Secondary battery manufacturing method

Abstract

To prevent a short circuit caused by the dissolution and precipitation of contaminated metal foreign matter.SOLUTION: A secondary battery manufacturing method includes the steps of repeatedly charging a secondary battery before initial charging at a voltage Va and discharging at a voltage Vb lower than the voltage Va, and initially charging the secondary battery.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a secondary battery. [Background technology]

[0002] Secondary batteries, such as lithium-ion secondary batteries and solid-state batteries, have a problem in that metallic foreign matter may be mixed into the battery during the manufacturing process, causing a short circuit. Specifically, when a battery containing metallic foreign matter is charged, the metallic foreign matter dissolves in the electrolyte or solid electrolyte layer, ionizes, and deposits on the negative electrode, which has a low potential. The metallic deposits grow in a dendritic pattern, penetrating the separator or solid electrolyte layer and reaching the positive electrode, causing a short circuit.

[0003] For this reason, defective products are prevented by preventing the introduction of metallic foreign matter and inspecting for short circuits. However, because the introduction of metallic foreign matter increases production losses, a manufacturing method has been proposed that prevents short circuits even when metallic foreign matter is introduced. For example, Patent Document 1 proposes that the initial charge is stopped at a state of charge (SOC) of 5% to 50% and the battery is left for one hour to seven days, which dissolves the metallic foreign matter on the positive electrode but prevents it from depositing on the negative electrode, thereby diffusing the metal ions generated from the metallic foreign matter and preventing short circuits. Furthermore, Patent Document 2 proposes that short circuits can be prevented by fluctuating the battery voltage up and down before charging to diffuse the dissolved metal ions in the electrolyte. Patent Document 3 also discloses a technology that improves charge / discharge cycle life by applying a constant DC charging pulse with a predetermined current value, with a rest period in between, a predetermined number of times before the initial charging process. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4661145 [Patent Document 2] Patent No. 7107649 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-244981 Summary of the Invention [Problem to be solved by the invention]

[0005] However, it has been found that the manufacturing methods proposed in Patent Document 1 or Patent Document 2, which prevent short circuits even when metal foreign matter is mixed in, are effective for metal foreign matter such as iron and stainless steel, which have a relatively slow dissolution-precipitation reaction, but are not effective for metal foreign matter such as copper, which has a relatively fast dissolution-precipitation reaction. Therefore, an object of the present invention is to provide a method for manufacturing a secondary battery that can prevent the occurrence of a short circuit due to the dissolution and precipitation of foreign matter, even if a metal such as copper, which undergoes a relatively fast dissolution and precipitation reaction, is mixed into the battery as foreign matter. [Means for solving the problem]

[0006] The present disclosure, which achieves the above-mentioned objectives, includes the following. <1> A method for manufacturing a secondary battery, comprising: a step of repeatedly charging a secondary battery before initial charging at a voltage Va and discharging at a voltage Vb lower than the voltage Va; and a step of initially charging the secondary battery. <2> The charging at the voltage Va is a battery voltage that makes the positive electrode potential a value equal to or lower than the dissolution potential of monovalent copper ions, and the discharging at the voltage Vb is a battery voltage that makes the negative electrode potential a value equal to or lower than the deposition potential of divalent copper ions. <1> 10. A method for producing the secondary battery according to claim 9. <3> Charging is performed for 1 to 10 seconds with the voltage Va set to 0.3 V to 1.0 V, and discharging is performed for 20 to 600 seconds with the voltage Vb set to 0.05 V to 0.2 V. <1> or <2> 10. A method for producing the secondary battery according to claim 9. <4> a step of applying a voltage higher than the voltage Va to the secondary battery after the step of repeatedly charging at the voltage Va and discharging at the voltage Vb and before the step of performing the initial charging, <1> ~ <3> 10. A method for producing a secondary battery according to claim 9, wherein the secondary battery is a battery having a diameter of 100 mm or less. <5> The secondary battery includes a metal piece disposed between a positive electrode layer and a negative electrode layer, and further includes a step of inspecting for the presence or absence of a short circuit after the step of performing the initial charging. <1> ~ <4> 10. A method for producing a secondary battery according to claim 9, wherein the secondary battery is a battery having a diameter of 100 mm or less. [Effects of the Invention]

