Non-aqueous electrolyte secondary battery and manufacturing method

Abstract

To provide a non-aqueous electrolyte secondary battery that can suppress the rapid decrease in capacity caused by charge-discharge cycles. [Solution] A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, further comprising a positive electrode current collector electrically connected to the positive electrode, the positive electrode current collector having layers of positive electrode mixture on its front and back sides, the positive electrode mixture being composed of a positive electrode active material and a binder, the positive electrode active material being composed of spinel-type lithium manganese oxide in a proportion of 50 parts by weight or more of the total amount of active material, the binder containing a predetermined amount of polytetrafluoroethylene relative to the total amount of active material, and the positive electrode current collector further having holes penetrating through its front and back sides, the non-aqueous electrolyte secondary battery.

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the same, and relates to the configuration of a current collector, an electrode mixture, and the like.

Background Art

[0002] Non-aqueous electrolyte secondary batteries typified by lithium-ion secondary batteries are used as the main power source for mobile devices such as portable communication devices and portable electronic devices because they are high-energy density and high-voltage power sources. In recent years, they have also come to be used as power sources for automobiles and large machine tools driven by direct current. Therefore, there is a demand for non-aqueous electrolyte secondary batteries capable of rapid charging and large current discharge.

[0003] As such a positive electrode active material for lithium-ion batteries, lithium cobalt oxide and composite oxides of nickel, cobalt, and manganese have generally been used. However, cobalt used in the positive electrode active material is a rare metal, which contributes to an increase in battery price. Therefore, development is being promoted with the policy of reducing the amount of cobalt used in the positive electrode active material. A material system mainly composed of inexpensive manganese based on materials has attracted attention as a positive electrode active material.

[0004] On the other hand, a positive electrode for a lithium-ion battery is usually manufactured by a wet method in which a highly fluid electrode mixture slurry having a solid content ratio of 60 to 40 parts by weight containing an active material, a binder, etc. is applied to the surface of a current collector which is a metal foil. In this case, a drying process for evaporating and removing the solvent contained in the coating film is required, which has been a factor in increasing battery costs. In recent years, a dry method has been proposed for manufacturing an electrode using a powdery electrode mixture having a solid content ratio of 80 parts by weight or more or without using a solvent. For example, Patent Document 1 describes a method for manufacturing an electrode in which an electrode mixture powder is supplied to the surface on which the core material is to be deposited, and the deposited layer is pressed in the thickness direction while heating. Also, a method has been proposed in which an electrode mixture is rolled and formed into a sheet shape, and this electrode mixture sheet is laminated on a metal thin film or a core material.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] U.S. Patent Application Publication No. 2007 / 0042274 Specification [Overview of the project] [Problems that the invention aims to solve]

[0006] In the dry method of electrode manufacturing, it is not possible to use a slurry in which polyvinylidene fluoride (PVdF), which is mainly used as a binder in lithium-ion batteries, is dissolved in N-methylpyrrolidone (NMP). Therefore, the adhesive properties obtained by dissolving the binder in a solvent cannot be utilized. Instead, fibrous PTFE is added to the mixture, and the mixture is tightly bonded to each other through a three-dimensional structure. However, because the core material and the mixture are bonded together by only a small force due to van der Waals forces, there is a problem that the electrode mixture sheet is prone to peeling from the current collector.

[0007] In particular, spinel-type lithium manganese oxide exhibits a larger change in lattice constant during charging and discharging compared to lithium cobalt oxide and lithium nickelate, which have a rock salt structure. When commonly used aluminum foil is used as the current collector, the expansion and contraction of the spinel-type lithium manganese oxide during the charge-discharge cycle easily causes delamination at the interface between the positive electrode mixture layer and the positive electrode current collector, resulting in a decrease in initial performance and rapid degradation due to increased resistance during the cycle.

[0008] To address these issues, for example, Patent Document 1 explores methods to stabilize the crystal structure and drastically improve the reduction in charge / discharge capacity by substituting some of the manganese elements in the crystal with other elements, but these methods are insufficient.

