２ / g or more, and ２ It is more preferable that the BET specific surface area of ​​the second graphite particles is 3.5 m / g or more. ２ When the pore size is 1 / g or more, the second graphite particles can be easily packed at a high density when the negative electrode mixture layer is produced by rolling, thereby increasing the capacity of the battery.

[0036] The volume-based median diameter (D50) of the first graphite particles and the volume-based median diameter (D50) of the second graphite particles may each be 1 μm or more, or 10 μm or more, and may be 30 μm or less, or 25 μm or less. The median diameter is the diameter at which the cumulative volume in the volume-based particle size distribution is 50%. The particle size distribution can be measured using a laser diffraction particle size distribution analyzer (e.g., MT3000II, manufactured by Microtrac-Bell Co., Ltd.) using water as a dispersion medium.

[0037] The specific surface area and median diameter of the graphite particles A may be within the ranges exemplified for the specific surface area and median diameter of the first graphite particles, respectively. The specific surface area and median diameter of the graphite particles B may be within the ranges exemplified for the specific surface area and median diameter of the second graphite particles, respectively.

[0038] The content of the heterocyclic compound (C) in the non-aqueous electrolyte may be 0.01% by mass or more, 0.1% by mass or more, or 0.5% by mass or more, and may be 10.0% by mass or less, 5.0% by mass or less, or 2.0% by mass or less. The content may be in the range of 0.01 to 10.0% by mass, 0.1 to 10.0% by mass, or 1.0 to 10.0% by mass. Within these ranges, the upper limit may be 5.0% by mass or less or 2.0% by mass or less. By setting the content in the range of 0.1 to 2.0% by mass, the effect of the addition of the heterocyclic compound (C) on the charge / discharge characteristics of the battery can be mitigated.

[0039] The non-aqueous electrolyte may contain only one type of compound as the heterocyclic compound (C), or may contain multiple types of compounds.

[0040] The electron-withdrawing group R contained in the heterocyclic compound (C) may have coordinate bonding properties with a metal ion. The electron-withdrawing group R contains oxygen and / or nitrogen. That is, the electron-withdrawing group R contains at least one selected from oxygen and nitrogen. The electron-withdrawing group R may contain only one of oxygen and nitrogen, or may contain both.

[0041] The electron-withdrawing group R is a carbonyl group (-C(=O)-), a nitrile group (-C≡N), a sulfonyl group (-S(=O) ２ The hydroxyl group, which is the electron-withdrawing group R, is bonded to a carbon atom constituting a saturated hydrocarbon group (e.g., an alkyl group or an alkylene group). The nitrile group may be contained in a thionitrile group. The sulfonyl group is a sulfonate ester bond (-S(=O) ２—O—). The C═O moiety contained in an isocyanate group and an isothiocyanate group is not usually considered to be a carbonyl group. Therefore, in this specification, the C═O moiety contained in an isocyanate group and an isothiocyanate group is not considered to be a carbonyl group.

[0042] The electron-withdrawing group R may be a carbonyl group, a nitrile group, a sulfonyl group, an isocyanate group, an isothiocyanate group, or a hydroxy group. The electron-withdrawing group R may be at least one selected from the group consisting of a carbonyl group, a nitrile group, a sulfonyl group, an isocyanate group, and an isothiocyanate group. The electron-withdrawing group R may be at least one selected from the group consisting of a carbonyl group, a sulfonyl group, an isocyanate group, and an isothiocyanate group.

[0043] The carbonyl group may be contained in at least one selected from the group consisting of an aldehyde group (-CHO), a ketone group, an amide bond (C(=O)-N), an ester bond (COO), and a carboxy group (-COOH). That is, the electron-withdrawing group R may be at least one selected from the group consisting of an aldehyde group, a carbonyl group contained in a ketone, an amide bond, an ester bond, and a carboxy group.

[0044] The number of electron-withdrawing groups R contained in the heterocyclic compound (C) may be 1, 2 or more, 5 or more, or 5 or less. The number of heterocycles (H) contained in the heterocyclic compound (C) may be 1, 2, 3 or less, or 3 or more.

[0045] The heterocycle (H) contains nitrogen and sulfur. The heterocycle (H) may or may not have aromaticity. The number of atoms constituting the heterocycle (H) may be in the range of 5 to 8, or in the range of 5 to 7, or may be 5 or 6. That is, the heterocycle (H) may be a five-membered ring, a six-membered ring, a seven-membered ring, or an eight-membered ring.

