Secondary battery testing method

Abstract

This secondary battery testing method includes a charging step (step S102) for charging a secondary battery to a prescribed charging state by constant voltage charging or constant voltage / constant current charging, an aging step (step S106) for aging the secondary battery charged in the charging step, voltage measuring steps (step S104 and step S108) for measuring the voltage of the secondary battery before and after aging, and a sorting step (step S110) for sorting the secondary battery on the basis of the voltages of the secondary battery measured in the voltage measuring steps, the testing method being characterized in that the prescribed charging state is set in a state of charge (SOC) region in which the slope of the tangent to an SOC-OCV curve indicating a relationship between the state of charge and the open circuit voltage (OCV) of the secondary battery is greater than 0.02.

Description

Secondary battery inspection method

[0001] The present disclosure relates to a method for inspecting a secondary battery.

[0002] A secondary battery such as a lithium ion secondary battery is produced, for example, by housing an electrode assembly in a battery case, in which a positive electrode and a negative electrode are wound or stacked with a separator interposed therebetween, injecting an electrolyte solution, and crimping or laser sealing the battery case.

[0003] An example of an inspection method for screening out defective batteries present among secondary batteries thus produced is a method in which secondary batteries that have been charged and discharged under predetermined conditions are left for a predetermined period of time to undergo aging, and then electrical characteristics such as the open circuit voltage, closed circuit voltage, and internal resistance of the secondary battery are measured, and the secondary batteries are screened based on the electrical characteristics. For example, Patent Documents 1 to 4 disclose inspection methods for screening out secondary batteries by measuring the voltage of the secondary batteries after aging.

[0004] JP 2014-17056 A JP 2004-288515 A JP 2005-158643 A JP 2001-228224 A

[0005] However, in the inspection method of measuring the voltage of a secondary battery to select defective batteries, there is a problem in that the aging period of the secondary battery required to maintain the accuracy of the voltage measurement of the secondary battery is long. A long aging period increases the inspection period for selecting defective batteries, which undesirably means that batteries undergoing inspection must be kept in stock for a long period of time.

[0006] Therefore, an object of the present disclosure is to provide a method for inspecting a secondary battery that can shorten the inspection period for the secondary battery.

[0007] A method for inspecting a secondary battery according to one aspect of the present disclosure includes a charging step of constant-voltage charging or constant-voltage-constant-current charging a secondary battery to a predetermined state of charge, an aging step of aging the secondary battery charged in the charging step, a voltage measurement step of measuring at least the voltage of the secondary battery after the aging step, and a sorting step of sorting the secondary battery based on the voltage of the secondary battery measured in the voltage measurement step, wherein the predetermined state of charge is set in a state of charge region where the slope of a tangent to an SOC-OCV curve showing the relationship between the state of charge (SOC) and open-circuit voltage (OCV) of the secondary battery is greater than 0.02.

[0008] According to one aspect of the present disclosure, it is possible to provide a method for inspecting a secondary battery that can shorten the inspection period for the secondary battery.

[0009] 1 is a flowchart of a method for inspecting a secondary battery according to an embodiment. 2 is a diagram showing an example of an SOC-OCV curve of a lithium ion secondary battery. 3 is a flowchart of a method for inspecting a secondary battery according to another embodiment.

[0010] An example of a method for inspecting a secondary battery according to the present disclosure will be described below.

[0011] 1 is a flowchart of a method for inspecting a secondary battery according to an embodiment. In the following description, a lithium-ion secondary battery is used as the inspection target; however, the inspection target secondary battery may be a secondary battery other than a lithium-ion secondary battery, such as an alkaline secondary battery. Furthermore, the number of secondary batteries inspected at one time is not particularly limited, but considering the inspection time, a number of 100 to 200 is preferable.

[0012] 1 includes an initial charge / discharge step (step S100), a charging step (step S102), a first voltage measurement step (step S104), an aging step (step S106), a second voltage measurement step (step S108), and a sorting step (step S110). Each step will be described below.

[0013] (Initial Charging and Discharging Step) In the initial charging and discharging step (step S100), the lithium ion secondary battery is initially charged and discharged. The initial charging and discharging is the first charging and discharging performed after the secondary battery is assembled. Although the initial charging and discharging is not an essential component of the inspection method of this embodiment, it is preferable to perform the initial charging and discharging in order to further improve the accuracy of the voltage measurement of the secondary battery that is performed thereafter. In particular, it is preferable to perform the initial charging and discharging for a lithium ion secondary battery.

