Non-aqueous secondary battery

Abstract

To provide a non-aqueous secondary battery that suppresses an increase in resistance in a positive electrode that uses an active material with a layered rock salt structure.SOLUTION: A non-aqueous secondary battery includes an electrode assembly in which a positive electrode sheet and a negative electrode sheet are laminated with a separator interposed therebetween, and a non-aqueous electrolyte containing a lithium salt. The positive electrode composite provided on the surface of the positive electrode substrate constituting the positive electrode sheet includes a positive electrode active material having a layered rock salt structure and a positive electrode binder containing fluorine. The specific surface area of the positive electrode active material is 2.5 m2 / g or more and 2.9 m2 / g or less. The ratio of the positive electrode binder contained in the positive electrode composite is 0.95 wt.% or more and 1.05 wt.% or less. The migration index of the positive electrode binder in the thickness direction of the positive electrode composite is 12 or more and 22 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] The positive electrode material of the nonaqueous secondary battery described in Patent Document 1 contains an active material with a layered rock salt structure. The active material with a layered rock salt structure has the characteristic of easily increasing the capacity of the nonaqueous secondary battery. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2020-80220 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when an active material with a layered rock salt structure is used as a positive electrode material, the crystalline structure of the active material surface changes from a layered rock salt structure to a rock salt structure during initial charging and aging, which can prevent Li ions from being inserted or extracted, resulting in an increase in resistance. Therefore, there is a need to suppress the increase in resistance in positive electrodes that use active materials with a layered rock salt structure. [Means for solving the problem]

[0005] The non-aqueous secondary battery that solves the above-mentioned problems is a non-aqueous secondary battery that includes an electrode assembly in which a positive electrode sheet and a negative electrode sheet are laminated with a separator interposed therebetween, and a non-aqueous electrolyte solution containing a lithium salt, wherein the positive electrode mixture provided on the surface of the positive electrode substrate that constitutes the positive electrode sheet includes a positive electrode active material having a layered rock salt structure and a positive electrode binder containing fluorine, and the specific surface area of ​​the positive electrode active material is 2.5 m 2 / g or more 2.9m 2 / g or less, the ratio of the positive electrode binder contained in the positive electrode mixture is 0.95 wt% or more and 1.05 wt% or less, and the migration index of the positive electrode binder in the thickness direction of the positive electrode mixture is 12 or more and 22 or less.

[0006] According to the above configuration, by causing the positive electrode binder to migrate into the positive electrode mixture, a coating is formed on the surface of the positive electrode active material by reaction between the positive electrode binder and the electrolyte, and this coating can suppress an increase in resistance due to a change in the crystalline structure on the surface of the positive electrode active material. Furthermore, by reducing the concentration of the positive electrode binder near the positive electrode substrate, the positive electrode binder is prevented from covering the positive electrode substrate, thereby suppressing an increase in resistance. Therefore, an increase in resistance in a positive electrode using an active material with a layered rock salt structure can be suppressed.

[0007] In the nonaqueous secondary battery, the positive electrode active material having a layered rock salt structure is preferably at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, and lithium nickel oxide.

[0008] In the nonaqueous secondary battery, the fluorine-containing positive electrode binder is preferably polyvinylidene fluoride. [Effects of the Invention]

[0009] According to the present invention, an increase in resistance in a positive electrode using an active material with a layered rock salt structure can be suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a perspective view showing a schematic configuration of a cell battery of an embodiment of a nonaqueous secondary battery. [Figure 2] FIG. 2 is a view showing a part of the electrode body of the embodiment in an expanded state. [Figure 3] FIG. 4 is a diagram showing changes in the positive electrode active material of the nonaqueous secondary battery according to the embodiment. [Figure 4] FIG. 4 is a diagram showing changes in the positive electrode active material of the nonaqueous secondary battery according to the embodiment. [Figure 5]3A and 3B are diagrams showing migration of a binder in a positive electrode of the nonaqueous secondary battery according to the embodiment. [Figure 6] FIG. 2 is a diagram showing a positive electrode of the nonaqueous secondary battery according to the embodiment. [Figure 7] FIG. 2 is a diagram showing a positive electrode of the nonaqueous secondary battery according to the embodiment. [Figure 8] FIG. 2 is a diagram showing a positive electrode of the nonaqueous secondary battery according to the embodiment. [Figure 9] 1 is a table showing examples and comparative examples of nonaqueous secondary batteries. DETAILED DESCRIPTION OF THE INVENTION