[0007] According to the present disclosure, it is possible to provide a method for manufacturing a secondary battery that can prevent short circuits caused by dissolution and precipitation of foreign metal matter mixed inside. According to the method for manufacturing a secondary battery of the present disclosure, it is possible to reduce losses in the manufacturing process by preventing the occurrence of short circuits caused by foreign metal matter mixed inside. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic cross-sectional view showing a main part of a lithium ion secondary battery. [Figure 2] 1 is a flowchart showing a method for manufacturing a nonaqueous lithium ion secondary battery according to an embodiment. [Figure 3] FIG. 10 is a characteristic diagram showing an example of a voltage pattern applied in a voltage application step. [Figure 4] FIG. 10 is a characteristic diagram showing another example of a voltage pattern applied in the voltage application step. [Figure 5] FIG. 10 is a characteristic diagram showing another example of a voltage pattern applied in the voltage application step. [Figure 6] 10 is a flowchart showing a method for manufacturing a nonaqueous lithium ion secondary battery according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described. The description is for illustrating the embodiments and is not intended to limit the scope of the present disclosure.

[0010] In this specification, a numerical range indicated using "to" indicates a range that includes the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In the present specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range. In addition, in the present specification, the upper or lower limit of a numerical range may be replaced with a value shown in the examples.

[0011] In this specification, the term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes, as long as the intended purpose of the process is achieved.

[0012] In this specification, when an embodiment is described with reference to drawings, the configuration of the embodiment is not limited to the configuration shown in the drawings. Furthermore, the sizes of components in each drawing are conceptual, and the relative size relationships between components are not limited to these.

[0013] In this specification, each component may contain a plurality of corresponding substances. When referring to the amount of each component in the composition in this embodiment, if a plurality of substances corresponding to each component are present in the composition, the amount refers to the total amount of the plurality of substances present in the composition unless otherwise specified.

[0014] The method for manufacturing a secondary battery according to the present disclosure includes the steps of: repeatedly charging a secondary battery before initial charging at a voltage Va and discharging at a voltage Vb lower than the voltage Va; and initially charging the secondary battery. According to the method for manufacturing a secondary battery according to the present disclosure, metallic foreign matter mixed in the secondary battery can be diffused and dissolved, and metal ions derived from the metallic foreign matter can be prevented from precipitating in a dendritic form, thereby preventing a short circuit.

[0015] [Secondary battery] One embodiment of the secondary battery according to the present disclosure is a lithium-ion secondary battery. As shown in FIG. 1 , the lithium-ion secondary battery 1 shown in this embodiment includes a positive electrode layer 2, a negative electrode layer 3, and an electrolyte layer 4 disposed between the positive electrode layer 2 and the negative electrode layer 3.

[0016] <Electrolytes> In the lithium-ion secondary battery 1, the electrolyte layer 4 may contain a liquid electrolyte but not a solid electrolyte, may contain a solid electrolyte but not a liquid electrolyte, or may contain both a liquid electrolyte and a solid electrolyte. When the electrolyte layer 4 contains a liquid electrolyte, it preferably has a separator for retaining the liquid electrolyte and preventing contact between the positive electrode layer 2 and the negative electrode layer 3. When the electrolyte layer 4 contains a solid electrolyte, the electrolyte layer 4 may optionally contain a binder or the like in addition to the solid electrolyte.

[0017] The solid electrolyte can be any solid electrolyte typically used in solid-state batteries. Crystalline nitrides, oxides, sulfides, and oxoacid salts, as well as amorphous glass-structured materials, can be used as such solid electrolytes. Specifically, sulfide solid electrolytes that can be used as the solid electrolyte include at least one selected from the group consisting of LiI-LiBr-Li3PS4, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-Li2O-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5, Li3PS4, LiCl-LiBr-Li3PS4, LiCl-LiBr-Li2S-P2S5, and LiCl-LiBr-Li2S-SiS2. Furthermore, oxide-based solid electrolytes include, for example, Li 0.34 La 0.56 TiO3, Li 3 / 8 Sr 7 / 16 Ta 3 / 4 M 1 / 4 O3 (M=Zr or Hf), Li7La3Zr2O 12 , Li 1.3 Al 0.7 Ti 1.3 (PO4)3, Li 1.5 Al 0.5 Ge 1.5 (PO4)3, Li 3.5 Ge 0.5 V 0.5 O, Li 2.88 PO 3.73 N 0.14 , and Li2.9 Si 0.45 PO 1.6 N 1.3 In addition to these, a complex hydride-based lithium ion conductor or a halide-based lithium ion conductor may also be used as the solid electrolyte.

