Non-aqueous electrolyte secondary battery

Abstract

To provide means for improving cycle durability in a non-aqueous electrolyte secondary battery having a high basis weight electrode.SOLUTION: A non-aqueous electrolyte secondary battery has a power generation element including a positive electrode in which a positive electrode active material layer containing a positive electrode active material is formed on the surface of a current collector, a negative electrode in which a negative electrode active material layer containing a negative electrode active material is formed on the surface of a current collector, and a separator containing an electrolytic solution. An active material carry amount per unit area of the positive electrode is 30 mg / cm2 or more, and the concentration of electrolyte salt in the electrolytic solution is 2.0 to 4.0 mol / L.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, the widespread use of various electric vehicles is expected to help solve environmental and energy problems. Secondary batteries are being developed as on-board power sources for driving motors and other applications, which are key to the widespread use of these electric vehicles. Non-aqueous electrolyte secondary batteries, which are expected to offer high energy density and high output, have been attracting attention as secondary batteries.

[0003] Here, Patent Document 1 discloses a technology aimed at maintaining the high-rate characteristics and low-temperature characteristics of a nonaqueous electrolyte secondary battery even when a low-porosity electrode is used due to an increase in energy density. Specifically, the technology described in Patent Document 1 aims to achieve a balance between the energy density, high-rate characteristics, and low-temperature characteristics of a nonaqueous electrolyte secondary battery by using an electrolyte solution in which the concentration of electrolyte salt exceeds the concentration that gives the conductivity peak in a battery using a low-porosity positive electrode. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-173821 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the inventors have conducted research and found that even in a battery using the technology described in Patent Document 1, if the amount of active material carried per unit area of ​​the positive electrode is above a certain level, the cycle durability may be significantly reduced. Thus, there is still room for improvement in the technology described in Patent Document 1.

[0006] Therefore, an object of the present invention is to provide a means for improving the cycle durability of a battery having a high-area-weight electrode. [Means for solving the problem]

[0007] The present inventors conducted extensive research to solve the above-mentioned problems. In the process, they discovered that even when the amount of active material loaded per unit area of ​​the positive electrode is high, the concentration gradient of lithium ions in the electrolyte solution across the thickness of the electrode can be suppressed by controlling the electrolyte salt concentration within a specific range. This suppresses the reaction distribution of the active material during charge and discharge, suppresses deterioration of the electrode material, thereby suppressing capacity loss and maintaining a high energy density even after cycle testing. Based on this finding, they discovered that by controlling both the amount of active material loaded per unit area of ​​the positive electrode and the electrolyte salt concentration within a specific range, it is possible to suppress a decrease in cycle durability even in batteries with high-area electrodes, leading to the completion of the present invention.

[0008] That is, according to one aspect of the present invention, there is provided a nonaqueous electrolyte secondary battery having a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material formed on the surface of a current collector, a negative electrode having a negative electrode active material layer containing a negative electrode active material formed on the surface of a current collector, and a separator containing an electrolytic solution. Here, in this nonaqueous electrolyte secondary battery, the amount of active material carried per unit area of ​​the positive electrode is 30 mg / cm. 2 The electrolytic solution is characterized in that the concentration of the electrolyte salt in the electrolytic solution is 2.0 to 4.0 mol / L. [Effects of the Invention]

[0009] According to the present invention, it is possible to improve the cycle durability of a battery having a high-area-weight electrode. [Brief explanation of the drawings]

[0010] [Figure 1]1 is a cross-sectional view schematically illustrating the overall structure of a flat (laminated) non-bipolar (internal parallel connection type) secondary battery (laminated secondary battery) according to one embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view schematically illustrating a bipolar secondary battery according to another embodiment of the present invention. [Figure 3] FIG. 1 is a graph showing the cycle durability of the batteries prepared in the examples. DETAILED DESCRIPTION OF THE INVENTION

[0011] One aspect of the present invention is a nonaqueous electrolyte secondary battery having a power generating element including a positive electrode having a positive electrode active material layer containing a positive electrode active material formed on the surface of a current collector, a negative electrode having a negative electrode active material layer containing a negative electrode active material formed on the surface of a current collector, and a separator containing an electrolytic solution, wherein the amount of active material carried per unit area of ​​the positive electrode is 30 mg / cm. 2 The nonaqueous electrolyte secondary battery has the above-mentioned configuration, and the concentration of the electrolyte salt in the electrolytic solution is 2.0 to 4.0 mol / L. The nonaqueous electrolyte secondary battery according to this embodiment can improve cycle durability in a battery having a high-area-weight electrode.

[0012] The following describes the above-mentioned embodiment of the present invention with reference to the drawings. However, the technical scope of the present invention should be defined based on the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for the convenience of explanation and may differ from the actual ratios. In this specification, the range "X to Y" means "X or more and Y or less." Furthermore, unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25°C) and a relative humidity of 40 to 50% RH.

[0013] FIG. 1 is a cross-sectional view schematically illustrating the overall structure of a flat (stacked) non-bipolar (internal parallel connection) secondary battery (hereinafter simply referred to as a "stacked secondary battery") according to one embodiment of the present invention.

[0014] As shown in FIG. 1 , the stacked secondary battery 10a of this embodiment has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually proceed, is sealed inside a laminate film 29. Here, the power generating element 21 has a configuration in which a positive electrode in which a positive electrode active material layer 13 is disposed on both sides of a positive electrode current collector 11′, an electrolyte layer 17 made of a separator containing an electrolytic solution, and a negative electrode in which a negative electrode active material layer 15 is disposed on both sides of a negative electrode current collector 11″ are laminated. Specifically, the negative electrode, electrolyte layer, and positive electrode are laminated in this order, with one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 facing each other with the electrolyte layer 17 interposed therebetween.

[0015] As a result, adjacent positive electrodes, electrolyte layers, and negative electrodes constitute a single cell layer 19. Therefore, the stacked secondary battery 10a shown in FIG. 1 can be said to have a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel. Although the positive electrode active material layer 13 is disposed on only one side of each of the outermost positive electrode current collectors located on both outermost layers of the power generating element 21, active material layers may be disposed on both sides. That is, instead of using a current collector exclusively for the outermost layer with an active material layer disposed on only one side, a current collector with active material layers on both sides may be used as the outermost current collector. Furthermore, by reversing the arrangement of the positive and negative electrodes from FIG. 1, the outermost negative electrode current collectors may be located on both outermost layers of the power generating element 21, and negative electrode active material layers may be disposed on one or both sides of the outermost negative electrode current collectors.

[0016] A positive electrode current collector 25 and a negative electrode current collector 27 that are electrically connected to the electrodes (positive and negative electrodes) are attached to the positive electrode current collector 11′ and the negative electrode current collector 11″, respectively, and are configured to be sandwiched between the ends of the laminate film 29 and extended to the outside of the laminate film 29. The positive electrode current collector 25 and the negative electrode current collector 27 may be attached to the positive electrode current collector 11′ and the negative electrode current collector 11″ of the electrodes by ultrasonic welding, resistance welding, or the like, via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as necessary.

