JP2020072059A - Inspection method of secondary cell - Google Patents

Abstract

To provide an inspection method of a secondary cell capable of detecting presence of micro short circuit of the secondary cell accurately.SOLUTION: An inspection method of a secondary cell includes a step of preparing a secondary cell in which a space between an electricity generation element including a positive electrode and a negative electrode and a battery case is insulated by an insulation film, a step of charging the secondary cell, a step of measuring a first voltage, i.e., the potential difference between the negative electrode and the battery case, while compressing for the secondary cell from the outside of the battery case, a step of maintaining the compressed state continuously, compression for the secondary cell is maintained for a predetermined time, a step of measuring a second voltage, i.e., the potential difference between the negative electrode and the battery case, for the secondary cell in which the compression time exceeded the maintenance time, and a step of determining that the secondary battery is a defective unit, when at least one of the first and second voltages is smaller than a predetermined threshold voltage.SELECTED DRAWING: Figure 5

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 as portable power sources for personal computers, mobile terminals, etc., and vehicle driving power sources for electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV), etc. Has been.

  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 hermetically sealed. .. When the battery case is made of metal, the electrode body is housed in the battery case while being covered with an insulating film, and insulation between the electrode body and the battery case is ensured. For this type of battery, charge carriers may deposit on the inner surface of the battery case due to the contact between the electrode body and the battery case during manufacturing, or metal foreign matter may enter the battery case during sealing or welding, and It is known that it can cause a short circuit. A battery in which a micro short-circuit has occurred is unlikely to have a problem immediately after production, but may have a significant capacity decrease due to long-term use, which may cause quality deterioration. Therefore, in the pre-shipment inspection of the battery, a short circuit test is performed to detect a battery in which a minute short circuit has occurred, as a defective product, and the product is not shipped.

Japanese Patent No. 5583480

  For example, in Patent Document 1, for a lithium ion secondary battery after assembly, a potential difference Δ between an outer can or a sealing plate and a negative electrode external terminal is measured after a charging process, and the potential difference Δ is predetermined. Disclosed is a method for inspecting a micro short circuit in which a product having a predetermined value or more is determined as a non-defective product. However, in this inspection method, there is a case where a minute short circuit occurs even in a battery determined to be a non-defective product, and improvement in detection accuracy is required.

  The present invention has been made in view of the above points, and an object thereof is to provide a method for inspecting a secondary battery that can accurately detect the presence or absence of a micro short circuit in the secondary battery.

  According to the inventor's earnest study, even if a battery is determined to be non-defective by a conventional short circuit test, if one or more batteries are applied with a binding pressure for the purpose of obtaining high output, for example. In some cases, a micro short circuit may occur later. It is considered that the subsequent minute 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 constraint pressure applied, it may be judged as a good product or a defective product due to different test timings for the same battery. .. Depending on the form of the battery or foreign matter, there are various things such as a micro short circuit immediately after restraint, a micro short circuit after restraint, or a micro short circuit that is resolved after a short circuit. It is thought that this is because there can be various short circuit forms. It should be noted that the battery in which the micro short circuit has been eliminated is an object to be detected as a defective product because the insulating film is damaged and a short circuit during the subsequent use of the battery is expected.

  Therefore, the method for inspecting a secondary battery disclosed here is a step of preparing a secondary battery in which a power generating element including a positive electrode and a negative electrode and a battery case are insulated by an insulating film, Charging the secondary battery, measuring the first voltage, which is the potential difference between the negative electrode and the battery case in a state where the secondary battery is compressed from the outside of the battery case, and compressing the secondary battery is predetermined. To maintain the compressed state so that the maintained time is maintained, and to measure the second voltage, which is the potential difference between the negative electrode and the battery case, of the secondary battery whose compressed time is equal to or longer than the maintained 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 of the compressed secondary battery is measured at two timings to check the presence or absence of a short circuit. The first time, it detects the secondary battery that is short-circuited immediately due to restraint. The second time, in addition to the secondary battery that is continuously short-circuited, it detects a secondary battery that is not short-circuited immediately after restraint but short-circuited for a while. The second detection is performed at a timing when a predetermined time (for example, 48 hours) has elapsed since the constraint was started. As a result, it is possible to accurately detect a minute short circuit that occurs subsequently due to the constraint. For example, it is possible to accurately detect a battery having a short circuit history, which is not detected only by checking whether or not one of the short circuits is present.

