Battery module and method for estimating the capacity of the battery module

Abstract

This invention improves the accuracy of estimating the capacity of secondary batteries in battery modules where multiple secondary batteries containing olivine-type structural active materials are stacked in series. [Solution] The battery module 10 has multiple secondary batteries 12 stacked in series. Each secondary battery 12 has a positive electrode 13 on the first surface 15 of a positive electrode current collector 14, which has a positive electrode active material layer 17 containing an olivine-type structural active material, and a negative electrode 19 on the first surface 21 of a negative electrode current collector 20, which has a negative electrode active material layer 24 arranged to face the positive electrode active material layer 17 of the positive electrode 13. When multiple secondary batteries 12 of the battery module 10 are simultaneously charged at a charge rate of 0.1C, the relationship between the cumulative charge capacity of the secondary batteries 12 and the voltage of the secondary batteries 12 satisfies the first condition. The first condition is the first ratio ((V 90 -V 70 ) / V 70 ) is 0.003 or higher.

Description

[Technical Field]

[0001] The present invention relates to a battery module and a method for estimating the capacity of a battery module. [Background technology]

[0002] Patent Document 1 discloses a flat-type battery module constructed by stacking multiple secondary batteries in series. The secondary battery comprises a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode. The positive electrode has a positive electrode active material layer formed on one side of a foil-shaped positive electrode current collector. The negative electrode has a negative electrode active material layer formed on one side of a foil-shaped negative electrode current collector, and the negative electrode active material layer is arranged to face the positive electrode active material layer of the positive electrode. The battery module disclosed in Patent Document 1 is formed by stacking multiple secondary batteries such that the positive electrode current collectors and negative electrode current collectors are in contact with each other.

[0003] Patent Document 2 discloses a battery module equipped with a positive electrode using olivine-type lithium iron phosphate (LiFePO4) as the positive electrode active material. Olivine-type structural active materials, such as olivine-type lithium iron phosphate, are positive electrode active materials with excellent thermal stability. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2017-16825 [Patent Document 2] Japanese Patent Publication No. 2019-185920 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] In the case of battery modules containing multiple rechargeable batteries, an inspection is performed to evaluate the capacity of each individual rechargeable battery before shipment. In this inspection, for example, all rechargeable batteries in the battery module are charged simultaneously. Then, for each rechargeable battery, the cumulative charge capacity of the rechargeable battery is estimated to evaluate the variability in the capacity of the rechargeable batteries. Battery modules that use olivine-type structural active materials such as olivine-type lithium iron phosphate as the positive electrode active material are difficult to estimate the capacity of with high accuracy. [Means for solving the problem]

[0006] An embodiment of a battery module that solves the above problems is described below. [Aspect 1] A battery module comprising multiple secondary batteries stacked in series, wherein each secondary battery comprises a positive electrode having a positive electrode active material layer containing an olivine-type structural active material and having a thickness of 200 μm or more on the first surface of the positive electrode current collector, and a negative electrode having a negative electrode active material layer arranged on the first surface of the negative electrode current collector so as to face the positive electrode active material layer of the positive electrode, wherein when the multiple secondary batteries of the battery module are simultaneously charged at a charge rate of 0.1C, the relationship between the cumulative charge capacity of the secondary batteries and the voltage of the secondary batteries satisfies a first condition, the first condition being that the capacity of the secondary battery at its design full charge is the design capacity C max The design capacity C max The voltage of the secondary battery at the point when the cumulative charge capacity is 70% and 90% of the total charge capacity is given by the voltage V. 70 , voltage V 90 The first ratio in this case ((V 90 -V 70 ) / V 70 A battery module in which the coefficient of measurement is 0.003 or higher.

[0007] [Aspect 2] When multiple secondary batteries in the battery module are simultaneously charged at a charge rate of 0.1C, the relationship between the cumulative charge capacity of the secondary batteries and the voltage of the secondary batteries satisfies the second condition, and the second condition is that the design capacity C maxThe voltages of the secondary battery at the integrated charge capacities of 90%, 95%, and 99.5% are respectively voltage V 90 , voltage V 95 , and voltage V 99.5 . When the second ratio ((V 95 - V 90 ) / (V 99.5 - V 90 [[ID=1 = 14]])) is 0.08 or more and 0.6 or less, the battery module according to aspect 1.

[0008] [Aspect 3] The area of the surface of the negative electrode active material layer facing the positive electrode active material layer is larger than the area of the surface of the positive electrode active material layer facing the negative electrode active material layer. The battery module according to aspect 1 or aspect 2.

[0009] [Aspect 4] The olivine - type structure active material is a polyanion - based compound having an olivine - type structure represented by the general formula LiM h PO4 (M is at least one element selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, Mo, and h is a numerical value satisfying 0 < h < 2). The battery module according to any one of aspects 1 to 3.

[0010] [Aspect 5] The negative electrode active material layer contains carbon. The battery module according to any one of aspects 1 to 4. [Aspect 6] A battery module in which multiple secondary batteries are stacked in series, and a capacity estimation method for estimating the capacity of the secondary batteries, wherein each of the secondary batteries comprises a positive electrode having a positive electrode active material layer containing an olivine-type structural active material and having a thickness of 200 μm or more on the first surface of the positive electrode current collector, and a negative electrode having a negative electrode active material layer arranged on the first surface of the negative electrode current collector so as to face the positive electrode active material layer of the positive electrode, and a charging step in which the multiple secondary batteries of the battery module are simultaneously charged at a charging rate of 0.05C or more while individually measuring the voltage of each secondary battery until any one of the secondary batteries reaches an arbitrarily set upper limit voltage, and an estimation step in which, after the charging step, the capacity of the secondary batteries that have not reached the upper limit voltage is estimated based on the detected voltage of the secondary battery and information obtained in advance showing the relationship between the cumulative charge capacity and voltage of the secondary battery. [Effects of the Invention]

[0011] According to the present invention, the accuracy of estimating the capacity of secondary batteries is improved in a battery module in which multiple secondary batteries containing an olivine-type structural active material are stacked in series. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a cross-sectional view of a battery module. [Figure 2] Figure 2 is an explanatory diagram of the positive electrode active material layer and the negative electrode active material layer in a secondary battery. [Figure 3] Figure 3 is a graph showing the charging curve of a secondary battery. [Figure 4] Figure 4 is a circuit diagram showing a battery module and a secondary battery testing device. [Figure 5] Figure 5 is a graph showing the charging curve of the secondary battery in Example 1. [Figure 6] Figure 6 is a graph showing the charging curve of the secondary battery of Comparative Example 1. [Modes for carrying out the invention]

[0013] An embodiment of the battery module of the present invention will be described below. <Battery Module> As shown in Figure 1, the battery module 10 comprises a stack 11. The stack 11 comprises a plurality of secondary batteries 12 connected in series. The plurality of secondary batteries 12 are stacked. The number of stacked secondary batteries 12 is, for example, 20 or more, or 25 or more. The number of stacked secondary batteries 12 is, for example, 70 or less. In the following description, the direction in which the secondary batteries 12 are stacked is referred to as the stacking direction. In addition, viewing in the stacking direction may be described as a plan view.

[0014] Each of the secondary batteries 12 comprises a positive electrode 13, a negative electrode 19, and a separator 27. The positive electrode 13 comprises a positive electrode current collector 14 and a positive electrode active material layer 17. The positive electrode current collector 14 is in the form of a sheet. The positive electrode current collector 14 comprises a first surface 15 and a second surface 16. The first surface 15 and the second surface 16 are opposite to each other in the thickness direction of the positive electrode current collector 14. The thickness of the positive electrode current collector 14 is, for example, 5 μm or more, preferably 10 μm or more. The thickness of the foil-shaped positive electrode current collector 14 is, for example, 100 μm or less, preferably 60 μm or less.

