Power storage device

Abstract

To provide a highly safe power storage device in which the capacitive maintenance rate and a high-rate property are maintained and also precipitation of metal lithium is suppressed.SOLUTION: In a power storage device, a positive electrode mixture material layer has a thickness of at least 40 μm and a negative electrode mixture material layer has a thickness of at least 50 μm. Also, the binder content is in the range of 0.1wt% to 5.0wt%, both inclusive, of the positive electrode mixture layer when the solid content weight of the positive electrode mixture material layer is 100wt%, and the binder content is in the range of 1.0wt% to 12.0wt%, both inclusive, of the negative electrode mixture layer when the solid content weight of the negative electrode mixture material layer is 100wt%.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a power storage device.

Background Art

[0002] Power storage devices such as lithium-ion secondary batteries, nickel-metal hydride batteries, and other secondary batteries or capacitors are preferably used as power sources for portable devices such as personal computers and mobile terminals, or as high-output power sources such as vehicle drive power sources for electric vehicles (EVs) due to their light weight and high energy density. Electrodes included in such power storage devices are provided with a current collector foil and a composite layer formed on the surface of the current collector foil. The composite layer is mainly composed of an active material. Typically, the composite layer also contains a conductive assistant and a binder. The binder contributes to maintaining the electrode structure, such as binding the active material to the current collector foil and binding the active materials together.

[0003] By the way, as power storage devices are expanded and developed in applications, it is required that they can be charged (or discharged) with a large current in a short time, and the development of power storage devices with excellent rapid charge and discharge characteristics (i.e., high rate characteristics) has been promoted. Japanese Patent Application Laid-Open No. 2014-10888 discloses a non-aqueous electrolyte secondary battery with a reduced resistance difference between the positive electrode and the negative electrode in order to enhance the high rate characteristics.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] On the other hand, there is also a demand for further increasing the capacity of the power storage device. For example, a technique has been used to increase the capacity of the power storage device by making the composite material layer of the negative electrode thicker than before. In order to improve the high-rate characteristics even for such an electrode with a thick composite material layer, the adoption of the technique described in Japanese Patent Application Laid-Open No. 2014-10888 has been considered.

[0006] However, the inventors have found that in the technique described in Japanese Patent Application Laid-Open No. 2014-10888, the capacity retention rate after cycling significantly decreases. Further, in an electrode with a thick negative electrode composite material layer, there is a possibility that a metal (for example, metallic lithium, etc.) serving as a charge carrier may precipitate during charging.

[0007] The present disclosure has been made in view of such circumstances, and its main object is to provide a highly safe power storage device that suppresses the precipitation of metallic lithium while maintaining the capacity retention rate and high-rate characteristics.

Means for Solving the Problems

[0008] The power storage device disclosed herein includes a positive electrode having a positive electrode composite material layer, a negative electrode having a negative electrode composite material layer, and a separator interposed between the positive electrode and the negative electrode. Further, the film thickness of the positive electrode composite material layer is 40 μm or more, and the film thickness of the negative electrode composite material layer is 50 μm or more. The binder content in the positive electrode composite material layer when the solid content weight of the entire positive electrode composite material layer is 100 wt% is 0.1 wt% or more and 5.0 wt% or less, and the binder content in the negative electrode composite material layer when the solid content weight of the entire negative electrode composite material layer is 100 wt% is 1.0 wt% or more and 12.0 wt% or less.

[0009] According to such a power storage device, the resistance value of the negative electrode increases, and the reactivity of charge carriers (such as lithium ions, etc.) in the negative electrode composite material layer decreases. As a result, when charge carriers move from the positive electrode to the negative electrode during charging, the charge carriers can be uniformly occluded in the entire negative electrode composite material layer. Consequently, the precipitation of a metal (such as metallic lithium, etc.) serving as a charge carrier can be suppressed. Also, the resistance value of the positive electrode decreases, and the capacity retention rate after charge-discharge cycles can be kept high. Such an effect enables both the improvement of the high-rate characteristics and the increase in capacity of the power storage device.

[0010] In a power storage device according to one aspect disclosed herein, the resistance value (A) of the positive electrode is 1 Ω / cm 2 or more and 30 Ω / cm 2 or less, and the resistance value (B) of the negative electrode is 3 Ω / cm 2 or more and 90 Ω / cm 2 or less, and satisfies the formula: 2 ≤ B / A ≤ 10. Thereby, the high-rate characteristics and the capacity retention rate can be more reliably kept high.

[0011] In a power storage device according to one aspect disclosed herein, the binder contained in the positive electrode composite material layer is PVDF, and the binder contained in the negative electrode composite material layer is SBR and / or CMC. Thereby, the resistance value of the power storage device is preferably controlled. Such an effect enables both the increase in capacity of the power storage device and the suppression of the precipitation of a metal serving as a charge carrier more reliably.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0013] <Definition of Terms> Hereinafter, a lithium-ion secondary battery, which is one of the typical embodiments of the power storage device in the present disclosure, will be described in detail with reference to the drawings. Matters other than those specifically mentioned in this specification and necessary for implementation (for example, the general configuration and manufacturing process of a power storage device that does not characterize the present disclosure, etc.) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and common general knowledge in the relevant field. In the following drawings, members and parts having the same function are denoted by the same reference numerals for explanation. Also, the dimensional relationships (length, width, thickness, etc.) in each figure do not reflect the actual dimensional relationships.