[0017] Fig. 2 is a cross-sectional view schematically illustrating a bipolar secondary battery according to another embodiment of the present invention. Bipolar secondary battery 10b shown in Fig. 2 has a structure in which a substantially rectangular power generating element 21, where charge and discharge reactions actually occur, is sealed inside a laminate film 29, which is a battery exterior. In this specification, a bipolar lithium-ion secondary battery may also be simply referred to as a "bipolar secondary battery," and an electrode for a bipolar lithium-ion secondary battery may also be simply referred to as a "bipolar electrode."

[0018] As shown in FIG. 2 , the power generating element 21 of the bipolar secondary battery 10b of this embodiment has a plurality of bipolar electrodes 23, each having a positive electrode active material layer 13 electrically coupled to one surface of a current collector 11 and a negative electrode active material layer 15 electrically coupled to the other surface of the current collector 11. The bipolar electrodes 23 are stacked with an electrolyte layer 17 interposed between them to form the power generating element 21. The bipolar electrodes 23 and the electrolyte layers 17 are alternately stacked such that the positive electrode active material layer 13 of one bipolar electrode 23 faces the negative electrode active material layer 15 of another bipolar electrode 23 adjacent to the first bipolar electrode 23 with the electrolyte layer 17 interposed therebetween. That is, the electrolyte layer 17 is sandwiched between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the first bipolar electrode 23.

[0019] Adjacent positive electrode active material layers 13, electrolyte layers 17, and negative electrode active material layers 15 constitute a single cell layer 19. Therefore, it can be said that the bipolar secondary battery 10b has a configuration in which the cell layers 19 are stacked. In addition, a seal portion (insulating layer) 31 is disposed on the outer periphery of the cell layer 19. This prevents a liquid junction due to leakage of the electrolyte solution from the electrolyte layer 17, and prevents contact between adjacent current collectors 11 within the battery and short circuits caused by slight misalignment of the edges of the cell layers 19 in the power-generating element 21. The positive electrode side outermost current collector 11a, which is the outermost layer of the power-generating element 21, has the positive electrode active material layer 13 formed on only one surface. The negative electrode side outermost current collector 11b, which is the outermost layer of the power-generating element 21, has the negative electrode active material layer 15 formed on only one surface.

[0020] 2, a positive electrode current collector (positive electrode tab) 25 is disposed adjacent to the outermost current collector 11a on the positive electrode side, and extends from the laminate film 29, which is the battery outer casing. On the other hand, a negative electrode current collector (negative electrode tab) 27 is disposed adjacent to the outermost current collector 11b on the negative electrode side, and similarly extends from the laminate film 29.

[0021] The number of times that cell layers 19 are stacked is adjusted according to the desired voltage. In addition, in bipolar secondary battery 10b, the number of times that cell layers 19 are stacked may be reduced as long as sufficient output can be ensured even if the thickness of the battery is made as thin as possible. In bipolar secondary battery 10b as well, it is preferable to use a structure in which power generating element 21 is vacuum-encapsulated in laminate film 29, which is the battery exterior, and positive electrode current collector 25 and negative electrode current collector 27 are exposed to the outside of laminate film 29, in order to protect against external impacts and environmental deterioration during use.

[0022] The main components of the nonaqueous electrolyte secondary battery according to this embodiment will be described below.

[0023] [Current collector] The current collector has a function of mediating the transfer of electrons from the electrode active material layer. There are no particular limitations on the material constituting the current collector. For example, metals and conductive resins can be used as the material constituting the current collector.

[0024] Specifically, examples of the metal include aluminum, nickel, iron, stainless steel, titanium, and copper. Other examples include clad materials of nickel and aluminum, and clad materials of copper and aluminum. Furthermore, foils in which aluminum is coated on a metal surface may also be used. Among these, aluminum, stainless steel, copper, and nickel are preferred from the viewpoints of electronic conductivity, battery operating potential, and adhesion of the negative electrode active material to the current collector by sputtering.

[0025] The latter conductive resin may be a resin in which a conductive filler is added to a non-conductive polymer material.

[0026] Examples of non-conductive polymeric materials include polyethylene (PE; high-density polyethylene (HDPE), low-density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), polyimide (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethyl acrylate (PMA), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), and polystyrene (PS). Such non-conductive polymeric materials can have excellent potential resistance or solvent resistance.

[0027] The conductive filler can be any material that is conductive. Examples of materials with excellent conductivity, potential resistance, or lithium ion blocking properties include metals and conductive carbon. While there are no particular limitations on the metal, it is preferable to use at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or an alloy or metal oxide containing such a metal. Furthermore, there are no particular limitations on the conductive carbon. Preferably, the conductive carbon contains at least one selected from the group consisting of acetylene black, Vulcan®, Black Pearl®, carbon nanofiber, Ketjen Black®, carbon nanotubes, carbon nanohorns, carbon nanoballoons, and fullerenes.

[0028] The amount of conductive filler added is not particularly limited as long as it is an amount that can impart sufficient conductivity to the current collector, and is generally 5 to 80 mass % relative to the total mass of the current collector (100 mass %).

[0029] The current collector may have a single layer structure made of a single material, or may have a laminate structure made of an appropriate combination of layers made of these materials. From the viewpoint of reducing the weight of the current collector, it is preferable that the current collector include at least a conductive resin layer made of a resin having electrical conductivity. Furthermore, from the viewpoint of blocking the movement of lithium ions between the cell layers, a metal layer may be provided on a part of the current collector.

[0030] [Cathode active material layer] The positive electrode active material layer contains a positive electrode active material that can release ions such as lithium ions during charging and absorb ions such as lithium ions during discharging.

[0031] Examples of positive electrode active materials include lithium-transition metal composite oxides such as LiMn2O4, LiCoO2, LiNiO2, LiMnO2, Li(Ni-Mn-Co)O2, Li(Ni-Co-Al)O2, and those in which a portion of the transition metal is replaced with another element, as well as lithium-transition metal phosphate compounds and lithium-transition metal sulfate compounds. In some cases, two or more positive electrode active materials may be used in combination. From the viewpoint of capacity and output characteristics, lithium-transition metal composite oxides, particularly lithium-transition metal composite oxides having a layered rock salt structure such as LiCoO2, LiNiO2, LiMnO2, Li(Ni-Mn-Co)O2, Li(Ni-Co-Al)O2, and those in which a portion of the transition metal is replaced with another element, are preferably used as the positive electrode active material. More preferably, a composite oxide containing lithium and nickel is used, and even more preferably, Li(Ni-Mn-Co)O2 and a composite oxide in which a portion of the transition metal is substituted with another element (hereinafter simply referred to as "NMC composite oxide") or Li(Ni-Co-Al)O2 and a composite oxide in which a portion of the transition metal is substituted with another element (hereinafter simply referred to as "NCA composite oxide") is used, and particularly preferably, NMC composite oxide is used. NMC composite oxides and NCA composite oxides have a layered crystal structure in which lithium atomic layers and transition metal atomic layers are alternately stacked with oxygen atomic layers interposed between them, and contain one Li atom per atom of the transition metal M. The amount of Li that can be extracted is twice that of spinel-type lithium manganese oxides, i.e., the supply capacity is doubled, resulting in high capacity.