[0015] An example of a positive electrode current collector 14 is an aluminum current collector in which the surface forming the first surface 15 is made of aluminum. The aluminum current collector may be a single object made entirely of aluminum, or it may be a composite having a part made of aluminum and a part made of a material other than aluminum. An example of the single object is aluminum foil. An example of the composite is a multilayer structure in which the layer forming the first surface 15 is an aluminum layer, or a substrate in which the surface including the first surface 15 is coated with an aluminum film.

[0016] The positive electrode current collector 14 may be formed of a material other than aluminum. Examples of materials other than aluminum include metal materials, conductive resin materials, and conductive inorganic materials. Examples of the metal materials include copper, nickel, titanium, and stainless steel (e.g., SUS304, SUS316, SUS301, SUS304, etc. defined in JIS G 4305:2015). Examples of the conductive resin materials include resin obtained by adding a conductive filler to a conductive polymer material or a non-conductive polymer material as needed.

[0017] The first surface 15 of the positive electrode current collector 14 may be coated with a known protective layer such as a carbon coating layer. The first surface 15 of the positive electrode current collector 14 may be treated by a known method such as plating.

[0018] The positive electrode active material layer 17 is provided on the first surface 15 of the positive electrode current collector 14. The positive electrode active material layer 17 contains a positive electrode active material capable of occluding and releasing lithium ions as charge carriers. The positive electrode active material includes an olivine-type structure active material. The olivine-type structure positive electrode active material is a polyanion-based compound having an olivine-type structure represented by the general formula LiM h PO4. In the general formula LiM h PO4, M is at least one element selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, Mo, and h is a numerical value satisfying 0 < h < 2. Examples of the olivine-type structure positive electrode active material include olivine-type lithium iron phosphate (LiFePO4) and olivine-type lithium manganese iron phosphate (LiMnFePO4). These olivine-type structure positive electrode active materials can be used alone or in combination of two or more. The olivine-type structure positive electrode active material may be coated with a carbon-based material on the particle surface to improve the electric conductivity.

[0019] The positive electrode active material may be a combination of an olivine-type structural active material and other positive electrode active materials other than the olivine-type structural active material. In this case, the mass ratio of the olivine-type structural active material to the total positive electrode active material is, for example, 90% or more, preferably 95% or more, and more preferably 98% or more. The above mass ratio is, for example, 99% or less. The other positive electrode active material is not particularly limited and any material usable as a positive electrode active material for a lithium-ion secondary battery is used. The other positive electrode active material can be used alone or in combination of two or more types. In this embodiment, all positive electrode active materials are olivine-type lithium iron phosphate (LiFePO4).

[0020] The positive electrode active material layer 17 may contain other components besides the positive electrode active material as needed. Examples of other components include binders, dispersants, conductive additives, electrolytes (polymer matrix, ion-conducting polymer, electrolyte solution, etc.), and electrolyte support salts (lithium salts) to enhance ion conductivity. The types and contents of other components are not particularly limited, and conventionally known knowledge regarding lithium-ion secondary batteries may be referenced as appropriate. The content ratio of the positive electrode active material in the positive electrode active material layer 17 is, for example, 97% by mass or more and 99% by mass or less.

[0021] The negative electrode 19 comprises a negative electrode current collector 20 and a negative electrode active material layer 24. The negative electrode current collector 20 is in the form of a sheet. The negative electrode current collector 20 comprises a first surface 21 and a second surface 22. The first surface 21 and the second surface 22 are opposite to each other in the thickness direction of the negative electrode current collector 20. The thickness of the negative electrode current collector 20 is, for example, 4 μm or more, preferably 6 μm or more. The thickness of the negative electrode current collector 20 is, for example, 20 μm or less, preferably 10 μm or less.

[0022] An example of a negative electrode current collector 20 is a copper current collector in which the surface forming the first surface 21 is made of copper. The copper current collector may be a single object made entirely of copper, or it may be a composite having a part made of copper and a part made of a material other than copper. An example of the single object is copper foil. An example of the composite is a multilayer structure in which the layer forming the first surface 21 is a copper layer, or a substrate in which the surface including the first surface 21 is coated with a copper film.

[0023] The negative electrode current collector 20 may be made of a material other than copper. Examples of materials other than copper include metallic materials, conductive resin materials, and conductive inorganic materials. Examples of metallic materials include aluminum, nickel, titanium, and stainless steel (e.g., SUS304, SUS316, SUS301, SUS304, etc. as specified in JIS G 4305:2015). Examples of conductive resin materials include conductive polymer materials or resins to which conductive fillers are added as needed.

[0024] The negative electrode active material layer 24 is provided on the first surface 21 of the negative electrode current collector 20. The negative electrode active material layer 24 contains a negative electrode active material capable of intercepting and releasing charge carriers such as lithium ions. The negative electrode active material contains carbon. Examples of carbon include natural graphite, artificial graphite, hard carbon (difficult-to-graphitize carbon), and soft carbon (easily graphitized carbon). Examples of artificial graphite include highly oriented graphite and mesocarbon microbeads. These carbons can be used alone or in combination of two or more types.

[0025] The negative electrode active material may be a combination of carbon and other negative electrode active materials. In this case, the mass percentage of carbon in the total negative electrode active material is, for example, 90% or more, preferably 95% or more, and more preferably 98% or more. The above mass percentage is, for example, 99% or less. The other negative electrode active materials are not particularly limited and any materials usable as negative electrode active materials for lithium-ion secondary batteries can be used. Examples of other negative electrode active materials include silicon and tin. The other negative electrode active materials can be used alone or in combination of two or more types. In this embodiment, all negative electrode active materials are graphite.

[0026] The negative electrode active material layer 24 may contain other components besides the negative electrode active material as needed. Examples of other components include binders, dispersants, conductive additives, electrolytes (polymer matrix, ion-conducting polymer, electrolyte solution, etc.), and electrolyte support salts (lithium salts) to enhance ion conductivity. The types and contents of other components are not particularly limited, and conventionally known knowledge regarding lithium-ion secondary batteries may be referenced as appropriate. The content ratio of the negative electrode active material in the negative electrode active material layer 24 is, for example, 95% by mass or more and 98% by mass or less.

[0027] The laminate 11 includes a bipolar electrode 25. The bipolar electrode 25 comprises a current collector 26, a positive electrode active material layer 17 provided on one side of the current collector 26, and a negative electrode active material layer 24 provided on the other side of the current collector 26. The current collector 26 is formed by integrating a positive electrode current collector 14 and a negative electrode current collector 20 that are in contact with each other. For example, in the bipolar electrode 25, the current collector 26 is formed by joining the positive electrode current collector 14 and the negative electrode current collector 20 with their second surfaces 16 and 22, respectively, superimposed. The bipolar electrode 25 is formed by considering the positive electrode current collector 14 and the negative electrode current collector 20, which are in contact with each other, as a single current collector 26.

