Non-aqueous electrolyte secondary battery

Abstract

The present invention relates to a nonaqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery includes a positive electrode, a negative electrode, and an electrolyte solution. A positive electrode active material layer is disposed on a surface of a positive electrode substrate. The positive electrode active material layer includes a first layer and a second layer. The first layer is disposed between the positive electrode substrate and the second layer. The positive electrode active material layer includes a positive electrode active material, carbon black, and fibrous carbon. The mass fraction of the carbon black in the first layer is higher than the mass fraction of the carbon black in the second layer. In the first layer, the mass fraction of the carbon black is 1.5% to 5% relative to the mass of the positive electrode active material. The mass fraction of the fibrous carbon in the second layer is higher than the mass fraction of the fibrous carbon in the first layer. In the second layer, the mass fraction of the fibrous carbon is 0.2% to 1% relative to the mass of the positive electrode active material.

Description

Technical Field

[0001] This technology relates to non-aqueous electrolyte secondary batteries. Background Technology

[0002] Japanese Patent Publication No. 2018-500714 discloses a multi-layered electrode structure. Summary of the Invention

[0003] Generally, the positive electrode active material of non-aqueous electrolyte secondary batteries (hereinafter referred to as "batteries") tends to have low electronic conductivity. To date, conductive materials such as carbon black have been used to improve the electronic conductivity of the positive electrode active material.

[0004] Generally, the positive electrode active material is composed of particles. Therefore, the positive electrode active material layer becomes porous. This is because the gaps between the particles are not completely filled. The electrolyte permeates into the voids within the positive electrode active material layer. The electrolyte can form ion conduction pathways within the positive electrode active material layer.

[0005] Adding carbon black to the positive electrode active material layer can create electronic conduction pathways within it. However, on the other hand, it may reduce ion conduction pathways within the positive electrode active material layer. At the surface of the positive electrode active material layer, it is believed that the carbon black blocks the voids, thus hindering electrolyte penetration. To promote electrolyte penetration, reducing the density of the surface layer is also considered. However, if the surface layer becomes low-density (i.e., if the porosity of the surface layer increases), there is a tendency for the electronic conduction pathways at the surface layer to be easily interrupted.

[0006] The surface layer is close to the negative electrode. Due to a deficiency in either ion or electron conduction pathways, the charge-discharge reaction tends to concentrate on the surface. This concentration can accelerate the degradation of the positive electrode active material (e.g., particle cracking), potentially reducing cycle durability.

[0007] To balance ion and electron conduction pathways, increasing the surface porosity and the amount of carbon black was considered. However, increased porosity may reduce energy density. Furthermore, carbon black essentially does not contribute to battery capacity. The interaction between increased porosity and increased carbon black content could potentially reduce energy density to a non-negligible degree.

[0008] The aim of this technology is to combine cycle durability and energy density.

[0009] The following describes the structure and effects of this technology. However, the mechanism of action in this specification is speculative and does not limit the scope of this technology.

[0010] [1] A non-aqueous electrolyte secondary battery consists of a positive electrode, a negative electrode, and an electrolyte.

[0011] The positive electrode comprises a positive electrode substrate and a positive electrode active material layer. The positive electrode active material layer is disposed on the surface of the positive electrode substrate. The positive electrode active material layer comprises a first layer and a second layer. The first layer is disposed between the positive electrode substrate and the second layer.

[0012] The positive electrode active material layer comprises positive electrode active material, carbon black, and fibrous carbon. The mass fraction of carbon black in the first layer is higher than that in the second layer. In the first layer, the mass fraction of carbon black relative to the mass of the positive electrode active material is 1.5% to 5%. The mass fraction of fibrous carbon in the second layer is higher than that in the first layer. In the second layer, the mass fraction of fibrous carbon relative to the mass of the positive electrode active material is 0.2% to 1%.

[0013] The positive electrode active material layer of this technology has a multi-layer structure. That is, the positive electrode active material layer includes a first layer (lower layer) and a second layer (upper layer). The second layer can constitute the surface layer of the positive electrode active material layer.

[0014] The positive electrode active material layer contains two conductive materials: carbon black and fibrous carbon. Compared to carbon black, fibrous carbon can form long-distance electron conduction pathways. Fibrous carbon is relatively abundant in the second layer (upper layer). Therefore, it is expected that even if the porosity of the second layer increases to some extent, the electron conduction pathways will not be easily interrupted. That is, the surface layer is expected to possess both ion and electron conduction pathways.

[0015] However, if the mass fraction of fibrous carbon in the overall positive electrode active material layer increases, there is a tendency for cycle durability to decrease. The detailed mechanism is still unclear, but it is possible that the fibrous carbon acts as a catalyst, promoting the dissolution of metals from the positive electrode active material.

[0016] In the positive electrode active material layer of this technology, fibrous carbon is relatively abundantly disposed in the second layer (upper layer), and carbon black mainly serves as the electron conduction pathway for the first layer (lower layer), thereby reducing reaction concentration in the upper layer and suppressing the mass fraction of fibrous carbon in the overall positive electrode active material layer. Therefore, improved cycle durability is expected in this technology.

