JP7011782B2 - Secondary battery inspection method - Google Patents

Description

The present invention relates to a method for inspecting a secondary battery.

In recent years, batteries such as lithium-ion batteries have been suitably used for portable power sources such as personal computers and mobile terminals, and vehicle drive power sources for electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV) and the like. Has been done.

Such a battery generally has a structure in which an electrode body in which a positive electrode and a negative electrode are laminated via a separator and a non-aqueous electrolyte are housed in a battery case and airtightly sealed. .. When the battery case is made of metal, the electrode body is housed in the battery case in a state of being covered with an insulating film, and the insulation between the electrode body and the battery case is ensured. For this type of battery, the contact between the electrode body and the battery case during manufacturing causes charge carriers to precipitate on the inner surface of the battery case, and metal foreign matter invades the battery case during sealing or welding, resulting in minute amounts. It is known that it can cause a short circuit. A battery with a minute short circuit is unlikely to cause any problems immediately after manufacturing, but the capacity may decrease significantly with long-term use, which may cause a decrease in quality. Therefore, in the pre-shipment inspection of batteries, a short-circuit test is performed to detect a battery having a minute short circuit as a defective product and prevent it from being shipped.

Japanese Patent No. 5583480

For example, in Patent Document 1, the potential difference Δ between the outer can or the sealing plate and the negative electrode external terminal is measured after the charging step of the assembled lithium ion secondary battery, and the potential difference Δ is predetermined. It discloses a method for inspecting a minute short circuit in which a product having a value equal to or higher than a predetermined value is judged to be a non-defective product. However, in this inspection method, even a battery judged to be a non-defective product may have a minute short circuit, and improvement in detection accuracy is required.

The present invention has been made in view of this point, and an object of the present invention is to provide a method for inspecting a secondary battery, which can accurately detect the presence or absence of a minute short circuit in the secondary battery.

According to the diligent study of the present inventor, even if the battery is judged to be a non-defective product by the conventional short circuit test, when one or more batteries are subjected to the confining pressure for the purpose of obtaining high output, for example. In some cases, a minute short circuit may occur later. It is considered that the subsequent micro short circuit induced by such restraint can be detected by performing a short circuit test with the restraint pressure applied to the battery. However, it was found that even if a short-circuit test is performed with a confining pressure applied, it may be judged as a non-defective product or a defective product because the test timing is different for the same battery. .. Depending on the form of the battery or foreign matter, there are various types, such as those that cause a minute short circuit immediately after restraint, those that lead to a minute short circuit shortly after restraint, and those that eliminate the minute short circuit after a while even if the minute short circuit occurs. It is considered that this is because there may be a short circuit form. It should be noted that the battery in which the minute short circuit has been resolved is a target to be detected as a defective product because the insulating film is damaged and a short circuit is expected when the battery is used thereafter.

Therefore, the method for inspecting a secondary battery disclosed here is a step of preparing a secondary battery in which a power generation element including a positive and negative electrodes and a battery case are insulated with an insulating film, for the secondary battery. The process of charging the secondary battery, the process of measuring the first voltage, which is the potential difference between the negative electrode and the battery case while the secondary battery is compressed from the outside of the battery case, and the compression of the secondary battery are predetermined. A step of continuously maintaining the compressed state so as to have the maintained maintenance time, and a step of measuring the second voltage, which is the potential difference between the negative electrode and the battery case, for the secondary battery whose compression time is longer than the above maintenance time. , And a step of determining that the secondary battery is defective when at least one of the first voltage and the second voltage is smaller than a predetermined threshold voltage.

In this secondary battery inspection method, the voltage between the negative electrode and the battery case is measured twice at the compressed secondary battery to confirm the presence or absence of a short circuit. The first time, a secondary battery that is short-circuited immediately due to restraint is detected. The second time, in addition to the secondary battery that is continuously short-circuited, the secondary battery that is short-circuited for a while without short-circuiting immediately after restraint is detected. The second detection is performed at a timing when a predetermined time (for example, 48 hours) has elapsed from the start of restraint. As a result, it is possible to accurately detect a minute short circuit that occurs later due to restraint. For example, it is possible to accurately detect a battery having a short circuit history, which cannot be detected only by confirming the presence or absence of a short circuit in either one.