[0028] Of the positive electrodes 13, the positive electrode 13 provided at one end of the laminate 11 is designated as the positive electrode terminal electrode 13A. Of the negative electrodes 19, the negative electrode 19 provided at the other end of the laminate 11 is designated as the negative electrode terminal electrode 19A. The positive electrode terminal electrode 13A and the negative electrode terminal electrode 19A sandwich a plurality of bipolar electrodes 25 from the stacking direction. The second surface 16 of the positive electrode current collector 14 of the positive electrode terminal electrode 13A constitutes the outer surface of the laminate 11. The second surface 22 of the negative electrode current collector 20 of the negative electrode terminal electrode 19A constitutes the outer surface of the laminate 11. The secondary battery 12 provided at one end of the laminate 11 comprises the positive electrode terminal electrode 13A and the negative electrode active material layer 24 of the bipolar electrode 25 adjacent to the positive electrode terminal electrode 13A. The secondary battery 12 provided at the other end of the laminate 11 comprises a negative terminal electrode 19A and a positive electrode active material layer 17 of a bipolar electrode 25 adjacent to the negative terminal electrode 19A. The secondary battery 12 provided between the secondary battery 12 at one end of the laminate 11 and the secondary battery 12 at the other end of the laminate 11 comprises a positive electrode active material layer 17 and a negative electrode active material layer 24 of a bipolar electrode 25 adjacent to each other.

[0029] The battery module 10 includes a positive electrode conductive plate 31 and a negative electrode conductive plate 32. The positive electrode conductive plate 31 and the negative electrode conductive plate 32 are made of a metal material such as aluminum, copper, or stainless steel.

[0030] The positive electrode current-carrying plate 31 is electrically connected to the second surface 16 of the positive electrode current collector 14 provided on the positive electrode terminal electrode 13A. The negative electrode current-carrying plate 32 is electrically connected to the second surface 22 of the negative electrode current collector 20 provided on the negative electrode terminal electrode 19A. The battery module 10 performs charging and discharging through the positive electrode terminal 31a provided on the positive electrode current-carrying plate 31 and the negative electrode terminal 32a provided on the negative electrode current-carrying plate 32.

[0031] The battery module 10 includes a encapsulant 40. The encapsulant 40 is rectangular in shape. The encapsulant 40 comprises a first portion 41 and a second portion 42. The first portion 41 is located between the current collectors 26, between the positive electrode current collector 14 of the positive terminal electrode 13A and the current collector 26 of the bipolar electrode 25 adjacent to the positive terminal electrode 13A, and between the negative electrode current collector 20 of the negative terminal electrode 19A and the current collector 26 of the bipolar electrode 25 adjacent to the negative terminal electrode 19A. The second portion 42 is located outside the periphery of the current collectors 26, the positive electrode current collector 14 of the positive terminal electrode 13A, and the negative electrode current collector 20 of the negative terminal electrode 19A. The encapsulant 40 encapsulates the laminate 11.

[0032] Inside the secondary battery 12, a space is formed surrounded by a seal 40, a positive electrode 13, and a negative electrode 19. A separator 27 and an electrolyte are housed in this space. The peripheral portion of the separator 27 is embedded in the seal 40.

[0033] The separator 27 is positioned between the positive electrode active material layer 17 and the negative electrode active material layer 24. The separator 27 prevents short circuits caused by contact between the two electrodes by isolating the positive electrode active material layer 17 and the negative electrode active material layer 24, while allowing charge carriers such as lithium ions to pass through.

[0034] The electrolyte is, for example, a liquid electrolyte (electrolyte solution). Examples of liquid electrolytes include liquid electrolytes containing a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Known lithium salts such as LiClO4, LiAsF6, LiPF6, LiBF4, LiCF3SO3, LiN(FSO2)2, and LiN(CF3SO2)2 can be used as the electrolyte salt. Known solvents such as cyclic carbonates, cyclic esters, linear carbonates, linear esters, and ethers can be used as the non-aqueous solvent. Two or more of these known solvent materials may be used in combination.

[0035] The electrolyte is not limited to a liquid electrolyte, but may also be a solid electrolyte such as a polymer gel electrolyte containing an electrolyte held in a polymer matrix. Furthermore, the separator 27 itself may be composed of an electrolyte such as a polymer electrolyte or an inorganic electrolyte.

[0036] The battery module 10 is equipped with voltage detection wires 51. The voltage detection wires 51 are, for example, long foil-like wires. One voltage detection wire 51 is provided on each of the current collectors 26 of the bipolar electrode 25, the positive electrode current collector 14 of the positive electrode terminal electrode 13A, and the negative electrode current collector 20 of the negative electrode terminal electrode 19A. For example, the voltage detection wires 51 are joined to each of the current collectors 26 of the bipolar electrode 25, the positive electrode current collector 14 of the positive electrode terminal electrode 13A, and the negative electrode current collector 20 of the negative electrode terminal electrode 19A by ultrasonic welding. This makes it possible to detect the voltage of the secondary battery 12 by two voltage detection wires 51 that are adjacent to each other in the stacking direction.

[0037] A portion of the voltage detection wire 51 is covered by the encapsulant 40. The voltage detection wire 51 extends outward from the encapsulant 40. A portion of the voltage detection wire 51 is exposed to the outside of the encapsulant 40. That is, the voltage detection wire 51 is brought out to the outside of the encapsulant 40 so that the voltage of the secondary battery 12 can be detected outside the encapsulant 40.

[0038] [Details on the positive electrode active material layer and negative electrode active material layer] Figure 2 is a schematic side view illustrating only the positive electrode 13 and negative electrode 19 of a single secondary battery 12.

[0039] The positive electrode 13 and negative electrode 19 constituting a single secondary battery 12 are arranged such that the positive electrode active material layer 17 and the negative electrode active material layer 24 face each other in the stacking direction. The opposing directions of the positive electrode 13 and the negative electrode 19 coincide with the stacking direction. The positive electrode active material layer 17 has a positive electrode opposing surface 17A which faces the negative electrode active material layer 24. The negative electrode active material layer 24 has a negative electrode opposing surface 24A which faces the positive electrode active material layer 17.

[0040] The planar shape of the positive electrode active material layer 17 in a plan view from the stacking direction is, for example, rectangular. The planar shape of the positive electrode active material layer 17 may also be a polygon, a circle, or other shape. The planar shape of the negative electrode active material layer 24 in the above planar view is, for example, rectangular. The planar shape of the negative electrode active material layer 24 may also be a polygon, a circle, or other shape.

[0041] The thickness of the positive electrode active material layer 17 is 200 μm or more. The thickness of the positive electrode active material layer 17 is preferably 240 μm or more, and more preferably 270 μm or more. The thickness of the positive electrode active material layer 17 is, for example, 600 μm or less, preferably 530 μm or less, and more preferably 470 μm or less.

[0042] The area of ​​the positive electrode opposing surface 17A of the positive electrode active material layer 17 is, for example, 200 m². 2 The above, preferably 300m 2 The above, and more preferably 400m 2 That's all. The area of ​​the positive electrode opposing surface 17A is, for example, 20,000 m². 2 The following, preferably 18,000 m 2 The following applies:

[0043] The basis weight of the positive electrode active material layer 17 is, for example, 50 mg / cm³. 2 The above is preferable to 60 mg / cm³. 2 The above is preferable to 70 mg / cm³. 2 That concludes the explanation. The basis weight of the positive electrode active material layer 17 is, for example, 90 mg / cm³. 2 The following, preferably 80 mg / cm³ 2 The following applies:

[0044] The density of the positive electrode active material layer 17 is, for example, 1.5 g / cm³. 3 The above is preferable, preferably 1.7 g / cm³. 3 The above is more preferable, and more preferably 1.8 g / cm³ 3 That concludes the explanation. The density of the positive electrode active material layer 17 is, for example, 2.3 g / cm³. 3 The following, preferably 2.2 g / cm³ 3 The following applies:

[0045] The thickness of the negative electrode active material layer 24 is, for example, 120 μm or more, preferably 140 μm or more. The thickness of the negative electrode active material layer 24 is, for example, 450 μm or less, preferably 360 μm or less.

