Non-aqueous electrolyte secondary battery

Abstract

Provided is a nonaqueous electrolyte secondary battery, which is a nonaqueous electrolyte secondary battery in which a nonaqueous electrolyte contains lithium bis(oxalato)borate, and which has reduced initial resistance and high resistance to lithium metal deposition. The nonaqueous electrolyte secondary battery disclosed herein includes a nonaqueous electrolyte, and an electrode body including a positive electrode, a negative electrode, and a separator. The negative electrode includes a negative electrode active material layer. The nonaqueous electrolyte contains lithium bis(oxalato)borate. The Na content in the negative electrode active material layer is 311 μg / g or less, as determined by laser ablation ICP mass spectrometry.

Description

TECHNICAL FIELD

[0001] The present application relates to a nonaqueous electrolyte secondary battery. BACKGROUND

[0002] In recent years, nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries are preferably used for portable power sources such as personal computers, mobile terminals, and the like, power sources for driving vehicles such as electric vehicles (BEV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and the like.

[0003] It is known that lithium bis(oxalato)borate (LiBOB) is added to the nonaqueous electrolyte of a nonaqueous electrolyte secondary battery. By the addition of LiBOB, a good film is formed at the negative electrode, and dissolution of transition metal from the positive electrode active material can be prevented, whereby resistance increase can be suppressed. On the other hand, Na as an impurity is mixed in the nonaqueous electrolyte secondary battery. This mixed-in Na can react with LiBOB to generate sodium bis(oxalato)borate (NaBOB).

[0004] Therefore, in order to reduce the amount of NaBOB generated in the nonaqueous electrolyte secondary battery, a technique is known in which an electrode is washed with an electrolyte containing LiBOB. For example, Patent Document 1 discloses a technique in which a laminated electrode body is produced using an electrode containing Na as an impurity, one end of the laminated electrode group in a direction orthogonal to the lamination direction is dipped in an electrolyte containing LiBOB, the electrolyte is allowed to permeate toward the other end opposite to the one end, and thereafter, a region of the laminated electrode body including the other end is removed. According to this technique, when the electrolyte permeates in the electrode body, Na contained in the electrode reacts with LiBOB, and the generated NaBOB moves toward the other end of the electrode body along with the permeation of the electrolyte. By removing the region including the other end, NaBOB can be removed to some extent.

[0005] PRIOR ART DOCUMENTS

[0006] PATENT DOCUMENTS

[0007] Patent Document 1: Japanese Patent Application Publication No. 2018-26297 SUMMARY

[0008] However, the present inventors and others have conducted intensive studies, and as a result, have found that in the above-described conventional technique, there is room for improvement in terms of reduction of initial resistance and improvement of metal Li deposition resistance.

[0009] In view of the above, an object of the present application is to provide a nonaqueous electrolyte secondary battery that is a nonaqueous electrolyte secondary battery in which the nonaqueous electrolyte contains lithium bis(oxalato)borate, and that has reduced initial resistance and high metal Li deposition resistance.

[0010] The present inventors have intensively studied the amount of Na in each component of the battery. As a result, it has been found that the amount of Na can be greatly reduced by improving the thickener and the binder used for the negative electrode. Furthermore, the present inventors have made further studies, and as a result, it has been found that, among the Na contained in the components of the battery, the Na contained in the negative electrode greatly adversely affects the characteristics of the battery.

[0011] Therefore, the nonaqueous electrolyte secondary battery disclosed herein has a nonaqueous electrolyte and an electrode body including a positive electrode, a negative electrode, and a separator. The negative electrode has a negative electrode active material layer. The nonaqueous electrolyte contains lithium bis(oxalato)borate. The Na content in the negative electrode active material layer is 311 μg / g or less as measured by laser ablation ICP mass spectrometry.

