JP2015060670A - Method for manufacturing nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing nonaqueous electrolyte secondary battery, capable of properly actuating a current interruption mechanism while maintaining battery performance at a high level.SOLUTION: Provided is a method for manufacturing a nonaqueous electrolyte secondary battery including a porous heat resistant layer disposed between a positive electrode active material layer and a separator, the porous heat resistant layer containing inorganic fillers. The manufacturing method includes the steps of: forming a positive electrode active material layer containing N-methylpyrrolidone (NMP); disposing a porous heat resistant layer between the positive electrode active material layer and a separator to form an electrode body; housing the electrode body into a battery case; subjecting the battery case in which the electrode body has been housed to a heat treatment at a temperature range of 60°C or above, thereby causing NMP in the positive electrode active material layer to be adsorbed to the porous heat resistant layer; and injecting a nonaqueous electrolyte into the battery case after the heat treatment.

Description

  The present invention relates to a method for manufacturing a secondary battery (nonaqueous electrolyte secondary battery) provided with a nonaqueous electrolyte.

  Lithium ion secondary batteries and other non-aqueous electrolyte secondary batteries are becoming increasingly important as on-vehicle power supplies or personal computers and portable terminals. In particular, a lithium ion secondary battery that is lightweight and has a high energy density is preferably used as a high-output power source for mounting on a vehicle (for example, Patent Document 1). When such a nonaqueous electrolyte secondary battery is overcharged, excessive charge carriers are released from the positive electrode, and excessive charge carriers are inserted into the negative electrode. For this reason, both the positive electrode and the negative electrode are thermally unstable. When both the positive electrode and the negative electrode become thermally unstable, the organic solvent of the electrolytic solution is eventually decomposed, and a rapid exothermic reaction occurs, thereby impairing the stability of the battery.

  To solve this problem, for example, the battery case is equipped with a current interruption mechanism that interrupts charging when the gas pressure inside the battery exceeds a predetermined pressure, and gas is generated when a predetermined overcharge state is reached in the electrolyte. A non-aqueous electrolyte secondary battery to which a gas generating agent is added is disclosed. As such a gas generating agent, for example, cyclohexylbenzene (CHB) or biphenyl (BP) is used. CHB and BP activate the polymerization reaction during overcharge and generate hydrogen gas. Thereby, the pressure in a battery case becomes high, an electric current interruption mechanism act | operates, and an overcharge electric current is interrupted | blocked.

JP 2007-012598 A

  By the way, in order to operate the current interruption mechanism efficiently, it is effective to increase the addition amount of the gas generating agent. However, since the gas generating agent works as a resistance component of the battery, if the amount of the gas generating agent added is increased, the battery performance may be a factor. On the other hand, if the addition amount of the gas generating agent is decreased in order to keep the battery performance high, the generation of gas may be slowed during overcharging, and the operation of the current interruption mechanism may be delayed. As described above, in the conventional configuration in which the gas is generated by relying only on the gas generating agent, it is difficult to appropriately operate the current interrupting mechanism while keeping the battery performance high. The present invention solves the above problems.

  The manufacturing method provided by the present invention includes a positive electrode in which a positive electrode active material layer is held on a positive electrode current collector, a negative electrode in which a negative electrode active material layer is held on a negative electrode current collector, and a gap between the positive electrode and the negative electrode. An electrode body including a separator interposed in the battery body, a battery case housing the electrode body, an external terminal provided in the battery case and connected to the electrode body, and housed in the battery case and predetermined. When the internal pressure of the non-aqueous electrolyte containing a gas generating agent that reacts at a voltage higher than the voltage and generates gas and the battery case becomes higher than a predetermined pressure, the electrical connection between the electrode body and the external terminal A non-aqueous electrolyte secondary battery comprising a current blocking mechanism for cutting off a simple connection, and a porous heat-resistant layer including an inorganic filler disposed between the positive electrode active material layer and the separator. . This manufacturing method includes a step of forming a positive electrode active material layer containing N-methylpyrrolidone (NMP). Further, the method includes a step of producing an electrode body by disposing a porous heat-resistant layer between the positive electrode active material layer and the separator. Moreover, the process which accommodates the said electrode body in a battery case is included. Moreover, the battery case which accommodated the said electrode body is heat-processed in the temperature range of 60 degreeC or more, and the process which makes NMP in the said positive electrode active material layer adsorb | suck to the said porous heat-resistant layer is included. Furthermore, a step of injecting a non-aqueous electrolyte into the battery case after the heat treatment is included.