FIG. 4 is a partially cutaway perspective view illustrating the configuration of a secondary battery to be inspected by the inspection method according to the embodiment. It is a graph which shows the measurement example of the negative electrode-battery case voltage before (A) restraint, (B) restraint immediately after, and (C) restraint 24 hours after. (A)-(E) is a cross-sectional schematic diagram explaining the state of the secondary battery and a foreign material by restraint. It is a graph which shows the aspect 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 flowchart 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 the matters particularly referred to in the present specification, which are necessary for carrying out the present invention (for example, the configuration and manufacturing process of a battery that does not characterize the present invention) are the same as those in the related art. It can be understood as a design matter of a person skilled in the art based on the technology. The present invention can be carried out based on the contents disclosed in this specification and the common general technical knowledge in the field. Further, in the following drawings, the same reference numerals are given to the members / sites that have the same effect. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not faithfully reflect the actual dimensional relationships.

  In the present specification, the term “secondary battery” generally refers to a power storage device that can be repeatedly charged and discharged, and is a term that includes so-called storage batteries and power storage elements such as electric double layer capacitors. Further, in the present specification, the “lithium ion battery” refers to a secondary battery that utilizes lithium ions as charge carriers and realizes charging / discharging as lithium ions move 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 shows a lithium ion battery 1 as an example of a secondary battery. Reference sign W in the drawing indicates the width direction of the lithium ion battery 1, and reference sign T indicates the thickness direction of the lithium ion battery 1. FIG. 3 is a schematic cross-sectional view in the thickness direction showing the restrained state and the state inside the battery 1. Hereinafter, each step of the inspection method of the lithium ion battery 1 will be described together with the constituent elements of the battery.

1. Step of Preparing Secondary Battery In step S1, the secondary battery 1 in which the power generation element including the positive electrode and the negative electrode and the battery case 10 are insulated by the insulating film 25 is prepared.
The battery case 10 includes a case body 11 having an opening 11a on one surface, and a lid member 12 that seals the opening 11a of the case body 11. The battery case 10 is made of metal, which is thin but has high strength as compared with a resin material or the like. Examples of the metal forming the battery case 10 include iron, copper, aluminum, titanium, and alloys containing these (for example, steel) and the like. For example, aluminum or aluminum alloy, which is lightweight and easy to process, is preferable. The case body 11 of this example has a flat bottomed rectangular 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 portion 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 to charge and extract electric charge in the battery 1, for example, to measure a potential difference between the negative electrode and the battery case 10.

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

  The negative electrode includes a negative electrode current collector and a porous negative electrode active material layer formed on the surface thereof. A metal foil such as a copper foil is preferably used for the negative electrode current collector. In this example, the negative electrode active material layer is provided on both surfaces of the negative electrode current collector. Further, in the width direction W, the negative electrode active material layer is formed 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 materials conventionally used as a negative electrode active material in lithium ion batteries can be used without particular limitation. Examples thereof include carbon-based materials such as graphite carbon and amorphous carbon, silicon, lithium transition metal oxide, and lithium transition metal nitride. 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), in addition to the above-described negative electrode active material.

  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 minute holes. 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 include a heat resistant layer (HRL).

  These positive electrode and negative electrode can be manufactured according to known methods. For example, a paste containing the constituent components of each active material layer is prepared and applied on a strip-shaped current collector. At this time, the paste is not applied to the ends of the positive electrode and the negative electrode in the width direction of the current collector, and the exposed portion of the current collector is provided. The positive electrode and the negative electrode thus manufactured are laminated with two strip-shaped separators interposed one by one. At this time, the stacking position of the positive electrode, the negative electrode, and the separator is adjusted so that the exposed portion of the positive electrode current collector and the exposed portion of the negative electrode current collector protrude in different width directions. The wound electrode body 20 can be obtained by winding the positive electrode, the negative electrode, and the separator, which are laminated in this way, so that the width direction is a winding axis and the cross section has an oval shape. With respect to such an electrode body 20, the electrode stacking direction can be taken as the elliptical minor axis direction of the cross section of the wound 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 the electrode body 20 and the battery case 10. Due to the presence of the insulating film 25, it is possible to 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 forming the insulating film 25 is not particularly limited, and is made of various materials having electrical insulation. From the viewpoint of cost, flexibility, processability, etc., typically, for example, a polyolefin resin such as polypropylene (PP) or polyethylene (PE), or a flexible resin such as polyphenylene sulfide, polyether ether ketone, nylon, etc. A sheet-shaped resin can be preferably used. The thickness of the insulating film 25 may differ depending on the battery size and the like. The thickness of the insulating film 25 may be, for example, about 40 μm or more, typically 50 μm or more. Furthermore, 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, the 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 appropriately about 250 μm or less, for example, preferably about 200 μm or less, and may be about 180 μm or less.