[0009] Therefore, the present invention aims to provide a non-aqueous electrolyte secondary battery and a manufacturing method that can suppress the rapid decrease in capacity due to charge-discharge cycles, in order to address the conventional problems described above. [Means for solving the problem]

[0010] As a result of various studies, the inventors have found that the above objective can be achieved by the present invention described below.

[0011] In other words, the non-aqueous electrolyte secondary battery according to the present invention is A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode comprises a positive electrode current collector electrically connected to the positive electrode, The positive electrode current collector has layers of positive electrode mixture on both its front and back surfaces. The aforementioned positive electrode mixture is composed of a positive electrode active material and a binder. The positive electrode active material consists of spinel-type lithium manganese oxide in a proportion of 50 parts by weight or more relative to the total amount of active material. The aforementioned binder contains a predetermined amount of polytetrafluoroethylene relative to the total amount of active material. The positive electrode current collector further has holes that penetrate through both its front and back sides. Furthermore, the holes that penetrate both the front and back surfaces of the positive electrode current collector may be formed in expanded metal, perforated metal, or protruding portions of metal foil.

[0012] In the non-aqueous electrolyte secondary battery according to the present invention, the positive electrode active material is composed of spinel-type lithium manganese oxide, and the binder is composed of polytetrafluoroethylene (PTFE). The positive electrode current collector further has holes that penetrate through its front and back surfaces. Here, even if the electrode (also called the electrode body) is manufactured by a dry method, for example, since a structure is used in which the positive electrode current collector has holes that penetrate through its front and back surfaces, the layers of the composite material present on the front and back surfaces of the current collector adhere firmly to each other through these through-holes provided on the front and back surfaces by the PTFE fibers. As a result, the peeling, decrease in initial performance, and rapid deterioration due to resistance increase during cycles described above can be reduced, and the rapid decrease in capacity due to charge-discharge cycles can be suppressed.

[0013] Preferably, the binder may further contain polytetrafluoroethylene in an amount of 1 to 10 parts by weight relative to the total amount of active material. More preferably, the binder may further contain polytetrafluoroethylene in a proportion of 3 to 5 parts by weight based on the total amount of the active material. Preferably, the positive electrode mixture is further composed of a conductive material. It is preferable that the layer of the positive electrode mixture contains the conductive material. The conductive material is included to enhance the conductivity between the active materials. Preferably, the positive electrode active material may further be composed of spinel-type lithium manganate in a proportion of 90 parts by weight or more based on the total amount of the active material.

[0014] The binder may further contain polytetrafluoroethylene in a proportion of 3 to 5 parts by weight based on the total amount of the active material.

[0015] The manufacturing method of any of the above non-aqueous electrolyte secondary batteries according to the present invention The positive electrode mixture is further composed of a conductive material. A mixing step of mixing the positive electrode active material, the binder, and the conductive material as the positive electrode mixture, and then adding water to obtain a predetermined solid content ratio; A sizing and drying step of performing the operations of first sizing, drying, and second sizing of the positive electrode mixture obtained in the mixing step in this order. In addition, the positive electrode mixture obtained in the sizing and drying step is passed between two rolls having different peripheral speed ratios and rolled to produce a positive electrode mixture sheet, The positive electrode mixture sheet and the core material may be pressed. As described above, a non-aqueous electrolyte secondary battery having the above-described various effects can be manufactured.

Effects of the Invention

[0016] The non-aqueous electrolyte secondary battery and the manufacturing method according to the present invention can suppress a rapid capacity decrease due to charge and discharge cycles.

Brief Description of the Drawings

[0017] [Figure 1]It is an enlarged front view showing an example of expanded metal employed in one embodiment of the present invention. [Figure 2] It is an enlarged front view showing an example of punching metal employed in the above embodiment. [Figure 3] It is an enlarged front view showing an example of a metal foil having holes penetrating the front and back surfaces in a positive electrode current collector employed in one embodiment of the present invention in a protruding portion.

Mode for Carrying Out the Invention

[0018] Hereinafter, one embodiment of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments.

[0019] The non-aqueous electrolyte secondary battery according to this embodiment and the non-aqueous electrolyte secondary battery manufactured by the manufacturing method according to this embodiment will be described by taking a lithium-ion secondary battery as an example.