  According to the manufacturing method of the present invention, since the NMP in the positive electrode active material layer is adsorbed to the porous heat-resistant layer by heat-treating the battery case containing the electrode body in a temperature range of 60 ° C. or higher, a predetermined amount (for example, 2000 ppm or higher) The nonaqueous electrolyte secondary battery provided with a porous heat-resistant layer containing NMP) can be produced. In such a nonaqueous electrolyte secondary battery, a large amount of gas is generated by vaporizing NMP in the porous heat-resistant layer during overcharge. Therefore, when the overcharge state is reached, the internal pressure quickly increases, and the current interrupt mechanism can be appropriately operated.

FIG. 1 is a diagram illustrating an example of the structure of a non-aqueous electrolyte secondary battery. FIG. 2 is a schematic view showing a part of a cross section of the wound electrode body cut in the radial direction. FIG. 3 is a graph showing the amount of overcharge gas generated in Test Example 1. FIG. 4 is a graph showing the battery characteristics of Test Example 1. FIG. 5 is a graph showing the relationship between the amount of NMP and the amount of gas generated in the porous heat-resistant layer.

Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.
In the present specification, the “secondary battery” refers to a general power storage device that can be repeatedly charged and discharged, and is a term including a so-called storage battery such as a lithium ion secondary battery and a power storage element such as an electric double layer capacitor. The “non-aqueous electrolyte secondary battery” refers to a battery provided with a non-aqueous electrolyte (typically, an electrolyte containing a supporting salt (supporting electrolyte) in a non-aqueous solvent). The “lithium ion secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged and discharged by the movement of lithium ions between positive and negative electrodes. The electrode active material refers to a material capable of reversibly occluding and releasing chemical species (lithium ions in a lithium ion secondary battery) serving as a charge carrier.

  Hereinafter, a non-aqueous secondary battery according to an embodiment of the present invention will be described with reference to the drawings. In addition, the same code | symbol is attached | subjected suitably to the member and site | part which show | play the same effect | action. Each drawing is schematically drawn and does not necessarily reflect the real thing. Each drawing shows an example only and does not limit the present invention unless otherwise specified. Hereinafter, embodiments of the present invention will be described by taking as an example the case where the present invention is applied to a lithium ion secondary battery, but it is not intended to limit the application target of the present invention.

  FIG. 1 shows a lithium ion secondary battery 100 according to an embodiment of the present invention. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 20 and a battery case 30. As shown in FIG. 1, a lithium ion secondary battery 100 according to an embodiment of the present invention includes a flat rectangular battery case 20 having a flat wound electrode body 20 together with a liquid electrolyte (electrolytic solution) (not shown). That is, it is accommodated in the exterior container 30.

  The battery case 30 has a box-shaped (that is, bottomed rectangular parallelepiped) case body 32 having an opening at one end (corresponding to the upper end in a normal use state of the battery), and the opening attached to the opening. It is comprised from the cover body 34 which consists of a rectangular-shaped plate member which plugs up a part. The material of the battery case 30 is exemplified by aluminum, for example. As shown in FIG. 1, a positive electrode terminal 42 and a negative electrode terminal 44 for external connection are formed on the lid 34. A safety valve 36 is formed between both terminals 42 and 44 of the lid 34.

  The wound electrode body 20 includes a long sheet-like positive electrode (positive electrode sheet 50) and a long sheet-like negative electrode (negative electrode sheet 60) similar to the positive electrode sheet 50, in total, two long sheet-like separators (separators). 70).