  When housing the electrode body 20 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 housed in the case body 11. For example, the lid member 12 is provided with positive and negative connection terminals 32 and 42 that are electrically connected to the positive and negative external terminals 30 and 40, respectively, and the positive and negative current collectors of the electrode body 20 are connected to the positive and negative connection terminals 32 and 42. Is fixed (welded). The electrode body 20 supported by the lid member 12 in this manner is arranged so that the lid member 12 is located below, and the remaining five directions except the lid member 12 side may be covered with the insulating film 25. .. The insulating film 25 may be processed into, for example, a bag shape corresponding to the outer shape of the electrode body 20, or may be configured to cover the electrode body 20 by folding a single film. Then, the case body 11 is covered from above the electrode body 20 supported by the lid member 12 with the opening 11a facing downward. The case body 11 and the lid member 12 can be hermetically sealed by, for example, laser welding. Thereby, 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 including the non-aqueous electrolyte solution as the non-aqueous electrolyte, the electrolyte solution is injected into the battery case 10 through the injection hole 15 provided in the lid member 12. The non-aqueous electrolytic 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 the non-aqueous solvent 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 that one or more kinds of the fluorinated chain-like or cyclic carbonates are included. Preferable examples of the lithium salt include LiPF ₆ and LiBF ₄ . The concentration of the lithium salt in the electrolytic solution can be 0.8 to 1.3 mol / L, for example. The injection hole 15 after the injection is sealed by, for example, an injection plug 16 that hermetically closes the 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. Charging Step In step S2, the secondary battery 1 is charged. The assembled lithium-ion battery 1 can have a function as a battery by appropriately performing a predetermined initial charge / discharge process. The initial charging condition is, for example, constant current (CC) charging up to 3.8 V at 0.1 A. Although not essential, in order to ensure high quality of the lithium ion battery 1, typically, an aging treatment may be performed after the initial charge / discharge treatment. Examples of the aging treatment include charging the battery to a state of charge (SOC) of 50% and leaving it to stand for one day in an environment of 65 ° C. Then, in order to perform the short circuit test from the next step, it is preferable that the secondary battery 1 be prepared in a charged state so that the SOC becomes 10% to 100%. In the lithium-ion secondary battery, the battery voltage changes as the SOC changes. When the SOC is in the range of 10% to 100%, the degree of change in the battery voltage is smaller than that when the SOC is less than 10%, and the potential of the negative electrode hardly changes. Therefore, a battery in which the negative electrode external terminal 40 and the battery case 10 are short-circuited can be detected. 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 SOC.

3. First Voltage Measuring Step In step S3, the secondary battery 1 is compressed from the outside of the battery case 10, and the first voltage V1 which is the potential difference between the negative electrode and the battery case 10 is measured. The secondary battery 1 for applications requiring high-rate output characteristics may be used by applying compressive stress in the stacking direction of the electrode bodies 20. Typically, when a plurality of secondary batteries 1 are assembled into a battery pack to expand the output and capacity, a 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. While restraining. In such a secondary battery 1, the battery case 10 is deformed in the thickness direction in a crushing direction according to the compressive stress at the time of the restraint.

  At this time, as shown in FIG. 3A, when the conductive foreign matter M exists between the insulating film 20 and the battery case 10, as shown in FIG. As described above, the conductive foreign matter M may be pressed against the insulating film 20, the insulating film 20 may be broken, and the battery case 10 and the electrode body 20 may be short-circuited. For example, the outermost negative electrode of the electrode body 20 and the case body 11 are likely to be short-circuited. At this time, the lithium ions dissolved in the non-aqueous electrolyte from the negative electrode move to the case body 11, and the potential of the case decreases. If the battery case 10 is made of aluminum or an aluminum alloy, for example, lithium may easily alloy with the aluminum component and corrode the battery case 10. Therefore, the secondary battery 1 including the battery case 10 that has been short-circuited even once should cause a safety problem such as liquid leakage and should be detected as a defective product.

