Non-aqueous electrolyte secondary batteries

Abstract

The non-aqueous electrolyte secondary battery disclosed herein is characterized in that it comprises an electrode body having a positive electrode and a negative electrode, and a non-aqueous electrolyte, the positive electrode comprising a positive electrode collector (52) and a positive electrode active material layer (54) formed on the collector, the average film thickness of the positive electrode active material layer being greater than 100 μm, the positive electrode active material layer comprising a positive electrode active material having an average particle size of less than 10 μm, and carbon nanotubes and other conductive carbon materials as conductive materials; the average length of the carbon nanotubes being 1 μm to 2 μm, and the average diameter being less than 10 nm; wherein the conductive material is dispersed in a cross-sectional electron microscope image of the positive electrode active material layer.

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte secondary battery. Background Art

[0002] Non-aqueous electrolyte secondary batteries such as lithium-ion secondary batteries are preferably used as high-output power supplies for vehicles or power supplies for personal computers and mobile terminals because they are lightweight and have high energy density compared to existing batteries. As a typical structure of the positive electrode and negative electrode (hereinafter referred to as "electrode" when the positive and negative electrodes are not particularly distinguished) in such a secondary battery, there can be cited a structure in which an electrode active material layer with an electrode active material as the main component is formed on one or both sides of a foil-shaped electrode collector. The above-mentioned electrode active material layer is formed in the following manner: solid components such as an electrode active material, a binding material (binder), and a conductive material are dispersed in a prescribed solvent, and the slurry (paste) electrode material prepared thereby is applied to the surface of the collector to form a coating film, and after the coating film is dried, a pressurized pressure is applied to form a prescribed density and thickness.

[0003] As a guideline for the high performance of secondary batteries, high energy density is achieved by increasing the density of the above-mentioned electrode active material layer. When the density of the electrode active material layer is increased, for example, there is a tendency that the gas that can be generated during charge and discharge is retained in the electrode active material layer and is difficult to be discharged to the outside. Since the gas is retained in the electrode active material layer in this way, it sometimes becomes an important factor in the degradation of the performance of the secondary battery. For example, Patent Document 1 discloses an electrode in which a gas flow path is provided on the surface and / or inside of the electrode active material layer to suppress gas retention.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-98211 Summary of the Invention

[0007] However, in order to achieve further high energy density of secondary batteries, an electrode active material with a smaller average particle size (e.g., less than 10 μm) is used to further increase the density of the electrode active material layer, or the film thickness of the electrode active material layer is thickened. When such a smaller electrode active material is used and the film thickness of the electrode active material layer is further thickened, a high energy density can be achieved. On the other hand, as described above, there is a tendency for the gas that can be generated during charge and discharge to be easily retained. According to the research of the inventors, it was found that in the initial charge and discharge when the gas is significantly generated, the behavior of the charge and discharge curve caused by gas retention and the reduction of charge and discharge efficiency occurred.

[0008] The present invention has been made in view of the above circumstances, and a main object of the present invention is to provide a non-aqueous electrolyte secondary battery having excellent charge and discharge efficiency even in the initial charge and discharge.

[0009] In order to achieve the above-mentioned purpose, a non-aqueous electrolyte secondary battery disclosed herein is provided. The non-aqueous electrolyte secondary battery disclosed herein is characterized in that it is a non-aqueous electrolyte secondary battery comprising an electrode body having a positive electrode and a negative electrode, and a non-aqueous electrolyte. The positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the current collector. The average film thickness of the positive electrode active material layer is 100 μm or more. The positive electrode active material layer comprises a positive electrode active material having an average particle size of 10 μm or less, and carbon nanotubes and other conductive carbon materials as conductive materials. The average length of the carbon nanotubes is 1 μm to 2 μm, and the average diameter is 10 nm or less. Here, the conductive material is dispersed in the cross-sectional electron microscope image of the positive electrode active material layer.

[0010] According to the above configuration, even when a positive electrode active material layer containing a positive electrode active material having a small average particle size is formed with an average film thickness of 100 μm or more, the conductive material can be dispersed to form a conductive network, thereby appropriately discharging gases generated during charge and discharge to the outside of the positive electrode active material layer. This can achieve a secondary battery with excellent charge and discharge efficiency even during initial charge and discharge when gas generation is relatively significant.

[0011] In a preferred embodiment of the secondary battery disclosed herein, in an electron microscopic image of a 50 μm square cross-section of the positive electrode active material layer, when the cross-sectional electron microscopic image is divided equally into n regions, the area occupied by the conductive material in each region is represented by S1, S2, ..., Sn (%) (where n is a natural number from 6 to 8), and the deviation of the area occupied by the conductive material in each region, S1, S2, ..., Sn (%), is within 15%. In another preferred embodiment, the average value of the area occupied by the conductive material, S1, S2, ..., Sn (%), is 6 to 9%.