It is a partially cutaway perspective view illustrating the configuration of the secondary battery to be inspected by the inspection method according to one embodiment. It is a graph which shows the measurement example of the voltage between a negative electrode and a battery case (A) before restraint, (B) immediately after restraint, and (C) 24 hours after restraint. (A) to (E) are schematic cross-sectional views illustrating the state of the secondary battery and the foreign matter due to restraint. It is a graph which shows the mode of the time-dependent change of the voltage between the negative electrode of a secondary battery and a battery case by restraint. It is a flow chart of the inspection method of the secondary battery which concerns on one Embodiment.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings. It should be noted that matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention (for example, battery configurations and manufacturing processes that do not characterize the present invention) are conventionally described in the art. It can be grasped as a design matter of a person skilled in the art based on technology. The present invention can be carried out based on the contents disclosed in the present specification and the common general technical knowledge in the art. Further, in the following drawings, members / parts having the same action are described with the same reference numerals. In addition, the dimensional relationships (length, width, thickness, etc.) in each figure do not faithfully reflect the actual dimensional relationships.

In the present specification, the "secondary battery" generally refers to a power storage device capable of being repeatedly charged and discharged, and is a term including a so-called storage battery and a power storage element such as an electric double layer capacitor. Further, the term "lithium ion battery" as used herein refers to a secondary battery that uses lithium ions as a charge carrier and realizes charging and discharging with the movement of lithium ions between the positive and negative electrodes.

The battery inspection method according to the present embodiment includes steps S1 to S6 shown in FIG. 5 in this order. Steps S3 to S6 correspond to a so-called short circuit test.
FIG. 1 is a lithium ion battery 1 as an example of a secondary battery. In the figure, reference numeral W indicates the width direction of the lithium ion battery 1, and reference numeral T indicates the thickness direction of the lithium ion battery 1. FIG. 3 is a schematic cross-sectional view in the thickness direction showing a restrained state and a state inside the battery 1. Hereinafter, each step of the inspection method of the lithium ion battery 1 will be described together with the components of the battery.

1. 1. Preparation step of secondary battery In step S1, a secondary battery 1 in which a power generation element including a positive electrode and a negative electrode and a battery case 10 are insulated by an insulating film 25 is prepared.
The battery case 10 includes a case main body 11 having an opening 11a on one surface, and a lid member 12 for sealing the opening 11a of the case main body 11. The battery case 10 is made of metal having high strength even if it is thin, as compared with a resin material or the like. Suitable examples of the metal constituting the battery case 10 include iron, copper, aluminum, titanium, and an alloy containing these (for example, steel). For example, it is preferably aluminum or an aluminum alloy that is lightweight and easy to process. The case body 11 of this example has a flat bottomed square tube shape. The lid member 12 includes a liquid injection hole 15 sealed with a liquid injection plug 16, a safety valve 18, a positive electrode external terminal 30, and a negative electrode external terminal 40. The positive electrode external terminal 30 and the negative electrode external terminal 40 are provided one by one at the end of the battery case 10 in the width direction W so as to project to the outside of the case. The positive electrode external terminal 30 and the negative electrode external terminal 40 are used for charging and extracting electric charges from the battery 1 and, for example, measuring the potential difference between the negative electrode and the battery case 10.

The electrode body 20 of this example is an example of a power generation element in the present invention. The electrode body 20 includes a positive electrode, a negative electrode, and a separator. The positive electrode and the negative electrode are laminated and wound in a state of being insulated from each other by a separator to form an electrode body 20.
The positive electrode includes a positive electrode current collector and a porous positive electrode active material layer formed on the surface thereof. For the positive electrode current collector, for example, a metal foil such as an aluminum foil is preferably used. In this example, positive electrode active material layers are provided on both sides of the positive electrode current collector. Further, in the width direction W, the positive electrode active material layer is formed to be narrow so that the positive electrode current collector on the side of the positive electrode external terminal 30 is exposed. The positive electrode active material layer contains granular positive electrode active material. As the positive electrode active material, one or more of the substances conventionally used as the positive electrode active material in the lithium ion battery can be used without particular limitation. Examples include lithium nickel cobalt manganese composite oxides (eg LiNi _1/3 Co _1/3 Mn _1/3 O ₂ etc.), lithium nickel composite oxides (eg LiNiO ₂ etc.), lithium cobalt composite oxides. Examples include lithium transition metal composite oxides such as (eg, LiCoO ₂ etc.) and lithium nickel manganese composite oxides (eg LiNi _0.5 Mn _1.5 O ₄ etc.). In addition to the above-mentioned positive electrode active material, the positive electrode active material layer may contain a conductive material such as acetylene black (AB) and a binder such as polyvinylidene fluoride (PVDF) and styrene butadiene rubber (SBR).