[0014] In this specification, the "power storage device" is a concept that includes a device in which a charge carrier moves between a pair of electrodes (a positive electrode and a negative electrode) to cause a charge and discharge reaction. That is, the power storage device includes batteries such as secondary batteries (for example, lithium-ion secondary batteries, nickel-metal hydride batteries, nickel-cadmium batteries), and capacitors (physical batteries) such as lithium-ion capacitors and electric double layer capacitors. Also, in this specification, the "lithium-ion secondary battery" refers to a power storage device that uses lithium ions as charge carriers and realizes repeated charge and discharge by the movement of charges associated with lithium ions between the positive and negative electrodes.

[0015] When a numerical range is described in this specification as "A to B (where A and B are arbitrary numerical values)", it means "A or more and B or less", and also includes the meanings of "more than A and less than B", "more than A and B or less", and "A or more and less than B".

[0016] <Lithium-Ion Secondary Battery> FIG. 1 is a schematic longitudinal sectional view of a lithium-ion secondary battery 1 according to an embodiment. In the following description, the reference numerals L, R, U, and D in the drawings represent the left, right, top, and bottom of the lithium-ion secondary battery 1. However, these are merely directions for convenience of explanation and do not limit the installation form of the lithium-ion secondary battery 1 in any way.

[0017] <Battery case> As shown in FIG. 1, in the lithium-ion secondary battery 1 according to the present embodiment, the shape of the battery case 10 is a rectangular parallelepiped shape and a flat square shape. However, the shape of the battery case 10 is not limited to this, and for example, it may be a cylindrical shape. The battery case 10 includes a battery case body 11 that houses the electrode body 20 and an electrolytic solution (not shown), and a sealing plate (lid body) 12 that seals the opening of the main body. The battery case body 11 and the sealing plate 12 are welded and sealed by laser welding or the like. The material of the battery case 10 may be the same as that used in conventional power storage devices of this type, and there is no particular limitation. As an example, a lightweight and highly thermally conductive metal material such as aluminum is used as the material of the battery case 10. However, it is also possible to change the configuration of the battery case 10. For example, a flexible laminate film may be used as the battery case.

[0018] In the present embodiment, the battery case body 11 is composed of a bottom wall, a pair of short side walls that extend from the bottom wall and face each other, and a long side wall. In the present embodiment, the sealing plate 12 of the battery case 10 is provided with a thin safety valve 13 that is set to release the internal pressure when the internal pressure of the battery case rises above a predetermined level, and an injection port (not shown) for injecting the electrolytic solution. In addition, a positive electrode external terminal 14 and a negative electrode external terminal 15 for external connection are provided outside the battery case. These electrode terminals are electrically connected to the electrode body 20 housed in the battery case via internal terminals 16 and 17.

[0019] The external terminals 14 and 15 are made of metal. As the positive electrode external terminal 14, for example, aluminum or an aluminum alloy may be used. As the negative electrode external terminal 15, for example, copper or a copper alloy may be used.

[0020] The internal terminals 16 and 17 are made of metal. As the positive electrode internal terminal 16, for example, aluminum or an aluminum alloy can be used from the viewpoint of improving the bonding strength with the non-formed portion 31a of the positive electrode composite material layer. As the negative electrode internal terminal 17, for example, copper or a copper alloy can be used from the viewpoint of improving the bonding strength with the non-formed portion 41a of the negative electrode composite material layer.

[0021] <Electrolyte> As the electrolyte, a non-aqueous electrolyte in which a supporting salt is dissolved in a suitable non-aqueous solvent can be used. A conventionally known non-aqueous electrolyte can be adopted without particular limitation. As an example of the non-aqueous solvent, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), etc. can be used. Also, as an example of the supporting salt, a lithium salt (for example, LiBOB, LiPF6, etc.) can be used.

[0022] <Electrode body> The positive electrode 30 and the negative electrode 40 of the lithium ion secondary battery 1 according to the present embodiment have the composite material layers 31 and 41 of the configuration disclosed herein. FIG. 3 is a schematic cross-sectional view showing the laminated structure in the laminated electrode body 20. The positive electrode 30 includes a positive electrode current collector foil 32 and a positive electrode composite material layer 31. Also, the negative electrode 40 includes a negative electrode current collector foil 42 and a negative electrode composite material layer 41. A separator 50 is interposed between the positive electrode 30 and the negative electrode 40.