[0012] According to the above configuration, the conductive material is dispersed to form a more appropriate conductive network, thereby providing a non-aqueous electrolyte secondary battery with improved charge and discharge efficiency even in the initial charge and discharge.

[0013] In a preferred embodiment of the secondary battery disclosed herein, the other conductive carbon material is carbon black.

[0014] By using carbon black, which has excellent conductivity, as a conductive material, the discharge capacity can be increased and the charge and discharge efficiency can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1It is an explanatory diagram schematically showing a lithium ion secondary battery according to one embodiment.

[0016] Figure 2 This is a graph showing the initial charge and discharge curves of Example 1.

[0017] Figure 3 This is a graph showing the initial charge and discharge curves of Example 2.

[0018] Figure 4 This is a graph showing the initial charge and discharge curves of Example 3.

[0019] Explanation of symbols

[0020] 20 wound electrode body

[0021] 30 battery housing

[0022] 32 Safety valve

[0023] 42 Positive terminal

[0024] 42a Positive electrode collector plate

[0025] 44 Negative terminal

[0026] 44a Negative electrode collector plate

[0027] 50 positive electrode sheet

[0028] 52 positive electrode collector

[0029] 54 positive electrode active material layer

[0030] 56 Positive electrode current collector exposed portion

[0031] 60 negative electrode sheet

[0032] 62 negative electrode collector

[0033] 64 Negative electrode active material layer

[0034] 66 Negative electrode current collector exposed part

[0035] 70 Isolators

[0036] 100 lithium-ion secondary batteries DETAILED DESCRIPTION

[0037] Hereinafter, appropriate embodiments of the technology disclosed herein will be described with reference to the accompanying drawings as appropriate. It should be noted that matters other than those specifically mentioned in this specification and necessary for implementation (for example, the general composition and construction process of non-aqueous electrolyte secondary batteries) can be grasped as design matters for those skilled in the art based on the prior art in this field. The technology disclosed herein can be implemented based on the contents disclosed in this specification and the technical common sense in this field. In addition, in the following drawings, the same symbols are marked for components and parts that play the same role, and repeated descriptions are sometimes omitted or simplified. In addition, the dimensional relationships (length, width, height, etc.) do not reflect the actual dimensional relationships.

[0038] In addition, the description "A to B (wherein A and B are arbitrary values)" showing a range in this specification means A or more and B or less.

[0039] In this specification, the term "secondary battery" refers to all electrical storage devices that can be repeatedly charged and discharged. It is a concept that includes so-called storage batteries (chemical batteries) such as lithium-ion secondary batteries and nickel-metal hydride batteries, and capacitors (physical batteries) such as double-layer capacitors. "Non-aqueous electrolyte secondary batteries" refer to all batteries that use non-aqueous electrolytes as charge carriers and can be repeatedly charged and discharged with the migration of charge carriers between the positive and negative electrodes. "Electrode active material (i.e., positive electrode active material or negative electrode active material)" refers to a compound that can reversibly absorb and release chemical substances (lithium ions in lithium-ion secondary batteries) that serve as charge carriers.

[0040] The non-aqueous electrolyte secondary battery disclosed herein comprises an electrode assembly having a positive electrode and a negative electrode, and a non-aqueous electrolyte. While not intended to be particularly limiting, the disclosed technology will be described in detail below using a lithium-ion secondary battery comprising a flat, wound electrode assembly and a non-aqueous electrolyte as an example.

[0041] Figure 1 The lithium-ion secondary battery 100 shown is constructed by accommodating a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a sealable box-shaped battery case 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, as well as a thin-walled safety valve 32 set in a manner to release the internal pressure of the battery case 30 when the internal pressure rises above a specified level. In addition, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte. The positive terminal 42 is electrically connected to the positive electrode collector plate 42a. The negative terminal 44 is electrically connected to the negative electrode collector plate 44a. The material of the battery case 30 is preferably a metal material with high strength, light weight and good thermal conductivity. As such a metal material, for example, aluminum, stainless steel, etc. can be cited.

[0042] The wound electrode body 20 typically comprises a long sheet of positive electrode (hereinafter referred to as the positive electrode sheet 50) and a long sheet of negative electrode (hereinafter referred to as the negative electrode sheet 60) stacked and wound in the longitudinal direction with a long sheet of separator 70 interposed therebetween. The positive electrode sheet 50 comprises a positive electrode active material layer 54 formed along the longitudinal direction on one or both sides of a long sheet of positive electrode current collector 52. The negative electrode sheet 60 comprises a negative electrode active material layer 64 formed along the longitudinal direction on one or both sides of a long sheet of negative electrode current collector 62.

[0043] The positive electrode collector exposure portion 56 (i.e., the portion where the positive electrode collector 52 is exposed without forming the positive electrode active material layer 54) and the negative electrode collector exposure portion 66 (i.e., the portion where the negative electrode collector 62 is exposed without forming the negative electrode active material layer 64) formed in a manner protruding outward from both ends of the wound electrode body 20 in the winding axis direction are respectively joined to the positive electrode collector plate 42a and the negative electrode collector plate 44a.