  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) as shown in FIG. Immediately after compression (B), V1 drops to about 1.4V. In this way, for example, when the threshold voltage VS for determining the presence or absence of a short circuit is set to 2.0 V or the like, if the first voltage V1 immediately after compression (B) is lower than the threshold voltage VS, the battery concerned. It is possible to detect that there is a short circuit. 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 broken and the battery case 10 and the electrode body 20 are short-circuited. May not be. The potential difference between the negative electrode (negative electrode external terminal 40) and the battery case 10 at this time is, for example, V0 = V1 = about 2.8 V before and after compression, as shown in FIG. possible. At this time, since the first voltage V1 is higher than the threshold voltage VS, it is possible to detect 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 measurement timing of the first voltage V1 may be immediately after compression, or may be, for example, the time from the compression until about 12 hours have elapsed. The first voltage V1 may be measured, for example, for 0 to 12 hours after applying the compressive stress.

  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 condition of the plurality of batteries. For example, the magnitude of the compressive stress may be substantially the same as the compressive stress when constraining to construct the assembled battery or the like (for example, 85% to 110%), or the secondary battery 1 that leads to a short circuit may be more effective. In order to detect at an early timing, it may be set higher (for example, about 110% to 200%) than the compressive stress at the time of restraint. 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 that fixes these restraint plates at predetermined intervals can be preferably used. For example, the secondary battery 1 is sandwiched between the two constraining plates in the thickness direction T, and a predetermined compressive stress is applied from the outside of the constraining plates. Then, in such a compressed state, the two constraining plates are fixed by a constraining member. This makes it possible to load the secondary battery 1 in a predetermined compressed state simply and stably. The secondary battery 1 may restrain a plurality of batteries with a set of restraining jigs.

4. Maintaining Compressed State In step S4, the compressed state is continuously maintained so that the compression of the secondary battery 1 takes a predetermined maintenance time. Here, for example, the secondary battery 1 in a compressed state by the restraint jig may be left as it is. At this time, as shown in FIG. 3B, the secondary battery 1 that has been in the short-circuited state at the time of measuring the first voltage may sometimes maintain the short-circuited state as it is. For example, as shown in FIG. As described above, when the conductive foreign matter M is pressed for a long time, the electrode body 20 is deformed, the conductive foreign matter M penetrates into the electrode body 20, and the short circuit between the electrode body 20 and the battery case 10 is eliminated. There are cases where Further, as shown in FIG. 3D, even in the secondary battery 1 that did not result in a short circuit at the time of measuring the first voltage, the insulating film 20 is crushed by compression after that, and the secondary battery 1 may be compressed as shown in FIG. As shown in, there may be a short circuit. Although the maintenance time did not result in a short circuit immediately after the restraint, it can be set to such a time that the conductive foreign matter M that can induce a short circuit during the subsequent use of the battery causes a short circuit. This maintaining time is, for example, a size to be detected while maintaining the above-mentioned compressed state in a state where the conductive foreign matter M having a size expected to be mixed is mixed in advance for each secondary battery to be inspected. It is possible to set a sufficient time for detecting a short-circuit state in the secondary battery in which the conductive foreign matter M is mixed, and for confirming a decrease in the second voltage described later. The maintenance time cannot be generally stated because it depends on the configuration of the battery (for example, the material and thickness of the separator), but it may be set to, for example, 36 hours or longer, preferably 48 hours or longer.

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 that has been compressed for at least the above maintenance time. The second voltage V2 may be measured by measuring the potential difference between the external terminal 40 and the battery case 10, similarly to the measurement of the first voltage V1. For example, as shown in FIG. 2 (B), the secondary battery that is in the short-circuited state when the second voltage V2 is measured has an inter-terminal voltage that is V0 = about 2.8 V before compression (A), for example. (B) V2 is reduced to about 1.4V. At this time, since 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 that has been in a short-circuit state at the time of measuring the first voltage but has eliminated the short-circuit at the time of measuring the second voltage has, for example, (B) first voltage V1 = What has been lowered to about 1.4V can be restored 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 is impossible to determine whether or not the battery was short-circuited in the past with only the second voltage. In step S5, the second voltage V2 after maintaining the compressed state is thus measured.