  The positive electrode sheet 50 includes a strip-shaped positive electrode current collector 52 and a positive electrode active material layer 54. For the positive electrode current collector 52, for example, a strip-shaped aluminum foil having a thickness of about 15 μm is used. A positive electrode active material layer non-forming portion 52 a is set along an edge portion on one side in the width direction of the positive electrode current collector 52. In the illustrated example, the positive electrode active material layer 54 is held on both surfaces of the positive electrode current collector 52 except for the positive electrode active material layer non-forming portion 52 a set in the positive electrode current collector 52. The positive electrode active material layer 54 includes a positive electrode active material 56 (see FIG. 2), a conductive material, and a binder.

As the positive electrode active material, a material used as a positive electrode active material of a lithium ion secondary battery can be used. Examples of cathode active materials include lithium transition metal oxides such as LiNiCoMnO ₂ (lithium-nickel-cobalt-manganese composite oxide). For example, a conductive material such as acetylene black (AB) can be mixed with the positive electrode active material. In addition to the positive electrode active material and the conductive material, a binder such as polyvinylidene fluoride (PVdF), styrene butadiene rubber (SBR), or carboxymethyl cellulose (CMC) can be added. A positive electrode active material layer forming composition (paste) can be prepared by dispersing these in a suitable dispersion medium and kneading. The positive electrode active material layer 54 is formed by applying the positive electrode active material layer forming composition to the positive electrode current collector 52, drying it, and pressing it to a predetermined thickness.

  The negative electrode sheet 60 includes a strip-shaped negative electrode current collector 62 and a negative electrode active material layer 64. For the negative electrode current collector 62, for example, a strip-shaped copper foil having a thickness of about 10 μm is used. On one side in the width direction of the negative electrode current collector 62, a negative electrode active material layer non-formation part 62a is set along the edge. The negative electrode active material layer 64 is held on both surfaces of the negative electrode current collector 62 except for the negative electrode active material layer non-forming portion 62 a set in the negative electrode current collector 62. The negative electrode active material layer 64 includes a negative electrode active material, a thickener, a binder, and the like.

  As the negative electrode active material, one type or two or more types of materials conventionally used in lithium ion secondary batteries can be used without any particular limitation. Preferable examples include carbon-based materials such as graphite carbon. And like a positive electrode, this negative electrode active material is disperse | distributed to a suitable dispersion medium with binders, such as PVDF, SBR, and CMC (it can function also as a thickener), and is kneaded, The composition for negative electrode active material layer formation A product (paste) can be prepared. The negative electrode active material layer 64 is formed by applying this negative electrode active material layer forming composition to the negative electrode current collector 62, drying it, and pressing it to a predetermined thickness.

  The separator 70 is a member that separates the positive electrode sheet 50 and the negative electrode sheet 60. In this example, the separator 70 is composed of a strip-shaped base material having a predetermined width and having a plurality of minute holes. Examples of the base material include a sheet material having a single layer structure (for example, a single layer structure of polyethylene) made of a porous polyolefin-based resin, or a sheet material having a laminated structure (for example, a three-layer structure of polypropylene, polyethylene, and polypropylene). Can be used. A porous heat-resistant layer 80 is formed on one side of the separator 70. This will be described later.

  The wound electrode body 20 is attached to electrode terminals 42 and 44 attached to the battery case 30 (in this example, the lid body 34). The wound electrode body 20 is housed in the battery case 30 in a state where the wound electrode body 20 is flatly pushed and bent in one direction orthogonal to the winding axis. In the wound electrode body 20, the positive electrode active material layer non-formed portion 52 a of the positive electrode sheet 50 and the negative electrode active material layer non-formed portion 62 a of the negative electrode sheet 60 protrude on the opposite sides in the winding axis direction. Among these, one electrode terminal 42 is fixed to the positive electrode active material layer non-forming portion 52 a of the positive electrode current collector 52 via the positive electrode current collector plate 42 a, and the other electrode terminal 44 is the negative electrode current collector 62. The negative electrode active material layer non-forming portion 62a is fixed via a negative electrode current collector plate 44a. The wound electrode body 20 is accommodated in the flat internal space of the case body 32. The case body 32 is closed by the lid 34 after the wound electrode body 20 is accommodated.

