JP4529436B2 - Electrode plate for lithium ion secondary battery and lithium ion secondary battery - Google Patents

Description

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

  In a chemical battery such as a lithium ion secondary battery, there is a separator between a positive electrode and a negative electrode that electrically insulates each electrode plate and further holds an electrolytic solution. In lithium ion secondary batteries, microporous films mainly made of polyethylene or polypropylene are currently used.

  However, film separators made of these resins generally tend to shrink at high temperatures. Therefore, when an internal short circuit or a sharply shaped protrusion such as a nail penetrates the battery, the short circuit reaction heat that occurs instantaneously causes the separator to contract and the short circuit part to expand, generating a greater amount of reaction heat. , Had the problem of promoting abnormal overheating.

  Therefore, in order to improve safety including the above problems, a technique for applying and forming a porous coating film containing solid fine particles on the surface of the active material layer has been proposed (see Patent Document 1).

Moreover, although the structure differs, the technique which forms a porous insulating film in the negative electrode surface by sputtering is proposed (refer patent document 2).
Japanese Patent No. 3371301 JP-A-6-36800

  However, in the conventional technique, a porous protective film is formed by applying a mixture of a resin binder, solid fine particles, and a solvent on the electrode plate, so that the porous film is formed thin (for example, 3 μm or less). It was difficult. The thickness is at least about 5 μm, and depending on the thickness, the battery capacity may be reduced, or the ion conductivity may be lowered to deteriorate the charge / discharge characteristics.

  As for the technique for forming the porous insulating film on the negative electrode surface by sputtering as in Patent Document 2, the porous insulating film is formed on the surface of the pure lithium or lithium alloy negative electrode, and the thickness is preferably 5 to 100 nm. Has been.

  However, in the negative electrode active material layer composed of the carbon-based negative electrode active material particles and the binder that are currently most widely used as the negative electrode plate of the lithium ion secondary battery, the average particle diameter of the active material particles is, for example, A surface of about 20 μm is used, and the surface of the negative electrode active material layer has irregularities on the order of several μm. Even when an attempt was made to form a porous insulating film having a thickness of 5 to 100 nm on the surface of the negative electrode active material layer having such irregularities by sputtering or the like, it was impossible to cover the negative electrode surface.

  Moreover, even if the insulating film can be partially formed, it is difficult to ensure safety in the case of internal short circuit or nail penetration, which is the subject of the present invention, with a thickness of 5 to 100 nm.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a lithium ion secondary battery having high capacity, high characteristics, and excellent internal short circuit and nail penetration safety.

  In order to solve the above problems, in the lithium ion secondary battery of the present invention, a positive electrode plate including a positive electrode active material layer containing lithium composite oxide particles having a particle size of 1 to 40 μm, or a particle size of 1 to 40 μm. A porous insulating layer having a thickness of 0.3 μm to 3 μm is deposited on at least one of the active material layers of the negative electrode plate including the negative electrode active material layer containing the negative electrode active material particles.

  By setting the porous insulating layer in the above thickness range, an insulating layer can be formed even on the uneven surface of the active material layer made of solid particles, which can improve the safety in internal short circuit and nail penetration tests. it can. Moreover, if it is the thickness of the said range, battery capacity will not decrease or charging / discharging characteristics will not be deteriorated.

  In the said structure, it is preferable to interpose a separator between positive-negative electrode plates. Safety can be further enhanced by the combined use of the porous insulating layer and the separator.

  In particular, the separator is more preferably provided with a polyolefin microporous film. Since the polyolefin-based microporous film is closed at a high temperature (so-called shutdown), a battery with higher safety is obtained.

  The porous insulating layer is desirably deposited by any one of sputtering, ion plating, and CVD (chemical vapor deposition). In order to form an insulating layer having a thickness of 0.3 to 3 μm, these methods are excellent in film thickness controllability and productivity.

The porous insulating layer is an island-shaped film having micropores . Ionic conductivity increases, and charge / discharge characteristics tend to be improved.

  The material for the porous insulating layer is preferably a substance selected from alumina, silica, and titanium oxide. These substances can be made into a thin film by the above-described film forming method and have excellent insulating properties. It is also suitable for mass production because of its low cost.