[0017] Furthermore, by incorporating a small amount of fibrous carbon, sufficient electronic conduction pathways can be formed. By replacing a portion of the carbon black with fibrous carbon, the total amount of conductive material incorporated can be reduced. That is, in this technology, the reduction in energy density associated with the incorporation of conductive materials can be mitigated.

[0018] As can be seen from the above, this technology aims to combine cycle durability and energy density.

[0019] [2] In the first layer, the mass fraction of fibrous carbon relative to the mass of the positive electrode active material may be, for example, 0.05% or less. In the second layer, the mass fraction of carbon black relative to the mass of the positive electrode active material may be, for example, 1% or less.

[0020] In the above configuration [2], for example, it is expected that at least one of cycle durability and energy density will be improved.

[0021] [3] The first layer may have, for example, a porosity of 15% to 20%. The second layer may have, for example, a porosity of 25% to 35%.

[0022] In the above configuration [3], for example, it is expected that at least one of cycle durability and energy density will be improved.

[0023] [4] The ratio of the thickness of the second layer to the thickness of the first layer can be, for example, 0.2 to 0.8.

[0024] In the above [4] configuration, for example, an increase in energy density is expected. Hereinafter, the "ratio of the thickness of the second layer to the thickness of the first layer" will also be referred to as "thickness ratio" or "T2 / T1", etc.

[0025] [5] The positive electrode active material may have a chemical composition, for example, represented by the following formula (I).

[0026] Li 1-a Ni x Me 1-x O2…(I)

[0027] In the above formula (I), "a" satisfies the relationship -0.3 ≤ a ≤ 0.3. "x" satisfies the relationship 0.7 ≤ x ≤ 1.0. "Me" represents at least one of the following: cobalt (Co), manganese (Mn), aluminum (Al), zirconium (Zr), boron (B), magnesium (Mg), iron (Fe), copper (Cu), zinc (Zn), tin (Sn), sodium (Na), potassium (K), barium (Ba), strontium (Sr), calcium (Ca), tungsten (W), molybdenum (Mo), niobium (Nb), titanium (Ti), silicon (Si), vanadium (V), chromium (Cr), and germanium (Ge).

[0028] Positive electrode active materials with the chemical composition of formula (I) above are, for example, called "high-nickel materials." This is because of the high molar fraction of nickel (Ni). High-nickel materials tend to have a large specific capacity. By using high-nickel materials, an increase in energy density is expected. However, high-nickel materials also tend to be prone to particle cracking due to reaction concentration. The higher the molar fraction of Ni, the more pronounced this tendency becomes. This technology is considered particularly effective against particle cracking in high-nickel materials.

[0029] The above and other objects, features, aspects and advantages of this technology become clear from the following detailed description of the technology as understood in conjunction with the accompanying drawings. Attached Figure Description

[0030] Figure 1 A schematic diagram illustrating an example of the configuration of a non-aqueous electrolyte secondary battery in this embodiment.

[0031] Figure 2 This is a schematic diagram illustrating an example of the configuration of the electrode body in this embodiment.

[0032] Figure 3 A schematic cross-sectional view is shown to illustrate an example of the positive electrode in this embodiment. Detailed Implementation

[0033] The following describes the implementation of this technology (also referred to as "this implementation" in this specification). However, the following description does not limit the scope of this technology. For example, regarding the effects described in this specification, the scope of this technology is not limited to achieving all of these effects.

[0034] In this specification, the expressions "comprise," "include," "have," and their variations [e.g., "be composed of," "encompass," "involve," "contain," "carry," "support," "hold," etc.] are open-ended. Open-ended expressions may include additional elements beyond the essential elements, or they may not contain any additional elements. The expression "consist of" is a closed-ended expression. The expression "consist essentially of" is a semi-closed-ended expression. A semi-closed-ended expression indicates that additional elements may be included beyond the essential elements without hindering the purpose of this technology. For example, elements commonly conceived in the art to which this technology pertains (e.g., unavoidable impurities) may be included as additional elements.

[0035] In this specification, the expressions “may…” and “can…” are not used in a mandatory sense, meaning “must…”, but rather in a permissive sense, meaning “there is a possibility of…”.

[0036] In this specification, the singular forms ("a", "an" and "the") include the plural forms unless otherwise specified. For example, "particle" can include not only "a single particle" but also "a collection of particles (powder, powder, particle swarm)".

[0037] In this specification, unless otherwise specified, numerical ranges such as "1.5% to 5%" and "1.5 to 5%" include both upper and lower limits. That is, "1.5% to 5%" and "1.5 to 5%" represent a numerical range of "above 1.5% and below 5%". Furthermore, any value arbitrarily selected from the numerical range can be set as a new upper and lower limit. For example, a new numerical range can be set by arbitrarily combining values ​​within the numerical range with values ​​described in other parts of this specification, tables, figures, etc.