[0011] [Present embodiment] An embodiment of a nonaqueous secondary battery and a method for manufacturing a nonaqueous secondary battery will be described below with reference to Figures 1 to 4. A lithium ion secondary battery will be described as an example of a nonaqueous secondary battery.

[0012] [Lithium-ion secondary battery 10] As shown in Figure 1, a lithium-ion secondary battery 10 is a cell battery that is combined with a plurality of lithium-ion secondary batteries 10 and sealed in a resin or metal case to form a battery pack. The battery pack is used in hybrid vehicles and electric vehicles.

[0013] The lithium-ion secondary battery 10 includes a battery case 11 and a lid 12. The battery case 11 has a rectangular parallelepiped shape with an opening on the upper side. The lid 12 seals the opening of the battery case 11. The battery case 11 and the lid 12 are made of a metal such as aluminum or an aluminum alloy. The lithium-ion secondary battery 10 forms a sealed battery container by attaching the lid 12 to the battery case 11.

[0014] The lid 12 is provided with two external terminals, a positive electrode terminal 13A and a negative electrode terminal 13B. The positive electrode external terminal 13A and the negative electrode external terminal 13B are used for charging and discharging power. An electrode assembly 20 is housed inside the battery case 11. An inserting film (not shown) is inserted between the battery case 11 and the electrode assembly 20. The positive electrode side current collector 20A, which is the end of the electrode assembly 20 on the positive electrode side, is electrically connected to the positive electrode external terminal 13A via a positive electrode side current collector 14A. The negative electrode side current collector 20B, which is the end of the electrode assembly 20 on the negative electrode side, is electrically connected to the negative electrode external terminal 13B via a negative electrode side current collector 14B. A nonaqueous electrolyte is injected into the battery case 11 through an inlet (not shown). The shapes of the positive electrode external terminal 13A and the negative electrode external terminal 13B are not limited to those shown in FIG. 1 and may be any shape.

[0015] [Electrode body 20] 2, the electrode assembly 20 is a flat wound body obtained by winding a laminate in which a long positive electrode sheet 21 and a negative electrode sheet 24 are stacked with a separator 27 interposed therebetween. The positive electrode sheet 21, the negative electrode sheet 24, and the separator 27 are stacked so that their respective longitudinal directions coincide with the longitudinal direction D1. In the laminate before winding, the positive electrode sheet 21, the separator 27, the negative electrode sheet 24, and the separator 27 are stacked in this order.

[0016] [Positive electrode sheet 21] The positive electrode sheet 21 includes a positive electrode current collector 22 and a positive electrode composite layer 23. The positive electrode current collector 22 is a foil-like positive electrode base material formed in a long shape. The positive electrode composite layer 23 is provided on each of two opposing surfaces of the positive electrode current collector 22. The positive electrode current collector 22 includes, at one end in the width direction D2, a positive electrode-side uncoated portion 22A where the positive electrode composite layer 23 is not formed and the positive electrode current collector 22 is exposed.

[0017] A metal foil made of aluminum or an alloy containing aluminum as a main component is used for the positive electrode current collector 22. The positive electrode current collector 22 functions as a current collector for the positive electrode. In the wound state, the positive electrode-side uncoated portion 22A of the positive electrode current collector 22 has opposing surfaces that are pressed against each other to form the positive electrode-side current collecting portion 20A.