As the electrolytic solution (non-aqueous electrolytic solution), the same non-aqueous electrolytic solution conventionally used for lithium ion secondary batteries can be used without any particular limitation. Such a nonaqueous electrolytic solution typically has a composition in which a supporting salt is contained in a suitable nonaqueous solvent. As the non-aqueous solvent, for example, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, or the like can be used. Moreover, as said support salt, lithium salts, such as LiPF ₆ , can be used, for example.

  The non-aqueous electrolyte contains, for example, a gas generating agent that reacts and generates gas when the battery voltage becomes equal to or higher than a predetermined voltage. As such a gas generating agent, for example, cyclohexylbenzene (CHB) or biphenyl (BP) can be used. For example, when CHB or BP is overcharged at about 4.35V to 4.6V, the polymerization reaction is activated to generate hydrogen gas. The amount of the gas generating agent added to the non-aqueous electrolyte is preferably about 0.05% by mass or more and 4.0% by mass or less, for example.

  Further, as described above, in the lithium ion secondary battery 100, a gas generating agent is added to the electrolytic solution. For example, when the battery is overcharged at about 4.35V to 4.6V, gas is generated. The pressure inside becomes high. The current interruption mechanism 90 is a mechanism that interrupts the current path when the pressure in the battery case becomes abnormally high. In this embodiment, as shown in FIG. 1, the current interruption mechanism 90 is constructed inside the positive electrode terminal 42 so that the conduction path of the battery current in the positive electrode is interrupted.

  Hereinafter, the lithium ion secondary battery 100 will be described in more detail. FIG. 2 schematically shows a cross section of the positive electrode sheet 50 and the separator 70 overlapped in the wound electrode body 20 taken along the winding axis direction (for example, the width direction of the positive electrode sheet 50).

  As shown in FIG. 2, in the wound electrode body 20, a porous heat resistant layer 80 is disposed between the positive electrode active material layer 54 and the separator 70. In this embodiment, the porous heat-resistant layer 80 is formed on the surface of the separator 70. The porous heat-resistant layer 80 is formed on the surface facing the positive electrode active material layer 54 on the front and back of the separator 70. The porous heat resistant layer 80 contains an inorganic filler, a binder, and N-methylpyrrolidone (NMP).

  The material that can constitute the inorganic filler used for the porous heat-resistant layer 80 is preferably a material that has high electrical insulation and a higher melting point than the separator 70. Examples thereof include inorganic compounds such as alumina, boehmite, magnesia, titania, silica, zirconia, zinc oxide, iron oxide, ceria, and yttria. These inorganic materials may be used individually by 1 type, and may be used in combination of 2 or more types. The shape (outer shape) of the inorganic filler is not particularly limited. From the viewpoints of mechanical strength, manufacturability, etc., generally spherical inorganic filler particles can be preferably used.

  The binder used for the porous heat-resistant layer 80 is preferably a binder having high affinity with the separator 70. In the case where the composition for forming the porous heat-resistant layer 80 is an aqueous solvent, it is preferable to use a binder that is dispersed or dissolved in the aqueous solvent. Preferable examples include binders in the form of solutions or emulsions such as acrylic polymers, aramid resins, and polyvinylidene fluoride (PVdF). The porous heat-resistant layer 80 may contain a thickener (for example, CMC).

  The porous heat-resistant layer 80 contains N-methylpyrrolidone (NMP) in addition to the inorganic filler and binder described above. Such NMP is held, for example, on the surface of the inorganic filler particles or in the gaps between the particles. The content of NMP is 500 ppm or more, preferably 2000 ppm or more, based on the total solid content of the porous heat-resistant layer 80 (that is, the total mass of the inorganic filler, the conductive material, and the binder). This makes it possible to appropriately increase the internal pressure during overcharge as compared with a conventional lithium secondary battery that does not contain NMP or has a NMP content of less than 500 ppm. That is, according to the above configuration, NMP in the porous heat-resistant layer 80 is vaporized during overcharge, so that gas is generated from other than the gas generating agent such as CHB or BP. Therefore, when the overcharge state occurs, the internal pressure quickly increases, and the current interrupt mechanism 90 (FIG. 1) can be appropriately operated. According to this configuration, it is possible to obtain an internal pressure increase assist effect without increasing the amount of the gas generating agent added (without deteriorating battery performance). Therefore, according to this configuration, a lithium secondary battery having high performance and excellent stability can be provided.