[0046] The heterocycle (H) may satisfy the following conditions (1) and / or (2), or may satisfy the following conditions (1) and (3): (1) The heterocycle (H) is composed of one nitrogen atom, one sulfur atom, and multiple carbon atoms; (2) The nitrogen atom forms a double bond with one adjacent carbon atom that constitutes the heterocycle (H); and (3) The nitrogen atom forms a single bond with each of the two adjacent carbon atoms that constitute the heterocycle (H).

[0047] The heterocycle (H) may be a thiazole ring represented by the following formula:

[0048]

[0049] The heterocycle (H) may be a thiomorpholine ring represented below.

[0050]

[0051] The heterocycle (H) may be a thiazepine ring represented by the following formula: The thiazepine ring may be a 1,3-thiazepine ring or a 1,4-thiazepine ring.

[0052]

[0053] The heterocycle (H) may contain or be at least one selected from the group consisting of a thiazole ring, a thiomorpholine ring, and a thiazepine ring.

[0054] The structure of the heterocyclic compound (C) other than the heterocycle (H) and the electron-withdrawing group R is not limited as long as the effects of the present disclosure are obtained. The portion other than the heterocycle (H) and the electron-withdrawing group R may be composed only of hydrocarbons. Examples of hydrocarbons include hydrocarbon groups (including hydrocarbon chains). Examples of hydrocarbons include aliphatic hydrocarbons and aromatic hydrocarbons. The heterocyclic compound (C) may contain an ether bond, a thioether bond, nitrogen not included in the electron-withdrawing group R, etc.

[0055] The molecular weight of the heterocyclic compound (C) may be 100 or more, or 130 or more, and may be 400 or less, or 370 or less.

[0056] As the heterocyclic compound (C), a compound that dissolves in the non-aqueous solvent of the non-aqueous electrolyte is preferably used.

[0057] Examples of the heterocyclic compound (C) include the compounds shown in Table 1 below. The heterocyclic compound (C) may be at least one selected from the group consisting of the 15 compounds shown in Table 1.

[0058]

[0059] The heterocyclic compound (C) may include at least one selected from the group consisting of 4,5-dimethyl-1,3-thiazole-2-carbaldehyde, 4,5,6,7-tetrahydro-1,3-benzothiazole-2-carbaldehyde, 2-ethylthiomorpholine-4-carbaldehyde, and 2,3-dimethylthiomorpholine-4-carbaldehyde, and these compounds are preferred in that they provide high effects.

[0060] Among the heterocyclic compounds (C) described above, 2-ethylthiomorpholine-4-carbaldehyde and 2,3-dimethylthiomorpholine-4-carbaldehyde are preferred because of their high effect of capturing metal ions.

[0061] The heterocyclic compound (C) may be a commercially available compound, or may be synthesized according to a known synthesis method.

[0062] The content of the heterocyclic compound (C) in the non-aqueous electrolyte can be determined, for example, using gas chromatography under the following conditions. Instrument used: GC-2010 Plus, manufactured by Shimadzu Corporation Column: HP-1 (film thickness 1 μm, inner diameter 0.32 mm, length 60 m), manufactured by J&W Corporation Column temperature: heated from 50°C to 90°C at a heating rate of 5°C / min, maintained at 90°C for 15 minutes, then heated from 90°C to 250°C at a heating rate of 10°C / min, and maintained at 250°C for 15 minutes Split ratio: 1 / 50 Linear velocity: 30.0 cm / sec Injection port temperature: 270°C Injection volume: 1 μL Detector: FID 290°C (sens. 10 １ )

[0063] From one perspective, an example of a nonaqueous electrolyte according to this embodiment includes a nonaqueous solvent, an electrolyte salt, and a heterocyclic compound (C1). The heterocyclic compound (C1) includes at least one atomic group Z containing oxygen and / or nitrogen, and a heterocycle (H). The atomic group Z can be any of the atomic groups exemplified as the electron-withdrawing group R. The atomic group Z may include at least one selected from the group consisting of a carbonyl group, a nitrile group, a sulfonyl group, and a hydroxy group, or may be at least one of the atomic groups. The hydroxy group, which is the atomic group Z, is bonded to a carbon atom constituting a saturated hydrocarbon group.

[0064] The nonaqueous electrolyte secondary battery according to this embodiment includes a positive electrode containing a positive electrode active material, a negative electrode facing the positive electrode, and a nonaqueous electrolyte. The nonaqueous electrolyte secondary battery may include other components. For example, the nonaqueous electrolyte secondary battery typically further includes a separator and an exterior body. The separator is disposed between the positive electrode and the negative electrode. The exterior body houses the nonaqueous electrolyte and an electrode group including the positive electrode, the negative electrode, and the separator. There are no particular limitations on the positive electrode, the negative electrode, the separator, and the exterior body, and known materials may be used.