[0018] The liquid electrolyte can be any non-aqueous electrolyte typically used in non-aqueous lithium-ion secondary batteries. The non-aqueous electrolyte may be a composition containing a supporting salt in a non-aqueous solvent. Examples of the non-aqueous solvent include organic electrolytes, fluorine-based solvents, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and combinations of two or more thereof.

[0019] Examples of the supporting salt include materials selected from the group consisting of lithium compounds (lithium salts) of Li(FSO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, and combinations of two or more thereof.

[0020] Examples of binders used in the solid electrolyte include butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, etc. Only one type of binder may be used alone, or two or more types may be used in combination.

[0021] The separator used for the liquid electrolyte may be any separator commonly used in non-aqueous lithium-ion secondary batteries, such as those containing resins such as polyethylene (PE), polypropylene (PP), polyester, and polyamide. The separator may have a single-layer structure or a multi-layer structure. Examples of multi-layer separators include a two-layer structure of PE / PP, or a three-layer structure of PP / PE / PP or PE / PP / PE. The separator may be made of a nonwoven fabric such as a cellulose nonwoven fabric, a resin nonwoven fabric, or a glass fiber nonwoven fabric.

[0022] <Positive electrode> The positive electrode layer 2 includes a positive electrode current collector 5 and a positive electrode active material layer 6 containing a positive electrode active material. The positive electrode current collector 5 is not particularly limited and may be in the form of a foil, plate, mesh, punched metal, or foam. Examples of metals constituting the positive electrode current collector 5 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, from the viewpoint of ensuring oxidation resistance, the positive electrode current collector 5 may contain Al.

[0023] The positive electrode active material is not particularly limited, and conventionally known materials can be used as appropriate. Examples of the positive electrode active material include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiCoMn)O2, Li(NiCoAl)O2, and LiFePO4. The positive electrode active material particles may be Hi-Nickel (a positive electrode active material with a high Ni content), a Li-Ni-Co-Mn composite oxide, or a ternary positive electrode active material.

[0024] <Negative electrode> The negative electrode layer 3 includes a negative electrode current collector 7 and a negative electrode active material layer 8 containing a negative electrode active material. The negative electrode current collector 7 is not particularly limited and may be in the form of a foil, plate, mesh, punching metal, or foam. Examples of the metal constituting the positive electrode current collector 5 include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel, and the like. In particular, from the viewpoints of ensuring reduction resistance and being difficult to alloy with lithium, the negative electrode current collector 7 may contain at least one metal selected from the group consisting of Cu, Ni, and stainless steel.

[0025] The negative electrode active material is not particularly limited, and conventionally known materials can be appropriately used. Examples of the negative electrode active material include graphite, Si, SiOx (0 < x < 2), and Li4Ti5O 12 and the like can be mentioned.

[0026] <Other configurations> FIG. illustrates only the positive electrode layer 2, the negative electrode layer 3, and the electrolyte layer 4 among the components of the lithium-ion secondary battery 1 according to this embodiment in a simplified manner. The lithium-ion secondary battery 1 according to this embodiment includes components other than the positive electrode layer 2, the negative electrode layer 3, and the electrolyte layer 4. For example, although not shown in the figure, the lithium-ion secondary battery 1 includes an insulating film enclosure that houses the positive electrode layer 2, the negative electrode layer 3, and the electrolyte layer 4, a positive electrode terminal member that conducts with the positive electrode current collector 5, a negative electrode terminal member that conducts with the negative electrode current collector 7, and a battery case that houses the insulating film enclosure that houses the positive electrode layer 2, the negative electrode layer 3, and the electrolyte layer 4 while exposing the ends of these positive electrode terminal member and negative electrode terminal member to the outside. When the lithium-ion secondary battery 1 is a non-aqueous lithium-ion secondary battery using a liquid electrolyte, a battery case having an opening for injecting the liquid electrolyte can be used.

[0027] In the lithium-ion secondary battery 1, among such components, the positive electrode terminal member is made of the same metal as the positive electrode current collector 5, preferably Al. Also, the negative electrode terminal member is made of at least one metal selected from the group consisting of the same metals as the negative electrode current collector 7, preferably Cu, Ni, and stainless steel.