[0046] The basis weight of the negative electrode active material layer 24 is, for example, 20 mg / cm³. 2 The above is preferable, with a concentration of 30 mg / cm³. 2 That concludes the explanation. The basis weight of the negative electrode active material layer 24 is, for example, 50 mg / cm³. 2 The following is preferable: 40 mg / cm³ 2 The following applies:

[0047] The density of the negative electrode active material layer 24 is, for example, 1.1 g / cm³. 3 The above is preferable, preferably 1.2 g / cm³. 3 That concludes the explanation. The density of the negative electrode active material layer 24 is, for example, 1.6 g / cm³. 3 The following is preferred, preferably 1.4 g / cm³ 3 The following applies:

[0048] The area of ​​the negative electrode facing surface 24A of the negative electrode active material layer 24 is, for example, 200 m². 2 The above, preferably 300m 2 That concludes the explanation. The area of ​​the negative electrode facing surface 24A is, for example, 26,000 m². 2 The following, preferably 23,000 m 2 The following applies:

[0049] Preferably, the area of ​​the negative electrode facing surface 24A of the negative electrode active material layer 24 is larger than the area of ​​the positive electrode facing surface 17A of the positive electrode active material layer 17. For example, the ratio of the area of ​​the negative electrode facing surface 24A to the area of ​​the positive electrode facing surface 17A is 1.05 or more and 1.3 or less. In this case, when the area of ​​the negative electrode facing surface 24A is larger than the area of ​​the positive electrode facing surface 17A, a protruding portion 24A1 is formed on the outer periphery of the negative electrode facing surface 24A of the negative electrode active material layer 24, which protrudes outward from the outer edge of the positive electrode facing surface 17A of the positive electrode active material layer 17 in the above plan view.

[0050] The protruding portion 24A1 is preferably formed over the entire circumferential surface 24A of the negative electrode active material layer 24. In other words, the negative electrode active material layer 24 is preferably formed to be slightly larger than the positive electrode active material layer 17, and in a plan view, it is preferable that the entire formation region of the positive electrode active material layer 17 is located within the formation region of the negative electrode active material layer 24.

[0051] The protruding length of the protruding portion 24A1 is, for example, 1 mm or more, preferably 5 mm or more. The protruding length is, for example, 20 mm or less. [Charging characteristics of secondary batteries] Each of the secondary batteries 12 in the battery module 10 has charging characteristics that satisfy the following first condition regarding the relationship between the cumulative charge capacity and voltage during charging. In addition, each of the secondary batteries 12 in the battery module 10 has charging characteristics that satisfy the following second condition in addition to the first condition. The second condition is optional, and the charging characteristics of the secondary batteries 12 do not have to satisfy the second condition. Note that the voltage of the secondary batteries 12 described in the explanation of charging characteristics refers to CCV (Closed Circuit Voltage).

[0052] The first and second conditions will be explained with reference to Figure 3. The graph in Figure 3 is a charging curve that shows the relationship between the cumulative charging capacity and voltage of the secondary batteries 12 when multiple secondary batteries 12 of the battery module 10 are simultaneously charged at a charging rate of 0.1C.

[0053] As shown in Figure 3, the charging curve of secondary battery 12, in which an olivine-type structural active material is used as the positive electrode active material, shows a flat region R1 at the end of the charging period, when it is close to full charge, where the change in voltage is significantly smaller than the change in cumulative charge capacity. In addition, after the flat region R1 at the end of the charging period, a steep region R2 appears where the change in voltage is significantly larger than the change in cumulative charge capacity. For reference, Figure 6 shows a graph. The graph in Figure 6 is a charging curve representing the relationship between cumulative charge capacity and voltage of a secondary battery of Comparative Example 1, which is described later and corresponds to a conventional structure, in which an olivine-type structural active material is used as the positive electrode active material. In the graph in Figure 6, the flat region R1 and the steep region R2 are clearly visible.

[0054] The first condition is a condition that defines the charging characteristics in the flat region R1. Regarding the first condition, the design capacity C of the secondary battery 12 when fully charged is defined as the design capacity C. max This is defined as: Design capacity C max This is a value that can be determined from the configuration of the secondary battery 12, such as the type of active material and the content of the active material. And the design capacity C max The cumulative charging capacity that is 70% and 90% of the capacity is C 70 , capacity C 90 This is defined as follows: In the charging curve of Figure 3, the capacity C 70 , capacity C 90 The voltages of the secondary battery 12 at that time are, respectively, voltage V 70 , voltage V 90 This is defined as follows: Capacity C 70 and capacity C 90 This is the capacity within the range of the flat region R1.

[0055] The first condition is the first ratio ((V 90 -V 70 ) / V 70 The first ratio is preferably 0.005 or more, and more preferably 0.007 or more. The first ratio is, for example, 0.3 or less. Voltage difference (V 90 -V 70 ) has a capacity of C 70 From capacity C 90This refers to the voltage increase during charging, i.e., the voltage increase in the flat region R1. Therefore, satisfying the first condition means that the secondary battery 12 has a capacity C 70 From capacity C 90 While it is being charged to the voltage V 70 This means that a voltage increase of a certain percentage or more relative to the reference point has occurred. In other words, satisfying the first condition means that the flat region R1 has a gradual upward curve.

[0056] As an example, the positive electrode active material of secondary battery 12 is olivine-type lithium iron phosphate (LiFePO4), and the negative electrode active material is carbon such as graphite, and the design capacity C max Let's assume that the capacity is between 3800mAh and 4000mAh. In this case, the design capacity C max The resulting cumulative charging capacity C 100 The voltage V in this case 100 For example, the voltage range is between 3.6V and 3.8V, and one example value is 3.75V.

[0057] The second condition is the defining condition for the charging characteristics in the steep region R2. Regarding the second condition, design capacity C max The cumulative charging capacity that is 90%, 95%, and 99.5% of the capacity is given by Capacity C. 90 , capacity C 95 , capacity C 99.5 This is defined as follows: Capacity C 95 and capacity C 99.5 This is the capacity within the steep region R2.

[0058] The second condition is the second ratio ((V 95 -V 90 ) / (V 99.5 -V 90 The ratio is 0.08 or more and 0.6 or less. The second ratio is preferably 0.09 or more, and more preferably 0.1 or more. The second ratio is preferably 0.4 or less, and more preferably 0.3 or less.

[0059] Voltage difference (V 99.5 -V 90 ) has a capacity of C 90from capacitance C 99.5 means the amount of voltage rise during charging from this point until capacitance C 95 -V 90 ), that is, the amount of voltage rise from the end of the flat region R1 to just before the end of the steep region R2. The voltage difference (V 90 from capacitance C 95 means the amount of voltage rise during charging until capacitance C 90 ), that is, the amount of voltage rise from the end of the flat region R1 to the beginning of the steep region R2. Therefore, the second ratio is the ratio of the amount of voltage rise from voltage V 99.5 to voltage V 90 in the amount of voltage rise from voltage V 95 to voltage V

[0060] When the second ratio is 0.08 or more, it means that in the charging curve, in the range from voltage V 90 to voltage V 99.5 , the voltage also rises by a certain ratio or more based on the capacitance difference (V 99.5 -V 90 ). When the second ratio is 0.6 or less, it means that in the charging curve, the degree of voltage rise in the steep region R2 in the range from voltage V 90 to voltage V 99.5 is small, that is, less than a certain ratio based on the capacitance difference (V 99.5 -V 90 ). Therefore, satisfying the second condition means that in the steep region R2 of the charging curve, the voltage does not rise sharply in either the range before voltage V 95 or the range after voltage V 95 , but the voltage rises in both ranges. Therefore, when the second condition is satisfied, it can be said that the rise of the steep region R2 is gentle.