[0012] According to such a configuration, a nonaqueous electrolyte secondary battery is provided, which is a nonaqueous electrolyte secondary battery in which the nonaqueous electrolyte contains lithium bis(oxalato)borate, and in which the initial resistance is reduced and the metal Li deposition resistance is high.

[0013] In a preferred embodiment of the nonaqueous electrolyte secondary battery disclosed herein, the positive electrode has a positive electrode active material layer. The proportion (%) of the Na content in the negative electrode active material layer to the total of the Na content in the negative electrode active material layer, the Na content in the positive electrode active material layer, and the Na content in the separator is 33% or less. According to such a configuration, the initial resistance becomes even smaller, and the metal Li deposition resistance becomes even higher.

[0014] In a preferred embodiment of the nonaqueous electrolyte secondary battery disclosed herein, when the resistance distribution is measured in the short edge direction of the main surface of the negative electrode active material layer, the proportion of the resistance value at the position with the highest resistance to the resistance value at the position with the lowest resistance is 1.10 or less. According to such a configuration, the initial resistance becomes even smaller, and the metal Li deposition resistance becomes even higher.

[0015] In a preferred embodiment of the nonaqueous electrolyte secondary battery disclosed herein, the negative electrode active material layer contains a negative electrode active material, a binder, and a thickener. The thickener is a carboxymethylcellulose salt, and at least a part of the cations in the carboxymethylcellulose salt is Li ion. According to such a configuration, the initial resistance becomes even smaller, and the metal Li deposition resistance becomes even higher.

[0016] In a preferred embodiment of the nonaqueous electrolyte secondary battery disclosed herein, the negative electrode active material layer contains a negative electrode active material and an acrylic binder that does not contain Na. According to such a configuration, the initial resistance becomes even smaller, and the metal Li deposition resistance becomes even higher.

[0017] In one preferred embodiment of the non-aqueous electrolyte secondary battery disclosed herein, the electrode body is a wound electrode body. This configuration further enhances the initial resistance reduction effect. Attached Figure Description

[0018] Figure 1 This is a cross-sectional view schematically illustrating the internal structure of a lithium-ion secondary battery according to one embodiment of the present invention.

[0019] Figure 2 This is a schematic exploded view showing the configuration of the wound electrode body of a lithium-ion secondary battery according to one embodiment of the present invention.

[0020] Symbol Explanation

[0021] 20. Winded electrode body

[0022] 30 Battery casing

[0023] 36 Safety valve

[0024] 42 Positive extremes

[0025] 42a Positive Current Collector

[0026] 44 Negative extremes

[0027] 44a Negative Current Collector

[0028] 50 Positive Electrode Sheets (Positive Electrode)

[0029] 52 Positive current collector

[0030] 52a Non-forming portion of the positive electrode active material layer

[0031] 54 Positive electrode active material layer

[0032] 60 Negative electrode plate (negative electrode)

[0033] 62 Negative current collector

[0034] 62a Non-forming portion of the negative electrode active material layer

[0035] 64 Negative Electrode Active Material Layer

[0036] 70. Insulation plate (isolation component)

[0037] 80 Non-aqueous electrolyte

[0038] 100 Lithium-ion Secondary Battery Detailed Implementation

[0039] Hereinafter, an embodiment of the present application will be described with reference to the drawings. It should be noted that matters not mentioned in the present specification and matters necessary for the implementation of the present application can be understood as design matters of those skilled in the art based on the prior art in the field. The present application can be implemented based on the content disclosed in the present specification and technical common sense in the field. In addition, in the following drawings, the same symbols are assigned to components and portions that play the same roles, and descriptions thereof will be omitted. In addition, the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship.

[0040] It should be noted that the "secondary battery" in the present specification refers to a storage device that can be repeatedly charged and discharged, and is a term including storage elements such as so-called storage batteries and electric double layer capacitors. In addition, the "lithium ion secondary battery" in the present specification refers to a secondary battery that uses lithium ions as charge carriers and realizes charge and discharge by migration of charges of lithium ions between positive and negative electrodes.