[0023] In this embodiment, the electrode body 20 is a laminated electrode body in which rectangular positive electrodes 30 and rectangular negative electrodes 40 are alternately stacked via a rectangular separator. FIG. 2 is an exploded perspective view schematically showing a cell unit constituting the laminated electrode body 20 which is one of the embodiments. In the following description, the symbols LR, T, and UD in the drawings represent the width direction, thickness direction, and depth direction of the laminated electrode body 20. However, these are merely directions for convenience of explanation and do not limit the installation form of the lithium ion secondary battery 1 in any way. Note that the electrode body 20 is not limited thereto, and may be a wound electrode body in which a positive electrode and a negative electrode are wound via a separator. Also, the number of electrode bodies 20 is not particularly limited. A plurality of electrode bodies (for example, stacking a plurality of the above cell units, etc.) may be accommodated in the battery case 10.

[0024] <Positive electrode and positive electrode composite material layer> As shown in FIG. 2, the positive electrode 30 includes a rectangular positive electrode current collector foil 32 and a positive electrode composite material layer 31 formed on the surface of the positive electrode current collector foil 32. Note that the positive electrode composite material layer 31 may be formed on one or both sides (here, both sides) of the positive electrode current collector foil 32. Also, as shown in FIGS. 1 and 2, the positive electrode 30 may have a non-formed portion 31a of the positive electrode composite material layer where the positive electrode current collector foil 32 is exposed without the positive electrode composite material layer 31 being formed. The non-formed portion 31a of the positive electrode composite material layer is provided so as to protrude from one end of the laminated electrode body 20. The positive electrode internal terminal 16 can be joined to the non-formed portion 31a of the positive electrode composite material layer.

[0025] The positive electrode current collector foil 32 disclosed here is rectangular, but its shape and dimensions are not particularly limited and may be appropriately determined according to the battery design. As the material of the positive electrode current collector foil 32, a known positive electrode current collector foil used for a power storage device may be used and is not particularly limited. The material of the positive electrode current collector foil 32 is, for example, aluminum or an aluminum alloy. Regarding the thickness pT of the positive electrode current collector foil 32, considering the balance between the capacity density of the power storage device and the strength of the current collector foil, the lower limit is preferably 5 μm or more, more preferably 8 μm or more, and most preferably 10 μm or more. Also, the upper limit is preferably 50 μm or less, more preferably 35 μm or less, and most preferably 20 μm or less.

[0026] The positive electrode composite material layer 31 disclosed herein includes at least a positive electrode active material 35 and a binder 60. Further, a conductive assistant, an inorganic filler, or the like may be added as long as the effects of the technology according to the present disclosure are not significantly impaired. The positive electrode composite material layer 31 is prepared, for example, by dispersing the positive electrode active material 35 and the binder 60 (and a conductive assistant, an inorganic filler, or the like used as necessary) in an appropriate solvent (such as ion-exchanged water or an organic solvent) to prepare a paste-like (slurry-like) composition. By applying an appropriate amount of the composition to the surface of the positive electrode current collector foil 32 and drying it, the positive electrode composite material layer 31 can be formed.

[0027] As the positive electrode active material 35 of the positive electrode composite material layer 31 disclosed herein, a positive electrode active material used for the positive electrode of a general lithium-ion secondary battery can be used. For example, the positive electrode active material 35 is a lithium composite metal oxide having a layered rock salt structure, a spinel structure, or an olivine structure. Specifically, LiCoO2, LiNiO2, LiFeO2, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (NCM), LiNi 0.5 Mn 1.5 O4, LiNi 0.8 Co 0.15 A l0.05 O2 (NCA), LiCrMO4, LiMn2O4, LiFePO4 (LFP), etc. may be mentioned. These positive electrode active materials 35 may be used alone or in combination of two or more. Among these, from the viewpoint of improving the cycle characteristics of the power storage device, the positive electrode active material 35 is preferably NCM.

[0028] The positive electrode active material 35 may be in a particulate form. In that case, the average particle size of the positive electrode active material 35 is not particularly limited. The average particle size of the positive electrode active material 35 is typically 30 μm or less. If the average particle size is small, the electrode density increases. Therefore, from the viewpoint of increasing the capacity of the power storage device, the upper limit value of the average particle size of the positive electrode active material 35 is preferably 25 μm or less, and more preferably 20 μm or less. The average particle size of the active material can be determined by the 50 volume% particle size (D50 particle size) based on the laser diffraction / scattering method.

[0029] The film thickness PT on one side of the positive electrode composite material layer 31 disclosed herein is preferably 40 μm or more. From the viewpoint of improving the capacity density of the power storage device, the lower limit value of the film thickness PT on one side of the positive electrode composite material layer 31 may be 45 μm or more, more preferably 50 μm or more, and most preferably 60 μm or more. The upper limit value may be 100 μm or less, 90 μm or less, or 80 μm or less.

[0030] The positive electrode composite material layer 31 according to the present embodiment may have voids as long as the effects of the technology according to the present disclosure are not significantly impaired. When the positive electrode composite material layer 31 has voids, the porosity of the positive electrode composite material layer 31 can be 5% or more and 35% or less. Since the composite material layer is likely to crack when the porosity is high, the porosity of the positive electrode composite material layer 31 is preferably 30% or less. Further, from the viewpoint of increasing the capacity of the power storage device, the resistance also increases, so the porosity of the positive electrode composite material layer 31 is more preferably 20% or less, and most preferably 10% or less.