[0065] There is no limitation on the shape of the nonaqueous electrolyte secondary battery, and it may be cylindrical or prismatic. There is no limitation on the form of the electrode group of the nonaqueous electrolyte secondary battery, and it may be wound or stacked.

[0066] Examples of the components of the nonaqueous electrolyte secondary battery (B) are described below, but the components of the secondary battery (B) are not limited to the following examples.

[0067] (Non-aqueous electrolyte) The non-aqueous electrolyte contains a non-aqueous solvent, an electrolyte salt, and a heterocyclic compound (C). The heterocyclic compound (C) may be any of the compounds described above.

[0068] (Non-aqueous Solvent) Examples of non-aqueous solvents include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain 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). Examples of chain carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate (MA), ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous electrolyte may contain only one non-aqueous solvent, or may contain two or more non-aqueous solvents.

[0069] (Electrolyte Salt) As the electrolyte salt, a lithium salt is suitable. Examples of the lithium salt include LiClO ４ , LiBF ４ , LiPF ６ , LiAlCl ４ , LiSbF ６ , LiSCN, LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiB １０ Cl １０ Examples of the lithium salts include lithium carboxylates, lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of the borates include lithium difluorooxalate borate and lithium bis(oxalate) borate. Examples of the imide salts include lithium bisfluorosulfonylimide (LiN(FSO ２ ) ２ ), lithium bistrifluoromethanesulfonyl imide (LiN(CF ３ SO ２ ) ２ The non-aqueous electrolyte may contain only one type of electrolyte salt, or may contain two or more types of electrolyte salts.

[0070] The concentration of the electrolyte salt in the non-aqueous electrolyte may be 0.5 mol / L or more and 2 mol / L or less.

[0071] The non-aqueous electrolyte may contain other additives, such as at least one selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, and vinylethylene carbonate.

[0072] The non-aqueous electrolyte may also contain a compound other than the heterocyclic compound (C) as an additive for suppressing dissolution and precipitation reactions of metal ions. Examples of such compounds include an isocyanate compound having an isocyanate group and / or a nitrile compound having two or more nitrile groups. The isocyanate compound is reductively decomposed at the negative electrode to form a film on the surface of the negative electrode active material (e.g., a carbon material such as graphite), thereby suppressing the reductive decomposition of the non-aqueous electrolyte. This can suppress the reductive precipitation reaction of metal ions. On the other hand, the nitrile compound is oxidized at the positive electrode to form a film on the positive electrode active material. This can suppress the dissolution of metal ions constituting the positive electrode active material into the non-aqueous electrolyte. However, the heterocyclic compound (C) of the present disclosure is significantly superior to isocyanate compounds and nitrile compounds in terms of suppressing dissolution and precipitation reactions of metal ions.

[0073] (Positive Electrode) The positive electrode includes a positive electrode active material. The positive electrode typically includes a positive electrode current collector and a positive electrode mixture layer held on the positive electrode current collector. In one example of a method for forming the positive electrode mixture layer, first, the components of the positive electrode mixture are dispersed in a dispersion medium to prepare a positive electrode slurry. Next, the positive electrode slurry is applied to the surface of the positive electrode current collector to form a coating film, and the coating film is then dried to form the positive electrode mixture layer. The dried coating film may be rolled as necessary. The positive electrode mixture includes a positive electrode active material as an essential component, and may also include a binder, a thickener, and the like as optional components.

[0074] (Positive Electrode Active Material) The positive electrode active material is not particularly limited as long as it can be used as a positive electrode active material for a non-aqueous electrolyte secondary battery (e.g., a lithium ion secondary battery). A preferred positive electrode active material is, for example, a lithium transition metal composite oxide having a layered rock salt structure and containing Ni and at least one selected from the group consisting of Co, Mn, and Al. Hereinafter, the lithium transition metal composite oxide may be referred to as a "composite oxide HN."

[0075] From the viewpoint of obtaining a high capacity, the proportion of Ni in the metal elements other than Li contained in the composite oxide HN is desirably 80 atomic % or more. The proportion of Ni in the metal elements other than Li may be 85 atomic % or more, or may be 90 atomic % or more. The proportion of Ni in the metal elements other than Li may be 95 atomic % or less.

[0076] The composite oxide HN has the formula: Li α Ni(1-x1-x2-yz)Co ｘ１ Mn ｘ２ Al ｙ M ｚ O ２＋β The element M may be an oxide represented by the formula: The element M is an element other than Li, Ni, Co, Mn, Al, and O (oxygen).