[0061] As shown in Figure 2, the charging characteristics of the secondary battery 12 can be adjusted by increasing the thickness of the positive electrode active material layer 17. During charging, charge carriers such as lithium ions are released from the positive electrode-facing surface 17A of the positive electrode active material layer 17 and absorbed into the negative electrode active material layer 24. At this time, if the positive electrode active material layer 17 is thick, charge carriers such as lithium ions are more easily released from the surface portion 17B of the positive electrode active material layer 17, which is close to the positive electrode-facing surface 17A, while charge carriers are less easily released from the deep portion 17C, which is farther from the positive electrode-facing surface 17A, because the distance to the positive electrode-facing surface 17A is longer. As a result, unevenness in the charging depth occurs between the surface portion 17B and the deep portion 17C of the positive electrode active material layer 17.

[0062] By increasing the unevenness in the depth of charge in the positive electrode active material layer 17, the charging characteristics of the secondary battery 12 can be adjusted in a direction that satisfies the first and second conditions. In this embodiment, the thickness of the positive electrode active material layer 17 is set to 200 μm or more in order to create unevenness in the depth of charge in the positive electrode active material layer 17. The unevenness in the depth of charge caused by the increased thickness of the positive electrode active material layer 17 becomes particularly noticeable when the secondary battery 12 is charged at a high charge rate of 0.1C or higher.

[0063] Furthermore, the charging characteristics of the secondary battery 12 can also be adjusted by making the negative electrode facing surface 24A of the negative electrode active material layer 24 larger than the positive electrode facing surface 17A of the positive electrode active material layer 17. As described above, during charging, the charge carrier is released from the positive electrode facing surface 17A of the positive electrode active material layer 17 and absorbed into the negative electrode active material layer 24. More specifically, the charge carrier released from the positive electrode facing surface 17A is absorbed into the negative electrode active material layer 24 from the negative electrode facing surface 24A.

[0064] Here, if the negative electrode facing surface 24A is larger than the positive electrode facing surface 17A, a protruding portion 24A1 is formed on the negative electrode facing surface 24A that protrudes outward from the outer edge of the positive electrode facing surface 17A. In this case, the charge carrier released from the central portion 17D of the positive electrode facing surface 17A is absorbed into the central portion 24D, which is the opposing portion of the negative electrode facing surface 24A. The charge carrier released from the outer peripheral portion 17E of the positive electrode facing surface 17A is absorbed into both the outer peripheral portion 24E and the protruding portion 24A1, which are the opposing portions of the negative electrode facing surface 24A. As a result, the outer peripheral portion 17E of the positive electrode facing surface 17A has a wider range for accepting charge carriers on the negative electrode facing surface 24A compared to the central portion 17D of the positive electrode facing surface 17A. Therefore, the release of charge carriers is promoted in the outer peripheral portion 17E of the positive electrode active material layer 17 than in the central portion 17D. As a result, unevenness in the charging depth occurs between the central and outer portions of the positive electrode active material layer 17 as a whole.

[0065] The charging characteristics of the secondary battery 12 can also be adjusted in a direction that satisfies the first and second conditions by increasing the unevenness in the charging depth between the central and outer portions of the positive electrode active material layer 17. Furthermore, the unevenness in the charging depth caused by increasing the size of the negative electrode facing surface 24A of the negative electrode active material layer 24 becomes more pronounced when the secondary battery 12 is charged at a high charging rate of 0.05C or higher, preferably 0.1C or higher.

[0066] <Manufacturing method for battery modules> Next, an example of a manufacturing method for the battery module 10 will be described. The manufacturing method for the battery module 10 includes an assembly step of assembling the battery module 10 and an inspection step of evaluating the capacity of the multiple secondary batteries 12 in the battery module 10.

[0067] The assembly process is not particularly limited, and conventionally known knowledge regarding lithium-ion secondary batteries can be referenced as appropriate. The inspection process is a process of evaluating the variation in capacity of each secondary battery 12 that makes up the battery module 10. The inspection process is, for example, a pre-shipment inspection of the assembled battery module 10, and is performed to evaluate the performance of the battery module 10 and to check for any defects.

[0068] Referring to Figure 4, the inspection apparatus 60 for performing the above inspection process will be described. The inspection device 60 includes a capacity estimation device 61. The capacity estimation device 61 includes a voltage detection circuit 62. The voltage detection circuit 62 is, for example, an integrated circuit. The voltage detection circuit 62 has multiple ports, and a voltage detection line 51 is connected to each port. The voltage detection circuit 62 detects the voltage of each secondary battery 12 from the potential input from the voltage detection line 51.

[0069] The capacity estimation device 61 includes an estimation device 63. The estimation device 63 is a device that estimates the capacity of each secondary battery 12 based on the detected voltage value of each secondary battery 12. The estimation device 63 includes a processor 64 and a storage unit 65. The processor 64 is, for example, a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), or a DSP (Digital Signal Processor). The storage unit 65 includes RAM (Random Access Memory) and ROM (Read Only Memory). The storage unit 65 stores a program for performing capacity testing and master data. The estimation device 63 estimates the capacity of each secondary battery 12 based on the detected voltage value of each secondary battery 12 and the master data.

[0070] The memory unit 65 stores program code or instructions configured to cause the processor 64 to perform processing. The memory unit 65, i.e., the computer-readable medium, includes any available medium accessible by a general-purpose or dedicated computer. The estimation device 63 may be composed of hardware circuits such as an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array). The estimation device 63, which is a processing circuit, may include one or more processors operating according to a computer program, one or more hardware circuits such as ASICs or FPGAs, or a combination thereof.

[0071] The inspection device 60 includes a charging device 71. The charging device 71 charges each secondary battery 12 in the battery module 10 by applying a DC voltage to the battery module 10. Charging of the battery module 10 is performed by connecting the charging device 71 to the positive terminal 31a and the negative terminal 32a. Therefore, the charging device 71 simultaneously charges multiple secondary batteries 12 connected in series in the battery module 10.

[0072] Next, I will explain the inspection process. The inspection process includes a charging process in which the voltage of each of the multiple secondary batteries 12 in the battery module 10 is measured individually while simultaneously charging them. Charging in the charging process continues until the detected voltage of one of the multiple secondary batteries 12 in the battery module 10 first reaches the upper limit voltage. Hereinafter, the charging state of the battery module 10 at the moment when the voltage of one of the secondary batteries 12 first reaches the upper limit voltage will be referred to as the specific charging state.

[0073] The upper voltage limit is a value arbitrarily set based on the design value of the secondary battery 12, etc. Since the multiple secondary batteries 12 in the battery module 10 all have basically the same design capacity, the upper voltage limit is set as a common value for all secondary batteries 12. The upper voltage limit is, for example, the voltage of the secondary battery 12 when the cumulative charge capacity is set to SOC (State of Charge) 100%. The upper capacity limit is, for example, the design capacity C max The resulting cumulative charging capacity C 100 In this case, the upper limit voltage is the voltage V 100 It is. Voltage V 100 For example, this is 3.75V.

[0074] Furthermore, the SOC 0% of the secondary battery 12 is a minimum capacity arbitrarily set based on the design values ​​of the secondary battery 12. The minimum capacity at which SOC 0% occurs is set to a value higher than, for example, the capacity in a completely discharged state. The maximum capacity at which SOC 100% occurs and the minimum capacity at which SOC 0% occur are set as common values ​​for all secondary batteries 12 included in the battery module 10.

