Non-aqueous electrolyte secondary battery
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- PANASONIC ENERGY CO LTD
- Filing Date
- 2026-01-23
- Publication Date
- 2026-07-30
Smart Images
Figure JP2026002245_30072026_PF_FP_ABST
Abstract
Description
Nonaqueous electrolyte secondary battery
[0001] This invention relates to a non-aqueous electrolyte secondary battery.
[0002] Non-aqueous electrolyte secondary batteries, such as lithium-ion batteries, are used in a variety of applications due to their high capacity and other characteristics. A non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, a non-aqueous electrolyte (e.g., electrolyte solution), and a separator. Various proposals have been made regarding such non-aqueous electrolyte secondary batteries.
[0003] Patent Document 1 discloses a non-aqueous electrolyte secondary battery comprising a negative electrode having a negative electrode current collector, a first negative electrode mixture layer provided on the surface of the negative electrode current collector, and a second negative electrode mixture layer provided on the surface of the first negative electrode mixture layer, wherein the first negative electrode mixture layer and the second negative electrode mixture layer contain graphite particles, the ratio (S2 / S1) of the void ratio between graphite particles in the second negative electrode mixture layer to the void ratio (S1) between graphite particles in the first negative electrode mixture layer to the void ratio (S2) in the second negative electrode mixture layer to the void ratio (S1) in the first negative electrode mixture layer to the packing density (D2) in the second negative electrode mixture layer to the packing density (D1) in the first negative electrode mixture layer to the packing density (D2 / D1) in the second negative electrode mixture layer to the packing density (D2) in the first negative electrode mixture layer to the packing density (D2) in the second negative electrode mixture layer to the packing density (D1) in the first negative electrode mixture layer to be 0.9 to 1.1.
[0004] Patent Document 1 discloses that by setting the porosity ratio S2 / S1 in the negative electrode within the above range, it is possible to suppress the decrease in battery capacity due to repeated rapid charging caused by poor electrolyte permeability in a non-aqueous electrolyte secondary battery. Furthermore, Patent Document 1 discloses that by setting the porosity ratio S2 / S1 within the above range and the packing density ratio D2 / D1 within the above range, it is possible to suppress the decrease in rapid charging cycle characteristics while maintaining high capacity in a non-aqueous electrolyte secondary battery.
[0005] International Publication No. 2021 / 106730
[0006] Incidentally, as mentioned above, improving the fluid permeability (hereinafter sometimes simply referred to as fluid permeability) of the electrolyte (e.g., a non-aqueous electrolyte) at the negative electrode can cause the electrolyte contained in a non-aqueous electrolyte secondary battery to be supplied unevenly to the negative electrode, potentially reducing the amount of electrolyte supplied to the positive electrode. In this case, the uneven distribution of the electrolyte can easily lead to deterioration of the positive electrode active material. When the positive electrode active material deteriorates, the internal resistance of the non-aqueous electrolyte secondary battery increases. Furthermore, if the amount of electrolyte supplied to the positive electrode is insufficient, the capacity at the positive electrode cannot be fully realized during repeated charging and discharging cycles, thus reducing the cycle characteristics of the non-aqueous electrolyte secondary battery.
[0007] Therefore, the object of this disclosure is to provide a non-aqueous electrolyte secondary battery that can suppress an increase in internal resistance and a decrease in cycle characteristics.
[0008] One aspect of the present invention relates to a non-aqueous electrolyte secondary battery. The non-aqueous electrolyte secondary battery comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode has a negative electrode current collector and a negative electrode mixture layer disposed on at least one surface of the negative electrode current collector. The negative electrode mixture layer has a first negative electrode mixture layer disposed on at least one surface of the negative electrode current collector and a second negative electrode mixture layer disposed on the surface of the first negative electrode mixture layer, where the porosity of the first negative electrode mixture layer is ε1 (%) and the porosity of the second negative electrode mixture layer is ε2 (%), then ε1 and ε2 satisfy the relationships ε1 < ε2, ε2 ≥ 10%, and 1.3 ≤ ε2 / ε1 ≤ 5.0. The positive electrode comprises a first positive electrode active material and a second positive electrode active material having a volume-based median diameter D50 smaller than that of the first positive electrode active material. When the volume-based median diameter D50 of the first positive electrode active material is D1 (μm) and the volume-based median diameter D50 of the second positive electrode active material is D2 (μm), D1 and D2 satisfy the relationship 2.0 ≤ D1 / D2 ≤ 15.0. When the content of the first positive electrode active material is W1 (mg) and the content of the second positive electrode active material is W2 (mg), W1 and W2 satisfy the relationship W1:W2 = 1:9 to 9:1.
[0009] According to this disclosure, it is possible to provide a non-aqueous electrolyte secondary battery that can suppress an increase in internal resistance and a decrease in cycle characteristics.
[0010] This is a schematic cross-sectional view showing a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure.
[0011] The embodiments of this disclosure will be described below with examples, but this disclosure is not limited to the examples described below. In the following description, specific numerical values and materials may be given as examples, but other numerical values, materials, etc. may be applied as long as the effects of this disclosure are obtained. Notwithstanding, known components may be applied to components of parts that are characteristic of this disclosure. In this specification, when "the range of numerical values A to numerical values B" is used, that range includes numerical values A and B.
[0012] In the following explanation, when examples are given for the lower and upper limits of numerical values related to specific physical properties or conditions, any combination of either of the given lower limits and any of the given upper limits is permitted, as long as the lower limit does not exceed the upper limit. When multiple materials are given as examples, unless otherwise specified, one type may be selected and used alone, or two or more types may be used in combination.
[0013] This disclosure includes any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims. In other words, any combination of two or more claims that can be arbitrarily selected from the claims set forth in the attached claims is possible, as long as it does not result in a technical inconsistency.
[0014] [Non-aqueous electrolyte secondary battery] A non-aqueous electrolyte secondary battery according to the embodiment of this disclosure comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte.
[0015] In the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on at least one surface of the negative electrode current collector. In the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, the negative electrode mixture layer comprises a first negative electrode mixture layer disposed on at least one surface of the negative electrode current collector and a second negative electrode mixture layer disposed on the surface of the first negative electrode mixture layer. In the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, when the porosity of the first negative electrode mixture layer is ε1 (%) and the porosity of the second negative electrode mixture layer is ε2 (%), ε1 and ε2 satisfy the relationships ε1 < ε2, ε2 ≥ 10%, and 1.3 ≤ ε2 / ε1 ≤ 5.0.
[0016] In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, the positive electrode includes a first positive electrode active material and a second positive electrode active material having a median diameter D50 based on volume smaller than that of the first positive electrode active material. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, when the median diameter D50 based on volume of the first positive electrode active material is D1 (μm) and the median diameter D50 based on volume of the second positive electrode active material is D2 (μm), D1 and D2 satisfy the relationship of 2.0 ≤ D1 / D2 ≤ 15.0. In the non-aqueous electrolyte secondary battery according to an embodiment of the present disclosure, when the content of the first positive electrode active material is W1 (mg) and the content of the second positive electrode active material is W2 (mg), W1 and W2 satisfy the relationship of W1:W2 = 1:9 to 9:1.
[0017] In the secondary battery according to an embodiment of the present disclosure, (i) when the porosity of the first negative electrode binder layer is ε1 (%) and the porosity of the second negative electrode binder layer is ε2 (%) ε1 and ε2 satisfy the relationships of ε1 < ε2, ε2 ≥ 10%, and 1.3 ≤ ε2 / ε1 ≤ 5.0, (ii) when the median diameter D50 based on volume of the first positive electrode active material is D1 (μm) and the median diameter D50 based on volume of the second positive electrode active material is D2 (μm), D1 and D2 satisfy the relationship of 2.0 ≤ D1 / D2 ≤ 15.0, and (iii) when the content of the first positive electrode active material is W1 (mg) and the content of the second positive electrode active material is W2 (mg), it is important that W1 and W2 satisfy the relationship of W1:W2 = 1:9 to 9:1. The reasons are described below.
