Electrode plate of lithium ion secondary battery, and lithium ion secondary battery as well as manufacturing method of the same

The present invention relates to a lithium ion secondary battery having excellent safety such as internal short circuit safety and heat resistance.

In chemical batteries such as lithium ion secondary batteries, there is a separator between the positive electrode and the negative electrode that electrically insulates each electrode plate and further holds an electrolytic solution. Currently, a microporous film mainly made of polyethylene, polypropylene, or the like is used in a lithium ion secondary battery. However, film-like separators made of these resins generally tend to shrink at high temperatures. Therefore, when an internal short circuit occurs or when a sharp protrusion such as a nail penetrates the battery, the separator shrinks due to the instantaneous heat of short circuit, and the short circuit expands, generating even more heat of reaction. , Had the problem of promoting abnormal overheating. Therefore, in order to improve safety including the above problems, a technique for coating and forming a porous coating film containing solid fine particles on the surface of an active material layer has been proposed. (See Patent Document 1)<patcit num="1"><text>Japanese Patent No. 3371301</text></patcit>

<p> However, in the conventional technique, the battery functions as a battery by passing ions through the pores of the porous protective film formed by applying a mixture of the resin binder and the solid fine particles on the electrode plate. Therefore, even if it is a porous membrane, it hinders ionic conduction and increases the internal resistance of the battery. As a result, there are problems that the charge / discharge characteristics are deteriorated and the capacity is reduced. Further, this technique is aimed at suppressing the peeling of the active material, and has a problem that the safety at the time of internal short circuit or nail piercing cannot be guaranteed.</p><p> The present invention solves the above problems, and an object of the present invention is to provide a lithium ion secondary battery having high capacity and high characteristics, and excellent in internal short circuit and nail piercing safety.</p>

<p> In order to solve the above problems, in the lithium ion secondary battery of the present invention, a positive electrode plate having a positive electrode active material layer containing lithium composite oxide particles or a negative electrode having a negative electrode active material layer containing negative electrode active material particles. A porous insulating layer containing an inorganic oxide filler and a resin binder is formed on at least one of the active material layers of the plate, and the porous insulating layer has a fine formed region and a non-formed region. ..</p><p> In this configuration, since the porous insulating layer has fine non-formed regions, higher ionic conductivity can be realized during normal use. Further, in the case of an internal short circuit, the minute formed region is prevented from being directly short-circuited between the positive and negative electrodes, so that it is insulated or the short-circuit current is suppressed to a low level, so that safety can be improved.</p><p> In the above configuration, it is desirable that the forming region of the porous insulating layer occupies an area of 20 to 90% of the surface of the active material. Within this range, both high ionic conductivity and short-circuit safety can be achieved.</p><p> In the above configuration, the safety can be further enhanced by using the porous insulating layer and the film separator in combination. In particular, it is more preferable that the separator includes a polyolefin-based microporous film. Since the polyolefin-based microporous film closes holes (so-called shutdown) at high temperatures, it becomes a safer battery.</p><p> In the above configuration, it is desirable that the formed or non-formed region of the porous insulating layer has an island-like pattern. It is also desirable that the formed or non-formed region of the porous insulating layer has a linear pattern. Further, it is also desirable that the formed or non-formed region of the porous insulating layer has a lattice pattern.</p><p> With these configurations, the formed region and the non-formed region of the porous insulating layer can be formed stably and uniformly, and it becomes easy to control the formed region to 20 to 90% of the surface of the active material. ..</p><p> In the method for producing a lithium ion secondary battery of the present invention, at least one of a step of mixing an inorganic oxide filler, a resin binder and a solvent to form a slurry, and at least one of a positive electrode active material layer and a negative electrode active material layer. It is desirable to have a step of applying a pattern to the surface of the slurry by inkjet printing.</p><p> In the other method, at least a step of mixing an inorganic oxide filler, a resin binder and a solvent to form a slurry, and a step of applying the slurry to at least one surface of the positive electrode active material layer or the negative electrode active material layer. It is desirable to have a step of leveling the coated slurry to provide a coated region and a non-coated region of the slurry.</p><p> As a method for applying this slurry, a conventionally known method can be used, but spray coating is preferable because it facilitates coating in the process.</p><p> By these methods, the fine formed region and the non-formed region of the porous insulating layer can be easily and accurately formed. It also facilitates controlling the formation region to 20-90% of the surface of the active material.</p><p> In the present invention, the presence of the porous insulating layer on the active material layer described above improves safety in an internal short circuit or nail piercing test. When there is no porous insulating layer, if a hole is opened in the separator due to foreign matter or the like and a short circuit occurs between the positive and negative electrodes, an excessive current may flow to the short circuit point and Joule heat may be generated. In that case, the heat melts or shrinks the separator around the short-circuit point to expand the hole, further expands the short-circuit area and continues to generate Joule heat, and by repeating this, the temperature of the battery continues to rise, resulting in abnormal heat generation and appearance. May cause deformation.</p><p> In the lithium ion secondary battery of the present invention, when a hole is opened in the separator and the positive and negative electrodes are short-circuited, the porous film insulating layer exists even if the separator melts or shrinks and the hole expands. The short-circuit area of is not widened. Therefore, the generation of Joule heat does not expand, and abnormal heat generation does not occur. In addition, since the temperature near the short-circuit point reaches 500 ° C instantaneously, the positive electrode current collector made of aluminum is blown, and the short-circuit between the positive and negative electrodes is eliminated. Further, even if the short-circuit current cannot be completely cut off due to the non-formed region of the porous membrane insulating layer, the short-circuit current can be reduced, so that the generation of Joule heat is suppressed and the temperature rise is prevented. be able to. On the other hand, in addition to the ion conduction being carried out through the pores of the porous membrane, the microscopic non-forming region of the porous membrane insulating layer also becomes an ion conduction path, so that the ion conductivity is very high as a whole. Therefore, the charge / discharge characteristics are good without increasing the internal resistance of the battery, and the capacity is not reduced.</p><p> Due to the above actions and effects, the lithium ion secondary battery of the present invention is a battery having excellent safety in an internal short circuit or nail piercing test and having high characteristics.</p>