6. Determination In step S6, when at least one of the first voltage V1 and the second voltage V2 is lower than a predetermined threshold voltage VS, it is determined that the secondary battery 1 is defective. 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 good product. The state of the determination 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 and the threshold voltage is classified into four types of Examples 1 to 4. Then, for example, the secondary battery 1 categorized in Example 1 is short-circuited during both the first voltage measurement and the second voltage measurement, and thus should be determined as a defective product. The secondary battery 1 categorized in Example 2 was short-circuited at the time of measuring the first voltage, but was eliminated at the subsequent measurement of the second voltage. Such a secondary battery 1 can be determined as a non-defective product only by measuring the second voltage. However, even if the secondary battery 1 is short-circuited even once, the insulating film 20 is damaged or charge carriers are deposited on the battery case 10. Therefore, it should be judged as a defective product. For example, the insulating film 20 having a hole as large as a pinhole is likely to be damaged from this pinhole as a starting point, and may be broken from the pinhole as a starting point when the insulating film 20 is thermally contracted at a high temperature. According to the inspection method disclosed herein, even 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 manner in which the voltage between the negative electrode of the secondary battery and the battery case changes with time due to restraint. In the battery of Example 2 which was short-circuited immediately after the application of the compressive stress due to the restraint, the voltage between the negative electrode and the battery case became smaller as time passed from 3 hours to 12 hours after compression, and the short circuit occurred after about 15 hours. It has been confirmed that it has been resolved. According to the study by the present inventors, even in the case of a battery that has once been short-circuited as described above, the short-circuiting is often solved over time depending on the hardness of the electrode body 20 around the conductive foreign matter M and the like.

  The secondary battery 1 categorized in Example 3 is a battery that was not short-circuited when measuring the first voltage V1, but was short-circuited when measuring the second voltage V2 thereafter. It can be confirmed that the battery of Example 3 shown in FIG. 4 was not short-circuited after 45 hours from compression, but was short-circuited after 48 hours. The timing of such a short circuit tends to occur within about 48 hours, although it depends on the battery condition, the compression condition, 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 a short circuit may occur if the compressed state continues thereafter, and thus should be determined to be a defective product. .. According to the inspection method disclosed herein, even a battery that is short-circuited with such a delay can be determined as a defective product, which is preferable.

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

  As described above, according to the technology disclosed herein, it is possible to highly accurately detect a minute short circuit that may be generated later due to the restraint of the secondary battery used for restraining use. 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 passed, and is measured as the second voltage V2 at the timing at which 48 hours or more have passed 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 with high accuracy as a defective product. The threshold voltage VS cannot be generally stated because it may vary depending on the configuration of the battery (combination of positive and negative active materials) and the scale of the short circuit, but is 0.2 V to 2.5 V, for example, about 2 V. Preferably.

  As a result, it is possible to select and ship the secondary battery having no history of short circuit due to restraint. Here, the lithium-ion battery 1 determined as a non-defective product has a smaller occurrence of a micro short circuit when restrained than a battery determined as a non-defective product by a conventional inspection method. Therefore, for example, when used as a battery for a restricted use, the possibility of a short circuit is significantly reduced, and the battery can be provided as a highly reliable battery.

  Although the present invention has been described in detail above, the above-described embodiments and examples are merely examples, and the invention disclosed herein includes various modifications and changes of the specific examples described above. For example, the electrode body is not limited to the wound 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 with a separator interposed therebetween. Further, the technique disclosed herein can be particularly suitably applied to a battery using a non-aqueous electrolytic 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 25 Insulating Film

Claims (1)

A step 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;
Charging the secondary battery,
Measuring a first voltage, which is a potential difference between the negative electrode and the battery case, in a state where the secondary battery is compressed from the outside of the battery case;
Compression of the secondary battery, so as to have a predetermined maintenance time, the step of continuously maintaining the compression state,
Measuring a second voltage, which is a potential difference between the negative electrode and the battery case, for the secondary battery in which the compression is equal to or longer than the maintenance time; and
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,
A method for inspecting a secondary battery, including:

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