[0044] The positive electrode sheet 50 includes a positive electrode active material layer 54 on a positive electrode current collector 52. Examples of the positive electrode current collector 52 include metal materials with good conductivity, such as aluminum, nickel, titanium, and stainless steel. Aluminum (e.g., aluminum foil) is particularly preferred. The thickness of the positive electrode current collector 52 is not particularly limited, but is, for example, 5 μm to 35 μm, preferably 7 μm to 20 μm.

[0045] The positive electrode active material contained in the positive electrode active material layer 54 typically has an average particle size of 10 μm or less. For example, the average particle size may be 0.5 μm to 10 μm, or 1 μm to 9 μm. By ensuring that the average particle size of the positive electrode active material is within this range, the contact area with the conductive material can be appropriately ensured, thereby improving electron conductivity and forming a good conductive path within the positive electrode active material layer 54.

[0046] In this specification, the “average particle size” refers to the particle size corresponding to 50% by volume of the cumulative frequency from the side of the smaller particle size in the volume-based particle size distribution based on the usual laser diffraction scattering method (D 50 , also called median particle size).

[0047] The positive electrode active material is not particularly limited as long as the average particle size is 10 μm or less, and positive electrode active materials that have been used in the positive electrode of such secondary batteries can be used. Specifically, lithium transition metal composite oxides such as lithium nickel composite oxides, lithium phosphate compounds such as lithium iron phosphate (LiFePO4), etc. can be mentioned. Among them, in the technology disclosed herein, lithium phosphate compounds are preferably used as the positive electrode active material.

[0048] Lithium phosphate compounds have the general formula Li 1+yThe composition represented by MPO4 contains lithium and at least one transition metal element as constituent elements, and is a phosphate with an olivine-type crystal structure. In the above general formula, y is a value satisfying 0.05≤y≤0.3, and M can be at least one metal element selected from Fe, Mn, Co, Ni, Mg, Zn, Cr, Ti and V. Such lithium phosphate compounds can use one or more of the ones used in non-aqueous electrolyte secondary batteries in the past without particular limitation. As specific examples, lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium nickel phosphate (LiNiPO4) and the like can be cited. Among them, lithium iron phosphate (LiFePO4) is preferably included. Since lithium iron phosphate forms a stable structure with phosphoric acid, it is not easy to release oxygen even at high temperatures and has excellent thermal stability. In addition, since iron, which is more abundant in resources than other compounds, is used, the price is relatively cheap. The above-mentioned lithium iron phosphate can also be further coated with a carbon film.

[0049] The conductive material included in the positive electrode active material layer 54 typically includes carbon nanotubes and other conductive carbon materials. According to extensive research conducted by the present inventors, the inclusion of carbon nanotubes and other conductive carbon materials allows a secondary battery with excellent charge and discharge efficiency to be achieved even when the positive electrode active material layer is formed with an average film thickness of 100 μm or more and includes a positive electrode active material with a small average particle size (e.g., an average particle size of 10 μm or less). The reasons for this are not particularly limited, but are presumed to be as follows.

[0050] Although conductive materials can be used in such secondary batteries in the past, they are unevenly dispersed in the electrode active material layer, which causes uneven reactions. For example, the smaller the average particle size of carbon black, which can be used as a general conductive material, the higher the purity and the better the conductivity. On the other hand, the cohesive force becomes larger and there is a tendency for it to be difficult to disperse evenly. In addition, when using a positive electrode active material with a smaller average particle size (for example, less than 10 μm), the density of solid particles (typically active materials) can be appropriately increased by pressurizing the electrode active material layer. On the other hand, since the gaps (voids) between the active materials become very small, the gas that can be generated during charge and discharge is trapped inside the electrode active material layer and becomes difficult to discharge. Since the gas is trapped inside the electrode active material layer, the contact area between the electrode active material and the non-aqueous electrolyte is reduced, which hinders the deintercalation and insertion of Li ions. Therefore, it is speculated that the discharge capacity is reduced and the charge and discharge efficiency is reduced. In particular, the charge and discharge efficiency is easily reduced during the initial charge and discharge when the generation of gas is obvious.

[0051] In contrast, the disclosed technology has been found to combine conductivity and dispersibility by including carbon nanotubes and other conductive carbon materials as conductive materials, forming a conductive and porous network within the positive electrode active material layer. This enables the realization of a non-aqueous electrolyte secondary battery with excellent charge and discharge efficiency, by suppressing gas retention within the electrode active material layer even during the initial stages of charge and discharge, when gas generation is particularly pronounced.

[0052] Carbon nanotubes (hereinafter referred to as CNTs) are conductive and can maintain a high porosity (e.g., about 30 to 70%) even when agglomerated due to a high tap density. Therefore, a network with suitable conductivity is easily formed within the electrode active material layer, and when gas is generated, the gas can be discharged to the outside of the active material layer through the network. This can improve discharge capacity and charge-discharge efficiency.