[0018] The positive electrode mixture layer 23 is a hardened product of a liquid positive electrode mixture paste. The positive electrode mixture paste includes a positive electrode active material, a positive electrode solvent, a positive electrode conductive material, and a positive electrode binder. The positive electrode mixture layer 23 is formed by drying the positive electrode mixture paste and evaporating the positive electrode solvent. Therefore, the positive electrode mixture layer 23 includes a positive electrode active material, a positive electrode conductive material, and a positive electrode binder.

[0019] The positive electrode active material is a lithium-containing composite oxide capable of absorbing and releasing lithium ions, which are charge carriers in the lithium-ion secondary battery 10. The lithium-containing composite oxide is an oxide containing lithium and metal elements other than lithium. In this embodiment, a positive electrode active material having a layered rock salt structure is used. For example, the positive electrode active material is at least one of lithium nickel cobalt manganese oxide (LiNiCoMnO2), lithium cobalt oxide (LiCoO2), and lithium nickel oxide (LiNiO2). The specific surface area of ​​the positive electrode active material is 2.5 m 2 / g or more 2.9m 2 / g or less.

[0020] As shown in FIG. 3, in the case of positive electrode active material 31 having a layered rock salt structure, the crystal structure of the surface of positive electrode active material 31 changes from a layered rock salt structure to a rock salt structure during initial charging and aging, making it impossible to insert and extract Li ions, and increasing the reaction resistance.

[0021] The positive electrode solvent is an NMP (N-methyl-2-pyrrolidone) solution, which is an example of an organic solvent. The positive electrode conductive material may be, for example, carbon black such as acetylene black or ketjen black, carbon fiber such as carbon nanotube or carbon nanofiber, or graphite. The positive electrode binder is an example of a resin component contained in the positive electrode mixture paste. The positive electrode binder is a positive electrode binder containing fluorine. For example, polyvinylidene fluoride (PVDF) is used as the positive electrode binder. The ratio of the positive electrode binder contained in the positive electrode mixture is 0.95 wt% or more and 1.05 wt% or less.

[0022] 4, a positive electrode active material 31 having a layered rock salt structure has a coating 33 formed on the surface of the positive electrode active material 31 by a reaction between a positive electrode binder 32 containing fluorine and the nonaqueous electrolyte. This coating 33 can suppress changes in the crystal structure of the positive electrode active material 31.

[0023] The distribution of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 can be evaluated by the migration index. In this embodiment, the migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is 12 or more and 22 or less. The migration index can be calculated by analyzing a cross section of the positive electrode mixture layer 23 using SEM-EDX (Scanning Electron Microscope-Energy Dispersive X-ray spectrometry). The measurement procedure is as follows. First, a sample for cross-sectional observation is cut out from the positive electrode mixture layer 23, and the cross section is cleaned using a cross-section polisher (CP) or the like. Next, the positive electrode binder 32 is modified with a predetermined element or compound. After modifying the positive electrode binder 32, the cross section is subjected to area analysis using SEM-EDX to map the positive electrode binder 32. At this time, as shown in FIG. 5, the cross section is divided into two equal parts in the thickness direction, with the positive electrode substrate side being the first region and the surface side of the positive electrode composite layer 23 being the second region. The migration index can be calculated by dividing the integrated value of the detected intensity of the positive electrode binder 32 in the second region by the integrated value of the detected intensity of the positive electrode binder 32 in the first region. If the positive electrode binder 32 is uniformly distributed, the migration index will be a value close to 1.0. In FIG. 5, the positive electrode binder 32 is denoted as F for convenience.

[0024] Here, the effects on positive electrode mixture layer 23 of the amount of positive electrode binder 32 contained in positive electrode mixture layer 23 and the migration of positive electrode binder 32 in the thickness direction of positive electrode mixture layer 23 will be described.