[0018] In a non-aqueous electrolyte secondary battery, it is important to improve the liquid wettability of the non-aqueous electrolyte in the positive electrode and the negative electrode in order to improve the cycle characteristics. For example, it is important to allow the non-aqueous electrolyte to sufficiently penetrate into each of the positive electrode binder layer and the negative electrode binder layer.
[0019] However, while studies have been conducted to improve the fluid flow at the negative electrode in non-aqueous electrolyte secondary batteries, as described in Patent Document 1 above, it is difficult to say that sufficient studies have been conducted to improve the fluid flow at the positive electrode. If the fluid flow at the positive electrode cannot be improved, the non-aqueous electrolyte may be supplied unevenly to the negative electrode, and the amount of non-aqueous electrolyte supplied to the positive electrode may decrease. In this case, the uneven distribution of the non-aqueous electrolyte can easily lead to the deterioration of the positive electrode active material contained in the positive electrode. When the positive electrode active material deteriorates, the internal resistance of the non-aqueous electrolyte secondary battery increases. In addition, if the amount of non-aqueous electrolyte supplied to the positive electrode decreases, the capacity at the positive electrode cannot be fully expressed during repeated charge and discharge cycles, thus reducing the cycle characteristics of the non-aqueous electrolyte secondary battery.
[0020] Here, in the non-aqueous electrolyte secondary battery according to the embodiment of this disclosure, as described above, in the positive electrode, (ii) when the volume-based median diameter D50 of the first positive electrode active material is D1 (μm) and the volume-based median diameter D50 of the second positive electrode active material is D2 (μm), D1 and D2 satisfy the relationship 2.0 ≤ D1 / D2 ≤ 15.0. And, because this relationship is satisfied, it is considered that the second positive electrode active material, which has a smaller volume-based median diameter D50, can be suitably arranged between the first positive electrode active material, which has a larger volume-based median diameter D50. Furthermore, in the positive electrode, (iii) when the content of the first positive electrode active material is W1 (mg) and the content of the second positive electrode active material is W2 (mg), W1 and W2 satisfy the relationship W1:W2 = 1:9 to 9:1. Furthermore, by satisfying this relationship, it is believed that the second positive electrode active material can be arranged between the first positive electrode active material particles in a suitable ratio. This is thought to suppress unevenness in the size of the voids formed between the first positive electrode active material particles. In other words, it is thought to suppress the coexistence of non-aqueous electrolyte diffusion pathways that are narrow enough to become bottlenecks and widened non-aqueous electrolyte diffusion pathways between the first positive electrode active material particles. Therefore, it is thought that the diffusivity of the non-aqueous electrolyte can be improved in the positive electrode, and consequently, the liquid flowability of the non-aqueous electrolyte can be improved.
[0021] Further, in the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, as described above, in the negative electrode, when the porosity of the first negative electrode mixture layer is ε1 (%) and the porosity of the second negative electrode mixture layer is ε2 (%), ε1 and ε2 satisfy the relationship of ε1 < ε2, ε2 ≥ 10%, and 1.3 ≤ ε2 / ε1 ≤ 5.0. By satisfying this relationship, it is considered that sufficient liquid circulation can be ensured in the second negative electrode mixture layer while sufficiently ensuring the capacity in the first negative electrode mixture layer. That is, it is considered that the capacity can be sufficiently ensured in the negative electrode mixture layer while sufficiently ensuring the liquid circulation in the negative electrode mixture layer. In the non-aqueous electrolyte secondary battery of the present disclosure, as described above, since the liquid circulation of the non-aqueous electrolyte in the positive electrode is improved, even if the liquid circulation of the non-aqueous electrolyte in the negative electrode is improved, it is considered that the non-aqueous electrolyte can be sufficiently suppressed from being supplied to the negative electrode in a biased manner.
[0022] Therefore, in the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, the increase in the internal resistance of the non-aqueous electrolyte secondary battery due to the deterioration of the positive electrode active material caused by the decrease in the supply of the non-aqueous electrolyte is suppressed. Further, in the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure, in addition to being able to sufficiently exhibit the capacity of the positive electrode during repeated charge and discharge, in the negative electrode, while sufficiently ensuring the liquid circulation in the negative electrode mixture layer, the capacity can be sufficiently ensured in the negative electrode mixture layer, so that the deterioration of the cycle characteristics of the non-aqueous electrolyte secondary battery can be suppressed.
[0023] Hereinafter, the configuration of the non-aqueous electrolyte secondary battery according to the embodiment of the present disclosure will be specifically described.
[0024] (Positive electrode) As described above, the positive electrode includes the first positive electrode active material and the second positive electrode active material having a median diameter D50 smaller than that of the first positive electrode active material on a volume basis.
[0025] The volume-based median diameter D50 refers to the particle size at which the cumulative frequency of the smallest particle size accounts for 50% in the volume-based particle size distribution, and is also called the median diameter. The particle size and particle size distribution of the first positive electrode active material, and the particle size and particle size distribution of the second positive electrode active material, can be measured using a laser diffraction particle size distribution analyzer with water as the dispersion medium. For example, the "MT3000II" manufactured by Microtrac-Bell Co., Ltd. can be used as a laser diffraction particle size distribution analyzer.
[0026] When the volume-based median diameter D50 of the first positive electrode active material is denoted as D1 (μm) and the volume-based median diameter D50 of the second positive electrode active material is denoted as D2 (μm), then D1 and D2 satisfy the relationship 2.0 ≤ D1 / D2 ≤ 15.0, as described above.
[0027] Preferably, D1 satisfies the relationship 8 μm ≤ D1 ≤ 17 μm, and D2 satisfies the relationship 2 μm ≤ D2 ≤ 6 μm. By satisfying the above relationship between D1 and D2, in the positive electrode mixture layer, second positive electrode active material with a smaller volume-based median diameter D50 can be suitably arranged between first positive electrode active material with a larger volume-based median diameter D50. This further suppresses unevenness in the size of voids formed between the first positive electrode active material. In other words, it further suppresses the mixing of non-aqueous electrolyte diffusion pathways that are narrow enough to become bottlenecks and wide non-aqueous electrolyte diffusion pathways between the first positive electrode active material. Therefore, the diffusivity of the non-aqueous electrolyte can be further improved in the positive electrode mixture layer.
[0028] The second positive electrode active material may be secondary particles formed by the aggregation of large primary particles with an average particle size of 0.5 μm or more, or it may be particles composed of substantially single particles. Particles composed of substantially single particles mean particles in which the grain boundaries of primary particles cannot be confirmed when observed at appropriate magnification using a scanning electron microscope (SEM). When the second positive electrode active material is secondary particles, the average particle size of the primary particles is 0.5 μm to 3 μm. On the other hand, the first positive electrode active material may be secondary particles formed by the aggregation of small primary particles with an average particle size of 0.3 μm or less.
[0029] When the second positive electrode active material consists of secondary particles, grain boundaries of primary particles can be observed in the particle cross-section observed by SEM. For example, the second positive electrode active material is composed of 100 or fewer primary particles. The second positive electrode active material may consist of several to several dozen primary particles, or 2 to 5 primary particles. In contrast, the first positive electrode active material is composed of 10,000 to 5,000,000 primary particles. The particle size of the primary particles is measured as the Ferret diameter of the region (primary particle) surrounded by grain boundaries in the SEM image of the particle cross-section of the first or second positive electrode active material. The average particle size of the primary particles is obtained as the arithmetic mean of the particle sizes of 100 primary particles.
[0030] In the positive electrode, when the content of the first positive electrode active material is W1 (mg) and the content of the second positive electrode active material is W2 (mg), W1 and W2 satisfy the relationship W1:W2 = 1:9 to 9:1 as described above.
[0031] The first positive electrode active material and the second positive electrode active material are both materials capable of intercalating and releasing lithium ions. The first positive electrode active material and the second positive electrode active material may both be composite oxides containing lithium and a transition metal. In other words, the first positive electrode active material and the second positive electrode active material may both be lithium transition metal composite oxides. The transition metal may include at least one selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), copper (Cu), chromium (Cr), titanium (Ti), niobium (Nb), zirconium (Zr), vanadium (V), tantalum (Ta), and molybdenum (Mo).