<p> As described above, according to the present invention, it is possible to provide a lithium ion secondary battery having high safety in an internal short circuit or nail piercing test, high capacity, and excellent charge / discharge characteristics and heat resistance.</p>

Hereinafter, the best mode for carrying out the present invention will be described in detail.

As shown in FIG. 1, in the lithium ion secondary battery of the present invention, the separator 3 is interposed between the positive electrode 1 and the negative electrode 2, and is inorganic on the surface of either the positive electrode active material layer 1a or the negative electrode active material layer 2a. A porous insulating layer 4 containing an oxide filler and a resin binder is deposited. As shown in FIG. 1 (b), when the vicinity of the surface of the active material layer is enlarged, the porous insulating layer 4 has a fine formed region 5 and a non-formed region 6.

The patterns of the formed region and the non-formed region include those in which the formed region is an island pattern as shown in FIG. 2 (a), for example, a grid pattern (Fig. 2 (b)), and a linear pattern. It may be something (Fig. 2 (c)). Of course, the non-formed region may have a pattern as shown in FIG. Further, the linear pattern does not necessarily have to be a straight line, and may be a curved line or a zigzag linear pattern. In the island-shaped pattern that serves as the formation region, the shape of the island does not matter, and conversely, the sea portion may serve as the formation region. For example, the sea portion as shown in FIG. 2 (d) is the formation region, and of course, an irregular pattern may be used.