[0025] 6, when the amount of positive electrode binder 32 contained in the positive electrode mixture layer 23 is large, a coating 33 is also formed on the surface of the positive electrode active material 31 located on the positive electrode current collector 22 side, even if the migration of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is insufficient. On the other hand, when the amount of positive electrode binder 32 contained in the positive electrode mixture layer 23 is too large, the diffusibility of Li ions decreases and the resistance increases. Furthermore, the concentration of the positive electrode binder 32 on the surface of the positive electrode current collector 22 increases, and the positive electrode binder 32 covers the surface of the positive electrode current collector 22, reducing the electronic conductivity between the positive electrode current collector 22 and the positive electrode active material 31 and increasing the resistance.

[0026] 7, when the amount of positive electrode binder 32 contained in positive electrode mixture layer 23 decreases and the migration index of positive electrode binder 32 in the thickness direction of positive electrode mixture layer 23 is low, the crystal structure on the surface of positive electrode active material 31 changes from a layered rock salt structure to a rock salt structure, and resistance increases. In addition, positive electrode binder 32 covers the surface of positive electrode current collector 22, and electronic conductivity between positive electrode current collector 22 and positive electrode active material 31 decreases, and resistance increases.

[0027] As shown in FIG. 8 , it is desirable to reduce the amount of positive electrode binder 32 contained in positive electrode mixture layer 23 while allowing the positive electrode binder 32 to migrate in the thickness direction of positive electrode mixture layer 23, thereby reducing the concentration of positive electrode binder 32 on the positive electrode current collector 22 side. In this way, a coating 33 is formed on the surface of positive electrode active material 31 near the surface of positive electrode mixture layer 23, thereby suppressing changes in the crystal structure of positive electrode active material 31. Furthermore, because positive electrode binder 32 does not cover the surface of positive electrode current collector 22, the electronic conductivity between positive electrode current collector 22 and positive electrode active material 31 is not reduced. Therefore, an increase in resistance in a positive electrode using positive electrode active material 31 with a layered rock salt structure can be suppressed.

[0028] The positive electrode sheet 21 may have an insulating layer at the boundary between the positive electrode uncoated portion 22A and the positive electrode composite layer 23. The insulating layer contains an inorganic component having insulating properties and a resin component that functions as a binder. The inorganic component is at least one selected from the group consisting of powdered boehmite, titania, and alumina. The resin component is at least one selected from the group consisting of PVDF, PVA, and acrylic.

[0029] [Negative electrode sheet 24] The negative electrode sheet 24 includes a negative electrode current collector 25 and a negative electrode composite layer 26. The negative electrode current collector 25 is a foil-like negative electrode base material formed in a long strip. The negative electrode composite layer 26 is provided on each of two opposing surfaces of the negative electrode current collector 25. The negative electrode current collector 25 includes a negative electrode-side uncoated portion 25A at one end in the width direction D2, opposite the positive electrode-side uncoated portion 22A, where the negative electrode composite layer 26 is not formed and the negative electrode current collector 25 is exposed.

[0030] A metal foil made of copper or an alloy mainly containing copper is used for the negative electrode current collector 25. The negative electrode current collector 25 functions as a current collector for the negative electrode. When the negative electrode uncoated portion 25A is wound, opposing surfaces of the negative electrode uncoated portion 25A are pressed against each other to form the negative electrode current collector 20B.

[0031] The negative electrode mixture layer 26 is a hardened product of a liquid negative electrode mixture paste. The negative electrode mixture paste includes a negative electrode active material, a negative electrode solvent, a negative electrode thickener, and a negative electrode binder. The negative electrode mixture layer 26 is formed by drying the negative electrode mixture paste and evaporating the negative electrode solvent. Therefore, the negative electrode mixture layer 26 includes the negative electrode active material, and further includes a negative electrode thickener and a negative electrode binder as additives. The negative electrode mixture layer 26 may further include an additive such as a conductive material.