[0032] The lithium transition metal composite oxide may contain metals other than transition metals. The metals other than transition metals may include at least one selected from the group consisting of aluminum (Al), magnesium (Mg), calcium (Ca), strontium (Sr), zinc (Zn), and silicon (Si). Furthermore, the first and second positive electrode active materials may contain boron (B) in addition to metals.
[0033] From the viewpoint of increasing capacity, the transition metal may contain Ni. The lithium transition metal composite oxide may contain Ni and at least one selected from the group consisting of Co, Mn, Al, Ti, and Fe. From the viewpoint of achieving high output in addition to high capacity, the lithium transition metal composite oxide may contain Ni and at least one selected from the group consisting of Co, Mn, and Al. When the lithium transition metal composite oxide further contains Co in addition to Li and Ni, the phase transition of the lithium transition metal composite oxide containing Li and Ni is suppressed during charging and discharging, improving the stability of the crystal structure. This makes it easier to improve the cycle characteristics of the non-aqueous electrolyte secondary battery. When the lithium transition metal composite oxide contains at least one of Mn and Al in addition to Li, Ni, and Co, the thermal stability of the non-aqueous electrolyte secondary battery is improved.
[0034] From the viewpoint of facilitating the achievement of higher capacity, in lithium transition metal composite oxides, the atomic ratio of Ni to the total amount of metals other than Li (Ni / Me) may be 0.3 or more and less than 1, or 0.5 or more and less than 1.
[0035] From the viewpoint of improving cycle characteristics and achieving high output, the lithium transition metal composite oxide may include a composite oxide having a layered rock salt type crystal structure and containing at least one of Ni and Co, or it may include a composite oxide having a spinel type crystal structure and containing Mn. From the viewpoint of increasing capacity, the lithium transition metal composite oxide may also be a composite oxide having a layered rock salt type crystal structure and containing Ni and a metal other than Ni, with the above Ni / Me ratio being 0.3 or more and less than 1 (hereinafter also referred to as nickel-based composite oxide).
[0036] Nickel-based composite oxides are positive electrode active materials that exhibit high potential, but their crystal structure is relatively unstable. Therefore, nickel-based composite oxides are prone to degradation due to nickel leaching upon contact with non-aqueous electrolytes. As a result, non-aqueous electrolyte secondary batteries using nickel-based composite oxides as positive electrode active materials tend to have reduced cycle characteristics. Consequently, when using nickel-based composite oxides as positive electrode active materials, covering at least a portion of the surface of the nickel-based composite oxide with a coating material significantly improves cycle characteristics. Furthermore, covering at least a portion of the surface of the nickel-based composite oxide with a coating material allows for the full utilization of its high-capacity characteristics.
[0037] In addition to improving cycle characteristics, lithium transition metal composite oxides may have a composition represented by the following compositional formula (1) from the viewpoint of achieving higher capacity and higher power output. In compositional formula (1), the following conditions are satisfied: 0.3 ≤ x1 < 1, 0 < y1 ≤ 0.5, and 0 < 1 - x1 - y1 ≤ 0.35. M2 may be at least one of A1 and Mn. Furthermore, in compositional formula (1), the following conditions may also be satisfied: 0.5 ≤ x1 < 1, and 0 < y1 ≤ 0.35. LiNi ｘ１ Co ｙ１ M2 １－ｘ１－ｙ１ O ２ ... (1)
[0038] Lithium transition metal composite oxides may have a composition represented by the following compositional formula (2). In compositional formula (2), the condition 0.3 ≤ x² < 1 is satisfied, and M3 may be at least one selected from the group consisting of Co, Mn, Al, Ti, and Fe. When x² is within the above range, a good balance can be obtained between the effects of Ni and the effects of element M3. In compositional formula (2), x² may be 0.5 or greater, or 0.75 or greater. LiNi ｘ２ M3 １－ｘ２ O ２ ... (2)
[0039] The lithium transition metal composite oxide may have a composition represented by the following compositional formula (3). In the following compositional formula (3), 2.7 ≤ x3 ≤ 3.3, 0.9 ≤ y3 ≤ 2.2, and 0.9 ≤ z3 ≤ 3.3 are satisfied, and M4 may be at least one selected from the group consisting of Ni, Co, Mn, Fe, and V. Li ｘ３ M4 ｙ３ (PO ４ ) ｚ３ ・・・・・(3)
[0040] The positive electrode may include a positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector. In this case, both the first positive electrode active material and the second positive electrode active material are included in the positive electrode mixture layer. The positive electrode mixture layer may include a binder, a conductive agent, and the like in addition to the first positive electrode active material and the second positive electrode active material. The positive electrode mixture layer can be formed, for example, by applying a positive electrode mixture slurry in which a positive electrode mixture is dispersed in a dispersion medium onto at least one surface of the positive electrode current collector to form a coating film, and then drying this coating film. The dried coating film may be rolled as necessary. The positive electrode mixture layer may be formed on only one surface of the positive electrode current collector or on both surfaces. As the dispersion medium, for example, N-methyl-2-pyrrolidone (NMP) or the like can be used.
[0041] As described above, when the positive electrode includes a positive electrode mixture layer, when the porosity of the positive electrode mixture layer is ε3 (%), it is preferable that the porosity ε3 satisfies the relationship of ε3 ≤ 12%. The porosity ε3 may satisfy the relationship of ε3 ≤ 10%. Also, the porosity ε3 may satisfy the relationship of 3.0% ≤ ε3, or may satisfy the relationship of 5.0% ≤ ε3. By the porosity ε3 satisfying the above relationship, the diffusibility of the non-aqueous electrolyte in the positive electrode mixture layer can be further improved, and consequently, the liquid wettability of the non-aqueous electrolyte can be further improved. Also, sufficient capacity can be ensured in the positive electrode mixture layer.
[0042] The porosity ε3 of the positive electrode mixture layer is a two-dimensional value obtained from the ratio of the area of voids to the cross-sectional area of each region in the cross-section of the positive electrode mixture layer. The porosity ε3 of the positive electrode mixture layer can be measured according to the following procedure.
[0043] (1) Disassemble the battery to be evaluated and cut out the positive electrode to expose the cross-section of the positive electrode mixture layer. One method for exposing the cross-section is to cut out a part of the positive electrode and process it with an ion milling device (e.g., Hitachi High-Tech Corporation, IM4000PLUS) to expose the cross-section of the positive electrode mixture layer. (2) Use a SEM to take backscattered electron images of the exposed cross-section of the positive electrode mixture layer for each region of the positive electrode mixture layer. The magnification for taking the backscattered electron images is, for example, 800x. Perform the following processes (3) and (4) for each region of the positive electrode mixture layer and calculate the porosity of each. (3) Import the cross-sectional images obtained in (2) above into a computer and perform binarization processing using image analysis software (e.g., ImageJ, National Institutes of Health, USA) to obtain a binarized image in which the particle cross-sections in the cross-sectional image are made black and the voids present in the particle cross-sections are made white. (4) In the binarized image obtained in (3) above, the area of the voids is calculated by excluding the voids inside the particles (pores not connected to the particle surface) and pores with a width of 3 μm or less that are connected to the particle surface from the voids that have been converted to white. The void ratio ε3 is calculated based on the following formula: Void ratio ε3 (%) = Area of voids / Area of each region in the cross-section of the positive electrode mixture layer × 100 (5) The calculation of the void ratio according to (3) and (4) above is performed three times, and the arithmetic mean of the three calculated values is taken as the void ratio of the positive electrode mixture layer.
[0044] Various resin materials can be used as binders, such as fluororesins, polyolefin resins, polyamide resins, polyimide resins, acrylic resins, and vinyl resins. Examples of fluororesins include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF). The binder may be used alone or in combination of two or more types.
[0045] Examples of conductive agents include carbon black, conductive fibers, and carbon fluoride. Examples of carbon black include acetylene black and Ketjen black. Examples of conductive fibers include carbon fibers and metal fibers, with examples of carbon fibers including carbon nanotubes. Conductive agents may be used individually or in combination of two or more.