The negative electrode includes a negative electrode current collector and a porous negative electrode active material layer formed on the surface thereof. For the negative electrode current collector, for example, a metal foil such as a copper foil is preferably used. In this example, the negative electrode active material layer is provided on both sides of the negative electrode current collector. Further, in the width direction W, the negative electrode active material layer is formed to be narrow so that the negative electrode current collector on the negative electrode external terminal 40 side is exposed. The negative electrode active material layer contains a negative electrode active material. As the negative electrode active material, one kind or two or more kinds of substances conventionally used as a negative electrode active material in a lithium ion battery can be used without particular limitation. Examples thereof include carbon-based materials such as graphite carbon and amorphous carbon, silicon, lithium transition metal oxides, and lithium transition metal nitrides. In addition to the above-mentioned negative electrode active material, the negative electrode active material layer may contain a binder such as polyvinylidene fluoride (PVDF) and styrene butadiene rubber (SBR), and a thickener such as carboxymethyl cellulose (CMC).

The separator is a porous member that electrically insulates the positive electrode and the negative electrode. In this example, the separator is composed of a microporous sheet having a plurality of micropores. As the separator, for example, a single-layer structure separator made of a porous polyolefin resin or a multi-layer structure separator can be used. The separator may be provided with a heat resistant layer (HRL).

These positive electrodes and negative electrodes can be manufactured according to a known method. For example, a paste containing the constituents of each active material layer is prepared and applied onto a band-shaped current collector. At this time, the paste is not applied to the widthwise ends of the positive electrode and the negative electrode current collectors, and exposed portions of the current collectors are provided. The positive electrode and the negative electrode thus produced are laminated via two strip-shaped separators one by one. At this time, the overlapping positions of the positive electrode, the negative electrode, and the separator are adjusted so that the exposed portion of the positive electrode current collector and the exposed portion of the negative electrode current collector project in different directions in the width direction. The wound electrode body 20 can be obtained by winding the positive electrode, the negative electrode, and the separator laminated in this way so that the winding axis is in the width direction and the cross section is oval. With respect to such an electrode body 20, the stacking direction of the electrodes can be taken in the elliptical short axis direction of the cross section of the wound type electrode body 20.

The insulating film 25 is arranged between the electrode body 20 which is a power generation element and the battery case 10, and electrically insulates between the electrode body 20 and the battery case 10. The presence of the insulating film 25 can prevent the electrode body 20 and the battery case 10 from coming into contact with each other and short-circuiting during and after manufacturing. The material constituting the insulating film 25 is not particularly limited, and is composed of various materials having electrical insulating properties. From the viewpoint of cost, flexibility, processability, etc., it is typically made of a polyolefin resin such as polypropylene (PP) or polyethylene (PE), polyphenylene sulfide, polyetheretherketone, nylon or the like. A sheet-like resin can be preferably used. The thickness of the insulating film 25 may vary depending on the body shape of the battery and the like. The thickness of the insulating film 25 may be, for example, about 40 μm or more, and typically 50 μm or more. Further, the thickness of the insulating film 25 may be 100 μm or more, 120 μm or more, and 140 μm or more. However, if the insulating film is too thick, heat dissipation of self-heating associated with charging and discharging of the battery may be hindered, which is not preferable. Therefore, the thickness of the insulating film 25 is preferably about 250 μm or less, preferably about 200 μm or less, and may be about 180 μm or less.