The reason why the fine formed region and the non-formed region, which are important constituent requirements of the present invention, are provided is to achieve both the insulating property between the positive and negative electrodes and the fact that the ion conduction is not hindered. The insulating action of the porous insulating layer and its safety effect related to the safety at the time of internal short circuit are as described in (Means for solving the problem), but in order to prevent the short circuit expansion between the positive and negative electrodes, It is desirable that the microscopic formed region and the non-formed region have a fine pattern of 500 μm or less. More preferably, it is 100 μm or less. For example, in the case of a linear pattern, it is desirable that the line width (that is, the formed region) is 500 μm or less and the line spacing (that is, the non-formed region) is 500 μm or less. When the non-formed region has an island pattern, it is desirable that the size of the island is 500 μm or less. If the pattern is 500 μm or more, the short-circuit current flowing through the non-formed region is excessive, so that heat generation increases, which leads to abnormal heat generation of the battery and safety cannot be ensured. Further, from the viewpoint of maintaining the surface uniformity of the occlusion and release of lithium in the electrode plate during normal use, a fine pattern is desirable.

Further, it is desirable that the formation region of the porous insulating layer occupies an area of 20 to 90% of the surface of the active material. Within this range, both high ionic conductivity and short-circuit safety are compatible. Here, if it is less than 20%, it becomes difficult to secure the insulating property, so that the short-circuit safety becomes insufficient, and if it is more than 90%, there is a high possibility that ion conduction is hindered and the charge / discharge characteristics are adversely affected.

The porous insulating layer is obtained by applying a slurry of an inorganic filler and a resin binder together with a solvent so that a formed region and a non-formed region are formed on the active material layer, and drying. For coating on the active material layer, for example, a continuous coating method such as gravure coating or die coating, a drawing method using an inkjet nozzle, a spray coating method, or the like can be used.

For example, when a gravure coat is used, a formed region and a non-formed region can be formed on the active material layer with a pattern corresponding to the shape and pattern of the engraved groove holding the slurry of the gravure roll. In the drawing method using an inkjet nozzle, a linear pattern can be drawn on the active material layer by arranging the nozzles at desired intervals and ejecting the slurry while scanning the electrode plate. Further, if the nozzle side is also scanned, a curved or zigzag linear pattern can be formed. Further, if the discharge is intermittent, a dot-shaped island pattern can be formed.

Further, it is not always necessary to apply the pattern, and it is possible to provide a fine formed region and a non-formed region. For example, the active material layer composed of the active material particles 7 and the binder, which will be described later, has an uneven surface. For example, when a porous insulating layer is formed as shown in FIG. 3, the surface thereof is formed. A fine formed region and a non-formed region derived from the unevenness can be provided. To do this, a slurry containing an inorganic filler, a resin binder, and a solvent is thinly applied onto the active material layer, and then the coating film is leveled by allowing it to stand, and a porous insulating layer is formed only in the recesses of the active material layer. It is formed on the convex portion. The formed region and the non-formed region thus obtained are preferable because they are very fine in the range of about 1 μm to 50 μm.

Inorganic fillers include oxides such as alumina, silica, and titanium oxide, as well as heat resistance such as silicon nitride, silicon carbide, and calcium carbonate, and are electrochemically stable within the range of use of lithium ion secondary batteries. Anything that is can be used. Further, this inorganic filler may be used by mixing or multi-layering a plurality of types. As the resin binder, a binder having heat resistance and electrolytic solution resistance is used, and among them, a rubber-like polymer containing, for example, an acrylonitrile unit, which has high heat resistance and rubber elasticity, is preferable. The porous insulating layer containing such a material as a binder has an advantage that it can be produced while maintaining a high yield because cracks and peeling do not occur when the positive and negative electrodes are wound around the positive and negative electrodes via a separator.

Although the thickness of this porous membrane layer is not particularly limited, the total with the combined separator thickness is about the same as the current separator specification (15 to 30 μm) from the viewpoint of maintaining the design capacity while demonstrating the utility of the porous membrane layer described above. That is, it is more preferably 0.5 to 20 μm.