[0028] [Secondary battery manufacturing process 1] As one embodiment of the method for manufacturing a secondary battery according to the present disclosure, a method for manufacturing a nonaqueous lithium-ion secondary battery will be described. As shown in Fig. 2, the method for manufacturing a nonaqueous lithium-ion secondary battery includes an assembly step S1 for assembling a nonaqueous lithium-ion secondary battery, a liquid injection step S2 for injecting a liquid electrolyte (nonaqueous electrolyte) into the assembled nonaqueous lithium-ion secondary battery, a voltage application step S3 for repeatedly charging at a voltage Va and discharging at a voltage Vb lower than the voltage Va, an initial charging step S4 for initially charging the nonaqueous lithium-ion secondary battery that has undergone the voltage application step S3, and a short-circuit inspection step S5 for detecting the presence or absence of an internal short circuit in the lithium-ion secondary battery.

[0029] In the method for manufacturing a secondary battery according to the present disclosure, when manufacturing a lithium-ion secondary battery containing a solid electrolyte but not a liquid electrolyte, the liquid injection step S2 is not performed, and the voltage application step S3 is performed after the assembly step S1. That is, the liquid injection step S2 described above is an optional step, not an essential step, in the method for manufacturing a secondary battery according to the present disclosure. Furthermore, regardless of whether the electrolyte is a liquid electrolyte or a solid electrolyte, the short-circuit inspection step S5 is an optional step, not an essential step, in the manufacturing method according to the embodiment shown in FIG. 2.

[0030] <Assembly process S1> In the method for manufacturing the nonaqueous lithium ion secondary battery shown in Fig. 2, the assembly step S1 includes a step of fabricating the lithium ion secondary battery shown in Fig. 1. The lithium ion secondary battery shown in Fig. 1 can be manufactured by applying a known method.

[0031] For example, a cathode active material constituting the cathode active material layer 6 is dispersed in a solvent to obtain a cathode layer slurry. The solvent used in this case is not particularly limited, and water or various organic solvents can be used, including N-methylpyrrolidone (NMP). The cathode layer slurry is applied to the surface of the cathode current collector 5 using a doctor blade or the like, and then dried. This produces the cathode layer 2 in the lithium-ion secondary battery 1 shown in FIG. 1.

[0032] Similarly, the negative electrode active material and the like that constitute the negative electrode active material layer 8 are dispersed in a solvent to obtain a slurry for the negative electrode layer. The solvent used in this case can be the same as that for the positive electrode layer 2. The negative electrode layer 3 can be produced by the same procedure as that for the positive electrode layer 2.

[0033] Next, the positive electrode layer 2 and the negative electrode layer 3 are laminated so as to sandwich a separator which becomes the electrolyte layer 4, to obtain a laminate having, in this order, the negative electrode current collector 7, the negative electrode active material layer 8, the separator, the positive electrode active material layer 6, and the positive electrode current collector 5. Other members such as a positive electrode terminal member and a negative electrode terminal member can be attached to the laminate as needed.

[0034] Next, the laminate is housed in a battery case. The laminate can be wound as needed, compressed into a flat shape, and processed into a shape that can be housed in the battery case. The positive electrode terminal member and the negative electrode terminal member can be fixed to the battery case in a state where the laminate is housed in the battery case.

[0035] <Liquid injection process S2> In the method for manufacturing a nonaqueous lithium ion secondary battery according to this embodiment, a liquid electrolyte (nonaqueous electrolyte solution) is then injected into the assembled nonaqueous lithium ion secondary battery in a liquid injection step S2. When the above-described stack is housed in a battery case, the liquid electrolyte can be injected into the battery case through an opening formed in the battery case. According to the liquid injection step S2, the liquid electrolyte is allowed to stand for a predetermined time (e.g., 0.5 hours or more) after injection, thereby allowing the liquid electrolyte to impregnate the separator included in the above-described stack, and an electrolyte layer 4 can be formed.

[0036] <Voltage application step S3> The method for producing a nonaqueous lithium-ion secondary battery according to this embodiment then performs a voltage application step S3, which involves repeatedly charging at a voltage Va and discharging at a voltage Vb lower than the voltage Va. This voltage application step S3 can diffuse and dissolve metallic foreign matter that has become mixed into the laminate composed of the positive electrode layer 2, the electrolyte layer 4, and the negative electrode layer 3, and can prevent metal ions derived from the metallic foreign matter from precipitating in a dendritic form.