[0038] In this specification, when a compound is represented by a stoichiometric formula such as "LiCoO2", the stoichiometric formula is merely a representative example. The composition ratio may be non-stoichiometric. For example, when lithium cobalt oxide is represented as "LiCoO2", unless otherwise specified, lithium cobalt oxide is not limited to a composition ratio of "Li / Co / O = 1 / 1 / 2", and can contain Li, Co, and O in any composition ratio. Furthermore, doping or substitution of trace elements is also permissible.

[0039] The geometric terms used in this specification (such as "parallel") should not be interpreted in a strict sense. For example, "parallel" may deviate slightly from the strict meaning of "parallel." The geometric terms used in this specification may include tolerances and errors, such as those related to design, operation, and manufacturing. Dimensional relationships in the drawings may sometimes differ from actual dimensional relationships. To aid in understanding this technology, dimensional relationships (length, width, thickness, etc.) in the drawings may sometimes be altered. Furthermore, some components may be omitted.

[0040] <Non-aqueous electrolyte secondary battery>

[0041] Figure 1 A schematic diagram illustrating an example of the configuration of a non-aqueous electrolyte secondary battery in this embodiment.

[0042] Battery 100 can be used for any purpose. For example, it can be used as a main power source or auxiliary power source in electric vehicles. Multiple batteries 100 can be connected to form a battery module or battery pack. Battery 100 can have a rated capacity, for example, from 1 Ah to 200 Ah.

[0043] The battery 100 includes an outer packaging body 90. The outer packaging body 90 is square (flat rectangular). However, square is just one example. The outer packaging body 90 can have any shape. The outer packaging body 90 can be, for example, cylindrical or bag-shaped. The outer packaging body 90 can be made of, for example, an Al alloy. The outer packaging body 90 houses the electrode body 50 and electrolyte (not shown). The outer packaging body 90 may include, for example, a sealing plate 91 and an outer packaging can 92. The sealing plate 91 plugs the opening of the outer packaging can 92. The sealing plate 91 and the outer packaging can 92 can be joined by, for example, laser welding.

[0044] A positive terminal 81 and a negative terminal 82 are provided on the sealing plate 91. An injection port (not shown) and a gas vent valve (not shown) may be further provided on the sealing plate 91. Electrolyte can be injected into the interior of the outer packaging 90 through the injection port. A positive current collector 71 is used to connect the electrode body 50 to the positive terminal 81. The positive current collector 71 can be, for example, an Al plate. A negative current collector 72 is used to connect the electrode body 50 to the negative terminal 82. The negative current collector 72 can be, for example, a Cu plate.

[0045] Figure 2 This is a schematic diagram illustrating an example of the configuration of the electrode body in this embodiment.

[0046] The electrode body 50 is a wound type. The electrode body 50 includes a positive electrode 10, a separator 30, and a negative electrode 20. That is, the battery 100 includes a positive electrode 10, a negative electrode 20, and an electrolyte. The positive electrode 10, the separator 30, and the negative electrode 20 are all strip-shaped sheets. The electrode body 50 may include multiple separators 30. The electrode body 50 is formed by sequentially stacking and winding the positive electrode 10, the separator 30, and the negative electrode 20 into a vortex shape. One of the positive electrode 10 or the negative electrode 20 can be held by the separator 30. Both the positive electrode 10 and the negative electrode 20 can be held by the separator 30. The electrode body 50 can be formed into a flat shape after winding. Furthermore, the wound type is one example. The electrode body 50 can also be, for example, a stacked type.

[0047] "positive electrode"

[0048] The positive electrode 10 includes a positive electrode substrate 11 and a positive electrode active material layer 12. The positive electrode substrate 11 is a conductive sheet. The positive electrode substrate 11 may be, for example, an Al alloy foil. The positive electrode substrate 11 may have a thickness of, for example, 10 μm to 30 μm. The positive electrode active material layer 12 is disposed on the surface of the positive electrode substrate 11. The positive electrode active material layer 12 may be disposed on only one side of the positive electrode substrate 11, for example. The positive electrode active material layer 12 may be disposed on both the front and back sides of the positive electrode substrate 11, for example. In the width direction of the positive electrode 10 ( Figure 2 In the X-axis direction, the positive electrode substrate 11 may be exposed at one end. The positive electrode current collector 71 may be connected to the exposed portion of the positive electrode substrate 11.

[0049] For example, an intermediate layer (not shown) may be formed between the positive electrode active material layer 12 and the positive electrode substrate 11. In this embodiment, when an intermediate layer is present, it is also considered that the positive electrode active material layer 12 is disposed on the surface of the positive electrode substrate 11. The intermediate layer may be thinner than the positive electrode active material layer 12. The intermediate layer may have a thickness of, for example, 0.1 μm to 10 μm. The intermediate layer may contain, for example, a conductive material, an insulating material, etc.

[0050] (Positive electrode active material layer)

[0051] The positive electrode active material layer 12 may have a thickness of, for example, 10 μm to 200 μm. The positive electrode active material layer 12 may have a thickness of, for example, 50 μm to 150 μm. The positive electrode active material layer 12 may have a thickness of, for example, 50 μm to 100 μm.