When the electrode body 20 is housed in the case body 11, the electrode body 20 is fixed to the lid member 12 in advance, the electrode body 20 is covered with the insulating film 25, and then the electrode body 20 is housed in the case body 11. For example, positive and negative connection terminals 32 and 42 that are electrically connected to positive and negative external terminals 30 and 40 are arranged on the lid member 12, and positive and negative current collectors of the electrode body 20 are arranged on the positive and negative connection terminals 32 and 42, respectively. Is fixed (welded). The electrode body 20 supported by the lid member 12 in this manner may be arranged so that the lid member 12 is located below, and the remaining five directions except the side of the lid member 12 may be covered with the insulating film 25. .. The insulating film 25 may be processed into a bag shape corresponding to the outer shape of the electrode body 20, for example, or may be configured to cover the electrode body 20 by folding a single film. Then, the case body 11 is covered with the opening 11a facing downward from above the electrode body 20 supported by the lid member 12. The case body 11 and the lid member 12 can be hermetically sealed by, for example, laser welding. As a result, the electrode body 20 can be housed in the case body 11 while being insulated by the insulating film 25. Further, the stacking direction of the electrodes in the electrode body 20 coincides with the thickness direction T of the battery case 10.

In the lithium ion battery 1 provided with the non-aqueous electrolytic solution as the non-aqueous electrolyte, the electrolytic solution is injected into the inside of the battery case 10 from the liquid injection hole 15 provided in the lid member 12. The non-aqueous electrolyte solution contains a non-aqueous solvent and an electrolyte-supporting salt. The electrolytic solution typically exhibits a liquid state at room temperature (typically 0 to 25 ° C.). The electrolyte supporting salt is, for example, a lithium salt. The non-aqueous solvent and the lithium salt are not particularly limited, and may be the same as those used in the electrolytic solution of the conventional secondary battery. Preferable examples of non-aqueous solvents include aprotic solvents such as carbonates, esters and ethers. Among them, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), chain carbonates such as diethyl carbonate (DEC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC), and these carbonates are fluorine. It is preferable to contain one or more of the fluorinated chain or cyclic carbonates. Preferable examples of the lithium salt include, for example, LiPF ₆ , LiBF ₄ , and the like. The concentration of the lithium salt in the electrolytic solution can be, for example, 0.8 to 1.3 mol / L. The liquid injection hole 15 after the liquid injection is sealed by, for example, a liquid injection plug 16 that airtightly closes the liquid injection hole 15. The lid member 12 and the liquid injection plug 16 may be firmly fixed by welding, for example. Thereby, the lithium ion battery 1 can be constructed.

2. 2. Charging step In step S2, the secondary battery 1 is charged. The assembled lithium-ion battery 1 can be provided with a function as a battery by appropriately performing a predetermined initial charge / discharge treatment. The initial charging condition is exemplified by constant current (CC) charging up to 3.8 V at 0.1 A, for example. Further, although not essential, in order to ensure the high quality of the lithium ion battery 1, an aging treatment may be typically performed after the initial charge / discharge treatment. As an example of the aging treatment, for example, the battery is charged to a state of charge (SOC) of 50% and left in an environment of 65 ° C. for one day. Then, the secondary battery 1 should be in a charged state so that the SOC is 10% to 100% in order to perform a short-circuit test from the next step. In a lithium ion secondary battery, when the SOC changes, the battery voltage also changes accordingly. When the SOC is in the range of 10% to 100%, the degree of change in the battery voltage is smaller than when the SOC is less than 10%, and the potential of the negative electrode hardly changes. Therefore, it becomes possible to detect a battery in which the negative electrode external terminal 40 and the battery case 10 are short-circuited. In the charging step, the voltage (initial voltage V0) of the secondary battery 1 after charging (in other words, before the short-circuit test) can be grasped at the time of adjusting the SOC.

3. 3. First voltage measuring step In step S3, the first voltage V1, which is the potential difference between the negative electrode and the battery case 10, is measured with respect to the secondary battery 1 in a state of being compressed from the outside of the battery case 10. The secondary battery 1 for applications that require high-rate output characteristics and the like may be used by applying compressive stress in the stacking direction of the electrode body 20. Typically, when a plurality of secondary batteries 1 are combined into a combined battery to expand the output and capacity, compressive stress is applied to the plurality of secondary batteries 1 in the electrode stacking direction, that is, in the thickness direction T of the battery case 10. Restrain while. In such a secondary battery 1, the battery case 10 is deformed in the thickness direction in the direction of being crushed in response to the compressive stress at the time of restraint.