[0046] As the positive electrode current collector, a non-porous conductive substrate (e.g., metal foil) or a porous conductive substrate (e.g., mesh, net, and perforated sheet) can be used. Examples of materials constituting the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium. The thickness of the positive electrode current collector is not particularly limited, but is preferably 1 to 50 μm, and more preferably 5 to 20 μm.
[0047] (Negative electrode) As described above, the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on at least one surface of the negative electrode current collector. As described above, the negative electrode mixture layer comprises a first negative electrode mixture layer disposed on at least one surface of the negative electrode current collector and a second negative electrode mixture layer disposed on the surface of the first negative electrode mixture layer. When the porosity of the first negative electrode mixture layer is ε1 (%) and the porosity of the second negative electrode mixture layer is ε2 (%), ε1 and ε2 satisfy the following relationships as described above: ε1 < ε2, ε2 ≥ 10%, and 1.3 ≤ ε2 / ε2 ≤ 5.0.
[0048] The porosity ε1 of the first negative electrode mixture layer and the porosity ε2 of the second negative electrode mixture layer can be determined in the same manner as for the positive electrode mixture layer. The first negative electrode mixture layer and the second negative electrode mixture layer can be distinguished by differences in the size of the voids in the cross-section of the negative electrode mixture layer. For example, the formation region of the first negative electrode mixture layer and the formation region of the second negative electrode mixture layer can be determined in the cross-section of the negative electrode mixture layer by differences in the size of the voids. Then, for the formation region of the first negative electrode mixture layer and the formation region of the second negative electrode mixture layer, the porosity ε1 and porosity ε2 can be determined, respectively, in the same manner as for the positive electrode mixture layer.
[0049] The negative electrode mixture layer can be formed, for example, by applying a first negative electrode mixture slurry, in which the first negative electrode mixture corresponding to the first negative electrode mixture layer is dispersed in a dispersion medium, to at least one surface of the negative electrode current collector to form a first coating film, and then applying a second negative electrode mixture slurry, in which the second negative electrode mixture corresponding to the second negative electrode mixture layer is dispersed in a dispersion medium, to the surface of the first coating film to form a second coating film, and then drying the laminate of the first and second coating films. The laminate of the first and second coating films after drying may be rolled as needed. The negative electrode mixture layer may be formed on only one surface of the negative electrode current collector or on both surfaces. As the dispersion medium, for example, water or NMP can be used.
[0050] The porosity ε2 of the second negative electrode mixture layer preferably satisfies the relationship 10% ≤ ε2 ≤ 30%, more preferably 12% ≤ ε2 ≤ 25%, and even more preferably 12% ≤ ε2 ≤ 20%. By having the porosity ε2 within the above range, the porosity of the second negative electrode mixture layer can be made sufficiently large, thereby improving the liquid circulation of the non-aqueous electrolyte in the second negative electrode mixture layer.
[0051] The porosity ε1 of the first negative electrode mixture layer preferably satisfies the relationship ε1 ≤ 6.0%. The porosity ε1 may also satisfy the relationship ε1 ≤ 5.0%. By satisfying the above relationship for the porosity ε1, sufficient volume can be secured in the first negative electrode mixture layer. Furthermore, the porosity ε1 may also satisfy the relationship 1.0% < ε1, 2.0% ≤ ε1, or 3.0% ≤ ε1. By satisfying the above relationship for the porosity ε1, a certain degree of liquid circulation can be secured in the first negative electrode mixture layer. And, by satisfying the above relationship for both the porosity ε1 of the first negative electrode mixture layer and the porosity ε2 of the second negative electrode mixture layer, sufficient volume can be secured in the negative electrode mixture layer while sufficiently improving the liquid circulation of the non-aqueous electrolyte in the negative electrode mixture layer.
[0052] The porosity ε1 of the first anode composite layer and the porosity ε2 of the second anode composite layer can be adjusted by using multiple anode active materials with different hardnesses and then adjusting the mass ratio of these multiple anode active materials. For example, among anode active materials, graphite with low intraparticle porosity (e.g., graphite with intraparticle porosity of 2.5% to 10%) and silicon-containing materials have relatively high hardness. Therefore, even if the anode composite layer (a laminate of the first anode composite layer and the second anode composite layer) is formed by rolling, the voids formed between these active materials are not easily crushed. Thus, by using such hard anode active materials, the values of porosity ε1 and ε2 can be increased. On the other hand, among anode active materials, graphite with high intraparticle porosity (e.g., graphite with intraparticle porosity of 12% to 20%) has relatively low hardness, so when the anode composite layer is formed by rolling, the voids formed between these active materials are easily crushed. Therefore, by using such a low-hardness negative electrode active material, the porosity values ε1 and ε2 can be reduced. Furthermore, by incorporating a high-hardness negative electrode active material and a low-hardness negative electrode active material in an appropriate mass ratio into the first negative electrode mixture layer and the second negative electrode mixture layer, the porosity values ε1 and ε2 can be adjusted to desired values. The negative electrode active material will be described later.
[0053] High-hardness negative electrode active materials such as graphite and silicon-containing materials with low intraparticle porosity may have a Vickers hardness of 300 Hv or higher, or 500 Hv or higher. The Vickers hardness may be 1000 Hv or lower, or 700 Hv or lower. The Vickers hardness is measured by embedding the negative electrode active material in a thermosetting resin and exposing the cross-section of the negative electrode active material with 400-grit abrasive paper. Furthermore, the cross-section is polished to a mirror finish using 2000-grit abrasive paper. Then, the Vickers hardness of the polished cross-section is measured using a Vickers hardness tester with a load of 1 kg and a holding time of 15 seconds.
[0054] The negative electrode mixture layers (first negative electrode mixture layer and second negative electrode mixture layer) contain a material having the property of intercalating and releasing lithium ions. The negative electrode mixture layers contain, for example, a negative electrode active material.
[0055] The negative electrode active material includes, for example, a carbon material that intercalates and releases lithium ions. Examples of such carbon materials include graphite, easily graphitizable carbon (soft carbon), and difficult-to-graphitize carbon (hard carbon). The graphite may be natural graphite or artificial graphite. Among the above carbon materials, graphite is preferred because it has excellent stability during charging and discharging and low irreversible capacity.
[0056] The negative electrode active material may contain an alloying material. An alloying material is a material containing a metal that can form an alloy with lithium. Examples of alloying materials include silicon, tin, silicon alloys, tin alloys, and silicon-containing materials. As the silicon-containing material, a composite material comprising a lithium ion conducting phase and silicon particles dispersed in this lithium ion conducting phase may be used. Examples of the lithium ion conducting phase include silicate phases such as lithium silicate phase, silicon oxide phases in which 95% by mass or more is silicon dioxide, and carbon phases.
[0057] A combination of alloying material and carbon material may be used as the negative electrode active material. In this case, the mass ratio of the carbon material to the total mass of the alloying material and carbon material may be, for example, 80% by mass or more, or 90% by mass or more.
[0058] The negative electrode active material may contain lithium titanium oxide. Lithium titanium oxide is Li ４ Ti ５ O １２ Li ７ Ti ５ O １２ , and LiTi ２ O ４ The negative electrode active material may include at least one selected from the group consisting of the following. In addition to lithium titanium oxide, the negative electrode active material may include TiO ２ It may further contain [the following]. Furthermore, lithium titanium oxide may be used in combination with a carbon material.
[0059] The negative electrode mixture layer may contain, in addition to the negative electrode active material, a binder, a conductive agent, and a thickener. The binder and conductive agent may be those exemplified in the positive electrode. Alternatively, a rubber material such as styrene-butadiene copolymer rubber (SBR) may be used as the binder. Examples of thickeners include carboxymethylcellulose (CMC) and its modified forms (such as Na salts).
[0060] The negative electrode current collector is not particularly limited. For example, a conductive sheet can be used as the negative electrode current collector. Examples of conductive sheets include metal foils such as copper foil, copper alloy foil, and stainless steel foil with copper vapor deposition. Alternatively, a resin sheet with copper vapor deposition (for example, a polyethylene terephthalate sheet) can also be used as the conductive sheet.