[0037] Examples of foreign metal matter include foreign matter containing metals with a relatively slow dissolution / precipitation reaction, such as iron or stainless steel, and foreign matter containing metals with a relatively fast dissolution / precipitation reaction compared to iron or stainless steel. Specifically, foreign metal matter includes metals with a relatively fast dissolution / precipitation reaction compared to iron or stainless steel, such as copper. As described above, the lithium-ion secondary battery 1 uses copper for the negative electrode current collector 7 or the negative electrode terminal member, and metal pieces containing copper may be mixed in as foreign metal matter. For example, in the assembly step S1, foreign metal matter may be mixed in between the positive electrode layer 2 and the electrolyte layer 4 of the laminate or within the electrolyte layer 4.

[0038] 3, the voltage application step S3 involves repeatedly charging at a voltage Va and discharging at a voltage Vb for a time Ta and a time Tb, respectively, thereby diffusing the copper ions dissolved on the positive electrode layer 2 over a wide area and expanding the area of ​​deposition on the negative electrode layer 3. Therefore, by performing the voltage application step S3, even if copper is deposited, it can be made to have a flat shape rather than a dendritic shape.

[0039] In particular, in the voltage application step S3, charging at a voltage Va is preferably a battery voltage that sets the positive electrode potential to a value equal to or lower than the dissolution potential of monovalent copper ions, and more preferably a battery voltage that sets the positive electrode potential to a value lower than the dissolution potential of monovalent copper ions. Furthermore, in the voltage application step S3, discharging at a voltage Vb is preferably a battery voltage that sets the negative electrode potential to a value equal to or lower than the deposition potential of divalent copper ions, and more preferably a battery voltage that sets the negative electrode potential to a value lower than the deposition potential of divalent copper ions. Here, the dissolution potential of monovalent copper ions is 3.55 V relative to the lithium potential. Furthermore, the deposition potential of divalent copper ions is 3.37 V relative to the lithium potential.

[0040] In the voltage application step S3, the positive electrode potential and the negative electrode potential can be measured by a conventionally known method. One example is a method in which an electrode made of a material exhibiting a stable, known potential is installed in a lithium-ion battery. This electrode, also called a reference electrode, can be made of lithium metal, a material exhibiting a stable, known potential. The positive electrode potential and the negative electrode potential can then be measured using this reference electrode as a reference.

[0041] In the voltage application step S3, charging with a voltage Va that sets the positive electrode potential to a value equal to or lower than the dissolution potential of monovalent copper ions dissolves copper from the copper-containing foreign metal material, but generates divalent copper ions rather than monovalent copper ions. Compared to monovalent copper ions, the dissolution rate of divalent copper ions is slower, and their deposition rate in the negative electrode layer 3 is also slower. Therefore, by charging with a voltage Va that sets the positive electrode potential to a value equal to or lower than the dissolution potential of monovalent copper ions, even if copper deposits on the negative electrode layer 3 side, it will be flat rather than dendritic, as it is called, dendrite-like.

[0042] Furthermore, in the voltage application step S3, discharging at a voltage Vb that sets the negative electrode potential to a value equal to or less than the deposition potential of divalent copper ions brings the negative electrode layer 3 closer to the potential at which divalent copper ions are generated, thereby suppressing copper deposition on the negative electrode layer 3. Discharging at a voltage Vb that sets the negative electrode potential to a value equal to or less than the deposition potential of divalent copper ions prevents divalent copper ions from dissolving from the copper-containing negative electrode current collector 7. Furthermore, while discharging at a voltage Vb that sets the negative electrode potential to a value equal to or less than the deposition potential of divalent copper ions, divalent copper ions generated on the positive electrode layer 2 can be diffused over a wider area. As a result, even if copper ions are deposited on the negative electrode layer 3, they form a thin, plate-like deposit, more reliably preventing short circuits.

[0043] More specifically, in the voltage application step S3, it is preferable to perform charging with a voltage Va of 0.3 V to 1.0 V and a time Ta of 1 to 10 seconds, and then repeatedly perform discharging with a voltage Vb of 0.05 V to 0.2 V and a time Tb of 20 to 600 seconds. The period for repeating charging and discharging under these conditions can be, for example, 1 hour to 72 hours, preferably 12 hours to 48 hours, and more preferably 22 hours to 26 hours.