[0053] As CNT, it can be a single-layer carbon nanotube (SWNT) composed of a cylindrical graphene sheet, it can also be a two-layer carbon nanotube (DWNT) formed by two different SWNTs, or it can be a multi-layer carbon nanotube (MWNT) formed by three or more different SWNTs. From the viewpoint of ensuring the void ratio during agglomeration, multi-layer carbon nanotubes (MWNT) are preferred. They can be used alone or in combination of two or more. Carbon nanotubes can be manufactured by arc discharge method, laser ablation method, chemical vapor deposition method, etc.

[0054] To achieve proper dispersion and aggregation, the average length of CNTs is typically 1 to 2 μm. This average length range allows for the formation of a network with suitable conductivity between the electrode active materials. Furthermore, the average diameter of CNTs is typically 10 nm or less. For example, it can be 1 to 10 nm, or 2 to 9 nm.

[0055] In addition, the average length and average diameter of CNTs can be, for example, values ​​obtained by measurement based on electron microscope observation.

[0056] The purity of CNTs is not particularly limited; however, since higher purity results in more ideal conductivity, it is typically preferably 85% or higher. It is preferably 90% or higher, and more preferably 95% or higher. The upper limit of CNT purity is not particularly limited; however, for ease of production, it can typically be 99% or lower, or 98% or lower.

[0057] In addition, the "purity of CNTs" in this specification can be determined by thermogravimetric analysis (TGA).

[0058] As other conductive carbon materials, any conductive carbon material used as a conductive material in such secondary batteries can be used without particular limitation. Specifically, carbon black, coke, activated carbon, graphite (natural graphite and its modified form, artificial graphite), carbon fiber (PAN-based carbon fiber, pitch-based carbon fiber), fullerene, graphene, etc. can be mentioned. Among them, carbon black, which has very excellent conductivity, can be preferably used.

[0059] There is no particular restriction on the properties of carbon black. The smaller the average particle size, the larger the specific surface area, which can ensure a larger contact area with the above-mentioned positive electrode active material, which is beneficial to improving conductivity. On the other hand, when the average particle size is too small, there is a tendency for the volume to become larger, so the energy density may be reduced. From the above viewpoints, the average particle size of carbon black can typically be in the range of 1 to 200 nm (for example, 10 to 100 nm). Specific examples of the above-mentioned carbon black include acetylene black, furnace black, Ketjen black, thermal black, etc. Among them, acetylene black is preferred because of its excellent conductivity, which contributes to the high output of secondary batteries.

[0060] To achieve both conductivity and dispersibility, the mass ratio of carbon nanotubes to other conductive carbon materials can be appropriately adjusted. While not particularly limited, the mass ratio of carbon nanotubes (CNTs) to other conductive carbon materials is preferably CNT:other conductive carbon materials = 90:10 to 50:50. Inclusion of these mass ratios further enhances the aforementioned effects.

[0061] In the technology disclosed herein, it is required that the above-mentioned conductive material be dispersed in the cross-sectional image of the active material layer obtained by an electron microscope (a scanning electron microscope (SEM) or a transmission electron microscope can be used). The dispersed state refers to a state in which, for example, a conductive material, an adhesive material (binder), etc. are not concentrated in a certain part, that is, they exist in an arbitrary range within the electrode at an average volume ratio relative to the active material, and the conductive material is continuously connected in the thickness direction and the surface direction of the electrode. By dispersing the conductive material, the gas generated in the electrode active material layer can be properly discharged, and a network that can ensure conductivity is formed. Therefore, a secondary battery with excellent charge and discharge efficiency can be achieved.

[0062] In a preferred embodiment, when a 50 μm square cross-sectional electron microscope image of the positive electrode active material layer is divided into n equal parts (n is a natural number of 6 to 8) and the ratio of the area occupied by the conductive material in each region (S1 to Sn) is calculated, the deviation of the area ratio S1 to Sn is preferably within 15%. It is more preferably within 14%, and particularly preferably within 12%. According to the results of in-depth research conducted by the present inventors, it was found that when the deviation of the area ratio (S1 to Sn) occupied by the conductive material in each region is within the above range, the conductive material is appropriately dispersed and a conductive network is appropriately formed.

[0063] The average area ratio (S1 to Sn) of the conductive material in the positive electrode active material layer 54 is preferably 6 to 9%. Within this range, the balance between the positive electrode active material and the conductive material in the positive electrode active material layer 54 is appropriately adjusted, thereby improving the output characteristics of the secondary battery.