[0032] As described above, the NMC composite oxide and the NCA composite oxide also include composite oxides in which part of the transition metal element is substituted by other metal elements. Examples of the other elements in that case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, Cu, Ag, Zn, etc. Preferably, they are Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr. More preferably, they are Ti, Zr, P, Al, Mg, Cr. From the viewpoint of improving cycle characteristics, even more preferably, they are Ti, Zr, Al, Mg, Cr. However, the other metal elements that can substitute the transition metal element of the NCA composite oxide are those other than Al.

[0033] Since the NMC composite oxide has a high theoretical discharge capacity, preferably, it has a composition represented by the general formula (1): Li a Ni b Mn c Co d M x O2 (wherein, in the formula, a, b, c, d, x satisfy 0.9 ≦ a ≦ 1.2, 0 < b < 1, 0 < c ≦ 0.5, 0 < d ≦ 0.5, 0 ≦ x ≦ 0.3, and b + c + d + x = 1. M is at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr). Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. From the viewpoint of cycle characteristics, in the general formula (1), it is preferable that 0.4 ≦ b ≦ 0.92, for example, 0.75 ≦ b ≦ 0.92. The composition of each element can be measured by, for example, inductively coupled plasma (ICP) optical emission spectrometry.

[0034] Generally, nickel (Ni), cobalt (Co), and manganese (Mn) are known to contribute to capacity and output characteristics from the viewpoints of improving the purity and electron conductivity of materials. Ti, etc. partially substitute transition metals in the crystal lattice. From the viewpoint of cycle characteristics, it is preferable that a part of the transition elements is substituted by other metal elements, and particularly preferably 0 < x ≦ 0.3 in the general formula (1). Since the crystal structure is stabilized by the solid solution of at least one selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr, as a result, it is considered that the capacity degradation of the battery can be prevented even when charge and discharge are repeated, and excellent cycle characteristics can be realized.

[0035] As a more preferable embodiment, in the general formula (1), it is preferable that b, c, and d are 0.44 ≦ b ≦ 0.92, 0.27 ≦ c ≦ 0.31, and 0.19 ≦ d ≦ 0.26 from the viewpoint of improving the balance between capacity and life characteristics. For example, LiNi 0.5 Mn 0.3 Co 0.2 O2 has the advantage that it can produce a compact and high-capacity battery by having a larger capacity per unit weight and being able to improve the energy density compared to LiCoO2, LiMn2O4, LiNi 1 / 3 Mn 1 / 3 Co 1 / 3 O2, etc. that have proven results in general consumer batteries, and is also preferable from the viewpoint of cruising range. In terms of having a larger capacity, LiNi 0.8 Mn 0.1 Co 0.1 O2, LiNi 0.8 Co 0.1 Al 0.1 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, etc. are preferable

[0036] The average particle size of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, but from the viewpoint of achieving high output, it is preferably 1 to 100 μm, more preferably 1 to 20 μm. In this specification, the average particle size is the median diameter (D50) measured by a particle size distribution measuring device using a laser diffraction scattering method.

[0037] The content of the positive electrode active material in the positive electrode active material layer is not particularly limited, but is preferably in the range of 60 to 99 mass %, and more preferably in the range of 80 to 98 mass %, relative to 100 mass % of the total solid content.

[0038] The positive electrode active material layer contains the positive electrode active material as described above, and further contains other additives such as a conductive aid and a binder, as necessary.

[0039] The conductive additive functions to form an electron conduction path (conductive passage) in the positive electrode active material layer. In particular, it is preferable that at least a portion of the conductive additive forms a conductive passage that electrically connects the first main surface of the positive electrode active material layer that contacts the electrolyte layer side to the second main surface that contacts the current collector side. This configuration further reduces the electron transfer resistance in the thickness direction of the positive electrode active material layer. As a result, the high-rate output characteristics of the battery can be further improved. Note that whether or not at least a portion of the conductive additive forms a conductive passage that electrically connects the first main surface of the positive electrode active material layer that contacts the electrolyte layer side to the second main surface that contacts the current collector side can be confirmed by observing a cross section of the positive electrode active material layer using an SEM or an optical microscope.

[0040] Examples of the conductive additive include particulate carbon materials and fibrous carbon materials. In this specification, particulate carbon materials refer to carbon materials having a primary particle aspect ratio of 20 or less. In addition, in this specification, fibrous carbon materials refer to carbon materials having an aspect ratio of 1000 or more. In this specification, the aspect ratios of particulate carbon materials and fibrous carbon materials respectively refer to the average value of the ratio of the major axis length to the minor axis length of 100 carbon materials randomly selected using a scanning electron microscope (SEM).

[0041] Examples of particulate carbon materials include carbon powders such as acetylene black, carbon black, channel black, thermal black, and Ketjen Black (registered trademark). Of these, acetylene black is preferred from the viewpoint of electrical conductivity. The average particle size (primary particle size) of the particulate carbon material (preferably acetylene black) is not particularly limited, but is preferably 10 to 500 nm, more preferably 10 to 100 nm, even more preferably 15 to 75 nm, and particularly preferably 20 to 60 nm. Furthermore, the aspect ratio (aspect ratio of primary particles) of the particulate carbon material (preferably acetylene black) is 20 or less as described above, but is preferably 10 or less, more preferably 5 or less, even more preferably 2 or less, and particularly preferably 1.5 or less (the lower limit is 1).

[0042] Examples of fibrous carbon materials include carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes), carbon nanofibers, vapor-grown carbon fibers, electrospun carbon fibers, polyacrylonitrile-based carbon fibers, and pitch-based carbon fibers. Among these, carbon nanotubes are preferred from the viewpoint of electrical conductivity. The diameter (minor axis) of the fibrous carbon material (preferably carbon nanotubes) is not particularly limited, but is preferably 1 to 100 nm, more preferably 2 to 50 nm, and even more preferably 5 to 20 nm. The length (major axis) of the fibrous carbon material (preferably carbon nanotubes) is not particularly limited, but is preferably 1 to 100 μm, more preferably 2 to 50 μm, and even more preferably 5 to 30 μm. Furthermore, the aspect ratio of the fibrous carbon material (preferably carbon nanotubes) is 1,000 or more as described above, but is preferably 1,500 or more, and more preferably 1,800 or more. The upper limit of the aspect ratio of the fibrous carbon material (preferably carbon nanotubes) is not particularly limited, but may be, for example, 20,000 or less, 10,000 or less, 5,000 or less, or 4,000 or less.