At this time, as shown in FIG. 3A, when the conductive foreign matter M is present between the insulating film 20 and the battery case 10, it is shown in FIG. 3B due to the deformation of the battery case 10. As described above, the conductive foreign matter M may be pressed against the insulating film 20, the insulating film 20 may be torn, and the battery case 10 and the electrode body 20 may be short-circuited. For example, the negative electrode on the outermost circumference of the electrode body 20 and the case body 11 are likely to be short-circuited. At this time, the lithium ions dissolved from the negative electrode into the non-aqueous electrolyte move to the case body 11, and the potential of the case drops. For example, when the battery case 10 is made of aluminum or an aluminum alloy, lithium may easily alloy with the aluminum component and corrode the battery case 10. Therefore, the secondary battery 1 provided with the battery case 10 that has been short-circuited even once should be detected as a defective product because safety problems such as liquid leakage may occur.

As shown in FIG. 2, the potential difference between the negative electrode (negative electrode external terminal 40) and the battery case 10 at this time is, for example, V0 = about 2.8 V before compression (A), for example. Immediately after compression (B), V1 = about 1.4V. In this way, for example, if the threshold voltage VS for determining the presence or absence of a short circuit is set to 2.0 V or the like, when the first voltage V1 immediately after compression (B) is lower than the threshold voltage VS, the battery is concerned. Can be detected as short-circuited. On the other hand, as shown in FIG. 3D, even when the conductive foreign matter M is pressed against the insulating film 20 by compression, the insulating film 20 is not torn and the battery case 10 and the electrode body 20 are short-circuited. May not be. At this time, the potential difference between the negative electrode (negative electrode external terminal 40) and the battery case 10 is, for example, V0 = V1 = about 2.8V before and immediately after compression, as shown in FIG. 2 (A). possible. At this time, since the first voltage V1 is higher than the threshold voltage VS, it can be detected that the battery is not short-circuited. In step S3, the first voltage V1 immediately after applying the compressive stress is measured in this way. The timing of measurement of the first voltage V1 may be immediately after compression, or may be, for example, the time from compression until about 12 hours have elapsed. The first voltage V1 may be measured, for example, between 0 and 12 hours after the compressive stress is applied.

The magnitude of the compressive stress applied to the secondary battery 1 can be appropriately set according to the application of the battery, for example, the constraint conditions of a plurality of batteries. For example, the magnitude of the compressive stress may be substantially the same as the compressive stress when constrained to construct an assembled battery or the like (for example, 85% to 110%), or the secondary battery 1 leading to a short circuit may be used. In order to detect at an early timing, it may be set higher than the compressive stress at the time of restraint (for example, about 110% to 200%). For applying the compressive stress to the secondary battery 1, for example, a restraint jig including a pair of restraint plates and a restraint member for fixing these restraint plates at predetermined intervals can be preferably used. For example, the secondary battery 1 is sandwiched between two restraint plates in the thickness direction T, and a predetermined compressive stress is applied from the outside of the restraint plates. Then, in such a compressed state, the space between the two restraining plates is fixed by the restraining member. As a result, a predetermined compressed state can be easily and stably loaded on the secondary battery 1. The secondary battery 1 may restrain a plurality of batteries by a set of restraining jigs.