[0041] Hereinafter, the present application will be described in detail taking a flat square-shaped lithium ion secondary battery provided with a wound electrode body as an example, but is not intended to limit the present application to the scheme described in this embodiment.

[0042] Figure 1 The lithium ion secondary battery 100 illustrated is a sealed battery constructed by housing a flat square-shaped wound electrode body 20 and a non-aqueous electrolyte 80 in a flat square-shaped battery case (i.e., an exterior container) 30. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 provided in a manner to release the internal pressure of the battery case 30 when the internal pressure rises to a predetermined level or more. In addition, the battery case 30 is provided with an injection port (not illustrated) for injecting the non-aqueous electrolyte 80. The positive electrode terminal 42 is electrically connected to the positive electrode current collector 42a. The negative electrode terminal 44 is electrically connected to the negative electrode current collector 44a. As the material of the battery case 30, for example, a lightweight metal material having good thermal conductivity such as aluminum can be used.

[0043] As Figure 1 and Figure 2As shown, the wound electrode body 20 has a configuration in which the positive electrode sheet 50 and the negative electrode sheet 60 are overlapped with two long strip-shaped separator sheets 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed on one or both surfaces (here, both surfaces) of a long strip-shaped positive electrode current collector 52 in the longitudinal direction. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed on one or both surfaces (here, both surfaces) of a long strip-shaped negative electrode current collector 62 in the longitudinal direction. A positive electrode active material layer non-formed portion 52a (i.e., a portion in which the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 being formed thereon) and a negative electrode active material layer non-formed portion 62a (i.e., a portion in which the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 being formed thereon) are formed in a manner in which they protrude outward from both ends in the winding axis direction (i.e., the sheet width direction orthogonal to the above-described longitudinal direction) of the wound electrode body 20. The positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a are respectively joined to the positive electrode current collector plate 42a and the negative electrode current collector plate 44a.

[0044] As the positive electrode current collector 52 constituting the positive electrode sheet 50, for example, an aluminum foil or the like can be given. As the positive electrode active material included in the positive electrode active material layer 54, for example, a lithium transition metal oxide (e.g., LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNiO2, LiCoO2, LiFeO2, LiMn2O4, LiNi 0.5 Mn 1.5 O4, etc.), a lithium transition metal phosphate compound (e.g., LiFePO4, etc.), or the like can be given.

[0045] The positive electrode active material layer 54 can include components other than the active material, such as a conductive material, a binder, or the like. As the conductive material, for example, a carbon black such as acetylene black (AB), and other carbon materials (e.g., graphite, etc.) can be appropriately used. As the binder, for example, polyvinylidene fluoride (PVDF), etc. can be used.

[0046] The separator 70 is a porous member, and as the separator, a porous sheet (film) composed of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, polyamide, etc. can be appropriately used. The above-described porous sheet can be a single layer structure, or a laminated structure of two or more layers (e.g., a three-layer structure in which a PP layer is laminated on both surfaces of a PE layer).

[0047] A heat-resistant layer (HRL) can be provided on the surface of the separator 70. The HRL can be the same as the heat-resistant layer provided in the separator of a known nonaqueous electrolyte secondary battery. For example, a ceramic particle such as alumina, silica, boehmite, magnesium oxide, titanium dioxide, etc., and a binder such as PVDF, etc. are included.

[0048] As the negative electrode current collector 62 constituting the negative electrode sheet 60, for example, a copper foil or the like can be given. As the negative electrode active material contained in the negative electrode active material layer 64, for example, a carbon material such as graphite, hard carbon, soft carbon or the like can be used. The negative electrode active material layer 64 can contain components other than the active material, such as a binder, a thickening agent or the like.