[0044] In the method for manufacturing a nonaqueous lithium-ion secondary battery according to the present embodiment, an example in which charging at a voltage Va and discharging at a voltage Vb are repeated using the voltage pattern shown in FIG. 3 has been described. However, the voltage pattern used when repeatedly charging at a voltage Va and discharging at a voltage Vb is not limited to the example shown in FIG. 3 , and may be the voltage pattern shown in FIG. 4 or FIG. 5 . The voltage patterns shown in FIGS. 4 and 5 have in common the fact that a gradient is provided in the voltage change when switching between the voltage Va and the voltage Vb. By providing a gradient in the voltage change when switching between the voltage Va and the voltage Vb, voltage overshoot due to a sudden voltage change can be prevented and voltage fluctuation can be suppressed. In particular, the voltage pattern shown in FIG. 5 can shorten the switching time from the voltage Va to the voltage Vb (i.e., the time between Ta and Tb) compared to the voltage pattern shown in FIG. 4 , thereby reducing the time required for the voltage application step S3.

[0045] <Initial charging process S4> In the method for producing a nonaqueous lithium ion secondary battery of this embodiment, next, in an initial charging step S4, an initial charge is performed on the lithium ion secondary battery 1. The initial charging step S4 can be performed, for example, by connecting a charging device to the lithium ion secondary battery 1 and charging it by constant current constant voltage (CCCV) charging at a constant current of 1 C until the battery voltage reaches 4.1 V and maintaining the battery voltage at 4.1 V for 2 minutes.

[0046] If the voltage application step S3 is not performed and the initial charging step S4 is performed after the liquid injection step S2, conditions are met at the start of the initial charging step S4 for dissolving the copper contained in the foreign metal matter and for copper to precipitate on the negative electrode layer 3. When a voltage for initial charging is applied in the initial charging step S4, the dissolution and precipitation of copper proceeds more quickly, and copper precipitates as dendrites on the negative electrode layer 3. The dendrites penetrate the electrolyte layer 4 and reach the positive electrode layer 2, causing a short circuit.

[0047] However, in the method for producing a nonaqueous lithium ion secondary battery of this embodiment, the voltage application step S3 described above is performed before the initial charging step S4, and therefore the copper contained in the foreign metal matter is dissolved as divalent copper ions and smoothly precipitates on the negative electrode layer 3 side. Therefore, in the initial charging step S4, the formation of copper-containing dendrites on the negative electrode layer 3 side is suppressed, and the occurrence of the above-mentioned short circuit can be reliably prevented.

[0048] <Short circuit inspection process S5> In the manufacturing method for a lithium-ion secondary battery according to this embodiment, the lithium-ion secondary battery 1 is subsequently inspected for the presence or absence of a short circuit in a short-circuit inspection step S5 after initial charging. The short-circuit inspection step S5 can be performed, for example, by allowing the lithium-ion secondary battery 1 to self-discharge by leaving the positive and negative terminals of the battery 1 open, and measuring the amount of voltage drop in the battery voltage. Specifically, the lithium-ion secondary battery 1 is allowed to self-discharge as described above, and a voltage drop ΔV (=V1-V2) is calculated from a battery voltage V1 measured after one day and a battery voltage V2 measured after three days. The calculated voltage drop ΔV is then compared with a predetermined reference voltage drop ΔVr. If the voltage drop ΔV is greater than the reference voltage drop ΔVr (ΔV > ΔVr), the battery is determined to be defective and have an internal short circuit. If the voltage drop ΔV is equal to or less than the reference voltage drop ΔVr (ΔV ≦ ΔVr), it is determined that no internal short circuit has occurred. In the short circuit inspection process S5, a lithium ion secondary battery 1 that is determined to have an internal short circuit is deemed to be a defective product, and a lithium ion secondary battery 1 that is determined not to have an internal short circuit can be deemed to be a good product.

[0049] At this time, by disposing a metal piece between the positive electrode layer 2 and the negative electrode layer 3 of the lithium-ion secondary battery 1, it is possible to confirm whether the above-mentioned voltage application step S3 is functioning properly. If the voltage application step S3 does not function properly, dendrites originating from the metal piece are formed, and it is determined in the short-circuit inspection step S5 that a short circuit has occurred. On the other hand, if the voltage application step S3 is functioning properly, the formation of dendrites originating from the metal piece is inhibited. Therefore, for a lithium-ion secondary battery 1 having a metal piece between the positive electrode layer 2 and the negative electrode layer 3, if it is confirmed in the short-circuit inspection step S5 that no short circuit has occurred, it can be determined that the voltage application step S3 performed in the previous step is functioning properly.