[0014] In the initial charge / discharge, it is preferable to charge the lithium ion secondary battery until the state of charge (SOC) reaches 100% and discharge it until the SOC reaches 0%. The charge in the initial charge / discharge may be charge to a state of charge less than 100%. Furthermore, the discharge in the initial charge / discharge does not have to be until the state of charge reaches 0%, but in order to improve the accuracy of voltage measurements of the secondary battery performed thereafter, it is preferable to discharge it until the state of charge reaches at least 5% or less, more preferably 1% or less, and even more preferably 0%.

[0015] (Charging Step) Next, the charging step (step S102) is carried out, in which the lithium ion secondary battery is charged at a constant voltage or at a constant voltage and a constant current until it reaches a predetermined state of charge. When the charging step is carried out without initial charging and discharging, the state of charge of the secondary battery is checked, and if the state of charge is high, the secondary battery is preferably discharged until its state of charge is 5% or less, more preferably 1% or less, and even more preferably 0% before starting the charging step.

[0016] The predetermined state of charge is a region of the state of charge where the slope of the tangent to the SOC-OCV curve, which shows the relationship between the state of charge (SOC) and the open circuit voltage (OCV) of the secondary battery, is greater than 0.02. The region of the state of charge where the slope of the tangent to the SOC-OCV curve is greater than 0.02 is observed in a low state of charge region, and is observed at least in a region where the state of charge of the secondary battery is 20% or less.

[0017] The SOC-OCV curve of a lithium-ion secondary battery can be obtained using a good lithium-ion secondary battery that does not have a micro-internal short circuit. Specifically, the SOC-OCV curve of a lithium-ion secondary battery can be obtained by measuring the integrated discharge capacity at each point during discharge of the secondary battery from a fully charged state (SOC = 100%) to a fully discharged state (SOC = 0%) and the open-circuit voltage of the secondary battery after resting. The integrated discharge capacity from a fully charged state at each point during discharge of the lithium-ion secondary battery is defined as Q(j) (j = 1, 2, ...), the open-circuit voltage when the integrated discharge capacity is Q(j) is defined as OCV(j), and the discharge capacity of the lithium-ion secondary battery from a fully charged state to a fully discharged state is defined as Qmax. The SOC(j) at each point during discharge is defined as SOC(j) = (Qmax - Qb(j)) / Qmax × 100. The SOC-OCV curve is obtained based on the plot data of the SOC(j) and open circuit voltage OCV(j) at each time point. When creating the SOC-OCV curve, the secondary battery is charged and discharged by, for example, constant current charging (0.5 C, 4.2 V cut) - constant voltage charging (4.2 V, 0.02 C cut) until the SOC reaches 100%, followed by a 10-minute pause, followed by a predetermined period of 0.002 C constant current discharge and pause, repeated until the discharge voltage reaches 2.75 V.

[0018] FIG. 2 is a diagram showing an example of an SOC-OCV curve for a lithium-ion secondary battery. As shown in the SOC-OCV curve for a lithium-ion secondary battery in FIG. 2, the slope of the tangent to the SOC-OCV curve (d(OCV) / d(SOC)) is large in the region where the secondary battery's state of charge is low, and the slope of the tangent to the SOC-OCV curve (d(OCV) / d(SOC)) decreases as the secondary battery's state of charge increases. In the SOC-OCV curve in FIG. 2, the region within the dashed line represents the region where d(OCV) / d(SOC) is greater than 0.02, and when the state of charge is higher than the dashed line, d(OCV) / d(SOC) becomes smaller than 0.02. For example, when using the SOC-OCV curve shown in FIG. 2, the state of charge to be charged in step S102 is set within the range of 0.5% to 9% state of charge, and the lithium-ion secondary battery is subjected to constant voltage charging or constant voltage-constant current charging up to the set state of charge.

[0019] The conditions for the constant voltage charging in step S102 are not particularly limited, but in the case of a lithium ion secondary battery, charging is preferably performed at a voltage between 3.1 V and 3.5 V. Furthermore, the constant voltage-constant current charging in step S102 is a method in which constant voltage charging is performed followed by constant current charging. The conditions for the constant voltage charging in the constant voltage-constant current charging in step S102 are not particularly limited, but in the case of a lithium ion secondary battery, charging is preferably performed at a voltage between 2.9 V and 3.3 V, and the conditions for the constant current charging are not particularly limited, but charging at a constant current of 0.2 C or more and 1.0 C or less is preferred.

[0020] By performing such constant voltage charging or constant voltage-constant current charging, variations in SOC among secondary batteries are eliminated when testing multiple secondary batteries, and the accuracy of voltage measurement for each secondary battery can be maintained even if the aging period for measuring the voltage of the secondary battery in the subsequent voltage measurement step is short. As a result, it becomes possible to screen out defective batteries in a shorter period of time. Furthermore, by starting charging of the lithium-ion secondary battery with constant voltage charging in step S102, the charging time is shortened, further shortening the testing period.