4. Step of maintaining the compressed state In step S4, the compressed state is continuously maintained so that the compression of the secondary battery 1 has a predetermined maintenance time. Here, for example, the secondary battery 1 compressed by the restraint jig may be left as it is. At this time, as shown in FIG. 3B, the secondary battery 1 that was in the short-circuited state at the time of the first voltage measurement may maintain the short-circuited state as it is, and is shown in FIG. 3C, for example. As described above, the electrode body 20 is deformed by being pressed against the conductive foreign matter M for a long time, and the conductive foreign matter M penetrates into the electrode body 20 to eliminate the short circuit between the electrode body 20 and the battery case 10. May occur. Further, as shown in FIG. 3 (d), even if the secondary battery 1 does not cause a short circuit at the time of the first voltage measurement, the insulating film 20 is subsequently crushed by compression, and the like, FIG. 3 (e). As shown in, it may lead to a short circuit. The maintenance time does not lead to a short circuit immediately after restraint, but can be set to such a time that the conductive foreign matter M that can induce a short circuit when the battery is used thereafter causes a short circuit. This maintenance time is, for example, a size to be detected by maintaining the above-mentioned compressed state in a state where a conductive foreign substance M having a size expected to be mixed in advance is mixed in each secondary battery to be inspected. It can be set as a time sufficient to detect a short-circuit state of the secondary battery mixed with the conductive foreign matter M and to confirm a decrease in the second voltage, which will be described later. The maintenance time cannot be unequivocally determined because it depends on the battery configuration (for example, the material and thickness of the separator), but it may be set to, for example, 36 hours or more, preferably 48 hours or more.

5. Second voltage measurement step In step S5, the second voltage V2, which is the potential difference between the negative electrode and the battery case 10, is measured for the secondary battery 1 whose compression is equal to or longer than the above-mentioned maintenance time. The measurement of the second voltage V2 may be performed by measuring the potential difference between the external terminal 40 and the battery case 10 in the same manner as the measurement of the first voltage V1. In the secondary battery that is in a short-circuited state when the second voltage V2 is measured, for example, as shown in FIG. 2B, the terminal voltage that was V0 = about 2.8V before compression (A) is, for example. (B) V2 is reduced to about 1.4V. At this time, when the second voltage V2 becomes lower than the threshold voltage VS, it is possible to detect that the battery is short-circuited. On the other hand, as shown in FIG. 3C, the secondary battery 1 which was in a short-circuited state at the time of the first voltage measurement but the short-circuit was resolved at the time of the second voltage measurement is, for example, (B) the first voltage V1 =. What had dropped to about 1.4V can be recovered to V2 = about 2.8V, as shown in FIG. 2 (C). At this time, since the second voltage V2 is higher than the threshold voltage VS, it is detected that the battery is not short-circuited, and it cannot be determined whether or not the battery has been short-circuited in the past only by the second voltage. In step S5, the second voltage V2 after maintaining the compressed state is measured in this way.

6. In the determination step S6, when at least one of the first voltage V1 and the second voltage V2 is smaller than the predetermined threshold voltage VS, it is determined that the secondary battery 1 is a defective product. In other words, when both the first voltage V1 and the second voltage V2 are equal to or higher than the threshold voltage VS, it is determined that the secondary battery 1 is a non-defective product. The state of the judgment is shown in Table 1 below.

As shown in Table 1, in this evaluation method, the relationship between the first voltage V1 and the second voltage V2 with the threshold voltage is classified into four examples 1 to 4. Then, for example, the secondary battery 1 categorized in Example 1 should be determined to be a defective product because it was short-circuited during both the first voltage measurement and the second voltage measurement. The secondary battery 1 categorized in Example 2 had a short circuit at the time of the first voltage measurement, but the short circuit was resolved at the time of the subsequent second voltage measurement. Such a secondary battery 1 can be judged to be a good product only by measuring the second voltage, but in the secondary battery 1 that has been short-circuited even once, the insulating film 20 is damaged or a charge carrier is deposited on the battery case 10. It should be judged as a defective product because it may be defective. For example, the insulating film 20 having a hole as large as a pinhole is likely to be damaged from the pinhole as a starting point. For example, when the insulating film 20 is thermally shrunk at a high temperature, the insulating film 20 may be broken from the pinhole as a starting point. According to the inspection method disclosed herein, it is preferable that such a battery that is not apparently short-circuited can be determined as a defective product in consideration of its history.

Note that FIG. 4 is a graph showing the mode of the time-dependent change in the voltage between the negative electrode and the battery case of the secondary battery due to restraint. In the battery of Example 2 which was short-circuited immediately after the application of compressive stress due to restraint, the voltage between the negative electrode and the battery case became smaller as time passed, 3 hours and 12 hours after compression, and the short circuit occurred approximately 15 hours later. It has been confirmed that it has been resolved. According to the study of the present inventor, even if the battery is short-circuited as described above, the short-circuit is often resolved with the passage of time depending on the hardness of the electrode body 20 around the conductive foreign substance M and the like.