[0049] Na from impurities of the positive electrode active material, a binder of the positive electrode active material layer 54, impurities within the HRL of the separator 70, a binder and a thickening agent of the negative electrode active material layer 64, and the like can exist inside the lithium ion secondary battery 100. This Na reacts with LiBOB to generate NaBOB, which can cause adverse effects on the battery characteristics such as initial resistance. Through intensive studies by the present inventors and the like, it was found that, as shown in the results of the examples and comparative examples described later, among the Na contained in the components constituting the battery, the Na contained in the negative electrode greatly adversely affects the battery characteristics. Therefore, in the present embodiment, the Na content in the negative electrode active material layer 64 is 311 μg / g or less as measured by laser ablation ICP mass spectrometry. When the Na content is in such a range, the initial resistance is significantly reduced, and furthermore, the metal Li deposition resistance is significantly improved. From the viewpoints of the smaller initial resistance and the higher metal Li deposition resistance, the Na content in the negative electrode active material layer 64 is preferably 200 μg / g or less, more preferably 100 μg / g or less, further preferably 50 μg / g or less, and most preferably 10 μg / g or less.

[0050] On the other hand, the Na content in the positive electrode active material layer 54 as measured by laser ablation ICP mass spectrometry is not particularly limited, and can be 100 μg / g or more, 150 μg / g or more, or 180 μg / g or more, and can be 300 μg / g or less or 250 μg / g or less. In addition, the Na content in the separator 70 as measured by laser ablation ICP mass spectrometry is not particularly limited, and can be 100 μg / g or more, 150 μg / g or more, or 200 μg / g or more, and can be 300 μg / g or less or 250 μg / g or less.

[0051] Note that the laser ablation ICP mass spectrometry can be performed using a publicly known laser ICP mass spectrometry (LA-ICP-MS) device.

[0052] The negative electrode active material layer 64 is not particularly limited in composition as long as the Na content is 311 μg / g or less.

[0053] As one of the methods for reducing the content of Na in the negative electrode active material layer 64, a method for reducing the content of Na as an impurity in the binder can be given. As the binder used in the negative electrode active material layer, styrene butadiene rubber (SBR) is most commonly used. However, SBR contains NaOH used at the time of synthesis thereof as an impurity. Therefore, as the binder, by using a binder synthesized without using a Na-containing component, the content of Na in the negative electrode active material layer 64 can be reduced. Specifically, as the binder, by using styrene butadiene rubber synthesized using LiOH instead of NaOH, the content of Na in the negative electrode active material layer 64 can be reduced.

[0054] In addition, in the research by the present inventors, it was found that by improving the thickening agent used for the negative electrode, the amount of Na can be greatly reduced. Specifically, as the thickening agent used in the negative electrode active material layer, carboxymethyl cellulose (CMC) is most commonly used, and since NaOH is used at the time of synthesis thereof, a part of the carboxyl group forms a salt with Na ions. Therefore, the CMC used in the negative electrode contains Na. That is, the CMC used as the thickening agent in the negative electrode active material layer can also be referred to as a Na salt of CMC. Therefore, as the thickening agent, by using a thickening agent synthesized without using a Na-containing component, the content of Na in the negative electrode active material layer 64 can be reduced. Specifically, by using CMC synthesized using LiOH as the thickening agent, the content of Na in the negative electrode active material layer 64 can be reduced. The CMC synthesized using this LiOH is a salt of CMC, and can be referred to as a salt in which at least a part of the cations contains Li, and as the thickening agent, a lithium salt of CMC is preferable. Among the lithium salts of CMC, a salt in which 80 to 90 mol% of the carboxyl group forms a salt with Li is preferable.

[0055] In addition, by using a binder that has both the functions of a thickening agent and a binder and is synthesized without using a Na-containing component, the content of Na in the negative electrode active material layer 64 can be reduced. The binder synthesized without using a Na-containing component can be referred to as a binder that does not contain Na. As an example of such a binder, an acrylic binder synthesized without using a Na-containing component (i.e., an acrylic binder that does not contain Na) can be given. Therefore, one preferable mode of the negative electrode active material layer 64 contains a negative electrode active material and an acrylic binder that does not contain Na, and a more preferable mode contains only a negative electrode active material and an acrylic binder that does not contain Na.