[0031] As the binder 60 of the positive electrode composite material layer 31 disclosed herein, binders used for the positive electrodes of general lithium-ion secondary batteries and lithium-ion capacitors can be used. For example, the binder 60 used in a non-aqueous paste includes vinyl halide resins such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride (PVDC), and polyalkylene oxides such as polyethylene oxide (PEO). Further, when an aqueous paste is used, a water-soluble polymer material or a water-dispersible polymer material can be preferably employed. For example, the binder 60 used in an aqueous paste includes polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), and the like. As will be described later, since the content of the binder 60 in the positive electrode composite material layer 31 is less than the content of the binder 60 in the negative electrode composite material layer 41, a binder having high strength and adhesiveness even in a small amount is preferable. Therefore, it is preferable to use a polymer containing fluorine as a constituent element as the binder, and PVDF or PTFE is preferable as the binder 60 used for the positive electrode composite material layer 31. These binders 60 may be used alone or in combination of two or more.

[0032] The content of the binder 60 in the positive electrode composite material layer 31 disclosed herein is preferably 0.1 wt% or more and 5.0 wt% or less when the total weight of the solid content of the positive electrode composite material layer 31 is 100 wt%. From the viewpoint of reducing the total resistance of the positive and negative electrode composite material layers 31 and 41 and improving the capacity retention rate of the power storage device, the upper limit value is preferably 4.5 wt% or less, more preferably 3.0 wt% or less, and even more preferably 1.5 wt% or less. Further, the lower limit value may be 0.1 wt% or more, 0.3 wt% or more, or 0.5 wt% or more.

[0033] Examples of the conductive auxiliary agent contained in the positive electrode composite material layer 31 disclosed herein include carbon black, acetylene black, carbon nanotubes (CNT), and metal powders such as silver, gold, or copper. Examples of the inorganic filler include aluminum hydroxide, silica, alumina, zirconia, titania, boehmite, or magnesia.

[0034] The resistance value of the positive electrode composite material layer 31 is preferably 1 Ω / cm 2 or more and 30 Ω / cm 2 or less. If the resistance value of the positive electrode composite material layer 31 is low, excellent output characteristics can be exhibited. Therefore, the upper limit value of the resistance value of the positive electrode composite material layer 31 is preferably 25 Ω / cm 2 or less, more preferably 20 Ω / cm 2 or less, even more preferably 15 Ω / cm 2 or less. Also, the lower limit value may be 1 Ω / cm 2 or more, may be 2 Ω / cm 2 or more, may be 3 Ω / cm 2 or more.

[0035] <Negative electrode and negative electrode composite material layer> As shown in FIG. 2, the negative electrode 40 includes a rectangular negative electrode current collector foil 42 and a negative electrode composite material layer 41 formed on the surface of the negative electrode current collector foil 42. The negative electrode composite material layer 41 may be formed on one side or both sides (here, both sides) of the negative electrode current collector foil 42. Also, as shown in FIGS. 1 and 2, the negative electrode 40 may have a non-formed portion 41a of the negative electrode composite material layer where the negative electrode current collector foil 42 is exposed without the negative electrode composite material layer 41 being formed. In part, it may have a non-formed portion 41a of the negative electrode composite material layer where the negative electrode current collector foil is exposed without the negative electrode composite material layer being formed. The non-formed portion 41a of the negative electrode composite material layer is provided so as to protrude from one end of the laminated electrode body 20. The negative electrode internal terminal 17 can be joined to the non-formed portion 41a of the negative electrode composite material layer.

[0036] The negative electrode current collector foil 42 according to the present embodiment is rectangular, but the shape and dimensions are not particularly limited and may be appropriately determined according to the battery design. The material of the negative electrode current collector foil 42 may be a known negative electrode current collector foil used in a power storage device and is not particularly limited. The material of the negative electrode current collector foil 42 is, for example, copper or a copper alloy. The film thickness nT of the negative electrode current collector foil 42 is preferably 3 μm or more, more preferably 5 μm or more, and most preferably 8 μm or more in view of the balance between the capacity density of the power storage device and the strength of the current collector. Also, the upper limit value is preferably 40 μm or less, more preferably 25 μm or less, and most preferably 15 μm or less.

[0037] The negative electrode composite material layer 41 disclosed herein includes at least a negative electrode active material 45 and a binder 60. Further, a conductive auxiliary agent, an inorganic filler, or the like may be added as long as the effects of the technology according to the present disclosure are not significantly impaired. The negative electrode composite material layer 41 is prepared, for example, by dispersing a negative electrode active material and a binder (and a conductive auxiliary agent, an inorganic filler, etc. used as necessary) in an appropriate solvent (such as ion-exchanged water or an organic solvent) to prepare a paste-like (slurry-like) composition. By applying an appropriate amount of the composition to the surface of the negative electrode current collector foil and drying it, the negative electrode composite material layer 41 can be formed.