[0061] In the non-aqueous electrolyte secondary battery according to the embodiment of this disclosure, when the liquid absorption time of the positive electrode mixture layer measured using propionate is A1 (seconds) and the liquid absorption time of the negative electrode mixture layer measured using propionate is A2 (seconds), it is preferable that the liquid absorption times A1 and A2 satisfy the relationship A1 / A2 ≤ 4.5. A1 / A2 may also satisfy the relationship A1 / A2 ≤ 4.0 or A1 / A2 ≤ 3.5. Furthermore, A1 / A2 may also satisfy the relationship 0.8 ≤ A1 / A2 or 1.0 ≤ A1 / A2. By A1 / A2 satisfying the above relationship, it is possible to suppress the excessive improvement in liquid absorption of the negative electrode mixture layer compared to the positive electrode mixture layer. This makes it possible to suitably balance the liquid flow properties of the non-aqueous electrolyte in the negative electrode mixture layer and the liquid flow properties of the non-aqueous electrolyte in the positive electrode mixture layer. Therefore, excessive uneven distribution of non-aqueous electrolytes in the negative electrode mixture layer can be suppressed. The absorption time A1 of the positive electrode mixture layer can be measured according to the following procedure. The absorption time A2 of the negative electrode mixture layer can also be measured in the same manner as the absorption time A1 of the positive electrode mixture layer.
[0062] (1) Disassemble the non-aqueous electrolyte secondary battery to be evaluated (non-aqueous electrolyte secondary battery after discharge), cut out the positive electrode to an appropriate size (for example, 4 cm x 6 cm in plan dimensions) to obtain a positive electrode test specimen, and then wash this positive electrode test specimen with dimethyl carbonate (DMC). Specifically, after placing the positive electrode test specimen in a container, pour in enough DMC to fully immerse the positive electrode test specimen into the container and leave it for about 30 minutes to perform the above washing. (2) After removing the washed positive electrode test specimen from the container, dry the washed positive electrode test specimen. Drying may be carried out by heating at a predetermined temperature, or by leaving the washed positive electrode test specimen at room temperature (25 ± 2°C) for about 10 minutes. (3) After drying, 1 μL of propionate (PC) is dropped onto the surface of the positive electrode mixture layer on the positive electrode test specimen using a pipette. The time (in seconds) from immediately after dropping until the PC is visually removed from the surface of the positive electrode mixture layer is measured. Note that the boiling point of PC is 242°C, so the amount removed due to volatilization can be ignored. (4) The time (in seconds) until the PC is visually removed is measured at six locations on the surface of the positive electrode mixture layer on one positive electrode test specimen, and the arithmetic mean of the six measured values is taken. This gives the absorption time A1 (in seconds) of the positive electrode mixture layer.
[0063] (Non-aqueous electrolyte) A non-aqueous electrolyte contains a non-aqueous solvent and an electrolyte. A non-aqueous electrolyte is also called a non-aqueous electrolyte solution. A non-aqueous electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the non-aqueous electrolyte may be, for example, 0.5 mol / L or more and 2.0 mol / L or less. By setting the concentration of the lithium salt within the above range, a non-aqueous electrolyte with excellent ionic conductivity and appropriate viscosity can be obtained. However, the concentration of the lithium salt is not limited to the above range.
[0064] Examples of non-aqueous solvents include cyclic carbonate esters, linear carbonate esters, cyclic carboxylic acid esters, and linear carboxylic acid esters. Examples of cyclic carbonate esters include propylene carbonate (PC) and ethylene carbonate (EC). Cyclic carbonate esters may also include cyclic carbonate esters having carbon-carbon unsaturated bonds. Examples of such cyclic carbonate esters include fluorinated cyclic carbonate esters (e.g., fluoroethylene carbonate (FEC)), vinylene carbonate (VC), and vinylethylene carbonate. Examples of linear carbonate esters include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of linear carboxylic acid esters include methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and propyl propionate. The non-aqueous solvent may be used alone or in combination of two or more types.
[0065] Various known lithium salts can be used as the lithium salt. For example, LiClO ４ LiBF ４ LiPF ６ LiAlCl ４ LiSbF ６ , LiSCN, LiCF ３ SO ３ LiCF ３ CO ２ LiAsF ６ LiB １０ Cl １０It is preferable to use lithium lower aliphatic carboxylates, LiCl, LiBr, LiI, borates, and imide salts. Examples of borates include lithium bis(1,2-benzenediolate(2-)-O,O')borate, lithium bis(2,3-naphthalenediolate(2-)-O,O')borate, lithium bis(2,2'-biphenyldiolate(2-)-O,O')borate, and lithium bis(5-fluoro-2-oleate-1-benzenesulfonic acid-O,O')borate. Examples of imide salts include lithium bis(fluorosulfonyl)imide (LiN(FSO) ２ ) ２ ), bistrifluoromethanesulfonate lithium (LiN(CF ３ SO ２ ) ２ ), trifluoromethanesulfonic acid nonafluorobutanesulfonic acid lithium (LiN(CF ３ SO ２ ) (C ４ F ９ SO ２ )), and bispentafluoroethanesulfonate lithium (LIN(C) ２ F ５ SO ２ ) ２ Examples include the following. Lithium salts may be used individually or in combination of two or more types.
[0066] The non-aqueous electrolyte secondary battery according to the embodiments of this disclosure may include a solid electrolyte in addition to the non-aqueous electrolyte. That is, the non-aqueous electrolyte secondary battery according to the embodiments of this disclosure may have a hybrid structure in which the non-aqueous electrolyte and the solid electrolyte are used in combination. Examples of solid electrolytes include halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, and organic polymer solid electrolytes. In this specification, "halide solid electrolyte" means a solid electrolyte containing a halogen element as the main component of the anion, "sulfide solid electrolyte" means a solid electrolyte containing sulfur as the main component of the anion, and "oxide solid electrolyte" means a solid electrolyte containing oxygen as the main component of the anion. Furthermore, the main component of the anion means the anion with the largest mole number among all the anions constituting the solid electrolyte.
[0067] (Separator) The non-aqueous electrolyte secondary battery according to the embodiments of this disclosure preferably includes a separator. In the non-aqueous electrolyte secondary battery, the separator is arranged to be interposed between the positive electrode and the negative electrode.
[0068] As the separator, a porous sheet having ion permeability and insulating properties can be used. As the porous sheet, for example, a thin film, woven fabric, and nonwoven fabric having microporous properties can be used. The material constituting the separator is not particularly limited, and for example, polymer materials can be used. Examples of polymer materials include polyolefin resins, polyamide resins, and cellulose. Examples of polyolefin resins include polyethylene resins, polypropylene resins, and copolymers of ethylene and propylene. The separator may contain additives (such as inorganic fillers) as needed. The thickness of the separator is not particularly limited and may be 10 μm or more, or 15 μm or more. The thickness of the separator may be 30 μm or less, or 20 μm or less.
[0069] In the following, an example of a non-aqueous electrolyte secondary battery according to one embodiment of this disclosure will be described with reference to the drawings.
[0070] Figure 1 is a schematic longitudinal cross-sectional view showing a non-aqueous electrolyte secondary battery according to one embodiment of the present disclosure. As shown in Figure 1, the non-aqueous electrolyte secondary battery 10 is a cylindrical battery comprising a cylindrical case, a wound electrode group 14, and a non-aqueous electrolyte (not shown). The electrode group 14 is housed in the battery case and is in contact with the non-aqueous electrolyte.
[0071] The battery case includes a case body 15, which is a bottomed cylindrical metal container, and a sealing body 16 that seals the opening of the case body 15. A gasket 27 is placed between the case body 15 and the sealing body 16. The placement of the gasket 27 ensures that the battery case is airtight. Inside the case body 15, insulating plates 17 and 18 are placed at both ends of the electrode group 14 in the direction of the winding axis, respectively.