[0043] The optional binder used in the positive electrode active material layer is not particularly limited, and examples thereof include thermoplastic polymers such as polybutylene terephthalate, polyethylene terephthalate, polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are substituted with other halogen elements), polyethylene, polypropylene, polymethylpentene, polybutene, polyether nitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene-propylene-diene copolymer, styrene-butadiene-styrene block copolymer and hydrogenated products thereof, styrene-isoprene-styrene block copolymer and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polychloroethylene ...TPE), tetrafluoroethylene-hexafluoropropylene copolymer (TPE), tetrafluoroethylene-hexafluoropropylene copolymer (TPE), tetrafluoroethylene-hexafluoropropylene copolymer (TPE), tetrafluoroethylene-hexafluoropropylene copolymer (TPE), tetrafluoroethylene-hexafluoropropylene copolymer (TPE), tetrafluoroethylene-hexafluoropropylene copolymer (TPE), tetrafluoroethylene-hexafluoroethylene copolymer (TPE), tetrafluoroethylene-hexafluoropropylene copolymer (TPE), tetrafluoroethylene-hexafluoroethylene copolymer (TPE), tetrafluoroethylene-hexafluoropropylene copolymer (TPE), tetrafluoroethylene-hexafluoroethylene copolymer (TPE), tetrafluoroethylene-hexafluoroethylene copolymer (TPE), tetrafluoroethylene-hexafluoroethylene copolymer (TPE), tetrafluoroethylene-hexafluoroethylene copolymer (TPE), tetrafluoroethylene-hexafluoroethylene copolymer (TPE), tetrafluoroethylene-hexafluoroethylene copolymer (TPE), tetrafluoroethylene-hexafluoro Examples of such fluororesins include polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinyl fluoride (PVF); vinylidene fluoride-based fluororubbers such as vinylidene fluoride-hexafluoropropylene-based fluororubbers (VDF-HFP-based fluororubbers), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-HFP-TFE-based fluororubbers), vinylidene fluoride-pentafluoropropylene-based fluororubbers (VDF-PFP-based fluororubbers), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene-based fluororubbers (VDF-PFP-TFE-based fluororubbers), vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoroethylene-based fluororubbers (VDF-PFMVE-TFE-based fluororubbers), and vinylidene fluoride-chlorotrifluoroethylene-based fluororubbers (VDF-CTFE-based fluororubbers); and epoxy resins. Among these, polyvinylidene fluoride, polyimide, styrene-butadiene rubber, carboxymethyl cellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

[0044] In this embodiment, the compounding ratio of other additives, such as a conductive additive and a binder, that may be contained in the positive electrode active material layer is not particularly limited, and can be adjusted by appropriately referring to known knowledge about non-aqueous electrolyte secondary batteries.

[0045] The nonaqueous electrolyte secondary battery of this embodiment is a battery using a high-area electrode. That is, in the battery of this embodiment, the amount of active material carried per unit area of ​​the positive electrode is 30 mg / cm. 2 The upper limit of the amount of the active material carried is not particularly limited, but for example, it is 100 mg / cm 2 or less, preferably 50 mg / cm 2 or less, more preferably 40 mg / cm 2 The amount of the active material carried is 50 mg / cm or less. 2 Below 40mg / cm 2 When the thickness is less than 1 / 2, the electrode does not become too thick due to the appropriate amount of active material carried, and the transportability of the electrolyte can be further improved. Therefore, the concentration distribution of the electrolyte can be suppressed, and the reaction can proceed more uniformly. As a result, the cycle durability can be further improved.

[0046] The coating amount (basis weight) of the positive electrode active material layer on one side is not particularly limited as long as it provides the above-mentioned predetermined amount of active material carried. For example, 2 or more, preferably 30 mg / cm 2 The upper limit of the coating amount on one side is not particularly limited, but is, for example, 100 mg / cm 2 The following is the result.

[0047] The porosity of the positive electrode active material layer is not particularly limited, but from the viewpoint of achieving high density, it is, for example, 27% or less, preferably 25% or less. Within this range, the effect of the present invention, that is, excellent cycle durability, can be more significantly achieved even in a high-capacity electrode. Furthermore, a porosity of 20% or more is preferable because it is less likely to cause a decrease in cycle durability due to a decrease in the transportability of the electrolyte. The porosity of the electrode active material layer can be determined by the following method: (1) The weight per unit area of ​​the electrode active material layer is measured, and the weight per unit area of ​​each material is calculated from the blending ratio of the materials used; (2) Measure the thickness [A] of the electrode active material layer; (3) Using the weight of each material obtained in (1) and the density of each material, calculate the thickness [B] of the electrode active material layer when the porosity is 0%; (4) The pore volume of the electrode active material layer is calculated from the difference (AB) between the measured thickness [A] of the electrode active material layer and the calculated thickness [B] of the electrode active material layer.

[0048] The thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be referred to as appropriate. The thickness of the positive electrode active material layer is preferably 80 to 200 μm, more preferably 90 to 150 μm, and even more preferably 90 to 120 μm. When the thickness of the positive electrode active material layer is 80 μm or more, it becomes easy to retain the positive electrode active material to exhibit sufficient capacity (energy density). On the other hand, when the thickness of the positive electrode active material layer is 200 μm or less, the discharge rate characteristics can be further improved.

[0049] The positive electrode active material layer can be formed by a method of applying (coating) a normal slurry.

[0050] [Negative electrode active material layer] The negative electrode active material layer contains a negative electrode active material that can release ions such as lithium ions during discharge and absorb ions such as lithium ions during charge. The type of negative electrode active material is not particularly limited, but examples include carbon materials, metal oxides, and metal active materials. Examples of carbon materials include graphite, such as natural graphite and artificial graphite, hard carbon, and soft carbon. Examples of metal oxides include Nb2O5, Li4Ti5O 12and the like. Furthermore, silicon-based negative electrode active materials and tin-based negative electrode active materials may also be used. Here, silicon and tin belong to the 14th group of elements, and are known to be negative electrode active materials that can significantly improve the capacity of non-aqueous electrolyte secondary batteries. These simple substances can absorb and release a large number of charge carriers (lithium ions, etc.) per unit volume (mass), and therefore become high-capacity negative electrode active materials. Here, it is preferable to use Si simple substance as the silicon-based negative electrode active material. Similarly, SiO 2 disproportionated into two phases, an Si phase and a silicon oxide phase, is also used. x It is also preferable to use silicon oxides such as (0.3≦x≦1.6). In this case, the range of x is more preferably 0.5≦x≦1.5, and even more preferably 0.7≦x≦1.2. Furthermore, an alloy containing silicon (silicon-containing alloy-based negative electrode active material) may be used. On the other hand, examples of negative electrode active materials containing tin element (tin-based negative electrode active materials) include simple Sn, tin alloys (Cu—Sn alloy, Co—Sn alloy), amorphous tin oxide, tin silicon oxide, etc. Among these, examples of amorphous tin oxide include SnB 0.4 P 0.6 O 3.1 Examples of tin silicon oxides include SnSiO3. A lithium-containing metal may also be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is an active material containing lithium, and examples thereof include metallic lithium and lithium-containing alloys. Examples of lithium-containing alloys include alloys of Li with at least one of In, Al, Si, and Sn. In some cases, two or more negative electrode active materials may be used in combination. Of course, negative electrode active materials other than those mentioned above may also be used. In terms of high capacity, the negative electrode active material preferably contains a carbon material, metallic lithium, a silicon-based negative electrode active material, or a tin-based negative electrode active material, and more preferably contains a carbon material or a silicon-based negative electrode active material.