[0050] Here, the metal piece may be a metal fragment made of copper. The location of the metal piece is not particularly limited, and the metal piece may be located inside the positive electrode active material layer 6, between the positive electrode active material layer 6 and the electrolyte layer 4, or inside the electrolyte layer 4.

[0051] [Secondary battery manufacturing process 2] Another embodiment of the method for manufacturing a secondary battery according to the present disclosure is a method for manufacturing a lithium-ion secondary battery shown in Fig. 6. Among the steps included in the method for manufacturing a lithium-ion secondary battery shown in Fig. 6, the same steps as those in the method for manufacturing a lithium-ion secondary battery shown in Fig. 2 described above are denoted by the same reference numerals, and detailed description thereof will be omitted. In the method for manufacturing a lithium-ion secondary battery according to this embodiment, a second voltage application step S6 is performed after the voltage application step S3 and before the initial charging step S4.

[0052] <Second voltage application step S6> In the method for producing a nonaqueous lithium ion secondary battery of this embodiment, a second voltage application step S6 is performed after the voltage application step S3, in which a voltage higher than the voltage Va in the voltage application step S3 is applied. Specifically, the second voltage application step S6 prevents the precipitation of dendrites derived from foreign metals such as iron and stainless steel, which undergo a slower dissolution-precipitation reaction than copper, according to the procedure disclosed in Japanese Patent No. 7107649.

[0053] For example, the second voltage application step S6 precharges the lithium-ion secondary battery 1 to a battery voltage (e.g., 1.5 V) that is lower than the battery voltage reached in the initial charge step S4, but where the positive electrode potential is higher than the dissolution potential of iron and the negative electrode potential is higher than the deposition potential of iron ions dissolved in the liquid electrolyte. After that, the battery temperature of the lithium-ion secondary battery 1 is raised to 30°C or higher, and the battery voltage is fluctuated up and down (e.g., −3.2 V to 3.2 V) to promote the diffusion of metal ions derived from the foreign metals dissolved in the liquid electrolyte. The second voltage application step S6 prevents the formation of dendrites derived from iron, which undergoes a slower dissolution-precipitation reaction than copper, and thus prevents the occurrence of short circuits.

[0054] In the method for producing a nonaqueous lithium-ion secondary battery of this embodiment, the voltage application step S3 prevents dendrite formation originating from copper-containing foreign metal substances, and the second voltage application step S6 prevents dendrite formation originating from iron-containing foreign metal substances. The method for producing a nonaqueous lithium-ion secondary battery of this embodiment may include one or more other voltage application steps similar to the second voltage application step S6 after the voltage application step S3 and the second voltage application step S6 and before the initial charging step S4 in order to prevent dendrite formation originating from other foreign metal substances whose dissolution and deposition reactions are slower than those of iron.

[0055] As a result, in the method for manufacturing a lithium ion secondary battery of this embodiment, the formation of dendrites derived from copper, iron, other metals, etc. can be effectively prevented, and as a result, the occurrence of short circuits can be reliably prevented. [Example]

[0056] Hereinafter, the secondary battery according to the present disclosure will be described with reference to examples, but the technical scope of the present disclosure is not limited to the following examples.

[0057] <Battery manufacturing> In the present examples and comparative examples, laminated lithium-ion secondary batteries (hereinafter simply referred to as "batteries") were manufactured. These batteries contained an electrode assembly, an electrolyte, and the like inside a bag-shaped exterior case made of laminated film. The electrode assembly consisted of one rectangular positive electrode plate and one rectangular negative electrode plate stacked together with one rectangular separator in between. The positive electrode plate was formed by providing a rectangular positive electrode active material layer measuring 45 mm x 45 mm at a predetermined position on one main surface (the main surface facing the negative electrode plate) of a rectangular positive electrode current collector made of aluminum foil. A positive electrode tab made of a strip-shaped aluminum plate was joined to the positive electrode plate and extended from the inside to the outside of the exterior case. A Li-Ni-Co-Mn composite oxide was used as the positive electrode active material.