Further, the secondary battery 1 categorized in Example 3 is a battery that was not short-circuited when the first voltage V1 was measured, but was short-circuited when the second voltage V2 was subsequently measured. It can be confirmed that the battery of Example 3 shown in FIG. 4 was not short-circuited 45 hours after compression, but was short-circuited 48 hours later. The timing of such a short circuit tends to occur by about 48 hours, although it depends on the battery conditions, compression conditions, and the like. Therefore, the measurement of the second voltage V2 is preferably after 48 hours. For example, it may be between 48 hours and 72 hours. The secondary battery 1 as in Example 3 is determined to be a non-defective product only by measuring the first voltage V1, but should be determined to be a defective product because a short circuit may occur if the compressed state continues thereafter. .. According to the inspection method disclosed herein, a battery short-circuited with such a delay is also preferable because it can be determined as a defective product.

The secondary battery 1 categorized in Example 4 is not short-circuited during the first voltage measurement and the second voltage measurement, and can be said to be a good product. According to the inspection method disclosed here, it is preferable because the secondary battery 1 having no history of short circuit can be discriminated as a non-defective product. According to the study of the present inventor, among the batteries short-circuited due to the compressive stress due to restraint, the battery short-circuited immediately after restraint and which maintains the short-circuited state even after a sufficient time has elapsed is the battery of Example 1. The characteristic was that the ratio of was extremely small. For example, in the short circuit test shown in FIG. 4, no battery categorized as Example 1 was found. In this respect, the inspection method disclosed herein has special significance that cannot be obtained by the conventional short circuit inspection method.

As described above, according to the technique disclosed herein, it is possible to detect a minute short circuit that may occur later due to the restraint with high accuracy in the secondary battery for the purpose of being restrained and used. For example, the voltage between the negative electrode and the battery case is measured as the first voltage V1 at the timing immediately after compression until 12 hours have elapsed, and as the second voltage V2 at the timing when 48 hours or more have elapsed immediately after compression. When either one of the first voltage V1 and the second voltage V2 is lower than the threshold voltage VS, the battery can be determined to be a defective product with high accuracy. The threshold voltage VS cannot be unequivocally determined because it can vary depending on the battery configuration (combination of positive and negative active materials), the scale of the short circuit, etc., but it is, for example, 0.2V to 2.5V above, for example, about 2V. Is preferable.

This makes it possible to select and ship secondary batteries that have no history of short circuits due to restraint. Here, the lithium ion battery 1 determined to be a non-defective product suppresses the occurrence of a minute short circuit even when it is restrained, as compared with the battery determined to be a non-defective product by the conventional inspection method. Therefore, for example, when the battery is used in a restrained manner, the possibility of a short circuit is greatly reduced, and the battery can be provided with high reliability.

Although the present invention has been described in detail above, the above-described embodiments and examples are merely examples, and the inventions disclosed herein include various modifications and modifications of the above-mentioned specific examples. For example, the electrode body is not limited to the winding type, and may be a so-called laminated type electrode body in which a plurality of plate-shaped positive electrodes and negative electrodes are laminated via a separator. Further, the technique disclosed herein can be particularly preferably applied to a battery using a non-aqueous electrolyte solution as an electrolyte, but can also be applied to a battery using a solid electrolyte.

1 Battery 10 Battery case 11 Case body 12 Lid member 20 Electrode body 25 Insulation film

Claims (1)

A process of preparing a secondary battery in which a power generation element including a positive electrode and a negative electrode and a battery case are insulated with an insulating film.
The process of charging the secondary battery,
A step of measuring a first voltage, which is a potential difference between the negative electrode and the battery case, in a state of being compressed from the outside of the battery case with respect to the secondary battery.
A step of continuously maintaining the compressed state so that the compression of the secondary battery has a predetermined maintenance time.
A step of measuring a second voltage, which is a potential difference between the negative electrode and the battery case, for the secondary battery whose compression is longer than the maintenance time, and
A step of determining that the secondary battery is a defective product when at least one of the first voltage and the second voltage is smaller than a predetermined threshold voltage.
How to inspect secondary batteries, including.

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