[0032] The negative electrode active material is a material capable of absorbing and releasing lithium ions. Examples of the negative electrode active material include carbon materials such as graphite, non-graphitizable carbon, graphitizable carbon, and carbon nanotubes. One example of the negative electrode solvent is water. One example of the negative electrode thickener is CMC (carboxymethyl cellulose), which contains a sodium salt. One example of the negative electrode binder is the same as the positive electrode binder. One example of the negative electrode binder is SAR (styrene-acrylic acid copolymer), which contains a sodium salt.

[0033] [Separator 27] The separator 27 prevents contact between the positive electrode sheet 21 and the negative electrode sheet 24, and also holds the nonaqueous electrolyte between the positive electrode sheet 21 and the negative electrode sheet 24. When the electrode assembly 20 is immersed in the nonaqueous electrolyte, the nonaqueous electrolyte permeates from the ends of the separator 27 in the width direction D2 toward the center.

[0034] The separator 27 is a nonwoven fabric made of polypropylene, etc. Examples of the separator 27 that can be used include porous polymer membranes such as porous polyethylene membranes, porous polyolefin membranes, and porous polyvinyl chloride membranes, and ion-conductive polymer electrolyte membranes.

[0035] [Nonaqueous electrolyte] The nonaqueous electrolyte is a composition containing a supporting salt in a nonaqueous solvent. The nonaqueous solvent may be one or more materials selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, etc. The supporting salt may be one or more lithium compounds (lithium salts) selected from the group consisting of LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, LiBOB (lithium bis(oxalatoborate)), etc.

[0036] In this embodiment, ethylene carbonate is used as the nonaqueous solvent. LiBOB is added to the nonaqueous electrolyte as a lithium salt of the film-forming agent. For example, LiBOB is added to the nonaqueous electrolyte so that the concentration of LiBOB in the nonaqueous electrolyte is 0.001 to 0.1 mol / L.

[0037] Next, the effects of this embodiment will be described. (1) By causing the positive electrode binder 32 to migrate into the positive electrode mixture, a coating 33 is formed on the surface of the positive electrode active material 31 as a result of a reaction between the positive electrode binder 32 and the electrolyte, and this coating 33 can suppress an increase in resistance due to a change in the crystal structure on the surface of the positive electrode active material 31. Furthermore, by reducing the concentration of the positive electrode binder 32 near the positive electrode current collector 22, the positive electrode binder 32 is prevented from covering the positive electrode current collector 22, thereby suppressing an increase in resistance. Therefore, an increase in resistance in a positive electrode using an active material with a layered rock salt structure can be suppressed.

[0038] (Other embodiments) The above embodiment can be modified as follows: The above embodiment and the following modifications can be combined with each other within the scope of technical compatibility.

[0039] In the above embodiment, the electrode body 20 is a wound body obtained by winding a laminate in which a positive electrode sheet 21 and a negative electrode sheet 24 are stacked with a separator 27 interposed therebetween. However, the electrode body may also be a laminate in which a plurality of positive electrode sheets 21 and a plurality of negative electrode sheets 24 are alternately stacked with a separator 27 interposed therebetween.

[0040] The lithium-ion secondary battery 10 may be installed in an automatic transport vehicle, a special-purpose vehicle for loading and unloading, an electric vehicle, a hybrid vehicle, a computer, or other electronic device, or may be part of other systems. For example, it may be installed in a moving object such as a ship or an aircraft, or it may be a power supply system that supplies power from a power plant via a substation to a building or home where a secondary battery is installed.

[0041] [Example] Next, examples and comparative examples of the lithium ion secondary battery 10 will be described with reference to Fig. 9. Note that these examples and comparative examples are not limited to non-aqueous secondary batteries.

[0042] 9, lithium-ion secondary batteries 10 were prepared in examples and comparative examples in which the combination of the migration index of positive electrode binder 32 in the thickness direction of positive electrode mixture layer 23 and the specific surface area of ​​positive electrode active material 31 was changed. The amount of positive electrode binder 32 contained in positive electrode mixture layer 23 in each example and comparative example was 1.0 [wt%]. Then, for each example and comparative example, the battery resistance [%] was evaluated using Example 2 as the reference.