  In the present invention, the presence of the porous insulating layer on the active material layer described above improves safety in internal short-circuiting and nail penetration tests. When there is no porous insulating layer, when a hole is opened in the separator due to foreign matter or the like and the positive and negative electrodes are short-circuited, an excessive current flows to the short-circuit point, and Joule heat may be generated. In that case, the heat causes the separator around the short-circuit point to melt or shrink, expanding the hole, further expanding the short-circuit area, and continuing to generate Joule heat. May cause deformation.

  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 is present even if the separator is melted or contracted to enlarge the hole. The short-circuit area does not increase. Therefore, the generation of Joule heat does not expand and does not lead to abnormal heat generation. In addition, since the temperature in the vicinity of the short circuit point instantaneously reaches 500 ° C., the positive electrode current collector made of aluminum is melted and the short circuit between the positive and negative electrodes is eliminated. Even if the porous film insulating layer is thin and the short-circuit current cannot be completely cut off, the short-circuit current can be reduced, and the generation of Joule heat can be suppressed and the temperature rise can be prevented.

  Due to the above effects, the lithium ion secondary battery of the present invention is excellent in safety in an internal short circuit or nail penetration test, and becomes a battery with high characteristics.

  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 penetration test, a high capacity, and excellent charge / discharge characteristics and heat resistance.

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

  FIG. 1 is a cross-sectional view schematically showing the configuration of a lithium ion secondary battery of the present invention, and FIG. 2 is an enlarged cross-sectional view of a part of the electrode plate surface.

  As shown in FIG. 1, in the lithium ion secondary battery of the present invention, a separator 3 is interposed between the positive electrode 1 and the negative electrode 2, and on either the surface of the positive electrode active material layer 1a or the negative electrode active material layer 2a, A porous insulating layer 4 is deposited. As will be described later, since the active material layer is composed of the active material particles 5 and the binder, the surface thereof is uneven. When the insulating film is deposited thinly as shown in FIG. A porous insulating layer 4 having a large amount of 6 can be obtained.

  The thickness of the insulating film is preferably 0.3 to 3 μm so as to be porous so as to cover the surface of the active material layer and serve as an insulating layer and to conduct ions. If it is less than 0.3 μm, it is too thin with respect to the irregularities on the surface of the active material layer, so that insulation cannot be obtained. If it is thicker than 3 μm, the insulation is sufficient, but the film is deposited by tracing the unevenness, and the micropores are small and few, or are blocked, so that the ion conductivity cannot be ensured.

  As a method of depositing the porous film insulating layer 4, a conventionally known thin film forming process can be used. In order to deposit a 0.3 to 3 μm insulating layer with high accuracy, it is particularly preferable to use any one of a sputtering method, an ion plating method, and a CVD method. In any of the methods, the active material layer may be damaged at a high temperature of 200 ° C. or higher, so that the film formation at a low temperature is necessary. In addition, an appropriate film forming method and method can be selected depending on productivity, positive and negative electrode plate shapes serving as base materials, and film forming materials. For example, in the case of a sputtering method, for example, RF sputtering, DC magnetron sputtering, ECR sputtering, a counter target method, or the like can be applied, and reactive sputtering, oxygen-assisted sputtering, or the like can be appropriately used depending on the material.

  As the porous insulating layer, a material that does not melt or change its shape at about 500 ° C. is used. In addition to inorganic oxides such as alumina, silica, and titanium oxide, ceramic materials such as silicon nitride, TiN, and SiC are also used. it can.

  For the positive electrode, lithium cobaltate and its modified products (such as those obtained by eutectic aluminum and magnesium), lithium nickelate and its modified products (such as those in which nickel is partially substituted with cobalt), and lithium manganate as active materials And particles of complex oxides such as modified products thereof. The particle diameter is about 1 μm to 40 μm. More preferably, it is about 5-20 micrometers. As the binder, a combination of polytetrafluoroethylene (PTFE) / modified acrylonitrile rubber particle binder with carboxymethyl cellulose (CMC) / polyethylene oxide (PEO) / soluble modified acrylonitrile rubber having a thickening effect, polyvinylidene fluoride ( PVDF) and modified products thereof are used. Further, acetylene black, ketjen black, various graphites and the like are added as a conductive agent.