  As the porous heat-resistant layer 80 disclosed herein, it is appropriate that the content X of the NMP satisfies 500 ppm ≦ X, preferably satisfies 2000 ppm ≦ X, and satisfies 3000 ppm ≦ X. Are more preferable, those satisfying 5000 ppm ≦ X are more preferable, and those satisfying 7000 ppm ≦ X are particularly preferable. If the content X of NMP is too small, the above-described internal pressure increase assist effect may be insufficient. On the other hand, if the content of NMP is too large, the production becomes difficult, and the effect of assisting the increase in internal pressure is slowed down. For example, the porous heat-resistant layer 80 in which the NMP content X satisfies 500 ppm ≦ X ≦ 10000 ppm (particularly 2000 ppm ≦ X ≦ 8000 ppm) is suitable from the viewpoint of achieving both a high internal pressure increase assist effect and manufacturability. is there. The NMP content can be grasped by gas chromatography mass spectrometry (GC-MS method).

  Next, the manufacturing method of the nonaqueous electrolyte secondary battery described above will be described. This manufacturing method can also be grasped as a method of causing the porous heat-resistant layer 80 to contain NMP. This manufacturing method includes a step of forming a positive electrode active material layer 54 containing N-methylpyrrolidone (NMP) (positive electrode active material layer forming step), and a porous heat-resistant layer between the positive electrode active material layer 54 and the separator 70. 80 is arranged to produce electrode body 20 (electrode body production process), electrode body 20 is housed in battery case 30 (electrode body housing process), and battery case 30 housing electrode body 20 is 60 It includes a step (heat treatment step) of heat treatment in a temperature range of not lower than ° C. and a step (pour step) of pouring a non-aqueous electrolyte into the battery case 30 after the heat treatment.

  In the positive electrode active material layer forming step, the positive electrode active material layer 54 containing NMP is formed. In this embodiment, a positive electrode active material layer forming composition in which a positive electrode active material, a binder, and a conductive material are dispersed in NMP is prepared. The composition is applied to the positive electrode current collector 52, dried, and pressed to a predetermined thickness to form the positive electrode active material layer 54. At that time, by appropriately selecting conditions such as drying temperature and drying time when the composition is dried, the positive electrode active material satisfying a predetermined range (for example, 2000 ppm to 10,000 ppm, preferably 2500 ppm to 5000 ppm) of NMP. Layer 54 can be formed. Although it does not specifically limit as solid content rate in the said composition, It is about 60 mass%-about 80 mass% in general.

  In the electrode body manufacturing step, the electrode body 20 is manufactured by disposing the porous heat-resistant layer 80 between the positive electrode active material layer 54 and the separator 70. In this embodiment, a porous heat-resistant layer forming composition in which an inorganic filler and a binder are dispersed in an appropriate solvent (for example, water) is prepared. The porous heat-resistant layer 80 is formed by applying the composition to the surface of the separator 70 and drying it. Thereafter, the wound electrode body 20 is manufactured by overlapping and winding two separators 70 with a porous heat-resistant layer 80 and separately prepared positive electrode sheet 50 and negative electrode sheet 60. At that time, the porous heat-resistant layer 80 and the positive electrode active material layer 54 of the positive electrode sheet 50 may be overlapped so as to face each other.

  In the electrode body housing step, the wound electrode body 20 is housed in the battery case 30. In this embodiment, the wound electrode body 20 is accommodated in the main body 32 from the upper end opening of the case main body 32. Then, after the wound electrode body 20 is accommodated in the case body 32, the upper end opening of the case body 32 is closed by the lid body 34. And the joint of the cover body 34 and the case main body 32 is good to seal by laser welding.