[0052] The positive electrode active material layer 12 may have, for example, a concentration of 3.3 g / cm³. 3 Up to 3.8 g / cm 3 The average density. The positive electrode active material layer 12 may have, for example, 3.5 g / cm³. 3 Up to 3.7 g / cm 3 The average density of the positive electrode active material layer 12 in this specification is obtained by dividing the mass of the positive electrode active material layer 12 by its apparent volume. The average density is measured in a sample cut from the positive electrode 10. The sample may have, for example, a diameter of 10 cm³. 2 The left and right planar dimensions. Furthermore, the density of the positive electrode active material layer 12 can vary locally.

[0053] The positive electrode active material layer 12 comprises a positive electrode active material and a conductive material. The conductive material comprises carbon black and fibrous carbon. That is, the positive electrode active material layer 12 comprises a positive electrode active material, carbon black, and fibrous carbon. The positive electrode active material layer 12 may further comprise, for example, a binder. For example, the positive electrode active material layer 12 may consist essentially of 0.1 to 10% by mass of binder, 0.1 to 10% by mass of conductive material, and the balance being the positive electrode active material.

[0054] (Positive electrode active material)

[0055] The positive electrode active material is a particle. The positive electrode active material can have any size. The positive electrode active material can have a D50, for example, from 1 μm to 30 μm. In this specification, "D50" is defined as the particle size at which the cumulative frequency from the smallest particle size in a volumetric particle size distribution reaches 50%. The volumetric particle size distribution can be determined using a laser diffraction particle size distribution measuring device.

[0056] The positive electrode active material can reversibly adsorb and release lithium ions. The positive electrode active material can have any crystal structure. For example, it can have a layered rock salt structure, a spinel structure, or a olivine structure.

[0057] The positive electrode active material can have any chemical composition. The chemical composition of the positive electrode active material can be determined using methods such as ICP-AES (inductively coupled plasma-atomic emission spectrometry).

[0058] The positive electrode active material may have a chemical composition, for example, represented by the following formula (I).

[0059] Li 1-a Ni x Me 1-x O2…(I)

[0060] In equation (I) above, "a" satisfies the relationship -0.3 ≤ a ≤ 0.3. "x" satisfies the relationship 0.7 ≤ x ≤ 1.0. "Me" represents at least one of the following: Co, Mn, Al, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Ti, Si, V, Cr, and Ge.

[0061] The positive electrode active material may have a chemical composition, for example, represented by the following formula (II).

[0062] Li 1-a Ni x Co y Mn 1-x-y O2…(II)

[0063] In equation (II) above, "a" satisfies the relationship -0.3 ≤ a ≤ 0.3. "x" satisfies the relationship 0.7 ≤ x ≤ 1.0. "y" satisfies the relationship 0 ≤ y ≤ 0.3.

[0064] The positive electrode active material may include, for example, materials selected from LiNiO2 and LiNi 0.9 Co 0.1 O2, LiNi 0.9 Mn 0.1 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2, LiNi 0.7 Co 0.1 Mn 0.2 O2, LiNi 0.6 Co 0.3 Mn 0.1 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.6 Co 0.1 Mn 0.3 O2, LiNi 0.5 Co 0.3 Mn 0.2 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.5 Co 0.1 Mn0.4 O2 and LiNi 0.5 Co 0.4 Mn 0.1 At least one of the following: O2.

[0065] The positive electrode active material may contain, for example, materials selected from LiNi. 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.7 Co 0.2 Mn 0.1 O2 and LiNi 0.6 Co 0.2 Mn 0.2 At least one of the following: O2.

[0066] (Carbon black)

[0067] Carbon black is a conductive material. Carbon black exhibits electronic conductivity. Carbon black is an aggregate of particles. Carbon black is a type of amorphous carbon. Carbon black can be essentially composed of carbon. Carbon black may contain at least one of, for example, selected from acetylene black, Ketjen black (registered trademark), furnace black, channel black, and pyrolytic black.

[0068] (Fibrous carbon)

[0069] The fibrous carbon described in this specification has a diameter of 0.1 nm to 1 μm and a length that is more than 100 times that diameter. The fibrous carbon may have a diameter of, for example, 0.5 nm to 20 nm, or a diameter of 5 nm to 15 nm. The fibrous carbon may have a length of, for example, more than 1 μm, or a length of more than 10 μm. The fibrous carbon may have a length of, for example, less than 500 μm, or a length of less than 100 μm. The diameter and length of the fibrous carbon can be measured using, for example, SEM (scanning electron microscope) or TEM (transmission electron microscope). The diameter and length can each be the arithmetic mean of more than 100 fibrous carbon particles.

[0070] Fibrous carbon is a conductive material. Fibrous carbon exhibits electronic conductivity. Fibrous carbon can be essentially composed of carbon. Partially or entirely graphitized, fibrous carbon can contain at least one of, for example, carbon nanotubes (CNTs), vapor-grown carbon fibers (VGCF), polyacrylonitrile (PAN)-based carbon fibers, and pitch-based carbon fibers.