[0051] In one embodiment, the negative electrode active material more preferably includes a carbon material and a silicon-based negative electrode active material. Because silicon-based negative electrode active materials have a higher capacity than carbon materials, the energy density can be increased by combining them with a silicon-based negative electrode active material. In this case, the content of the silicon-based negative electrode active material is preferably greater than 0 and less than 5% by mass relative to the total amount of the negative electrode active material. In particular, the content of the silicon-based negative electrode active material is preferably greater than 0 and less than 5% by mass relative to the total amount of the carbon material and the silicon-based negative electrode active material. This can achieve high capacity and excellent cycle durability.

[0052] The negative electrode active material may be in the form of, for example, particles (spherical, fibrous), thin film, etc. When the negative electrode active material is in the form of particles, the average particle diameter is, for example, in the range of 1 nm to 100 μm, preferably in the range of 10 nm to 50 μm, more preferably in the range of 100 nm to 20 μm, and even more preferably in the range of 1 to 20 μm.

[0053] The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably within the range of 60 to 99 mass %, and more preferably within the range of 80 to 98 mass %, for example.

[0054] In addition, the negative electrode active material layer may further contain other additives such as a conductive aid and a binder, as required, in the same manner as described above for the positive electrode active material layer.

[0055] The porosity of the negative electrode active material layer is not particularly limited, but from the viewpoint of achieving a good balance between high density and excellent cycle durability, it is, for example, 20 to 50%, preferably 22 to 40%, and more preferably 22 to 32%.

[0056] In the nonaqueous electrolyte secondary battery of this embodiment, the thickness of the negative electrode active material layer is not particularly limited, but is preferably 90 to 250 μm, more preferably 100 to 200 μm, and even more preferably 100 to 150 μm. The thicker the negative electrode active material layer, the more negative electrode active material can be retained to exhibit sufficient capacity (energy density). On the other hand, the thinner the negative electrode active material layer, the more improved the discharge rate characteristics can be.

[0057] The negative electrode active material layer can be formed by a method of applying (coating) a normal slurry.

[0058] [Electrolyte layer] The electrolyte layer of the nonaqueous electrolyte secondary battery according to this embodiment has a structure in which a separator is impregnated with an electrolytic solution.

[0059] The electrolyte (liquid electrolyte) functions as a lithium ion carrier. The electrolyte (liquid electrolyte) that constitutes the electrolyte layer is in the form of an electrolyte salt (lithium salt) dissolved in an organic solvent. Examples of organic solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), and γ-butyrolactone (GBL). Among these, from the viewpoint of further improving the rapid charging characteristics and output characteristics, the organic solvent is preferably a chain carbonate, more preferably at least one selected from the group consisting of diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), and more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

[0060] Examples of electrolyte salts (lithium salts) include Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C2F5SO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, and LiCF3SO3. Among these, from the viewpoint of battery output and charge-discharge cycle characteristics, the electrolyte salt (lithium salt) preferably contains a salt of an imide group-containing anion (i.e., contains an imide group-containing anion as a counter anion of the lithium ion), and more preferably Li(FSO2)2N (LiFSI). Furthermore, if the salt contains a salt of an imide group-containing anion, the lithium ion transport number is high, thereby efficiently transporting the lithium ions within the electrode and suppressing reaction distribution. This can further improve the cycle durability of the battery.

[0061] The electrolyte may further contain additives other than the above-mentioned components. Specific examples of such compounds include vinylene carbonate, methyl vinylene carbonate, dimethyl vinylene carbonate, phenyl vinylene carbonate, diphenyl vinylene carbonate, ethyl vinylene carbonate, diethyl vinylene carbonate, vinyl ethylene carbonate, 1,2-divinyl ethylene carbonate, 1-methyl-1-vinyl ethylene carbonate, 1-methyl-2-vinyl ethylene carbonate, 1-ethyl-1-vinyl ethylene carbonate, and 1-ethyl-2-vinyl ethylene carbonate. Examples of the additive include ethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethyl ethylene carbonate, allyloxymethyl ethylene carbonate, acryloxymethyl ethylene carbonate, methacryloxymethyl ethylene carbonate, ethynyl ethylene carbonate, propargyl ethylene carbonate, ethynyloxymethyl ethylene carbonate, propargyloxyethylene carbonate, methylene ethylene carbonate, and 1,1-dimethyl-2-methylene ethylene carbonate. These additives may be used alone or in combination of two or more. The amount of additive used in the electrolyte solution can be adjusted as appropriate.

[0062] In the nonaqueous electrolyte secondary battery according to this embodiment, the amount of active material carried per unit area is 30 mg / cm 2 The positive electrode described above is used, and the concentration of the electrolyte salt in the electrolytic solution is 2.0 to 4.0 mol / L.

[0063] While batteries using high-area electrodes, which increase the amount of active material per unit area by increasing the thickness of the electrode active material layer, are expected to have higher energy densities, when the electrolyte salt concentration in the electrolyte solution is around 1 mol / L, which is the concentration that generally gives peak conductivity, the transport of lithium ions in the electrode is insufficient. As a result, lithium ions are depleted in the electrode active material layer on the lithium ion absorbing side during charge and discharge, resulting in reduced cycle durability and reduced rate characteristics.

[0064] Furthermore, increasing the electrolyte salt concentration in the electrolyte solution increases the viscosity of the electrolyte solution and the lithium ion concentration on the lithium ion release side, which reduces the lithium ion transportability, resulting in a significant decrease in cycle durability.

[0065] In response to this, the inventors investigated the electrolyte concentration distribution and lithium ion concentration distribution across the electrode thickness during charge and discharge in batteries using high-area-weight electrodes, including through battery reaction simulations. As a result, they found that in batteries using a specified high-area-weight electrode, controlling the electrolyte salt concentration of the electrolyte within a specified range can suppress the lithium ion concentration gradient across the electrode thickness. Therefore, it is believed that the electrode reaction proceeds not only near the separator but also in the region close to the current collector in the electrode active material layer, improving cycle durability. In other words, it is believed that the improved lithium ion transport properties within the electrode and the suppression of the reaction distribution within the electrode suppress decomposition of the active material and electrolyte, thereby maintaining capacity. As a result, a battery with high energy density can be obtained even after cycle testing.

[0066] In the nonaqueous electrolyte secondary battery according to this embodiment, the concentration of the electrolyte salt in the electrolyte solution is 2.0 to 4.0 mol / L. If the concentration of the electrolyte salt in the electrolyte solution is less than 2.0 mol / L, the low lithium ion concentration causes depletion of the electrolyte solution on the lithium ion absorption side, resulting in a decrease in cycle durability. On the other hand, if the concentration exceeds 4.0 mol / L, the lithium ion concentration on the lithium ion release side increases and the viscosity of the electrolyte solution increases, which can reduce the lithium ion transportability. As a result, cycle durability decreases. The concentration of the electrolyte salt in the electrolyte solution is preferably 2.4 mol / L or more, more preferably 2.6 mol / L or more. Furthermore, the concentration of the electrolyte salt in the electrolyte solution is preferably 3.5 mol / L or less, more preferably 3.4 mol / L or less. Within the above range, the cycle durability of a battery using a high-area-weight electrode can be further improved. Furthermore, this is preferable because the rate characteristics of the battery are excellent.