[0077] In the above formula, α, which indicates the atomic ratio of lithium, may satisfy 0.95≦α≦1.05, where α increases or decreases with charge and discharge. In (2+β), which indicates the atomic ratio of oxygen, β may satisfy −0.05≦β≦0.05.

[0078] 1-x1-x2-y-z (=v), which represents the atomic ratio of Ni, is 0.8 or more, and may be 0.85 or more, 0.90 or more, or 0.95 or more. v, which represents the atomic ratio of Ni, may be 0.98 or less, or 0.95 or less. x1, which represents the atomic ratio of Co, may be 0.1 or less (0≦x1≦0.1). x2, which represents the atomic ratio of Mn, may be 0.1 or less (0≦x2≦0.1). y, which represents the atomic ratio of Al, may be 0.1 or less (0≦y≦0.1).

[0079] The value z, which indicates the atomic ratio of the element M, may satisfy the relationship 0≦z≦0.10. The element M may be at least one element selected from the group consisting of Ti, Zr, Nb, Mo, W, Fe, Zn, B, Si, Mg, Ca, Sr, Sc, and Y. When the element M contains at least one element selected from the group consisting of Nb, Sr, and Ca, it is believed that the surface structure of the composite oxide HN is stabilized, the resistance is reduced, and metal elution is further suppressed.

[0080] The composite oxide HN may be a secondary particle formed by agglomeration of a plurality of primary particles. The particle size of the primary particles may be 0.05 μm or more and 1 μm or less. The average particle size of the secondary particles of the composite oxide HN may be 3 μm or more and 30 μm or less, or 5 μm or more and 25 μm or less. In this specification, the average particle size means the above-mentioned median diameter (D50).

[0081] (Others) As the binder, for example, a resin material is used. Examples of the binder include fluororesin, polyolefin resin, polyamide resin, polyimide resin, acrylic resin, vinyl resin, and rubber-like material (for example, styrene butadiene copolymer (SBR)). One type of binder may be used alone, or two or more types may be used in combination.

[0082] Examples of thickeners include cellulose derivatives such as cellulose ether. Examples of cellulose derivatives include carboxymethyl cellulose (CMC) and its modified forms, methyl cellulose, etc. One type of thickener may be used alone, or two or more types may be used in combination.

[0083] Examples of conductive materials include carbon nanotubes (CNTs), carbon fibers other than CNTs, and conductive particles (for example, carbon black).

[0084] The dispersion medium used in the positive electrode slurry is not particularly limited, but examples thereof include water, alcohol, N-methyl-2-pyrrolidone (NMP), and mixed solvents thereof.

[0085] The positive electrode current collector may be, for example, a metal foil. The positive electrode current collector may be porous. Examples of porous current collectors include nets, punched sheets, and expanded metals. Examples of materials for the positive electrode current collector include stainless steel, aluminum, aluminum alloys, and titanium. The thickness of the positive electrode current collector is not particularly limited and may be in the range of 1 to 50 μm (for example, in the range of 5 to 30 μm).

[0086] (Negative Electrode) The negative electrode includes a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector. The negative electrode mixture layer includes a negative electrode active material.

[0087] The negative electrode mixture layer contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a thickener, and a conductive agent.

[0088] (Negative electrode active material) As the negative electrode active material, metallic lithium, a lithium alloy, or the like may be used, but a material capable of electrochemically absorbing and releasing lithium ions is preferably used. Examples of such materials include carbonaceous materials and Si-containing materials. The negative electrode mixture layer may contain one type of negative electrode active material or a combination of two or more types.

[0089] Examples of carbonaceous materials include graphite, easily graphitizable carbon (soft carbon), and hardly graphitizable carbon (hard carbon). One type of carbonaceous material may be used alone, or two or more types may be used in combination. Among these, graphite is preferred as the carbonaceous material because it has excellent charge / discharge stability and a small irreversible capacity. Examples of graphite include natural graphite, artificial graphite, and graphitized mesophase carbon particles.

[0090] Examples of the Si-containing material include simple Si, silicon alloys, silicon compounds (such as silicon oxides), and composite materials in which a silicon phase is dispersed in a lithium ion conductive phase (matrix). Examples of silicon oxides include SiO ｘ The x may be, for example, 0.5≦x<2, or 0.8≦x≦1.6. The lithium ion conductive phase may be SiO ２At least one selected from the group consisting of a lithium silicate phase, a silicate phase (for example, a lithium silicate phase), and a carbon phase may be used.

[0091] As described above, the negative electrode mixture layer of the nonaqueous electrolyte secondary battery (B) contains graphite as a negative electrode active material. The negative electrode mixture layer of the secondary battery (B) may further contain a Si-containing material as a negative electrode active material.