[0071] (Adhesive)

[0072] The adhesive may contain any components. The adhesive may contain at least one of, for example, polyvinylidene fluoride (PVdF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), polytetrafluoroethylene (PTFE), and polyacrylic acid (PAA).

[0073] Multi-layered Structure

[0074] Figure 3 A schematic cross-sectional view is shown to illustrate an example of the positive electrode in this embodiment.

[0075] The positive electrode active material layer 12 has a multilayer structure. That is, the positive electrode active material layer 12 includes a first layer 1 and a second layer 2. The first layer 1 is disposed between the positive electrode substrate 11 and the second layer 2.

[0076] The positive electrode active material layer 12 may also include additional layers (not shown) provided that it includes a first layer 1 and a second layer 2. These additional layers may have a different composition than the first layer 1 and the second layer 2. For example, an additional layer may be formed between the first layer 1 and the second layer 2. For example, an additional layer may be formed between the first layer 1 and the positive electrode substrate 11. For example, an additional layer may be formed between the surface of the positive electrode active material layer 12 and the second layer 2.

[0077] The second layer 2, also known as the "upper layer," is disposed on the surface side of the positive electrode active material layer 12, compared to the first layer 1. The second layer 2 may be exposed on the surface of the positive electrode active material layer 12. The second layer 2 can form the surface of the positive electrode active material layer 12. The first layer 1, also known as the "lower layer," is disposed on the side of the positive electrode substrate 11, compared to the second layer 2. The first layer 1 may be in contact with the surface of the positive electrode substrate 11. The first layer 1 can be formed on the surface of the positive electrode substrate 11.

[0078] (Conductive material composition in the thickness direction)

[0079] Layer 2 has a different conductive material composition than Layer 1. The mass fraction of fibrous carbon and carbon black in each layer is a value relative to the mass of the positive electrode active material in each layer. For example, the mass fraction (percentage) of fibrous carbon in Layer 2 is obtained by dividing the total mass of fibrous carbon contained in Layer 2 by the total mass of the positive electrode active material contained in Layer 2.

[0080] Between layer 1 (lower layer) and layer 2 (upper layer), fibrous carbon is relatively more abundant in layer 2 (upper layer). That is, the mass fraction of fibrous carbon in layer 2 is higher than that in layer 1. The mass fraction of fibrous carbon in layer 2 is 0.2% to 1%. The mass fraction of fibrous carbon in layer 2 can be, for example, 0.4% to 0.8%.

[0081] The ratio of the mass fraction of fibrous carbon in layer 1 to that in layer 2 can be, for example, 0 to 0.25. The ratio of the mass fraction of fibrous carbon in layer 1 to that in layer 2 can be, for example, 0 to 0.1.

[0082] The second layer 2 may or may not contain carbon black. The mass fraction of carbon black in the second layer 2 may be, for example, less than 1%. This is expected to result in, for example, an increase in energy density. The mass fraction of carbon black in the second layer 2 may be, for example, 0% to 0.5%.

[0083] Between layer 1 (lower layer) and layer 2 (upper layer), carbon black is relatively more concentrated in layer 1 (lower layer). That is, the mass fraction of carbon black in layer 1 is higher than that in layer 2. The mass fraction of carbon black in layer 1 is 1.5% to 5%. The mass fraction of carbon black in layer 1 can be, for example, 2% to 4%.

[0084] The ratio of the mass fraction of carbon black in the second layer 2 to the mass fraction of carbon black in the first layer 1 can be, for example, 0 to 0.7. The ratio of the mass fraction of carbon black in the second layer 2 to the mass fraction of carbon black in the first layer 1 can be, for example, 0 to 0.3.

[0085] Layer 1 may or may not contain fibrous carbon. The mass fraction of fibrous carbon in Layer 1 may be, for example, less than 0.05%. Therefore, for example, an improvement in cycle durability is expected. The mass fraction of fibrous carbon in Layer 1 may be, for example, 0% to 0.05%, 0% to 0.02%, or 0.02% to 0.05%.

[0086] (Methods for determining the composition of conductive materials)

[0087] The composition of the conductive material can be determined using TG-DTA (thermogravimetry-differential thermal analysis). First, a specified area of ​​positive electrode active material layer 12 is recovered. A powder sample is prepared from the specified area of ​​positive electrode active material layer 12. The powder sample can be approximately 0.01 g. The TG-DTA curve of the powder sample is measured. The measurement is performed under atmospheric conditions. The heating rate can be approximately 5 °C / min. Based on the mass reduction from approximately 100 °C to approximately 400 °C in the TG-DTA curve, the mass fraction of carbon black in the overall positive electrode active material layer 12 is determined. Based on the mass reduction from approximately 400 °C to approximately 700 °C in the TG-DTA curve, the mass fraction of fibrous carbon in the overall positive electrode active material layer 12 is determined.