[0072] The case body 15 has, for example, a stepped portion 21. The stepped portion 21 is formed, for example, by partially pressing the side wall of the case body 15 from the outside. The stepped portion 21 may be formed in an annular shape on the side wall of the case body 15 along the circumferential direction of a virtual circle defined by the case body 15. In this case, the sealing body 16 is supported, for example, by the opening side surface of the stepped portion 21.
[0073] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26. These members are stacked in this order in the sealing body 16. The sealing body 16 is installed in the opening of the case body 15 such that the cap 26 is located on the outside of the case body 15 and the filter 22 is located on the inside of the case body 15.
[0074] The lower valve body 23 and the upper valve body 25 are connected at their respective centers. An insulating member 24 is positioned between the peripheral edge of the lower valve body 23 and the peripheral edge of the upper valve body 25. The filter 22 and the lower valve body 23 are connected at their respective peripheral edges. The upper valve body 25 and the cap 26 are connected at their respective peripheral edges. Each of the above-mentioned components constituting the sealing body 16 has, for example, a disc shape or a ring shape. Each of the above-mentioned components is electrically connected to one another, except for the insulating member 24.
[0075] The lower valve body 23 has a ventilation hole. Therefore, if the internal pressure of the battery case rises due to abnormal heat generation or other reasons, the upper valve body 25 bulges towards the cap 26 and separates from the lower valve body 23. This disconnects the electrical connection between the lower valve body 23 and the upper valve body 25. If the internal pressure rises further, the upper valve body 25 ruptures, and gas is released through the opening formed in the cap 26.
[0076] The electrode group 14 includes a positive electrode 11, a negative electrode 12, and a separator 13. The positive electrode 11, the negative electrode 12, and the separator 13 are all strip-shaped. The width direction of the strip-shaped positive electrode 11 and negative electrode 12 is, for example, parallel to the winding axis direction of the electrode group 14. The separator 13 is positioned between the positive electrode 11 and the negative electrode 12. The positive electrode 11 and the negative electrode 12 are wound in a spiral shape with the separator 13 interposed between them.
[0077] In the non-aqueous electrolyte secondary battery 10, when observing a cross-section of the electrode group 14 in a direction perpendicular to the winding axis, the positive electrode 11 and the negative electrode 12 are alternately stacked in the radial direction of a virtual circle defined by the case body 15, with a separator 13 interposed between them.
[0078] The positive electrode 11 is electrically connected to a cap 26, which functions as a positive electrode terminal, via a positive electrode lead 19. The positive electrode lead 19 extends from the positive electrode 11 to the filter 22 through a through hole formed in the insulating plate 17. One end of the positive electrode lead 19 is connected, for example, near the center of the positive electrode 11 in the longitudinal direction, and the other end of the positive electrode lead 19 is welded, for example, to the electrode group 14 side of the filter 22. The positive electrode 11 used is a positive electrode for a secondary battery according to the embodiment of this disclosure.
[0079] The negative electrode 12 is electrically connected to the case body 15, which functions as a negative electrode terminal, via a negative electrode lead 20. One end of the negative electrode lead 20 is connected, for example, to the end of the negative electrode 12 in the longitudinal direction, and the other end of the negative electrode lead 20 is welded, for example, to the inner bottom surface of the case body 15.
[0080] In the above, as an example of a non-aqueous electrolyte secondary battery, the configuration shown in Figure 1, specifically, a configuration in which an electrode group formed by winding a positive electrode and a negative electrode with a separator in between, and an electrolyte are housed in an outer casing (battery case), is described. However, the configuration of a non-aqueous electrolyte secondary battery is not limited to the configuration shown in Figure 1. That is, a non-aqueous electrolyte secondary battery is not limited to an example of a cylindrical configuration, but may also be configured as a rectangular, coin-shaped, button-shaped, or laminate-shaped configuration. Furthermore, the electrode group of a non-aqueous electrolyte secondary battery is not limited to an example of a wound type configuration. For example, the electrode group of a non-aqueous electrolyte secondary battery may be formed as a laminated type by stacking the positive electrode and negative electrode with a separator in between.
[0081] (Note) The following technologies are disclosed in accordance with the above description. (Technology 1) A positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode mixture layer comprises a first negative electrode mixture layer disposed on at least one surface of the negative electrode current collector and a second negative electrode mixture layer disposed on the surface of the first negative electrode mixture layer, where the porosity of the first negative electrode mixture layer is ε1 (%) and the porosity of the second negative electrode mixture layer is ε2 (%), such that ε1 and ε2 satisfy the relationships ε1 < ε2, ε2 ≥ 10%, and 1.3 ≤ ε2 / ε1 ≤ 5.0, wherein the positive electrode comprises a first positive electrode active material and a second positive electrode active material having a volume-based median diameter D50 smaller than that of the first positive electrode active material. A non-aqueous electrolyte secondary battery in which, when the volume-based median diameter D50 of the first positive electrode active material is D1 (μm) and the volume-based median diameter D50 of the second positive electrode active material is D2 (μm), D1 and D2 satisfy the relationship 2.0 ≤ D1 / D2 ≤ 15.0, and when the content of the first positive electrode active material is W1 (mg) and the content of the second positive electrode active material is W2 (mg), W1 and W2 satisfy the relationship W1:W2 = 1:9 to 9:1. (Technology 2) A non-aqueous electrolyte secondary battery in which, when the ε1 satisfies the relationship ε1 ≤ 6.0%, the non-aqueous electrolyte secondary battery in which, when the ε1 satisfies the relationship 8 μm ≤ D1 ≤ 17 μm. (Technology 4) A non-aqueous electrolyte secondary battery according to any one of Techniques 1 to 3, wherein D2 satisfies the relationship 2 μm ≤ D2 ≤ 6 μm. (Technology 5) A non-aqueous electrolyte secondary battery according to any one of Techniques 1 to 4, wherein the positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, the first positive electrode active material and the second positive electrode active material are contained in the positive electrode mixture layer, and when the porosity of the positive electrode mixture layer is ε3 (%), ε3 satisfies the relationship ε3 ≤ 12%.(Technical 6) A non-aqueous electrolyte secondary battery according to Technical 5, wherein when the liquid absorption time of the positive electrode mixture layer measured using propion carbonate is A1 (seconds) and the liquid absorption time of the positive electrode mixture layer measured using propion carbonate is A2 (seconds), A1 and A2 satisfy the relationship A1 / A2 ≤ 4.5.
[0082] The present disclosure will be described below in detail based on examples and comparative examples, but the present disclosure is not limited to the following examples.
[0083] (Example 1) (1) Preparation of the negative electrode A first negative electrode slurry was prepared by mixing the negative electrode active material, polyacrylic acid (PAA), styrene-butadiene copolymer rubber (SBR), carboxymethylcellulose (CMC), carbon nanotubes (CNT), and an appropriate amount of water. The negative electrode active material contained a silicon-containing material, graphite A with high particle porosity, and graphite B with low particle porosity. Silicon-carbon composite particles (GSS manufactured by Giga Solar Materials) were used as the silicon-containing material. The mass ratio of these materials was silicon-containing material:graphite A:graphite B = 10:50:40. In the first negative electrode mixture slurry, the mass ratio of the negative electrode active material, PAA, SBR, CMC, and CNT was set to negative electrode active material:PAA:SBR:CMC:CNT = 100:1:1:0.1.
[0084] The second anode mixture slurry was prepared in the same manner as the first anode mixture slurry, except that the mass ratio of silicon-containing material, graphite A, and graphite B in the anode active material was changed to silicon-containing material:graphite A:graphite B = 10:90:0.
[0085] A first negative electrode mixture slurry was applied to one side of a strip-shaped electrolytic copper foil (negative electrode current collector) to form a first coating film. Then, a second negative electrode mixture slurry was applied on the first coating film to form a second coating film, thereby obtaining a laminate in which the electrolytic copper foil, the first coating film, and the second coating film were laminated in this order. Next, this laminate was dried, and the dried laminate was rolled. In this way, a negative electrode was obtained in which the first negative electrode mixture layer and the second negative electrode mixture layer were laminated on a strip-shaped electrolytic copper foil in this order. Note that the negative electrode mixture layer was formed on only one side of the electrolytic copper foil. For the negative electrode according to Example 1, the porosity ε1 (%) of the first negative electrode mixture layer and the porosity ε2 (%) of the second negative electrode mixture layer were measured according to the method described in the above embodiment section. The measurement results are shown in Table 1 below. The ratio of the porosity ε2 to the porosity ε1 (ε2 / ε1) is also shown in Table 1 below.