For the positive electrode, lithium cobalt oxide and its modified product (such as co-crystallized aluminum and magnesium), lithium nickel oxide and its modified product (such as those in which nickel is partially substituted with cobalt) and lithium manganate are used as active materials. And particles such as composite oxides such as modified products thereof. Its particle size is about 5 μm to 30 μm. Binders include polytetrafluoroethylene (PTFE), modified acrylonitrile rubber particle binder combined with carboxymethyl cellulose (CMC), polyethylene oxide (PEO), and soluble modified acrylonitrile rubber, which have a thickening effect, and polyvinylidene fluoride (polyvinylidene fluoride). PVDF) and its variants are used. In addition, acetylene black, Ketjen black, various types of graphite, etc. are added as conductive agents. These positive electrode materials are applied onto a current collector as a mixture slurried with a solvent such as N-methylpyrrolidone (NMP), and after a drying and rolling process, a positive electrode plate having an active material layer is completed. .. The surface of the positive electrode active material layer thus obtained has an uneven shape derived from the shape of the active material particles, for example, as shown in FIG.

For the negative electrode, particles of various natural graphites, silicon-based composite materials such as artificial graphite and silicide, and particles of various alloy composition materials are used as active materials. The particle size is generally about 2 μm to 15 μm. As the binder, various binders such as PVDF and its modified product can be used. These negative electrode materials also undergo the same process as the positive electrode to become a negative electrode plate, and the surface of the active material layer thereof also has an uneven shape derived from the shape of the active material particles as shown in FIG. 3, for example, like the positive electrode.

For electrolyte, LiPF as salt₆And LiBF₄Various lithium compounds such as these can be used. Further, as a solvent, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) can be used alone or in combination. It is also possible to add vinylene carbonate (VC), cyclohexylbenzene (CHB), etc. in order to form a good film on the positive and negative electrodes and to guarantee stability during overcharging.

The separator is not particularly limited as long as it has a composition that can withstand the range of use of the lithium ion secondary battery, but a microporous film of an olefin resin such as polyethylene (PE) or polypropylene (PP) is used alone or in combination. It is generally used, and is preferable as an embodiment. Although the thickness of this separator is not particularly limited, the total with the combined porous film thickness is about the same as the current separator specification (15 to 30 μm), that is, from the viewpoint of maintaining the design capacity while exerting the utility of the porous film layer described above. More preferably, it is 10 to 25 μm.

If necessary, an inexpensive separator such as a non-woven fabric can be used. Further, it is preferable to use a separator having excellent heat resistance containing, for example, an aramid resin, because the safety is further improved.

In the present embodiment, FIG. 1 shows an example in which the insulating porous layer 4 is provided on the negative electrode active material layer, but the insulating porous layer may be provided on either the positive electrode or the negative electrode, or both. ..

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example 1) (Preparation of positive electrode) 3 kg of lithium cobalt oxide, 1 kg of PVDF # 1320 (N-methylpyrrolidone (NMP) solution having a solid content of 12% by weight) manufactured by Kureha Chemical Co., Ltd., 90 g of acetylene black and an appropriate amount of NMP. A positive electrode paste was prepared by stirring with a double-arm kneader. This paste was applied to a 15 μm thick aluminum foil, dried, rolled to a total thickness of 160 μm, and then slit to a width that could be inserted into a cylindrical 18650 case to obtain a positive electrode plate.

(Preparation of negative electrode) On the other hand, 3 kg of artificial graphite is combined with 75 g of styrene-butadiene copolymer rubber particle binder BM-400B (solid content 40% by weight), CMC 30 g and an appropriate amount of water manufactured by Nippon Zeon Corporation. The negative electrode paste was prepared by stirring with a kneader. This paste was applied to a copper foil having a thickness of 10 μm, dried, rolled to a total thickness of 180 μm, and then slit to a width that could be inserted into a cylindrical 18650 case to obtain a negative electrode plate.

(Preparation of Porous Insulation Layer) Double-armed kneading of 950 g of alumina with a median diameter of 0.3 μm together with 625 g of polyacrylonitrile-modified rubber binder BM-720H (solid content 8% by weight) manufactured by Nippon Zeon Corporation and an appropriate amount of NMP. The mixture was stirred with a combination machine to prepare a slurry for forming a porous insulating layer containing an inorganic filler and a resin binder.