[0088] Next, the second layer 2 (upper layer) is separated from the positive electrode active material layer 12 using adhesive tape or the like. The adhesive tape can be, for example, Capton tape. A powder sample is prepared from the remaining portion of the positive electrode active material layer 12 (i.e., the first layer 1). Following the steps described above, a TG-DTA curve is measured. The mass fraction of carbon black in the first layer 1 and the mass fraction of fibrous carbon in the first layer 1 are then determined.

[0089] The mass fraction of carbon black in the second layer 2 is obtained by subtracting the mass fraction of carbon black in the first layer 1 from the mass fraction of carbon black in the entire positive electrode active material layer 12. Similarly, the mass fraction of fibrous carbon in the second layer 2 is obtained by subtracting the mass fraction of fibrous carbon in the first layer 1 from the mass fraction of fibrous carbon in the entire positive electrode active material layer 12.

[0090] (Porosity)

[0091] The porosity can differ between layer 1 (upper layer) and layer 2 (lower layer). For example, layer 2 (upper layer) can have a higher porosity than layer 1 (lower layer). Thus, for example, an improvement in at least one of cycle durability and energy density is expected.

[0092] The second layer 2 may have a porosity of, for example, 23.5% or more. The second layer 2 may have a porosity of, for example, 25% to 35%. The second layer 2 may have a porosity of, for example, 27.9% or less.

[0093] Layer 1 may have a porosity of, for example, less than 22%. Layer 1 may have a porosity of, for example, between 15% and 20%. Layer 1 may have a porosity of, for example, more than 17.1%.

[0094] The porosity of each layer was measured in cross-sectional SEM images of the positive electrode active material layer 12. The cross-sectional SEM images were obtained at a section parallel to the thickness direction of the positive electrode active material layer 12. Porosity was distinguished from other parts by binarizing the cross-sectional SEM images. The area fraction of the porosity within layer 1 was considered the porosity of layer 1. That is, the porosity was calculated by dividing the total area of ​​the porosity within layer 1 by the total area of ​​layer 1. Similarly, the porosity of layer 2 was calculated. The porosity was measured in at least five cross-sectional SEM images. The arithmetic mean of the five or more cross-sectional SEM images was used.

[0095] The porosity of each layer can be adjusted, for example, by the following steps: A first slurry is coated onto the surface of the positive electrode substrate 11 and dried to form a first layer 1. The first layer 1 is then compressed, for example, using a calender or the like, to give it a specified density. A second slurry is coated onto the surface of the compressed first layer 1 and dried to form a second layer 2. The second layer 2 is then compressed, for example, using a calender or the like, to give it a specified density. By compressing the first layer 1 and the second layer 2 to give them different densities, the porosity of each layer can be adjusted. For example, the layers can be compressed so that the second layer 2 has a lower density than the first layer 1. Therefore, the second layer 2 can have a higher porosity than the first layer 1.

[0096] (Thickness ratio)

[0097] The thickness ratio (T2 / T1) is obtained by dividing the thickness T2 of the second layer 2 by the thickness T1 of the first layer 1. The thickness ratio can be, for example, 0.2 to 0.8. An increase in energy density is expected with a thickness ratio of 0.2 to 0.8. The thickness ratio can also be, for example, 0.3 to 0.5. The thickness of each layer is measured in a cross-sectional SEM image of the positive electrode active material layer 12. The cross-sectional SEM image is obtained at a section parallel to the thickness direction of the positive electrode active material layer 12. The thickness of each layer is measured at at least five locations. The arithmetic mean of the thicknesses at these five locations is used.

[0098] (Composition of positive electrode active material in the thickness direction)

[0099] The second layer 2 may contain, for example, the same positive electrode active material as the first layer 1. The second layer 2 may also contain, for example, a different positive electrode active material than the first layer 1. For example, when the chemical composition of the positive electrode active material is represented by the above formula (I) or (II), the positive electrode active material contained in the second layer 2 may have a smaller "x (molar fraction of Ni)" than the positive electrode active material contained in the first layer 1. Therefore, it is expected that, for example, even if a small amount of reaction concentration occurs in the second layer 2 (the upper layer), the degradation (particle cracking) of the positive electrode active material will be mitigated.

[0100] In the second layer 2, the "x" in the above equation (I) or (II) can satisfy, for example, the relationship 0.7 ≤ x < 0.8. In the second layer 2, the "x" in the above equation (I) or (II) can satisfy, for example, the relationship 0.7 ≤ x ≤ 0.75. In the first layer 1, the "x" in the above equation (I) or (II) can satisfy, for example, the relationship 0.8 ≤ x ≤ 1.0. In the first layer 1, the "x" in the above equation (I) or (II) can satisfy, for example, the relationship 0.85 ≤ x ≤ 1.0.