[0038] As the negative electrode active material 45 of the negative electrode composite material layer 41 disclosed herein, a negative electrode active material used for the positive electrode of a general lithium-ion secondary battery can be used. For example, the negative electrode active material includes carbon materials such as soft carbon (easily graphitizable carbon), amorphous carbon materials, graphite, hard carbon (difficultly graphitizable carbon), carbon nanotubes, metal oxide materials such as silicon oxide, titanium oxide, vanadium oxide, lithium titanium composite oxides, metal nitride materials such as lithium nitride, lithium cobalt composite nitrides, and silicon compounds. These positive electrode active materials may be used alone or in combination of two or more. Among these, from the viewpoint of improving the energy density, the negative electrode active material 45 is preferably graphite.

[0039] The negative electrode active material 45 may be in a particulate form. In that case, the average particle size of the negative electrode active material 45 is not particularly limited. The average particle size of the negative electrode active material 45 is typically 30 μm or less. If the average particle size is small, the electrode density increases. Therefore, from the viewpoint of increasing the capacity of the power storage device, the upper limit value of the average particle size of the negative electrode active material is preferably 25 μm or less, and more preferably 20 μm or less.

[0040] The film thickness NT of the negative electrode composite material layer 41 disclosed herein is preferably 50 μm or more. From the viewpoint of improving the capacity density of the power storage device, the lower limit value of the film thickness of the negative electrode composite material layer is preferably 45 μm or more, more preferably 60 μm or more, and most preferably 70 μm or more. The upper limit value may be 150 μm or less, 130 μm or less, or 110 μm or less.

[0041] The negative electrode composite material layer 41 may have voids as long as the effects of the technology according to the present disclosure are not significantly impaired. When the negative electrode composite material layer 41 has voids, the porosity of the negative electrode composite material layer 41 can be 5% or more and 35% or less. If the porosity is high, the resistance value increases and the composite material layer is also likely to crack. Therefore, the porosity of the negative electrode composite material layer 41 is preferably 30% or less. Further, from the viewpoint of increasing the capacity of the power storage device, the porosity of the negative electrode composite material layer 41 is more preferably 20% or less, and most preferably 10% or less.

[0042] As the binder 60 of the negative electrode composite material layer 41 disclosed herein, binders used for the negative electrodes of general lithium-ion secondary batteries and lithium-ion capacitors can be used. For example, binders used in non-aqueous pastes include vinyl halide resins such as polyvinylidene fluoride (PVDF) and polyvinylidene chloride (PVDC), and polyalkylene oxides such as polyethylene oxide (PEO). When using an aqueous paste, a water-soluble polymer material or a water-dispersible polymer material can be preferably adopted. For example, binders used in aqueous pastes include polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), etc. As will be described later, since the content of the binder 60 in the negative electrode composite material layer 41 is higher than the content of the binder 60 in the positive electrode composite material layer 31, those with low cost and less environmental load are preferred. Therefore, CMC or SBR is preferred as the binder used for the negative electrode composite material layer. These binders may be used alone or in combination of two or more.

[0043] The content ratio of the binder 60 in the negative electrode composite material layer 41 disclosed herein may typically be 1.0 wt% or more and 12.0 wt% or less, assuming that the total weight of the solid content of the negative electrode composite material layer 41 is 100 wt%. From the perspective of reducing the total resistance of the positive and negative electrode composite material layers and improving the capacity retention rate of the power storage device, the upper limit value is preferably 10.0 wt% or less, more preferably 8.0 wt% or less, and even more preferably 6.0 wt% or less. Also, from the perspective of suppressing the precipitation of metallic lithium, the lower limit value is preferably 1.5 wt% or more, more preferably 3.0 wt% or more, and most preferably 4.5 wt% or more. Also, as the thickness of the composite material layer increases, the mechanical strength increases. On the other hand, a composite material layer with a large thickness tends to lack flexibility and is likely to crack during the manufacturing process of the power storage device. Since the composite material layer of the present disclosure is provided with the binder 60 having a high content ratio as described above, it is possible to reduce the occurrence of cracks during the manufacturing process of the power storage device.

[0044] Examples of the conductive assistant included in the negative electrode composite material layer 41 include metal powders such as silver, gold, or copper. Examples of the inorganic filler include aluminum hydroxide, silica, alumina, zirconia, titania, boehmite, or magnesia.

[0045] The resistance value of the negative electrode composite material layer 41 is preferably 3 Ω / cm 2 or more and 90 Ω / cm 2 or less. From the perspective of maintaining battery performance, the upper limit value of the resistance value of the negative electrode composite material layer is preferably 80 Ω / cm 2 or less, more preferably 70 Ω / cm 2 or less, and even more preferably 60 Ω / cm 2 or less. Also, from the perspective of suppressing the precipitation of metallic lithium, the lower limit value is preferably 5 Ω / cm 2 or more, more preferably 7 Ω / cm 2 or more, and most preferably 10 Ω / cm 2 or more.