[0064] The deviation of the area ratio (S1 to Sn) occupied by the conductive material in the positive electrode active material layer and the average value of the area ratio can be calculated, for example, as follows. An electron microscope is used to obtain multiple (for example, 5 or more) 50 μm square cross-sectional images of the positive electrode active material layer. In the multiple cross-sectional images obtained, the carbon element is mapped using an electron probe microanalyzer (EPMA) to determine the area where the conductive material exists (typically, the area where the carbon concentration is 12% or more). In this way, the area where the conductive material exists can be distinguished from the area where other solid components (typically the positive electrode active material) exist. The above cross-sectional image can be divided into n equal parts (n is a natural number of 6 to 8) and the area ratio (S1 to Sn) occupied by the conductive material in each area can be calculated. The area ratio is calculated in the same way in the above multiple cross-sectional images, and the average value of the area ratio is calculated, thereby calculating the average value of the area ratio in this specification. In addition, the deviation of the area ratio can be calculated from the average value of the calculated area ratio. Thereby, the variation in the area ratio occupied by the conductive material in the positive electrode active material layer and the average value of the area ratio can be determined.

[0065] In another preferred embodiment, in the cross-sectional image of the positive electrode active material layer obtained above, a plurality of arbitrary straight lines are set and the length d1 (μm) of the area where the conductive material exists and the length d2 (μm) of the area where the other solid components exist are measured on each straight line, and the ratio of d1 to d2 (d1 / d2) is 0.1 to 0.3. In addition, although not particularly limited, d1 can be about 0.5μm to 3μm. According to the above-mentioned configuration, the gas generated in the electrode active material layer is appropriately discharged, and the contact barrier between the electrode active material and the non-aqueous electrolyte is suppressed. Thus, the discharge capacity and the charge and discharge efficiency can be improved.

[0066] The length d1 (μm) of the area where the conductive material exists and the length d2 (μm) of the area where other solid components exist can be calculated, for example, as follows. A plurality of (e.g., 5 or more) cross-sectional images of the positive electrode active material layer are obtained as described above. In the plurality of cross-sectional images, an electron probe microanalyzer (EPMA) is used for analysis as described above to distinguish the area where the conductive material exists from the area where other solid components exist. Arbitrary multiple straight lines (e.g., diagonal lines) are set on the cross-sectional image and the respective average values ​​of d1 and d2 existing on the multiple straight lines are calculated. The values ​​of d1 and d2 are calculated in the same manner in the above-mentioned multiple cross-sectional images, and their average values ​​are obtained, thereby obtaining the values ​​of d1 and d2 here. In addition, the ratio of d1 to d2 here (d1 / d2) can be obtained from these values ​​of d1 and d2.

[0067] The positive electrode active material layer 54 may include any component other than the above-mentioned positive electrode active material and conductive material as needed. As the above-mentioned arbitrary component, a binder etc. can be cited. As a binder, a polymer that can be dissolved or dispersed in the solvent used can be used. For example, polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC) etc. can be used.

[0068] From the perspective of energy density, the content of the positive electrode active material in the positive electrode active material layer 54 (i.e., the ratio of the positive electrode active material to the total mass of the positive electrode active material layer) is preferably approximately 60% by mass or more. For example, it is more preferably 75% by mass to 90% by mass, and further preferably 80% by mass to 90% by mass. In addition, the content of the conductive material in the positive electrode active material layer 54 is preferably, for example, 1% by mass to 10% by mass, and more preferably 1% by mass to 8% by mass. The content of the binder in the positive electrode active material layer 54 is preferably, for example, 0.5% by mass to 5% by mass, and more preferably 1% by mass to 3% by mass. In addition, when various additives such as a thickener are included, the content of the additive in the positive electrode active material layer 54 is preferably, for example, 7% by mass or less, and more preferably 5% by mass or less.

[0069] From the perspective of achieving high capacity, the average thickness of the positive electrode active material layer 54 (the thickness of each side of the positive electrode collector 52) is required to be 100 μm or more. The average thickness of the positive electrode active material layer 54 can be 100 μm to 200 μm, or 110 μm to 190 μm. From the perspective of achieving high energy density, the mass (unit area weight) of the positive electrode active material layer 54 set per unit area of ​​the positive electrode collector 52 can be 20 mg / cm 2 Above (e.g. 25 mg / cm 2 Above, typically 27 mg / cm 2In addition, the mass (weight per unit area) of the positive electrode active material layer 54 provided per unit area of ​​the positive electrode current collector 52 can be typically 60 mg / cm per side of the positive electrode current collector 52. 2 Below (e.g. 55 mg / cm 2 In addition, from the perspective of achieving gas discharge and energy density, the density of the positive electrode active material layer 54 is preferably 2.0 g / cm 3 ~3.0g / cm 3 , more preferably 2.1 g / cm 3 ~2.5g / cm 3 .

[0070] The negative electrode sheet 60 includes a negative electrode active material layer 64 on a negative electrode current collector 62. The negative electrode current collector 62 can be made of a metal material with good conductivity, such as copper, a copper-based alloy, nickel, titanium, or stainless steel. Copper (e.g., copper foil) is particularly preferred. The thickness of the negative electrode current collector 62 can be, for example, approximately 5 μm to 20 μm, and preferably 8 μm to 15 μm.