[0056] The content of the negative electrode active material in the negative electrode active material layer 64 is not particularly limited, and is preferably 70% by mass or more, more preferably 80% by mass or more, and further preferably 90% by mass or more. The content of the binder in the negative electrode active material layer 64 is not particularly limited, and is preferably 0.1% to 8% by mass, more preferably 0.2% to 3% by mass, and further preferably 0.3% to 2% by mass. The content of the thickening agent in the negative electrode active material layer 64 is not particularly limited, and is preferably 0.3% to 3% by mass, more preferably 0.4% to 2% by mass.

[0057] The proportion (%) of the content of Na in the negative electrode active material layer 64 with respect to the total of the content of Na in the positive electrode active material layer 54, the content of Na in the negative electrode active material layer 64, and the content of Na in the separator 70 is, for example, 45% or less, preferably 33% or less, more preferably 10% or less, further preferably 5% or less, and most preferably 3% or less, from the viewpoint of a smaller initial resistance and higher resistance to precipitation of metal Li.

[0058] The proportion of the resistance value of the position of the highest resistance with respect to the resistance value of the position of the lowest resistance is, for example, 1.17 or less, preferably 1.10 or less, more preferably 1.07 or less, and further preferably 1.05 or less, when the resistance distribution is measured along the short side direction (i.e., the width direction) of the main surface of the negative electrode active material layer 64, from the viewpoint of a smaller initial resistance and higher resistance to precipitation of metal Li. Note that, in the wound electrode body 20, the position of the highest resistance is usually in the central portion in the winding axis direction (specifically, a region from the center to ±20%, and particularly a region from the center to ±10%).

[0059] Note that the resistance distribution measurement can be performed by measuring the resistance value at a prescribed interval (for example, 5 mm intervals from the end portion to 30% of the total width of the negative electrode active material layer 64, and 2 mm intervals in the central portion (40% of the remaining portion)) along the short side direction of the main surface of the negative electrode active material layer 64 using the AC impedance method.

[0060] The nonaqueous electrolyte 80 contains lithium bis(oxalato)borate (LiBOB). In addition, the nonaqueous electrolyte 80 typically contains a nonaqueous solvent and a supporting salt. As the nonaqueous solvent, various kinds of organic solvents such as carbonates, ethers, esters, nitriles, sulfones, lactones, and the like used in electrolytes of general lithium-ion secondary batteries can be used without particular limitation. Among them, carbonates are preferred, and as specific examples thereof, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), and the like can be given. Such a nonaqueous solvent can be used alone as one kind or in combination of two or more kinds.

[0061] As the supporting salt, for example, lithium salts such as LiPF6, LiBF4, LiClO4, and the like (LiPF6is preferred) can be appropriately used. The concentration of the supporting salt is preferably 0.7 mol / L to 1.3 mol / L.

[0062] The content of LiBOB in the nonaqueous electrolyte 80 is, for example, 0.1% by mass or more, preferably 0.3% by mass or more, and more preferably 0.5% by mass or more. On the other hand, the content of LiBOB in the nonaqueous electrolyte 80 is, for example, 1.5% by mass or less, preferably 1.0% by mass or less, and more preferably 0.7% by mass or less.

[0063] Note that the above-described nonaqueous electrolyte 80 can contain various additives such as a gas generating agent such as biphenyl (BP), cyclohexylbenzene (CHB), and the like; a coating film forming agent such as vinylene carbonate (VC) and the like; a dispersant; a thickening agent; and the like, as long as the effects of the present application are not significantly impaired.