[0058] The negative electrode plate was formed by providing a rectangular negative electrode active material layer (47 mm x 47 mm) at a predetermined position on one main surface (the main surface facing the positive electrode plate) of a rectangular negative electrode current collector made of copper foil. A negative electrode tab made of a strip of copper plate was joined to the negative electrode plate and extended from the inside to the outside of the exterior case. Graphite was used as the negative electrode active material.

[0059] In these batteries, a disk-shaped copper piece (metallic foreign object) having a diameter of 100 μm and a thickness of 4 μm was placed between the center of the positive electrode active material layer of the positive electrode plate and the separator.

[0060] <Examples 1 to 10, Comparative Examples 1 and 2> The batteries fabricated as described above were subjected to the voltage application step S3 shown in Fig. 2 under the conditions shown in Table 1, and then to the initial charging step S4 and short-circuit inspection step S5 (Examples 1 to 10). For comparison, a battery that was not subjected to the voltage application step S3 was designated Comparative Example 1, and a battery that was subjected to a cycle of applying a constant current of 0.1 C for 3 seconds and then stopping the current for 200 seconds, repeated for 24 hours, was designated Comparative Example 2. The voltage application conditions and the results of the short-circuit inspection step S5 for Examples 1 to 10 and Comparative Examples 1 and 2 are shown in Table 1.

[0061] [Table 1]

[0062] As shown in Table 1, in Comparative Example 1, the voltage application step S3 was not performed, so copper foreign metal matter remained on the positive electrode at the start of the initial charging step S4. As the battery voltage increased, the copper foreign metal matter dissolved and immediately precipitated on the negative electrode, and the precipitated copper grew into dendrites within the pores of the separator. As a result, in Comparative Example 1, a short circuit occurred within a few minutes of the start of the initial charging step S4. In Comparative Example 2, a current of 0.1 C was applied instead of the voltage application step S3, so the applied voltage fluctuated naturally, and the charge amount increased over time, causing the voltage to rise. Therefore, in Comparative Example 2, copper foreign metal matter dissolved and precipitated on the negative electrode, and the precipitated copper grew into dendrites within the pores of the separator. As a result, in Comparative Example 2, a short circuit occurred during the initial charging step S4.

[0063] In contrast to Comparative Examples 1 and 2, in Examples 1 to 10, the precipitation of copper ions from the copper foreign metal particles is suppressed and the copper ions in the electrolyte are presumably diffused by repeatedly charging the battery before the initial charge at a voltage Va and discharging it at a voltage Vb lower than the voltage Va. Therefore, it is presumed that copper is precipitated evenly on the negative electrode over an area wider than the outline of the copper foreign metal particles, thereby preventing short circuits. [Explanation of symbols]

[0064] 1... lithium ion secondary battery, 2... positive electrode layer, 3... negative electrode layer, 4... electrolyte layer, 5... positive electrode current collector, 6... positive electrode active material layer, 7... negative electrode current collector, 8... negative electrode active material layer

Claims

1. a step of repeatedly charging the secondary battery before initial charging at a voltage Va and discharging at a voltage Vb lower than the voltage Va; performing an initial charge on the secondary battery; A method for manufacturing a secondary battery comprising the steps of:

2. 2. The method for producing a secondary battery according to claim 1, wherein charging at the voltage Va is a battery voltage that makes the positive electrode potential a value equal to or lower than a dissolution potential of monovalent copper ions, and discharging at the voltage Vb is a battery voltage that makes the negative electrode potential a value equal to or lower than a deposition potential of divalent copper ions.

3. 2. The method for producing a secondary battery according to claim 1, wherein charging is performed for 1 to 10 seconds with the voltage Va set to 0.3 V to 1.0 V, and discharging is performed for 20 to 600 seconds with the voltage Vb set to 0.05 V to 0.2 V.

4. 2. The method for manufacturing a secondary battery according to claim 1, further comprising the step of applying a voltage higher than the voltage Va to the secondary battery after the step of repeatedly charging at the voltage Va and discharging at the voltage Vb and before the step of performing the initial charging.

5. 2. The method for manufacturing a secondary battery according to claim 1, wherein the secondary battery comprises a metal piece disposed between a positive electrode layer and a negative electrode layer, and further comprises a step of inspecting for the presence or absence of a short circuit after the step of performing the initial charging.

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