[0092] The proportion of the Si-containing material in the negative electrode active material may be 1% by mass or more, or 10% by mass or more, and may be 3% by mass or less, or 7% by mass or less.

[0093] The binder, thickener, conductive agent, and dispersion medium used in the negative electrode slurry are not particularly limited, and the materials exemplified for the positive electrode may be used.

[0094] The negative electrode current collector may be, for example, a metal foil. The negative electrode current collector may be porous. Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloy, copper, and copper alloy. The thickness of the negative electrode current collector is not particularly limited and may be in the range of 1 to 50 μm (for example, in the range of 5 to 30 μm).

[0095] The negative electrode mixture layer may be formed by applying a negative electrode slurry, in which the components of the negative electrode mixture are dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the applied film. The dried coating film may be rolled as necessary.

[0096] The negative electrode mixture layer may be composed of multiple layers to differentiate the internal porosity of the graphite particles between the inner and outer layers. By varying the internal porosity of the graphite particles in each layer, the average internal porosity of the graphite particles in the inner layer and the average internal porosity of the graphite particles in the outer layer can be adjusted. Graphite particles with various internal porosities are commercially available, so it is possible to use graphite particles with an appropriate internal porosity. Graphite particles with different internal porosities may also be produced by known methods.

[0097] (Separator) It is desirable to interpose a separator between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. As the separator, a microporous thin film, woven fabric, nonwoven fabric, etc. can be used. As the material of the separator, polyolefin such as polypropylene or polyethylene is preferred.

[0098] The electrode group of the nonaqueous electrolyte secondary battery (B) may be a wound electrode group. In a wound electrode group, a positive electrode and a negative electrode are wound with a separator interposed therebetween. Alternatively, the electrode group may be a stacked electrode group. A stacked electrode group is formed by stacking a flat positive electrode and a flat negative electrode in one direction with a separator interposed therebetween. The shape of the nonaqueous electrolyte secondary battery (B) is not limited, and may be a cylindrical type, a prismatic type, a coin type, a button type, a laminate type, or the like.

[0099] (Method for manufacturing non-aqueous electrolyte secondary battery (B)) The method for manufacturing the non-aqueous electrolyte secondary battery (B) is not particularly limited except for the method for forming the negative electrode mixture layer. In one example of the manufacturing method, first, an electrode group is formed using the negative electrode formed by the above method, a positive electrode, and a separator. Next, the electrode group and the non-aqueous electrolyte are enclosed in an exterior body, thereby manufacturing the secondary battery (B).

[0100] An example of an embodiment according to the present disclosure will be specifically described below with reference to the drawings. The embodiment described below can be modified based on the above description. Furthermore, the matters described below may be applied to the above embodiment. Furthermore, in the embodiment described below, matters that are not essential to the invention according to the present disclosure may be omitted.

[0101] Embodiment 1 In Embodiment 1, an example of a prismatic nonaqueous electrolyte secondary battery will be described as an example of the nonaqueous electrolyte secondary battery (B).

[0102] Fig. 1 is a schematic diagram showing a partially exploded perspective view of a nonaqueous electrolyte secondary battery according to Embodiment 1. The nonaqueous electrolyte secondary battery 10 shown in Fig. 1 includes a battery case 4 in the shape of a rectangular tubular battery with a bottom, and an electrode group 1 and a nonaqueous electrolyte (not shown) housed in the battery case 4. The nonaqueous electrolyte includes a heterocyclic compound (C). The negative electrode mixture layer of the negative electrode includes the graphite particles described above.

[0103] The electrode group 1 includes a strip-shaped negative electrode, a strip-shaped positive electrode, and a separator disposed therebetween. The negative electrode current collector of the negative electrode is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector of the positive electrode is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. The periphery of the sealing plate 5 is fitted into the open edge of the battery case 4, and the fitting portion is laser welded. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The sealing plate 5 has an injection hole for a non-aqueous electrolyte. The injection hole is closed with a seal plug 8 after the non-aqueous electrolyte is injected.

[0104] A partial cross section of an example of a negative electrode is shown schematically in Fig. 2. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode mixture layer 22 formed on both sides of the negative electrode current collector 21. When the negative electrode mixture layer 22 is divided into two equal parts in the thickness direction, the portion on the negative electrode current collector 21 side is an inner layer 22a, and the portion on the outer surface side is an outer layer 22b.

[0105] The anode mixture layer 22 may be formed by stacking multiple anode mixture layers using different anode slurries. For example, the inner layer 22a may be formed using a first anode slurry, and the outer layer 22b may be formed using a second anode slurry. In this case, the internal porosity of the graphite in the first anode slurry and the internal porosity of the graphite in the second anode slurry are selected so that the internal porosity of the graphite satisfies the above condition. For example, the content of the first graphite particles and the second graphite particles in the first anode slurry and the content of the first graphite particles and the second graphite particles in the second anode slurry may be selected so that the above condition is satisfied.