[0020] The lithium-ion secondary battery, which is the non-aqueous electrolyte secondary battery of this embodiment, includes at least a positive electrode, a negative electrode, and a non-aqueous electrolyte. A lithium-ion secondary battery usually further includes a separator and the like. An electrode for a lithium-ion battery has a current collector that is a metal foil, and the surface of the current collector contains an active material, a binder, and the like. That is, an electrically positive electrode current collector is connected to the positive electrode, and an electrically negative electrode current collector is connected to the negative electrode. The positive electrode current collector of this embodiment has a layer of a positive electrode mixture on the front and back surfaces, and the positive electrode mixture is composed of a positive electrode active material and a binder.

[0021] The positive electrode active material of this embodiment is composed of spinel-type lithium manganese oxide, and the binder contains polytetrafluoroethylene. Preferably, the positive electrode active material is composed of spinel-type lithium manganese oxide in a proportion of 50 parts by weight or more relative to the total amount of active material. More preferably, the positive electrode active material is composed of spinel-type lithium manganese oxide in a proportion of 90 parts by weight or more relative to the total amount of active material. Examples of spinel-type lithium manganese oxide content will be described in detail later. Preferably, the binder contains polytetrafluoroethylene in a proportion of 1 part by weight or more and 10 parts by weight or less relative to the total amount of active material. More preferably, the binder contains polytetrafluoroethylene in a proportion of 3 parts by weight or more and 5 parts by weight or less relative to the total amount of active material. Examples of binder content will be described in detail later.

[0022] The positive electrode mixture in this embodiment may also contain a material called a conductive material or conductive additive. Preferably, the conductive material is present in an amount of 2.7 parts by weight or more and 3.3 parts by weight or less relative to the total amount of active material. Examples of conductive material content ratios will be described in detail later.

[0023] One of the features of this embodiment is that the positive electrode current collector further has a hole that penetrates through both sides. The hole 100 that penetrates through both sides in the positive electrode current collector is formed, for example, in the expanded metal illustrated in Figure 1 (through hole 101), in the perforated metal illustrated in Figure 2 (through hole 102), or in the protruding portion 200 of the metal foil illustrated in Figure 3 (through hole 103). It should be noted that the through hole 103 in Figure 3 can also be said to be formed in a recessed portion 200A (not shown) if the front and back sides are reversed.

[0024] In the manufacturing method for a lithium-ion secondary battery, which is a non-aqueous electrolyte secondary battery of this embodiment, preferably, when the positive electrode mixture is further composed of a conductive material, the manufacturing method includes a mixing step in which the positive electrode active material, the binder, and the conductive material are mixed as the positive electrode mixture, and then water is added to obtain a predetermined solid content ratio; and a sizing and drying step in which the positive electrode mixture obtained in the mixing step is subjected to first sizing, drying, and second sizing in that order. In this manufacturing method, the positive electrode mixture obtained in the sizing and drying step may be further rolled by passing it between two rolls with different peripheral speed ratios to produce a positive electrode mixture sheet, and the positive electrode mixture sheet and the core material may be pressed together. Examples of manufacturing methods will be described in detail later. [Examples]

[0025] <Example 1> [Fabrication of the positive electrode] Next, we will describe the comparative experiment. Specifically, a lithium-containing transition metal oxide represented by the chemical formula LiMn2O4 was used as the positive electrode active material. This active material, acetylene black as a conductive material, and polytetrafluoroethylene (PTFE) as a binder were mixed in a mass ratio of 93:3:4. The mixture was then adjusted with water to a solid content of 90%, and the mixture was placed in a mixer or granulator and mixed at room temperature, at a rotation speed of 14,000 rpm, for 2 minutes. The wet-granulated positive electrode material was sized using a sizing machine with a mesh of 6 to 12 mesh. The sized mixture was dried in a dryer to a predetermined moisture content, and then sized again using a sizing machine with a mesh of 24 mesh. This mixing process resulted in the PTFE particles becoming fibrous, and a positive electrode mixture was obtained in which the active material, fibrous PTFE, and acetylene black were uniformly dispersed.