[0046] <Separator> The separator 50 according to this embodiment is a porous sheet having insulating properties, but its shape and dimensions are not particularly limited and may be appropriately determined according to the battery design. Typically, since the separator 50 insulates the positive electrode and the negative electrode, the dimensions of the separator 50 are larger than those of the positive electrode sheet 30 and the negative electrode sheet 40. Also, as the material of the separator 50, a known separator used in a power storage device may be used and is not particularly limited. For example, as the separator 50, polyolefins such as polyethylene or polypropylene, polyesters, celluloses, or resins such as polyamides are preferably used. Further, a heat-resistant layer may be provided on the surface of the separator 50 as long as the effects of the technology according to the present disclosure are not significantly impaired. The lower limit of the film thickness ST of the separator 50 is preferably 5 μm or more, more preferably 8 μm or more, and most preferably 10 μm or more, in consideration of the balance between the capacity density of the power storage device and the strength of the current collector. Also, the upper limit of the film thickness of the separator 50 is preferably 40 μm or less, more preferably 30 μm or less, and most preferably 20 μm or less.

[0047] <Total resistance of the electrode body> Here, the total resistance value of the positive electrode 30 and the negative electrode 40 of the power storage device disclosed herein is not particularly limited as long as the effects of the technology according to the present disclosure are not significantly impaired. When the total resistance value of the positive electrode 30 and the negative electrode 40 exceeds 100 Ω / cm 2 the input / output of the power storage device is significantly reduced, and the battery performance cannot be stabilized. Therefore, the upper limit of the total resistance value of the positive electrode 30 and the negative electrode 40 may be 100 Ω / cm or less, preferably 90 Ω / cm or less, more preferably 80 Ω / cm or less, and most preferably 60 Ω / cm or less. The lower limit of the total resistance value of the positive electrode 30 and the negative electrode 40 is preferably 8 Ω / cm or more, more preferably 10 Ω / cm or more, and most preferably 15 Ω / cm or more. 2 or less, preferably 90 Ω / cm 2 or less, more preferably 80 Ω / cm 2 or less, and most preferably 60 Ω / cm 2 or less. The lower limit of the total resistance value of the positive electrode 30 and the negative electrode 40 is preferably 8 Ω / cm or more, more preferably 10 Ω / cm or more, and most preferably 15 Ω / cm or more. 2 or more, more preferably 10 Ω / cm 2 or more, and most preferably 15 Ω / cm 2 or more.

[0048] In the power storage device of the present disclosure, when the resistance value of the positive electrode 30 is A and the resistance value of the negative electrode 40 is B, it is preferable that B / A (that is, the resistance ratio of the positive and negative electrodes) satisfies 1.0 ≦ B / A ≦ 20.0. From the viewpoint of improving the capacity retention rate, the lower limit value of the resistance ratio of the positive and negative electrodes is preferably 2.0 or more, more preferably 2.5 or more, and still more preferably 3.0 or more. Further, from the viewpoint of maintaining battery performance, the upper limit value of the resistance ratio of the positive and negative electrodes is preferably 15.0 or less, more preferably 10.0 or less, and still more preferably 5.5 or less.

[0049] A power storage device including the positive electrode 30 and the negative electrode 40 having the above resistance values can suppress the precipitation of metallic lithium without degrading the high-rate characteristics and the capacity retention rate. Generally, the larger the thickness of the negative electrode composite layer, the larger the number of lithium ions that can be occluded in the negative electrode, and thus the higher the capacity of the power storage device. However, in a power storage device with too low a resistance value (too high lithium ion conductivity), a large amount of lithium ions rapidly move from the positive electrode to the negative electrode during charging. As a result, the lithium ions do not reach the vicinity of the current collector foil of the negative electrode, and metallic lithium precipitates at the interface with the separator. On the other hand, in the power storage device of the present disclosure, by containing a certain amount of binder, the resistance values of the positive and negative electrodes are adjusted. Since the resistance value of the negative electrode is relatively high, precipitation of metallic lithium at the interface between the separator and the negative electrode can be suppressed and it can be made to evenly spread throughout the negative electrode composite layer. Further, in the power storage device of the present disclosure, the resistance value of the positive electrode is made lower than that of the negative electrode. Thereby, the total resistance value of the entire power storage device is reduced, and the high-rate characteristics can be maintained. Also, the resistance ratio between the negative electrode and the positive electrode is limited. Thereby, the capacity retention rate of the power storage device can be improved.

[0050] Specifically, the negative electrode 40 of the power storage device of the present disclosure includes a thick negative electrode composite material layer 41 with a film thickness of 50 μm or more on one side. Also, the positive electrode 30 also includes a thick positive electrode composite material layer 31 with a film thickness of 40 μm or more on one side. As a result, the energy density of the power storage device increases, and a high-capacity power storage device is obtained. Further, the negative electrode composite material layer 41 of this power storage device has a high negative electrode resistance value by containing 1.0 wt% or more of a binder. Thereby, during charging, it is possible to suppress the precipitation as metallic lithium at the interface with the separator before lithium ions reach the vicinity of the current collector foil of the negative electrode. The content of the binder in the positive electrode composite material layer 31 of this power storage device is suppressed to 5.0 wt% or less. Also, the content of the binder in the negative electrode composite material layer 41 is suppressed to 12.0 wt% or less. As a result, the total resistance value of the positive and negative electrodes decreases, and the high-rate characteristics can be maintained at a high level. Also, the resistance ratio of the positive and negative electrodes is preferably controlled, and the capacity retention rate of the power storage device is improved.