[0021] (Voltage Measurement Steps: First and Second Voltage Measurement Steps, and Aging Step) Next, the process proceeds to the first voltage measurement step (step S104), where the voltage V1 of the lithium ion secondary battery is measured before the aging step (step S106). In the aging step (step S106), the lithium ion secondary battery is left in an open circuit state for a predetermined time. Then, the process proceeds to the second voltage measurement step (step S108), where the voltage V2 of the lithium ion secondary battery after the aging step (step S106) is measured.

[0022] The aging temperature in the aging step (step S106) is not particularly limited, but is preferably in the range of 15°C to 60°C, more preferably 20°C to 40°C, in order to stabilize the battery characteristics.

[0023] The aging period in the aging step (step S106) is determined appropriately based on the specifications of the secondary batteries to be tested. As described above, the charging step (step S102) eliminates SOC variations among the secondary batteries, thereby shortening the aging period.

[0024] (Sorting Step) In the sorting step (step S110), a voltage difference ΔV between the voltage V1 measured in the first voltage measurement step (step S104) and the voltage V2 measured in the second voltage measurement step (step S108) is calculated, and the lithium-ion secondary batteries are sorted based on the voltage difference ΔV. The sorting is performed, for example, by classifying the lithium-ion secondary battery as a non-defective product if the voltage difference ΔV is within a range of a sorting criterion, and classifying the lithium-ion secondary battery as a defective product if the voltage difference ΔV is outside the range of the sorting criterion. Examples of the sorting criterion include ΔVA±3σ, which is obtained by calculating the average value ΔVA of the voltage differences ΔV of each lithium-ion secondary battery and the standard deviation σ. If the voltage difference ΔV of a lithium-ion secondary battery is within the range of ΔVA±3σ, the lithium-ion secondary battery is classified as a non-defective product, and if the voltage difference ΔV of a lithium-ion secondary battery is outside the range of ΔVA±3σ, the lithium-ion secondary battery is classified as a defective product.

[0025] 3 is a flowchart of a method for inspecting a secondary battery according to another embodiment of the present invention, which includes an initial charge / discharge step (step S200), a charging step (step S202), an aging step (step S204), a voltage measurement step (step S206), and a sorting step (step S208).

[0026] The initial charge / discharge step (step S200) and the charging step (step S202) are the same as the initial charge / discharge step (step S100) and the charging step (step S102) described above.

[0027] (Voltage Measurement Step) In the voltage measurement step (step S206), the voltage V of the lithium ion secondary battery after the aging step (step S204) is measured. The aging temperature in the aging step (step S204) is not particularly limited, but is preferably in the range of 15°C to 60°C, and more preferably in the range of 20°C to 40°C, for example, in terms of stabilizing the battery characteristics.

[0028] (Sorting Step) In the sorting step (step S208), lithium ion secondary batteries are sorted based on the voltage V measured in the voltage measurement step (step S206). For example, the sorting criterion is VA±3σ, which is obtained by calculating the average value VA of the voltage V of each lithium ion secondary battery and the standard deviation σ. If the voltage V of a lithium ion secondary battery is within the range of VA±3σ, the lithium ion secondary battery is classified as a non-defective product, and if the voltage V of a lithium ion secondary battery is outside the range of VA±3σ, the lithium ion secondary battery is classified as a defective product.

[0029] According to the secondary battery inspection method shown in Fig. 3, voltage measurement is required only once after the aging step (step S204), and therefore it is possible to determine the quality of a lithium-ion secondary battery in a shorter time than with the secondary battery inspection method shown in Fig. 1. However, in terms of improving the accuracy of determining the quality of a lithium-ion secondary battery, it is preferable to sort lithium-ion secondary batteries based on the voltage difference ΔV before and after aging of the lithium-ion secondary battery, as in the secondary battery inspection method shown in Fig. 1.

[0030] A lithium ion secondary battery can be obtained by, for example, winding or stacking a positive electrode and a negative electrode with a separator between them, and housing the electrode assembly together with an electrolyte in a housing such as a battery can or a laminate. Known materials can be used for the positive electrode, negative electrode, separator, and electrolyte, for example, as follows.

[0031] <Positive Electrode> The positive electrode includes a positive electrode current collector such as a metal foil and a positive electrode composite layer formed on the positive electrode current collector. The positive electrode current collector can be a foil of a metal such as aluminum that is stable in the potential range of the positive electrode, or a film with such a metal disposed on the surface layer. The positive electrode composite layer includes, for example, a positive electrode active material, a conductive material, and a binder.