[0075] The upper limit capacity is, for example, the design capacity C. max The resulting cumulative charging capacity C 100 In this case, the upper limit voltage is the voltage V 100 That is the case. As an example, the positive electrode active material of secondary battery 12 is olivine-type lithium iron phosphate (LiFePO4), and the negative electrode active material is carbon such as graphite, and the design capacity C max Assume that the capacity is between 3800mAh and 4000mAh. In this case, the upper limit voltage is voltage V. 100 In that case, the value would be, for example, between 3.6V and 3.8V, with one example being 3.75V. Also, if the lower limit voltage is taken as the voltage V0 of the secondary battery 12 when the lower limit capacity Cmin is 2.5V and 3.2V, with one example being 3.0V.

[0076] During the charging process, the inspection device 60 simultaneously charges the multiple secondary batteries 12 of the battery module 10 using the charging device 71. While charging, the inspection device 60 continuously or intermittently acquires the voltage of each of the multiple secondary batteries 12 using the voltage detection circuit 62. The inspection device 60 then stops charging by the charging device 71 when it detects that the voltage of any one of the secondary batteries 12 has reached the upper limit voltage. At the time of stopping, the charge state of the battery module 10 is a specific charge state.

[0077] The charging rate in the charging process is 0.05C or higher, preferably 0.08C or higher, and more preferably 0.1C or higher. By increasing the charging rate in the charging process, the flat region R1 that appears at the end of charging in the charging curve of the secondary battery 12 can be made closer to a gentle rising curve. For example, the charging rate in the charging process is 1.5C or lower, preferably 0.5C or lower, and more preferably 0.3C or lower.

[0078] Furthermore, the charging rate during the charging process may be constant or may change during the charging process. For example, it is preferable that the charging rate is 0.3C or less when any one of the secondary batteries 12 reaches an arbitrarily set upper limit voltage. In this case, the charging process may be performed at a constant charging rate of 0.3C or less, or the charging process may be performed by switching the charging rate from more than 0.3C to 0.3C or less during the charging process. A specific example of switching the charging rate is to set the charging rate to more than 0.3C and 1.5C or less when the SOC of the battery module 10 or the SOC of any one of the secondary batteries 12 is below a specific value, and to set the charging rate to 0.05C or more and 0.3C or less when it is above the specific value. The specific value mentioned above is, for example, a value of 65% or more and 90% or less, and an example of a specific value is 80%.

[0079] In the charging process, it is preferable to carry out the charging under a constant temperature environment, for example, by using a constant temperature chamber. The charging temperature is, for example, 20°C to 25°C. An example of a charging temperature is 25°C (room temperature). The charging temperature may also change in multiple stages or continuously.

[0080] The inspection process includes an estimation process that, after the charging process, estimates the capacity of secondary batteries 12 that have not reached the upper limit voltage based on the detected voltage of the secondary battery 12 and information obtained in advance showing the relationship between the cumulative charge capacity and voltage of the secondary battery 12.

[0081] When multiple secondary batteries 12 of the battery module 10 are charged simultaneously, theoretically, multiple secondary batteries 12 configured in the same way will reach their upper voltage limit at the same time. However, due to individual differences among the multiple secondary batteries 12, there will be variations in the timing at which the multiple secondary batteries 12 reach their upper voltage limit. Therefore, if charging continues until all of the multiple secondary batteries 12 reach their upper voltage limit, the secondary battery 12 that reaches its upper voltage limit first will be overcharged beyond its upper voltage limit. To suppress this overcharging, the charging process stops when one of the secondary batteries 12 first reaches its upper voltage limit. In this specific charging state, the secondary batteries other than the one that has reached its upper voltage limit have not yet reached it. The estimation process estimates the State of Charge (SOC) of the other secondary batteries 12 that have not reached their upper voltage limit in this specific charging state.

[0082] In the estimation process, the estimation device 63 uses a voltage detection circuit 62 to obtain the voltage of each of the secondary batteries 12 in the battery module 10 that are in a specific charge state and have not reached the upper limit voltage. Next, for each of the secondary batteries 12, the estimation device 63 refers to master data and estimates the State of Charge (SOC) of the secondary battery 12 from the detected voltage values ​​obtained. The value estimated in the estimation process may be the uncharged capacity of the secondary battery 12.

[0083] The master data is data showing the relationship between SOC and voltage for a secondary battery 12 of the same specifications as the secondary battery 12 provided in the battery module 10 to be measured. The master data is acquired in advance and stored in the storage unit 65. The master data is information showing the relationship between SOC and voltage of the secondary battery 12 that has been acquired in advance.

[0084] The method for obtaining master data is as follows: First, a secondary battery 12 of the same specifications as the secondary battery 12 installed in the battery module 10 to be measured is prepared as a measurement sample. The secondary battery 12 of the measurement sample is charged under the same conditions as the charging process until it reaches the upper limit voltage. During the charging of the measurement sample, the voltage of the secondary battery 12 is acquired at regular intervals, and the State of Charge (SOC) of the secondary battery 12 at the time the voltage is acquired is calculated. This yields master data, which is a set of data representing the relationship between the SOC and voltage of the secondary battery 12 at each point in time while the secondary battery 12 is being charged until it reaches the upper limit voltage. The master data is, for example, a charging curve with SOC on the horizontal axis and voltage on the vertical axis. The charging curve as master data has the same shape as the charging curve shown in Figure 3. Note that the shape of the charging curve changes when the charging rate is changed. The master data may also be a table representing the relationship between voltage and SOC at each point in the charging curve.

[0085] The SOC (State of Charge) is estimated using master data by comparing the detected voltage with the master data. Specifically, the SOC of the secondary battery 12 is estimated to be the SOC of the secondary battery 12 when the voltage in the master data is the same as the detected voltage. This estimation process is performed for all other secondary batteries 12. By comparing the estimated SOCs for the other secondary batteries 12, the variation in the capacity of each secondary battery 12 forming the battery module 10 can be evaluated. This estimation process can also be applied as a method for estimating the capacity of the battery module.

[0086] <effect> As shown in Figure 6, in the case of a conventional secondary battery structure using an olivine-type structural active material as the positive electrode active material, a flat region R1 appears in the charging curve at the end of charging where the voltage hardly changes while the accumulated charge capacity changes. In the flat region R1, even a small change in voltage causes a large change in the accumulated charge capacity. Therefore, when estimating the accumulated charge capacity from the detected voltage, if the detected voltage is in the flat region R1, the error in the detected voltage will have a significant impact on the estimation of the accumulated charge capacity. As a result, the accuracy of the secondary battery capacity estimation decreases.

[0087] The secondary battery 12 of the battery module 10 in this embodiment has a first ratio ((V 90 -V 70 ) / V 70 The value is 0.003 or greater. As shown in Figure 3, satisfying the first condition means that the flat region R1 is a gently rising curve. Therefore, in the charging curve, even in the flat region R1, the difference in cumulative charging capacity is more clearly expressed as a difference in voltage. As a result, the influence of the error in the detected value on the estimation of cumulative charging capacity is reduced. Consequently, the accuracy of secondary battery capacity estimation is improved.

[0088] Furthermore, as shown in Figure 6, in the case of a conventional secondary battery structure using an olivine-type structural active material as the positive electrode active material, a steep region R2 appears after the flat region R1 at the end of charging, where the change in voltage is significantly larger than the change in cumulative charge capacity. This steep region R2 also reduces the accuracy of estimating the capacity of the secondary battery when estimating the cumulative charge capacity from the detected voltage.