  In the heat treatment step, the battery case 30 containing the wound electrode body 20 is heat treated in a temperature range of 60 ° C. or higher. By this heat treatment, NMP in the positive electrode active material layer 54 is vaporized and adsorbed on the porous heat-resistant layer 80. The temperature of the heat treatment may be a temperature range in which NMP in the positive electrode active material layer 54 is vaporized and adsorbed on the porous heat-resistant layer 80. For example, the temperature of the heat treatment is suitably about 60 ° C. or higher (eg, 60 ° C. to 100 ° C.), preferably 60 ° C. to 80 ° C. The heat treatment time may be of a length that allows NMP in the positive electrode active material layer 54 to vaporize and be sufficiently adsorbed to the porous heat-resistant layer 80. For example, the heat treatment time is generally about 10 hours or longer (for example, 10 hours to 50 hours), preferably 20 hours or longer (for example, 20 hours to 36 hours).

  In the liquid injection process, a non-aqueous electrolyte is injected into the battery case 30 after the heat treatment. In this embodiment, the electrolytic solution is injected into the battery case 30 from the liquid injection hole provided in the lid 34. Thereafter, a metal sealing cap is attached to the liquid injection hole (for example, by welding) to seal the battery case 30. In this way, the lithium ion secondary battery 100 can be manufactured (constructed).

  According to this manufacturing method, it is possible to appropriately manufacture the lithium ion secondary battery 100 including the porous heat-resistant layer 80 containing NMP in a predetermined amount (for example, 2000 ppm or more, preferably 2500 ppm or more). In addition, according to the above manufacturing method, NMP in the positive electrode active material layer 54 is adsorbed to the porous heat-resistant layer 80, so that the amount of NMP remaining in the positive electrode active material layer 54 during the manufacturing process is reduced. Therefore, the performance degradation resulting from the residual NMP in the positive electrode active material layer 54 is suppressed, and a higher performance lithium ion secondary battery 100 can be manufactured.

  An evaluation cell (lithium ion secondary battery) was constructed using the production method according to the present invention, and the amount of gas generated during an overcharge test of the battery was evaluated. A specific method will be described below.

<Test Example 1>
Here, the positive electrode of the evaluation cell was produced as follows. First, LiNiCoMnO ₂ powder as a positive electrode active material, AB as a conductive material, and PVdF as a binder are mixed in NMP so that the mass ratio of these materials is 93: 4: 3. A layer composition was prepared. The positive electrode active material layer composition is coated on both sides of a long sheet-like aluminum foil (positive electrode current collector: thickness 12 to 15 μm) and dried to form a positive electrode active material on both sides of the positive electrode current collector. A positive electrode sheet provided with a layer was produced. The positive electrode active material layer was dried by passing it through a drying furnace at 100 ° C. and then at 120 ° C. at 6 m / s. After drying, the density of the positive electrode active material layer was pressed to a 2.6~3g / cm ^3. The coating amount of the positive electrode active material layer composition was adjusted so that the total amount of both surfaces was about 26 mg / cm ² (solid content basis).

The negative electrode of the evaluation cell was produced as follows. First, amorphous coated graphite as a negative electrode active material, SBR as a binder and CMC as a thickener are dispersed in water so that the mass ratio of these materials is 98: 1: 1, and the negative electrode active material layer A composition was prepared. This negative electrode active material layer composition was applied to both sides of a long sheet-like copper foil (negative electrode current collector: thickness 10 μm) to produce a negative electrode sheet having a negative electrode active material layer provided on both sides of the negative electrode current collector did. After drying, the negative electrode active material layer was pressed so as to have a density of 1.1 to 1.3 g / cm ³ . Moreover, the coating amount of the negative electrode active material layer composition was adjusted so as to be about 14.7 mg / cm ² (solid content basis) for both surfaces.

  The separator with a porous heat-resistant layer of the evaluation cell was produced as follows. A porous heat-resistant layer forming composition was prepared by dispersing alumina powder as an inorganic filler and acrylic polymer as a binder in water such that the mass ratio of these materials was 99: 1 in terms of solid content. . This porous heat-resistant layer forming composition was applied to the surface of a separator (a porous polyethylene (PE) sheet having a thickness of 18 μm was used) and dried to form a porous heat-resistant layer.

As a non-aqueous electrolyte for the evaluation cell, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3: 4: 3, and LiPF as a supporting salt. ₆ was used at a concentration of about 1 mol / liter. In addition, 4% by mass of CHB was added as a gas generating agent to the non-aqueous electrolyte.