[0064] The lithium-ion secondary battery 100 configured as described above can be used for various purposes. As appropriate uses, a driving power source mounted on a vehicle such as an electric vehicle (BEV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like can be given. The lithium-ion secondary battery 100 can also be typically used in the form of a battery pack in which a plurality of cells are connected in series and / or in parallel.

[0065] Note that a square lithium-ion secondary battery 100 provided with a wound electrode body 20 is described as an example. The electrode body 20 provided in the lithium-ion secondary battery 100 can also be a laminated electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated with separators. However, in the wound electrode body 20, when the non-aqueous electrolyte 80 is impregnated into the wound electrode body 20 during the manufacturing process of the lithium-ion secondary battery 100, the non-aqueous electrolyte 80 is impregnated from both sides of the open end portion of the wound electrode body 20. Therefore, in the wound electrode body 20, the central portion in the winding axis direction of the wound electrode body 20 is likely to accumulate NaBOB. Therefore, the wound electrode body 20 is more likely to be adversely affected by NaBOB than the laminated electrode body. Specifically, for the wound electrode body 20, the resistance is likely to increase in the central portion. Therefore, when the electrode body 20 provided in the lithium-ion secondary battery 100 is a wound electrode body, the initial resistance reduction effect becomes apparent. In addition, when the electrode body 20 provided in the lithium-ion secondary battery 100 is a wound electrode body, it is also difficult to remove NaBOB using the technology described in Patent Document 1.

[0066] The configuration of the lithium-ion secondary battery 100 is not limited to the above-described configuration, and the lithium-ion secondary battery 100 can also be configured as a cylindrical lithium-ion secondary battery, a laminated lithium-ion secondary battery, or the like. In addition, the technology disclosed herein can also be applied to non-aqueous electrolyte secondary batteries other than lithium-ion secondary batteries.

[0067] Hereinafter, an embodiment related to the present application will be described, but the present application is not intended to be limited to the scheme disclosed in the embodiment.

[0068] Preparation of Negative Electrode

[0069] As the binder A, styrene butadiene rubber (SBR) synthesized using NaOH as a neutralizing agent was prepared. In addition, as the binder B having a small Na content, styrene butadiene rubber synthesized using LiOH as a neutralizing agent was prepared.

[0070] As the thickener A, carboxymethyl cellulose (sodium salt) synthesized using NaOH was prepared. In addition, as the thickener B having a small Na content, carboxymethyl cellulose (lithium salt obtained by forming a salt with Li from 88 mol% of carboxyl groups) synthesized using LiOH was prepared.

[0071] In addition, as a binder having both the functions of a binder and a thickener, an acrylic binder synthesized without using a Na-containing component was prepared.

[0072] Natural graphite (C), used as the negative electrode active material, a binder, and a thickener are mixed with deionized water at a mass ratio of C:binder:thickener = 98:1:1 to prepare a slurry for forming the negative electrode active material layer. This slurry is then coated in strip form onto both sides of a long strip of copper foil and dried, followed by pressure to form a negative electrode sheet. It should be noted that when using the aforementioned acrylic binder, the natural graphite (C) and acrylic binder are used at a mass ratio of C:acrylic binder = 98:2.

[0073] At this point, regarding binders and thickeners, four types of negative electrode sheets A to D are produced: a combination of binder A and thickener A, a combination of binder B and thickener A, a combination of binder A and thickener B, and an acrylic binder only.

[0074] A portion of the negative electrode active material layer of the obtained negative electrode sheet was cut off. Using this as a sample, laser ablation ICP mass spectrometry was performed to determine the Na content in the negative electrode active material layer. The results showed that the Na content in the negative electrode active material layer of negative electrode sheet A was 420 μg / g, the Na content in the negative electrode active material layer of negative electrode sheet B was 311 μg / g, the Na content in the negative electrode active material layer of negative electrode sheet C was 191 μg / g, and the Na content in the negative electrode active material layer of negative electrode sheet D was 9 μg / g.