This slurry was gravure-coated onto the negative electrode active material layer in a grid pattern as shown in FIG. 2 (b). The line width X was 200 μm, the line spacing Y was 300 μm, and the porous insulating layer forming region occupied 64% of the total negative electrode surface area. The thickness of the porous insulating layer was 6 μm. (Battery production) These positive and negative electrodes are wound around a 20 μm thick polyethylene microporous film as a separator, cut to a predetermined length, inserted into an electric tank can, and used as an EC / DMC / EMC mixed solvent. LiPF₆A cylindrical 18650 lithium-ion secondary battery having a design capacity of 2000 mAh was prepared by adding 5.5 g of an electrolytic solution in which 1 M and 3% by weight of VC were dissolved and sealing the battery. (Example 2) The gravure coat pattern was changed from that of Example 1, and the batteries of Example 2 were produced in the same manner as the others. The film thickness of the porous insulating layer was 6 um, the line width X of the lattice pattern was 100 μm, the line spacing Y was 400 μm, and the porous insulating layer forming region occupied 36% of the total negative electrode surface area. (Example 3) The gravure coat pattern was changed from that of Example 1, and the batteries of Example 3 were produced in the same manner as the others. The film thickness of the porous insulating layer was 6 um, the line width X of the lattice pattern was 300 μm, the line spacing Y was 200 μm, and the porous insulating layer forming region occupied 84% of the total negative electrode surface area.

(Example 4) The slurry for forming a porous insulating layer prepared in the same manner as in Example 1 was thinly applied onto the active material layer using a die coater on the negative electrode plate, and then allowed to stand to level the coating film. I let you. A porous insulating layer was formed in the recess of the active material layer of the negative electrode, and its thickness was 3 μm. A porous insulating layer was not formed on the convex portion of the active material layer, and the formed region occupied 60% of the total surface area of the negative electrode.

Batteries were produced in the same manner as in Example 1 in other steps. (Example 5) The slurry for forming a porous insulating layer prepared in the same manner as in Example 1 was linearly applied onto the negative electrode active material layer using inkjet printing as shown in FIG. 2 (b). The facing negative electrode plates were moved at 1.4 m / sec while discharging the slurry at 60 times / sec from the nozzles arranged at intervals of 800 μm of the inkjet head. Since the negative electrode moving speed is sufficiently small with respect to the discharge frequency, the discharged slurry is drawn as a continuous straight line on the negative electrode. The line width X was 500 μm, the line spacing Y was 300 μm, the porous insulating layer forming region was 63% of the total negative electrode surface area, and the thickness of the porous insulating layer was 4 μm.

A battery was produced in the same manner as in Example 1 except for the above. (Example 6) The negative electrode moving speed of Example 5 was changed to 2 m / sec, and the others were carried out in the same manner. Since the negative electrode moving speed is high with respect to the discharge frequency, the discharged slurry is drawn on the negative electrode in a discontinuous island shape as shown in FIG. 2 (a). The size of the drawn island was about 500 μm, the porous insulating layer forming region was 40% of the total negative electrode surface area, and the thickness of the porous insulating layer was 4 μm.

A battery was produced in the same manner as in Example 1 except for the above. (Example 7) The slurry for forming a porous insulating layer prepared in the same manner as in Example 1 was applied onto the negative electrode active material layer by spray coating. After thinly applying it on the active material layer, it was allowed to stand to level the coating film. A porous insulating layer was formed in the recess of the active material layer of the negative electrode, and its thickness was 2 μm. A porous insulating layer was not formed on the convex portion of the active material layer, and the formed region occupied 55% of the total surface area of the negative electrode.

A battery was produced in the same manner as in Example 1 except for the above. (Comparative Example 1) Comparative Example 1 is a battery prepared by producing a positive electrode and a negative electrode according to Example 1 and without forming a porous insulating layer. (Comparative Example 2) A battery manufactured by the same method as in Example 1 in which the line width X of the lattice pattern is 50 μm and the line spacing Y is 450 μm so that the formation region of the porous insulating layer is 19%. Was designated as Comparative Example 2. (Comparative Example 3) A battery was manufactured by the same method as in Example 1 except that the line width X of the lattice pattern was 350 μm and the line spacing Y was 150 μm so that the formation region of the porous insulating layer was 91%. Was designated as Comparative Example 3.