  These positive electrode materials are applied onto a current collector as a slurry mixed with a solvent such as N-methylpyrrolidone (NMP), and after drying and rolling, a positive electrode plate having an active material layer is completed. . The surface of the positive electrode active material layer obtained in this way 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 graphite, silicon-based composite materials such as artificial graphite and silicide, and various alloy composition materials are used as the active material. The particle diameter is generally about 1 μm to 40 μm. More preferably, about 5 to 30 μm is used. Various binders such as PVDF and its modification can be used as the binder.

  These negative electrode materials also undergo a process similar to that of the positive electrode to become a negative electrode plate, and the surface of the active material layer has an uneven shape derived from the shape of the active material particles as in the positive electrode.

For the electrolytic solution, it is possible to use various lithium compounds such as LiPF ₆ and LiBF ₄ as a salt. Further, ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) can be used alone or in combination as a solvent. In addition, vinylene carbonate (VC), cyclohexylbenzene (CHB), and modified products thereof can be used in order to form a good film on the positive and negative electrodes and to ensure stability during overcharge.

  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 / polypropylene is generally used singly or in combination. Yes, and preferred as an embodiment. Although the thickness of the separator is not particularly limited, from the viewpoint of maintaining the design capacity while demonstrating the effect of the porous film layer described above, the total sum of the combined porous film thickness is about the same as the current separator specification (15 to 30 μm), that is, More preferably, it is 10-25 micrometers.

  Note that an inexpensive separator such as a nonwoven fabric can be used as necessary. For example, it is preferable to use a separator having an excellent heat resistance containing an aramid resin or the like because the safety is further improved.

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

  Hereinafter, the present invention will be described more specifically with reference to examples.

Example 1
(Preparation of positive electrode)
As active material particles, 3 kg of lithium cobaltate powder is combined with 1 kg of PVDF # 1320 (N-methylpyrrolidone (NMP) solution having a solid content of 12% by weight), 90 g of acetylene black and an appropriate amount of NMP. The mixture was stirred with a kneader to prepare a positive electrode paste. The particle sizes of the lithium cobalt oxide powder were d50 = 7.4 μm, d10 = 5.2 μm, and d90 = 9.5 μm. This paste was applied to and dried on a 15 μm thick aluminum foil, rolled to a total thickness of 160 μm, and then slit into a width that could be inserted into a cylindrical mold 18650 to obtain a positive electrode plate.

(Preparation of negative electrode)
On the other hand, 3 kg of artificial graphite as negative electrode active material particles, a double-arm type together with 75 g of styrene-butadiene copolymer rubber particle binder BM-400B (solid content 40 wt%) manufactured by Nippon Zeon Co., Ltd., 30 g of CMC and an appropriate amount of water. The mixture was stirred with a kneader to prepare a negative electrode paste. The particle diameter of the artificial graphite was d50 = 17.5 μm, d10 = 6.5 μm, d90 = 34 μm. This paste was applied to and dried on a 10 μm thick copper foil, rolled to a total thickness of 180 μm, and then slit to a width that could be inserted into a cylindrical mold 18650 to obtain a negative electrode plate.

(Preparation of porous insulation layer)
A porous insulating layer was deposited on the negative electrode active material layer as shown in FIG. 1 or 2 using a commercially available scraping film forming apparatus. Using an alumina sintered material as a target, RF sputtering film formation was performed at an RF power of 3 kW / cm ² and an Ar gas pressure of 1 mm Torr, and a porous insulating layer made of alumina having a thickness of 1 μm was deposited.

(Preparation of battery) These positive and negative electrodes were wound with a 20 μm thick polyethylene microporous film as a separator, cut into a predetermined length, inserted into a battery case, and mixed with an EC / DMC / EMC mixed solvent. An electrolytic solution in which 1M LiPF ₆ and 3% by weight of VC were dissolved was added and sealed to prepare a cylindrical 18650 lithium ion secondary battery having a design capacity of 2000 mAh.

(Examples 2 and 3)
As shown in Table 1, the batteries of Examples 2 and 3 were fabricated in the same manner as in Example 1 except that the alumina film thickness was 0.3 μm and 3 μm.

(Example 4)
A titanium oxide film was deposited by ion plating. A porous insulating layer made of titanium oxide having a thickness of 1 μm was deposited on the negative electrode active material layer by arc ion plating using titanium as an evaporation source and oxygen introduced. A battery was fabricated in the same manner as in Example 1 except for the above.