[0075] <Preparation for the Positive Electrode>

[0076] LiNi will be used as the positive electrode active material 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (LNCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder are mixed with N-methylpyrrolidone (NMP) in a mass ratio of LNCM:AB:PVdF = 90:8:2 to prepare a slurry for forming the positive electrode active material layer. This slurry is then coated in strips onto both sides of a long strip of aluminum foil and dried. Pressure is then applied to produce a positive electrode sheet A with a high Na content.

[0077] In addition, the positive electrode A was cleaned for 30 minutes with a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of EC:DMC:EMC = 3:3:4. This positive electrode was then used as a positive electrode B with a low Na content.

[0078] A portion of the positive electrode active material layer of the obtained positive electrode sheet was cut off. Using this as a sample, laser ablation ICP mass spectrometry was performed to determine the Na content in the positive electrode active material layer. The results showed that the Na content in the positive electrode active material layer of positive electrode sheet A was 183 μg / g, and the Na content in the positive electrode active material layer of positive electrode sheet B was 88 μg / g.

[0079] Preparation of separators

[0080] Two kinds of separators having different Na contents were prepared. Specifically, a separator having HRL provided on a porous polyolefin sheet of a three-layer structure of PP / PE / PP was prepared as a separator A having a large Na content. In addition, the separator A was washed with a mixed solvent containing EC, DMC and EMC at a volume ratio of EC:DMC:EMC = 3:3:4 for 30 minutes, and a separator obtained after the washing was prepared as a separator B having a small Na content.

[0081] A part of the prepared separator was cut out. As a test sample, laser ablation ICP mass spectrometry was performed using a laser ICP mass spectrometer to measure the Na content in the separator. As a result, the Na content in the separator A was 202 μg / g, and the Na content in the separator B was 65 μg / g.

[0082] Production of lithium-ion secondary batteries for evaluation

[0083] The positive electrode sheet, the negative electrode sheet and two pieces of the above prepared separators were stacked and wound, and then pressed from the side direction to make a flat-shaped wound electrode body. The Na contents of the respective components used are shown in Table 1.

[0084] Next, the positive electrode terminal and the negative electrode terminal were connected to the wound electrode body, and housed in a square battery case having an electrolyte injection port. Then, a nonaqueous electrolyte solution was injected from the electrolyte injection port of the battery case, and the injection port was hermetically sealed. It should be noted that, as the nonaqueous electrolyte solution, a nonaqueous electrolyte solution obtained by dissolving LiPF6 as a supporting salt at a concentration of 1.1 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) at a volume ratio of EC:DMC:EMC = 3:3:4, and further adding LiBOB so as to reach 0.5 mass% was prepared.

[0085] Then, activation treatment was performed to obtain lithium-ion secondary batteries for evaluation of each of the examples and the comparative examples.

[0086] Na content of negative electrode / Na content of entire body

[0087] The proportion of the Na content in the negative electrode active material layer with respect to the Na content in the positive electrode active material layer, the total of the Na content in the negative electrode active material layer and the Na content in the separator was calculated using the results of the above laser ablation ICP mass spectrometry.

[0088] Resistance distribution measurement

[0089] After disassembling the fabricated lithium-ion secondary batteries for evaluation in a dry environment glove box, the wound electrode bodies were taken out. Next, the innermost circumference of the negative electrode of the wound electrode body was cut out in an appropriate size, immersed in an EMC for about 10 minutes and cleaned, and a test body for resistance measurement was prepared. Then, the reaction resistance of the surface of the negative electrode active material layer formed on the test body was measured along the width direction of the negative electrode active material layer using an alternating current impedance method. The resistance measurement based on the alternating current impedance method was performed according to the method disclosed in Japanese Patent Application Publication No. 2014-25850. At this time, the resistance values were obtained at 5 mm intervals from the end portion to the 30% portion of the negative electrode active material layer, and the resistance values were obtained at 2 mm intervals for the central portion (the remaining 40% portion).