[0067] In addition, a typical electrode thickness, for example, the amount of active material carried per unit area of ​​the positive electrode is 30 mg / cm 2 Less than 25 mg / cm 2 When the electrolyte salt concentration is below 4.0 mol / L, the higher the electrolyte salt concentration in the electrolyte solution, the more solvation progresses, and decomposition of the electrolyte solution is suppressed, improving cycle durability. Even when the electrolyte salt concentration exceeds 4.0 mol / L, cycle durability is less likely to decrease.

[0068] Although not particularly limited, the nonaqueous electrolyte secondary battery according to this embodiment has an electrolyte ionic conductivity of preferably 3 mS / cm or more, more preferably 4 mS / cm or more, in the electrolyte solution. This allows for efficient transport of lithium ions within the electrode and suppresses reaction distribution. This can further improve the cycle durability of the battery. Although the upper limit of the electrolyte ionic conductivity is not particularly limited, it is substantially 15 mS / cm or less.

[0069] In the nonaqueous electrolyte secondary battery according to this embodiment, a separator is used in the electrolyte layer. The separator has the functions of retaining the electrolyte solution to ensure lithium ion conductivity between the positive electrode and the negative electrode, and of acting as a partition between the positive electrode and the negative electrode.

[0070] Examples of the form of the separator include a porous sheet separator made of polymer or fiber that absorbs and retains the electrolyte, and a nonwoven fabric separator.

[0071] As a separator made of a porous sheet of polymer or fiber, for example, a microporous material (microporous membrane) can be used. Specific forms of the porous sheet made of polymer or fiber include, for example, polyolefins such as polyethylene (PE) and polypropylene (PP); laminates of multiple layers of these (e.g., a laminate with a three-layer structure of PP / PE / PP); hydrocarbon resins such as polyimide, aramid, and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP); and microporous (microporous membrane) separators made of glass fiber. The micropore diameter of the microporous (microporous membrane) separator is preferably a maximum of 1 μm or less (usually a pore diameter of about several tens of nanometers).

[0072] The nonwoven fabric separator may be made of any of the conventional materials, such as cotton, rayon, acetate, nylon, polyester, polyolefins such as PP and PE, polyimide, aramid, etc., either singly or in combination. The bulk density of the nonwoven fabric is not particularly limited as long as it can provide sufficient battery characteristics with the impregnated polymer gel electrolyte.

[0073] The thickness of the separator may be the same as that of the electrolyte layer, and is, for example, 5 to 200 μm, preferably 10 to 100 μm, and more preferably 10 to 25 μm. The porosity of the separator is not particularly limited, and is, for example, 30 to 70%.

[0074] Furthermore, the separator is preferably a separator in which a heat-resistant insulating layer is laminated on a porous substrate (a separator with a heat-resistant insulating layer). The heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. The separator with a heat-resistant insulating layer is highly heat-resistant, with a melting point or thermal softening point of 150°C or higher, preferably 200°C or higher. The presence of the heat-resistant insulating layer alleviates the internal stress of the separator that increases with temperature rise, thereby suppressing thermal shrinkage. As a result, short circuits between battery electrodes can be prevented, resulting in a battery configuration that is less susceptible to performance degradation due to temperature rise. Furthermore, the presence of the heat-resistant insulating layer improves the mechanical strength of the separator with a heat-resistant insulating layer, making it less likely to rupture. Furthermore, the heat-shrinkage suppression effect and high mechanical strength make the separator less likely to curl during the battery manufacturing process.

[0075] The inorganic particles in the heat-resistant insulating layer contribute to the mechanical strength and thermal shrinkage suppression effect of the heat-resistant insulating layer. Materials used as inorganic particles are not particularly limited. Examples include oxides (SiO2, Al2O3, ZrO2, TiO2), hydroxides, and nitrides of silicon, aluminum, zirconium, and titanium, as well as composites thereof. These inorganic particles may be derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, and mica, or may be artificially produced. These inorganic particles may be used alone or in combination of two or more types. Among these, silica (SiO2) or alumina (Al2O3) is preferred from the viewpoint of cost, and alumina (Al2O3) is more preferred.

[0076] The weight of the heat-resistant particles is not particularly limited, but is preferably 5 to 15 g / m 2 Within this range, sufficient ion conductivity can be obtained and heat resistance strength can be maintained, which is preferable.

[0077] The binder in the heat-resistant insulating layer functions to bond the inorganic particles together and between the inorganic particles and the resin porous substrate layer, ensuring stable formation of the heat-resistant insulating layer and preventing peeling between the porous substrate layer and the heat-resistant insulating layer.

[0078] The binder used in the heat-resistant insulating layer is not particularly limited, and examples of compounds that can be used as binders include carboxymethyl cellulose (CMC), polyacrylonitrile, cellulose, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), isoprene rubber, butadiene rubber, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), and methyl acrylate. Of these, it is preferable to use carboxymethyl cellulose (CMC), methyl acrylate, or polyvinylidene fluoride (PVDF). These compounds may be used alone or in combination of two or more.

[0079] The binder content in the heat-resistant insulating layer is preferably 2 to 20% by mass relative to 100% by mass of the heat-resistant insulating layer. When the binder content is 2% by mass or more, the peel strength between the heat-resistant insulating layer and the porous substrate layer can be increased, improving the vibration resistance of the separator. On the other hand, when the binder content is 20% by mass or less, an appropriate amount of space between the inorganic particles is maintained, ensuring sufficient lithium ion conductivity.

[0080] The heat shrinkage rate of the separator with heat-resistant insulation layer is 2gf / cm at 150℃. 2 It is preferable that both the MD and TD are 10% or less after being held for 1 hour under these conditions. By using such a highly heat-resistant material, it is possible to effectively prevent the separator from shrinking even when the heat generation amount increases and the internal temperature of the battery reaches 150°C. As a result, it is possible to prevent short circuits between the battery electrodes, resulting in a battery configuration that is less susceptible to performance degradation due to temperature rise.

[0081] [Positive and negative current collector plates] The material constituting the current collector plates (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collector plates for lithium-ion secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferred as constituent materials of the current collector plates. From the viewpoints of light weight, corrosion resistance, and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The positive electrode current collector plate 27 and the negative electrode current collector plate 25 may be made of the same material or different materials.

[0082] [Positive and negative leads] Although not shown, the current collectors (11′, 11″) may be electrically connected to the current collector plates (25, 27) via positive and negative electrode leads. Materials used in known lithium ion secondary batteries may be used as the constituent materials of the positive and negative electrode leads. The parts removed from the exterior are preferably covered with a heat-resistant, insulating heat-shrinkable tube or the like to prevent contact with peripheral devices or wiring, resulting in electrical leakage and affecting products (e.g., automobile parts, particularly electronic devices).