[0086] (2) Preparation of the positive electrode As the first positive electrode active material, LiNi ０．６ Co ０．２ Mn ０．２ A lithium transition metal composite oxide having the following composition and a volume-based median diameter D50 of 15 μm (D1 = 15 μm) was prepared. ０．６ Co ０．２ Mn ０．２ Lithium transition metal composite oxides having the above composition are also called NCMs. Furthermore, the NCMs had a layered rock salt-type crystalline structure. In addition, an NCM with a volume-based median diameter D50 of 4 μm (D2 = 4 μm) was prepared as the second cathode active material.
[0087] Next, the positive electrode active materials (first positive electrode active material and second positive electrode active material), acetylene black (AB), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of positive electrode active material:AB:PVDF = 90:7:3 to prepare a positive electrode mixture. In the positive electrode mixture, when the content of the first positive electrode active material was W1 (mg) and the content of the second positive electrode active material was W2 (mg), W1 and W2 were blended to satisfy the relationship W1:W2 = 8:2. N-methyl-2-pyrrolidone (NMP) was added to this positive electrode mixture and then stirred to prepare a positive electrode mixture slurry.
[0088] A positive electrode mixture slurry was applied to the surface of a strip of aluminum foil to form a coating film, which was then dried. Next, the coating film was rolled to form a positive electrode mixture layer on the surface of the strip of aluminum foil. The positive electrode mixture layer was formed on only one side of the aluminum foil. Table 1 below shows the D1 of the first positive electrode active material, the D2 of the second positive electrode active material, the ratio of D1 to D2 (D1 / D2), the ratio of the content W1 of the first positive electrode active material to the content W2 of the second positive electrode active material (W1:W2), and the porosity ε3 (%) for the positive electrode mixture layer according to Example 1. D1, D2, and the porosity ε3 were measured according to the method described in the above embodiment section.
[0089] (3) Preparation of non-aqueous electrolytes A mixed solvent obtained by mixing fluoroethylene carbonate (FEC) and dimethyl carbonate (DMC) in a volume ratio of FEC:DMC = 2:8 is mixed with LiPF ６ The LiPF was dissolved to obtain a non-aqueous electrolyte. ６ The concentration was set to 1 mol / L.
[0090] (4) Fabrication of a non-aqueous electrolyte secondary battery An aluminum positive electrode lead was attached to the positive electrode obtained as described above. Similarly, an aluminum negative electrode lead was attached to the negative electrode obtained as described above. Next, in a low dew point atmosphere of -50°C, the positive electrode and negative electrode were wound in a spiral shape with a separator in between to fabricate a wound electrode group. A polyethylene thin film was used as the separator.
[0091] The above-mentioned wound electrode group was housed inside a bag-shaped outer casing made of a laminate sheet having an Al layer, and the above-mentioned non-aqueous electrolyte was injected, after which the outer casing was sealed. When housing the wound electrode group in the outer casing, a portion of the positive electrode lead and the negative electrode lead were exposed to the outside of the outer casing. In this way, a non-aqueous electrolyte secondary battery according to Example 1 was manufactured.
[0092] (Example 2) A non-aqueous electrolyte secondary battery according to Example 2 was prepared in the same manner as in Example 1, except that the content W1 (mg) of the first positive electrode active material and the content W2 (mg) of the second positive electrode active material in the positive electrode mixture were blended to satisfy the relationship W1:W2 = 5:5. Table 1 below shows the D1, D2, the ratio of D1 to D2 (D1 / D2), the ratio of W1 to W2 (W1:W2), and the porosity ε3 (%) for the positive electrode mixture layer according to Example 2. Also, Table 1 below shows the porosity ε1 (%), porosity ε2 (%), and the ratio of porosity ε2 to porosity ε1 (ε2 / ε1) for the negative electrode mixture layer according to Example 2.
[0093] (Example 3) A non-aqueous electrolyte secondary battery according to Example 3 was prepared in the same manner as in Example 1, except that the content W1 (mg) of the first positive electrode active material and the content W2 (mg) of the second positive electrode active material in the positive electrode mixture were blended to satisfy the relationship W1:W2 = 2:8. Table 1 below shows the D1, D2, the ratio of D1 to D2 (D1 / D2), the ratio of W1 to W2 (W1:W2), and the porosity ε3 (%) for the positive electrode mixture layer according to Example 3. Also, Table 1 below shows the porosity ε1 (%), the porosity ε2 (%), and the ratio of the porosity ε2 to the porosity ε1 (ε2 / ε1) for the negative electrode mixture layer according to Example 3.
[0094] (Example 4) A non-aqueous electrolyte secondary battery according to Example 4 was manufactured in the same manner as in Example 1, except that the linear pressure during coating rolling was reduced when manufacturing the positive electrode mixture layer. Table 1 below shows the D1, D2, the ratio of D1 to D2 (D1 / D2), the ratio of W1 to W2 (W1:W2), and the porosity ε3 (%) for the positive electrode mixture layer according to Example 4. Table 1 below also shows the porosity ε1 (%), the porosity ε2 (%), and the ratio of the porosity ε2 to the porosity ε1 (ε2 / ε1) for the negative electrode mixture layer according to Example 4.
[0095] (Comparative Example 1) A non-aqueous electrolyte secondary battery according to Comparative Example 1 was prepared in the same manner as in Example 1, except that only the first positive electrode active material was used as the positive electrode active material. That is, in the non-aqueous electrolyte secondary battery according to Comparative Example 1, the positive electrode mixture layer did not contain the second positive electrode active material. Table 1 below shows the D1, D2, the ratio of D1 to D2 (D1 / D2), the ratio of W1 to W2 (W1:W2), and the porosity ε3 (%) for the positive electrode mixture layer according to Comparative Example 1. Also, Table 1 below shows the porosity ε1 (%), porosity ε2 (%), and the ratio of porosity ε2 to porosity ε1 (ε2 / ε1) for the negative electrode mixture layer according to Comparative Example 1. In Comparative Example 1, since only one type of positive electrode active material was used, D1 / D2 was set to 1.0. The same applies to Comparative Examples 2 to 4 below.
[0096] (Comparative Example 2) A non-aqueous electrolyte secondary battery according to Comparative Example 2 was prepared in the same manner as in Example 1, except that only the second positive electrode active material was used as the positive electrode active material. That is, in the non-aqueous electrolyte secondary battery according to Comparative Example 2, the positive electrode mixture layer did not contain the first positive electrode active material. Table 1 below shows the D1, D2, the ratio of D1 to D2 (D1 / D2), the ratio of W1 to W2 (W1:W2), and the porosity ε3 (%) for the positive electrode mixture layer according to Comparative Example 2. Also, Table 1 below shows the porosity ε1 (%), porosity ε2 (%), and the ratio of porosity ε2 to porosity ε1 (ε2 / ε1) for the negative electrode mixture layer according to Comparative Example 2.
[0097] (Comparative Example 3) A non-aqueous electrolyte secondary battery according to Comparative Example 3 was prepared in the same manner as in Example 1, except that only NMC (first positive electrode active material) with D1 of 11 μm was used as the positive electrode active material. That is, in the non-aqueous electrolyte secondary battery according to Comparative Example 3, the positive electrode mixture layer did not contain the second positive electrode active material. Table 1 below shows the D1, D2, the ratio of D1 to D2 (D1 / D2), the ratio of W1 to W2 (W1:W2), and the porosity ε3 (%) for the positive electrode mixture layer according to Comparative Example 3. Also, Table 1 below shows the porosity ε1 (%), porosity ε2 (%), and the ratio of porosity ε2 to porosity ε1 (ε2 / ε1) for the negative electrode mixture layer according to Comparative Example 3.