[0071] The negative electrode active material layer 64 contains at least a negative electrode active material. As the negative electrode active material, one or more materials known as negative electrode active materials for this type of secondary battery can be used. Preferred examples include carbon materials such as graphite, hard carbon, and soft carbon. The negative electrode active material is typically granular. The average particle size of the granular negative electrode active material is not particularly limited, but is suitably 50 μm or less, typically 20 μm or less, for example, 1 μm to 20 μm.

[0072] In addition to the above-mentioned negative electrode active material, the negative electrode active material layer 64 may also contain materials that can be used as constituent components of the negative electrode active material layer in general non-aqueous electrolyte secondary batteries as needed. Examples of such materials include binders and various additives. As a binder, for example, styrene-butadiene rubber (SBR) can be used. In addition, various additives such as thickeners, dispersants, and conductive materials can be used as appropriate. For example, carboxymethyl cellulose (CMC) and methyl cellulose (MC) can be used as thickeners.

[0073] From the perspective of energy density, the content of the negative electrode active material in the negative electrode active material layer 64 is preferably 60% by mass or greater. For example, it is more preferably 90% by mass to 99% by mass, and even more preferably 95% by mass to 99% by mass. Furthermore, when a binder is used, the content of the binder in the negative electrode active material layer 64 is preferably, for example, 1% by mass to 10% by mass, and more preferably 1% by mass to 5% by mass. When a thickener is used, the content of the thickener in the negative electrode active material layer 64 is preferably, for example, 1% by mass to 10% by mass, and more preferably 1% by mass to 5% by mass.

[0074] From the perspective of achieving high capacity, the average thickness of the negative electrode active material layer 64 can be 100 μm to 200 μm, or 110 μm to 190 μm. From the perspective of achieving high energy density, the mass (unit area weight) of the negative electrode active material layer 64 provided per unit area of ​​the negative electrode current collector 62 can be 5 mg / cm2 per side of the negative electrode current collector 62. 2 Above (e.g. 7 mg / cm 2 Above, typically 10 mg / cm 2 In addition, the mass (weight per unit area) of the negative electrode active material layer 64 provided per unit area of ​​the negative electrode current collector 62 can be typically 20 mg / cm per side of the negative electrode current collector 62. 2 Below (e.g. 15 mg / cm 2 In addition, the density of the negative electrode active material layer 64 can be 1.0 g / cm 3 ~2.0g / cm 3 In addition, the density of the negative electrode active material layer 64 can be 1.1 g / cm 3 ~1.8g / cm 3 .

[0075] The capacity ratio of the positive electrode and the negative electrode can be adjusted according to the difference in the charge carrier acceptance characteristics. Specifically, the positive electrode capacity C c (Ah) and negative electrode capacity C a (Ah) ratio (C a / C c ) is suitably 1.0 to 2.0, preferably 1.5 to 1.9, and more preferably 1.2 to 1.3. c (Ah) is defined as the product of the theoretical capacity per unit mass of the positive electrode active material (Ah / g) and the mass (g) of the positive electrode active material. a (Ah) is similarly defined as the product of the theoretical capacity per unit mass of the negative electrode active material (Ah / g) and the mass (g) of the negative electrode active material.

[0076] Examples of separator 70 include porous sheets (films) made of resins such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. These porous sheets may have a single-layer structure or a laminated structure of two or more layers (e.g., a three-layer structure with PP layers laminated on both sides of a PE layer). Separator 70 may also include a heat-resistant layer (HRL).

[0077] As a non-aqueous electrolyte, a liquid substance (non-aqueous electrolyte) in which an auxiliary salt (e.g., lithium salt, sodium salt, magnesium salt, etc., or lithium salt in lithium-ion secondary batteries) is dissolved or dispersed in a non-aqueous solvent is typically used. Alternatively, a polymer may be added to the non-aqueous electrolyte to form a solid (typically a so-called gel).

[0078] As the auxiliary salt, any auxiliary salt conventionally used in secondary batteries of this type can be used without particular limitation. For example, lithium salts such as LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, and LiC(CF3SO2)3 can be used. Among them, LiPF6 is preferably used. The auxiliary salt concentration can be, for example, 0.7 mol / L to 1.3 mol / L.

[0079] As the non-aqueous solvent, carbonates, esters, ethers, nitriles, sulfones, lactones and the like can be used without particular limitation. Specifically, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), dimethyl trifluorocarbonate (TFDMC) and the like can be preferably used. Such non-aqueous solvents can be used alone or in combination of two or more.

[0080] It should be noted that the non-aqueous electrolyte of the non-aqueous secondary battery of this embodiment may also contain various additives such as gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB), film forming agents, dispersants, and thickeners, as long as the effects of the present invention are not significantly impaired.