[0083] [Sealing part] The sealing portion 31 is a component specific to bipolar secondary batteries (series-stacked batteries) and has the function of preventing leakage of the electrolyte solution from the electrolyte layer. In addition, it can also prevent contact between adjacent current collectors in the battery and short circuits caused by slight irregularities at the edges of the stacked electrodes.

[0084] The constituent material of the sealing portion is not particularly limited, but may be polyolefin resins such as polyethylene and polypropylene, epoxy resins, rubber, polyimides, etc. Among these, it is preferable to use polyolefin resins from the viewpoints of corrosion resistance, chemical resistance, film-forming properties, economy, etc.

[0085] [Battery exterior materials] As the battery exterior material, a known metal can case can be used, or a bag-shaped case using an aluminum-containing laminate film 29 that can cover the power-generating element as shown in Figures 1 and 2 can be used. The laminate film can be, for example, a three-layer laminate film formed by laminating PP, aluminum, and nylon in this order, but is not limited to these. A laminate film is desirable from the viewpoint of achieving high output and excellent cooling performance, making it suitable for use in batteries for large devices such as EVs and HEVs. Furthermore, an aluminum-containing laminate film is more preferable for the exterior body because it allows for easy adjustment of the collective pressure applied to the power-generating element from the outside.

[0086] The nonaqueous electrolyte secondary battery according to this embodiment has a high capacity and excellent cycle durability, and therefore the stacked secondary battery according to this embodiment is suitable for use as a power source for driving EVs and HEVs.

[0087] In particular, the nonaqueous electrolyte secondary battery according to this embodiment has the advantage that it can improve cycle durability even in the case of a high-area-weight electrode. Therefore, the nonaqueous electrolyte secondary battery according to one embodiment of the present invention has a positive electrode capacity density per unit area of ​​one side of 60 Ah / m 2 It is preferable that the positive electrode capacity density per unit area of ​​one side is 65 Ah / m or more. It is preferable that the positive electrode capacity density per unit area of ​​one side is the above-mentioned even after a durability test of 100 cycles. In the nonaqueous electrolyte secondary battery according to one embodiment of the present invention, the positive electrode capacity density per unit area of ​​one side is 65 Ah / m or more. 2 The positive electrode capacity density per unit area of ​​one side is not particularly limited, but is preferably 150 Ah / m 2 The following is the result.

[0088] In a nonaqueous electrolyte secondary battery according to an embodiment of the present invention, the electrode energy density of the positive electrode is preferably 870 Wh / L or more. It is preferable that the above-mentioned electrode energy density of the positive electrode is maintained even after a 100-cycle durability test. In a nonaqueous electrolyte secondary battery according to an embodiment of the present invention, the electrode energy density of the positive electrode is more preferably 900 Wh / L or more. The electrode energy density is not particularly limited, but is, for example, 1200 Wh / L or less. The electrode energy density can be determined by the procedure described in the Examples.

[0089] [Method of manufacturing non-aqueous electrolyte secondary battery] The method for producing the nonaqueous electrolyte secondary battery according to this embodiment is not particularly limited, but as an example, first, electrodes (positive and negative electrodes) are produced. One method for producing the electrodes is to prepare an electrode active material slurry, apply the electrode active material slurry to the surface of a current collector, dry it, and then press it to produce an electrode active material layer. The resulting positive and negative electrodes are then stacked with a separator interposed between them, and placed inside a pack made of an aluminum laminate film or the like, which serves as an exterior. Thereafter, an electrolyte solution is poured into the pack, which is then vacuum-sealed to produce a battery.

[0090] [Battery pack] A battery pack is made up of multiple batteries connected together. Specifically, it is made up of at least two batteries connected in series, parallel, or both. By connecting them in series or parallel, it is possible to freely adjust the capacity and voltage.

[0091] A small, detachable assembled battery can be formed by connecting multiple batteries in series or in parallel. Furthermore, a large-capacity, high-output assembled battery (such as a battery module or battery pack) can be formed by further connecting multiple such small, detachable assembled batteries in series or in parallel, suitable for use as a vehicle drive power source or auxiliary power source, which require high volumetric energy density and high volumetric power density. The number of batteries to be connected to form a battery assembly and the number of stacked small assembled batteries to form a large-capacity assembled battery can be determined based on the battery capacity and output of the vehicle (electric vehicle) in which the battery will be installed.

[0092] [vehicle] A battery or a battery pack formed by combining a plurality of such batteries can be mounted on a vehicle. The present invention makes it possible to construct a long-life battery with excellent long-term reliability. Therefore, by incorporating such a battery, a plug-in hybrid electric vehicle with a long EV driving range or an electric vehicle with a long driving range per charge can be constructed. For example, a battery or a battery pack formed by combining a plurality of such batteries can be used in a hybrid vehicle, a fuel cell vehicle, or an electric vehicle (all of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, and light vehicles), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles) to produce a vehicle with a long life and high reliability. However, the application is not limited to automobiles, and the battery pack can also be applied to various power sources for other vehicles, such as trains, and can also be used as an on-board power source for uninterruptible power supplies and the like. [Example]

[0093] The present invention will be explained in more detail below using examples and comparative examples, but it should be understood that the present invention is not limited to the following examples.

[0094] "Making a Battery" [Example 1] <Preparation of electrolyte> The electrolyte solution of this example was prepared by dissolving a lithium salt, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), in a non-aqueous solvent, dimethyl carbonate (DMC), at a concentration of 2.0 mol / L. The electrolyte ionic conductivity of this electrolyte solution was 10.77 mS / cm. The electrolyte ionic conductivity of the electrolyte solution was determined by AC impedance measurement. Measurements were performed at frequencies from 500 kHz to 1 Hz, an applied voltage of 10 mV, and a constant temperature of 25°C.

[0095] <Preparation of positive electrode> Positive electrode active material LiNi 0.8 Mn 0.1 Co 0.1A solid mixture consisting of 97% by mass of O2 (NCM), 1% by mass of carbon nanotubes as a conductive additive, and 2% by mass of polyvinylidene fluoride (PVDF) as a binder was prepared. First, the positive electrode active material and the conductive additive were kneaded together. An appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to the mixture, and the mixture was mixed at 2000 rpm for 2 minutes using a planetary stirring mixer / kneader "Awatori Rentaro" (ARE-310, manufactured by Thinky Corporation). A binder dissolved in NMP was added to the mixture, and the mixture was mixed at 2000 rpm for 4 minutes using the same mixer / kneader. If necessary, further NMP was added to adjust the viscosity to prepare a positive electrode active material slurry. Next, the resulting positive electrode active material slurry was applied to one side of an aluminum foil (thickness 15 μm) current collector using a doctor blade and dried on a hot plate at 80 °C for 1 hour. The resulting laminate was then pressed using a roll press to adjust the porosity of the positive electrode active material layer to 25%. The laminate was then placed in a vacuum dryer and dried at 130°C for 8 hours under vacuum conditions to produce a positive electrode with a positive electrode active material layer thickness of 92µm. The coating weight (amount of coating per unit area) of the positive electrode active material layer was 31.2mg / cm. 2 The amount of active material carried per unit area of ​​the positive electrode was 30.26 mg / cm 2 It was.