[0106] The thickness Dx of the first layer 22x formed with the first anode slurry may be the same as, smaller than, or larger than the thickness Dy of the second layer 22y formed with the second anode slurry. FIG. 3 shows a cross section of the anode mixture layer 22 in which the first layer 22x is thinner than the second layer 22y. Even in this case, the inner layer 22a and the outer layer 22b obtained by dividing the anode mixture layer 22 into two equal parts in the thickness direction satisfy the above-mentioned conditions. The anode mixture layer 22 may be formed of three or more layers. Even in this case, the inner layer 22a and the outer layer 22b obtained by dividing the anode mixture layer 22 into two equal parts in the thickness direction satisfy the above-mentioned conditions.

[0107] (Additional Notes) The above description discloses the following technology: (Technology 1) A non-aqueous electrolyte secondary battery comprising: a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector and containing a negative electrode active material, the negative electrode mixture layer contains graphite particles as the negative electrode active material, when the negative electrode mixture layer is divided into equal halves in a thickness direction into an inner layer on the negative electrode current collector side and an outer layer on the outer surface side, the average value of internal porosity of the graphite particles present in the outer layer is smaller than the average value of internal porosity of the graphite particles present in the inner layer, the non-aqueous electrolyte comprises a heterocyclic compound, a heterocycle in the heterocyclic compound contains nitrogen and sulfur, and the heterocyclic compound has an electron-withdrawing group R containing oxygen and / or nitrogen. (Technology 2) The nonaqueous electrolyte secondary battery according to Technology 1, wherein the graphite particles include first graphite particles and second graphite particles, the internal porosity of the first graphite particles is smaller than the internal porosity of the second graphite particles, and the content R1out of the first graphite particles in the outer layer is larger than the content R1inn of the first graphite particles in the inner layer. (Technology 3) The nonaqueous electrolyte secondary battery according to Technology 2, wherein the internal porosity of the first graphite particles is 5% or less, and the internal porosity of the second graphite particles is in the range of 8 to 20%. (Technology 4) The nonaqueous electrolyte secondary battery according to any one of Technology 1 to 3, wherein the electron-withdrawing group R includes at least one selected from the group consisting of a carbonyl group, a nitrile group, a sulfonyl group, an isocyanate group, an isothiocyanate group, and a hydroxy group, and the hydroxy group is bonded to a carbon constituting a saturated hydrocarbon group. (Technology 5) The nonaqueous electrolyte secondary battery according to Technology 4, wherein the carbonyl group is included in at least one selected from the group consisting of an aldehyde group, a ketone group, an amide bond, an ester bond, and a carboxy group. (Technology 6) The nonaqueous electrolyte secondary battery according to any one of Technology 1 to Technology 5, wherein the heterocycle includes at least one selected from the group consisting of a thiazole ring, a thiomorpholine ring, and a thiazepine ring.(Technology 7) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 6, wherein the heterocyclic compound includes at least one selected from the group consisting of 4,5-dimethyl-1,3-thiazole-2-carbaldehyde, 4,5,6,7-tetrahydro-1,3-benzothiazole-2-carbaldehyde, 2-ethylthiomorpholine-4-carbaldehyde, and 2,3-dimethylthiomorpholine-4-carbaldehyde. (Technology 8) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 7, wherein the content of the heterocyclic compound in the nonaqueous electrolyte is in the range of 0.1% by mass to 2.0% by mass. (Technology 9) The nonaqueous electrolyte secondary battery according to any one of Technologies 1 to 8, wherein the negative electrode mixture layer further includes a Si-containing material as the negative electrode active material.

[0108] EXAMPLES The present disclosure will be specifically described below based on examples. The present disclosure is not limited to the following examples. In these examples, a plurality of non-aqueous electrolyte secondary batteries were fabricated and evaluated.

[0109] (Battery A1) Battery A1 was fabricated by the following procedure. (1) Fabrication of Positive Electrode Positive electrode active material particles (LiNi ０．８８ Co ０．０９ Al ０．０３ O ２ 100 parts by mass of ethylenediamine fluoride (E1), 1 part by mass of carbon nanotubes, 1 part by mass of polyvinylidene fluoride, and an appropriate amount of N-methyl-2-pyrrolidone (NMP) were mixed to prepare a positive electrode slurry. Next, the positive electrode slurry was applied to an aluminum foil to form a coating film. Next, the coating film was dried and then rolled. In this way, a positive electrode including an aluminum foil and a positive electrode mixture layer formed on the aluminum foil was obtained.