These batteries were evaluated by the methods shown below. The results are shown in Table 1 together with the configuration conditions.

<tables num="1"><img file="JP2005174792A_D0001.tif" /></tables>

(Battery charge / discharge characteristics) The completed battery was run-in and discharged twice, stored for 7 days in a 45 ° C environment, and then the following two charge / discharge tests were performed in a 20 ° C environment.

(1) Perform constant current charging with a charging current of 1400mA until the charging voltage reaches 4.2V, and then perform constant voltage charging with that 4.2V until the charging current reaches 100mA. After that, a constant current discharge is performed with a discharge current of 400 mA and a discharge end voltage of 3 V.

(2) Perform constant current charging with a charging current of 1400mA until the charging voltage reaches 4.2V, and then perform constant voltage charging with that 4.2V until the charging current reaches 100mA. After that, the discharge current is 4000 mA, the discharge end voltage is set to 3 V, and constant current discharge is performed.

Table 1 shows the charge / discharge capacity at this time. (Nail piercing safety) For the battery after battery charge / discharge characteristics evaluation, first perform constant current charging with a charging current of 1400mA until the charging voltage reaches 4.25V under a 20 ° C environment, and leave it at 4.25V. Constant voltage charging was performed until the charging current reached 100 mA.

For the charged battery, we observed the heat generation state when a 2.7 mm diameter iron round nail was passed through at a speed of 5 mm / sec in a 20 ° C environment. Table 1 shows the temperatures reached after 1 second and 90 seconds in the vicinity of the penetration point of this battery.

The evaluation results are described below in order.

In the nail piercing test, it can be seen that the overheating of Comparative Example 1 in which the porous membrane insulating layer does not exist is remarkable, whereas the overheating of the batteries of Examples 1 to 7 is significantly suppressed after the nail piercing. .. When the batteries of Examples were disassembled and examined after the test, a porous insulating layer was present on the active material layer in the same manner as before the test in all the batteries, and the melting of the separator remained in a slight range. Was there. From this, it is considered that the porous insulating layer did not shrink even when the heat was generated by the nail piercing short circuit, and the expansion of the short circuited portion could be suppressed, so that a large overheating could be prevented.

In the battery of Comparative Example 2, the temperature rise 90 seconds after nailing is large as compared with the Example, and the safety effect is not sufficient. It is considered that this is because the short-circuit current could not be completely blocked because the formed region of the porous membrane insulating layer was as small as 19%.

Further, in the battery of Comparative Example 3, the safety effect was sufficient as in the Example from the result of the nail piercing test, but the capacity at the time of discharging 4000 mA was reduced. It is considered that this is because the formation region of the porous membrane insulating layer is as large as 91%, so that the ionic conductivity is lowered.

The lithium ion secondary battery of the present invention is useful as a portable power source or the like having excellent safety.

<figref num="1">(a) Cross-sectional view schematically showing the configuration of the lithium ion secondary battery of the present invention (b) Enlarged view of FIG. 1 (a)</figref><figref num="2">(A) Schematic diagram for explaining the island pattern of the porous insulating layer of the present invention (b) Schematic diagram for explaining the lattice pattern of the porous insulating layer of the present invention (c) Line of the porous insulating layer of the present invention Schematic diagram for explaining the pattern (d) Schematic diagram for explaining the irregular island pattern of the porous insulating layer of the present invention.</figref><figref num="3">A cross-sectional view schematically showing the vicinity of the electrode plate surface of the lithium ion secondary battery of the present invention.</figref>

Code description

1 Positive electrode 1a Positive electrode active material layer 2 Negative electrode 2a Negative electrode active material layer 3 Separator 4 Porous membrane insulating layer 5 Porous membrane insulating layer forming region 6 Porous membrane insulating layer non-forming region 7 Active material particles