[0051] As described above, the preferred embodiments of the present disclosure have been described based on the drawings, but such descriptions are not limiting matters, and of course, various modifications are possible.

Examples

[0052] Hereinafter, examples and comparative examples will be shown to specifically describe the power storage device of the present disclosure. Note that the present disclosure is not intended to be limited to what is shown in such examples.

[0053] <Example 1: Fabrication of Positive Electrode Sheet> As the positive electrode active material, 97.5 wt% of NCM powder, 1.5 wt% of PVDF as the binder, and 1.0 wt% of CNT as the conductive assistant were mixed to prepare a paste for forming a positive electrode composite layer. Note that N-methyl-2-pyrrolidone (NMP) was used as the solvent. Next, an aluminum foil with a thickness of 13 μm was prepared. The paste for forming the positive electrode composite layer was applied to both sides of the aluminum foil. At this time, the thickness of the aluminum foil and the positive electrode composite layers on both of its sides (i.e., the average film thickness of the positive electrode) was adjusted to be 140 μm and then coated. A comma coater (registered trademark) was used for the coating. After the coating of the positive electrode composite layer, it was air-dried and then heat-dried at 120 °C. Then, leaving the conductive part, it was cut to be 45 cm in length and 35 cm in width to produce a positive electrode sheet.

[0054] <Example 1: Fabrication of a negative electrode sheet> As the negative electrode active material, 96.5 wt% of graphite particles, 3.0 wt% of SBR as the binder, and 0.5 wt% of CMC were mixed to prepare a paste for forming a negative electrode composite layer. Note that ion-exchanged water was used as the solvent. Next, a copper foil with a thickness of 8 μm was prepared. The paste for forming the negative electrode composite layer was applied to both sides of the copper foil. Also, at this time, the thickness of the copper foil and the negative electrode composite layers on both of its sides (i.e., the average film thickness of the negative electrode) was adjusted to be 180 μm and then coated. After the coating of the negative electrode composite layer, it was air-dried and then heat-dried at 120 °C. Then, leaving the conductive part, it was cut to be 50 cm in length and 40 cm in width to produce a negative electrode sheet.

[0055] <Fabrication of a laminated three-electrode cell> In the measurement of the resistance value shown below, a laminated three - electrode cell composed of a positive electrode sheet, a reference electrode (RE), a negative electrode sheet, a separator, and a non - aqueous electrolyte was used. The reference electrode was fabricated using a platinum wire and included a conductive part and a coating part coated with LFP. As the separator, two porous polyolefin sheets made of polyethylene with a length of 53 cm, a width of 43 cm, and a thickness of 16 μm were prepared. The above - mentioned positive electrode sheet, separator, reference electrode, separator, and negative electrode sheet were stacked in this order, and sealed with an aluminum / PP laminated film so that each conductive part was exposed, thereby fabricating a laminated three - electrode cell. The non - aqueous electrolyte used was a solution in which a supporting salt (LiPF6) was dissolved at a concentration of about 1.15 mol / L in an organic mixed solvent (EC:EMC:DMC = 3:3:4). Terminals were connected to the reference electrode and the negative electrode of the laminated three - electrode cell, and preliminary charging was performed at 0.6C until the state of charge (SOC) reached 50%.

[0056] <Measurement of Resistance Value> At 25°C, an alternating current impedance measurement method (frequency range: 10000 Hz to 10 mHz, applied voltage: 30 mV) was performed to measure the resistance value. The resistance values from 1000 Hz to 50 mHz were taken as the positive electrode resistance value / negative electrode resistance value, and each resistance value (Ω / cm 2 ) was calculated by dividing by the area (cm 2 ) of the above - mentioned positive and negative electrode sheets.

[0057] <Fabrication of Laminated Two - Electrode Cell> In the measurement of the capacity retention rate shown below, a laminated two - electrode cell composed of a positive electrode sheet, a negative electrode sheet, a separator, and a non - aqueous electrolyte was used. As the separator, one porous polyolefin sheet made of polyethylene with a length of 53 cm, a width of 43 cm, and a thickness of 16 μm was prepared. The above - mentioned positive electrode sheet, separator, and negative electrode sheet were stacked in this order, and sealed with an aluminum / PP laminated film so that each conductive part was exposed, thereby fabricating a laminated two - electrode cell. The non - aqueous electrolyte used was a solution in which a supporting salt (LiPF6) was dissolved at a concentration of about 1.15 mol / L in an organic mixed solvent (EC:EMC:DMC = 3:3:4).