[0096] <Preparation of negative electrode> A solid material consisting of 95.5% by mass of graphite (negative electrode active material), 0.5% by mass of carbon nanotubes (conductive additive), and 4% by mass of polyvinylidene fluoride (PVDF) (binder) was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP), a slurry viscosity adjusting solvent, was added to this solid material and mixed to prepare a negative electrode active material slurry. The resulting negative electrode active material slurry was then applied to one side of a copper foil (thickness 10 μm) current collector, followed by drying and pressing in the same manner as above to prepare a negative electrode (thickness 118 μm) of this example.

[0097] <Preparation of test cell> The positive electrode obtained above is 12 cm 2 , negative electrode 13cm 2Then, aluminum foil with an aluminum terminal was laminated onto the positive electrode current collector (aluminum foil), and copper foil with a nickel terminal was laminated onto the negative electrode current collector (copper foil).

[0098] Next, a separator (manufactured by Celgard, made of polypropylene (PP), 25 μm thick, 55% porosity) was inserted on the electrode active material layer side of the positive and negative electrodes to form a laminate. This laminate was sandwiched between heat-sealed aluminum laminate films (150 μm thick) that served as exterior bodies, and the electrolyte solution prepared above was poured into them. After that, the inside of the exterior body was depressurized to a vacuum using a vacuum sealer, the depressurization was temporarily released and returned to atmospheric pressure, and then the pressure was reduced again to a vacuum of 99.7% and sealed, thereby producing a pouch-type lithium-ion battery (test cell) having a power generation element in which the positive electrode active material layer and the negative electrode active material layer were stacked so that they faced each other with the separator interposed therebetween.

[0099] <Evaluation of test cells (measurement of cycle durability)> The test cell was subjected to a charge-discharge cycle durability test. Specifically, to apply pressure uniformly across the electrode reaction surface, the electrode portion of the cell was sandwiched between rubber plates, which were then sandwiched between aluminum plates and fixed with bolts.

[0100] The conditions for the charge-discharge test are as follows: Temperature: 25℃; Voltage range: 2.5V-4.3V; Initial charge / discharge: Charge 0.05C 4.3V CCCV, cut off at 0.01C current value; Discharge 0.1C 2.5V CC; Cycle test: Charge 0.33C 4.3V CCCV, cut at 0.025C current value; Discharge 0.33C 2.5V CC.

[0101] The electrode energy density at the initial stage and after 100 cycles was calculated using the following formula.

[0102]

number

[0103] In the above formula, the positive electrode capacity density (Ah / m 2 ) is the capacity of the positive electrode active material (Ah / g) × the amount of active material carried per unit area of ​​the positive electrode (g / m 2 ) The average voltage (V) was calculated by calculating the total energy (mWh) from the area of ​​the discharge curve (curve with capacity on the horizontal axis and voltage on the vertical axis) and dividing this by the resulting discharge capacity (mAh). The electrode layer thickness (m) represents the thickness of (positive electrode current collector - positive electrode active material layer - separator - negative electrode active material layer - negative electrode current collector).

[0104] [Examples 2 to 12, Comparative Examples 1 to 14] In Example 1, the composition of the electrolyte, the coating amount on one side of the positive electrode active material layer, the porosity, and the thickness were changed as shown in Table 1 below. In addition, in the negative electrode, the porosity and thickness of the negative electrode active material layer were changed as shown in Table 1 below. Except for the above, batteries of Examples 2 to 12 and Comparative Examples 1 to 14 were fabricated in the same manner as in Example 1, and cycle durability tests were conducted. The results are shown in Table 1 below. In Table 1, the positive electrode capacity density is the capacity density at the time of the first discharge. Furthermore, FIG. 3 shows the results of cycle durability for the batteries of Examples 1 to 6, 9 to 12, and Comparative Examples 1 to 8, and 11 to 14.

[0105] In Example 12, the electrolyte solution of Example 12 was prepared by dissolving LiPF6, a lithium salt, at a concentration of 2.0 mol / L in an equal volume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), which are non-aqueous solvents.

[0106] Furthermore, in Examples 7 and 8 and Comparative Examples 9 and 10, the negative electrode active material was changed from 95.5 mass % graphite to 90.5 mass % graphite and 5 mass % SiO.

[0107] [Table 1]

[0108] From the results shown in Table 1, it can be seen that the batteries of each example in which the electrolyte salt concentration was 2.0 to 4.0 mol / L had an active material loading per unit area of ​​the positive electrode of 30 mg / cm.2 Even when using electrodes with such a high area density, the electrode energy density after 100 cycles was high, demonstrating excellent cycle durability. [Explanation of symbols]

[0109] 10a stacked secondary battery, 10b Bipolar secondary battery 11 current collector, 11' Positive electrode current collector 11” negative electrode current collector 11a: outermost current collector on the positive electrode side; 11b: outermost current collector on the negative electrode side; 13 positive electrode active material layer, 15 negative electrode active material layer, 17 electrolyte layer, 19 cell layer, 21 power generation elements, 23 Bipolar electrodes, 25 Positive current collector plate (positive tab), 27 negative electrode current collector plate (negative electrode tab), 29 Laminating film, 31 Seal part.

Claims

1. a positive electrode having a positive electrode active material layer containing a positive electrode active material formed on the surface of a current collector; a negative electrode having a negative electrode active material layer containing a negative electrode active material formed on the surface of a current collector; a separator containing an electrolytic solution; A non-aqueous electrolyte secondary battery having a power generating element comprising: The amount of active material carried per unit area of ​​the positive electrode is 30 mg / cm ２ More than 46.56mg / cm ２ is as follows: The concentration of the electrolyte salt in the electrolytic solution is 2.0 to 4.0 mol / L, the porosity of the positive electrode active material layer is 25% or less, The nonaqueous electrolyte secondary battery includes an organic solvent, the organic solvent consisting solely of a chain carbonate.

2. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the concentration of the electrolyte salt in the electrolytic solution is 2.4 to 3.5 mol / L.

3. The amount of active material carried per unit area of ​​the positive electrode is 40 mg / cm ２ 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein:

4. 4. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electrolyte salt comprises a salt of an imide group-containing anion.

5. 5. The nonaqueous electrolyte secondary battery according to claim 1, wherein the electrolyte solution has an electrolyte ion conductivity of 4 mS / cm or more.

6. 6. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material comprises a carbon material or a silicon-based negative electrode active material selected from the group consisting of simple silicon, silicon oxides, and silicon-containing alloy-based negative electrode active materials.

7. 7. The nonaqueous electrolyte secondary battery according to claim 6, wherein the negative electrode active material comprises the carbon material and the silicon-based negative electrode active material, and the content of the silicon-based negative electrode active material is more than 0 and not more than 5 mass% with respect to the total amount of the negative electrode active material.

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