[0032] Examples of the positive electrode active material include lithium transition metal composite oxides, and specifically, lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel manganese composite oxide, lithium nickel cobalt composite oxide, and the like can be used. These lithium transition metal composite oxides may also be doped with Al, Ti, Zr, Nb, B, W, Mg, Mo, and the like.

[0033] Examples of the conductive material include carbon powder such as carbon black, acetylene black, ketjen black, and graphite.

[0034] Examples of the binder include fluorine-based polymers, rubber-based polymers, etc. Examples of the fluorine-based polymers include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and modified products thereof, and examples of the rubber-based polymers include ethylene-propylene-isoprene copolymers, ethylene-propylene-butadiene copolymers, styrene-butadiene copolymers, and carboxymethyl cellulose.

[0035] <Negative electrode> The negative electrode includes a negative electrode current collector such as a metal foil and a negative electrode composite layer formed on the negative electrode current collector. The negative electrode current collector can be a foil of a metal such as copper that is stable in the potential range of the negative electrode, or a film with such a metal disposed on the surface layer. The negative electrode composite layer includes, for example, a negative electrode active material and a binder.

[0036] The negative electrode active material can be a carbon material capable of absorbing and releasing lithium ions, such as graphite, non-graphitizable carbon, graphitizable carbon, fibrous carbon, coke, and carbon black. Furthermore, non-carbon materials can be silicon, tin, and alloys or oxides thereof. The binder can be the same as that used for the positive electrode.

[0037] <Electrolyte> The electrolyte has, for example, ion conductivity (for example, lithium ion conductivity). The electrolyte may be a liquid electrolyte (electrolytic solution) or a solid electrolyte.

[0038] The liquid electrolyte (electrolytic solution) contains, for example, a non-aqueous solvent and an electrolyte salt dissolved in the non-aqueous solvent. Examples of the non-aqueous solvent include esters, ethers, nitriles, amides, and mixed solvents of two or more of these. Examples of the non-aqueous solvent include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and mixed solvents of these. The non-aqueous solvent may contain a halogen-substituted compound (e.g., fluoroethylene carbonate) in which at least a portion of the hydrogen atoms of these solvents are substituted with halogen atoms such as fluorine. Examples of the electrolyte salt include LiPF ６ Lithium salts such as

[0039] Furthermore, examples of the solid electrolyte that can be used include solid or gel-like polymer electrolytes, inorganic solid electrolytes, and the like. The polymer electrolyte includes, for example, a lithium salt and a matrix polymer, or a non-aqueous solvent, a lithium salt, and a matrix polymer. For example, a polymer material that absorbs a non-aqueous solvent and gels is used as the matrix polymer. Examples of the polymer material include fluororesin, acrylic resin, and polyether resin. For example, the inorganic solid electrolyte can be a material known in all-solid-state lithium ion secondary batteries (e.g., oxide-based solid electrolytes, sulfide-based solid electrolytes, halogen-based solid electrolytes, and the like). While the above-exemplified electrolytes are non-aqueous electrolytes, the electrolyte is not limited to non-aqueous electrolytes and may be an aqueous electrolyte.

[0040] <Separator> A porous sheet or the like having ion permeability and insulating properties is used as the separator. Specific examples of the porous sheet include a microporous thin film, a woven fabric, a nonwoven fabric, etc. Suitable materials for the separator include olefin-based resins such as polyethylene and polypropylene, and cellulose.

[0041] Hereinafter, the present disclosure will be further described with reference to examples and comparative examples, but the present disclosure is not limited to the following examples.

[0042] Example 1 A total of 100 lithium ion secondary batteries were prepared, including 5 defective lithium ion secondary batteries that had a micro-internal short circuit and 95 non-defective lithium ion secondary batteries that had no micro-internal short circuit.

[0043] The 100 lithium ion secondary batteries were charged in a temperature environment of 25°C by constant current charging (0.5 C, 4.2 V cut) - constant voltage charging (4.2 V, 0.02 C cut), and then discharged at a constant current of 0.2 C until the battery voltage reached 2.75 V (initial charge / discharge step).

[0044] Next, in a temperature environment of 25°C, the lithium ion secondary battery after the initial charge / discharge step was charged at a constant voltage of 3.25 V until the state of charge of the lithium ion secondary battery reached 3% (charging step). Note that the SOC-OCV curve had been measured in advance using a good lithium ion secondary battery, and it was confirmed that the slope of the tangent to the SOC-OCV curve at a state of charge of 3% was greater than 0.02.