[0090] <Initial Resistance Ratio>

[0091] Each lithium-ion secondary battery for evaluation was adjusted to have an SOC of 60%. It was left in an environment at -10°C and discharged for 10 seconds. The discharge current rate was 1C, 3C, 5C, and 10C, and the voltage after discharge at each current rate was measured. The IV resistance was calculated from the current rate and the voltage, and the average value thereof was taken as the battery resistance. The ratio of the resistance of the other batteries when the resistance of the lithium-ion secondary battery of Comparative Example 1 was taken as "100" was calculated. The results are shown in Table 1.

[0092] <Resistance to Metal Lithium Deposition>

[0093] Each lithium-ion secondary battery for evaluation was left in an environment at -10°C, and a cycle in which charging was performed at a prescribed current value for 5 seconds, resting for 10 minutes, discharging for 5 seconds, and resting for 10 minutes was repeated 1000 times. Thereafter, each lithium-ion secondary battery was disassembled, and the presence or absence of deposition of metal lithium on the negative electrode was observed. The maximum current value in the current value at which metal lithium was not deposited on the negative electrode was taken as the limiting current value. The ratio of the limiting current value of the other lithium-ion secondary batteries when the limiting current value of the lithium-ion secondary battery of Comparative Example 1 was taken as "100" was calculated. The results are shown in Table 1.

[0094] [Table 1]

[0095]

[0096] From the results of Table 1, it is known that Examples 1 to 3 in which the Na content in the negative electrode active material layer is reduced have a small initial resistance and high metal Li deposition resistance as compared with the Comparative Examples. Further, it is known that the smaller the Na content in the negative electrode active material layer, the smaller the initial resistance and the higher the metal Li deposition resistance. On the other hand, from the comparison of Comparative Examples 1 to 4, it is known that even if the Na content in the positive electrode active material layer is reduced, there is no effect on the initial resistance and the metal Li deposition resistance. Further, it is known that even if the Na content in the separator is reduced, there is no effect on the initial resistance and the metal Li deposition resistance. Further, it is known that even if both the Na content in the positive electrode active material layer and the Na content in the separator are reduced, there is no effect on the initial resistance and almost no effect on the improvement of the metal Li deposition resistance.

[0097] From the above, it is known that the nonaqueous electrolyte secondary battery according to the present disclosure has a small initial resistance and high metal Li deposition resistance.

[0098] The above detailed description has been given of specific examples of the present disclosure, but these are merely examples and do not limit the scope of the claimed invention. The technology described in the scope of the claimed invention includes solutions obtained by various modifications and changes to the above-described specific examples.

Claims

1. A nonaqueous electrolyte secondary battery comprising: a nonaqueous electrolyte, and an electrode body including a positive electrode, a negative electrode, and a separator, the negative electrode includes a negative electrode active material layer, the negative electrode active material layer contains a negative electrode active material, a binder, and a thickening agent, at least one of the following is satisfied: the thickening agent is a carboxymethyl cellulose salt synthesized using LiOH, or the binder is a styrene butadiene rubber synthesized using LiOH; the positive electrode includes a positive electrode active material layer, a proportion of the Na content in the negative electrode active material layer with respect to a total of the Na content in the positive electrode active material layer, the Na content in the negative electrode active material layer, and the Na content in the separator is 5% or less, the nonaqueous electrolyte contains lithium bis(oxalato)borate, the Na content in the negative electrode active material layer is 50 μg / g or less as measured by laser ablation ICP mass spectrometry.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein When resistance distribution measurement is performed in a short edge direction of a major surface of the negative electrode active material layer, a proportion of a resistance value of a position with the highest resistance with respect to a resistance value of a position with the lowest resistance is 1.10 or less.

3. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein In the carboxymethyl cellulose salt synthesized using LiOH, 80 to 90 mol% of carboxyl groups form a salt with Li.

4. The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the electrode body is a wound electrode body.

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