[0089] The secondary battery 12 of the battery module 10 in this embodiment has a second ratio ((V 95 -V 90 ) / (V 99.5 -V 90 The second condition is that the rise of the steep region R2 is gradual. Therefore, even in the steep region R2 of the charging curve, small differences in the cumulative charging capacity are less likely to appear as large differences in voltage. As a result, the accuracy of estimating the capacity of the secondary battery is improved.

[0090] <Effects> According to this embodiment, the following effects can be obtained. (1) The battery module 10 consists of multiple secondary batteries 12 stacked in series. Each secondary battery 12 has a positive electrode 13 on the first surface 15 of a positive electrode current collector 14, which has a positive electrode active material layer 17 containing an olivine-type structural active material, and a negative electrode 19 on the first surface 21 of a negative electrode current collector 20, which has a negative electrode active material layer 24 arranged to face the positive electrode active material layer 17 of the positive electrode 13. When multiple secondary batteries 12 of the battery module 10 are simultaneously charged at a charge rate of 0.1C, the relationship between the cumulative charge capacity of the secondary batteries 12 and the voltage of the secondary batteries 12 satisfies the first condition. The first condition is the first ratio ((V 90 -V 70 ) / V 70 The value is 0.003 or greater. According to the above configuration, the accuracy of estimating the capacity of the secondary battery 12 based on the method of estimating the cumulative charge capacity from the detected voltage value is improved.

[0091] (2) When multiple secondary batteries 12 of the battery module 10 are charged simultaneously at a charge rate of 0.1C, the relationship between the cumulative charge capacity of the secondary batteries 12 and the voltage of the secondary batteries 12 satisfies the second condition. The second condition is the second ratio ((V 95 -V 90 ) / (V 99.5 -V 90 The value is between 0.08 and 0.6. According to the above configuration, the accuracy of estimating the capacity of the secondary battery 12 based on the method of estimating the cumulative charge capacity from the detected voltage value is improved.

[0092] (3) The thickness of the positive electrode active material layer 17 is 300 μm or more. In this case, during charging, it is possible to create unevenness in the charging depth between the surface and deep parts of the positive electrode active material layer 17. This makes it easy to form a secondary battery 12 that satisfies the first and second conditions.

[0093] (4) The area of ​​the negative electrode facing surface 24A in the negative electrode active material layer 24 that faces the positive electrode active material layer 17 is larger than the area of ​​the positive electrode facing surface 17A in the positive electrode active material layer 17 that faces the negative electrode active material layer 24. In this case, during charging, unevenness in the charging depth can be created between the central part and the outer part of the positive electrode active material layer 17. This makes it easy to form a secondary battery 12 that satisfies the first and second conditions.

[0094] (5) The olivine-type structural active material is the one with the general formula LiM h This is a polyanionic compound having an olivine-type structure represented by PO4. In this case, a flat region R1 and a steep region R2 are easily formed in the charging curve of the secondary battery 12. Therefore, the effect of improving the accuracy of capacity estimation described in (1) above is obtained more significantly.

[0095] (6) The negative electrode active material layer 24 contains carbon as the negative electrode active material. In this case, a flat region R1 and a steep region R2 are easily formed in the charging curve of the secondary battery 12. Therefore, the effect of improving the accuracy of capacity estimation described in (1) above is obtained more significantly.

[0096] (7) The secondary battery 12 has olivine-type lithium iron phosphate (LiFePO4) as the positive electrode active material and carbon as the negative electrode active material. The secondary battery 12 has a voltage (upper limit voltage) of 3.6V to 3.8V when the capacity is set to SOC 100%, and a voltage (lower limit voltage) of 2.5V to 3.2V when the capacity is set to SOC 0%. In other words, an example of a secondary battery 12 is a secondary battery 12 that is usually used in the range from a lower limit voltage of 2.5V to 3.2V to an upper limit voltage of 3.6V to 3.8V. In this case, the effect of improving the accuracy of capacity estimation described in (1) above can be obtained more significantly. [Examples]

[0097] The following describes an embodiment that further elaborates on the above embodiment. <Example 1> As a current collector, a 30 μm thick aluminum foil was prepared, with an 8 μm thick copper foil laminated to one side of its surface. A positive electrode mixture was applied in a film-like manner to the aluminum foil surface of the current collector using the doctor blade method. The positive electrode mixture consisted of LiFePO4 (positive electrode active material), carbon nanotubes (conductive additive), carboxymethylcellulose (binding agent), and styrene-butadiene rubber (binding agent) in a solid content mass ratio of 98.25:0.05:0.4:1.3, with water as the solvent. The applied positive electrode mixture was heat-treated at 50°C to dry and solidify the mixture, thereby forming a positive electrode active material layer. The thickness of the positive electrode active material layer was 360 μm, and the area of ​​the positive electrode-facing surface of the positive electrode active material layer was 450 cm². 2 That was the case.

[0098] Subsequently, a negative electrode mixture was applied in a film-like manner to the copper foil surface of the current collector using the doctor blade method. The negative electrode mixture consisted of graphite (negative electrode active material), carbon nanotubes (conductive additive), carboxymethylcellulose (binding agent), and styrene-butadiene rubber (binding agent) in a solid content mass ratio of 96.99:0.01:0.4:2.6, with water as the solvent. The applied negative electrode mixture was heat-treated at 50°C to dry and solidify it, thereby forming a negative electrode active material layer. The thickness of the negative electrode active material layer was 300 μm. The negative electrode-facing surface of the negative electrode active material layer was formed to be larger than the positive electrode-facing surface of the positive electrode active material layer in a plan view. In this way, a bipolar electrode having a positive electrode active material layer and a negative electrode active material layer was fabricated.

[0099] Using the fabricated bipolar electrodes and polyethylene separator, a battery module with the structure shown in Figure 1 was constructed, and this was designated as Example 1. The battery module was fabricated such that, in a plan view, the entire area where the positive electrode active material layer is formed is located within the area where the negative electrode active material layer is formed. The battery module of Example 1 comprises 30 secondary batteries connected in series. The electrolyte was prepared by dissolving lithium hexafluorophosphate at a concentration of 1.6 M in a mixed solvent of methyl propionate and ethylene carbonate in a volume ratio of 15:85. Acid-modified polyethylene resin was used as the encapsulant.

[0100] Each secondary battery constituting Example 1 has a design capacity C max It is designed to have a capacity of 3912mAh. <Comparative Example 1> A battery module for Comparative Example 1 was fabricated in the same manner as in Example 1, except that the thickness of the positive electrode active material layer and the negative electrode active material layer in the bipolar electrode were different, and this was designated as Comparative Example 1. The thickness of the positive electrode active material layer of the bipolar electrode used in Comparative Example 1 was 360 μm.

[0101] Each secondary battery comprising Comparative Example 1 has a design capacity C, similar to Example 1. max It is designed to have a capacity of 3912mAh. <Charging Test 1> All secondary batteries constituting the battery module of Example 1 were charged simultaneously. Charging was performed at a constant current of 0.1C until the voltage reached 3.75V. During charging, the voltage of any one secondary battery composed of two bipolar electrodes was measured at predetermined intervals. The cumulative charge capacity of the secondary battery at each point in time when the voltage was measured was calculated. The results are shown in the graph of Figure 5. Figure 5 is a charging curve showing the relationship between the cumulative charge capacity and voltage of the secondary battery of Example 1.