[0026] The resulting cathode mixture was rolled between two rolls to produce a cathode mixture sheet. The peripheral speed ratio of the two rolls was set to 1:3, and the thickness of the cathode mixture sheet was adjusted to 80 μm by performing multiple stretching processes.

[0027] The resulting positive electrode mixture sheets are placed on both sides of the core material, and the laminate of the positive electrode mixture sheets and the core material has a mixture density of 3.5 g / cm³. 2The material was pressed in this manner. This pressing resulted in a positive electrode in which the positive electrode mixture sheet was firmly bonded to the surface of the core material. As the core material, an aluminum expanded metal with a thickness of 30 μm and an opening of 50% was used, as an example shown in Figure 1. The positive electrode of Example 1 was prepared in the same manner as in Example 1, except that the obtained positive electrode mixture had a solid content concentration of 90%.

[0028] [Fabrication of the negative electrode] First, flake-shaped artificial graphite was crushed and classified so that the average particle size was approximately 20 μm. Next, 100 parts by weight of flake-shaped artificial graphite was mixed with 1 part by weight of styrene / butadiene rubber (binding agent) and 100 parts by weight of an aqueous solution containing 1% by weight of carboxymethylcellulose to obtain a negative electrode mixture slurry.

[0029] Subsequently, this negative electrode mixture slurry was applied to both sides of an 8 μm thick copper foil (negative electrode current collector) and dried. Then, it was pressed to a thickness of 0.2 mm to obtain the negative electrode.

[0030] [Adjustment of non-aqueous electrolytes] A mixed solvent containing ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate in a volume ratio of 2:1:7 was to which 1 wt% vinylene carbonate was added. (1.4 mol / m³) 3 LiPF6 was dissolved at the specified concentration to prepare a non-aqueous electrolyte.

[0031] [Separator] A porous polyethylene film with a thickness of 16 μm and a porosity of 42% was used as the separator. Next, the positive electrode and negative electrode were laminated with the separator in between to create an electrode body. Then, the electrolyte and the electrode body were inserted into the laminate, and the laminate was sealed to create a battery with a capacity of 100 mAh.

[0032] <Example 2> The active material, acetylene black, and PTFE were mixed in a mass ratio of 93:3:4. The mixture was then placed in a mixer (Mechanofusion, manufactured by Hosokawa Micron) and mixed at room temperature, a rotation speed of 14,000 rpm, and for 2 minutes. This mixing process caused the PTFE particles to fibrousize, resulting in a cathode mixture in which the active material, fibrous PTFE, and acetylene black were uniformly dispersed. The resulting cathode mixture had a solid content concentration of 100%.

[0033] The obtained cathode mixture was rolled by passing it between two rolls to produce a cathode mixture sheet. The cathode of Example 2 was produced in the same manner as in Example 1, except that the peripheral speed ratio of the two rolls was 1:3 and the thickness of the cathode mixture sheet was adjusted to 80 μm by performing multiple stretching processes.

[0034] <Example 3> The battery of Example 3 was prepared in the same manner as in Example 1, except that the mixing ratio of the active material, acetylene black, and PTFE was mixed in a mass ratio of 95:3:2.

[0035] <Example 4> The battery of Example 4 was prepared in the same manner as in Example 1, except that the mixing ratio of the active material, acetylene black, and PTFE was mixed in a mass ratio of 88:3:9.

[0036] <Example 5> A battery for Comparative Example 5 was fabricated in the same manner as in Example 1, except that a 30 μm thick, 50% aperture-ratio aluminum perforated metal, as shown as an example in Figure 2, was used as the core material.

[0037] <Example 6> The battery of Example 6 was fabricated in the same manner as in Example 1, except that a 30 μm aluminum foil (three-dimensional processed foil) with 30 μm protruding openings on both sides of the core material was used as the core material, as shown as an example in Figure 3.

[0038] <Comparative Example 1> A battery of Comparative Example 1 was prepared in the same manner as in Example 1, except that the mixing ratio of the active material, acetylene black, and PTFE was mixed in a mass ratio of 96.5:3:0.5.

[0039] <Comparative Example 2> The battery of Example 4 was prepared in the same manner as in Example 1, except that the mixing ratio of the active material, acetylene black, and PTFE was mixed in a mass ratio of 82:3:15.