[0098] (Comparative Example 4) A non-aqueous electrolyte secondary battery according to Comparative Example 4 was prepared in the same manner as in Comparative Example 3, except that the mass ratio of silicon-containing material, graphite A, and graphite B in the first negative electrode mixture slurry was changed to silicon-containing material:graphite A:graphite B = 10:72:18, and the mass ratio of silicon-containing material, graphite A, and graphite B in the second negative electrode mixture slurry was changed to silicon-containing material:graphite A:graphite B = 10:68:22. Table 1 below shows the D1, D2, the ratio of D1 to D2 (D1 / D2), the ratio of W1 to W2 (W1:W2), and the porosity ε3 (%) for the positive electrode mixture layer according to Comparative Example 3. Furthermore, Table 1 below shows the porosity ε1 (%), porosity ε2 (%), and the ratio of porosity ε2 to porosity ε1 (ε2 / ε1) for the negative electrode mixture layer related to Comparative Example 4.
[0099]
[0100] [Evaluation] <Initial Capacity> Charge and discharge tests were conducted using the non-aqueous electrolyte secondary batteries according to each example (Examples 1-4 and Comparative Examples 1-4) as follows. Constant current charging was performed with a current of 0.5C until the voltage reached 4.2V, and then constant voltage charging was performed with a voltage of 4.2V until the current reached 0.05C. Next, constant current discharge was performed with a current of 0.2C until the voltage reached 2.5V. The rest period between charging and discharging was 60 minutes. The charging and discharging were performed in an environment of 25°C. The discharge capacity during the first discharge performed in this manner was determined as the initial capacity. The results are shown in Table 2 below. Table 2 below shows the ratio of the initial capacities of the non-aqueous electrolyte secondary batteries according to each example, with the initial capacity of the non-aqueous electrolyte secondary battery according to Example 1 set to 100.
[0101] <Internal Resistance> For each example of the non-aqueous electrolyte secondary battery, a cycle test was conducted with the above charge-discharge cycle counted as one cycle. After the 50th charge-discharge cycle, charging was performed to bring the SOC to 50%, and the difference in voltage drop ΔV at the start of constant current discharge at a current I of 0.3C was measured. The value of ΔV / I was then calculated, and this calculated value was taken as the internal resistance. The results are shown in Table 2 below. Note that Table 2 below shows the ratio of the internal resistance of each example of the non-aqueous electrolyte secondary battery when the internal resistance of the non-aqueous electrolyte secondary battery of Example 1 is set to 100.
[0102] <Room Temperature Cycle Maintenance Rate> For each example of the non-aqueous electrolyte secondary battery, the above charge-discharge cycle was repeated 300 times, with one cycle being the charge-discharge cycle described above. The discharge capacity C1 at the 300th cycle was then determined. The ratio of the discharge capacity C1 to the initial capacity (C0) X1 = C1 / C0 was then calculated, and this calculated value was defined as the room temperature cycle maintenance rate. The results are shown in Table 2 below. In Table 2 below, the room temperature cycle maintenance rate X1 of each example of the non-aqueous electrolyte secondary battery is shown as a ratio, with the room temperature cycle maintenance rate X1 of the non-aqueous electrolyte secondary battery of Example 1 set to 100.
[0103] Table 2 below also shows the absorption time A1 of the positive electrode mixture layer measured using propylene carbonate, the absorption time A2 of the negative electrode mixture layer measured using propylene carbonate, and the ratio of A1 to A2 (A1 / A2). In Table 2 below, the absorption time A1 of the non-aqueous electrolyte secondary battery according to Example 1 is set to 100, and the ratio of the absorption time A1 of each example of the non-aqueous electrolyte secondary battery is shown. The same applies to the absorption time A2. The absorption times A1 and A2 were measured by the method described in the Embodiments section above.
[0104]
[0105] Table 2 shows that the non-aqueous electrolyte secondary batteries of Examples 1 and 4 exhibited particularly good results in both internal resistance evaluation and room-temperature cycle retention rate evaluation. Furthermore, while the non-aqueous electrolyte secondary batteries of Examples 2 and 3 showed slightly inferior internal resistance evaluation results compared to those of Examples 1 and 4, their room-temperature cycle retention rate evaluation results were good. In other words, the non-aqueous electrolyte secondary batteries of each example achieved both suppression of internal resistance increase and suppression of cycle performance deterioration. In contrast, the non-aqueous electrolyte secondary batteries of Comparative Examples 1, 3, and 4 showed good internal resistance evaluation results, but their room-temperature cycle retention rate decreased significantly, and the non-aqueous electrolyte secondary battery of Comparative Example 2 showed good room-temperature cycle retention rate evaluation results, but its internal resistance increased significantly.
[0106] Furthermore, it can be seen that the non-aqueous electrolyte secondary batteries according to each embodiment also show good results in initial capacity evaluation. Among the non-aqueous electrolyte secondary batteries according to each embodiment, it can be seen that the non-aqueous electrolyte secondary battery according to Example 1 shows particularly good evaluations in all aspects: internal resistance, room temperature cycle maintenance rate, and initial capacity.
[0107] Although the present invention has been described in relation to preferred embodiments at present, such disclosure should not be interpreted restrictively. Various modifications and alterations will undoubtedly become apparent to those skilled in the art in the field to which the invention pertains by reading the above disclosure. Accordingly, the appended claims should be interpreted as encompassing all modifications and alterations without departing from the true spirit and scope of the invention.
[0108] The non-aqueous electrolyte secondary battery described herein can be used in applications where it is required to suppress an increase in internal resistance and a decrease in cycle characteristics.
[0109] 10: Non-aqueous electrolyte secondary battery, 11: Positive electrode, 12: Negative electrode, 13: Separator, 14: Electrode group
Claims
1. A positive electrode comprises a positive electrode, a negative electrode, and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode current collector and a negative electrode mixture layer disposed on at least one surface of the negative electrode current collector, the negative electrode mixture layer comprises a first negative electrode mixture layer disposed on at least one surface of the negative electrode current collector and a second negative electrode mixture layer disposed on the surface of the first negative electrode mixture layer, when the porosity of the first negative electrode mixture layer is ε1 (%) and the porosity of the second negative electrode mixture layer is ε2 (%), then ε1 and ε2 satisfy the relationships ε1 < ε2, ε2 ≥ 10%, and 1.3 ≤ ε2 / ε1 ≤ 5.0, and the positive electrode comprises a first positive electrode active material and a second positive electrode active material having a volume-based median diameter D50 smaller than that of the first positive electrode active material. A non-aqueous electrolyte secondary battery, wherein when the volume-based median diameter D50 of the first positive electrode active material is D1 (μm) and the volume-based median diameter D50 of the second positive electrode active material is D2 (μm), D1 and D2 satisfy the relationship 2.0 ≤ D1 / D2 ≤ 15.0, and when the content of the first positive electrode active material is W1 (mg) and the content of the second positive electrode active material is W2 (mg), W1 and W2 satisfy the relationship W1:W2 = 1:9 to 9:
1.
2. The non-aqueous electrolyte secondary battery according to claim 1, wherein ε1 satisfies the relationship ε1 ≤ 6.0%.
3. The non-aqueous electrolyte secondary battery according to claim 1, wherein D1 satisfies the relationship 8 μm ≤ D1 ≤ 17 μm.
4. The non-aqueous electrolyte secondary battery according to claim 3, wherein D2 satisfies the relationship 2 μm ≤ D2 ≤ 6 μm.
5. The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer disposed on at least one surface of the positive electrode current collector, the first positive electrode active material and the second positive electrode active material are contained in the positive electrode mixture layer, and when the porosity of the positive electrode mixture layer is ε3 (%), ε3 satisfies the relationship ε3 ≤ 12%, the non-aqueous electrolyte secondary battery according to claim 1.
6. The non-aqueous electrolyte secondary battery according to claim 5, wherein when the liquid absorption time of the positive electrode mixture layer measured using propionate is A1 (seconds) and the liquid absorption time of the negative electrode mixture layer measured using propionate is A2 (seconds), A1 and A2 satisfy the relationship A1 / A2 ≤ 4.5.