(Example 5)
A silica film was deposited by CVD. A porous insulating layer made of silica having a film thickness of 1 μm was deposited on the positive electrode active material layer by a plasma-assisted CVD method using tetraethyl orthosilicate (TEOS) as a material. A battery was fabricated in the same manner as in Example 1 except for the above.

(Comparative Example 1)
A positive electrode and a negative electrode manufactured according to Example 1 and a battery manufactured without forming a porous insulating layer is referred to as Comparative Example 1.

(Comparative Examples 2 and 3)
As shown in Table 1, comparative examples 2 and 3 were prepared by forming batteries with an alumina film thickness of 0.1 μm and 5 μm, respectively, and producing the batteries by the same method as in Example 1.

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

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

  (1) The constant current charge is performed until the charge voltage reaches 4.2V with the charge current of 1400 mA, and the constant voltage charge is performed until the charge current reaches 100 mA while maintaining the 4.2V. Thereafter, constant current discharge is performed with a discharge current of 400 mA and an end-of-discharge voltage of 3V.

  (2) Constant current charging is performed until the charging voltage reaches 4.2 V at a charging current of 1400 mA, and constant voltage charging is performed until the charging current reaches 100 mA while maintaining 4.2 V. Thereafter, constant current discharge is performed with a discharge current of 4000 mA and a discharge end voltage of 3V.

  The charge / discharge capacity at this time is shown in Table 1.

(Nail penetration safety)
For the battery after the battery charge / discharge characteristics evaluation, under a 20 ° C. environment, first, constant current charging was performed at a charging current of 1400 mA until the charging voltage reached 4.25 V, and the charging current was kept at 4.25 V to 100 mA. Constant voltage charging was performed until

  Regarding the battery after charging, a heat generation state was observed when a 2.7 mm diameter iron round nail was penetrated at a speed of 5 mm / second in a 20 ° C. environment. Table 1 shows the temperatures reached after 1 second and 90 seconds in the vicinity of the penetration portion of the battery.

  Hereinafter, the evaluation results will be described in order.

  In the nail penetration test, the overheating of Comparative Example 1 in which no porous membrane insulating layer is present is significant, whereas in the batteries of Examples 1 to 5, the overheating after the nail penetration is significantly suppressed. . When the batteries of the examples were disassembled and examined after the test, the porous insulating layer was present on the active material layer in the same manner as before the test, and the melting of the separator remained in a slight range. It was. From this, it is considered that the porous insulating layer did not contract even in the heat generated by the nail penetration short-circuit, and the expansion of the short-circuited portion could be suppressed, so that it was possible to prevent significant overheating.

  In the battery of Comparative Example 2, the temperature rise 90 seconds after the nail penetration is large, and the safety effect is not sufficient. This is probably because the short-circuit current could not be prevented because the porous insulating layer was as thin as 0.1 μm.

  Further, in the battery of Comparative Example 3, the safety effect was sufficient from the result of the nail penetration test as in the Example, but the capacity at the time of 4000 mA discharge was lowered. This is considered to be because the porous film insulating layer is as thick as 5 μm, so that the number of pores is reduced and the ion conductivity is lowered.

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

Sectional drawing which showed the structure of the lithium ion secondary battery of this invention typically Sectional drawing which expanded a part of electrode plate surface shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 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 Active material particle 6 Micropore

Claims (4)

Lithium ions having an active material layer containing active material particles having a particle diameter of 1 to 40 μm, a porous insulating layer is deposited on the surface of the active material layer, and the thickness of the porous insulating layer is 0.3 μm to 3 μm An electrode plate for a secondary battery ,
The electrode for a lithium ion secondary battery, wherein the porous insulating layer is a film having an island structure deposited by any one of a sputtering method, an ion plating method, and a CVD method and having micropores Board . 2. The electrode plate for a lithium ion secondary battery according to claim 1 , wherein the porous insulating layer is made of only a material selected from alumina, silica, and titanium oxide. A positive electrode plate including a positive electrode active material layer including lithium composite oxide particles; a negative electrode plate including a negative electrode active material layer including negative electrode active material particles; a separator interposed between the positive and negative electrode plates; and a nonaqueous solvent. A lithium ion secondary battery comprising an electrolyte solution, wherein the positive electrode plate or the negative electrode plate is an electrode plate for a lithium ion secondary battery according to claim 1 or 2. battery. The lithium ion secondary battery according to claim 3, wherein the separator is a polyolefin microporous film.

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