[0040] <Comparative Example 3> A battery for Comparative Example 3 was prepared in the same manner as in Example 1, except that a 30 μm aluminum (unprocessed) foil was used as the core material.

[0041] <Comparative Example 4> A battery for Comparative Example 4 was prepared in the same manner as in Example 1, except that a 30 μm aluminum (unprocessed) foil was used as the core material, and the active material, acetylene black, and PTFE were mixed in a mass ratio of 82:3:15.

[0042] <Comparative Example 5> A lithium-containing transition metal oxide represented by the chemical formula LiMnO2 was used as the positive electrode active material. This active material, acetylene black, and an N-methylpyrrolidone solution of PVdF were mixed in a mass ratio of 93:3:2 for their respective solid contents. The solid content of the resulting mixture slurry was 50 parts by weight.

[0043] The resulting mixture slurry was coated onto 30 μm (unprocessed) aluminum foil, dried, and then the resulting positive electrode mixture sheet was processed to have a mixture density of 3.5 g / cm³. 3 The device was pressed in this manner. This pressing process resulted in a positive electrode in which the positive electrode mixture sheet was firmly bonded to the surface of the core material.

[0044] <<Evaluation Tests and Results>> Table 1 lists the prototype batteries. In addition, the weight ratios of the composite materials in Examples 1-6 and Comparative Examples 1-4 are listed in the order of positive electrode active material: acetylene black: PTFE. On the other hand, for Comparative Example 5, the weight ratios are listed in the order of positive electrode active material: acetylene black: PVdF.

[0045] [Table 1]

[0046] [Drying time for cathode mixture slurry] The drying time for the mixture containing the granulated positive electrode active material, acetylene black, PTFE, and water described in Example 1, and the mixture slurry containing the positive electrode active material, acetylene black, PVdF, and NMP described in Comparative Example 5 were compared. The drying time was defined as the point at which the change rate at 100°C became 0.5% or less, using a heat-drying type moisture meter.

[0047] [Charge / Discharge Test] The fabricated non-aqueous electrolyte secondary battery was evaluated for its 0.2C capacity and load characteristics at 1CA under the following test conditions at room temperature. I. 0.2C discharge capacity: charge CCCV / 0.2C / 4.2V, discharge CCCV / 0.2C / 2.5Vcut, temperature 25℃ II. 1C discharge capacity: charge CCCV / 0.2C / 4.2V, discharge CCCV / 1.0C / 2.5Vcut, temperature 25℃ Table 2 shows the results of the study.

[0048] [Table 2]

[0049] <Comparison of Examples 1, 3, and 4 and Comparative Examples 1 and 2> Compared to the battery in Comparative Example 1, the batteries in Examples 1, 3, and 4 have a PTFE content of 1 part by weight or more, and therefore maintain excellent discharge capacity because the adhesion between the active materials is firmly maintained by the PTFE fibers.

[0050] In Comparative Example 1, the amount of PTFE added was 1 part by weight or less, and sufficient adhesion between the active materials due to the PTFE fibers could not be ensured, making it impossible to form a film and fabricate electrodes. However, since the aforementioned film formation is difficult when the amount of PTFE is 2 parts by weight or less, it is preferable for the amount of PTFE to be 3 parts by weight or more from the perspective of productivity. On the other hand, in Comparative Example 2, the amount of PTFE was 15 parts by weight, which is an excessive amount and is thought to have inhibited the conductivity between the active materials, leading to a significant decrease in rate characteristics.

[0051] <Comparison of Examples 1 and 2> Compared to the battery in Example 1, the battery in Example 2 did not contain water in the granulated mixture, but the same performance was obtained regardless of the presence or absence of water. Furthermore, including 10% water during granulation offers significant environmental benefits in terms of process, as it eliminates dust scattering and other issues.

[0052] <Comparison of Examples 1, 5, and 6, and Comparative Examples 3 and 4> Compared to the battery in Example 1, the battery in Example 5 used perforated metal with an aperture ratio of 50% instead of expanded metal. Compared to expanded metal, perforated metal has structurally inferior uniformity in the distance between each active material particle and the core material, resulting in a slight decrease in performance. The battery in Example 6 used three-dimensional processed foil. Compared to expanded metal, three-dimensional processed foil was superior in current collection of the active material on the electrode surface, and an improvement trend in performance was observed compared to expanded metal.