[0102] All secondary batteries constituting the battery module of Comparative Example 1 were charged simultaneously. Charging was performed at a constant current of 0.1C until the voltage reached 3.75V. During charging, the voltage of any one secondary battery composed of two bipolar electrodes was measured at predetermined intervals. The cumulative charge capacity of the secondary battery at each point in time when the voltage was measured was calculated. The results are shown in the graph in Figure 6. Figure 6 is the charging curve showing the relationship between the cumulative charge capacity and voltage of the secondary battery of Comparative Example 1.

[0103] From the charging curve of the secondary battery of Example 1 shown in Figure 5, and the charging curve of the secondary battery of Comparative Example 1 shown in Figure 6, the design capacity C max The voltage V is the voltage of the secondary battery when the cumulative charge capacity is 70%, 90%, 95%, and 99.5% of the total charge capacity. 70 , voltage V 90 , voltage V 95 , voltage V 99.5 We calculated the following. Then, using the calculated voltages, we calculated the first ratio ((V 90 -V 70 ) / V 70 ) and the second ratio ((V 95 -V 90 ) / (V 99.5 -V 90 The following was calculated. The results are shown in Table 1. V in Table 1 70 , V 90 , V 95 , V 99.5 The unit of the numerical value is [V]. Also, below, the design capacity C max The cumulative charging capacity that is 70%, 90%, 95%, and 99.5% of the total capacity is given by Capacity C. 70 , capacity C 90 , capacity C 95 , capacity C 99.5 It may be written as follows.

[0104] [Table 1] As shown in Figure 6 and Table 1, the secondary battery of the battery module of Comparative Example 1 has a first ratio of less than 0.003. This indicates a flat region, capacity C 70 From capacity C 90This indicates that the voltage of the secondary battery hardly changes while it is being charged to this level. Furthermore, the secondary battery of the battery module in Comparative Example 1 has a second ratio value smaller than the range of 0.08 to 0.6. This indicates that the capacity C is in a steep region. 90 From capacity C 99.5 During the interval, capacity C 90 From capacity C 95 While charging, the voltage of the secondary battery remains almost unchanged, and the capacity C 95 From capacity C 99.5 This indicates that the voltage of the secondary battery rises sharply while it is being charged to this level.

[0105] In contrast, as shown in Figure 5 and Table 1, the secondary battery of the battery module in Example 1 has a first ratio of 0.003 or higher. This indicates that the capacity C is in a flat region. 70 From capacity C 90 This indicates that the voltage of the secondary battery rises above a certain level even while it is being charged. From this result, it can be seen that in Example 1, the difference in cumulative charge capacity in the flat region is more clearly expressed as a voltage difference compared to Comparative Example 1.

[0106] Furthermore, the secondary battery of the battery module in Example 1 has a second ratio of 0.08 to 0.6. This indicates that the capacity C is in the steep region. 90 From capacity C 99.5 During the interval, capacity C 90 From capacity C 95 While charging up to capacity C 95 From capacity C 99.5 This indicates that the voltage of the secondary battery rises above a certain level in both ranges while it is being charged. From these results, it can be seen that, compared to Comparative Example 1, Example 1 has a gentler steep rise in voltage in the steep region, and as a result, small differences in cumulative charge capacity are less likely to appear as large differences in voltage.

[0107] <Charging Test 2> A charging test was performed on the battery module of Example 1 in the same manner as in Charging Test 1, except that the charging rate was changed from 0.1C to 0.22C. From the obtained charging curve (not shown), the voltage V 70 , voltage V 90 , voltage V 95 , voltage V 99.5 The following was determined. Then, the first and second ratios were calculated using the determined voltages. The results are shown in Table 2. Table 2 also shows the results when the charging rate is 0.1C.

[0108] [Table 2] As shown in Table 2, the first and second ratios increase as the charging rate rises from 0.1C to 0.22C. The increase in the first ratio indicates that the capacity C is within the flat region. 70 From capacity C 90 This indicates that the change in cumulative charging capacity within this range is even larger. The fact that the second ratio is large within the range of 0.08 to 0.6 indicates that it is within the steep region, capacity C 90 From capacity C 99.5 This indicates that the sharp voltage rise that occurs during this period is further mitigated. [Explanation of Symbols]

[0109] 10…Battery module 11…Laminate 12…Secondary battery 17...Cathode active material layer 24...Negative electrode active material layer

Claims

1. A battery module in which multiple secondary batteries are stacked in series, Each of the aforementioned secondary batteries is A positive electrode having a positive electrode active material layer containing an olivine-type structural active material and having a thickness of 200 μm or more on the first surface of the positive electrode current collector, The negative electrode comprises a negative electrode on the first surface of the negative electrode current collector, the negative electrode active material layer being arranged to face the positive electrode active material layer of the positive electrode, When multiple secondary batteries in the battery module are simultaneously charged at a charge rate of 0.1C, the relationship between the cumulative charge capacity of the secondary batteries and the voltage of the secondary batteries satisfies the first condition. The first condition is, The design capacity C of the aforementioned secondary battery when fully charged is defined as the design capacity. ｍａｘ year, The design capacity C ｍａｘ The voltage of the secondary battery at the point when the cumulative charge capacity is 70% and 90% of the total charge capacity is given by the voltage V. ７０ Voltage V ９０ The first ratio in this case ((V ９０ -V ７０ ) / V ７０ A battery module in which the ratio is 0.003 or higher.

2. When multiple secondary batteries in the battery module are simultaneously charged at a charge rate of 0.1C, the relationship between the cumulative charge capacity of the secondary batteries and the voltage of the secondary batteries satisfies the second condition. The second condition is, The design capacity C ｍａｘ When the integrated charge capacity reaches 90%, 95%, and 99.5% respectively, the voltages of the secondary battery are voltage V ９０ , voltage V ９５ , voltage V ９９．５ . When the second ratio ((V ９５ - V ９０ ) / (V ９９．５ - V ９０ )) is 0.08 or more and 0.6 or less, the battery module according to claim 1.

3. The battery module according to claim 1 or claim 2, wherein the area of ​​the surface of the negative electrode active material layer facing the positive electrode active material layer is greater than the area of ​​the surface of the positive electrode active material layer facing the negative electrode active material layer.

4. The olivine-type structural active material has the general formula LiM ｈ PO ４ A battery module according to claim 1 or claim 2, wherein the polyanionic compound has an olivine-type structure represented by (M is at least one element selected from the group consisting of Mn, Fe, Co, Ni, Cu, Mg, Zn, V, Ca, Sr, Ba, Ti, Al, Si, B, Te, Mo, and h is a numerical value satisfying 0 < h < 2).

5. The battery module according to claim 1 or claim 2, wherein the negative electrode active material layer contains carbon.

6. A capacity estimation method for estimating the capacity of a secondary battery in a battery module in which multiple secondary batteries are stacked in series, Each of the aforementioned secondary batteries is A positive electrode having a positive electrode active material layer containing an olivine-type structural active material and having a thickness of 200 μm or more on the first surface of the positive electrode current collector, The negative electrode comprises a negative electrode on the first surface of the negative electrode current collector, the negative electrode active material layer being arranged to face the positive electrode active material layer of the positive electrode, A charging process in which, with respect to the multiple secondary batteries of the battery module, the voltage of each secondary battery is measured individually, and the batteries are simultaneously charged at a charging rate of 0.05C or higher until any one of the secondary batteries reaches an arbitrarily set upper limit voltage; A method for estimating the capacity of a battery module, which includes an estimation step of estimating the capacity of a secondary battery that has not reached the upper limit voltage after the charging step, based on a detected value of the voltage of the secondary battery and information obtained in advance showing the relationship between the cumulative charge capacity and voltage of the secondary battery.

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