[0081] The non-aqueous electrolyte secondary battery disclosed herein can be used for various purposes. For example, the non-aqueous electrolyte secondary battery disclosed herein is characterized in that high energy density is achieved, and the charge and discharge efficiency is also excellent in the initial charge and discharge. Therefore, using such a feature, it can be preferably used for, for example, a power source (driving power supply) for a motor of a vehicle such as a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), or an electric vehicle (BEV). It should be noted that the above-mentioned non-aqueous electrolyte secondary battery can typically be used in the form of a battery pack formed by connecting a plurality of batteries in series and / or in parallel.

[0082] Hereinafter, examples of the non-aqueous electrolyte secondary battery disclosed herein will be described. However, the technology disclosed herein is not intended to be limited to the technical aspects shown in the following examples.

[0083] <Making the positive electrode>

[0084] (Example 1)

[0085] The average particle size (D 50 ) is 6μm LiFePO4 powder (LFP), carbon nanotubes (CNT) with an average length of 2μm and an average diameter of 8nm as a conductive material, acetylene black (AB) as a conductive material and polyvinylidene fluoride (PVdF) as a binder. These materials are weighed in a mass ratio of LFP:CNT:AB:PVdF=92:2.5:2.5:3, mixed with N-methylpyrrolidone (NMP) as a solvent, and a positive electrode slurry is prepared with a solid content of 50%. The positive electrode slurry is applied to both sides of a long aluminum foil (positive electrode collector) in a manner of more than 100μm, and dried at 80°C for 15 minutes. After drying, pressurization is performed so that the density of the positive electrode active material layer is 2.2g / cm 3 In this way, a positive electrode sheet including a positive electrode active material layer on a positive electrode current collector is obtained.

[0086] (Example 2)

[0087] The average particle size (D 50 ) 6μm LiFePO4 powder (LFP), carbon nanotubes (CNTs) with an average length of 2μm and an average diameter of 8nm as a conductive material, and polyvinylidene fluoride (PVdF) as a binder. These materials were weighed to a mass ratio of LFP:CNT:PVdF = 91:6:3, mixed with N-methylpyrrolidone (NMP) as a solvent, and prepared to a solid content of 50% to prepare a positive electrode slurry. Using this positive electrode slurry, a positive electrode sheet was obtained in the same manner as in Example 1, except for the above.

[0088] (Example 3)

[0089] The average particle size (D 50) 6μm LiFePO4 powder (LFP), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder. These materials were weighed to a mass ratio of LFP:AB:PVdF = 89:8:3, and mixed with N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode slurry with a solid content of 50%. Using this positive electrode slurry, a positive electrode sheet was obtained in the same manner as in Example 1, except for the above.

[0090] (Reference example)

[0091] The average particle size (D 50 ) 6μm LiFePO4 powder (LFP), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder. These materials were weighed to a mass ratio of LFP:AB:PVdF = 90:7:3, mixed with N-methylpyrrolidone (NMP) as a solvent, and a positive electrode slurry was prepared to a solid content of 50%. This positive electrode slurry was applied to the current collector to an average film thickness of 100μm or less. A positive electrode sheet was obtained in the same manner as in Example 1, except that the above procedures were repeated.

[0092] <Evaluation of positive electrode active material layer>

[0093] Using a scanning electron microscope (SEM), five 50 μm square cross-sectional images (300x magnification) of the positive electrode active material layers of Examples 1 to 3 and the reference example were obtained. The resulting multiple observation images were analyzed using an electron probe microanalyzer (EPMA) to map the carbon element. Regions with a carbon concentration of 12% or more were identified as areas containing conductive material, and these areas were distinguished from areas containing other solid components.

[0094] The cross-sectional image was divided into 6 equal parts, and the deviation of the area ratio (S1 to S6) occupied by the conductive material in each area was calculated. The deviation value of the area ratio in multiple cross-sectional images was obtained, and the average value of the deviation was calculated. When the deviation of the values ​​of S1 to S6 is within 15%, it can be evaluated that the dispersion state of the positive electrode active material and the conductive material is good. When the dispersion state of the conductive material is good, it is recorded as "○", and when it is uneven, it is recorded as "×". The results are shown in Table 1.

[0095] In addition, the average value of the area ratio occupied by the conductive material was calculated for each example. The results are shown in Table 1.

[0096] In the multiple cross-sectional images obtained and analyzed using an electron probe microanalyzer (EPMA), the length of the area where the conductive material exists (d1) and the length of the area where other solid components exist (d2) were measured. A diagonal line was set on the cross-sectional image, and the length d1 (μm) of the conductive material and the length d2 (μm) of the other solid components were measured. The average values ​​of d1 and d2 existing on the diagonal line were calculated, and the ratio of d1 to d2 (d1 / d2) was determined. The values ​​of d1 and d2 in multiple cross-sectional images were obtained, and the average value of the ratio of d1 to d2 (d1 / d2) was calculated. The results are shown in Table 1.