  In Example 1, the positive electrode sheet and the negative electrode sheet were wound through two separators to produce a wound body, and the wound body was crushed from the lateral direction to produce a flat wound electrode body. . In that case, it arrange | positioned so that the porous heat resistant layer formed in the surface of a separator might oppose a positive electrode active material layer (electrode body preparation process). The wound electrode body thus obtained was housed in a box-shaped battery case, and the opening of the battery case was sealed with a lid. And the battery case which accommodated the winding electrode body was heat-processed at 60 degreeC for 24 hours in the airtight oven (heat treatment process). Then, electrolyte solution was inject | poured in the battery case from the injection hole, the metal sealing cap was attached to the injection hole, and the battery case was sealed. Thus, the lithium ion secondary battery of the evaluation cell was constructed.

  In Comparative Example 1, in the heat treatment step described above, the wound electrode body was left as it was (that is, the open electrode body that was not accommodated in the battery case) was heat-treated in a sealed oven. And the wound electrode body after heat processing was accommodated in the battery case with the non-aqueous electrolyte. Otherwise, an evaluation cell was constructed under the same conditions as in Example 1.

  In Comparative Example 2, in the electrode body manufacturing process described above, the porous heat-resistant layer formed on the separator surface was disposed so as to face the negative electrode active material layer. Otherwise, an evaluation cell was constructed under the same conditions as in Example 1.

  In Comparative Example 3, an evaluation cell was constructed without performing the above-described heat treatment step. Otherwise, an evaluation cell was constructed under the same conditions as in Example 1.

  In Comparative Example 4, an evaluation cell was constructed by changing the drying conditions of the positive electrode active material layer. Specifically, the positive electrode active material layer is dried by applying the composition for the positive electrode active material layer on both sides of the positive electrode current collector in a strip shape, and then using a drying furnace at 100 ° C. and then 120 ° C. in 1.5 m / s ( This was carried out by passing at a speed 1/4 times that of Example 1). Since this positive electrode active material layer is firmly dried, the amount of residual NMP is smaller than that in Example 1. Otherwise, an evaluation cell was constructed under the same conditions as in Example 1.

  The amount of NMP contained in the porous heat-resistant layer and the positive electrode active material layer of each evaluation cell of Example 1 and Comparative Examples 1 to 4 was measured by the GC-MS method. The results are shown in Table 1.

  The amount of overcharge gas generated in each evaluation cell of Example 1 and Comparative Examples 1 to 4 was grasped by attaching an internal pressure sensor to the square cell, making the SOC 140% overcharged, and comparing the internal pressure at that time. . The results are shown in Table 1 and FIG. In FIG. 3, the relative value of each example when the overcharge gas generation amount of the comparative example 4 is set to 100 is shown. In Table 1, a case where the amount of generated overcharge gas is better than that of Comparative Example 4 is indicated by ◯, and a case where it is insufficient is indicated by ×.

Moreover, the battery characteristic of each cell for evaluation of Example 1 and Comparative Examples 1 to 4 was examined. In particular,
The SOC was adjusted to 60% SOC in a 25 ° C. test tank, and the resistance when charged with a constant current of 8 C was measured. The results are shown in Table 1 and FIG. In FIG. 4, the relative value of each example when the overcharge gas generation amount of the comparative example 4 is set to 100 is shown. In Table 1, the case where the battery characteristics did not change as compared with Comparative Example 4 was indicated by ◯, and the case where the battery characteristics deteriorated was indicated by x.

  As shown in Table 1, FIG. 3 and FIG. 4, in Example 1 where heat treatment was performed with the porous heat-resistant layer facing the positive electrode active material layer and the electrode body housed in the battery case, the battery characteristics were Without loss, the amount of gas generated during overcharge increased. On the other hand, in Comparative Example 1 in which the electrode body was heat-treated in an open state, NMP in the positive electrode active material layer was not sufficiently adsorbed on the porous heat-resistant layer, and thus the gas generation amount tended to decrease. In Comparative Example 2, since the porous heat-resistant layer was opposed to the negative electrode active material layer, NMP in the positive electrode active material layer was not sufficiently adsorbed to the porous heat-resistant layer, and the gas generation amount tended to decrease. In Comparative Example 2, since the separator resin base material was opposed to the positive electrode active material layer, NMP in the positive electrode active material layer swelled the resin base material, and the battery resistance increased. In Comparative Example 3, although the amount of gas generated during overcharge increased, the battery resistance increased because the amount of NMP in the positive electrode active material layer was large. From this result, in order to increase the amount of gas generated while keeping the battery performance high, heat treatment is performed with the porous heat-resistant layer facing the positive electrode active material layer and the electrode body accommodated in the battery case ( It was confirmed that it was effective to adsorb NMP to the porous heat-resistant layer.