[0043] [Comparative Example 1] The migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is set to 2.0, and the specific surface area of ​​the active material is set to 2.4 [m 2 / g].

[0044] Comparative Example 2 The migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is set to 2.0, and the specific surface area of ​​the active material is set to 2.8 [m 2 / g].

[0045] Comparative Example 3 The migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is set to 13.3, and the specific surface area of ​​the active material is set to 2.3 [m 2 / g].

[0046] [Example 1] The migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is set to 21.2, and the specific surface area of ​​the active material is set to 2.5 [m 2 / g].

[0047] [Example 2] The migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is set to 12.2, and the specific surface area of ​​the active material is set to 2.8 [m 2 / g].

[0048] [Example 3] The migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is set to 20.0, and the specific surface area of ​​the active material is set to 2.9 [m 2 / g].

[0049] [Example 4] The migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is set to 12.2, and the specific surface area of ​​the active material is set to 2.5 [m 2 / g].

[0050] [evaluation] The battery resistance [%] was evaluated for each of the above examples and comparative examples. In Comparative Examples 1 and 2, in which the migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is significantly low, the battery resistance is greater than 110%. 2 / g], the battery resistance is greater than 110[%]. 2 / g or more is desirable.

[0051] The migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is greater than 12, and the specific surface area of ​​the positive electrode active material is 2.5 [m 2 / g] or more, the battery resistance is close to 100[%]. The maximum migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 in Examples 1 to 4 is 21.2, and the maximum specific surface area of ​​the positive electrode active material in Examples 1 to 4 is 2.9[m 2 / g]. Therefore, the migration index of the positive electrode binder 32 in the thickness direction of the positive electrode mixture layer 23 is 12 or more and 22 or less, and the specific surface area of ​​the positive electrode active material is 2.5 [m 2 / g] or more 2.9[m 2 / g] or less. In Examples 1 to 4, excellent results were obtained. [Explanation of symbols]

[0052] 10...Lithium-ion secondary battery 11...Battery case 12...lid body 13A...Positive external terminal 13B…Negative external terminal 14A...Positive electrode side current collecting member 14B...Negative electrode side current collecting member 15...Extrapolation film 20...Electrode body 20A...Positive electrode current collector 20B...Negative electrode side current collector 21...Positive electrode sheet 22...Positive electrode current collector 22A: Uncoated area on the positive electrode side 23...Positive electrode mixture layer 24...Negative electrode sheet 25...Negative electrode current collector 25A...Negative electrode uncoated area 26…Negative electrode composite material layer 27...Separator 31...Cathode active material 32...Positive electrode binder 33...Coating

Claims

1. A non-aqueous secondary battery comprising an electrode assembly in which a positive electrode sheet and a negative electrode sheet are laminated with a separator interposed therebetween, and a non-aqueous electrolyte solution containing a lithium salt, The positive electrode mixture provided on the surface of the positive electrode substrate constituting the positive electrode sheet includes a positive electrode active material having a layered rock salt structure and a positive electrode binder containing fluorine, The specific surface area of ​​the positive electrode active material is 2.5 m ２ / g or more 2.9m ２ / g or less, A ratio of the positive electrode binder contained in the positive electrode mixture is 0.95 wt % or more and 1.05 wt % or less, The migration index of the positive electrode binder in the thickness direction of the positive electrode mixture is 12 or more and 22 or less. Non-aqueous secondary battery.

2. The positive electrode active material having a layered rock salt structure is at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, and lithium nickel oxide. The nonaqueous secondary battery according to claim 1 .

3. The positive electrode binder containing fluorine is polyvinylidene fluoride. The nonaqueous secondary battery according to claim 1 or 2.

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