[0101] "negative electrode"

[0102] The negative electrode 20 may include, for example, a negative electrode substrate 21 and a negative electrode active material layer 22. The negative electrode substrate 21 is a conductive sheet. The negative electrode substrate 21 may be, for example, a Cu alloy foil. The negative electrode substrate 21 may have a thickness of, for example, 5 μm to 30 μm. The negative electrode active material layer 22 may be disposed on the surface of the negative electrode substrate 21. The negative electrode active material layer 22 may, for example, be disposed only on one side of the negative electrode substrate 21. The negative electrode active material layer 22 may be disposed on, for example, both the front and back sides of the negative electrode substrate 21. In the width direction of the negative electrode 20 ( Figure 2 Along the X-axis direction, the negative electrode substrate 21 may be exposed at one end. The negative electrode current collector 72 may be joined to the exposed portion of the negative electrode substrate 21.

[0103] The negative electrode active material layer 22 may have a thickness of, for example, 10 μm to 200 μm. The negative electrode active material layer 22 contains a negative electrode active material. The negative electrode active material may contain any composition. The negative electrode active material may contain, for example, materials selected from graphite, soft carbon, hard carbon, Si, SiO, Si-based alloys, Sn, SnO, Sn-based alloys, and Li4Ti5O. 12 At least one of them.

[0104] In addition to the negative electrode active material, the negative electrode active material layer 22 may further include, for example, a binder. The negative electrode active material layer 22 may, for example, consist substantially of 0.1 to 10% by mass of a binder and the balance being the negative electrode active material. The binder may contain any components. The binder may contain, for example, at least one selected from carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR).

[0105] Separator

[0106] At least a portion of the separator (partition) 30 is located between the positive electrode 10 and the negative electrode 20. The separator 30 separates the positive electrode 10 and the negative electrode 20. The separator 30 may have a thickness of, for example, 10 μm to 30 μm.

[0107] The separator 30 is a porous sheet. Electrolyte permeates through the separator 30. The separator 30 may have an air permeability of, for example, from 100 s / 100 mL to 400 s / 100 mL. In this specification, "air permeability" refers to "air resistance" as specified in JIS P 8117:2009. Air permeability can be determined using the Grie test method.

[0108] The separator 30 is electrically insulating. The separator 30 may contain, for example, a polyolefin resin. The separator 30 may be substantially composed of a polyolefin resin, for example, at least one selected from polyethylene (PE) and polypropylene (PP). The separator 30 may have, for example, a single-layer structure. The separator 30 may be substantially composed of a PE layer, for example. The separator 30 may have, for example, a multi-layer structure. The separator 30 may be formed by, for example, sequentially stacking a PP layer, a PE layer, and another PP layer. A heat-resistant layer (ceramic particle layer), for example, may be formed on the surface of the separator 30.

[0109] Electrolyte

[0110] The electrolyte comprises a solvent and a supporting electrolyte. The solvent is aprotic. The solvent may contain any components. The solvent may contain at least one of, for example, ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), 1,2-dimethoxyethane (DME), methyl formate (MF), methyl acetate (MA), methyl propionate (MP), and γ-butyrolactone (GBL).

[0111] The supporting electrolyte is soluble in a solvent. The supporting electrolyte may, for example, comprise at least one selected from LiPF6, LiBF4, and LiN(FSO2)2. The supporting electrolyte may have a molar concentration, for example, from 0.5 mol / L to 2.0 mol / L. The supporting electrolyte may have a molar concentration, for example, from 0.8 mol / L to 1.2 mol / L.

[0112] In addition to the solvent and supporting electrolyte, the electrolyte may further contain any additives. For example, the electrolyte may contain 0.01% to 5% additives by mass fraction. The additives may include, for example, at least one selected from vinylene carbonate (VC), lithium difluorophosphate (LiPO2F2), lithium fluorosulfonate (FSO3Li), and lithium bis(oxalato)borate (LiBOB).

[0113] Example

[0114] The following describes embodiments of the present technology (also referred to as "the present embodiments" in this specification). However, the following description does not limit the scope of the present technology.

[0115] <The Manufacturing of the Positive Electrode>

[0116] No.1

[0117] The following materials have been prepared.

[0118] Positive electrode active material: LiNi 0.8 Co 0.1 Mn 0.1 O2

[0119] Fibrous carbon: CNT

[0120] Carbon black: Acetylene black

[0121] Adhesive: PVdF (powder)

[0122] Dispersion medium: N-methyl-2-pyrrolidone

[0123] Positive electrode substrate: Al foil

[0124] A first slurry was prepared by mixing 100 parts by weight of positive electrode active material, 0.05 parts by weight of fibrous carbon, 4 parts by weight of carbon black, 0.9 parts by weight of binder, and dispersion medium. The first slurry was then coated onto the surface of a positive electrode substrate and dried to form a first layer. The first layer was then compressed using a calender. The compressed first layer had a density of 3.7 g / cm³. 3 The density.

[0125] A second slurry was prepared by mixing 100 parts by mass of positive electrode active material, 0.8 parts by mass of fibrous carbon, 1 part by mass of carbon black, 0.9 parts by mass of binder, and dispersion medium. The second slurry was then coated onto the surface of the first layer and dried to form the second layer. The second layer was then compressed using a calender. The compressed second layer had a density of 3.5 g / cm³. 3 The density is such that a positive electrode active material layer is formed. The thickness ratio (T2 / T1) is 1.0. A positive electrode active material layer is also formed on the back side of the positive electrode substrate. Through the above operations, the positive electrode described in No.1 is manufactured.