[0058] <Measurement of Capacity Retention Rate> The above laminate-type bipolar cell was subjected to a soaking heat treatment in a thermostat at 25°C for 1 hour. Charging and discharging were repeated twice between 4.2 V and 3.0 V at a constant current of 0.1C. The discharge capacity obtained in the second charge-discharge cycle was defined as the initial discharge capacity. Next, after charging to 4.2 V at a constant current of 3C, discharging was performed at a constant current of 1C until 3.0 V. The above 3C charging and 1C discharging were defined as one cycle, and this was repeated 100 times. Thereafter, charging and discharging were performed once between 4.2 V and 3.0 V at a constant current of 0.1C, and the obtained discharge capacity was defined as the discharge capacity after cycling. The capacity retention rate (%) was calculated by dividing the discharge capacity after cycling by the initial discharge capacity and multiplying by 100.

[0059] <Examples 2 to 5 and Comparative Examples 1 to 5> As shown in Table 1, it was the same as Example 1 except that the blending ratio of the constituent component (binder) was changed.

[0060]

Table 1

[0061] From the above test results, in Comparative Examples 1 to 4 where the resistance ratio of the positive and negative electrodes was low, the capacity retention rate decreased. On the other hand, in Examples 1 to 5 where the resistance ratio of the positive and negative electrodes was high (that is, the binder content rate of the negative electrode was large), the capacity retention rate was high, and it was found that the content rates of the binders of the positive and negative electrodes could suppress the precipitation of metallic lithium while improving the capacity retention rate. Also, the capacity retention rate of Comparative Example 5 where the resistance ratio of the positive and negative electrodes was extremely high was high. However, in Comparative Example 5, the total resistance of the positive and negative electrodes was also extremely high, and although not shown in detail, the input / output of the power storage device (that is, the high-rate characteristics) decreased significantly, and the battery performance could not be stabilized.

[0062] Although specific examples of the technology disclosed herein have been shown in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

[0063] The technology disclosed herein may omit each component and each process mentioned herein as appropriate, or may be combined as appropriate, as long as no particular problem occurs. Further, this specification includes the disclosures described in the following sections.

[0064] Item 1: A power storage device including a positive electrode having a positive electrode composite material layer, a negative electrode having a negative electrode composite material layer, and a separator interposed between the positive electrode and the negative electrode, wherein the film thickness of the positive electrode composite material layer is 40 μm or more, the film thickness of the negative electrode composite material layer is 50 μm or more, the binder content in the positive electrode composite material layer when the solid content weight of the entire positive electrode composite material layer is 100 wt% is 0.1 wt% to 5.0 wt% or less, and the binder content in the negative electrode composite material layer when the solid content weight of the entire negative electrode composite material layer is 100 wt% is 1.0 wt% to 12.0 wt% or less.

[0065] Item 2: The resistance value (A) of the positive electrode is 1 Ω / cm 2 ~30 Ω / cm 2 or less, the resistance value (B) of the negative electrode is 3 Ω / cm 2 ~90 Ω / cm 2 or less, and the power storage device according to Item 1 satisfies the following formula (1); 2 ≦ B / A ≦ 10.

[0066] Item 3: The binder contained in the positive electrode composite material layer is PVDF, and the binder contained in the negative electrode composite material layer is SBR and / or CMC. The power storage device according to Item 1 or 2.

Explanation of Reference Numerals

[0067] 1 Lithium-ion secondary battery 10 Battery case 11 Battery case body 12 Sealing plate 13 Safety valve 14 Positive electrode external terminal 15 Negative electrode external terminal 16 Positive electrode internal terminal 17 Negative electrode internal terminal 20 Electrode body 30 Positive electrode 31 Positive electrode composite material layer Non-formed portion of the positive electrode composite material layer Positive current collector foil 32 Positive active material 35 Negative electrode 40 Negative electrode composite material layer 41 Non-formed portion of the negative electrode composite material layer Negative current collector foil 42 Negative active material 45 Separator 50 Binder 60

Claims

1. A power storage device comprising a positive electrode having a positive electrode composite material layer, a negative electrode having a negative electrode composite material layer, and a separator interposed between the positive electrode and the negative electrode, The film thickness of the positive electrode composite material layer is 40 μm or more, The film thickness of the negative electrode composite material layer is 50 μm or more, and When the solid content weight of the entire positive electrode composite material layer is 100 wt%, the binder content in the positive electrode composite material layer is 0.1 wt% or more and 5.0 wt% or less, A power storage device in which, when the solid content weight of the entire negative electrode composite material layer is 100 wt%, the binder content in the negative electrode composite material layer is 1.0 wt% or more and 12.0 wt% or less.

2. The resistance value (A) of the positive electrode is 1 Ω / cm ２ or more and 30 Ω / cm ２ or less, and The resistance value (B) of the negative electrode is 3 Ω / cm ２ or more and 90 Ω / cm ２ or less, and The following formula (1); 1 ≤ B / A ≤ 20 The power storage device according to claim 1, which satisfies the above.

3. The binder contained in the positive electrode composite material layer is PVDF, The power storage device according to claim 1 or 2, wherein the binder contained in the negative electrode composite material layer is SBR and / or CMC.

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