  Furthermore, in order to confirm the influence of the amount of NMP in the porous heat-resistant layer on the gas generation amount, the following test was performed.

<Test Example 2>
In this example, an evaluation cell was constructed in the same procedure as in Example 1 described above except that the amounts of NMP in the porous heat-resistant layer and the positive electrode active material layer were varied as shown in Table 2. The amount of NMP in the porous heat-resistant layer and the positive electrode active material layer was adjusted by appropriately changing the conditions for drying the positive electrode active material layer. The results are shown in Table 2 and FIG. FIG. 5 is a graph showing the relationship between the amount of NMP and the amount of gas generated in the porous heat-resistant layer. In FIG. 5, the relative value of each example when the amount of overcharge gas generation when the NMP amount of the porous heat-resistant layer is 0 ppm is 100 is shown. Moreover, in Table 2, the thing with especially favorable overcharge gas generation amount is represented by (double-circle), and the favorable thing is represented by (circle).

  As shown in Table 2 and FIG. 5, the gas generation amount tended to increase as the amount of NMP in the porous heat-resistant layer increased. In particular, in Examples 4 to 6 in which the amount of NMP in the porous heat-resistant layer was 2000 ppm, the amount of gas generated was significantly increased as compared with Examples 2 and 3. From this result, from the viewpoint of increasing the amount of gas generated during overcharge, the amount of NMP in the porous heat-resistant layer is preferably 2000 ppm or more, more preferably 5500 ppm or more, and more preferably 7000 ppm or more. Particularly preferred.

  Although the lithium ion secondary battery according to one embodiment of the present invention has been described above, the secondary battery according to the present invention is not limited to any of the above-described embodiments, and various modifications can be made. For example, in the above-described embodiment, the case where the porous heat-resistant layer 80 is formed on the surface of the separator 70 is exemplified, but the present invention is not limited to this. The porous heat-resistant layer 80 may be formed on the surface of the positive electrode active material layer 54. Even in this case, the above-described effects can be obtained.

20 wound electrode body 30 battery case 50 positive electrode sheet 54 positive electrode active material layer 60 negative electrode sheet 64 negative electrode active material layer 70 separator 80 porous heat-resistant layer 100 lithium ion secondary battery

Claims (1)

An electrode body comprising: a positive electrode in which a positive electrode active material layer is held on a positive electrode current collector; a negative electrode in which a negative electrode active material layer is held on a negative electrode current collector; and a separator interposed between the positive electrode and the negative electrode; ,
A battery case that houses the electrode body;
An external terminal provided in the battery case and connected to the electrode body;
A non-aqueous electrolyte containing a gas generating agent that is contained in the battery case, reacts at a voltage equal to or higher than a predetermined voltage, and generates a gas;
When the internal pressure of the battery case becomes higher than a predetermined pressure, a current interruption mechanism that interrupts electrical connection between the electrode body and the external terminal,
A method for producing a non-aqueous electrolyte secondary battery in which a porous heat-resistant layer containing an inorganic filler is disposed between the positive electrode active material layer and the separator,
The following steps:
Forming a positive electrode active material layer containing N-methylpyrrolidone (NMP);
A step of producing an electrode body by disposing a porous heat-resistant layer between the positive electrode active material layer and the separator;
Housing the electrode body in a battery case;
Adsorbing NMP in the positive electrode active material layer to the porous heat-resistant layer by heat-treating a battery case containing the electrode body in a temperature range of 60 ° C. or higher; and
A method for producing a non-aqueous electrolyte secondary battery, comprising a step of injecting a non-aqueous electrolyte into the battery case after the heat treatment.

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