[0045] Next, the voltage V1 of each lithium ion secondary battery after the charging step was measured (first voltage measurement step). The lithium ion secondary batteries after the first voltage measurement step were subjected to an aging step, and the voltage V2 of each lithium ion secondary battery was measured (second voltage measurement step).

[0046] For each lithium-ion secondary battery, the voltage V2 was measured and the voltage difference ΔV between voltage V1 and voltage V2 was determined, and the average value ΔVA and standard deviation σ were calculated. The lithium-ion secondary batteries were then sorted based on the selection criteria (ΔVA±3σ) (sorting step). The aging step and sorting step were repeated until the voltage difference ΔV of all five defective lithium-ion secondary batteries fell outside the range of the selection criteria (ΔVA±3σ), and the aging period required to completely reject the defective batteries was measured. As a result, all defective batteries were rejected two days after the start of aging. From this, it can be determined that the aging period required to reject all defective batteries using the inspection method of Example 1 is at least two days.

[0047] Example 2 The lithium ion secondary batteries were inspected under the same conditions as in Example 1, except that in the charging step, the lithium ion secondary batteries after the initial charge / discharge step were charged at a constant voltage of 3.15 V until the current value reached 0.01 C in a temperature environment of 25° C., and then at a constant current of 1 C until the state of charge of the lithium ion secondary batteries reached 3%. As a result, two days after the start of aging, the voltage differences ΔV of all five defective lithium ion secondary batteries fell outside the range of the selection criterion (ΔVA±3σ). From this, it can be determined that the aging period required to reject all defective batteries using the inspection method of Example 2 is at least two days.

[0048] Comparative Example 1 The lithium ion secondary battery was inspected under the same conditions as in Example 1, except that in the charging step, the lithium ion secondary battery after the initial charge / discharge step was charged at a constant current of 1 C in a temperature environment of 25° C. until the state of charge of the lithium ion secondary battery reached 30%. It was confirmed that the slope of the tangent to the SOC-OCV curve at a state of charge of 30% was smaller than 0.02.

[0049] In Comparative Example 1, six days after the start of aging, the voltage differences ΔV of all five defective lithium ion secondary batteries fell outside the range of the selection criterion (ΔVA±3σ). From this, it can be determined that the aging period required to reject all defective lithium ion secondary batteries using the inspection method of Comparative Example 1 is at least six days.

[0050] Comparative Example 2 The lithium ion secondary battery was inspected under the same conditions as in Example 1, except that in the charging step, the lithium ion secondary battery after the initial charge / discharge step was charged at a constant current of 1 C in a temperature environment of 25° C. until the state of charge of the lithium ion secondary battery reached 3%.

[0051] In Comparative Example 2, three days after the start of aging, the voltage differences ΔV of all five defective lithium ion secondary batteries fell outside the range of the selection criterion (ΔVA±3σ). In other words, it can be determined that the aging period required to reject all defective lithium ion secondary batteries using the inspection method of Comparative Example 2 is at least three days.

[0052] As described above, by performing a charging step in which a secondary battery is charged at a constant voltage or at a constant voltage and a constant current until the secondary battery reaches a state of charge set within a region in which the slope of the tangent to the SOC-OCV curve of the secondary battery is greater than 0.02, it is possible to shorten the aging period required to discard defective products. Furthermore, by starting charging of the secondary battery with constant voltage charging in the charging step, it is possible to shorten the charging time. In this way, shortening the aging period and charging time significantly shortens the inspection period.

Claims

1. A method for inspecting a secondary battery, comprising: a charging step of constant-voltage charging or constant-voltage-constant-current charging a secondary battery to a predetermined state of charge; an aging step of aging the secondary battery charged in the charging step; a voltage measurement step of measuring at least the voltage of the secondary battery after the aging step; and a sorting step of sorting the secondary battery based on the voltage of the secondary battery measured in the voltage measurement step, wherein the predetermined state of charge is set in a state of charge region where the slope of the tangent to an SOC-OCV curve showing the relationship between the state of charge (SOC) and open circuit voltage (OCV) of the secondary battery is greater than 0.

02.

2. A method for inspecting a secondary battery as described in claim 1, wherein the voltage measurement step includes a first voltage measurement step of measuring a voltage V1 of the secondary battery before the aging step, and a second voltage measurement step of measuring a voltage V2 of the secondary battery after the aging step, and the sorting step involves sorting the secondary batteries based on a voltage difference ΔV between the voltage V1 and the voltage V2.

3. The method for inspecting a secondary battery according to claim 1 or 2, further comprising an initial charge / discharge step of charging / discharging the secondary battery before the charging step.

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