[0053] Comparative Example 3 used a core material without openings, resulting in a significant decrease in performance due to the inability to ensure proper adhesion between the core material and the composite material. Although Comparative Example 4 increased the amount of PTFE compared to Comparative Example 3, no effect of the increase was observed.

[0054] <Comparison between Example 1 and Comparative Example 5> The drying time for the mixture containing granulated positive electrode active material, acetylene black, PTFE, and water described in Example 1 was 5 minutes. On the other hand, the drying time for the mixture slurry containing positive electrode active material, acetylene black, PVdF, and NMP described in Comparative Example 5 was 20 minutes, indicating that approximately four times the drying time is required.

[0055] As shown in the results above, the lithium-ion secondary battery and manufacturing method according to this embodiment can be said to suppress the rapid decrease in capacity due to charge-discharge cycles.

[0056] The present invention is not limited to the embodiments described above, and various additions, modifications, or deletions are possible without departing from the spirit of the invention. Therefore, such additions, modifications, or deletions are also included within the scope of the present invention. [Explanation of symbols]

[0057] 100 Holes penetrating the front and back surfaces of the positive electrode current collector 101, 102, 103 Through-holes (holes that penetrate through the front and back of the positive electrode current collector) 200 Convex part (projection part)

Claims

1. A non-aqueous electrolyte secondary battery comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte, The positive electrode comprises a positive electrode current collector electrically connected to the positive electrode, The positive electrode current collector has layers of positive electrode mixture on both its front and back surfaces. The aforementioned positive electrode mixture is composed of a positive electrode active material and a binder. The positive electrode active material is composed of spinel-type lithium manganese oxide in a proportion of 50 parts by weight or more relative to the total amount of active material. The aforementioned binder contains a predetermined amount of polytetrafluoroethylene relative to the total amount of active material. The positive electrode current collector further has a hole that penetrates through both the front and back sides. Nonaqueous electrolyte secondary battery.

2. A non-aqueous electrolyte secondary battery according to claim 1, The binder further contains polytetrafluoroethylene in a proportion of 1 part by weight or more and 10 parts by weight or less relative to the total amount of active material. Nonaqueous electrolyte secondary battery.

3. A non-aqueous electrolyte secondary battery according to claim 1, The binder further contains polytetrafluoroethylene in a proportion of 3 to 5 parts by weight relative to the total amount of active material. Nonaqueous electrolyte secondary battery.

4. A non-aqueous electrolyte secondary battery according to claim 1, The aforementioned positive electrode mixture is further composed of a conductive material, The positive electrode mixture layer contains the conductive material. Nonaqueous electrolyte secondary battery.

5. A non-aqueous electrolyte secondary battery according to claim 1, The hole that penetrates the front and back surfaces of the positive electrode current collector is formed in an expanded metal, perforated metal, or a protruding portion of a metal foil. Nonaqueous electrolyte secondary battery.

6. A non-aqueous electrolyte secondary battery according to claim 1, The positive electrode active material is further composed of spinel-type lithium manganese oxide in a proportion of 90 parts by weight or more relative to the total amount of active material. Nonaqueous electrolyte secondary battery.

7. A method for manufacturing a non-aqueous electrolyte secondary battery according to any one of claims 1 to 6, The aforementioned positive electrode mixture is further composed of a conductive material, The positive electrode mixture comprises a mixing step of mixing the positive electrode active material, the binder, and the conductive material, and then adding water to obtain a predetermined solid content ratio. The process includes a grain sizing and drying step in which the positive electrode mixture obtained in the mixing step is subjected to first grain sizing, drying, and second grain sizing in that order, Manufacturing method.

8. A method for manufacturing a non-aqueous electrolyte secondary battery according to claim 7, further, The positive electrode mixture obtained in the aforementioned grain sizing and drying process is rolled by passing it between two rolls with different peripheral speed ratios to produce a positive electrode mixture sheet. The positive electrode mixture sheet and the core material are pressed together. Manufacturing method.

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