[0110] (2) Preparation of Negative Electrode A first layer negative electrode slurry was prepared by mixing 98 parts by mass of the negative electrode active material (second graphite particles), 1 part by mass of a sodium salt of carboxymethyl cellulose (CMC-Na), 1 part by mass of SBR, and an appropriate amount of water. Further, 98 parts by mass of the negative electrode active material (graphite particles), 1 part by mass of a sodium salt of carboxymethyl cellulose (CMC-Na), 1 part by mass of SBR, and an appropriate amount of water were mixed to prepare a second layer negative electrode slurry. The graphite particles used were a mixture of first graphite particles and second graphite particles in a mass ratio of 50:50. In preparing the first layer negative electrode slurry and the second layer negative electrode slurry, the first graphite particles had an internal porosity Vr1 of 3%, and the second graphite particles had an internal porosity Vr2 of 15%.

[0111] Next, the first layer negative electrode slurry was applied to a copper foil (negative electrode current collector) to form a first layer. Next, the second layer negative electrode slurry was applied to the first layer to form a second layer. Next, a laminate consisting of the negative electrode current collector, the first layer, and the second layer was rolled. In this way, a negative electrode including copper foil and a negative electrode mixture layer formed on the copper foil was obtained. The thickness of the first layer and the thickness of the second layer were the same. That is, the first layer was the inner layer described above, and the second layer was the outer layer described above.

[0112] (3) Preparation of non-aqueous electrolyte (electrolyte solution) An electrolyte solution (nonaqueous electrolyte) was prepared by dissolving LiPF6 and compound (C), a heterocyclic compound, in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (EC:EMC = 3:7 (volume ratio)). The concentration of LiPF6 in the electrolyte solution was 1.0 mol / L. 4,5-dimethyl-1,3-thiazole-2-carbaldehyde was used as compound (C). The content (concentration) of compound (C) in the electrolyte solution was the value shown in Table 2.

[0113] The structure of 4,5-dimethyl-1,3-thiazole-2-carbaldehyde used as the heterocyclic compound (C) is shown below.

[0114]

[0115] (4) Fabrication of a Non-Aqueous Electrolyte Secondary Battery The positive electrode was cut into a predetermined shape. Next, a portion of the positive electrode mixture layer was scraped off to expose the positive electrode current collector, forming a connection area with the tab lead. In this way, a positive electrode including a region that functions as a positive electrode (size: 20 mm × 20 mm) and a connection area with the tab lead was obtained. Next, the exposed portion of the positive electrode current collector was connected to the positive electrode tab lead. Next, a predetermined region on the periphery of the positive electrode tab lead was covered with an insulating tab film. In this way, a positive electrode for evaluation was obtained.

[0116] The negative electrode was cut into the same shape as the positive electrode. Next, the same processing as the positive electrode was performed to obtain a negative electrode including a region that functions as a negative electrode and a region that connects to the tab lead. Next, the exposed portion of the negative electrode current collector was connected to the negative electrode tab lead. Next, a predetermined region on the periphery of the negative electrode tab lead was covered with an insulating tab film. In this way, a negative electrode for evaluation was obtained.

[0117] A battery was fabricated using the evaluation positive and negative electrodes. First, the positive and negative electrodes were arranged with the positive electrode mixture layer and the negative electrode mixture layer facing each other via a separator to obtain an electrode group. A polyethylene separator (12 μm thick) was used as the separator. Next, a rectangular (60 mm × 90 mm) Al laminate film (100 μm thick) was cut and folded in half. Next, the 60 mm long edge of the folded laminate film was heat-sealed to form a 60 mm × 45 mm cylindrical shape. The fabricated electrode group was then placed into the cylinder. Next, the end face of the Al laminate film was aligned with the heat-sealed resin of each tab lead and sealed. Next, nonaqueous electrolyte was injected from the short side of the Al laminate film that was not heat-sealed, impregnating each mixture layer with the nonaqueous electrolyte. Finally, the end face of the Al laminate film on the injected side was sealed. In this manner, a battery A1 (nonaqueous electrolyte secondary battery) for evaluation was obtained.

[0118] (Batteries A2 to A3, Batteries C1 to C3) Batteries A2 to A3 and Batteries C1 to C3 were produced by the same method and under the same conditions as those for producing Battery A1, except that the ratio of the first graphite particles to the second graphite particles in the first layer (inner layer) and the second layer (outer layer), the internal porosity Vr1 of the first graphite particles, the internal porosity Vr2 of the second graphite particles, and the concentration of the heterocyclic compound (C) in the electrolyte were set to the values ​​shown in Table 2. The heterocyclic compound (C) used was the same compound as the heterocyclic compound (C) used in producing Battery A1. No heterocyclic compound (C) was added to the electrolyte of Batteries C1 and C2.