[0097] Production of lithium-ion secondary batteries

[0098] Prepare graphite (C) as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, and carboxymethyl cellulose (CMC) as the thickener. Weigh these materials in a mass ratio of C:SBR:CNC=98:1:1, mix with distilled water as a solvent, and prepare a negative electrode slurry with a solid content of 52%. Apply the negative electrode slurry on both sides of a long copper foil (negative electrode collector). Dry the negative electrode slurry on the negative electrode collector at 80°C for 15 minutes. After drying, pressurize so that the density of the negative electrode active material layer reaches 1.3g / cm 3 In this way, a negative electrode sheet including a negative electrode active material layer on a negative electrode current collector is obtained.

[0099] As a separator, a porous polyolefin sheet having a three-layer structure of PE / PP / PE was prepared.

[0100] The prepared positive electrode sheets (Examples 1 to 3 and Reference Example) and negative electrode sheets are stacked with the prepared separator to produce an electrode body. Next, the positive terminal and the negative terminal are connected to the electrode body and housed in a laminated case together with a non-aqueous electrolyte. It should be noted that as a non-aqueous electrolyte, a solution is used in which LiPF6 as an auxiliary salt is dissolved at a concentration of 1.0 mol / L in a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:40:30. Thus, the lithium ion secondary batteries of Examples 1 to 3 and Reference Example are obtained.

[0101] <Evaluation of lithium-ion secondary batteries>

[0102] The lithium-ion secondary battery of each example was placed in an environment of 25°C. Constant current (CC) charging was performed at a rate of 0.33C until 4.0V and then rested for 10 minutes. Constant current (CC) discharge was performed at a rate of 0.33C until 2.5V and then rested for 10 minutes. The charge capacity from the start of the initial charge to the end and the discharge capacity from the start of the discharge to the end were calculated. The value obtained by dividing the above charge capacity by the weight (g) of the positive electrode active material layer was taken as the charge capacity (mAh / g) of the positive electrode. In addition, the value obtained by dividing the discharge capacity by the weight (g) of the positive electrode active material layer was taken as the discharge capacity (mAh / g) of the positive electrode. The ratio of the discharge capacity to the charge capacity was calculated as the charge and discharge efficiency (%). The results are shown in Table 1.

[0103] Note that "1C" refers to the current value (current density) that allows charging to the battery capacity (Ah) predicted from the theoretical capacity of the active material in one hour. Therefore, for example, 1 / 3C means the current value that allows charging to that battery capacity in three hours, and 20C means the current value that allows charging to that battery capacity in 1 / 20 of an hour.

[0104] In addition, the initial charge and discharge curve (battery voltage transition) was obtained for each lithium ion secondary battery to evaluate the presence or absence of abnormal voltage behavior. The results are shown in Tables 1 and Figures 2-4 .

[0105] [Table 1]

[0106]

[0107] According to Table 1 and Figure 2 It can be seen that in Example 1, where the positive electrode active material and the conductive material are well dispersed, there is no abnormal voltage behavior during the first charge even with an average film thickness of 100 μm or more, and the charge and discharge efficiency reaches over 90%.

[0108] On the other hand, in Examples 2 and 3, we see Figure 3 、 Figure 4 The voltage behavior shown is abnormal. In addition, the charge and discharge efficiency is less than 90%.

[0109] Therefore, even if a positive electrode active material layer containing a positive electrode active material with an average particle size of less than 10 μm is formed with an average film thickness of more than 100 μm, a non-aqueous electrolyte secondary battery with excellent charge and discharge efficiency in the initial charge and discharge can be achieved by including carbon nanotubes with an average length of 1 μm to 2 μm and an average diameter of less than 10 nm and other conductive carbon materials, and the conductive materials are dispersed in the positive electrode active material layer.

[0110] While specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. The technology described in the claims encompasses various modifications and variations of the specific examples described above.

Claims

1. A non-aqueous electrolyte secondary battery, characterized in that: An electrode body having a positive electrode and a negative electrode, and a non-aqueous electrolyte, The positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The average film thickness of the positive electrode active material layer is greater than 110 μm. The positive electrode active material layer contains a positive electrode active material having an average particle size of 10 μm or less, carbon nanotubes as a conductive material, and other conductive carbon materials, wherein the other conductive carbon material is carbon black. The mass ratio of the carbon nanotubes to the carbon black is 90:10 to 50:50, The average length of the carbon nanotubes is 1 μm to 2 μm, The average diameter of the carbon nanotubes is less than 10 nm, In the electron microscope image of a 50 μm square cross section of the positive electrode active material layer, when the cross section electron microscope image is evenly divided into n equal parts, the area occupied by the conductive material in each region is denoted as S1, S2, ..., Sn (%), wherein n is a natural number of 6 to 8, The deviation of the areas S1, S2, ..., Sn (%) occupied by the conductive material in each region is within 15%.

2. The nonaqueous electrolyte secondary battery according to claim 1, wherein The average value of the areas S1, S2, ...Sn (%) occupied by the conductive materials is 6 to 9%.

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