[0126] "No.2 to No.15"

[0127] Aside from changing the composition of the conductive materials in the first and second layers, various positive electrodes were manufactured in the same manner as the positive electrode involved in No. 1 (see Table 1 below). Furthermore, the positive electrode active material layers involved in No. 8 and No. 9 are single-layer structures. The positive electrode active material layers involved in the other samples are two-layer structures.

[0128] No. 16, No. 17

[0129] Apart from changing the thickness ratio, various positive electrodes were manufactured in the same manner as the positive electrode involved in No. 15 (see Table 1 below).

[0130] No. 18, No. 19

[0131] Except for altering the chemical composition of the positive electrode active material, the positive electrode was manufactured in the same manner as the positive electrode involved in No.1 (see Table 1 below).

[0132] <Evaluation>

[0133] Test batteries (non-aqueous electrolyte secondary batteries) containing various positive electrodes manufactured as described above were produced. The initial discharge capacity of the test batteries was measured. The energy density of the test batteries was determined from the initial discharge capacity.

[0134] Charge-discharge cycles were performed on the test battery. The capacity retention was determined by dividing the discharge capacity of the 300th cycle by the discharge capacity of the 1st cycle.

[0135] Table 1

[0136]

[0137] <Results>

[0138] In the test batteries involved in No.1 to No.17, the chemical composition of the positive electrode active material is common. In the test batteries involved in No.1 to No.17, a capacity retention of 72.2% or more and an energy density of 635 Wh / L or more after 300 cycles are considered to have both cycle durability and energy density.

[0139] The test batteries involved in No. 1 to No. 3 and No. 11 to No. 17 possess both cycle durability and energy density. These test batteries meet the conditions (a) to (e) below.

[0140] (a) The positive electrode active material layer has a multi-layer structure.

[0141] (b) The mass fraction of carbon black in the first layer is higher than that in the second layer.

[0142] (c) The mass fraction of carbon black in the first layer is 1.5% to 5%.

[0143] (d) The mass fraction of fibrous carbon in the second layer is higher than that in the first layer.

[0144] (e) The mass fraction of fibrous carbon in the second layer is 0.2% to 1%.

[0145] None of the test batteries involved in No. 4 to No. 10 possess both cycle durability and energy density. These test batteries do not meet any one or more of the conditions (a) to (e) above.

[0146] In the test cells involved in No.15 to No.17, a tendency was found that the smaller the thickness ratio, the higher the energy density.

[0147] In the test batteries involved in No.1, No.18, and No.19, a positive electrode active material (general formula: LiNi) was found. x Coy Mn 1-x-y The larger the "x" value of O2, the higher the energy density tends to be. Conversely, the smaller the "x" value, the higher the cycle durability tends to be. In No. 18 and No. 19, it is believed that both cycle durability and energy density are achieved.

[0148] This embodiment and example are illustrative in all respects. This embodiment and example are not restrictive. The scope of this technology includes all changes within the meaning and scope equivalent to the claims. For example, it was contemplated from the outset that arbitrary components could be extracted from this embodiment and example and combined arbitrarily.

Claims

1. A non-aqueous electrolyte secondary battery, which includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode substrate and a positive electrode active material layer. The positive electrode active material layer is disposed on the surface of the positive electrode substrate. The positive electrode active material layer comprises a first layer and a second layer. The first layer is disposed between the positive electrode substrate and the second layer. The positive electrode active material layer comprises positive electrode active material, carbon black, and fibrous carbon. The mass fraction of carbon black in the first layer is higher than the mass fraction of carbon black in the second layer. In the first layer, the mass fraction of the carbon black is 1.5% to 5% relative to the mass of the positive electrode active material. The mass fraction of fibrous carbon in the second layer is higher than that in the first layer. In the second layer, the mass fraction of the fibrous carbon is 0.2% to 1% relative to the mass of the positive electrode active material. The first layer has a porosity of 17.1% or more and 22% or less, and the second layer has a porosity of 23.5% or more and 27.9% or less.

2. The non-aqueous electrolyte secondary battery according to claim 1, wherein, In the first layer, the mass fraction of the fibrous carbon is less than 0.05% relative to the mass of the positive electrode active material, and in the second layer, the mass fraction of the carbon black is less than 1% relative to the mass of the positive electrode active material.

3. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The ratio of the thickness of the second layer to the thickness of the first layer is 0.2 to 0.

8.

4. The non-aqueous electrolyte secondary battery according to claim 1 or 2, wherein, The positive electrode active material has a chemical composition represented by formula (I): Li 1-a Ni x Along with 1-x O2 …(I) In the above formula (I), a satisfies the relationship -0.3≦a≦0.3, x satisfies the relationship 0.7≦x≦1.0, and Me represents at least one of the following: Co, Mn, Al, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Ti, Si, V, Cr, and Ge.

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