[0119] The internal resistance of the batteries A1 to A3 and C1 to C3 was measured. Table 2 shows some of the manufacturing conditions for the batteries and the measured internal resistance. It is preferable that the internal resistance is low.

[0120]

[0121] Batteries A1 to A3 are nonaqueous electrolyte secondary batteries (B) according to the present disclosure. Batteries C1 to C3 are comparative examples. In batteries A1 to A3 and C1, the average internal porosity VAout of the graphite particles present in the outer layer is smaller than the average internal porosity VAinn of the graphite particles present in the inner layer. In batteries C2 and C3, the average internal porosity VAout of the graphite particles present in the outer layer is the same as the average internal porosity VAinn of the graphite particles present in the inner layer.

[0122] As shown in Table 2, the resistance could be reduced by adding compound (C) to the electrolyte. Furthermore, the resistance could be significantly reduced by making the average value of the internal porosity VAout of the graphite particles in the outer layer smaller than the average value of the internal porosity VAinn of the graphite particles in the inner layer. Note that the same effect was obtained when compound (C) was changed to any of 4,5,6,7-tetrahydro-1,3-benzothiazole-2-carbaldehyde, 2-ethylthiomorpholine-4-carbaldehyde, and 2,3-dimethylthiomorpholine-4-carbaldehyde.

[0123] The present disclosure can be used in non-aqueous electrolyte secondary batteries. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be interpreted as limiting. Various modifications and alterations will undoubtedly become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Therefore, the appended claims should be construed to cover all modifications and alterations without departing from the true spirit and scope of the present invention.

[0124] 1: Electrode group 5: Sealing plate 6: Negative electrode terminal 7: Resin gasket 10: Non-aqueous electrolyte secondary battery 20: Negative electrode 21: Negative electrode current collector 22: Negative electrode mixture layer 22a: Inner layer 22b: Outer layer

Claims

1. A non-aqueous electrolyte secondary battery comprising: a positive electrode, a negative electrode, and a non-aqueous electrolyte; the negative electrode comprises a negative electrode current collector; and a negative electrode mixture layer disposed on the negative electrode current collector and containing a negative electrode active material; the negative electrode mixture layer contains graphite particles as the negative electrode active material; when the negative electrode mixture layer is divided into two equal halves in a thickness direction into an inner layer on the negative electrode current collector side and an outer layer on an outer surface side, the average value of the internal porosity of the graphite particles present in the outer layer is smaller than the average value of the internal porosity of the graphite particles present in the inner layer; the non-aqueous electrolyte comprises a heterocyclic compound; a heterocycle in the heterocyclic compound contains nitrogen and sulfur; and the heterocyclic compound has an electron-withdrawing group R containing oxygen and / or nitrogen.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the graphite particles include first graphite particles and second graphite particles, the internal porosity of the first graphite particles is smaller than the internal porosity of the second graphite particles, and the content R1out of the first graphite particles in the outer layer is greater than the content R1inn of the first graphite particles in the inner layer.

3. The nonaqueous electrolyte secondary battery according to claim 2, wherein the internal porosity of the first graphite particles is 5% or less, and the internal porosity of the second graphite particles is in the range of 8 to 20%.

4. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the electron-withdrawing group R comprises at least one selected from the group consisting of a carbonyl group, a nitrile group, a sulfonyl group, an isocyanate group, an isothiocyanate group, and a hydroxy group, and the hydroxy group is bonded to a carbon constituting a saturated hydrocarbon group.

5. The nonaqueous electrolyte secondary battery according to claim 4, wherein the carbonyl group is contained in at least one selected from the group consisting of an aldehyde group, a ketone group, an amide bond, an ester bond, and a carboxy group.

6. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the heterocycle includes at least one selected from the group consisting of a thiazole ring, a thiomorpholine ring, and a thiazepine ring.

7. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the heterocyclic compound includes at least one selected from the group consisting of 4,5-dimethyl-1,3-thiazole-2-carbaldehyde, 4,5,6,7-tetrahydro-1,3-benzothiazole-2-carbaldehyde, 2-ethylthiomorpholine-4-carbaldehyde, and 2,3-dimethylthiomorpholine-4-carbaldehyde.

8. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of said heterocyclic compound in said non-aqueous electrolyte is in the range of 0.1% by mass to 2.0% by mass.

9. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the negative electrode mixture layer further contains a Si-containing material as the negative electrode active material.

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